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Life cycle engineering methodology applied to material selection, a fender case study
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Journal of Cleaner Production 16 (2008) 1887e1899 www.elsevier.com/locate/jclepro
Life cycle engineering methodology applied to material selection, a fender case study Ineˆs Ribeiro a,1, Paulo Pec¸as a,*, Arlindo Silva b,2, Elsa Henriques a,3 a b
IDMEC, Instituto Superior Te´cnico, TULisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal ICEMS, Instituto Superior Te´cnico, TULisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Received 8 January 2008; received in revised form 14 January 2008; accepted 15 January 2008 Available online 4 March 2008
Abstract Materials selection is a multidisciplinary activity, which integrates a large number of knowledge fields and professional domains. In fact a material selection decision should capture not only the functional performance required for the application but should also consider the economical and environmental impacts originated all along the product life cycle. In this paper a life cycle engineering (LCE) approach is proposed to support material selection, integrating the performance of the material for the specific application in technological, environmental and economical dimensions throughout the duration of the product. The methodology proposed compares a set of candidate materials and, through the aggregation of the three dimensions (technical, economical and environmental), identifies the ‘‘best material domains’’. These ‘‘best material domains’’ are presented in a ternary diagram, which allows a global comparison of the candidate materials and supports an informed decision as regards the selection of the ‘‘best material’’ according to different business scenarios and corporate strategies. The methodology was applied to a case study aiming the use of new metallic materials (high strength steels and aluminium alloys) for an automobile fender currently made of mild steel and the evaluation of potential benefits as regards the global performance of the material. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Life cycle engineering; Life cycle cost; Life cycle assessment; Materials selection; Automobile fender
1. Introduction Nowadays, to meet market needs, the development of a product cannot be focused only on the classic approaches, such as technical and economic performances [1]. In the last decade, environmental problems have emerged as an important public concern. Consequently, strategies and development methods to promote products as ecological as possible have been incorporated in product design. In this context, to have a broader view of the product’s environmental impacts, an innovative approach which extends to the end of the product’s useful * Corresponding author. Tel.: þ351 21 8417316/9573; fax: þ351 21 8419058. E-mail addresses: [email protected] (I. Ribeiro), [email protected] (P. Pec¸as), [email protected] (A. Silva), [email protected] (E. Henriques). 1 Tel.: þ351 21 8417316; fax: þ351 21 8419058. 2 Tel.: þ351 21 8417723/342; fax: þ351 21 8474045. 3 Tel.: þ351 21 8417316/556; fax: þ351 21 8419058. 0959-6526/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2008.01.002
life and retirement have been developed, in opposition to the conventional approach, where the product’s sale is considered as the final analytical step [2]. Life cycle engineering (LCE) emerged in response to the need to develop life cycles causing the lowest possible environmental impacts, while still offering economic viability. LCE refers to ‘‘Engineering activities which include: the application of technological and scientific principles to the design and manufacture of products, with the goal of protecting the environment and conserving resources, while encouraging economic progress, keeping in mind the need for sustainability, and at the same time optimizing the product life cycle and minimizing pollution and waste’’ [3]. Therefore, LCE can be defined as a decision making methodology that considers performance, environmental, and cost dimensions throughout the duration of a product, guiding design engineers towards informed decisions [4,5]. LCE differs from other life cycle methodologies in this point; while LCE incorporates these three dimensions of analysis, life cycle
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assessment (LCA) covers environmental aspects, life cycle cost (LCC) covers economic aspects and life cycle management (LCM) includes economic and environmental aspects. Life cycle engineering includes not only conventional tools, as technical performance analysis based on mechanical, electrical, and chemical properties [1], but also life cycle tools to analyse economic performance (LCC) and environmental performance (LCA). LCC and LCA were chosen from a large number of available tools for their suitability for product assessment [6]. Several authors have applied LCE to different case studies in automotive [7,8], construction [9], and computer industry [10,11], to name only a few, targeting the individual evaluation of technical, economic and environmental aspects of products or systems. Life cycle cost generally refers to ‘‘all the costs associated with a product throughout the product’s life’’ [12]. Its objective is to cover the assessments of costs in all steps of the product’s life cycle, including the costs that are not normally expressed in the product market price [13], such as costs incurred during the usage and disposal. LCC is essentially an evaluation tool in the sense that it gets on to important metrics for choosing the most cost-effective solution from a series of alternatives [12]. Life cycle assessment is a structured method to quantify potential environmental impacts of products or services over their full life cycle [14,15]. Although LCA had emerged in the early 1970s, its methodological basis at that time was chaotic and conflicting results were produced. As an attempt to organize and harmonize LCA developments, SETAC (1980s), ISO (1990s) and UNEP/SETAC (1990s) established a terminology, a standard methodology [16e19] and, as LCA is still a field of innovation and discovery, became a privileged space to submit developments for discussion [14]. Presently, LCA consists of four steps: definition of the goal and scope of the study, construction of the product life cycle model with all environmental inflows and outflows (life cycle inventory stage e LCI), evaluation of the environmental relevance of all the inflows and outflows (life cycle impacts assessment stage e LCIA) and, finally, the interpretation of the results [20]. There are several methods for LCIA stage compatible with ISO requirements [15] and therefore, most experts prefer to select a published method instead of developing a new one [20]. Material selection is an important application area of LCE. As material selection is part of product design, decisions taken during this stage largely influence the product’s costs and environmental impacts for its entire life cycle [21]. When selecting a material for a specific set of functionalities the relevant engineering properties of the material are identified and correlated to the design requirements. Normally, the selection is carried out considering the values of such properties altogether with economic considerations. For example, in mechanical design mechanical properties are the most important for material selection [22], but the influence of the selected material on the product’s final cost must be controlled in order to get a viable design solution as regards both technical performance and economical suitability. Therefore, material selection can
be regarded as a multi-objective problem, being the optimal selection and the best match found between the available materials profiles and the requirements of the design [22,23]. In fact, according to Field et al. ‘‘the four main factors upon which the designers rely when considering materials choice are the relationship between materials specifications and technical performance of the product, the economic performance of the product, the environmental performance of the product and the practice of industrial design embedded in the product and its functionality’’ [24]. Certainly this is not an easy task and in practice material selection methods frequently disregard economic and environmental aspects or at least only consider them at late stages of product development. Material properties charts are probably the most common and visual way of selecting materials for a given application [25]. This method allows for the selection of a material, or set of candidate materials, by comparing two engineering properties at a time. A certain twist of the method can make it possible to compare indirectly four properties at a time. The method also performs a process selection, knowing in advance the material to be used and the shape of the part. Cost is not a very realistic variable in this method. In fact, even knowing that it is fundamental to evaluate the impact of the material in the product final cost, only a relative cost of each raw material is considered in the selection. As the relative cost of raw materials is only a parcel of such impact, the effect of materials, in a certain production volume, on manufacturing process and on its cost is naturally neglected. Bearing in mind that materials selection regardless of the objective is at its basis a decision making process [26], another way of selecting materials is by using decision matrices. Several approaches are available. Some are qualitative [27] and some are quantitative [28]. They normally use some kind of weighting to account for some criteria being more important than others, but they are very similar in all aspects. Value analysis is another method available to select materials [29]. Once again, this is a general method, used for decision making. It is usually employed with a whole product in mind, not just a part of it, since the main objective is to enhance the value of the product to the customer. The method works with a design already in place, so it is of little use when the decision of what material to use is done when there is still no completely defined design. The outputs of the method are recommendations for modifications to the product e possibly materials substitution. These approaches do not fully capture the entire materials selection framework, because no one encompasses all the technological, economical, environmental, and current practice details needed to make an accurate and robust selection as mentioned by Field et al. [24]. The life cycle engineering approach proposed in this paper aims to include in the same framework the dimensions of technology, economy, environment and current practice in a single decision making tool to support materials comparison and achieve an informed selection. The assessment of economical, technical and environmental criteria is made by gathering information from different dimensions of analysis into a global
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evaluation. The global evaluation presented, unlike the classic approach of LCE studies, illustrates the best material choices for different scenarios, that is, for different practices and even corporate strategies.
rough assumptions are normally made. The influence of such inaccurate assumptions in the final result is not always clear. To study the robustness of the final result, the ternary materials selection diagrams can easily handle any sensitivity analysis.
2. LCE model
2.1. Technical evaluation
The first stage of the LCE model proposed in this study (Fig. 1) is to define the boundaries of the problem under analysis and to collect specific data for material application and product life cycle. The next step is to evaluate individually the product from an economic, environmental and technical point of view. These evaluations use distinctive methods. Economical and environmental evaluations are performed from a life cycle perspective, using LCC and LCA, respectively. Technical evaluation is performed using a conventional approach based on decision matrices, with the materials analysed based on their properties. For each material and for each dimension of evaluation (technical, economic, and environmental) a single indicator is obtained, allowing the direct incorporation of the technical, economical and environmental performances into a multi-criteria decision problem. The final result is a global evaluation, presented in a ternary diagram, clearly showing the possible choices according to the importance given to the three dimensions of analysis. This ternary materials selection diagram illustrates the ‘‘best materials’’ for different criteria weights. In fact the ternary diagrams identify not only the best materials according to a set of weights attributed to technical, economic, and environmental dimensions, but also the domain (range of weights) of each ‘‘best material’’. Within this approach the design engineer overcomes the difficult task related to the materialization of the relative importance of the three dimensions into a set of weights. In addition, considering that materials selection (and particularly data collection around the product specific conditions) takes place in a preliminary design phase, several
Within the LCE model applied to material selection, the technical evaluation refers to material properties and their contribution to the technical requirements of the product. Fig. 2 outlines the steps of this evaluation, resulting in a score for each candidate material under evaluation, which is a relative quantification of the appropriateness of each material for the technical performance of the product. The first step is to identify the product requirements, Ri, and their relative importance, wRi, for the product performance (requirements weights). The ability of a specific material to fulfil the requirements depends on its properties. Therefore, it is necessary to determine the relevant properties and correlate them with the requirements. So, the material properties are related to the requirements through a matrix of indices, W, where each index Wij reflects the contribution of the property Pj to accomplish the requirement Ri. Most of these indices are zero, meaning that there is no correlation between the property and the requirement, but the sum of the indexes in eachP line of the W matrix P have to yield the same number, i.e., W ¼ X ¼ W2j ¼ 1j P W3j ¼ ., which means that the value of X must be preset and distributed through one or several properties correlated with the requirement. The absolute importance, aPj, of each material property Pj can then be calculated: aPj ¼ W1j $wR1 þ W2j $wR2 þ .. þ Wnj $wRn
ð1Þ
Finally, the material property weight (the relative importance) will be: wPj ¼
aPj aP1 þ aP2 þ . þ aPj þ . þ aPm
ð2Þ
Taking now these material property weights, all the required information to compare the technical performance of a set of materials is present. The final step is the construction of a global comparison table in which each material with its quantified properties is ranked against all the others. Since each material property has its own value and units, the properties must be adimensionalized. The adimensional properties are then weighted according to the material property weight, wPj. Finally, adding all the weighted adimensional material properties, a weighted material index, wMIk, is achieved for each material. The potentially ‘‘best material’’ will be the one with the highest wMI. 2.2. Economic evaluation (LCC model)
Fig. 1. Overview of the life cycle engineering model for materials selection.
Life cycle cost model (Fig. 3) allows the assessment of the product’s life cycle cost. The first step of the model is to gather information from all processes during the entire life cycle stages. With data related to the resources consumption along all these stages it is possible to quantify the different cost
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Fig. 2. Technical evaluation methodology.
items involved, from labour and machine use to materials and energy spending, and correlate them to each life cycle stage. In fact, along product life several costs are associated to different entities. For example, production costs are only supported by the industry in question, while the in-use costs are supported by the customers/users. Therefore, if the LCC analysis is being performed by the industry to support a material selection decision, these values cannot be assumed as having the same importance. For the industry, a reduction in the production cost is normally perceived as more important than a reduction of the in-use costs. So, the adimensionalization and weighting of the costs must be done. To define such weighting system is not easy, as it depends on the companies’ strategy and involves subjective and controversial issues. The weights applied in this study are a weighting system used in other similar study [8], in which the weights are applied according to the product life
cycle stages: material acquisition e 2, production e 6.5, use e 0.75, and end-of-life e 0.75. The comparisons between materials are performed considering as reference the material that results in a product with a lower life cycle cost, having this material a 100% score. 2.3. Environmental evaluation (LCA model) At this stage an environmental impact assessment of the several candidate materials is performed, using all data previously collected for the product’s specific conditions. A cradle to grave approach is used, according to LCA standards [16e19]. The methodology for the impact assessment proposed (Fig. 4), considers 11 environmental impact categories, in the following three areas: Human Health (HH), Ecosystem Quality (EQ) and Resources (R). The methodology aggregates all the
Fig. 3. LCC model.
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Fig. 4. Eco-Indicator 99 methodology.
emissions and resources consumption from the life cycle into these impact categories and, afterwards, weights the scores into a single value, called the ‘‘eco-indicator 99’’ (EI 99) [30]. The weighting coefficients were applied according to the hierarchic/average (H/A) perspective, which is a moderate perspective generally accepted by the scientific community, attributing 40e40e20% of weight to the three considered impact areas, HHeEQeR, respectively [31]. These areas have specific separated units. Disability-Adjusted Life Years (DALY) is the health indicator, Potentially Disappeared Fraction multiplied by an area unit, m2, and a time unit, year (PDF m2 year) translates the damage to ecosystems and Mega Joule (MJ) surplus is the measure of resource depletion. More detailed information can be found elsewhere [16e19,30]. Fig. 5. LCE model application.
3. LCE model application In this part, the application of the life cycle engineering model previously presented is exemplified through a case study. The global objective is to support the materials selection decision for an automobile front fender, considering not only the technical performance, but also economical issues and environmental implications (Fig. 5). 3.1. Fender specific conditions The fender is already in production in an annual production volume of 100 000 units (assembled in 50 000 vehicles), so it is considered that any new material selected should modify the automobile aesthetics or the fender assembly in the car body system. To analyse materials for an automobile fender using an LCE model, it is necessary to pre-select some candidate materials. This pre-selection was made under some considerations. Only metallic materials were considered for the sake of simplicity. Other material classes can also be considered,
maintaining exactly the same conditions of the LCE model. The fender is already in the market so the mild steel presently used was included. The two other steels chosen are ultra high strength ones. Three aluminium alloys commonly used in automotive external body parts in Europe, United States and Japan [32] were also included in the analysis (Table 1). 3.1.1. Data origin The materials in focus have different properties, which might result in different design features. However, as the fender assembly system and aesthetics are frozen, the only design feature that is allowed to change with different materials is the fender thickness. Because there are no standards or formal rules that an automobile fender is obliged to satisfy, to perform the quick estimation of the fender thickness the current mild steel fender was used as a benchmark against which all the other candidate materials are compared. So a simple structural analysis and frequency analysis were performed to guarantee that the fenders of all the candidate materials
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Table 1 Candidate materials and relevant design features Materials
Material label
Thickness [mm]
Surface area [m2]
Weight [kg]
HX220YD þ z100MCO DOCOL 600DP DOCOL 1000DP Al 6010-T4 Al 2036-T4 GZ45/30-30
St-1 St-2 St-3 Al-1 Al-2 Al-3
0.65 0.50 0.35 1.00 0.90 1.00
0.34
1.73 1.33 0.93 0.92 0.84 0.93
Note: weight ¼ density (surface area thickness).
have an equivalent level of strengths and strains when subjected to an equal load, and have natural frequencies far enough from the most relevant exciting ones. The results achieved for the fender’s thicknesses are presented in Table 1. In order to assess input and output streams of material production for the several materials, the SimaPro7 software was used. SimaPro7 uses LCA concepts and methodologies [33], incorporating an extensive database for materials production inflows and outflows. The SimaPro7 database was used with some adaptations to this case study. Nowadays recycling metals is an important issue and most metals are recycled. The recycled materials provide an improvement on energy consumption during material production when compared with primary material (using only raw material). However, especially in the aluminium case, the existent scraps in the market are not sufficient for the current material demands. Therefore, in the present case study, different recycled material rates were considered: 30 and 70% for aluminium and steel, respectively [34,35]. The costs incurred during the annual production were calculated using a process cost model and necessary data were provided by an automotive assembly plant and body manufacturer. Environmental impacts related to this stage consider the energy consumed and CO2 emissions due to this consumption. Considering the fender use stage, the vehicle consumption considered was 7.6 l/100 km, with a CO2 emission rate of 182 g/km. In order to obtain values for the economic and environmental evaluation of the dismantling stage, an inquiry was made to an end-of-life vehicles management company. An average cost for dismantling one vehicle was obtained, 50 euros, and a proportional calculation was made considering the automobile weight and the weight of two fenders. For the environmental evaluation, also the average energy consumption was estimated, considering the main machine power of a dismantling plant and its dismantling rate. As a result, energy consumption and CO2 emissions related to this energy could be obtained. 3.1.2. Fender life cycle To develop a life cycle approach for material selection, it is necessary to analyse the product’s life cycle. Fig. 6 illustrates the life cycle of a generic product. This case study aims to select a material for an automobile fender, being therefore necessary to analyse the generic life cycle considering the fender’s life cycle characteristics.
Fig. 6. Generic life cycle of a product.
Some phases of the generic life cycle were not considered in the study: the assembly/packaging phase, since it is not expected to change with the materials under analysis; and the scenarios of landfill, incineration and re-use for the final disposal, since they are not valid for the fender case. The raw material acquisition phase consists mainly on the extraction of iron ore in the steel case and bauxite in the aluminium case. Other resources are required for these materials production and are included in this study in terms of environmental impacts. The cost of raw material acquisition is not directly included as it is assumed to be in the material cost (for the different steels or aluminium alloys), therefore already included in LCC model. On the material manufacture stage, the manufacture procedures of steel and aluminium are considered. The comparison is based on the material cost, energy consumption and CO2e emissions. One main factor on this stage is the material’s recycling rate. During material production, two distinctive production processes can be adopted; primary material production (from raw materials) and secondary material production (from scraps). In Figs. 7 and 8 the main difference between these processes in terms of energy consumption and CO2 emissions, both for aluminium and for steel, is shown. As illustrated, secondary aluminium made by aluminium scraps
Fig. 7. Energy consumed in material production vs recycling rate.
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battery, catalyser and fluids are first removed, airbags and pyrotechnical components are neutralised. Afterwards, some parts are removed if re-using is possible and plastics, tires and glass are recycled. Finally, the materials left are shredded, in order to obtain single elements. These particles pass through a magnetic field to separate ferrous metals, such as steel. Aluminium and nonferrous metals are temporarily magnetic induced (Eddy Current) to separate them from nonmetallic materials. A final treatment based on fluids with different densities (dense media) is required to obtain aluminium with adequate purity. The end for the nonmetallic particles left is the landfill. Therefore, in the fender case, landfill is not considered as the materials in study are metallic; re-use was also discarded as this option although possible is truly unlikely. Fig. 8. CO2 emitted in material production vs recycling rate.
(no bauxite extraction needed) requires less energy and emits less greenhouse gases than the secondary steel. In a near future, as aluminium recycling rate rises, economical and environmental performance is expected to improve. The next stage is the fender manufacturing, which includes four technological processes (Fig. 9). In the blanking stage the blanks are sheared from the metal coil, generating scraps and rejected parts as waste, consuming energy, labour, and production facilities. The blanks are then taken to a rinsing process, where they are washed and lubricated. In this process, energy, production facilities, water and a washing solution are consumed. Afterwards, the blanks are stamped (stamping process), requiring energy, labour and production facilities. In this process some parts are completed, others rejected, and some, with minor faults, are manually reworked in the finishing stage. If these faults cannot be eliminated, the part is rejected after finishing. Both in stamping and in finishing, scraps are generated. In the fender use stage, the fuel consumed over the lifespan of 50 000 vehicles (two fenders per vehicle) was considered as a material as well as an energetic resource. The weight is a key factor in reducing fuel consumption. For every 100 kg weight reduction there is a cut of 0.6 l/100 km in fuel consumption leading to proportionally lower exhaust gas emissions and running costs [36]. The final stage is the end-of-life/dismantling, which is a process with energy and labour consumption. Generally, car’s
3.2. Technical performance The evaluation of the materials technical performance follows the procedure explained in Section 2.1. The technical evaluation considers the most important requirements that must be fulfilled by the fender material (Table 2). Such requirements have different levels of importance for the performance of the fender and this effect was modelled through the attribution of an importance weight. Such attribution might not be an easy task, especially if the number of requirements is quite high. In such case a procedure that follows a pairwise comparison can be a good approach. Each requirement was then correlated to one or more technological properties of the candidate materials. To each requirement a total of 15 points were distributed among the material properties, considering their relevance to the requirement. The final weight of each material property represents its importance to the fender technical performance. The analysis of Table 2 reveals that yield strength, Young’s modulus and density are the most important properties for the fender performance, as their final weights are higher than others. This result is inline with the common sense in typical material selection processes, where strength and rigidity at low density are the usual assumptions. Finally, the values of the material properties were collected, adimensionalized and compared with the best material in each material property (Table 3) and multiplied by the importance weight of each property. The total score of each material represents its technical performance evaluation, being 100% the highest possible score for a material. The material with highest score in performance evaluation is St-3. In opposite, the worst score was attributed to Al-1. Globally, steel performs better for the functions required for a fender than aluminium. 3.3. Economic evaluation
Fig. 9. Fender production processes (scraps e material not used; rejected parts e parts with major faults).
Economic evaluation was performed using the LCC model. The life cycle costs of the annual fender production are presented in Table 4. The material acquisition includes the cost of the annual material input for the fender production and the benefit of the scraps re-sold to the metal industry. The fabrication costs can be divided into two main cost groups: variable, which includes energy and labour, and fixed,
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1894 Table 2 Weights applied for the material properties Fender requirements Resistant Light Corrosion resistant Stiff Allow forming processes Easy to handle Easy to dismantle Sum Weighting (%)
Weight (%) 25 15 5 20 15
Yield strength (Rp) 10 4
Density (r)
5 4
7
10
5
Ductility (strain at rupture)
Strain hardening exponent
Corrosion resistance
Hardness (HV)
Ferromagnetism
Coefficient of anisotropy
8
5
15 5
10 10 100 e
Young’s modulus (E )
2 3.10 21.8
4.05 28.4
5
5 7 3.25 22.8
8 8 0.75 5.3
which includes machine, tooling, building and fixed overhead cost. They are estimated based on process cost models, considering that the fenders of the new candidate materials require the same process flow (Fig. 9) and will be fabricated in the same production line already used for the current fender. The costs associated with each step of the process flow are derived from a combination of engineering principles and empirical data considering the current manufacturing practice. Factor inputs include design specifications, material parameters, processing parameters (e.g., equipment parameters, space
0.75 5.3
0.75 5.3
0.80 5.6
0.80 5.6
0.75 5.3
requirements, power consumption), production parameters (e.g., production volumes, scrap rates, down times, maintenance time), and economic parameters (e.g. cost factors, cost of capital associated with investments). Inputs are transformed into estimates of fixed and variable costs for each manufacturing step. In the absence of accurate and site-specific data, which is the case for the new candidate materials, the machine and tooling costs are predicted based on the design specifications of the product using regressions derived from empirical data.
Table 3 Technical performance evaluation Properties
Weight (%)
St-1
St-2
St-3
Al-1
Al-2
Al-3
Yield strength (Rp)
21.8
Value [MPa] Adimensional Score
220 31.4 6.8
350 50.0 10.9
700 100.0 21.8
170 24.3 5.3
190 27.1 5.9
155 22.1 4.8
Young’s modulus (E )
28.4
Value [GPa] Adimensional Score
207 100.0 28.4
207 100.0 28.4
207 100.0 28.4
69 33.3 9.5
71 34.3 9.7
70 33.8 9.6
Density (r)
22.8
Value [kg/m3] Adimensional Score
Ductility (strain at rupture)
5.3
Value [kN m/kg] Adimensional Score
Strain hardening exponent
5.3
Value [N m/kg] Adimensional Score
Corrosion resistance
5.3
Hardness
7.85 34.5 7.9
7.85 34.5 7.9
2.71 100.0 22.8
2.75 98.5 22.5
2.74 98.9 22.6
16 50.0 2.6
5 15.6 0.8
24 75.0 3.9
24 75.0 3.9
30 93.8 4.9
0.17 58.6 3.1
0.15 51.7 2.7
0.14 48.3 2.5
0.22 75.9 4.0
0.23 79.3 4.2
0.29 100.0 5.3
Value Adimensional Score
0.5 50.0 2.6
0.5 50.0 2.6
0.5 50.0 2.6
1 100.0 5.3
1 100.0 5.3
1 100.0 5.3
5.6
Value [HV] Adimensional Score
124 31.5 1.8
197 50.0 2.8
394 100.0 5.6
88 22.3 1.3
107 27.2 1.5
98 24.9 1.4
Ferromagnetism
5.6
Value Adimensional Score
1 100.0 5.6
1 100.0 5.6
1 100.0 5.6
0 0.0 0.0
0 0.0 0.0
0 0.0 0.0
Coefficient of anisotropy
5.3
Value Adimensional Score
1.5 45.3 2.4 63.9
1.6 42.5 2.2 65.8
1.6 42.5 2.2 77.5
0.7 97.1 5.1 57.1
0.7 97.1 5.1 58.2
0.7 100.0 5.3 59.1
Total score
7.85 34.5 7.9 32 100.0 5.3
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Table 4 Fenders annual life cycle costs (in V) Costs [V]
St-1
St-2
St-3
Al-1
Al-2
Al-3
Material acquisition Production Labour Energy Fixed costs Total production
375 485
363 948
304 374
489 926
447 441
495 349
39 120 8749 974 780 1 022 649
39 128 9666 1 092 556 1 141 350
39 130 11 497 1 266 251 1 316 878
39 130 8752 1 123 526 1 171 408
39 130 8752 1 123 526 1171 408
39 130 8752 1 123 526 1 171 408
Use Dismantling
1 844 444 5905
1 411 590 4519
988 750 3165
979 607 3136
894 659 2864
990 451 3171
The fender life cycle costs were adimensionalized and weighted (Table 5). Fig. 10 presents graphically the LCC results. The results show that the material that incurs in a smaller life cycle cost is St-1, which is mainly due to both the smaller fabrication costs of the fender and the higher importance given to them. In contrast, St-3 is the material with higher fabrication costs mainly because requirements regarding the stamping process (tooling and press machine specifications and process performance) are tougher, which results in a more expensive fabrication. It should be noted that the fender forming requires even a press machine tool with a larger tonnage capacity than the one already installed and used in the production of the current fender. However, although St-3 has a higher specific market price (price per mass unit), the total acquisition cost for the fender is lower, which is the result of smaller thickness required for an equivalent technical performance of the fender. The material acquisition costs are higher in the aluminium cases, even though the aluminium fenders are lighter, leading to lower in-use costs.
3.4. Environmental evaluation Environmental evaluation was performed using the LCA model. The major consumptions and emissions over the fender life cycle are presented in Table 6. The exhaustive results obtained in SimaPro7 software were incorporated in the environmental evaluation, but not presented in the table. As the results achieved for the three major impact categories (Fig. 11) and the respective EI’99 show, the steel currently used St-1 and St-3 are the ones with higher and lower environmental impacts, respectively. Even though the fender production stage for St-3 results in larger environmental damages, it performs better for the overall life cycle.
3.5. Global evaluation Finally, with the results obtained from the economic, environmental and technical performance dimensions, an integrated and global evaluation can be performed. The outcome values from the individual dimensions were adimensionalized to allow the attribution of importance weights (dimension weights). The sum of the three dimension weights must be 100%. Different combinations of weights might result in a different ‘‘best material for the application’’ and a slight modification of such weights might deeply modify this ‘‘best material’’. In fact, the difficulty to attribute importance weights to the dimensions of analysis that closely reflect a corporation strategy for the product, and the sensibility of the results achieved to such weights are the major drawbacks normally pointed to a global evaluation based on weights attribution. To overcome this disadvantage and have a clear view of the possible ‘‘best materials’’ correlated to its domain of weights, the global evaluation is performed through a ternary diagram, where each axis represents one dimension of analysis. The diagram illustrates not only the ‘‘best material’’ for a particular set of importance weights but also the domain of weights for each ‘‘best material’’. For example, if minimizing economic costs of the product is the strategic goal, then the scenario required can be characterized by point A (Fig. 12). In this scenario, a high weight is given to the economic dimension (90%), a low criteria weight is given to the technical performance dimension (10%) and a null importance is given to environmental dimension. And the ‘‘best material’’ is St-1. On the other hand, if the company’s strategy is a more balanced one, then a good scenario would be characterized by point B, where 60% is the importance given to economic, 30% to technical and 10% to environmental issues. In this scenario, the ‘‘best material’’ would be St-3. Point C illustrates a more extreme
Table 5 Adimensionalized costs, weights applied to life cycle stages and LCC results Life cycle stages
Adimensionalized costs
Weights
St-1
St-2
St-3
Al-1
Al-2
Al-3
Material acquisition Fender production Fender use Dismantling
0.76 0.78 1.00 1.00
0.73 0.87 0.77 0.77
0.61 1.00 0.54 0.54
0.99 0.89 0.53 0.53
0.90 0.89 0.49 0.49
1.00 0.89 0.54 0.54
LCC results
8.06
8.25
8.52
8.56
8.31
8.59
2.00 6.50 0.75 0.75
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Fig. 11. Environmental evaluation based on LCA model.
Fig. 10. Economical evaluation based on LCC model.
environmental strategy (50, 40 and 10% weights given to environment, technical performance and economics, respectively), resulting in the same ‘‘best material’’ e St-3. In this case study and according to the analysis, St-1 and St3 are the only two materials appearing in the ternary diagram. Therefore, whatever the set of weights those are the materials to consider for the application e St-1 points out to a strategy more focused on cost issues over lifetime, while St-3 will point out to a strategy associated to the performance of the material and environmental concerns. 3.5.1. Sensitivity analysis The thickness required for the St-3 (0.35 mm), resulting from the structural and frequency analysis, is not currently available on the market. However, as a methodological approach and as the steel supplier informally announced its availability in the near future, the required value was used in the previous analysis. In order to identify the impact differences obtained by the thicknesses available in the current real scenario, 0.50 mm, a sensitivity analysis was performed. The result for economic evaluation is illustrated in Fig. 13 and as expected, with a higher thickness, the material costs for the third steel increase. The production costs suffered the
highest raise due to the higher press tonnage required for this material with a 0.5-mm thickness. The increase of the costs is so accentuated that this steel, with this thickness, is the material that incur higher costs. Following the same approach as in economical evaluation, a sensitivity analysis was also performed for the environmental evaluation. As expected (Fig. 14), with the increase of fenders weight using St-3, the environmental impacts increased and the material with a lower EI’99 indicator changed to St-2. St-1 remained the material with higher environmental impacts. The global evaluation, considering the sensitivity analysis performed, led to some changes (Fig. 15). As the amount of material increased, the St-3 environmental performance decreased and as a result, St-2 appeared as the best choice for some scenarios, in particular if an extremely environmental or a more balanced scenario is chosen. St-1 remained the material with higher score if reducing economical costs is the goal of the design. All the evaluations were based on the assumption that aluminium has a recycling rate much lower than steel (70% for steel, 30% for aluminium). Therefore, it is interesting to perform a sensitivity analysis considering a future scenario, where aluminium achieves steels recycling rate (70%). Note
Table 6 Consumptions and emissions over the fender life cycle Consumptions and emissions Material manufacture Material [ton] Energy [TJ] CO2e [ton] Fender production Energy [TJ] CO2e [ton] Fender use Fuel [m3] Energy [TJ] CO2e [ton] Fender dismantling Energy [TJ] CO2e [ton]
St-1
St-2
623 10.83 1742
477 8.29 1333
0.55 6.22
St-3
Al-1
Al-2
Al-3
335 5.83 938
331 33.59 3066
302 30.67 2800
334 33.96 3100
0.60 7.31
0.72 8.69
0.55 6.62
0.55 6.62
0.55 6.62
1346 6.42 3224
1030 4.92 2467
722 3.44 1728
715 3.41 1712
653 3.12 1564
723 3.45 1731
0.03 0.35
0.02 0.27
0.02 0.19
0.02 0.18
0.01 0.17
0.02 0.19
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Fig. 14. Environmental evaluation of the fender life cycle with thicknesses currently available.
criteria weights result in different material choices, especially if economical performance is important to the selection. Fig. 12. Global evaluation of fender designs based on performance, environmental and cost criteria. Weight criteria: A e 90% Econ. Perf., 10% Tech. Perf., 0% Env. Perf.; B e 60% Econ. Perf., 30% Tech. Perf., 10% Env. Perf.; C e 40% Econ. Perf., 10% Tech. Perf., and 50% Env. Perf.
that the recycling rate considered in this study is the amount of scrap used for the production of material (lower in the aluminium case due to insufficient amount of aluminium scrap in the market). Considering St-3 with a thickness of 0.35 mm, the minimum required, the global evaluation will have no changes and the ternary diagram, representing the possible choices, will be the same as the diagram in Fig. 12. The reason for these results is that St-3 remains the higher scored in environmental and technical evaluation, and St-1 remains the most economic one. If the same sensitivity analysis is performed considering St-3 with a thickness of 0.50 mm, the results will be different (Fig. 16). Four materials can be the ‘‘best choice’’, depending on the importance given to the evaluation dimensions. This variety results from the materials similar scores on different dimensions. While in the first evaluation, St-3 performed better in environmental and technical dimensions, in this case no material is much better than the other, making the material selection more difficult. Slightly different
Fig. 13. Economical evaluation of the fender life cycle with thicknesses currently available.
4. Conclusions In this paper a material selection methodology was proposed using a life cycle engineering (LCE) approach. In a unifying framework the methodology supports the integration of different dimensions of selection. Starting from a set of candidate materials elected for a particular application based on the design team experience and expertise, a comparison process is launched including the technical/functional requirements of the material in the application and the economical and environmental impacts all over the product life cycle. The performance of the candidate materials over the three dimensions (technical, economical and environmental) is then calculated, based on a technical evaluation, a life cycle cost analysis and a life cycle environmental assessment. The ‘‘best material’’ for each combination of the importance assigned to each dimension is identified. The ‘‘best material domains’’, presented in a ternary
Fig. 15. Global evaluation of fender designs with thicknesses available.
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References
Fig. 16. Global evaluation of fender designs with thicknesses available and 70% recycled aluminium.
diagram, are the outcome from the integrated methodology allowing the design team to select the ‘‘best material’’ according to their practice and corporate strategy. To demonstrate the application of the proposed methodology a case study was presented based on an automotive front fender, which is currently made of mild steel (HX220DP þ z100MCO). The global objective was to analyse the possibility of using a different metallic material for the fender, as high strength steel or aluminium alloys, to redesign it accordingly (if necessary), and to evaluate the global performance of each material integrating the three dimensions of analysis. The results revealed that the comparisons between fender materials based on economic, environmental and technical dimensions greatly depend on the importance weights attributed to the dimensions evaluated. The global evaluation developed in the study illustrated the possible material choices for different importance weights, which are attributed according to the pursuing strategy. Moreover, as far as the ‘‘best material’’ for the application is highly dependent on these importance weights, it is crucial to decide cautiously the importance given to the technical, economical and environmental decision aspects. Another important issue is the importance of the data collected in an early stage of the analysis. As demonstrated in the sensitivity analysis, one single data value can modify the entire evaluation and can lead to different choices. Thus, it is important to obtain accurate information about the product life cycle in order to avoid unwise decisions. Finally it should be remarked that in this paper, only metals were considered as alternatives. In fact, the screening of the set of candidate materials, highly dependent on the experience and knowledge of the design team, is a critical issue to achieve a good material for the application. Material selection methods like the one proposed by Ashby [25] is a useful approach for the initial screening, feeding the methodology proposed in this paper with a consistent set of candidate materials.
[1] Environmental Protection Agency. Life cycle engineering guidelines. EPA; 2001. Report 600/R-01/101. [2] Giudice F, La Rosa G, Risitano A. Materials selection in the Life Cycle Design process: a method to integrate mechanical and environmental performances in optimal choice. Materials and Design 2005;26(1): 9e20. [3] Jeswiet JA. Definition of life cycle engineering. In: Bley H, editor. Proceedings of the thirty-sixth CIRP international seminar on manufacturing systems. Saarbru¨cken; 2003. p. 17e20. [4] Wanyama W, Ertas A, Zhang H-C, Ekwaro-Osire S. Life-cycle engineering: issues, tools and research. International Journal of Computer Integrated Manufacturing 2003;16(4):307e16. [5] Betz M, Schuckert M, Herrmann C. Life cycle engineering as decision making support in the electronics industry. In: Proceedings of the custom integrated circuits conference. IEEE; 1998. [6] Finnveden G, Moberg A. Environmental systems analysis tools an overview. Journal of Cleaner Production 2004;13(12):1165e73. [7] Schmidt W-P, Taylor A. Ford of Europe’s product sustainability index. Organization for Economic Co-operation and Development 2006. [8] Saur K, Fava JA, Spatari S. Life cycle engineering case study automobile fender designs. Environmental Progress 2000;19(2):72e82. [9] Banaitiene N, Banaitis A, Kaklauskas A, Zavadskas EK. Evaluating the life cycle of a building: a multivariant and multiple criteria approach. Omega 2006;36(3):429e41. [10] Ebert C. The impacts of software product management. The Journal of Systems and Software 2007;80(6):850e61. [11] Penoyer JA, Burnett G, Fawcett DJ, Liou S-Y. Knowledge based product life cycle systems: principles of integration of KBE and C3P. ComputerAided Design 2000;32(5e6):311e20. [12] Shtub A, Bard JF, Globerson S. Project management e processes, methodologies and economics. Pearson Prentice Hall; 2005. [13] Krozer Y. Life cycle costing for innovations in product chains. Journal of Cleaner Production 2006;16(3):310e21. [14] Johansson K, Perzon M, Fro¨ling M, Mossakowska A. Sewage sludge handling with phosphorus utilization e life cycle assessment of four alternatives. Journal of Cleaner Production 2007;16(1):135e51. [15] Udo de Haes HA, Heijungs R. Life-cycle assessment for energy analysis and management. Applied Energy 2007;84(7e8):817e27. [16] International Standard Organisation. ISO 14040-environmental management e life cycle assessment e principles and framework. International Standard Organisation; 1997. [17] International Standard Organisation. ISO 14041-environmental management e life cycle assessment e goal and scope definition and inventory analysis. International Standard Organisation; 1998. [18] International Standard Organisation. IS0 14042-environmental management e life cycle assessment e life cycle impact assessment. International Standard Organisation; 2000. [19] International Standard Organisation. IS0 14043-environmental management e life cycle assessment e life cycle interpretation. International Standard Organisation; 2000. [20] Goedkoop M, Schryver A, Oele M. Introduction to LCA with SimaPro 7, 2007. PRe´ Consultants. [21] Kurk F, Eagan P. The value of adding design-for-the-environment to pollution prevention assistance options. Journal of Cleaner Production 2008;16(6):722e6. [22] Dehghan-Manshadi B, Mahmudi H, Abedian A, Mahmudi R. A novel method for material selection in mechanical design: combination of non-linear normalization and a modified digital logic method. Materials and Design 2005;28(1):8e15. [23] Ashby F. Multi-objective optimization in material design and selection. Acta Materialia 1999;48(1):359e69. [24] Field III R, Clark P, Ashby F. Market drives for materials and process development in the 21st century. MRS Bulletin 2001:716e25. [25] Ashby M. Materials selection in mechanical design. ButterwortheHeinemann; 1999.
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[31] Ermolaeva S, Castro G, Kandachar V. Materials selection for an automotive structure by integrating structural optimization with environmental impact assessment. Materials and Design 2004;25(8):689e98. [32] Siegert K, Wagner S. Formability characteristics of aluminium sheet. Training in aluminium application technologies (TALAT); 1994. [33] SimaPro7 software. PRe´ Consultants. Available from: http://www.pre.nl. [34] Aluminium packaging recycling organisation. Available from: http:// www.alupro.org.uk. [35] Steel Recycling Institute. Available from: http://www.recycle-steel.org. [36] European Aluminium Association. Aluminium in the automobile industry. Available from: http://www.eaa.net; 1996.
Journal of Cleaner Production 16 (2008) 1887e1899 www.elsevier.com/locate/jclepro
Life cycle engineering methodology applied to material selection, a fender case study Ineˆs Ribeiro a,1, Paulo Pec¸as a,*, Arlindo Silva b,2, Elsa Henriques a,3 a b
IDMEC, Instituto Superior Te´cnico, TULisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal ICEMS, Instituto Superior Te´cnico, TULisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Received 8 January 2008; received in revised form 14 January 2008; accepted 15 January 2008 Available online 4 March 2008
Abstract Materials selection is a multidisciplinary activity, which integrates a large number of knowledge fields and professional domains. In fact a material selection decision should capture not only the functional performance required for the application but should also consider the economical and environmental impacts originated all along the product life cycle. In this paper a life cycle engineering (LCE) approach is proposed to support material selection, integrating the performance of the material for the specific application in technological, environmental and economical dimensions throughout the duration of the product. The methodology proposed compares a set of candidate materials and, through the aggregation of the three dimensions (technical, economical and environmental), identifies the ‘‘best material domains’’. These ‘‘best material domains’’ are presented in a ternary diagram, which allows a global comparison of the candidate materials and supports an informed decision as regards the selection of the ‘‘best material’’ according to different business scenarios and corporate strategies. The methodology was applied to a case study aiming the use of new metallic materials (high strength steels and aluminium alloys) for an automobile fender currently made of mild steel and the evaluation of potential benefits as regards the global performance of the material. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Life cycle engineering; Life cycle cost; Life cycle assessment; Materials selection; Automobile fender
1. Introduction Nowadays, to meet market needs, the development of a product cannot be focused only on the classic approaches, such as technical and economic performances [1]. In the last decade, environmental problems have emerged as an important public concern. Consequently, strategies and development methods to promote products as ecological as possible have been incorporated in product design. In this context, to have a broader view of the product’s environmental impacts, an innovative approach which extends to the end of the product’s useful * Corresponding author. Tel.: þ351 21 8417316/9573; fax: þ351 21 8419058. E-mail addresses: [email protected] (I. Ribeiro), [email protected] (P. Pec¸as), [email protected] (A. Silva), [email protected] (E. Henriques). 1 Tel.: þ351 21 8417316; fax: þ351 21 8419058. 2 Tel.: þ351 21 8417723/342; fax: þ351 21 8474045. 3 Tel.: þ351 21 8417316/556; fax: þ351 21 8419058. 0959-6526/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2008.01.002
life and retirement have been developed, in opposition to the conventional approach, where the product’s sale is considered as the final analytical step [2]. Life cycle engineering (LCE) emerged in response to the need to develop life cycles causing the lowest possible environmental impacts, while still offering economic viability. LCE refers to ‘‘Engineering activities which include: the application of technological and scientific principles to the design and manufacture of products, with the goal of protecting the environment and conserving resources, while encouraging economic progress, keeping in mind the need for sustainability, and at the same time optimizing the product life cycle and minimizing pollution and waste’’ [3]. Therefore, LCE can be defined as a decision making methodology that considers performance, environmental, and cost dimensions throughout the duration of a product, guiding design engineers towards informed decisions [4,5]. LCE differs from other life cycle methodologies in this point; while LCE incorporates these three dimensions of analysis, life cycle
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assessment (LCA) covers environmental aspects, life cycle cost (LCC) covers economic aspects and life cycle management (LCM) includes economic and environmental aspects. Life cycle engineering includes not only conventional tools, as technical performance analysis based on mechanical, electrical, and chemical properties [1], but also life cycle tools to analyse economic performance (LCC) and environmental performance (LCA). LCC and LCA were chosen from a large number of available tools for their suitability for product assessment [6]. Several authors have applied LCE to different case studies in automotive [7,8], construction [9], and computer industry [10,11], to name only a few, targeting the individual evaluation of technical, economic and environmental aspects of products or systems. Life cycle cost generally refers to ‘‘all the costs associated with a product throughout the product’s life’’ [12]. Its objective is to cover the assessments of costs in all steps of the product’s life cycle, including the costs that are not normally expressed in the product market price [13], such as costs incurred during the usage and disposal. LCC is essentially an evaluation tool in the sense that it gets on to important metrics for choosing the most cost-effective solution from a series of alternatives [12]. Life cycle assessment is a structured method to quantify potential environmental impacts of products or services over their full life cycle [14,15]. Although LCA had emerged in the early 1970s, its methodological basis at that time was chaotic and conflicting results were produced. As an attempt to organize and harmonize LCA developments, SETAC (1980s), ISO (1990s) and UNEP/SETAC (1990s) established a terminology, a standard methodology [16e19] and, as LCA is still a field of innovation and discovery, became a privileged space to submit developments for discussion [14]. Presently, LCA consists of four steps: definition of the goal and scope of the study, construction of the product life cycle model with all environmental inflows and outflows (life cycle inventory stage e LCI), evaluation of the environmental relevance of all the inflows and outflows (life cycle impacts assessment stage e LCIA) and, finally, the interpretation of the results [20]. There are several methods for LCIA stage compatible with ISO requirements [15] and therefore, most experts prefer to select a published method instead of developing a new one [20]. Material selection is an important application area of LCE. As material selection is part of product design, decisions taken during this stage largely influence the product’s costs and environmental impacts for its entire life cycle [21]. When selecting a material for a specific set of functionalities the relevant engineering properties of the material are identified and correlated to the design requirements. Normally, the selection is carried out considering the values of such properties altogether with economic considerations. For example, in mechanical design mechanical properties are the most important for material selection [22], but the influence of the selected material on the product’s final cost must be controlled in order to get a viable design solution as regards both technical performance and economical suitability. Therefore, material selection can
be regarded as a multi-objective problem, being the optimal selection and the best match found between the available materials profiles and the requirements of the design [22,23]. In fact, according to Field et al. ‘‘the four main factors upon which the designers rely when considering materials choice are the relationship between materials specifications and technical performance of the product, the economic performance of the product, the environmental performance of the product and the practice of industrial design embedded in the product and its functionality’’ [24]. Certainly this is not an easy task and in practice material selection methods frequently disregard economic and environmental aspects or at least only consider them at late stages of product development. Material properties charts are probably the most common and visual way of selecting materials for a given application [25]. This method allows for the selection of a material, or set of candidate materials, by comparing two engineering properties at a time. A certain twist of the method can make it possible to compare indirectly four properties at a time. The method also performs a process selection, knowing in advance the material to be used and the shape of the part. Cost is not a very realistic variable in this method. In fact, even knowing that it is fundamental to evaluate the impact of the material in the product final cost, only a relative cost of each raw material is considered in the selection. As the relative cost of raw materials is only a parcel of such impact, the effect of materials, in a certain production volume, on manufacturing process and on its cost is naturally neglected. Bearing in mind that materials selection regardless of the objective is at its basis a decision making process [26], another way of selecting materials is by using decision matrices. Several approaches are available. Some are qualitative [27] and some are quantitative [28]. They normally use some kind of weighting to account for some criteria being more important than others, but they are very similar in all aspects. Value analysis is another method available to select materials [29]. Once again, this is a general method, used for decision making. It is usually employed with a whole product in mind, not just a part of it, since the main objective is to enhance the value of the product to the customer. The method works with a design already in place, so it is of little use when the decision of what material to use is done when there is still no completely defined design. The outputs of the method are recommendations for modifications to the product e possibly materials substitution. These approaches do not fully capture the entire materials selection framework, because no one encompasses all the technological, economical, environmental, and current practice details needed to make an accurate and robust selection as mentioned by Field et al. [24]. The life cycle engineering approach proposed in this paper aims to include in the same framework the dimensions of technology, economy, environment and current practice in a single decision making tool to support materials comparison and achieve an informed selection. The assessment of economical, technical and environmental criteria is made by gathering information from different dimensions of analysis into a global
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evaluation. The global evaluation presented, unlike the classic approach of LCE studies, illustrates the best material choices for different scenarios, that is, for different practices and even corporate strategies.
rough assumptions are normally made. The influence of such inaccurate assumptions in the final result is not always clear. To study the robustness of the final result, the ternary materials selection diagrams can easily handle any sensitivity analysis.
2. LCE model
2.1. Technical evaluation
The first stage of the LCE model proposed in this study (Fig. 1) is to define the boundaries of the problem under analysis and to collect specific data for material application and product life cycle. The next step is to evaluate individually the product from an economic, environmental and technical point of view. These evaluations use distinctive methods. Economical and environmental evaluations are performed from a life cycle perspective, using LCC and LCA, respectively. Technical evaluation is performed using a conventional approach based on decision matrices, with the materials analysed based on their properties. For each material and for each dimension of evaluation (technical, economic, and environmental) a single indicator is obtained, allowing the direct incorporation of the technical, economical and environmental performances into a multi-criteria decision problem. The final result is a global evaluation, presented in a ternary diagram, clearly showing the possible choices according to the importance given to the three dimensions of analysis. This ternary materials selection diagram illustrates the ‘‘best materials’’ for different criteria weights. In fact the ternary diagrams identify not only the best materials according to a set of weights attributed to technical, economic, and environmental dimensions, but also the domain (range of weights) of each ‘‘best material’’. Within this approach the design engineer overcomes the difficult task related to the materialization of the relative importance of the three dimensions into a set of weights. In addition, considering that materials selection (and particularly data collection around the product specific conditions) takes place in a preliminary design phase, several
Within the LCE model applied to material selection, the technical evaluation refers to material properties and their contribution to the technical requirements of the product. Fig. 2 outlines the steps of this evaluation, resulting in a score for each candidate material under evaluation, which is a relative quantification of the appropriateness of each material for the technical performance of the product. The first step is to identify the product requirements, Ri, and their relative importance, wRi, for the product performance (requirements weights). The ability of a specific material to fulfil the requirements depends on its properties. Therefore, it is necessary to determine the relevant properties and correlate them with the requirements. So, the material properties are related to the requirements through a matrix of indices, W, where each index Wij reflects the contribution of the property Pj to accomplish the requirement Ri. Most of these indices are zero, meaning that there is no correlation between the property and the requirement, but the sum of the indexes in eachP line of the W matrix P have to yield the same number, i.e., W ¼ X ¼ W2j ¼ 1j P W3j ¼ ., which means that the value of X must be preset and distributed through one or several properties correlated with the requirement. The absolute importance, aPj, of each material property Pj can then be calculated: aPj ¼ W1j $wR1 þ W2j $wR2 þ .. þ Wnj $wRn
ð1Þ
Finally, the material property weight (the relative importance) will be: wPj ¼
aPj aP1 þ aP2 þ . þ aPj þ . þ aPm
ð2Þ
Taking now these material property weights, all the required information to compare the technical performance of a set of materials is present. The final step is the construction of a global comparison table in which each material with its quantified properties is ranked against all the others. Since each material property has its own value and units, the properties must be adimensionalized. The adimensional properties are then weighted according to the material property weight, wPj. Finally, adding all the weighted adimensional material properties, a weighted material index, wMIk, is achieved for each material. The potentially ‘‘best material’’ will be the one with the highest wMI. 2.2. Economic evaluation (LCC model)
Fig. 1. Overview of the life cycle engineering model for materials selection.
Life cycle cost model (Fig. 3) allows the assessment of the product’s life cycle cost. The first step of the model is to gather information from all processes during the entire life cycle stages. With data related to the resources consumption along all these stages it is possible to quantify the different cost
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Fig. 2. Technical evaluation methodology.
items involved, from labour and machine use to materials and energy spending, and correlate them to each life cycle stage. In fact, along product life several costs are associated to different entities. For example, production costs are only supported by the industry in question, while the in-use costs are supported by the customers/users. Therefore, if the LCC analysis is being performed by the industry to support a material selection decision, these values cannot be assumed as having the same importance. For the industry, a reduction in the production cost is normally perceived as more important than a reduction of the in-use costs. So, the adimensionalization and weighting of the costs must be done. To define such weighting system is not easy, as it depends on the companies’ strategy and involves subjective and controversial issues. The weights applied in this study are a weighting system used in other similar study [8], in which the weights are applied according to the product life
cycle stages: material acquisition e 2, production e 6.5, use e 0.75, and end-of-life e 0.75. The comparisons between materials are performed considering as reference the material that results in a product with a lower life cycle cost, having this material a 100% score. 2.3. Environmental evaluation (LCA model) At this stage an environmental impact assessment of the several candidate materials is performed, using all data previously collected for the product’s specific conditions. A cradle to grave approach is used, according to LCA standards [16e19]. The methodology for the impact assessment proposed (Fig. 4), considers 11 environmental impact categories, in the following three areas: Human Health (HH), Ecosystem Quality (EQ) and Resources (R). The methodology aggregates all the
Fig. 3. LCC model.
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Fig. 4. Eco-Indicator 99 methodology.
emissions and resources consumption from the life cycle into these impact categories and, afterwards, weights the scores into a single value, called the ‘‘eco-indicator 99’’ (EI 99) [30]. The weighting coefficients were applied according to the hierarchic/average (H/A) perspective, which is a moderate perspective generally accepted by the scientific community, attributing 40e40e20% of weight to the three considered impact areas, HHeEQeR, respectively [31]. These areas have specific separated units. Disability-Adjusted Life Years (DALY) is the health indicator, Potentially Disappeared Fraction multiplied by an area unit, m2, and a time unit, year (PDF m2 year) translates the damage to ecosystems and Mega Joule (MJ) surplus is the measure of resource depletion. More detailed information can be found elsewhere [16e19,30]. Fig. 5. LCE model application.
3. LCE model application In this part, the application of the life cycle engineering model previously presented is exemplified through a case study. The global objective is to support the materials selection decision for an automobile front fender, considering not only the technical performance, but also economical issues and environmental implications (Fig. 5). 3.1. Fender specific conditions The fender is already in production in an annual production volume of 100 000 units (assembled in 50 000 vehicles), so it is considered that any new material selected should modify the automobile aesthetics or the fender assembly in the car body system. To analyse materials for an automobile fender using an LCE model, it is necessary to pre-select some candidate materials. This pre-selection was made under some considerations. Only metallic materials were considered for the sake of simplicity. Other material classes can also be considered,
maintaining exactly the same conditions of the LCE model. The fender is already in the market so the mild steel presently used was included. The two other steels chosen are ultra high strength ones. Three aluminium alloys commonly used in automotive external body parts in Europe, United States and Japan [32] were also included in the analysis (Table 1). 3.1.1. Data origin The materials in focus have different properties, which might result in different design features. However, as the fender assembly system and aesthetics are frozen, the only design feature that is allowed to change with different materials is the fender thickness. Because there are no standards or formal rules that an automobile fender is obliged to satisfy, to perform the quick estimation of the fender thickness the current mild steel fender was used as a benchmark against which all the other candidate materials are compared. So a simple structural analysis and frequency analysis were performed to guarantee that the fenders of all the candidate materials
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Table 1 Candidate materials and relevant design features Materials
Material label
Thickness [mm]
Surface area [m2]
Weight [kg]
HX220YD þ z100MCO DOCOL 600DP DOCOL 1000DP Al 6010-T4 Al 2036-T4 GZ45/30-30
St-1 St-2 St-3 Al-1 Al-2 Al-3
0.65 0.50 0.35 1.00 0.90 1.00
0.34
1.73 1.33 0.93 0.92 0.84 0.93
Note: weight ¼ density (surface area thickness).
have an equivalent level of strengths and strains when subjected to an equal load, and have natural frequencies far enough from the most relevant exciting ones. The results achieved for the fender’s thicknesses are presented in Table 1. In order to assess input and output streams of material production for the several materials, the SimaPro7 software was used. SimaPro7 uses LCA concepts and methodologies [33], incorporating an extensive database for materials production inflows and outflows. The SimaPro7 database was used with some adaptations to this case study. Nowadays recycling metals is an important issue and most metals are recycled. The recycled materials provide an improvement on energy consumption during material production when compared with primary material (using only raw material). However, especially in the aluminium case, the existent scraps in the market are not sufficient for the current material demands. Therefore, in the present case study, different recycled material rates were considered: 30 and 70% for aluminium and steel, respectively [34,35]. The costs incurred during the annual production were calculated using a process cost model and necessary data were provided by an automotive assembly plant and body manufacturer. Environmental impacts related to this stage consider the energy consumed and CO2 emissions due to this consumption. Considering the fender use stage, the vehicle consumption considered was 7.6 l/100 km, with a CO2 emission rate of 182 g/km. In order to obtain values for the economic and environmental evaluation of the dismantling stage, an inquiry was made to an end-of-life vehicles management company. An average cost for dismantling one vehicle was obtained, 50 euros, and a proportional calculation was made considering the automobile weight and the weight of two fenders. For the environmental evaluation, also the average energy consumption was estimated, considering the main machine power of a dismantling plant and its dismantling rate. As a result, energy consumption and CO2 emissions related to this energy could be obtained. 3.1.2. Fender life cycle To develop a life cycle approach for material selection, it is necessary to analyse the product’s life cycle. Fig. 6 illustrates the life cycle of a generic product. This case study aims to select a material for an automobile fender, being therefore necessary to analyse the generic life cycle considering the fender’s life cycle characteristics.
Fig. 6. Generic life cycle of a product.
Some phases of the generic life cycle were not considered in the study: the assembly/packaging phase, since it is not expected to change with the materials under analysis; and the scenarios of landfill, incineration and re-use for the final disposal, since they are not valid for the fender case. The raw material acquisition phase consists mainly on the extraction of iron ore in the steel case and bauxite in the aluminium case. Other resources are required for these materials production and are included in this study in terms of environmental impacts. The cost of raw material acquisition is not directly included as it is assumed to be in the material cost (for the different steels or aluminium alloys), therefore already included in LCC model. On the material manufacture stage, the manufacture procedures of steel and aluminium are considered. The comparison is based on the material cost, energy consumption and CO2e emissions. One main factor on this stage is the material’s recycling rate. During material production, two distinctive production processes can be adopted; primary material production (from raw materials) and secondary material production (from scraps). In Figs. 7 and 8 the main difference between these processes in terms of energy consumption and CO2 emissions, both for aluminium and for steel, is shown. As illustrated, secondary aluminium made by aluminium scraps
Fig. 7. Energy consumed in material production vs recycling rate.
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battery, catalyser and fluids are first removed, airbags and pyrotechnical components are neutralised. Afterwards, some parts are removed if re-using is possible and plastics, tires and glass are recycled. Finally, the materials left are shredded, in order to obtain single elements. These particles pass through a magnetic field to separate ferrous metals, such as steel. Aluminium and nonferrous metals are temporarily magnetic induced (Eddy Current) to separate them from nonmetallic materials. A final treatment based on fluids with different densities (dense media) is required to obtain aluminium with adequate purity. The end for the nonmetallic particles left is the landfill. Therefore, in the fender case, landfill is not considered as the materials in study are metallic; re-use was also discarded as this option although possible is truly unlikely. Fig. 8. CO2 emitted in material production vs recycling rate.
(no bauxite extraction needed) requires less energy and emits less greenhouse gases than the secondary steel. In a near future, as aluminium recycling rate rises, economical and environmental performance is expected to improve. The next stage is the fender manufacturing, which includes four technological processes (Fig. 9). In the blanking stage the blanks are sheared from the metal coil, generating scraps and rejected parts as waste, consuming energy, labour, and production facilities. The blanks are then taken to a rinsing process, where they are washed and lubricated. In this process, energy, production facilities, water and a washing solution are consumed. Afterwards, the blanks are stamped (stamping process), requiring energy, labour and production facilities. In this process some parts are completed, others rejected, and some, with minor faults, are manually reworked in the finishing stage. If these faults cannot be eliminated, the part is rejected after finishing. Both in stamping and in finishing, scraps are generated. In the fender use stage, the fuel consumed over the lifespan of 50 000 vehicles (two fenders per vehicle) was considered as a material as well as an energetic resource. The weight is a key factor in reducing fuel consumption. For every 100 kg weight reduction there is a cut of 0.6 l/100 km in fuel consumption leading to proportionally lower exhaust gas emissions and running costs [36]. The final stage is the end-of-life/dismantling, which is a process with energy and labour consumption. Generally, car’s
3.2. Technical performance The evaluation of the materials technical performance follows the procedure explained in Section 2.1. The technical evaluation considers the most important requirements that must be fulfilled by the fender material (Table 2). Such requirements have different levels of importance for the performance of the fender and this effect was modelled through the attribution of an importance weight. Such attribution might not be an easy task, especially if the number of requirements is quite high. In such case a procedure that follows a pairwise comparison can be a good approach. Each requirement was then correlated to one or more technological properties of the candidate materials. To each requirement a total of 15 points were distributed among the material properties, considering their relevance to the requirement. The final weight of each material property represents its importance to the fender technical performance. The analysis of Table 2 reveals that yield strength, Young’s modulus and density are the most important properties for the fender performance, as their final weights are higher than others. This result is inline with the common sense in typical material selection processes, where strength and rigidity at low density are the usual assumptions. Finally, the values of the material properties were collected, adimensionalized and compared with the best material in each material property (Table 3) and multiplied by the importance weight of each property. The total score of each material represents its technical performance evaluation, being 100% the highest possible score for a material. The material with highest score in performance evaluation is St-3. In opposite, the worst score was attributed to Al-1. Globally, steel performs better for the functions required for a fender than aluminium. 3.3. Economic evaluation
Fig. 9. Fender production processes (scraps e material not used; rejected parts e parts with major faults).
Economic evaluation was performed using the LCC model. The life cycle costs of the annual fender production are presented in Table 4. The material acquisition includes the cost of the annual material input for the fender production and the benefit of the scraps re-sold to the metal industry. The fabrication costs can be divided into two main cost groups: variable, which includes energy and labour, and fixed,
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Weight (%) 25 15 5 20 15
Yield strength (Rp) 10 4
Density (r)
5 4
7
10
5
Ductility (strain at rupture)
Strain hardening exponent
Corrosion resistance
Hardness (HV)
Ferromagnetism
Coefficient of anisotropy
8
5
15 5
10 10 100 e
Young’s modulus (E )
2 3.10 21.8
4.05 28.4
5
5 7 3.25 22.8
8 8 0.75 5.3
which includes machine, tooling, building and fixed overhead cost. They are estimated based on process cost models, considering that the fenders of the new candidate materials require the same process flow (Fig. 9) and will be fabricated in the same production line already used for the current fender. The costs associated with each step of the process flow are derived from a combination of engineering principles and empirical data considering the current manufacturing practice. Factor inputs include design specifications, material parameters, processing parameters (e.g., equipment parameters, space
0.75 5.3
0.75 5.3
0.80 5.6
0.80 5.6
0.75 5.3
requirements, power consumption), production parameters (e.g., production volumes, scrap rates, down times, maintenance time), and economic parameters (e.g. cost factors, cost of capital associated with investments). Inputs are transformed into estimates of fixed and variable costs for each manufacturing step. In the absence of accurate and site-specific data, which is the case for the new candidate materials, the machine and tooling costs are predicted based on the design specifications of the product using regressions derived from empirical data.
Table 3 Technical performance evaluation Properties
Weight (%)
St-1
St-2
St-3
Al-1
Al-2
Al-3
Yield strength (Rp)
21.8
Value [MPa] Adimensional Score
220 31.4 6.8
350 50.0 10.9
700 100.0 21.8
170 24.3 5.3
190 27.1 5.9
155 22.1 4.8
Young’s modulus (E )
28.4
Value [GPa] Adimensional Score
207 100.0 28.4
207 100.0 28.4
207 100.0 28.4
69 33.3 9.5
71 34.3 9.7
70 33.8 9.6
Density (r)
22.8
Value [kg/m3] Adimensional Score
Ductility (strain at rupture)
5.3
Value [kN m/kg] Adimensional Score
Strain hardening exponent
5.3
Value [N m/kg] Adimensional Score
Corrosion resistance
5.3
Hardness
7.85 34.5 7.9
7.85 34.5 7.9
2.71 100.0 22.8
2.75 98.5 22.5
2.74 98.9 22.6
16 50.0 2.6
5 15.6 0.8
24 75.0 3.9
24 75.0 3.9
30 93.8 4.9
0.17 58.6 3.1
0.15 51.7 2.7
0.14 48.3 2.5
0.22 75.9 4.0
0.23 79.3 4.2
0.29 100.0 5.3
Value Adimensional Score
0.5 50.0 2.6
0.5 50.0 2.6
0.5 50.0 2.6
1 100.0 5.3
1 100.0 5.3
1 100.0 5.3
5.6
Value [HV] Adimensional Score
124 31.5 1.8
197 50.0 2.8
394 100.0 5.6
88 22.3 1.3
107 27.2 1.5
98 24.9 1.4
Ferromagnetism
5.6
Value Adimensional Score
1 100.0 5.6
1 100.0 5.6
1 100.0 5.6
0 0.0 0.0
0 0.0 0.0
0 0.0 0.0
Coefficient of anisotropy
5.3
Value Adimensional Score
1.5 45.3 2.4 63.9
1.6 42.5 2.2 65.8
1.6 42.5 2.2 77.5
0.7 97.1 5.1 57.1
0.7 97.1 5.1 58.2
0.7 100.0 5.3 59.1
Total score
7.85 34.5 7.9 32 100.0 5.3
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Table 4 Fenders annual life cycle costs (in V) Costs [V]
St-1
St-2
St-3
Al-1
Al-2
Al-3
Material acquisition Production Labour Energy Fixed costs Total production
375 485
363 948
304 374
489 926
447 441
495 349
39 120 8749 974 780 1 022 649
39 128 9666 1 092 556 1 141 350
39 130 11 497 1 266 251 1 316 878
39 130 8752 1 123 526 1 171 408
39 130 8752 1 123 526 1171 408
39 130 8752 1 123 526 1 171 408
Use Dismantling
1 844 444 5905
1 411 590 4519
988 750 3165
979 607 3136
894 659 2864
990 451 3171
The fender life cycle costs were adimensionalized and weighted (Table 5). Fig. 10 presents graphically the LCC results. The results show that the material that incurs in a smaller life cycle cost is St-1, which is mainly due to both the smaller fabrication costs of the fender and the higher importance given to them. In contrast, St-3 is the material with higher fabrication costs mainly because requirements regarding the stamping process (tooling and press machine specifications and process performance) are tougher, which results in a more expensive fabrication. It should be noted that the fender forming requires even a press machine tool with a larger tonnage capacity than the one already installed and used in the production of the current fender. However, although St-3 has a higher specific market price (price per mass unit), the total acquisition cost for the fender is lower, which is the result of smaller thickness required for an equivalent technical performance of the fender. The material acquisition costs are higher in the aluminium cases, even though the aluminium fenders are lighter, leading to lower in-use costs.
3.4. Environmental evaluation Environmental evaluation was performed using the LCA model. The major consumptions and emissions over the fender life cycle are presented in Table 6. The exhaustive results obtained in SimaPro7 software were incorporated in the environmental evaluation, but not presented in the table. As the results achieved for the three major impact categories (Fig. 11) and the respective EI’99 show, the steel currently used St-1 and St-3 are the ones with higher and lower environmental impacts, respectively. Even though the fender production stage for St-3 results in larger environmental damages, it performs better for the overall life cycle.
3.5. Global evaluation Finally, with the results obtained from the economic, environmental and technical performance dimensions, an integrated and global evaluation can be performed. The outcome values from the individual dimensions were adimensionalized to allow the attribution of importance weights (dimension weights). The sum of the three dimension weights must be 100%. Different combinations of weights might result in a different ‘‘best material for the application’’ and a slight modification of such weights might deeply modify this ‘‘best material’’. In fact, the difficulty to attribute importance weights to the dimensions of analysis that closely reflect a corporation strategy for the product, and the sensibility of the results achieved to such weights are the major drawbacks normally pointed to a global evaluation based on weights attribution. To overcome this disadvantage and have a clear view of the possible ‘‘best materials’’ correlated to its domain of weights, the global evaluation is performed through a ternary diagram, where each axis represents one dimension of analysis. The diagram illustrates not only the ‘‘best material’’ for a particular set of importance weights but also the domain of weights for each ‘‘best material’’. For example, if minimizing economic costs of the product is the strategic goal, then the scenario required can be characterized by point A (Fig. 12). In this scenario, a high weight is given to the economic dimension (90%), a low criteria weight is given to the technical performance dimension (10%) and a null importance is given to environmental dimension. And the ‘‘best material’’ is St-1. On the other hand, if the company’s strategy is a more balanced one, then a good scenario would be characterized by point B, where 60% is the importance given to economic, 30% to technical and 10% to environmental issues. In this scenario, the ‘‘best material’’ would be St-3. Point C illustrates a more extreme
Table 5 Adimensionalized costs, weights applied to life cycle stages and LCC results Life cycle stages
Adimensionalized costs
Weights
St-1
St-2
St-3
Al-1
Al-2
Al-3
Material acquisition Fender production Fender use Dismantling
0.76 0.78 1.00 1.00
0.73 0.87 0.77 0.77
0.61 1.00 0.54 0.54
0.99 0.89 0.53 0.53
0.90 0.89 0.49 0.49
1.00 0.89 0.54 0.54
LCC results
8.06
8.25
8.52
8.56
8.31
8.59
2.00 6.50 0.75 0.75
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Fig. 11. Environmental evaluation based on LCA model.
Fig. 10. Economical evaluation based on LCC model.
environmental strategy (50, 40 and 10% weights given to environment, technical performance and economics, respectively), resulting in the same ‘‘best material’’ e St-3. In this case study and according to the analysis, St-1 and St3 are the only two materials appearing in the ternary diagram. Therefore, whatever the set of weights those are the materials to consider for the application e St-1 points out to a strategy more focused on cost issues over lifetime, while St-3 will point out to a strategy associated to the performance of the material and environmental concerns. 3.5.1. Sensitivity analysis The thickness required for the St-3 (0.35 mm), resulting from the structural and frequency analysis, is not currently available on the market. However, as a methodological approach and as the steel supplier informally announced its availability in the near future, the required value was used in the previous analysis. In order to identify the impact differences obtained by the thicknesses available in the current real scenario, 0.50 mm, a sensitivity analysis was performed. The result for economic evaluation is illustrated in Fig. 13 and as expected, with a higher thickness, the material costs for the third steel increase. The production costs suffered the
highest raise due to the higher press tonnage required for this material with a 0.5-mm thickness. The increase of the costs is so accentuated that this steel, with this thickness, is the material that incur higher costs. Following the same approach as in economical evaluation, a sensitivity analysis was also performed for the environmental evaluation. As expected (Fig. 14), with the increase of fenders weight using St-3, the environmental impacts increased and the material with a lower EI’99 indicator changed to St-2. St-1 remained the material with higher environmental impacts. The global evaluation, considering the sensitivity analysis performed, led to some changes (Fig. 15). As the amount of material increased, the St-3 environmental performance decreased and as a result, St-2 appeared as the best choice for some scenarios, in particular if an extremely environmental or a more balanced scenario is chosen. St-1 remained the material with higher score if reducing economical costs is the goal of the design. All the evaluations were based on the assumption that aluminium has a recycling rate much lower than steel (70% for steel, 30% for aluminium). Therefore, it is interesting to perform a sensitivity analysis considering a future scenario, where aluminium achieves steels recycling rate (70%). Note
Table 6 Consumptions and emissions over the fender life cycle Consumptions and emissions Material manufacture Material [ton] Energy [TJ] CO2e [ton] Fender production Energy [TJ] CO2e [ton] Fender use Fuel [m3] Energy [TJ] CO2e [ton] Fender dismantling Energy [TJ] CO2e [ton]
St-1
St-2
623 10.83 1742
477 8.29 1333
0.55 6.22
St-3
Al-1
Al-2
Al-3
335 5.83 938
331 33.59 3066
302 30.67 2800
334 33.96 3100
0.60 7.31
0.72 8.69
0.55 6.62
0.55 6.62
0.55 6.62
1346 6.42 3224
1030 4.92 2467
722 3.44 1728
715 3.41 1712
653 3.12 1564
723 3.45 1731
0.03 0.35
0.02 0.27
0.02 0.19
0.02 0.18
0.01 0.17
0.02 0.19
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Fig. 14. Environmental evaluation of the fender life cycle with thicknesses currently available.
criteria weights result in different material choices, especially if economical performance is important to the selection. Fig. 12. Global evaluation of fender designs based on performance, environmental and cost criteria. Weight criteria: A e 90% Econ. Perf., 10% Tech. Perf., 0% Env. Perf.; B e 60% Econ. Perf., 30% Tech. Perf., 10% Env. Perf.; C e 40% Econ. Perf., 10% Tech. Perf., and 50% Env. Perf.
that the recycling rate considered in this study is the amount of scrap used for the production of material (lower in the aluminium case due to insufficient amount of aluminium scrap in the market). Considering St-3 with a thickness of 0.35 mm, the minimum required, the global evaluation will have no changes and the ternary diagram, representing the possible choices, will be the same as the diagram in Fig. 12. The reason for these results is that St-3 remains the higher scored in environmental and technical evaluation, and St-1 remains the most economic one. If the same sensitivity analysis is performed considering St-3 with a thickness of 0.50 mm, the results will be different (Fig. 16). Four materials can be the ‘‘best choice’’, depending on the importance given to the evaluation dimensions. This variety results from the materials similar scores on different dimensions. While in the first evaluation, St-3 performed better in environmental and technical dimensions, in this case no material is much better than the other, making the material selection more difficult. Slightly different
Fig. 13. Economical evaluation of the fender life cycle with thicknesses currently available.
4. Conclusions In this paper a material selection methodology was proposed using a life cycle engineering (LCE) approach. In a unifying framework the methodology supports the integration of different dimensions of selection. Starting from a set of candidate materials elected for a particular application based on the design team experience and expertise, a comparison process is launched including the technical/functional requirements of the material in the application and the economical and environmental impacts all over the product life cycle. The performance of the candidate materials over the three dimensions (technical, economical and environmental) is then calculated, based on a technical evaluation, a life cycle cost analysis and a life cycle environmental assessment. The ‘‘best material’’ for each combination of the importance assigned to each dimension is identified. The ‘‘best material domains’’, presented in a ternary
Fig. 15. Global evaluation of fender designs with thicknesses available.
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References
Fig. 16. Global evaluation of fender designs with thicknesses available and 70% recycled aluminium.
diagram, are the outcome from the integrated methodology allowing the design team to select the ‘‘best material’’ according to their practice and corporate strategy. To demonstrate the application of the proposed methodology a case study was presented based on an automotive front fender, which is currently made of mild steel (HX220DP þ z100MCO). The global objective was to analyse the possibility of using a different metallic material for the fender, as high strength steel or aluminium alloys, to redesign it accordingly (if necessary), and to evaluate the global performance of each material integrating the three dimensions of analysis. The results revealed that the comparisons between fender materials based on economic, environmental and technical dimensions greatly depend on the importance weights attributed to the dimensions evaluated. The global evaluation developed in the study illustrated the possible material choices for different importance weights, which are attributed according to the pursuing strategy. Moreover, as far as the ‘‘best material’’ for the application is highly dependent on these importance weights, it is crucial to decide cautiously the importance given to the technical, economical and environmental decision aspects. Another important issue is the importance of the data collected in an early stage of the analysis. As demonstrated in the sensitivity analysis, one single data value can modify the entire evaluation and can lead to different choices. Thus, it is important to obtain accurate information about the product life cycle in order to avoid unwise decisions. Finally it should be remarked that in this paper, only metals were considered as alternatives. In fact, the screening of the set of candidate materials, highly dependent on the experience and knowledge of the design team, is a critical issue to achieve a good material for the application. Material selection methods like the one proposed by Ashby [25] is a useful approach for the initial screening, feeding the methodology proposed in this paper with a consistent set of candidate materials.
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