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The influence of surface roughness on the mechanical strength properties of machined short-fibre-reinforced thermoplastics
Composites Science and Technology 60 (2000) 107±113
The in¯uence of surface roughness on the mechanical strength properties of machined short-®bre-reinforced thermoplastics Else Eriksen* Institute of Mechanical Engineering, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg, Denmark Received 4 January 1999; received in revised form 10 June 1999; accepted 30 July 1999
Abstract When a component is manufactured by machining, the surface roughness often diers from what is prescribed in standards on the preparation of test specimens for mechanical tests. If the surface roughness in¯uences the strength, the properties of a machined part are therefore lower than the values given by the material supplier. The present study investigates the in¯uence of the surface roughness of three machined short-®bre-reinforced thermoplastics (SFRTP) on the following strength properties: Charpy impact, monotonic bending and ¯exural fatigue. The strengths were found to be independent of the roughness when using a 5% level of signi®cance. If this turns out to be true for other materials as well, the standards may need reassessment to avoid unnecessary work and unnecessary doubt about the use of the data. Furthermore, the production time can be lowered and there may be new advantageous applications of SFRTP. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Short-®bre composites; B. Strength; Machining
1. Introduction Most products made from short-®bre-reinforced thermoplastics (SFRTP) are manufactured to their ®nal form in one process by injection moulding. However, for some geometries or production numbers it is more advantageous to use machining either from extruded standard geometries or from parts moulded with a simpler geometry than the ®nal one. Machining is still a relatively new manufacturing method for SFRTP, and the theories and experience from metals and unreinforced plastics cannot be applied because of the dierent microstructure and thermal properties. It has been shown that it seems to be possible to set up guide lines for the manner in which the surface quality of machined SFRTP is in¯uenced by the production parameters [1]. Such knowledge is only valuable, however, if it is known which quality to aim at. This will depend on the functions of the surface and will therefore be related to the properties of the surface itself or the way it in¯uences the bulk properties of the component.
* Tel.: +45-9635-9323; fax:+45-9815-1411. E-mail address: [email protected] (E. Eriksen).
In addition, the surface quality may be decisive for the extent to which both types of properties are in¯uenced by the environment. Much research aims at describing how the mechanical properties of SFRTP are related to the microstructure. A comprehensive understanding of these relationships can be used to optimise the structure to ®nd the best properties for a certain application. However, if some of the mechanical properties are in¯uenced by the surface quality, a model based purely on the microstructure may be worthless for use in the machining industry. This will be the case for the strength properties if defects in the surface cause crack growth when the part is loaded. In standards on mechanical tests it is prescribed that test specimens made by machining must be manufactured very carefully. For example, ISO 2818 Plastics Ð Preparation of test specimens by machining prescribes that ``the machined surfaces and edges of the ®nished specimens shall be free of visible ¯aws, scratches or other imperfections when viewed with a low-power magnifying glass (approximately5 magni®cation)''. Such prescriptions indicate that the strength properties are, at least in some cases, expected to be highly in¯uenced by the surface quality of the test specimens. Since this careful treatment diers considerably from an ordinary industrial
0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(99)00102-5
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E. Eriksen / Composites Science and Technology 60 (2000) 107±113
procedure, the data for the mechanical properties given by the material suppliers may dier from the properties of industrially manufactured components. If this is the case, they may be of limited value for design purposes. On the other hand, if there is no signi®cant in¯uence, the test standards may need reassessment to avoid unnecessary work and unnecessary doubt about the validity of the data. Some research has dealt with the in¯uence of the surface quality on the mechanical strength properties of ®brereinforced plastics (FRP). Tagliaferri et al. investigated how the hole quality of drilled glass-®bre-reinforced (GFR) epoxy in¯uences the tensile properties [2]. The tensile strength was in no way aected by the cut ®nish, whereas the bearing strength was markedly decreased for an increasing damage around the holes. Persson et al. investigated the eects of the hole quality for carbon®bre-reinforced (CFR) epoxy [3]. The static and fatigue strengths of pin-loaded laminates were measured, and in both cases the machining defects were found to reduce the strengths signi®cantly. The bending strength of CFR epoxy machined by dierent methods resulting in different surface qualities was studied by Arola and Ramulu [4]. They found no dependence on the surface quality. It was observed that the failure predominantly occurred on the compression side of the specimens. That may be the reason why the surface roughness did not in¯uence the strength, as the compression forces closed the possible defects from the machining. These results cannot be transferred to SFRTP for which the fracture is supposed to start on the tensile side of the specimens. Fatigue tests of GFR epoxy showed that the fatigue life is very dependent on the surface quality [5]. This was found by studying the eect of polishing the edge of the specimens, which increased the fatigue strength signi®cantly. Other studies were carried out to investigate the fatigue failure process and the eects of reinforcing thermoplastics with short ®bres [6,7]. In both these studies the specimens were manufactured by machining, and to eliminate the eect from the machining process itself and the resulting surface roughness, the specimens were annealed and polished before the fatigue tests were carried out. This treatment indicates that the authors expected an in¯uence from the surface roughness of the machined SFRTP specimens. The present study was carried out to investigate how the following strength properties of three SFRTP were in¯uenced by the roughness of the machined surfaces: Charpy impact, monotonic bending and fatigue bending. The standardised roughness parameter, Ra, was used to characterise the surface quality, as this is the parameter most often used in the manufacturing industry. Furthermore, it is a quantitative measure and thereby made the comparisons between the specimens easier. The results from the impact and monotonic bending tests were analysed by use of a t test, and the
strengths were found to be independent of the roughness when using a 5% level of signi®cance. For the fatigue tests the independence was seen by the naked eye. It was therefore concluded that the standards for mechanical testing may need to be changed to avoid unnecessary work and unnecessary doubt about the validity of data obtained from test specimens without surface defects. If insensitivity of the surface quality turns out to be characteristic for a number of SFRTP, a shorter production time is possible, and there may also be more advantageous applications of these materials than the present. 2. Materials and methods Three thermoplastics reinforced with short glass ®bres were used in the study: Polyoxymethylene (POM) with 26 wt% ®bres; Polypropylene (PP) with 30 wt% ®bres and Styrene acrylonitrile (SAN) with 35 wt% ®bres. They will be named by the names of the matrix materials in the following. The Charpy impact strength was investigated for all three materials; the monotonic bending strength was investigated for POM and SAN; and the fatigue bending test was only carried out for the SAN specimens. Tensile tests were also made but many of the specimens broke outside the gauge length, and the results are therefore not included in the following. The POM and PP specimens were injection moulded as standard tensile test specimens (ISO 3167), and the middle part (the gauge length) was used for the bending and impact tests. The specimens were machined on both sides by turning after being fastened radially on a circular disc mounted on a Cazeneuve 590 HBY lathe (see Fig. 1). New wolfram carbide inserts with the numbers TPMR160304-F1 TP30 and TPMR160308-F1 TP30 were used. The rotational speed used was 300 rev minÿ1, which resulted in cutting speeds between 188 and 302 m minÿ1 for the narrow part of the specimens. Three roughness levels of the machined surfaces were obtained
Fig. 1. The mounting of the POM and PP specimens on a circular disc. The enlargement shows the direction of the tool marks.
E. Eriksen / Composites Science and Technology 60 (2000) 107±113
by the combinations of the feed rate and the tool radius shown in Table 1. Below, specimens from the three levels will be referred to as `low', `medium' and `high'. The corresponding symbols used in the ®gures are included in the table. The SAN specimens were milled from injection moulded quadratic plates with the dimensions 80806 mm. The specimens were taken from positions where the ®bres were as much aligned in one direction as possible. Such positions were found by polishing several sections of the moulded plates. Two perpendicular orientations were studied. 1.7 mm of material was removed from each side resulting in a ®nal thickness of 2.6 mm. The geometry described in the standard DIN 53 442 was used for the fatigue test. For the impact and monotonic bending tests, specimens with a rectangular cross-section corresponding to the middle part of the fatigue specimens were used, i.e. a width of 10 mm. The surfaces being characterised were machined with a TOS FN20 milling machine. The cutter had a diameter of 20 mm and only one cutting edge was used. The rotational speed was 800 rev minÿ1 which corresponded to a cutting speed of 50.3 mm minÿ1. The inserts used were the same as for turning, but only those with a tool radius of 0.4 mm were used. The three dierent roughness levels were obtained by varying the feed rate. These settings were used: 80, 160 and 250 mm minÿ1. The outer geometries were milled with an EMCOtronic machine with Komet Q36 18000.01.21 K10 inserts. The tool had a diameter of 12 mm, a speed of 1100 rev minÿ1 and a feed rate of 130 mm minÿ1. The roughnesses of the specimens were measured with a Perthometer SP6 instrument with a stylus tip radius of 5 mm. Both M and RC ®ltering was used and not all the measurements were carried out with the evaluation lengths and cut-o values speci®ed in ISO 4288. For the actual values used, see Ref. [1]. However, all measurements used for one test type were identical and the differences between the instruments and measurements are therefore believed not to have in¯uenced the conclusions of the results. The roughness was measured ®ve or six times on each specimen and the mean value was used in the results. The Charpy impact tests were carried out on unnotched specimens using a Zwick 5102 equipment. Two
pendulums with maximum swings of 5 an 10 kpcm (0.491 and 0.982 J) were used. The bending tests were carried out on an Instron 5568 machine with a 50 kN load cell and using a cross head speed of 1 mm minÿ1. The equipment for the bending tests was designed for the study to allow for large deformations of the specimens. The design was based on ASTM D790-91. It was a four-point test to ensure that a large part of the specimen was loaded with the maximum load. The rollers had a radius of 4 mm and the support span was 48 mm. The fatigue test were carried out in equipment which was built for the study. The design was based on DIN 53 442. The equipment is shown in Fig. 2 and a full description can be found in Ref. [1]. The equipment is driven by a motor with frequency control, and the rotation of the eccentric disc is transferred to a bending of the specimen through the connecting rod. This results in a deformation controlled test mode, whereby the risk of problems with accelerating degradation is minimised. The design ensures a constant bending moment and no longitudinal stresses in the test specimens. The mean deformation and the amplitude are adjusted by varying the length and the fastening position of the connecting rod. The load transmitted to the measuring arm is measured by the load cell which is based on a full bridge strain gauge. From the dimensions of the equipment and the specimen, it is possible to calculate the stress level in the test specimen. The failure criteria used was complete fracture, which resulted in a considerable decrease in the force measured. The tests were carried out with a zero mean deformation and a frequency of approximately 1 Hz. As the tests were deformation controlled, the different stinesses for the two ®bre orientations resulted in dierent stress levels for the two types of specimens.
Table 1 The combinations of the tool radius and the feed rates used to obtain three roughness levels of the POM and PP specimens Tool radius (mm)
Feed rate (mm revÿ1)
Roughness level
Symbol
0.8 0.8 0.4
0.025 0.250 0.250
Low Medium High
^ & ~
109
Fig. 2. The fatigue equipment.
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E. Eriksen / Composites Science and Technology 60 (2000) 107±113
Table 2 The deformation levels and the corresponding maximum bending stresses used in the fatigue tests Main ®bre orientation
Deformation level
Corresponding maximum stress (MPa)
Parallel to the length direction
1 2 3 4
22 31 41 46
Perpendicular to the length direction
2 4
24 37
No measurable heating of the specimens occurred. The deformation levels (resulting from four dierent settings of the connecting rod) and their corresponding maximum bending stresses in the specimens are shown in Table 2. The calculation of the stresses was based on the load measured at the beginning of the tests and the following equation: max=M ymax Iÿ1, where M is the bending moment, ymax is half of the thickness of the specimen and I is the moment of inertia. The smallest cross section of the specimens was used in the calculations, even though some of them broke at other positions. 3. Results In the following ®gures, the strength values on the yaxes dier between the materials, but the lengths of the intervals are of equal sizes to ease comparisons. The symbols used in the ®gures indicate the three roughness levels to which the specimens were machined. The results from the impact and monotonic bending tests are shown in Figs. 3 and 4. Two sample t tests were carried out to compare the strengths of the specimens with low and high roughnesses for each case. Separate variance tests were used. The p values calculated are given in Table 3. The results from the fatigue tests are shown in Fig. 5. They are shown with reference to the deformation levels and not to the stresses. This was done as the deformation was constant throughout a test, whereas the load could not be read precisely and decreased slightly during crack growth. The results for the three dierent surface roughnesses tested at the same deformation level are shown above each other because of the overlapping. The results show that the number of cycles to fracture was not in¯uenced by the surface roughness for the material and the conditions tested. Neither was there any signi®cant dierence between the number of cycles to failure for the two ®bre orientations when the results were compared in relation to the deformation levels.
Fig. 3. Charpy impact strength as a function of the roughness; ^low, &medium, high: (a) POM; (b) PP; (c) SAN with the main ®bre orientation in the load direction; (d) SAN with the main ®bre orientation perpendicular to the load direction.
4. Discussion The impact and monotonic bending strengths of the three materials studied were found to be independent of the surface roughness at a 5% level of signi®cance. For the fatigue tests of the SAN specimens, the independence was seen by the naked eye from Fig. 5. The independence of the surface roughness may be caused by a low bond strength between the ®bres and the matrix for the materials studied; this was seen from SEM images of the fracture surfaces. The matrix materials used may also be relatively insensitive to notches. These two properties result in most of the fracture energy being
E. Eriksen / Composites Science and Technology 60 (2000) 107±113
111
Fig. 5. Fatigue tests; relations between the deformation level and the number of cycles to fracture. ^low, &medium, high: (a) SAN with the main ®bre orientation in the load direction; (b) SAN with the main ®bre orientation perpendicular to the load direction.
Fig. 4. Monotonic bending strength as a function of the roughness; ^low, &medium, high: (a) POM; (b) SAN with the main ®bre orientation in the load direction; (c) SAN with the main ®bre orientation perpendicular to the load direction. Table 3 The probabilities (p values) calculated from two-sample t tests comparing the strengths of specimens with low and high roughness Test
Charpy impact Monotonic bending
POM
PP
SAN Fibres in the load direction
Fibres perpendicular to the load direction
0.955
0.279
0.710
0.169
0.069
±
0.839
0.069
used for crack propagation and hence a low in¯uence from the surface quality. Therefore, the results cannot be applied to composites with a stronger bond or with a more notch sensitive matrix material before further studies have been carried out. The ®ndings cannot exclude the possibility that there is a dierence between specimens without any defects (as prescribed in the standards) and specimens with machining marks, even with very low roughnesses. This would mean that the signi®cance of
the surface defects is related to whether or not they are present but is independent of their size. If this is the case, it emphasises the problems related to applying the results from standardised tests to industrially machined components. Another possible reason for the unexpected lack of in¯uence from the surface roughness is that the Ra-value may be inappropriate to describe the surface details of signi®cance for the strength. For some of the mechanical tests a relatively large scatter in the strength was found for the same roughness. This may be caused by dierences in other parameters as, for example, the size of closed cracks, the size of the deepest valleys of the roughness pro®le or some still unde®ned parameters. At a 10% level of signi®cance, the monotonic bending strength of POM and of SAN with the ®bres perpendicular to the load direction showed a signi®cant dependence on the roughness. This may indicate that for some materials the roughness in¯uences some strength properties and not others. However, some of the roughness levels used in the study were larger than what is normally applied to industrial components, and that may be the reason for the signi®cant in¯uence measured. More investigations of this aspect are necessary before ®nal conclusions can be made. A part of the purpose of the study was to investigate, whether the prescriptions in the standards for mechanical tests are sensible. If the strength of most materials turns out to be in¯uenced by the surface roughness, that
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may be a sucient reason to keep the standards as they are. For some materials this may, however, cause unnecessary preparation time for mechanical testing and cause doubt about the mechanical properties of components with machining marks. A possible solution will be to distinguish between groups of materials with different prescriptions for the test. If the standards are modi®ed in this way, they must include procedures on how to determine which of the groups a speci®c material belongs to. The determination of such procedures will need further investigations and thorough considerations. For example, it must be considered how much a new material may dier from one that is already designated to a group, before the procedure for the grouping is to be carried out. Furthermore it must be ensured that it is possible to make reliable comparisons between material data measured by dierent suppliers who may have used specimens with dierent surface qualities. SFRTP are complex materials mainly as a result of the anisotropic material structure which is determined by the manufacturing process. Therefore many aspects have to be taken into account when these materials are used for new applications. Lack of knowledge about how to get the best use of the advantages of SFRTP may result in unsuccessful attempts leading to limited or at least not optimised use of the materials. Lack of knowledge about the signi®cance of the surface roughness may cause the designers to consult the standards for mechanical tests to get some information about the material behaviour. These standards may cause unnecessary doubts about the obtainable strength properties of the materials if they are to be machined. If it can be proved that the strength is not in¯uenced by the surface roughness for the materials under consideration for a speci®c purpose, the eort can be concentrated on other aspects, such as the ®bre orientation which is known to in¯uence the properties signi®cantly. The possibility to concentrate on one or a few new aspects will ease a change-over to manufacturing with SFRTP. In the manufacturing industry it is often desired to keep the production time as low as possible. For machining processes this is done by an appropriate setting of the production parameters, which will often in¯uence the resulting surface quality. Therefore, it must be known which roughness can be excepted. From the present results it seems as if the surface quality is not as crucial as expected, and a higher production rate than what is currently used may therefore be possible. These conclusions are expected to be valid for other machining processes than those used in the present study, as long as the same type of chip formation process takes place. However, if the machining results in cracks extending into the material, the strength may be lowered extensively without any signi®cant change in the roughness. Further studies are necessary, before any conclusions can be made on the machining methods, in which the mate-
rial is not removed by conventional cutting processes, for example water- or laser cutting. For these methods, other parameters than the surface roughness may also be more appropriate to describe the surface quality. For metals the fatigue strength is highly in¯uenced by defects in the surface. Since this in¯uence was not found in the present study, it may be advantageous to use SFRTP instead of metals for applications where the fatigue properties are crucial. This can be valuable not only due to the independence of the surface quality when the component is taken into use, but also because defects caused by wear or accidental knocks or scratches in service are less critical. 5. Conclusions In the present study it was investigated how mechanical strength properties of three thermoplastics reinforced with short ®bres were in¯uenced by the roughness of the machined surfaces. The following strength properties were included: Charpy impact, monotonic bending and fatigue bending. The results are expected to be valid for machining processes with the same type of chip formation, whereas other processes may show other behaviours. Furthermore, the results cannot be applied to other environments before further investigations are made. The following conclusions can be drawn from the present study: . The mechanical strength properties were found to be independent of the surface roughness at a 5% level of signi®cance. This may not be the case for materials with a stronger bonding between the matrix and the ®bres or composites which have more notch sensitive matrix materials. . For some materials the in¯uence from the surface roughness may depend on the type of loading. . The standards for mechanical testing by use of machined specimens may need to be changed. The purpose will be to avoid unnecessary work in the preparation of the specimens and unnecessary doubt about whether the properties measured are valid for industrially machined components. . A shorter production time is possible if the demands on the surface quality can be lowered. . There may be new advantageous applications of SFRTP if defects caused by wear or knocks and scratches during use are less critical than for metals. Acknowledgements The study was carried out when the author was a Ph.D. student at the Department of Production, Aalborg University, Denmark.
E. Eriksen / Composites Science and Technology 60 (2000) 107±113
References [1] Eriksen E. Machining of short ®bre reinforced thermoplastics. Ph.D. thesis, Department of Production, Aalborg University, Denmark, 1997. [2] Tagliaferri V, Caprino G, Diterlizzi A. Eect of drilling parameters on the ®nish and mechanical properties of GFRP composites. Int J Mach Tools Manufact 1990;30:77±84. [3] Persson E, Eriksson I, Zackrisson L. Eects of hole machining defects on strength and fatigue life of composite laminates. Composites Part A 1997;28A:141±51.
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[4] Arola D, Ramula M. Machining-induced surface texture eects on the ¯exural properties of a graphite/epoxy laminate. Composites 1994;25:822±34. [5] Curtis T. The fatigue behaviour of ®brous composite materials. Journal of Strain Analysis 1989;24:47±56. [6] Nisitani H, Noguchi H, Kim Y-H. Fatigue process in short carbon®ber reinforced polyamid 6.6 under rotating-bending and torsional fatigue. Eng Fract Mech 1993;45:497±512. [7] Noguchi H, Kim Y-H, Nisitani H. On the cumulative fatigue damage in short carbon ®ber reinforced poly-ether-ether-ketone. Eng Fract Mech 1995;51:457±68.
The in¯uence of surface roughness on the mechanical strength properties of machined short-®bre-reinforced thermoplastics Else Eriksen* Institute of Mechanical Engineering, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg, Denmark Received 4 January 1999; received in revised form 10 June 1999; accepted 30 July 1999
Abstract When a component is manufactured by machining, the surface roughness often diers from what is prescribed in standards on the preparation of test specimens for mechanical tests. If the surface roughness in¯uences the strength, the properties of a machined part are therefore lower than the values given by the material supplier. The present study investigates the in¯uence of the surface roughness of three machined short-®bre-reinforced thermoplastics (SFRTP) on the following strength properties: Charpy impact, monotonic bending and ¯exural fatigue. The strengths were found to be independent of the roughness when using a 5% level of signi®cance. If this turns out to be true for other materials as well, the standards may need reassessment to avoid unnecessary work and unnecessary doubt about the use of the data. Furthermore, the production time can be lowered and there may be new advantageous applications of SFRTP. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Short-®bre composites; B. Strength; Machining
1. Introduction Most products made from short-®bre-reinforced thermoplastics (SFRTP) are manufactured to their ®nal form in one process by injection moulding. However, for some geometries or production numbers it is more advantageous to use machining either from extruded standard geometries or from parts moulded with a simpler geometry than the ®nal one. Machining is still a relatively new manufacturing method for SFRTP, and the theories and experience from metals and unreinforced plastics cannot be applied because of the dierent microstructure and thermal properties. It has been shown that it seems to be possible to set up guide lines for the manner in which the surface quality of machined SFRTP is in¯uenced by the production parameters [1]. Such knowledge is only valuable, however, if it is known which quality to aim at. This will depend on the functions of the surface and will therefore be related to the properties of the surface itself or the way it in¯uences the bulk properties of the component.
* Tel.: +45-9635-9323; fax:+45-9815-1411. E-mail address: [email protected] (E. Eriksen).
In addition, the surface quality may be decisive for the extent to which both types of properties are in¯uenced by the environment. Much research aims at describing how the mechanical properties of SFRTP are related to the microstructure. A comprehensive understanding of these relationships can be used to optimise the structure to ®nd the best properties for a certain application. However, if some of the mechanical properties are in¯uenced by the surface quality, a model based purely on the microstructure may be worthless for use in the machining industry. This will be the case for the strength properties if defects in the surface cause crack growth when the part is loaded. In standards on mechanical tests it is prescribed that test specimens made by machining must be manufactured very carefully. For example, ISO 2818 Plastics Ð Preparation of test specimens by machining prescribes that ``the machined surfaces and edges of the ®nished specimens shall be free of visible ¯aws, scratches or other imperfections when viewed with a low-power magnifying glass (approximately5 magni®cation)''. Such prescriptions indicate that the strength properties are, at least in some cases, expected to be highly in¯uenced by the surface quality of the test specimens. Since this careful treatment diers considerably from an ordinary industrial
0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(99)00102-5
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E. Eriksen / Composites Science and Technology 60 (2000) 107±113
procedure, the data for the mechanical properties given by the material suppliers may dier from the properties of industrially manufactured components. If this is the case, they may be of limited value for design purposes. On the other hand, if there is no signi®cant in¯uence, the test standards may need reassessment to avoid unnecessary work and unnecessary doubt about the validity of the data. Some research has dealt with the in¯uence of the surface quality on the mechanical strength properties of ®brereinforced plastics (FRP). Tagliaferri et al. investigated how the hole quality of drilled glass-®bre-reinforced (GFR) epoxy in¯uences the tensile properties [2]. The tensile strength was in no way aected by the cut ®nish, whereas the bearing strength was markedly decreased for an increasing damage around the holes. Persson et al. investigated the eects of the hole quality for carbon®bre-reinforced (CFR) epoxy [3]. The static and fatigue strengths of pin-loaded laminates were measured, and in both cases the machining defects were found to reduce the strengths signi®cantly. The bending strength of CFR epoxy machined by dierent methods resulting in different surface qualities was studied by Arola and Ramulu [4]. They found no dependence on the surface quality. It was observed that the failure predominantly occurred on the compression side of the specimens. That may be the reason why the surface roughness did not in¯uence the strength, as the compression forces closed the possible defects from the machining. These results cannot be transferred to SFRTP for which the fracture is supposed to start on the tensile side of the specimens. Fatigue tests of GFR epoxy showed that the fatigue life is very dependent on the surface quality [5]. This was found by studying the eect of polishing the edge of the specimens, which increased the fatigue strength signi®cantly. Other studies were carried out to investigate the fatigue failure process and the eects of reinforcing thermoplastics with short ®bres [6,7]. In both these studies the specimens were manufactured by machining, and to eliminate the eect from the machining process itself and the resulting surface roughness, the specimens were annealed and polished before the fatigue tests were carried out. This treatment indicates that the authors expected an in¯uence from the surface roughness of the machined SFRTP specimens. The present study was carried out to investigate how the following strength properties of three SFRTP were in¯uenced by the roughness of the machined surfaces: Charpy impact, monotonic bending and fatigue bending. The standardised roughness parameter, Ra, was used to characterise the surface quality, as this is the parameter most often used in the manufacturing industry. Furthermore, it is a quantitative measure and thereby made the comparisons between the specimens easier. The results from the impact and monotonic bending tests were analysed by use of a t test, and the
strengths were found to be independent of the roughness when using a 5% level of signi®cance. For the fatigue tests the independence was seen by the naked eye. It was therefore concluded that the standards for mechanical testing may need to be changed to avoid unnecessary work and unnecessary doubt about the validity of data obtained from test specimens without surface defects. If insensitivity of the surface quality turns out to be characteristic for a number of SFRTP, a shorter production time is possible, and there may also be more advantageous applications of these materials than the present. 2. Materials and methods Three thermoplastics reinforced with short glass ®bres were used in the study: Polyoxymethylene (POM) with 26 wt% ®bres; Polypropylene (PP) with 30 wt% ®bres and Styrene acrylonitrile (SAN) with 35 wt% ®bres. They will be named by the names of the matrix materials in the following. The Charpy impact strength was investigated for all three materials; the monotonic bending strength was investigated for POM and SAN; and the fatigue bending test was only carried out for the SAN specimens. Tensile tests were also made but many of the specimens broke outside the gauge length, and the results are therefore not included in the following. The POM and PP specimens were injection moulded as standard tensile test specimens (ISO 3167), and the middle part (the gauge length) was used for the bending and impact tests. The specimens were machined on both sides by turning after being fastened radially on a circular disc mounted on a Cazeneuve 590 HBY lathe (see Fig. 1). New wolfram carbide inserts with the numbers TPMR160304-F1 TP30 and TPMR160308-F1 TP30 were used. The rotational speed used was 300 rev minÿ1, which resulted in cutting speeds between 188 and 302 m minÿ1 for the narrow part of the specimens. Three roughness levels of the machined surfaces were obtained
Fig. 1. The mounting of the POM and PP specimens on a circular disc. The enlargement shows the direction of the tool marks.
E. Eriksen / Composites Science and Technology 60 (2000) 107±113
by the combinations of the feed rate and the tool radius shown in Table 1. Below, specimens from the three levels will be referred to as `low', `medium' and `high'. The corresponding symbols used in the ®gures are included in the table. The SAN specimens were milled from injection moulded quadratic plates with the dimensions 80806 mm. The specimens were taken from positions where the ®bres were as much aligned in one direction as possible. Such positions were found by polishing several sections of the moulded plates. Two perpendicular orientations were studied. 1.7 mm of material was removed from each side resulting in a ®nal thickness of 2.6 mm. The geometry described in the standard DIN 53 442 was used for the fatigue test. For the impact and monotonic bending tests, specimens with a rectangular cross-section corresponding to the middle part of the fatigue specimens were used, i.e. a width of 10 mm. The surfaces being characterised were machined with a TOS FN20 milling machine. The cutter had a diameter of 20 mm and only one cutting edge was used. The rotational speed was 800 rev minÿ1 which corresponded to a cutting speed of 50.3 mm minÿ1. The inserts used were the same as for turning, but only those with a tool radius of 0.4 mm were used. The three dierent roughness levels were obtained by varying the feed rate. These settings were used: 80, 160 and 250 mm minÿ1. The outer geometries were milled with an EMCOtronic machine with Komet Q36 18000.01.21 K10 inserts. The tool had a diameter of 12 mm, a speed of 1100 rev minÿ1 and a feed rate of 130 mm minÿ1. The roughnesses of the specimens were measured with a Perthometer SP6 instrument with a stylus tip radius of 5 mm. Both M and RC ®ltering was used and not all the measurements were carried out with the evaluation lengths and cut-o values speci®ed in ISO 4288. For the actual values used, see Ref. [1]. However, all measurements used for one test type were identical and the differences between the instruments and measurements are therefore believed not to have in¯uenced the conclusions of the results. The roughness was measured ®ve or six times on each specimen and the mean value was used in the results. The Charpy impact tests were carried out on unnotched specimens using a Zwick 5102 equipment. Two
pendulums with maximum swings of 5 an 10 kpcm (0.491 and 0.982 J) were used. The bending tests were carried out on an Instron 5568 machine with a 50 kN load cell and using a cross head speed of 1 mm minÿ1. The equipment for the bending tests was designed for the study to allow for large deformations of the specimens. The design was based on ASTM D790-91. It was a four-point test to ensure that a large part of the specimen was loaded with the maximum load. The rollers had a radius of 4 mm and the support span was 48 mm. The fatigue test were carried out in equipment which was built for the study. The design was based on DIN 53 442. The equipment is shown in Fig. 2 and a full description can be found in Ref. [1]. The equipment is driven by a motor with frequency control, and the rotation of the eccentric disc is transferred to a bending of the specimen through the connecting rod. This results in a deformation controlled test mode, whereby the risk of problems with accelerating degradation is minimised. The design ensures a constant bending moment and no longitudinal stresses in the test specimens. The mean deformation and the amplitude are adjusted by varying the length and the fastening position of the connecting rod. The load transmitted to the measuring arm is measured by the load cell which is based on a full bridge strain gauge. From the dimensions of the equipment and the specimen, it is possible to calculate the stress level in the test specimen. The failure criteria used was complete fracture, which resulted in a considerable decrease in the force measured. The tests were carried out with a zero mean deformation and a frequency of approximately 1 Hz. As the tests were deformation controlled, the different stinesses for the two ®bre orientations resulted in dierent stress levels for the two types of specimens.
Table 1 The combinations of the tool radius and the feed rates used to obtain three roughness levels of the POM and PP specimens Tool radius (mm)
Feed rate (mm revÿ1)
Roughness level
Symbol
0.8 0.8 0.4
0.025 0.250 0.250
Low Medium High
^ & ~
109
Fig. 2. The fatigue equipment.
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Table 2 The deformation levels and the corresponding maximum bending stresses used in the fatigue tests Main ®bre orientation
Deformation level
Corresponding maximum stress (MPa)
Parallel to the length direction
1 2 3 4
22 31 41 46
Perpendicular to the length direction
2 4
24 37
No measurable heating of the specimens occurred. The deformation levels (resulting from four dierent settings of the connecting rod) and their corresponding maximum bending stresses in the specimens are shown in Table 2. The calculation of the stresses was based on the load measured at the beginning of the tests and the following equation: max=M ymax Iÿ1, where M is the bending moment, ymax is half of the thickness of the specimen and I is the moment of inertia. The smallest cross section of the specimens was used in the calculations, even though some of them broke at other positions. 3. Results In the following ®gures, the strength values on the yaxes dier between the materials, but the lengths of the intervals are of equal sizes to ease comparisons. The symbols used in the ®gures indicate the three roughness levels to which the specimens were machined. The results from the impact and monotonic bending tests are shown in Figs. 3 and 4. Two sample t tests were carried out to compare the strengths of the specimens with low and high roughnesses for each case. Separate variance tests were used. The p values calculated are given in Table 3. The results from the fatigue tests are shown in Fig. 5. They are shown with reference to the deformation levels and not to the stresses. This was done as the deformation was constant throughout a test, whereas the load could not be read precisely and decreased slightly during crack growth. The results for the three dierent surface roughnesses tested at the same deformation level are shown above each other because of the overlapping. The results show that the number of cycles to fracture was not in¯uenced by the surface roughness for the material and the conditions tested. Neither was there any signi®cant dierence between the number of cycles to failure for the two ®bre orientations when the results were compared in relation to the deformation levels.
Fig. 3. Charpy impact strength as a function of the roughness; ^low, &medium, high: (a) POM; (b) PP; (c) SAN with the main ®bre orientation in the load direction; (d) SAN with the main ®bre orientation perpendicular to the load direction.
4. Discussion The impact and monotonic bending strengths of the three materials studied were found to be independent of the surface roughness at a 5% level of signi®cance. For the fatigue tests of the SAN specimens, the independence was seen by the naked eye from Fig. 5. The independence of the surface roughness may be caused by a low bond strength between the ®bres and the matrix for the materials studied; this was seen from SEM images of the fracture surfaces. The matrix materials used may also be relatively insensitive to notches. These two properties result in most of the fracture energy being
E. Eriksen / Composites Science and Technology 60 (2000) 107±113
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Fig. 5. Fatigue tests; relations between the deformation level and the number of cycles to fracture. ^low, &medium, high: (a) SAN with the main ®bre orientation in the load direction; (b) SAN with the main ®bre orientation perpendicular to the load direction.
Fig. 4. Monotonic bending strength as a function of the roughness; ^low, &medium, high: (a) POM; (b) SAN with the main ®bre orientation in the load direction; (c) SAN with the main ®bre orientation perpendicular to the load direction. Table 3 The probabilities (p values) calculated from two-sample t tests comparing the strengths of specimens with low and high roughness Test
Charpy impact Monotonic bending
POM
PP
SAN Fibres in the load direction
Fibres perpendicular to the load direction
0.955
0.279
0.710
0.169
0.069
±
0.839
0.069
used for crack propagation and hence a low in¯uence from the surface quality. Therefore, the results cannot be applied to composites with a stronger bond or with a more notch sensitive matrix material before further studies have been carried out. The ®ndings cannot exclude the possibility that there is a dierence between specimens without any defects (as prescribed in the standards) and specimens with machining marks, even with very low roughnesses. This would mean that the signi®cance of
the surface defects is related to whether or not they are present but is independent of their size. If this is the case, it emphasises the problems related to applying the results from standardised tests to industrially machined components. Another possible reason for the unexpected lack of in¯uence from the surface roughness is that the Ra-value may be inappropriate to describe the surface details of signi®cance for the strength. For some of the mechanical tests a relatively large scatter in the strength was found for the same roughness. This may be caused by dierences in other parameters as, for example, the size of closed cracks, the size of the deepest valleys of the roughness pro®le or some still unde®ned parameters. At a 10% level of signi®cance, the monotonic bending strength of POM and of SAN with the ®bres perpendicular to the load direction showed a signi®cant dependence on the roughness. This may indicate that for some materials the roughness in¯uences some strength properties and not others. However, some of the roughness levels used in the study were larger than what is normally applied to industrial components, and that may be the reason for the signi®cant in¯uence measured. More investigations of this aspect are necessary before ®nal conclusions can be made. A part of the purpose of the study was to investigate, whether the prescriptions in the standards for mechanical tests are sensible. If the strength of most materials turns out to be in¯uenced by the surface roughness, that
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may be a sucient reason to keep the standards as they are. For some materials this may, however, cause unnecessary preparation time for mechanical testing and cause doubt about the mechanical properties of components with machining marks. A possible solution will be to distinguish between groups of materials with different prescriptions for the test. If the standards are modi®ed in this way, they must include procedures on how to determine which of the groups a speci®c material belongs to. The determination of such procedures will need further investigations and thorough considerations. For example, it must be considered how much a new material may dier from one that is already designated to a group, before the procedure for the grouping is to be carried out. Furthermore it must be ensured that it is possible to make reliable comparisons between material data measured by dierent suppliers who may have used specimens with dierent surface qualities. SFRTP are complex materials mainly as a result of the anisotropic material structure which is determined by the manufacturing process. Therefore many aspects have to be taken into account when these materials are used for new applications. Lack of knowledge about how to get the best use of the advantages of SFRTP may result in unsuccessful attempts leading to limited or at least not optimised use of the materials. Lack of knowledge about the signi®cance of the surface roughness may cause the designers to consult the standards for mechanical tests to get some information about the material behaviour. These standards may cause unnecessary doubts about the obtainable strength properties of the materials if they are to be machined. If it can be proved that the strength is not in¯uenced by the surface roughness for the materials under consideration for a speci®c purpose, the eort can be concentrated on other aspects, such as the ®bre orientation which is known to in¯uence the properties signi®cantly. The possibility to concentrate on one or a few new aspects will ease a change-over to manufacturing with SFRTP. In the manufacturing industry it is often desired to keep the production time as low as possible. For machining processes this is done by an appropriate setting of the production parameters, which will often in¯uence the resulting surface quality. Therefore, it must be known which roughness can be excepted. From the present results it seems as if the surface quality is not as crucial as expected, and a higher production rate than what is currently used may therefore be possible. These conclusions are expected to be valid for other machining processes than those used in the present study, as long as the same type of chip formation process takes place. However, if the machining results in cracks extending into the material, the strength may be lowered extensively without any signi®cant change in the roughness. Further studies are necessary, before any conclusions can be made on the machining methods, in which the mate-
rial is not removed by conventional cutting processes, for example water- or laser cutting. For these methods, other parameters than the surface roughness may also be more appropriate to describe the surface quality. For metals the fatigue strength is highly in¯uenced by defects in the surface. Since this in¯uence was not found in the present study, it may be advantageous to use SFRTP instead of metals for applications where the fatigue properties are crucial. This can be valuable not only due to the independence of the surface quality when the component is taken into use, but also because defects caused by wear or accidental knocks or scratches in service are less critical. 5. Conclusions In the present study it was investigated how mechanical strength properties of three thermoplastics reinforced with short ®bres were in¯uenced by the roughness of the machined surfaces. The following strength properties were included: Charpy impact, monotonic bending and fatigue bending. The results are expected to be valid for machining processes with the same type of chip formation, whereas other processes may show other behaviours. Furthermore, the results cannot be applied to other environments before further investigations are made. The following conclusions can be drawn from the present study: . The mechanical strength properties were found to be independent of the surface roughness at a 5% level of signi®cance. This may not be the case for materials with a stronger bonding between the matrix and the ®bres or composites which have more notch sensitive matrix materials. . For some materials the in¯uence from the surface roughness may depend on the type of loading. . The standards for mechanical testing by use of machined specimens may need to be changed. The purpose will be to avoid unnecessary work in the preparation of the specimens and unnecessary doubt about whether the properties measured are valid for industrially machined components. . A shorter production time is possible if the demands on the surface quality can be lowered. . There may be new advantageous applications of SFRTP if defects caused by wear or knocks and scratches during use are less critical than for metals. Acknowledgements The study was carried out when the author was a Ph.D. student at the Department of Production, Aalborg University, Denmark.
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