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The Influence of the Joint Thickness Upon the Impact Strength of Block Adhesive Joints
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Transportation Research Procedia 00 (2018) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Transportation Research Procedia 35 (2018) 80–89 www.elsevier.com/locate/procedia
International Conference on Air Transport – INAIR 2018
The Influence of the Joint Thickness Upon the Impact Strength of Block Adhesive Joints Andrzej Komoreka*, Jan Godzimirski b, Wojciech Kucharczykc a
Polish Air Force Academy, Faculty of Aviation, 35 Dywizjonu 303 Street, 08-521 Dęblin, Poland
b
Military University of Technology, Faculty of Mechatronics and Aviation, 2 Urbanowicza Street 01-479 Warsaw, Poland
c
Faculty of Mechanical Engineering, University of Technology and Humanities 54 Stasieckiego Street, 26-600 Radom, Poland
Abstract The research described in this article relates to one of the aspects of methodology of investigating impact loading in block bonded joints. The authors checked experimentally the effect of the thickness of adhesive joints upon impact strength of the connection. The experimental research was completed with preliminary dynamic numerical calculations of the test cases. The experimental testing was conducted with a pendulum hammer designed to examine adhesive joints. The maximum energy of the pendulum used in the investigation equalled 15 J. In order to determine the impact strength of the examined connections, it was used a dependency according to which the energy used to detach the upper element of the sample in such a test, that is the energy lost by the pendulum, is the measure of impact strength of the adhesive joint. The elements of the samples were made of steel S235 or an aluminum alloy 2017A. In order to bond the samples, was used Epidian 57 epoxide resin with Z1 hardener. The models for numerical computations were prepared on the basis of samples used in the experimental research. The prepared models, taking into account the initial and boundary conditions were subjected to dynamic numerical computations by means of the finite element method with the use of the Explicit Dynamics module in the ANSYS programme. The aim of the numerical calculations was to compare the compliance of the results of the experimental and analytical investigations as well as the assessment of the impact of the size of boundary conditions, which were subjected to parametrization, upon the values of occurring stresses during the conducted impact loading simulations. The results of the experimental investigation indicate that along with increasing the thickness of the adhesive joint, impact strength of the connection is reduced, regardless of the bonded material. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)
* Corresponding author. Tel.: +48 261-518-86 E-mail address: [email protected] 2352-1465 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – INAIR 2018. 2352-1465 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – INAIR 2018. 10.1016/j.trpro.2018.12.015
Andrzej Komorek et al. / Transportation Research Procedia 35 (2018) 80–89
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Komorek, Godzimirski, Kucharczyk/ Transportation Research Procedia 00 (2018) 000–000
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© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – INAIR INAIR 2018. 2018. Keywords: adhesive joint; impact loading; pendulum hammer; numerical calculations
1. Introduction Aircraft manufacturers aim at achieving the lowest weight of airship. One way to reduce weight is to join structural elements by an adhesive, since in mechanical joints the mass of the connecting elements is several times bigger than the weight of the adhesive forming the same connections; besides bonding does not weaken the connected elements. However, effective use of adhesive joints largely depends upon their properties. One of the relevant strength parameters of adhesive joints exploited in connecting components in aircraft constructions is impact strength, which is discussed in this article. The research problem concerns the influence of the thickness of the adhesive joint upon the strength of the connection that was subjected to impact loading. In the tests we used the methodology of investigating impact strength of block samples in which the adhesive joint is damaged after impact shear loading. Using this technology, the energy lost in the destruction of the sample, which is the measure of impact strength of the connection Taylor (1996), can be determined from the difference in the height of the pendulum prior to and after the impact. This test method is difficult to repeat due to the need to keep very precise behaviour parameters of the samples as well as ensuring repeatibility of the test conditions Komorek and Przybyłek (2015), Adams and Harris (1996), da Silva et al. (2012), Adams et al. (1997). The method of testing impact strength of block adhesive joints, provided that the conditions of the test results are repeated, allows obtaining impact strength findings of connections which are prepared with different glue compositions. However, it is difficult to utilize the results obtained in practice. In order to obtain results that may find wider application in the design and implementation of structural bonding, research is conducted into impact strength of adhesive lap joints Karachalios et.al. (2013), Casas-Rodriguez et al. (2007), Goglio and Rosetto (2008), Belingardi et al. (2005), Harris and Adams (1985). The main objective of the investigation was to determine the relationship between thickness and impact strength of the adhesive joint in the shear impact test. In addition, we compared impact strength of adhesive joints in steel and duralumin elements. Having conducted impact loading tests, the fractures were inspected with regard to the type of the tear and accuracy of the damage. The experimental studies were completed with dynamic numerical calculations of the test cases. 2. Methodology of experimental research In order to conduct the investigation was made 10 series of adhesively bonded block samples (Fig. 1).
Fig. 1. Dimensions of the samples used in the investigation.
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The aluminum alloy 2017A or steel S235 were selected for bonding elements. The adhesive material, which was used to make the bonding was Epidian 57 with Z-1 hardener. It was prepared 5 series of duralumin samples and 5 series of steel samples, each including 10 pieces. The samples in particular batches varied in the thickness of the joints. Different thicknesses of the joints was obtained through applying spacers in the form of a sewing thread which was placed in between the bonded elements. Prior to bonding, the metal parts were cleaned up; their surface was given proper structure through abrasive blasting, with copper slag as an abrasive medium. Next the samples were washed with petroleum naphtha to remove the grease present in the samples as well as the dust remaining after abrasive blasting. The washed samples were placed in the chamber of the laboratory dryer at a temperature of 90°C in order to vaporize the petroleum naphtha from their surface. The prepared elements were bonded as soon as possible to avoid accidental soiling of the bonded surfaces or falling of dust, oxides or moisture. While fixing the items for bonding, was paid particular attention to proper positioning of the elements against each other, since even slight irregularities in the geometry of the samples lead to significant changes in the obtained findings. The bonded series of samples with an identical joint thickness were placed on a base plate and clamped down at a pressure of 40 kPa for the time of curing, which equalled 7 days at ambient temperature (21°C). After curing the joints, it was performed a visual inspection of the quality of the obtained connections and removed excess glue and threads outside the bonded surfaces. Removing threads soaked in glue is particularly relevant since they may be caught up during an impact loading test by a detached upper element and consequently stop the dropping tool, which will translate into a higher value of impact loading of the connection. The excess glue may result in raising the endurance of the joint (in practice, they are not used unless necessary). Measuring the thickness was performed using the indirect method - the thickness of the glue layer was calculated on the basis of other measurements of the dimensions of the sample. In the measurements was used a digital caliper. The obtained joint thicknesses have been shown in Table 1. Table 1. Thickness range of sample joints used in the investigation. No 1a. 2a. 3a. 4a. 5a. 1b. 2b. 3b. 4b. 5b.
Adhesive
Bonded material
The scope of the obtained joint thicknesses [mm]
The average thickness of the joint in series [mm]
2017A
0.04 – 0.09 0.15 – 0.21 0.30 – 0.34 0.30 – 0.37 0.62 – 0.67
0.07 0.19 0.32 0,32 0.65
S235
0.01 – 0.03 0.19 – 0.25 0.27 – 0.34 0.35 – 0.43 0.59 – 0.67
0.02 0.23 0.31 0.39 0.63
Epidian 57/Z1
The thickness of the joints in steel samples and samples of aluminum alloy 2017A were almost identical (Table 1). Only for the thinnest series of joints, was received smaller thicknesses for S235 rather than 2017A. 3. Experimental studies and discussion of the results The investigation was conducted on a special machine designed for testing block adhesive joints and lap adhesive joints. The maximum energy of the pendulum used in the investigation equalled 15 J, and the speed in the lowest position was equal to 2.96 m/s. The investigation was conducted by applying impact load according to the scheme shown in Figure 2 Komorek and Godzimirski (2016). During the investigation, it was paid particular attention to maintaining a constant distance between the impactor and the adhesive joint, due to a significant impact of this parameter upon the obtained results.
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Fig. 2 A way of applying load to the sample
The results of the conducted impact loading testing for duralumin samples have been presented in Figure 3.
Fig. 3. The effect of thickness on impact loading of the connection of the elements made with aluminum alloy 2017A.
The highest impact strength was obtained for the samples with the smallest thickness of the joint, whereas the lowest one for connections with the greatest joint thickness. It is possible to observe a large discrepancy among particular results in each series. The impact strength of the samples where the average joint thickness equalled 0.65 mm was more than three times lower than in the samples whose average joint thickness was 0.07 mm. The results of the testing conducted for steel samples have been presented in Fig. 4. In the case of dural samples (Fig. 3), in a series with an average joint thickness of 0.07 mm there was achieved the highest average impact strength, whereas for steel samples the highest impact strength was obtained for the series of an average joint thickness equal to 0.23 mm (Fig. 4). With similar thicknesses of the joints which connect duralumin elements (Figure 3) and steel elements (Fig. 4), the impact strength of steel samples was lower than in duralumin samples, thus it is possible to formulate a hypothesis of weaker adhesion in steel samples. While analysing the results presented in Figures 3 and 4, it becomes obvious that the strength of the adhesive connection decreases along with the rise in thickness of the adhesive joint. Moreover, in each case, the average value of impact strength for duralumin samples has a greater value than for the same joint thicknesses in steel samples.
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Figure 4. The effect of the joint thickness upon impact loading of the connections of steel elements
4. Analysis of damage The analysis of damage to adhesive joints has been made in order to determine the character of the damage, regularities and similarities between the thicknesses of the adhesive joints under investigation. The analysis was conducted by means of a visual macroscopic and microscopic methods. The investigation made with a macroscopic method cannot be applied to the smallest joint thicknesses due to the fact that the scale of the destroyed adhesive does not to allow seeing any details or drawing conclusions. The joints whose thickness is below 0.1 mm were examined by means of the other method, which uses the electron microscope. The most frequent character of the cracking of larger joint thicknesses is shown in Figure. 5.
Fig. 5. Cracking of an adhesive joint after impact loading tests.
It was noted that most of the joints become fractured as in Figure 5, thus on the upper element there remains approximately 25-50% of the adhesive volume, similarly to the lower sample’s element, and the remainder of the glue is dispersed inside the test machine chamber. Another regularity is the manner of separating the adhesive joint. On the upper element there is more adhesive on the edge adjacent to applying impact loading, while on the bottom parts of the samples there is more glue residue away from the edge of the impact. The last point of the analysis is an assessment of the nature of the damage in order to evaluate whether the fracture occurred through tearing off a joint from one of the glued surfaces, or whether there is a thin layer of the adhesive glue on both bonded surfaces. Through a visual inspection it is possible to observe larger aggregations of the glue, however in cases of thin joints such an assessment is extremely difficult and ambiguous. The nature of the destruction of the joint was evaluated by means of a microscopic method, using the electron microscope HITACHI TM3030. The microscopic photos of the surface of the joint’s fractures in the samples are shown in Figures 6 and 7.
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Fig. 6. Image of breaking the joint connecting the elements of the alloy 2017A (electron microscope, magnification x 60).
Figure 6 shows the surface of the joint’s fracture connecting the elements made with 2017A alloy. In the photo it is possible to observe regular glue belts with a thickness of approximately 0.1 mm and a large number of small glue aggregations. The damage is cohesive-adhesive in it nature.
Fig. 7. The image of breaking the joint connecting the steel elements (electron microscope, magnification x 60).
The breaking of the thinnest steel sample joint has been depicted in Fig. 7. By visual inspection it was estimated that for this sample in the most part there occurred adhesive tear off with several glue aggregates, which are dark grey in the photo. The microscopic observations enabled to make a statement that in the assessed areas with regard to the glue residues, they remained little glue. 5. The analysis of FEM stresses in adhesive joints The calculations were carried out in the program Ansys, using the Explicit Dynamics module. It was built a simple numerical model of the sample, which was divided into 8-node hexahedral finite elements, dedicated for dynamic computations (impact strength) (Fig. 8). In the modelling was used the contact connection bonded-type Solid to Solid. The model was developed on the basis of the sample used in experimental research.
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Fig. 8. 3D model of the sample with a bonded cuboid element. Since was known the mass of the pendulum hammer and its initial energy, it was possible to calculate the speed at which the impactor strikes the adhesive sample, i.e. 2,960 m/s. In order to reflect the impactor’s mass, it was increased the density of the material to an extent that was declared for its manufacture so that it could be equal to the mass of the actual device. Having declared the Young’s modulus of the impactor's material as that of steel, it underwent excessive deformations, thus the impactor was modelled as an element consisting of two parts - one piece hitting directly the sample was given steel properties; the other had the modulus of elasticity greater, by one order of magnitude (2,000 GPa), so as to ensure adequate stiffness of the striking element (Figure 9). Finally, the models of the tested samples were dynamically impact loaded with a cuboid impactor 10x25x3 mm, with the speed of 2,960 mm/s and the density of 4.6 106 kg/m3, corresponding to the actual impact energy of the dropping hammer used in the experiment - 15 J. The calculations were carried out for the direction of the hammer as illustrated in the diagram (Fig. 2). The calculations were carried out for connections of steel or duralumin elements made with Epidian 57/Z1 adhesive. In the computations the authors took into account linear properties of an isotropic polymer material and nonlinear characteristics of the metal elements adopted from the software material database of ANSYS. It was compared the distributions of Max Principal Stresses in the joints for the same computational times, assuming that higher stress values should correspond to lower impact loading. The Max Principal Stresses hypothesis is best suited for analysis when testing glue joints.
Fig. 9. The impactor model consisting of two parts of varying stiffness.
It was also examined how the courses of principal stresses in the joints of dynamically impact loaded block samples change if the bonded elements are made with different materials and the adhesive is characterized with various stiffness (Figures 10, 11).
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Fig. 10. Max Principal Stresses and their time characteristics FME-determined for steel samples, loaded with the energy of 15 J and with different thicknesses of the joints: a - 0.1 mm, b – 0.2 mm, c-0.4 mm, d - 0.6 mm.
Fig. 11. Max Principal Stresses and their time characteristics FME-determined for duralumin samples, loaded with the energy of 15 J and with different thicknesses of the joints: a - 0.1 mm, b – 0.2 mm, c-0.4 mm, d - 0.6 mm.
Increasing the thickness of the joints affects the decrease in maximum stresses and extends the time of impact loading the joint, stretching the chart. A similar effect is obtained by replacing steel elements with duralumin ones. The analysis of the stress tensor component values with Max Principal Stresses close to the adhesive strength to stretching (approximately 83.5 MPa) brings nothing to the analysis - the components are comparable (Fig. 12, 13).
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Stresses [MPa]
88
9
100 80 60 40 20 0 0
0,2
0,4
0,6
0,8
Thickness of the adhesive joint [mm] SigI
Sigx
Sigy
Tauxy
Fig. 12. The values of the stress components calculated in the joints of the steel elements connections in the function of the joint's thickness (SigI –Max Principal Stresses, Sigy – normal stresses perpendicular to the joint's surface).
Stresses [MPa]
100 80 60 40 20 0
0
0,2
0,4
0,6
0,8
Thickness of the adhesive joint [mm] SigI
Sigx
Sigy
Tauxy
Fig. 13. The values of the stress components calculated in the joints of the duralumin element connections in the function of the joint's thickness (SigI –Max Principal Stresses, Sigy – normal stresses perpendicular to the joint's surface).
It seems that the drop in impact strength which is linked with the rise in the joints’ thickness (resulting from the experimental research) is related to the fall in the strength of the joints along with the increase in their thickness, similarly to the static investigations (Fig. 14)
Fig. 14. Dependence between static strength of the adhesive to tearing off and thickness of the adhesive joint.
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6. Conclusions 1. The results of the experimental research indicate a significant impact of the thickness of the joint upon the impact strength of the adhesive connections - increasing the thickness of the joint results in a decrease in the recorded impact strength, irrespective of the material of the bonded elements. The obtained experimental results are not confirmed by the results of numerical calculations, according to which the change in the thickness of the joint exerts no significant effect on the values of the stress tensor components. 2. The drop in impact strength along with the rise in the thickness of the joint probably results from a lower shear stress values of the joints with greater thickness. 3. The fall in impact strength associated with increasing the thickness of the joint may also be the result of increasing the distance of the impactor from the lower surface of the joint, however it needs to be proved in further analytical studies. 4. The findings of impact loading tests performed on the very same test station are characterised with large discrepancies, which is caused by dimensional tolerance of particular samples and requires their modification in order to eliminate the likelihood of uneven load. 5. The macroscopic analysis pointed to a regular nature of the destruction. The method of microscopic observation confirmed the adhesive-cohesive nature of breaking the adhesive connections with different joint thicknesses. 6. It appears that the methodology used in the preparation of the sample’s surface for bonding enables to obtain greatly improved adhesion of the aluminum alloy samples rather than that of the steel samples. 7. References Taylor A (1996) Impact Testing of Adhesive Joints. MTS Adhesive Project 2 AEA Technology 5 (2). Komorek A, Przybyłek P (2015) Initial research of impact strength in adhesive joints. Solid State Phenomena 237:160-165, DOI: 10.4028/www.scientific.net/SSP.237.160 Adams RD, Harris JA (1996) A critical assessment of the block impact test for measuring the impact strength of adhesive bonds. Int J Adhes Adhes 16: 61-71 https://doi.org/10.1016/0143-7496(95)00050-X LFM da Silva, DA Dillard, B Blackman, RD Adams (2012) Testing Adhesive Joints: Best Practices. Wiley & Sons Weinheim Adams, RD, Comyn, J, Wake WC (1997) Structural Adhesive Joints in Engineering. Chapman & Hall, London Karachalios EF, Adams RD, da Silva LFM (2013) Single lap joints loaded in tension with ductile steel adherends. Int J Adhes Adhes 43:96-108 https://doi.org/10.1016/j.ijadhadh.2013.01.017 Casas-Rodriguez JP, Ashcroft IA, Silberschmidt VV (2007) Damage evolution in adhesive joints to impact fatigue. J Sound Vib 308:467-478 https://doi.org/10.1016/j.jsv.2007.03.088 Goglio L, Rosetto M (2008) Impact rupture of structural adhesive joints under different stress combinations. Int J Impact Eng 35:635–643 https://doi.org/10.1016/j.ijimpeng.2007.02.006 Belingardi G, Goglio L, Rossetto M (2005) Impact behaviour of bonded built-up beams: experimental results. Int J Adhes Adhes 25:173–180 Harris JA, Adams RD (1985) An assessment of the impact performance of bonded joints for use in high energy absorbing structures. Proceedings of the Institution of Mechanical Engineers 199 C2:121-131, Komorek A, Godzimirski J (2016) The selected aspects of the research into impact loading of adhesive joints in block samples - comparison of different ways of applying the load. Maintenance Problems 4: 77-91
ScienceDirect
Available online at www.sciencedirect.com
Transportation Research Procedia 00 (2018) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Transportation Research Procedia 35 (2018) 80–89 www.elsevier.com/locate/procedia
International Conference on Air Transport – INAIR 2018
The Influence of the Joint Thickness Upon the Impact Strength of Block Adhesive Joints Andrzej Komoreka*, Jan Godzimirski b, Wojciech Kucharczykc a
Polish Air Force Academy, Faculty of Aviation, 35 Dywizjonu 303 Street, 08-521 Dęblin, Poland
b
Military University of Technology, Faculty of Mechatronics and Aviation, 2 Urbanowicza Street 01-479 Warsaw, Poland
c
Faculty of Mechanical Engineering, University of Technology and Humanities 54 Stasieckiego Street, 26-600 Radom, Poland
Abstract The research described in this article relates to one of the aspects of methodology of investigating impact loading in block bonded joints. The authors checked experimentally the effect of the thickness of adhesive joints upon impact strength of the connection. The experimental research was completed with preliminary dynamic numerical calculations of the test cases. The experimental testing was conducted with a pendulum hammer designed to examine adhesive joints. The maximum energy of the pendulum used in the investigation equalled 15 J. In order to determine the impact strength of the examined connections, it was used a dependency according to which the energy used to detach the upper element of the sample in such a test, that is the energy lost by the pendulum, is the measure of impact strength of the adhesive joint. The elements of the samples were made of steel S235 or an aluminum alloy 2017A. In order to bond the samples, was used Epidian 57 epoxide resin with Z1 hardener. The models for numerical computations were prepared on the basis of samples used in the experimental research. The prepared models, taking into account the initial and boundary conditions were subjected to dynamic numerical computations by means of the finite element method with the use of the Explicit Dynamics module in the ANSYS programme. The aim of the numerical calculations was to compare the compliance of the results of the experimental and analytical investigations as well as the assessment of the impact of the size of boundary conditions, which were subjected to parametrization, upon the values of occurring stresses during the conducted impact loading simulations. The results of the experimental investigation indicate that along with increasing the thickness of the adhesive joint, impact strength of the connection is reduced, regardless of the bonded material. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)
* Corresponding author. Tel.: +48 261-518-86 E-mail address: [email protected] 2352-1465 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – INAIR 2018. 2352-1465 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – INAIR 2018. 10.1016/j.trpro.2018.12.015
Andrzej Komorek et al. / Transportation Research Procedia 35 (2018) 80–89
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Komorek, Godzimirski, Kucharczyk/ Transportation Research Procedia 00 (2018) 000–000
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© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – Selection and peer-review under responsibility of the scientific committee of the International Conference on Air Transport – INAIR INAIR 2018. 2018. Keywords: adhesive joint; impact loading; pendulum hammer; numerical calculations
1. Introduction Aircraft manufacturers aim at achieving the lowest weight of airship. One way to reduce weight is to join structural elements by an adhesive, since in mechanical joints the mass of the connecting elements is several times bigger than the weight of the adhesive forming the same connections; besides bonding does not weaken the connected elements. However, effective use of adhesive joints largely depends upon their properties. One of the relevant strength parameters of adhesive joints exploited in connecting components in aircraft constructions is impact strength, which is discussed in this article. The research problem concerns the influence of the thickness of the adhesive joint upon the strength of the connection that was subjected to impact loading. In the tests we used the methodology of investigating impact strength of block samples in which the adhesive joint is damaged after impact shear loading. Using this technology, the energy lost in the destruction of the sample, which is the measure of impact strength of the connection Taylor (1996), can be determined from the difference in the height of the pendulum prior to and after the impact. This test method is difficult to repeat due to the need to keep very precise behaviour parameters of the samples as well as ensuring repeatibility of the test conditions Komorek and Przybyłek (2015), Adams and Harris (1996), da Silva et al. (2012), Adams et al. (1997). The method of testing impact strength of block adhesive joints, provided that the conditions of the test results are repeated, allows obtaining impact strength findings of connections which are prepared with different glue compositions. However, it is difficult to utilize the results obtained in practice. In order to obtain results that may find wider application in the design and implementation of structural bonding, research is conducted into impact strength of adhesive lap joints Karachalios et.al. (2013), Casas-Rodriguez et al. (2007), Goglio and Rosetto (2008), Belingardi et al. (2005), Harris and Adams (1985). The main objective of the investigation was to determine the relationship between thickness and impact strength of the adhesive joint in the shear impact test. In addition, we compared impact strength of adhesive joints in steel and duralumin elements. Having conducted impact loading tests, the fractures were inspected with regard to the type of the tear and accuracy of the damage. The experimental studies were completed with dynamic numerical calculations of the test cases. 2. Methodology of experimental research In order to conduct the investigation was made 10 series of adhesively bonded block samples (Fig. 1).
Fig. 1. Dimensions of the samples used in the investigation.
Andrzej Komorek et al. / Transportation Research Procedia 35 (2018) 80–89 Komorek, Godzimirski, Kucharczyk / Transportation Research Procedia 00 (2018) 000–000
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The aluminum alloy 2017A or steel S235 were selected for bonding elements. The adhesive material, which was used to make the bonding was Epidian 57 with Z-1 hardener. It was prepared 5 series of duralumin samples and 5 series of steel samples, each including 10 pieces. The samples in particular batches varied in the thickness of the joints. Different thicknesses of the joints was obtained through applying spacers in the form of a sewing thread which was placed in between the bonded elements. Prior to bonding, the metal parts were cleaned up; their surface was given proper structure through abrasive blasting, with copper slag as an abrasive medium. Next the samples were washed with petroleum naphtha to remove the grease present in the samples as well as the dust remaining after abrasive blasting. The washed samples were placed in the chamber of the laboratory dryer at a temperature of 90°C in order to vaporize the petroleum naphtha from their surface. The prepared elements were bonded as soon as possible to avoid accidental soiling of the bonded surfaces or falling of dust, oxides or moisture. While fixing the items for bonding, was paid particular attention to proper positioning of the elements against each other, since even slight irregularities in the geometry of the samples lead to significant changes in the obtained findings. The bonded series of samples with an identical joint thickness were placed on a base plate and clamped down at a pressure of 40 kPa for the time of curing, which equalled 7 days at ambient temperature (21°C). After curing the joints, it was performed a visual inspection of the quality of the obtained connections and removed excess glue and threads outside the bonded surfaces. Removing threads soaked in glue is particularly relevant since they may be caught up during an impact loading test by a detached upper element and consequently stop the dropping tool, which will translate into a higher value of impact loading of the connection. The excess glue may result in raising the endurance of the joint (in practice, they are not used unless necessary). Measuring the thickness was performed using the indirect method - the thickness of the glue layer was calculated on the basis of other measurements of the dimensions of the sample. In the measurements was used a digital caliper. The obtained joint thicknesses have been shown in Table 1. Table 1. Thickness range of sample joints used in the investigation. No 1a. 2a. 3a. 4a. 5a. 1b. 2b. 3b. 4b. 5b.
Adhesive
Bonded material
The scope of the obtained joint thicknesses [mm]
The average thickness of the joint in series [mm]
2017A
0.04 – 0.09 0.15 – 0.21 0.30 – 0.34 0.30 – 0.37 0.62 – 0.67
0.07 0.19 0.32 0,32 0.65
S235
0.01 – 0.03 0.19 – 0.25 0.27 – 0.34 0.35 – 0.43 0.59 – 0.67
0.02 0.23 0.31 0.39 0.63
Epidian 57/Z1
The thickness of the joints in steel samples and samples of aluminum alloy 2017A were almost identical (Table 1). Only for the thinnest series of joints, was received smaller thicknesses for S235 rather than 2017A. 3. Experimental studies and discussion of the results The investigation was conducted on a special machine designed for testing block adhesive joints and lap adhesive joints. The maximum energy of the pendulum used in the investigation equalled 15 J, and the speed in the lowest position was equal to 2.96 m/s. The investigation was conducted by applying impact load according to the scheme shown in Figure 2 Komorek and Godzimirski (2016). During the investigation, it was paid particular attention to maintaining a constant distance between the impactor and the adhesive joint, due to a significant impact of this parameter upon the obtained results.
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Fig. 2 A way of applying load to the sample
The results of the conducted impact loading testing for duralumin samples have been presented in Figure 3.
Fig. 3. The effect of thickness on impact loading of the connection of the elements made with aluminum alloy 2017A.
The highest impact strength was obtained for the samples with the smallest thickness of the joint, whereas the lowest one for connections with the greatest joint thickness. It is possible to observe a large discrepancy among particular results in each series. The impact strength of the samples where the average joint thickness equalled 0.65 mm was more than three times lower than in the samples whose average joint thickness was 0.07 mm. The results of the testing conducted for steel samples have been presented in Fig. 4. In the case of dural samples (Fig. 3), in a series with an average joint thickness of 0.07 mm there was achieved the highest average impact strength, whereas for steel samples the highest impact strength was obtained for the series of an average joint thickness equal to 0.23 mm (Fig. 4). With similar thicknesses of the joints which connect duralumin elements (Figure 3) and steel elements (Fig. 4), the impact strength of steel samples was lower than in duralumin samples, thus it is possible to formulate a hypothesis of weaker adhesion in steel samples. While analysing the results presented in Figures 3 and 4, it becomes obvious that the strength of the adhesive connection decreases along with the rise in thickness of the adhesive joint. Moreover, in each case, the average value of impact strength for duralumin samples has a greater value than for the same joint thicknesses in steel samples.
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Figure 4. The effect of the joint thickness upon impact loading of the connections of steel elements
4. Analysis of damage The analysis of damage to adhesive joints has been made in order to determine the character of the damage, regularities and similarities between the thicknesses of the adhesive joints under investigation. The analysis was conducted by means of a visual macroscopic and microscopic methods. The investigation made with a macroscopic method cannot be applied to the smallest joint thicknesses due to the fact that the scale of the destroyed adhesive does not to allow seeing any details or drawing conclusions. The joints whose thickness is below 0.1 mm were examined by means of the other method, which uses the electron microscope. The most frequent character of the cracking of larger joint thicknesses is shown in Figure. 5.
Fig. 5. Cracking of an adhesive joint after impact loading tests.
It was noted that most of the joints become fractured as in Figure 5, thus on the upper element there remains approximately 25-50% of the adhesive volume, similarly to the lower sample’s element, and the remainder of the glue is dispersed inside the test machine chamber. Another regularity is the manner of separating the adhesive joint. On the upper element there is more adhesive on the edge adjacent to applying impact loading, while on the bottom parts of the samples there is more glue residue away from the edge of the impact. The last point of the analysis is an assessment of the nature of the damage in order to evaluate whether the fracture occurred through tearing off a joint from one of the glued surfaces, or whether there is a thin layer of the adhesive glue on both bonded surfaces. Through a visual inspection it is possible to observe larger aggregations of the glue, however in cases of thin joints such an assessment is extremely difficult and ambiguous. The nature of the destruction of the joint was evaluated by means of a microscopic method, using the electron microscope HITACHI TM3030. The microscopic photos of the surface of the joint’s fractures in the samples are shown in Figures 6 and 7.
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Fig. 6. Image of breaking the joint connecting the elements of the alloy 2017A (electron microscope, magnification x 60).
Figure 6 shows the surface of the joint’s fracture connecting the elements made with 2017A alloy. In the photo it is possible to observe regular glue belts with a thickness of approximately 0.1 mm and a large number of small glue aggregations. The damage is cohesive-adhesive in it nature.
Fig. 7. The image of breaking the joint connecting the steel elements (electron microscope, magnification x 60).
The breaking of the thinnest steel sample joint has been depicted in Fig. 7. By visual inspection it was estimated that for this sample in the most part there occurred adhesive tear off with several glue aggregates, which are dark grey in the photo. The microscopic observations enabled to make a statement that in the assessed areas with regard to the glue residues, they remained little glue. 5. The analysis of FEM stresses in adhesive joints The calculations were carried out in the program Ansys, using the Explicit Dynamics module. It was built a simple numerical model of the sample, which was divided into 8-node hexahedral finite elements, dedicated for dynamic computations (impact strength) (Fig. 8). In the modelling was used the contact connection bonded-type Solid to Solid. The model was developed on the basis of the sample used in experimental research.
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Fig. 8. 3D model of the sample with a bonded cuboid element. Since was known the mass of the pendulum hammer and its initial energy, it was possible to calculate the speed at which the impactor strikes the adhesive sample, i.e. 2,960 m/s. In order to reflect the impactor’s mass, it was increased the density of the material to an extent that was declared for its manufacture so that it could be equal to the mass of the actual device. Having declared the Young’s modulus of the impactor's material as that of steel, it underwent excessive deformations, thus the impactor was modelled as an element consisting of two parts - one piece hitting directly the sample was given steel properties; the other had the modulus of elasticity greater, by one order of magnitude (2,000 GPa), so as to ensure adequate stiffness of the striking element (Figure 9). Finally, the models of the tested samples were dynamically impact loaded with a cuboid impactor 10x25x3 mm, with the speed of 2,960 mm/s and the density of 4.6 106 kg/m3, corresponding to the actual impact energy of the dropping hammer used in the experiment - 15 J. The calculations were carried out for the direction of the hammer as illustrated in the diagram (Fig. 2). The calculations were carried out for connections of steel or duralumin elements made with Epidian 57/Z1 adhesive. In the computations the authors took into account linear properties of an isotropic polymer material and nonlinear characteristics of the metal elements adopted from the software material database of ANSYS. It was compared the distributions of Max Principal Stresses in the joints for the same computational times, assuming that higher stress values should correspond to lower impact loading. The Max Principal Stresses hypothesis is best suited for analysis when testing glue joints.
Fig. 9. The impactor model consisting of two parts of varying stiffness.
It was also examined how the courses of principal stresses in the joints of dynamically impact loaded block samples change if the bonded elements are made with different materials and the adhesive is characterized with various stiffness (Figures 10, 11).
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Fig. 10. Max Principal Stresses and their time characteristics FME-determined for steel samples, loaded with the energy of 15 J and with different thicknesses of the joints: a - 0.1 mm, b – 0.2 mm, c-0.4 mm, d - 0.6 mm.
Fig. 11. Max Principal Stresses and their time characteristics FME-determined for duralumin samples, loaded with the energy of 15 J and with different thicknesses of the joints: a - 0.1 mm, b – 0.2 mm, c-0.4 mm, d - 0.6 mm.
Increasing the thickness of the joints affects the decrease in maximum stresses and extends the time of impact loading the joint, stretching the chart. A similar effect is obtained by replacing steel elements with duralumin ones. The analysis of the stress tensor component values with Max Principal Stresses close to the adhesive strength to stretching (approximately 83.5 MPa) brings nothing to the analysis - the components are comparable (Fig. 12, 13).
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Fig. 12. The values of the stress components calculated in the joints of the steel elements connections in the function of the joint's thickness (SigI –Max Principal Stresses, Sigy – normal stresses perpendicular to the joint's surface).
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Fig. 13. The values of the stress components calculated in the joints of the duralumin element connections in the function of the joint's thickness (SigI –Max Principal Stresses, Sigy – normal stresses perpendicular to the joint's surface).
It seems that the drop in impact strength which is linked with the rise in the joints’ thickness (resulting from the experimental research) is related to the fall in the strength of the joints along with the increase in their thickness, similarly to the static investigations (Fig. 14)
Fig. 14. Dependence between static strength of the adhesive to tearing off and thickness of the adhesive joint.
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6. Conclusions 1. The results of the experimental research indicate a significant impact of the thickness of the joint upon the impact strength of the adhesive connections - increasing the thickness of the joint results in a decrease in the recorded impact strength, irrespective of the material of the bonded elements. The obtained experimental results are not confirmed by the results of numerical calculations, according to which the change in the thickness of the joint exerts no significant effect on the values of the stress tensor components. 2. The drop in impact strength along with the rise in the thickness of the joint probably results from a lower shear stress values of the joints with greater thickness. 3. The fall in impact strength associated with increasing the thickness of the joint may also be the result of increasing the distance of the impactor from the lower surface of the joint, however it needs to be proved in further analytical studies. 4. The findings of impact loading tests performed on the very same test station are characterised with large discrepancies, which is caused by dimensional tolerance of particular samples and requires their modification in order to eliminate the likelihood of uneven load. 5. The macroscopic analysis pointed to a regular nature of the destruction. The method of microscopic observation confirmed the adhesive-cohesive nature of breaking the adhesive connections with different joint thicknesses. 6. It appears that the methodology used in the preparation of the sample’s surface for bonding enables to obtain greatly improved adhesion of the aluminum alloy samples rather than that of the steel samples. 7. References Taylor A (1996) Impact Testing of Adhesive Joints. MTS Adhesive Project 2 AEA Technology 5 (2). Komorek A, Przybyłek P (2015) Initial research of impact strength in adhesive joints. Solid State Phenomena 237:160-165, DOI: 10.4028/www.scientific.net/SSP.237.160 Adams RD, Harris JA (1996) A critical assessment of the block impact test for measuring the impact strength of adhesive bonds. Int J Adhes Adhes 16: 61-71 https://doi.org/10.1016/0143-7496(95)00050-X LFM da Silva, DA Dillard, B Blackman, RD Adams (2012) Testing Adhesive Joints: Best Practices. Wiley & Sons Weinheim Adams, RD, Comyn, J, Wake WC (1997) Structural Adhesive Joints in Engineering. Chapman & Hall, London Karachalios EF, Adams RD, da Silva LFM (2013) Single lap joints loaded in tension with ductile steel adherends. Int J Adhes Adhes 43:96-108 https://doi.org/10.1016/j.ijadhadh.2013.01.017 Casas-Rodriguez JP, Ashcroft IA, Silberschmidt VV (2007) Damage evolution in adhesive joints to impact fatigue. J Sound Vib 308:467-478 https://doi.org/10.1016/j.jsv.2007.03.088 Goglio L, Rosetto M (2008) Impact rupture of structural adhesive joints under different stress combinations. Int J Impact Eng 35:635–643 https://doi.org/10.1016/j.ijimpeng.2007.02.006 Belingardi G, Goglio L, Rossetto M (2005) Impact behaviour of bonded built-up beams: experimental results. Int J Adhes Adhes 25:173–180 Harris JA, Adams RD (1985) An assessment of the impact performance of bonded joints for use in high energy absorbing structures. Proceedings of the Institution of Mechanical Engineers 199 C2:121-131, Komorek A, Godzimirski J (2016) The selected aspects of the research into impact loading of adhesive joints in block samples - comparison of different ways of applying the load. Maintenance Problems 4: 77-91