Experimental estimation of the mechanical and fracture properties of a new epoxy adhesive
- J. P. R. Monteiro^{1},
- R. D. S. G. Campilho^{1}Email author,
- E. A. S. Marques^{2} and
- L. F. M. da Silva^{3}
Received: 17 November 2015
Accepted: 14 December 2015
Published: 22 December 2015
Abstract
The automotive industry is currently increasing its use of high performance structural adhesives in order to reduce vehicle weight and increase the crash resistance of automotive structures. To achieve these goals, the high performance adhesives employed in the automotive industry must not only have high mechanical strength but also large ductility, enabling them to sustain severe dynamic loads. Due to this complex behaviour, the design process necessary to engineering structures with these materials requires a complete knowledge of their mechanical properties. In this work, the mechanical properties of a structural epoxy, Sikapower^{®} 4720, were determined. Tensile tests were performed to determine the Young’s modulus (E) and tensile strength (σ _{f}). Shear tests were performed to determine the shear modulus (G) and the shear strength (τ _{f}). Tests were also performed to assess the toughness of the adhesive. For mode I toughness determination (G _{Ic}), the double-cantilever beam (DCB) test was employed. For determination of toughness under mode II (G _{IIc}), the end-notched flexure (ENF) test was performed. The data obtained from the DCB and ENF tests was analysed with the compliance calibration method (CCM), corrected beam theory (CBT) and compliance-based beam method (CBBM) techniques. The test results were able to fully mechanically characterize the adhesive and demonstrate that the adhesive has not only high mechanical strength but combines this with a high degree of ductility, which makes it adequate for use in the automotive industry.
Keywords
Background
The reduction of structural weight and the enhancement of vehicle safety are currently two of the most important research subjects for the automotive industry. The demand for lighter and safer structures has led the designers to increasingly employ alternative joining methods, replacing the more commonly used spot welding. Adhesive bonding is one of these methods and its use has expanded significantly, driven by the development of improved high performance adhesives and bonding techniques. While previous adhesives were relatively strong but brittle, the adhesives currently used for structural bonding by the automotive industry are designed with the aim of providing the joint with high ductility and high mechanical strength [1]. These materials are commonly referred as crash resistant adhesives due to their ability to plastically deform but still maintain the structure firmly bonded under significantly large loads, therefore ensuring that the structure has a large degree of energy absorption. Modern automotive structures combine multiple bonded materials and use adhesive layers with complex geometry. To efficiently design such structures, the use of finite element method (FEM) techniques is fundamental. One of the most accurate methods to model adhesive layers are cohesive zone models (CZM) to simulate adhesive failure and associated debonding. CZM are a very powerful tool for studying the behavior of adhesive joints. Cohesive elements can be easily added to FEM models. Needleman [2], Tvergaard et al. [3] and Camacho et al. [4] were among the first to adapt this technique for use in adhesive joints. A CZM improves on classical continuum mechanics modelling and can describe the fracture process and location. By using both strength and energy parameters to simulate the nucleation and advance of a fracture crack [5], these elements can fully simulate the crack progression in adhesive layers. The relationship between the stresses and displacements is governed by a traction separation law.
The experimental campaign described in this work enabled the estimation of E, G, σ _{f}, τ _{f}, G _{Ic} and G _{IIc}. The tensile properties of the specimen (E and σ _{f}) were determined using the bulk tensile testing of “dog bone” specimens. This almost universal test is standardized under ISO 527:1997 [6] and its ASTM equivalent D638-03 [7]. To measure the shear properties of the adhesive (G and τ _{f}), the thick adherend shear test (TAST) was employed. This test follows the standard ISO 11003-2:1993 [8]. Another method commonly employed to assess τ _{f} of adhesives is the torsion test, standardized under ASTM E143-02 [9]. The determination of G _{Ic} is usually performed with the DCB specimen [10], although other common specimen geometries exist such as the tapered double-cantilever beam (TDCB) or the single edge notch bend (SENB) specimens. The DCB test is widely used because it requires relatively simple specimens and it has well defined testing procedures. Several methodologies exist that allow the derivation of G _{Ic} from this testing data, resulting from a linear elastic fracture mechanics analysis. During a DCB test it is assumed that a crack will stably propagate when the tensile strain energy release rate (G _{I}) equals G _{Ic}. The CCM is based on the Irwin-Kies [11] equation and requires the calculation of the compliance (C) relatively to the crack length (a). The compliance is given by C = δ/P, where δ is the displacement and P is the applied load. As an alternative, the DBT uses the classical beam theory equations to assess the compliance [12] and the CBT improves on it by taking account the effects of crack tip rotation and deflection [13]. All these methods require the constant measurement of the crack location, which might be difficult or yield imprecise results. As an alternative, the CBBM uses the concept of the crack equivalent [14]. This means that it derives the crack location solely from C at any given moment, negating the need to visually monitor the crack progression as required by other methods [15]. The determination of G _{IIc} can be performed using three different tests by the theoretically steady-state value of shear strain energy release rate (G _{II}) that is attained during crack propagation. The ENF test, the end-loaded split (ELS) and the four point end-notched flexure (4ENF) test. Among these alternatives, the ENF is the most commonly used, as it does not exhibit the friction problems found in the 4ENF test and avoids the excessively large displacements found in the ELS test. The ENF has also the advantage of using a specimen mostly similar to the one used in the DCB tests, differing only in the loading direction. The ENF test is simply a three-point flexure test on a pre-cracked specimen. During the ENF test the relative displacement of the upper and lower specimens introduces a shearing load in a pre-cracked adhesive layer. The data from these tests can be analyzed using the same methods used for the analysis of the DCB test results.
Saldanha et al. [16] have used similar methods to perform a characterization procedure on a high elongation, high toughness epoxy adhesive. They found that such adhesive combined the high tensile strength and shear strength typical of epoxy adhesives with the high toughness of polyurethane adhesives. Similarly, García et al. [17] characterized a toughened epoxy adhesive to use in the FEM. Their experimental procedure used only tensile and shear tests to build a continuum damage model that was able to accurately simulate the mechanical behavior of complete joints. Much of the work on characterizing toughened, high elongation epoxy adhesives focuses on the study of the fracture properties, where there are significant improvements to be found. A variety of specimen types are used in these tests. Jin et al. [18] studied the mode I fracture behavior of a self-healing toughened adhesive by the TDCB test. Kim et al. [19] performed a similar characterization for a nanoparticle reinforced epoxy but used the simpler single SENB specimens for this purpose. All these tests found improvements in toughness over standard epoxy formulations.
In this work, the mechanical properties of a structural epoxy, Sikapower^{®} 4720, were determined. Tensile tests were performed to determine E and σ _{f}. Shear tests were performed to determine G and τ _{f}. Tests were also performed to assess the toughness of the adhesive. For G _{Ic} characterization, the DCB test was employed. For determination of G _{IIc}, the ENF test was performed. The data obtained from the DCB and ENF tests was analysed with the CCM, CBT and CBBM techniques. Comparison of the Sikapower^{®} 4720 with another epoxy adhesive was also undertaken. The Araldite^{®} 2015 was chosen for this purpose because of being a direct competitor in terms of applications and being established in the market. With this work, complete data for the numerical design of bonded structures with this novel adhesive is provided, enabling the optimization of the joints and the subsequent cost and weight reduction of the structures.
Methods
Tensile tests
Shear tests
Fracture tests
Detailed explanations of the method can be found in the work of Campilho et al. [26]. The value of a _{eq} is estimated from the current specimen compliance and taking into consideration the damage zone, E _{f} is a corrected flexural modulus to account for stress concentrations at the crack tip and stiffness variability between specimens, and G _{AD} is the shear modulus of the adherends.
Detailed explanations of the method can be found in Ref. [27]. Equally to the DCB tests, E _{f} is an equivalent flexural modulus obtained from the specimen’s initial compliance and value of a _{0}.
Results and discussion
Tensile tests
Bulk mechanical properties in tension of the adhesive Sikapower^{®} 4720
Specimen | P _{max} (N) | δ _{max} (mm) | σ _{y} (MPa) | σ _{f} (MPa) | ε _{f} (%) | E (MPa) |
---|---|---|---|---|---|---|
1 | 822.512 | 1.428 | 22.600 | 27.746 | 1.762 | 2162.285 |
2 | 886.882 | 1.861 | 20.048 | 27.212 | 1.755 | 2013.104 |
3 | 840.769 | 1.690 | 26.366 | 28.399 | 1.815 | 1988.688 |
4 | 822.810 | 1.943 | 21.979 | 26.228 | 2.183 | 1992.987 |
5 | 842.490 | 2.265 | 25.030 | 28.420 | 2.584 | 2160.623 |
6 | 803.614 | 1.755 | 23.691 | 27.109 | 1.741 | 1997.173 |
Average | 836.513 | 1.824 | 23.286 | 27.519 | 1.973 | 2052.477 |
Standard deviation | 28.485 | 0.279 | 2.252 | 0.845 | 0.343 | 84.818 |
Tensile comparative evaluation between the Sikapower^{®} 4720 and the Araldite^{®} 2015 [30]
Properties | σ _{f} (MPa) | ε _{f} (%) | E (MPa) | σ _{y} (MPa) |
---|---|---|---|---|
SikaPower^{®} 4720 | 27.519 ± 0.845 | 1.973 ± 0.343 | 2052.477 ± 84.818 | 23.286 ± 2.252 |
Araldite^{®} 2015 | 21.63 ± 1.61 | 4.77 ± 0.15 | 1850 ± 210 | 12.63 ± 0.61 |
Shear tests
TAST mechanical properties in shear of the adhesive Sikapower^{®} 4720
Specimen | P _{max} (N) | δ _{max} (mm) | τ _{y} (MPa) | τ _{f} (MPa) | γ _{f} (%) | G (MPa) |
---|---|---|---|---|---|---|
1 | 2865.967 | 0.071 | 15.213 | 22.928 | 10.936 | 697.567 |
2 | 3073.479 | 0.213 | 14.817 | 24.588 | 30.669 | 727.154 |
3 | 3108.062 | 0.142 | 13.539 | 24.864 | 21.460 | 770.566 |
4 | 3171.353 | 0.217 | 14.502 | 25.371 | 29.351 | 819.194 |
5 | 2994.543 | 0.149 | 16.309 | 23.956 | 23.333 | 739.212 |
Average | 3042.681 | 0.159 | 14.876 | 24.341 | 23.150 | 750.738 |
Standard deviation | 117.606 | 0.059 | 1.012 | 0.941 | 7.859 | 46.356 |
Shear comparative evaluation between the Sikapower^{®} 4720 and the Araldite^{®} 2015 [30]
Properties | τ _{f} (MPa) | γ _{f} (%) | G (MPa) | υ | τ _{y} (MPa) |
---|---|---|---|---|---|
SikaPower^{®} 4720 | 24.341 ± 0.941 | 23.150 ± 7.859 | 750.738 ± 46.356 | 0.367 | 14.876 ± 1.012 |
Araldite^{®} 2015 | 17.9 ± 1.8 | 43.9 ± 3.4 | 560 ± 210 | 0.33^{a} | 14.6 ± 1.3 |
Tensile fracture tests
Values of G _{Ic} obtained by the different data reduction methods from the DCB tests
Specimen | P _{max} (N) | δ _{max} (mm) | G _{Ic} (N/mm) | ||
---|---|---|---|---|---|
CCM | CBT | CBBM | |||
1 | 238.825 | 16.287 | 1.249 | 1.317 | 1.363 |
2 | 206.584 | 23.846 | 1.395 | 1.443 | 1.401 |
3 | 276.072 | 14.413 | 1.350 | 1.447 | 1.598 |
4 | 210.687 | 19.150 | 1.059 | 1.174 | 1.186 |
5 | 239.236 | 19.022 | 1.067 | 1.120 | 1.173 |
6 | 243.150 | 19.948 | 1.084 | 1.168 | 1.040 |
Average | 235.759 | 18.778 | 1.201 | 1.278 | 1.294 |
Standard deviation | 23.025 | 2.958 | 0.138 | 0.132 | 0.182 |
After the analysis of all DCB specimens, no significant differences were found in the R-curves when comparing all considered data reduction methods. The measured values of G _{Ic} for each specimen were close between data reduction methods. No data is available from the manufacturer regarding this parameter. Evaluated against the CBBM, which is regarded as the most reliable method by not requiring measurement of a and including the FPZ effects in the results, the observed differences to the other methods were 7.2 % (CCM) and 1.2 % (CBT). Between specimens of the same method, the percentile deviations were 11.5 % (CCM), 10.3 % (CBT) and 14.1 % (CBBM). The current adhesive has G _{Ic} = 1.294 ± 0.182 N/mm (CBBM values) compared to G _{Ic} = 0.43 ± 0.02 N/mm of the Araldite^{®} 2015 [30], which corresponds to an excess of nearly three times.
Shear fracture tests
Values of G _{IIc} obtained by the different data reduction methods from the ENF tests
Specimen | P _{max} (N) | δ _{max} (mm) | G _{IIc} (N/mm) | ||
---|---|---|---|---|---|
CCM | CBT | CBBM | |||
1 | 1110.912 | 17.990 | 5.183 | 3.327 | 4.467 |
2 | 1035.342 | 17.507 | 4.473 | 3.886 | 4.544 |
3 | 933.773 | 18.847 | 3.729 | 3.858 | 4.327 |
4 | 989.140 | 15.350 | 4.671 | 2.824 | 4.151 |
5 | 1119.458 | 17.650 | 5.428 | 3.195 | 4.283 |
6 | 866.329 | 12.257 | – | – | – |
7 | 950.081 | 13.593 | 3.317 | 2.773 | 3.503 |
8 | 1010.304 | 15.953 | 3.843 | 2.984 | 4.367 |
Average | 1001.917 | 16.143 | 4.378 | 3.264 | 4.235 |
Standard deviation | 86.818 | 2.303 | 0.783 | 0.459 | 0.347 |
In accordance with the previous tests, the ENF tests also revealed a high repeatability. The G _{IIc} results of each test agreed well between methods. Equally to G _{Ic}, comparison with reference values cannot be carried out. The percentile differences to the CBBM, once again considered the most robust method, are 3.3 % for the CCM and 22.9 % for the CBT. The discrepancy for the CBT is linked to the energy dissipated in the FPZ, which is not accounted for in beam theories. Comparing the different specimens for each method, the percentile deviations were 17.9, 14.1 and 8.2 % for the CCM, CBT and CBBM, by this order. The CBBM results of the adhesive Sikapower^{®} 4720 gave G _{IIc} = 4.235 ± 0.347 N/mm, which is a slightly smaller value than that obtained for the Araldite^{®} 2015, of G _{IIc} = 4.70 ± 0.34 N/mm [30].
Conclusions
The main objective of this work was the complete mechanical and fracture characterization of a new epoxy adhesive (Sikapower^{®} 4720). Bulk tensile and TAST tests were performed to obtain the tensile and shear mechanical properties, respectively. The bulk tensile tests gave the following values: E = 2052.477 ± 84.818 MPa, σ _{y} = 23.286 ± 2.252 MPa, σ _{f} = 27.519 ± 0.845 MPa, and ε _{f} = 1.973 ± 0.343 %. From the manufacturer’s data, only σ _{f} (24 MPa), E (1900 MPa) and ε _{f} (3 %) were available. The biggest difference was found in ε _{f}, justified by small experimental defects in the specimens that could compromise the full ductility of the specimens to develop. The TAST tests resulted in G = 750.738 ± 46.356 MPa, τ _{y} = 14.876 ± 1.012 MPa, τ _{f} = 24.341 ± 0.941 and γ _{f} = 23.150 ± 7.859 %. The only comparison with the manufacturer’s data regards τ _{f} (14 MPa), which corresponds to a significant difference to the obtained value in this work. However, the manufacturer’s value was empirically defined by the von Mises criterion which, as it is known, does not apply to toughened adhesives. The availability of E and G permits the calculation of ν for isotropic materials as 0.367, which is within the interval of expected values for structural adhesives, i.e., between 0.3 and 0.5 [1]. The G _{Ic} values, obtained by DCB tests, gave 1.201 ± 0.138 N/mm (CCM), 1.278 ± 0.132 N/mm (CBT) and 1.294 ± 0.182 N/mm (CBBM), corresponding to a good correspondence between methods. The ENF tests provided the G _{IIc} estimations as 4.378 ± 0.783 N/mm (CCM), 3.264 ± 0.459 N/mm (CBT) and 4.235 ± 0.347 N/mm (CBBM). As previously mentioned, the CBT under predicted the other methods. It was not possible to compare G _{Ic} and G _{IIc} with the manufacturer’s values due to the absence of information. The comparison of the obtained results with the Araldite^{®} 2015 revealed better properties in all parameters except ε _{f}, γf and G _{IIc}, in this last parameter by a very short difference.
Declarations
Authors’ contributions
JPRM—carried out the experimental tests and analyses. RDSG, EASM and LFMS wrote the manuscript. All authors read and approved the final manuscript.
Acknowledgements
The authors would like to thank Sika^{®} Portugal for supplying the adhesive Sikapower^{®} 4720.
Competing interests
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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