Fatigue Crack Growth Behavior of Bonded Aluminum ... - Science Direct

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ScienceDirect Procedia Engineering 160 (2016) 270 – 277

XVIII International Colloquium on Mechanical Fatigue of Metals (ICMFM XVIII)

Fatigue crack growth behavior of bonded aluminum joints A.A.M.A. Camposa, A.M.P. de Jesus b,*, J.A.F.O. Correiab, J.J.L. Moraisa a

University of Tras-os-Montes e Alto Douro, CITAB, Department of Engineering, School of Sciences and Technology, 5000-801 Vila Real, Portugal b University of Porto, INEGI, Engineering Faculty, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

Abstract The current paper presents a research aiming at characterizing the fatigue behavior of adhesively bonded aluminum joints. In particular, Double Cantilever Beam (DCB) specimens and End Notch Flexure (ENF) specimens built using the 6061-T651 aluminum alloy substrate and the Araldite 2015 epoxy adhesive were used to evaluate the pure mode I and mode II fatigue crack propagation rates. Besides the fatigue crack growth rates the monotonic quasi-static fracture behaviors of the adhesive joints are also characterized for both loading conditions. Numerical finite element models of the joints are also proposed for the evaluation of the compliance calibration curves, avoiding the need of direct fatigue crack growth measurements, which is a very complex task mainly for the ENF tests. Critical fracture energies (GIc and GIIc) from experimental tests were in the same order of magnitude of the values published in the literature. Concerning the fatigue crack growth behavior, it was verified a higher fatigue crack growth resistance under pure mode II loading. A good agreement was found between the experimental fatigue crack growth data and the modified Paris law that accounts for fatigue crack propagation regimes I and II. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the University of Oviedo. Peer-review under responsibility of the University of Oviedo Keywords: Adhesive joints; Fatigue crack growth; AA6061-T651, Araldite 2015; Epoxy resin; DCB; ENF; Compliance.

1. Introduction Adhesively bonded joints are currently attracting a great interest by the research community. They are competing against other traditional joining techniques (ex: bolting, welding), showing significant potential advantages, such as the contribution for weight reduction, possibility of joining dissimilar materials and contribution to reduce the stress

* Corresponding author. Tel.: +351225081740; fax: +351225081445. E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the University of Oviedo

doi:10.1016/j.proeng.2016.08.890

A.A.M.A. Campos et al. / Procedia Engineering 160 (2016) 270 – 277

concentrations, due to a more uniform stress redistribution. One of the main concerns when adhesive joints are being used in structural applications is related to their long term mechanical behavior which is not fully understood nor appropriately modelled. In particular, the fatigue behavior of adhesively bonded joints is a main concern when structural applications are envisaged, mainly those covering dynamic loading. Therefore, research aiming at investigating the fatigue behavior of adhesively bonded joints is essential to allow fully exploitation of this joining process in future challenging structural applications such as in transportation industry (e.g. automotive, aerospace). The durability of adhesive joints is dependent of various conditions such as environmental variables (temperature and moisture), aging of the adhesives, dynamic loads, strain rate (e.g. creep) effects and fatigue damage. The fatigue behavior of adhesively bonded joints can be influenced by geometric factors, the combination of different materials of the adherents and adhesives, type of loading, surfaces preparation and curing cycles. Each of these parameters have deserved attention in the literature [1], but due the multiplicity and complexity of the problems, the research on fatigue behavior of adhesively bonded joints still is at an early stage, when compared with the research history in the field of metals fatigue. Similarly to the fatigue studies performed in metals, the fatigue behavior of adhesively bonded joints can be decoupled into two distinct phases: the fatigue crack initiation and the fatigue crack propagation. The fatigue crack initiation corresponding to the macroscopic crack initiation may represent a significant fraction of the joint life [2, 3]. The fatigue crack propagation has been strongly investigated and its importance will depend on the extension of the glued interface, the load level and load type. The studies on fatigue crack propagation of adhesively bonded joints are more common, covering distinct crack propagation mode mixities [1, 4 – 12]. The efficiency of adhesively bonded joints is higher for polymer or reinforced polymer subtracts since there is generally higher compatibility (e.g. similar stiffness) between subtracts and adhesives, resulting in relatively higher strengths, including the fatigue resistance. When metallic subtracts are envisaged, very often industry proposes the use of hybrid joints in order to increase the mechanical reliability of the joints [13,14]. The current paper presents a study on the fatigue behavior of aluminum adhesively bonded joints. The 6061-T651 aluminum alloy and the Araldite 2015 epoxy adhesive were selected for this study. The fatigue crack propagation is investigated using specimen geometries that are typically proposed for static fracture properties evaluation of adhesive joints: the Double Cantilever Beam (DCB) specimens and the End Notch Flexure (ENF) specimens. The first geometry is proposed to characterize the pure mode I fatigue crack propagation behavior; the second is used to characterize the pure mode II fatigue crack propagation rates. In addition to the fatigue crack propagation tests, the monotonic fracture behavior of the adhesive joints are also characterized using the same geometries. Numerical finite element models of the joints were used for the assessment of compliance calibration curves, avoiding the need of direct fatigue crack growth measurements which is a very complex task, mainly in the ENF tests. The information from these monotonic tests is very important to establish the testing conditions for the fatigue tests. This includes the critical fracture energies corresponding to pure mode crack propagation tests (GIc and GIIc). Concerning the fatigue crack growth test data, an attempt is performed to identify the fatigue crack propagation regime I (near threshold) and II (stable crack growth) and the Paris equation [15] is updated to correlate the experimental data. 2. Experimental details For the current investigation the 6061-T651 aluminum alloy was selected for the preparation of the DCB and ENF specimens which are machined in two halves bonded using Araldite® 2015 epoxy adhesive. The selected aluminum alloy shows high mechanical strength and corrosion resistance. The main mechanical properties of the aluminum alloy are: Young modulus of 64.85 GPa (evaluated in this research); Poisson coefficient of 0.33; yield strength of 276 MPa and ultimate tensile strength of 310 MPa. The Araldite® 2015 epoxy adhesive has been fully investigated in the literature [16] and the following typical properties can be referred: Young modulus of 1.85 ± 0.21 GPa; Poisson coefficient of 0.33; ultimate tensile strength of 30 MPa and shear strength of 20 MPa. Figures 1 and 2 represent the geometry of the DCB and ENF specimens selected for the present study. The dimensions of the specimens resulted from a limited amount of available aluminum. For both specimen geometries a bond line thickness of 0.2 mm was adopted and controlled during the specimens preparation. Regarding the DCB tests, both monotonic and fatigue tests started with an initial crack length, a0=45mm. In addition, the fatigue tests

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were preceded by compliance calibration tests using the following crack lengths (a 0=45 mm; a1=72.5 mm; a2=100 mm). Concerning the ENF tests, monotonic and fatigue tests were performed taking into account the geometry given in the Figure 2. For these tests experimental calibration of the specimen compliance was not performed. Two loading configurations were considered in the tests with distinct spans (V m – minor span and VM – major span). For both cases, the initial crack length (a0) was kept equal to 45 mm. L = 259 27.5

a0 = 45

B = 18 ea = 10

27.5

h = 10

15

6 x I3.5

P

(Dimensions in mm) Fig. 1. DCB specimen geometry.

P

h = 10

B = 18

a0 = 45 (0.5454L) ea = 10

47

2L(Vm) = 165 2L (VM) = 213 (Dimensions in mm) Fig. 2. Geometry of the ENF specimens.

The surfaces of specimens to be bonded were preconditioned using sand blasting at 4 Bars pressure and subsequently degreased with acetone and cleaned with compressed air. The curing of the adhesive was performed during a minimum of 16 hours at room temperature and under a compressive loading, followed by a minimum 15 days cure at room temperature, without any compressive loading. All tests were performed in a servo-hydraulic machine INSTRON 8801 rated to 100kN at room temperature and ambient humidity in the range 50-70%. The monotonic tests of DCB and ENF specimens were performed at a displacement rate of 2 mm/min. Two monotonic tests were performed for the DCB specimens and three for the ENF specimens (two with the minor span and one with the major span). The compliance calibration curve for the monotonic specimens was evaluated by means of Finite Element Analysis. Monotonic tests were used to establish load-displacement (P-G) curves, from which the mode I and mode II critical energy release rates (GIc e GIIc) can be derived, using the Irwin-Kies relation. The fatigue tests were performed at load control, under sinusoidal waveform, with a stress ratio, R=0.1, and a frequency of 1Hz. As regards the DCB fatigue tests, two test series with 6 specimens each, at 60 and 65% of the ultimate monotonic loads were successfully tested. For these tests the compliance calibration curves were assessed experimentally using 3 preliminary cyclic tests performed at lower maximum loads to avoid damage, and for three distinct crack lengths, as depicted in the Figure 1. The fatigue crack growth rates, da/dN, were estimated using the 7 points polynomial incremental technique as recommended by the ASTM E647 standard [17]. Fatigue tests were performed on ENF specimens using the same type of loading selected for the DCB specimens. However, the maximum loads of the tests were redefined and two testing series were proposed: for the specimens with larger span one specimen was tested at 42.9% of the average maximum monotonic load and two specimens were tested at 45% of the same reference monotonic load. The second test series included two specimens with smaller span that were tested at 38.36% of the reference monotonic load. For these tests the compliance calibration

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curves were evaluated using again finite element models. The ASTM E647 standard was again applied to derive filtered da/dN data.

3. Experimental Results and Discussion 3.1. Monotonic test results Figure 3a represents the experimental load-displacement curves resulted from the DCB monotonic tests. Figure 3b represents the typical failure surfaces where a typical adhesive failure is observed. The crack length was not measured by direct observation. Instead, a compliance calibration curve was obtained by numerical analysis and calibrated using the experimental compliance measured for initial crack length. The resulting compliance calibration curve is illustrated in the Figure 4a. Figure 5 illustrates the finite element model adopted for the compliance calibration curve evaluation. Elastic analyses with elastic properties of the materials referred in the previous Section 2 were used. 3D models were used using 3D brick elements and the loading was applied using rigid-to-flexible surface-to-surface contact elements. Using the Irwin-Kies relation, the strain energy release rates were evaluated and plotted in the form of the R-resistance curves, as presented in the Figure 4b. The Irwin-Kies relation has the following form: ‫ܩ‬ൌ

ܲଶ ݀‫ܥ‬ ʹ‫ܽ݀ ܤ‬

(1)

The application of the Irwin-Kies relation requires a correspondence between the experimental load and the compliance, which may be derived using the following relation: ‫ܥ‬ൌ

ߜ ܲ

(2)

For each point of the graph of the Figure 3a one may compute the compliance using equation (2) and make the computation of equation (1), using the relation presented in Figure 4a, resulting the R-resistance curve of Figure 4b. An average critical energy release rate of 0.59N/mm was obtained for the adhesive joint under pure mode I fracture. Monotonic ENF test data was analyzed following a similar procedure adopted for the DCB tests. Figures 6 and 7 summarize the results of the analyses. Again compliance calibration curves were computed using a numerical model presented in the Figure 5b. Adhesive failures were observed. The R-resistance curves showed higher scatter than observed for the DCB tests. The average fracture energy for pure mode II cracking was 5.3 N/mm, which is significantly higher than verified for the pure mode I cracking, but still in the range found in the literature for the two materials combination.

a) Load – displacement curves. Fig. 3. Monotonic test results of DCB specimens.

b) Failure surfaces.

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a) Compliance calibration curves.

b) R-resistance curves.

Fig. 4. R-resistance curves evaluation for DCB specimens (pure Mode I cracking).

a) FEM mesh of the DCB specimen.

b) FEM mesh of the ENF specimen.

Fig. 5. FEM models used to derive the compliance calibration curves.

a) Load – displacement curves.

b) Failure surfaces.

Fig. 6. Monotonic test results of ENF specimens.

a) Compliance calibration curves.

b) R-resistance curves.

Fig. 7. R-resistance curves evaluation for ENF specimens (pure Mode II cracking).


3.2. Fatigue crack propagation results Figures 8 and 9 summarize the results from the fatigue crack propagation tests performed using DCB and ENF specimens, respectively. The compliance was monitored during the tests and their evolution illustrated in Figures 8a, 8b and 9a. Concerning the DCB tests, an experimental-based compliance calibration technique was used; however for the ENF tests the numerical model was applied again. For DCB tests the compliance increases at continuously increasing rates; however for the ENF tests the compliance shows a sigmoidal shape with acceleration and retardation stages. There is a stepped increasing in the compliance that corresponds to unstable crack propagation. Figures 8c, 8d and 9b show the fatigue crack growth rates that were obtained for pure mode I and mode II crack propagation modes. The data was correlated with a modified Paris equation that accounts for the fatigue crack propagation threshold. Besides an average trend line, conservative relations are represented in the graphs. Table 1 presents the resulting fatigue crack propagation relations. For DCB tests it is verified that the maximum load selected for the fatigue tests have an influence on the fatigue crack propagation rates: the increase in the maximum load tends to increase the fatigue crack growth rates, for the same energy release range. Comparing the mode I fatigue crack propagation rates with the mode II fatigue crack propagation rates, one may conclude the lower fatigue crack propagation rates for the pure mode II (shift right of the crack propagation curves). In general the fatigue crack propagation rates show a very high scatter which is not common in metals fatigue. Despite the specimens have been prepared with very precise and repetitive procedures, it was not possible to avoid this significant scatter between specimen results. Figure 10 illustrates the fracture surfaces of two specimens tested under fatigue loading. One may conclude that adhesive failure is observed for both cases. In addition the ENF test shows the adhesive burn due to intense friction observed during the tests, before the crack acceleration.

a) Compliance curves (60% of ultimate load).

b) Compliance curves (65% of ultimate load).

c) da/dN curves (60% of ultimate load).

d) da/dN curves (65% of ultimate load).

Fig. 8. Pure mode I fatigue crack propagation curves.

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a) Compliance curves.

b) da/dN curves. Fig. 9. Pure mode II fatigue crack propagation curves.

4. Conclusions This paper presented the results of a fatigue study performed in an adhesively bonded aluminum alloy using an epoxy adhesive. The study was focused on fatigue crack propagation and included both monotonic and fatigue (cyclic) tests, using both DCB and ENF tests. The pure mode I tests revealed average critical fracture energy of 0.59 N/mm which contrasted from the critical fracture energy for mode II which was 5.3 N/mm, i.e. about 10 times higher. Concerning the fatigue crack growth rates, it was observed that mode II fatigue crack growth demands for more crack driving energy than mode I fatigue crack growth, which is consistent with the observed monotonic fracture energies. For the DCB tests, it was observed that besides the range of energy release rate, the maximum energy value (maximum load) also influences the fatigue crack propagation rates. Higher loads led to higher fatigue crack growth rates, for the same load ranges, which is consistent with mean stress effects. A rough estimation of the fatigue crack propagation thresholds would be 0.5 N/mm for pure mode II crack propagation and 0.19 N/mm for pure mode I propagation. The fatigue crack propagation tests revealed significant scatter between repetitions. This aspect is probability one of the main difficulties to allow a damage tolerance design for adhesive joints. The joining of the aluminum alloy with the epoxy resin resulted in all cases in adhesive failure rather than cohesive which is typical of metallic subtracts joined with moderate rigid adhesives. ENF fatigue crack propagation tests are not ideal tests since friction developed between crack faces may led to spurious crack driving energies. Also, the crack propagation in this test was not uniform: it shows some instability before the crack arrest at the central part of the specimen.


Table 1. Fatigue crack propagation data. Loading

DCB (60% Fu) DCB (65% Fu) ENF

Modified Paris relation

average da/dN=3.0('G-0.15)2.5 da/dN =0.065('G-0.2)1.2 da/dN=0.08('G-0.52)1.7

a) DCB test.

upper boundary da/dN =6.4('G-0.15)2.5 da/dN =0.072('G-0.2)1.2 da/dN =0.149('G-0.52)1.7

b) ENF test. Fig. 10. Fracture surfaces for the fatigue specimens.

Acknowledgements The authors acknowledge the Portuguese Science Foundation (FCT) for the financial support through the postdoctoral grant SFRH/BPD/107825/2015. Authors gratefully acknowledge the funding of SciTech - Science and Technology for Competitive and Sustainable Industries (NORTE-01-0145-FEDER-000022), R&D project cofinanced by Programa Operacional Regional do Norte (“NORTE2020”), through Fundo Europeu de Desenvolvimento Regional (FEDER). References [1] M. M. A. Wahab, Fatigue in Adhesively Bonded Joints: A Review, International Scholarly Research Network, Volume 2012, Article ID 746308, 25 Pages. [2] V. Shenoy, I. A. Ashcroft, G.W. Critchlow, A.D. Crocombe, M.M.A. Wahab, An investigation into the crack initiation and propagation behaviour of bonded singlelap joints using backface strain, International Journal of Adhesion and Adhesives, Volume 29, Issue 4, 1999, Pages 361–371. [3] A.D. Crocombe, C.Y. Ong, C.M. Chan, M.M.A. Wahab, I.A. Ashcroft, Investigating fatigue damage evolution in adhesively bonded structures using backface strain measurement, The Journal of Adhesion, Volume 78, Issue 9, 2002, Pages 745–776. [4] F. Moroni, A. Pirondi, A procedure for the simulation of fatigue crack growth in adhesively bonded joints based on the cohesive zone model and different mixed-mode propagation criteria, Engineering Fracture Mechanics, Volume 78, Issue 8, 2011, Pages1808–1816. [5] A. Pirondi, F. Moroni, Simulation of mixed mode I/II fatigue crack propagation in adhesive joints with a modified cohesive zone model, Journal of Adhesion Science and Technology, Volume 25, Issue 18, 2011, Pages 2483–2499. [6] A. Pirondi, F. Moroni, A progressive damage model for the prediction of fatigue crack growth in bonded joints, The Journal of Adhesion, Volume 86, Issue 5-6, 2010, Pages 501–521. [7] S. Azari, M. Papini, J.A. Schroeder, J.K. Spelt, The effect of mode ratio and bond interface on the fatigue behavior of a highly-toughened epoxy, Engineering Fracture Mechanics, Volume 77, Issue 3, 2010, Pages 395–414. [8] A. Pirondi, F. Moroni, An investigation of fatigue failure prediction of adhesively bonded metal/metal joints, International Journal of Adhesion and Adhesives, Volume 29, Issue 8, 2009, Pages 796–805. [9] X.X. Xu, A.D. Crocrombe, P.A. Smith, Fatigue behaviour of joints bonded with either filled, or filled and toughened, adhesive, International Journal of Fatigue, Volume 16, Issue 7, 1994, Pages 469–477. [10] X.X. Xu, A.D. Crocombe, P.A. Smith, Frequency effect on fatigue-crack growth-rate in joints bonded with either filled or filled and toughened adhesive, Vide Science Technique et Applications, Volume 272, 1994, Pages 232–236. [11] M.F.S.F de Moura, J.P.M. Gonçalves, Development of a cohesive zone model for fatigue/fracture characterization of composite bonded joints under mode II loading, International Journal of Adhesion & Adhesives, Volume 54, 2014, Pages 224–230. [12] Y. Du, L. Shi, Effect of vibration fatigue on modal properties of single lap adhesive joints, International Journal of Adhesion & Adhesives, Volume 53, 2014, Pages 72–79. [13] P.C. Wang, S.K. Chisholm, G. Banas, F.V. Lawrence, The role of failure mode, resistance spot weld and adhesive on the fatigue behavior of weld/bonded aluminum, Welding Journal, Volume 74, Issue 2, 1995, Pages S41–S47. [14] F. Moroni, A. Pirondi, F. Kleiner, Experimental analysis and comparison of the strength of simple and hybrid structural joints, International Journal of Adhesion and Adhesives, Volume 30, Issue 5, 2010, Pages 367–379. [15] P. Paris, F. Erdogan, A Critical Analysis of Crack Propagation Laws, Journal of Basic Engineering, Volume 85, Issue 4, 1963, Pages 528533. [16] A.C.C. Leitão, Aplicação de um método de correlação de imagem para a determinação da tenacidade à fratura em corte de adesivos estruturais, MSc Thesis, Instituto Superior de Engenharia do Porto, 2005. [17] ASTM E647, Standard Method for Measurement of Fatigue Crack Growth. Rates, in Annual Book of ASTM Standards, Section 3, Volume 03.01, 1996.

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