Fatigue testing, bonded joints

Fatigue Test. Fatigue testing places a given load repeatedly on a bonded joint. Lap shear or other specimens are tested on a machine capable of inducing cyclic loading... 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

Significant scatter is often evident in time to failure data obtained from stress rupture tests conducted on either neat materials or on bonded joints. This scatter may obscure trends and frustrate the user. Results are typically plotted as load level versus the time to failure, a form that is analogous to S-N plots used in fatigue tests (see Durability Fatigue). In keeping with the principles of polymer physics, the time to failure axis should be plotted on a log scale, as illustrated in Fig. 1. Many creep-rupture models for homogeneous materials are based on forms like... 

Fatigue tests on hybrid joints bonded with the FT-EP adhesive were also carried out, by using a tension-tension fatigue test (Croccolo et al., 2010 Sekercioglu et al., 2005) on a resonant testing machine. 

A tension-tension fatigue test was performed on the hybrid joints bonded with the FT-EP at 35 pm of interference and the FT-EP bonded samples with clearance, in order to investigate the influence of the interference on the fatigue behavior of the joints. 

Another basic major advantage is that the cyclic-fatigue fracture-mechanics data may be gathered in a relatively short time-period, but may be applied to other designs of bonded joints and components, whose lifetime may then be predicted over a far longer time-span. Obviously, the fracture-mechanics tests need to be conducted under similar test conditions and environments as the joints, or components, whose service-life is to be predicted. This is important since the fracture-mechanics test specimens do need to exhibit a similar mechanism and locus of failure (e.g. cohesively through the adhesive layer, or interfacially between the adhesive and substrate, or through the oxide layer on the metallic substrate, etc.) as observed in the joints, or components, whose lifetime is to be ranked and predicted. 

Variation in force and strain with time in a typical impact in the impact fatigue testing of a bonded lap joint (Casas-Rodriguez et al. 2007)... 

Ashcroft lA, Abdel Wahab MM, Crocombe AD, Hughes DJ, Shaw SJ (2001) The effect of environment on the fatigue of bonded composite joints. Part 1 testing and fractography. Compos A 32 45-58... 

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