PSI - Issue 57

Cristian Bagni et al. / Procedia Structural Integrity 57 (2024) 859–871 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 5. Variation of the maximum peel stress with the size of the solid elements used to model the adhesive: (a) lap shear and (b) coach peel.

A decrease of the specimen stiffness during a fatigue test is related to the nucleation and propagation of one or more cracks in the specimen. The stiffness of a specimen can be defined as: ( ) = ∆ ∆ ( ) (1) where ∆ is the applied load range and ∆ ( ) is the displacement range defined as follows: ∆ = max − min (2) ∆ ( ) = max ( )− min ( ) (3) It is worth noting that while ∆ is constant during constant amplitude load tests, ∆ varies with the number of cycles, , as the crack(s) propagates. In particular, ∆ increases (and therefore decreases) as the crack(s) propagates and the crack opening increases. If the position transducer of the test rig is not accurate enough (the accuracy should be at least in the region of 0.02mm), the displacement should be measured using an additional device with higher accuracy (e.g., deflectometer, extensometer, etc.). According to this criterion, when the stiffness has decreased by a given value, the material is considered to have failed, and the number of cycles is recorded. Since the magnitude of the stiffness described above can change when testing specimens made of different materials, with different geometries or when the maximum load is changed, it is good practice to use a normalised stiffness defined as: ( )= ( ) ( 0 ) ∙ 100 [%] (4) where 0 is a given number of cycles chosen according to two conditions:  The cyclic load must be stable and at the target load.  The value of the normalisation stiffness ( 0 ) must not be decreasing due to the propagating fatigue crack (must be taken in the fatigue crack initiation stage). This is not always possible; for example, the stiffness of coach peel specimens, due to their geometry, tends to decrease from the beginning of the test. In this case, 0 is chosen as close as possible to the beginning of the test and ensuring that the first condition is met.