PSI - Issue 57

Cristian Bagni et al. / Procedia Structural Integrity 57 (2024) 598–610 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Lap shear and coach peel are the most common specimen geometries used when testing adhesive joints. More complex specimen geometries can be tested if required, however, these are usually more difficult and expensive to test. When deriving fatigue parameters of adhesively bonded joints it is essential to choose the most appropriate failure criterion, or in other words to appropriately define the number of cycles to failure. Three possible failure criteria are: • Specimen separation . This corresponds to when the test stops either because the specimen is fully separated or because the displacement transducer reaches a maximum allowable displacement, set to protect the test equipment. This last condition can leave the specimen physically intact, but with significant cracks and the specimen is considered ‘separated’ for fatigue purposes. This is definitely the easiest failure criterion to adopt as it does not require any additional data analysis effort. However, it is inherently non-conservative. • Crack initiation . This can be detected in different ways as described in Section 1, such as by measuring back face strain or visual inspection of video recordings of the tests. The different techniques used to detect crack initiation might have a significant impact on the cost of the tests, the time needed to set up each test and analyse the collected data, and the accuracy of the results, etc. Furthermore, using crack initiation as the failure criterion might be over-conservative. • Stiffness drop . This is based on the variation of the specimen stiffness throughout the test duration, as described below in more detail, and calculated based on displacement data captured during the test. 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