Damage Evolution in Composite Bonded Joints under Fatigue Loading

The fatigue behaviour of single lap bonded joints under fatigue loading was already investigated and a suitable life prediction model was presented and validated [1-2]. The model is based on the actual damage evolution under fatigue load: a crack initiation followed by its propagation up to a critical length [2, 3]. In the present paper the evolution of fatigue damage in single lap joints is extensively analysed and discussed, with particular reference to the influence of layer orientation at adhesive-adherend interface, corner geometry at the end of the overlap area and load level. Single lap bonded joints were manufactured from autoclave-moulded laminates (Seal Texipreg CC206, T300 twill 2x2 carbon fibre fabric/ET442 toughened epoxy matrix) and bonded with a two-part epoxy adhesive 9323 B/A by 3M. The laminate lay-ups were [45/02]s and [452/0]s in order to have a 45° oriented layer at the adhesive interface, whereas the previous joints [1,2] were obtained from [0]6 laminates. Different overlap lengths (20 and 40 mm) and corner geometry (square edge and fillet) were investigated under tension-tension fatigue loading. The damage evolution has been investigated at macroscopic and microscopic level, in the former case by measuring the stiffness decrease with different techniques (extensometer and LVDT) and then by correlating the stiffness trend with that of the average crack length, as reported in [4], in the latter by analysing cracks and delaminations on the polished edges of the joints via optical microscopy. The microscopic observations allowed the crack initiation to be detected and the propagation phase to be investigated: usually in the presence of 45° layer, two different propagation paths can be found: i) initiation and crack propagation path at the adhesive-adherend interface, as shown in Figure 1; ii) fatigue damage evolution involving interlaminar/intralaminar crack propagation paths, as shown in Figure 2. In general, it was found that the presence of 45° layer at the adhesive interface makes the crack paths much more complicated and the life spent in the crack propagation phase longer with respect to the joints having a 0° interface. Fig. 1: Crack initiation and propagation path observed at the interface between adhesive and adherend in a square edge joint (lay-up [452/0]s; σmax=65 MPa, w=20 mm, Nf=588737 cycles). intralaminar interlaminar Fig. 2: Typical damage evolution in the case of 45° interface: crack initiation and interlaminar/intralaminar delamination in a square edge joint (lay-up [452/0]s; σmax=95 MPa, w=20 mm, Nf=78209 cycles). To describe the shape of the crack front through the joint width and to assess the actual delamination extent, after interrupted fatigue test, some joints will be analysed by enhanced xrays and then cut lengthwise in thin slices at different distance from the edges, polished and observed again by using the optical microscopy. Moreover, to evaluate the influence of different layer orientation on the adhesive-adherend adhesion and to investigate the morphology of the fracture surface, the results of an extensive SEM fractographic analysis will be reported and discussed.