Static and fatigue propagation of buckle-driven delaminations under bending and compressive loads

Abstract

Impact damage to composites is a major concern for many industrial sectors such as aerospace, automotive and marine. Damage such as fibre breakage, matrix fracture and sub-layer delaminations may occur, where the latter is considered the most critical under compressive loading. Compression after impact (CAI) in composites has been extensively studied by researchers to predict the strength of the impacted laminate. So far, however, various numerical techniques were developed to predict the CAI strength. There is a considerable demand for analytical approximate models that could be utilised in the early stage of the design process rather than implementing time-consuming numerical methods. The present work focuses on buckle-driven delamination of impact-damaged composites. The study included experimental and analytical investigations for two different laminate configurations: sandwich composites under edge compression and solid laminates under pure bending.

The first part of the work investigated the behaviour of impacted foam-core sandwich specimens with composite laminated faces under uni-axial compression. The applicability of two pre-developed analytical models to predict the failure strain under static and fatigue loading was examined. Both models assumed that the thin sub-laminate damage existed within the laminate. This could be modelled as a plate structure prone to buckle and the propagation of such damage depended on its buckling strain. The propagation occurred in the load direction. An elliptical plate pattern resembling the damage morphology was incorporated into the static model and showed good agreement. Another modelling strategy was utilised to predict the fatigue failure strain by modelling it as a circle encompassing the whole damage and compared against the experimental fatigue trends for the studied laminate.

In order to verify the analytical model results, an initial experimental testing programme was carried out to establish a suitable setup that fulfilled the assumptions of both models. The main testing programme was then performed on the finalised specimen configurations under static and fatigue loading. The comparison between experimental and analytical models showed good agreement. Testing of specimens with two different face laminates, [(∓45C )2,(90C ,90G)4,(∓45C )2] and [(±45C )2,(0C ,0G)4,(±45C )2] showed two distinctive face delamination behaviours: opening and closing. A Digital Image Correlation (DIC) system was used to monitor the face damage propagation during loading. Two distinct failure behaviours were observed for each specimen leading to different fatigue resistance: the [(∓45C )2,(90C ,90G)4,(∓45C )2] laminate showed an opening propagation of a delamination while the [(±45C )2,(0C ,0G)4,(±45C )2] laminate showed closing propagation. The failure modes and critical levels of static load causing face damage propagation were determined experimentally in both static and fatigue testing.

The second and third parts of the work were concerning bending of delaminated solid laminates. Through-width delaminated specimens were initially tested under four-point static and fatigue bending. A pre-developed non-linear kinematic model was initially utilised to explore the deformation behaviour of the thin sublaminate. The analytical model was then extended to include a mixed-mode propagation strategy in order to predict the propagation moment for three different lay-up configurations.

Fractography analysis was performed to distinguish between Mode-I and Mode-II contribution to the final failure of specimens. Comparison between experimental results and analysis showed agreement to within 5% in static propagation moment for two different materials. It is concluded that static fracture is almost entirely driven by Mode-II effects. This result was unexpected since it arises from a buckling mode that opens the delamination. For this reason, and because of the excellent repeatability of the experiments, the method of test may be a promising means of establishing the critical value of Mode-II fracture toughness, GIIC of the material. Fatigue testing on similar samples showed that buckled delamination resulted in a fatigue threshold that was over 80% lower than the static propagation moment. Such an outcome highlights the significance of predicting snap-buckling moment and subsequent propagation for design purposes.

The final part was to experimentally investigate impacted composite plates under static and fatigue bending. The initial study included the establishment of a threshold impact energy level on the studied laminate. Static and fatigue testing were then performed. Specimens with circular embedded delamination were investigated to study the behaviour of single delaminations as a possible idealised methodology to understand delaminated two-dimensional plates under bending.

Date of Award	1 Jan 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Michele Meo (Supervisor) & Richard Butler (Supervisor)