Modelling Fracture and Fatigue Failure of Laminated Composites

Abstract

Delamination, matrix cracking and fibre breakage are the main fracture modes in composite materials/structures. Currently, the most popular models for simulating these fracture failures are built on a progressive damage evaluation approach. Damage initiates and grows in the material, resulting in progressive stiffness degradation of the material. A complete loss of material stiffness indicates a completed material failure. However, further development is still needed to improve the behaviour of the existing models. For example, most of the cohesive zone models (CZM) for delamination do not consider the effect of through thickness compression on delamination growth. A few existing CZM models and the cohesive surface contact model in ABAQUS combines cohesive damage and friction to include the effect of through thickness compression on delamination growth. In the present work, the behaviour of these models is investigated. Numerical simulation shows that the change of the interface stiffness affects the prediction significantly. A very large interface stiffness is required to achieve a converged prediction and it is revealed that this causes a physically unacceptable damage evolution. To overcome the above issue, a new approach is developed based on a fracture energy concept. A Representative Interface Area (RIA) is first analysed and the damaged area in the RIA is then described in terms of cohesive energy dissipation rate. This damaged area is finally employed to combine damage and friction in CZM. The developed model is validated by comparing numerical predictions with experimental measurements for delamination failure in a composite specimen. The findings showed that the final failure load when using developed model based on 〖(τ〗_2=(1-ω)k_2 δ_2+A_ τ_f) is very close to the experimental results with percentage differences of about 2% and 7% for specimen [(0/90)5/03/(90/0)5] under compression loads of 10 kN and 20 kN respectively and only 1.68% for specimen [(0/90/0)4/0/(0/90/0)4] under compression load of 20 kN. This research also investigates the intra-laminar damage of composite laminates. The matrix cracking and fibre breakage failure based on strain failure criteria have been derived and implemented in ABAQUS software via UMAT (User Material Subroutine). Simulating impact damage in composite laminates is performed to validate the proposed damage models. A comparison between proposed model and the Hashin’s damage model available in ABAQUS has been made. The present model provides more accurate damage prediction than the existing model in ABAQUS and also can capture intact zone in the delamination area when using enhancement factor (η=0.75) and friction coefficient (f_c≥0.5) for laminate [03/903]s. While the delamination lobes can be separated in laminate [903/03]s when using friction coefficient is 0.9 and enhancement factor is 0.75. Attempt has also been made to develop a fatigue cohesive zone model for delamination under cyclic loading in this study. The influence of friction on the delamination propagation is included. Fatigue delamination growth in cut-ply specimen is simulated and predictions are compared with the available experimental results in the literature. The results showed that the delamination length after 1500 cycles is 18.4 mm of the frictionless model. Whereas, the delamination length at the same number of cycles 15.2 mm. It can be observed that the proposed model successfully an enhanced fatigue life.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhenmin Zou (Supervisor) & Lei Ren (Supervisor)