Ahmad Amiri Rad

Grade:  Ph.D.

Ph.D. Thesis:

Modeling Delamination Growth in Composite Laminates Under Fatigue Loading

Year: Sept. 2011-Nov. 2016.

Abstract:

In this thesis fatigue-driven delamination growth in laminated composites is studied. Delamination is one of the most common modes of failure in laminated composites. Delamination can be initiated by impact, interlaminar stresses near free edges and flaws in production process and its growth can lead to loss of stiffness and final failure of the structure. Three models are proposed in this thesis for simulation of delamination growth under fatigue loading. All these models are based on the cohesive zone method. Cohesive zone method is chosen since it does not require remeshing after each step of crack growth. This reduces the simulation time and helps the automation of the process. Cohesive zone method also helps more accurate calculation of the energy release rate in the laminated composites compared to conventional fracture mechanics methods like virtual crack closure technique. In the first model a two-scale continuum damage mechanics model is combined with the cohesive zone method. In this model material behavior is considered ductile at the micro-scale and brittle at the macro-scale. A bi-linear traction-separation law represents the material response at the process zone ahead of the crack tip at the macro-scale. A new two-parameter damage evolution equation is proposed which uses accumulated plastic strains at the micro-scale to calculate fatigue damage growth rate. This model is a step towards physical models for fatigue-driven delamination which are not solely based on macroscopic observations. In the second model a link has been established between the damage growth rate in the cohesive zone method and the crack growth rate of the Paris law. This is beneficial since no new material parameters are introduced and well known constants of Paris law are used. The model unlike similar previous models does not use an equivalent length and shows improved accuracy. Delamination growth in mode I, mode II and mixed-mode specimens are predicted accurately using the proposed damage growth equation. In this model one global value is calculated for the energy release rate and there is no need for calculation of this value at each integration point in the cohesive zone. In the third model, a new approach has been proposed which provide a robust tool for modeling delamination growth in 3D problems. In this approach the cohesive zone method and the level set method are combined. The cohesive zone method is used for calculation of the energy release rate and the level set method is used to track the crack front. Crack growth rate is calculated at the crack front using Paris law and is propagated through the domain using the fast marching method. A new damage evolution relation is proposed which creates a vertical traction-separation response. This helps more accurate calculation of the energy release rate irrespective of the element size at the crack front. Damage is updated in the integration points according to the calculated level set function. 3D crack growth in simulated benchmarks shows ability and accuracy of the model in predicting fatigue-driven delamination growth. Fatigue under variable amplitude loading is also discussed and performance of the proposed models under this kind of loading is studied. It is shown that the use of cohesive zone length in the damage evolution equation helps the simulation of crack acceleration effect seen in laminated composites.

Keywords: Laminated Composites, Fatigue, Delamination, Cohesive Zone Method, Fracture Mechanics, Damage Mechanics