Computational modeling of progressive failure and damage in composite laminates

Current and future aerospace systems utilize an ever-increasing amount of fiber reinforced composite laminates in various mission critical structural components making it imperative to understand their damage tolerance capacity under a multitude of loading envelopes. Their comparatively low strength under predominantly axial compressive loading severely limits the design loads of such structures. In the current work, a mechanism based lamina level computational methodology is developed for progressive failure analysis (PFA) of composite laminates beyond initial failure. A combination of analytical and micromechanical studies are used to identify the underlying mechanism of failure under predominantly compressive loading. Under such loading, the class of carbon fiber reinforced laminates considered in this thesis fails by fiber kinking. Results from an analytical study dispel the notion of a fixed compressive strength and show that it is a function of the in-situ geometric and material properties and stress state. These observations and finite element based micromechanical studies have identified the in-situ fiber rotation in the presence of initial fiber misalignment and the degradation of the in-situ shear modulus due to microcracking as the two main drivers of the kinking failure mechanism. A previously developed thermodynamics based lamina constitutive model is utilized to develop a PFA methodology for laminated composites. Laminae are assumed to be damaged by microcraking that is manifested in the degradation of the shear modulus and the transverse modulus. The amount of irrecoverable energy, expressed as a thermodynamic state variable S, provides a measure of the damage state inside a lamina. Lamina level coupon tests are used to obtain relations between S and the degrading moduli. These relations in conjunction with the lamina elastic constants and the geometric information such as the lamina thickness and the lamina lay-up are used as the PFA inputs. Damage accumulates in a lamina when an energy balance condition is satisfied and the in-situ secant stiffnesses in shear and in the transverse direction are degraded. The complete laminate response under loading is modeled by the classical lamination theory. Beyond reaching a critical value of S, denoted as S* (obtained experimentally), all lamina moduli are degraded steeply. For numerical implementation, in essence, this models an abrupt catastrophic event in a finite number of gradual steps. This method is demonstrated via numerical examples of single and multi element meshes under uniaxial compression that generate the characteristic load-deflection curves observed for detailed micromechanical studies. The maximum load predictions are seen to match the results obtained with rigorous micromechanical analyses, and corresponding laboratory experiments. The PFA developed in this thesis is demonstrated, verified and validated by modeling the responses of composite panels tested by the author and elsewhere. The PFA predictions are very close to the experimental observations.