Damage Detection And Residual Mechanical Performance Prediction For Composite Laminates

Composite materials have seen a very wide range of applications in engineering industry due to its high specific stiffness and specific strength.Compared to traditional metallic materials,composite materials are not only significantly lighter but also more durable and resistant to corrosion,which makes it a very promising material in advanced aerospace and astronautic industry.Despite all these advantages though,composite materials still suffer from various damages induced during both manufacturing and service.These accumulated damages would notably change the mechanical properties of composite materials,usually degrade them to shorten the service time.It is therefore of great interests to be able to evaluate the accumulated damages in early stage of a composite structure’s service time and further predict the remaining properties to not only ensure the safety but also prolong the service time of the structure for economic purpose.In this thesis,multiple models are proposed from theoretical level to study the mechanical properties of both pristine and damaged composite laminates in hopes to combine with non-destructive testing methods to predict the residual strength and fatigue life of multiple composite structures.The main content of the thesis includes:1.An investigation into the effect of drilling-induced delamination on tensile strength of composite laminates.Based on continuum damage mechanics a finite element model for the purpose of tensile strength predicting of open-hole composite laminates is constructed to investigate the strength decreasing caused by initial delamination induced by mechanical drilling.According to the different failure mechanisms of fiber and matrix,four different failure criteria are used correspondingly.Cohesive elements are also adopted to model the interface layers.Comparisons of models with initial delamination and without were made,where models with initial delamination of the same size from an ultrasonic C scan of experimental samples have given closer predictions of the residual strength to experimental results than those without.2.Developments of single-integration-point fatigue cohesive elements with local algorithm.Tradition cohesive elements are extended to fatigue analysis by combing with a modified Paris law.A local crack tip element identification approach is proposed to ensure the overall accuracy as well as an energy release rate correction approach.Both approaches are based on local information of the crack tip element only and can be implemented into single-integration-point linear elements.Verifications on both DCB and 4ENF models were conducted,where results indicate that the proposed model can achieve reasonable accuracy with significantly improved mesh sensitivity.3.Developments of four-integration-point fatigue cohesive elements with non-local algorithm.By adopting a non-local algorithm,fatigue cohesive elements are extended to four-integration-point elements.An average approach and a new local damage rate approach are proposed accordingly to correct the energy release rate of the crack tip elements.Based on the features of fourintegration-point elements,a dynamic fatigue characteristic length method is also proposed to increase the accuracy of fatigue characteristic length calculation.Furthermore,a new initiation model is proposed and incorporated into the cohesive element model,so it can be applied on more complicated three-dimensional multi-interface cases.The model was validated with both fatigue propagation damage dominated cases and fatigue initiation damage domninated cases and good agreement was obtained.4.Fatigue life prediction of open-hole composite laminates and the effect of drilling-induced delamination on residual fatigue life.The aforementioned four-integration-point fatigue cohesive elements were used to model composite laminates with two different lay-up sequences.Predicted fatigue lives have matched experimental results quiet accurately,indicating that the proposed fatigue cohesive elements are suitable for applications in complex models.A further numerical analysis of the effect of drilling-induced delamination on the residual fatigue life was also conducted,where preliminary results show that initial delamination can have large impacts on fatigue life and must be taken into considerations.5.Lamb-wave based fatigue life characterization and prediction model for composite laminates.Under fatigue loadings,laminates show three major damage mechanisms: fiber breakage,matrix cracks and delamination.A velocity/stiffness degradation model is proposed based on these three damage mechanisms to characterize fatigue damages within laminates.Controlled experiments were conducted to validate the model,where wave phase velocity was measured using laser ultrasonic method.Results indicate that the proposed modal gives accurate characterizations.Furthermore,with a residual velocity criterion,residual fatigue life prediction was made,where good accuracy was also obtained.