Failure Mechanism Analysis For Edge Impact Damage And Compression-after-impact Behavior Of Stiffened Composite Panels

Stiffened composite panels are widely used in the field of aeronautical engineering.Their damage tolerance characteristics are particularly significant to ensure the security of wing-relevant structures.According to the multi-scale damage analysis,the experimental and numerical studies on the edge impact damage and compression-after-impact behavior of stiffened composite panels are carried out.The main content of this thesis is stated as follows:(1)Experimental investigation on edge impact and compression-after-edge-impact behavior of composite panels,stiffened with both T-shaped and I-shaped stiffeners,is conducted.Different damage types and shapes are discovered from four critical locations of impact after careful inspection by visual and nondestructive detection.The residual compression strength and strain distribution are also measured.Besides,failure mechanism is illustrated through the analysis of experimental results.(2)Based on intralaminar failure mechanism of composite materials,a novel intralaminar progressive damage evolution analysis model,which includes damage characterization,damage initiation criterion and damage evolution law,is established with the help of continuum damage mechanics approach for 3D and plane stress states.With considering affects of the fracture plane angle,material constitutive relation of damage states in the fracture plane coordinates is established by introducing damage state variable matrix in the material principal coordinates.The onset of damage is evaluated by the Puck criterion and the evolution of damage is controlled by equivalent strain on the fracture plane.From the viewpoint of strain energy release,the material is assumed to exhibit a linear strain-softening behavior.The measured results of tensile and compressive tests about four configurations of notched composite laminates show the validity of the proposed model.(3)Motivated by experimental evidence which indicates that intralaminar matrix cracks,as the initial damage mode,propagate into the nearby interface and then coalesce into one interface crack,namely delamination,a saturation crack density parameter is employed to scale the occurrence of interlaminar delamination during low-velocity impact events.The specific value of the saturation crack density can be gained by the micro mechanical FE model based on the RVE technique.The relevant low velocity damage response parameters of the laminates [03/45/-45]S and [45/0/-45/90]4S are predicted with the proposed model at various impact energies.A good agreement is achieved with experimentally obtained data and validates the predictive capacity of the FE model established for low-velocity impact damage analysis of composites.Furthermore,results for different mesh-densitiesindicate that the approach by introducing a characteristic length of the element can alleviate the mesh dependency at the stage of material damage evolution.(4)Studies on the edge impact damage characterization between the T-shaped and I-shaped stiffeners are carried out.For free edge impact of the T-shaped stiffener,its physical controlling mechanism is sufficiently like the one arising from the progressive crushing process,while that of a I-shaped stiffener in the edge impact event is similar to the one observed during a skin impact under high energy levels.(5)Taking into account the similarity between the edge impact response of the T-shaped stiffener and progressive crushing state of composite laminates,complex damage induced by edge impact is able to incorporate into numerical compression model.With the help of a composite damage model comprising the CDM model and surface-based cohesive contact model,the residual strength of the composite panel stiffened with T-shaped stiffeners is predicted.Rather good correlation is obtained between experimental and numerical results in terms of compressive strength,load-strain curves of representative locations,strain distribution and ultimate damage modes for a range of configurations.(6)The numerical method to predict residual compression strength of I-stiffened composite panels subjected by edge impact is proposed.According the edge impact damage characterization of I-shaped stiffener,typical edge impact damage,including maximum delamination,permanent indentation deformation around the dent and intralaminar damage modes,is incorporated as initial damage in the strength analysis.In particularly,for R-sections of the impacted stiffener,interfacial delamination,which may cause premature failure and to some extent reduce the load carrying capacity,is efficiently considered by an engineering estimation method.A good correlation between the experimental and numerical results shows correctness and effectiveness of the proposed mechanical method.Meanwhile,numerical results reveal the compressive failure mechanism of I-stiffened composite panels subjected to edge impact.