The Method Of Analysis For Vacuum-assisted Wet-layup Flush Repaired Composite Laminates

With the rapid development of aerospace, wind power generation, high-speed train, shipindustry, automobile industry, the composite laminates, which possess high specific strength,high specific stiffness, outstanding designability, excellent fatigue resistance and otheradvantages, have been more and more widely used. However, damages that may affect the safetyperformance of composite structures will inevitably produce in composite laminates during theprocess of the manufacturing, transportation and use, thus affects the safety performance ofcomposite structures. Therefore, the repairs for damaged composite laminates has a great valuein practicality. Once repaired, the structure shall satisfy the structural stiffness and strengthrequirements. This paper mainly studies the assessment of the vacuum-assisted wet-layupscarf-patch and stepped-patch repaired composite laminates.The study started from the continuum damage constitutive model for composites of each ply,which took into account the fiber tensile and compressive failure, the matrix tensile andcompression damage, tensile and compression failure in the thickness direction and the shearnonlinear phenomena in1-2plane and1-3plane. The stress discontinuity problem before andafter damage was solved by using the Hashin criterion based on strain. At the same time,considering the zero thickness characteristics of secondary curing interface, the motherboard andthe composite patch are bonded to each other through the contact. The bilinear cohesive damagemodel was used to simulate this adhesive contact property. The pure mode and mixed mode ofthe damage variable formula were described, respectively.Following a crack extension theoretical analysis on the double cantilever beam (DCB) andend notched flexure (ENF) specimens using bilinear cohesive damage model, the accuracy ofthis theoretical analysis was verified by comparing results to data obtained by the beam theory,modified beam theory and elastic foundation beam model. The influence of the changes of thedamage model parameters on the test load displacement curve and the length of the cohesivezone is discussed, This provides a theoretical guidance for the selection of parameters of bilinearcohesive damage model.Based on genetic algorithm, the identification method of constitutive model parameter for amechanical problem was established. The fitness function was defined by using the errorbetween the load displacement curves, obtained by the experiment and analysis using theparameter identification model. This parameter identification method was implemented byMATLAB program. The open hole tensile testing specimens as specified in ASTM, whose layupwere (00,900,450,-450)s, were used for parameter identification of the shear nonlinear factors and the damage rate impact factors in the single-layer composite damage constitutive model. TheDCB and ENF specimens were selected as the bilinear damage parameter identification model ofsecondary curing interface for the interface stiffness, the interface point strength, interfacialfracture toughness values. Numerical simulation analysis were carried out by assuming the ’realvalue’ to verify the reliability and validity of the parameter identification model.According to the process of the vacuum-assisted wet-layup flush repaired compositelaminates, three different grinding angles of scarf repaired and different step width to thicknessratioe of stepped repaired specimens were produced. The elastic strain at specified points and theultimate tensile load were obtained by experiment. The failure process and the final failure modeof these two kinds of repaired method were obtained by observation during the trial process.Through the microscopic analysis of the configurations of the parent material and the patchin the vacuum-assisted wet-layup flush repaired laminates, the models of the section for the scarfand stepped repair were established reasonably. And three-dimensional finite element models ofthese two kinds of repaired mode were built respectively. The composite continuous damageconstitutive model is used to simulate layer destruction, and the contact analysis is based onbilinear cohesive damage model to simulate the failure of secondary curing interface. The failureprocess of each method was studied. These models were verified by comparing with theexperimental results. Finally the effect of the repair parameters, such as grinding angle, stepwidth to thickness ratio, on the repair result were studied.