| Advanced composite materials have been used in the aerospace,ship and other industries due to notable performance,such as high specific modulus and strength,easy to design and good fatigue resistance.It is popular to manufacture whole large composite components with various curing moulds to achieve good quality,however,the manufacturing process-induced distortions after demoulding are inevasible due to the effects of extrinsic process and intrinsic anisotropy in the thermal strain and shrinkage strains,etc.Unnecessary process-induced distortions not only bring the difficulties in engineering structure assembly,but also influence the structural safety.In order to reduce the effect of process-induced distortions,cure cycle and mould design need to be iteratively optimized and trial and error compensation design is required,which take too much time and cost.It is important to understand the mechanisms,formation and control of the process-induced distortions.There are many parameters causing process-induced distortions and the effects of the various parameters are not fully understood.This thesis mainly focuses on the investigation of process-induced distortions in composite structures with numerical simulation and theoretical derivation.The distribution of temperature and degree of cure along through-thickness direction of composite parts and the effects of mechanical properties development during curing on the process-induced distortions are investigated by numerical simulations,then new constitutive models and analytical solution for process-induced distortions are developed.Firstly,the numerical simulations for the heat transfer and cure kinetic for AS4/8552 composite parts cured in the invar,aluminum and steel mould are presented respectively to investigate the effects of part thickness and mould types on the distribution of temperature and degree of cure along the through-thickness direction of composite parts.The results show that the gradient of temperature and degree of cure in the through-thickness direction can be neglected during curing for plane composite parts with a thickness of less than 5.4mm.The temperature distribution along the through-thickness direction measured by the FBG sensor embedded in the composite parts confirmed that the numerical results are reliable and true.Consequently the gradient of temperature and degree of cure for thin composites parts can be ignored in the numerical and analytical studies to predict the process-induced distortions.Subsequently,the popular mechanical constitutive models and their characterizations are presented,including viscoelastic,Path-dependent and CHILE model.It is revealed that the viscoelastic and path-dependent models only describe the behaviour of thermorheological simple materials(TSMs).The aforementioned constitutive models are implemented into the numerical simulations of the C-shaped composite parts made from AS4/8552 materials to investigate the process-induced distortions.The numerically simulated spring-in angles of the C-shaped composite parts are compared with the experimental and analytical results obtained by Wisnom et al.The results show that the difference among the spring-in angles predicted by different constitutive models is small and the numerically simulated spring-in is very close to the analytical one for the cross-ply C-shaped parts.By comparison,the path-dependent model exhibits a balance between the accuracy and efficiency for predicting the process-induced distortions.However,the analytical and numerical predictions of the spring-in are significantly less than experimental results for the unidirectional C-shaped parts.The results indicate that the analytical solution by Wisnom et al.is not suitable for the prediction of process-induced distortions of unidirectional C-shaped parts since it is questionable to assume the unidirectional parts under plane stress condition during curing.Meanwhile,the numerically predicted spring-in and residual stresses by the viscoelastic and path-dependent model are nearly identical,therefore it is effective and reasonable to assume that the shift factor is a step function that approaches to zero in the rubbery state and to infinity in the glassy state during curingTo eliminate the limitation that the current viscoelastic constitutive model only reflects the mechanical behavior of TSMs during curing,the thermorheological complex materials(TCMs)constitutive model and the corresponding incremental formulation are developed by introducing the dependency of the initial and balanced stiffness on temperature together with the consideration that the shift factor and strain rate can be regarded as constants in a very small time increment.Subsequently,by adoption of the verified assumption that the shift factor as a step function approaches to zero in the rubbery state and to infinity in the glassy state respectively to the TCMs constitutive model for simplification,a new path-dependent constitutive model is developed.The two constitutive models are then implemented in FE-program and the numerical results are validated by analytical testing cases.Numerical simulations with matured techniques are useful for predict the distortions of composite parts,but it is difficult to identify the fundamental mechanism of process-induced distortions and the controlling curing process parameters.The analytical solution can normally provide information to explain the underlying mechanisms.However,the analytical solution presented by Wisnom et al.cannot accurately predict the spring-in of unidirectional C-shaped composite parts.Consequently a new analytical solution for spring-in of C-shaped composite parts is developed in this thesis to take into account the effects of non-mechanical strain in the length direction with the assumption of plane strain condition in the rubbery state instead of plane stress condition.The new analytical solution can describe the effects of mechanical properties,cure shrinkage strain and thermal strain in the rubbery state as well as thermal strain and cure shrinkage strain in the glassy state on spring-in.The composite materials properties predicted based on the RVE numerical model are used in the analytical solution and numerical simulation of the C-shaped composite parts to predict the spring-in angles of unidirectional and cross-ply C-shaped AS4/8552 composite parts.The predicted spring-in using the analytical solution is compared with the experimental and numerical values.The results show that analytically predicted values match well with experimental results and are also in good agreement with numerically predicted values under plane strain condition,with maximum a difference of 5%.The good correlation with experimental and numerical results shows the validity of the new analytical solution.Meanwhile,numerical simulation of the development of the hoop stress in the symmetrical plane of C-shaped parts reveals that the residual stress is remained in the unidirectional parts after release.Further investigation demonstrates that the residual stress is induced by the not fully released hoop stress generated in the rubbery after release.Considering the L-shaped composite parts are more widely utilized in the engineering structures,a new analytical solution is first derived for spring-in of L-shaped composite parts taking into accounting the length of straight section in the last part of this thesis.This analytical solution reveals that effects of length of straight section,mechanical properties,cure shrinkage strain and thermal strain in the rubbery state as well as thermal strain and cure shrinkage strain in the glassy state on the spring-in of L-shaped composite parts.The experimental results of unidirectional and cross-ply AS4/8552 L-shaped parts with R/t=5,10 and 134 mm straight section length agree well with the predicted spring-in using the developed analytical solution with assumption that the part is under plane strain condition in the rubbery state.Furthermore the numerically predicted spring-in angles for L-shaped parts with R/t=5,10,15,20 and 0mm,10 mm,57mm,109 mm,134mm straight section length are very close to analytically predicted values,with a maximum difference of 5%.The good correlation shows the validity of the analytical solution for the spring-in of L-shaped parts developed in the thesis. |
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