Shuxin Li

Comparison of phenomenological and crystallographic models for single crystal nickel base superalloys. II. Numerical simulations

A numerical procedure to determine the material constants for phenomenological and crystallographic material models is proposed, based on a group of generic equations. These models have been developed to describe the anisotropic behaviour of single crystal nickel base superalloys. The procedure includes a number of local optimisation routines to determine each group of material constants using experimental results. By using this procedure for both models, the intrinsic characteristics of the models are compared. Overall, both models can describe the fundamental deformation features of single crystal alloys. However differences are observed in the detailed predictions of the stress and strain curves. The differences between the two models are dependent on crystal orientation. The orientations where the two models predict similar behaviour are consistent with the regions where either octahedral or cubic slip dominates the inelastic deformation. Outside these regions the differences between the models are the greatest.

Modeling of the characteristics of fiber-reinforced composite materials damaged by matrix-cracking

Abstract A continuum damage mechanics (CDM) approach is used to develop a constitutive model for describing the characteristics of fiber-reinforced composite materials damaged by matrix cracking. In this model, the matrix cracks are assumed to be an array of parallel cracks along fibers in the plies, and the average measurement of the damage effects by this special damage state on the macroscopic mechanical properties of the composite materials is represented by a vectorial internal state variable (ISV). The general constitutive relationships for a damaged composite lamina and laminate, together with the damage evolution law, are developed, and the non-linear deformation characteristics of damaged composite material are investigated. The general model is finally verified by its application to a symmetric composite laminate for which there is a favorable comparison between the model prediction and the experimental results.

MEASUREMENT OF THE TRANSVERSE LOCAL STRAIN RESPONSE OF SINGLE CRYSTAL SUPERALLOYS

Abstract The experimental and theoretical results of an investigation into the characteristics of the anisotropic transverse mechanical behaviour of single crystal superalloys are presented. The local transverse strain measurements have been made at 15-degree increments using a diametral extensometer rotating about the main axis of the specimen. Measurements were conducted for nickel base single crystal alloys SRR99 and SC16 to determine the influence of temperature on the anisotropic transverse local strain fields of single crystal superalloys. A general procedure has been proposed to analyse the experimental data. The results indicate that the transverse mechanical behaviour of the single crystal alloys depend on temperature, crystal orientation and the transverse measurement position.

Numerical analysis on the flow–compaction behavior and the effect of interface permeability in thick composite plates during autoclave processing

A multi-field coupled model was developed to simulate the flow–compaction behavior of thick composite laminates manufactured by the autoclave process based on Darcy’s law and the effective compaction stress theory. The model was verified by comparing the predictions with the experiment results of a thick unidirectional laminate. The results show that the resin flow and compaction of fiber bed start from the top surface and gradually spread into the interior region, and the non-uniform resin flow along the thickness direction causes a gradient distribution of fiber volume fraction in the thick composite part. A cross-plied composite laminate model with a thin interlaminar layer was constructed, and the effect of the interlaminar transverse permeability on the flow–compaction behavior of the thick cross-plied laminate was numerically analyzed. The results indicate that the thick cross-plied composite laminate with high interlaminar transverse permeability has the similar flow–compaction process with that of the thick unidirectional laminate. An interlaminar layer with low transverse permeability impedes the resin flowing out from the interior of the thick cross-plied composite laminate and causes a lower fiber volume fraction compared with that in a unidirectional laminate.

Numerical Analysis on Process-Induced Residual Stress in Thick Semi-Cylindrical Composite Shell Using a State-Dependent Viscoelastic Model

A two-step state-dependent analysis model corresponding to the flow state before gelation and the viscoelastic state after gelation was proposed for the prediction of the residual stresses developed in polymer matrix composites (PMCs) during cure. The evolution of the viscoelasticity of PMCs during cure was analyzed based on the non-dimensional viscoelasticity parameters 1/Dem, and the gelation time was determined at the condition of Min(1/Dem) = 102. The proposed model was used to predict the process-induced residual stress and distortion in a thick semi-cylindrical composite shell. By comparing the predictions with those of the original cure-dependent viscoelastic model, the proposed model was verified and the computing time and memory storage were significantly reduced. The results show that during the cure of thick semi-cylindrical composite shell, the residual stresses caused by the cure shrinkage at high cure temperature are quickly relaxed, and most of the residual stress develops during cool-down. The spring-in deformation of the semi-cylindrical shell when release from the mould is mainly caused by the significant hoop stress gradient in the thickness direction developed during cure. Obvious warpage is not present in the composite semi-cylindrical shell due to the high bending stiffness.