Analysis of viscoelastic laminated composite plates

A micromechanics analysis is performed for the determination of the five independent elastic moduli of unidirectional fiber composites. By considering viscoelastic phases and by using the correspondence principle and the inversion of the Laplace transform, the five time-dependent functions which characterize the effective behavior of viscoelastic composites are established. The predicted time-dependent behavior is applied for the analysis of viscoelastic laminated plates. The resulting viscoelastic effects are shown, and comparison between the results obtained within the classical laminated plate theory and the first-order shear deformation theory is discussed.     

The nonlinear lamb problem

The two-dimensional problem of finite amplitude wave propagation in a nonlinear half-space excited by a normal and tangential time-dependent line loads is considered. A numerical scheme is developed and applied. The reliability of the numerical procedure is examined in some special cases by comparison with the corresponding analytical solutions which can be deduced for those cases. Vertical and horizontal displacements in the nonlinear half-space are presented and compared with the behavior of those obtained in the corresponding linear problem. It is noted that the symmetry of the horizontal displacements and the antisymmetry of the vertical displacements which are present in the linear halfspace for the case of tangential loading are completely destroyed in the nonlinear problem and that significant vertical displacements are obtained at stations beneath the load.     

Niobium addition effect in molds at last cooling step on EN-GJL250 gray cast iron: Microstructural changes and electrochemical behavior

The purpose of this study was to determine the impact of niobium addition as an inoculation element on the microstructure and electrochemical properties of EN-FGL250 gray cast iron. Niobium additions are in a powder form and have a 0.5 mm particle size at dfferent proportions of 1 wt.% and 3 wt.%. The addition was done during casting of the metal in the mold at the last cooling step of the melt cast iron. These additions have a significant impact on the phenomenon of solidifi cation as the metal powder deposited in the sand molds creates new centers of germination and absorbs a lot of heat. The cooling rate directly affects the microstructure and electrochemical behavior. This is confirmed by SEM observations and electrochemical tests. Furthermore, the addition of niobium transforms the microstructure of gray cast iron from cellular structure into totally dendritic structure. As a consequence, the niobium addition affected the shape and size of graphite, thus considerably reducing the corrosion current density by increasing the polarization resistance Rp.     

The over-all instantaneous properties of metal-matrix composites

A micromechanical analysis is presented for the determination of the instantaneous effective properties of metal-matrix composites, which consist of elastic fibers reinforcing elastic-viscoplastic matrices. At any stage of loading, the current behavior of the initially isotropic matrix exhibits anisotropy that is load-level- and history-dependent. It is shown that the knowledge of the local instantaneous properties of the inelastic-matrix and elastic-fiber constants provides, in conjunction with the micromechanical analysis, the over-all instantaneous stiffnesses of the metal-matrix composite. The method is illustrated for the prediction of the instantaneous properties of unidirectional and laminated composite materials.     

The effective thermomechanical behavior of inelastic fiber-reinforced materials

A continuum theory is presented for the prediction of the average behavior of unidirectional fiber-reinforced materials in which both constituents are thermoelastic in the linear region and thermo-inelastic in the nonlinear region. The resulting effective constitutive equations are given by a set of temperature dependent incremental relations from which the response of the composite to a given mechanical and thermal loading can be determined. The derived theory is applied to investigate the overall behavior of unidirectional graphite fibers reinforcing an aluminum alloy matrix under various types of applied stresses and temperature changes. In particular, the effect of residual stresses developed when the composite is cooled is investigated.     

Elastoplasticity theory for porous materials

A continuum theory is derived for the modeling of elastoplastic work-hardening porous materials. The theory provides a set of constitutive relations which, using the properties of the inelastic matrix, determines by an incremental procedure the overall response of the porous solid to various types of loading. In the elastic region, effective elastic moduli of the porous material are obtained. Comparison with theoretical and experimental results are given.     

Plastic buckling of ARALL plates

The plastic bifurcation buckling loads of ARALL (aramid aluminum laminate) plates are predicted. The plastic behavior of the plate is caused by the significant plasticity effects of the aluminum strips. The critical load level at which the ARALL plate loses its stability is determined from the material properties of the elastoplastic aluminum alloy strips and the elastic unidirectional aramid/epoxy composite layers, in conjunction with their geometric characteristics.     

A 2-dimensional mixture theory for biaxially fiber reinforced composites with application to dynamic crack problems

A 2-dimensional mixture theory is developed for wave propagation in a laminated medium in which every layer is made of a fiber reinforced composite material with the angle of reinforcement alternating from layer to layer. The developed theory contains as special cases the 2-dimensional mixture theory for a laminated medium made of isotropic layers, as well as the equivalent modulus theories for bi-directionally and unidirectionally fiber reinforced composites. The developed mixture theory is applied to the problem of a semi-infinite crack in the composite which is under dynamic loading. The induced fracture mode of the crack is of mixed type and contains both Mode I and Mode II types of opening. A numerical method of solution is applied to the four coupled mixture equations of motion in the average displacements and results are given for the dynamic stress fields in the composite.     

Effective constitutive equations for fiber-reinforced viscoplastic composites exhibiting anisotropic hardening

Effective elastic-viscoplastic stress-strain relations are derived for fiber-reinforced composites whose constituents are elastic-viscoplastic materials displaying anisotropic hardening. The derivation is based on a recently developed high-order continuum theory with microstructure for the modeling of viscoplastic composites, and is generalized here to incorporate anisotropic hardening effects. A specific reduction of the theory gives the effective rate-dependent elastic-plastic behavior of the composite which exhibits plastic anisotropy. In the special case of perfectly elastic constituents, the approximate overall moduli of the fiber-reinforced composite are obtained. Rate-dependent average stress-strain curves are given for numerous modes of cyclic loading of the composite. The effective behavior of periodically bilaminated viscoplastic composites is determined as a special case.     

A continuum theory for fiber-reinforced elastic-viscoplastic composites

A higher order continuum theory with microstructure is derived for the modeling of the 3-dimensional motion of fiber-reinforced composites in which both the matrix and fibers constituents are assumed to be elastic-viscoplastic work-hardening materials. The fibers are unidirectional with rectangular cross section and are imbedded in the matrix in the form of a doubly periodic array. The derivation of the theory is systematic and can be applied to various types of non-elastic composites to the desired degree of expansion. An appropriate reduction of the theory gives the average behavior of the viscoplastic composite in the form of effective rate-dependent stress-strain curves. In the special case of perfectly elastic constituents the reduction gives the approximate effective moduli of the composite.