Key Engineering Materials Vols. 417-418

Abstract: An implicit numerical algorithm for the integration of the primary/secondary creep constitutive law at the material point level is derived. For the presented constitutive model, the experimentally obtained material parameters for the medium density polyethylene (MDPE) are taken from literature. The derived algorithm in conjunction with the Jacobian matrix is implemented in the finite element (FE) code ABAQUS by using the user subroutine CREEP. The accuracy of the proposed algorithm is demonstrated.

Abstract: An experimental analysis on a set of strengthened masonry walls has been carried out by means of cyclic loading tests in order to simulate the creep effects. The damage evolution of specimens reinforced by traditional or innovative methods is evaluated by the Acoustic Emission (AE) technique. The AE time dependence during fracture propagation is analysed through a power law. In addition, the AE frequency analysis is used to obtain information on the criticality of the ongoing process.

Abstract: The overall elastic moduli of a solid are changed when the solid is damaged by cracks. For a finite solid, the size influence has been investigated and it has been found that for a given crack density, increasing crack size reduce the overall moduli [1]. For an infinite solid, it is obviously impossible to make the computation with all cracks. Classical methods suggest computing the overall moduli with the solution of the crack opening displacement of one single crack. The interaction between cracks is neglected or taken into account approximately. In this paper, the overall moduli of two dimensional infinite solids with cracks are computed numerically. From numerical simulations, it has been found that the interaction between cracks can be neglected if the distance between them is three times larger than the crack size. So one can compute the opening displacement on one crack with the presence of cracks nearby and use the crack opening displacement to compute the overall moduli. The numerical values are smaller than those of the method of diluted distribution but greater than those of the differential scheme and the self consistent method. They are also slight greater than the numerical results of bounded cracked solids. For small values of crack density however, the numerical results of both infinite solids and bounded solids are close to the estimation of the differential scheme.

Abstract: The occurrence of small scale plasticity can be modeled physically by force doublets embedded in an elastic medium and therefore the plasticity problem can be treated by the superposition of elastic solutions. This idea for the treatment of an inelastic strain is reviewed and generalized to develop a versatile program for two-dimensional elastic-plastic problems based on Body Force Method. In the present study, a treatment of an elastic-perfect plastic body is discussed in detail. The increment of the density of force doublets, which has one to one correspondence to the increment of plastic strain, can be determined from Prandtl-Reuss equation. It was also found the Delaunay triangulation is useful and convenient for the automated elastic-plastic analysis.

Abstract: Experimental results on the ballistic limits and failure mechanisms that lead to perforation in aluminum alloy target plates of 1-6 mm thickness by solid spherical projectile are presented. The projectile was launched between velocities of 150 m/s to 700 m/s by using the 10mm smooth bore gas gun. In this study, the attention was focused on the peculiarities of penetration process when the impact point approaches closely to the free or firmly fixed edge of the plate. The results were compared with the case of central impact and with others under changing the boundary condition, thickness and the target material properties.

Abstract: The presence of damage or cracks modifies the elastic properties of a sample, introducing nonlinear components in the elastic response to an ultrasonic excitation. Among other effects, we discuss here conditioning and memory, i.e. the transition of the sample to a new (non equilibrium) elastic state when perturbed by a strain, even at a relatively low amplitude. A quantification of these phenomena can be the basis for a novel Nondestructive Method to evaluate the integrity of a structural or mechanical component.

Abstract: The influence of specimen thickness on fatigue crack behaviour has been investigated. To this aim the fatigue crack propagation rate has been measured on two different types of test specimens with varying thickness. The change of stress singularity exponent for the crack front due to vicinity of the free surface is considered. To explain the effect of stress singularity changes on obtained experimental results a methodology based on generalized stress intensity factor and strain energy density concept has been used. It is shown that for materials with Poisson’s ratio of about 0.3 the free surface effect does not play a decisive role for specimens with a low level of in-plane constraint but can influence fatigue crack propagation rate in the case of geometries with a high level of the constraint.

Abstract: The mechanism of the directional coarsening of ' phases (rafting) of Ni-base superalloy under an uni-axial strain was analyzed by molecular dynamics (MD) analysis. The stress-induced anisotropic diffusion of Al atoms perpendicular to the interface was observed clearly in a Ni(001)/Ni3Al(001) interface structure, The reduction of the diffusion of Al atoms perpendicular to the interface is thus, effective for improving the creep and fatigue resistance of the alloy. It was also found that the dopant elements in the superalloy also affected the strain-induced diffusion of Al atoms. Pd was one of the most effective elements which restrain Al atoms from moving around the interface.

Abstract: HTPB propellant is the high filled particulate elastomeric matrix composite. Debonding of particle/matrix interfaces can significantly affect the macroscopic behavior of composite propellant. How to model the propellant material and describe damage processes to discover damage mechanism has been a long-standing question. This paper used the bilinear cohesive law with different parameter values for particle/matrix interfaces to study interface debonding. By analyzing the damage evolution in two model particulate composite systems with finite element method, the scheme was shown to capture effects associated with the interface strength and the interactions between particles.

Abstract: This paper presents an experimental investigation of a new method for damage detection based on the most fundamental concept in continuum mechanics: strain compatibility. Compliance with this principle implies a deformed material is free from discontinuities, which are indicative of many types of structural damage. Therefore the principle of strain compatibility, in its ability to identify discontinuities, is very promising as a new foundation for future research into non-destructive evaluation and structural health monitoring technologies. The proposed method has many advantages compared to existing damage detection techniques, such as its invariance to material properties, type and intensity of loading, and the geometry of the structure. In this paper, a proposed formulation of the strain compatibility equation for beam structures, which is invariant to loading intensity, is presented. An experimental investigation of the proposed algorithm was conducted on a delaminated cantilever beam, utilising a PSV-3D scanning laser vibrometer. The experiment demonstrated that the strain compatibility technique can accurately locate delamination damage in composite beam structures.