Advanced Materials Research Vols. 33-37

Abstract: In this paper, the local approach was used to analyze the geometry dependence of coating specimens for interface brittle fracture initiation, and a definition of the fracture process zone was proposed in the paper. The results showed that the interface fracture behavior of two types of specimens with notch had been predicted from the test results of pre-crack specimens based on the local approach for interface brittle fracture, and the predicted distribution of the critical load for the notched specimens gave a good agreement with the test results. It indicated that the local approach not only can be used to describe the interface fracture behavior, but also can be used in the integrity evaluation for interface between different materials.

Abstract: A steady-state subsonic interface crack propagating between an elastic solid and a rigid substrate with crack face contact is studied. Two cases with respective to the contact length are considered, i.e., semi-infinite and finite crack face contact. Different from a stationary or an open subsonic interface crack, stress singularity at the crack tip in the present paper is found to be non-oscillatory. Furthermore, in the semi-infinite contact case, the singularity of the stress field near the crack tip is less than 1/2. In the finite contact case, no singularity exists near the crack tip, but less than 1/2 singularity does at the end of the contact zone. In both cases, the singularity depends on the linear contact coefficient and the crack speed. Asymptotic solutions near the crack tip are given and analyzed. In order to satisfy the contact conditions, reasonable region of the linear contact coefficient is found. In addition, the solution predicts a non-zero-energy dissipation rate due to crack face contact.

Abstract: Interfacial stress distribution of bonded quarter-planes subjected to a concentrated force was re-investigated based on Bogy’s solution[1]. The characteristic length of the singular interface end, δ, was defined, and found varying in a very large size scale with the index of stress singularity from millimeter to nanometer or even smaller scale. The influences the characteristic length scale on the initial debonding of the interface end is a new question worth to pay attention. Photoelasticity experiment was employed to verify whether the initial debonding is always located at interface end with stress singularity. The test results show that the initial debonding does not start from singular interface end if the index of stress singularity is small enough.

Abstract: Nonlinear finite element model analysis of the casing plug joints of steel tubular has been realized by ANSYS software. The law of load-carrying capability and stiffness of joint are separately gained by changing the ratio of length and diameter (R/L) and the ratio of the casing length and the main tube length (l/L). The influence of the casing thickness on the load-carrying capability and stiffness are also discussed. The results indicated that the load-carrying capability and stiffness of the joints both increase with the ratio(R/L) increment and the ratio of the casing length and main tube length (l/L). When the main tube thickness is equal to casing thickness, the load-carrying capacity of joints achieves the most.

Abstract: The failure in an adhesive-bonded structure starts at the interface, and the interfacial fracture is of interest whenever adhesion between different materials is concerned. One of primary factors limiting the application of adhesive-bonded joints to structural design is the lack of a good evaluation tool for adhesion strength to predict the load-bearing capacity of boned joints. The adhesion strength of composite/steel bonding has been evaluated using interfacial fracture mechanics characterization. The energy release rate of a composite/steel interfacial crack was compared with the fracture toughness of the interface, which was measured from bi-material end notched flexure (ENF) specimens, to predict the failure loads of bi-material lap joints. Fracture toughness, IIc G , was regarded as a property of the interface rather than a property of the adhesive. The results show that interfacial fracture mechanics characterization of adhesion strength can be a practical engineering tool for predicting the load-bearing capacities of adhesive-bonded joints.

Abstract: An axisymmetric problem of interaction of a rigid rotating flat ended punch with a transversely isotropic linear piezoelectric half-space is considered. The contact zone consists of an inner circular adhesion region surrounded by an outer annular slip region with Coulomb friction. Beyond the contact region, the surface of the piezoelectric half-space is free from load. With the aid of the Hankel integral transform, this mixed boundary value problem is formulated as a system of dual integral equations. By solving the dual integral equations, analytical expressions for the tangential stress and displacement, and normal electric displacement on the surface of the piezoelectric half-space are obtained. An explicit relationship between the radius of the adhesion region, the angle of the rotation of the punch, material parameters, and the applied loads is presented. The obtained results are useful for characterization of piezoelectric materials by micro-indentation and micro-friction techniques.

Abstract: Adhesives are widely used in our life and industrial world. However, it is difficult to characterize their mechanical properties because those strongly depend on environmental and mechanical conditions such as temperature, humidity or strain rate. In this paper, we focus on the strain rate dependence of the interfacial strength and investigate the interfacial strength by peel tests under several peel rates. The results show that, in lower rate region (under 1.0 mm/s), the interfacial strength is constant and, in transition region (1.0 to 10 mm/s) the interface strength increased with the peel rate. In middle rate region (10 to 103 mm/s), the interfacial strength is constant again. Over 103 mm/s region, the interfacial strength drops and became lower than those in middle rate cases. From the observation of peeling front by a high speed video camera, the deformation behavior of adhesives changes with the peel rate.􀀁Finite element analysis by using cohesive zone model is also conducted, and influence of the rate dependency of adhesive and base material is discussed.

Abstract: Alumina have been investigated for its distinguished characteristics to be widely used in armor application. One-stage light gas gun were utilized to study the dynamic mechanical properties of alumina subjected to shock loading. Manganin gauges were employed to obtain the stress-time histories at the different Lagrange locations in alumina target. The Hugoniot curves of stress and pressure versus specific volume were fitted based on the experimental data. The compressive behaviors for AD90 alumina are shown to be from elastic to “plastic” below 12GPa and under more higher pressure it will be transferred to similar-fluid state. Adopting LS-DYNA finite element code the dynamic failure of alumina ceramic armor are simulated. It is concluded that nucleation and growth of great number of radial and axial cracks and lateral cracks play dominant role in fractured ceramic target under impact loading.

Abstract: The Mode I and Mode II dynamic fracture toughness (DFT) values, KId and KIId for high strength steels 40Cr and 30CrMnSiNi2A were evaluated using a combined experimental-numerical method. The tests were performed on three point bend (3PB) and shear specimens with Hopkinson pressure bar. The time of crack initiation was determined by a strain gauge. With the assistance of 3-D transient finite element analysis, the temporal evolution of the dynamic stress intensity factor under different loading rates was obtained, and the DFT was determined by the fracture initiation time. In Mode I tests, a brittle transgranular fracture was found in 40Cr, while evidence for ductile fracture was observed on the failure surface of 30CrMnSiNi2A. In Mode II tests, both tensile cracks and adiabatic shear bands were found for the two steels. The effect of ligament size on fracture toughness was discussed. The correlations of KId and KIId with loading rate were also investigated. The micromechanisms of loading rate effect on the DFT were discussed for both modes.

Abstract: The dynamic fracture experiments were conducted on the heat treated magnesium alloys; AZ31B-O, AZ31B-200 °C, and AZ31B-430 °C. Cross shaped specimens with the crack on their center were used for the experiments. Dynamic fracture behavior near a crack tip under equal and unequal biaxial stress was observed by the caustics method. From the observation, the stress intensity factor and the fracture toughness value were calculated. As a result, the effect of heat treatment was found. However, no clear relation such as correlation between dynamic stress intensity factor and heat treatment temperature was deduced.