Key Engineering Materials Vols. 345-346

Abstract: The cohesive zone model and the local failure criterion in the form of the average stress limitation in the cohesive zone ahead of the crack/notch tip have been employed to describe the failure assessment diagram for solids with a crack or notch under uniaxial and biaxial loading. The local stress on the crack extension line has been described by the exact elastic Westergaard’s solution taking into account the stress parallel to the crack plane. The cohesive stress ahead of the crack or notch tip is treated according to von Mises yield criterion as a property of the material, the applied stress and the stress biaxial ratio. Effect of the biaxial ratio is discussed for plates with a crack and notch. The failure assessment curve for a plate with a crack shows no significant differentiation among the three loading cases for plane stress, whereas the biaxial assessment curves move outward from the uniaxial case for plane strain.

Abstract: We present a continuum theory which predicts the steady state propagation of cracks. The theory overcomes the usual problem of a finite time cusp singularity of the Grinfeld instability by the inclusion of elastodynamic effects which restore selection of the steady state tip radius and velocity. We developed a sharp interface method and a phase field approach to investigate the model. The simulations confirm analytical predictions for fast crack propagation.

Abstract: A torsional transient wave was assumed acting at infinity on the piezoelectric body with an embedded penny-shaped crack. Appropriate governing equations and boundary conditions have been built within the three-dimensional electroelasticity. The total displacement field was simply considered as the combination of two parts, one related to incident waves inducing an oscillating motion, and another with the scattered waves. An electric impermeable crack was assumed to simplify the mathematical analysis. The problem was formulated in terms of integral transforms techniques. Hankel transform were applied to obtain the dual integral equations, which were then expressed to Fredholm integral equations of the second kind.

Abstract: This paper investigated the initiation and propagation characteristics of impact-induced damage in carbon-fiber-reinforced-plastic (CFRP) laminates with different stacking sequences and thicknesses under low-velocity impact. Impact force histories were measured with a drop-weight impact tester. A strain gauge was attached on the back face of CFRP laminates to measure exactly when a matrix crack on its back face was initiated. It was found from fractographic observation that impact-induced damage in CFRP laminates was initiated at the matrix crack on the back face of CFRP laminates due to bending deformation during impact. Finite element analysis was conducted using the impact forces derived from the experimental results of the impact test. Its results clarified that the tensile stress normal to the fiber on the back face of the specimen was the criterion to initiate impact damage in CFRP laminates.

Abstract: A finite geometrically similar element method is proposed to determine the dynamic stress intensity factor. A group of geometrically similar elements is automatically generated layer by layer around the point of singularity. The large number of degrees of freedom around the tip of singularity is transformed to a small set of generalized coordinates by means of the series expansion formulas of the displacement field. By taking advantage of the same stiffness and similar mass of similarly shaped elements, the combined stiffness matrix of super-element is obtained directly. The small set of generalized coordinates can be obtained through solving the equation, and then the dynamic stress intensity factor of V-notch (crack) will be obtained. There are some advantages for this method such as good adaptability, high precision and good convenience.

Abstract: The fracture problems of medium carbon steel under extra-low cycle axial fatigue loading were studied using artificial neural network in this paper. The training data were used in the formation of training set of artificial neural network. The artificial neural network model exhibited excellent comparison with the experimental results. It was concluded that predicted fracture design parameters by the trained neural network model seem more reasonable compared to approximate methods. Training artificial neural network model was introduced at first. And then the Training data for the development of the neural network model was obtained from the experiments. The input parameters, notch depth and tip radius of the notch, and the output, the cycle times of fracture were used during the network training. The neural network architecture is designed. The artificial neural network model was developed using back propagation architecture with three layers jump connections, where every layer was connected or linked to every previous layer. The number of hidden neurons was determined according to special formula. The performance of system is summarized at last. The result show that the training model has good performance, and the experimental data and predicted data from artificial neural network are in good coherence.

Abstract: The load separation method was employed to measure the pl η -factor, the growing crack length and the applied J-integral during the course of the test of small curved CT specimen of Z-2.5Nb pressure tube material. The effect of the notch tip radius of the notched (reference) CT specimen on the separation parameter was analyzed to predict the crack growing length in the precracked specimen. To avoid the effect of load relaxation in the reference specimen on the separation parameter and the crack growing length estimation, the load was assumed to be maximum and constant value behind the peak load in the reference curve.