International Journal of Damage Mechanics current issue

Professor T.H. Lin Memorial Issue in The International Journal of Damage Mechanics

Sun, L., Ju, J. W. — 2008-07-07

PQR Model-based Micromechanical Analysis of Hysteresis Loops for Single Crystal Fatigue: Aspects of Multi-Axial Loading, Geometric Effects and Creep

Dongdong Wang, , Tung Hua Lin, — 2008-07-07

The micromechanical theory of fatigue crack initiation, namely the PQR model with a gating mechanism proposed by Lin (1992), is systematically summarized and further generalized to arbitrary loading conditions which may occur in reality. All possible loading cases and slip mechanisms are considered for a FCC single crystal. The Schmid factors for both primary and secondary slip systems are presented. A 3D boundary element method is adopted to compute the stress influence coefficients for the residual stress field. The hysteresis loops, which are due to a general two phase sequential loading process, i.e., a uni-axial loading followed by a multi-axial loading applied by combined shear and axial loadings, are given to illustrate the fatigue development and demonstrate the proposed methodology. The results show that the two stage loading may increase the magnitudes of intrusion and extrusion. Subsequently the geometry influence for intrusion is studied. The effect of geometry variation caused by intrusion is taken into account through updating the stress influence coefficients as well as the applied resolved stress when intrusion grows. This process is continued until the saturated state of fatigue crack initiation is reached. Comparison of geometry influence on intrusion growth is shown as well. It turns out that the geometric change could become an important factor for the intrusion growth. Finally by replacing the plastic strain with creep strain the present model is easily extended to investigate the steady hysteresis loops for single crystal creep at elevated temperature. The results compare favorably with the available experimental data.

Micromechanical Elastoplastic Damage Modeling of Progressive Interfacial Arc Debonding for Fiber Reinforced Composites

Ju, J.W., Ko, Y.F. — 2008-07-07

This study presents a new micromechanical elastoplastic progressive damage model to predict the effective transverse mechanical behavior and interfacial arc microcrack evolution of fiber-reinforced composites. The partial debonding process at the fiber—matrix interfaces is represented by the growing debonding angles of arc microcracks. Progressive partially debonded cylindrical isotropic fibers are replaced by equivalent orthotropic yet perfectly bonded elastic cylindrical fibers. The equivalent orthotropic elastic moduli are constructed to characterize the reduction of the load-transfer capacity in the debonded directions. The effective elastic moduli of four-phase composites are derived by using a micromechanical formulation. In order to characterize the overall transverse elastoplastic damage behavior, an effective yield criterion is derived on the basis of the ensemble-area averaging procedure and the first-order effects of eigenstrains upon yielding. The proposed effective yield criterion, coupling with the overall plastic flow rule and the hardening law, comprises the analytical framework for the prediction of effective elastoplastic-damage responses of ductile matrix composites containing randomly located yet aligned cylindrical fibers. The Weibull's probabilistic function is utilized to characterize the varying probability of progressive interfacial arc microcracks, governed by the internal stresses of fibers and the interfacial bonding strength. The proposed micromechanical elastoplastic-damage model is then applied to the transverse uniaxial and transverse biaxial tensile loading with varied stress ratios. Comparisons between the present predictions and available experimental data, as well as other numerical simulations, are performed to elucidate the potential of the proposed formulation.

A Note on Short-time Response of Two-dimensional Lattices during Dynamic Loading

Mastilovic, S. — 2008-07-07

The disordered 2D lattices are used extensively to study damage evolution and fracture of inhomogeneous or multi-phase systems. The present note addresses their initial elastic response during dynamic loading. Namely, a transition from short-time values of modulus of elasticity and Poisson's ratio to respective long-time values, which is not accompanied by the corresponding change of stiffness tensor components. The study is performed on three 2D truss-type lattices. It is demonstrated that the difference between the two sets of elastic properties is a result of combining effects of the initial lateral inertia and the disorder of the system.

Flexure Toughness of Polymer Fiber-reinforced Cementitious Materials

Punyamurtula, V. K., Yu Qiao, — 2008-07-07

In this article, the toughening effect of polymer fibers in cementitious materials is analyzed through an energy method. When a crack front encounters a fiber array, additional fracture work is required to overcome the barrier effect. The influences of fibers, matrix, and crack length on the critical energy release rate are collectively described by a single system parameter.