Yuguo Sun

Analysis of a penny-shaped crack in magnetoelectroelastic materials

Cracks in magnetoelectroelastic fracture have been extensively assumed to be impermeable or permeable to the electromagnetic fields. Past analysis suggested that even in piezoelectric cracking, the permeability of the medium inside the crack has a substantial influence on the crack-tip fields and the crack opening displacement. This paper performs a self-consistent analysis for magnetoelectroelastic cracking. An internal penny-shaped crack in a magnetoelectroelastic material under remote magnetoelectromechanical loads is investigated. Effects of electric and magnetic permeabilities inside the cracks in magnetoelectroelastic media are clarified. It is found that the self-consistent approach and the traditional ideal assumptions (impermeable and permeable assumptions) to the crack-face electric and magnetic boundary conditions produce substantial differences for the crack-tip quantities, such as the field intensity factors, energy release rate, and crack opening displacement.

Analysis of shear localization in porous materials based on a lower bound approach

The conditions for shear localization in porous materials are examined based on the lower bound approach proposed by the present authors. The influence of void nucleation and material inhomogeneity on the critical strain to localization is investigated and an improved plastic strain controlled nucleation criterion is proposed which makes it possible to include the influence of hydrostatic stress and avoid the ambiguity caused by the non-normality flow rule. The constitutive behavior of porous materials (including the yield loci, the void growth rate and the stress-strain curve) is also examined and comparison is made between the theoretical result and the experiment. Finally the instability and fracture of sintered CP Ti alloy and AISI4340 steels is analyzed and results are compared with the experiment.

A Theoretical Model of Conductive Cracks in Piezoelectric Materials under Electromechanical Cycling

This article develops an analytical model for the conductive cracks in piezoelectric ceramics under electromechanical cycling. An electrical yielding strip and a mechanical yielding strip are assumed to develop at the crack tips when the medium is subjected to external electromechanical loads. The yielding strip is assumed to have constant stress and constant electric field. Based on these assumptions and extending the accumulated plastic displacement criterion for crack propagation in traditional structural materials to piezoelectric ceramics, a fatigue crack growth law is derived. The law, similar to the well known Paris law, is a fourth-power function of the effective stress intensity factor. Graphical results for the effect of electric load on the effective crack tip stress intensity factor and crack growth rate are provided. The law is given in closed-form so that it might be used in the future for the fatigue and reliability analysis and design of piezoelectric materials.

Experimental investigation of compressive damage mechanisms on 3-D braided composites by acoustic emission

In this paper, the Acoustic emission (AE) technique is applied to clarify the damage mechanisms of 3-D carbon/epoxy braided composites with different braiding angles under the compressive loading. The AE responses for the three kinds of specimens are significantly different due to the different damage mechanisms. The results reveal that the curves of cumulative AE counts can be used to analyze the damage processes of 3-D braided composites more exactly than that of only using the load-time curves. Through analyzing both the cumulative AE counts and the load-time curves at the same time, the damage processes can be divided into different stages and be described in detail. The present paper reveals that the AE technique is a viable and effective tool for analyzing the fracture process and damage mechanisms in the 3-D braided composite materials under the compressive testing.