Notch effects in tensile behavior of AM60 magnesium alloys

The deformation and failure behavior of an AM60 magnesium alloy was investigated using tensile test on circumferentially notched specimens with different notch radii. The strain and stress triaxiality corresponding to the failure point were evaluated using both analytical and finite element analyses. Combining with systematical observations of the fracture surfaces, it is concluded that deformation and failure of AM60 magnesium alloy are notch (constraint) sensitive. The failure mechanisms change from ductile tearing to quasi cleavage with the increase of constraint

A Self-Consistent Model For Directional Dependence Of Crack Growth

Fracture growth is considered as the competition between cleavage and dislocation self-organization in elastic-plastic solids. A self-consistent model is proposed to bridge the responses at relevant length scales, an elastic enclave in the immediate vicinity of crack tip, an array of disclination dipoles and macroscopic plastic deformation. The directional dependence of crack growth is studied. In the continuum limit, the flow stress is expressed by a spatial coupling in terms of a second-order gradient of the rotation strength of disclination dipoles. An estimate of the core size and the crack-tip shielding ratio is given by identification of the macroscopic plastic fields, the elastic field and the constitutive flow stress from the micromechanics consideration, on the boundary of elastic core. Strong dependence of apparent fracture toughness on the intrinsic surface energy and the ductile-to-brittle transition are examined

Experimental Study On Z-Pinned DCB Mode I Delamination

An experimental investigation on mode I delamination of z-pinned double-cantilever-beams (DCB) and corresponding pin pullout behaviour is presented. The effects of loading rate on delamination crack opening and z-pin bridging mechanisms is reported. Optical micrographs of z-pins after pullout show that a higher loading rate causes more splitting damage in the pins. Comparison of fracture load shows that the fracture load rises with increasing loading rate. Z-pin pullout tests were also conducted to give a detailed description of z-pin bridging mechanisms

Fatigue Crack Growth And Piezoelectric Property Decay Induced By Cyclic Electric Fields For An Actuation Piezoceramic

Degradation of piezoelectric properties of piezomaterials has long been a concern in the applications of actuators and sensors. In this work, alternating electric field induced fatigue crack growth and effect of cyclic electric field on piezoelectric property decay were characterized for a polarized PZT-PIC151. The results show that a relatively high alternating electric field drives the pre-existing microcracks to grow very fast initially due to the superposition of electrostriction induced stress and residual stress at the crack tip, then slow down gradually to becoming dormant. The butterfly loop evolution shows that cyclic electric field strongly degrades the piezoelectric properties due to the frequent domain switching. The output strain decays more than 50% after 106 electric cycles at 0.9 Ec for PIC 151 pellet bonded on an aluminum beam

Thermal-stress resistance and fracture toughness of two tool ceramics

The thermal-stress resistance and fracture strength behaviour of two oxide ceramics (a hot-pressed pure Al 2 O 3 and a composite ceramic NTK-HC2) subjected to severe thermal shocks have been investigated. The damage resistance parameter ( K IC / σ f ) 2 for both ceramics is also determined for a wide range of temperatures (25° to 800° C) and cross-head rates (10 −2 cm min −1 ∼ 2.5 m sec −1 ). Fracture strength behaviour of these two oxide ceramics is shown to follow Hasselman's model where the instantaneous strength loss at the critical quenching temperature may be calculated using appropriate ( K IC / σ f ) 2 values to give good agreement with experimental results. Repeated shocks show some further degradation in the retained strength for both ceramics so that these materials are susceptible to thermal fatigue. It is found that both materials possess similar resistance to crack initiation (i.e. similar Δ T c and retained strength after shocking through Δ T c ) but the pure oxide ceramic has higher resistance to crack propagation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44673/1/10853_2004_Article_BF00540875.pd

Saturation Dislocation Microstructures In A Copper Single Crystal During Fatigue In HClO4 Aqueous Solution

A copper single crystal was tested at room temperature in air and in a 0.1M HClO4 solution under the symmetric tension-compression load mode, with loading axis parallel to the [013] direction. The dislocation structures were characterised using the electron channeling contrast (ECC) technique of scanning electron microscopy (SEM) and using transmission electron microscopy (TEM). The results show that the saturation dislocation structures in samples subjected to corrosion fatigue in the 0.1M HClO4 aqueous solution manly had the form of cells, dislocation wall-like and veins, which differ from the dislocation structures of dislocation wall-like and veins in the air environment

Fracture Mechanics of Quasicrystalline Materials

The existence of phason field - the unusual field parameter - in quasicrystals leads to the essential difference of the elasticity between the materials with the conventional structural materials including crystals. In the new solid phase, there are two different displacement vectors - phonon and phason fields. Connecting with this, there are two different strain tensors and stress tensors in the new material. This challenges the solid mechanics as well as the fracture mechanics of conventional structural materials. In this paper, some exact solutions for crack problems under different configurations and states of motion in different quasicrystal systems will be exhibited. These results reveal the characteristics of fracture mechanics of the new material in which the correlation and difference features of the new branch with the traditional fracture mechanics are demonstrated