Modelling of spall damage in ductile materials and its application to the simulation of the plate impact on copper

A statistical model of dynamic spall damage due to void nucleation and growth is proposed for ductile materials under intense loading, which takes into account inertia, the elastic-plastic effect, and initial void size. To some extent, void interaction could be accounted for in this approach. Based on this model, the simulation of spall experiments for copper is performed by using the Lagrangian finite element method. The simulation results are in good agreement with experimental data for the free surface velocity profile, stress record behind copper target, final porosity, and void concentrations across the target. The influence of elastic-plastic effect upon the damage evolution is explored. The correlation between the damage evolution and the history of the stress near the spall plane is also analyzed.

EXPERIMENTAL STUDY OF MICROHOLE GROWTH AND COALFSCENCE IN DUCTILE MATERIALS

The growth and coalescence of two microholes in copper foil were studied experimental ly by in situ tensile tests under a,scanning electronic microscope.Two microholes of 15-35μm in di- ameter were arranged in different distances and orientations.It was found that the mechanisms of mi crohole evolution were represented by slipping band creation,and then crack initiation and propagation along the slipping bands,in ligament.The process of microhole growth and coalescence was influenced by the inter-center distance and orientation of microholes.The critical surface of microholes at coales- cence is about 2—2.5 times that of the initial one.The variation of both the inter center distance and orientation depends on the initial angle.

THE MICRO-VOID GROWTH MODEL IN DUCTILE MATERIAL

Based on the unbounded matrix, a void growth model is proposed inthis paper. In this mod- el, extending the Griffith criterion ofenergy conservation during the expanding of cracks to the case ofimpact loading, we propose an assumption that the strain energy inmatrix can transform into its surface energy and kinetic energyduring the micro-void growth. The void growth velocities of the OFHCcopper and LY12 alu- minim are calculated, and the result of whichagrees with that of Seaman

Determination of the Uniaxial Stress-Strain Relations of Ductile Materials by Small Disk Specimens

In this work,the small lateral-compression testing based on energy equivalent(SLTEE)method is put forward to determine the stress-strain curves of materials utilizing small disk specimens.Numerical simulations of small lateral-compression testing with imaginary materials are conducted to examine the validity of the SLT-EE method.The results demonstrate that the stress-strain curves determined by the SLT-EE method coincide with the curves input by finite element analysis.In order to predict the stress-strain curves of materials with different dimensions,a modified SLT-EE method is successfully proposed by introducing a correction factor/.Finally,the small disk compression experiments of Q345B,304,7075 and 6061 are performed.The stress-strain curves of the four materials predicted by the SLT-EE method show agreement with the tension results.Furthermore,the mechanical properties of in-service hollow components are also determined utilizing the same method successfully.

Micro/nano Indentation and Single Grit Diamond Grinding Mechanism on Ultra Pure Fused Silica

The existing research about ductile grinding of fused silica glass was mainly focused on how to carry out ductile regime material removal for generating very ＂smoothed＂ surface and investigate the machining-induced damage in the grinding in order to reduce or eliminate the subsurface damage.The brittle/ductile transition behavior of optical glass materials and the wear of diamond wheel are the most important factors for ductile grinding of optical glass.In this paper,the critical brittle/ductile depth,the influence factors on brittle/ductile transition behavior,the wear of diamond grits in diamond grinding of ultra pure fused silica（UPFS） are investigated by means of micro/nano indentation technique,as well as single grit diamond grinding on an ultra-stiff machine tool,Tetraform ＂C＂.The single grit grinding processes are in-process monitored using acoustic emission（AE） and force dynamometer simultaneously.The wear of diamond grits,morphology and subsurface integrity of the machined groves are examined with atomic force microscope（AFM） and scanning electron microscope（SEM）.The critical brittle/ductile depth of more than 0.5 μm is achieved.When compared to the using roof-like grits,by using pyramidal diamonds leads to higher critical depths of scratch with identical grinding parameters.However,the influence of grit shapes on the critical depth is not significant as supposed.The grinding force increased linearly with depth of cut in the ductile removal regime,but in brittle removal regime,there are large fluctuations instead of forces increase.The SEM photographs of the cross-section profile show that the median cracks dominate the crack patterns beneath the single grooves.Furthermore,The SEM photographs show multi worn patterns of diamond grits,indicating an inhomogeneous wear mechanism of diamond grits in grinding of fused silica with diamond grinding wheels.The proposed research provides the basal technical theory for improving the ultra-precision grinding of UPFS.

Dense dislocations induced ductile SnTe thermoelectric semiconductor over a wide range of temperatures

Most inorganic thermoelectric semiconductors are intrinsically brittle,restricting the application of ther-moelectric materials.Therefore,developing ductile thermoelectric materials is crucial to thermoelectric technology applications.In this work,single-phase SnTe bulks with dense dislocations were prepared by melting quenching combined with spark plasma sintering.The resulting SnTe thermoelectric materials exhibited a large compressive strain of∼7.5%at room temperature,originating from high-density pre-existing mobile dislocations.The initiation of localized slip bands and preferred slip system were also identified by first-principles simulation.Detail microstructural characterizations reveal that the thermal activated dislocation emission and migration lead to higher compressive strains at intermediate tem-peratures.At 673 K,the deformation mechanism changed from dislocation mediated to grain boundary mediated plasticity,resulting in an ultra-high compressive strain of∼42%.In sum,new insights into the mechanical behavior of SnTe thermoelectric material over a wide range of temperatures were provided.This work offers the dislocation engineering strategy to design ductile thermoelectric materials for flexi-ble electronics and energy systems.

Competing failure mechanisms of thin metal films on polymer substrates under tension

The ductility of thin metal films on polymer substrates reported in recent experiments has a huge disparity,ranging from less than 1 % up to more than 50 %.To reveal the underpinning origins for such a large variation,this paper reports a systematic computational study of two competing failure mechanisms:metal film necking and grain boundary cracking.The quantitative results suggest that strong grain boundaries and metal/polymer interfacial adhesion are keys to achieve high ductility of polymer-supported metal films.

EVALUATION OF DUCTILE FRACTURE CRITERIA IN A GENERAL THREE-DIMENSIONAL STRESS STATE CONSIDERING THE STRESS TRIAXIALITY AND THE LODE PARAMETER

This paper is concerned with evaluation of various ductile fracture criteria in a general three-dimensional stress state of stress triaxiality, the Lode parameter and the equiva- lent plastic strain to fracture. Evaluation is carried out by comparing fracture loci constructed by fracture criteria to experimental results of A12024-T351. Comparison demonstrates that the Modified Mohr-Coulomb criterion and a newly proposed criterion provide sufficient predictabil- ity of fracture strain. Moreover, evaluation is emphasized on the predicted cut-off value for stress triaxiality. The evaluation demonstrates that the Cockcroft-Latham, Brozzo, Oh, Ko-Huh and the new criteria coupled a reasonable cut-off value for ductile materials.