Quasi-static tensile deformation and fracture behavior of a highly particle-filled composite using digital image correlation method

Polymer bonded explosives (PBXs) are highly particle-filled composite materials.This paper experimentally studies the tensile deformation and fracture behavior of a PBX simulation by using the semi-circular bending (SCB) test.The deformation and fracture process of a pre-notched SCB sample with a random speckle pattern is recorded by a charge coupled device camera.The displacement and strain fields on the observed surface during the loading process are obtained by using the digital image correlation method.The crack opening displacement is calculated from the displacement fields,the initiation and propagation of the crack are analyzed.In addition,the damage evolution and fracture mechanisms of the SCB sample are analyzed according to the strain fields and the correlation coefficient fields at different loading steps.

Modification of Recrystalization Grain Structure of Ytria Dispersion Strengthened MA956 Alloy by Pre-deformation

The effects of cold deformation on recrystallization characteristics of a mechanically alloyed yttria dispersion strengthened alloy (Fe 20%Cr 4.5%Al 0.5%Ti 0.01%C 0.5%Y 2O 3 (wt%)) are studied. Experimental techniques employed include optical and transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and laser processing methods. In addition, a recrystallization grain growth model is presented to explain the observed reduction of recrystallized grain aspect ratio as a result of cold forging.

Effect of ECAP on the Microstructure and Mechanical Properties of a High-Mg_2Si Content Al-Mg-Si Alloy

A high-Mg2Si content Al alloy was extruded by equal channel angular pressing（ECAP） for 8 passes at 250 ℃ and an ultrafine-grained structure with an average grain size of about 1.5 μm was achieved.The coarse skeleton-shaped Mg2Si phase presenting in the as-cast alloy are significantly fragmented into fine rod-shaped as well as equiaxed particles mostly less than about 230 nm and become relatively dispersed.The tensile strength 192.8 MPa and the elongation up to 31.3% at ambient temperature are attained in the 8-pass ECAPed alloy versus 163.3 MPa and 9.1% in the as-cast alloy.High-temperature creep test at 250 ℃ reveals that the ECAPed sample exhibits a high elongation close to 100% at a relatively high creep rate 7.64×10-5 s-1,compared to the elongation 56% at a low strain rate 1.74×10-7 s-1 in the as-cast alloy.

Stress－rupture Property and Microstructure of Yttria Dispersion Strengthened Alloy MA956

The streas-rupture property and corresponding deformed microstructures of an yttria dispersionstrengthened alloy bar MA956 ( Fe-20 % , Cr-4. 5 % , Al-0. 5% , Ti-0. 5% , Y_2O_3(wt% ) ) were investi-gated by using tensile stress-rupture testing, fractography (optical and scanning electron micrscopy(SEM)) , and optical metallography. It has been found that the longitudinal strength is much higher thanthe transverse strength, as a result of fracture occurring along grain boundaries in the latter orientationwhen the load direction is normal to the recrystallization grain growth direction. For the same reason, lon-gitudinally tested samples fractured in a ductile manner while transversely tested ones had brittle fracture.It is also found that pre-anneal decreases the longitudinal strength, since it reduces the grin size and there-fore increace the amount of grain boundarings. In addition, the effect of grain size on crack propagation wasexamined .

Effect of Frequency on Fatigue Lifetime of Extruded Mg-3%Al-1%Zn Alloy

Samples prepared from as-extruded magnesium alloy Mg-3%Al-1%Zn （AZ31） billets were utilized in low-cycle fatigue tests in order to investigate the frequency-dependent fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1 Hz and 10 Hz in air. The microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）.When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10 Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at these two frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.

Enhancing the high-temperature creep properties of Mo alloys via nanosized La_(2)O_(3) particle addition

Molybdenum(Mo) alloys with different La_(2)O_(3)particle additions(0.6,0.9,1.5 wt.%) were prepared by powder metallurgy to investigate the effect of La_(2)O_(3)particles on microstructural evolution and creep behavior of the alloy.Pure Mo,annealed at 1500℃ for 1 h,presented a fully recrystallized microstructure characterized by equiaxed grains.The alloys doped with La_(2)O_(3)particles(Mo-La_(2)O_(3)alloys),on the other hand,exhibited fibrous grains elongated in the rolling direction of the plate.In contrast to the shape of the grains,the average grain size of the alloys was found to be insensitive to the addition of La_(2)O_(3)particles.Nanosized La_(2)O_(3)particles with diameters ranging from 65 to 75 nm were distributed within the grain interior.Tensile creep tests showed that dislocation creep was the predominant deformation mode at intermediate creep rate(10^(-7)s^(-1)-10^(-4)s^(-1)) in the present alloys.The creep stress exponent and activation energy were found to decrease with increasing temperature,particularly within the low creep rate regime(<10^(-7)s^(-1)).The Mo-La_(2)O_(3)alloys exhibited remarkably greater apparent stress exponent and activation energy than pure Mo.A creep constitutive model based on the interaction between particles and dislocations was utilized to rationalize the nanoparticle-improved creep behavior.It was demonstrated that low relaxed efficiency of dislocation line energy,which is responsible for an enhanced climb resistance of dislocations,is the major creep strengthening mechanism in the Mo-La_(2)O_(3)alloys.In addition,the area reduction and creep fracture mode of the Mo-La_(2)O_(3)alloys were found to be a function of the creep rate and temperature,which can be explained by the effect of the two parameters on the creep and fracture mechanisms.

FINITE ELEMENT SIMULATION FOR YIELD STRESS OF HARD POLY(VINYL CHLORIDE)/ACRYLONITRILE-BUTADIENE-STYRENE BLENDS AT DIFFERENT CROSSHEAD SPEEDS

Hard poly（vinyl chloride） （PVC）/acrylonitrile-butadiene-styrene （ABS） blends were prepared using injection- molding and influence of crosshead speed on mechanical properties was examined. Based on morphology parameters obtained from transmission electron microscopy photography and the material parameters from true stress-strain curves of neat PVC and ABS, yield stresses of the blends at different crosshead speeds were simulated employing a two-dimensional nine-particle model based on the finite element analysis （FEA）. The FEA results were compared with the experimental yielding stress and the good agreement validated the simulation approach. The FEA approach allowed establishing a yielding criterion related to local yielding of the interstitial matrix between ABS particles.