Study On Deformation Mechanism And Strain Rate Effect Of Foam Ti Based On In-situ X-ray Imaging

Titanium-based foams exhibit excellent mechanical and biocompatible properties,and corrosion resistance,thus making them attractive for aeronautical,biomedical(implant),and chemical(catalysis)engineering applicaitons However,there are many pending questions regarding their structure-property relationships,due to the absence of in situ,microscopic measurements on deformation.That restricts the extensive application of titanium foam in many aspects.For instance,Ti-based foams in the above applications are concurrently exposed to impact loading.The mechanical properties of Ti-based foams under high-rate loading have been rarely reported.The origin of strain-rate sensitivity for Ti-based foams is under debate.In addition,the influence of pore size on the mechanical properties of Ti-based foams remains to be clarified.In this paper,effects of the strain rate and pore size on the mechanical behavior and deformation mechanisms of Ti-based foams are investigated.The microstructure and crystal structure of foamed Ti are characterized by scanning electron microscopy,X-ray diffractometer and computed tomography.The stress-strain curves under a wide range of strain rates are measured with material testing system and split Hopkinson pressure bar.To fully discuss the microscopic deformation mechanisms and the origin of the strain rate effect,theultrafast,in situ X-ray phase contrast imaging is implemented to capture the dynamic deformation process of foam Ti.Strain fields across the sample under dynamic and quasi-static loading are obtained with the X-ray digital image correlation method.The stress-strain curves of foam Ti with different pore sizes show that the pore size has significant effects on the yield strength and strain hardening of foam Ti.The yield strength of foam Ti with a pore size of 20 ?m is 50% higher than that of 80 ?m,and the former shows stronger strain hardening.The bulk stress-strain curves under a series of strain rates show that porous Ti exhibits enhanced strain-rate sensitivity under high-rate loading.In the lower strainrate range,the strain rate sensitivity of foam Ti with 20 ?m and 80 ?m pores is 0.022 and 0.008,respectively.The strain rate sensitivity increases to 0.22 and 0.23,respectively,under higher strain-rate range.The X-ray image sequence shows that pore compaction occurs firstly in the pore-rich area.The dynamic deformation process of porous Ti appears similar to that under quasi-static loading.Strain mapping shows that curved compaction bands grow continuously with increasing external loading.The strain pattern differs for foam samples with different pore sizes and under different loading rates.For foam Ti with large pores,compaction bands are sparsely distributed across the sample,yielding a more heterogeneous strain distribution.The strain distribution is much more homogeneous in the strain fields of samples with smaller pores.The compaction bands appear narrower and denser in dynamic strain fields than those in quasi-static fields.The intersected strain localizations result in a more uniform strain field under dynamic loading,which results in strain-rate hardening of porous Ti.The dynamic and quasi-static mean Poisson's ratio values both increase with bulk strains.The mean Poisson's ratio under dynamic loading is about 15% lower than that under quasi-static loading at the same bulk strain,as a result of local inertial confinement which also contributes to the rate hardening of porous titanium.