Study On Effects Of Microstructure And Thickness Of Specimen On Small Punch Testing

Small punch test (SPT) technique which is used for quantifying the properties of bulk materials with greatly reduced volume has been extensively studied recently. However, some size effects on mechanical, physical and chemical properties of material have been observed at small scale (or microscale) and macroscale. So far very few studies have been devoted to verify if the miniature specimen SPT is able to measure the mechanical properties of in-service component correctly and appropriately due to observed mechanical size effects. In this study, the existing correlation equations for correlating yield load Fp, maximum load Fm in SPT to yield strength ReL and ultimate tensile strength Rm in conventional tensile test (CTT) were compared and the rationalities of them were discussed. Evolution of stress state of specimen during SPT was analyzed through finite element model and stress linearization method. CTT and SPT were carried out on A350forging flange, which has anisotropic mechanical properties, to study specimen size effect on mechanical behavior at different scales and to study the corresponding sampling direction that is able to adopted to evaluate the properties of anisotropic material with different specimen orientation and different specimen sizes between SPT and CTT. Additionally, two kinds of316L austenitic stainless steel (ASS) SPT specimens with same thickness but distinct texture were prepared by means of unidirectional rolling and multi-step cross rolling, and25sorts of316L ASS samples with nearly the same crystallographic texture but different thickness and grain size were prepared using different thermo-mechanical process. SPT is used to evaluate the effects of specimen thickness, grain size and ratio of thickness to grain size on mechanical properties of316L ASS. The present work provided a critical ratio of thickness to grain size of SPT sample for engineering application, as well as its mechanism and a basic understanding of the size effect of specimen on SPT, based on which the reliability of SPT in relating the mechanical properties of small specimens to those of bulk counterparts was enhanced. The conclusions obtained in this study are as follows.(1) The evolution of stress state of deformation process of specimen in SPT was clarified. The results showed that at initial stage the elastic bending stress is predominant and then the stress state dominated by membrane stress with the decreasing of elastic bending stress and the increasing of punch displacement. The reason for introducing the specimen thickness h2to the equation for correlating Fp with ReL, and the reason for introducing h to equation for correlating Fm with Rm were provided respectively. As a result, the coefficients of determination of linear regression between Fp and ReL, as well as Fm and Rm, were improved.(2) The fractographic features of CTT specimens machined along different orientations of anisotropic A350forging flange are different. The CTT longitudinal specimen showed brittle fracture mode, while CTT circumferential specimen showed ductile fracture mode. The results revealed that SPT samples showed more ductile features compared with CTT samples and the SPT is not sensitive to the difference of properties between specimens along different sampling orientation in the anisotropic material.(3) The results of study of size effects observed in SPT specimens with different grain size and thickness showed that the SPT is sensitive not only to thickness (h), but also to the ratio of thickness to grain size (h/dg), though a size effect exists between SPT and standard-sized samples. Such an effect becomes more significant especially for the samples with relative larger grain size and smaller specimen thickness.(4) A size effect criteria on SPT and critical thickness-grain size-ratio and thickness for AISI316L ASS was provided through data analysis. The yield strength and ultimate tensile strength correlated from the SPT results of ASS316L showed the grain size, thickness and specimen size dependencies significantly in the SPT. When the value of h/dg is larger than～8and h≥200μm, the yield strength and ultimate tensile strength correlated with yield load and maximum load respectively in SPT tend to be constants.