Analysis on the deformation and fracture behavior of carbon steel by in situ tensile test

The deformation and fracture behaviors of low-carbon steel, medium-carbon steel, and high-carbon steel were studied on internal microstructure using the scanning electron microscopy in situ tensile test. The microstructure mechanism of their deformation and fracture behavior was analyzed. The results show that the deformation and fracture behavior of low-carbon steel depends on the grain size of ferrite, the deformation and fracture behavior of medium-carbon steel depends on the size of ferrite grain and pearlite lump, and the deformation and fracture behavior of high-carbon steel depends on the size of pearlite lump and the pearlitic interlamellar spacing.

Forming Limit Stress Diagram Prediction of Aluminum Alloy 5052 Based on GTN Model Parameters Determined by In Situ Tensile Test

The conventional forming limit diagram （FLD） is described as a plot of major strain versus minor strain. However, FLD is dependent on forming history and strain path. In the present study, a forming limit stress-based diagram （FLSD） has been adopted to predict the fracture limit of aluminum alloy （AA） 5052-O1 sheet. Nakazima test is simulated by plastic constitutive formula derived from the modified Gurson-Tvergaard-Needleman （GTN） model. An in situ tensile test with scanning electron microscope （SEM） is proposed to determine the parameters in GTN model. The damage evolution is observed and recorded, and the parameters of GTN model are identified through counting void fraction at three damage stages of AA5052-O 1. According to the experimental results, the original void volume fraction, the volume fraction of potential nucleated voids, the critical void volume fraction, the void volume fraction at the final failure of material are assigned as 0.002 918, 0.024 9, 0.030 103, 0.048 54, respectively. The stress and strain are obtained at the last loading step before crack. FLSD and FLD of AA5052-O 1 are plotted. Compared with the experimental Nakazima test and uniaxial tensile test, the predicted results show a good agreement. The parameters determined by in situ tensile test can be applied to the research of the forming limit for ductile metals.

Microstructure, property and deformation and fracture behavior of 800 MPa complex phase steel with different coiling temperatures

The microstructure characteristics and properties(especially hole expansion property)of 800 MPa hot-rolled complex phase steel with different coiling temperatures were studied.The microstructure consisted of polygonal ferrite and precipitates when the steel was coiled at 550℃,and when the steel was coiled between 460–520℃,the microstructure was composed of granular bainite and martensite and austenite(M/A)islands.The morphology of the crack was analyzed by scanning electron microscopy,and the in situ scanning electron microscope tensile test was used to find out the fracture mechanism and deformation behavior of the steel with different coiling temperatures.When the steel was coiled at 550℃,the cracks initiated at the ferrite grain boundary and propagated through the grains or along the grain boundaries.When the steel was coiled at 520℃,the cracks first initiated at the junction of ferrite and M/A island and then propagated through the grains.The steel coiled at 520℃ has quite good mechanical properties and relatively high hole expansion ratio.