The Research On Measuring Device And Method For Large Deformation In High Temperature Superplastic Tensile Test

Superplastic tensile test has always been a basis for the research of superplasticity,which is the most common and most simple experimental method for studying the superplasticity of materials and determining the material parameters.It is also an important way to reveal the superplastic deformation essence,connect the microdeformation mechanism and macroscopic deformation regularity of superplasticity.In superplastic tensile test,the sample is extended in a high temperature furnace,thus extensometer cannot be used to measure the elongation ?L.In general,the displacement of activity beam is regarded as the ?L to calculate the mechanical parameters such as stress,strain and strain rate,which can meet the accuracy requirements in the uniform deformation stage.However,along with sample entering geometric instability state and appearing necking,deformation of the specimen becomes uneven.Therefore,using the ?L to calculate mechanical parameters will produce a greater deviation after necking.In addition,the displacement of the beam does not reflect ?L accurately,which contains the deformation outside the gauge length,the slipping which may occur between fixture and clamping part,and some gaps that may exist between the force transmission links.Hence,a greater deviation will appear in the tensile test of some small elongation materials.Therefore,it is necessary to measure the deformation and cross-sectional changes accurately to obtain the tension test results within gauge length precisely.In addition,due to the inability to measure the generation and development of sample necking during the tension test,it has been a difficulty in studying the superplastic tensile instability especially the fracture process.The presented invention relates to a device and method for large deformation visual measurement of high temperature superplastic tensile test.This paper designs a high-temperature superplastic tensile test A non-contact measuring device and method for large deformation in the high-temperature superplastic tensile test was presented.(1)The device was consisted of computer,precision cross table,stand column,two performance digital cameras with the same parameters,adjustable platform and strip light source,in which the stand column and strip light source is fixed on the cross table.cameras and adjustable platform were united together,and all installed in the stand column,the distance between two cameras was adjustable.(2)Based on the above device,the pinhole imaging principle was used to calibrate cameras and stitch pictures.The deformation process of specimen in tensile test was recorded by cameras in real time,meanwhile,the Halcon software was used to save,extract and process these images.Finally,the contours of the samples,the longitudinal and lateral deformation,the generation and development process of necking were outputted.(3)The above method was used to measure the deformation process of round bar and plate specimen during tension test at room temperature,and the positions and changes of the necking at room temperature are calculated.It was testified that the measurement accuracy was improved greatly compared with the beam displacement method.(4)The procreant reasons of deviation in the room temperature experiment were summarized and analyzed.According to the characteristics of the high temperature and large deformation,the correction formula was deduced to compensate the system error for high-temperature superplastic tensile test,and the corresponding self-calibration method was put forward,and the corresponding solution was put forward for the random error.(5)A meshing measurement method was proposed,and the corresponding formulas of the superplastic mechanical parameters for the bar and plate specimens were deducted.