Study On Fracture Toughness Test Method And Fracture Behavior Of Single Edge Notched Tensile Sheet Specimen

High toughness sheet metal is widely used in workpiece structure design.It is of great significance to study the fracture toughness and fracture behavior of sheet metal for lightweight development and safety performance evaluation of applied structures.Single-notch tensile specimens（SENT）are simple and fast to measure the fracture toughness of materials,and the fracture toughness assessment obtained from the test is less conservative.These specimens have been used in the study of foundation fracture of sheet metals.Crack tip opening displacement（CTOD）can be used to characterize the fracture toughness of the sheet metal if the initiation process of the sample SENT can be accurately measured.Therefore,how to accurately observe crack initiation and CTOD is the key problem to obtain the fracture toughness of sheet metal.In this paper,the samples of SENT thin plate with thickness of 1mm were taken as the research object,and the fracture behavior of SENT thin plate samples was studied by digital image correlation（DIC）technology.A new test method was proposed to observe crack initiation and measure crack growth,CTOD,etc.,and the test method of CTOD resistance curve and crack initiation toughness of SENT thin plate samples were studied.The main research contents of this paper are as follows:（1）The tensile fracture test with DIC technology and universal testing machine was used to observe the fracture behavior of SENT sample of thin plate,and the full-field longitudinal displacement and longitudinal strain of SENT sample of thin plate were analyzed.It was found that there were obvious differences in the displacements above and below the cracks during crack propagation,and the change of crack tip location was the main reason for such differences.（2）Two parallel lines symmetrical to the crack were placed in the DIC longitudinal displacement nemogram.The distribution curve of the longitudinal displacement difference of the sample in the direction of crack growth was obtained by calculating the displacement difference between the two lines.The critical point of slope change of the curve was used to determine the location of the crack tip.Then,by tracking the displacement of the critical point on the normalized curve of longitudinal displacement difference at different times,the crack growth amount of the thin plate SENT sample was measured successfully.This method is simpler and faster than other methods.（3）In order to measure the crack tip opening displacement of SENT samples of thin plate,the DIC virtual extensiometer technology was used to measure the opening displacement of the outer end of the initial crack（CMOD）,and then the CTOD calculation formula suitable for SENT samples of thin plate was derived based on the properties of similar triangles.The calculated resultδof CTOD was compared with CTOD-δ₅.The resistance curve of SENT samples of thin plate was obtained by fitting.The results show that the measurement method in this paper is accurate enough and can be used to test the resistance curve of SENT samples of thin plate and analyze the fracture behavior.（4）In order to study the fracture behavior of SENT samples of thin plate,fracture toughness tests were carried out on samples with three different initial crack lengths,and the measurement method proposed in this paper was used to measure the crack growth amount and crack tip opening displacement during the fracture process.Finally,the CTOD resistance curve and crack initiation toughness of the three samples were obtained.The results show that the fracture toughness of SENT specimens decreases with the increase of the initial crack length,and the stress behavior of SENT specimens is more stable in the fracture process.