Test method for measuring limit tip cold bending angle and equivalent fracture strain

Abstract

The invention discloses a test method for measuring a limit tip cold bending angle and equivalent fracture strain, which comprises the following steps: preparing a bending sample, setting an initial fixed loading stroke, determining existence and absence of a crack state, determining an effective sample, and measuring the limit cold bending angle. The method for measuring the equivalent fracture strain comprises the following steps: spraying speckles on the side surface of a sample, obtaining an effective sample for DIC data processing, calculating the deformation of the edge side of the effective sample, measuring the arc length and calculating the equivalent fracture strain. According to the invention, a technical scheme of combining fixed stroke loading, a DIC technology and theoretical calculation is adopted, and the limit cold bending angle and the equivalent fracture strain are accurately measured. According to the method, the failure moment of the material is accurately controlled, the intrinsic parameters of the material are measured, the test cost is reduced, and the method has important engineering practical significance and wide engineering application prospects.

Description

Technical field [0001] The present invention belongs to the field of mechanical properties testing of metal materials, in particular the test method involving the determination of the ultimate sharp cold bend angle and equivalent fracture strain of advanced high-strength steels. Background [0002] With the continuous improvement of vehicle crash safety standards, in order to cope with the technical challenges brought by safety regulations, the proportion of advanced high-strength steels applied in the lightweight design of automotive bodies is increasing, especially the large number of hot stamped parts. However, in a car collision, for example, the bending deformation of A-column and B-pillar is a typical flat strain state failure under bending deformation, and the higher the material strength, the greater the risk of material failure. Therefore, the bendability of the material has become one of the key indicators to measure the mechanical properties of advanced high-strength s...

AI Technical Summary

Problems solved by technology

[0004] However, there are at least two problems with VDA238-100: first, the test termination condition, stop loading until the pressure value reaches the closing threshold, and the closing threshold refers to the drop of 30N or 60N after the pressure head reaches the maximum load; second, Measurement results, the limit sharp cold bending angle is a physical parameter, not an intrinsic parameter of the material

If the purpose of the test is to quantitatively evaluate the bending resistance of the material, the VDA238-100 technical standard is difficult to meet

[0005] For example, in view of the measurement results, people prefer to obtain the equivalent fracture strain under extreme sharp cold bending. Therefore, on the basis of the VDA238-100 technical standard, the numerical image correlation method technology, DIC technology for short, is introduced to measure the bending surface of the sample. Strain distribution is a common technical solution, as detailed in reference ("Evaluation of the VDA238-100Tight Radius Bending Test using Digital Image Correlation StrainMeasurement", doi:10.1088 / 1742-6596 / 896 / 1 / 012075) The test method, however, still has the following technical problems. First, in order to cooperate with the DIC technology to measure the curved surface of the sample, a new test system needs to be developed. Compared with the VDA238-100 technical standard, only one set of tooling device needs to be added. Obviously, it is greatly improved. Increased test equipment cost; Second, the reference to the bending surface fracture strain measured by DIC technique has no correlation with the limit sharp cold bending angle, weakening the interpretability of the results

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