Wrinkle generation determination index acquisition method, wrinkle generation determination method, wrinkle generation determination device, and wrinkle generation determination program for press formed part

A wrinkle generation determination device for a press formed part determining presence or absence of wrinkle generation in a press forming process of the press formed part by obtaining an index for determining the presence or absence of wrinkle generation under a biaxial stress condition in the press forming process of the press formed part by a biaxial stress test, the biaxial stress test applying a load in in-plane biaxial directions including at least a compressive load in an in-plane uniaxial direction to a measurement part in an intersecting cross shape of a cruciform test piece for biaxial test, the load being applied to induce out-of-plane buckling in the measurement part, the wrinkle generation determination device comprising:

• an in-plane biaxial strain and load relationship acquisition unit configured to change a load ratio in the in-plane biaxial directions acting on the measurement part in the biaxial stress test and acquire, for each the load ratio, a relationship between an in-plane biaxial strain and the load, the in-plane biaxial strain being generated in the measurement part in a process of inducing the out-of-plane buckling in the measurement part;
• a stable behavior limit strain acquisition unit configured to calculate, for each the load ratio, a primary differential coefficient of a strain in the in-plane uniaxial direction on which a larger compressive load acts, obtain a point at which the primary differential coefficient of the strain calculated becomes local maximum as a stable behavior limit point, and acquire an in-plane biaxial strain at the stable behavior limit point in the measurement part as a stable behavior limit strain;
• a stable behavior limit line acquisition unit configured to plot, on two-dimensional coordinates, the stable behavior limit strain in the in-plane biaxial directions acquired for each the load ratio and connect the stable behavior limit strain plotted to obtain a stable behavior limit line to be an index for wrinkle generation under the biaxial stress condition;
• a wrinkle generation starting strain acquisition unit configured to obtain a point at which a polarity of an increment of a strain with respect to an increment of the compressive load is reversed in a relationship between an in-plane uniaxial strain on which a larger compressive load acts and the load as a wrinkle generation starting point, and acquire an in-plane biaxial strain at the wrinkle generation starting point in the measurement part as a wrinkle generation starting strain;
• a wrinkle generation start line acquisition unit configured to plot, on two-dimensional coordinates, the wrinkle generation starting strain in the in-plane biaxial directions obtained for each the load ratio and connect the wrinkle generation starting strain plotted to obtain a wrinkle generation start line under the biaxial stress condition;
• a press formed part in-plane biaxial strain calculation unit configured to perform a press forming simulation on the press formed part so as to obtain the in-plane biaxial strain in the press forming process of the press formed part;
• a wrinkle generation determination map acquisition unit configured to acquire a wrinkle generation determination map indicating the stable behavior limit line and the wrinkle generation start line on two-dimensional coordinates having in-plane biaxial strains as coordinate axes;
• and
• a press formed part wrinkle generation determination unit configured to perform plotting of the in-plane biaxial strain in a predetermined portion of the press formed part on the wrinkle generation determination map obtained, determine that there is a high risk of wrinkle generation in the predetermined portion of the press formed part when the plotting is located in a region between the stable behavior limit line and the wrinkle generation start line, and determine that there is wrinkle generation in the predetermined portion of the press formed part when the plotting is located on a compression side of the wrinkle generation start line.

A non-transitory computer-readable recording medium on which an executable program for determining presence or absence of wrinkle generation in a press forming process of a press formed part by obtaining an index for determining the presence or absence of wrinkle generation under a biaxial stress condition in the press forming process of the press formed part by a biaxial stress test, the biaxial stress test applying a load in in-plane biaxial directions including at least a compressive load in an in-plane uniaxial direction to a measurement part in an intersecting cross shape of a cruciform test piece for biaxial test, the load being applied to induce out-of-plane buckling in the measurement part, the program causing a processor of a computer to execute:

• changing a load ratio in the in-plane biaxial directions acting on the measurement part in the biaxial stress test and acquiring, for each the load ratio, a relationship between an in-plane biaxial strain and the load, the in-plane biaxial strain being generated in the measurement part in a process of inducing the out-of-plane buckling in the measurement part;
• calculating, for each the load ratio, a primary differential coefficient of a strain in the in-plane uniaxial direction on which a larger compressive load acts, obtaining a point at which the primary differential coefficient of the strain calculated becomes local maximum as a stable behavior limit point, and acquiring an in-plane biaxial strain at the stable behavior limit point in the measurement part as a stable behavior limit strain;
• plotting, on two-dimensional coordinates, the stable behavior limit strain in the in-plane biaxial directions acquired for each the load ratio and connecting the stable behavior limit strain plotted to obtain a stable behavior limit line to be an index for wrinkle generation under the biaxial stress condition;
• obtaining a point at which a polarity of an increment of a strain with respect to an increment of the compressive load is reversed in a relationship between an in-plane uniaxial strain on which a larger compressive load acts and the load as a wrinkle generation starting point, and acquiring an in-plane biaxial strain at the wrinkle generation starting point in the measurement part as a wrinkle generation starting strain;
• plotting, on two-dimensional coordinates, the wrinkle generation starting strain in the in-plane biaxial directions obtained for each the load ratio and connecting the wrinkle generation starting strain plotted to obtain a wrinkle generation start line under the biaxial stress condition;
• performing a press forming simulation on the press formed part so as to obtain the in-plane biaxial strain in the press forming process of the press formed part;
• acquiring a wrinkle generation determination map indicating the stable behavior limit line and the wrinkle generation start line on two-dimensional coordinates having in-plane biaxial strains as coordinate axes;
• and
• performing plotting of the in-plane biaxial strain in a predetermined portion of the press formed part on the wrinkle generation determination map obtained, determining that there is a high risk of wrinkle generation in the predetermined portion of the press formed part when the plotting is located in a region between the stable behavior limit line and the wrinkle generation start line, and determining that there is wrinkle generation in the predetermined portion of the press formed part when the plotting is located on a compression side of the wrinkle generation start line.