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US-12618758-B2 - Wrinkle generation determination index acquisition method, wrinkle generation determination method, wrinkle generation determination device, and wrinkle generation determination program for press formed part

US12618758B2US 12618758 B2US12618758 B2US 12618758B2US-12618758-B2

Abstract

A wrinkle generation determination method for a press formed part includes acquiring, for each load ratio in in-plane biaxial directions, a relationship between an in-plane biaxial strain generated in a process of inducing out-of-plane buckling in a measurement part and a load, acquiring an in-plane biaxial strain at a point where a primary differential coefficient of an in-plane uniaxial strain on which a compressive load acts becomes local maximum as a stable behavior limit strain, and acquiring a stable behavior limit line that will be an index for wrinkle generation under a biaxial stress condition by plotting, on two-dimensional coordinates, the stable behavior limit strain acquired for each load ratio in a biaxial stress test in which a load in the in-plane biaxial directions is applied to a cruciform test piece for biaxial test so as to induce out-of-plane buckling in the measurement part.

Inventors

  • Yuki OGIHARA

Assignees

  • JFE STEEL CORPORATION

Dates

Publication Date
20260505
Application Date
20220412
Priority Date
20210730

Claims (5)

  1. 1 . A method for acquiring a wrinkle generation determination index for a press formed part, the method obtaining an index for determining presence or absence of wrinkle generation under a biaxial stress condition in a 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 method comprising: acquiring an in-plane biaxial strain and load relationship by 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; acquiring a stable behavior limit strain by 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 the stable behavior limit strain; and acquiring a stable behavior limit line by 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 the stable behavior limit line to be an index for wrinkle generation under the biaxial stress condition.
  2. 2 . The method according to claim 1 , further comprising: acquiring, for each the load ratio, a wrinkle generation starting strain by 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 the wrinkle generation starting strain; and acquiring a wrinkle generation start line by plotting, on two-dimensional coordinates, the wrinkle generation starting strain in the in-plane biaxial directions acquired for each the load ratio and connecting the wrinkle generation starting strain plotted to obtain the wrinkle generation start line under the biaxial stress condition.
  3. 3 . A wrinkle generation determination method for a press formed part determining presence or absence of wrinkle generation in a press forming process of the press formed part by using the stable behavior limit line and a wrinkle generation start line obtained by the method according to claim 2 , the wrinkle generation determination method comprising: calculating an in-plane biaxial strain in the press formed part by 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 determining wrinkle generation on the press formed part by 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 from the wrinkle generation start line.
  4. 4 . 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.
  5. 5 . 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.

Description

FIELD The present invention relates to a wrinkle generation determination index acquisition method for a press formed part so as to obtain an index for determining the presence of wrinkle generation under a biaxial stress condition in a press forming process of the press formed part, and a wrinkle generation determination method, a wrinkle generation determination device, and a wrinkle generation determination program for a press formed part so as to determine the presence of wrinkle generation on the press formed part based on the index. BACKGROUND Conventionally, as a material test of a metal sheet, an FLD (forming limit diagram) test is performed to measure a press-formable region without fracturing a thin metal sheet (Non Patent Literature 1, etc.). By using a forming limit diagram obtained by the FLD test, it is possible to objectively and quantitatively evaluate a risk of occurrence of bulging fracture, drawing fracture, and the like during press forming of the thin metal sheet. In addition, as the material test of the metal sheet, a material test in which a compressive load is applied to a thin metal sheet is also performed (Patent Literature 1, Patent Literature 2, Patent Literature 3, etc.). For example, in the material test disclosed in Patent Literature 3, it is said that the compressive load in an in-plane biaxial direction is applied to a thin test piece of the thin metal sheet, whereby a mechanical property of the thin metal sheet under a biaxial compressive stress condition can be accurately measured. Further, it is expected to contribute to improvement of prediction accuracy of CAE (computer aided engineering) analysis (press forming simulation) in the press forming process by measuring the mechanical property of the thin metal sheet under a compressive stress condition in the material tests where the compressive load is applied to the thin metal sheet. In general, when the thin metal sheet receives the compressive load during press forming, a phenomenon (out-of-plane buckling) in which the thin metal sheet suddenly undergoes out-of-plane deformation and protrudes from a target shape of the press formed part occurs. As a result, wrinkles (hereinafter also referred to as “press wrinkles”) may be generated in the press formed part. This press wrinkle generation mechanism is classified into (1) an elastic or plastic buckling behavior of the thin metal sheet and (2) an excess metal or thickening due to excessive or uneven material inflow of the thin metal sheet. Among them, the elastic or plastic buckling behavior in (1) may be caused by, for example, the compressive stress or the like generated in a flange portion due to shrinkage flanging in the press forming process of a press formed part having a hat-shaped cross section including a punch bottom portion, a side wall portion, and a flange portion, or may be caused by non-uniform deformation of the thin metal sheet due to non-uniform load, non-axisymmetric load, or the like. On the other hand, the excess metal or thickening in (2) is considered to occur at a portion where the shape of the press formed part rapidly changes (see Non Patent Literature 2). In addition, press wrinkles are more likely to be generated as a thickness of the thin metal sheet is thin and a material strength is low. In bulge forming, since excess metal of a bulge forming surface due to the inflow of the material from a periphery to a forming portion is likely to be a direct cause of press wrinkles, there has been a problem that it is difficult to obtain a press formed part having a target shape while preventing press wrinkle generation. Therefore, in order to obtain a press formed part in which press wrinkle generation is prevented, it is necessary to clarify causes of wrinkle generation on the press forming process and to determine press forming conditions according to the mechanical property of the thin metal sheet, the target shape of the press formed part, and the like. As a method of predicting press wrinkle generation, there is a technique of performing press forming simulation by an elasto-plastic finite element method or the like to apply shading according to a degree of compressive stress or compressive strain when a press formed part is displayed on a computer screen during or after forming obtained by the press forming simulation, whereby visually determining the presence of wrinkle generation. Furthermore, some techniques have been proposed in which a wrinkle generation mechanism is estimated and an index for quantitatively determining the presence of wrinkle generation is obtained based on strain, stress, or the like calculated by the press forming simulation. For example, Patent Literature 4 discloses a technique of evaluating the presence of wrinkle generation by obtaining an equivalent stress and an equivalent strain of each element in the press forming process by the press forming simulation of a sheet-like material based on the elasto-plastic finite ele