Tomoyuki Hakoyama

Biaxial Tensile Test of High Strength Steel Sheet for Large Plastic Strain Range

Deformation behavior of high strength steel with a tensile strength of 590 MPa under biaxial tension was investigated for a work equivalent plastic strain range of 0.002 0.16. The test material was bent and laser welded to fabricate a tubular specimen with an inner diameter of 44.6mm and wall thickness of 1.2 mm. Using a servo-controlled tension-internal pressure testing machine, many linear stress paths in the first quadrant of stress space were applied to the tubular specimens. Moreover, biaxial tensile tests using a cruciform specimen were performed to precisely measure the deformation behavior of the test material for a small strain range following initial yielding. True stress-true plastic strain curves, contours of plastic work in stress space and the directions of plastic strain rates were measured and compared with those calculated using selected yield functions. The plastic deformation behavior up to an equivalent plastic strain of 0.16 was successfully measured. The Yld2000-2d yield function most closely predicts the general work contour trends and the directions of plastic strain rates of the test material.

Effect of Biaxial Work Hardening Modeling for Sheet Metals on the Accuracy of Forming Limit Analyses Using the Marciniak-Kuczyński Approach

A servo-controlled tension-internal pressure testing machine with an optical 3D deformation analysis system (ARAMIS\(^\circledR \), GOM) was used to measure the multiaxial plastic deformation behavior of a high-strength steel sheet with a tensile strength of 590 MPa for a strain range from initial yield to fracture. Tubular specimens were fabricated by roller bending and laser welding the as-received flat sheet materials. Many linear stress paths in the first quadrant of the stress space were applied to the tubular specimens to measure the forming limit curve (FLC), forming limit stress curve (FLSC), and forming limit plastic work per unit volume (FLPW) of the as-received sheet material in addition to the contours of plastic work and the directions of the plastic strain rates. Differential hardening behavior was observed; the shapes of the work contours constructed in the principal stress space changed with an increase in plastic work. The observed differential hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function as functions of the reference plastic strain. Marciniak-Kuczynski-type forming limit analyses were performed using both the differential hardening model and isotropic hardening models based on the Yld2000-2d yield function. It was found that the material model that is capable of reproducing both the work contours and the directions of the plastic strain rates measured for a strain range close to the fracture limit can give a more effective constitutive model for accurately predicting the FLC, FLSC, and FLPW.

Measurement of the forming limit stress curve using a multi-axial tube expansion test with a digital image correlation system

A servo-controlled tension-internal pressure testing machine with an optical 3D deformation analysis system (ARAMIS) was used to measure the multi-axial plastic deformation behavior of a high-strength steel sheet for a range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to a tubular specimen using an electrical, closed-loop servo-control system for axial force and internal pressure. Tubular specimens with an inner diameter of 44.6 mm were fabricated from a high-strength steel sheet with a tensile strength of 590 MPa and a thickness of 1.2 mm by roller bending and laser welding. Several linear and non-linear stress paths in the first quadrant of the stress space were applied to the tubular specimens in order to measure the forming limit curve (FLC) and forming limit stress curve (FLSC) of the as-received test material, in addition to the contours of plastic work and the directions of plastic strain rates. The contours of plas...

Effect of the determination method of the material parameters on the accuracy of the hole expansion simulation for cold rolled steel sheet

Hole expansion forming of a cold rolled steel sheet is investigated both experimentally and analytically to clarify the effects of material models on the predictive accuracy of finite element analyses (FEA). The multiaxial plastic deformation behavior of a cold rolled steel sheet with a thickness of 1.2 mm was measured using a servo-controlled multiaxial tube expansion testing machine for the range of strain from initial yield to fracture. Tubular specimens were fabricated from the sheet sample by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the contours of plastic work in stress space up to a reference plastic strain of 0.24 along with the directions of plastic strain rates. The anisotropic parameters and exponent of the Yld2000-2d yield function (Barlat et al., 2003) were optimized to approximate the contours of plastic work and the directions of plastic strain rates. The hole expansion forming simulatio...

Numerical biaxial tensile test for sheet metal forming simulation of aluminium alloy sheets based on the homogenized crystal plasticity finite element method

The simulation of the stretch forming of A5182-O aluminum alloy sheet with a spherical punch is performed using the crystal plasticity (CP) finite element method based on the mathematical homogenization theory. In the simulation, the CP constitutive equations and their parameters calibrated by the numerical and experimental biaxial tensile tests with a cruciform specimen are used. The results demonstrate that the variation of the sheet thickness distribution simulated show a relatively good agreement with the experimental results.

Fracture Prediction for High-strength Steel Sheet Subjected to Draw-bending Using Forming Limit Stress Criterion

A fracture criterion for sheet metals subjected to draw-bending is investigated using the concept of the forming limit stress criterion. The test material used is a 1.0-mm-thick high- strength steel sheet with a tensile strength of 590MPa. The specimen undergoes bendingunbending under tension when passing over the die profile. The drawing speed was set to 5-100 mm • s-1. The magnitude of true stress σDB when a specimen fractured has been precisely determined. Moreover, multiaxial tube expansion tests of the test material are performed to measure the forming limit stress σPT of the test material under plane-strain tension. It is found that σDB is larger than σPT by 2.8-6.3%. Therefore, it is concluded that the forming limit stress criterion is effective as a fracture criterion in draw-bending.

Effect of Equivalent Stress-Strain Relation and Differential Hardening on Accuracy of Forming Limit Analyses

Multiaxial tube expansion tests (MTETs) were performed to measure the multiaxial plastic deformation behavior of a cold rolled interstitial-free (IF) steel sheet for a range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to tubular specimens using an electrical, closed-loop servo-control system for an axial force and an internal pressure. Tubular specimens with an inner diameter of 44.6 mm were fabricated from a cold rolled IF steel sheet with a thickness of 0.7 mm by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit strains and forming limit stresses of the as-received sheet sample, in addition to the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work. The observed differential hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al, 2003) as functions of the reference plastic strain. The hydraulic bulge tests were also conducted to measure the forming limit strain and forming limit stress for equibiaxial tension and to determine the equivalent stress-equivalent plastic strain relation for a larger strain range. The forming limit curve and forming limit stress curve were calculated using the Marciniak-Kuczyński-type approach. The calculated results were in fair agreement with the measurement.

Fracture prediction of hole expansion forming using forming limit stress criterion

The fracture prediction of a hole expansion forming for a cold rolled steel sheet is investigated using the concept of forming limit stress criterion. Multiaxial tube expansion tests (MTET) (T. Kuwabara and F. Sugawara, 2014), in which linear paths in the first quadrant of the stress space are applied to tubular specimens, are performed to measure the contours of plastic work and the directions of the plastic strain rates of the test material. The anisotropic parameters and the exponent of the Yld2000-2d yield function (F. Barlat et al., 2003) are optimized to approximate the material test data. Marciniak-Kuczynski type forming limit analyses (Z. Marciniak and K. Kuczynski, 1967) are performed to determine the forming limit stress surface (FLSS) in a stress space. It is concluded that the FLSS calculated using an appropriate material model calibrated using the MTET is effective in predicting the necking timing in the hole expansion forming of the test material.

Development of draw-bending testing method using digital image correlation system

A novel draw-bending testing method using a non-contact optical 3D deformation measuring system (ARAMIS®, GOM) was developed to investigate the deformation process of draw-bending. Furthermore, the accuracy of the fracture prediction based on a fracture criterion using the concept of forming limit stress was also investigated. The experimental apparatus is capable of applying draw-bending to a sheet specimen with a forming speed of 100 mm/s, which is comparable to those in real press forming operations. Specimens undergo bending-unbending under tension when passing over the die profile radius. The test materials is a 590MPa high-strength steel sheet. In order to evaluate the effect of the stress variation in the width direction of a specimen on the accuracy of the calculated draw-bending fracture stress σDB, the strain components on the upper surface of the specimen were measured using the ARAMIS® software. High-speed CMOS cameras were used to take synchronized images. The development of the strain field ...

Error assessment in post-necking strain hardening behaviour identification of mild steel sheet

The information hidden in the diffuse neck of a tensile test on a thin metal sheet can be extracted using a special case of the non-linear virtual fields method yielding the so-called post-necking strain hardening behaviour. The method, however, requires a number of assumptions which are scrutinized in this paper. To eliminate experimental errors which could potentially hamper the assessment, virtual test data (i.e. strain fields at different load steps) is generated using a FE model of the tensile test. The identification strategy is then used to retrieve the reference strain hardening behaviour used in the FE simulation. This approach is used to study the necessity of incorporating rate-dependent plasticity in the identification procedure. Additionally, the necessary plane stress condition in the diffuse neck is studied.