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Ductile Fracture from Spot Weld and Flange Edge in Advanced High Strength Steels.

INTRODUCTION

Achieving an optimized combination of crash safety performance and a lightweight structure has been an important challenge in automotive body engineering. Applications of high strength steels to auto body structures have been considered as an effective solution to this problem, which is feasible at relatively low cost in volume production in the automotive industry [1, 2, 3]. High strength steels must provide not only higher strength but also sufficient formability in the press stamping processes. The strength of steel materials have traditionally been limited to 440MPa considering the formability of automotive parts. Recently, advanced high strength steels with improved formability have been developed by optimizing the metallic microstructures [4]. As a result, advanced high strength steels with strengths over 590MPa are now widely used in crucial parts for safety performance. Moreover, the range of applications of high strength steels in auto body structures is expanding, and ultra-high strength steels with strengths ranging from 980MPa to 1470MPa have been developed considering the optimum balance of material strength and formability [5].

While advanced high strength steels both reduce body weight and improve crashworthiness [6, 7], fracture of these materials in a crash event has been a critical issue for the reliable design of structural components. High strength steels are often more prone to necking or failure in crash deformation than conventional mild steels because the elongation of the steel decreases as its strength increases [8, 9, 10]. In addition, spot-welded joints are a relatively weak point in high strength steel parts. Fracture is prone to initiate from spot-welded joints, and the initial fracture then extends over a large area in the body part [11,12, 13]. Figure 1 shows an example of a fracture from a spot-weld and flange edge in an automotive component.

In optimization of materials and structures in body components, the finite element method (FEM) has been considered a powerful tool for estimating crash performance. However, continual study and improvement of the material models used in the numerical simulations is still needed due to the ever-increasing demands for more accurate predictions of deformation and material failure [14, 15, 16].

In this study, static bending tests and tensile tests were carried out by using a strain measurement system with digital images to investigate the fracture behaviors of spot-welded parts. The parts were made of a variety of steel sheets with tensile strengths from 590MPa to 1470MPa. The material thicknesses was 1.6mm considering the typical thickness of steel sheets used in automotive parts. These experiments clarified the effects of the material strength and the shearing condition on fracture behavior. The strain criterion for fracture of spot welds was investigated by using the strain measurement results with the aim of understanding the fracture behavior of spot-welded parts. A FE simulation was also demonstrated as a tool for simulating the fracture of spot-welded specimens.

EXPERIMENTAL PROCEDURE

Materials

In these experiments, several grades of sheet steels were selected in order to investigate the effect of material properties on fracture behavior. Table 1 shows the mechanical properties of the steel sheets used in this study. The mechanical properties, including yield strength, tensile strength and elongation, were examined by static tensile tests based on the applicable Japan Industrial Standard (JIS). The geometry of the specimen used in the tests had a gauge length of 50mm and a width of 25mm. The tensile tests were performed with a constant velocity of 10mm/min. The material thicknesses of 1.6mm was selected considering the steel sheets commonly used in automotive body structures. Steel grades (indicated by the tensile strength) are also listed in Table 1. Steel sheets with a wide range of tensile strengths from 590MPa to 1470MPa were selected for the study. The steels with tensile strengths from 590MPa to 980MPa have a dual-phase micro-structure consisting of a soft ferrite phase and a hard martensite phase. The 1470MPa steel is a full-martensite steel, which is the highest strength steel currently used in commercial steel sheets for automotive applications.

Strain Measurement Method

A digital imaging strain analysis system was developed based on the computer vision principle [17] illustrated in Figure 2. Two digital cameras were installed in a tensile testing machine to measure the strain distribution during the tensile tests. Figure 3 (a) shows the two-camera system for static tensile testing with computer-controlled digital cameras. Figure 3 (b) shows the specifications of the digital camera. Dots with a diameter of 0.5 mm were printed with a pitch of 1 mm on specimens by the electrolytic etching method shown in Figure 3 (c). Although the smaller pattern is better for the strain measurement, the size of the dots was determined based on the limitation of etching precision. The center of the grid was determined by fitting the elliptic curves for each image shown in Figure 4. Using this method, the center of the distorted dots was determined accurately for the large deformation after the strain localization. The 3D coordinate points of the dots on the specimen were calculated using the center of the fitted ellipses. The strain distribution was calculated by tracking the 3D reconstructed points during tensile tests.

Tensile Test of Spot-Welded Coupon

Tensile tests were performed with JIS No.5 specimen having a spot-welded nugget at the center of the coupon. Figure 5(a) illustrates the shape and geometry of the tensile specimen. The specimen and a tab plate were jointed with a spot-welded nugget. Figure 5(b) shows a photograph of the spot-welded specimen fixed in the tensile test machine. The grids for measurement of the strain distribution were printed on the surface of the specimen. Figure 5(c) shows the printed grid around the spot-welded nugget. The grid was printed on the spot-welded specimen. Before grid printing, the surface of nugget was polished with sandpaper to remove the contamination caused by welding. The tensile speed was 2mm/min. During the tensile test, the global strain in the gauge length of 50mm and tensile force were recorded by a digital recorder. Tensile tests of the specimens without spot welding were also carried out with the four high strength steels to investigate the effect of the spot-welded nugget on deformation and fracture behavior, depending on the material strength level. The fracture strain around the nugget and in the base metal was investigated by the strain analysis method in order to discuss the fracture behavior of spot-welded joint in advanced high strength steels.

Three-Point-Bending Test of T-Shape Spot-Welded Specimen

A three-point-bending test method was proposed in this study in order to simulate the fracture around a spot-welded nugget and near the flange edge in crash deformation. The main feature of the method is simultaneous evaluation of spot weld fracture and edge fracture with a simple test specimen and test equipment. Strain measurement during the bending test is also possible by using the strain measurement system shown in Figure 3. Figure 6 shows the dimensions of the specimen, which is called as "T-Shaped spot-welded specimen". Two L-shaped parts and a top plate are welded at the flange and top portion. The spot pitch in flange area is 30mm. The L-shaped parts are produced by a bending process with the bending radius of 5 mm. The flange edge is shaped by a machining process or a shearing process to investigate the effect of the edge condition on fracture. In the specimens prepared by shearing, the edge surface is not smooth, and includes a burr and fractured surface.

Figure 7 shows the three-point-bending method. Figure 7(a) shows the dimension of the equipment. The T-shaped spot-welded specimen is supported at the two fulcrums. The fulcrums can be rotated freely depending on the bending process. The axis of rotation is located at the top side of the flange in order to generate tensile strain in the flange area. In this study, the span of the two fulcrums was set at 150mm. A punch with a circular cylindrical surface was positioned at the center of the specimen. The punch radius used in this test was 32.5mm. Figure 7(b) shows an example of the three-point-bending test. The center of the flange around the spot weld was deformed by the bending process. The fulcrums were rotated at the center of the joint. The grid was printed on the flange surface in order to analyze the strain distribution and fracture strain in the bending test. The punch speed was set to 5mm/min. The punch force and punch displacement were measured by the load cell and displacement sensor in the test machine.

EXPERIMENTAL RESULT

Tensile Test of Spot-Welded Sample

The strain distributions during the tensile tests of the four materials are presented in Figure 8. In this figure, the strain distributions in the base material without spot welding and the spot-welded specimen just before fracture are compared for each steels. The contour colors indicate the maximum principal strain in the specimens calculated based on the dimensions of the printed grid by using the image analysis technique described above. In the case of material No.1/590MPa, the strain was concentrated at an area distant from the spot-welded nugget. In the spot-welded specimens, fracture occurred in the base material. With the higher strength materials, including No.2/780MPa, No.3/980MPa and No.4/1470MPa, the fracture occurred near the spot-welded nugget. Figure 8 shows the higher strain around the spot-welded nugget just before fracture for these three material. The strain distribution decreased as the strength of the steel increased. The strain level in base material of No.4/1470MPa was lower than that in the other materials.

Figure 9 shows photographs of the fractures after the tensile test. In the case of No.1/590Ma, fracture did not occur in the spot-welded specimen, but plastic deformation was observed around the spotwelded nugget. In materials No.2-4, fracture initiated near the spot-welded nugget, and the crack then propagated from the center of the specimen to the base metal.

Figure 10 shows the nominal strain-nominal stress curves for the four steel grades. In this figure, the solid lines indicate the results of the tensile tests of the spot-welded specimens, and the dot lines indicate the results of the tensile tests of the base material without spot welding. The maximum stress increased as the strength of the steel grades increased. The stress curves dropped sharply after the maximum stress at the moment when fracture occurred. The fracture point of the spot-welded specimens were lower than those of the base material specimens without spot welds. In particular, spot-welded specimen of steel No.4/1470MPa showed as lower maximum stress and lower elongation compared with those of the base material without spot welding.

Three-Point-Bending Test of T-Shaped Spot-Welded Specimen

Figure 11 shows the results of the three-point-bending test. The contour colors indicate the maximum principal strain in the specimens during bending deformation. The bottom figure for each material indicates the strain distribution just before fracture. In No.1/590MPa, the strain concentrated in the edge area. In the case of No.1/590MPa, the bending test was stopped without fracture at the stroke of 40mm. In No.2/780MPa, higher strain was located to the left and right sides of the nugget and edge at the center of the specimen, and fracture occurred from the edge of center.

In materials No.3/980MPa and No.4/1470MPa, the maximum strain concentrated near the nugget, and fracture occurred near the nugget. The strain level at the moment of fracture decreased with increasing material strength. Material No.4/ 1470MPa showed a lower strain distribution compared with the other steel grades.

Figure 12 shows the fractures after the three-point-bending test. This figure shows the effects of the material strength and shared or machined edge condition on the fracture behavior at the spot weld and the flange edge. In the case of steel No.1/590MPa, fracture occurred in the sheared edge, while fracture did not occur in the machining edge. In material No.2/780MPa, fracture occurred at the edge area before the fracture of the spot-welded nugget. Thus, the existence of the sheared edge promoted fracture in the three-point-bending test.

Figure 13 shows the force stroke curves in the three-point-bending test. In this figure, the solid lines indicate the results of the machined edge specimen and the dotted lines indicate the result of the sheared edge specimens. The maximum force increased as the strength of the steel grades increased, while the stroke at fracture decreased with increasing strength. In the case of No.1/590MPa, the fracture stroke of the sheared edge specimen was 28mm, while the machined edge specimen showed no fracture up to a stroke of 40mm. With the other high strength steels, including materials No.2 and 3, the fracture stroke was significantly shorter with the sheared edge compared with the machined edge. In the case of No.4/1470MPa, the edge condition showed the small effect on the fracture stroke. Figure 14 shows the relationship between the fracture stroke and tensile strength. The influence of the edge condition decreased in the higher strength materials because the spot weld became the week point for fracture. Conversely, with the lowest strength steel, For No.1/590MPa, the edge condition was the major point of fracture behavior under three-point-bending deformation.

DISCUSSION OF FRACTURE IN SPOT-WELDED SPECIMENS

Hardness Distribution around Spot-Welded Nugget

The hardness in the spot-welded nugget and heat affected zone (HAZ) was measured by the Vickers hardness test with a weight of 50 gf. Figure 15 shows the hardness distribution around the spotwelded nuggets for the four steel grades. In lowest strength steel, No.1/590MPa, the hardness of the nugget is higher than that of the base metal, whereas in the highest strength steel, No.4/1470MPa, the hardness of the nugget and the base metal are almost the same. In the outer area of the nugget, hardness showed a lower value than that of the base metal due to the annealing effect of the temperature in the spot welding process. The decrease of hardiness is about 180 Hv. The specimens of No.3/980MPa and No.2/780MPa show lower hardnesses around the spot-welded nugget. In the high strength steels No.2, 3 and 4, fractures shown in three-point-bending test occurred from the HAZ.

Strain Concentration in HAZ and Edge in Three-Point-Bending Test

The strain history at the fracture point was analyzed by using the experimental strain data measuring system in order to understand the limit strain at the HAZ and the flange edge in the three-point-bending test. Figure 16 shows the strain history in the HAZ and edge in the three-point-bending test with machined edge. In this graph, the dotted lines show the force-stroke curve, the red lines show the strain history at the strain concentration point in the HAZ, and the blue lines show the strain history at the edge. The strains increased as the punch stroke increased. In No. 1/590MPa and No 2/780MPa, the strain in the edge were higher than those in the HAZ. In No. 3/980MPa, the strain levels in the HAZ and the edge were almost the same during deformation, and in No. 4/1470MPa, the strain in the HAZ was higher than that at the edge. The sharp drop in the force-stroke curves represents the moment of fracture. Comparing the force-stroke curves, the limit strain, which indicates fracture strain, can be determined for the HAZ and edge point of each material.

Figure 17 shows the limit strain at the HAZ and the base material for the four steel grades. The upper graph shows the relationship between tensile strength and limit strain of base material of the tensile test and limit strain at edge produced by machining in three-point bending test. The lower graph in figure 17 shows the limit strain at the HAZ near the spot-welded nugget. In these graphs, the limit strains determined by strain analysis in the tensile test and the three-point-bending test of the spot-welded specimens are plotted with circles and cross marks, respectively. The fitting curves present the limit strain criteria for estimating the fracture of the spot-welded joint samples. The limit strain decreased as the material strength increased. When the effect of element size on the limit strain can be considered, the criteria can be used in FE-simulations to estimate the fracture of spot-welded parts made from high strength steel,.

FE Simulation for Three-Point-Bending Test

A FE simulation was carried out to verify the limit strain determined by strain measurement in the three-point-bending tests. Figure 18 shows FE model of three-point-bending test with T-shaped spot-welded specimen and the test equipment. The equipment, including the punch and the two fulcrums was modeled by rigid elements. The spot-welded specimen was modeled by 3-dimensional solid elements. Figure 18(a) shows the overview of the FE model. Figure 18(b) shows the detailed model of the spot-welded nugget, which considers the change in material properties due to the temperature of the spot welding process. The shape of the nugget and size of the HAZ was modeled based on the photograph of the spot welded shown in Figure 15. The material properties in the HAZ were modeled based on the hardness information measured by the Vickers test shown in Figure 15. The minimum mesh size is 0.1mm in the HAZ area. In this study, the stress-strain curves were modified by the ratio of hardness difference in comparison with the hardness of the base material.

An explicit analysis in LS-DYNA was used for this calculation. The simulation did not consider the fracture of material in this study. Figure 19 shows the result of the simulation of the three-point-bending test with material No. 4/1470MPa. Figure 19(a) shows the deformation and strain distribution around the spot-welded nugget. The contour color shows the maximum principal strain distribution at the moment of fracture in the experiment, that is, the stroke of 5.2mm. The strain distribution was very close to the experimental result shown in Figure 11. Figure 19(b) shows the force -stroke curve and strain history in the HAZ in comparison with the experimental results. The calculated force was close to the experimental result. Because the fracture was not calculated in the FE simulation, the calculated force increased after the fracture point.

The calculated strain history in the HAZ around the spot-welded nugget was also close to the measured data obtained in the bending experiment up to the fracture point. After fracture, the measured strain increased rapidly due to the local strain concentration at the fracture point in the HAZ.

SUMMARY/CONCLUSIONS

An experimental using a strain measurement system with digital images was proposed in order to investigate the fracture behavior at the spot weld and the flange edge of high strength steels for automotive body parts. The effect of the material properties of the spot weld and the condition (shared or machined) of the flange edge surface on fracture behavior was successfully clarified by the simple tensile tests with spot-welded coupons and bending tests of T-shaped spot-welded specimens.

1. In high strength steels with tensile strength of 980MPa and over, the heat affected zone (HAZ) is a weak point for crash deformation because strain concentrates in the softened material in the HAZ.

2. Strain measurements by digital images clarified the criterion for fracture in the HAZ and the base material. This criterion, namely, the limit maximum principal strain, was determined by the experimental results.

3. A simulation indicated the possibility of fracture estimation of spot weld by using a detailed FE model which considers the change of material properties in HAZ due to temperature of spot welding.

4. Further study is expected to improve the accuracy of the fracture prediction based on the experimental results obtained by the proposed simple experimental method.

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CONTACT INFORMATION

Kentaro Sato

JFE Steel Corporation

1 Kokan-cho, Fukuyama-city, Hiroshima Prefecture, 721-8510, Japan kent-sato@jfe-steel.co.jp

Kentaro Sato

JFE Steel Corp.

Takayuki Futatsuka

JFE Steel Corp

Jiro Hiramoto

JFE Steel Corp.

Kei Nagasaka, Akira Akita, and Takeshi Kashiyama

Suzuki Motor Corp

doi:10.4271/2017-01-0365
Table 1. Mechanical properties of high strength steels measured by JIS
tensile test protocol.

No.   Steel     Thickness    YS      TS      El
      grade      (mm)       (MPa)   (MPa)   (%)

1      590MPa    1.6          383     633    33
2      780MPa    1.6          543     814    22
3      980MPa    1.6          691    1015    17
4     1470MPa    1.6         1267    1496     8
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Author:Sato, Kentaro; Futatsuka, Takayuki; Hiramoto, Jiro; Nagasaka, Kei; Akita, Akira; Kashiyama, Takeshi
Publication:SAE International Journal of Engines
Date:Apr 1, 2017
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