High Strain Rate Testing

High Strain Rate Testing

A dynamic tensile test was conducted to evaluate the mechanical properties of spot welds under automotive collision conditions. The actual tensile shear strengths of steel sheets with nominal tensile strengths ranging from 270 MPa to 780 MPa were investigated.

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Test Method

Figure 1 presents the dynamic tensile test machine and illustrates a schematic diagram of the tensile shear test specimen. A 1.6 mm thick steel sheet was placed on top of the tensile shear test specimen and spot welded, with nugget diameters of 5.5√t (7.0 mm) used for both. In the dynamic tensile test, a cone was dropped at high speed onto the specimen to apply a tensile load and determine the breaking point. The tensile speed was adjusted by varying the drop height of the cone, with a maximum speed of 2.4 m/s. For comparison, a static tensile test was conducted at a tensile speed of 1.6 × 10-4 m/s.

 

Figure 1: Dynamic tensile shear test equipment (left) and test specimen (right)

 

Results

Figure 2 shows the relationship between tensile shear strength and tensile speed for the steel sheet with a rated tensile strength of 590 MPa. Tensile shear strength tended to increase with tensile speed, with values of approximately 22 kN and 25.5 kN under static and dynamic loading conditions, respectively. All specimens exhibited plug fracture as the failure mode

 

 

Figure 2: Relationship between tensile shear strength and tensile speed (steel sheet with a rated tensile strength of 590 MPa)

Figure 3 illustrates the effect of the tensile strength of the base material on the rate of increase in dynamic strength relative to static strength. Plug fracture remained the consistent failure mode across all cases. For the steel sheet with a rated tensile strength of 270 MPa, dynamic strength increased by approximately 60% compared to static strength. In contrast, the sheet with a rated tensile strength of 780 MPa showed an increase of only about 14%. These results indicate a tendency for the rate of increase in dynamic strength relative to static strength to decrease as the rated tensile strength of the steel increases. This is consistent with the general trend of mild steel strength increasing with strain rate, while strain rate sensitivity diminishes for higher-strength steels.

Figure 3: Relationship between dynamic and static tensile shear strengths of spot welds and base material strength

Source

Dynamic Tensile Shear Strength of Spot-Welded Joints: Experimental Investigation and Results Hiroki Fujimoto, Welding & Joining Research Laboratories, Nippon Steel Corporation

 

 

Microstructural Evolution and Effect on Joint Strength in Laser Welding of DP to Aluminium

Microstructural Evolution and Effect on Joint Strength in Laser Welding of DP to Aluminium

Joining Dissimilar Materials, Laser Welding

Given the use of many different metals in the Body in White construction, it is important to understand the effects of dissimilar welding AHSS. Researchers at Indian Institute of Technology Madras in Chennai, India and Centre of Laser Processing of Materials in Hyderabad, India developed tests to study the resulting microstructure from laser welding 2.5 mm thick DP600 steel to 3 mm thick AA6061 aluminium alloy using a laser beam diameter of 1.5 mm.I-1  They discovered a softening in the steel HAZ due to a tempering effect and an increase in hardness in the aluminum HAZ due to the presence of aluminium intermetallic phases present. Maximum shear strength was observed when the thickness of intermetallics was reduced to 8-11 microns. They concluded that best quality welds were made under power densities and interaction times of 1.98kW/mm2, 0.15s and 2.26 kW/mm2, 0.187s.

The laser power was varied from 3 kW to 4.5 kW and the scanning speed of 8 mm/s, 10 mm/s, and 12 mm/s. Power density and interaction time were two parameters they used to compare trials where:

 

Power density (Pd) =  disimilar-materials-equation1

and

interaction time (It) = disimilar-materials-equation2

 

The resulting welding parameters are shown in Table 1 below. Figure 1 shows the microstructure of the fusion boundary and HAZ on the DP600 side of the welded joint. Figure 2 shows the microstructure of the weld interface on the AA 6061 side. Figure 3 displays the hardness data with (a) representing 3.5 kW and 10 mm/s, (b) representing 3.5 kW and 8 mm/s, and (c) representing 4 kW and 8 mm/s. Figure 4 represents the Shear Stress-Strain of the welds given different IMC thickness.

 

Table 1: Welding Parameters

Table 1: Welding Parameters.I-1

 

Figure 1: Weld Metal, DP 600 Base Metal, and HAZ microstructure

Figure 1: Weld Metal, DP 600 Base Metal and HAZ microstructure.I-1

 

Figure 2: Fe-Al interface microstructure

Figure 2: Fe-Al interface microstructure.I-1

 

Figure 3: Microhardness Plot

Figure 3: Microhardness Plot.I-1

 

Figure 4: Load vs. Displacement

Figure 4: Load vs. Displacement.I-1