Tension Tests for Resistance Welds
Tension-Shear Test. The tension-shear test is the most widely used method for determining the strength of resistance spot welds. It is also used for evaluation of weld schedules for ferrous and nonferrous alloys. The test specimen in Fig. 10.4 is made by overlapping suitable size coupons and making a spot weld in the center of the overlapped area. A tensile test machine is used to make the test.
The test is used mainly to establish ultimate shear strength when the specimen is tested in tension. When this test is used in combination with the cross-tension test (Fig. 10.5), the cross-tension strength/tension-shear strength ratio is referred to as a measure of ductility.
When gages less than about 1 mm (0.04 in.) are tested, a plug will usually be pulled from one sheet. This condition is typical of the fracture due to the eccentric loading caused by the overlapped sheets. As the thickness of the sheets or strength increases, the weld will fracture by shearing across the nugget (weld metal) at the interface.
When the thickness becomes large such as 4.8 mm (0.19 in.) and greater, the wedge grips of the test machine should be offset to reduce the eccentric loading which is accentuated
by the thickness of the specimen. A more precise shear load will be imposed on the spot weld, thus minimizing a tension or peeling component.
Fig. 10.4 Test specimen for tension shear
Thickness up to 4.8 mm (0.19 in.)
Fig. 10.5 Cross-tension test
The tension-shear test is commonly used in production assurance testing because it is an easy and inexpensive test to perform. Coupons welded at regular intervals are tested to a prior established standard of test results.
Fig. 10.6 Test jig for cross-tension specimens
The reader is directed to Recommended Practices for Resistance Welding. AWS C1.1, for more details with respect to test specimen dimensions and test fixtures as well as statistical methods for evaluating resistance weld test results. This publication is also applicable for the direct-tension test described in the next section.
Direct-Tension Test. The direct-tension spot weld test is used to measure the strength of welds for loads applied in a direction normal to the spot weld interface. This test used mostly for weld schedule development and as a research tool for the weldability of new materials. The direct-tension test can be applied to ferrous and nonferrous alloys of all thicknesses. The direct - tension test specimen is used to determine the relative notch sensitivity of spot welds.
There are two types of specimens used for the direct-tension test. The cross-tension specimens of Fig 10.5 can be used for all alloys and all thicknesses. When the metal gage is less than 1 mm (0.04 in.), it is necessary to reinforce the specimen to prevent excessive bending. Test jig for cross-tension specimens is shown in Fig. 10.6 for thicknesses up to 4.9 mm and Fig.
10.7 for greater thicknesses.
Peel Test. A variation of the direct-tension test is the peel test which is commonly used as a production control test. The test is shown in Fig 10.7(6). The size of the plug or button is measured or correlated with weld sizes having known strengths that are produced by satisfactory production weld schedules. This weld test is fast and inexpensive to perform. Howerver, high strength or thicker specimens may fracture at the interface without producing a plug.
Fig. 10.7 Jig for cross-tension test (t > 4.8 mm)