Chisel Test (for Resistance and Spot Welds)

The chisel test is a destructive testing method that evaluates the integrity of resistance spot, projection and seam welds under the peel force generated by driving a chisel-shaped tool between the welded sheets. The basic physical principle of the method is to apply a progressive opening/separating force to the area surrounding the weld nugget and to observe where and how failure occurs. In a sound weld, separation takes place not in the nugget itself but in the surrounding base material; a "button" (plug) is torn from one sheet, leaving a hole in the other. In an insufficient or cold weld, by contrast, the sheets separate cleanly along the faying surface (interfacial failure) and no button is formed; this is direct evidence that the nugget diameter or the depth of fusion is inadequate.

Two basic forms are used in practice. In the partial chisel test the chisel is driven between the sheets at a distance of approximately one sheet thickness from the weld and a partial separation is created around the nugget in order to check whether a button forms; this is convenient for rapid setting verification on the production line. In the full chisel test the sheets are forced apart until they are completely separated and the nugget is fully exposed so that its diameter can be measured. In the related peel test, one of the sheets is folded back by hand or in a vice in order to pull the nugget out. The aim of all three approaches is the same: to reveal the true load-bearing diameter of the weld and its failure mode.

The equipment of the method is simple but its geometry is critical. A chisel made of hardened steel with a wedge angle and edge thickness defined in the standards; a hammer or a pneumatic/hydraulic press for controlled impact; and callipers, a diameter comparator or a profile template to measure the button or nugget diameter once the sheets have been opened. Because the insertion depth, angle and distance of the chisel from the weld directly affect the results, checking the tool geometry and the measuring equipment before the test is mandatory. Suitable leverage and clamping ensure that the force is transmitted symmetrically to the nugget; otherwise incorrect failure modes may be read.

The application-specific depth of the method lies in defining the acceptance criteria in terms of the nugget/button diameter. The minimum acceptable diameter is most often related to the sheet thickness (the 4√t or 5√t criterion is common in practice) and, if the measured value falls below this threshold, the weld is rejected. Not only the diameter but also the failure mode is decisive in the evaluation: button (plug) failure indicates a ductile and acceptable nugget, whereas interfacial failure, a partial button, porosity or cracking are signs of an inadequate weld. Since nugget size and failure behaviour vary in materials such as thin automotive sheet, galvanised surfaces or high-strength steels, the test parameters are adapted to the material and thickness.

Reporting covers the form of test (partial/full chisel or peel), the measured button/nugget diameter, the observed failure mode, any cracking/porosity findings and the accept/reject decision according to the relevant standard. Because it requires no complex equipment and its result can be read directly by eye, the chisel test is a reliable and economical tool for verifying spot welding parameters (current, force, time, electrode condition) and for periodic quality monitoring in series production. TÜV AUSTRIA SILA KALİTE performs the chisel test within the scope of its TS EN ISO/IEC 17025 accreditation using defined procedures and calibrated measuring equipment; test results are documented with traceable records and an accredited test report.