Stripping (Peel) Test (for Resistance and Spot Welds)

The stripping test (peel test) is a destructive testing method used to determine the mechanical strength and failure character of the weld spot (nugget) in sheet specimens joined by resistance spot and projection welding. The basic principle of the method is to attempt to separate two overlapped sheets from one another immediately adjacent to the weld spot, in the shear/stripping direction. During this separation the weld nugget is subjected to a loading mode different from that of the tensile-shear test: the load is transmitted to the joint interface as a peel load containing a component perpendicular to the sheet plane. Because this loading stresses the weakest bond between the weld nugget and the base material, it is highly discriminating in revealing the true integrity and ductility behaviour of the weld.

The test is applied in two basic ways. In the manual (chisel/stripping) method the specimen is clamped in a vice; one of the sheets is held fixed while the other is bent back and pulled with pliers, a vice or a chisel so as to be peeled away from the weld spot. In the mechanised stripping method a specimen of standard dimensions is mounted in a tensile machine and separated at a controlled speed; in this way both the failure load and the failure mode are obtained under repeatable conditions. In both cases the purpose is the same: to observe whether the weld spot withstands loading before the surrounding base metal fails, and how the failure develops.

In terms of equipment, a vice, a stripping chisel/pliers and fixtures that hold the specimen securely are sufficient for manual stripping; for mechanised stripping a tensile testing machine with suitable grips, a specimen fixture and, where required, load–displacement recording are used. After the test the diameter of the failed weld spot (button/nugget diameter) is measured with callipers or a diameter gauge. Two quantities are prominent in the evaluation: the diameter of the pulled-out nugget and the failure mode. In a ductile weld the nugget tears out of the base metal and leaves a complete "button" in one of the sheets; in a brittle or inadequate weld, interfacial failure is observed, which is generally not accepted.

In application-specific terms, the stripping test is interpreted together with minimum nugget diameter criteria related to sheet thickness; in many applications the minimum acceptable nugget diameter is assessed against a threshold proportional to the square root of the sheet thickness (for example a given coefficient × √t). The test therefore provides not only "failed/did not fail" information but also engineering feedback as to whether the welding parameters (current, electrode force, weld time) have been correctly selected. It is a practical method frequently used for welding procedure approval and in-process quality control in fields such as automotive body manufacturing, white goods, sheet metal work and projection welded nut/stud connections where thin sheets are joined. Since the tendency towards interfacial failure increases in high-strength and coated sheets, the stripping test is particularly valuable for monitoring weld quality in these materials.

At the reporting stage the failure load (in the mechanised test), the diameter of the failed nugget, the failure mode (button failure / partial button / interfacial failure), the specimen and sheet data and the test conditions are documented; the results are evaluated as conforming/non-conforming against the acceptance criteria of the relevant standard. This output forms the basis for decisions in the verification of welding procedure specifications (WPS) and in the release of production batches. TÜV AUSTRIA SILA KALİTE performs the stripping test on resistance and projection welds within the scope of its TS EN ISO/IEC 17025 accreditation, using defined test procedures and calibrated equipment; the results obtained are therefore traceable and repeatable and are recognised both nationally and internationally.