Peel Test (for Resistance and Spot Welds)

The peel test is a destructive testing method based on forcing thin sheet components joined by resistance and spot welding apart from one another (peeling) by means of a force applied perpendicular to the joint plane. In spot welding, two sheets are melted by the resistive heat generated by the high current passed between the electrodes, forming a localised molten pool; when this pool solidifies it produces the lens-shaped "weld nugget" that bonds the sheets together. The mechanical strength of the weld depends directly on the diameter and internal soundness of this nugget. The physical purpose of the peel test is to open the joint in a controlled manner in order to reveal how the weld fails and to measure the nugget size directly. The desired failure mode is "nugget/button pull-out", in which the nugget tears out of one of the sheets and leaves a round hole (button) in the other; a planar interfacial failure, where the weld separates along the faying surface, is an indication of insufficient fusion or inadequate strength.

Two basic approaches are used in practice. In the manual peel and chisel test, one end of the welded specimen is clamped in a vice and the free sheet is bent back by hand, with pliers, or with the aid of a chisel/wedge inserted along the weld line, thus opening the joint; peeling is continued until the nugget leaves one of the sheets. In the mechanised peel test, the specimen is mounted in a tensile machine that pulls at a constant speed and controlled angle; this method records both the failure mode and, where required, the peel force in a repeatable manner. In both cases the aim is to extract the nugget without destroying it and to make its geometry visible.

For evaluation, the button left by the pulled-out nugget or the hole formed in the sheet is measured with callipers or a dedicated nugget gauge. Since the nugget is usually not perfectly round, the diameter is read in two mutually perpendicular directions and averaged; a hand magnifier or low-magnification stereo microscope may be used for measurement accuracy. Although the test equipment is simple (vice, chisel, pliers, callipers, gauge), the reliability of the result depends on specimen preparation and operator technique: sheet thickness, weld spacing and edge distances must comply with the specimen dimensions given in the relevant standard.

The method is widely used for verifying welding parameters (current, time, electrode force) and for start-of-production quality control in manufacturing with series spot welding, particularly automotive body sheets, white goods panels, sheet metal enclosures and galvanised/coated thin sheets. The resulting nugget diameter is compared in most specifications with a minimum value related to the square root of the sheet thickness (for example 4√t or 5√t); in addition, the failure mode (button failure / partial button / interfacial failure) is classified. Findings such as weld expulsion, surface indentation, porosity or insufficient penetration are also recorded.

At the reporting stage the specimen identification, material and sheet thickness, welding parameters, measured nugget diameters, failure type, acceptance criterion and result (conforming/non-conforming) are combined with the required photographic records in an inspection report. In this way the peel test provides concrete, traceable evidence of strength both for parameter approval before production and as a periodic monitoring tool in series production. TÜV AUSTRIA SILA KALİTE performs the peel test on resistance and spot welded joints within the scope of its TS EN ISO/IEC 17025 accreditation, in accordance with the relevant national and international standards, and documents the results in a traceable inspection report.