Pressure Test (for Resistance and Spot Welds)

The pressure test (commonly known as the "pillow test") is a destructive testing method based on inflating panel specimens, in which two sheets are joined to one another by resistance welding (spot or seam welding), by applying a controlled fluid pressure inside the closed volume, and thereby verifying the strength of the weld line under this pressure. The specimen is welded around its perimeter so as to form a leak-tight pocket between the two sheets; when water or air is introduced, the sheets bulge like a pillow and apply a coaxial peel and tensile load to the weld seam and the base material. As the pressure is increased in steps, the weld either withstands the specified target pressure or begins to tear from a given point. This behaviour allows the nugget diameter, the depth of fusion and the continuity of the seam to be evaluated under a loading close to real service conditions.

The test is carried out by clamping the edges of the prepared panel specimen in a fixture (between flanges/plates) and applying pressure through a central feed channel. The free central region of the specimen bulges outwards under pressure; during this, the weld line — unlike the region where the base metal begins to yield — is subjected to peel loading in the joint plane. The objective is either to verify leak-free and tear-free resistance up to a predefined acceptance pressure (accept/reject test) or to pressurise the specimen until it fails, thereby determining the ultimate burst pressure and the fracture mode (button/nugget pull-out, interfacial separation or base metal tearing). The desired behaviour is for the weld to pull a "button" out of the base material, leaving it in the sheet; a clean separation in the joint plane, on the other hand, indicates insufficient fusion.

On the equipment side, the method comprises a calibrated pressure source (hydraulic hand pump, motorised test unit or pneumatic regulator), a traceable pressure gauge/transducer, a clamping fixture providing leak-tightness, and an indicator/data logger recording pressure against time. The use of hydraulic (water) pressure is preferred for safety reasons, as little energy is released at the moment of tearing; in tests carried out with gas, a protective enclosure is mandatory. The loading rate is kept under control, because sudden pressure increases can mask the true weld strength. The specimen geometry, sheet thickness, weld spacing and free diameter are fixed in accordance with the standard; otherwise the results are not comparable.

In application-specific terms the method is decisive particularly for leak-tight bodies produced by resistance seam welding and for double-walled panels joined with a spot weld pattern. In fuel tanks, radiator and heat exchanger cells, solar collector absorber panels, white goods boiler and compressor bodies and thin sheet constructions, both weld strength and leak-tightness can be evaluated within the same test. The method is frequently used to verify welding parameters (current, electrode force, weld time) and as a supporting specimen test for WPS/procedure approval before series production; it is also effective in detecting weld quality that declines over time due to electrode wear or the effect of galvanising/coating.

At the reporting stage the ultimate burst pressure obtained, the resistance status against the acceptance pressure, the location and mode of fracture, the sheet thickness and material type and the specimen identification are recorded; where possible a pressure–deformation curve is included. Acceptance criteria are generally defined in terms of the minimum burst pressure and the required fracture mode (button pull-out from the base metal, no interfacial separation) specified by the product specification or the relevant welding standard. In this way the test produces not merely "pass/fail" information but numerical and visual evidence of the true load-bearing capacity of the weld line, which is an output directly usable for the calibration of production parameters and for reliability in the field.

TÜV AUSTRIA SILA KALİTE performs the pressure (pillow) test on resistance and spot welds within the scope of its TS EN ISO/IEC 17025 accreditation, with traceably calibrated equipment and competent personnel; the results are issued as an accredited test report, independently documenting the conformity of the specimen to the relevant standard and product specification.