Testing of welded joints
The testing of resistance welded joints is one of the most sensitive topics in quality assurance. With non-destructive testing methods, no clear statements about the quality can be achieved for spot, projection and roller seam welding. For this reason, simple workshop tests and test procedures are used in practice. In parallel, non-destructive and “inline” methods are being developed.
In the simple workshop procedure, peel, roll-off and chisel tests are used to assess the weld quality. The weld specimens are thus destroyed and the type of fracture and the size of the torn-out spot are used to evaluate the welded joint. A distinction is made in the type of break between unbuttoning and shearing. Especially with the new high-strength steel grades, however, the statement is not unambiguous, as mixed fractures can occur. An expert group of the DVS is also currently dealing with this topic.
Other mechanical-technological test methods are the shear tensile test, the head tensile test, the torsion test or the fatigue test. Depending on the specific load or quality requirements, these procedures are used.
The main non-destructive testing methods used are the chisel test and subsequent ultrasonic testing. Both methods have their problems. The chisel test can no longer be used for higher-strength and thicker materials. Ultrasonic testing causes major problems with coated materials, is dependent on the subjective assessment of the sound sequences and the coupling of the probes is not always possible.
Due to these problems and the mere random evaluation of the quality, resistance welding, as with other manufacturing processes, is also moving towards “inline” monitoring and testing of the welded joints. This can be realised very effectively with the PQS weld system. A major German car manufacturer has declared the PQS weld method as a recommended test method. The advantage here is the complete evaluation of all welds through the recording of welding current and voltage and the evaluation through corresponding algorithms. 100 % of all welds are evaluated. More in one of the next issues of Schweißzeit.
You can find more information on this in the DVS leaflets, in the technical book Resistance Pressure Welding by Mr Manfred Krause and, as always, from your HWH partner.