The quality of a product is identified by its stability, its endurance and capability to withstand real working conditions. The composition of metal decides its properties and composition is decided as per the application. Take the example of gold, which is widely used as connectors in semiconductors or connecting jacks, it has very ductile nature when used in raw form. To make it fit to use in different applications, it is alloyed with copper. This adds rigidity in the metal and can be easily moulded for desired applications. Now, to get the desired results, you must perform the test on alloy which is suitable for the application, instead of on the purest form of metal.
Destructive and non-destructive tests are two categories defining test procedures. As the name suggests, the test sample gets destroyed in destructive testing. Say, you are checking the tensile strength of metal bar using Starrett FMS series, it breaks or deforms the sample. Whereas, in later one, the test is either performed on a sample or on end product. Say, you are estimating the thickness of alloy wheels using Magna Mike 8600. It’s simply a gauge that tells the operator about the uniformity in the structure without damaging it. It’s like pass/ fail test.
You cannot compare NDT and DT, as they serve their purpose for different industries. Like, without exposing the metal into real condition, how can you predict its behaviour? The application of metal and relative industry standards decides which tests are significant for your sample. Sometimes, you don’t need to check the composition of raw materials, but in certain applications, you cannot proceed further without assessing the quality of raw material. This is the responsibility of QC department. They are accountable for preparing the list of tests that are mandated for the perfect production. And, if something is going faulty, where is the process lacking? It is always good to refer international testing standards for this.
Non-Destructive Testing of Metals
Thickness gauging – In automotive and aerospace industry, even tiny components play an important role. We cannot underestimate their significance by their size. Even a defective small fastener can lead to product failure. When we are saying ‘products’, we are not talking about assembled product, we are referring to modules or parts that are manufactured separately by different vendors and assembled together to make the final product. So, every small part has to be as per the specifications.
Before starting the bulk manufacturing, tests are performed on samples like dimensions of fasteners, the thickness of metal fixtures and so on. Magna Mike 8600, which is a wall thickness gauge, that measure the thickness of surfaces and grooves of sample product. It has a magnetic probe and target ball. Both the components are placed on either side of the surface of sample product. The probe scans the surface and ball moves along with due to the magnetic field produced by the probe. The distance between probe and ball is sensed by a ‘hall effect sensor’ and equivalent values are displayed on the screen.
Endurance test – This is again a non-destructive form for certain components. Take the example of springs. The stamina to bear the push and pull force is determined. Also called proof test, the tester exerts periodic push and pulls for a certain count to check if the test sample can withstand the real life working conditions or not.
Destructive Testing of Metals
When we say destructive testing, this gives an impression of the destruction of the end product, which is not true. As per the testing standards, destructive testing is performed on samples. For every test, shape and size of the sample is predefined.
Tensile Test – Apart from tension bearing capacity, you can estimate elongation, brittleness and ductility of the sample with this test. Starrett FMS series, which is a tensile testing machine has a fixture to clamp the sample. A piece of the sample can be cut out or mould out from the raw material that is going to be used for mass production. This sample is subjected to tension until deformity occurs or it breaks, as per the test requirements. Here, the operator can estimate whether the material is brittle, which means if it breaks without any deformation. Similarly, necking can be observed in some metals, whether elongated before fracture. Grips are a substantial part of the tensile testing machine. It should not allow the sample to slip, to break from clamped point or assures self-tightening when metal expands from clamping point.
Corrosion resistance testing – Metals have a tendency to lose its properties in different environmental conditions, if not coated aptly. To understand the behaviour of metal, it is subjected to a controlled environment that simulates real life conditions at an accelerated rate. Samples are exposed to salt laden controlled environment for defined hours. Behaviour can be assessed by visual inspection.