Testing Materials — Destructive & Non-Destructive
Why Test Materials?
Before an engineer specifies a material for a product, they need evidence that it will perform as required. Material testing provides that evidence. Some tests are carried out during the design phase, to check that a chosen material meets the specification. Others are used in production quality control, to check that every batch of material supplied to the factory has the properties expected. The type of test chosen depends on what property needs to be measured — and whether the sample being tested can be sacrificed.
Destructive testing (DT) damages or destroys the sample to obtain a measurement. It cannot be used on a finished product that is going into service. Non-destructive testing (NDT) measures properties without permanently harming the sample — the part can still be used after testing.
Destructive Testing: Tensile Test
A tensile test measures tensile strength. The sample is machined into a standardised ‘dog-bone’ shape so that the test section has a known cross-sectional area. It is gripped in a tensile testing machine and a gradually increasing pulling force is applied until the sample fractures. The maximum force sustained before fracture, divided by the original cross-sectional area, gives the tensile strength. The sample is destroyed in the process.
The tensile test also reveals ductility. As the force increases, the sample stretches. The engineer measures how much the sample elongates before it fractures: a ductile material will neck (form a visible waist) and stretch significantly; a brittle material will fracture suddenly with very little stretching. Plotting force against extension produces a stress–strain curve, which shows the elastic limit, yield point and ultimate tensile strength of the material.
Hardness Testing
Hardness testing uses an indenter — a hard ball or diamond-tipped cone — pressed into the surface of the sample under a controlled force. The size of the indent left in the surface is measured. A smaller indent means a harder material. Common hardness scales include the Brinell test (steel ball) and the Vickers test (diamond pyramid). The indenter leaves a mark in the surface. Whether that makes the test unsuitable depends on whether the component can still be used after marking.
Destructive Testing: Toughness Test
A Charpy impact test measures toughness. A notched sample is placed horizontally on two supports and struck by a heavy pendulum. (Note: in the related Izod test the specimen is clamped vertically at one end — this is the key difference between the two methods.) The height the pendulum swings to after impact tells the engineer how much energy the material absorbed — a tough material absorbs more energy and the pendulum does not swing as high on the follow-through. The sample is fractured in the test. This test is used to check that structural steels maintain their toughness at low temperatures, since some metals become brittle in cold conditions.
Malleability test: A sample is repeatedly hammered or rolled while being observed for cracking. A malleable material can be worked without cracking; a brittle material will crack early in the test. The test is destructive because the sample is deformed in the process.
Ductility test (cup and cone / wire draw): The tensile test already provides ductility data through percentage elongation measurements. A separate bend test — bending a sample through a set angle — also checks ductility: a ductile sample bends without cracking, a brittle one fractures.
Elasticity test: A spring or sample is loaded to a known force and the extension is measured. The load is then removed. If the sample returns to its original length, it behaved elastically within the test range. If the sample remains permanently deformed, it was loaded beyond its elastic limit. The test can be conducted without fracturing the sample if it is kept below the elastic limit.
Conductivity Testing
Electrical conductivity is tested by passing a known current through a sample of defined dimensions and measuring the voltage across it. Using Ohm’s law (V = IR), the resistance can be calculated. With the sample’s dimensions, this gives resistivity; conductivity is the reciprocal of resistivity. A multimeter is a common workshop tool used to measure resistance in materials and components. This test does not destroy the sample, so it is a non-destructive test.
Non-Destructive Testing: The Core Principle
NDT methods leave the test sample intact and usable. This matters enormously in engineering practice because many parts — the welds on a bridge, the fuselage skin of an aircraft, the gas pipe below a city street — cannot be sacrificed for a destructive test. NDT allows engineers to check for internal flaws, cracks or weaknesses without cutting the part open. The most important NDT methods at this level are visual inspection and ultrasonic testing.
Scenario: A manufacturer produces a batch of steel cable for a new suspension bridge. They want to verify that the cable meets its tensile strength specification before installation. After installation, the bridge maintenance team wants to check the cables annually for internal corrosion and micro-cracks.
Before installation — destructive tensile test: A sample of cable from the batch is pulled to fracture in a tensile testing machine. This confirms the tensile strength meets the specification. The tested sample is destroyed, but because it is taken from the production batch rather than from the actual bridge cable, this is acceptable. The tested sample cannot be used in the bridge.
After installation — non-destructive ultrasonic test: Sound waves are passed through the installed cable. Any crack or void inside the cable reflects the sound waves back differently from solid metal. The maintenance team can detect internal flaws without cutting the cable out of the bridge. The cable remains in service throughout the test.
Key principle: Use DT to characterise material properties before the product goes into service. Use NDT to inspect finished parts and products that must remain intact and in service.
The fundamental reason to choose NDT over DT is that the in-service part cannot be sacrificed. A weld in a live gas pipeline cannot be cut out for a destructive test; an ultrasonic scan checks for cracks non-destructively. Top-band exam answers connect the choice of test to whether the product can be sacrificed.