Destructive testing is often performed when the quality of a component or structure must be ensured. It is also used to verify material selection decisions and determine the allowable material discontinuity limits, acceptable environments and acceptable stresses.
Comparing Destructive and Nondestructive Testing
While nondestructive testing is often more affordable, it does not yield the same type of information that destructive testing can. Destructive testing is important because it makes it possible to not only determine possible failure modes but also to evaluate those failures before, after and even while they are occurring. Different types of destructive tests give the evaluator different kinds of information. Therefore, it is important to be familiar with the various types of destructive tests and what data is available from them. (For more information on how to effectively prepare for testing, see 6 Ways to Prevent Failure Analysis Frustration.)
Tensile testing is a very useful and commonly practiced form of destructive testing. During tensile testing, a material is machined to a specific size. The material is made thicker on the ends with a thinner portion in the middle. The two thicker ends are then placed into separate clamps. The ends are thicker than the center so that the failure occurs in the middle rather than near the clamps. Once secured, the specimen is gradually pulled apart at a fixed speed until it fractures.
Stress, strain and elongation data are all captured during this time. Since the material was made to a specific size, the material's strength can be determined in load/area units. The strain and elongation values are used to determine the material's modulus of elasticity as well as the yield strength. The amount of elongation that occurs after the yield limit is reached and before final fracture gives the tester a sense of the ductility of the material. (A deeper dive into the role of corrosion can be found in the article Effect of Corrosion on a Material's Tensile Strength and Ductility.)
The toughness of a material is a measure of how well it can withstand an impact. This is an important data point to collect for materials that are in motion or in an environment where objects around it are in motion.
The two most prominent types of toughness testing are the Charpy V-notch test and the Izod impact test. Both are similar in the way they are performed. First, a specimen is prepared by machining it into a rectangular shape with a notch placed in the center. This notch serves as a stress concentrator that promotes material fracture. The notched specimen is then loaded into a test machine. The machine has a pendulum that is released when testing begins. The force of gravity causes the pendulum to impact and fracture the specimen.
Measurement data is recorded prior to the test, including the size and shape of the specimen, and the size and radius of the notch. If these measurements are inaccurate then the values derived from the toughness test will be of no use. The pendulum weight must be known prior to testing. The most important measurement to be recorded during the test is the amount of energy that the specimen absorbs from the pendulum, which can be determined by measuring the distance the pendulum traveled after the impact occurred. With this data the material toughness value can be calculated. Toughness values from these tests are typically measured in foot-pounds/inch (ft-lb/in) or Joules/centimeter (J/cm).
A fatigue failure occurs when a material undergoes an excessive number of cycles that cause one or more cracks to propagate and lead to a fracture. The level of stress itself is not enough to cause a material failure, but the repetition of the stress causes changes in the microstructure or macrostructure. This can ultimately promote crack initiation (formation) and propagation. (Discover a possible remedy in Nitriding for Corrosion and Wear Fatigue Resistance.)
Since fatigue failure can happen slowly over time, it is necessary to know the time frame in which a failure may occur so that the structure or component can either be repaired or removed from service before a failure is allowed to occur.
Two common types of fatigue are high cycle fatigue and low cycle fatigue. High cycle fatigue testing is used to calculate the number of load cycles a material can undergo when the stress applied is under the elastic limit. Low cycle fatigue calculates the number of cycles when the load is above the material’s elastic limit and plastic deformation is occurring.
Fatigue testing is used to estimate the time until a fatigue failure may occur. Since it is impractical to actually test a material for a duration equal to the actual amount of time it will be in service, a fatigue test will often use an accelerated means of testing. Generally, a fatigue test will place a material under a constant load. In addition to the constant load, a fluctuating load is placed on the material, which simulates the cycles the material will undergo when in service. The loads may be applied in several different ways: they can be used to simulate torsional load, tensile load and compressive load, to name a few. Many different loads are applied over several different tests for a given type of material. The number of cycles from each test are recorded and used to calculate the number of cycles that the material can withstand before failure.