Stress testing is a form of deliberately intense or thorough testing, used to determine the stability of a given system, critical infrastructure or entity. It involves testing beyond normal operational capacity, often to a breaking point, in order to observe the results.

Reasons can include:

Reliability engineers often test items under expected stress or even under accelerated stress in order to determine the operating life of the item or to determine modes of failure.

The term "stress" may have a more specific meaning in certain industries, such as material sciences, and therefore stress testing may sometimes have a technical meaning – one example is in fatigue testing for materials.

In animal biology, there are various forms of biological stress and biological stress testing, such as the cardiac stress test in humans, often administered for biomedical reasons. In exercise physiology, training zones are often determined in relation to metabolic stress protocols, quantifying energy production, oxygen uptake, or blood chemistry regimes.

Fatigue testing is a specialised form of mechanical testing that is performed by applying cyclic loading to a coupon or structure. These tests are used either to generate fatigue life and crack growth data, identify critical locations or demonstrate the safety of a structure that may be susceptible to fatigue. Fatigue tests are used on a range of components from coupons through to full size test articles such as automobiles and aircraft.

Fatigue tests on coupons are typically conducted using servo hydraulic test machines which are capable of applying large variable amplitude cyclic loads. Constant amplitude testing can also be applied by simpler oscillating machines. The fatigue life of a coupon is the number of cycles it takes to break the coupon. This data can be used for creating stress-life or strain-life curves. The rate of crack growth in a coupon can also be measured, either during the test or afterward using fractography. Testing of coupons can also be carried out inside environmental chambers where the temperature, humidity and environment that may affect the rate of crack growth can be controlled.

Because of the size and unique shape of full size test articles, special test rigs are built to apply loads through a series of hydraulic or electric actuators. Actuators aim to reproduce the significant loads experienced by a structure, which in the case of aircraft, may consist of manoeuvre, gust, buffet and ground-air-ground (GAG) loading. A representative sample or block of loading is applied repeatedly until the safe life of the structure has been demonstrated or failures occur which need to be repaired. Instrumentation such as load cells, strain gauges and displacement gauges are installed on the structure to ensure the correct loading has been applied. Periodic inspections of the structure around critical stress concentrations such as holes and fittings are made to determine the time detectable cracks were found and to ensure any cracking that does occur, does not affect other areas of the test article. Because not all loads can be applied, any unbalanced structural loads are typically reacted out to the test floor through non-critical structure such as the undercarriage.