Fatigue testing

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 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.
Airworthiness standards generally require a fatigue test to be carried out for large aircraft prior to certification to determine their "safe life". Small aircraft may demonstrate safety through calculations, although typically larger "scatter" or safety factors are used because of the additional uncertainty involved.
Coupon tests.
Fatigue tests are used to obtain material data such as the rate of growth of a fatigue crack that can be used with crack growth equations to predict the fatigue life. These tests usually determine the rate of crack growth per cycle formula_1 versus the stress intensity factor range formula_2, where the minimum stress intensity factor formula_3 corresponds to the minimum load for formula_4 and is taken to be zero for formula_5, and formula_6 is the stress ratio formula_7. Standardised tests have been developed to ensure repeatability and to allow the stress intensity factor to be easily determined but other shapes can be used providing the coupon is large enough to be mostly elastic.
Coupon shape.
A variety of coupons can be used but some of the common ones are:
Instrumentation.
The following instrumentation is commonly used for monitoring coupon tests: 
Full scale fatigue tests.
Full-scale tests may be used to: 
Fatigue tests can also be used to determine the extent that widespread fatigue damage may be a problem.
Test article.
Certification requires knowing and accounting for the complete load history that has been experienced by a test article. 
Using test articles that have previously been used for static proof testing have caused problems where "overloads" have been applied and that can retard the rate of fatigue crack growth.
The test loads are typically recorded using a data acquisition system acquiring data from possibly thousands of inputs from instrumentation installed on the test article, including: strain gages, pressure gauges, load cells, LVDTs, etc.
Fatigue cracks typically initiate from high stress regions such as stress concentrations or material and manufacturing defects. It is important that the test article is representative of all of these features.
Cracks may initiate from the following sources:
Loading sequence.
A representative block of loading is applied repeatedly until the "safe life" of the structure has been demonstrated or failures occur which need to be repaired.
The size of the sequence is chosen so that the maximum loads which may cause retardation effects are applied sufficiently often, typically at least ten times throughout the test, so that there are no sequence effects.
The loading sequence is generally filtered to eliminate applying small non-fatigue damaging cycles that would take too long to apply. Two types of filtering are typically 
used:
The testing rate of large structures is typically limited to a few Hz and needs to avoid the resonance frequency of the structure.
Test rig.
All components that are not part of the "test article" or instrumentation are termed the "test rig". The following components are typically found in "full scale fatigue tests":
Instrumentation.
The following instrumentation is typically used on a fatigue test:
It is important to install any strain gauges on the test article that are also used for monitoring fleet aircraft. This allows the same damage calculations to be performed on the test article that are used to track the fatigue life of fleet aircraft. This is the primary way of ensuring fleet aircraft do not exceed the safe-life determined from the fatigue test.
Inspections.
Inspections form a component of a fatigue test. It is important to know when a detectable crack occurs in order to determine the certified life of each component in addition to minimising the damage to surrounding structure and to develop repairs that have minimal impact on the certification of the adjacent structure. Non-destructive inspections may be carried out during testing and destructive tests may be used at the end of testing to ensure the structure retains its load carrying capacity.
Certification.
Test interpretation and certification involves using the results from the fatigue test to justify the safe life and operation of an item. The purpose of certification is to ensure the probability of failure in service is acceptably small. The following factors may need to be considered:
Airworthy standards typically require that an aircraft remains safe even with the structure in a degraded state due to the presence of fatigue cracking.