Fatigue testing
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 on coupons are typically conducted using servo hydraulic test machines which are capable of applying large variable amplitude cyclic loads.
[2] Constant amplitude testing can also be applied by simpler oscillating machines.
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.
Airworthiness standards generally require a fatigue test to be carried out for large aircraft prior to certification to determine their safe life.
These tests usually determine the rate of crack growth per cycle
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.
The minimum size of the specimen is given by[4] The stress intensity range for this coupon is[4] where
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.
Cracks may initiate from the following sources: 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.
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.
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.
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.
[11] 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.
