Fatigue testing

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 da/dN versus the stress intensity factor range \Delta K = K_\max - K_\min, where the minimum stress intensity factor K_\min corresponds to the minimum load for R > 0 and is taken to be zero for R\le 0, and R is the stress ratio R= K_\min/K_\max. 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. In cases where crack initiation from subsurface inclusions dominates, extreme value theory may be used in determining the effect of the coupon size. Coupon shape A variety of coupons can be used but some of the common ones are: • compact tension coupon (CT). The compact specimen uses the least amount of material for a specimen that is used to measure crack growth. The single edge coupon is an elongated version of the compact tension coupon. Instrumentation The following instrumentation is commonly used for monitoring coupon tests: • Strain gauges are used to monitor the applied loading or stress fields around the crack tip. They may be placed beneath the path of the crack or on the back face of a compact tension coupon. • An extensometer or displacement gauge can be used to measure the crack tip opening displacement at the mouth of a crack. This value can be used to determine the stress intensity factor which will change with the length of the crack. Displacement gauges can also be used to measure the compliance of a coupon and the position during the loading cycle when contact between the opposite crack faces occurs in order to measure crack closure. • Applied test loads are usually monitored on the test machine with a load cell. • A travelling optical microscope can be use for measurement of the position of the crack tip. == Full scale fatigue tests ==

Full-scale tests may be used to: • Validate the proposed aircraft maintenance schedule. • Demonstrate the safety of a structure that may be susceptible to widespread fatigue damage. • Generate fatigue data • Validate expectations for crack initiation and growth pattern. • Identify critical locations • Validate software used to design and manufacture the aircraft. 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: • Fretting, typically from high cycle count dynamic loads. • Mis-drilled holes or incorrectly sized holes for interference fit fasteners. • Material treatment and defects such as broken inclusions. • Stress concentrations such as holes and fillets. • Scratches, impact damage. 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: • deadband filtering eliminates small cycles that completely fall within a certain range such as +/-3g. • rise-fall filtering eliminates small cycles that are less than a certain range such as 1g. The testing rate of large structures is typically limited to a few Hz and needs to avoid the resonance frequency of the structure. The purpose of certification is to ensure the probability of failure in service is acceptably small. The following factors may need to be considered: • number of tests • symmetry of the test structure and the applied loading • installation and certification of repairs • scatter factors • material and manufacturing process variability • environment • criticality Airworthy standards typically require that an aircraft remains safe even with the structure in a degraded state due to the presence of fatigue cracking. == Notable fatigue tests ==

• Cold proof loading tests of the F-111. These tests involved applying static limit loads to aircraft which had been chilled to reduce the critical fracture size. Passing the test meant that there were no large fatigue cracks present. When cracks were present, the wings failed catastrophically. • de Havilland Comet suffered a series of catastrophic failures that ultimately proved to be fatigue despite being fatigue tested. • Fatigue tests on 110 Mustang wing sets were carried out to determine the scatter in fatigue life. • The novel No Highway and movie No Highway in the Sky were about the fictional fatigue test of the fuselage of a passenger aircraft. • Fatigue tests have also been used to grow fatigue cracks that are too small to be detected. ==References==