Several AISI type 321 stainless steel welded oil tank assemblies used on helicopter engine systems began to leak in service. One failure, a fracture on the aft side of a spot weld, was submitted for analysis. SEM fractography examination revealed fatigue failure. The failure initiated at an overload fracture near the root of the weld and was followed by mode III fatigue crack propagation (tearing) around the periphery of the weld. The initial overload fracture was caused by a high external load, which produced a concentrated stress and fracture at the weld root. The subsequent fatigue fracture was caused by engine vibrations during operation of the aircraft. Fracture characteristics indicated that the fatigue would not have occurred if the initial damage had not taken place.

A Marine Corps helicopter crash was investigated. Efforts were directed to the failure of one of the main rotor blades that had apparently separated in the air. The apparent failure of a blade integrity monitor (BIM) system was also considered. The rotor blade comprised a long, hollow 6061-T651 aluminum alloy extrusion and 26 fiberglass “pockets” that provided the trailing-edge airfoil shape. Visual examination of the fracture surface of the aluminum extrusion indicated fatigue crack growth followed by ductile overload separation. Examination of the fatigue fracture region revealed several pits that appeared to have acted as fracture origin sites. Time to failure was determined using fracture mechanics. It was concluded that failure was caused by a fatigue crack that grew to critical length without detection. The crack originated at pits that resulted from the use of an improperly designed heating element used to cure fiberglass repairs.

There was a fracture of an anchor link that rides on a post with a spherical surface. No bevel or radius was called for, leaving an extremely sharp edge. A fatigue crack grew from the top sharp edge. The casting had good toughness and strength (1448 MPa, or 210 ksi, tensile strength). This fatigue failure was the result of poor design, which incorporated a notch that acted as a stress raiser, leading to fatigue-crack growth. A generous radius was recommended to avoid similar failures.

The 4340 steel main rotor yoke of a helicopter failed during a hovering exercise. Visual examination of the yoke revealed no evidence of gross external damage. Visual fracture surface examination, macrofractography, scanning electron micrography, and metallography of a section cut from the yoke in the region of the cracking indicated that the failure was caused by fatigue-crack initiation and growth from severe corrosion damage to a pillow-block bolt hole. Corrosion occurred because of failure of the protection scheme. An upgraded corrosion protection scheme for the bolt holes was recommended, along with nondestructive inspection of the region at intervals determined by fractographic analysis of the fatigue crack growth.

Examination of several fighter aircraft main landing gear legs revealed unusual cracking in the hard chromium plating that covered the sliding section of the inner strut. The cracking was associated with cracks in the 35 NCD 16 steel beneath the plating. A detailed investigation revealed that the cracking was caused by the combination of incorrect grinding procedure, the presence of hydrogen, and fatigue. The grinding damage generated tensile stresses in the steel, which caused intergranular cracking during the plating cycle. The intergranular cracks were initiation sites for fatigue crack growth during service. It was recommended that the damaged undercarriage struts be withdrawn from service pending further analysis and development of a repair technique.

In 1979, during a routine bridge inspection, a fatigue crack was discovered in the top flange plate of one tie girder in a tied arch bridge crossing the Mississippi River. Metallographic analysis indicated a banding or segregation problem in the middle of the plate, where the carbon content was twice what it should have been. Based on this and results of ultrasonic testing, which revealed that the banding occurred in 24-ft lengths, it was decided to close the bridge and replace the defective steel. The steel used in the construction of this bridge was specified as ASTM A441, commonly used in structural applications. Testing showed an increase in hardness and weight percent carbon and manganese in the banded region. Further testing revealed that the area containing the segregation and coarse grain structure had a lower than expected toughness and a transition temperature 90 deg F higher than specified by the ASTM standards. The fatigue crack growth rate through this area was much faster than expected. All of these property changes resulted from increased carbon levels, higher yield strength, and larger than normal grain size.

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, KID, is obviously lower than static fracture toughness, KIC, and has the same trend as KIC. The ratio of KID/sigma YD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.

A fracture mechanics based failure analysis and life prediction of a large centrifugal fan made from low-carbon, medium-strength steel was undertaken following shortcomings in attempts to explain its fatigue life from start stop cycles alone. Measurements of the fracture toughness and flaw size at failure, coupled with quantitative SEM fractography using striation spacing methods, revealed that the cyclic stress amplitudes just prior to failure were much larger than expected, in this particular case. Subsequent improvements in fan design and fabrication have effectively alleviated the problem of slow, high cycle fatigue crack growth, at normal operating stresses in similar fans.

A piece of wheel flange separated from the main landing gear wheel of a C130 aircraft as it taxied on a runway. The wheel was a 2014-T61 aluminum alloy forging and had been in service nearly 20 years. Fractographic evidence indicated that the initial crack growth was caused by high-cycle fatigue. The crack grew to approximately 8 in. in length before final catastrophic fracture. Fatigue analyses accurately predicted the cyclic life demonstrated by the failed wheel since its last inspection, assuming an initial crack length of 13 to 25 mm (0.5 to 1.0 in.). It was recommended that the inspection interval be reduced to one-third of its original duration for the current level of inspection reliability, or that inspection procedures be improved in order that cracks substantially smaller than 13 mm (0.5 in.) can be reliably detected.

This article commences with a summary of fatigue processes and mechanisms. It focuses on fractography of fatigue. Characteristic fatigue fracture features that can be discerned visually or under low magnification are described. Typical microscopic features observed on structural metals are presented subsequently, followed by a brief discussion of fatigue in nonmetals. The article reviews the various macroscopic and microscopic features to characterize the history and growth rate of fatigue in metals. It concludes with a description of fatigue of polymers and composites.

Fatigue failure of engineering components and structures results from progressive fracture caused by cyclic or fluctuating loads. Fatigue is an important potential cause of mechanical failure, because most engineering components or structures are or can be subjected to cyclic loads during their lifetime. This article focuses on fractography of fatigue. It provides an abbreviated summary of fatigue processes and mechanisms: fatigue crack initiation, fatigue crack propagation, and final fracture,. Characteristic fatigue fracture features that can be discerned visually or under low magnification are then described. Typical microscopic features observed on structural metals are presented subsequently, followed by a brief discussion on fatigue in polymers and polymer-matrix composites.

Two blade-detachment failures in large (600 kW) wind turbine generators were investigated. In the first case, bolt failures were established as the initial failure event. A fatigue crack reached a critical length, fast fracture developed and was then arrested as the bolt unloaded. Crack growth resumed when loading increased with cracking or fracture of adjacent bolts. The problem was identified as one of insufficient preload on the bolts. In the second failure on a different unit, a retaining nut on a blade assembly split, allowing a roller bearing to slide off a shaft and a blade to separate at its attachment hub. The failure was observed to be by fatigue. It was determined that pieces of the outer retaining rib (or flange) on the bearing inner cage had fractured by fatigue and were trapped between the nut and the bearing, producing excessive cyclic loading on the nut by a wedging action as the blade pitch adjusted during a revolution. Fatigue of the rim occurred as a result of inadequate lubrication in the bearing, which led to load transfer across the rollers, onto the rim.

Several compressor disks in military fighter and trainer aircraft gas turbine engines cracked prematurely in the bolt hole regions. The disks were made of precipitation-hardened AM355 martensitic stainless steel. Experimental and analytical work was performed on specimens from the fifth-stage compressor disk (judged to be the most crack-prone disk in the compressor) to determine the cause of the failures. Failure was attributed to high-strain low-cycle fatigue during service. It was also determined that the cyclic engine usage assumed in the original life calculations had been under estimated, which led to low-cycle fatigue cracking earlier than expected. Fracture mechanics analysis of the disks was carried out to assess their damage tolerance and to predict safe inspection intervals.

Thermomechanical fatigue (TMF) refers to the process of fatigue damage under simultaneous changes in temperature and mechanical strain. This article reviews the process of TMF with a practical example of life assessment. It describes TMF damages caused due to two possible types of loading: in-phase and out-of-phase cycling. The article illustrates the ways in which damage can interact at high and low temperatures and the development of microstructurally based models in parametric form. It presents a case study of the prediction of residual life in a turbine casing of a ship through stress analysis and fracture mechanics analyses of the casing.

Thermomechanical fatigue (TMF) is the general term given to the material damage accumulation process that occurs with simultaneous changes in temperature and mechanical loading. TMF may couple cyclic inelastic deformation accumulation, temperature-assisted diffusion within the material, temperature-assisted grain-boundary evolution, and temperature-driven surface oxidation, among other things. This article discusses some of the major aspects and challenges of dealing with TMF life prediction. It describes the damage mechanisms of TMF and covers various experimental techniques to promote TMF damage mechanisms and elucidate mechanism coupling interactions. In addition, life modeling in TMF conditions and a practical application of TMF life prediction are presented.