This article describes three design-life methods or philosophies of fatigue, namely, infinite-life, finite-life, and damage tolerant. It outlines the three stages in the process of fatigue fracture: the initial fatigue damage leading to crack initiation, progressive cyclic growth of crack, and the sudden fracture of the remaining cross section. The article discusses the effects of loading and stress distribution on fatigue cracks, and reviews the fatigue behavior of materials when subjected to different loading conditions such as bending and loading. The article examines the effects of load frequency and temperature, material condition, and manufacturing practices on fatigue strength. It provides information on subsurface discontinuities, including gas porosity, inclusions, and internal bursts as well as on corrosion fatigue testing to measure rates of fatigue-crack propagation in different environments. The article concludes with a discussion on rolling-contact fatigue, macropitting, micropitting, and subcase fatigue.

Failure occurred by fatigue cracking of links from chains which were used to replace the ropes on grabs of the multirope type. In the first example, the links were made from high tensile steel rod. The fracture in the side of the link was duplex in appearance one half of the surface being discolored, indicative of a preexisting crack of the fatigue type, whilst the remaining portion was brightly crystalline, resulting from brittle fracture at the time of the mishap. In the second example, the fracture took place at a similar location adjacent to one of the butt welds situated at the mid-length of the sides. Brinell hardness values confirmed that the link was made from the higher tensile grade of material. The cracks were due to fatigue, there being no indications that the weld was initially defective.

Crane collapse due to bolt fatigue and fatigue failure of a crane support column, crane tower, overhead yard crane, hoist rope, and overhead crane drive shaft are described. The first four examples relate to the structural integrity of cranes. However, equipment such as drive and hoist-train components are often subject to severe fatigue loading and are perhaps even more prone to fatigue failure. In all instances, the presence of fatigue cracks at least contributed to the failure. In most instances, fatigue was the sole cause. Further, in each case, with regular inspection, fatigue cracks probably would have been detected well before final failure.

Fatigue failures may occur in components subjected to fluctuating (time-dependent) loading as a result of progressive localized permanent damage described by the stages of crack initiation, cyclic crack propagation, and subsequent final fracture after a given number of load fluctuations. This article begins with an overview of fatigue properties and design life. This is followed by a description of the two approaches to fatigue, namely infinite-life criterion and finite-life criterion, along with information on damage tolerance criterion. The article then discusses the characteristics of fatigue fractures followed by a discussion on the effects of loading and stress distribution, and material condition on the microstructure of the material. In addition, general prevention and characteristics of corrosion fatigue, contact fatigue, and thermal fatigue are also presented.

JIS SCM435 steel bolts that connected the slewing ring to the base carrier on a truck crane failed during the lifting of steel piles. The bolts were double-ended stud types and had been in operation for 5600 h. Failure occurred in the root of the external thread that was in contact with the first internal thread in the slewing ring. Examination of plastic carbon replicas indicated that failure was the result of fatigue action. Failure was attributed to overloading during service and increased stress concentration on a few bolts due to nonuniform separations around the slewing ring. A design change to achieve equal separation between bolt holes was recommended.

Several crankshaft failures occurred in equipment that was being used in logging operations in subzero temperatures. Failure usually initiated at a cracked pin oil hole, and the failure origin was approximately 7.6 mm (0.3 in.) from the shaft surface. The holes were produced by gun drilling, giving rise to surface defects. The fracture surface was characteristic of fatigue in that it was flat, relatively shiny, and exhibited beach marks. The crack surface was at a 45 deg angle to the axis of the shaft, indicating dominant tensile stresses. The material was the French designation AFNOR 38CD4 (similar to AISI type 4140H) and was in the quenched-and-tempered condition, with a yield strength of about 760 MPa (110 ksi). It was treated to have compressive surface stresses, and the prior-austenite grain size was ASTM 8. Analysis (visual inspection, stress analyses, and macrographs) supported the conclusion that failure was caused by fatigue stress caused by surface defects in the oil holes. Recommendation includes drilling the oil holes by a technique that essentially eliminates surface defects.

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.

This paper reviews several fatigue failures from the waterwall, superheater, and economizer portions of the boiler, their causes and how they were mitigated and monitored. Some cases required simple field modifications by cutting or welding, repair of existing controls, and/or changes in maintenance. Nondestructive inspections by visual, magnetic particle, ultrasonic, and radiographic methods for detecting and monitoring damage are discussed. These failures are presented to provide hindsight that will help others in increasing the success rate for anticipating and analyzing the remaining life of other units.

An intermediate shaft (3 in. diam), part of a camshaft drive on a large diesel engine, broke after two weeks of service. Failure occurred at the end of the taper portion adjacent to the screwed thread. The irregular saw-tooth form of fracture was characteristic of failure from torsional fatigue. A second shaft carried as spare gear was fitted and failure took place in a similar manner in about the same period of time. Examination revealed that the tapered portion of the Fe-0.6C carbon steel shaft had been built up by welding prior to final machining. A detailed check by the engine-builder established that the manufacture of these two shafts had been subcontracted. It was ascertained that the taper portions had been machined to an incorrect angle and then subsequently built-up and remachined to the correct taper. The reduction in fatigue endurance following welding was due to heat-affected zone cracking, residual stresses, the lower fatigue strength of the weld deposited metal, and weld defects.

Fundamentals of fatigue failure are outlined. Addressed are fatigue crack characteristics, basic crack types, unidirectional bending, alternate bending, rotary bending, torsion, direct stress, and combined stress. Stress cycle, endurance limits, under and overstressing, stress concentration, and surface condition are discussed. Sections are devoted to fatigue crack assessment, corrosion relation to fatigue failure, and the micro-mechanisms of fatigue failure. Materials considered include steels. Photographs of service failures are used to illustrate features alluded to in the text.

Nuclear power plants typically experience two or three high-cycle fatigue failures of stainless steel socket-welded connections in small bore piping during each plant-year of operation. This paper discusses fatigue-induced failure in socket-welded joints and the strategy Texas Utilities Electric Company (TU Electric) has implemented in response to these failures. High-cycle fatigue is invisible to proven commercial nondestructive evaluation (NDE) methods during crack initiation and the initial phases of crack growth. Under a constant applied stress, cracks grow at accelerating rates, which means cracks extend from a detectable size to a through-wall crack in a relatively short time. When fatigue cracks grow large enough to be visible to NDE, it is likely that the component is near the end of its useful life. TU Electric has determined that an inspection program designed to detect a crack prior to the component leaking would involve frequent inspections at a given location and that the cost of the inspection program would far exceed the benefits of avoiding a leak. Instead, TU Electric locates these cracks by visually monitoring for leaks. Field experience with fatigue-induced cracks in socket-welded joints has confirmed that visual monitoring does detect cracks in a timely manner, that these cracks do not result in catastrophic failures, and that the plant can be safely shut down in spite of a leaking socket-welded joint in a small bore pipe. Historical data from TU Electric and Southwest Research Institute are presented regarding the frequency of failures, failure locations, and the potential causes. The topics addressed include 1) metallurgical and fractographic features of fatigue cracks at the weld toe and weld root; 2) factors that are associated with fatigue, such as mechanical vibration, internal pulsation, joint design, and welding workmanship; and 3) implications of a leaking crack on plant safety. TU Electric has implemented the use of modified welding techniques for the fabrication of socket-welded joints that are expected to improve their ability to tolerate fatigue.

Failure of structural polymeric materials under cyclic application of stress or strain is a subject of industrial importance. The understanding of fatigue mechanisms (damage) and the development of constitutive equations for damage evolution, leading to crack initiation and propagation as a function of loading or displacement history, represent a fundamental problem for scientists and engineers. This article describes the approaches to predict fatigue life and discusses the difference between thermal and mechanical fatigue failure of polymers.

The drive shaft in a marine propulsion system broke, stranding a large vessel along the Canadian seacoast. The shaft was made from quenched and tempered low-alloy steel. Fractographic investigation revealed that the shaft failed under low rotating-bending variable stress. Fatigue propagation occurred on about 95% of the total cross section of the shaft, under both low-cycle and high-cycle fatigue mechanisms. It was found that the fillet radius at the fracture’s origin was smaller than the one provisioned by design. As a result, the stresses at this location exceeded the values used in the design calculations, thus causing the initiation of the cracking. Moreover, although the shaft had been quenched and tempered, its actual hardness did not have the optimal value for long-term fatigue strength.

A gas-fired, ASTM A-106 Grade B carbon steel vaporizer failed on three different occasions during attempts to bring the vaporizer on line. Dye penetrant examination indicated the presence of multiple packets of ductile cracks on the inside of the coil radius at the bottom of the horizontal axis coils. Visual examination of the inside of the tubing indicated the presence of a carbonaceous deposit resulting from decomposition of the heat-exchanging fluid. Subsequent metallographic examination and microhardness testing indicated that the steel was heated to a temperature above the allowable operating temperature for the fluid. The probable cause for failure is thermal fatigue due to the localized overheating. Flow conditions inside the tubing should be reexamined to ensure suitable conditions for annular fluid flow.

An axle shaft in an open-pit mining truck hauling overburden failed after operating for 27,000 h. Previous failures had resulted from longitudinal shear, but this had not, bringing material quality into question. Chemical analysis verified that the part was SAE4340 Ni-Cr-Mo alloy steel and thus met material specification. The failure was a result of torsional fatigue in the tensile plane, originating from one of several gouges around the splined radius of the shaft. The fatigue crack progressed for a large number of cycles before final fracture. The shaft met metallurgical requirements and should have withstood normal operating conditions. The spacing of the gouge marks coincided with the spacing of the splines, indicative of careless assembly with the mating wheel gear.

The fracture of a clevis which formed part of the derricking gear of a large crane showed well developed conchoidal markings. These were associated with a principal origin on the outer surface at about the mid-width of the section. A number of secondary origins were apparent along this same edge. Failure was initiated at the extrados which suggested that a discrepancy in the size of the pin may have contributed to failure. Microscopic examination of a section through the main origin did not reveal any material defects or the presence of weld repairs which could have led to the premature failure. Furthermore, there were no indications that corrosion had contributed to the fatigue cracking.

In a shaft subjected to reversed torsional stresses, failure resulted from the gradual development of fatigue cracks from opposite sides of the shaft. These broke out from origins located adjacent to the fillets at the start of the square section. The remaining uncracked material which fractured at the time of the mishap was in the form of a narrow strip, situated slightly to one side of the center of the shaft. The material was a mild steel in the normalized or annealed condition, having a carbon content of approximately 0.3%. The cracking was characteristic of that resulting from torsional fatigue. Because it occurred on two different planes at 45 deg to the axis of the shaft it was due to reversals of torsional stress rather than fluctuations of unidirectional torque. Following this failure, the shafts of six other similar cranes were tested ultrasonically. Cracks to varying degree were found in all the shafts. Timely replacement was possible and the likelihood of serious accidents removed.

A bull gear from a coal pulverizer at a utility failed by rolling-contact fatigue as the result of continual overloading of the gear and a nonuniform, case-hardened surface of the gear teeth. The gear consisted of an AISI 4140 Cr-Mo steel gear ring that was shrunk fit and pinned onto a cast iron hub. The wear and pitting pattern in the addendum area of the gear teeth indicated that either the gear or pinion was out of alignment. Beach marks observed on the fractured surface of the gear indicated that fatigue was the cause of the gear failure. Similar gears should be inspected carefully for signs of cracking or misalignment. Ultrasonic testing is recommended for detection of subsurface cracks, while magnetic particle testing will detect surface cracking. Visual inspection can be used to determine the teeth contact pattern.

An AMS 4126 (7075-T6) aluminum alloy impeller from a radial inflow turbine fractured during commissioning. Initial examination showed that two adjacent vanes had fractured through airfoils in the vicinity of the vane leading edges, and one vane fractured through an airfoil near the hub in the vicinity of the vane trailing edge. Some remaining vanes exhibited radial and transverse cracks in similar locations. Binocular and scanning electron microscope examinations showed that the cracks had been caused by high-cycle fatigue and had progressed from multiple origins on the vane surface. Structural analysis indicated that the fatigue loading probably had been caused by forced excitation, resulting in the impeller vibrating at its resonant frequency. It was recommended that the impeller design, control systems, and material of construction be changed.

A medium-carbon helical spring was installed in a machine assembly that was welded into its final location. Weld spatter was not prevented from landing on the wire surface by any shield. An elongated drop and two tiny droplets of metal were observed a short distance from the fracture. No droplets were revealed at the origin of the fracture, but it was assumed that a drop of molten metal landed at the origin. Adherence of the spatter drop was expected to have been affected by the opening and closing of the fatigue crack. Weld spatter bead was concluded to have caused the fatigue fracture.