Two shafts that transmit power from the engine to the propeller of a container ship failed after a short time in service. The shafts usually have a 25 year lifetime, but the two in question failed after only a few years. One of the shafts, which carries power from a gearbox to the propeller, is made of low alloy steel. The other shaft, part of a clutch mechanism that regulates the transmission of power from the engine to the gears, is made of carbon steel. Fracture surface examination of the gear shaft revealed circumferential ratchet marks with the presence of inward progressive beach marks, suggesting rotary-bending fatigue. The fracture surfaces on the clutch shaft exhibited a star-shaped pattern, suggesting that the failure was due to torsional overload which may have initiated at corrosion pits discovered during the examination. Based on the observations, it was concluded that rotational bending stresses caused the gear shaft to fail due to insufficient fatigue strength. This led to the torsional failure of the corroded clutch shaft, which was subjected to a sudden, high level load when the shaft connecting the gearbox to the propeller failed.

The shaft of an exciter that was used with a diesel-driven electric generator broke at a fillet after ten hours of service following resurfacing of the shaft by welding. The fracture surface contained a dull off-center region of final ductile fracture surrounded by regions of fatigue that had been subjected to appreciable rubbing. The fracture appeared to be typical of rotary bending fatigue under conditions of a low nominal stress with a severe stress concentration. It appeared that the fatigue cracks initiated in the surface-weld layer. The weld deposit in the original keyway displays a lack of fusion at the bottom corner. Fatigue fracture of the shaft resulted from stresses that were created by vibration acting on a crack or cracks formed in the weld deposit because of the lack of preheating and postheating. Rebuilding of exciter shafts should be discontinued, and the support plate of the exciter should be braced to reduce the amount of transmitted vibration. Also, the fillet in the exciter shaft should be carefully machined to provide an adequate radius.

A copper alloy C51000 (phosphor bronze, 5%A) failed prematurely during life testing of several such springs. The wire used for the springs was 0.46 mm (0.018 in.) in diam and was in the spring-temper condition. The springs were revealed to be subjected to cyclic loading, in the horizontal and vertical planes during the testing. The fracture was revealed to have occurred in bend 2. An indentation, presumably caused by the bending tool during forming, at the inner surface of the bend where fracture occurred was revealed by microscopic examination. Spiral marks produced on springs during rotary straightening were observed. A crack that had originated at the surface at the inside bend and had propagated toward the outside of the bend was revealed by microscopy of a longitudinal section taken through bend 2. The small bend radius was interpreted to contribute to spring fatigue as a result of result in straining at the bend zone. The spring was concluded to have failed in fatigue. It was recommended that the springs should be made of wire free from straightener marks and the bending tool should be redesigned so as not to indent the wire.

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.

This article discusses failures in shafts such as connecting rods, which translate rotary motion to linear motion, and in piston rods, which translate the action of fluid power to linear motion. It describes the process of examining a failed shaft to guide the direction of failure investigation and corrective action. Fatigue failures in shafts, such as bending fatigue, torsional fatigue, contact fatigue, and axial fatigue, are reviewed. The article provides information on the brittle fracture, ductile fracture, distortion, and corrosion of shafts. Abrasive wear and adhesive wear of metal parts are also discussed. The article concludes with a discussion on the influence of metallurgical factors and fabrication practices on the fatigue properties of materials, as well as the effects of surface coatings.

In addition to failures in shafts, this article discusses failures in connecting rods, which translate rotary motion to linear motion (and conversely), and in piston rods, which translate the action of fluid power to linear motion. It begins by discussing the origins of fracture. Next, the article describes the background information about the shaft used for examination. Then, it focuses on various failures in shafts, namely bending fatigue, torsional fatigue, axial fatigue, contact fatigue, wear, brittle fracture, and ductile fracture. Further, the article discusses the effects of distortion and corrosion on shafts. Finally, it discusses the types of stress raisers and the influence of changes in shaft diameter.

A service water pump in a nuclear reactor failed when its shaft gave way. The fracture originated in the threaded portion of the sleeve nut on the drive-end side of the shaft. Results of the failure analysis showed that the cracking initiated at the thread root as a result of corrosion fatigue. Crack propagation occurred either by corrosion or mechanical fatigue. Evidence was found indicating high rotary bending stresses on the shaft during operation. The nonstandard composition of the En 8 steel used in the shaft and irregular maintenance reduced the life of the shaft. Recommendations included use of a case-hardened En 8 steel with the correct composition and regular maintenance of the pump.

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.

The main shaft in a locomotive turbocharger fractured along with an associated bearing sleeve. Visual and fractographic examination revealed that the shaft fractured at a sharp-edged groove between two journals of different cross-sectional area. The dominant failure mechanism was low-cycle rotation-bending fatigue. The bearing sleeve failed as a result of abrasive and adhesive wear. Detailed metallurgical analysis indicated that the sleeve and its respective journal had been subjected to abnormally high temperatures, increasing the amount of friction between the sleeve, bearing bush, and journal surface. The excessive heat also softened the induction-hardened case on the journal surface, decreasing its fatigue strength. Fatigue crack initiation occurred at the root fillet of the groove because of stress concentration.

This article discusses the common causes of failures of springs, with illustrations. Design deficiencies, material defects, processing errors or deficiencies, and unusual operating conditions are the common causes of spring failures. In most cases, these causes result in failure by fatigue. The article describes the operating conditions of springs, common failure mechanisms, and presents an examination of the failures that occur in springs.

This article focuses on the mechanisms and common causes of failure of metal components in lifting equipment in the following three categories: cranes and bridges, particularly those for outdoor and other low-temperature service; attachments used for direct lifting, such as hooks, chains, wire rope, slings, beams, bales, and trunnions; and built-in members such as shafts, gears, and drums.

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.

Gears can fail in many different ways, and except for an increase in noise level and vibration, there is often no indication of difficulty until total failure occurs. This article reviews the major types of gears and the basic principles of gear-tooth contact. It discusses the loading conditions and stresses that effect gear strength and durability. The article provides information on different gear materials, the common types and causes of gear failures, and the procedures employed to analyze them. Finally, it presents a chosen few examples to illustrate a systematic approach to the failure examination.

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.

This article first reviews variations within the most common types of gears, namely spur, helical, worm, and straight and spiral bevel. It then provides information on gear tooth contact and gear metallurgy. This is followed by sections describing the important points of gear lubrication, the measurement of the backlash, and the necessary factors for starting the failure analysis. Next, the article explains various gear failure causes, including wear, scuffing, Hertzian fatigue, cracking, fracture, and bending fatigue, and finally presents examples of gear and reducer failure analysis.

Distortion often is observed in the analysis of other types of failures, and consideration of the distortion can be an important part of the analysis. This article first considers that true distortion occurs when it was unexpected and in which the distortion is associated with a functional failure. Then, a more general consideration of distortion in failure analysis is introduced. Several common aspects of failure by distortion are discussed and suitable examples of distortion failures are presented for illustration. The article provides information on methods to compute load limits, errors in the specification of the material, and faulty process and their corrective measures to meet specifications. It discusses the general process of material failure analysis and special types of distortion and deformation failure.

Distortion failure occurs when a structure or component is deformed so that it can no longer support the load it was intended to carry. Every structure has a load limit beyond which it is considered unsafe or unreliable. Estimation of load limits is an important aspect of design and is commonly computed by classical design or limit analysis. This article discusses the common aspects of failure by distortion with suitable examples. Analysis of a distortion failure often must be thorough and rigorous to determine the root cause of failure and to specify proper corrective action. The article summarizes the general process of distortion failure analysis. It also discusses three types of distortion failures that provide useful insights into the problems of analyzing unusual mechanisms of distortion. These include elastic distortion, ratcheting, and inelastic cyclic buckling.

Heat exchangers are devices used to transfer thermal energy between two or more fluids, between a solid surface and a fluid, or between a solid particulate and a fluid at different temperatures. This article first addresses the causes of failures in heat exchangers. It then provides a description of heat-transfer surface area, discussing the design of the tubular heat exchanger. Next, the article discusses the processes involved in the examination of failed parts. Finally, it describes the most important types of corrosion, including uniform, galvanic, pitting, stress, and erosion corrosion.

Erosion occurs as the result of a number of different mechanisms, depending on the composition, size, and shape of the eroding particles; their velocity and angle of impact; and the composition of the surface being eroded. This article describes the erosion of ductile and brittle materials with the aid of models and equations. It presents three examples of erosive wear failures, namely, abrasive erosion, erosion-corrosion, and cavitation erosion.

Rolling-element bearings use rolling elements interposed between two raceways, and relative motion is permitted by the rotation of these elements. This article presents an overview of bearing materials, bearing-load ratings, and an examination of failed bearings. Rolling-element bearings are designed on the principle of rolling contact rather than sliding contact; frictional effects, although low, are not negligible, and lubrication is essential. The article lists the typical characteristics and causes of several types of failures. It describes failure by wear, failure by fretting, failure by corrosion, failure by plastic flow, failure by rolling-contact fatigue, and failure by damage. The article discusses the effects of fabrication practices, heat treatment and hardness of bearing components, and lubrication of rolling-element bearings with a few examples.