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.

Turbine buckets in a 37.5-MW gas turbine made of Udimet 500 superalloy failed in service. The power plant was located 1 km (0.6 miles) from the Pacific Ocean and operated on No. 2 diesel fuel, which was supplied by tanker ship. Turbine bucket failures occurred on three units after 2500 to 6400 h of operation. Investigation (visual inspection, metallographic examination, and stress analysis) supported the conclusion that the differing microstructure of the airfoil resulted in changes in mechanical properties. Because normal operation includes cycling of loads and temperatures, the shroud tip fractured due to thermomechanical fatigue in its degraded state. Recommendations included special chromium or silicon-rich coating to minimize corrosion in gas turbines operating in a marine environment with operating temperatures in the range of type 2 corrosion (650 to 750 deg C, or 1200 to 1380 deg F). Additionally, it was suggested that fuel delivery, handling, and treatment be high quality, to maintain fuel contamination within design limits, and inlet air filtration must be designed for the coastal site. Also, changing the bucket tip by increasing its thickness and changing the casting technique would reduce the stress and make the design more tolerant of corrosion.

Type 347 stainless steel moderator circuit branch piping in a pressurized hot water reactor was experiencing frequent leakage. Investigation of the problem involved failure analysis of leaking pipe specimens, analytical stress analysis, and determination of “leak-before-break” conditions using fracture mechanics and thermal fatigue simulation tests. Failure analysis indicated that cracking had been initiated by thermal fatigue. Data from the analysis were used in making the leak-before-break predictions. It was determined that the cracks could grow to two-thirds of the circumferential length of the pipe without catastrophic failure. A thin stainless steel sleeve was inserted in the branch pipe to resolve the problem.

A turbine vane made of cast cobalt-base alloy AMS 5382 (Stellite 31; composition: Co-25.5Cr-10.5Ni-7.5W) was returned from service after an undetermined number of service hours because of crack indications on the airfoil sections. This alloy is cast by the precision investment method. Analysis (visual inspection, 100x/500x metallographic examination of sections etched with a mixture of ferric chloride, hydrochloric acid, and methanol, and bend tests) supported the conclusions that cracking of the airfoil sections was caused by thermal fatigue and was contributed to by low ductility due to age hardening, subsurface oxidation related to intragranular carbides, and high residual tensile macrostresses. No further conclusions could be drawn because of the lack of detailed service history, and no recommendations were made.

A 2–3 mm thick electroformed nickel mold showed early cracking under thermal load cycles. To determine the root cause, investigators obtained monotonic and cyclic properties of electroformed nickel at various temperatures and identified possible fatigue mechanisms. With the help of finite element modeling, they analyzed the material as well as the design and in-service application of the mold. They discovered that overconstraining the mold, while it was in service, caused excessive thermal stresses which accelerated crack initiation and propagation. Investigators also proposed remedies to prevent additional failures.

During disassembly of an engine that was to be modified, a fractured turbine blade was found. When the fracture was examined at low magnification, it was observed that a fatigue fracture had originated on the concave side of the leading edge and had progressed slightly more than halfway from the leading edge to the trailing edge on the concave surface before ultimate failure occurred in dynamic tension. Analysis (including visual inspection, SEM, and 250x/500x micrographic examination) supported the conclusions that the blades failed due to thermal fatigue. Recommendations included application of a protective coating to the blades, provided the coating was sufficiently ductile to avoid cracking during operation to prevent surface oxidation. Such a coating would also alleviate thermal differentials, provided the thermal conductivity of the coating exceeded that of the base metal. It was also determined that directionally solidified blades could minimize thermal fatigue cracking by eliminating intersection of grain boundaries with the surface. However, this improvement would be more costly than applying a protective coating.

This article presents a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis was carried out on elements thought to have influenced the failure process: a) the stress increase from the loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale), b) influence of the tip shroud microstructure (e.g., a presence of equiaxed and columnar grains, their distribution and orientation), c) evidence of the transgranular initiation, and d) intergranular creep mechanism propagation. The most probable cause of the bucket damage was the combination of increased stresses due to corrosion-induced thinning of the tip shroud and unfavorable microstructures in the tip shroud region.

Several failures occurred in 64-mm schedule 80 type 304 stainless steel (ASME SA-312, grade TP304) piping in a steam-plant heat-exchanger system near tee fittings at which cool water returning from the heat exchanger was combined with hot water from a bypass. Various portions of the piping were subjected to temperatures ranging from 29 to 288 deg C. Each of the failures were revealed to consist of transgranular cracking in and/or close to the circumferential butt weld joining the tee fitting to the downstream pipe leg, where the hot bypass water mixed with the cool return water. The transgranular cracks suggested that thermal fatigue was a more likely cause of failure than SCC. It was concluded by temperature measurements that circumferential temperature gradients, in combination with inadequate flexibility in the piping system as a whole, had caused the failures. The tee fitting was redesigned to alleviate the thermal stress pattern.

An experimental high-temperature rotary valve was found stuck due to growth and distortion after approximately 100 h. Gas temperatures were suspected to have been high due to overfueled conditions. Both the rotor and housing in which it was stuck were annealed ferritic ductile iron similar to ASTM A395. Visual examination of the rotor revealed unusually heavy oxidation and thermal fatigue cracking along the edge of the gas passage. Material properties, including microstructure, composition, and hardness, of both the rotor and housing were evaluated to determine the cause of failure. The microstructure of the rotor was examined in three regions. The shaft material, the heavy section next to the gas passage and the thin edge of the rotor adjacent to the gas passage. The excessive gas temperatures were responsible for the expansion and distortion that prevented rotation of the rotor. Actual operating temperatures exceeded those intended for this application. The presence of transformation products in the brake-rotor edge indicated that the lower critical temperature had been exceeded during operation.

After some 87,000 h of operation, failure took place in the bend of a steam pipe connecting a coil of the third superheater of a steam generator to the outlet steam collector. The unit operated at 538 deg C and 135 kPa, producing 400 t/h of steam. The 2.25Cr-1Mo steel pipe in which failure took place was 50.8 mm in diam with a nominal wall thickness of 8 mm. It connected to the AISI 321 superheater tube by means of a butt weld and was one of 46 such parallel connecting tubes. The Cr-Mo tubing was situated outside the heat transfer zone of the superheater. The overall sequence of failure involved overheating of the Cr-Mo outlet tubes, heavy oxidation, oxide cracking on thermal cycling, thermal fatigue cracking plus oxidation, creep-controlled crack growth, and rapid plastic deformation and rupture. This failure was indicative of excess temperature of the steam coming from the heat transfer zone of the coil. It showed that many damage mechanisms may combine in the transition from fracture initiation to final failure. The presence of grain boundary sliding as an indication of creep damage was useful in the characterization of the stress level as high and showed that the process of creep was not operative throughout the life of the equipment.

Following leakage which developed within the furnace of a horizontal multi-tubular type boiler, examination revealed a series of cracks adjacent to the stiffening rings in the first plain furnace ring. The fire-side surface of the sample was coated with a layer of oxide scale. Microscopical examination of sections through the cracks showed them to be filled with oxide and to be of the multi-branched type, having blunt terminations. The general nature of the cracks was characteristic of cracking from thermal or corrosion fatigue, as results from the operation of varying stresses in an oxidizing or corrosive environment. The cracking in this particular case was due principally to the inordinately large gap between the components. Additionally, several of the sealing welds of the tubes to the back tube plate were cracked in a radial manner, and it would appear that in addition, abnormal thermal conditions may well have been experienced intermittently in service.

Cracks on the outer surface near a hanger lug were revealed by visual inspection of a type 316 stainless steel main steam line of a major utility boiler system. Cracking was found to have initiated at the outside of the pipe wall or immediately beneath the surface. The microstructure of the failed pipe was found to consist of a matrix precipitate array (M23C6) and large s-phase particles in the grain boundaries. A portable grinding tool was used to prepare the surface and followed by swab etching. All material upstream of the boiler stop valve was revealed to have oriented the cracking normally or nearly so to the main hoop stress direction. Residual-stress measurements were made using a hole-drilling technique and strain gage rosettes. Large tensile axial residual stresses were measured at nearly every location investigated with a large residual hoop stress was found for locations before the stop valve. It was concluded using thermal stress analysis done using numerical methods and software identified as CREPLACYL that one or more severe thermal downshocks might cause the damage pattern that was found. The root cause of the failure was identified to be thermal fatigue, with associated creep relaxation.

An ASTM A 504 carbon steel railway car wheel that was used on a train in a metropolitan railway system failed during service, causing derailment. The wheel was completely fractured from rim to hub. Macrofractography of the fracture surface showed road grime, indicating that the crack had existed for a considerable time prior to derailment and initiated in the flange. Failure propagated from the flange across the rim and down the plate to the bore of the hub. Two zones that exhibited definite signs of heating were observed. The fracture initiation site was typical of fatigue fracture. No defects were found that could have contributed to failure. The wheel conformed to the chemical, microstructural, and hardness requirements for class A wheels. Failure was attributed to repeated severe heating and cooling of the rim and flange due to brake locking or misapplication of the hand brake. It was recommended that the brake system on the car be examined and replaced if necessary.

To determine the effect of severe service on cast 357 aluminum pistons, a metallurgical evaluation was made of four pistons removed from the engine of the Hawk-Offenhauser car which had been driven by Rich Muther in the first Ontario, California 500 race. The pistons were studied by visual inspection, hardness traverses, radiography, dye penetrant inspection, chemical analysis, macrometallography, optical microscopy, and electron microscopy. The crown of one piston had a rough, crumbly deposit, which was detachable with a knife. Two pistons had remains of carbonaceous deposits. The fourth was severely hammered. It was concluded that the high temperatures developed in this engine created an environment too severe for 357 aluminum. Surfaces were so hot that the low-melting constituent melted. Then, the alloy oxidized rapidly to form Al2O3, an abrasive which further aggravated problems. The temperature in much of the piston was high enough to cause softening by overaging, lowering strength.

This article reviews the applied aspects of creep and stress-rupture failures. It discusses the microstructural changes and bulk mechanical behavior of classical and nonclassical creep behavior. The article provides a description of microstructural changes and damage from creep deformation, including stress-rupture fractures. It also describes metallurgical instabilities, such as aging and carbide reactions, and evaluates the complex effects of creep-fatigue interaction. The article concludes with a discussion on thermal fatigue and creep fatigue failures.

A failure analysis was conducted on a flow-sensing device that had cracked while in service. The polysulfone sensor body cracked radially, adjacent to a molded-in steel insert. This article describes the investigative methods used to conduct the failure analysis. The techniques utilized included scanning electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermomechanical analysis, and melt flow rate determination. It was the conclusion of the investigation that the part failed via brittle fracture, with evidence also indicating low cycle fatigue associated with cyclic temperature changes from normal service. The design of the part and the material selection were significant contributing factors because of stresses induced during molding, physical aging of the amorphous polysulfone resin, and the substantial differential in coefficients of thermal expansion between the polysulfone and the mating steel insert.