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crack nucleation
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Series: ASM Handbook
Volume: 11
Publisher: ASM International
Published: 15 January 2021
DOI: 10.31399/asm.hb.v11.a0006775
EISBN: 978-1-62708-295-2
...Abstract Abstract This article focuses on characterizing the fracture-surface appearance at the microscale and contains some discussion on both crack nucleation and propagation mechanisms that cause the fracture appearance. It begins with a discussion on microscale models and mechanisms...
Abstract
This article focuses on characterizing the fracture-surface appearance at the microscale and contains some discussion on both crack nucleation and propagation mechanisms that cause the fracture appearance. It begins with a discussion on microscale models and mechanisms for deformation and fracture. Next, the mechanisms of void nucleation and void coalescence are briefly described. Macroscale and microscale appearances of ductile and brittle fracture are then discussed for various specimen geometries (smooth cylindrical and prismatic) and loading conditions (e.g., tension compression, bending, torsion). Finally, the factors influencing the appearance of a fracture surface and various imperfections or stress raisers are described, followed by a root-cause failure analysis case history to illustrate some of these fractography concepts.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.design.c9001582
EISBN: 978-1-62708-233-4
... rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation...
Abstract
This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200 to 250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.petrol.c9001603
EISBN: 978-1-62708-228-0
... and propagate along the through-thickness direction. The presence of extensive decarburization and formation of a soft ferrite band within the fusion zone may have contributed to the nucleation of the cracks. Crack propagation was aided by the presence of exogenous inclusions entrapped within the fusion zone...
Abstract
The genesis of failure of 6.1 mm thick electric resistance welded API 5L X-46 pipes during pretesting at a pressure equivalent to 90% of specified minimum yield strength was investigated. Cracks were found to initiate on the outer surface of the pipes in the fusion zone and propagate along the through-thickness direction. The presence of extensive decarburization and formation of a soft ferrite band within the fusion zone may have contributed to the nucleation of the cracks. Crack propagation was aided by the presence of exogenous inclusions entrapped within the fusion zone. Analysis of these inclusions confirmed the presence of Fe, Si, Ca, and O, indicating slag entrapment to be the most probable culprit.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.conag.c0047352
EISBN: 978-1-62708-221-1
... cracking is significant because the user advised that the liners were not backed in the installation. Cracking was present in the microstructures of both liners. These cracks tend to fracture the brittle carbide phase first; once nucleated, the sharp cracks can propagate and grow to critical dimensions...
Abstract
Two broken ball-mill liners from a copper-mine ore operation were submitted for failure analysis. These liners failed prematurely, having reached less than 20% of their expected life. The chemical composition of the liners was within specifications for high-chromium white cast iron. The two broken liners were sand blasted for visual inspection and subsequent metallography and hardness testing. Many cracks were found externally and on the undersides. There were also signs of mechanical damage that occurred inside the mill before detection of the failures. The underside cracking is significant because the user advised that the liners were not backed in the installation. Cracking was present in the microstructures of both liners. These cracks tend to fracture the brittle carbide phase first; once nucleated, the sharp cracks can propagate and grow to critical dimensions, which eventually induces complete failure to the load-bearing section. The premature failure of these liners was caused by severe localized overstress conditions due to localized impact in service. Proper backing of shell liners should be ensured to reduce the effect of impact forces in the ball mill.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.design.c0046155
EISBN: 978-1-62708-233-4
... was the primary cause of failure. Rubbing of the faying surfaces worked the interference area on the post until small tears developed. These small tears became stress-concentration points that nucleated fatigue cracks. Recommendations included rounding the edge of the box in the area of contact with the post...
Abstract
A structure had been undergoing fatigue testing for several months when a post-like member heat treated to a tensile strength of 1517 to 1655 MPa (220 to 240 ksi) ruptured. The fracture occurred in the fillet of the post that contacted the edge of a carry-through box bolted to the member. At failure, the part was receiving a second set of loads up to 103.6% of design load. Visual investigations showed rubbing and galling of the fillet. Microscopic and metallographic examination revealed beach marks on the fracture surface and evidence of cold work and secondary cracking in the rubbed and galled area. Electron fractography confirmed that cracking had initiated at a region of tearing and that the cracks had propagated by fatigue. Mechanical properties of all specimens exceeded the minimum values specified for the post. This evidence supports the conclusion that fatigue was the primary cause of failure. Rubbing of the faying surfaces worked the interference area on the post until small tears developed. These small tears became stress-concentration points that nucleated fatigue cracks. Recommendations included rounding the edge of the box in the area of contact with the post to ensure a tangency fit.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.aero.c9001538
EISBN: 978-1-62708-217-4
... micrograph revealed this area also displayed an intergranular texture. One of a group of small grinding cracks on the ID of the cylinder nucleated the failure. Other evidence indicated the cracks developed when the cylinder was ground during overhaul. Aircraft components Cylinders Grinding cracks...
Abstract
Examination of a cracked nose landing gear cylinder made of AISI 4340 Cr-Mo-Ni alloy steel proved that the part started to fail on the inside diam. When the nucleus of the stress-corrosion crack was studied in detail, iron oxide was found on the fracture surface. A 6500x micrograph revealed this area also displayed an intergranular texture. One of a group of small grinding cracks on the ID of the cylinder nucleated the failure. Other evidence indicated the cracks developed when the cylinder was ground during overhaul.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001337
EISBN: 978-1-62708-215-0
...Abstract Abstract A carbon-molybdenum (ASTM A209 Grade T1) steel superheater tube section in an 8.6 MPa (1250 psig) boiler cracked because of long-term overheating damage that resulted from prolonged exposure to metal temperatures between 482 deg C (900 deg F) and 551 deg C (1025 deg F...
Abstract
A carbon-molybdenum (ASTM A209 Grade T1) steel superheater tube section in an 8.6 MPa (1250 psig) boiler cracked because of long-term overheating damage that resulted from prolonged exposure to metal temperatures between 482 deg C (900 deg F) and 551 deg C (1025 deg F). The outer diameter of the tube exhibited a crack (fissure) oriented approximately 45 deg to the longitudinal axis and 3.8 cm (1.5 in.) long. The inner diameter surface showed a fissure in the same location and orientation. Microstructure at the failure near the outer diameter surface exhibited evidence of creep cracking and creep void formation at the fissure. A nearly continuous band of graphite nodules was observed on the surface of the fissure. In addition to the graphite band formation, the microstructure near the failure exhibited carbide spheroidization from long-term overheating in all the tube regions examined. It was concluded that preferential nucleations of graphite nodules in a series of bands weakened the steel locally, producing preferred fracture paths. Formation of these graphite bands probably expedited the creep failure of the tube. Future failures may be avoided by using low-alloy steels with chromium additions such as ASTM A213 Grade T11 or T22, which are resistant to graphitization damage.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.rail.c0046238
EISBN: 978-1-62708-231-0
... in operation. Visual examination revealed beach marks typical of fatigue cracks that had nucleated at the base of the longitudinal oil hole. Micrographs of sections revealed a remelt zone and an area of untempered martensite within the region of the cracks. However, review of inspection procedures disclosed...
Abstract
Within about one month, several knuckle pins (AMS 6470 steel failed, and required to have a minimum case hardness of 92 h15N, a case depth of 0.4 to 0.5 mm (0.017 to 0.022 in.), and a core hardness of 285 to 341 HRB) used in engines failed over a range of 218 to 463 h in operation. Visual examination revealed beach marks typical of fatigue cracks that had nucleated at the base of the longitudinal oil hole. Micrographs of sections revealed a remelt zone and an area of untempered martensite within the region of the cracks. However, review of inspection procedures disclosed the pins had been magnetic-particle inspected by inserting a probe into the longitudinal hole. Evidence found supports the conclusions that the knuckle pins failed by fatigue fracture. The circular cracks at the longitudinal holes were the result of improper technique in magnetic-particle inspection. Thermal transformation of the metal also causes a stress concentration that may lead to fatigue failure. Recommendations included insulating the conductor to prevent arc burning at the base of the longitudinal oil hole. Also, a borescope or metal monitor could be used to inspect the hole for evidence of arc burning from magnetic-particle inspection.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c9001534
EISBN: 978-1-62708-220-4
.... Diffusion of hydrogen to an inclusion site results in either decohesion of the inclusion/matrix interface or nucleation of a crack at the inclusion site. Because cracking by hydrogen embrittlement is diffusion controlled, it takes time (in days or months) for the cracking to appear. The maximum...
Abstract
A detailed failure analysis was conducted on an ammonia refrigerant condenser tube component that failed catastrophically during its initial hours of operation. Evidence collected clearly demonstrated that the weld between a pipe and a dished end contained a sharp unfused region at its root (lack of penetration). Component failure had started from this weld defect. The hydrogen absorbed during welding facilitated crack initiation from this weld defect during storage of the component after welding. Poor weld toughness at the low operating temperature facilitated crack growth during startup, culminating in catastrophic failure as soon as the crack exceeded critical length.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c9001613
EISBN: 978-1-62708-235-8
...” seen in Fig. 3 . This type of nucleation is typical for H 2 -induced cracking/blistering in electroplated carbon and low alloy steels. Observation of the fracture surface at high magnification near the ID clearly demonstrates the transition in the fracture mode ( Fig. 4 ). The region of brittle...
Abstract
The electroplated tappet adjusting screws used in diesel engines failed during initial bend testing. The analysis of the failure showed that the fracture was nucleated from the subsurface of the screw. The fracture surface was intergranular at the ID and OD region and microvoid coalescence in the center. The improper baking after electroplating of the screw led to H2-induced blistering/cracking. The high strength of the threaded region of the adjusting screw increased the failure propensity.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.aero.c0047835
EISBN: 978-1-62708-217-4
... the center of the piston-pin-bushing bore. The fracture was nucleated at an electroetched numeral 5 on one of the flange surfaces. A notch, caused by arc erosion during electroetching, was revealed by metallographic examination of a polished-and-etched section through the fracture origin. A remelted zone...
Abstract
An articulated rod (made from 4337 steel (AMS 6412) forging, quenched and tempered to 36 to 40 HRC) used in an overhauled aircraft engine was fractured after being in operation for 138 h. Visual examination revealed that the rod was broken into two pieces 6.4 cm from the center of the piston-pin-bushing bore. The fracture was nucleated at an electroetched numeral 5 on one of the flange surfaces. A notch, caused by arc erosion during electroetching, was revealed by metallographic examination of a polished-and-etched section through the fracture origin. A remelted zone and a layer of untempered martensite constituted the microstructure of the metal at the origin. Small cracks, caused by the high temperatures developed during electro-etching, were observed in the remelted area. It was concluded that fatigue fracture of the rod was caused by the notch resulting from electroetching and thus electroetched marking of the articulated rods was discontinued as a corrective measure.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.machtools.c0047779
EISBN: 978-1-62708-223-5
... overstress as the smooth-textured fatigue zone was relatively large compared with the crystalline textured coarse final-fracture zone. The fatigue crack was nucleated by the nonmetallic inclusion that intersected the surface and initiated in the 0.25 mm radius fillet at a change in section due to stress...
Abstract
Two A6 tool steel (free machining grade) shafts, parts of a clamping device used for bending 5.7 cm OD tubing on an 8.6 cm radius, failed simultaneously under a maximum clamping force of 54,430 kg. The shaft was imposed with cyclic tensile stresses due to the clamping force and unidirectional bending stresses resulting from the nature of operation. Nonmetallic oxide-sulfide segregation was indicated by microscopic examination of the edge of the fracture surface. Both smooth and granular areas were revealed on visual examination of the fracture. The shaft was subjected to a low overstress as the smooth-textured fatigue zone was relatively large compared with the crystalline textured coarse final-fracture zone. The fatigue crack was nucleated by the nonmetallic inclusion that intersected the surface and initiated in the 0.25 mm radius fillet at a change in section due to stress concentration. To minimize this stress concentration, a larger radius fillet shaft at the critical change in section was suggested as corrective measure.
Series: ASM Handbook Archive
Volume: 11
Publisher: ASM International
Published: 01 January 2002
DOI: 10.31399/asm.hb.v11.a0003538
EISBN: 978-1-62708-180-1
... concepts. bending brittle fracture compression failure deformation ductile crack nucleation ductile fracture ductility fractography manufacturing imperfections metals microvoid coalescence notched specimen plastic flow root cause failure analysis single-crystal cleavage models specimen...
Abstract
This article provides a description of the microscale models and mechanisms for deformation and fracture. Macroscale and microscale appearances of ductile and brittle fracture are discussed for various specimen geometries and loading conditions. The article reviews the general geometric factors and materials aspects that influence the stress-strain behavior and fracture of ductile metals. It highlights fractures arising from manufacturing imperfections and stress raisers. The article presents a root cause failure analysis case history to illustrate some of the fractography concepts.
Book Chapter
Series: ASM Handbook Archive
Volume: 11
Publisher: ASM International
Published: 01 January 2002
DOI: 10.31399/asm.hb.v11.a0003544
EISBN: 978-1-62708-180-1
... to a better understanding of both crack nucleation in regions of localized strain and the subsequent crack growth mechanisms outside the plastic zone in the qualification of component designs or to conduct failure analyses. Infinite-Life Criterion (<italic>S</italic>-<italic>N</italic> Curves) The safe...
Abstract
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.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.marine.c9001657
EISBN: 978-1-62708-227-3
... fatigue striations and microscopic crack growth bands, Figure 4 . A specific fatigue crack nucleation site could not be readily identified, but the orientation of the crack growth bands and striations in area A, Figure 3 , suggested that fatigue crack initiation occurred at the trailing edge tip area...
Abstract
The circumstances surrounding the in-service failure of a cast Ni-base superalloy (Alloy 713LC) second stage turbine blade and a cast and coated Co-base superalloy (MAR-M302) first stage air-cooled vane in two turbine engines used for marine application are described. An overview of a systematic approach, analyzing the nature of degeneration and failure of the failed components, utilizing conventional metallurgical techniques, is presented. The topographical features of the turbine blade fracture surface revealed a fatigue-induced crack growth pattern, where crack initiation had taken place in the blade trailing edge. An estimate of the crack-growth rate for the stage II fatigue fracture region coupled with the metallographic results helped to identify the final mode of the turbine blade failure. A detailed metallographic and fractographic examination of the air-cooled vane revealed that coating erosion in conjunction with severe hot-corrosion was responsible for crack initiation in the leading edge area.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.auto.c0046981
EISBN: 978-1-62708-218-1
... was metallographically examined. The area of the stem in which the fatigue cracks nucleated exhibited corrosion pits. Such pits act as notches and are likely sites for nucleation of fatigue cracks. The corrosion pits were the result of intergranular attack by the corrosive environment and the depletion of chromium from...
Abstract
The exhaust valve of a truck engine failed after 488 h of a 1000 h laboratory endurance test. The valve was made of 21-2 valve steel in the solution treated and aged condition and was faced with Stellite 12 alloy. The failure occurred by fracture of the underhead portion of the valve. Analysis (visual inspection, electron probe x-ray microanalysis, hardness testing, 4.5x fractograph) supported the conclusions that failure of the valve stem occurred by fatigue as a result of a combination of a nonuniform bending load, which caused a mild stress-concentration condition, and a high operating temperature in a corrosive environment. When the microstructure near the stem surface was examined, it was apparent that carbide spheroidization had occurred. Also, there was a coarsening of the carbide network within the austenite grains. The microstructure indicated that the underhead region of the valve was heated to about 930 deg C (1700 deg F) during operation. The cause of fatigue fracture, therefore, was a combination of non-uniform bending loads and overheating. No recommendations were made.
Series: ASM Failure Analysis Case Histories
Volume: 1
Publisher: ASM International
Published: 01 December 1992
DOI: 10.31399/asm.fach.v01.c9001072
EISBN: 978-1-62708-214-3
... impeller. The vanes were fillet welded to both the disk and the cover. Applications Specimen Selection The cracks were adjacent to the fillet welds. They were near the outer diameter of the cover but, with one or two exceptions, did not extend to the outer diameter edge. They appeared to nucleate...
Abstract
Radial cracking occurred adjacent to 11 vanes in a 19-vane impeller operating in a chemical plant environment. The impeller vanes were fillet welded to both the disk and the cover Cracks were next to the fillet welds and near the cover outer diameter They generally did not extend to the outer diameter. The entire impeller surface was tested by the dry magnetic particle method. Visual and microstructural examinations revealed intergranular cracking. Energy-dispersive spectroscopy of corrosion products contained in the cracks disclosed the presence of chlorine and sulfur The failure was attributed to stress-corrosion cracking caused by a corrosive atmosphere.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001351
EISBN: 978-1-62708-215-0
... revealed several pits in the regions where cracklike defects were observed. Fine cracks originated at pit bottoms and extended across the thickness of the convolutes, suggesting that pits acted as sites for crack nucleation. The high level of hardness of the parent material indicated that the bellows had...
Abstract
A number of AISI 347 stainless steel bellows intended for use in the control rod drive mechanism of a fast breeder reactor were found to be leaking before being placed in service. The bellows, which had been in storage for one year in a seacoast environment, exhibited a leak rate on the order of 1 x 10−7 cu cm/s (6 x 10−8 cu in./s). Optical metallography revealed numerous pits and cracks on the surfaces of the bellow convolutes, which had been welded to one another using an autogenous gas tungsten arc welding process. Microhardness measurements indicated that the bellows had not been adequately stress relieved. It was recommended that a complete stress-relieving treatment be applied to the formed bellows. Improvement of storage conditions to avoid direct and prolonged contact of the bellows with the humid, chloride-containing environment was also recommended.
Series: ASM Handbook Archive
Volume: 11
Publisher: ASM International
Published: 01 January 2002
DOI: 10.31399/asm.hb.v11.a0003565
EISBN: 978-1-62708-180-1
... reducing metal-to-metal contact Adhesion: a more severe wear regime characterized by material smearing and transfer Abrasion: equally severe, one-body or two-body modes lead to the formation of ploughing grooves Surface fatigue: more severe wear again, leading to subsurface crack nucleation...
Abstract
This article discusses the generic features of impact wear on metals, ceramics, and polymers. It describes normal impact wear and compound impact wear, as well as the features of impact wear testing apparatus such as ballistic impact wear apparatus and pivotal hammer impact wear apparatus. Most mechanical components continue to be functional beyond the zero wear limit, and their usefulness is normally connected with the loss of a specific depth of material. The article reviews the zero impact wear model and some measurable impact wear models. It presents a case study illustrating the impact of wear failure on automotive engine inlet valves and seat inserts.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001280
EISBN: 978-1-62708-215-0
.... Nondestructive Evaluation Surface Examination Metallography Chemical Analysis/Identification Mechanical Properties Figure 4 clearly shows that the HK-40 reformed tubing failed by a cracking process initiating at the weld root. The mechanism of crack nucleation was related to the coalescence...
Abstract
An HK-40 alloy tubing weld in a reformer furnace of a petrochemical plant failed by leaking after a shorter time than that predicted by design specifications. Leaking occurred because of cracks that passed through the thickness of the weldment. Analysis of the cracked tubing indicated that the sulfur and phosphorus contents of the weld metal were higher than specified, the thickness was narrower at the weld, and the mechanical resistance of the weld metal was lower than specified. Cracking initiated at the weld root by coalescence of creep cavities. Propagation and expansion was aided by internal carburization. Quality control of welding procedures and filler metal was recommended.