The high-strength aluminum alloy Al–5.5Zn–2.5Mg–1.5Cu (AA 7075) is extensively used in aerospace applications. An injector
body of a gas generator for a turbine rotor in a liquid propulsion system was fabricated out of AA 7075 forgings in the T7352
condition, and, subsequent to a qualification hot test, one of the injector discs showed the presence of cracks on the top
face of the body. Detailed metallurgical investigation indicated that the failure was caused by stress-corrosion cracking
A criterion for predicting the workability limits for internal britde failure was developed for cold heading of 1038 steel.
The criterion considers internal defects caused by microstructural changes generated by adiabatic shear. This transformation
is termed the transformed adiabatic shear band (TASB) phenomena. The defect that develops is the formation ofbritde martensite as a result ofthe temperature rise and fall inside the adiabatic
shear band (ASB). In this work, the material is considered to have a TASB defect when the temperature inside the ASB exceeds
the phase transformation temperature (AC3). The empirical formulas provided by Andrews were used to determine transformation temperatures. Microhardness testing and etching with 2% Nital and Le Pera etchants
were performed on the sectioned specimens to locate and study the TASB. In order to simulate the cold heading process, a drop
weight compression test was used and modeled with finite-element analysis (implemented within ABAQUS/Explicit).
Silicomanganese grade billets are the most commonly used steels for manufacture of automobile leaf springs. However, Cr-Mn-B
grade steel known by trade name of SUP 11A grade is replacing the conventional silicomanganese grades such as 60Si7 or 65Si7
steels because it has become a competitive alternative in the market. Three heats of SUP11A grade spring steel were made through
BOF-VAD-CC route and continuously cast into 125×125mm billets. Some of the billets contained blowholes and piping. Rolling
of selected billets into 85×15mm flats revealed occasional slivers, seams, and a few shallow hairline surface cracks. A
detailed metallurgical investigation was carried out to understand the genesis of these defects. A pearlite-free ferritic
microstructure near the cracks combined with the presence of dispersed inclusions resulting from internal oxidation in the
vicinity of cracks and the presence of scales within the shallow discontinuous short-length longitudinal cracks indicated
that these defects resulted from pre-existing subsurface blowholes lying within 1mm of billet surface. Reduction of the gas
content of liquid steel in the mold, optimization of electromagnetic stirring (EMS) current, and control of superheat are
some of the broad measures identified to improve the cast quality of SUP 11A spring steel billets and minimize the rejection
of rolled flats.
The purpose of this investigation was to determine the root cause of the differences noted in the fatigue test
data of main rotor spindle assembly retaining rods fabricated from three different vendors, as part of a “Second
Source” evaluation process. ARL performed dimensional verification, accessed overall workmanship, and
measured the respective surface roughness of the rods in an effort to identify any discrepancies. Next, mechanical
testing was performed, followed by optical and electron microscopy, and chemical analysis. Finally, ARL
performed laboratory heat treatments at the required aging temperature and follow-up mechanical testing.
Spline actuators made of investment cast 17-4 PH (precipitation hardening) stainless steel were found to contain micro-cracks.
The cracked actuators were subjected to optical and scanning electron microscopy and hardness testing, which revealed that
the failure occurred due to fatigue crack initiation and growth after electrical discharge machining (EDM). The rehardened
layer produced by the EDM remained after machining, and the cracks and surface irregularities associated with this layer provided
sites for crack initiation and growth, which ultimately caused rejection of parts. Close dimensional tolerances on actuators
require post-heat treatment EDM. Thickness of the recast layer was measured to be about 38–55μm, and precipitation in vicinity
of the machined surface is a potential source for corrosion. Post-machining polishing by means of fluidized bed granules was
employed to remove recast layer and associated precipitates. Test results proved that removal of surface layers improved the
microstructure and the resistance to crack formation. The post-EDM polishing and subsequent annual inspections proved that
problem was solved.
KeywordsElectrical machining–Failure analysis–Microstructure–17-4 PH stainless steel
Analysis of failed sections from a 31km long pipeline show that premature failure was caused by microbial-influenced corrosion.
This case history summarizes the failure analysis and demonstrates the need for extreme care when using untreated water to
hydrotest a pipeline.
A Mazda Miata crankshaft and timing belt pulley bolt failed in service. This caused extensive damage to the engine. The crankshaft
and bolt were analyzed to determine the cause of failure. Using visual examination and other means, it was determined that
the crankshaft and bolt failed by fatigue. The crankshaft failure initiated at the keyway, while the timing belt pulley bolt
initiated in the threads. Inadequate clamping force during installation of the timing belt pulley bolt is thought to have
KeywordsCasting–SEM–Shafts–Torsion–Energy dispersive spectroscopy–Failure analysis–Fatigue failure
This study deals with the analysis of an ink-producing machine rotor part composed of WC–2Ni–1Co which failed in brittle manner
during service. The part was made by powder metallurgy techniques and is being used in ink-grinding machines due to its high
hardness and wear resistance. Similar parts had worked satisfactorily for many ink compositions, but the part under investigation
failed prematurely. Investigation was considered important because the part is expensive, and other identical components frequently
failed after a short service life. Moreover, replacement of the part requires complete dismantling of the machine which reduces
the production rate. Spectroscopic analysis, density, optical and scanning electron microscopy, SEM–EDS analysis, fractography,
X-ray diffraction, and microhardness measurements were carried out on failed parts to find out the root causes of the failure.
Results revealed that the part cracked due to combined effects of selective dissolution of metal binder-caused corrosive action
of ink solution and hydrogen-induced deterioration of WC and Ni–Co phases. Localized removal of binder phase left the hard
WC phase unsupported. Cracks were found initiating from the root of the machined slot which acted as a stress concentration
point and resulted in brittle fracture.
This paper reports the details of a test method that uses elements of elastic-plastic fracture mechanics to assess fracture
resistance of zirconium (Zr)-2.5 wt.% niobium (Nb) pressure tubes for a pressurized heavy water reactor. The fracture properties
were evaluated on curved specimens, and the effect of certain trace elements on the fracture properties was determined. Significant
reduction of trace impurities, produced by using four-stage melting practices rather than the conventional two-stage process,
was observed to cause considerable improvement in the fracture resistance of the alloy. Scanning electron microscopy (SEM)
of the fracture surfaces of the test specimens confirmed this observation.
Characterization of macrostructure, microstructure, hardness, precipitate distribution, residual stress, and cyclic deformation
behavior of 2024-T351 friction stir welded joints has been conducted. Inhomogeneous microparameters governing the nonuniform
residual stresses and cyclic strength are discussed. The cyclic strength of the weld microregimes is controlled by grain size
and distribution of precipitates achieved during the weld process. The comprehensive information of micro-and macromechanics
is used to assist in understanding the mechanism that governed the fatigue crack initiation, propagation, and life of the
Duplex stainless steel (DSS) grades are used in pulp mills for their superior properties and resistance to general corrosion.
However, stress corrosion cracking (SCC) of DSS equipment has been experienced in different pulp mills. The susceptibility
of DSS grades to SCC can be mainly attributed to the various heating processes involved during the manufacturing of industrial
equipments, especially welding. It is generally understood that heating cycles during welding may affect the dual microstructure
(ferrite/austenite ratio) of the steel, making it more prone to cracking in aggressive environments such as chlorides and
caustics and further exposure to high temperatures. Welded 2205 DSS failed in white liquor (mainly NaOH+Na2S) was examined for SCC crack morphology and microstructure. Heat-treated 2205 DSS samples were tested in simulated white
liquor to see the effect of microstructure on SCC susceptibility. Austenite is more susceptible to SCC than ferrite, but the
SCC susceptibility primarily depends on the composition of the alloy and the chemistry of the exposure environment.
The susceptibility of austenitic stainless steels to the formation of two distinct weld defects, solidification cracking and
lack of penetration, is related to the chemical composition of the base and filler material. The propensity for cracking is
determined primarily by the solidification mode and the amount of residual tramp elements such as phosphorous and sulfur.
High sulfur levels can lead to weld centerline cracking and heat affected zone (HAZ) cracking while very low sulfur levels
(less than ∼50 ppm) in types 304L and 316L are associated with lack of penetration weld defects and a distinct loss in puddle
control during fusion welding. A calculated Creq to Nieq ratio of 1.52 to 1.9 is recommended to control the primary mode of solidification and prevent solidification cracks in type
304L while the Creq/Nieq ratio of 1.42 to 1.9 is recommended for type 316L stainless steel. A lower limit of 50 ppm sulfur is recommended to avoid
possible lack of penetration. These ranges should be validated by welding trials for specific weld processes and applications.
The room temperature burst pressure of 316L stainless steel burst discs exhibited increases of about 10% over 90 days. This
increase may be associated with a strain-aging phenomenon requiring the presence of carbon since tensile property instability
in worked austenitic stainless steels has been reported.[1–5] The cold worked material directly beneath the score root on the burst disc could undergo the strain aging process, thus causing
the observed increase in burst strength. Characterization and analysis were therefore undertaken to identify the controlling
phenomena in the small heterogeneous volume that controls rupture of the burst disc. Optical metallography and magnetic measurements
confirmed the presence of martensite. Nanoindentation hardness measurements were correlated with finite element simulation
of the as-formed mechanical properties. A representative portion of the microstructure was then recreated through cold rolling,
and subjected to real-time and accelerated thermal aging treatments and mechanical activation analysis. Saturation of strengthening
was observed, and a low temperature martensite reversion anneal was found to prevent or reverse the aging process. The results
are consistent with previous observations of strain aging, although in this instance the effects are observed over a 10,000-fold
greater aging time. Aging mechanisms are discussed, incorporating the phenomenologies of activation enthalpy and aging kinetics.
A model explaining the sensitivity of aging rate to extreme cold work-induced dislocation densities and cold work-induced
vacancy content is proposed.
Several boiler superheater tubes showed circumferential cracking at weld seams after 2years in noncontinuous service (several
shutdowns). On-site inspection revealed that several tubes were cracked and leaked; while many others were cracked, however,
the severity was less pronounced. Two types of superheater tubes samples were collected: one with butt-welded tubes and the
other with fillet-welded sleeve. The latter was found to be out of the boiler fireplace, and the sleeve was used as tubing
support to the boiler shell. Detailed investigation showed that the butt-welded tubes contained circumferential fatigue cracks
that initiated from the internal surface. The cracks initiated in the heat-affected zone and propagated as a result of tube
vibration. The variations in the tube internal diameter and tube wall thickness are expected to play a role in tube fatigue
failure. On the other hand, tubes with fillet-weld sleeve showed circumferential cracking as a result of fatigue crack initiation
from weld defects on the tube external surface. The high vibration during several unscheduled shutdowns in addition to several
other factors such as variations in tube inside diameter, wall thickness, and weld defects resulted in the initiation and
propagation of fatigue cracks and premature failure. White deposits, similar to those observed when boiler tubes failed by
caustic exposure, were seen in the vicinity of the tube cracks. Therefore, it was difficult to confirm whether the boiler
tubes failed because of the fatigue cracks or because of the caustic salts (pH control chemical).
Early in the shuttle Columbia crash investigation, item 33767 was one of several “Pathfinder” components selected from the Columbia debris that exhibited damage patterns similar to those observed on the left wing airframe components, the components in which
initial failure was thought to have occurred. “Pathfinder Analysis” sought to answer academic questions regarding the maximum
heat attained and heating direction/duration and identify debris imposed on this fuselage section during Columbia breakup and re-entry. Traditional failure analysis techniques provided useful information on debris constituents and damage
sequence and were successful in identifying heat effects, such as the presence of large thermal gradients across the component,
and the existence of several failure modes that included hot tensile failure, hot bending failure, and rapid overload fracture.
A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery installation, failed
by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid
(HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter
was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched.
Although nominal chemical composition of the component was found within the specified range, electron dispersive analysis
by X-ray (EDXA) indicated significant segregation of sulfur and chlorine on grain boundaries. Failure was attributed to hypochlorous-acid
(HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O2, and the oxygen entered the tube during shutdowns/startups of the installation. Residual stresses, originating from the in
situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation
of sulfur on grain boundaries enhanced the susceptibility of the material to cracking.
Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning.
The HRSG was in a combined cycle, gas-fired, combustion turbine electric power plant. Alloy 430, a 17% chromium (Cr) ferritic
stainless steel, was selected because of its resistance to chloride and sulfuric acid dewpoint corrosion under conditions
potentially present in the HRSG low-pressure feedwater economizer. Intergranular corrosion and cracking were found in the
weld metal and heat-affected zones (HAZs). The hardness in these regions was up to 35 HRC, and the weld had received a postweld
heat treatment (PWHT). Metallographic examination revealed that the corroded areas contained undertempered martensite. Fully
tempered weld areas with a hardness of 93 HRB were not attacked. No evidence of corrosion fatigue was found. Uneven temperature
control during PWHT was the most likely cause of failure.
The genesis of failure of 6.1 mm thick electric resistance welded (ERW) API 5L X-46 pipes during pretesting at a pressure
equivalent to 90% of specified minimum yield strength (SMYS) 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.
This paper describes the remote ultrasonic (UT) examinations of a high-level radioactive waste (HLW) storage tank at the Savannah
River Site in South Carolina. The inspections, carried out by E.R. Holland, R.W. Vande Kamp, and J.B. Elder, were performed
from the contaminated, annular space of the 46-year-old, inactive, 1.03 million gallon waste storage tank. A steerable, magnetic
wheel wall crawler was inserted into the annular space through small (6 in., or 150 mm, diameter) holes/risers in the tank
top. The crawler carried the equipment used to simultaneously collect data with up to four UT transducers and two cameras.
The purpose of this inspection was to verify corrosion models and to investigate the possibility of previously unidentified
corrosion sites or mechanisms. The inspections included evaluation of previously identified leak sites, thickness mapping,
and crack detection scans on specified areas of the tank. No indications of reportable wall loss or pitting were detected.
All thickness readings were above minimum design tank-wall thickness, although several small indications of thinning were
noted. The crack detection and sizing examinations revealed five previously undetected indications, four of which were only
partially through-wall. The cracks that were examined were found to be slightly longer than expected but still well within
the flaw size criteria used to evaluate tank structural integrity.
Metal dusting can cause a significant amount of expensive damage to ammonia, hydrogen, carbon monoxide and methanol plants. One of the means to limit this development has been to design the equipment parts, considered subject to metal dusting attack with Alloy 601. This will typically mean parts subjected to a reformed gas at 600°C (1,110°F). The risk of stress relaxation cracking of Alloy 601 at exactly this temperature has not previously been reported, but it must be considered a major risk that has to be dealt with in the design phase.
The theoretical basis of detecting wire rope anomalies using permanent magnetic field has been fully established. The local
faults (LF) such as broken wires and loss of metal area (LMA) signals are provided by nondestructive evaluation instruments.
These signals represent the electronic equivalent of the mechanical anomalies present in the wire rope. The saturating magnetic
field of the instrument makes the anomalies visible to the magnetic sensors placed around the rope. The condition of a haulage
rope (construction 6X19) in a monocable continuously moving passenger ropeway has been studied using this nondestructive method,
and the results are presented in this paper.
Aluminum alloys are frequently preferred materials for aerospace applications due to their high-specific density, high-specific
stiffness, and ease of fabrication. In one such application, adaptors used in the torroidal shaped water tank of liquid propulsion
system, were made of an AFNOR 7020 (Al–4.5Zn–1.5Mg) aluminum alloy extrusion. These pressurization adaptors, in T6 temper
condition, were initially shrink fitted to the openings provided in water tank main body and later circumferentially welded,
manual TIG, to configure the tank. Four such adaptors were welded at different locations of a water tank. During one of the
qualification tests, cracks were noticed near to the weld fusion line of one of the adaptor. Detailed metallographic investigation
on the cracked adaptor revealed that cracking was due to combined effect of locked-in stresses in the material and anodic
dissolution of solute rich phases present at elongated grain boundaries of the HAZ: a typical case of Stress Corrosion Cracking
Two out of three struts of a drilling tool’s 6.75″ Flow Diverter failed during operation. This failure resulted in invasion
of mud into the electronic module of the tool. During drilling, high stick slip was observed. Measurement While Drilling (MWD)
signal was good and Logging While Drilling (LWD) data was used to pick the oil/water contact point. At the end of the run,
cracks were observed on the radii of the struts. Failure investigations were performed to identify the cause of the cracks.
Material characterization, microstructural examinations, and fractography by SEM technique revealed that fatigue under compression
forces was the cause of the cracking of the struts. Finite elemental analysis (FEA) was also used to determine the magnitude
and the area of stress concentration. Failure mechanism has been explained. A few issues related to integrity of the material
have also been raised.
The quality advantages of continuous casting (concast) have enabled the production of a wide range of billets for various
end applications, including IS:7887 Gr. 3 billets for the fastener industry. This paper discusses the influence of the internal
quality of these concast billets on the processing of a wide range of products for the fastener industry. Internal soundness,
inclusion volume fraction, and cleanliness were found to have a strong influence on the cracking susceptibility of self-tapping
screws and nuts. A high incidence of hairline cracks on nut surfaces was found to be due to a combination of a high volume
fraction of inclusions (0.54%) and the presence of complex manganese (Mn) (aluminate-silicate)-type inclusions.
The observation that carrier panel fasteners recovered from the Columbia appeared to have fractured with minimal macroscopic ductility led to concern that stress-corrosion cracking (SCC) or other
age-related degradation processes in the A286 bolts may have been the cause of failure during re-entry. To assess this hypothesis,
several recovered fasteners having a brittle fracture appearance and several fasteners that fractured in a more ductile mode
were analyzed. The results provided the information necessary to discount the “stress-corrosion” hypothesis and conclude that
the fasteners failed by some unidentified tensile load after being weakened by high-temperature grain growth and liquid aluminum
In industrial applications failures of mechanical parts made of carbon and alloyed steels may develop either during heat treatment
steps or final finishing operations. Such failures have high impact costs for manufacturers, since heat treated steel products,
in general, are high value products which increase in value with each step in the production process until the final life-cycle
manufacturing steps are completed. This work highlights the selection of steels to avoid premature ruptures developing during
either the heat treatment steps or finishing operations with emphasis on the role of banding in the failure process. Failure
does not have to involve fracture but may simply imply a decrease in performance of surface treated components as consequence
of surface properties, even in the presence of correct heat treatment parameters. The root causes for banding in steels, are
described in literature, and banding has major effects on final product properties (and causal relationships). Therefore,
the causes of banding are studied and classified. Conclusions suggest that microstructural defects such as (micro)segregation
bands and other defects such as slag and oxides inclusions are developed in the early fabricating cycle steps and can cause
premature failure of either semi-finished or finished products.
KeywordsSegregation-Heat treatments-Toughness-Banding-Anisotropy-Martensite-Transformation plasticity-Failure-Austenite
Three basic modes of failure are known for oil tanks: manufacturing defects, mechanical damage, and corrosion. Most of the
tanks currently in use and manufactured in or prior to the 90s, are subject to leaking caused by internal corrosion. Corrosion-induced
leakage always results in environmental damage by underground and/or aboveground oil contamination. Considering the costs
related to decontamination and to replacement of residential tanks, the possibility of corrosion should be carefully addressed.
To accomplish this task, the most common issues related to tank corrosion must be recognized and understood. This article
is an overview of the corrosion mechanisms in aboveground residential tanks made of non-coated mild steel.
KeywordsPitting corrosion-Oil tanks-Water-Heating oil
The high stress wear behavior of steels is dependent on a number of experimental factors, such as load, sliding distance,
speed, abrasive size, and so forth. With the aid of computer-based regression analysis, this study investigates the role of
each experimental variable on wear rate. Correlations are formed between the predicted and experimentally observed values.
Increases in failures due to internal board shorting by conductive filament formation have driven glass and laminate manufacturers
to consider screens and qualification tests to assess the hollow fiber concentration of circuit cards. This article describes
the hollow fiber problem, methods to deter conductive filament formation, a hollow fiber screening technique, and the impact
of next generation technology on conductive filament failure mechanism.
A double-walled, hemispherical metal beam exit window made of alloy 718, used at the Los Alamos Neutron Science Center (LANSCE),
developed a crack during service, leading to leakage of coolant. The window had been exposed to radiation damage from 800
MeV protons and a cyclic stress from 600 MPa tensile to near zero induced by numerous temperature cycles calculated to be
from 400 to 30 °C. The window was activated to > 200 Sv/h. It was determined, through analysis using remote handling techniques
and hot cells, that the crack initiated near a spot weld used to affix thermocouples to the window surface. In addition to
analysis of the crack, some of the irradiated material from the window was used to measure mechanical properties. Hot cell
techniques for preparation of samples and testing have been developed at both Forschungszentrum Juelich (FZJ) and Los Alamos
National Laboratory (LANL). Important and useful data on radiation effects can be obtained from components used in service
because they have experienced true operating conditions of radiation, strain, and temperature.
On 3 February 2003, the space shuttle Columbia and its crew were lost during re-entry from a successful scientific mission. The physical root cause of the space shuttle Columbia accident was determined to be a breach in the thermal protection system (TPS) on the leading edge of the left wing. After a thorough investigation of the incident, the Columbia Accident Investigation Board (CAIB) set forth their recommendations which are focused on three major areas: the accident, why the accident occurred, and a look ahead.
A metallurgical and mechanical failure analysis was applied as part of a vehicle accident reconstruction of a multi-vehicle
collision. One of these vehicles was a coal-hauling tractor-trailer. Examination of the trailer involved in the incident revealed
a fatigue fracture to a primary lateral stiffener, along with a significant misalignment of the stiffener. Stress and fatigue
analysis indicated that the misalignment severely degraded the fatigue life of the stiffener. Evaluation of the structural
dynamics of the trailer after the fatigue fracture indicated decreased lateral stability. The decreased stability caused by
fracture of the lateral stiffener allowed rollover of the trailer to occur while negotiating a curve. The failure sequence
developed in this investigation proved consistent with all physical damage observed on the trailer and with witness accounts
of the incident. The failure scenario developed in this investigation is compared with other conclusions made by other investigators
to show that those conclusions are not consistent with all of the available evidence.
The space shuttle Columbia launched from the Kennedy Space Center (KSC) in January of 2003. During ascent, between one and three pieces of material—likely
insulating foam from the external tanks—impacted the leading edge of the left side of the orbiter. Upon re-entry back to earth,
the Columbia began to disintegrate, leaving an enormous primary debris field stretching over eastern Texas and western Louisiana. Tens
of thousands of volunteers were mobilized to help with the recovery of the Columbia remnants. Once the debris was delivered to KSC, several hundred scientists, engineers, and technicians helped analyze the
debris and identify its original location on the orbiter. A Materials and Processes Team performed extensive failure analysis
and chemical identification to help determine the most likely breach location resultant from the strike that occurred during
liftoff, and the path that the impinging plasma generated during re-entry followed once it penetrated the wing of the Columbia. A combination of qualitative and quantitative analytical methods, ranging from radiographic nondestructive examination (NDE)
and X-ray diffraction to scanning electron microscope with energy-dispersive spectroscopy (SEM/EDS) and electron probe microanalysis
(EPMA), were used to help determine the breach location and the plasma path within the wing itself.
Aircraft accidents are usually complex, and hence their investigation requires expertise from several fields. The challenge
for an investigating team is to analyze the bits and pieces of information gathered in the course of the investigation through
background information, interrogation, and material evidence and stitch them into a descriptive picture to arrive at the possible
cause(s) of the accident. During this process, the investigating team is extremely dependent on laboratories where failed
components/structures are subjected to detailed study. The laboratory findings are the decisive factors in ensuring a successful
investigation. Any mistakes committed at this stage can, apart from wasteful work, lead to confusion and wrong direction of
investigation. This paper presents an investigation that in the author’s opinion should have been conclusive, but remained
inconclusive. The shortcomings in the investigation are discussed.
Acoustic Emission (AE) technology applied to condition monitoring is gaining acceptance as a useful complimentary tool. This
article demonstrates the use of traditional AE parameters, the Enegry Index and Kolmogorov–Smirnov test (KS-test) to detect,
locate, and monitor natural defect initiation and propagation in a conventional rolling element thrust bearing. To undertake
this task a special purpose test-rig was built to allow for accelerated natural degradation of a bearing race. It is concluded
that sub-surface initiation and subsequent crack propagation can be detected using a range of data analysis techniques on
AE’s generated from natural degrading bearings. The article also investigates the source characterization on AE signals associated
with such a bearing while in operation.
Vibration monitoring of rolling element bearings is possibly the most established diagnostic technique for rotating machinery.
The application of Acoustic Emission (AE) for bearing diagnosis is gaining ground as a complementary diagnostic tool, however,
limitations in the successful application of the AE technique have been partly due to the difficulty in processing, interpreting
and classifying the acquired data. Furthermore, the extent of bearing damage has eluded the AE diagnostician. The investigation
reported in this paper was centered on the application of the Acoustic Emission technique for identifying the size of a defect
on a radially loaded bearing. An experimental test-rig was designed such that defects of varying sizes could be seeded onto
the outer race of a test bearing. It was concluded that AE can provide an indication of the defect size, thus allowing the
user to monitor the rate of degradation on the bearing.
Over the last four to five decades, applications of fiber-reinforced plastics (FRP) have seen tremendous growth. In spite
of the complexity of their behavior, development of composite pressure chambers has significantly contributed to the continuing
increase in the strength-to-weight ratio of a significant amount of hardware and testing techniques to evaluate that hardware
are continually being developed. This article presents an overview of the developments toward burst pressure prediction of
FRP pressure chambers using acoustic emission (AE) technique. Burst strength prediction methods and related research in this
context are also briefly discussed. The historical background of FRP pressure chamber/vessel testing using AE is also covered
in this review.
KeywordsAerospace fasteners–Catastrophic failure–Characterization–Composite materials–Non-destructive testing
Concrete walkways are relatively low cost alternatives to bridges and culverts. They provide good and dependable ways across streams and rivers. This article discusses failures of concrete walkways in selected locations in Nigeria. The selected locations in Katsina state were visited before raining season started and during raining season. Concrete walkways across selected rivers (streams) were examined and their failures were assessed. Soil samples at the location of the walkway were collected and classified. The study revealed that earlier failures of culverts/bridges led to the construction of these concrete walkways in 1985 because the walkways were cheaper and required lower technology than construction of bridges. The soils at the site were mainly sandy–loamy soil with an average of 75.5% sand, 9.85% clay, and 14.65% silt. Failures of the concrete walkways were due to rapid and varied flow in the middle of the river channel, the presence of a hydraulic jump, the lack of resistance to turbulent flow, the presence of impulse forces, high Froude number (F
r), and the lack of cohesion between the soil and concrete materials. It was then concluded that provision must be made against turbulent flow and cohesive materials must be provided in the construction of walkways to prevent failure of walkways.
Precipitation-hardened stainless steel of 15-5PH grade was used for the fabrication of forward cylinder used in advanced linear
actuator of aircraft. The component had undergone a series of qualification tests. During one of the accelerated fatigue tests,
an external leak was observed after experiencing more than 1,50,000 cycles of fluctuating stress. Subsequent to disassembly,
the leak was found to be because of the hair line cracks in the forward cylinder. The cracked component was subjected to a
detailed metallurgical investigation to understand the cause of failure. The failure analysis revealed that the fracture initiated
at the hard chrome layer and propagated under cyclic loading. This premature failure of the component was attributed to the
surface roughness and the presence of the hard chrome layer at the corner contour of the component. This article brings out
the details of the investigation carried out and the results achieved.
Keywords15-5PH stainless steel–Hard chrome layer–Effect of surface roughness on fatigue–Fatigue failure
Determination of the failure mechanisms of mechanical devices is the key to the design of reliable products. This paper reports
an investigation on creep and fatigue failure of microelectromechanical (MEMS) thermal actuators. Finite element modeling
is used to predict thermomechanical behavior of actuators under low to moderate voltage differences. The modeling results
are compared with experimental results to evaluate the models. Two probable failure modes associated with thermal actuators,
that is, fatigue and creep, are investigated, and it is found that creep is the dominant failure mechanism. The creep behaviors
of several U-shape and double hot arm thermal MEMS actuators are examined, and their deformation-time curves are obtained
numerically and experimentally. The curves follow a typical three-stage creep curve usually observed in metals. The creep
life cycles of the devices are compared on the basis of their stress and temperature distributions. This study shows that
actuators with the maximum temperature occurring at the location where the high stress is induced have shorter life spans
than those experiencing the high stress away from the maximum temperature location. It is concluded that the double hot arm
actuators with equal length have longer creep life than the U-shape (single hot arm) actuators.
This paper reports on an investigation in which several standard Microelectromechanical Systems (MEMS) elements consisting
of thermal actuators, inchworm drives, and comb drives were subjected to vibration loading representative of the environment
seen in space applications. Finite-element analysis of the MEMS devices showed that sufficient margins existed under the expected
environmental loading. Vibration testing, however, resulted in several failures in the devices, and analysis showed that progressive
failure initiated from large local displacements. Debris transport and entrapment was another source of failure leading to
shorting of thermal actuators. The results illustrate the importance of debris control and packaging design for reliable MEMS
operation. Suggestions for improving the reliability of MEMS devices through practical layout and packaging guidelines are
Adaptors used in check valve inlet pressure sensors were fabricated from austenitic stainless steel. Recently, during pretest
operation and during hydrogen pressurization of an engine, a leak was noticed through the adaptor. Subsequent to disassembly,
the leak was attributed to a crack in the adaptor. Detailed metallurgical investigations were carried out to understand the
cause of cracking. Synergistic effect of chloride ions present in marine environment, the presence of residual stresses and
sensitized grains of the material led to intergranular stress corrosion cracking. This article highlights details of investigation.
KeywordsAustenitic stainless steel–Sensitization–Anodic dissolution of chromium-depleted region–Failure analysis
This investigation characterizes five surgical stainless steel piercings and one niobium piercing that caused adverse reactions
during use, culminating with the removal of the jewelry. Chemical composition shows that none of the materials are in accordance
with ISO (International Organization for Standardization) standards for surgical implant materials. Additionally, none of
the stainless steel piercings passed the pitting-resistance criterion of ISO 5832-1, which implies that %Cr + 3.3 × %Mo >26.
Under microscopic examination, most of the jewelry revealed the intense presence of linear irregularities on the surface.
The lack of resistance to pitting corrosion associated with the poor surface finishing of the stainless steel jewelry may
induce localized corrosion, promoting the release of cytotoxic metallic ions (such as Cr, Ni, and Mo) in the local tissue,
which can promote several types of adverse effects in the human body, including allergic reactions. The adverse reaction to
the niobium jewelry could not be directly associated with the liberation of niobium ions or the residual presence of cytotoxic
elements such as Co, Ni, Mo, and Cr. The poor surface finish of the niobium jewelry seems to be the only variable of the material
that may promote adverse reactions.
Stainless steel fasteners are often chosen to replace carbon steel fasteners in many engineering applications. These fasteners
offer some advantages over their carbon steel counterparts—most notably, increased corrosion resistance. However, stainless
steel fasteners have a tendency to gall, or cold weld, when in contact with a mating stainless steel fastener of similar composition
when the proper precautions are not taken. Galling can vary in severity, and may be only a nuisance when minor, but lead to
fastener failure in the extreme. This paper describes the galling of a threaded stainless steel ejector rod from a laser guided
training round (LGTR) used by the U.S. Navy. Also presented are the precautions that should have been taken to avoid galling.
A failure analysis case study is presented for a two-piece aerosol containing tetrafluoroethane, commonly referred to as Refrigerant
134a. A gentleman was preparing to recharge the air conditioning system of an automobile when the bottom exploded off the
aerosol container, propelling the body of the aerosol container like a rocket, which hit the man in the eye and blinded him
in that eye. The aerosol was never connected to the air conditioner, therefore backpressure from the air conditioner (AC)
compressor was ruled out as a cause for the explosion. The objective of the study was to determine why the aerosol exploded.
Several recently developed test methods were used, including two types of heat-to-burst tests and a puncture chamber to measure
the pressure-versus-temperature behavior of aerosols. More common test methods were also used, such as water bath pressure
tests, hydro pressure burst tests, pneumatic pressure burst tests, hardness measurements, weight measurements, metallography,
scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and an accident scenario recreation. A semi-empirical
correlation between the hardness and weights of the container bottoms was used to determine the explosion temperature and/or
pressure. This semi-empirical correlation agrees in principle with an analysis of the explosion pressures using finite-element
analysis (FEA). The root cause for the explosion was determined to be a lack of strength of the bottom of the two-piece aerosol
coupled with heating the aerosol to temperatures significantly above room temperature.
This paper explains how three-piece aerosol cans made from tin-plated steel are designed and manufactured, how they behave
structurally, and provides information on doing failure analyses of aerosol cans. Particular emphasis is placed on the metallurgical
aspects of the side-seam welding process used to create the body cylinders, the process of attaching the ends, and explains
how three-piece aerosol cans made from tin-plated steel behave structurally. The paper also addresses a number of representations
and conclusions presented in a paper by M. Fox and R. Hastings entitled “Pressurized 3-Piece Steel Container Explosions and
Failure Mechanisms” published in the June 2003 issue of Practical Failure Analysis. Aerosol cans may fail, although not necessarily explode, by internal or external corrosion, by mechanical abuse (puncture,
for example), or by being overheated. Understanding the design, manufacture, and structural behavior of these pressure vessels
will aid in the formulation of conclusions as to the root causes of aerosol can failures.
This paper presents a continuation of an open public discussion about the failure analysis of a relatively simple mechanical
device: the three-piece steel aerosol container. In the June 2003 issue of Practical Failure Analysis, M. Fox and R. Hastings published a paper on the failure of aerosol containers. In August 2004, R.C. Daehn and J.J. Blum
published a paper in the Journal of Failure Analysis and Prevention that was critical of the Fox-Hastings article. The present manuscript addresses some of the criticisms of the June 2003 paper,
using the methods of the August 2004 paper. Also discussed is the fact that not many things are manufactured perfectly 100%
of the time and the observation that quality-assurance tests do not always represent failure mechanisms in the real world.