Severe wear and aseptic loosening in an uncemented acetabular prosthesis have been observed in a revision surgery carried out at King Edwards VII hospital by L. Taylor and P. Heaton-Adegbile, twelve years following the primary total hip replacement operation. The superior-lateral wall of the polyethylene liner and part of the titanium cup were found to be completely worn out, such that the ceramic head was in direct articulation with the titanium cup.A three-dimensional finite element model was developed. The polyethylene liner was modelled with the outer surface of the liner fully constrained to represent the much stiffer metal cup. Contact analyses were performed between the articulating surfaces under physiological loading conditions, including normal walking, climbing upstairs and downstairs, using the finite element software ANSYS. The results show high initial contact pressure along the periphery of the liner due to the oversize of the femoral head. The maximum contact pressure was found in the superior-posterior quadrant, which correlates well with the location and the direction of the wear. Both wear particles and stress shielding may have contributed to the periprosthetic bone loss and ultimately the late loosening. Reduction of the interference between the liner and the femoral head seems to be effective in the reduction of the initial contact pressure.
During a flight over sea, the right-hand trailing-edge flap (TEF) from an RAAF F/A-18 separated. Although the flap was not recovered, the mode of failure suggested that the outboard hinge lug was first to fail. A fleet-wide inspection of the TEF outer hinge lugs disclosed two other cracked hinge lugs. Examination of these cracks revealed that they had propagated as the result of a combination of corrosion and fatigue, in that the corrosive environment appeared to have accelerated the fatigue crack growth rate. Laboratory experiments indicated that, in a salt solution, the 7050 aluminium alloy from which the lugs were manufactured is susceptible to corrosion fatigue and that the observed cracking matched that on the cracked lugs: corrosion fatigue was therefore suspected to have been a contributing factor in the unexpected failure. Quantitative fractography on the fatigue cracks, combined with several assumptions and deductions, including information about the failure size of the crack gained from witness marks found on the monoball bearing of the failed TEF, led to an estimate of the fatigue crack growth rate. This crack growth rate and the short service life clearly indicated the limitations of the then-current non-destructive inspection (NDI) technique, and a more sensitive inspection method was developed to assist in the maintenance of adequate RAAF flying hours between inspections. This failure investigation provided an excellent example of how fractography, laboratory experiments and NDI can be combined to determine a possible cause of failure and to establish remedial measures for the fleet.
The Cr–Mo steels which are widely used for pressure vessels in refineries and petrochemical plants, have a potential for hydrogen and temper embrittlement. During long-time service the embrittlement leads to decrease of the critical flaw size of brittle fracture and/or to the reduction of the remaining life of a pressure vessel. In this investigation the effect of high temperature and high pressure hydrogen on a vessel, made of 3Cr–1Mo low-alloy steel is studied. Inspections show that the only detected crack in the base metal is originally formed by welding defects and calculations show that it will not grow up. Therefore, it is predicted that the operation of the pressure vessel in normal condition and under regular supervision can be continued.
The premature failure of nickel 200 sintered filter cartridges used to separate the solid catalytic particles has been investigated. The failure is due to the formation of holes and transversal cracks, which occur preferentially on the upper part of the cartridges, near the welding. Microstructural and fractographic characterisation indicated that the failure started on the internal surface of the cartridges, which presented intense secondary intergranular cracking associated with corrosive attack. Fractographic examination on the surface of the transversal crack near the external surface indicated the action of a ductile intergranular fracture mechanism, while near the internal surface it featured a faceted and jagged topography, with cracks propagating inter and transgranularly. The results suggested that the premature of the cartridge failure was promoted by an environmental-assisted mechanism (stress corrosion or hydrogen embrittlement).
A fractured vitallium plate used as a fixation device was investigated for determining the basic mechanism/s responsible for premature failure. Microhardness tests, tensile tests and extensive scanning electron microscopy were carried out to establish the dominant mechanism of fracture. From the detailed fracture surface analysis and the background information provided on the implant, it was determined that the implant failed by the mechanism of corrosion fatigue at the interface of implant plate and screw used in the bone fixation device.
This paper presents the results obtained in the study of the cracking problems on the driving gearboxes of series 2600 locomotives. These components were under big vibration and severe fatigue conditions. The cracks in these components appear in two distinct parts: in the upper zone of the cover and in the frontal central zone of the body of the housing. The loading data was obtained in service, in two routine journeys: Lisboa–Porto intercity train, with maximum speed of 160 km/h and Entroncamento–Guarda freight train, with a maximum speed of 120 km/h. Finally from a synthesis of all the data obtained a set of conclusions are presented to prevent further failures in the driving gearboxes.
The objective of this work was to identify the conditions and mechanisms for failure of burst disks employed in a batch supercritical water oxidation (BSCWO) reactor and to recommend solutions for avoiding burst-disk failure. Two alloy C-276 burst disks failed prematurely during successive operations of a BSCWO reactor designed to destroy simulated chemical munitions. Analysis of temperature data from the two operations led to a hypothesis where an aggressive subcritical environment developed in the burst-disk housing, while the environment in the reactor interior was in the supercritical state. The hypothesis that a detrimental local environment developed in the burst-disk housing was tested by conducting experiments in bench-scale reactors. Results from the bench-scale experiments confirmed that burst disks failed when a local environment formed in the burst-disk housing in a temperature range from 300 to 330 °C. These temperatures caused supercritical water from the reactor interior to condense on the burst disk, and hydrochloric acid vapor formed from the oxidation of chlorinated hydrocarbons then dissolved in the elevated-temperature subcritical water. Electron microscopy analysis revealed evidence of an environment-assisted failure with extensive secondary cracking near the main fracture-surface. The microscopy evidence was consistent with a scenario where acidic chlorinated solution promoted dealloying of Ni and Fe and cracking of the burst-disk. The cracking was severe enough to reduce the cross-sectional area of the burst disk so that failure occurred at pressures well below the rated pressure.
The fatigue crack initiation lives of notches in AISI 304L austenitic stainless steel weldments were improved after the deep cryogenic treatment of specimens at liquid nitrogen temperature (−185 °C). During this treatment, a significant change in microstructure is developed. Strain induced martensitic transformation occurs. During this transformation, the weld metal expands. This expansion relieves the tensile residual stresses and induces compressive stresses around the welded part. However, it was observed that the fatigue crack propagation properties were not changed due to this treatment. Center crack tension type specimens were used in this investigation. Constant amplitude fatigue experiments with R = 0 were carried out with a frequency of 30 Hz.
During 3 months storing of 50% KOH in a 40-m3 cistern the content of iron in KOH increased above the allowed limit of 10 mg/l. The analysis of circumstances revealed that the only possible source for iron is corrosion of basic material and/or corrosion of welds. Chemical analysis of base material and weld confirmed that both corresponded to declared stainless steel AISI 316 Ti. Metallographic analysis showed the presence of martensite in the outer and inner surface layers with pitting and intercrystalline corrosion. Shotblasting caused plastic deformation and the appearance of deformation induced martensite in both surface layers of the cistern. The presence of martensite diminished corrosion resistance and enabled the start of the corrosion process which increased the content of iron in 50% KOH during the storing period.
The paper presents a methodology for shape optimisation of structures with fracture strength as the design objective. There have been limited applications of durability based optimisation to realistic structures. To overcome this, we present a 3D biological algorithm that uses fracture parameters as the design criteria. Damage tolerance optimisation is illustrated via the problem of optimal design of ‘a through-hole in a rectangular block under biaxial loading’. A wide range of 3D flaws were considered to investigate the effect of crack parameters on optimal shapes. It was found that the optimum hole shapes were approximately elliptical with the aspect ratios being dependent on crack sizes, structural geometry and boundary conditions. It has been shown that the fracture strength optimised shape can be quite different from the corresponding stress optimised solution. This emphasises the need to explicitly consider fracture strength as the design objective. In all cases, a significant reduction in the maximum stress intensity factor was achieved with the generation of a ‘near uniform’ fracture critical surface. The effect of crack aspect ratio on the optimal solutions was also investigated and it was found that the relative dominance of the stress intensity factors at the deepest and surface points of the cracks governs the optimal shapes. The design space near the ‘optimal’ region was found to be relatively flat. This is beneficial as a significant structural performance enhancement can be achieved without the precise identification of the local/global optimum solution.
In this paper, the effect of the tensile pre-strain on ratcheting process in 430 Stainless Steel was examined by performing a series of fully tensile–compression cyclic loading tests on the tested material with three tensile pre-strain levels or no prior deformation. The experimental results indicate that a compressive cyclic creep occurs in all applied cases for the tested material without pre-strain. Clearly, the observation reflects that the tested material exists the an-isotropic in tension and compression. Furthermore, for the tested material with various value of pre-strain, cyclic creep can also be found and the direction of creep deformation is always opposite to that of the given pre-strain. From an experimental observation on the residue deformation produced by cyclic creep, it is found that the stable total compressive creep strain scaled with increasing tensile pre-strains at the same stress amplitude condition. The material with greater compressive creep strain responded to the same applied tensile pre-strain level with higher controlled stress amplitude. Exploring the effects of the tensile pre-strain on the stable stress–strain response, this paper has been focused on those items. They are the strain range, Δε, plastic strain range, Δεp, and plastic strain energy density, ΔWP at half-life. Experimental results show that the material with different tensile pre-strains or without pre-strain had a higher response when the applied stress amplitude range was increased. Those indicated material responses have increased at increasing tensile pre-strain. In observation the effects of tensile pre-strain on the fatigue, it is found that the effect of the tensile pre-strain is to reduce the cycles to failure. A decreasing fatigue life is observed with increasing the tensile pre-strain level. In the domain of high-cycle fatigue life (in the range of 105–107 cycles), the effect of tensile pre-strain on degrading fatigue life is obvious and the degree of reducing fatigue life is governed by the magnitude of the stress amplitude. Moreover, the damage parameter based on the total plastic strain energy, Wf=∑ΔWp, can produce satisfactory life prediction results for 430 Stainless Steel with a tensile pre-strain effect under fully reversed tension–compression loading condition.
The anisotropic plastic behavior and the fracture of as-received and hydrided Cold-Worked Stress Relieved Zircaloy-4 cladding tubes are investigated under thermal–mechanical loading conditions representative of Pellet–Clad Mechanical Interaction during Reactivity Initiated Accidents in Pressurized Water Reactors. In order to study the combined effects of temperature, hydrogen content, loading direction and stress state, Axial Tensile, Hoop Tensile, Expansion Due to Compression and hoop Plane Strain Tensile tests are performed at room temperature, 350 °C and 480 °C on the material containing various hydrogen contents up to 1200 wt. ppm (hydrides are circumferential and homogeneously distributed). These tests are combined with digital image correlation and metallographic and fractographic observations at different scales. The flow stress of the material decreases with increasing temperature. The material is either strengthened or softened by hydrogen depending on temperature and hydrogen content. Plastic anisotropy depends on temperature but not on hydrogen content. The ductility of the material decreases with increasing hydrogen content at room temperature due to damage nucleation by hydride cracking. The plastic strain that leads to hydride fracture at room temperature decreases with increasing hydrogen content. The influence of stress triaxiality on hydride cracking is negligible in the studied range. The influence of hydrogen on material ductility is negligible at 350 °C and 480 °C since hydrides do not crack at these temperatures. The ductility of the material increases with increasing temperature. The evolution of material ductility is associated with a change in both the macroscopic fracture mode of the specimens and the microscopic failure mechanisms.
This paper reports an investigation that was carried out on two damaged crankshafts. They were diesel van crankshafts that were sent to be ground, after a life of about 300,000 km each. Some journals were damaged on each crankshaft. After grinding, and assembling on the diesel van, the crankshafts lasted about 1000 km each, and the journals were damaged again. The crankshafts were then sent to be investigated. Different laboratory tests were carried out in order to discover what could have been the cause of the damage. Different typical crankshaft failures were assessed, and will be discussed in this paper. The cause of the damaged journals was found to be a wrong grinding process that originated small thermal fatigue cracks at the center of the journals, on both crankshafts. These almost invisible cracks, with sharp edges, acted as knives originating a very quick damaging of the journal bearings, and as a consequence damaged the journals themselves.
The current methods of fatigue life estimation do not model the interaction of stress with microstructural discontinuities. An experimental study of fatigue crack nucleation in 7075-T6 and 7079-T6 aluminum alloy sheet was performed to obtain more information on the role of discontinuities in the microstructure and to determine the relative contributions of each type of discontinuity. Some specimens were machined from recently manufactured material without any coating (cladding or anodizing). Some were fabricated from old clad-anodized material from an aircraft fuselage and old anodized material from a stock of unused wing panels. In the uncoated materials, large constituent particles and the surface roughness were found to be important factors in fatigue crack nucleation. In the coated materials, discontinuities related to the cladding and anodizing appeared to be the controlling factors in crack nucleation and particles did not play any role. The coated specimens showed a low scatter and a relatively low fatigue life. Multiple nucleation sites were observed in the coated materials.
Al alloy AZ5G (AFNOR 7020) forgings, blocks, rods, rings etc. have been successfully used for the fabrication of liquid propulsion engine components of satellite launch vehicles. Of late, a few components have failed at various stages like fabrication, storage, assembly, pressure testing etc. One particular component which encountered cracking was analysed in detail. Detailed metallurgical analyses carried out on this component revealed high levels of residual stresses and unacceptable microstructure in the form of remnant cast structure with coring as causes for the failures. Residual stress from the thermal quenching and machining acted synergistically with the improperly worked condition of the material. EPMA analysis of the samples revealed segregation and non uniform distribution of the second phase particles throughout the matrix in the failed material. This paper highlights the investigations carried out on these cracked components/raw material and the remedial measures suggested to avoid their recurrence in future. Remedial measures suggested are also listed.
This paper provides an analysis of tread throw of B737-800 airplane tire. The plane experienced tire tread throw during takeoff. Laboratory observation shows that there exist numerous dense beach marks around one shoulder area of the tire. These beach marks shows that the repair exposed cords area around tire shoulder is the origin of tread separation of the incident tire. It is believed that the poor adhesion of the repair exposed cords area caused fatigue accumulative damage to develop into a tread separation. The structure form of the main landing gear of B737-800 airplane leads to unbalanced loadings on the double-tire (wheel) of the main landing gear, this gave a promotion to tread throw of the tire. For this reason, repair limits for damages at tire shoulder should be controlled more strictly for tires of B737-800 airplane. A large deep foreign object damage (FOD) cut found on the center tread rib is ruled out to be the cause of the tire tread throw according to the sequencing crack method called T-junction procedure in wreckage analysis. Tire FOD cut simulation test was carried out to confirm this viewpoint about FOD cut.
Al–Li alloys are characterized by a strong anisotropy in mechanical and microstructural properties with respect to the rolling direction. In the present paper, 4 mm sheets of 2198 Al–Li alloy were joined via friction stir welding (FSW) by employing a rotating speed of 1000 mm/min and a welding speed of 80 mm/min in parallel and orthogonal direction with respect to the rolling one. The joints mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests were performed by using a resonant electro-mechanical testing machine under constant amplitude control up to 250 Hz sinusoidal loading. The fatigue tests were conducted in axial control mode with R = σmin/σmax = 0.33, for all the welding and rotating speeds used in the present study.
The survivability of a Formula driver in an accident is achieved by a combination of the crash resistance of the car and its ability to absorb energy. This has been achieved by providing a survival cell (the chassis), which is extremely resistant to damage, around which energy absorbing devices are placed at strategic points on the vehicle. Since the late 1980s the controlling body of Formula 1, FIA, has introduced a series of regulations to ensure that the cars conform to stringent safety requirements and build quality. Each vehicle must satisfy a list of requirements, in the form of officially witnessed tests, before it is allowed to race. There are two groups of tests that must be passed. The first is a series of static loads applied to the chassis, which guarantees the strength and integrity of the survival cell. The second series defines the position, and effectiveness of the energy absorbing structures. In order to keep the weight of the car to a minimum, some of the energy absorbing devices are also required to perform a structural function in addition to their ability to absorb energy by controlled disintegration. The Rear Impact Structure (RIMP), for example, sits on the differential cover at the rear of the gearbox. The function of this device is to react the downforce generated by the rear wing assembly as well as to provide protection in the event of a crash. Engineering structures are generally designed such that they do not fail. Producing a component, which is specifically intended to fail in a catastrophic, but controlled manner, presents a unique set of problems to the designer. When the same piece is also required to perform as a load-bearing member, the engineering problems are far more acute. The design process from materials selection to a finished component is discussed for an F1 RIMP. Particular reference is made to the design and analysis process and how it deals with the conflicting aspects of controlled fracture and load bearing durability. The analysis and testing process required to prove the piece fit for purpose and homologated for competition are also covered.
The environmental factors such as hot-wet and pre-compression may lead to degradation in energy absorption properties of polystyrene foam in protective helmets. In this paper, an experimental study on the mechanical properties of Expanded Polystyrene (EPS) foam was conducted using high precision micro-force compression tests. Tests were conducted with foam specimens from actual helmets at pre-compression strain ranging from 10% to 50% and hot-wet conditions at 60 °C with different relative humidity (RH) levels between 40% and 85%. The load–displacement behavior of EPS foam was carefully recorded for determining the compressive stress–strain curves. The test results indicated that when the pre-compression strain exceeded 10%, the elastic stiffness and plastic yielding region (plateau region) decreased and the plastic yielding point increased dramatically. When the pre-compression strain reached 50%, the plateau region vanished from the stress–strain curves. High humidity has a significant effect on lowering the plastic yield point, but has little effect on the plateau region range. The stress–strain curves from these tests were successfully implemented into a validated helmet finite element model to simulate guided helmet free drop experiments. Headform peak decelerations and head injury criterion (HIC) values were obtained from simulation and analyzed to estimate the energy absorption degradation in EPS foam. The finite element analysis predictions showed that pre-compression significantly degrades the energy absorption properties of EPS foam. The maximum permissible pre-compression strain should not exceed 10%. High humidity levels lead the helmet to transmit a lower HIC value but wider HIC duration to the head.
This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS) under lap shear loading condition. DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. Static weld strength tests using lap shear samples were performed on the joint populations with various fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with conventionally required fusion zone size of cannot produce nugget pullout mode for both the DP800 and TRIP800 welds under lap shear loading. Moreover, failure mode has strong influence on weld peak load and energy absorption for all the DP800 welds and the TRIP800 small welds: welds failed in pullout mode have statistically higher strength and energy absorption than those failed in interfacial fracture mode. For TRIP800 welds above the critical fusion zone level, the influence of weld failure modes on peak load and energy absorption diminishes. Scatter plots of peak load and energy absorption versus weld fusion zone size were then constructed, and the results indicate that fusion zone size is the most critical factor in weld quality in terms of peak load and energy absorption for both DP800 and TRIP800 spot welds.
While a wheel was being replaced on the subject aircraft, a crack was found on the rear axle bore of the left-hand main landing gear truck beam. This part had been overhauled 11 months earlier. One year later, while the subject aircraft was being parked, two loud bangs were heard coming from the right-hand main landing gear. Upon inspection, the right-hand truck beam was found cracked longitudinally at two locations on the rear axle bore. Microscopic examination revealed two crack origins, one on each side of the bore. Both cracks propagated from corrosion pits under the chromium plating in a stable intergranular mechanism. The final overload fracture produced quasi-cleavage features. Nital etch, following removal of the chromium plating, revealed areas of overtempered and untempered martensite indicative of heat damage incurred during abusive grinding. The hardness of the material in the heat-affected areas and in the areas adjacent to the origins was lower than that of the surrounding tempered martensite structure. These heat-affected areas were located in the chromium plating runout plateau adjacent to the counterbore transition radius and exhibited numerous thermally induced secondary cracks. It was concluded that the cause of failure was improper overhaul process, which left grinding burns and cracks beneath the chromium coating. Subsequently, electrolyte that penetrated through these cracks promoted the formation of pits beneath the coating, which served as preferred sites for failure initiation.
In the present work, the failure of galvanized iron pipes accessories has been analyzed. These accessories are part of a fire protection system in an industrial building. It has been observed the confluence of several circumstances, as defects in the design or in the execution of the installation, false alarms with filling of the pipe installation, geometry of the accessories made in malleable white-heart cast iron and inherent superficial defects of this type of accessories, that have lead to the sudden failure of an elevated number of such accessories during one of the fillings of the installation. An exhaustive macro and micrographic analysis has been done, in order to identify the combined fracture micro-mechanisms observed for this material.
The sport of bungee jumping has become very popular worldwide over the past decade. The author was recently commissioned to investigate a fatal accident, which occurred during a bungee jump. As a result of this investigation, it has emerged that the equipment used for bungee jumping has evolved in an empirical way, using a mixture of braided rubber cords originally intended for aeronautical applications, and ancillary equipment designed for climbing protection. Many sporting organisations and government agencies have established codes of practice for bungee jumping. However, these codes are essentially empirical, and are not based on a quantitative materials engineering analysis of the forces generated in the load train in relation to the strength of the components. The fatal accident is presented as a detailed case study, in which the load/extension characteristics of the bungee rope and the end attachment webbing are measured, and used as the critical inputs to an energy-based analysis of the complete jumping process. It is shown that the bungee rope was unable to absorb all the potential energy of the falling jumper, with the result that the jumper broke away from the bottom end of the rope. The paper also discusses the urgent need for a quantitative code for the design and use of bungee jumping equipment, based on rigorous materials engineering analysis.
The engineering forensic analysis of a traffic accident involving a truck and a bus is presented. Fractographic, metallographic and mechanical studies and numerical models of load transfer and crack propagation were made. A mechanical failure due to fatigue crack propagation was the immediate cause of the accident. However, a series of other factors contributed to the accident, which are also discussed.
This paper describes the background to a major accident involving a Royal Australian Navy helicopter, and discusses the scientific and technical aspects involved in the on-site accident investigation. This particular investigation was a particularly complex one which highlights a number of difficulties involved in conducting technical investigations in a remote location, and the paper will focus on describing the broad progress of the on-site investigation while highlighting the key issues and those where it was possible to overcome major hurdles and reach sound conclusions.
The concrete substructure of the Sleipner A platform sank during a controlled ballast test operation in August 1991. The accident, the investigation methodology, the course of events, the technical cause as well as supporting model tests are described.
In 1998 a very severe railroad accident occurred in Germany. The case went to court for negligent homicide after a preliminary investigation had been performed. The accident had been caused by fracture of a wheel and the manufacturer of the wheel and the railroad company were accused. The defendants engaged a number of experts to investigate the different technical aspects of the accident for their defence. In spring 2003 the court decided to employ an unique procedure, to hear all experts consecutively to get the best possible overview of the different opinions and possibly find the real cause of the accident. After the court had heard the testimony of these 13 experts from 5 different countries it decided to discontinue the case since the guilt of the accused was deemed to be very small, if there was any guilt at all and that further technical investigations and expert testimony would most probably bring the court no nearer to a conviction. A failure analysis proper was not the subject of the court procedure and therefore a complete investigation was not carried out. The result of the hearing was in the opinion of most experts, that the accident could not be explained by the results of the investigations performed. Rather a singular incident or technological material phenomenon could have initiated the fatigue crack, which then caused the accident. Since all realistic aspects of the accident had been thoroughly investigated by the experts, only speculations on such incidents or phenomena were possible.
The catastrophic collapse of Big Blue on the Milwaukee Brewers baseball stadium retractable roof project could be the most awesome lift accident of all time. The crane, a Lampson TransiLift III with a 340 ft main boom and a 200 ft jib, was setting a 100 × 180 × 16 ft open truss panel roof section weighing close to 500 tons at a lift height of 230 ft. With 11 diesel engines, 6 miles of wire rope and 1150 ton counter weight, the 2100 ton crane was a massive machine, indeed. The accident occurred during 26 mph average winds with gusts in the mid 30’s. Three ironworker fatalities and hundreds-of-million dollar damages resulted from this mishap. The ensuing litigation pitted co-defendants Mitsubishi Heavy Industries, the crane lessee/operator versus Lampson, the crane designer/builder, on totally disparate theories for cause and origin of the failure. The paper describes the comprehensive engineering analyses undertaken to disprove the Mitsubishi theories of failure as confirmed by jury verdict. Among the topics discussed are: wind tunnel testing, structural analyses of the boom, metallurgy of failed parts from a critical king-pin assembly, and soils engineering work related to ground loads and displacements during the lift. Crucial role of the SAE J1093, 2% design side load criterion and Lampson’s justification for an 85% crawler crane stability criterion are presented.
Military helicopters provide ideal conditions for the nucleation and propagation of failure damage. Very few modern machines are expected to perform under such diverse loading and environmental conditions. It is often assumed that failures are a direct consequence of these extreme operating conditions. This paper aims to demonstrate that the originating causes of failure can be introduced at any point in the life of a component, including the design, materials processing, manufacturing or during the service life of the component. Solutions to prevent recurrence of problems cannot be developed without identification of the root cause. Details of four investigations will be presented. Each case study will demonstrate a physical cause residing in an action/inaction occurring during one (or more) of the key life stages of a component, Design, Processing, Manufacture and Service.
An integrated database and expert system has been developed for identifying the failure mechanism of mechanical components. The system comprises six major modules: database and management system, case maintenance; knowledge acquisition and editing; expert system; explanation and test-recommendation facilities; and user interface. Part I of a two-part paper details the knowledge acquisition and editing module, as presented here. Part II describes the remaining modules and also gives test results (Liao TW, Zhan Z-H, Mount CR. Engineering Failure Analysis 1999;6:387–406). The method used for automated knowledge acquisition is an inductive learning algorithm, which was modified from PRISM (Cendrowska J. Int J. Man–Machine Studies 1987;27:349–70) to handle noisy and missing data. Using the algorithm, a total of 48 rules were induced from 477 training examples gathered for the identification of 15 different failure mechanisms such as brittle fracture, fatigue, and stress corrosion cracking. Fifty-nine attributes were used to distinguish one failure mechanism from the others. They include pitted, beach marks, microvoids, etc. The knowledge editing function is provided to allow the verification of induced rules by the human expert.
The failure of the conductor of the 460 kV overhead transmission line located along the crossing of the Paraná River was investigated. As a result of this failure, a blackout took place in the southern states of Brazil in January of 2002, reaching approximately 67 million inhabitants. Investigation of the external aluminium layer of the ACSR conductor near the fracture showed typical static deformation marks and dynamic fretting wear tangential marks, both associated with the presence of Al2O3 debris. Additionally, encrusted silicon particles were observed on the external surface of the strands. The internal Al layer showed elliptical deformation marks also associated with Al2O3 debris. Both Al2O3 and Si particles are efficient abrasive material, which associated with slight relative motion of metallic surfaces (clamp/strand and strand/strand) can promote fretting wear. Two types of fracture surface of the Al strands were identified: 45° and quasi-normal surfaces, the former being the predominant type. Delamination and particle detachment were the mechanisms of superficial degradation observed on the Al strands, indicating that the rupture of the strands occurred under a gross slip fretting regime induced by sub-conductor oscillation. Inspection of the internal surface of as-cast Al–10%Si spacer clamps revealed different stages of intense circumferential wear caused by the preferential cracking and particle detachment of the eutectic constituent of the as-cast microstructure. This intense wear reduces the clamping contact pressure, which allows higher displacement amplitude and leads to the critical fretting regime. Finally, a few suggestions are discussed to minimise the occurrence of future failures.
An austenitic stainless steel adaptor used in the command pressure distributor assembly of a test stand for liquid fuel engines cracked during one of the ground tests. The failed component was studied through metallographic techniques to analyse the cause of failure. The study revealed that the crack initiated at a location containing a brittle iron precipitate rich in silicon and propagated by vibration-induced fatigue through the tube wall.
The deployment of peacekeeping forces in conflict areas has shown that some armour systems are not sufficient to meet the latest threats.This applies particularly to lightweight vehicles whose armours give protection only against low calibre ammunitions.The development, production and fielding of add-on armours gives the answers to the demand for mission adjustable protection systems.Add-on armours represent a new protection philosophy, because they are produced as a separate kit, designed to achieve different protection levels. They have several advantages: Separate transport of armour and vehicle, they can be screwed to the main skull structure by crews and they are easily repairable or upgraded.Advanced add-on armours are produced by a clever combination of ceramic tiles backed by metal or composite plates.The utilization of advanced ceramics began in the 1960s when the US Army was demanding lightweight body armours for helicopter crews. Nowadays a wide spectrum of advanced ceramics is currently used for armour production, including alumina, silicon carbide, titanium diboride and boron carbide.Ceramics possess a high protection potential due a moderate density combined to a high compressive strength. But they are too brittle to be used without a ductile material backing.The design of ceramic add-on armours is a difficult task due to the high number of parameters involved: material selection, thicknesses of different materials, impact obliquity, etc. A design based exclusively on experimental tests is therefore expensive in money and time.This paper summarizes the utilization of analytical and numerical computation of ceramic/metal and ceramic/composite add-on armour failure process as valuable tools for armour design optimisation. Some examples are presented showing a good agreement between analytical, numerical and experimental results of residual mass and residual velocity of kinetic energy projectiles after perforation of the add-on armour.
The article presents outcomes from finite element and X-ray diffraction analyses of the cold hole expansion process fulfilled by means of solid mandrel gradually passing through the hole. Regardless of the manner of accomplishing – split sleeve; split mandrel; with a mating tapered split sleeve; spherical mandrelling – an axial gradient of the residual stresses is obtained due to the interaction between the layers caused by an axial force flow which passes through the mandrel, the workpiece and the support to forma closed circuit. To predict the fatigue life it is necessary to know the residual stresses on the entrance and the exit faces immediately to the hole periphery which are considered to be potential crack initiation sites. It has been proved that there are two key moments in the building of an adequate finite element model. The first is modeling of a realistic contact mandrel-workpiece with or without mediator and workpiece-support. The second is a suitable constitutive model of the workpiece material. Six hardening models of low-carbon steel obtained on the basis of a symmetric strain-controlled experiment and half-cycle test data from unidirectional tension experiment have been used consecutively in the finite element model of the process. On the basis of a comparison with experimental results obtained by means of an X-ray diffraction technique it has been established that the nonlinear kinematic hardening model obtained by a strain-controlled cyclic test to achieve a stabilized cycle secures finite element results close to the experimental ones. The chosen finite element model has been compared with simplified ones following the residual circumferential normal stress criterion and its advantages have been proved.
A dilemma encountered in engineering practice is a proliferation of newly designed (mostly high-strength and/or corrosion-resistant) steels and alloys that are unusable in industry as they are highly susceptible to failure under operating conditions including environmentally assisted cracking. The problem of materials failure has several sources, the most significant of which is how engineers select which material to use in which industry. As a rule, selection is based solely on assessing the mechanical properties of materials with little or no consideration of how these mechanical properties will interface with specific operating parameters found within different industries. The functional design, selection and use of materials aimed at preventing in-service failures depends, therefore, on finding testing methods, standards and approaches appropriate to real operating conditions guided by analysis of material performance under those conditions.
The influence of hydrogen charging on the room-temperature tensile properties and fracture behaviour of two dissimilar weld joints has been investigated. The weld joints were either ferritic/ferritic (T91/STN15128) or ferritic/austenitic (T91/TP316H). The tensile tests were carried out using the samples with a circumferential notch. The position of notch was individually located in different weld joint regions, either in the heat-affected zones (HAZ) or weld metal (WM). The application of hydrogen charging had detrimental effect on strength and plasticity of the weld joint T91/STN15128. The most significant deterioration of the notch tensile properties was measured for the STN15128 HAZ. The hydrogen charging had only small influence on the strength of the weld joint T91/TP316H but remarkable detrimental effects on the plasticity. In the PWHT state (without hydrogen charging) all regions of the studied weld joints fractured by ductile dimple tearing. The failure initiated on the secondary phase particles and/or inclusions as well. In contrast, the failure after hydrogen charging initiated in the vicinity of sizeable particles and showed a transition from the ductile dimple tearing to the transgranular cleavage and/or quasi-cleavage fracture mode.
The retaining ring of an aero-engine failed in fatigue mode. Metallurgical investigations revealed that there was a high density of inclusions in the material which are not deformable. During deformation, they disintegrated and formed voids which could act as sharp notches during fatigue loading. Axial surface cracks were visible on the surface of the ring. It is proposed, as found in previous failure analysis studies, that surface cracks trap lubricants used during the drawing process of the wires, and carburize the region during subsequent annealing. These regions can also be potential sites of fatigue crack initiation.
The paper presents a failure case of an aero-engine combustor liner. Macroscopic and microscopic observations, microstructure investigation, chemical analysis and hardness measurement have been performed to investigate the damage mechanism and failure causes of notches and cracks. The results show that the failure modes of notches, axial-direction crack along the combustor liner and crack at the edge of welding spot are respectively attributed to high-temperature ablation, mixed crack of ablation and thermal fatigue crack. The bad configuration of crossover tube is the main reason for the failure of combustor liner.
This paper presents the failure analysis of the turbine disc of an aero engine, installed in a certain type of aircraft. From the visual examination of the fractured surface, it was possible to observe beach marks, typical of fatigue failure. A non-linear finite element method was utilized to determine the stress state of the disc/blade segment under operating conditions. High stress zones were found at the region of the lower fir-tree slot, where the failure occurred. A computation were also performed with excessive rotational speed. Attention of this study is devoted to the mechanisms of damage of the turbine disc and also the critical high stress areas.
In spite of the high levels of reliability of modern aeroengine components resulting from rigid standards and practices, failures of compressor and turbine blades during normal operational environments are common situations which compromise the flight safety. The investigation of real failures affecting these components allows gaining a deeper knowledge concerning the mechanisms of crack initiation and propagation which, in turn, can be used in order to prevent future incidents or accidents.This paper presents the analysis of two in service failures involving blades breakage belonging to different compressor stages. Crack growth mechanisms were evaluated based on the visual inspection of the affected components and with both macroscopic and microscopic observations of the fracture surfaces. The origins of crack initiation were evaluated through the examination of crack path and beach marks on the fracture surface.Mechanical analyses were carried out to identify the possible causes of the failures by examining anomalies in the mechanical behaviour of the materials such as hardness tests, chemical composition and surface coating analysis through SEM observations.The analysis of the different fracture surfaces shows that crack propagation is mainly related with fatigue mechanisms whilst crack initiation can be attributed to distinct causes, either the presence of defects in the surface of the blade due to impact of debris or intrinsic material defects or some degrading mechanisms affecting the internal microstructure of the material. In some cases the blades were in service after having being object of an overhaul procedure, which can justify some crack initiation cases that caused in service failures.
Many aeronautical fastners are exposed to cyclic stresses during service. Therefore, such parts are usually designed for limited fatigue lifetime. Various combinations of process type and sequence may be employed to produce threads, each resulting in different fatigue properties. Specifications of aircraft bolts often require production of threads by heat treatment followed by rolling, in order to improve the fatigue properties. Unfortunately, these specifications are not always followed to the letter. Therefore, for either quality assurance or failure analysis purposes, it is important to be able to determine unambigiously the process by which threads were produced. The objectives of this work were to study the effect of varied thread manufacturing process type and sequence on the mechanical properties of AISI 4340 stud bolts, and to develop a laboratory procedure for distinguishing between them. Threads were produced on heat-treated and non-heat-treated stud bolts either by machining or cold-rolling. The non-heat-treated bolts were subsequently heat-treated. All bolts were then subjected to mechanical testing (static tensile, dynamic fatigue, hardness and microhardness tests), metallographic and fractographic examinations. While the fatigue properties were significantly affected by the manufacturing process used, no effects on the tensile strength of the bolt were observed. Metallographic inspection and microhardness testing, but not fractographic inspection, were found to be effective for distinguishing between different manufacturing procedures.
The attention of the present study was focused on the aid provided by lock-in thermography for non-destructive evaluation of aerospace materials and structures. The experimental analysis was performed by testing several specimens, which were made of different materials employed in the fabrication of aircraft (composites, hybrid composites, sandwiches, metals) and which included the most commonly encountered kinds of damage (delamination, impact damage, fatigue failure). (c) 2005 Elsevier Ltd. All rights reserved.
The use of AlBeMet AM162, an aluminum–beryllium metal matrix composite, is an effective way to reduce the size and weight of many structural aerospace components that are currently made out of aluminum and titanium alloys. These savings, which are essential for today's technologies, are primarily due to the material's high modulus and low density combined with its better than average specific strength. The raw material costs are significantly higher than they are for traditional engineering alloys, and the available billet sizes are more limited than they are for aluminum and titanium alloys. Although the material costs are higher for the aluminum–beryllium composite, ease of machining makes it financially competitive when compared to equivalent parts made out of titanium alloys. Due to toxic nature of beryllium, however, debris-generating operations must be strictly regulated to limit worker's exposure to the material and protect them from potentially fatal diseases.
The effect of isothermal ageing on two high temperature, bismaleimide composite materials, a novel CSIRO CBR 320/328 composite and a commercial CIBA GEIGY Matrimid® 5292 composite, was examined at 204 and 250 °C. Delamination is a major cause of failure in composite materials, therefore, the Mode I interlaminar fracture toughness (GIC) of both materials was measured using the double cantilever beam (DCB) test. Chemical degradation of the matrix was monitored concurrently using Fourier transform infrared (FTIR) and Raman spectroscopy. Chemical changes at the core of both of these materials were found to occur concomitantly with the observed changes in interlaminar fracture toughness. FTIR analysis of both matrix materials revealed the predominant degradation mechanism to be the oxidation of the methylene group bridging two aromatic rings common to the structure of both resins, and was substantiated by the ingrowth of a broad peak centred at 1600 cm−1 . In addition to this, the pyromellitic anhydride unit present only in the CBR 320/328 composites was found to be highly resistant to the effects of ageing, whereas the saturated imide, common to the cured structures of both materials, was observed to degrade. Raman spectroscopy indicated that the predominant degradation mechanism of the composites differed at the two ageing temperatures.
Some features of the problem of pipeline materials degradation and failure under in-service conditions, including environmentally assisted cracking, are examined. As the practice of cathodic protection results in hydrogen charging of carbon steels used as components of oil and natural gas transmission pipelines, close attention is paid to the role of hydrogen embrittlement (hydrogen-induced cracking) in pipeline safety and reliability.
Transport is a vital cog in the world economy. However, the cost of new aircraft has meant that there are now aging fleets whose continued airworthiness requires special analysis and improved crack detection techniques. This paper highlights the need for research into the field of structures containing both multi-site and widespread fatigue damage, topics which have recently received worldwide attention. To adequately explain the work that has been completed in these fields, a brief introduction to some of the more common terminology will also be given.
In Australia, the post-war years saw rapid economic growth, and was the period during which much of the current infrastructure was constructed. However, there are now doubts on its durability and structural integrity. In the aeronautical scene, composite repairs have been shown to be a very effective way of maintaining the operational performance of damaged aircraft structures, and they may well be one method for addressing the problems of aging infrastructure. To this end, the present paper evaluates the ability of externally bonded composite (fibre glass) wraps to increase the load-carrying capacity of both damaged and undamaged concrete beams. Particular attention is paid to the failure mechanisms, and to verifying the feasibility of using glass-reinforced plastic wraps to repair degrded concrete beams.
This research conducted an analysis on three failed rocket motors, which are of different types and were used at different times, in order to identify the root cause of cracks occurring in the wing attachment of the Aim-9 (Air Interceptor Missile) rocket motors. Through visual inspection, it was found that the corrosion severity was in proportion to the failure rate in each crack surface. Chemical analysis, microscopic examination, and residual stress determination on the crack surfaces have been carried out to find out the cause of crack initiation and the mechanism of crack growth. The crack origin was covered with mud cracks produced by an interaction of corrosion and stress, and the cracks were propagated along the grain boundary in the longitudinal direction. Also, corrosion-promoting components were found on the crack surface, and the tensile residual stress acted on the channel area of the rocket motor. As a result of the analysis, it is concluded that stress corrosion cracking occurred due to the interaction of corrosive environment and tensile residual stress, which are latent in the rocket motor body.
An in-service failure of a thin-walled titanium bleed-air duct from a wide-bodied commercial aircraft has been investigated. Cracking had occurred in the heat-affected zone (HAZ) adjacent to a circumferential weld joining two sections of the duct which was manufactured from commercially pure (grade 3) titanium sheet. Specimens were cut from the duct to include an intact weld and tested under known conditions (overload, fatigue, sustained loading) for comparison with the failed duct. Metallographic observations showed that cracking occurred through an acicular α HAZ, and fractographic observations revealed brittle, cleavage-like cracking with occasional areas of fatigue striations for both the failed duct and fatigued specimens. These observations, and the absence of sustained-load cracking in test specimens, suggested that the in-service failure had occurred primarily by fatigue. Observations also indicated that small, cleavage-like cracks had been present in the ducts prior to service, although whether these cracks were caused by overload tearing, stress-corrosion cracking, hot-salt cracking or fatigue was not clear. Other possible causes of cleavage-like cracking, e.g. the presence of hydrides as proposed for previous failures, contamination of HAZs by oxygen/nitrogen, are discussed. Possible ways of preventing further failures are then outlined.