Bending overload and thermal fatigue fractures in a cast exhaust manifold

The left exhaust manifold of a Ford Y-Block engine failed at two different locations after nearly fifty years of service. A study was conducted in order to determine if the main causes and the sequence of failure of this component were related to its operation. A detailed analysis comprising materials characterization, fractography and a thorough finite element modeling was carried out. The results showed that the normal operation of the engine induced several thermal fatigue cracks, which were primarily located in the outermost runners of the component. One of these cracks propagated unstably during the operation, producing one of the fractures. The other one was generated during an overhaul, when a bending load and an improper assembly condition propagated a preexisting crack. In addition, the presence of a mixed graphite distribution comprising A, B and D flake graphite compromised the mechanical properties of the exhaust manifold, and aided in the process of crack propagation.     

Catastrophic failure of a fan in a diesel engine cooler

The fan used to cool a diesel engine fractured catastrophically after approximately 100 h of operation. The fan failed at a spider, which was resistance spot welded to a shim placed between two circular spiders of 3 mm thickness. The detailed analysis of the fracture indicated that the premature failure of the fan was due to inadequate bonding between the sheets at the weld nugget. The fracture was initiated from the nugget-plate interface. The inadequate penetration and lack of fusion between the steel sheets during resistance spot welding led to poor weld strength and the fracture during operation. The propensity to crack initiation and failure was accentuated by improper cleaning of the surfaces prior to welding and to inadequate nugget-to-sheet edge distance.     

Failure of an intermediate gearbox of a helicopter

An intermediate gearbox of a helicopter failed resulting in an accident. A systematic failure analysis was conducted to find out the cause of failure. Examination revealed that fatigue fracturing of the driving gear was responsible for the gearbox failure. The teeth of the gear were severely damaged by spalling. Fractographic study revealed multiple fatigue crack initiation at the tooth root regions. It was established that the failure was caused due to improper assembly of the gear. A detailed analysis of the failure is presented in this paper.     

Liquation cracking of Al-6.3 Cu alloy propellant storage tank – A case study

Rings and sheets of aluminium alloy AA 2219 of Al–Cu series are being used for the fabrication of propellant storage tank. Under development phase, one such tank failed in the course of qualification. Detailed metallurgical analyses were carried out on this failed tank, which revealed the presence of large number of incipiently melted & solidified particles as a result of welding/repair welding, along the grain boundaries and also within the grains nearer to weld fusion line. This has resulted in the formation of continuous film along the grain boundaries of already coarser grains of the ring. Such film which formed by liquation at the weld fusion line/HAZ, under the influence of residual stresses due to welding under fixture constraints, especially near the weld repair region, initiated the crack formation. The crack once initiated, found easy propagation through film along the grain boundaries (GB). Electron probe micro analysis (EPMA) of these particles confirmed the film as eutectic, low-melting compound and their non-uniform distribution.This paper highlights the investigations carried out on the failed tank.     

Corrosion Fatigue of a Pump Bearing Journal after Exposure to Two-Phase Flow

A failure of a pressure relief valve in one of the CANDU (acronym for CANadian Deuterium Uranium Reactors, a registered trademark of AECL) reactors during routine testing resulted in the Heat Transport (HT) pumps operating under a Loss Of Coolant Accident (LOCA) condition for a short time. It took 50 min of operation before the reactor could be safely shutdown. During a part of this 50 min period, HT pumps were exposed to two-phase flow conditions. When the reactor was restarted following this incident, one of the HT pumps was operating with high shaft and frame vibrations that reached the shut down limit in a very short time. The reactor was shut down and a detailed inspection of the pump showed that the pump bearing journal on one of the 4 HT pumps had failed and had a large crack extending across its entire length. This paper summarizes the results of extensive investigations carried out to establish the root cause of the journal failure. The failure analysis included a finite-element analysis of the shrink fitted journal, metallurgical analysis, scanning electron fractography of the failed journal, and a design review of the assembly fits. Based on these investigations, the journal failure was determined to be due to corrosion fatigue. The corrective actions implemented to prevent the susceptibility of this component to future failures were detailed and required significant effort and expertise to develop and implement.     

Failure analysis of induction hardened injector body

The failure analysis of induction hardened injector body was carried out to identify surface defects. Producer revealed the defects using nondestructive testing method and requested a detailed analysis to determine the cause of their origin. Metallographic and fracture analysis were performed to study the material microstructure and fracture surface. Metallographic analysis proved the existence of a crack, initiated from the front face of component. Microstructure of the crack vicinity as well as hardness was significantly different with increasing distance from the face of component. Microstructure near the front face consisted of coarse martensite, while finer martensitic structure was observed with increasing distance from the front face. Hardness showed decreasing tendency with increasing distance from the front face. Fractographic analysis revealed the intergranular cleavage fracture near the front face of component. With increasing distance from the front face, quasicleavage fracture was observed with increasing areal fraction characterized by ductile fracture. Due to significant difference in microstructure and corresponding difference in hardness within a small area of component can be assumed, that the crack initiation occurred due to internal stress of the material caused by heat treatment. It is necessary to optimize the parameters of induction hardening process with respect to the different thicknesses of the product.     

Failure analysis of a cylinder sleeve from a turbocharged diesel engine

This work presents the conclusions of a failure analysis performed on a V12 Turbo Charged diesel engine cylinder sleeve. The failure analysis carried out established the most relevant factors that caused damage to the cylinder sleeve. An examination of the internal surface of the cylinder sleeve revealed an elevated number of cavities close to the top center area, which acted as stress concentrators reducing the resistance of the component, creating crack nucleation spots. Additionally there were internal differences in the microstructure of the component, which indicate that different cooling conditions occurred during its manufacture, providing a secondary failure mechanism, due to material fragility.     

Fatigue failure of a Slickline wire induced by corrosion pits

A Slickline wire failed after nearly 400 h of service. In order to find out the main causes and the sequence of this failure, a detailed analysis was carried out on a fracture fragment of this component. This analysis revealed that the operation of the wire produced a series of superficial discontinuities, comprising corrosion pits, fatigue cracking and wear grooves, that provided several stress raisers which served as the initiation point for the failure. Additionally, the manufacturing process introduced some longitudinal cracks that helped in the propagation of the final fracture. Finally, due to the presence of dimples in the last portion of the failure, it could be concluded that the ductility of the material was not compromised.     

Failure analysis on high temperature superheater Inconel® 800 tube

This paper presents failure analysis on a super alloy Inconel^® 800 superheater tube in Kapar Power Station Malaysia. Visual inspection, microscopic examinations and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The failed high temperature superheater (HTSH) tube was found snapped into two parts, heavily distorted shape and bent at several points. Microstructures of the failed tube showed that creep crack initiated at both external and internal surfaces of the tube and propagated as grain boundary creep cavities coalesced to form intergranular cracks. The severe geometry of tube causing steam flow starvation is identified to have caused increasing tube metal temperature resulting in overheating of the failed tube. Creep rupture is revealed as the cause of failure of the superheater tube.     

钛/铁复合板平底球形薄壁件充液拉深工艺研究

目的 选用充液拉深先进成形技术制备钛/铁复合板平底球形薄壁件,并研究其充液拉深变形行为,以解决传统拉深工艺制备平底球形薄壁件极易产生褶皱的问题。方法 对钛/铁复合板平底球形薄壁件在不同液压力、压边间隙及凸模与板料间摩擦因数等工艺参数下的充液拉深过程进行数值模拟。对数值模拟结果进行分析,讨论工艺参数对零件成形性能的影响以及抑制起皱的机理。最后在不同拉深工艺下进行成形试验,制备钛/铁复合板平底球形薄壁件并与数值模拟结果进行对比。结果 数值模拟和成形试验结果表明,传统拉深工艺制备的钛/铁复合板平底球形薄壁件出现了明显的褶皱,采用充液拉深工艺可以有效解决零件侧壁起皱的问题。增大液压力、减小压边间隙或增大凸模与板料间摩擦因数会导致零件减薄率的提高并降低零件侧壁起皱的风险。在压边间隙1.5 mm、液压力25 MPa的条件下,采用充液拉深工艺可以制备出侧壁无褶皱的平底球形薄壁件。结论 通过充液拉深工艺可以有效解决钛/铁复合板平底球形薄壁件成形过程中起皱的问题。     

Failure Analysis of Roller Cone Bit Bearing Based on Mechanics and Microstructure

The service life of the roller cone bit mainly depends on the bearing. In order to figure out the cause and mechanism of bearing failure, mechanics and microstructure analysis of failed roller cone bit bearings are carried out. The results show that the bearing failure mainly includes wear (including adhesive wear and abrasive wear), plastic deformation, crack, fracture and burn. The main reasons for these failures are: abrasives and temperature rise caused by the cuttings and the lubrication failure; stress concentration, shock and vibration due to uneven load and fit clearance; and initial cracks or deficiencies because of unqualified surface treatment. In addition, investigation indicates that seal failure can bring degeneration on the bearing surface, which reduces the hardness of the bearing surface and thus accelerates the failure of the bearing.     

Quantitative Failure Analysis Using a Simple Finite Element Approach

Failure analysis encompasses the examination of a failed component and assessment of the failure situation in order to determine the causes of failure. Metallurgical and manufacturing defects often initiate a crack that subsequently propagates under service conditions, leading to premature failure and/or catastrophic fracture of the component. Traditional failure analysis performed on failed section often constitutes only of qualitative metallurgical and microstructure studies in establishing the causes of failure. Since the failed parts have likely been subjected to tensile overload, localized fatigue damage and/or excessive creep strains, the mechanics aspects of the failure should also be considered in the failure analysis. This includes load and stress analyses, fatigue life calculations, and failure simulations. In this respect, finite element (FE) analysis offers a simple, yet effective approach in establishing the causes of failure by predicting the internal states of strains and stresses in the material relative to the strength of the material during the fracture event. Qualitative metallurgical and fractographic results complement such FE-based prediction of the failure.     

Failure mode and fatigue behavior of weld-bonded lap-shear specimens of magnesium and steel sheets

Failure modes and fatigue behaviors of ultrasonic spot welds in lap-shear specimens of magnesium AZ31B-H24 and hot-dipped-galvanized mild steel sheets with and without adhesive were investigated. The spot welded specimens failed from the kinked crack growth mode. The adhesive-bonded specimens failed from the cohesive failure through the adhesive and the kinked crack growth through the magnesium sheet. The weld-bonded specimens failed from the cohesive failure through the adhesive, the interfacial failure through the spot weld, and the kinked crack growth through the magnesium sheet. The estimated fatigue lives for the adhesive-bonded and weld-bonded specimens failed from the kinked crack growth mode are lower than the experimental results.     

Cracking of AFNOR 7020 aluminium alloy component: A metallurgical investigation

Aluminium alloy of Al–4.5Zn–1.5Mg system, belonging to age hardenable, high strength category is being used for the fabrication of various components. The Al alloy component was part of a conduit line for filling the liquid chemical to its storage tank. The component consisted of an extruded tube, TIG welded to a bulb, which was fabricated from Al alloy forgings. A crack was observed on the conduit tube at the region where tube was welded to the bulb. Detailed investigations were carried out on the cracked component, which revealed insufficient working of the ingot during forging, resulting in remnant cast structure along with a heavy network of low melting point compound throughout the material. The cracks, which were initiated under the stresses induced during thermal treatment, machining and assembly, were found propagating through the eutectic network.

This paper highlights the investigations carried out on the failed components.     

Mechanical and anisotropic behaviors of 7075 aluminum alloy sheets

Formability of 7075 aluminum alloy sheets was studied after annealing of 71% cold worked (CW) samples at different temperatures (270–450 °C). Uniaxial tensile test, deep drawing and Erichsen test were carried out at room temperature to evaluate formability parameters.     

Metallurgical investigations and corrosion behavior of failed weld joint in AISI 1518 low carbon steel pipeline

In this paper, failure analysis was carried out based on the available documents, metallographic studies and corrosion behavior of the welded joint pipe sample made of AISI 1518 low carbon steel. Nondestructive evaluations including penetration test (PT) and radiographic test (RT) were performed on the as-received pipeline and results indicated the presence of micro- and macro-cracks. Optical microscopic images and scanning electron microscopy (SEM) micrographs revealed various microstructures in the base metal (BM), heat affected zone (HAZ) and weld metal (WM). The microstructural variations may result in galvanic feature and lead to failure and rupture of the weld joint during the service. Microhardness measurements showed that hardness value was about 260 HV in the WM, while it declined in the HAZ and BM. Qualitative chemical analyses such as X-ray diffraction pattern (XRD) and SEM equipped with energy dispersive spectroscopy (EDS) confirmed the presence of corrosive media during weld joint rupture. Additionally, SEM and optical investigations indicated that micro-cracks were formed in HAZ due to residual stress as a consequence of improper welding condition. Surface fracture studies showed that the crack initiation, crack growth and finally crack propagation took place in the WM/HAZ interface. Electrochemical studies were conducted on the BM, HAZ and WM to investigate corrosion behavior of the failed joint sample. Finally, a proper corrosion mechanism is proposed based on the failure analyses and electrochemical studies.     

Investigation of a Boeing 747 wing main landing gear trunnion failure

A loud noise was heard from the vicinity of the port wing landing gear during pushback of a Boeing 747-300 from the terminal at Sydney (Australia) airport. Inspection showed that one of the wing landing gear trunnion fork assemblies had failed. Detailed investigation revealed that the trunnion had failed by fatigue cracking. Deep machining grooves were found at the root of an internal radius that had not been shot-peened as required, and a chemical surface process during manufacture had resulted in shallow intergranular attack at the bottom of these grooves. It is probable that the critical cracking started from some of these grooves. In addition, the wall thickness at the failure location was significantly less than the minimum required in the drawings.Since the deep machining grooves, the lack of peening and the intergranular attack were all consequences of manufacturing, the fatigue cracking probably started shortly after the component entered service. This implies that fatigue cracking was present during all the trunnion overhauls, but was not detected by non-destructive inspections during the overhauls. Quantitative fractography was used to produce a crack growth curve based on fracture surface markings thought to represent the overhaul timings. The crack growth curve suggested that the fatigue cracking was large enough to be detected by inspection during the last overhaul, if not the one before. However, it was probably not easy to detect the cracking. This investigation therefore highlights the difficulties that can be encountered when inspection is the last (or only) line of defence against failure owing to unexpected manufacturing deficiencies.     

Numerical simulation of thermal fatigue behavior in a cracked disc of AISI H-11 tool steel

Thermal fatigue is one of the key reasons of material failure in components which are exposed to high temperature cycles. Prediction of crack initiation site and crack propagation speed during thermal cycling helps us predict the life of a component in service. In this research numerical simulation of crack propagation due to thermal cycling on a circular disc has been carried out. The thermal profiles used in the simulation are taken from actual thermal fatigue experiments. The effect of the length of cracks and interaction between adjacent cracks has been investigated. 50 sets of numerical simulation models with different crack numbers and crack lengths have been simulated. The variation in Stress Intensity Factor (SIF), hoop stress and Crack Mouth Opening Displacement (CMOD) has been plotted as a function of primary/secondary crack lengths and number of cracks. Envelopes of possible crack growth have been identified and correspond well to the experimental observations. Results show a significant drop in hoop stress, SIF and CMOD with increase in number of cracks, thus limiting the number of cracks possible in a thermal fatigue crack network.     

Fatigue and fretting of mixed metal self-piercing riveted joint

The fatigue behavior of self-piercing rivet (SPR) joints joining differing thicknesses of AA6111-T4 aluminum and HSLA340 steel sheets in lap shear geometry was investigated in this paper. Crack initiation in the aluminum sheet was the dominant failure mode, while unexpected rivet shank failure tended to occur at high loading levels. Fretting wear was also observed at interface between aluminum and steel sheets as well as between the rivet and sheets under sinusoidal cyclic tension–tension loading. An Energy Dispersive X-ray (EDX) analysis of fretting debris revealed the presence of oxides of aluminum and zinc. Fretting was shown to be critical to crack initiation. For initiations in the aluminum sheet, micro cracks were found to nucleate early in the fatigue life, and crack initiation life was found to be much shorter than crack growth life.     

Failure of cylindrical steel silos composed of corrugated sheets and columns and repair methods using a sensitivity analysis

The paper deals with failure of large cylindrical steel silos composed of horizontally corrugated sheets with vertical stiffeners. The failure reasons were discussed. A linear buckling and a non-linear analysis with geometric and material non-linearity were carried out with a perfect and an imperfect silo shell (with different initial geometric imperfections) by taking into account axisymmetric and non-axisymmetric loads imposed by a bulk solid following Eurocode 1. The 3D FE calculations were carried out with the commercial finite element code “Abaqus”. The calculated buckling forces were compared with the allowable one given by Eurocode 3. Repair methods of silos against buckling were proposed. A sensitivity analysis was performed for a silo to predict the location and profile type of strengthening elements.