Failure analysis of an aircraft engine cylinder head

The piston engine of the training aircraft malfunctioned during the flight due to the cracking of its cylinder head (CH), which is manufactured from an aluminum casting alloy. Based on the fractographic examination of the mating fracture surfaces, the characteristic ratchet and beach marks were observed indicating the occurrence of fatigue failure. The crack was initiated from multiple origins located on the inner flange fillet on the exhaust side of the CH. The metallography examination has shown that the fatigue was promoted from pre-existing material defect due to an elevated presence of shrinkage pores at the crack initiation zone and was most likely associated with the manufacturing process of casting. The finite element (FE) method, utilized to determine the stress state of the CH subjected to gas pressure, also confirmed that the crack origin was located at the most stress area.     

Thermal and Mechanical Failure Analysis of a Two-Stroke Motocross Engine Piston

The failure analysis of an aluminum two-stroke single-cylinder 250 cc motocross engine piston with significant material cracking was performed using both computational and theoretical approaches revealing several contributing factors to the cracking. A main central crack in the piston skirt is the direct result of mechanical fatigue imposed by the contact loads exerted on the piston during cold-start situations. Two symmetric secondary cracks also observed on the piston skirt region are similarly caused by the resulting contact of the piston skirt against the engine cylinder wall. Although thermal fatigue is considered, theoretical calculations dismiss the likelihood that thermal stresses develop as a result of the piston-cylinder wall contact under normal operating conditions. However, under extreme temperatures due to cold start or altered air/fuel ratios, thermal fatigue plays a more likely role. A finite element analysis confirms the critical stress locations resulting from the contact of the piston skirt against the engine cylinder wall, and analyses of the fracture surfaces confirm the initiation and propagation of the fatigue cracks.     

Failure Analysis of a Vehicle Engine Crankshaft

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.     

Cause and effect assessment after a complex failure of a large ethylene compressor

A rusted cylinder liner and excessive wear of piston rings forced several maintenance disassemblies in a 1000 kW ethylene reciprocating compressor. Several months later the compressor failed due to growth of cracks in the crosshead of one of the cylinders. The initiation site was located in material defects near a stress raiser. In order to identify the root cause of the failure, crack growth time calculations were required. The applied stress field near the initiation sites and along fatigue paths were FEM estimated. Stresses vary steeply and become partly compressive along a large part of one of the fatigue crack paths. A recently developed weight function based numerical method was used to assess total fatigue crack growth time; this method also predicts the shapes of the crack front during propagation. Fatigue crack initiation was traced to a disassembly six months before final failure, which was found to be a joint result of non-conformities in manufacture and maintenance.     

Thermo-mechanical fatigue damage and failure of modern high performance diesel pistons

This study resulted from an engineering failure investigation related to diesel engine piston failures which occurred during a bench dynamometer engine durability test programme. The test programme aimed at evaluating the effects of various fuel types on the durability of fuel system components in passenger car diesel engines. A number of unexpected cylinder head, turbocharger and piston failures were experienced during the course of the test programme. This study focused on the cause of the piston failures experienced during these tests.Analyses of the fractured pistons revealed that thermo-mechanical fatigue initiation occurred as a result of primary silicon phase cracking and subsequent micro-crack formation due to excessive thermo-mechanical loading. Progressive formations of such micro-cracks lead to flaws that were of sufficient magnitude to initiate propagation by high cycle fatigue mechanisms.The investigation also revealed that the excessive thermo-mechanical piston loading was caused by over-fuelling and a combination of elevated and poorly controlled post intercooler air temperature. There was no evidence to suggest that the failures were related to the test fuel formulations.     

Investigation of In-Flight Shutdown of Turboprop Engine Due to Electrical Discharge Damage

A turboprop training aircraft experienced an in-flight shutdown failure with complete seizure of its propeller. Disassembly of the mishap engine revealed that many of the engine components were severely damaged. The laboratory investigation of the failed engine components determined that mechanical failure of the driveshaft bearing in the gearbox was the principal contributing factor that led to in-flight complete seizure of the propeller shaft. Microscopic examination of the failed bearing remnants found electrical arc-induced pittings which played a role as crack initiation sites resulting in premature rolling contact fatigue cracking during continued engine operation. The investigation established clear evidence of electrical discharge damage (EDD) on engine components connecting from the starter-generator to the failed input driveshaft bearing. The evidence of EDD observed in multiple elements located along the electrical current path and the residual magnetism measurement suggested that the starter-generator is highly associated with the source of the EDD.     

镍基单晶合金涡轮叶片开裂原因

某发动机高压涡轮叶片为镍基单晶合金叶片,在室温下进行振动疲劳试验后发现叶片开裂,通过宏观观察、金相检验和扫描电镜分析等方法对叶片开裂的原因进行了分析。结果表明:进气边叶根和榫头伸根的开裂形式均为疲劳开裂;进气边叶根气膜孔内壁存在多处小缺口及榫头伸根亚表面存在疏松缺陷,这些缺陷部位容易形成裂纹源,促进了裂纹的萌生,裂纹扩展后最终导致开裂失效。     

Fracture and Wear Failure of a Locomotive Turbocharger-Bearing Sleeve

A failure investigation was conducted on a locomotive turbocharger-bearing sleeve. The failed bearing sleeve is made of 38CrMoAl steel and the external and internal surfaces are nitrided. The fracture took place at the transition fillet between the cylinder and the plate of the bearing sleeve. Multiple origin fatigue fracture was the dominant fracture mechanism. Serious wear took place on the external surface of the bearing sleeve. Metallurgical examination indicated that the depth of the nitrided layer on the surfaces of the bearing sleeve was generally below the acceptable limit set by the manufacturer. The thin nitrided layer on the external surface of cylinder was rapidly worn from the cylinder in service and the soft underlying matrix was exposed. At this point, the wear on the cylinder surface increased intensely and ultimately lead to increased friction between the external surface of the bearing sleeve and the bearing bush. Fatigue crack initiation occurred at the root fillet between the cylinder and the plate because of stress concentration in that area and the increased frictional forces. In addition, the insufficient nitrided layer depth in the fillet region facilitated propagation of fatigue crack. Appropriate nitriding process should be conducted on the bearing sleeve to obtain a sufficient hardening depth thus inhibiting surface wear and preventing fillet fatigue failure.     

Failure Investigation of an Aircraft Crankshaft Gear Connection

Improper assembly of an aircraft crankshaft can have serious consequences. If an adequate joint clamping force is not applied to the connection between the crankshaft and crankshaft gear during assembly, relative motion in the system could create flexural loads on connection components, and cause damage such as cyclic fatigue cracking, shear overstress fracture, and plastic deformation. Many factors can contribute to insufficient joint clamping, including poor joint seating, the presence of a foreign object on the faying surface, and failure to apply proper torque during assembly. This paper reviews a case involving a crankshaft gear connection, which separated while the subject aircraft was in flight, causing the engine to fail and the aircraft to crash. To determine the root cause of the failure, a metallurgical analysis was performed.     

Truck Diesel Engine Crankshaft Failure Analysis

A diesel engine crankshaft fractured in service after 76010 km of operation. The fracture took place on the first crankpin, and the fracture surface has a 45° inclination with respect to the axial. The results indicate that fatigue is the dominant failure mechanism of the crankshaft. It was observed that the fatigue crack initiated at the fillet region of the first crankpin-web. This crankpin is the one among the six crankpins which bear operational load. Absence of the induction hardening case in the fillet region decreased the fatigue strength and led to fatigue initiation and propagation in the weakened region. Although hard-rolling process was conducted in the fillet region, the depth of hard-rolling layer was insufficient to produce the desired residual compressive stress in the fillet region, and therefore the fillet could not offer resistance to the applied load. In addition, the presence of network-like ferrite in the microstructure facilitated the fatigue crack to be initiated and propagated.     

Failure analysis of an aircraft nose landing gear piston rod end

A failure investigation has been conducted on a piston rod end used in a hydraulic actuating cylinder of an aircraft landing gear. The failed piston rod end was found to be broken. An evaluation of the failed piston rod end was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope (SEM) equipped with EDX facility. A stress analysis is also carried out by the finite element technique for the determination of highly stressed regions on the piston rod end. The results indicated that the piston rod end failed by fatigue with cracks initiated at the surface close to the mechanically damaged region due to high stress concentrations.     

Failure Analysis of a Cracked Gasoline Engine Cylinder Head

This article presents a failure analysis on a gasoline engine cylinder head made of aluminum alloy, which has been used in passenger cars. During an endurance test, a crack initiated from the interior wall of a hole in the center of the cylinder head and then propagated through the thickness of the cylinder head. The metallurgical examinations are conducted in the crack origin zone. The results show that there are many casting pores due to poor quality of casting in the failed cylinder head which has certainly played a crucial role in initiating the crack. Finite element analysis of the cylinder head is performed to identify the stress components. Modeling of a bolt for the hole shows that the plastic stresses are occurred. Moreover, the lower strength of the material due to high assembly stress caused the failure in the cylinder head.     

Failure Analysis on Diesel-Engine Valve Springs

A diesel engine used in a truck had a trouble when servicing. Inspection indicated that four exhaust and intake valve springs and two exhaust and intake valves were fractured. Fractographic studies indicated that fatigue fracture is the main failure mechanism for all of the four valve springs. Under the action of the maximum normal stress, the fatigue crack initiated in the spring wire of coil 1.3-1.5 from the upper end of the spring. This region is also the most damaged location by contact friction wear. The fracture of the intake and exhaust valve stems also suggests fatigue failure probably as a result of the failure of the associated valve springs.     

Failure analysis of the exhaust valve stem from a Waukesha P9390 GSI gas engine

A failure occurred of the exhaust valve stem from a Waukesha P9390 GSI gas engine. The valve failed as a result of overheating. The significant hardness loss and the extensive surface oxidation and fretting/galling on the valve stem were indicative of the overheating. The fatigue properties of the alloy dropped due to the over aging and the over temperature. This led to multiple fatigue crack initiation followed by rapid crack propagation to failure.     

An air crash due to failure of compressor rotor

A fighter aircraft crashed in an accident. Initial investigations pointed out that the accident was due to the failure of compressor rotor. The engine had a nine-stage axial flow compressor in which the nine disks were joined together through riveting. After accident a number of blades and broken pieces of outer ring of the 9th stage compressor disc were found scattered on the runway and near the crashed aircraft. On the removal of the engine from the aircraft, the mid casing was found ruptured. Initial investigation concluded that the failure of 9th stage disc of compressor rotor caused the aircraft crash.Failure analysis of 9th stage disk showed that the disk failed due to fatigue which started from one of the six holes present on the disk. The origin of fatigue crack was machining marks, which were present on the surface of the disk. The crack traveled ∼30 mm in length before final catastrophic failure. Cracks were also observed around other holes on the disk; they were starting from machining marks. The failure of ring and blades were subsequent.     

Analyses of the multiple cracking behaviour of brittle hollow cylinders under internal pressure

In order to elucidate the multiple cracking behaviour of brittle hollow cylinders under static internal pressure, two-dimensional dynamic finite element analyses have been performed firstly for graphite hollow cylinders with inner and outer diameter of 16 and 22 mm, respectively, under internal gas pressure. In the analyses, propagation speed of the primary crack was set to be extremely high by instantaneously releasing the nodes that defined the path of the primary crack, and internal pressure was preserved after the primary cracking. The analyses showed that the stress was enhanced due to stress waves generated by the primary cracking. The initial stress enhancement was observed at the side position of the cylinder, which was located at approximately ±90° with respect to the primary cracking site. This implied that secondary cracking could occur at the side positions. Fracture modes of the cylinders might depend on the following parameters: (1) propagation speed of the primary crack, (2) pressure drop rate after the primary cracking, (3) medium to generate internal pressure, (4) geometry of a cylinder, (5) mechanical properties of brittle materials, and (6) presence of a notch. Thus, the effect of the above parameters on the behaviour of the multiple cracking was also analysed. It was found that secondary cracking would still occur at the side positions if (i) the crack propagation speed was between 70% and 100% of the theoretical crack propagation speed, (ii) the pressure drop rate was below 10⁷ Pa/s, (iii) wall thickness of the cylinder was changed, and (iv) other brittle materials were employed. Also, it was found that multiple cracking would not be observed if liquid pressure was employed instead of gas pressure, because of fluid-structure interaction. In addition, the position of the secondary cracking would be shifted by introducing a notch on the outer surface of the cylinder. These results were in good accordance with experiments formerly reported.     

Failure Analysis and Robust Optimization of an Exhaust Manifold Diffuser Plate

Diffuser plates in exhaust system manifolds are designed to provide uniform flow pattern within the manifold for maximum utilization of the catalytic converter substrate during high-temperature applications. In this paper, failure analysis of a diffuser which survived only 20% duration of a manifold crack test and various design optimization studies of the diffuser plate using computer-aided engineering (CAE) analyses are presented. During the manifold crack test, the failure occurred at the inner and outer periphery of the diffuser. Metallurgical failure analysis coupled with CAE thermal fatigue analysis of the component concluded that thermal fatigue was the root cause of the failure. The new recommended robust design showed considerable improvement in the thermal durability of the diffuser plate assembly.     

Fatigue in the Aerospace Industry: Striations

In the aircraft engine industry, as in others, fracture surfaces are mined as a rich source of information. When present, fatigue striations can be used to judge the cracking mode, measure the crack growth rate, and estimate the number of propagation and initiation cycles. With training and practice, a failure analyst can learn to measure striations on fracture surfaces. Each individual striation is not counted, because there are thousands on a typical fracture and that would not be practical. Striations are measured from photographs taken in several locations across a fracture and their locations are recorded. It is best to start photographing selected areas at the deepest part of the fracture and work back toward the origin. Striations are usually much easier to see at the end of a fatigue zone due to several factors. The density of the striations (cycles per in. or lines per in.) is plotted as a function of crack depth. This data is fitted with a curve. The area under the striations/in. versus crack depth plot is an estimate of the total number of cycles the crack has propagated. Once the number of crack propagating cycles has been estimated, this data can be used to determine the number of cycles for crack initiation if the number of total cycles on the part is known.     

Analysis of a diesel generator cylinder failure

This paper analyses a catastrophic cylinder failure of a four stroke 14 V diesel engine of an electrical power plant when running to nominal speed of 600 rpm. The rated power of the engine was 7.5 MW and before failure had accumulated 80,000 h in service operating mainly at full load. As a result, the piston and liner of cylinder 4 were broken; the crankcase and main crankshaft bearings next to this cylinder were also damaged. The mechanical properties of the liner (grey cast iron) and piston body (aluminium alloy) including tensile properties and Brinell hardness were evaluated. No signs of fatigue failure were identified in liner and piston. A finite element model of the liner has shown that the most heavily loaded areas match the fractured zones.     

Marine main engine crankshaft failure analysis: A case study

A case study of a catastrophic failure of a web marine crankshaft and a failure analysis under bending and torsion applied to crankshafts are presented. A microscopy (eye seen) observation showed that the crack initiation started on the fillet of the crankpin by rotary bending and the propagation was a combination of cyclic bending and steady torsion. The crack front profile approximately adopts a semi-elliptical shape with some distortion due to torsion and this study is supported by a previous research work already published by the authors. The number of cycles from crack initiation to final failure of this crankshaft was achieved by recording of the main engine operation on board, taking into account the beachmarks left on the fatigue crack surface. The cycles calculated by the linear elastic fracture mechanics approaches showed that the propagation was fast which means that the level of bending stress was relatively high when compared with total cycles of main engine in service. Microstructure defects or inclusion were not observed which can conclude that the failure was probably originated by an external cause and not due to an intrinsic latent defect. Possible effects of added torsional vibrations which induce stresses are also discussed. Some causes are analyzed and reported here but the origin of the fatigue fracture was not clearly determined.