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.     

GH2132三齿二级涡轮盘榫齿裂纹分析

对GH2132三齿二级涡轮盘在使用过程中第一榫齿产生的裂纹进行了光学金相和扫描电镜分析。结果表明，该裂纹属于高周疲劳裂纹，疲劳源区无冶金缺陷及加工刀痕。对失效件的化学成分、力学性能以及显微组织进行了检测，其结果均符合技术条件要求。疲劳裂纹产生的原因，是由于二级涡轮盘与二级涡轮叶片榫齿配匹不均匀，在发动机工作时，由于热应力作用，两种材料的线膨胀系数不一样(GH2132合金线膨胀系数大，而GH4037合金线膨胀系数小)，使之对盘的榫齿产生相当大的压应力，导致在榫齿配合面上产生了压陷，在交变载荷作用下，在离压陷边缘0．5mm处产生了疲劳裂纹。     

Numerical stress and crack initiation analysis of the compressor blades after foreign object damage subjected to high-cycle fatigue

This paper presents results of the complex stress and crack initiation analysis of the PZL-10 W turbo-engine compressor blade subjected to high cycle fatigue (HCF). A nonlinear finite element method was utilized to determine the stress state of the blade during the first mode of transverse vibration. In this analysis, the numerical models without defects and also with V-notches were defined. The quality of the numerical solution was checked by the convergence analysis. Obtained results were next used as an input data into crack initiation (ε–N) analyzes performed for the load time history equivalent to one cycle of the transverse vibration. In the fatigue analysis the different methods such as: Neuber elastic–plastic strain correction, linear damage summation and Palmgreen–Miner rule were utilized. As a result of ε–N analysis, the number of load cycles to the first fatigue crack appearing in the compressor blades was obtained. Moreover, the influence of the blade vibration amplitude on the number of cycles to the crack initiation was analyzed. Values of the fatigue properties of the blade material according to Baumel–Seeger and Muralidharan methods were calculated. The influence of both the notch radius and values of the UTS of the blade material on the fatigue behavior of the structure was also considered. In the last part of work, the finite element results were compared with the results of an experimental vibration HCF tests performed for the compressor blades.     

Fretting fatigue failure mechanism of automotive shock absorber valve

Fretting fatigue is a complex mechanical failure phenomenon, in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. This study proposes a methodology to analyze the fretting fatigue failure mechanism of automotive shock absorber valve by means of experimental and numerical approaches. A servo hydraulic test set-up is used to simulate fretting fatigue under real working conditions. Moreover, a 3-D finite element model is developed to analyze the contact status and stress distribution at contact interface between connected components, i.e. washer-disc contact. The experimental test results depict that fretting damage appears at contact interface between washer and disc, which causes the initial crack nucleation and advancing the crack up to the final fracture of valve disc. Stress field, obtained by numerical simulation, is used to monitor some fretting fatigue features such as the distribution of relative slip amplitude, contact pressure and different stress fields at contact interfaces. Eventually, the crack initiation site is estimated by monitoring variation of equivalent multiaxial damage stress at contact interface.     

Failure analysis of generator rotor fan blades

The failure analysis of a generator rotor fan blade was investigated by mechanical analysis and metallurgical examination of fracture surface. Fracture took place at the airfoil root, surface examination showed that the blade had cracked by a high cycle fatigue mechanism. However, there was no evidence of material defect. A series of analytical, finite element and experimental analysis was utilized to determine the steady-state stresses and dynamic characteristic of the blade. Possibly the failure was due to aerodynamical disturbances that resulted in a state of resonant condition of vibration. The simulation of blade with final crack showed the stress intensity factor (SIF) under these condition exceed the critical SIF and final fracture could be occurred under analyzed stresses.     

Analysis of a crankshaft fatigue failure

The reason of the crankshaft fracture of the air compressor has been analyzed through the chemical composition, mechanical properties, macroscopic feature, microscopic structure and theoretical calculation methods. The analysis results show that the crankshaft which has obvious fatigue crack belongs to fatigue fracture. The fatigue crack initiated from the fillet region of the lubrication hole because of the high bending stress concentration which is caused by both the small fillet and the misalignment of main journals. The crankshaft fatigue fracture was only attributed to the initiation and propagation of the fatigue cracks on the lubrication hole under cyclic bending and torsion. The high bending loading bending level is the root cause of the failure.     

Numerical life estimation after fatigue failure of a complex component

Failure of a large ethylene‐reciprocating compressor was found to be due to fatigue growth of cracks in the crosshead of one of the cylinders, initiated at material defects near stress raisers. Total fatigue crack growth time was required in order to identify the cause of the failure. The applied stress field near the initiation sites and along fatigue path was estimated using FEM. The stresses were found to vary steeply and become partly compressive along a large part of the fatigue crack path. A weight function based on numerical method was developed, which was able to predict exactly the shape of the crack front during propagation. Fatigue crack initiation was traced to a disassembly 6 months before final failure. This failure was found to be jointly the result of non‐conformities in manufacture and maintenance.     

Failure analysis and retrofit design of low pressure 1st stage blades for a steam turbine

Cracking of blade fingers occurred in a few numbers of low pressure 1st stage blades for a certain type of steam turbines. In order to find out the failure mechanism, one of the cracked blades has been inspected. The inspection results showed that the cracking of the blade finger was caused by high cycle fatigue and surface defect coming from rough machining induced the initiation of crack. Vibratory modes of the blade group have been calculated and measured using a 3D finite element S/W and impact test, respectively. The results showed that resonance of the second type group axial vibration mode with nozzle passing frequency was the source of high cycle fatigue load. To avoid the dangerous resonance, the blade groups have been modified into 10 blades per group without any change of vane and dovetail. The new blade groups have been operated safely more than 2 years since the modification.     

Investigations on fretting fatigue in aircraft engine compressor blade

An investigation of several cracked blade tangs in the military aircraft engine compressor was conducted to identify the root cause of the failure. These cracks were found during the scheduled maintenance with fluorescent penetration inspection. The engine compressor blade made of Ti–6Al–4V is attached to compressor rotor by means of inserting retaining pin through rotor and blade tang. By analyzing the fracture surface of the failed blade tang, it is found that the crack in the blade tang was initiated by fretting fatigue and propagated under low cycle fatigue. Stress analysis of the blade using a non-linear finite element method is coincident with the results of fractography. The clearance between retaining pin and tang hole caused small amplitude of sliding motion leading to fretting wear during engine operation. Consequently, the damaged area due to fretting wear acts as a stress raiser inside tang hole and contributes to accelerate fretting fatigue.     

Fatigue crack growth simulation in a first stage of compressor blade

A first-stage rotary compressor blade of a Model GE-F6 gas turbine failed due to vibration in early March 2008. Initial investigations showed that pitting on the pressure side of the blade caused micro cracks, leading to larger cracks due to high cycle fatigue. To assess this failure, a series of experimental, numerical, and analytical analyses were conducted. Fractography of the fractured surface of the blade indicated that two semi-elliptical cracks incorporated and formed a single crack. In this study, static and dynamic stress analyses were performed in Abaqus software. Moreover, fracture mechanics criterion was accomplished to simulate fatigue crack growth. This was carried out using a fracture analysis code for 3-dimensional problems (Franc3D) in two states. Firstly, stress intensity factors (SIFs) for one semi-elliptical surface crack and then SIFs for two semi-elliptical surface cracks were taken into account. Finally, the Paris and Forman–Newman–De Koning models were used to predict fatigue life. Since stress level and crack shape in both conditions are the same and the SIF at the crack tip reaches the fracture toughness of the blade, SIFs results indicate that insertion of a second crack has no effect on the final SIF, however, the second crack facilitates the process of reaching the critical length. So, fatigue life in two-crack condition is less than in the one-crack state.     

Fatigue Failure of LP Compressor Blade in an Aero Gas Turbine Engine

Failure of low-pressure compressor rotor blade in an aero gas turbine engine is analyzed to determine its root cause. Forensic and metallurgical investigations are carried out on the blade failed. The failure of the first stage rotor blade is found to be the first in the chain of events that led to the engine failure. The mode of failure in the blade is found to be fatigue and has originated from the mounting lug fillet region due to high stress concentrations. The failure has caused extensive damages in low-pressure compressor module and also in downstream modules as a secondary effect. Remedial measures are also suggested to prevent such failures.     

Cement kiln dust induced corrosion fatigue damage of gas turbine compressor blades – A failure analysis

Compressor of one of the gas turbines installed in a power plant was stopped under emergency conditions. Primary investigation showed that almost all of the first stage blades and some of the next stages were severely damaged. In this study, one of the first stage broken blades was failure analyzed. The results showed that the corrosion pits were formed on the compressor blade surface due to the presence of Cl and S elements in the compressor inlet air. Since the power plant located in the vicinity of a cement company and also an oil refining company, the inlet air of compressor had large amounts of Cl and S containing compounds. The corrosion pits acted as stress concentration sites, and facilitated fatigue crack initiation and propagation, leading to final fracture of the blades.     

Similarities of stress concentrations in contact at round punches and fatigue at notches: implications to fretting fatigue crack initiation

A linear elastic model of the stress concentration due to contact between a rounded flat punch and a homogeneous substrate is presented, with the aim of investigating fretting fatigue crack initiation in contacting parts of vibrating structures including turbine engines. The asymptotic forms for the stress fields in the vicinity of a rounded punch-on-flat substrate are derived for both normal and tangential loading, using both analytical and finite element methods. Under the action of the normal load, P , the ensuing contact is of width 2 b which includes an initial flat part of width 2 a . The asymptotic stress fields for the sharply rounded flat punch contact have certain similarities with the asymptotic stress fields around the tip of a blunt crack. The analysis showed that the maximum tensile stress, which occurs at the contact boundary due to tangential load Q , is proportional to a mode II stress intensity factor of a sharp punch divided by the square root of the additional contact length due to the roundness of the punch, Q /(√( b − a )√ π b ). The fretting fatigue crack initiation can then be investigated by relating the maximum tensile stress with the fatigue endurance stress. The result is analogous to that of Barsom and McNicol where the notched fatigue endurance stress was correlated with the stress intensity factor and the square root of the notch-tip radius. The proposed methodology establishes a 'notch analogue' by making a connection between fretting fatigue at a rounded punch/flat contact and crack initiation at a notch tip and uses fracture mechanics concepts. Conditions of validity of the present model are established both to avoid yielding and to account for the finite thickness of the substrate. The predictions of the model are compared with fretting fatigue experiments on Ti–6Al–4V and shown to be in good agreement.     

Experimental crack propagation and failure analysis of the first stage compressor blade subjected to vibration

This paper presents results of experimental vibration tests of the helicopter turbo-engine compressor blades. The blades used in investigation were retired from maintenance under technical inspection of engine. Investigations were conducted for selected undamaged blades, without existence of preliminary cracks or corrosion pits. The blades during experiment were entered into transverse vibration. The crack propagation process was conducted in resonance condition. During the fatigue test, the growth of crack was monitored. In the second part of work, a nonlinear finite element method was utilized to determine the stress state of the blade during vibration. In this analysis a first mode of transverse vibration were considered. High maximum principal stress zone was found at the region of blade where the crack occurred.     

微动疲劳中的应力状态参数和微动磨损参数的研究

本文对微动疲劳中的力学参数作出了研究。微动接触面上的力学参数可分为应力状态参数（SSP）和微动磨损参数（FWP）两类，并将应力状态参数综合为当量应力σ-1E,而将微动磨损参数用摩擦功W来表示．对桥式微动疲劳试件和燕尾型榫联接试件的数值分析表明，在微动接触面上疲劳断裂处的σ-1E和W值较大。因此，有可能使用了σ-1E和W值作为预测微动疲劳失效的两个基本参数。     

Prediction of fretting fatigue crack initiation in double lap bolted joint using Continuum Damage Mechanics

In fretting fatigue, the combination of small oscillatory motion, normal pressure and cyclic axial loading develops a noticeable stress concentration at the contact zone leading to accumulation of damage in fretted region, which produces micro cracks, and consequently forms a leading crack that can lead to failure. In fretting fatigue experiments, it is very difficult to detect the crack initiation phase. Damages and cracks are always hidden between the counterpart surfaces. Therefore, numerical modeling techniques for analyzing fretting fatigue crack initiation provide a precious tool to study this phenomenon. This article gives an insight in fretting fatigue crack initiation. This is done by means of an experimental set up and numerical models developed with the Finite Element Analysis (FEA) software package ABAQUS. Using Continuum Damage Mechanics (CDM) approach in conjunction with FEA, an uncoupled damage evolution law is used to model fretting fatigue crack initiation lifetime of Double Bolted Lap Joint (DBLJ). The predicted fatigue lifetimes are in good agreement with the experimentally measured ones. This comparison provides insight to the contribution of damage initiation and crack propagation in the total fatigue lifetime of DBLJ test specimens.     

Investigation on the failure of air compressor

The cause for the failure of an air compressor has been investigated. It was found that a pre-existing fatigue crack was present at the root of the impeller blade. Transients and unsteady operation of the equipment prior to the accident are thought to have grown the fatigue crack to its critical size, thereby causing an imbalance in the impeller rotation and leading to failure.     

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...     

Effects of shot-peening intensity on fretting fatigue crack-initiation behaviour of Ti–6Al–4V

The effects of shot‐peening intensity on fretting fatigue crack‐initiation behaviour of titanium alloy, Ti–6Al–4V, were investigated. Three intensities, 4A, 7A and 10A with 100% surface coverage, were employed. The contact geometry involved a cylinder‐on‐flat configuration. Residual stress and improvement in fretting fatigue life were directly related to shot‐peening intensity. The magnitude of compensatory tensile stress and its location away from the contact surface increased with increasing intensity. The relaxation of residual stress occurred during fretting fatigue which increased with increasing the number of cycles. An analysis using a critical plane‐based fatigue crack‐initiation model showed that stress relaxation during the fretting fatigue affects life and location of crack initiation. Greater relaxation of the residual stress caused greater reduction of fatigue life and shifted the location of crack initiation from inside towards the contact surface. Modified shear stress range (MSSR) parameter was able to predict fretting fatigue crack‐initiation location, which agreed with the experimental counterparts. Also, the computed parameter showed an appropriate trend with the experimental observations of the measured fretting fatigue life based on the shot‐peening intensity.     

Mechanics of two-stage crack growth in fretting fatigue

Motivated by experimental observations, we carry out a numerical analysis of the two-stage crack growth under fretting fatigue by using an efficient and accurate boundary element method. To start with, the variation of stress field during a loading cycle is analyzed. Various values of friction coefficient in the contact zone are considered, which is shown to considerably affect the stress field. Then, by assuming crack initiation to occur in the shear mode, a surface-breaking crack is introduced to the specimen at the location of highest shear-stress amplitude. The crack-tip stress intensity factors (SIFs) are calculated for various crack lengths and at various crack angles ranging from 25° to 45° about the contact surface. It is shown that, for a loading ratio of 0.5, the cyclic mode-II SIF amplitude decreases with increasing crack length, whilst its mean value increases. It suggests that the (first-stage) shear crack would sooner or later become dormant, or switch to another mode that can provide continuous support of growth. Then, the first-stage shear crack is manually kinked into a second-stage opening crack, and the follow-on driving force is analyzed. It is shown that the kinking event is only favored after the first-stage crack has grown to a certain length. The present study thus provides insights in the mechanics of two-stage crack growth that has been frequently observed in a typical dovetail joint under fretting fatigue. It also suggests an improved experimental setup to quantitatively investigate the fretting fatigue in dovetail joints.