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.     

基于流固耦合的导叶式离心泵强度分析

运用顺序耦合和双向流固耦合方法对导叶式离心泵进行了强度分析。通过顺序流固耦合方法,对叶轮进行了静应力强度分析,并与双向流固耦合方法得到的结果进行了比较。同时,对双向流固耦合结果中最大等效应力节点A和最大变形区域的节点B在叶轮旋转一周过程中的等效应力变化的时域图以及频域图进行了分析。结果表明,最大等效应力出现在叶轮前盖板、叶片背面和叶轮出口边的交界处（节点A）;叶片进口边中部以及与前后盖板的交界处和叶片出口与后盖板的交界处这些地方有应力集中,可能发生强度破坏。后盖板出口处且正好在流道中部位置变形最大（节点B）,可能发生刚度破坏。对于静力学分析,顺序耦合与双向耦合的结果基本一致。节点A的变形量小于节点B,但交变应力的幅值却远大于节点B。疲劳裂纹的扩展速度主要取决于交变应力幅值的大小,因此,在节点A处更易发生疲劳破坏。计算结果对导叶式离心泵叶轮结构优化设计提供了有效依据。     

Deformation and life analysis of composite flywheel disk systems

In this study an attempt is made to put into perspective the problem of a rotating disk, be it a single disk or a number of concentric disks forming a unit. An analytical model capable of performing an elastic stress analysis for single/multiple, annular/solid, anisotropic/isotropic disk systems, subjected to pressure surface tractions, body forces (in the form of temperature-changes and rotation fields) and interfacial misfits is summarized. Results of an extensive parametric study are presented to clearly define the key design variables and their associated influence. In general the important parameters were identified as misfit, mean radius, thickness, material property and/or load gradation, and speed; all of which must be simultaneously optimized to achieve the ‘best’ and most reliable design. Also, the important issue of defining proper performance/merit indices (based on the specific stored energy), in the presence of multiaxiality and material anisotropy is addressed. These merit indices are then utilized to discuss the difference between flywheels made from PMC and TMC materials with either an annular or solid geometry.Finally two major aspects of failure analysis, that is the static and cyclic limit (burst) speeds are addressed. In the case of static limit loads, a lower (first fracture) bound for disks with constant thickness is presented. The results (interaction diagrams) are displayed graphically in designer friendly format. For the case of fatigue, a representative fatigue/life master curve is illustrated in which the normalized limit speed versus number of applied cycles is given for a cladded TMC disk application.     

Fatigue Failure of Extrusion Dies: Effect of Process Parameters and Design Features on Die Life

Analysis of die failure plays an important role in the prediction and prevention of die failure, and subsequently in improving economics of any metal-forming process. Industrial experience has shown that fracture is the most common mode of failure in the case of hot aluminum extrusion dies. The purpose of the present work is to implement fatigue damage models in a Finite Element code for identification of critical process parameters and die design features in the case of fatigue being the dominating failure mode. For the maximum number of billets extruded by the die before fatigue crack initiation (fatigue life cycles in extrusion), Morrow’s stress- and strain-life damage models are implemented for axisymmetric flat extrusion die. With the help of finite element software ABAQUS, extrusion process is simulated and dynamic stress and strain values were obtained by first identifying the potential fatigue location in the die. The evaluation of applicability of the damage models is done for specific hot extrusion die made of H13 steels with Al-6063 as billet material. By considering temperature and strain rate as process parameters and bearing length and fillet radius of the die as geometric features, different simulation runs are performed to investigate the effect of process and design features on the useful die life. Morrow’s stress life model shows a good correspondence between computed and actual failure of dies. By establishing correlations of die life with process and design parameters under different conditions, it was shown that the present investigation is a useful guideline at die design and extrusion process stages.     

On the assessment of fatigue life of marine diesel engine crankshafts

The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.     

Failure mechanism in SMC subjected to alternating stresses

Failure mechanisms in randomly reinforced sheet moulding compounds subjected to fatigue testing were studied by relating the changes in characteristic mechanical properties to microscopic changes in the material. It was demonstrated that various mechanisms take place simultaneously to an extent that depends on the local microstructure and strength and that the collective interplay of these mechanisms is responsible for failure. As damage progresses, a uniform pattern of cracks is formed in the matrix, and Mode II fibre/matrix-interfacial failure occurs. The mechanisms concerned can be explained by calculating the forces transmitted between the fibres and the matrix. It was observed that different load amplitudes gave rise to equivalent damage patterns in the material. In the light of this fact and with the aid of the failure mechanisms identified, a method has been devised, by means of which the fatigue life can be estimated of SMC exposed to alternating loads ef any given amplitude.     

Fatigue reliability study on T‐welded component considering load shedding

In this paper, a coupled reliability method for structural fatigue evaluation considering load shedding is first proposed based on probabilistic fracture mechanics in which the uncertainties of the structural parameters are taken into account. Then, the method is applied to predict the fatigue reliability of the T‐welded structure to the case of considering load shedding or not. The compared results show that by considering the load shedding, the structural fatigue reliability might be improved with less conservativeness. The influence rules of the load‐shedding coefficient on the fatigue failure probability of the T‐welded component are investigated, and some interesting results are obtained. That is, the influences of load‐shedding coefficient on the fatigue failure probability can be divided into three regions, namely the high, medium and low fatigue failure areas. The last area is the most intriguing when we try to design a T‐welded structure. The thickness of T‐welded structure along the crack propagation direction is found to be one of the important design variables for the design of fatigue reliability, in which the low‐fatigue failure zone is used as one of the reliability constraints. The basic design frame of T‐welded structure is established to constrain the fatigue failure probability within the low‐fatigue failure area.     

DEM study of the effect of impeller design on mixing performance in a U-shape ribbon mixer

The mixing of powders in a U-shape mixer is significantly influenced by the mixer design, especially impellers, but the studies on the mixing processes are still insufficient. In this study, the effect of impeller designs on mixing performance in an industrial-scale U-shaped ribbon mixer is studied using DEM simulations. Three impeller designs are studied: 2-bladed impeller spiralling in the same direction (i.e., Design I) and the opposite direction (i.e., Design II), and 4-bladed impeller (i.e., Design III). Different particle mixing behaviours in three different impeller designs are studied in aspects of mixing status, particle path line, velocity distribution, and forces. The radial direction has the highest dispersion coefficient while the axial direction has the lowest dispersion coefficient. Most particles in the mixers are imposed a weak force. Design III shows the best mixing performance among the three with the front-by-back and top-by-bottom loading used. Design II shows a better mixing performance used than Design I and III with the side-by-side loading but takes a longer time to reach the stable status. This work evaluates the effect of different impeller designs on the mixing performance in an industrial-scale U-shaped ribbon mixer and provides an effective way to assist industrial design in an economical and safe manner.     

Cause of failure and optimization of a V-belt pulley considering fatigue life uncertainty in automotive applications

High accuracy of dimensions and strength in design requirements are required to produce reliable automotive components with consistent strength distribution. For example, a V-belt pulley is widely used to transmit power between rotational mechanical elements. However, due to defects from the manufacturing process and heterogeneity of materials, different kinds of failure damage may occur in pulleys of identical shape and material. Common applications in the automotive industry include crankshafts, water pumps, air-conditioner compressors and power steering pumps. Although the shape and the usage of pulleys are very simple, evaluating the pulley design is difficult because the loading conditions and installation environment are complicated. This paper focuses on the clutch pulley in the A/C compressor system of automotives and cause of failure was investigated. The applied stress distribution of the pulley under high-tension and torque was obtained by using finite element analysis (FEA) and based on theses results, the life of the pulley with variation in fatigue strength was estimated with a durability analysis simulator. The results for failure probabilities of 50% and 1% were compared with the fatigue life. Incidentally, the purpose of this study was to optimize the fatigue life of vehicle components from the stochastic point of view. The fatigue life was obtained by an approximation function, and the optimum design was verified by fatigue tests considering durability and validity. The design optimization of a V-belt pulley was performed using an approximation function, which improved the fatigue life. A new shape optimization procedure was presented to improve the fatigue life of the pulley in automotive applications and the shape control concept was introduced to reduce the shape design variables. Design of experiment (DOE) was employed to evaluate the design sensitivity of fatigue life with respect to shape design variables.     

Prediction of fatigue life of metallic structures with welded joints using automatic learning systems

Welded metallic joints are prone to fatigue damage, which may lead to sudden and catastrophic structural failure. In this research, fatigue failures of metallic structures with welded joints are analyzed using an approach based on automatic learning technology. A database of physics-based parameters, including material properties, loading histories, and stresses around potential cracking sites, is constructed based on experimental results and numerical analyses. Various automatic learning tools are used to search for the mathematical formulas and data patterns embedded in the database. The obtained rules and formulas can be used to support design of welded metallic structures. This approach provides a new way to locate fatigue-prone areas, predict fatigue lives, and may lead to designs of more fatigue resistant structures. It complements the classical deterministic and statistical fatigue failure predictions.     

Stress-Based Uniaxial Fatigue Analysis Using Methods Described in FKM-Guideline

The process of prevention of failure from structural fatigue is a process that should take place during the early development and design phases of a structure. In the ground vehicle industry, for example, the durability specifications of a new product are directly interweaved with the desired performance characteristics, materials selection, manufacturing methods, and safety characteristics of the vehicle. In the field of fatigue and durability analysis of materials, three main techniques have emerged: nominal stress-based analysis, local strain-based analysis, and fracture mechanics analysis. Each of these methods has their own strengths and domain of applicability??for example, if an initial crack or flaw size is known to exist in a structure, a fracture mechanics approach can give a meaningful estimate of the number of cycles it takes to propagate the initial flaw to failure. The development of the local strain-based fatigue analysis approach has been used to great success in the automotive industry, particularly for the analysis of measured strain time histories gathered during proving ground testing or customer usage. However, the strain life approach is dependent on specific material properties data and the ability to measure (or calculate) a local strain history. Historically, the stress-based fatigue analysis approach was developed first??and is sometimes considered an ??old?? approach??but the stress-based fatigue analysis methods have been continued to be developed. The major strengths of this approach include the ability to give both quantitative and qualitative estimates of fatigue life with minimal estimates on stress levels and material properties, thus making the stress-based approach very relevant in the early design phase of structures where uncertainties regarding material selection, manufacturing processes, and final design specifications may cause numerous design iterations. This article explains the FKM-Guideline approach to stress-based uniaxial fatigue analysis. The Forschungskuratorium Maschinenbau (FKM) was developed in 1994 in Germany and has since continued to be updated. The guideline was developed for the use of the mechanical engineering community involved in the design of machine components, welded joints, and related areas. It is our desire to make the failure prevention and design community aware of these guidelines through a thorough explanation of the method and the application of the method to detailed examples.     

Fatigue and fracture analysis of a seven‐wire stainless steel strand under axial and bending loads

Fatigue failure of cables and strands is a common and complex problem. Failure is typically caused by different combinations of time‐variable bending and axial forces. In addition to these loads, contact stresses between wires may play an important role in the fatigue failure of cables. The present work aims to provide deep insight into the fatigue failure of a seven‐wire stainless steel strand subjected to a combination of variable axial and bending loads. To avoid side effects in the analysis, fatigue failure of the strand close to the clamps is prevented. Several tests were performed with a new device specifically designed to avoid failure near the clamps. Thus, failure is always produced at the middle length of the specimen. Test simulations were performed by employing the finite element method. The numerical results were validated via comparisons with experimental data. Finally, life prediction curves were obtained.     

Premature Failure of Ductile Iron Pump Impeller in Cooling Tower System

The root cause of corrosion of a pump impeller in a cooling water system is investigated. The impeller material was made of ductile cast iron. The pump failure was encountered after eight months of operation. A detailed visual examination, microstructure examination, and water analysis were carried out to ascertain the probable cause of failure. Finally, the obtained results infer that the solid-particle containing fluid flow was responsible for erosion–corrosion in the impeller failure.     

A unified method of design for fatigue crack growth resistance in structural materials

It is well established that there are two fatigue crack tip driving forces – the cyclic, ΔK, and the static, K_max. In this study, the effects of each crack tip driving force on crack growth were evaluated for various structural materials. A unified method of design that allows for predicting the response of long and physically small fatigue cracks at positive stress ratios is introduced. Good agreement between predicted and experimental long and physically small fatigue crack growth data was obtained. The importance of this method in material and component design is discussed as part of a contemporary design philosophy.     

Fatigue Fracture of a Compressor Disc of an Aeroengine

There was an accident to a single engine aircraft. From the eye and ear witness accounts, it was established that the accident occurred because of engine failure. After preliminary examination of the wreckage of the crashed engine, a few suspected components were identified for detailed laboratory investigation. The objective was to establish the primary failure in the engine. While majority of the engine components submitted for laboratory analysis showed secondary damages due to either crash impact forces or post-accident fire, the fracture pattern in one of the compressor disks was different from other components of the engine. Fractographic study revealed that the failure of the disk was by fatigue mechanism. Subsequent investigation showed that the fatigue fracture of the compressor disk was the first in the chain of events that led to the engine failure. This finding was further substantiated through fracture mechanics calculations.     

Fatigue in bamboo

This paper describes the results of an experimental programme to determine the fatigue behaviour of bamboo. Bamboo is subjected to cyclic loading, both in the plant itself and subsequently when the material is used in load-bearing applications in the construction industry. However, there is currently no data in the literature describing fatigue in this material. We found that sections of bamboo culm loaded parallel to the culm axis did not undergo fatigue failure: samples either failed on the first loading cycle, or not at all. By contrast, fatigue was readily apparent in samples loaded in compression across the diameter of the culm. The number of cycles to failure increased as the cyclic load range decreased in a manner similar to that found in many engineering materials: fatigue occurred at applied loads as small as 40% of the ultimate strength. Two different species of bamboo were tested and found to have different ultimate strengths but similar high-cycle fatigue strengths. Finite element analysis was used to help understand the progression of fatigue damage and the effect of stress concentration features. Some tentative design rules are proposed to define stress levels for the safe use of bamboo, taking fatigue into account.     

Fatigue behaviour and lifing of two single crystal superalloys

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX-4 under conditions of creep and fatigue. A combined creep–fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep–fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high- and low-frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore-initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C. This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.     

Frequent Failures of Motor Shaft in Seawater Desalination Plant: Some Case Studies

The failure of a shaft from a motor in a pump or a compressor has been a phenomenon of common occurrence in seawater desalination plants. The origin of the problem in majority of cases is either the inability of the material to withstand the level of dynamic stresses to which shaft is subjected during operation and/or inadequacy of the design. The shortcoming in the design may be responsible for initiating localized corrosion which ultimately leads to failure of the component. The mode of failure of the shaft could be stress-related failure such as stress corrosion cracking, mechanical fatigue or corrosion fatigue, and/or localized corrosion such as crevice corrosion. This paper describes some recent case studies related to shaft failures in seawater desalination plants. The case studies include shearing of a shaft in brine recycle pump in which a combination of environment, design, and stresses played important role in failure. In another case, ingress of chloride inside the key slot was the main cause of the problem. The failure in a high pressure seawater pump in a SWRO plant occurred due to cracking in the middle of the shaft.     

Crankshaft failure analysis of a boxer diesel motor

This paper reports a failure mode analysis of a boxer diesel engine crankshaft. Crankshafts are components which experiment severe and complex dynamic loadings due to rotating bending combined with torsion on main journals and alternating bending on crankpins. High level stresses appear on critical areas like web fillets, as well as the effect of centrifugal forces and vibrations. Since the fatigue fracture near the crankpin-web fillet regions is one of the primary failure mechanisms of automotive crankshafts, designers and researchers have done the best for improving its fatigue strength. The present failure has occurred at approximately 2000 manufactured engines, and after about 95,000 km in service. The aim of this work is to investigate the damage root cause and understand the mechanism which led to the catastrophic failure. Recommendations for improving the engine design are also presented.     

Roles of microstructure,inclusion, and surface roughness on rolling contact fatigue of a wind turbine gear

Contact fatigue is a key feature limiting the service lives and reliabilities of gears. The gear contact fatigue failure mechanism has not been understood fundamentally due to the complexities of structural factors, material properties, and operating conditions. In this work, an integrated finite element model of a megawatt level wind turbine gear is established considering the real gear geometry, material microstructure heterogeneity, existence of nonmetallic inclusion, and the tooth surface roughness. The gear steel material properties are defined based on the crystal elasticity anisotropy framework. The modified Dang Van multiaxial criterion is utilized to estimate the material fatigue failure probability during gear engagement. With the developed model, the roles of microstructure, inclusion, and surface roughness on the gear contact fatigue behaviour are comparatively investigated. Additionally, the influences of different inclusion size and surface roughness profile on gear failure risk are investigated and discussed in detail.