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.     

Accelerated testing for long-term durability of GFRP laminates for marine use

Our proposed accelerated testing methodology for the long term durability of polymer composites is based on the time–temperature superposition principle to be held for the viscoelasticity of polymer matrix. The long term flexural fatigue life of plain woven glass fiber/vinyl-ester (GFRP) laminates for conventional marine use was predicted based on the proposed methodology. As results, the flexural fatigue strengths of GFRP laminates decreases strongly with increasing time and temperature as well as the number of cycles to failure. The long term fatigue strength at any time, temperature and number of cycles to failure can be predicted using the master curves of fatigue strength obtained based on our proposed accelerated testing methodology.     

Recent developments in assessing microstructure-sensitive early stage fatigue of polycrystals

Fatigue failure is a leading concern for many applications involving structures for transportation, manufacturing, medical devices, and electronic components. Recent advances in modeling and simulation, coupled with in situ experimental techniques, have enhanced the understanding required to distinguish and characterize mechanisms of fatigue crack formation and early growth at scales of underlying microstructure. In particular, microstructure substantially influences high cycle fatigue resistance and contributes to variability of the fatigue response. This paper reviews the confluence of recent experimental and modeling advances aimed at understanding and modeling of the formation and early growth of fatigue cracks with size on the order of dominant microstructure attributes.     

Fatigue life estimation of ultrasonic spot welded Mg alloy joints

Lightweight magnesium alloys are increasingly used in automotive and other transportation industries for weight reduction and fuel efficiency improvement. The structural application of magnesium components requires proper welding and fatigue resistance to guarantee their durability and safety. The objective of this investigation was to identify failure mode and estimate fatigue life of ultrasonic spot welded (USWed) lap joints of an AZ31B-H24 magnesium alloy. It was observed that the solid-state USWed joints exhibited a superior fatigue life compared with other welding processes. Fatigue failure mode changed from interfacial failure to transverse-through-thickness crack growth with decreasing cyclic load level, depending on the welding energy. Fatigue crack initiation and propagation occurred from both the notch tip inside the faying surface and the edge of sonotrode indentation-footprints due to the presence of stress concentration. A life prediction model for the spot welded lap joints developed by Newman and Dowling was adopted to estimate the fatigue lives of the USWed magnesium alloy joints. The fatigue life estimation, based on the fatigue crack growth model with the global and local stress intensity factors as a function of kink length and the experimentally determined kink angle, agreed fairly well with the obtained experimental results.     

Factors influencing transit fatigue of seamless pipes

Full‐scale fatigue tests simulating real pipe stock conditions in ships were carried out in order to reproduce failures of seamless pipes due to fatigue crack growth during shipping. Simple mechanical analyses were then used to assess the factors that influence transit fatigue. As seen in several actual failures, fatigue cracks were found to propagate from both sides of the pipe wall. It is possible to have transit fatigue (TF) when piling up a few pipes for transportation, if there is previous plastic indentation. Susceptibility to TF was found to be independent of the nature of discontinuities in the surfaces of the pipes. Precautions to reduce the probability of TF in sea shipment include preventing plastic indentation of the pipes and storing stiffer pipes in the bottom part of the hold.     

弹药公路运输动力学特性研究综述

公路运输过程中弹药的动力学特性是引起其损伤或失效的重要因素,也是研究弹药公路运输安全的基础。 从冲击和振动两个方面,对弹药公路运输动力学特性的研究现状进行了综述,并指出了目前这一领域有待进一步研究和完善的相关问题。     

Degradation in fatigue behavior of carbon fiber–vinyl ester based composites due to sea environment

This study focuses on the effect of confined and one sided sea water confinement on the cyclic fatigue behavior of carbon fiber reinforced vinyl ester composites that serve as facings materials for naval sandwich structures. Experimental results for facings yielded failures under much lower number of cycles when fatigued under immersed conditions surrounded by sea water than in air. Water penetrates the matrix resin through diffusion and fiber/matrix interface by capillary action through micro-cracks or inter-layer delaminations. During fatigue loading, its inability to drain during the downward (compressive) cyclic loading and the near incompressibility of water induces an internal pore water pressures which dominates the progressive failure mechanism. Sea water induced fatigue degradation data and resulting microstructure changes are obtained using high resolution X-ray micro-tomography along with the implications for marine composites.     

Fatigue failure of a star–ratchet gear

We present an analysis of the fatigue failure of an 18 tooth star–ratchet gear (SRG). The subject gear was implemented in the freewheel assembly of a mountain bicycle. After 6 years of service, the gear failed unexpectedly during a typical off-road ride. The unique geometry of SRGs precluded a simple comparison to existing gear lifetimes. Scanning Electron Microscopy (SEM) analysis of the failed gears showed crack initiation at the root of the gear teeth, followed by fatigue crack propagation and eventual chip-out. A biomechanical analysis of pedaling forces, coupled with explicit power data obtained from instrumented rides over the same trails, in conjunction with a Finite Element Analysis (FEA) of the gear, were used to determine stress amplitudes for fatigue calculations. Energy dispersive spectroscopy (EDS) determined the alloy composition of the gear and thus set the strength and fatigue properties of the gear. Basquin’s law, Goodman’s mean stress correction, and Miner’s rule were used to estimate the lifetime, in bike rides, of the gear. Our analysis led to an estimate of 2288 rides, while failure was reported after roughly 312 rides. Given the uncertainties in fatigue life estimation and service use, we find this estimate acceptable.     

Failure of a 17-4 PH stainless steel sailboat propeller shaft

In this study, a 17-4 PH precipitation hardening stainless steel propeller shaft failed in use when installed in a sailboat working in a marine environment. Failure analysis was conducted on the propeller shaft. Results indicate that the failure was caused by the fracture of the propeller shaft by torsional fatigue and stress corrosion cracking (SCC). SCC progressed transgranulary in the martensitic matrix.     

Fatigue of Magnesium Alloys

Magnesium alloys show a high specific strength and are therefore increasingly used for light‐weight constructions in transportation industry.^[1,2] To predict the behaviour of the material under the influence of cyclic loading it is vital to understand the fatigue behaviour of magnesium alloys. Only when understood properly, it is possible to fully apply the potential weight reduction by using magnesium alloys. A very important aspect in fatigue of magnesium alloys is the influence of a corrosive media and elevated temperatures, of which both are relevant in automotive applications. These two factors tend to have deleterious effects on magnesium alloys and therefore also have to be considered in investigations on the fatigue behaviour of magnesium alloys.     

Research needs and opportunities for reducing the adverse safety consequences of fatigue

Although there has been a significant amount of research on fatigue globally, it remains a major contributor to workplace and highway mortality and morbidity. Given its importance, a Hopkinton Conference was organized to review and discuss the state of knowledge in the area and to define future directions for research aimed at preventing or mitigating the consequences of fatigue. In all, five groups of international contributors produced six articles for this special issue, comprising state of the art reviews, along with a discussion of knowledge gaps and future research needs. In this concluding paper, we capture some of the major outcomes and recommendations from this process. These are organized into five topic areas: the link between fatigue and safety, demographic issues in fatigue, modeling and predicting fatigue, technological approaches to fatigue management, and organizational factors in fatigue management.     

Fatigue performance of friction stir welded marine grade steel

An extensive study on the fatigue performance of friction stir welded DH36 steel was carried out. The main focus of this experimental testing programme was fatigue testing accompanied by tensile tests, geometry measurements, hardness and residual stress measurements, and fracture surface examination. The S–N curve for friction stir butt welded joints was generated and compared with the International Institute of Welding recommendations for conventional fusion butt welds. Friction stir welds of marine grade steel exceeded the relevant rules for fusion welding. This newly developed S–N curve is being proposed for use in the relevant fatigue assessment guidelines for friction stir welding of low alloy steel. Fracture surfaces were examined to investigate the fatigue failure mechanism, which was found to be affected by the processing features generated by the friction stir welding tool.     

The immersed fatigue response of polymer composites

The effect of sea water on carbon/epoxy cross ply specimens is studied by consideration of fatigue data and failure modes. Tests were conducted using dry and saturated coupons fatigued in air, as well as pre-saturated coupons fatigued while immersed in sea water. The saturated coupons fatigued in air had the longest fatigue life, which was attributed to stress relief from sorption induced swelling. Saturated coupons fatigued in an immersed environment exhibited the shortest fatigue life, and also experienced significant deliminations prior to failure. Water trapped inside the transverse cracks during the load cycle provides a physical mechanism explaining this behavior. Analytical and numerical analyses are presented which show how moisture inside the transverse cracks can alter the coupon stress state and enhance delimination.     

Fracture Characteristics of Torsion-Bending Fatigue and Impact Fatigue Failure of Two Steel Pins in a Crawler Excavator

The arms of crawler excavators used in diverse quarries work under extremely harsh conditions. A common failure during their operation is the fracture of the steel pin joints that permit free movement of the two adjacent booms closer to the machine housing, as well as the motion of the backhoe bucket against its nearest boom. This work presents examples of two steel pin joint failures. The pins had a diameter of 90 mm, and the failures resulted in high stand-by time for repairs. The first failure was caused by the combination of bending and torsion fatigue, and the second fracture occurred because of impact fatigue. Optical metallography and scanning electron microscopy/electron dispersive x-ray analysis (SEM/EDS) analysis provided a better understanding of the failure mechanism of the two steel pins, since these particular components exhibited excellent fatigue characteristics in both mating fracture surfaces.     

Classical Fatigue Design Techniques as a Failure Analysis Tool

The classical method for designing against high-cycle fatigue fracture is based primarily on statistical models derived from laboratory experimental data. This paper considers a number of actual fatigue failures where the analyses of the failures, in part, made use of classical high-cycle fatigue resistance design methodology as an analytical tool. This paper uses failure analyses to demonstrate that the long-taught classical methodology is useful and accurate as both a design and an analysis tool. The usefulness and accuracy of the method is verified in that it is shown to have predicted actual failures, given known materials, manufacturing histories, and service operating conditions. Example analyses include: a fatigue-cracked roll from a paper-making machine, a fractured anvil on a steam powered forge, and a fractured shaft on a helical ribbon dryer.     

In situ monitoring in real time of fatigue-induced damage growth in composite materials by acoustography

There are growing concerns about the effects of accidental impact damage on the structural integrity of aerospace composites and about the possible growth of the damage due to in-service fatigue. There has been some success in the use of established methods (ultrasonic C-scan, thermography, X-rays) to monitor damage development during fatigue experiments by interrupting a test and removing the specimen for damage inspection but this stop-and-restart test procedure is far from satisfactory. Real-time damage monitoring in composite materials during fatigue has now become possible by the emergence of a new ultrasonic imaging technology, acoustography. The successful integration of acoustography and a servo-hydraulic fatigue test machine has resulted in a new measurement system which can be used for the in situ monitoring in real time of damage growth in composite specimens during long-term fatigue tests. Results are presented which show damage-area growth during fatigue cycling under high compressive loads. After an initial small enlargement (stage 1), damage grows at a constant rate (stage 2) until the third stage is reached when there is further growth at an increasing rate to final failure. However, a ‘fatigue limit’ has also been observed. At stresses below this fatigue limit, a zero damage-growth régime has been found in studies of >10⁶ fatigue cycles. The results obtained have important implications for the understanding of the effects of damage on fatigue life and for the design of ‘safe’ damage-tolerant structures.     

Fatigue characteristics of continuous welded rails and the effect of residual stress on fatigue-ratchetting interaction

ABSTRACT

The importance of ratchetting-fatigue interaction is garnering interest due to complex failure mechanism of rail welds under cyclic loading. The objective of this paper is to investigate the fatigue characteristics of continuous welded rails (CWRs) and the effect of residual stress on fatigue-ratchetting interaction. For this purpose, UIC60 rails have been modeled using a three-dimensional finite element model, including a combination of nonlinear kinematic and isotropic hardening. In addition, the interaction between cyclic loading and the effect of residual stress on fatigue is taken into consideration. Finite element model is validated against representative experimental findings. Smith–Watson–Topper (SWT) method is utilized in order to estimate the fatigue life of rail welds under static and cyclic loading. Lower fatigue life is predicted with increasing load due to the contact between rails and wheels. Simulation results also show that failure in the form of ratchetting occurs during the 10,236th cycle, while failure corresponds to the 15,290th cycle and the 145,161st cycle based on the SWT and Coffin-Manson fatigue models, respectively. These findings suggest that investigations on ratchetting and fatigue should be carried out simultaneously to estimate the failure of the CWRs.     

Mechanics and mechanisms of fatigue in a WC–Ni hardmetal and a comparative study with respect to WC–Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC–Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC–Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path.     

Monotony of road environment and driver fatigue: a simulator study

Studies have shown that drowsiness and hypovigilance frequently occur during highway driving and that they may have serious implications in terms of accident causation. This paper focuses on the task induced factors that are involved in the development of these phenomena. A driving simulator study was conducted in order to evaluate the impact of the monotony of roadside visual stimulation using a steering wheel movement (SWM) analysis procedure. Fifty-six male subjects each drove during two different 40-min periods. In one case, roadside visual stimuli were essentially repetitive and monotonous, while in the other one, the environment contained disparate visual elements aiming to disrupt monotony without changing road geometry. Subject's driving performance was compared across these conditions in order to determine whether disruptions of monotony can have a positive effect and help alleviate driver fatigue. Results reveal an early time-on-task effect on driving performance for both driving periods and more frequent large SWM when driving in the more monotonous road environment, which implies greater fatigue and vigilance decrements. Implications in terms of environmental countermeasures for driver fatigue are discussed.     

液压模块挂车刚柔耦合动态响应特性及摆臂疲劳分析

为了研究重型液压模块挂车动态响应特性及其摆臂疲劳强度的问题,基于有限元分析方法与刚柔多体动力学理论,建立挂车刚柔耦合多体动力学模型,以路面不平度作为仿真的激励信号,结合振动试验验证模型的准确性。对挂车在不同的运行工况下,进行动态响应仿真分析。将多体动力学仿真结果作为有限元分析的动载荷,计算获得疲劳分析所需要的应力时间历程,运用局部应力应变法对摆臂进行疲劳寿命预测。计算结果表明,摆臂应力集中部位出现在已发生断裂的断面位置及应力水平已进入塑性状态。挂车在B级、C级与D级路面下的运行,危险点的疲劳寿命均大于挂车使用年限。而在正弦形凹凸路面冲击下,危险点的疲劳寿命随正弦形幅值与车速增加而明显减短。根据仿真计算结果,可提出适用的运行工况以提高挂车运行安全性与运输效率。