Novel method for the fatigue strength assessment of heavy sections made by ductile cast iron in presence of solidification defects

The fatigue strength of ferritic, pearlitic, and solution strengthened ferritic ductile irons taken from heavy sections and characterized by long solidification times has been assessed. Starting from the idea of Murakami and co‐workers, a new model for the prediction of the fatigue strength is proposed. It allows a sound fatigue assessment of the fatigue strength of as‐cast ductile irons containing solidification defects, such as low nodule count, exploded, chunky and spiky graphite, or microshrinkage porosities. The newly developed equation validated by means of an extensive benchmarking with data taken from the literature has shown a very high potential for applications to thick‐walled components.     

Influence of nitriding on the fatigue behavior and fracture micromechanisms of nodular cast iron

Surface modification processes are increasingly used to fully exploit material potential in fatigue critical applications because fatigue strength is sensitive to surface conditions. Nitriding is extensively adopted with ferrous materials because it forms a hard and strong surface layer and a system of superficial compressive residual stresses. Fatigue, however, is strongly dependent also on defects and inhomogeneity. When nitriding is applied to nodular cast iron, the relatively thin hardened layer (about 300 μm) contains graphite nodules (diameter of the order of 30 μm), casting defects and a heterogeneous matrix structure. The paper presents and discusses the influence of nitriding on the fatigue response and fracture mechanisms of nodular cast iron. A ferritic nodular cast iron and a synthetic melt with different content of effective ferrite were initially gas-nitrided. Then, (i) structural analysis of nitrided layers, (ii) fatigue testing with rotating bending specimens, and (iii) fatigue fracture surface inspection were performed. Performance and scatter in fatigue performance is discussed by selective inspection of fracture surfaces and identification fracture micromechanisms. A semiempirical model explains observed trends in test results and is used for the process optimization. __________ Translated from Problemy Prochnosti, No. 1, pp. 85–88, January–February, 2008.     

Damaging micromechanisms characterization of a ferritic ductile cast iron

Damaging micromechanisms in a ferritic ductile cast iron were investigated by using a scanning electron microscope with a microtensile holder. Strain rate influence was investigated: tensile tests were performed according to a step by step experimental procedure and specimens surfaces were observed by means of a scanning electron microscope (SEM) during the test (“in situ” tests). Experimental results do not confirm matrix-graphite nodules debonding as the main damaging micromechanisms involved in ferritic ductile irons failure.     

Fracture of pearlitic ductile cast iron under different loading conditions

In this study, fracture surfaces of pearlitic ductile iron generated under impact, bending and fatigue tests were characterised and compared. The fracture mechanisms in each case were identified from the observation of scanning electron microscopy images, and the characteristic topographic features were quantitatively and qualitatively evaluated. Cleavage was the predominant fracture mechanism for impact loading at all test temperatures and for slow bending testing. On the other hand, under fatigue loading, the fracture surfaces showed a mix of cleavage and ductile striation areas. A complex but reliable methodology for the determination of the direction of propagation of the main crack was also proposed, based on a careful analysis of the river patterns along the cleavage planes.     

Ductile cast irons: Microstructure influence on the fatigue initiation mechanisms

In the last decades, the combination of high mechanical performances and low production costs increased the industrial interest on ductile cast irons. These grades are often used for applications where the fatigue resistance can be a critical issue (eg, machine frames for the wind‐power industry or crankshaft used in trucks) and the investigation of the main damaging mechanisms during both the crack initiation and the crack propagation stage could offer new perspectives about these alloys. Ductile cast irons can be considered as a natural composite, being characterized by graphite elements (nodules) embedded in a more or less ductile matrix (ranging from fully ferritic to pearlitic, from martensitic to austempered). In this work, the fatigue crack initiation mechanisms were investigated considering different matrix microstructure and the presence of a mechanical properties gradient in the graphite nodules.     

On the fatigue damage micromechanisms in Si‐solution–strengthened spheroidal graphite cast iron

Graphite nodules in spheroidal graphite cast iron (SGI) play a vital role in fatigue crack initiation and propagation. Graphite nodules growth morphology can go through transitions to form degenerated graphite elements other than spheroidal graphite nodules in SGI microstructure. These graphite particles significantly influence damage micromechanisms in SGI and could act differently than spheroidal graphite nodules. Most of the damage mechanism studies on SGI focused on the role of spheroidal graphite nodules on the stable crack propagation region. The role of degenerated graphite elements on SGI damage mechanisms has not been frequently studied. In this work, fatigue crack initiation and propagation tests were conducted on EN‐GJS‐500‐14 and observed under scanning electron microscope to understand the damage mechanisms for different graphite shapes. Crack initiation tests showed a dominant influence of degenerated graphite elements where early cracks initiated in the microstructure. Most of the spheroidal graphite nodules were unaffected at the early crack initiation stage, but few of them showed decohesion from the ferrite matrix and internal cracking. In the crack propagation region, graphite/ferrite matrix decohesion was the frequent damage mechanism observed with noticeable crack branching around graphite nodules and the crack passing through degenerated graphite elements. Finally, graphite nodules after decohesion acted like voids which grew and coalesced to form microcracks eventually causing rapid fracture of the remaining section.     

Correlation of the acoustic emission and the fracture toughness of ductile nodular cast iron

Salzbrenner has recently determined the fracture toughness of a series of ductile cast iron samples which were heat treated to produce a fully ferritic matrix. His results indicated that the fracture toughness is strongly dependent upon the average spacing between (or equally the diameter of) the spherical graphite nodules in the ferrite matrix. The acoustic emission generated during the uniaxial compressive deformation of nodular cast iron also depends strongly on the average diameter of the graphite nodules in the test sample. The present investigation was carried out to determine the correlations, if any, between the fracture toughness and the acoustic emission generated during compression of ductile cast iron. The acoustic emission generated during compression was determined using sample materials identical to those used by Salzbrenner. Excellent correlations between certain features of the measured acoustic emission and the fracture toughness were obtained. Data indicate that it should be possible to determine both the fracture toughness and the average size of the graphite nodules from the acoustic emission and load curve generated during a compression test of ductile cast iron.     

Life assessment of SiMo 4.06 cast iron under LCF and TMF loading conditions

Ductile cast irons are often used for turbocharged diesel engine components. These components are usually subjected to cyclic thermo-mechanical loading during their lifetime due to variable service conditions. A reliable damage assessment is required to guarantee service function, safety or light weight of the products. The first objective of this paper is to introduce the new test stand designed for uniaxial Low Cycle Fatigue (LCF) and Thermo-Mechanical Fatigue (TMF) tests. The second objective of the paper is to assess the lifetime of SiMo (Silicon Molybdenum) cast iron. Based on the experimental results, an appropriate energy-based fatigue criterion is proposed for predicting the LCF/TMF of complex engineering components. Reasonable correlation was achieved between the observed results and the predicted results.     

Fatigue life assessment of nodular cast iron containing casting defects

The fatigue behaviour of a nodular cast iron containing casting defects has been investigated in the high-cycle fatigue regime. In this paper, we propose a fatigue life assessment model for flawed materials based on a fracture mechanics approach which takes into account the position and size of the defect, short crack behaviour and the notch effect introduced by the defect. The fatigue behaviour of smooth samples, and long and short crack behaviour have been experimentally determined in order to identify the relevant mechanical parameters; these being introduced into the model. An experimental study has been made both in air and in vacuum in order to account for the position of the defect, noting that internal defects are supposed to be under vacuum conditions. Experimental results, which are based on a two-crack front-marking technique specially developed for this study, show that the propagation of natural cracks is controlled by the effective stress intensity factor in air as well as in vacuum. The K calculation for a short crack in the stress field of a notch is analysed using numerical elastic–plastic results. Comparison between experimental results and the computation of fatigue life for fatigue lives less than 10⁶ cycles shows that the fatigue behaviour of nodular cast iron is controlled by a propagation process. The model proposed is thus relevant for fatigue lives less than 10⁶ cycles so that the defect can be considered as a crack and the initiation stage neglected. Closer to the fatigue limit, this study shows that the initiation stage should be considered in the assessment of fatigue life of nodular cast iron, because a single macroscopic propagation assessment is not enough to describe the whole fatigue life. The defect cannot be considered as a pre-existent crack in the high-cycle fatigue range (>10⁶ cycles), and the initiation stage that contains microcrack propagation around the defect should be evaluated when assessing the high-cycle fatigue life of nodular cast iron.     

Large scale testing and statistical analysis of dynamic fracture toughness of ductile cast iron

The present paper deals with the experimental determination and statistical analysis of dynamic fracture toughness values of ductile cast iron. K_Id data from 140 mm thick single edge bend specimens of two dynamic fracture toughness test series on ductile cast iron from heavy-walled castings were analysed.At first, the statistical analysis of data at −40 °C was done based on ASME Code Case N-670 using a two-parameter Weibull distribution function. Weibull analyses of three samples covering different pearlite contents (?4%, ?9%, ?20%) were performed and characteristics of the distribution functions as well as two-sided confidence intervals were calculated. The calculated characteristics show that K_Id of ductile cast iron decreases with increasing pearlite content.In a second step, the applicability of the Master curve procedure according to ASTM E 1921 to ductile cast iron materials was investigated and it was formally used for statistical analysis of ductile cast iron dynamic fracture toughness data. Although the Master curve method was originally introduced for static fracture toughness data of ferritic steels, the successful individual analyses performed here support the engineering way taken to apply the method to ductile cast iron materials too. The results of both methods, the Master curve procedure and the ASME Code Case N-670, show acceptable congruity. At the same time, it is concluded from the present study that further investigations and experiments are required to improve precision and for verification before the results could be applied within component safety analyses.     

Failure analysis of a crankshaft made from ductile cast iron

This paper describes the failure analysis of a diesel engine crankshaft used in a truck, which is made from ductile cast iron. The crankshaft was found to break into two pieces at the crankpin portion before completion of warranty period. The crankshaft was induction hardened. An evaluation of the failed crankshaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that fatigue is the dominant mechanism of failure of the crankshaft. It was observed that the fatigue cracks initiated from the fillet region of the crankpin-web. The absence of the hardened case in the fillet region and the presence of free graphite and nonspheroidal graphite in the microstructure of the crankshaft made fatigue strength decrease to lead to fatigue initiation and propagation in the weaker region and premature fracture.     

Effect of heat treatment on ultrasonic velocity in ductile cast iron

Abstract

The measurement of the ultrasonic velocity is a common method in the foundry industry for the evaluation of the nodularity in ductile iron castings. Practical experience has shown that heat treatment can reduce the ultrasonic velocity compared to the as cast condition. Using ductile iron samples with different heat treatments in order to vary the ferrite and pearlite content respectively confirmed this decrease in the ultrasonic velocity compared to the as cast state. Further investigations showed that with all the heat treatments applied, irrespective of their effect on the microstructure, the density was decreased. The decrease in density correlated with the decrease in ultrasonic velocity for all heat treatments. The mechanisms involved in the reduction in the density are discussed.     

Weibull-based methodology for condition assessment of cast iron water mains and its application

A qualitative and quantitative understanding of how cast iron water distribution pipes fail in service would facilitate a targeted approach to the management of rehabilitation in the water industry. This paper proposes a technique for assessing the condition of pipes, based on strength characteristics obtained from small samples; this offers an alternative way of estimating the likelihood of failure to current methodologies based on pit‐depth measurements. Examination of recovered pipe samples indicates that the strength of the cast iron pipe reduces over time as a result of corrosion, although other time‐dependent processes, such as fatigue, may also contribute to this degradation. Taken with previous work, this paper suggests that the variation in strength of small samples removed from cast iron water distribution pipes can be characterized using Weibull methods. It is argued that the Weibull modulus provides a useful indicator of the condition of the pipe. Using scaling arguments, inherent in the Weibull methodology, it is then possible to use data from small samples to predict the likely strength characteristics of water distribution pipes in the ground, which is reasoned to be a good measure of the potential performance of the pipe in service. The Weibull approach is applied to a number of different data sets obtained from testing samples extracted from a range of pipes, which have seen service at various locations in the Thames Water region. One of these data sets was from locations where failure had occurred in service. It is shown that the use of Weibull analysis can identify pipes in the network that have degraded the most significantly. A methodology is suggested whereby this information taken with other performance indicators can be used to identify the local regions where rehabilitation is required most urgently. Alternatively, it can be used to identify those regions of the network, which are in good condition and unlikely to need repair or replacement work.     

Ni合金化球墨铸铁组织和性能研究

为获得兼具较高强度和良好低温冲击韧性的球墨铸铁铸件,向球墨铸铁中加入质量分数约0.5%的Ni进行合金化,并对其进行中温奥氏体化(880℃+3 h)和低温退火(720℃+4 h)处理.采用光学显微镜(OM)、扫描电子显微镜(SEM)对铸态和热处理态试样的显微组织和冲击断口形貌进行分析;利用万能试验机、布氏硬度计和摆锤式冲击试验机等对铸态和热处理态试样进行了室温拉伸、硬度检测、低温冲击等力学性能测试.结果表明:铸态球墨铸铁的微观组织由珠光体、铁素体和球状石墨及少量的渗碳体组成,其强度、硬度偏高,塑性、韧性较差;热处理态试样中的珠光体向铁素体转变后为铁素体和球状石墨,试样强度、硬度有所降低,塑性、韧性得到明显的改善;铸态试样呈现典型的脆性断裂特征,热处理态试样冲击断口处存在少量韧窝,断裂模式以解理断裂为主,伴有少量塑性变形的韧脆混合断裂,且在-40℃冲击功达到12.4 J;比较铸态与热处理态的冲击断口形貌可知,试样断裂方式由脆性断裂转变为韧脆混合断裂.     

Structural integrity and damage type in military aircraft

This paper discusses the nature of the damage which can cause unanticipated, early fleet failures in legacy military aircraft. This damage often takes a form quite unlike the notional cracking which is used in structural integrity design, but can lead to high costs, or reduced fleet availability, by requiring additional costly inspections and recovery programs in response to discovery of the damage. Using examples from Australian fleets the paper demonstrates that a lack of diagnostic and prognostic tools contributed to the impact of the damage on the fleet. Development of even simple prognostic and diagnostic tools could reduce the fleet impact and cost of discovered damage, and would ultimately allow the non‐crack damage to be incorporated into design to achieve a much more global level of damage tolerance. The paper discusses the key differences between these non‐crack damage forms and the more traditional crack‐like defect which is used in current damage tolerance based structural integrity management approaches for these aircraft. These differences are associated principally with damage variability and damage location, and they challenge some aspects of our existing structural integrity design methods such as reliance on testing and analysis of supposedly ‘representative’ example of aircraft structure. The larger challenge is to fully exploit the principles of damage tolerant structural design and management, and the paper argues that to achieve this we need to maintain a move towards a broader, risk‐based approach to structural integrity management. This longer‐term goal will also involve a reappraisal of the nature and distribution of damage, and a fundamental shift in our crack‐centric view of structural integrity, additional diagnostic and prognostic tools for such damage would be essential for developing this transition to a more global risk‐based damage management approach.     

An experimental and numerical analysis to correlate variation in ductility to defects and microstructure in ductile cast iron components

This paper describes a statistical test plan to determine tensile properties and fracture properties for ductile cast iron considered for the Swedish nuclear waste canisters and associated analysis. Large variations were found in the ductility between tested canister inserts and between specimens taken from different locations in each insert. A large number of tested tensile specimens were subsequently analysed by fractography and metallography to relate low ductility values to size and type of casting defects. Loss of ductility could be related to slag defects and to a lesser extent to high pearlite content, low nodularity and chunky graphite. Slag defects were modelled by an elasto-plastic fracture mechanics model for penny-shaped slag defects and semi-empirical models for the other defect types. The fracture model was incorporated into a probabilistic scheme to compute distribution of elongation for the inserts and the associated defect size. The computed ductility distribution agrees very well with measured data whereas the computed defect size distribution is underestimated. By including crack growth resistance and various aspect ratios of defects a much better agreement with observed defects can be achieved.     

Study of the fracture of ferritic ductile cast iron under different loading conditions

This work is a continuation of the studies presented in a recent paper by the authors, where the fracture surfaces of pearlitic ductile cast iron under different loading conditions were exhaustively analysed. In this study, fracture surfaces of ferritic ductile cast iron (or ferritic spheroidal graphite cast iron) generated under impact, bending and fatigue loading conditions were characterised and compared. The fracture surfaces were characterised qualitatively and quantitatively from the observation under a scanning electron microscope. The fracture mechanisms in each case were identified. For impact tests, as test temperature increases, the dominant fracture mechanism changes from brittle to ductile. For bending tests, a fully ductile fracture micromechanism dominates the surface. In fatigue tests, the surface shows a mix of flat facets that appear to be cleavage facets and ductile striations, but the typical fatigue striations are not easily found on the fracture surface. Methodologies for the determination of the macroscopic direction of main crack propagation in both ductile and brittle failure modes are proposed. These allow identifying main crack propagation direction with good approximation. The results are potentially useful to identify the nature of loading conditions in a fractured specimen of ferritic spheroidal graphite cast iron. The authors believe that it is necessary to extend the methodologies proposed in samples with different geometry and size, before they can be used to provide additional information to the classical fractographic analysis.     

Non-local regularization for FE simulation of damage in ductile materials

The paper presents an application of the non-local regularization to the finite element modelling of ductile damage and tearing. In order to model the damage growth in ductile materials of structural components, integral limiters have been introduced. These integral limiters, which are spatial averaging operators, can prevent the problem of mesh-sensitivity of the finite element computations. Hence, it was important to establish the characteristic length l_c for spatial averaging operators of the non-local regularization. More formally, the in-plane distance l_c and the out-of-plane FE dimension have been specified, characterizing the volume over which averaging of stress and strain was carried out to ensure that the continuum theory can represent the physical process of damage.

In order to check the reliability and transferability of the method, FE simulations of various testing examples have been carried out, namely ductile fracture in notched tensile specimens, ductile crack growth in C(T) specimens and application to the pipe-bending test.     

Mechanical behaviour and damage mechanisms analysis of a flax-fibre reinforced composite by acoustic emission

This article presents the mechanical characterization of an eco composite consisting of a thermoplastic matrix reinforced by flax fibres. Different configurations of specimens were tested with uniaxial tensile loading and their mechanical behaviours were discussed. Moreover, the acoustic emission technique was used to detect the appearance of damage mechanisms and to follow their evolution. In addition, a list of these mechanisms was established by means of macroscopic and microscopic observations. The acoustic emission records were post processed by the k-means unsupervised pattern recognition algorithm. Depending on the specimen configuration, three or four classes of events were obtained. The acoustic characteristics of these classes were compared. Then, a correlation between these AE events classes and the damage mechanisms observed was proposed. Their effects on the mechanical behaviour of the material were investigated by means of a variable called the Sentry Function.