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.     

Fractographic analysis of austempered ductile iron

This investigation involves a systematic study of the fracture surfaces of two grades of austempered ductile iron (ADI) broken under quasi‐static, dynamic and cyclic loading conditions. The study used electron microscopy, optical microscopy and image post‐processing. The results show that the predominating fracture mechanism in ADI upon impact loading changes from quasi‐cleavage to ductile (with little areas of cleavage facets) as the testing temperature increases. Noticeably, even at the lower temperatures tested, the fracture surface of ADI shows clear signs of ductile fracture mechanisms. In particular, graphite nodule cavities suffer marked plastic deformation. Fracture after bending tests at room temperature was characterized by a mix of quasi‐cleavage facets, deformation of the contour of nodular cavities and microvoid coalescence. In the case of fatigue fracture at room temperature, the fracture surfaces show a flat appearance which has notorious differences with those reported for other loading conditions, but the typical fatigue striations were not found. The particular features identified on the fatigue fracture surfaces can be used to identify fatigue failures. It was also shown that the determination of the direction of main crack propagation by using the experimental methodology proposed earlier by the authors is applicable to ADI fractured by impact and quasi‐static loads. The results provide information potentially useful to fractographic analyses of ADI, particularly in samples that fail in service under unknown conditions.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

Three-point bending fatigue behavior of WC–Co cemented carbides

WC–Co cemented carbides with different WC grain sizes and Co binder contents were sintered and fabricated. The three-point bending specimens with a single edge notch were prepared for tests. In the experiments, the mechanical properties of materials were investigated under static and cyclic loads (20 Hz) in air at room temperature. The fatigue behaviors of the materials under the same applied loading conditions are presented and discussed. Optical microscope and scanning electron microscopy were used to investigate the micro-mechanisms of damage during fatigue, and the results were used to correlate with the mechanical fatigue behavior of WC–Co cemented carbides. Experimental results indicated that the fatigue fracture surfaces exhibited more fracture origins and diversification of crack propagation paths than the static strength fracture surfaces. The fatigue fracture typically originates from inhomogeneities or defects such as micropores or aggregates of WC grains near the notch tip. Moreover, due to the diversity and complexity of the fatigue mechanisms, together with the evolution of the crack tip and the ductile deformation zone, the fatigue properties of WC–Co cemented carbides were largely relevant with the combination of transverse rupture strength and fracture toughness, rather than only one of them. Transverse rupture strength dominated the fatigue behavior of carbides with low Co content, whilst the fatigue behavior of carbides with high Co content was determined by fracture toughness.     

Fracture mechanisms near threshold conditions of nickel alloyed PM steel

The paper examined fractographically four nickel alloyed powder metallurgy (PM) steels with total porosity between 3 and 9%. Fracture surfaces were inspected on smooth rectangular specimens from constant stress amplitude tests under axial loading with zero mean stress, at 30 Hz frequency.

The area containing the fatigue crack origin was observed in a region located invariably at a specimen surface. In all cases, the fracture surface was composed of three different morphological appearances (regions) associated with changed proportions of particular fracture mechanisms: macrocrack initiation region where cracks propagated preferentially through particles and there was no influence of pores on the propagation paths; in the other regions (macrocrack growth and unstable crack growth) cracks propagated mainly through the sintering necks by ductile rupture from microvoid coalescence and transparticle cleavage fracture.     

梯度结构Al_2O_3-(W,Ti)C-TiN-Mo-Ni纳米金属陶瓷刀具材料的设计及制备

基于对高速硬切削时刀具应力和温度分布,以及刀具内部疲劳裂纹扩展仿真分析,提出一个组分含量分布和微观结构具有梯度特征的设计模型。通过韧性相的添加和梯度结构的引入,实现疲劳裂纹扩展速率的减缓,从而提高刀具寿命。采用二阶段热压烧结工艺制备出具有梯度结构的Al_2O_3-(W,Ti)C-TiN-Mo-Ni纳米复合刀具材料,并对其微观结构和力学性能进行研究。结果表明:所制备的梯度结构金属陶瓷材料表层硬度、内层的断裂韧度和抗弯强度分别达到19.258GPa,10.015MPa·m~(1/2)和1017.475MPa,满足高速硬切削刀具的性能要求。材料的断口出现韧窝和黏结相撕裂形成的断裂棱,有利于断裂韧度和抗弯强度的增强,从而提高刀具抗疲劳裂纹扩展能力。     

Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.     

Metallurgical investigation on fatigue failure of stainless steel chain in a continuous casting machine

Stainless steels strips (chains) are used for the connection of dam blocks in belt casting machines. Thermal cycling and repetitive stressing under complex loading conditions due to tension and bending are the most frequent function modes during production. Samples from fractured stainless steel strips used for the connection of dam blocks in a copper rod continuous casting line, were sent for failure investigation. Optical and scanning electron microscopy for structural and fractographic evaluation along with mechanical testing are used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the failure was caused by bending fatigue which assisted also by thermal cycling, initiated from the strip surface and followed by ductile final overload fracture. Final fracture occurred via ductile failure, when the remaining strip cross sectional area reaches a critical size, becoming unable to sustain the operating load. Review of the service history (operating conditions, e.g. process design, applied loads, thermal cycles), in combination to the examination of a potential substitution of the material to a more heat (and fatigue) resistant one are suggested as further fatigue damage preventive actions.     

Assessment of effect of ductile tearing on cleavage failure probability in ductile to brittle transition region

The fracture behaviour of ferritic and ferritic martensitic steels in ductile to brittle transition (DBT) region has been extensively studied in recent years and a probabilistic approach of master curve method is generally used to describe the fracture toughness of BCC steels in DBT region as a function of temperature. The assessment of cleavage failure probability however is still untouched in the upper region of ductile to brittle transition, although various extensions of master curve approach and various local approaches has been explored. Additionally the geometry and loading in tension and bending also adds up to the difficulties when cleavage failure is assisted with prior ductile tearing. In this work the cleavage fracture is investigated in upper region of DBT and a modified master curve approach is presented which can satisfactorily describe the fracture toughness as a function of temperature as well as amount of ductile tearing preceded by cleavage.     

Investigation on low cycle fatigue of nickel‐based single crystal turbine blade in different regions

Low cycle fatigue experiments of nickel‐based single crystal superalloy miniature specimens were carried out at 760 °C/1000 MPa and 980 °C/750 MPa. According to testing results, low cycle fatigue life is dependent on sampling position of turbine blade under same test conditions. Fracture surface morphology and longitudinal profile microstructure indicated that the fracture mechanism transformed from cleavage fracture to ductile fracture with the changing of medium temperature to high temperature due to the particle cutting at yield stress intensity. The scanning electron microscopy observation of original material demonstrated that the smaller precipitate size of samples have a shorter fatigue life. Meanwhile, the constitutive model considering size effect was built based on the crystal plastic theory. The finite element analysis demonstrated that the smaller precipitate size could dramatically reduce the plastic deformation suffering the same cycle loading.     

Static and impact fatigue behaviour of borosilicate glass

Failure of a borosilicate glass as a result of repeated impact has been studied. Impact fatigue study was conducted in an improved pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strength. For elimination of the effect of humidity, repeated impact tests were carried out under liquid nitrogen. Quasi-static measurements were determined under four-point bending. Using a square waveform as applicable to the present impact tests and fracture mechanics interpretation, the number of cycles to failure during impact fatigue tests were predicted from quasi-static fatigue measurements. It has been shown that repeated impact loading has a deleterious effect on the failure cycles compared to slow stressing. The role of an added mechanical effect during repeated impacts has been suggested in controlling the cyclic fatigue behaviour. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Mechanical properties of In-Situ Composites

In this paper, author's results of several years of research work on the mechanical properties of directionally solidified (In-Situ) composites are reviewed. Alloy systems investigated were the fibrous Al-Ni, Fe-MnS and the cobalt base superalloy Co-Cr-C and the lamellar Al-Cu and Co-W. The mechanical behavior of the above systems were studied under both static and dynamic loadings. Static loading involved tension, compression and 3-point bending and the dynamic loading involved rotating bending fatigue, fatigue crack propagation and strain controlled fatigue. It was found that the tensile fracture stress and toughness and the ultimate compressive stress were generally enhanced by increasing growth rate and/or temperature gradient. However, at very high growth rates, the properties were found to decrease due to misalignment of the structure. Models were suggested to describe the static behavior of the composites investigated. Good agreement was found between the model predictions and the experimented results which indicate that the static properties are structural sensitive. On the contrary, the fatigue life of the Al-Al₃ Ni was insensitive to structural changes caused by varying the growth rate. The fatigue crack propagation response of the Co-Cr-C composites was found to follow the Paris Erdogan relation. Examination of the fracture surface confirmed a brittle mode of fracture with fiber cleavage and matrix shearing to link up fiber breaks.     

Anomalous fracture behaviour in polyethylenes under fatigue and constant load

The fracture behaviour of two copolymers of polyethylene were studied under fatigue and constant load. There is an anomaly because one resin was stronger under fatigue loading and the other under a constant load. The fatigue parameters that were investigated were waveform, frequency,R, and time under maximum and minimum stress. The fatigue failure was described in terms of the stretching of the fibril in a craze under the maximum stress and bending of the fibril under the minimum stress, whereas only fibril stretching occurs under a constant load. The damage per cycle by stretching is much less than the damage per cycle by bending the fibril. It is proposed that molecular weight has a greater influence on fatigue fracture than on fracture under a constant load.     

Fracture surfaces and the associated failure mechanisms in ductile iron with different matrices and load bearing

Ductile iron (DI) is a family of cast alloys that covers a wide range of mechanical properties, depending on its matrix microstructure. For instance, ferritic matrices used in parts, such as automotive suspension components, demand high impact properties and ductility among some of their main requirements. On the other hand, pearlitic and martensitic matrices are used when hardness, strength and wear resistance are of particular concern. When it comes to very high strength parts, ausferritic matrices, typically austempered ductile iron (ADI), are widely used.DI has been employed to replace cast and forged steels in a large number of applications and its production has shown a sustained rate of growth over the last decades.Knowing about failure modes and fracture mechanisms associated to materials with the properties mentioned above is crucial, since they can be of great value for designers of mechanical components.This paper deals with the analysis of fracture surfaces of ductile cast iron generated under different conditions of load application, temperature and environments.The studies include the examination of fracture surfaces obtained by means of tensile tests, impact tests and by samples used to determine fracture toughness properties, where the zones of fatigue pre-crack and monotonic load condition were evaluated. A special case of ductile iron fracture is also examined.The study of the different surfaces permitted to establish patterns that contributed to unveil the fracture mechanisms of ductile iron with different matrices, nodule count, etc.     

Failure study of connecting shafts of a plug screw feeder in a paper production plant

The connecting shafts of two plug screw feeder units of a paper production plant failed within a relatively short period after the production start of the plant. To investigate the reasons for these failures, material characterizations of the broken shafts were performed. The program included tensile, impact and hardness tests and metallographical and fractographical investigations. Fracture surfaces of the failed shafts were studied using light and electron microscopy. Obvious striations and beach marks could be observed by fractographic examination. The fracture features observed on the fracture surfaces reveal that (a) the shafts had experienced fatigue crack growth before the final failure; (b) the crack growth initiated from a sharp undercut at the threaded part of the shaft; (c) the fatigue driving forces have combined torsional and bending loading, having torsional as the governing load.     

On the topography of fracture surfaces in bending–torsion fatigue

Fracture surfaces generated under combined bending–torsion fatigue loading in both the low-cycle fatigue and the high-cycle fatigue regions of specimens made of high-strength low-alloy Cr–Al–Mo steel are analysed in terms of topographical characteristics. Parameters reported here are the root mean square roughness, the number of peaks per unit length and the Hurst exponent quantifying various aspects of surface topography. As a main result of the analysis, the critical portion of torque beyond which the character of fracture topography significantly changes is estimated to be within the range of 40–50% of a total loading.     

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10⁶ cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.     

Effects of temperature on delamination growth in a carbon/epoxy composite under fatigue loading

This paper presents a study of delamination growth in HTA/6376C carbon fibre/epoxy laminates. Tests were conducted under Mode I, Mode II and mixed-mode static and fatigue loading at both ambient conditions and elevated temperature. The results show that the strain energy release rate threshold values for delamination growth under fatigue loading are significantly lower than the critical energy release rates in static tests. At elevated temperature, the threshold values in the fatigue loading were only about 10% of the critical values in the static tests. A fractographic analysis of the delamination growth revealed that the fracture surfaces generated at elevated temperature generally were similar to the fracture surfaces generated at room temperature. Nevertheless, some differences in morphology of the fracture surfaces were observed, and their effect on the static and fatigue delamination growth is discussed in detail.     

FAILURE MECHANISMS IN IMPACT FATIGUE OF METALS

Abstract Impact fatigue tests were performed with smooth and notched specimens of low carbon steels under various impact loading conditions. The characteristic failure mechanisms in impact fatigue was discriminated by comparison with those in non-impact, ordinary fatigue. The fatigue life of smooth specimens was uniquely related to the range of plastic strain at the middle of the fatigue life in both impact and non-impact fatigue, although the characteristics of micro-structural deformation and cyclic stress-strain relationships were markedly different. The growth rate of a fatigue crack in impact fatigue of notched specimens was higher than that in non-impact fatigue when compared at the same stress intensity factor. Fractographic observations with scanning electron microscopy and the X-ray diffraction technique revealed more abundant cleavage facets and a smaller spread of the plastic zone beneath the fracture surface made by impact fatigue. Both nucleation and propagation lives in notched specimens were much shorter in impact fatigue than in non impact fatigue when compared at the same values of nominal stress and stress concentration factor.     

Effects of cyclic torsional prestraining and overstrain on fatigue life and damage behavior of brass alloy

Many practical structural members and parts may be subjected to fluctuating plastic deformation by prestraining due to manufacturing and machining process (forming operation, straightening, etc.) and unintentional overstrains (misuse, accidents, under design, etc.). For this reason, the effect of the prestrain and periodic overstrains on fatigue life and damage behavior was being necessary considered for reasonable fatigue design. In this context, an experimental program was conducted to study the effects of overstrain and prestraining on fatigue life and damage behavior of brass alloy subjected to cyclic torsional loading. To establish baseline fatigue behavior, several virgin specimens were tested under fully-reversed strain control and constant amplitude fatigue torsional loading up to failure. The obtained experimental results showed that the fatigue life depends strongly on the strain amplitude and prestraining type (monotonic or cyclic). In addition, a beneficial effect in the fatigue life was observed for all tests with periodic overstrain. Cyclic fatigue fracture on a macroscopic scale revealed features reminiscent of locally ductile and brittle mechanisms. At the same time, microscopic analysis indicated a difference on fatigue fracture surface morphology between the conducted tests and those performed under constant amplitude loading.