Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs

This paper evaluates the fatigue life properties of low carbon grey cast iron (EN-GJL-250), which is widely used for automotive brake discs. Although several authors have examined mechanical and fatigue properties at room temperatures, there has been a lack of such data regarding brake discs operating temperatures. The tension, compression and low cycle fatigue properties were examined at room temperature (RT) and at brake discs’ working temperatures: 500 °C, 600 °C and 700 °C. The microstructure of the material was documented and analysed. Tensile stress–strain curves, cyclic hardening/softening curves, stress–strain hysteresis loops, and fatigue life curves were obtained for all the above-mentioned temperatures. It was concluded, that Young’s modulus is comparable with both tension and compression, but yield its strength and ultimate strength are approximately twice as great in compression than in tension. All the mechanical properties remained quite stable until 500 °C, where at 700 °C all deteriorated drastically. During fatigue testing, the samples endured at 500 °C on average at around 50% of cycles at room temperature. Similar to other materials’ properties, the cycles to failure have dropped significantly at 700 °C.     

Crack growth resistance of a gray cast iron

Gray cast iron exhibits a discontinuous crack growth process. No definite crack front is observed but fracture is induced by the progression and extension of a damaged zone whose extremities remain ill-defined. It was demonstrated, in an earlier study, that the crack resistance of a grey cast iron is therefore strongly dependent on the length of the cracked zone. In order to otbain a meaningful toughness value, i.e. a measurement of the resistance to crack propagation, it is necessary to use a specimen configuration in which the size and position of the damaged zone remain unaffected by the crack propagation. Such measurements have been made on a continuously cooled gray cast iron by using DCB specimens with circular side grooves. After the fracture had initially progressed to a relatively large extent, the loading points were moved forward and a new loading sequence was applied. Crack resistance was measured during successive loading sequences, applied after displacement of the loading points. Compliance measurements were used to evaluate the crack increase and an equivalent R-curve was drawn for each loading sequence. Except for the initial sequence the successive R-curves remained fairly identical. Moreover, a constant R-value was observed for a relatively large range of crack extensions. This demonstrates that stationary fracture conditions have been obtained. From these results it may be concluded that by using this particular procedure applied to a DCB specimen, a resistance to fracture propagation has been measured which is independent of the size of the cracked zone and which is representative of the microstructure.     

A methodology for the fatigue design of notched castings in gray cast iron

Fatigue failures of machine components remain a topic of relevant importance in the industrial world. They usually occur from geometrical features such as holes, notches, corners and grooves, whose actual influence is not well estimated in the design phase. Cast parts made in gray cast iron are typical examples of components difficult to design in fatigue because they are simultaneously characterized by complex geometries and microstructure. In this contribution the issue is discussed starting from the failure analysis of a cyclically pressurized hydraulic component. The work consists of an experimental procedure, i.e. the fatigue characterization of the material on specimens extracted from cast parts, and of a numerical design activity, i.e. the prediction of life time according to the critical distance method [Taylor D. Crack modelling: a technique for the fatigue design of components. Engng Fail Anal 1996;3(2):129-36]. The implication is that cracks and localized damage begin to appear in the microstructure of gray cast iron at sharp notches from the first cycles of loading. In order to obtain a correct prediction, the fatigue design should adopt fracture mechanics arguments to determine non-propagating conditions.     

Effect of heat treatment on fracture behavior of steel-wire-reinforced gray cast iron

In this experimental study, fracture toughness of heat-treated metal matrix composite was investigated. The gray cast iron was reinforced with steel wire of volume fraction of V_r = 0.05 and three-point bend specimens were manufactured to determine fracture toughness. Heat treatment was applied to the specimens at the normalization temperatures of 850°C and then cooled in three distinct environments (water, air, and furnace). Fracture toughness of the metal matrix composite was calculated by unloading compliance method. The study shows that the fracture toughness of the steel-wire-reinforced gray cast iron increases with the increase in cooling rate. Scanning electron microcopy (SEM) analyses were used to examine the microstructure and fracture surface. It is observed that the carbon diffuses from the gray cast iron to the steel wire and transition region having partially dissolved graphite was observed due to carbon diffusion, and it plays an important role in the fracture toughness depending on the cooling media.     

Probabilistic high cycle fatigue behaviour of nodular cast iron containing casting defects

Theoretical and experimental investigations were combined to characterize the influence of surface casting defects (shrinkages) on the high cycle fatigue (HCF) reliability. On fracture surfaces of fatigue samples, the defect is located at the surface. The shape used for the calculation is a spherical void with variable radius. Finite-element simulations were then performed to determine stress distribution around defects for different sizes and different loadings. Correlated expressions of the maximum hydrostatic stress and the amplitude of the shear stress were obtained by using the response surface technique. The loading representative point in the HCF criterion was then transformed into a scattering surface, which has been obtained by a random sampling of the defect sizes. The HCF reliability has been computed by using the Monte Carlo simulation method. Tension and torsion fatigue tests were conducted on nodular cast iron with quantification of defect size on the fracture surface. The S – N curves show a large fatigue life scattering; shrinkages are at the origin of the fatal crack leading to the final failure. The comparison of the computed HCF reliability to the experimental results shows a good agreement. The capability of the proposed model to take into account the influence of the range of the defect sizes and the type of its statistical distribution has been demonstrated. It is shown that the stress distribution at the fatigue limit is log-normal, which can be explained by the log-normal defect distribution in the nodular cast iron tested.     

Mechanical and wear behavior of quenched and tempered alloyed hypereutectic gray cast iron

The effect of tempering temperature (100–600 °C) on the hardness and wear resistance of a series of quenched and tempered hypereutectic alloyed gray cast irons has been studied in this work. Hardness was observed decreases with increase in tempering temperature and this trend is influenced by alloying additions and the volume of graphite flakes. Hardness of alloyed gray irons is also influenced by solid solution strengthening of tempered ferrite and carbide content and their distribution. The wear loss of alloyed cast irons was found to be lowest at a tempering temperature of 100 °C and 400 °C. The optimum tempering temperature is 400 °C with moderate hardness and low wear rate. This has been attributed to strengthening of the matrix at this temperature. Beyond 400 °C, the wear rate increases significantly due to carbide coagulation.     

Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron

An investigation was carried out to examine the influence of austempering heat treatments and the resultant microstructure of austempered ductile cast iron, on the fatigue crack growth rate, fatigue threshold, and high-cycle fatigue strength of the material. Two different approaches were used to study the fatigue behavior of this relatively new material, that is, a traditional S-N curve approach for determination of fatigue strength and a fracture mechanics-based approach for determination of the fatigue threshold. Compact tension and cylindrical specimens prepared from alloyed nodular ductile cast iron were given three different austempering heat treatments to produce three different microstructures. The fatigue threshold and high-cycle fatigue behavior of these specimens were studied in room temperature ambient atmosphere. The results of the present investigation demonstrate that the fatigue threshold of the material increases with increase in volume fraction of carbon-saturated austenite. The fatigue strength of the material, on the other hand, was found to increase with decrease in austenitic grain size. The crack growth process in the material was a combination of ductile striations and microvoid coalescence, and crack propagation by connecting the graphite nodules along its path.     

Lifetime prediction of cast iron materials under combined thermomechanical fatigue and high cycle fatigue loading using a mechanism-based model

In this paper the fatigue life of three cast iron materials, namely EN-GJS-700, EN-GJV-450 and EN-GJL-250, is predicted for combined thermomechanical fatigue and high cycle fatigue loading. To this end, a mechanism-based model is used, which is based on microcrack growth. The model considers crack growth due to low frequency loading (thermomechanical and low cycle fatigue) and due to high cycle fatigue. To determine the model parameters for the cast iron materials, fatigue tests are performed under combined loading and crack growth is measured at room temperature using the replica technique. Superimposed high cycle fatigue leads to an accelerated crack growth as soon as a critical crack length and thus the threshold stress intensity factor is exceeded. The model takes this effect into account and predicts the fatigue lives of all cast iron materials investigated under combined loadings very well.     

High-temperature properties of ferritic spheroidal graphite cast iron

The behaviour of fracture mode and intermediate temperature embrittlement of ferritic spheroidal graphite cast iron is influenced by many factors. From the experimental results, intermediate temperature embrittlement can be considered to be dominated by dynamic strain ageing and the triaxial stress field developed in the ferrite matrix amongst the graphite particles. In order to understand the effect of dynamic strain ageing on high-temperature properties, tensile properties, push-pull low-cycle fatigue properties, rotary bending fatigue properties and creep-rupture properties were investigated from room temperature to 500° C. It was found that all the properties investigated were influenced by dynamic strain ageing. The intermediate temperature embrittlement of ferritic spheroidal graphite cast iron found in different load conditions is reported.     

A comparison of methods for predicting the fatigue life of gray cast iron at elevated temperatures

High temperature fatigue life tests were compared with various life assessment methods for gray cast iron in the cylinder liners of marine engines. The plastic strain range‐based methods such as the universal slopes method, Mitchell's method, Bäumel and Seeger's method, and Ong's method were not predicted well. A method employing tensile energy density, which is defined as the sum of the tensile area of the plastic energy and the elastic energy in the hysteresis loop, was suggested as an improved alternative to the plastic strain range‐based methods. The life estimation equation using tensile energy density predicted well within the 3X scatter band at various temperature ranges of the gray cast iron.     

Stress relaxation behavior and low cycle fatigue behavior of bulk SAC 305

Bulk Sn_96.5Ag₃Cu_0.5 samples were mechanically tested to investigate the effect of temperature, frequency and applied stress on the low cycle fatigue and stress relaxation behavior and the corresponding microstructure. Samples were tested under a variety of parameters including applied stresses between 8 and 80 MPa, temperatures of 25, 50, 100 and 150 °C and frequencies of 1, 0.1 and 0.01 Hz, respectively. Samples used for the stress relaxation behavior exhibited plastic behavior with increased softening behavior with increased stress levels, increased temperature and lower frequencies. Bayesian analysis revealed that stress relaxation behavior could be expressed in general by the following expression: ?σ = AN ^B In the previous expression, Bayesian analysis showed that the testing frequency has an exponential dependency while the temperature has a power law dependency on the parameters A and b. The results of the low cycle fatigue study showed that life decreased with increased applied stress, decreased frequency and increased temperature. Bayesian analysis revealed that the low cycle fatigue behavior could be described by the following expression: ?σ = G(logN) ^m . Additionally, Bayesian analysis showed that the testing frequency and temperature both have a power law dependency on the parameters G and m.     

Low‐cycle fatigue behavior of a newly developed cast aluminum alloy for automotive applications

The drive for increasing fuel efficiency and decreasing anthropogenic greenhouse effect via lightweighting leads to the development of several new Al alloys. The effect of Mn and Fe addition on the microstructure of Al‐Mg‐Si alloy in as‐cast condition was investigated. The mechanical properties including strain‐controlled low‐cycle fatigue characteristics were evaluated. The microstructure of the as‐cast alloy consisted of globular primary α‐Al phase and characteristic Mg₂Si‐containing eutectic structure, along with Al₈(Fe,Mn)₂Si particles randomly distributed in the matrix. Relative to several commercial alloys including A319 cast alloy, the present alloy exhibited superior tensile properties without trade‐off in elongation and improved fatigue life due to the unique microstructure with fine grains and random textures. The as‐cast alloy possessed yield stress, ultimate tensile strength, and elongation of about 185 MPa, 304 MPa, and 6.3%, respectively. The stress‐strain hysteresis loops were symmetrical and approximately followed Masing behavior. The fatigue life of the as‐cast alloy was attained to be higher than that of several commercial cast and wrought Al alloys. Cyclic hardening occurred at higher strain amplitudes from 0.3% to 0.8%, while cyclic stabilization sustained at lower strain amplitudes of ≤0.2%. Examination of fractured surfaces revealed that fatigue crack initiated from the specimen surface/near‐surface, and crack propagation occurred mainly in the formation of fatigue striations.     

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.     

Effect of toughness on low cycle fatigue behavior of pipeline steels

The effect of toughness on the fatigue behavior of pipeline steels was investigated, including the fatigue crack propagation rate and low cycle fatigue test under the loading condition simulating the actual operation of pipelines. The results indicate that the toughness can strongly influence the fatigue behavior of pipeline steels (i.e., the steels with high toughness possess high resistance to fatigue crack propagation and high tolerance of damage, which are much beneficial to obtaining a long life for line pipe structures).     

Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy

In order to observe the influence of strontium (Sr) modification and hot isostatic pressing (HIP) on an aluminum–silicon cast alloy A357 (AlSi7Mg0.6), the microstructure and the high cycle fatigue behavior of three batches of materials produced by investment casting (IC) were studied. The parts were produced by an advanced IC proprietary process. The main process innovation is to increase the solidification and cooling rate by immersing the mold in cool liquid. Its advantage is to produce finer microstructures. Microstructural characterization showed a dendrite arm spacing (DAS) refinement of 40% when compared with the same part produced by conventional investment casting. Fatigue tests were conducted on hourglass specimens heat treated to T6, under a stress ratio of R = 0.1 and a frequency of 25 Hz. One batch of material was unmodified but two batches were modified with 0.007% and 0.013% Sr addition, from which one batch was submitted to HIP after casting. Results reported in S–N diagrams show that the addition of Sr and the HIP process improve the 10⁶ cycles fatigue strength by 9% and 34% respectively. Scanning electron microscopy (SEM) observation of the fracture surfaces showed a variety of crack initiation mechanisms. In the unmodified alloy, decohesion between the coarse Si particles and the aluminum matrix was mostly observed. On the other hand, in the modified but non HIP-ed alloy, cracks initiated from pores. When the same alloy was subjected to HIP, a competition between crystallographic crack initiations (at persistent slip bands) and decohesion/failure of intermetallic phases was observed. When compared to fatigue strength reported for components produced by permanent mold casting, the studied material are more resistant to fatigue even in the unmodified and non HIP-ed states.     

Thermomechanical behavior of PA6.6 composites subjected to low cycle fatigue

In this study, manifold experiments were conducted to investigate the thermomechanical behavior of short E-glass fiber-reinforced polyamide 6.6 composites subjected to low cycle fatigue loadings. Different hygrometric states, fiber configurations and loading rates were considered. Mechanical, thermal and energy responses of composite specimens were recorded using photomechanic techniques. The influence of water content, fiber orientation and loading rate on these thermomechanical responses was systematically analysed.The mechanical findings indicated that the ratcheting phenomenon was more pronounced for humid composites reinforced with fibers oriented transversely and subjected to a low loading rate. Moreover, the order of magnitude in self-heating was greater for transversal fiber composites conditioned at high relative humidity and subjected to a 10 Hz loading rate. From a thermodynamic standpoint, we also noticed that high proportions of the mean stored energy rate were obtained at a high loading rate, with values exceeded 64%. These values were noticeably altered by the water content and fiber angles, i.e. lower as the relative humidity increased and higher as the fiber angles increased.