Fatigue damage accumulation in grey cast iron

Observations and results of constant-amplitude, completely reversed strain-control fatigue tests are reported for a pearlitic grey cast iron. Both axial and torsional cyclic load cases have been investigated. A damage parameter based upon the product of the tensile stresses and strains (σ_maxΔε/2) gives good correlation of all the fatigue tests performed. The specimens employed were produced using a developmental ‘lost foam’ casting process. Defects present in the castings had a detrimental effect on fatigue life. A continuum damage model was used to account for the influence of the defects on fatigue life. Fatigue behaviour of grey cast iron castings containing inclusions and porosities has been quantified on the basis of smooth specimen uniaxial fatigue data.     

Influence of chunky graphite on mechanical and fatigue properties of heavy-section cast iron

This research work aimed to find out the influences of the different amounts of chunky graphite on mechanical and fatigue properties of GJS 800 ductile cast iron. Chunky graphite has been a problem of heavy section thick-walled ductile cast iron components. Chunky graphite is branched and interconnected as a network within eutectic cells and has been observed to form in thermal centres of heavy ductile cast iron sections during solidification. This research work proved that chunky graphite in the microstructure decreases the ultimate tensile strength, the elongation to fracture and fatigue life significantly, but does not influence on the yield stress of ductile cast iron GJS 800 substantially. Low nodular count and nodularity rate also decrease the fatigue life of ductile cast iron, and the difference of fatigue life of specimens containing chunky graphite or having low nodular count and nodularity rate is not large. Influence of the amount of chunky graphite on fatigue life increases until 20% chunky graphite content, and above that the fatigue life does not decrease substantially.     

Bruchzähigkeit und Ermüdungs-Rißwachstum von Gußeisen

Fracture Toughness and Fatigue Crack Growth of Cast Irons Fracture toughness, elastic moduli and fatigue crack growth rates in air and in vacuum were measured for 17 different cast irons. The graphite shape in the cast irons varied from flakes to nodules, the matrix varied from ferrite to pearlite to martensite. In the fatigue crack growth rate tests, using fracture mechanics methods, it was observed that the fatigue crack growth rate increases significantly as the cyclic stress intensity range approaches the critical value for stable crack growth. This phenomenon was used to determine the fracture toughness of the cast irons. Such toughness data agree well with literature data on the fracture toughness of cast irons. An extensive review of the effects of strength on the fracture toughness of commercial cast irons is presented. In cast irons with flake graphite, cyclic loading results in a reduced modulus of elasticity. This is attributed to the rupture of the graphite flakes under cyclic loading.     

Fatigue crack propagation damaging micromechanisms in ductile cast irons

Ductile iron discovery in 1948 gave a new lease on life to the cast iron family. In fact, these cast irons are characterized both by a high castability and by high toughness values, combining cast irons and steel good properties. Ductile cast irons are also characterized by high fatigue crack propagation resistance, although this property is still not widely investigated.In the present work, three different ferritic–pearlitic ductile cast irons, characterized by different ferrite/pearlite volume fractions, and an austempered ductile cast iron were considered. Their fatigue crack propagation resistance was investigated in air by means of fatigue crack propagation tests according to ASTM E647 standard, considering three different stress ratios (R = K_min/K_max = 0.1; 0.5; 0.75). Crack paths were investigated by means of a crack path profile analysis performed with an optical microscope. Crack surfaces were extensively analysed by means of a scanning electron microscope both considering a traditional procedure and performing a quantitative analysis of 3D reconstructed surfaces, mainly focusing graphite nodules debonding.     

A study of tensile properties of ferritic compacted graphite cast irons at intermediate temperatures

Results of an investigation of the influence of carbon and silicon content on tensile properties of ferritic compacted graphite cast irons from room temperature to 500° C are presented. The tensile flow stress plateau of ferritic compacted graphite cast irons occurs owing to dynamic strain ageing from 200 to 400° C with various carbon and silicon contents. Because the ferritic compacted graphite cast irons have irregularly shaped graphite particles, their fracture mechanism is attributed to a mixed fracture mode. With increasing silicon content, the fracture mechanism changes from dimple pattern to transgranular cleavage at room temperature. However, when the test temperature is above the raised transition temperature the specimens fail in ductile manner.     

Fracture mechanics behaviour of austenitic compacted graphite cast iron

The fracture mechanics behaviour of high-nickel austenitic compacted graphite cast iron was studied and the effects of graphite morphology, alloying elements and specimen thickness on the mechanical properties, plane stress fracture toughness, and fatigue crack growth rate were evaluated. It was found that the graphite morphology, i.e. the percentage of compacted graphite present, was the major determinant of all properties of the materials investigated. The irons with a greater amount of compacted graphite (the balance was nodular graphite in austenitic matrix) resulted in lower tensile strength, yield strength, elongation and K _c fracture toughness but higher crack-growth index values (poorer crack-growth resistance). For 25 mm thick specimens, K _c values of the austenitic compacted graphite cast irons in this study were in the range of 58–64 MPa m^1/2. This is higher than ferritic/pearlitic ductile iron of 43–53 Mpa m^1/2, and is compatible to Ni-resist austenitic ductile iron of 64.1 Mpa m^1/2. The addition of cobalt not only contributed to slightly higher values of mechanical properties, but also higher plane stress fracture toughness and better crack growth resistance. Optical microscopy, scanning electron microscopy and X-ray diffraction techniques were applied to correlate the microstructural features to the properties attained.     

Low-cycle fatigue behavior of AA2618-T61 forged disk

The cyclic stress–strain response and the low-cycle fatigue life behavior of an aluminum alloy AA2618-T61 forged disk were studied. Fully reversed strain-controlled tests were performed at 200 °C in air at a constant total strain rate and under the total strain ranges of 0.5–0.9%. Specimens cut from longitudinal direction of disk displayed cyclic hardening or softening which was dependent on the total strain range. The variation of low-cycle fatigue life with plastic strain amplitude followed a single-slope Coffin–Manson power-law relationship. Fracture of the samples was predominantly ductile fracture of high density microdimples.     

Long-life torsion fatigue with normal mean stresses

Relatively simple fatigue tests have been performed on two common engineering materials, cast ductile iron and low-carbon steel, using two stress states, cyclic torsion and cyclic torsion with static axial and hoop stresses. Tests were designed to discriminate between normal stress and hydrostatic stress as the most suitable mean stress correction term for high cycle fatigue analysis. Microscopy shows that cracks in low-carbon steel nucleate and grow on maximum shear planes, while for cast iron pre-existing flaws grow on maximum normal stress planes. The data illustrate that tensile normal stress acting on a shear plane significantly reduced fatigue life and is an appropriate input for fatigue analysis of ductile materials. Static normal stresses did not significantly affect the fatigue life for the cast iron because the net mean stress on the maximum normal stress plane was zero. Mean torsion significantly reduced the fatigue strength of the cast iron. A critical plane long-life parameter for nodular iron which accounts for both stress state and mean stress is proposed, and is found to accurately correlate experimental data.     

Cyclic Crack Resistance of Grey and High-Strength Cast Irons with Increased Phosphorus Content

We investigated the influence of phosphorus (0.02–0.76%) on the microstructure, short-term strength, cyclic crack resistance characteristics, and micromechanism of fatigue fracture of grey and high-strength cast irons. It was established that the low cyclic crack resistance of grey and high-strength cast irons with increased phosphorus contents (0.7–0.8%) is caused by the propagation of a fatigue crack via intergranular cleavage, initiated by a discontinuous or continuous network of precipitates of the ternary fine-grained phosphide eutectic along boundaries of ferrite grains. We showed that, from the viewpoint of cyclic crack resistance, it is admissible to alloy cast irons of the ferritic and ferritic–pearlitic class with phosphorus up to 0.3% when the phosphide eutectic forms in amounts of 3–5% for grey cast irons and 4–7% for high-strength cast irons without significant decrease in their resistance to brittle fracture.     

Vermicular graphite cast iron—current state of the art

Vermicular graphite cast iron is a new addition to the family of cast irons. Various methods for producing vermicular graphite cast iron are briefly discussed in this paper. The mechanical and physical properties of cast irons with vermicular graphite have been found to be intermediate between those of gray and ductile irons. Other properties such as casting characteristics, scaling resistance, damping capacity and machinability have been compared with those of gray and ductile irons. Probable applications of vermicular graphite cast irons are suggested.     

Effect of heating temperature and magnesium content on the thermal cyclic failure behaviour of ductile irons

Abstract

This study elucidates the effect of residual magnesium content and heating temperature on the thermal cyclic failure behaviour of ductile irons by applying repeated heating and cooling cycles. Five irons with different residual magnesium contents ranging from 0.038 to 0.066 wt-% were obtained by controlling the amount of nodulariser additions. The thermal fatigue cracking behaviour was investigated during thermal cycling from 25°C to 650, 700, 750, and 800°C, respectively. Experimental results indicate that the thermal fatigue cracking resistance of ductile iron decreases with increasing residual magnesium content. The maximum heating temperatures of 700°C and 750°C led to the most severe thermal fatigue cracking in the specimens containing 0.054 wt-% and 0.060 wt-% residual magnesium content. Recrystallisation of ferrite grain occurred when the thermal cycles exceeded a certain number after testing at 800°C, which deferred the initiation of thermal fatigue cracking.     

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.     

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.     

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.     

氢处理对铸造钛合金低周疲劳寿命及断裂韧性的影响

本文研究了氢处理对铸造钛合金的低周疲劳寿命及断裂韧性的影响,发现经氢处理后铸造钛合金粗大的魏氏组织转变成细小的等轴组织,其应变低周疲劳寿命大大延长,在低周疲劳过程中,应变量较小时,试样出现循环硬化现象,而在应变量较大时,出现循环软化现象,氢处理后材料的断裂形式由脆性断裂转为韧性断裂;断裂韧性提高。     

氢处理对铸造钛合金低周疲劳寿命及断裂韧性的影响

本文研究了氢处理对铸造钛合金的低周疲劳寿命及断裂韧性的影响,发现经氢处理后铸造钛合金粗大的魏氏组织转变成细小的等轴组织,其应变低周疲劳寿命大大延长。在低周疲劳过程中,应变量较小时,试样出现循环硬化现象;而在应变量较大时,出现循环软化现象。氢处理后材料的断裂形式由脆性断裂转为韧性断裂;断裂韧性提高。     

The influence of porosity on the fatigue life for sand and permanent mould cast aluminium

The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied.

Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R = −1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life.

The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.     

Effect of matrix structure on ultrasonic attenuation of ductile cast iron

The ultrasonic attenuation of ductile cast irons with different matrices was investigated by means of ultrasonic echo waves. In the ductile cast irons with ferritic and pearlitic matrix structures, both of the ultrasonic attenuation increased with frequency. For similar frequencies, the ultrasonic attenuation of the pearlitic matrix was larger than that of the ferritic matrix. Based on the theory of ultrasonic attenuation in the solid, the mechanisms of ultrasonic attenuation in the ductile cast irons with different matrices were analyzed. It indicated that in the ductile cast irons with transformation of matrix from the ferrite to the pearlite, the mechanism of ultrasonic attenuation varied in the range of present frequencies. In the ferritic matrix, the total ultrasonic attenuation was mainly attributed to the scattering loss which included the stochastic scattering and the Rayleigh scattering. On the contrary, the absorption loss predominated in the total ultrasonic attenuation of the ductile cast iron with pearlitic matrix.     

Schwingfestigkeitssteigerung durch Festwalzen

Improvement in Fatigue Properties by Means of Deep Rolling Deep rolling has an almost 60 years old tradition and today it is an important process especially for automobile structural parts. The development of deep rolling was greatly influenced by the Institute of Material Science of the Technical University in Darmstadt. After a short historical review the general mechanism of mechanical strengthening is discussed. Then new experimental results on ductile cast irons and a case hardened steel are given. The improvement in fatigue strength of notched specimens out of these materials is even higher than that observed on steel in previous investigations. However the experiments also indicate that mechanical strengthened parts obviously have no real fatigue limit. Measurements of crack propagation on nodular cast iron specimens show that the propagation rate of cracks in the notch root is extremely reduced by compressive residual stresses. However no real crack arrest could be observed.     

Comparison of fatigue properties of nodular cast iron production and Y-block castings

Monotonic, cyclic, low and high cycle axial fatigue and region I and II fatigue crack growth behaviour were obtained and compared for a nodular iron Y-block casting and a heavy, off-highway, ground vehicle production casting. Despite the greater size and incidence of micro-discontinuities in the production iron, the two castings had essentially the same monotonic, cyclic and fatigue properties. SEM fractography indicated the same fatigue mechanisms for both casting types. Crack closure did not completely account for mean stress effects found in both cast irons.