Strain-Controlled Fatigue Behavior of a Nodular Cast Iron in Real Off-Highway Axles: Effects of Casting Skin and Strain Ratio

Abstract

Nodular cast irons are widely adopted in off-highway vehicles, since they allow to obtain components with complex shapes as well as good mechanical properties. However, the fatigue strength of such components is a major issue, which is typically addressed by adopting the strain-life approach, since it considers the local strains at stress concentrators where fatigue cracks are prone to initiate. In the fatigue design of off-highway components, the detrimental effect of casting skin, due to both surface and subsurface features, must be accounted for in all cases where machining is not needed. Moreover, the local strain ratio at stress raisers can be different from the nominal one due to forward plasticity. In this paper, static tensile as well as strain-controlled fatigue tests have been executed on specimens taken from real EN-GJS-450-10 off-highway axles. Static tensile tests have been performed on specimens with machined surfaces, while fatigue tests at a strain ratio equal to −1 have been carried out on specimens with surfaces in both machined and as-cast conditions. In addition, machined specimens have been tested at strain ratios equal to 0.1 and 0.5 to investigate the material sensitivity to the mean strain. Finally, the high-cycle downgrading effect of the casting skin has been evaluated, and experimental data of machined specimens tested at different strain ratios have been summarized by using the Smith–Watson–Topper expression.

Linking the microstructure with strain-life curves for improved utilization of the lightweight potential of thick-walled nodular cast iron

“…To determine the parameters of the cyclic stress-strain curve in Figure 6 and Figure 7, the cyclic hardening coefficient K' and the cyclic hardening exponent n', literature data [13,14,15,32,33,34] is included in the evaluation alongside to the 44 present series of tests. This correlation has already been shown very similarly in [28] for nodular cast iron.…”

“…The experimental knee points of the damage parameter PRAM,Z,WS and PRAM,Z,WS are shown in Figure 11 and Figure 12 in comparison to the calculated knee points of the damage parameter using the MLR. The resulting relationship between the knee points of the damage parameter curve and the tensile strength Rm, elongation at break A, graphite particle density ρgraphite can be found in equation (14) and equation (15). The resulting parameters of this correlation can be found in Table 3 The factor for conversion to 2.5% probability of failure f2.5% is likewise chosen so that in the normal probability plot, Figure 14 for log(Nexperimental/Ncalculated) = 0 a 2.5 % probability of occurrence P = 2.5 % with the use of the lower percentile of the confidence bounds is achieved.…”

“…In these works, mainly wind energy related ferritic GJS have been investigated based on stress-controlled fatigue tests while defects and size effects according to [10] were considered. In further works from the literature [11,12,13,14,15] and works performed at LBF [7,8,16,17], the focus is also on strain-based approaches to better understand the cyclic deformation behavior as well as the strain-based fatigue behavior. The influence of locally different microstructures is not significant on the cyclic stress-strain behavior for large castings made of EN-GJS-400-18LT as well as on the material cyclic hardening [11,12].…”

Recent Advances in Cast Irons

“…The book collects manuscripts from cutting-edge academic researchers and consists of one review paper regarding the research progress in improving the corrosion wear resistance of a novel hypereutectic high-chromium cast iron (HCCI) [1] and eight experimental research papers focused on methods for enhancing the corrosion, mechanical and microstructural features of cast irons in general, i.e., nodular, gray, vermicular, austempered ductile iron (ADI) and HCCI [2][3][4][5][6][7][8][9].…”

“…Concerning nodular cast irons, relevant contributions have been proposed [2,3,5]. The paper by Vicente et al [2] concerns the applicability of the Hollomon-Jaffe parameter, usually used to establish an equivalence between time and temperature in a tempering treatment, to predict the hardness of nodular cast iron after quenching and tempering treatments.…”