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

Abstract

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.

Analysis of the correlation between micro-mechanical fields and fatigue crack propagation path in nodular cast iron

“…It is worth remarking that the present fatigue crack was grown under conditions that correspond to the mid Paris regime for the material considered [43]. For both ferritic and pearlitic SGIs, 2D microscopy observations indicate that the appearance of the crack surface is similar in the entire stable crack propagation regime [15], as well as at different fatigue ratios and maximum stress intensity factors [8]. Therefore, it is likely that some of the features of the crack identified in Figure 7 are characteristic of a rather wide range of stable crack propagation regimes.…”

“…These appear to act as crack attractors [12], leading to a higher graphite fraction on the fracture surface than in the bulk of the material [13]. Also, a crack might or might not propagate depending on the presence of clusters of nodules near its tip [14], which seem to be associated with load shielding effects [15]. These facts suggest that an analogy exists with particle-reinforced metal-matrix composites, where it has been clarified that the crack growth, including the occurrence of shielding and anti-shielding phenomena, is driven by the particle-induced microstructural stress field [16,17].…”

Analysis of the Correlation between Micro-Mechanical Fields and Fatigue Crack Propagation Path in Nodular Cast Iron

“…It is worth remarking that the present fatigue crack was grown under conditions that correspond to the mid Paris regime for the material considered [43]. For both ferritic and pearlitic SGIs, 2D microscopy observations indicate that the appearance of the crack surface is similar in the entire stable crack propagation regime [15], as well as at different fatigue ratios and maximum stress intensity factors [8]. Therefore, it is likely that some of the features of the crack identified in Figure 7 are characteristic of a rather wide range of stable crack propagation regimes.…”

“…The qualitative impression is that the crack front is initially straight and splits up after encountering such large nodules, hence generating a step with step wall parallel to the crack growth direction. This observation, combined with the fact that the nodules act as crack attractors [12,15], allows formulating a hypothesis for the mechanisms of formation of these steps, which is schematized in Figure 11 (a). The idea is to consider a straight horizontal crack front that moves along the Z-axis (i) and, at a certain point, reaches a nodule (ii).…”

Influence of solidification defects on the fatigue behaviour of heavy‐section silicon solution–strengthened ferritic ductile cast irons

“…In the work of Sujakhu et al, the fatigue damage micromechanisms of SSF DI have been carefully studied. Based on the experimental observations, it was found that the crack initiation is affected by the degenerated graphite particles and casting defects, while the crack propagation stage is characterized by the interface decohesion between the spheroidal graphite and the ferritic matrix.…”