Effects of Holding Time on Thermomechanical Fatigue Properties of Compacted Graphite Iron Through Tests with Notched Specimens

Abstract

In cylinder heads of compacted graphite iron (CGI), the heating and cooling cycles can lead to localized cracking due to thermomechanical fatigue (TMF). Traditionally, TMF behavior is studied by thermal cycling of smooth specimens. The resulting number of cycles to failure (N f ) constitutes a single parameter that can be used to predict actual service failures. Nevertheless, there are also some drawbacks of the conventional testing procedures, most noticeably the prolonged testing times and a considerable scatter in test results. To address these drawbacks, TMF tests were performed using notched specimens, resulting in shorter testing times with less scatter. In the case of cast iron, artificial notches do not necessarily change the TMF behavior since the inherent graphite particles behave as internal notches. Using a notch depth of 0.2 mm, the effect of prolonged holding times (HT) on TMF lifetime was studied and a clear effect was found. Extended holding times were also found to be accompanied by relaxation of compressive stresses, causing higher tensile stresses to develop in the subsequent low temperature stages of the TMF cycles. The lifetimes in notched CGI specimens can be predicted by the Paris' fatigue crack growth model. This model was used to differentiate between the individual effects of stress level and holding times on TMF lifetime. Microstructural changes were evaluated by analyzing quantitative data sets obtained by orientation contrast microscopy based on electron backscattered diffraction (EBSD).

Thermomechanical fatigue behavior of CGI 450 and CGI 500 cast irons

“…As shown in Figure 8 and Figure 9a for the CGI 500, cracks start from graphite particles at or close to the specimen surface, with crack growth through the pearlitic matrix by fatigue striation (Figure 9b) with a cleavage appearance (grey phase), and by graphite matrix interface (black phase), until final failure by mixture of ductile and cleavage modes [12,14,15]. The cavities or rounded features observed on the fracture surface are voids left by the graphite or from the graphite that stayed in the other half [14,16].…”

Thermo-Mechanical Fatigue Lifetime Assessment of Spheroidal Cast Iron at Different Thermal Constraint Levels

“…Fatigue-crack initiation and growth in graphitic cast irons is largely affected by the presence of graphite particles. Because of the likely fast initiation of TMF cracks in cast irons as a result of delamination at the graphite-metal interface, Ghodrat et al [3][4][5] evaluated TMF lifetime using a crack-growth model, and they proved that, in tension, the graphite particles can be considered as internal notches or defects, from which TMF cracks start to grow during the very first TMF cycles. In the presence of an external notch, the notch depth can be considered as the initial crack length.…”

“…It is acknowledged that, apart from TMF constraint levels, other factors also influence TMF lifetime, most notably high temperature effects such as creep and oxidation. However, the additional variation of test conditions identifying high temperature effects, such as using prolonged holding times [5], was considered to be beyond the scope of this research.…”

Fuzzy neural network analysis on the compacted graphite iron with improved tensile and heat transport properties

“…High cycle fatigue under the combined steady thermal and alternating firing loads demands high ultimate tensile strength [1]. Low cycle fatigue and thermo-mechanical fatigue due to start-operate-stop cycles are related to ductility and thermal transport [1][2][3]. Thus, high service life of these components demands compacted graphite irons with improved properties combining high tensile strength, ductility and thermal conductivity.…”