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). © 2012 The Author(s).