Cracking due to combined TMF and HCF loading in cast iron

Abstract

Critical locations in diesel engine cylinder heads may be susceptible to damage as a consequence of thermomechanical fatigue (TMF) and/or high vibratory firing pressure (high cycle fatigue, HCF) loading. Evidence is presented for a vermicular cast iron to show that superimposed HCF loading below a threshold strain amplitude of ∼0.03% does not reduce the crack initiation endurances due to a service-type TMF cycle with Tmax of 450°C and a dwell/hold time at peak mechanical strain in compression of 180s. For superimposed strain amplitudes above the threshold, there can be a significant reduction in the TMF crack initiation endurances. The threshold superimposed HCF strain amplitude is found to be temperature sensitive, reducing with increasing Tmax (e.g. to 480°C). The magnitudes of the thresholds are judged to be determined by the strain rate sensitivity of oxide scale cracking at these temperatures. Importantly, for TMF cycles with Tmax 450°C and low mechanical strain ranges (i.e. low levels of constraint), a limiting value for total mechanical strain range is identified to be responsible for significantly longer endurances (>104cycles). The limit is found to be similar for both TMF and TMF(HCF) testing conditions (i.e. insensitive to the superimposed HCF loading) and coincides with the isothermal fatigue-limit for HCF loading of the iron at the TMF cycle TTRD equivalent temperature with the same mean stress level encountered during the low constraint TMF tests.With the improved capabilities of modern digital controllers in mechanical testing units, the possibilities for complex cycle control such as that required for combined HCF and TMF testing is readily attainable. New-efficient algorithms for TMF and TMF(HCF) testing and data acquisition are introduced in the paper.