A Formalism to Model Environment-Sensitive Fatigue Crack Growth F. Bergner Dresden University of Technology, Institute of Materials Science, Germany ABSTRACT: The paper deals with the effect of 3.5% NaCl solution on long fatigue crack growth in the thin sheet wrought aluminium alloy 6013 T6. A formalism aimed to provide a quantitative model of the environmental effect is summarized and extended. The formalism is essentially based on the concept of a rate-controlling step and the application of the method of dimensional analysis to the individual steps. The sensitivity of the resulting model to distinct modifications of the underlying assumptions is discussed. INTRODUCTION The starting point of the present investigation is the observation of fatigue crack growth in the aluminium alloys 6013 T6 in 3.5% NaCl solution at rates in excess of those in both laboratory air and deionized water [1,2]. As stress corrosion cracking cannot be the reason for the increased fatigue crack growth rates [3], an alternative LEFM-based model was developed and applied to a set of measurements of constant-amplitude long fatigue crack growth in the range of frequencies, f, from 25 mHz to 40 Hz and in the K range from about 8 to about 30 MPa m1/2 [1]. The model is based on the assumption of a single multiple-step mechanism being responsible for the increased crack growth rates. Of these steps the slowest one is assumed to control the crack growth rate [4]. The expressions derived were then fitted to the measurements. However, as mentioned in [1] a good fit alone cannot be considered enough evidence for both the validity of the model assumptions and the interpretation of the rate-controlling steps. In the present paper, the formalism applied in [1] is investigated in some more detail. First, a list of assumptions along with comments is given. Both the formalism and its application to the aluminium alloy 6013 T6 are summarized. The model is then extended in order to empirically take into account the simultaneous contribution of more than one step to the rate control of environmental fatigue crack growth. Finally, the sensitivity of the resulting model to distinct modifications of the assumptions is considered.
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