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Title: Cumulative damage fatigue in notched mild steel specimens
Author: Ashby, David John
ISNI:       0000 0001 3430 2742
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1969
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The work presented is specifically concerned with improving cumulative fatigue damage life predictions in the region of long lives where most of the stress levels experienced by a specimen or structure are below the constant amplitude fatigue limit. Further, an attempt has been made to improve these predictions in such a manner as to be feasable in the design process, i.e. mathematical simplicity and minimum data reaquirements to enable predictions of improved accurancy to be made. Several existing cumulative damage prediction rules are examined with reference to their accuracy, areas of application and suitability for the design process. Results are presented for constant amplitude tests on four different specimen configurations and various forms of block loading tests with finally a triangular modulation of anplied load on a single specimen configuration in rotating bending. The cumulative damage experimental results are used to investigate the resultant effect of loads below the original constant amplitude fatigue limit. The observed behavioural patterns have been rationalized to form the basis of a cumulative damage rule. The main feature of the rule is the division of the fatigue life into two stages termed Stage A and Stage B which are defined as follows:- Stage A Initiation and Micro-crack propagation. Stage B Macro-crack propagation. A linear summation is performed on each stage simulating observed patterns of behaviour, using a modified S-N curve derived from the standard constant amplitude S-N curve. The length of each stage is determined by the length of that same stage in a constant amplitude test at the highest damaging stress level in the spectrum. The division of the life between Stage A and Stage B has heen shown to be a unique quantity dependant on specimen geometry and an empirical relationship has been derived relating the KT value of the notch to the slope of a life in Stage A versus Total life to failure plot. As a consequence of different behaviour in Stages A and B different cumulative damage behaviour is predicted for differing notch severities. Certain aspects of size effects are examined and discussed and the general areas of apnlication of the cumulative damage rule are outlined. Finally, the proposed rule is tested against other cumulative damage rules and experimental results presented in the cumulative damage literature. Suggestions are presented for further research work to further investigate and extend the areas of applicability of the proposed method.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)