Norman E. Dowling

Mean Stress Effects in Stress-Life and Strain-Life Fatigue

Various approaches to estimating mean stress effects on stress-life and strain-life behavior are compared with test data for engineering metals. The modified Goodman equation with the ultimate tensile strength is found to be highly inaccurate, and the similar expression of Morrow using the true fracture strength is a considerable improvement. However, the Morrow expression employing the fatigue strength coefficient ′ σ f may be grossly non-conservative for metals other than steels. The Smith, Watson, and Topper (SWT) method is a reasonable choice that avoids the above difficulties. Another option is the Walker approach, with an adjustable exponent γ that may be fitted to test data, allowing superior accuracy. Handling mean stress effects for strainlife curves is also discussed, including the issue of mathematical consistency with mean stress equations expressed in terms of stress. A new and mathematically consistent method for incorporating the Walker approach into strainlife curves is developed. With γ = 0.5, this result gives a new strain-based interpretation of the SWT method.

Fatigue loading history reconstruction based on the rainflow technique

Methods are considered of reducing a non-random fatigue loading history to a concise description and then of reconstructing a time history similar to the original. In particular, three methods of reconstruction based on a rainflow cycle counting matrix are presented. A rainflow matrix consists of the numbers of cycles at various peak and valley combinations. Two methods are based on a two-dimensional rainflow matrix, and the third on a three-dimensional rainflow matrix. Histories reconstructed by any of these methods produce a rainflow matrix identical to that of the original history, and the resulting time history is expected to produce a fatigue life similar to that for the original. The procedures described allow lengthy loading histories to be stored in compact form.

Estimation and correlation of fatigue lives for random loading

Abstract Where the constant amplitude stress vs life curve of a test specimen or component is a straight line when plotted on log-log coordinates, the Palmgren-Miner rule predicts a parallel straight line for random loading. This paper suggests several other methods of correlating the data between random and constant amplitude loading so that the parallel stress-life lines coincide. As an example, the methods are applied to random loading data recently reported in this journal by Stanzl et al . It is significant that an endurance limit may not appear for random loading until very long lives have been reached, and then only when all stresses are below the endurance limit derived from constant amplitude data.

Creep deformation of a soft magnetic iron–cobalt alloy

The U.S. Air Force is in the process of developing magnetic bearings, as well as an aircraft integrated power unit and an internal starter/generator for main propulsion engines. These developments are the driving force for the new emphasis on the development of high saturation, low loss magnets capable of maintaining structural integrity in high stress and high temperature environments. It is this combination of desired material characteristics that is the motivation of this effort to measure, model, and predict the creep behavior of such advanced magnetic materials. Hiperco® Alloy 50HS, manufactured by Carpenter Technology Corporation, is one of the leading candidates for these applications. Material specimens were subjected to a battery of mechanical tests in order to study and characterize their behaviors. Tensile tests provided stress versus strain behaviors that clearly indicated: a yield point, a heterogeneous deformation described as Luders elongation, the Portevin–LeChatelier effect at elevated te...

SOME COMMENTS ON METHODS OF REDUCING AND RECONSTRUCTING IRREGULAR FATIGUE LOADING HISTORIES

The methods of irregular fatigue loading history characterization and reconstruction are briefly reviewed for use in life prediction and efficient laboratory testing. The use of Autoregressive Moving Average (ARMA) models is introduced.

Design Criterion of Fatigue Analysis on Plastic Basis by ASME Boiler and Pressure Vessel Code

The paper investigates the current rules for fatigue analysis on plastic basis of the ASME Boiler and Pressure Vessel Code. It addresses the question: When these rules are used for a vessel or component, is it designed with respect to crack initiation or failure? Using available fatigue test data, the question is answered for blunt double-notch plate specimens made of AISI 4340 steel. When the rules are applied to these specimens, it is found that they are designed with respect to the initiation of a crack of a certain length that decreases as the strain range increases. The plate specimens are not designed with respect to failure. It is concluded that for geometries that exhibit a strain range gradient away from the surface, the number of cycles allowed by the current rules includes the initiation phase but not the propagation phase of a fatigue crack. If this result is unsatisfactory, new rules have to be written.

Design Criterion of Fatigue Analysis on Plastic Basis by ASME B&PV Code

The paper investigates the current rules for fatigue analysis on plastic basis of the ASME Boiler and Pressure Vessel Code. It addresses the question: When these rules are used for a vessel or component, is it designed with respect to crack initiation or failure? Using available fatigue test data, the question is answered for blunt double-notch plate specimens made of AISI 4340 steel. When the rules are applied to these specimens, it is found that they are designed with respect to the initiation of a crack of a certain length that decreases as the strain range increases. The plate specimens are not designed with respect to failure. It is concluded that for geometries that exhibit a strain range gradient away from the surface, the number of cycles allowed by the current rules includes the initiation phase but not the propagation phase of a fatigue crack. If this result is unsatisfactory, new rules have to be written.Copyright