Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approache
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INTRODUCTION
R¼
MOST
methods of processing structural metals introduce surface or internal residual stresses, which remain after all external forces, constraints, and thermal gradients are removed. These stresses result from the plastic deformation of material and phase transformation induced volume change, often a result of mechanical or thermomechanical processing. These processes are performed frequently to introduce beneficial compressive residual stresses, impeding initiation and growth of cracks, and therefore increasing crack growth threshold, decreasing growth rates, and increasing the failure stress intensity during fatigue crack growth (FCG). Conversely, tensile residual stresses, which counterbalance the beneficial stresses, have the opposite effect. As a result, residual stresses are a major source of material and component variability when processing parameters cannot be controlled sufficiently, generating bias in data generation and interpretation and ultimately leading to inaccuracies in structural analysis. In the absence of residual stress, the nominal stress intensity range (DK) and nominal stress ratio (R) are defined by Eqs. [1] and [2]. DK ¼ Kmax Kmin
½1
CHRISTOPHER J. LAMMI, Graduate Research Assistant, is with the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332. Contact e-mail: [email protected] DIANA A. LADOS, Assistant Professor, is with the Department of Material Science and Engineering, Worcester Polytechnic Institute, Worcester, MA 01609. Manuscript Submitted November 21, 2010. Article published online September 16, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
Kmin Kmax
½2
When the effects of residual stress are to be accounted for, the stress intensity caused by residual stresses Kres must be added to the minimum and maximum applied stress intensities, Kmin and Kmax, respectively, changing the nominal stress intensity K to Kapp = K + Kres by superposition.[1–6] Residual stresses thus also change the nominal stress ratio R. Through the application of Eqs. [1] and [2] for stress intensity factor range and stress ratio become Eqs. [3] and [4] with the consideration of residual stresses: DKapp ¼ ðKmax þ Kres Þ ðKmin þ Kres Þ Rapp ¼
Kmin þ Kres Kmax þ Kres
½3 ½4
where DKapp and Rapp are the applied crack tip stress intensity factor range and stress ratio with consideration of residual stresses. Compressive residual stresses, which produce negative Kres, decrease Kapp and Rapp, and increase crack closure during FCG tests. During FCG testing, compressive residual stresses normal to the crack propagation plane are of the greatest importance because of their direct role in crack closure. If compressive residual stresses are large enough or if a specimen is FCG tested at a low nominal stress ratio subject to microstructural closure mechanisms, there will be a change in the effective DK caused by closure. This is true when the value Kmin + Kres becomes close to zero or negative, implying crack closure caused by residual stresses.
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