Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Residual Strength and Fatigue Lifetime of ... - Solid Mechanics Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Figure 1.6: Schematic illustration of the CSDE method. 1.5 Fatigue Crack Propagation in Sandwich Structures Specimens with pre-cracks are normally used to study crack propagation rates for a given material. In these specimens one or more cracks are artificially created and by applying cyclic load the crack growth is measured. The Single Edge Notched Bending specimen (SENB) and the Compact Tension specimen (CT), as shown in Figure 1.7, are two standard specimen types used for measurement of mode I crack growth rate. Figure 1.7: The single edge notched bending specimen (SENB) and the compact tension specimen (CT). The resulting crack growth rate, da/dN, is usually plotted against the stress intensity factor K defined as Kmax-Kmin in a load cycle. 10
Figure 1.8: Typical fatigue crack growth rate vs. K diagram. The crack growth rate diagram is divided into an initiation phase (I), a stable crack growth phase (II) and an unstable crack growth phase (III), as shown in Figure 1.8. The initial phase includes non-continuous fracture processes with a very low crack growth rate. The stress intensity factor range in this phase approaches the fatigue crack growth threshold, Kth. The linear intermediate phase is the most interesting phase due to the linear relation between the logarithm of the crack propagation rate and the logarithm of the stress intensity factor. This phase covers a large range of stress intensity factors and crack propagation in this phase is generally more stable than in the two other phases. The linear phase of the diagram, also named the Paris regime, can be written as where m is the slope of the linear phase and c is the crack growth rate for K=1. 11 (1.20) In phase III the crack grows fast and in an unstable manner. The effect of the loading ratio, R=Fmin/Fmax, on the crack growth rate is shown in Figure 1.8. It is seen for a given crack growth rate that the K value increases with increased loading ratio. The influence of the loading ratio is due to the fact that the crack growth is mainly determined by the maximum stress intensity factor value for each fatigue cycle, Kmax, and its proximity to the fracture toughness of the material, Kc. In homogeneous materials due to the fact that the crack only experiences opening mode I loading, the crack growth rate diagram is unique. On the contrary, in an interface due to the existence of mixed-mode loading at the crack tip and the resistance of other layers toward kinking of the crack, different crack growth rate relations exist for different mode-mixities.
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- Page 14 and 15: Contents Preface Executive Summary
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Figure 1.6: Schematic illustration <strong>of</strong> the CSDE method.<br />
1.5 <strong>Fatigue</strong> Crack Propagation in S<strong>and</strong>wich Structures<br />
Specimens with pre-cracks are normally used to study crack propagation rates for a given<br />
material. In these specimens one or more cracks are artificially created <strong>and</strong> by applying cyclic<br />
load the crack growth is measured. The Single Edge Notched Bending specimen (SENB) <strong>and</strong> the<br />
Compact Tension specimen (CT), as shown in Figure 1.7, are two st<strong>and</strong>ard specimen types used<br />
for measurement <strong>of</strong> mode I crack growth rate.<br />
Figure 1.7: The single edge notched bending specimen (SENB) <strong>and</strong> the compact tension<br />
specimen (CT).<br />
The resulting crack growth rate, da/dN, is usually plotted against the stress intensity factor K<br />
defined as Kmax-Kmin in a load cycle.<br />
10
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