Abstract
In the current work an experimental study of the crack tip opening displacement (CTOD) is performed to evaluate the ability of this parameter to characterise fatigue crack growth. A methodology is developed to measure and to analyse the CTOD from experimental data. The vertical displacements measured by implementing Digital Image Correlation on growing fatigue cracks are used to measure the CTOD. Fatigue tests at R ratios of 0.1 and 0.6 were conducted on compact-tension specimens manufactured from commercially pure titanium. A sensitivity analysis was performed to explore the effect of the position selected behind the crack tip for the CTOD measurement. The analysis of a full loading cycle allowed identifying the elastic and plastic components of the CTOD. The plastic CTOD was found to be directly related to the plastic deformation at the crack tip. Moreover, a linear relationship between da/dN and the plastic CTOD for both tests was observed. Results show that the CTOD can be used as a viable alternative to ?K in characterising fatigue crack propagation because the parameter considers fatigue threshold and crack shielding in an intrinsic way. This work is intended to contribute to a better understanding of the different mechanisms driving fatigue crack growth and the address the outstanding controversy associated with plasticity-induced fatigue crack closure.
Highlights
Fatigue crack growth has been traditionally characterised by the Paris law [1], that relates the crack growth per cycle, da/dN to the stress intensity factor range, ΔK
The plastic crack tip opening displacement (CTOD) was found to be directly related to the plastic deformation at the crack tip
A n experimental evaluation of the CTOD has been performed to analyse the ability of this parameter to characterise fatigue crack growth
Summary
Fatigue crack growth has been traditionally characterised by the Paris law [1], that relates the crack growth per cycle, da/dN to the stress intensity factor range, ΔK. Two elastic-plastic parameters have been proposed to be related with crack tip plastic deformation, the crack tip opening displacement (CTOD) and the J contour integral. Both parameters describe crack tip conditions in elastic-plastic materials, and they can be used as a fracture criterion. CTOD was first observed by Wells [2] when he was attempting to measure KIC values in a number of structural steel Wells found that these materials were too tough to be characterised by LEFM. The degree of crack blunting increased in proportion to the toughness of the material This observation led Wells to propose the opening at the crack tip as a measurement of fracture toughness. The displacement at the first node behind the crack tip is generally used as an operational CTOD [7]
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