Digital Repository, Giornata IGF Forni di Sopra (UD) 2011

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Crack tip shielding effects, Part 1: direct measurement of the plastic enclave
E.A. Patterson, M. N. James

Last modified: 2011-02-25


The direct observation or measurement of the size and shape of plastic zones associated with a
propagating fatigue crack has been difficult. In earlier work the authors used the growth of fatigue cracks in
polycarbonate specimens combined with transmission photoelasticity to observe qualitatively the plastic enclave
ahead of a crack and the development of a plastic wake along the crack flanks. Recently this approach has been
extended to studying the effect of overloads on the size of the plastic enclave and the correlation with crack
growth rate. In more recent work thermoelastic stress analysis has been employed to map the size and shape of
the plastic zone ahead of cracks in aluminum compact tension specimens. The phase difference between the
measured temperature signal and the applied loading permitted the identification of the extent of plastic
behavior, or the limit of thermoelastic behavior. Experiments were performed with overloads for single cycles
and ten cycles and the change in size of the plastic enclave was defined and correlated to both stress intensity
factor and growth rate. The coefficients in Wheeler’s model were evaluated from direct measurements and used
to reliably predict crack behavior. Work is in progress to obtain the strain field around the crack tip, both
within and around the plastic zone using digital image correlation. The data from photoelasticity, thermoelastic
stress analysis and digital image correlation confirm the influence on the crack growth rate of the plastic enclave
associated with the crack tip and the plastic wake behind the tip. This influence occurs as a result of the plastic
enclave shielding the crack tip from the full effect of the applied loading and the shear stresses established
between the elastic and plastic regions in the wake. The traditional descriptions of the singularity-dominated,
elastic stress field do not appear to be capable of describing this complex situation. The experimental data
obtained with the above techniques has inspired the development of a new multi-parameter model in which the
stress field can be characterized by a new set of intensity factors which quantify crack driving and crack
retarding effects separately. As will be described in part II [1], the results from the new model correlate well
with experimental measurements.

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