The effects of the plastic wake zone on the conditions for fatigue crack propagation

Abstract

A B S T R A C T Closure is investigated with an optical interference technique which measures the transverse displacement associated with a propagating fatigue crack. The results of this study require that "closure" be devided into two phenomena, each of which affects the propagating mechanism differently. The plastic region is partitioned into the plastic wake zone behind the crack front and the active plastic zone ahead of the crack. The work to be presented deals with the effects of the plastic wake zone on fatigue crack propagation. It is found that the contact length between the crack faces can be several times the specimen thickness dimensions and that contact occurs only in the plane stress region of the specimen. It is concluded that the contact forces between the crack faces act like some external tensile load, modifying the Klmin of the loading cycles with low R-ratios. But, contact between the fracture faces (and the plastic wake zone) cannot account for any overload effects. The occurence of contact in the plane stress region of the specimen explains the observed thickness dependence of fatigue crack growth rates. Nomenclature0376--9429/80/020133-21500.20[0 Int. Journ. o[ Fracture, 16 (1980) 133-153

Micromechanisms of fatigue crack growth retardation following overloads

“…While it is well established that plasticity-induced crack closure can occur duri.ng Stage II plane stress constant amplitude loading without any abrasion marks or obliteration of striations (1,2,28), it is inconceiveab1e that the same mechanism of crack closure can remove the striation marks on the fracture -6-surface by an abrasion process (3) or enhance microstructure-sensitivity in the post-overload zone. In addition,there is existing experimental evidence indicating that plasticity-induced crack closure due to residual tensile displacements in the wake of the crack plays a hlore important role under plane stress conditions (11)(12)(13)(14)(15)). Yet, considerable crack growth reta-rdation does occur following single overloads at lower ilK levels whe-re'-predominantly plane strain conditions exist (19)(20)(21)(22).…”

“…Such effects, which are known to be of significance under plane stress conditions (11,15), have been examined experimentally (11)(12)(13)(14)(15), analytically (40), and numerically (41). There is presently some controversy as to the applicability of plasticity-induced crack closure to plane strain loading situations (15).…”

“…Although a large body of evidence indicates that the phenomenon of crack closure (11)(12)(13)(14)(15) and the extent of plastic zone directly ahead of the crack ti p (5) are of more si gni fi cance under pl ane stress conditions, experimental results for a low strength steel are presented identify and analyze clearly the underlying mechanisms), other types of load interactions are also briefly discussed. The implications of the proposed arguments are described not only from a mechanistic viewpoint, but also in terms of the measurement of crack growth rates, especially at low stress intensities.…”

A Review of Crack Closure

“…And even though in most fatigue crack analysis, the crack front is idealized in the manner shown in Figure 6, the actual crack front is very complicated -more complicated than I the curved crack front reported in Figure 8. The true nature of threedimensional crack front is evident from Figure 35 as reported in [63,89].…”

“…Even though overload retardation has been extensively investigated, only a few investigators [80][81][82][83][84][85][86][87][88][89][90][91][92][93][94] have used the closure concept to explain the observed effects. Only a few [27,32,65] If the K originating from the overload plastic zone were to be op responsible for the observed overload effects, the determination of K op produced by such small effects at regions close to the crack tip require precision measurements.…”