Elastic compliance expressions for compact type (CT) and WOL specimens have been formulated for a wide range of crack lengths (0.2 ⩽a/W ⩽ 0.975) using results from Newman's modified boundary collection techniques and Wilson's deep crack analysis. The location of the axis of rotation of the specimen arms at various crack lengths has been calculated and subsequently used in a proposed extrapolation technique to predict compliance at any location of the specimen convenient for measuring deflection during a crack growth test. The predicted compliances were found to be in excellent agreement with expreimental values for the two specimen types considered. Compliance expressions are also included for the center crack tension specimen.

Review and extension of compliance information for common crack growth specimens

Cyclic stress—strain response of two-phase alloys Part I. Microstructures containing particles penetrable by dislocations

https://escholarship.org/content/qt4h95q06g/qt4h95q06g.pdf?t=p0fucf

Micromechanisms of fatigue crack growth retardation following overloads

Sub-critical flaw growth.

Research within the past two decades has achieved a dramatic upsurge of improvements in the mechanical properties of engineering ceramics. These improvements have often been made through increased toughness by novel toughening mechanisms such as the stress induced phase transformation, microcracking, fibre/whisker crack bridging, etc. These may occur not only in the frontal process zone ahead of a sharp crack, but also in the following crack wake region. The consequences of these microfracture processes and mechanisms in the wake and the crack bridging regions are significant, for they result in very complex fracture processes and they create many critical issues and difficulties in the experimental determination of the fracture resistance of brittle materials. The lack of a physical basis for a fracture criterion in the present fracture mechanics framework adds further confusion to fracture mechanics studies. This paper is a state of the art review of the application of fracture mechanics to brit...

Fracture toughness testing of brittle materials

Metal fatigue has been well recognized as an important cause for failure of engineering structures. In most applications, fatigue damage results from the conjoint actions of the cyclically applied stress and the external (chemical) environment, and is therefore time dependent. Understanding of this load-environment interaction is essential to the formulation of rational life prediction procedures and to the development of realistic materials evaluation and qualification tests. Research over the past 15 years has led to the suggestion that the rate of fatigue crack growth in an aggressive environment, (da/dN)e, is the sum of three components—the rate of fatigue crack growth in an inert environment, (da/dN)r, which represents the contribution of “pure” fatigue, a cycle-dependent component, (da/dN)cf, that requires the synergistic interaction of fatigue and environmental attack, and the contribution by sustained-load crack growth (i.e., stress corrosion cracking) at K levels above K
Iscc, (da/dN)scc. Recent fracture mechanics and surface chemistry studies have provided a clearer understanding of the cycle-dependent term, and, hence, a more complete understanding of environment assisted fatigue crack growth. (da/dN)cf results from the reaction of the environment with fresh crack surfaces produced by fatigue, and is a function of the extent of reaction during one loading cycle. For highly reactive alloy-environment systems, this contribution depends also on the rate of transport of the aggressive environment to the crack tip. The experimental basis and the development of models for transport and surface reaction controlled fatigue crack growth are reviewed. Interpretation of the effects of partial pressure of the aggressive environment and cyclic load frequency in terms of surface reaction and transport processes is discussed. Implications in terms of service performance and life prediction procedures are considered.

Recent progress in understanding environment assisted fatigue crack growth

The importance of delay, or retardation in the rate of fatigue crack growth, produced by load interactions in variable amplitude loading on the accurate prediction of fatigue lives of engineering structures has been well recognized for some time. Heretofore, only a few simple loading combinations or spectra have been examined systematically. In this investigation the effects of a broad range of loading variables on delay in fatigue crack growth at room temperature are examined for a mill annealed Ti-6Al-4V alloy. The results are used to estimate crack growth behavior under programmed loads.

Delay in fatigue crack growth

Fatigue-crack propagation experiments were carried out within the framework of linear-elastic fracture mechanics to examine the effect of water and its constituents on the rate of crack growth in a high-strength aluminum alloy over a range of test temperatures from 295 to 380° K. Dehumidified high-purity (99.9995 percent purity) argon was used as an inert reference environment. The results showed that water accelerated the rate of fatigue-crack propagation by about a factor of 10 in this temperature range, whereas dry oxygen and dry hydrogen had a negligible effect. They confirm the findings of Hartman, and Bradshaw and Wheeler that the cause for the large increase in the rate of crack growth is the formation of hydrogen gas at high pressure in the region ahead of the crack tip, driven in by the reaction of water with the-freshly created aluminum crack surfaces, as suggested by Broom and Nicholson. The results showed further that fatigue-crack propagation in water, as well as in the dry environments, is controlled by thermally activated processes, with apparent activation energies that depend strongly on the crack-tip stress-intensity parameter, ΔK. The rate controlling process appears to be that associated with the creation of new crack surfaces in the range of crack growth rates 10−6 to 10−5 inch per cycle. The strong dependence of the apparent activation energy on ΔK suggests that a careful study of the kinetics of fatigue-crack growth and of the crack growth laws is in order. Such a study should incorporate both the mechanical and chemical variables involved.

Fatigue-crack propagation in a high-strength aluminum alloy

The kinetics of subcritical crack growth under sustained loading for an AISI 4340 steel tempered at 400°F in distilled water were determined. Crack growth experiments were carried out over a range of temperatures from 10–75°C, using the crack tip stress intensity factor, K, to characterize the mechanical driving force. Results show that crack growth in distilled water is controlled by a thermally activated process with apparent activation energies that depend on K. Crack growth occurred only above a threshold K level with growth rates showing a strong K dependence at lower values of K and attaining constant, rate limiting values at higher K levels. The rate limiting velocities correspond to an apparent activation energy of 8000±1000 cal/mole, in essential agreement with values reported for crack growth in similar steels. This apparent activation energy corresponds to that for hydrogen permeation in AISI 4340 steel and lends further support to the concept that the rate limiting process for crack growth in high strength steels in water is that of hydrogen permeation into the crack-tip region. Because of the differences in the apparent activation energies for crack growth in water and in gaseous hydrogen, the rate limiting step in the permeation process still needs to be defined.

The kinetics of subcritical crack growth under sustained loading

Compression, creep, stress relaxation and overloading effects on delay in fatigue crack growth and structural life predictions

R. P. Wei

Papers

Recent progress in understanding environment assisted fatigue crack growth

Delay in fatigue crack growth

Fatigue-crack propagation in a high-strength aluminum alloy

The kinetics of subcritical crack growth under sustained loading

An exploratory study of delay in fatigue-crack growth