After Griffith's explanation of the decrease in the strength of a loaded material containing a disbonded area (a crack), it took about another quarter century before “Fracture Mechanics” as an engineering discipline became established. At that time it was the catastrophic failure of the Liberty ships and the basic contributions by Irwin2 that created a high interest in the quantification of the process by which a crack will grow. A series of accidents involving bridges, pressure vessels, generator rotors, aircraft, etc., contributed greatly — not only to further research but also to the opinion that cracks are basically “bad” and should be avoided under all circumstances.

I started to become acquainted with cracks in the early 1970s. The U.S. Air. Force had recently lost a few F-lll aircraft in southeast Asia due to wing-box cracking. In reaction, the Air Force decided that the new bomber, the B1, should be designed according to fatigue and fracture criteria. Since the materials were specified mostly by metallurgical engineers, they had to become concerned with the effects, for example, of the materials' microstructures on fatigue and crack propagation. At the same time, the interest in crack formation began to evolve.

Furthermore, the capabilities for quantitative nondestructive evaluation of a structure in service increased sharply since it was realized that we have to live with cracks and that not all cracks are necessarily “bad.”

The propagation of small fatigue cracks differs considerably from that of long cracks in the same material. Small cracks tend to grow more rapidly than would be expected from long-crack data. Following Suresh and Ritchie, cracks are small when (1) their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics, LEFM, limitation); (2) their length is small compared to microstructural dimensions (a continuum mechanics limitation); or (3) they are merely physically small. The importance of understanding the anomalously rapid growth of small cracks has been the subject of recent reviews and two specialized conferences. The impact of small cracks on component design is to force the design of highly conservative structures.

Many investigators have tried to correct LEFM to account for crack shortness. Since the early work of Kitagawa and Takahashi and Smith showing the limitations of LEFM, many procedures to modify, correct, or replace LEFM have been proposed to predict short-crack growth rates. These include mechanisms based on crack closure stress and crack deflection, elastic-plastic approaches such as the J integral, or simply semi-empirical approaches. These methods have been reasonably successful when the crack length is a few times that of the relevant micro-structural size.

Fracture mechanics technology is an effective tool for characterizing the rates of fatigue crack propagation. Generally, fatigue crack growth rate (da/dN) in each loading cycle can be presented as a function of stress intensity range (ΔK), where ΔK = Kmax — Kmin, Kmax and Kmin are the maximum and the minimum stress intensities, respectively. A typical fatigue crack growth rate curve of da/dN versus ΔK can be divided into three regimes, i.e., Stage I (near-threshold), Stage II (Paris), and Stage III (fast) crack growth regions, as shown in Figure 1.

Depending on the region of crack growth, fatigue crack growth behavior can be sensitive to microstructure, environment, and loading conditions [e.g., R (load) ratio = Kmin / Kmax]. In the nearthreshold region, fatigue crack growth rates are very slow, ranging from approximately 10−10 to 10−8 m/cycle. In this region, the fatigue crack growth rate curve eventually reaches a threshold stress intensity range, ΔKth, below which the crack would not grow or grow at an extremely slow rate. Typically, the value of ΔKth is operationally defined as the stress intensity range which gives a corresponding crack growth rate of 10−10 m/cycle. In the nearthreshold region, the influence of microstructure, environment, and load ratio on the rates of crack propagation is very significant.

A material's susceptibility to cracking may be significantly affected by its chemical environment. Stress corrosion cracking (SCC), liquid metal embrittle-ment (LME), hydrogen embrittlement (HE), and corrosion fatigue are examples of environmental effects which cause ductility or endurance losses through environment-assisted cracking (EAC). Under certain conditions, virtually all commercially important materials are susceptible to one or more of the above embrittlement processes. Cracking may occur intergranularly, transgranularly, or in a mixed mode, depending on conditions. Much is known about the metallurgical and environmental conditions which promote environment-assisted cracking, and prudent control of these is often successful in mitigating or preventing cracking. However, in spite of our understanding of the factors controlling SCC, LME, and HE, the responsible mechanisms remain elusive.

This article will (1) review some of the important variables affecting these phenomena, such as stress, stress intensity, material microstructure, strain rate, electrochemical potential and pH, and (2) attempt to relate phenomeno-logical characteristics of environment-induced embrittlement to several mechanisms proposed for environment-assisted cracking, as they are understood today.

The problem of stress corrosion cracking is unquestionably the most costly of environmental cracking phenomena, with losses occurring in a wide variety of service environments. Liquid metal embrittlement is of concern in nuclear power and other industries. Hydrogen embrittlement, first recognized as an embrittler of iron in 1873, causes cracking problems in applications ranging from welding to oil drilling. In all, the list of situations in which environment-assisted cracking occurs is long and is likely to grow as materials are increasingly challenged by the severity of their service conditions.

Although cracks in structural components are, in general, highly undesirable, not all of them are detrimental to the life of the structure. To determine precisely how detrimental the crack actually is, one must characterize several crack parameters, particularly the location, size, shape, and orientation with respect to the applied stress. With these parameters and the magnitude of the applied stress, the driving force on the crack can be calculated by determining the stress intensity factor K or the stress intensity range ΔK. Most inspections today are performed in a scanning mode. If indications of some anomaly appear, the inspection is concentrated on these locations; and, if the response verifies the presence of a flaw, the size, shape, orientation of the flaw and the potential for future growth of the flaws are determined. This process is now termed “quantitative nondestructive evaluation” (QNDE).

This short review is devoted to recent advances in the characterization of cracks in various stages of development. The increase in sophistication in instrumentation and the use of computers for data analysis and interpretation has been dramatic and will certainly continue.

In the laboratory, compliance gauges and electrical potential drop methods are now extensively used for crack length and shape measurements. Acoustic methods are being widely used in industry as nondestructive testing and evaluation tools because they are relatively easy to apply to real structures.