Environmental effects in crack growth

The interdependence of material structure, electrochemical parameters and the response of a material to the application of stress can produce slow crack growth by a number of different mechanisms. The recognition of the importance of such factors as environment composition, especially within the confines of a pit or crack, and of plasticity, especially in relation to the rate at which deformation presents bare metal at the tips of cracks, is vital if reproducible data are to be obtained.

Experimental methods for fracture toughness measurement

Recommended methods for measuring fracture toughness parameters are discussed. The relevant Draft British Standards are: (a) Draft for Development 3:1971 ‘Methods for Plane Strain Fracture Toughness ( KI c) Testing’. (b) Draft for Development 19:1972 ‘Methods for Crack Opening Displacement (COD) Testing’. These documents are being revized and updated for publishing as full British Standards and any significant additions and alterations are noted. The situation with respect to non-standard tests is reviewed. The fracture toughness parameters under this heading include JI c, equivalent energy values, fracture propagation energy values and R-curve analysis. Particular attention is paid to the influence of strain rate on testing procedures and a crack monitoring technique is discussed.

The fracture toughness of metals

Problems in applying LEFM to the fracture of metals are discussed. Fracture toughness values for cleavage and fibrous modes are related to the micromechanics of fracture.

Yielding fracture mechanics

Yielding fracture mechanics seeks to find a relationship between applied stress, crack size and material toughness that is independent of the geometry of a component when fracture occurs after significant degree of yielding. The crack opening displacement, Δ, and the J contour integral are two proposals for describing the stresses and deformation at the tip of a sharp crack embedded in a region of yielding material. The concepts can be related in the form J = M σ YΔ where σ Y is the uniaxial yield stress, and M a factor with a value between about 1 and 2.5. The concepts are still under development. Either term can be chosen as a measure of the severity of crack tip deformation in a given material with the onset of crack growth in monotonic loading occurring at a critical value, Δ c or Jc, for a given thickness. Experimental evidence so far is in broad support of this picture but there remains uncertainty over the degree to which Δ c or Jc is independent of geometry and the extent to which stable crack growth prevents the usage of one simple criterion of fracture for all structural configurations.

Origins of the stress intensity factor approach to fracture

The development of energy balance approach to fracture is traced. Limitations of the approach are discussed and the justification for its application to practical problems is indicated.

Analysis and application of fatigue crack growth data

The phenomenon of metal fatigue has been studied for a long time, but it has only relatively recently been appreciated that most structures, particularly welded joints, contain crack-like flaws, so that virtually the whole fatigue life is occupied by fatigue crack growth. The fracture mechanics concept of stress intensity factor has proved particularly convenient for the analysis of fatigue crack growth data in a form which can be applied directly to engineering problems, and its use has led to a much better understanding of the fatigue behaviour of structures. The effects of interaction between different load levels are not yet fully understood; this is not necessarily a serious drawback as servo-hydraulic fatigue testing equipment permits the application of virtually any load history to structures or fatigue crack growth specimens.

Evaluation of stress intensity factors

Some of the more useful methods of evaluating stress intensity factors are presented in a concise form. The stress intensity factor is defined and compared with the more familiar stress concentration factor. The shape factor, the parameter which characterizes the shape of the crack, the orientation of the crack and the proximity of other boundaries, is introduced; the techniques for determining the shape factor are divided into theoretical and experimental. Each method is described with a minimum of mathematical detail; references are given to papers where the methods are more fully described and used to solve specific crack problems. The accuracy and usefulness of the methods is summarized.