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Stress concentration has an important effect of fatigue, including welded joints /6/.

The situations shown in Figs. 2-4 are typical locations for fatigue crack initiation, but also

direction of loading is important. Thus, planar discontinuities, such as plate laminations,

whose plane is parallel to the surfaces of a plate subjected to in-plane tensile stress

fluctuations, rarely cause degradation in the fatigue life, but can be detrimental when the

plate is subjected to tensile stress in the through-thickness direction. Also, unwedded

regions loaded perpendicular to their plane in partial penetration groove welds or when

lack of fusion occurs behave like cracks and, depending on the geometry of the weld-

ment, can cause crack initiation and propagation from the weld root.

4. THE APPLICATION OF FRACTURE MECHANICS PARAMETERS

Fracture may be defined as the mechanical separation of a solid by applied stress.

Fracture of engineering materials is classified as ductile or brittle. In a brittle material,

like glass, the energy required for fracture is simply the intrinsic surface energy of the

material. For alloys at room temperature, much more energy is spent for fracture because

of plastic deformation preceding final separation of metallic materials.

The application of fracture-mechanics technology has significantly improved design

of structures regarding safety and reliability, quantifying the parameters that affect struc-

tural integrity. These parameters include the magnitude and range of the applied stresses;

the size, shape orientation, and rate of propagation of the existing crack; and the fracture

toughness of the material. Linear-elastic fracture mechanics (LEFM) is applicable if the

crack tip in a body is sharp and negligible plastic deformation preceded fracture, like in

case of high-strength steels, titanium, and some aluminium alloys. Elastic-plastic fracture

mechanics (EPFM) is used when the crack is not sharp, so crack-tip plasticity can produ-

ce crack tip blunting, for materials such as lower-strength, higher-toughness steels.

Toughness is defined as the ability of a material to absorb energy. It is characterized

by the area under a stress-strain curve for a smooth tensile specimen loaded slowly to

fracture. Notch toughness represents the ability of a material to absorb energy in the

presence of a notch, usually by impact loading, using the Charpy V-notch specimen, the

dynamic-tear specimen, and fracture-toughness specimens loaded quasi- statically (K

) or

dynamically (K

). Traditionally, the notch-toughness of low- and intermediate-strength

steels have been characterized in terms of the transition from ductile to brittle behaviour

by decreasing test temperature

3,7,8

. Most structural steels can fail in either a ductile or

a brittle manner depending on temperature, loading rate, and constraint. The ductile-to-

brittle transition behaviour of steels is most frequently characterized by impact testing the

Charpy V-notch specimen, ASTM E 23. Notch toughness at each testing temperature may

be determined from the energy absorbed during fracture, from the present shear (tearing)

or cleavage on the fracture surface, or lateral expansion. An example of the ductile-to-

brittle transition with temperature for each of these parameters is presented in Fig. 5 /3/.

The rate of change from ductile to brittle behaviour depends on many parameters,

including strength and composition of the material. It has been customary to define a

single temperature, corresponding to consumed energy of 20 J, or to 50% of cleavage.

Fracture-toughness behaviour, or the ability of cracked material to withstand fracture,

is established by using fracture-mechanics concepts, in the form of stress-intensity factor:

K C a

(2)