Stress Corrosion Cracking

Stress Corrosion Cracking Definitions

Stress: The intensity of the internally distributed forces or components of forces that resist a change in the volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis of the original dimensions of the cross section of the specimen. Stress can be either direct (tension or compression) or shear. See also residual stress.

Stress concentration factor (Kt): A multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole; Kt, equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section. Also called theoretical stress concentration factor.

Stress-corrosion cracking (SCC): A cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Stress-corrosion cracking may occur in combination with hydrogen embrittlement.

Stress-intensity factor: A scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing a crack, the factor is called the critical stress-intensity factor, or the fracture toughness. Various subscripts are used to denote different loading conditions or fracture toughnesses:

Kc: Plane-stress fracture toughness. The value of stress intensity at which crack propagation becomes rapid in sections thinner than those in which plane-strain conditions prevail.

KI: Stress-intensity factor for a loading condition that displaces the crack faces in a direction normal to the crack plane (also known as the opening mode of deformation).

KIC: Plane-strain fracture toughness. The minimum value of KC for any given material and condition, which is attained when rapid crack propagation in the opening mode is governed by plane-strain conditions.

KId: Dynamic fracture toughness. The fracture toughness determined under dynamic loading conditions; it is used as an approximation of KIC for very tough materials.

KISCC: Threshold stress-intensity factor for stress-corrosion cracking. The critical plane-strain stress intensity at the onset of stress-corrosion cracking under specified conditions.

KQ: Provisional value for plane-strain fracture toughness.

Kth: Threshold stress intensity for stress-corrosion cracking. The critical stress intensity at the onset of stress-corrosion cracking under specified conditions.

Delta K: The range of the stress-intensity factor during a fatigue cycle. stress-intensity factor range, Delta K. In fatigue, the variation in the stress-intensity factor in a cycle, that is, Kmax - Kmin

Stress raisers: Changes in contour or discontinuities in structure that cause local increases in stress.

Stress ratio, A or R: The algebraic ratio of two specified stress values in a stress cycle. Two commonly used stress ratios are: (1) the ratio of the alternating stress amplitude to the mean stress, A = Sa/Sm; and (2) the ratio of the minimum stress to the maximum stress, R =Smin / Smax

Stress-relief cracking: Also called post-weld heat treatment cracking, stress-relief cracking occurs when susceptible alloys are subjected to thermal stress relief after welding to reduce residual stresses and improve toughness. Stress-relief cracking occurs only in metals that can precipitation-harden during such elevated-temperature exposure; it usually occurs at stress raisers, is intergranular in nature, and is generally observed in the coarse-grained region of the weld heat affected zone. See also cold cracking, hot cracking, and lamellar tearing.

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