LITTLE P.ENG. FOR ENGINEERING CONSULTANCY SERVICES

A plot of the cyclic stress capacity of a material is called a fatigue (or endurance) curve. These curves are generated through multiple cyclic tests at different stress levels. The number of cycles to failure usually increases as the applied cyclic stress decreases, often until a threshold stress (known as the endurance limit) is reached below which no fatigue failure occurs, regardless of the number of applied cycles. The endurance limit (for those metals that possess one)

Steels and other metals are made up of organized patterns of molecules, known as crystal structures. However, these patterns are not maintained throughout the steel producing an ideal homogenous material, but are found in microscopic isolated island-like areas called a grains. Inside each grain the pattern of molecules is preserved. From one grain boundary to the next the molecular pattern is the same, but the orientation differs. As a result, grain boundaries are high energ

Most of the current piping codes use a slight modification of the maximum shear stress theory for flexibility related failures. Repeating, the maximum shear stress theory predicts that failure occurs when the maximum shear stress in a body equals Syjeld/2, the maximum shear stress existing at failure during the uniaxial tensile test. Recapping, the maximum shear stress in a body is given by: For the differential element at the outer surface of the pipe, the principal stresses

To be useful, calculated stresses must be compared to material allowables. Material allowable stresses are related to strengths as determined by material uniaxial tensile tests, therefore calculated stresses must also be related to the uniaxial tensile test. This relationship can be developed by looking at available failure theories. There are three generally accepted failure theories which may be used to predict the onset of yielding in a material: 1OCTAHEDRAL SHEAR, or V

During operation, pipes are subject to all of these types of stresses. Examining a small cube of metal from the most highly stressed point of the pipe wall, the stresses are distributed as so: There are an infinite number of orientations in which this cube could have been selected, each with a different combination of normal and shear stresses on the faces. For example, there is one orientation of the orthogonal stress axes for which one normal stress is maximized, and anothe