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At some point, in the vicinity of 7,000 cycles, the (Syc + Syh) limitation intersects the fatigue curve for carbon and low alloy steel. The allowable stress range must therefore be reduced to fit the fatigue curve for cyclic applications with 7,000 cycles or more: SE <= F f (SYC + Syh) Where: f = cyclic reduction factor, as shown in the accompanying table Read More: Introduction to Pipe Stress Analysis To get our piping engineering services, please contact our engineering con

A. R. C. Markl investigated the phenomenon of fatigue failure of piping during the 1940's and 1950's, and published his results in papers such as "Piping Flexibility Analysis", published in 1955. He tested a number of configurations (straight pipe, and various fittings, such as pipe elbow, miter bend, unreinforced fabricated tee, welding tee, etc.) by using cyclic displacements to apply alternating bending stresses. Plotting the cycles to failure for each applied displacement

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