Vibration & Fatigue Analysis

Thermal / Fatigue Design & Analysis of Equipment and Welded Structures 
Fatigue Life Prediction of structures and systems ensures designs meet their requirements for safe-life or damage-tolerant design. Tom O’Donnell performs vibration, fatigue and failure analysis to ASME and other relevant Codes. Bill O’Donnell Sr. continues to serve as a Contributing Member of the ASME Boiler and Pressure Vessel Code Subgroup on Fatigue Strength – and has published numerous papers on fatigue and failure analysis. One such paper is Code Design and Evaluation for Cyclic Loading – ASME Sections III and VIII.

Fatigue of welded joints is a very complex problem. Simple procedures for fatigue properties of joints cannot be formulated. Additionally, during the welding process, residual stresses are created. The fatigue life of welded joints is mainly due to three factors:

• Notch effect due to weld filler metal
• Presence of welding imperfections
• Presence of residual stresses

Low Cycle / High Cycle Fatigue
Fatigue (from cyclic loading) is one of the most frequent causes of failure in pressure vessels, piping and process equipment. A vibrating component or system often leads to fatigue – which eventually leads to equipment malfunction, or catastrophic failure. Fatigue cracks usually initiate at the surface, and for this reason, the condition of the surface and the environment are important factors influencing fatigue behavior. Low cycle fatigue failures result when a material is stressed above its elastic limit. This is the limit a material my be stressed without permanent alteration of size or shape. High cycle fatigue failures occur under repetitive (e.g., cyclical, harmonic, and random vibration) stresses well below the yield strength of the material.

Fatigue analysis is performed per ASME Section III Class 1 and Section VIII Division 2. The fatigue design life evaluation procedures in Section III of the ASME Boiler and Pressure Vessel Code were originally developed in the U.S. Naval Nuclear Program.

Those involved were Bill O’Donnell, (Bernie) Langer, W.E. (Bill) Cooper and James (Jim) Farr – who, in the late 1950’s and early 1960’s developed the initial formulation of this technology in the Tentative Structural Design Basis for Reactor Pressure Vessels, which became known as “SDB-63.” Section III of the ASME Code “Vessels in Nuclear Service” was the first to include specific Code rules to prevent low cycle fatigue failure.