Vibration & Fatigue Analysis

Thermal / Fatigue Design & Analysis of Equipment and Structures

Fatigue Life Prediction of Structures and Systems ensures designs meet their requirements for safe-life or damage-tolerant design. Tom O’Donnell performs fatigue design/analysis – as well as vibration, fatigue and failure analysis to ASME and other relevant Codes. Bill O’Donnell Sr. has served 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.

Thermal Fatigue represents a particularly challenging failure mechanism in equipment subject to cyclic temperature variations. Unlike mechanical fatigue from external loads, thermal fatigue develops from repeated thermal expansion and contraction as equipment heats up and cools down during startup, shutdown, or process upsets. Components experiencing frequent thermal cycling—such as heat exchangers, boiler tubes, steam headers, and piping at mixing points—accumulate fatigue damage even when operating pressures remain constant. The combination of temperature gradients, material property changes at elevated temperatures, and constraint from adjacent components creates complex stress states that require specialized analysis to predict crack initiation and component life.

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

Fatigue Analysis
Fatigue from cyclic loading is one of the most frequent causes of failure in pressure vessels, piping and process equipment. Unlike static overload failures, fatigue damage accumulates gradually over thousands or millions of load cycles, making it particularly insidious because equipment can appear sound until catastrophic failure occurs. A vibrating component or system subjects materials to repeated stress reversals that progressively weaken the structure, eventually leading to crack initiation, propagation, and equipment failure.

Fatigue cracks typically initiate at the surface where stress concentrations are highest—particularly at welds, sharp corners, notches, or surface defects. The condition of the surface finish, presence of residual stresses from welding, and operating environment (including temperature, corrosive media, and loading frequency) are critical factors influencing fatigue behavior and component life. In corrosive environments, the combination of cyclic stresses and chemical attack can significantly accelerate crack growth, a phenomenon known as corrosion fatigue. Understanding these failure mechanisms allows engineers to design equipment with adequate fatigue resistance and implement modifications that extend service life before failures occur.

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.