Fracture Mechanics-Based Fatigue Analyses

Estimation of fatigue lives of machines and structures is a major concern with regards to service safety and costs. Numerous analytical techniques are available and can be used with varying degrees of success. Therefore, several important questions need to be answered before any fatigue analysis can get started. The first problem is the choice of the methodology best suited for the problem to be analyzed. Then, how to define appropriate inputs for a specific fatigue methodology? How can the structure be improved to meet the service conditions? This seminar will address those questions from the fracture mechanics perspective, relatively young branch of material science and mechanics.

Dr. Grzegorz (Greg) Glinka, D.Sc.
Professor, Department of Mechanical and Mechatronics Engineering, University of Waterloo

Seminar Description

Any fracture analysis or fatigue life estimation procedure consists of three sets of input data used for the fatigue analysis, for example:

• Loading/stress history;
• Material properties; and
• Geometry of the analyzed mechanical component or engineering structure.

The basics of the fracture mechanics theory, the derivation of all input data necessary for fracture mechanics analyses, and their physical meanings will be discussed in the first part of the course.

Secondly, the general rules concerning the static strength analysis of cracked bodies will be briefly discussed in order to make potential analysts aware of differences between the static and fatigue failure processes and stress parameters used in such analyses. Particular attention will be devoted to the nature of stress parameters used in fatigue analyses, for example, stresses normal to the potential crack plane, stress distributions, and stress intensity factors. Various methods of calculating stress intensity factors for notched and welded components will be discussed and illustrated with practical examples.

Finally, the Fracture Mechanics based fatigue analyses (da/dN - ∆K) will be discussed including the calculation of appropriate Stress Intensity Factors for cracks in geometrically complex machine components, evaluation of the residual stress effect, evaluation of the weld geometry and the effect of their scatter on the predicted fatigue life. Among others the weight function technique, particularly useful for calculating Stress Intensity Factors for non-classical crack problems, when combined with the Finite Element stress data will be presented. A technique for the fatigue crack growth analysis of planar irregular cracks in nonlinear stress filed will be discussed and the possibility for its application for the fatigue analysis of small inclusions or material imperfections.

The course will be concluded with failure analysis of several mechanical and structural cases encountered in practice.

Contents

1. Modes of Failure and Review of Classical Static Strength Criteria

• The classical static strength criteria
• Plastic failure
• Brittle fracture
• Fatigue fracture

2. Stresses in Notched Bodies

• Stress distributions in notched components
• Global and local geometrical effects on stress distributions
• The stress concentration phenomenon and stress concentration factors for geometrically complex objects (the stress multiaxiality, the ambiguity of the Kt)
• Shell FE versus the 3-D FE stress modelling of notched components
• Residual stresses in notched components

3. Fracture Mechanics Approach to Fatigue and Fracture Analysis

Principles of the Linear Elastic Fracture Mechanics

• The stress concentration near cracks
• The universal stress field near the crack tip
• The stress intensity factor
• The critical crack dimensions and the fracture toughness
• Calculation of stress intensity factors:
  • Handbooks
  • Superposition
  • Weight functions
  • Numerical methods
• Examples

The Fatigue Crack Growth Analysis

• Fatigue crack growth equations and the scatter
• Integration of fatigue crack growth expressions
• The effect of the initial crack size
• The effect of the local
• The effect of the stress spectrum
• Residual stress effect
• Non-classical fatigue crack growth problems (short cracks, cracks near notches)
• Fatigue crack growth analysis of small irregular planar cracks initiated from non-metallic inclusions or material imperfections
• Evaluation of the scatter in the da/dN-∆K method
• Reliability and the Monte-Carlo simulation in the da/dN-∆K method
• Example
• Recent activities in the field of fatigue crack growth modelling

Simple Fatigue Strength Improvement Methods

• Reduction of stress concentration
• Reduction of nominal stress
• Stiffness effects
• Local and global geometry improvements
• Macroscopic and microscopic analysis of failures

4. Software Tools and Relevant Websites - Summary

Presenter

Dr. Grzegorz (Greg) Glinka, D.Sc.

Professor, Department of Mechanical and Mechatronics Engineering, University of Waterloo

Dr. Glinka has been with the University of Waterloo, Ontario, Canada since 1989. He was a Post-Doctoral Fellow at The University of Iowa (USA) in 1978 and later he lectured at the University of Metz, France and at the University College London, England. He holds a PhD and D.Sc. from the Warsaw University of Technology. He has also acted as an expert of the United Nations and visiting professor at the Aalto University in Helsinki, Finland. Dr. Glinka is a specialist in fracture and fatigue of steel structures and mechanical engineering machinery. His research interests include fracture of materials, fatigue of structures, multiaxial fatigue and creep of engineering materials, computer aided design, FEM-elastic-plastic stress-strain analysis and reliability. His recent research activities concern modelling of fatigue crack growth under random loading and fatigue optimization of welded structures. Dr. Glinka has published over 190 related articles in technical journals and textbooks.