101-0121-00L  Fatigue and Fracture in Materials and Structures

SemesterAutumn Semester 2021
LecturersE. Ghafoori, A. Taras
Periodicityyearly recurring course
Language of instructionEnglish

AbstractThe fundamentals in fatigue and fracture mechanics, which are used in different engineering disciplines (e.g., for mechanical, aerospace, civil and material engineers) will be discussed. The focus will be on fundamental theories (based on fracture mechanics) that model fatigue damage and crack propagation.
ObjectiveIn this course, the students will learn:
• Mechanisms of fatigue crack initiations in materials.
• Linear elastic and elastic-plastic fracture mechanics.
• Modern computer-based techniques (using ABAQUS Finite Element Package) to simulate cracks in both bulk materials and bonded joints/interfaces.
• Laboratory fatigue and fracture tests on details with cracks.
ContentThe course starts with a discussion on the importance of fatigue and fracture in different engineering disciplines such as mechanical, aerospace, civil and material engineering domains. The preliminary topics that are covered in this course are:

I) Fatigue of materials:
• Mechanisms of fatigue crack initiation in (ductile and brittle) metals.
• Crack initiation under uni-axial high-cycle fatigue (HCF) loadings: Wöhler (S-N) curves, constant life diagram approach (mean-stress effects), rainflow analysis and Miner's damage rule.
• Crack initiation under multi-axial HCF loadings: multi-axial fatigue mechanisms, critical plane approach (critical distance theory), equivalent stress approach, proportional and non-proportional loading.

II) Fracture mechanics:
• ELinear elastic fracture mechanics (LEFM): limits of LEFM, stress intensity factors, crack opening displacement, mixed-mode fracture, etc.
• Elastic-plastic fracture mechanics: Irwin and Dugdale models, plastic zone shapes, crack-tip opening displacement and J-integral.
• Fatigue crack growth (FCG): FCG models, Paris' law, cyclic plastic zones, crack closure effects. This also includes FE modeling of the FCG and laboratory tests (at Empa).

III) Introduction to cohesive zone models (CZMs):
• Advantages and disadvantages of CZMs compared to fracture mechanics.
• Different bond-slip models for the bonded joints/interfaces.

IV) Computer laboratory to simulate cracks and debonding problems:
• Finite Element (FE) modeling of complex details with cracks.
• FE simulations of debonding problems using CZMs.
• Computer laboratory: FE training and exercises using (the student edition of) the ABAQUS FE Package.

V) Introduction to fatigue and fracture design in civil structures. Different methods for fatigue strengthening will be disscussed.

VI) Visits to the Empa (Swiss Federal Laboratories for Materials Science and Technology) in Dübendorf, and “Laboratory Competition”. The students will:
• Visit different small-scale and large-scale fatigue testing equipment.
• Get to know different ongoing fatigue- and fracture-related projects.
• Witness and help to conduct a fatigue test on a steel plate with a pre-crack and a fracture test on an adhesively-bonded joint.
• Compare the experimental results with their own calculations (from the fracture theories).
• “Laboratory Competition” at Empa: the students with the closest predictions will win the “Empa Laboratory Competition” and will be awarded by a prize.
Lecture notesLectures are based on the lecture slides and the handouts, which will be given to the students during the semester.
Literature1. Schijve J. “Fatigue of Structures and Materials”, 2008: New York: Springer.
2. Anderson T.L. “Fracture Mechanics - Fundamentals and Applications”, 3rd Edition, Taylor & Francis Group, LLC. 2005.
3. Budynas R.G., Nisbett J.K. “Shigley's Mechanical Engineering Design”, 2008, New York: McGraw-Hill.
Prerequisites / NoticeNote 1: A basic knowledge on mechanics of structures and structural analysis (i.e., stress-strain analysis and calculations of internal deformations, strains and stresses within structures) is recommended and will be helpful in the course.

Note 2: Laboratory demonstrations and fatigue/fracture tests at the Structural Engineering Research Laboratory of Empa in Dübendorf. This includes laboratory tours and showcasing the Empa large-scale 7-MN fatigue testing machine for bridge cables, different fatigue and fracture testing equipment for structural components, etc.