Unit 4 - Cracks and Fracture Mechanics Flashcards
What is the Paris-Erdogan Law in fatigue crack growth?
The Paris-Erdogan Law describes the rate of fatigue crack growth, relating it to the range of the stress intensity factor. It’s a key tool in predicting the life of structures under cyclic loading.
How do environmental factors influence fracture mechanics?
Environmental factors like temperature, humidity, and corrosive environments can accelerate crack growth and reduce material toughness. These factors are critical in designing and maintaining structures in different environments.
How does microstructure affect fracture toughness?
Microstructure, including grain size and phase distribution, influences how a material absorbs energy and resists crack propagation, directly affecting its fracture toughness.
What is the significance of the Stress Intensity Factor (SIF) in fracture mechanics?
The Stress Intensity Factor (SIF), denoted as K, describes the stress field near the crack tip. It’s a critical parameter in predicting whether a crack will grow or remain stable, based on the material’s fracture toughness.
Explain the concept of fracture toughness.
Fracture toughness is a material property that describes its resistance to fracture in the presence of a flaw. It’s denoted as K1C and indicates the critical stress intensity factor at which crack propagation occurs.
Describe the mixed-mode fracture in the context of fracture mechanics.
Mixed-mode fracture occurs when different loading modes (I, II, and III) contribute to crack propagation. For example, Mode I (opening) and Mode II (sliding) can combine, affecting the crack path and fracture characteristics.
What is the Griffith criterion in material science?
Griffith criterion states that a crack will grow if the energy supplied by the material exceeds that required to form new surfaces. If energy supplied is much greater than surface energy, catastrophic failure occurs.
What are stress concentration features and their importance?
Elastic stress concentration factor (Kt) describes local stress raising ability of stress raisers or discontinuities.
Examples include porosity, solid inclusions, notches, holes, and damage like weld defects.
Stress amplification depends on the size, shape, and orientation of the defect relative to applied stress.
What defines the fracture criterion in materials?
Stress Intensity Factor (SIF) describes the stress field ahead of a crack tip and defines the plastic zone size/shape.
Fracture occurs when stresses/strains reach a critical value: K1 = K1C.
K1C is a material-specific parameter called fracture toughness.
What are the key points about Stress Intensity Factor?
Three modes of crack displacement: Modes I, II, and III.
Stress concentration valid only for 3D features.
Near the crack tip, stress approaches infinity, diminishing to nominal applied stress away from the crack.
Stress intensity factor K introduced by Irwin, can be modified for local geometry effects.
At critical K1, failure occurs (fracture toughness).
K1C is a material parameter.
What is the Compact Tension (CT) test procedure for toughness measurements?
CT specimen design widely employed for toughness measurements.
Initial procedure involves fatigue pre-cracking.
Clip gauge monitors crack opening displacement.
Monotonic loading ramp applied until failure.
Load vs. displacement curves recorded to define load causing failure.
Onset of failure indicated by change in compliance.
