Unit 3 - Fracture Flashcards
What characterises brittle fracture?
Brittle fracture occurs with little to no plastic deformation, often featuring a flat and shiny fracture surface.
Describe ductile fracture.
Ductile fracture involves significant plastic deformation before failure, typically showing a dimpled rupture surface.
How does temperature affect fracture behaviour?
Higher temperatures generally promote ductile fracture, while lower temperatures increase the likelihood of brittle fracture.
Explain the difference between theoretical and measured strengths of materials.
Ideal/Theoretical Strength: maximum stress for failure in a perfect crystal (no defects).
Real Strength: influenced by defects like dislocations, grain boundaries, second phase constituents, and processing defects.
What is a stress concentration factor?
It’s a measure of the increase in stress caused by the presence of flaws or geometric discontinuities in a material.
Define fracture toughness
Fracture toughness is a material’s ability to resist fracture in the presence of flaws or cracks.
What role does strain rate play in fracture?
Higher strain rates tend to promote brittle fracture, while lower rates allow more time for plastic deformation, leading to ductile fracture.
What is shear fracture?
Shear fracture occurs due to high shear stresses, typically resulting in a rough and textured fracture surface
How do brittle and ductile fractures differ in appearance and behavior?
Brittle fractures occur with minimal plastic deformation, often appearing shiny or granular. Ductile fractures show considerable deformation, often with a ‘cup and cone’ appearance.
How does fracture appearance change with temperature?
The appearance of fractures changes with temperature, showing more ‘fibrous’ features at higher temperatures and ‘shiny’ facets (indicating microvoids vs. facets) at lower temperatures
What are the requirements and purposes of impact testing?
Toughness measurement, simulating severe service conditions.
Includes Charpy (CVN) and Izod standards.
Measures impact energy and notch toughness.
Used to define ductile to brittle transitions and temperature range for transitions
How are fractures in polycrystalline samples classified?
Transgranular: crack propagates through the grains.
Intergranular: crack propagates along grain boundaries.
