1. paper+for+4870+(rev).pdf Flashcards
What is a long-standing problem inherent to steel?
Its tendency toward brittleness at low temperatures and at high rates of deformation.
What is the problem with simply using higher strength material?
Higher strength materials are generally accompanied by ductility and toughness problems, enhancing the possibility of brittle fracture.
What are the two kinds of fractures?
Cup and cone fracture. Rapid crack growth.
What is a cup and cone fracture?
Driving force is the increase of tensile stress.
- Crystalline defect accumulations.
- Micro-voids can develop from large enough crystalline defects clusters.
- A crack at the centre of the specimen can develop form micro-voids under further deformation.
- The specimen shears at the edges because it cannot handle the load.
How does rapid crack growth fracturing occur?
Driving force is the internal elastic energy produced by the applied load. Usually begins with a material flaw.
What did Griffith assume about crack advancing?
The crack would advance when the incremental release of stored elastic energy d(W_E) in a body becomes greater than the increase in work d(W_S) required to create an increment of new crack surface.
Describe the Griffith model for crack propagation.
Specimen under tensile stress. Elliptical crack in the centre that is 2C in length.
What is the stored elastic energy density at the crack tip in the Griffith model?
where: 2C=crack length
What’s the symbol for surface energy per unit area of separated surface? What is the surface energy per unit width of a crack?
γ_s
4(γ_s)C
What is the Griffith criteria for crack growth?
What is σ_F?
Failure stress form fracture. The value of applied stress at which the stored elastic energy density is sufficient to cause unstable crack propagation.
What do Irwin and Orowan add to the Griffith analysis? Why?
They added γ_s, which accounts for plastic work energy. The original Griffith analysis assumed the material to be totally brittle and that it would experience unstable crack growth.
What is the equation for the failure stress from fracture which takes into consideration plastic deformation?
σ_F = sqrt[2 E ( γ + γ_p) / pi C]
Why is it important to include plastic deformation in calculated fracture failure stress?
Plastic deformation accounts for the great resistance of ductile metals to fracture.
What is the critical stress intensity factor?
where α is a geometric factor which takes into account crack and specimen geometries relative to applied stress.
What is a mode I fracture?
Opening fracture: Applied tensile strain is perpendicular to the plane of the fracture.
What is a mode II fracture?
In-Plane Shear Fracture: Applied shear strain is parallel to the plane of the fracture and perpendicular to the front of the fracture.
What is a mode III fracture?
Out-of-Plane Shear Fracture: applied tensile strain parallel with the front of the fracture and the fracture plane.
In a plane strain fracture toughness test, when are plane strain conditions met?
when: t >= 2.5 (K_IC / σ_ys)^2 where t=thickness of specimen
