Mechanical Failure Flashcards
Define stress.
A loading force applied to a cross sectional area (F/A).
Define strain.
A material’s response to stress and is the deformation in the direction of the stress.
(Extension / Length)
Define YM.
The measure of the stiffness of a material.
The ratio of stress to strain in a given direction.
Stress / Strain
FL / Ax
Define yield strength.
The pressure at which the materials begins to deform permanently or plastically.
Define tensile strength.
Maximum pressure a material can take before it fails.
Define ductility.
A material’s ability to deform under tensile stress.
Define malleability.
A material’s ability to deform under compressive stress.
Define brittle.
Materials that experience little deformation when stressed and fail without plastic deformation.
Define hardness / stiffness and give the unit.
The ability of a material to resist permanent deformation.
Determined by a standard test where the surface resistance to indentation / penetration is measured.
Pa
How is hardness determined?
A standard test where the surface resistance to indentation or penetration is measured.
What does the area under a stress-strain graph represent?
Toughness
What does a steep stress-strain line represent?
High YM –> Stiff
What does a gentle stress-strain line represent?
Low YM –> Elastic
What does the Charpy Test measure?
Indicates the energy a material absorbs during fracture.
Describe the Charpy test.
Specimen is hit by a swinging arm
Sample is fractured by impact. Impact energy indicates toughness.
Height of swinging arm indicates energy absorbed during fracture.
What does the Vickers’ Hardness Test observe?
A material’s ability to resist plastic deformation.
Describe the Vickers’ Hardness test.
Square based pyramidal indenter makes an indentation in the sample material.
Hardness number = Load / surface area of the indentation
How do materials fail?
Materials contain inclusions (material trapped inside minerals).
When load is applied onto material, inclusions act as stress concentrations.
Inclusions separate from matrix, causing voids to grow and then fracture.
What is the equation for the stress concentration factor?
K SC = maximum local stress / nominal load
What is fatigue?
Material fails due to cyclic loads below the material’s yield strength.
What is creep?
Slow, continuous deformation of a material at high temperatures, ending in fracture.
Describe the two stages of creep.
Steady-state: deformation over a long period of time.
Tertiary: Massive increase in strain over a small time before fracture.
How does temperature affect mechanical properties of a material?
At low temperatures, some materials can experience ductile to brittle transitions.
Define wear.
The removal of solid from a material due to impact from another material.
