Metal Fracture, fatigue and creep Flashcards
What are the two types of failure?
Ductile and Brittle
What are the causes of failure?
Tension, compression, shear, torsion, heat & pressure
Describe an indication of ductile fracture in metals.
A material that experiences a large amount of plastic deformation. In the case of metals, necking occurs in the gauge area.
What is the name given to a ductile fracture surface?
Cup and cone fracture.
Describe a Brittle fracture
A fracture that occurs after very little stretching/plastic deformation. Sudden fracture without any warning. Cracks propagate perpendicular to the surface.
Where to cracks propagate from?
Through the weakest part of materials, this is the grains.
What is Intergranular Fracture?
Cracks that propagate along grain boundaries.
What is transgranular fracture?
Cracks that propagate through grains.
What are fracture mechanics? And what is this based on?
It is a concept based on the fact that materials have flaws or defects. These are called stress concentrations.
How can stress concentrations be induced?
Geometry, voids, inclusions and imperfections
The sharper the crack tip…..
The higher the stress magnification
The duller the tip…….
The lower the stress magnification.
What are two impact tests we can use and what do they measure?
Charpy and Izod. they measure the impact energy.
what is fatigue?
a form of failure that occurs in structures subjected to dynamic and fluctuating stresses. usually under loads well below that of tensile strengths.
what does the S-N curve measure?
Stress vs the log of the number of cycles before failure.
what is the fatigue limit?
the fatigue limit is where the S-N Curve becomes straight. below this line, failure will not occur. it represents the largest value of fluctuating stress that will not cause failure for essentially and infinite number of cycles.
what is creep?
time dependent and permanent deformation of materials when subjected to a constant load or stress.
what are the 3 stages of creep? this can be shown on the generic creep curve
primary, secondary, tertiary
dimples surface fracture indicates a
ductile fracture
the process used to create compressive residual stresses in a material so that fatigue is delayed is called
shot peening.
for metals, creep deformation can only occur if the load on a material exceeds the materials yield strength.
false
charpy impact testing is used to measure
fracture toughness.
