What properties are important when stresses are applied?
tensile strength
compressive strength
modulus
flexural strength
What is tensile testing?
a longitudinal force is applied toa specimen of original length, elongating it, resulting in a reduced area until fracture occures
What is the curve like on a stress strain graph during elastic deformation
linear curve
What is the drawing phase in plastic deformation?
once the yield point is overcome, the metal ions start to slide past one another, “easy glide”
stress only increases slightly
What is the strain hardening phase in plsatic deformation?
stress starts to increase dramatically, and the number of dislocations also increase
no more easy glide
What is the necking phase in the plastic deformation?
cross sectional area at the neck region starts to reduce and the material is about to fracture
Explain why stress reduces during necking
The cross sectional area at the neck starts to decrease so the same force is distributed over a smaller area. In reality, the stress increases, but the curve does not take into account the reduced cross sectional area
What is true stress?
the force loaded divided by the cross sectional area at a specific instant. this cannot be calculated unless the thickness and widh are monitored continuously during the test
What is yield stress/strength
the stress and the yield point
What is maximum tensile strength?
when the stress is at its peak in the graph
Explain the stress and strain curve for carbon steels
the upper yield point signifies the first movement of dislocations in the crystal structure, and the ripple afterwards is caused by non-uniform plastic deformation at stress concentration regions. the lower yield point signifies the start of plastic deformation
How do you find the yield strength with no defined yield point?
using an offset yield strength or proof stress
How do you obtain proof stress?
draw a line parallel to the linear region on the curve offset by a particular strain
example: 0.2%
What are the differences between brittle and ductile metals?
ductile has an extensive plastic deformation before fracture whilst brittle materials have little plastic deformation
Explain why its important to know about the ductility of materials
to know the degree to which a material will deform plastically before fracture
specifying the degree of allowable deformation
What are the feature of brittle fracture?
no noticeable plastic deformation
crach propagation is very fast
often happens when materials are very cold
