Tensile Testing and Material Deformation

brittle materials?

show little or no plastic deformation before failure

formula for stress

stress = force/area

What is the glass transition temperature?

The temperature at which a thermoplastic polymer changes from a glassy, brittle material to a rubbery ductile one

what conditions must a tensile test be done in?

room temp must use the standard geometry sample dimensions

What is flexural strength?

ability to resist deformation under load

when a single crystal material is put under tensile stress, at what angle does the material break?

at 45 degrees this is to do with the close packed direction being easier for dislocations to occur than for an entire row of atom's bonds to break

When does dislocation slip occur?

at the yield point

formula for strain

change in length/original length

what is the tangent modulus? (in terms of youngs modulus)

choose a specific stress and find the gradient of the tangent at that point

what, in a tensile testing machine, measures the extension?

extensometer or strain gauge

what graph do you get from a tensile test?

force v extension this should be converted to a more useful stress v strain

What does Poisson's Ratio describe?

how a material will contract when it is put in tension and spread outwards when it is compressed the ratio is of the lateral to axial strain

what happens to youngs modulus at higher temps?

it decreases

what is dislocation slip?

permanently deforming a metal by displacing atoms relative to one another it allows planes of atoms to move at relatively low stress

what types of materials do not have an elastic region on the stress strain curve?

polymers

what happens to polycrystalline metals when in a tensile tester?

tend to end up with elongated grain structure this is because all the different grains are at a variety of angles, making it difficult for a whole row to slip

if a material seems to have two yield points, which is said to be the yield strength?

the lower one

yeild stress

the stress at which a material yields

why is it hard to test a cermaic in a tensile tester?

they are so brittle could break when gripped in place

how do you find the yield stress of a polymer?

use offset method

describe ceramics

very brittle dislocation slip is very hard without fracture good in compression but weak in tension

what is offset yield strength? when is it used?

when it is not clear to see on the graph where the material changes from elastic to plastic deformation, a line is drawn parallel to the elastic region around 0.002 away from the actual line where the lines cross is said to be the offset yield strength

what does the peak of the curve tell you?

where the neck of the material forms the tensile strength

what is the point where the elastic region ends called?

yield point, limit of proportionality

why is a stress strain graph better than a force extrension graph?

you dont have to worry about the test peice dimensions

what is the secant modulus? (in terms of youngs modulus)

you specify a level of stress that the material will be under and draw a line to the origin the gradient is the secant modulus this is used for when there is no elastic region (eg polymers)

what is the gradient of a stress strain curve? a larger gradient means?

youngs modulus steeper gradient is a larger youngs modulus and more stiff

what can a tensile testing machine tell you?

youngs modulus, tensile strength, yeild strength, ductility