Material Failure

What classification of material properties relate to mechanical failure?

Give examples of mechanical properties

• density

• young’s modulus

• specific strength

• compressive strength

• tensile strength

• yield strength

• ductility

• fatigue limit

• poisson’s ratio

• hardness

• fracture roughness

• toughness

What is applied physical force and what are the main types?

Every material responds to forces acting on them, this applied force is represented as stress

The main types are

1. Tension

2. Compression

3. Bending

4. Torsion

5. Bi-axial tension or compression

Define tension

forces pulling on the object

Define torsion

forces twisting the object axially

Define bending

opposing forces cause tension on one side of object and compression on other side

Define buckling

deformation under compression force

What are the components of stress / strain graphs?

Draw a stress vs strain graph for a ductile polymer and explain the reasons why the graph has such a specific shape

Draw a stress vs strain graph for metal materials which are under the tensile stress

How can the elastic modules (E) be calculated using the stress-strain graph?

• E = stress/strain

• On the elastic part of graph and can be calculated by E = rise / rest which is the slope

What is hardness / stiffness ?

• The ability of a material to resist permanent deformation, determined by a standard test where surface resistance to indentation or penetration is measured

• Measured in Pa

Define ‘creep’

Slow, continuous deformation of a material at high temperatures, ending in fracture

Define ‘wear’

The removal of solid from a material due to an impact with another material

Define ‘erosion’

The mechanical removal of solid from a material due to contact with a fluid containing hard particles

Define ‘adhesive wear’

Adhesion at the work hardened junction cause shear-off of materials when surfaces slide. Transfer of material from one surface to the other occurs

Describe the strength/toughness on the stress/strain graph

1. Strong, not tough

2. Strong and tough

3. Not strong or tough