Lesson 2

Material testing

Stress stress – internal force in a material which

tends to resist deformation when subjected to external forces

intensity of a stress unit depends on the size of the force acting on a unit area of the material

applied force stress =---------------------c.s.a. of a material

Types of stressesCompressive stressTensile stressShear stressTorsionBending

Compressive stressCompressive stress is the stress applied

to materials resulting in their compaction (decrease of volume).

Usually compressive stress is applied to bars, columns, etc.

Tensile stressTensile stress is the stress state leading

to expansion (volume and/or length of a material tends to increase). In the uniaxial manner of tension, tensile stress is induced by pulling forces across a bar, specimen, etc.

Shear stressShear stress is a stress state where the

shape of a material tends to change without particular volume change.

Torsionthe stress which resists a force tending to

twist the material (e.g. axle, screw, etc.)

BendingBending occurs when the force applied

tends to pull a horizontal bar out of its straight line.

StrainStrain = distortion of a material

(permanent change in size and shape due to stress)

Material testingTensile test – measures strength and ductility of a

materialA) a static increasing pull is applied until fracture results

(stress - strain curve)B) a dynamic load is applied giving data on fatigue impact

Impact test – measures the energy absorbed by a material when it is fractured

Hardness test – measures material resistance to indentation

Creep test – measures slow plastic deformation of a material under constant stress

Testing machine

Testing procedurea specimen of standard size = test piecegrip in jawsapply load gradually (tensile or

compressive)extend, extension, extendedoriginal length = L1new length = L2deformation = extension/original length

Stress – Strain Curve

Modulus of elasticitythe modulus of elasticity (elastic modulus)

of an object is defined as the slope of its stress-strain curve in the elastic deformation region

Yield point= the stress at which a material begins to

deform plastically

prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed

once the yield point is passed, some fraction of the deformation will be permanent and non-reversible

Proof stress (Offset yield point)when a yield point is not easily defined

based on the shape of the stress-strain curve an offset yield point is arbitrarily defined

the value for this is commonly set at 0.1 or 0.2% of the strain

Proof stress (Offset yield point)

Ultimate tensile strength (UTS)= the maximum stress that a material can

withstand while being stretched or pulled before necking (c.s.a. of a material is reduced)

Factor of safetydescribes the structural capacity of a

system beyond the expected loads or actual loads (= how much stronger the system is than it usually needs to be for an intended load)