Material testing Lesson 2

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 stresses Compressive stress Tensile stress Shear stress Torsion Bending

Compressive stress Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume). Usually compressive stress is applied to bars, columns, etc.

Tensile stress Tensile 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 stress Shear stress is a stress state where the shape of a material tends to change without particular volume change.

Torsion the stress which resists a force tending to twist the material (e.g. axle, screw, etc.)

Bending Bending occurs when the force applied tends to pull a horizontal bar out of its straight line.

Strain Strain = distortion of a material (permanent change in size and shape due to stress)

Material testing Tensile test – measures strength and ductility of a material A) 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 procedure a specimen of standard size = test piece grip in jaws apply load gradually (tensile or compressive) extend, extension, extended original length = L1 new length = L2 deformation = extension/original length

Stress – Strain Curve

Modulus of elasticity the 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 safety describes 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)