2Stressstress – internal force in a material which tends to resist deformation when subjected to external forcesintensity of a stress unit depends on the size of the force acting on a unit area of the materialapplied forcestress =c.s.a. of a material
3Types of stresses Compressive stress Tensile stress Shear stress TorsionBending
4Compressive stressCompressive stress is the stress applied to materials resulting in their compaction (decrease of volume).Usually compressive stress is applied to bars, columns, etc.
5Tensile 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.
6Shear stressShear stress is a stress state where the shape of a material tends to change without particular volume change.
7Torsionthe stress which resists a force tending to twist the material (e.g. axle, screw, etc.)
8BendingBending occurs when the force applied tends to pull a horizontal bar out of its straight line.
9StrainStrain = distortion of a material (permanent change in size and shape due to stress)
10Material 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 impactImpact test – measures the energy absorbed by a material when it is fracturedHardness test – measures material resistance to indentationCreep test – measures slow plastic deformation of a material under constant stress
12Testing procedure a specimen of standard size = test piece grip in jawsapply load gradually (tensile or compressive)extend, extension, extendedoriginal length = L1new length = L2deformation = extension/original length
14Modulus 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
15Yield point= the stress at which a material begins to deform plasticallyprior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removedonce the yield point is passed, some fraction of the deformation will be permanent and non-reversible
16Proof 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 definedthe value for this is commonly set at 0.1 or 0.2% of the strain