Material testing

1. Material testing Lesson 2

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

3. Types of stresses • Compressive stress • Tensile stress • Shear stress • Torsion • Bending

4. 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.

5. 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.

6. Shear stress • Shear stress is a stress state where the shape of a material tends to changewithout particular volume change.

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

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

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

10. 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

11. Testing machine

12. 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

13. Stress – Strain Curve

14. 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

15. 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

16. 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

17. Proof stress (Offset yield point)

18. 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)

19. 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)