Transcript Material testing Lesson 2 Stress  stress – internal force in a material which tends to resist deformation when subjected to external forces 

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
stress
applied force
= --------------------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)