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