Outline
Types Topic of Tests Definitions
Stress Strain Strain Ratio Area Reduction and Elongation
Terminology Your Responsibilities
CHE 294
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Types of Tests
What questions might we ask about the mechanical behavior of a material?
How does it stretch? How does it bend? How does it withstand cyclic loads? How does it withstand sudden impacts? How long does it last under a load?
Tensile Test Flex Test Fatigue Test Impact Test Creep Test
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Stress
Stress is a measure of the force applied to an area of the component.
Tensile Forces
applied load F A area force applied normal to area Engineering (Tensile) Stress = F / Ao (initial area) The area changes as the piece stretches. True Stress
Shear Forces
applied load F A area
force applied in the plane Shear Stress
=F/A
=F/A
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CHE/MAE 294 S97
Topic 10a Introduction
1
Strain
Strain is a measure of the deformation that occurs. applied load F A area Engineering Strain l length applied load F A area
= ( l lo ) / lo
True Strain d = dl / l
= ln ( l / lo )
Shear Strain
= tan
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Strain Ratio
Under tension loads, the ratio of transverse to axial deformation is Poisson's ratio.
transverse = strain
t (r ro) / ro
Poisson's ratio
axial strain
a = (l lo) / lo
Why does this have a negative sign? For ideal materials, < 1 / 3
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= t / a
Area Reduction and Elongation
These also measure the deformation of the material.
% area reduction = 100
(Ao A) Ao
(the relative amount of area reduction)
%elongation = 100
(lbreak l) l
(the amount left to stretch a component until it breaks)
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CHE/MAE 294 S97
Topic 10a Introduction
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Terminology
Flexible The component bends easily and returns to its original shape after bending. Stiff The component is difficult to bend without permanently deforming it (plastically). Brittle The component breaks before it bends ...