Lecture 6 Mechanical stress Flashcards
Concept of engineerig stress
When a load is placed on an object, a stress is induced on that object.
* Stress could be with many types depending on loading
e.g., tensile; compressive; shear; torsional; biaxial; hydrostaticoA
where F is the instantaneous force applied
perpendicular to the specimen cross section, and
its unit is N;
Ao is the original cross-sectional area in square
meter before any load is applied and its unit is mm2.
Units: N/m2 or Pa,
often using MPa
(1 MPa = 106 Pa = 106 N/m2)
* Engineering stress, σ, (often just
called stress) is defined by:
concept of engineering strain
When a load is placed on an object, the object is subject to change its
shape. Namely, strain is induced in the object under load.
* Strain is defined by how much the relative change in sample
dimensions due to load when compared to its original dimensions.0
0
l
l
l
ll
o
i
=
−
=
where li is instantaneous length (in mm) and lo is
the original length (in mm) before any load is
applied;
Δl is deformation elongation or change in length at
some instant as references to the original length.
Note: unitless,
sometimes in mm/mm but
often in a percentage, %
* Engineering strain, ε, (often just
called strain) is defined by:
Elastic Deformantion
Young’s modulus, E, (also known as modulus of elasticity or stiffness),
is a material’s resistance to elastic deformation. It has the same unit as stress,
N/m2 or Pa, but most uses GPa (1 GPa = 103 MPa = 109 N/m2).
* The greater the modulus is, the stiffer the material is, or a smaller elastic strain
resulted from a given stress
= E·
The tensile test
the most common method of
determining strength of a material under load
in a mechanical properties testing machine.
Elastic deformation
Young’s modulus, E, (also known as modulus of elasticity or stiffness),
is a material’s resistance to elastic deformation. It has the same unit as stress,
N/m2 or Pa, but most uses GPa (1 GPa = 103 MPa = 109 N/m2).
* The greater the modulus is, the stiffer the material is, or a smaller elastic strain
resulted from a given stress.
simple
tension
test
* For most metals that are stressed in tension and at relatively low levels, stress
and strain are proportional to each other through the Hooke’s law
