Stress:
It is a force that tends to deform the body on which it acts per unit area.It is measured in N/m2 and this unit is specifically called Pascal (Pa). A bigger unit of stress is the mega Pascal (MPa).
1 Pa = 1N/m2,
1MPa = 106 N/m2 =1N/mm2.
Compressive stress tends to squeeze a body, tensile stress to stretch (extend) it, and shear stress to cut it.
Three Basic Types of Stresses
Basically three different types of stresses can be identified. These are related to the nature of the deforming force applied on the body. That is, whether they are tensile, compressive or shearing.
Tensile Stress
Consider a uniform bar of cross sectional area
A subjected to an axial tensile force P. The
stress at any section x-x normal to the line
of action of the tensile force P is specifically
called tensile stress pt . Since internal resistance
R at x-x is equal to the applied force P, we
have,
pt = (internal resistance at x-x)/(resisting
area at x-x)
=R/A
=P/A.
Under tensile stress the bar suffers stretching
or elongation.
Compressive
Stress
If the bar is subjected to axial compression instead of axial tension, the stress developed at x-x is specifically called compressive stress pc. pc =R/A = P/A. Under compressive stress the bar suffers shortening. |
Shear Stress Consider the section x-x of the rivet forming joint between two plates subjected to a tensile force P as shown in figure. |
The
stresses set up at the section x-x acts along
the surface of the section, that is, along a direction
tangential to the section. It is specifically
called shear or tangential stress at the section
and is denoted by q.
q =R/A =P/A. |
Normal or Direct Stresses
When the stress acts at a section or normal to the plane of the section, it is called a normal stress or a direct stress. It is a term used to mean both the tensile stress and the compressive stress.
2.3. Simple and Pure Stresses
The three basic types of stresses are tensile, compressive and shear stresses. The stress developed in a body is said to be simple tension, simple compression and simple shear when the stress induced in the body is (a) single and (b) uniform. If the condition (a) alone is satisfied, the stress is called pure tension or pure compression or pure shear, as the case may be.
2.4. Volumetric Stress
Three mutually perpendicular like direct stresses of same intensity produced in a body constitute a volumetric stress. For example consider a body in the shape of a cube subjected equal normal pushes on all its six faces. It is now subjected to equal compressive stresses p in all the three mutually perpendicular directions. The body is now said to be subjected to a volumetric compressive stress p.
When the stress acts at a section or normal to the plane of the section, it is called a normal stress or a direct stress. It is a term used to mean both the tensile stress and the compressive stress.
2.3. Simple and Pure Stresses
The three basic types of stresses are tensile, compressive and shear stresses. The stress developed in a body is said to be simple tension, simple compression and simple shear when the stress induced in the body is (a) single and (b) uniform. If the condition (a) alone is satisfied, the stress is called pure tension or pure compression or pure shear, as the case may be.
2.4. Volumetric Stress
Three mutually perpendicular like direct stresses of same intensity produced in a body constitute a volumetric stress. For example consider a body in the shape of a cube subjected equal normal pushes on all its six faces. It is now subjected to equal compressive stresses p in all the three mutually perpendicular directions. The body is now said to be subjected to a volumetric compressive stress p.
Volumetric stress
produces a change in volume of the body without
producing any distortion to the shape of the
body.
Strain:
Bending Moment & Shear Force
Beam: Beam is a structural member which is acted upon by a systme of external loads at right angle to its axis.
Bending: Bending implies deformation of a bar produced by loads
perpendicular to its axis as well as forece-couples acting in a plane
passing through the axis of the bar.
Plane Bending: If the plane of loading passes through one of the
principal centroidal axis of inertia of the cross-section of the beam,
the bending is said to be plane or direct.
Oblique Bending: If the plane of loading does not pass through one of
the principal centroidal axes of inertia of the cross-section of the
beam, the bending is said to be oblique.
Bending moment: Algebric sum of the moments of all vertical forces either to the left or to the right of a section.
Shear force: Algebric sum of all vertical forces either to the left or to the right hand side of a section.
Tension & Compression
Tension and compression are directional terms to identify how forces are
acting upon or within a member. If a member (for example, a truss or a
guide rod) is in tension, then the overall forces are pulling away from
it; if the member is under compression, the forces acting upon it are
directed toward the member. Tension can be likened to pulling on the
ends of a rod, whereas compression can be likened to pushing on the ends
of the rod toward the middle.
Tension and compression tests are used to determine the strength of a
material and to develop the material's stress-strain diagram, which
shows the relationship between the stress placed on the material and the
strain experienced by the material.
Tension and compression are directional terms to communicate how forces
are acting upon or within a member. If a member (for example, a truss or
a guide rod) is in tension, then the overall forces are pulling away
from it; if the member is under compression, the forces acting upon it
are directed toward the member. Tension can be likened to pulling on the
ends of a rod, whereas compression can be likened to pushing on the
ends of the rod toward the middle. Tension and compression tests are
used to determine the strength of a material and to develop the
material's stress-strain diagram, which shows the relationship between
the stress placed on the material and the strain experienced by the
material.
Place a flexible object like an eraser, sponge, or small piece of bread between your thumb and index finger. Press your fingers together. One side of the object will bend inwards and shorten while the other will bend outwards and lengthen. The shorter side has been compressed, while the other side is under tension.
Place a flexible object like an eraser, sponge, or small piece of bread between your thumb and index finger. Press your fingers together. One side of the object will bend inwards and shorten while the other will bend outwards and lengthen. The shorter side has been compressed, while the other side is under tension.
http://www.cement.org/tech/cct_cement_characteristics.asp
http://www.buildinglime.org/Tate_Property.pdf
http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/stress.cfm
http://www.youtube.com/watch?v=HzDacklKnNc
http://blog.mechguru.com/machine-design/create-bending-moment-diagrams-in-four-simple-steps/
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