Stress – Strain Relationships Credit: Modified from: 20102/(elasticity).ppt

Stress – Strain RelationshipsStress – Strain RelationshipsCredit: Modified from: Credit: Modified from: http://faculty.ksu.edu.sa/11843/phy%20102/(elasticity).ppthttp://faculty.ksu.edu.sa/11843/phy%20102/(elasticity).ppt

All objects are deformable. It is possible All objects are deformable. It is possible to change the shape or the size of an to change the shape or the size of an object by applying external forces. object by applying external forces. However, internal forces in the object However, internal forces in the object resist deformation.resist deformation.

Stress and Strain:Stress and Strain:Stress:Stress: is a quantity that is proportional to is a quantity that is proportional to

the force causing a deformation. Stress is the force causing a deformation. Stress is the external force acting on an object per the external force acting on an object per unit cross sectional area.unit cross sectional area.

Strain:Strain: is a measure of the degree of is a measure of the degree of deformation. It is found that for sufficiently deformation. It is found that for sufficiently small stresses small stresses strain is proportional to strain is proportional to stress.stress.

The constant of the proportionality The constant of the proportionality depends on the material being deformed depends on the material being deformed and on the nature of deformationand on the nature of deformation

We call this proportionality constant the We call this proportionality constant the elastic modulus.elastic modulus.

The elastic modulus is therefore the ratio The elastic modulus is therefore the ratio of stress to the resulting strain.of stress to the resulting strain.

Elastic Modulus=Stress/Strain Elastic Modulus=Stress/Strain

In a very real sense it is a comparison of In a very real sense it is a comparison of what is done to a solid object (a force is what is done to a solid object (a force is applied) and how that object responds (it applied) and how that object responds (it deforms to some extent) deforms to some extent)

We consider three types of deformation We consider three types of deformation ::and define an elastic modulus for eachand define an elastic modulus for each

1.1. Young’s Modulus: which measures the Young’s Modulus: which measures the resistance of a solid to a change in its resistance of a solid to a change in its length length

2.2. Shear Modulus: which measures the Shear Modulus: which measures the resistance to motion of the planes of a resistance to motion of the planes of a solid sliding past each other solid sliding past each other

3.3. Bulk Modulus: which measures the Bulk Modulus: which measures the resistance of solids or liquids to changes resistance of solids or liquids to changes in their volumein their volume

Young’s Modulus:Young’s Modulus:Consider a long bar of cross sectional Consider a long bar of cross sectional

area A and initial length Li that is clamped area A and initial length Li that is clamped at one end. When an external force is at one end. When an external force is

applied perpendicular to the cross section applied perpendicular to the cross section internal forces in the bar resist distortion internal forces in the bar resist distortion

“stretching” but the bar attains an “stretching” but the bar attains an equilibrium in which its length Lf is greater equilibrium in which its length Lf is greater

than Li and in which the external force is than Li and in which the external force is exactly balanced by internal forces.exactly balanced by internal forces.

In such a situation the bar is said to be In such a situation the bar is said to be stressed. We define the tensile stress as stressed. We define the tensile stress as the ratio of the magnitude of the external the ratio of the magnitude of the external force F to the cross sectional area A. the force F to the cross sectional area A. the tensile strain in this case is defines as the tensile strain in this case is defines as the ratio of the change in length ratio of the change in length ΔΔL to the L to the original length Li.original length Li.

Y=tensile stress/ tensile strainY=tensile stress/ tensile strainY=(F/A)/(Y=(F/A)/(ΔΔL/Li)L/Li)

The Elastic Limit:The Elastic Limit:The elastic limit of a substance is defined as the maximum The elastic limit of a substance is defined as the maximum

stress that can be applied to the substance before it stress that can be applied to the substance before it

becomes permanently deformed. It is possible to exceed becomes permanently deformed. It is possible to exceed

the elastic limit of a substance by applying sufficiently large the elastic limit of a substance by applying sufficiently large

stress, as seen in in the figurestress, as seen in in the figure

Initially a stress strain curve is a straight Initially a stress strain curve is a straight line. As the stress increases, however the line. As the stress increases, however the curve is no longer a straight line.curve is no longer a straight line.

When the stress exceeds the elastic limit When the stress exceeds the elastic limit the object is permanently distorted and it the object is permanently distorted and it does not return to its original shape after does not return to its original shape after the stress is removed.the stress is removed.

What is Young’s modulus for the elastic solid whose What is Young’s modulus for the elastic solid whose stress strain curve is depicted in the figure ??stress strain curve is depicted in the figure ??

Young’s modulus is given by the ratio of stress to strain Young’s modulus is given by the ratio of stress to strain

which is the slope of the elastic behavior section of the which is the slope of the elastic behavior section of the

graph in slide 9 reading from the graph we note that a graph in slide 9 reading from the graph we note that a

stress of approximately 3x10⁸N/m² results in a strain of stress of approximately 3x10⁸N/m² results in a strain of

0.003. The slope, and hence Young’s modulus are 0.003. The slope, and hence Young’s modulus are

therefore 10x10¹ºN/m².therefore 10x10¹ºN/m².

::Shear ModulusShear Modulus

Another type of deformation occurs when Another type of deformation occurs when an object is subjected to a force an object is subjected to a force tangential to one of its faces while the tangential to one of its faces while the opposite face is held fixed by another opposite face is held fixed by another force. The stress in this case is called a force. The stress in this case is called a shear stress. shear stress.

If the object is originally a rectangular If the object is originally a rectangular block a shear stress results in a shape block a shear stress results in a shape whose cross section is a parallelogram. whose cross section is a parallelogram. To a first approximation (for small To a first approximation (for small distortions) no change in volume occurs distortions) no change in volume occurs with this deformation.with this deformation.

We define the shear stress as F/A, the We define the shear stress as F/A, the ratio of the tangential to the area of A of ratio of the tangential to the area of A of the force being sheared. the force being sheared.

The shear strain is defined as the ratio The shear strain is defined as the ratio ΔΔX/H where X/H where ΔΔX is the horizontal X is the horizontal distance that the sheared force moves distance that the sheared force moves and H is the height of the object.and H is the height of the object.

In terms of these quantities the shear In terms of these quantities the shear modulus is modulus is

S= shear stress/ shear strainS= shear stress/ shear strainS= (F/A)/ (S= (F/A)/ (ΔΔX/H) X/H)

Bulk Modulus:Bulk Modulus:Bulk modulus characterizes the response Bulk modulus characterizes the response of a substance to uniform squeezing or to of a substance to uniform squeezing or to a reduction in pressure when the object is a reduction in pressure when the object is placed in a partial vacuum. Suppose that placed in a partial vacuum. Suppose that the external forces acting on an object are the external forces acting on an object are at right angles to all its faces, and that at right angles to all its faces, and that they are distributed uniformly over all the they are distributed uniformly over all the faces. faces.

A uniform distribution of forces occur A uniform distribution of forces occur when an object is immersed in a fluid. An when an object is immersed in a fluid. An object subject to this type of deformation object subject to this type of deformation undergoes a change in volume but no undergoes a change in volume but no change in shape. The volume stress is change in shape. The volume stress is defined as the ratio of the magnitude of defined as the ratio of the magnitude of the normal force F to the area A. the normal force F to the area A. The quantity P=F/A is called the pressure. The quantity P=F/A is called the pressure. If the pressure on an object changes by If the pressure on an object changes by an amount an amount ΔΔP=P= Δ ΔF/A the object will F/A the object will experience a volume change experience a volume change ΔΔV.V.

The volume strain is equal to the change The volume strain is equal to the change in volume in volume ΔΔV divided by the initial volume V divided by the initial volume ViVi

B= volume stress/volume strainB= volume stress/volume strainB=-(B=-(ΔΔF/A)/(F/A)/(ΔΔ V/Vi) V/Vi)

B=- B=- Δ Δ P/(P/(ΔΔV/Vi)V/Vi)

When a solid is under uniform pressure it undergoes a change in volume but no change in shape. This cube is compressed on all sides by forces normal to its 6 faces.

::Prestressed ConcretePrestressed ConcreteIf the stress on a solid object exceeds a If the stress on a solid object exceeds a certain value, the object fractures. The certain value, the object fractures. The maximum stress that can be applied maximum stress that can be applied before fracture occurs depends on the before fracture occurs depends on the nature of the material and on the type of nature of the material and on the type of applied stress. applied stress.

For example concrete has a tensile For example concrete has a tensile strength of about 2 x 10ˆ6 N/m², a strength of about 2 x 10ˆ6 N/m², a compressive strength of 20 x 10ˆ6 N/m², compressive strength of 20 x 10ˆ6 N/m², and a shear strength of 2 x 10ˆ6 N/m.² and a shear strength of 2 x 10ˆ6 N/m.²

It is common practice to use large safety It is common practice to use large safety factors to prevent failure in concrete factors to prevent failure in concrete structures.structures.

Concrete is normally very brittle when it is Concrete is normally very brittle when it is cast in thin sections. Thus concrete slabs cast in thin sections. Thus concrete slabs tend to slab and crack at unsupported tend to slab and crack at unsupported areas as shown in figure A.areas as shown in figure A.

The slab can be strengthened by the use The slab can be strengthened by the use of steal rods to reinforce the concrete as of steal rods to reinforce the concrete as illustrated in figure B. Because concrete is illustrated in figure B. Because concrete is much stronger under compression much stronger under compression “squeezing” than under tension “squeezing” than under tension “stretching” or shear, vertical columns of “stretching” or shear, vertical columns of concrete can support very heavy loads, concrete can support very heavy loads, whereas horizontal beams of concrete whereas horizontal beams of concrete tend to sag and crack.tend to sag and crack.

However, a significant increase in shear However, a significant increase in shear strength is achieved if the reinforced strength is achieved if the reinforced concrete is prestressed as shown in concrete is prestressed as shown in figure C. As the concrete is being poured figure C. As the concrete is being poured the steal rods are held under tension by the steal rods are held under tension by external forces. external forces.

The external forces are released after the The external forces are released after the

concrete cures this results in a permanent concrete cures this results in a permanent

tension in the steel and hence a tension in the steel and hence a

compressive stress on the concrete. This compressive stress on the concrete. This

enables the concrete slab to support a enables the concrete slab to support a

much heavier load.much heavier load.

References:References:Physics For Scientists and Engineers Physics For Scientists and Engineers

(Serway . Beichner).(Serway . Beichner).