Mechanics of Materials: Strain » Mechanics of Slender Structures | Boston University
Strain is the ratio of change in dimension to the original dimension. Strain occurs in a body when it undergoes any kind of deformation due to. constitutive relationship of linear materials. ◇ Know how to compute normal and shearing strains and stresses in Stress & Strain: Axial Loading. • Suitability of. Strain is thus, a measure of the deformation of the material and is a So we have two types of strain i.e. normal stress & shear stresses. i.e. We will have the following relation. Therefore, a strain at any point in body can be characterized by two axial strains i.e Îx in x direction, Îy in y - direction and gxy the shear strain.
It is simply a ratio of two quantities with the same unit. Since in practice, the extensions of materials under load are very very small, it is often convenient to measure the strain in the form of strain x i.
Sign convention for strain: Tensile strains are positive whereas compressive strains are negative. The strain defined earlier was known as linear strain or normal strain or the longitudinal strain now let us define the shear strain. An element which is subjected to a shear stress experiences a deformation as shown in the figure below. The tangent of the angle through which two adjacent sides rotate relative to their initial position is termed shear strain.
As we know that the shear stresses acts along the surface.
The action of the stresses is to produce or being about the deformation in the body consider the distortion produced b shear sheer stress on an element or rectangular block This shear strain or slide is f and can be defined as the change in right angle.
So we have two types of strain i.
Shear Stress Strain
A material is said to be elastic if it returns to its original, unloaded dimensions when load is removed. Within the elastic limits of materials i. There will also be a strain in all directions at right angles to s. The final shape being shown by the dotted lines. It has been observed that for an elastic materials, the lateral strain is proportional to the longitudinal strain. The ratio of the lateral strain to longitudinal strain is known as the poison's ratio. Consider an element subjected to three mutually perpendicular tensile stresses sxsyand sz as shown in the figure below.
In the absence of shear stresses on the faces of the elements let us say that sxsysz are in fact the principal stress. The resulting strain in the three directions would be the principal strains.
We will have the following relation.
For Two dimensional strain: When a force acts parallel to the surface of an object, it exerts a shear stress. Let's consider a rod under uniaxial tension.
The rod elongates under this tension to a new length, and the normal strain is a ratio of this small deformation to the rod's original length. Strain is a unitless measure of how much an object gets bigger or smaller from an applied load. Shear strain occurs when the deformation of an object is response to a shear stress i. Mechanical Behavior of Materials Clearly, stress and strain are related.
Stress and strain are related by a constitutive law, and we can determine their relationship experimentally by measuring how much stress is required to stretch a material. This measurement can be done using a tensile test. In the simplest case, the more you pull on an object, the more it deforms, and for small values of strain this relationship is linear.
This linear, elastic relationship between stress and strain is known as Hooke's Law. If you plot stress versus strain, for small strains this graph will be linear, and the slope of the line will be a property of the material known as Young's Elastic Modulus.
What're Strain, Stress and Poisson's Ratio? | KYOWA
This value can vary greatly from 1 kPa for Jello to GPa for steel. In this course, we will focus only on materials that are linear elastic i.
From Hooke's law and our definitions of stress and strain, we can easily get a simple relationship for the deformation of a material. Intuitively, this exam makes a bit of sense: If the structure changes shape, or material, or is loaded differently at various points, then we can split up these multiple loadings using the principle of superposition.
Generalized Hooke's Law In the last lesson, we began to learn about how stress and strain are related — through Hooke's law. But, up until this point we've only considered a very simplified version of Hooke's law: In this lesson, we're going to consider the generalized Hooke's law for homogenousisotropicand elastic materials being exposed to forces on more than one axis. First things first, even just pulling or pushing on most materials in one direction actually causes deformation in all three orthogonal directions.
Let's go back to that first illustration of strain. This time, we will account for the fact that pulling on an object axially causes it to compress laterally in the transverse directions: This property of a material is known as Poisson's ratio, and it is denoted by the Greek letter nu, and is defined as: Or, more mathematically, using the axial load shown in the above image, we can write this out as an equation: Since Poisson's ratio is a ratio of two strains, and strain is dimensionless, Poisson's ratio is also unitless.
Poisson's ratio is a material property. Poisson's ratio can range from a value of -1 to 0. For most engineering materials, for example steel or aluminum have a Poisson's ratio around 0.
Incompressible simply means that any amount you compress it in one direction, it will expand the same amount in it's other directions — hence, its volume will not change. Physically, this means that when you pull on the material in one direction it expands in all directions and vice versa: We can in turn relate this back to stress through Hooke's law. This is an important note: In reality, structures can be simultaneously loaded in multiple directions, causing stress in those directions.
- Mechanics of Materials: Strain
A helpful way to understand this is to imagine a very tiny "cube" of material within an object. That cube can have stresses that are normal to each surface, like this: