# Finding Strain Without Extension: A Guide

• andrewh21
In summary, the conversation discusses finding the strain in a specimen given the stress and other parameters. The equations for engineering and true strain, as well as Hooke's law and Young's modulus, are mentioned. The correct calculation for the given stress and modulus is determined to be 2.25E-03.

## Homework Statement

hi guys i have a sheet of i have the W,L and T a force and also the elastic modules etc
now my issue is i need to find the strain i have the stress etc but the is no extension figure given i have the usual strain equation e=x/l
can anyone point me in the right direction in regards to the equation [/B]

e=x/l
strain= stress/e

## The Attempt at a Solution

if my stress is 160 mpa and my gpa is 71 am i correct in strain = stress/e
160*10 to the power of 6/71*10 to the power of 9 =2.25 *10 to the power -2[/B]

Last edited:
if my stress is 160 mpa and my gpa is 71 am i correct in strain = stress/e
160*10 to the power of 6/71*10 to the power of 9 =2.25 *10 to the power -2

You have denoted strain as e, and the modulus of Elasticity as e. That's confusing. Use E for youngs modulus. Then check your math. 160 MPa / 70 GPa equals ??

The engineering strain is equal to the change in length of a specimen divided by the original specimen length. True strain can be manipulated so that: true strain = LN (instantaneous length of sample / original length of specimen)

The engineering stress is equal to the force applied to the body divided by the original cross-sectional area of the specimen. During a tensile stress, the length increases and therefore the cross-sectional area reduces within the necking region until fracture. Thus the material should actually experience an increase in stress. And so you use the true stress value, as opposed to the engineering stress. The true stress can be calculated by the force being applied at time, t, divided by the cross-sectional area of the specimen at that instant. This equation can be manipulated to express the true stress in terms of the engineering stress and and engineering strain.

Therefore the true stress = eng stress * (1 + eng strain) for tensile tests. For compression tests, the cross-sectional area increases and so the change in area affects the stress, demanding compensation in the following manner: true stress = eng stress * (1 - eng strain)

Using Hooke's law, the stress can be mathematically expressed as the product of the modulus of elasticity and strain. This is valid only for the elastic region of a stress-strain graph and not for plastic. Therefore, Young's modulus/modulus of elasticity is equal to stress divided by strain. This equation can be used for either true or engineering stress and strain.

In terms of your calculation and if it is only the strain you are interested in calculating, then the strain is indeed given by strain = stress / Young's modulus for any specimen exhibiting elastic behavior. The strain value you are trying to calculate is the corresponding value for when 160MPa of stress is experienced in the specimen. You should find your calculation to equal 2.25E-03.

## 1. What is the purpose of "Finding Strain Without Extension: A Guide"?

The purpose of "Finding Strain Without Extension: A Guide" is to provide a comprehensive guide for scientists and researchers to accurately measure strain in materials without the use of extension methods. This can be useful in various fields, such as materials science, engineering, and geology.

## 2. What is strain and why is it important to measure?

Strain is a measure of the deformation or change in shape of a material in response to an applied force. It is important to measure strain because it can provide valuable information about the mechanical properties and behavior of a material, which can be used in the design and development of new materials and structures.

## 3. How is strain typically measured and why is it necessary to find alternative methods?

Strain is typically measured using extension methods, such as tensile or compression testing. However, these methods can be costly, time-consuming, and may not be suitable for certain materials. Therefore, it is necessary to find alternative methods for measuring strain in order to provide more efficient and accurate results.

## 4. What are some common techniques for finding strain without extension?

Some common techniques for finding strain without extension include optical methods, such as digital image correlation and moiré interferometry, and electrical methods, such as resistance strain gauges and piezoelectric sensors. Other techniques include acoustic emission, ultrasonic methods, and X-ray diffraction.

## 5. What are the limitations of using alternative methods for measuring strain?

While alternative methods for measuring strain can provide many benefits, they also have some limitations. These may include accuracy, sensitivity, and the ability to measure strain in different directions. It is important to carefully consider the limitations of each method and choose the most suitable one for the specific application.