This stress calculator will help you solve the problems in mechanics involving stress, strain, and Young's modulus. In a few simple steps, you will learn the stress vs. strain relationship for any material that remains elastic. We will also teach you how to calculate strain and how to apply the stress equation.
How to calculate strain and stress
Strain is defined as the measure of deformation - a proportion between the change of length and original length of an object. For example, if you take an elastic band and stretch it so that it is twice longer than initially, then the strain will be equal to 1 (100%).
The formula for strain is:
ε = ΔL/L₁ = (L₂ - L₁)/L₁
L₁ denotes the initial length,
L₂ - the final length, and
ΔL is the change in length. Note that strain is dimensionless.
Stress, on the other hand, is the measure of pressure that the particles of a material exert on each other. It is defined as the force acting on the object per unit area. It is different from the pressure, though; when calculating stress, the area considered must be so small that the analyzed particles are assumed to be homogeneous. If we take into account a bigger area, the calculated stress is usually the average value.
The stress equation is:
σ = F/A
F denotes the force acting on a body and
A denotes the area. Units of stress are the same as units of pressure - Pascals (symbol: Pa) or Newtons per squared meter.
Positive stress means that the object is in tension - it "wants" to elongate. Negative stress means that it is in compression and "wants" to become shorter.
Do you know?
There are two types of strain — engineering and true strain. Find out more in our true strain calculator
Young's modulus (stress vs strain)
If the material is linearly elastic, then the stress and strain are directly related with the following formula:
E = σ/ε
E is the modulus of elasticity, or the Young's modulus. It is a material constant, different for each substance.
What exactly is a linear elastic behavior of a material? If we apply stress to a material, strain increases proportionally. This may be true for some range of stress only - after we reach a certain value, the material may break or yield. Yielding is the increase of strain in a constant stress state.
An example of calculations
Let's assume we want to find the Young's modulus of steel. To do it, we prepared a steel rod that was pulled with a high force.
- We decide that the force used to pull the rod will be equal to 30 kN (
- We determine the dimensions of the rod. Let's assume the length of 2 m (2,000 mm) and a cross-sectional area of 1 cm² (
- We observed that the rod elongated by 3 mm.
- We calculate the strain is the rod according to the formula:
ε = ΔL/L₁ = 3/2000 = 0.0015.
- We calculate the stress, using the stress formula:
σ = F/A = 30*10³ / (1*10⁻⁴) = 300*10⁶ = 300 MPa.
- Finally, we divide the stress by strain to find the Young's modulus of steel:
E = σ/ε = 300*10⁶ / 0.0015 = 200*10⁹ = 200 GPa.
Modulus of elasticity units
The units of the Young's modulus are the same as the units of pressure and stress: Pascals or newtons per square meter. In SI units,
1 Pa = 1 N / 1 m² = 1 kg·m / s² / m² = 1 kg / (m·s²)
What does it mean if Young's modulus is high?
The higher the modulus of elasticity, or Young's modulus, the stiffer the material. This means it can withstand a greater amount of stress.
What type of stress acts on a pillar's particular cross-section due to segment above it?
The stress on a pillar's cross-section is negative or compressive stress, due to the weight of the segment above said cross-section.
How do I estimate the stress at a particular cross-section of a pillar?
To estimate the stress at a particular cross-section of a pillar:
- Find the weight of the segment above it.
- Substitute the value of weight for force in the formula for stress, σ = F/A, where F is the force and A is the area of cross-section.
- Yay! You just found out the stress acting on the cross-section!
What is the difference between yield and ultimate tensile strength?
A solid material's yield strength is the maximum tensile stress it can handle before permanent deformation occurs. Ultimate strength is the maximum stress it can withstand before failure.