Stress and Strain
A modulus of elasticity or young’s modulus (Elastizitätsmodul) coming from experiment measures a substance’s resistance to being deformed elastically when a stress is applied to it.
Yield strength is a point at which any material starts to deform. Yield strength value is an important factor for choosing material, which must perform in the elastic region under normal operating temperature and pressure. The yield strength is highly dependent on the temperature influencing the crystals.
Below the yield point, a material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible and is known as plastic deformation.
Upper yield point (Obere Streckgrenze): the upper yield strength represents the point at which a sudden leveling, or drop in stress or load occurs as a material transitions from elastic to plastic deformation. It also marks the beginning of yield point elongation (YPE).
As proof stress is determined for the materials whose yield point cannot be easily defined ( Aluminium, Magnesium, Copper), it is called as offset yield stress. The 0.2% offset yield strength is defined as the amount of stress that will result in a plastic strain of 0.2%. After 0.2% Strain, the material is not strong anymore.
Thermal shock: leads to excessive thermal gradients on materials which can cause fracture.
If thermal stress (modulus of elasticity* strain)> yield strength, there will be a fracture.
How to calculate thermal expansion due to blocking in a heat exchanger?
V1=1 m3, P1=1 atm, ID=20 mm OD=25 mm
V2=1.5 m3, P2=?
Solution:
- A= 3.14*(OD-ID)²/4
- Volume difference= V2-V1
- Length difference can be calculated
- P2=F/A
