What is residual stress?
The residual stress is an elastic force that can change the distance between atoms in a material without external loads. The residual stress is caused by the presence of non-uniform plastic deformation in a material, among others due to uneven heat treatment or differences in cooling rates in materials undergoing the process. Although the residual stress is not visually visible, in fact the residual stress also acts as a constant load which will add to the value of the work load given from outside.
The existence of residual stresses can benefit or adversely affect the strength of a component, depending on the size and type of load experienced by the component. The distribution of residual stresses which increases the used stresses can be detrimental, and the stresses which are against the direction of action of the used stresses can be advantageous. For cyclic loads, under tensile stresses, the residual surface tension of the tensile can be very dangerous at times as it helps initiate crack growth when subjected to fatigue stress. The manufacturing process may also leave compressive surface residual stresses capable of inhibiting cracking.
Several things that can cause residual stress are:
- Residual stress due to thermal stress. For example the residual stress in the welding process and heat treatment.
- Residual stress caused by phase transformations (such as carbon steel).
- Residual stress due to plastic deformation. Uneven deformation is caused by mechanical forces when cold working during rolling, forging, metal forming or other machining.
Residual stress characteristic
- Very high residual stresses occur around the weld and HAZ (heat affected zone).
- The maximum residual stress value can be equal to the yield stress.
- In materials undergoing phase transformation, for example low carbon steel, residual stresses may vary on the surface and interior of the parent metal.
Very high residual stresses usually occur in the weld area and the heat affected zone (HAZ). It is possible that the maximum residual stress exceeds the yield stress as in the case of metal hardening due to the build-up of dislocation (strain hardening). In materials undergoing phase transformation such as low carbon steel, residual stresses may vary on the surface and interior of the weld and parent metal.
What are the effects of residual stresses?
- In the welding process, residual stress can affect the mechanical properties of the weld structure such as brittle fracture, fatigue and cracking due to a combination of stress and corrosion.
- The effect of residual stress decreases as the working stress acting on the material increases.
- The effect of residual stress on the weld structure can be neglected if the working stress acting on the structure exceeds the yield stress.
- The effect of residual stress decreases after repeated loading.
The effect of residual stress caused by the welding process can affect the mechanical properties of the weld structure such as brittle fracture, fatigue and cracking due to a combination of stress and corrosion (stress-corrosion cracking).
The residual stress arises due to a large temperature difference while distortion occurs when the weld metal is allowed to move freely during the cooling process. The thicker the plates in the Butt Joint and T Joint welding, the smaller the distortion that occurs. This is consistent with the von missed stress analysis, which explains that the thicker the plate used, the more it can reduce the residual stress until it reaches a safe limit. The shape of the welding joint affects the amount of residual stress that occurs. Butt joints are subjected to greater distortion than those of T joints.
The method used to measure the residual stress:
- Cutting method (Sectioning technique)
The residual stress is measured using a strain gauge which works based on changes in electrical resistance.
- Drilling method (hole drilling technique)
The strain gauge is arranged in an angle position of 0o, 45o, and 900 and then a hole is made in the middle. The strain during drilling will be measured by the strain gauge. This strain comes from the relief of residual stresses.
- X-ray method
The working principle of X-ray measurement is based on the residual stress properties that can affect the crystal structure. When the x-rays hit the material, some of the rays are diffracted and form circular patterns that can be seen on the film
- simulation using engineering software
There are two methods that can be used to reduce the residual stresses
- Reduction of residual stress before and during welding. This is done by improving the welding design by the welding engineer. For example, by reducing the heat input or changing the design of the weld joint.
- Reduction of residual stress after welding
Relief of residual stress after welding usually uses the PWHT – Post Weld Heat Treatment method. Besides reducing residual stresses, the PWHT process also improves microstructure and avoids distortion and cracking.