In solid materials, residual stress is stresses that remain in them after their original cause of stress is removed. Residual stress can be useless and useful in a material. that is, it may be desirable or undesirable. Unintended residual stress in a material may cause it to fail prematurely, while it can be used in toughened glass to allow large, thin, crack- and scratch-resistant on smartphone glasses.
Today we’ll be looking at the definition, applications, types, causes, effects, diagram, control, and how to measure residual stresses.
What is residual stress?
Residual stress is the stress that remains in a material (common in a welded component) even in the absence of external loading or heat gradient. Residual stresses can result in significant plastic deformation, which may lead to distortion or warping of materials. They may affect susceptibility to fracture and fatigue in some cases.
Residual stresses are also said to be locked-in stresses within a metal object, even as the object is free of external forces. The stresses may result in one region of the metal being constrained by adjacent regions from contracting, expanding, or releasing elastic strains. Since residual stresses can be tensile or compressive, they can co-exist within a component.
What are the causes of residual stress?
Residual stresses occur when objects or components are stressed beyond their elastic limit, resulting in plastic deformation. The plastic deformation may be because of the following:
- Phase transformations during cooling from elevated temperatures.
- Surface treatments like enameling, electroplating PVD and CVD coating.
- Non-uniform plastic deformation during heating and cooling.
- Heterogeneity of a chemical or crystallographic order (nitriding or case hardening)
- Non-plastic deformation during mechanical processing like rolling, forming operations (bending or drawing), machining, and mechanical surface treatments (shot peening and roller burnishing).
The following are the three causes of residual stress:
The thermal variation is when an object is cooled from a temperature, this often takes place in a welded joint due to the intense heat used for the joining. Because of this, there is a large difference in the cooling rate throughout the body which results in localized variations in the surface and interior of the material. This different level of thermal contractions creates non-uniform stresses within an object.
During cooling, the surface cools at a quicker rate, and the heated material is compressed at the center. While the center part of the material takes time to cool, it’s constrained by the cooler outer material. This causes the inner portion to have residual tensile stress and the outer portion of the material will have residual compressive stress.
Read more: Hot and Cold Rolling Process
Residual stresses that occur due to plastic deformation are caused by mechanical processing. It occurs when the plastic deformation is non-uniform through the cross-section of the material undergoing a manufacturing process, such as bending, drawing, extruding, rolling, etc.
During the deformation process, one part of the material is elastic and the other is plastic. So, when the load is removed, the material tries to recover the elastic part of the deformation. However, it’s inhibited from fully recovering because of the adjacent plastically deformed material.
Phase transformation is another way residual stress can be caused. It occurs when a material undergoes a phase transformation, that is, a volume difference between the newly formed phase and the surrounding material which is yet to undergo the phase transformation. This volume difference cause expansion or contraction in the material, resulting in a residual stress
What are the effects of residual stress?
Residual stresses are beneficial in some situations, depending on whether the stress is tensile or compressive. Tensile residual stresses can be large enough to cause materials distortion or cracking. Also, tensile stresses are required in fatigue and stress corrosion cracking. This is because residual stresses are algebraically summed with applied stresses. The surface residual tensile stresses combined with applied tensile stress can reduce the reliability of the material. Also, residual tensile stresses are sometimes sufficient to cause stress corrosion cracking.
Generally, surface residual compressive stresses reduce the effects of applied tensile stresses. In fact, surface compressive stresses contribute to the improvement of fatigue strength and resistance to stress-corrosion cracking.
Just as earlier stated, residual stresses are either positive or negative, depending on the application. Positive effects are obtained when residual stresses are implemented in the designs of some applications, which can achieve by laser peening. A laser peening imparts compressive residual stress to the surface of a material. This toughens brittle surfaces or strengthens a thin section.
Generally, residual stresses can also result in negative effects. Although the stresses are often invisible to manufacturers, only if they result in significant distortion. Structural integrity can negatively be affected, for instance, welded thick-walled structures are more prone to brittle fracture than structures that are stress relieved.
Types of residual stresses
Below are the different types of residual stresses:
Tensile residual stresses:
Tensile residual stresses decrease the fatigue strength and result in fatigue failure. They are usually the side effects of production that result in aggressive grinding which causes crack growth. They can also introduce shrinking, fitting, bending, or torsion. Torsion always remains on cast components as residual stresses which may cause cracking on the component surface. Furthermore, stress corrosion cracking is an event that occurs where there are tensile residual stresses.
Compressive residual stresses:
Compressive residual stresses increase both fatigue strength and resistance to stress corrosion cracking. They can be intentionally formed by processes like shot peening, laser peening, low plasticity burnishing, and autofrettage. The material strain hardens or cold works the material. Mostly, the importance of inducing compressive residual stresses is to balance the detrimental effects of tensile stresses. The heat treatment process which is known as stress-relief annealing is also used to reduce the residual tensile stress.
Residual stresses can also be classified into three types:
Type-1 residual stress:
These types of residual stresses are known as macro-residual stress often developed in grains. This is as a result of any change in the equilibrium of the residual stress which will result in a change in macroscopic dimensions. Treatments or processes that cause the inhomogeneous distribution of strains will produce type-1 residual stresses.
Type-2 Residual Stress:
Types-2 residual stresses are micro-residual stress developed in one grain. They can of different sizes in different grains. Martensitic transformation is best in producing this residual stress. During the transformation process, an incomplete transformation of austenite is gotten. However, the volume of martensite is larger than the austenite, resulting in different forms of residual stresses.
Type-3 Residual Stress:
Type-3 residual stresses are sub-micro residual stress developed within several atomic distances of the grain. Their formation is caused by crystalline defects such as vacancies, dislocations, etc.
watch the video below to learn more about Residual stresses:
How to measure residual stresses?
There are different techniques that can be used in measuring residual stress, which is broadly categorized into Destructive, Semi-destructive, and Non-destructive. They are often used depending on the information required.
Let’s get to understand these residual stress measuring techniques!
The destructive measuring techniques of residual stress are performed by destroying the object or material to be measured. They are generally carried out for research and development purposes. And it’s a cheaper method of measuring and detecting residual stress when compared to non-destructive testing.
Destructive testing can be done in two ways:
- Contour method
Contour methods determine residual stress by cutting an object into two and measuring their surface height maps towards the free plane created by the cut. This method determines the deformations caused by residual stress distribution and is used to know the amount of residual stress through an elastic finite element model of the specimen. The result is a 2-D map of residual stress which is normal to the measurement plane.
- Slitting method
Slitting methods are techniques used for measuring the thickness of the residual stress normal to a plane cut through an object. This involves cutting a thin slit in increments of depth through the thickness of the workpiece. The resulting deformation measured is achieved by the slit depth. And the residual stress is calculated by the through-thickness position, which is determined by solving an inverse problem using measured deformations.
A semi-destructive residual stress measuring technique is similar to a destructive type. This is because they use a strain release principle to determine the residual stress. But only a small amount of material was removed rather than destroyed. It allows the structure to better maintain its integrity.
Semi-destructive testing is also performed in two ways:
- Deep hole drilling
Deep drilling is achieved by drilling a hole through the thickness of a material, measuring the diameter of the hole. Cutting a circular slot around the hole to remove a core of material from around the hole and then re-measuring the hole diameter. Residual stresses are discovered here by the geometric change.
- Centre hole drilling
Centre hole drilling techniques are done by drilling a small hole into an object. So, when the material containing residual stress is removed the remaining material reaches a new equilibrium state. With this, the deformations around the hole are associated. Strain gauges or optical methods are used to measure the deformations around the hole during analysis. The original residual stress in the material is calculated from the measured deformations.
Non-destructive is another method of measuring and testing residual stresses in a material. It involves measuring the effects of relationships between the residual stresses and their material changes in the crystal lattice spacing.
The non-destructive method can be achieved in three ways:
- Neutron diffraction
Neutrons are used to measure the crystal lattice spacing in a material. The neutrons existing in the object have comparable energy to the incident neutrons. This allows the residual stress to be determined from the lattice spacing.
- Synchrotron X-ray diffraction
A synchrotron is used to accelerate electromagnetic radiation to allow a true thickness that knows the material lattice spacing. A similar approach to neutron diffraction is used to calculate the residual stress.
- X-ray diffraction
Measurement of surface residual stress is achieved with this method since the X-ray only penetrates the object’s surface by a few hundred microns.
How to control residual stress
Controlling residual stress is common on material since it will be of benefit to requiring some sort of stress on applications. The materials are exposed to fatigue or stress corrosion cracking conditions or if the residual stresses are large enough to cause component deformation or cracking.
Controlling residual stresses can be achieved by mechanical treatments such as shot peening, light cold rolling, and stretching. Small amounts of compressing are used to induce compressive residual stress at the surface of a component. Stress relief heat treatment, control of the heat-treating process, and alloy selection are other methods of controlling residual stresses.
Since metal yield strength decreases as its temperature increases, metals can be stress relieved by heating to a temperature where the yield strength of the metal is the same or less than the magnitude of the residual stress. If this happens, the metal can undergo microscopic plastic deformation, which will release at least a portion of the residual stress. After the stress-relieving, the maximum residual stress left in the object will be equal to the yield strength of the material at the stress-relieving temperature.
Residual stresses can be reduced by using reduced cooling rates to step down temperature variations so that phase transformations can occur more uniformly throughout a component’s cross-section. Well, this will be based on the component processing perspective. In this case, alloys of slower cooling rates can be selected, while the desired phase transformations will still occur.
Read more: What are the types of casting processes?
Residual stresses have been seen to be positioned or negative on the material. They can occur and reduce the longevity of the material and also, they can purposely implement in an object to get some certain residual stresses. In this article, we’ve seen the definition, types, causes, effects, control, and how to measure residual stresses.
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