Residual Stress: Self-equilibrating internal stresses that remain in an object after the removal of external forces or uneven temperature fields. Mechanical processing and strengthening process can cause residual stress. Such as cold drawing, bending, cutting, rolling, shot blasting, casting, forging, welding and metal heat treatment, etc., due to uneven plastic deformation or phase transition may cause residual stress.
A. Exploration of the causes of residual stress:
Residual stresses are generated when the stress in a workpiece (especially a welded part) exceeds its elastic limit, leading to plastic deformation.
There are three main reasons for these stresses:
1. Thermal changes
2. Phase transformations
3. Machining
Metallic materials are subjected to varying levels of residual stresses during both mechanical and thermal processing. The existence of residual stress has a significant impact on the mechanical properties of the material, therefore, the detection of residual stress is of great significance to the heat treatment process, the surface strengthening treatment process, the effect of the stress relief process and the control and analysis of defective products.
B. Residual stress testing of the main industries and fields:
Aeronautics: aviation vehicle frame, engine casing, engine blades, thin-walled casting or forging parts, hydraulic rods, aircraft landing gear, and welded structural components, etc.;
Aerospace: spacecraft thin-walled parts, modules, brackets, etc.;
Rail transit: car body, bogie, chassis Beams, springs, fasteners, wheels, welded structural parts, etc.;
Machinery Manufacturing: booms, cantilever beams, screws, gears, racks, wind turbine shells and wind turbine towers, etc.
Automobile Manufacturing: engines, welded structures of frames, engine housings, engine blades;
Shipbuilding Industry: welded hulls, marine engine castings, engine blades, etc;
Thermal Spraying Industry: Surface Engineering, Thermal Spraying Surfaces, Ceramic Materials, Rare Earth Modified Materials;
Medical Materials: Medical Devices, Implanted Medical Stents, Artificial Bones etc.
C. Methods of Measuring Residual Stress:
There are a number of techniques that can be used to measure residual stress, which are broadly categorized into three areas: destructive, semi-destructive and non-destructive.
The methods usually used depend on the information required. Due to the complexity of some measurement techniques, measurements must be performed in specialized equipment. This is especially true for many non-destructive techniques.
The three main test method categories:
Destructive test methods include:
1. Contouring.
The contour method determines residual stress by cutting an object in two and measuring a surface height map along the free plane created by the cut.
The average contour determines the deformation caused by the redistribution of the residual stresses and is used to calculate the residual stresses from an elastic finite element model of the sample. The result is a two-dimensional residual stress map perpendicular to the measurement plane.
2. Segmentation method.
The longitudinal slitting method is a technique used to measure thickness residual stresses perpendicular to the plane through the object. It involves cutting a thin slit in increments of depth throughout the thickness of the workpiece and measuring the deformation with respect to the depth of the slit. The residual stress is then calculated from the through-thickness location determined by solving the inverse problem by means of the measured deformation.
Semi-destructive includes:
1. Deep hole drilling.
Deep hole drilling involves drilling a hole through the thickness of an object, measuring the diameter of the hole, cutting a circular groove around the hole to remove a core of material from around the hole, and then re-measuring the diameter of the hole. Residual stresses are determined by geometric changes.
2. Center hole drilling.
Center hole drilling works by drilling a small hole in the object. When the material containing the residual stresses is removed, the remaining material reaches a new equilibrium state which has an associated deformation around the hole. The deformation around the hole is measured during analysis using strain gauges or optical
methods. The original residual stress in the material is calculated from the measured deformation.
Non-destructive testing techniques include:
1. neutron diffraction.
Using neutrons to measure lattice spacing in an object. Neutrons leaving the object have an energy comparable to that of incident neutrons, allowing residual stresses to be determined from lattice spacing.
2. Synchrotron X-ray diffraction. A synchrotron is required to accelerate electromagnetic radiation to gain a thorough understanding of the lattice spacing of an object. The process uses a similar method for neutron diffraction to calculate residual stresses.
3. X-ray diffractometry. This process allows for the measurement of surface residual stresses because X-rays only penetrate a few hundred micrometers into the target surface.
D. Residual Stress Relief Methods:
Common controllable techniques can be used to redistribute or relieve residual stresses to meet design and use requirements:
1. Selections that can be made in the design of the fabrication and in the weld parameters can reduce the formation of residual stresses. For example, welding processing techniques that reduce the thermal gradient within an object will reduce the size of the resulting stresses.
2. Additional steps can be taken after the manufacturing process to significantly reduce the residual stresses contained in the object.
3. This can be accomplished through heat treatment or machining.
4. Post-welding heat treatment is often used to reduce or redistribute residual stresses in welded objects.
From a mechanical point of view, techniques such as shot peening, cold rolling and stretching, and vibratory aging can be used to obtain the desired results.
Residual stresses are harmful!
Metal structural components in the welding, commonly used melting welding method, in the metal filling process, in the joint parts left residual height, crater, biting edge and a variety of welding defects, resulting in severe stress concentration, but also produce a certain welding residual tensile stress.
Residual tensile stress is extremely unfavorable to the fatigue strength of the welded structure, easily lead to cracking, accelerated stress corrosion and welding deformation;
At the same time a lot of research has shown that there are welding defects in the weld toe, the defects are relatively sharp, resulting in a concentration of stress resulting in fatigue cracks sprouted early, resulting in welding cracking phenomenon.
Our team service:
Yunquan test hand in hand with the domestic industry technical experts to provide customers with leading technology, professional team testing services for all types of specimens, workpieces to provide professional residual stress testing technology services.
According to the difference of the customer's products, we can choose
1. to send samples to the laboratory for testing;
2. engineer on-site testing (large components that are not easy to cut)
According to the customer's products, we can provide the detailed testing program for the customer's demand of residual stress testing, and provide the authoritative test report according to the domestic and international testing standards, and provide the stress analysis and stress solution for the customer. analysis and stress solutions.
Residual stress testing service is for the machinery manufacturing industry products (casting/forging/welding) blanks - rough machining - heat treatment - semi-finishing - finishing - finished products, such as the whole process or an important process of residual stress testing services, metal or welding, and other important processes. Stress testing service, metal or non-metal can be tested.
Our advantages: professional team and advanced testing equipment
1. We can establish an exclusive residual stress database for customers, improve the enterprise product qualification testing standards, so that customers can fully understand the whole process or the key process processing, to avoid unnecessary losses.
2. Can provide customers with the use of reasonable heat treatment after the level of residual stress in the product, shorten the product development time, optimize the product structure and process, improve product safety performance, ensure quality, reduce costs, improve product quality control means, improve the yield of customer products, enhance product competitiveness.