Residual stress is stress that remain in the material after certain fabrication and heat treatment operations. Many products require this stress to be relieved prior to application due to the fact it will affect mechanical, chemical, physical performance of the products. For example, a hot rolled steel or aluminum plate will have residual stress, which will manifest in subsequent operations like machining by deflecting during or after machining. Another example is a cast or forged turbine blade will have residual stress which will impact on fatigue and corrosion performance. For all these products, whether they are semi-finished or finished, may require stress relieving operation to reduce the residual stress by stretching or annealing. Even after these operations often some stresses will be left in the product and it is important to know the stresses in the final state.
Two types of measurement methods namely destructive and nondestructive methods are available to measure residual stresses. Nondestructive techniques (NDT) invariably use energetic radiations lie X-ray, gamma-ray, neutron, ultrasonic, etc., to measure residual stresses. Destructive tests use mostly machining methods to measure residual stresses. The choice of methods depends on what type of stresses and where they must be measured. Residual stress in general is not homogeneous and constant. It will vary from surface to interior. Even in stress relieved materials, there will be a distribution of stresses from surface to interior. Nondestructive techniques are useful for measuring surface or near surface stresses or stresses in thin gage materials. In thicker materials due to absorption and extinction of energetic radiations, these techniques are not very useful. Mechanical methods like machining can be used for both thicker and thinner materials. Mechanical method involves basically milling the surfaces, measuring the deflection of the material using strain gages or other deflection measuring devices, back calculating stresses using plane stress or plane strain models.
Capability – At Touchstone Testing Laboratory, we have capability to measure residual stresses in thick plates, forging, casting, etc., using machining method. We conduct stress measurements routinely for aluminum, steel plate manufacturers using milling and deflection measurements using dial gages and displacement transducers.
Some relevant standards:
Touchstone is committed to providing its customers with quality, reliable test results. That is why we have undertaken the rigorous steps needed to meet and secure the most stringent of test lab accreditations including ISO/IEC 17025, NADCAP 7101 (Materials Test Lab) and NADCAP 7122-I (Non-Metallics Materials Testing).