Residual stresses are stresses that are locked into components and structures in the absence of any external load. Residual stresses can be caused by manufacturing processes or by the application of excessive loads that induce plastic material behaviour. Many manufacturing processes involve extreme levels of localised loads and temperatures that impart residual stresses into components. The magnitudes of residual stresses depend on the severity of the process. Effects can vary from 'near-surface' (caused by machining, grinding, etc) to the setting up of fundamental stress systems throughout the component or structure (caused by casting, forging, welding, heat-treatment, etc).
Residual stresses can have harmful effects on the performance of components. These are often tensile in nature, for example, within un-heat-treated castings and welds, which can lead to premature fatigue failures when combined with alternating service loads.
The image below shows a National Gas engine from 1899. The flywheel spokes are curved, rather than radial and straight. This shape introduces flexibility into the structure so that, during the casting process, thermal gradients between the rim and spokes do not lead to excessive tensile residual stresses, and possible cracking in the spokes.
Residual Stresses
Beneficial compressive residual stresses can be introduced into components to extend the component life. Examples of processes to achieve this include, shot-peening, rolling and burnishing.
Accordingly, in order to assess the likely performance of a component, knowledge of the residual stresses levels within that component is of fundamental importance.
Measurement of residual stresses cannot be carried out using conventional stress analysis procedures since the sensor measures only change in stresses (caused by applied loads). Such sensors are insensitive to the residual stresses accumulated during the history of the component.
A number of special methods have been developed to determine the magnitudes and distributions of residual stresses. These include:
Relaxation methods - centre hole drilling, slitting, ring core, deep hole drilling, contour and sectioning.
Diffraction methods - X-ray, Synchrotron and Neutron radiation.
Other methods - ultrasonic, magnetic, thermoelastic and photoelastic methods.
Here at Stresscraft Ltd, we specialise in centre hole drilling using 3-axis, hole drilling machines, highly developed strain gauges and the Integral Method
The origins of the hole drilling method date back to the 1930’s, when Josef Mathar produced papers describing measurements carried out on an I-section beam which utilised a modified end-mill to create the hole and a reflecting extensometer to measure the surface displacement. A host of later workers developed and improved the various activities of hole drilling stage-by-stage, including:
· The use of electrical resistance strain gauge rosettes to measure strains in three directions.
· Miniaturisation of the hole (to a diameter in the range 1 mm to 4 mm).
· Development of various methods of producing the hole, including the use of high speed dental burs, air turbines and medium speed electric motors, orbital drilling and numerical control of the drill head motion.
· Improvements in the methods of computing residual stresses from the relaxed strains, using theoretically (rather than experimentally) derived coefficients resulting, finally, in the Integral Method.
· Standardisation of many details of the method; establishment of the ASTM E837 “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method”.
Subsequent sections describe the method practiced at Stresscraft in greater detail.