Mechanical Strain Release
The mechanical methods, also known as Mechanical Strain Release (MSR) techniques, are destructive techniques which measure residual stresses using strain gages, strain gage rosettes o strain gage chains installed on the surface of the specimens.
The mechanical methods work on the basis of removing material from the component to release stresses by sectioning, turning or milling and hence disturb the state of residual stress equilibrium; the remaining component material then deforms to reinstate equilibrium. The resulting deformations or strains of the remaining component material are recorded using strain gages, strain gage rosettes or strain gage chains and compliance functions are used to back-calculate to the original residual stresses.
All these methods require the installation of one or more strain gage sensors and then, by a dedicated mechanical cutting, the material of the workpiece is removed.
The more typical destructive mechanical strain release methods are:
- Layering (Sach’s boring out or Layer removal)
The Sectioning Method
This method consists in installing one or more strain gage sensors in a specific area of the workpiece. Subsequently, the area to which the strain gage gauges are installed is progressively isolated from the remaining material by sectioning or milling and, as consequence, the relaxed strains are then used to evaluate the residual stresses which have been relieved. Cutting process needs to be carried out without avoiding alternations in the stress field. The sectioning method finds industrial application particularly for the determination of residual stresses inside railway rails made in steel according to the EN 13674-1.
The Slitting Method
An evolution of the sectioning is the Crack Compliance or Slitting method. This is an ideal technique for the measurement of residual stresses on a plane of interest. It requires the installation of 2 strain gages in the cutting surface (front) and opposite surface (rear). In this method, a linear slot is realized with step-by-step depth increments and strain values are then acquired. Data acquired from the strain gage placed on the front surface are used for the evaluation of the surface stresses, and data acquired from the strain gage located in the rear surface are used for the evaluation of the stress distribution in the depth using calculation coefficients developed by finite element model (FEM).
The Layering methods
These methods include the Sach’s boring out and the Layer Removal Method: the Sach’s boring out method can be used only in cylindrical parts while the layer removal method can be used only on flat planes. The methods consist in the installation of strain gage sensors on the surface of the workpiece: then the layers of materials are mechanically removed starting from the opposite side in which strain gages are installed and, at the end of each step of removal, relaxed strains are acquired. Dedicated calculation coefficients evaluated by finite element models (FEM) are used for the evaluation of the residual stresses.
Thanks to the experience of its engineers and technicians and the 20 years company’s background regarding strain gage installation also in uncommon conditions, SINT Technology is able to suggest the best technical solution for the evaluation of residual stresses by mechanical methods.
Further readings & Scientific Papers
- B. Prime, “Residual Stress Measurements by Successive Extension of a Slot: The Crack Compliance Method”, Applied Mechanical Reviews, 52(2):75-95, 1999.
- B. Prime, “Experimental Procedure for Crack Compliance (Slitting) Measurements of Residual Stress”, LA-UR-03-8629, Los Alamos National Laborotary Report, 2003.
- Sachs, “Nachweic innerer spannungen in stange und rohren”, Zitschrift fur Metalkunde, vol. 19, pp. 352–357, 1927.
- Tebedge, G. Alpsten and L. Tall, “Residual-stress Measurement by the Sectioning Method”, Experimental Mechanics 13(2):88-96, 1973.
- G. Treuting and W. T. Read Jr., “A Mechanical Determination of Biaxial Residual Stress in Sheet Materials”, Journal of Applied Physics 22:2, 130-134, 1951.
- Shadley, J.R., Rybicki, E.F. and Shealy, W.S. (1987), Application guidelines for the parting out step in a through thickness residual stress measurement procedure. Strain, 23: 157-166. doi:10.1111/j.1475-1305.1987.tb00640.x
- Schajer G.S., “Practical Residual Stress Measurement Methods”, Wiley, 2013.