MTS3000 - The system for automatically determining residual stress using strain gages

The strength behavior of components is influenced by their existing residual stresses without showing any visible signs. Therefore, the aim is to determine the mechanical stresses in the components. With the hole-drilling method for determining residual stresses, a small hole of 1.6mm diameter is drilled into the work piece and strain gages are used to measure the resulting strain.

SINT Technology offers both the required amplifier and the MTS3000 system which enable this process to be implemented comfortably. The system uses a stepping motor which permits drilling at 300,000 rpm. The strain changes arising due to the step-by-step drilling of the hole into the work piece will be detected by a strain gage rosette specifically designed for this process.

Signal processing is performed digitally. In addition to system control functions, the software package comprises four different evaluation algorithms which enable the mechanical stresses to be computed from the measured strain. The entire measurement process is PC-controlled. This ensures a high degree of measurement reliability as well as optimum reproducibility.

SINT Technology

Residual Stress: Measuring internal forces in the material

Residual stresses arise in material due to the effects of internal forces without any external forces acting. For example, Residual stresses arise by non-uniform cooling of cast components, by welding or forging of the parts, or by mechanical processing.

Two strain gage based technologies are often used for determining residual stresses:

• The ring core method
• The hole drilling method

The common feature of both methods is that after installation of the strain gage rosette on the workpiece, the residual stress state is disturbed by mechanical interference. After thisinterference, the residual stresses cause strains within the surface of the workpiece which are measured with strain gages, and afterwards are used for calculating the residual stress state.

With the ring core method, a ring-shaped groove is extracted around the respective strain gage.

With the hole drilling method, a milling cutter is used to drill a hole in the center of the strain gage rosette. For this purpose, two methods can be used alternatively:

• The integral method: The result of the integral method is the mean value of the residual stresses over the total depth.
• The high speed drilling method: SINT Technology offers the MTS3000 system for the high speed drilling method. A milling cutter rotating at 300,000 rpm and advanced by a stepper motor is used for hole drilling. The strain changes arising due to the step-by-step drilling of the hole into the workpiece are detected by a strain gage rosette specially designed for this method.