Residual stress on metals

The residual stresses are a mechanical quantity always present in all the metallic components.

Their origin can be attributed to multiple production processes and therefore it is necessary to know the residual stress level of the components in order to improve their fatigue strength.

At world level, the reference standard for measuring residual stresses by the strain-gage hole-drilling method is the ASTM E837-20: this standard is originally and mainly addressed to the application of this method on metals of different types and properties.

The method can be applied not only to standard metallic materials (construction ferritic steels, austenitic stainless steels, aluminium alloys, cast irons), but also to metals with improved mechanical properties for advanced engineering applications (Magnesium alloys, Nickel alloys, Titanium alloys).

The hole-drilling residual stress measurements can be carried out both in laboratory environment and in field applications. In certain particular applications, due to the size of the specimens or to a loading system reproducible only in operation, these measurements can necessarily be carried out directly outside laboratory conditions.

For the reasons above, a specific version of the device operating with an electric motor at high speed (> 50,000 RPM) has been developed. This modified system improves the portability of the system and the simplicity of use in field conditions, for example in remote areas or in absence of a pressure line necessary for using the standard drilling turbine. It has been demonstrated by scientific papers and worldwide experiences that a high-speed drilling technology avoids inducing any residual stress to the material under test.

The high-speed electric motor version supports endmills with diameters up to 4mm, and thus allows to carry out measurements even with larger strain gage rosettes: this guarantees to extend, in accordance with ASTM E837- 20 standard, the total depth of analysis of non- uniform residual stress up to 2mm.

The MTS3000-Restan system in the high-speed electric motor version has the same features of the system operating an air turbine: the measurement chain is therefore automatic and controlled by a dedicated electrical control unit. The entire drilling and data acquisition process is managed by a dedicated software, thus minimizing the influence of the operator in the measurement and consequently increasing the accuracy of the results.

Furthermore, especially in field operations, but also for laboratory applications, the system supports the acquisition of the temperature of the specimens and allows the correction of the strain gauge readings using the dummy strain gauge technique: in this way, even in case of not ideal external conditions, the system allows the correction of the thermal apparent strains and a higher quality of the results.

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