What are residual stresses and how are they caused
All stresses that exist in materials, also without the application of any external loads, are termed residual stresses. Residual stresses can originally exist in a component and naturally add to stresses induced by applied loads.
As a result, residual stresses influence the behavior of mechanical components and can impair the structural and dimensional stability, as well as the fatigue and fracture resistance of components. A residual tensile stress actually facilitates crack propagation and therefore reduces the fatigue life of a mechanical component.
Residual stresses limit the loading capacity and safety of mechanical components during operation and in certain circumstances it is neccesary to be able to quantify those stresses.
Residual stresses can be caused by the following main factors:
- non-uniform heating or cooling of a component during manufacturing and fabricating processes (e.g., casting, welding, molding, heat treatment)
- machining processes to remove shavings or plastic deformation (e.g., turning and forging)
- through or surface heat treatments (e.g., tempering, welding or grinding)
- sand-blasting or shot-peening.
The hole-drilling method for residual stress measurements
The hole-drilling method allows accurate experimental stress analyses at moderate costs. It consists in drilling a small hole (typically 1.8 – 2.0 mm) which changes the initial deformation allowing redistribution of the residual stresses locked in a material).
The strains that are caused in this way can be measured by a specially configured three-element strain gage rosette and then used in special calculation systems to determine the residual stresses that exist in a material.
Briefly summarized, the residual stress measurement procedure involves the following steps:
- A strain gage rosette with three radial grids is installed
- A through hole or a blind hole is drilled through the geometric center of the rosette
- Readings are made of the strains produced by relieving the residual stresses
With the strength of many years of experience in the field of strain gage measurements, mechanical design engineering and software development, SINT Technology has developed and patented a fully automated system for measuring residual stresses.
Residual stress measurements by the hole-drilling method
SINT Technology uses its own system, the MTS3000, also known as RESTAN (Residual Stress Analyzer), which allows residual stresses to be measured by the hole-drilling method and the data acquired by that technique to be processed by four possible calculation systems:
- the ASTM E837-08 Standard Test Method for uniform stresses
- the ASTM E837-08 Standard Test Method for non-uniform stresses
- the Integral Method
The first method allows the residual stress measurement in the case of uniform distribution. The other 3 methods allow measurement of residual stress variation in the depth. Consequently, the system provides a complete analysis of relieved strain distribution and of near-surface residual stress profiles on completion of testing.
Use of the MTS3000 system therefore offers two advantages:
- flexibility in the acquisition stage with automatic control of the selected strain gage amplifier system and selection of the most suitable testing conditions for specific requirements
- fast back-calculation of residual stress from recorded measurements using complex algorithms and allowing comparison with a number of calculation methods.
Thus, use of the RESTAN technology plus the presence of expert engineers allow the best results to be achieved for the most diverse requirements. SINT Technology engineers have both experience and expertise in measurement of residual stresses and technical knowledge of the RESTAN system, being directly involved in managing the quality of the product and in the setting, calibration and inspection and testing processes. SINT Technology is a ISO/IEC 17025 accredited laboratory which provides residual stress determination by the hole-drilling method, both for laboratory work and field tests. Furthermore, SINT Technology has experience on other mechanical methods for residual stress measurements, like the Ring-Core method, the Sachs’ method, layer removal and sectioning methods.
Residual stress measurements: Restan-Ringcore
In addition, SINT Technology has developed the RESTAN-RINGCORE system for determining residual stresses by the ring core method, which allows residual stresses to be determined up to a depth of 5 mm and is particularly suited to many applications not requiring the surface measurement. The method is applied on forgings or castings of rotating machinery components (compressors, turbines etc).
The system is completely automatic: during the measurement process an electronic control unit and control software control progression and drilling with a crown milling cutter.
The system can be used for determining residual stresses by the ring core method in accordance with the KWU standard.