Experimental Residual Stress Analysis

At OCTOGON, we rely on two different methods to perform residual stress measurements: the hole-drilling method and the X-ray diffraction (XRD) method.

Both methods are used to measure residual stresses in structural components and materials to determine load-bearing capacity, fatigue strength, or influences from manufacturing.

Why Residual Stress Measurements Are Important

Residual stresses are mechanical stresses that occur in structural components and materials but are not due to external operating loads. Causes for such residual stresses include manufacturing processes like casting, welding, heat treatments, or mechanical processing. These process-related stresses are in many cases crucial for the behavior of components regarding their operational safety. The problem is that residual and operating stresses can unfavorably superimpose and lead to failure without the intended limit load of the component being reached. Quantitative information about the magnitude and direction of these residual stresses thus determines the operational safety, quality, and performance of the component. A prerequisite for clarifying the causes and evaluating the influence on operational behavior is knowledge of the residual stress state. The determination of residual stresses is carried out by experimental methods. Over the past few years, a variety of methods have been developed that differ according to the respective tasks. Mechanical or X-ray methods are used. Our goal is to provide our customers with residual stress-optimized components.

Mechanical Hole-Drilling Method

The hole-drilling method can be classified as neither a completely destructive nor completely non-destructive measurement method. The intervention in the component to be examined is kept minimal, so that the load-bearing capacity is not impaired.

A special strain gauge rosette is applied to the component to be examined. Then, a small blind hole is drilled through the center of this strain gauge rosette.

This special strain gauge measures the deformations or spring-back at the drill hole, which result from the released residual stresses.

The specially developed strain gauge rosettes have three individual measuring grids arranged relative to each other to simplify the precise positioning of the drill.

The blind hole for releasing the residual stresses usually has a diameter of a few millimeters and is introduced either with a slow cutting speed, for example with an HSS end mill, or with high-speed compressed air turbines and carbide end mills.

The detection of strain using the hole-drilling method occurs in steps. From the obtained measurement data, the residual stresses over the drilling depth and the total strains are then back-calculated.

Process of a Residual Stress Analysis Using the Hole-Drilling Method

Customer-provided component, e.g., machine part or vehicle part Clarification of where residual stresses need to be determined Definition of strain gauge application points If necessary, mechanical adaptation of components, e.g., creation of a 3D CAD model Selection of strain gauges considering the component material and environmental conditions Execution of strain gauge application and wiring Wiring with measuring amplifiers according to customer requirements Calibration of the sensor with a calibration device specially developed for the application

X-Ray Diffraction Method

For near-surface residual stress measurements, we preferentially use the X-ray method with X-ray diffraction (XRD).

The X-ray diffraction determination of residual stresses is based on the principle of X-ray diffraction at the lattice planes of a crystalline material.

The basic idea of X-ray stress determination is to ascertain the changes in lattice plane spacing that occur in an elastically strained material compared to the stress-free state. The determined strains can be converted into stresses using corresponding elasticity constants.

This method is only suitable for crystalline materials. The X-radiation used has a low penetration depth, which allows for high depth resolution. Larger depth ranges can be achieved by stepwise electrochemical removal followed by subsequent measurements.

Process of a Residual Stress Analysis Using the X-Ray Diffraction Method (XRD)

Detailed material analysis of the supplied component Detailed incoming documentation Definition of relevant measurement positions Specification of the residual stress directions to be measured Electrochemical depth removal Reference measurements Meaningful results report

Residual Stress Measurements by OCTOGON

Together with our partners, we realize the economical and reliable determination and evaluation of the residual stress state of your components.

Our equipment also enables residual stress investigations on complex component geometries and various materials.

To provide you with a sound basis for decision-making, you will receive not only a meaningful results report but also detailed incoming documentation of your component.

Inquire today and we will be happy to provide you with a customized offer for the residual stress analysis of your structural and component parts.