Experimental Residual Stress Analyses

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 manufacturing influences.

Why Residual Stress Measurements Are Important

Residual stresses are mechanical stresses that occur in structural components and materials but are not due to external operational loads. Causes for such residual stresses include manufacturing processes such as casting, welding, heat treatments, or mechanical processing. In many cases, these process-induced stresses are crucial for the behavior of components regarding their operational safety. The problem is that residual and operational stresses can unfavorably superimpose and lead to failure, even without reaching the component's intended load limit. 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 assessing 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, which 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 either a destructive or non-destructive measurement method. The intervention in the component to be examined is kept small 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 springbacks at the borehole, which result from the released residual stresses.

The specially developed strain gauge rosettes have three individual measuring grids arranged with respect 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 is carried out step by step. From the acquired measurement data, the residual stresses over the drilling depth and the total strains are then recalculated.

Procedure for 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 Determination of strain gauge application points If necessary, mechanical adaptation of the 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 via X-ray diffraction (XRD).

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

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

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

Procedure for a Residual Stress Analysis Using the X-ray Diffraction Method (XRD)

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

Residual Stress Measurements from 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 allows for residual stress investigations on complex component geometries and various materials.

To provide you with a sound decision-making basis, you will receive not only a meaningful results protocol but also detailed initial 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 components and parts.