Residual Stress

Residual Stress Measurement

X-Ray Diffraction

Residual stresses are vital to the overall strength and life of critical components. Most processes involved in the manufacture of a component can cause residual stresses that will be either detrimental or beneficial to its strength and longevity. Lambda has a distinguished history and is well respected in the field of residual stress measurement with over 100 technical papers and authorship of American Society for Metals, “X-ray Diffraction Residual Stress Techniques”. Our residual stress measurement capabilities and expertise provide our clients with superior product knowledge and solutions to make stronger, longer lasting components. The high spatial and depth resolution possible with the x-ray diffraction method are ideally suited for the study of residual stresses from machining, shot peening, grinding, and similar surface treatments, or for high spatial resolution adjacent to fractures, welds, and similar features. Electrolytic material removal is required for subsurface measurement. The subsurface residual stress distributions are corrected for the penetration of the radiation into the subsurface stress gradient, and for stress relaxation caused by sectioning or layer removal by electropolishing in accordance with SAE HS784, ASTM E915 and E1426. Lambda has developed unique capabilities for automated subsurface residual stress measurement and stress contour mapping.

For additional, downloadable information regarding X-Ray Diffraction:

• Lambda Research Diffraction Notes, No. 19.
• Lambda Research Diffraction Notes, No. 20.
• Lambda Research Diffraction Notes, No. 27.

Features of X-Ray Diffraction Testing

Diffractometers

Lambda uses diffractometers that are custom developed and built in-house specifically for the measurement of residual stresses. Diffractometers are a Bragg-Brentano geometry with a large goniometer radius allowing for higher accuracy measurement compared to standard portable instruments. Our diffractometers are capable of accepting a large range of samples sizes. Our infinitely adjustable incident beam slit system produces a rectangular irradiated area for higher diffracted intensities on complex geometries for more accurate residual stress measurements. Sample holding fixtures allow for highly reproducible sample positioning eliminating errors resulting from sample displacement.

X-Ray Detectors

Lambda uses state of the art, high performance solid-state x-ray detectors for residual stress measurements. Our detectors have the highest energy resolution available, providing a high intensity diffraction peak with very low background radiation, for more accurate assessment of the diffraction peak position. Our detectors are mechanically step-scanned across the diffraction peak to accommodate extremely broad peaks typically found on hardened steel or highly cold worked components that position sensitive detectors cannot accurately fit.

Data Collection Software

Lambda has written data collection and reduction software for accurate and rapid data collection allowing for the proper corrections and peak fitting. Our software corrects the data for the penetration of the radiation into the subsurface stress gradient, and for stress relaxation caused by sectioning or layer removal by electropolishing, in accordance with SAE HS784, ASTM E915 and E1426. Diffraction peaks are fit with a Pearson VII function that is necessary for eliminating errors from varying blending and defocusing of the Ka doublet diffraction peak. Lambda was the first to develop and use the Pearson VII technique of fitting the Ka doublet. (For further information regarding Data Collection Software, download our paper, "The Use of Pearson VII Distribution Functions in X-Ray Diffraction Residual Stress Measurement".)

X-Ray Elastic Constant Determination

Lambda measures the x-ray elastic constants required to calculate the residual stress from the measured strain. It is necessary to determine the x-ray elastic constant for the specific set of crystallographic planes being measured. The bulk elastic properties can vary by as much as 80% from the x-ray elastic constants. Lambda was responsible for developing ASTM specification 1426 detailing the correct procedure for measurement of the x-ray elastic constant. Lambda has a database of x-ray elastic constants consisting of over 200 separate alloys. (For more information on X-Ray Elastic Constant Determination, see our paper "A Method of Determining the Elastic Properties of Alloys in Selected Crystallographic Directions for X-Ray Diffraction Residual Stress Measurement".)

Electropolishing Layer Removal

Subsurface residual stress measurements are recommended in order to fully understand the residual stress distribution. Electropolishing is the only viable means of removing material without inducing residual stresses. Lambda has several specially formulated electropolishing solutions to provide a finish and flatness necessary for accurate residual stress measurement. Proprietary techniques are used to provide an accurate and reproducible means of measuring the depth of removal. Electropolishing depths can be measured to a precision of 0.0001 in. (0.002 mm). Lambda provides rigorous corrections for residual stress relaxation from layer removal. Closed form solutions per SAE HS784 are used. Finite element based layer removal corrects are also available for more complex geometries in cases where the closed form solutions do not apply. (For a complete description of subsurface residual stress measurements, see our paper "Problems with Non-Destructive Surface X-Ray Diffraction Residual Stress Measurement".)

Material Property Prediction from Line Broadening

Lambda has developed methods of predicting material property information using x-ray diffraction line broadening. Line broadening data collected during a standard residual stress measurement can provide material information such as cold working and hardness. Percent cold work distributions can be obtained as a function of position or depth to assess the degree of damage to the surface during processing. Yield strength gradients, with depth and along the surface, can be estimated from the cold work distribution and used to calculate alteration of the residual stress fields during subsequent plastic deformation. Likewise, hardness can be measured as a function of position or depth along with the residual stress on hardened steel components. (See papers, "The Measurement of Subsurface Residual Stress and Cold Work Distributions in Nickel Base Alloys.", and "Current Applications of X-Ray Diffraction Residual Stress Measurement".)

Automated Mapping

Lambda has developed automated sample translation tables to map the residual stresses via x-ray diffraction. The translation system allows our technicians to easily measure a precise set of points automatically. Results can be plotted to provide a high resolution residual stress contour describing areas of high gradients and peak residual stresses.

Severely coarse grained components such as weldments and castings cannot typically be measured employing standard diffraction methods since a statistically significant amount of grains are required to define the diffraction peak. Lambda can use the translation tables to measure the residual stress in coarse grained alloys that cannot be measured using standard diffraction methods. (For more information on Residual Stress Contour Mapping, see Lambda Research Diffraction Notes, No. 19.)

Automated Depth Profiling

Lambda has designed and patented a device that performs automated residual stress depth profiling. The device accepts two samples. Residual stress measurements are made on one sample while the other sample is electropolished. The device alternately electropolishes and measures both samples to rapidly obtain residual stress vs. depth on both samples. The device lends itself to quality control of a large volume of samples or process development for a large design of experiment. (For more information on Automated Depth Profiling, see Lambda Research Diffraction Notes, No. 20 and No. 27.)

Principal Residual Stress Determination

Processes that are inherently directional, such as forming, grinding or turning may induce residual stress fields that vary markedly in magnitude and even sign with direction in the plane of the surface. Such a stress field cannot be adequately defined, and the maximum residual stress cannot be known, without a complete determination of the residual stress in all possible directions. A measurement in any direction could yield results far from the maximum stress. To completely characterize such a residual stress field Lambda can determine the principal residual stresses both at the surface and as a function of depth. Lambda can provide the principal residual stresses and their orientation. For further details on Principal Residual Stress Determination, see Lambda Research Diffraction Notes, No. 3)

Lambda can also determine the axial, circumferential and radial residual stress distributions in circumferential specimens. The radial residual stress component is calculated from the measured circumferential residual stress at the free surface. (See our paper, "The Use of X-Ray Diffraction to Determine the Triaxial Stress State in Cylindrical Specimens").

Supporting Services

Machine Shop

Lambda has a machine shop equipped with state of the art CNC mills and lathes as well as wire and sinker type electrical discharge machines. Large capacity vertical and horizontal band saws, abrasive chop saws, manual mills, drill presses and much more are also available. Lambda’s shop can machine and section all types of test component geometries. Titanium, nickel, iron and aluminum alloy components can be machined or sectioned. Customized fixtures can be designed and manufactured at our facility in support of our testing services.

Strain Gage Application

It is often necessary to instrument the test component with strain gage to characterize the residual stress. Fatigue testing, bulk sectioning, ring core and hole drilling (discussed in detail below) all require the monitoring of strains via strain gages. Lambda has a large supply of various strain gage patterns in stock and have trained technicians for professional strain gage installation.

Ring Core

The use of diffraction for measuring residual stresses is limited for coarse grain materials such as castings and weldments. Mechanical techniques, which involve removing material and monitoring strain relaxation, can often provide the only means of determining residual stresses in coarse grain components. Mechanical methods of measuring residual stress are often a more cost effective and efficient means of determining deep residual stresses in large castings or forgings. They also allow determination of the principal residual stresses as a function of depth.

The ring-core method is a mechanical technique used to quantify the principal residual stresses within a specified depth of material. The technique is based upon linear elastic theory and consists of dissecting a circular plug containing a strain gage. The change in strain is monitored by an on-line computer as a function of cut depth. The ring-core technique can be used on metals, ceramics, and polymers, where linear elastic theory can be assumed. Lambda has a larger stationary ring-core system and a portable system.

Ring core offers the following advantages over hole drilling:

• Higher strain sensitivity.
• Not limited to measuring residual stresses up to only 1/2 the yield strength.
• Less sensitive to errors involved in placement of cut.
• Less sensitive to residual stresses induced by creating cut.

For additional information on the measurement of residual stresses using the ring-core technique, see Lambda Diffraction Notes, No. 31.

Center Hole Drilling

The hole drilling method is a mechanical technique for measuring residual stresses. The method involves drilling a small hole on the inside of a specially designed strain gage and measuring the resulting strain relaxation. Residual stresses are computed from the relaxed strain and calibration coefficients determined for each specific strain gage geometry. Hole drilling is conducted per the ASTM E837 specification.

General Dissection

Measurement of the bulk residual stresses using the general dissection method is, in some applications, a sufficient means of characterizing the residual stresses. The general dissection method involves instrumenting a component with strain gages and measuring the strain relaxation resulting from dissecting the component. The strain relaxation is then used to compute the bulk residual stresses. This method is useful in determining stresses resulting from fit-up, casting or forging processes, where the stresses exist over a relatively large area of the component. Lambda has a number of saws and electrical discharge machines that are available for the general dissection method.

On-Site Residual Stress Measurements

Lambda offers on-site residual stress testing services. Measurements can be made at our clients' facilities on a wide range of components that cannot be readily shipped to Lambda’s facilities for measurement. Lambda provides x-ray diffraction, ring core and hole-drilling methods for on–site residual stress determination.

X-Ray Diffraction Method

• Determination of surface and/or subsurface residual stresses in metal alloy components. Field electropolishing support is available for subsurface measurement.
• Measurements can be made in a wide variety of alloys, including carbon steels, ferritic and austenitic stainless steels, nickel base alloys, aluminums, and titanium base alloys.
• Measurements are possible in many ceramic and composite manufacturing materials.
• Quantification of residual stresses due to processes, such as turning, grinding, milling, welding, shot peening, carburizing and induction hardening.

Ring-Core Method

• Determination of principal residual stresses as a function of depth in coarse grain castings and weldments.
• Higher sensitivity than hole drilling method.
• Measurements are possible in metallic components.

Center-Hole Drilling Method

• On-site determination of subsurface principal residual stress in components which are not readily measured using x-ray diffraction.
• Measurements are made per ASTM specification E837.
• Measurements can be made in all types of metals and non-metals. A polycrystalline material is not necessary for the hole drilling method.