In materials, there are always internal stresses (also known as retained stress) due to fabrication and treatment processes of materials and products. For example, during the hardening of steel significant internal stresses form. In addition, cold forging and rolling of metals will induce high stresses that are not uniformly distributed. Another well-known example is the induction of stresses by welding.
Residual or retained stresses are dangerous in construction materials because they contribute to external stresses. This can result in exceeding of the yielding point of the material, which the external stresses alone may not have done. Also, unwanted distortions can occur during mechanical treatment such as milling, which can lead to expensive corrective actions, including straightening or thermal stress relieving.
Obviously, residual stresses should be prevented or kept as low as possible. To determine the level of internal stress, several techniques are available, including strain gage measurement and ultrasonic measurement. Residual stress can be performed onsite as well as in a laboratory setting.
For over twenty years, Element has performed retained stress measurements regularly and has gained wide experience in this field. Element residual stress measurement experts have developed many innovative procedures for the benefits of our clients, including methods using the microscan and Rigaku in combination.
Strain Gage Technique to ASTM E837-08 Testing
The principle used to determine residual stress is based on stress-lessly drilling a hole in which the stress in the material surface around the hole relaxes (ASTM E837-08). This relaxed stress is a measure of the residual stress present at the appropriate location at that moment. The relaxed stress is measured indirectly by measuring the local strain using strain gages. Changing the occuring strain causes a change of resistance of the strain gage. This change can be accurately measured because the strain gage is configured in a Wheatstone bridge, and the bridge-voltage varies with the change of the resistance of the strain gauge. Because the change of the strain has a linear gradient in the elastic area, the strain is proportional to a certain stress (Hooke's law).
In theory and in practice it appears that when there is a hole in a flat plate, the stress caused by the stress concentrations—in a bi-directional-stress-plane—will increase by a factor 2. Based on this information it can be suggested that during hole-drilling the stress should remain lower than ½ the yield strength, so no plastic deformation around the hole occurs. Therefore ASTM E837-08 describes that this method is only valid if the calculated stress do not exceed 50% of the yield strength of the material. When this is no longer valid, the accuracy of this measurement is no longer guaranteed. In this case, the result of the measurement gives only qualitative information. It is also true that the more uniformly the stress is distributed in the material, the more reliable are the measurements generated by this technique.
X-ray residual stress measurement (RSM, RIGAKU stress analyzer)
Internal stresses can be quantified using RSM. The atoms (of a material) are all packed in a lattice. The distance between the planes of the lattice is an absolute
measure for the level of internal stress, and can be determined by using RSM. This method uses sharp focussed X-rays and a detector that are both mounted on a goniometer.
By varying the angles between the X-rays and the surface, and between the X-rays and the detector, a sophisticated computer program can determine the stresses in a quantitative way. This measurement will give the stress in exactly one direction at a time, therefore at least two measurements are needed to find the combined stress at a given position.
The advantage of RSM is that it is self-calibrating and gives the stress values directly in MPa (N/mm2) with high accuracy (depending on the absolute level of stress, an accuracy of several MPa's is feasible). The RSM method can be applied to all crystalline materials like metals and crystalline polymers. In most cases, the surface should be prepared by electrolytic polishing.
Barkhausen-noise analysis (American Stress Tech. Microscan)
Residual stress can be determined in a qualitative way using microscan measurements. This method can only be applied to ferro-magnetic materials. It uses a low frequency magnetic field that is induced in the surface locally. A very sensitive magnetic sensor detects the electromagnetic reaction of the atoms at the surface, the so called Barkhausen-noise. From this signal a computer program determines a qualitative value for the stress level, the Magnetic Parameter (MP).
The measurement will give a good impression of the stress situation of a product because positions of stress concentrations are easily found. The advantages of this method are the speed of measurement (several seconds per position), the good mobility, the non-destructive character of the process and the fact that preliminary surface preparation is not needed. The method is very useful to get a general view of the stress concentrations in a product.
Practical application of Residual Stress Measurement
lf a problem arises that could be caused by residual stress, an investigation plan can be made. The exact position in the manufacturing process where the problem occurs will be determined. It is important to make the influence on the stress of every process step visible, so corrective actions can be taken to improve the manufacturing process. This can be verified afterwards with repeated RSM.
In some cases the products are too large to measure a lot of positions using the Xray method. In that situation microscan measurements are very useful to find the positions with the highest stress level. These positions can be measured using X-ray measurement to find the exact stress level. Another possibility is to use the microscan method exclusively, but in that case the internal stresses can only be determined qualitatively.
To partly overcome this restriction, it is possible to calibrate the microscan equipment using a calibration procedure. This comprises of measurement of a test strip in a tensile machine at several constant loads with the Rigaku and the microscan, and comparing the results. It will result in a translation formula for the microscan values to real stress values. This formula is only valid for the material composition and structure involved.
An example of Residual Stress Measurement
An investigation has been made into drawn steel tubes that are used for manufacturing hydraulic telescopic cylinders. These tubes showed oval distortion during turning possibly because of residual stresses in tangential direction. In order to find the positions of these stresses -especially those of the highest stress valuesthe tubes were first analysed using the microscan method. The positions of ultimate stress values were marked and prepared using electrolytic polishing. After that, the stress level was quantified using the X-ray technique.
Following this, the tubes were mechanically stress relieved. Immediately after this treatment the stresses were again measured by X-rays. The effect of the treatment was immediately apparent.
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