Title of Invention

METHOD OF MEASUREMENT OF RESIDUAL STRESS

Abstract

Measurement of residual stress and ,in particular, to a simple and accurate direct measurement of residual stress which is the locked in stress present in free body on which there is no external load or thermal gradient. It is possible by way of the above method to sense / measure the actual compression / tension caused by residual stress by simple and effective use of strain gauge which makes it possible to carry out direct measurement of stress. The method also favours obtaining bulk residual stress of the sample by cutting the same into small slice such that the total stress of the sample is released. The method avoids the limitations of conventional residual stress measurement techniques including X-ray diffraction method, Barkhausen Noise method and the Ultrasonic method presently followed in the art.

Full Text

FIELD OF THE INVENTION The present invention relates to measurement of residual stress and ,in particular, to a simple and accurate direct measurement of residual stress which is the locked in stress present in free body on which there is no external load or thermal gradient. It is possible by way of the above method to sense / measure the actual compression / tension caused by residual stress by simple and effective use of strain gauge which makes it possible to carry out direct measurement of stress. The method also favours obtaining bulk residual stress of the sample by cutting the same into small slice such that the total stress of the sample is released. The method avoids the limitations of conventional residual stress measurement techniques including X-ray diffraction method, Barkhausen Noise method and the Ultrasonic method presently followed in the art BACKGROUND OF THE ART It is known that residual stress comprise of the locked in stress present in free body on which there is no external load or thermal gradient. It is usually caused by several stress generating conditions such as casting, welding, grinding, machining, heat treatment, rolling, non-uniform cooling of structural and straightening of structural. Importantly, it is found that there are several effects of such residual stresses wherein combined with applied load and environment it causes failure at load much below the designed load. Residual stress is also found to enhance the fatigue failure and even influences the dimensional stability. There are various well known methods for measurement of residual stress such as those discussed hereunder. X-ray diffraction method is one of the most widely used non-destructive method for evaluating residual stresses. It is based on lattice strains, the changes in the spacing between crystallographic lattice planes, which are caused by stress. The disadvantages of such method are several which include as the volume of the surface materia! interrogated by the X-ray beam is small, and the lattice strain which is measured reflects the combined influence for both micro and macro- residual stresses acting at that location. In materials having high variation of micro - stress gradients, the evaluation of macro- residual stress is not proper. The elastic constant of crystal vary with their orientation so that they calculated values differ substantially from the measured values. Also, the measured values are also not always available. Ultrasonic method for evaluating residual stresses are based upon the changes in the velocity of ultrasonic waves due to stress. The disadvantages of this method is higher order elastic constants are generally required in order to relate ultrasonic velocities to residual stress. These constants which are also dependent on the metallurgical texture must be experimentally determined for a particular material being examined. This method has a limited capability for detecting sharp stress gradients and it has little use for determining residual stress in material, such as, plastics, composites and certain non-metallic Barkhausen Noise method is now-a-days a very popular method for the determination of residual stress. When a ferromagnetic material is subjected to magnetic field it becomes magnetized in a rapid series of tiny steps for jumps. These discontinuous changes can be monitored with sensitive electro-magnetic or acoustic detectors. The measured signals, called "Barkhausen Noise0 are influenced by the residual stresses in the material. Under alternating magnetic field repetitive noise patterns can be obtained. The disadvantages of this method Include a theoretical relationship between Barkhausen Noise and residual stress is yet to be established and the use is limited to ferromagnetic materials. It would be apparent from the above that the conventional processes of residual stress measurement have, certain inherent limitations and not accurate to the desired extent. Also it is not readily possible to achieve direct measurement of stress and suffer from disadvantages , such as, the x-ray diffraction method which has got limitation of depth of penetration into the material, the Barkhausen Noise method which is dependent on grain size and magnetic properties of the sample and the ultrasonic method which is highly dependent on the velocity of propagation. OBJECTS OF THE INVENTION It is thus the basic object of the present invention to provide a simple and accurate method of measurement of residual stress i.e. locked in stress present in a free body on which there is no external load applied. Another object of the present invention is directed to a simple and accurate method of measurement of residual stress involving simple and effective slice (Transverse Template) cutting method which would favour avoiding limitations and draw backs associated with the presently available and practiced methods of residual stress measurement including X-ray diffraction method, Barkhausen Noise method and also the ultrasonic method. Another object of the present invention is to provide for a residual stress measurement method which would enable sensing / measuring the actual compression / tension caused by residual stress involving simple strain gauge which would favour direct stress measurement. Another object of the present invention is directed to a method for achieving bulk residual stress of samples wherein the sample is cut into small slice and in this way the total stress of the sample is released. Another object of the present invention is to provide for a simple and accurate method measurement of residual stress which would avoid the limitations of depth of penetration of materials such as experienced in conventional x-ray diffraction method . Yet another object of the present invention is to provide for a method of measurement of stress wherein the Barkhausen Noise limitations which are dependent on grain size and magnetic properties of the sample could be avoided and provide for a more reliable and simple method of measurement of residual stress. A further of the present invention is directed to a method of measurement of residual stress which would not be dependent on the velocity of propagation as found to be a limitation of the conventional ultrasonic method of residual stress -measurement, SUMMARY OF THE INVENTION Thus according to the basic object of the present invention there is provided a method for residua] stress measurement of samples comprising: providing the sample in desired form as a test sample for the measurement; measuring the stress in the test sample using strain gauge preferably mounted or glued to the sample with switch and balance unit and a strain indicator means to obtain the stress value indicative of strain before the cutting; transverse cutting of the test sample without any heat generation; measuring the after cut strain of the slice thus cut using strain gauge preferably mounted or glued to the sample with co-operative switch and balance unit; and identifying the residual stress using the Formula (l) σ = EΕ wherein E=2.06X 105N/mm2 Ε = Measured Value in µε (micro-strain) based on the strain before the cutting and said strain after cutting. Preferably, in the above method for residual stress measurement the said slice transverse template is cut in the center of the test sample. According to an aspect of the invention, the test sample can comprise a rail and a sample of one meter length taken from me rail discarding two meters from the end and a slice of 50-60 mm thick is cut in the center of the test sample. The strain gauges are preferably fixed with pressure sensitive adhesive and after curing connecting wires are soldered with strain gauge and terminals for subsequent installation testmg.The strain is measured using strain balancing bridge and digital strain indicator. As disclosed above, the method for residual stress measurement comprises identifying of the Residual Stresses following the formula (I) hereunder σ = Eε wherein E=2.06X 105N/mm2 Ε = Measured Value in us (micro-strain) It is thus possible by way of above disclosed method to achieve residual stress measurement by way of a simple slice (Transverse Template) cut in the test sample. As discussed above such a test sample can be from the rail and the method would enable simple and cost effective residual stress measurement using the above method. Importantly, the slice (Transverse Template) cutting method is found to be simple and accurate and in this method actual compression / tension caused by residual stress can be sensed / measured by strain gauge. The method therefore favours direct measurement of stress. Also, in the method it is possible to identify bulk residual stress of the sample as the sample is cut into small slice and in this way the total stress of the sample is released. The stresses of the slice are measured before and after cutting. The strain gauges are preferably fixed with pressure sensitive adhesive. After curing, the connecting wires are soldered with strain gauges and terminals. The installation is then ready for testing and the strain is measured preferably using strain balancing bridge and digital strain indicator. The detail of the invention, its object and advantages are explained hereunder in greater detail in relation to non-limiting exemplarity illustration in relation to the following accompanying figures. BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES Figure 1 is a schematic diagram illustrating the disposition of the test sample and the strain measuring units used in accordance with the invention; Figure 2 illustrates the complete set up used in measuring residual stress in accordance with the invention; Figures 3A and 3B illustrate the manner of cutting of sample to provide the Transverse Template used in stress measurement; The sample is next disconnected and slice of the sample is cut out without Temperature rise in the sample. Such cutting of slice from the test sample is further illustrated in detail in accompanying figure in 3A and 3B. After cutting of the slice (Transverse Template) again the strain is measured with the above mentioned set up as shown in figure 2 without any disturbance. The reading of the indicator provides the strain in micro strain. The indicator means Is further shown in figure 4. Finally the residual stress is calculated based on the stress values identified before and after cutting of the test sample following the formula 1 above. It is thus possible by way of the above invention to provide a simple and accurate method of measurement of residual stress, which would be reliable and can be readily applied. Importantly, the method is simple and accurate method of measurement of residual stress involving simple and effective slice (Transverse Template) cutting method which would favour avoiding limitations and draw backs associated with the presently available and practiced methods of residual stress measurement including X-ray diffraction method, Barkhausen Noise method and also the ultrasonic method. WE CLAIM: 1. A method for residual stress measurement of samples comprising: providing the sample in desired form as a test sample for the measurement; measuring the stress by measuring the initial strain in the test sample using strain gauge preferably mounted or glued to the sample with operative connection to switch and balance unit and a strain indicator means to obtain the stress value indicative of strain before the cutting; transverse cutting of a slice alongwith said attached strain gauges as transverse template from the test sample without any temperature rise whereby total stress of the sample is released; measuring the after cut strain of the slice thus cut using strain gauge attached to the sample with switch and balance unit and said strain indicator; and identifying the residual stress using the formula σ = Eε wherein E= 2.06X 105 N/mm2 Ε = Measured Value in µε (micro-strain) based on the strain before the cutting and said strain after cutting favouring achieving bulk residual stress of sample obtained therefrom. 2. A method for residual stress measurement as claimed in claim 1 wherein said slice transverse template is cut out from the center of the test sample. 3. A method for residual stress measurement as claimed in anyone of claims 1 or 2 wherein the test sample comprises a rail and a sample of one meter length is taken from the rail discarding two meters from the end and said slice of 50-60 mm thick is cut out from the center of the test sample. 4. A method for residual stress measurement as claimed in anyone of claims 1 to 3 wherein the strain gauges are fixed with pressure sensitive adhesive and aftercuring connecting wires are soldered with strain gauge and terminals for subsequent installation testing and stress measurement. 5. A method for residual stress measurement as claimed in anyone of claims 1 to 4 wherein strain is measured using strain balancing bridge and digital strain indicator. 6. A method for residual stress measurement as claimed in anyone of claims 1 to 5 wherein the Residual Stresses identified comprise σ = Ε X 10-6 X 2.06 x 104 kg/mm2; Σ=Ε X 2.06 x 10-2 kg/mm2. ABSTRACT Measurement of residual stress and ,in particular, to a simple and accurate direct measurement of residual stress which is the locked in stress present in free body on which there is no external load or thermal gradient. It is possible by way of the above method to sense / measure the actual compression / tension caused by residual stress by simple and effective use of strain gauge which makes it possible to carry out direct measurement of stress. The method also favours obtaining bulk residual stress of the sample by cutting the same into small slice such that the total stress of the sample is released. The method avoids the limitations of conventional residual stress measurement techniques including X-ray diffraction method, Barkhausen Noise method and the Ultrasonic method presently followed in the art.

Documents:

00109-kol-2006-abstract.pdf

00109-kol-2006-claims.pdf

00109-kol-2006-description complete.pdf

00109-kol-2006-drawings.pdf

00109-kol-2006-form 1.pdf

00109-kol-2006-form 2.pdf

00109-kol-2006-form 3.pdf

109-KOL-2006-(17-04-2013)-CLAIMS.pdf

109-KOL-2006-(17-04-2013)-CORRESPONDENCE.pdf

109-KOL-2006-(17-04-2013)-DRAWINGS.pdf

109-KOL-2006-(17-04-2013)-FORM-1.pdf

109-KOL-2006-(17-04-2013)-OTHERS.pdf

109-KOL-2006-CANCELLED PAGES.pdf

109-KOL-2006-CORRESPONDENCE.pdf

109-KOL-2006-EXAMINATION REPORT.pdf

109-KOL-2006-FORM 18.pdf

109-KOL-2006-GRANTED-ABSTRACT.pdf

109-KOL-2006-GRANTED-CLAIMS.pdf

109-KOL-2006-GRANTED-DESCRIPTION (COMPLETE).pdf

109-KOL-2006-GRANTED-DRAWINGS.pdf

109-KOL-2006-GRANTED-FORM 1.pdf

109-KOL-2006-GRANTED-FORM 2.pdf

109-KOL-2006-GRANTED-FORM 3.pdf

109-KOL-2006-GRANTED-SPECIFICATION-COMPLETE.pdf

109-KOL-2006-PA.pdf

109-KOL-2006-REPLY TO EXAMINATION REPORT.pdf