Volume 17 - Nondestructive Evaluation and Quality Control Part 5 doc

Fig. 27 Variations of the magabsorption signal and stress with strain for an annealed nickel plated 3 mm ( in.) diam aluminum rod These meager data show that with a calibration curve, the stress in nonferromagnetic materials can be measured using the magabsorption signals from a thin nickel plating on the nonferromagnetic material. The magabsorption versus stress graph for the nickel plated aluminum seems to obey nearly the same equation as does the nickel wire. To date, no measurements of the stress in bars of aluminum with small areas of nickel plating have been accomplished as yet, but are planned for the future. Residual Stresses and Magnetism. Asymmetries in magabsorption signals may be indicative of residual stresses or magnetism. In the case of residual magnetism, the magnetic domains are not entirely haphazard; instead they do some ordering in a particular direction (Fig. 4f). This will produce asymmetries in the magnitude of the magabsorption signal, depending on the orientation of the bias field, H B , with respect to the orientation of the residual magnetism. Similarly, residual stresses are indicated by the ratio of magabsorption signals from two orientations (0°, 90°) of the bias coil. When the 0° and 90° amplitudes are equal, the stress is zero whatever the amplitudes are. When the parallel/perpendicular ratio is greater than 1, the stress value is positive and is tensile; when the ratio is less than 1, the stress is negative or compressive. For example, in one investigation, magabsorption measurements were made on steel samples before and after turning, cutting, and shaping operations. For the turning operation, the sample was reduced in diameter with a cutting tool; for the cutting operation, a sample was reduced in thickness by a ram shaper; for the shaping operation, the sample was reduced in thickness by an end mill. With the turning operation, the ratio of the 0° to the 90° magnitude of the magabsorption decreased from 0.91 to 0.89 when a 130 m (5 mil) cut was made. When a 230 m (9 mil) cut was made, the magabsorption ratio decreased from 0.89 to 0.65. When a 500 m (20 mil) cut was made, the magabsorption ratio decreased from 0.65 to 0.60. These changes indicated that the turning operation was placing compressive stress on the testpiece. The testpiece used with the ram shaper was in tension along its length before being reduced in width. A reduction in width of 760 m (30 mils) by the ram shaper applied perpendicular to the length caused the surface magabsorption signal ratio to indicate compression after the reduction. With the end mill, the reduction in thickness resulted in the stress changing from tensile to compressive. Example 1: Magabsorption Measurement of Residual Stress in a Crankshaft Throw. Quantitative estimates of residual stress from magabsorption measurements were also performed on a large crank-shaft throw (Fig. 28) made of 5046 steel. The estimates first required the development of calibration curves as described below. Fig. 28 Magabsorption detector and three detector heads used to perform measurements on the throw of the crankshaft shown on the left. A closeup view of the detector heads is shown in Fig. 20. The calibration curves were developed from two samples made of the same material as the crankshaft throw (type 5046 steel). The graph of the parallel-versus-perpendicular peak-to-peak values of the magabsorption signals from two of the calibration samples are given in Fig. 29. Two straight lines at angles of 45 and 50° relative to the horizontal axis are also drawn in Fig. 29. The one at 45° is a zero-stress line where the parallel and perpendicular magabsorption signals are the same magnitude. The line at 50° is the calibration line to be used to determine the calibration constant for the estimate of residual stress from the magabsorption measurements. Five stress levels (A, B, C, D, and E in Fig. 29) were applied at the measuring point on each test bar, and the calibration constant was determined as described below. Fig. 29 Graph showing the plot of the parallel/perpendicular ratios for sample 6 (type 5046 steel) and sample 7 (type 5046 steel) Previous experiments have indicated that the intersections of the parallel and perpendicular magnitudes for magabsorption signals at one point for the residual stresses seldom occur along the same line as applied stresses. However, it has been indicated that the applied stress lines in Fig. 29 probably can be used to determine the residual stress values in general by following the rule: All points on a radial line from the origin at some angle with respect to the abscissa have the same value of residual stress. The 45° line should be the locus of points of zero stress where the parallel and perpendicular values of the magabsorption curve are equal. With the 45° line as a reference, residual or applied stress can be expressed mathematically as: (Eq 19) where K is the calibration constant for the material. When the parallel and perpendicular amplitudes are equal, the stress is zero whatever the amplitudes are. When the parallel/perpendicular ratio is greater than one, the stress value is positive and is tension; when the ratio is less than one, the stress is negative or compression. The calibration constant, K, is obtained from Fig. 29 by the following procedure: (a) draw a calibration line through the origin at an angle for which an applied stress can be assigned to the intersection of the parallel/perpendicular ratio for the applied stress and (b), calculate the value of the constant K by inserting the applied stress and the angle into Eq 19. Before this procedure could be used with the data in Fig. 29, the usable applied stress curve had to be chosen. One of the applied stress curves from sample No. 7 was chosen because its amplitude was the closest to the residual stress data, and because its curve shape was close to that obtained from the throw. The calibration proceeded as described. The line at 50° was chosen for the first calibration value because it intersected the lowest amplitude (dashed curve) for the data from sample 7 at nearly the value of applied stress level B (closed circle) (55 MPa, or 8 ksi). The 45° line passes through the No. 7 stress curve (dashed) nearly at level C (open triangle), or where an estimated value of 160 MPa (23.5 ksi) had been applied. Therefore, the line at an angle of 50° represents a differential applied stress relative to the 45° line of 160 - 55 = 105 MPa (23.5 - 8 = 15.5 ksi). When several values are taken from several lines both above and below the 45° line, the average value for a line 5° above or below 45° is 100 MPa (15 ksi). Therefore, if both the value of tan -1 50° and the stress equal to 100 MPa (15 ksi) are inserted into Eq 19, the value of K is found to be 20 MPa/degree (3 ksi/degree). This implies that for every degree of offset from the 45° line, points along the line at that offset resulting from signal amplitude measurements will have the same value of residual stress. When the value of K is used, calibration lines can be drawn through zero at useful angles relative to the 45° line. Using these calibration lines and marks, the values of the residual stresses for points on the crankshaft throw were determined. Residual stress values as high as 600 MPa (87 ksi) tension and 90 MPa (13 ksi) compression were determined. References cited in this section 3. W.L. Rollwitz, "Magnetoabsorption," Final Report, Research Project No. 712- 4, Southwest Research Institute, 1958 4. W.L. Rollwitz and A.W. Whitney, "Special Te chniques for Measuring Material Properties," Technical Report ASD-TDR-64-123, USAF Contract No. AF-33(657)-10326, Air Force Materials Laboratory, 1964 5. W.L. Rollwitz and J.P. Classen, "Magnetoabsorption Techniques for Measuring Material Properties," Technical Report AFML-TR-65-17, USAF Contract No. AF-33(657)- 10326, Air Force Materials Laboratory, 1965 6. W.L. Rollwitz and J.P. Classen, "Magnetoabsorption Techniques for Measuring Material Properties," Technical Report AFML-TR-66-76 (Part I), USAF Contract No. AF-33(657)- 10326, Air Force Materials Laboratory, 1966 7. W.L. Rollwitz, "Magnetoabsorption Techniques for Measuring Material Properties. Part II. Measurements of Residual and Applied Stress." Technical Report AFML-TR-66-76 (Part II), USAF Contract No. AF- 33(615)-5068, Air Force Materials Laboratory, 1968 11. W.L. Rollwitz, "Preliminary Magnetoabsorption Measurements of Stress in a Crankshaft Throw," Summary Report on Project 15-2438, Southwest Research Institute, 1970 Magabsorption NDE William L. Rollwitz, Southwest Research Institute References 1. W.E. Bell, Magnetoabsorption, Vol 2, Proceedings of the Conference on Magnetism and Magnetic Materials, American Institute of Physics, 1956, p 305 2. R.M. Bozorth, Magnetism and Electrical Properties, in Ferromagnetism, D. Van Nostrand, 1951, p 745- 768 3. W.L. Rollwitz, "Magnetoabsorption," Final Report, Research Project No. 712- 4, Southwest Research Institute, 1958 4. W.L. Rollwitz and A.W. Whitney, "Special Techniques for Measuring Material Properti es," Technical Report ASD-TDR-64-123, USAF Contract No. AF-33(657)-10326, Air Force Materials Laboratory, 1964 5. W.L. Rollwitz and J.P. Classen, "Magnetoabsorption Techniques for Measuring Material Properties," Technical Report AFML-TR-65-17, USAF Contract No. AF-33(657)- 10326, Air Force Materials Laboratory, 1965 6. W.L. Rollwitz and J.P. Classen, "Magnetoabsorption Techniques for Measuring Material Properties," Technical Report AFML-TR-66-76 (Part I), USAF Contract No. AF-33(657)-10326, Air Force Mater ials Laboratory, 1966 7. W.L. Rollwitz, "Magnetoabsorption Techniques for Measuring Material Properties. Part II. Measurements of Residual and Applied Stress." Technical Report AFML-TR-66-76 (Part II), USAF Contract No. AF- 33(615)-5068, Air Force Materials Laboratory, 1968 8. W.L. Rollwitz, Magnetoabsorption, Progress in Applied Materials Research, Vol 6, E.G. Stanford, J.H. Fearon, and W.J. McGonnagle, Ed., Heywood, 1964 9. W.L. Rollwitz, Sensing Apparatus for Use With Magnetoabsorption Apparatus, U.S. Patent 3,612,968, 1971 10. W.L. Rollwitz, J. Arambula, and J. Classen, Method of Determining Stress in Ferromagnetic Members Using Magnetoabsorption, 1974 11. W.L. Rollwitz, "Preliminary Magnetoabsorption Measurements of Stress in a Crankshaft Throw," Summary Report on Project 15-2438, Southwest Research Institute, 1970 Electromagnetic Techniques for Residual Stress Measurements H. Kwun and G.L. Burkhardt, Southwest Research Institute Introduction RESIDUAL STRESSES in materials can be nondestructively measured by a variety of methods, including x-ray diffraction, ultrasonics, and electromagnetics (Ref 1, 2, 3). With the x-ray diffraction technique, the interatomic planar distance is measured, and the corresponding stress is calculated (Ref 4). The penetration depth of x-rays is of the order of only 10 in. (400 in.) in metals. Therefore, the technique is limited to measurements of surface stresses. Its use has been generally limited to the laboratory because of the lack of field-usable equipment and concern with radiation safety. With ultrasonic techniques, the velocity of the ultrasonic waves in materials is measured and related to stress (Ref 5). These techniques rely on a small velocity change caused by the presence of stress, which is known as the acoustoelastic effect (Ref 6). In principle, ultrasonic techniques can be used to measure bulk as well as surface stresses. Because of the difficulty in differentiating stress effects from the effect of material texture, practical ultrasonic applications have not yet materialized. With electromagnetic techniques, one or more of the magnetic properties of a material (such as permeability, magnetostriction, hysteresis, coercive force, or magnetic domain wall motion during magnetization) are sensed and correlated to stress. These techniques rely on the change in magnetic properties of the material caused by stress; this is known as the magnetoelastic effect (Ref 7). These techniques, therefore, apply only to ferromagnetic materials, such as steel. Of the many electromagnetic stress-measurement techniques, this article deals with three specific ones: Barkhausen noise, non-linear harmonics, and magnetically induced velocity changes. The principles, instrumentation, stress dependence, and capabilities and limitations of these three techniques are described in the following sections. References 1. M.R. James and O. Buck, Quantitative Nondestructive Measurements of Residual Stresses, CRC Crit. Rev. Solid State Mater. Sci., Vol 9, 1980, p 61 2. C.O. Ruud, "Review and Evaluation of Nondestructive Methods for Residual Stress Measurement," Final Report, NP-1971, Project 1395-5, Electric Power Research Institute, Sept 1981 3. W.B. Young, Ed., Residual Stress in Design, Process, and Material Selection, Proceedings of the ASM Conference on Residual Stress in Design, Process, and Materials Selection, Cincinnati, OH, April 1987, ASM INTERNATIONAL, 1987 4. M.R. James and J.B. Cohen, The Measurement of Residual Stresses by X- Ray Diffraction Techniques, in Treatise on Materials Science and Technology Experimental Methods, Vol 19A, H. Herman, Ed., Academic Press, 1980, p 1 5. Y. H. Pao, W. Sachse, and H. Fukuoka, Acoustoelasticity and Ultrasonic Measurement of Residual Stresses, in Physical Acoustics: Principles and Methods, Vol XVII, W.P. Mason and R.M. Thurston, Ed., Academic Press, 1984, p 61-143 6. D.S. Hughes and J.L. Kelly, Second-Order Elastic Deformation of Solids, Phys. Rev., Vol 92, 1953, p 1145 7. R.M. Bozorth, Ferromagnetism, Van Nostrand, 1951 Electromagnetic Techniques for Residual Stress Measurements H. Kwun and G.L. Burkhardt, Southwest Research Institute Barkhausen Noise The magnetic flux density in a ferromagnetic material subjected to a time-varying magnetic field does not change in a strictly continuous way, but rather by small, abrupt, discontinuous increments called Barkhausen jumps (after the name of the researcher who first observed this phenomenon), as illustrated in Fig. 1. The jumps are due primarily to discontinuous movements of boundaries between small magnetically saturated regions called magnetic domains in the material (Ref 7, 8, 9). An unmagnetized macroscopic specimen consists of a great number of domains with random magnetic direction so that the average bulk magnetization is zero. Under an external magnetic field, the specimen becomes magnetized mainly by the growth of volume of domains oriented close to the direction of the applied field, at the expense of domains unfavorably oriented. The principal mechanism of growth is the movement of the walls between adjacent domains. Because of the magnetoelastic interaction, the direction and magnitude of the mechanical stress strongly influence the distribution of domains and the dynamics of the domain wall motion and therefore the behavior of Barkhausen jumps (Ref 8). This influence, in turn, is used for stress measurements. Because the signal produced by Barkhausen jumps resembles noise, the term Barkhausen noise is often used. Fig. 1 Hysteresis loop for magnetic material showing discontinuities that produce Barkhausen noise. Source: Ref 2 Instrumentation. The arrangement illustrated in Fig. 2 is used for the Barkhausen noise technique (Ref 8). A small C- shaped electromagnet is used to apply a controlled, time-varying magnetic field to the specimen. The abrupt movements of the magnetic domains are typically detected with an inductive coil placed on the specimen. The detected signal is a burst of noiselike pulses, as illustrated in Fig. 3. Certain features of the signal, such as the maximum amplitude or root mean square (rms) amplitude of the Barkhausen noise burst or the applied magnetic field strength at which the maximum amplitude occurs, are used to determine the stress state in the material (Ref 1, 8, 9, 10, 11, and 12). Fig. 2 Arrangement for sensing the Barkhausen effect Fig. 3 Schematic showing the change in magnetic field H with time, variation in flux density over the same period, and the generation of the Barkhausen noise burst as flux density changes. Source: Ref 14 In addition to inductive sensing of the magnetic Barkhausen noise, magnetoacoustic Barkhausen activity can also be detected with an acoustic emission sensor (Ref 13). This phenomenon occurs when Barkhausen jumps during the magnetization of a specimen produce mechanical stress pulses in a manner similar to the inductive Barkhausen noise burst shown in Fig. 3. It is caused by microscopic changes in strain due to magnetostriction when discontinuous, irreversible domain wall motion of non-180° domain walls occurs (Ref 14, 15). This acoustic Barkhausen noise is also dependent on the stress state in the material and can therefore be used for stress measurements (Ref 15, 16, 17, 18, and 19). Stress Dependence. The magnetic Barkhausen effect is dependent on the stress as well as the relative direction of the applied magnetic field to the stress direction. To illustrate this, Fig. 4 shows a typical stress dependence of the inductively detected Barkhausen noise in a ferrous material. In the case where the magnetic field and the stress are parallel, the Barkhausen amplitude increases with tension and decreases with compression (Ref 8, 10, 20, and 21). In the case where the two are perpendicular, the opposite result is obtained. The behavior shown in Fig. 4 holds for materials with a positive magnetostriction coefficient; for materials with a negative magnetostriction, the Barkhausen amplitude exhibits the opposite behavior. Fig. 4 Typical stress dependenc e of Barkhausen noise signal amplitude with the applied magnetic field parallel (curve A) and perpendicular (curve B) to the stress direction For a given stress, the dependence of the Barkhausen amplitude on the angle between the magnetic field and stress directions is proportional to the strain produced by the stress (Ref 21). Because the Barkhausen noise is dependent on the strain, Barkhausen measurements can be used as an alternative to strain gages (Ref 20, 21). A typical stress dependence of the acoustic Barkhausen noise is illustrated in Fig. 5, in which the magnetic field is applied parallel to the stress direction. As shown, the amplitude of the acoustic signal decreases with tension. Under compression, it increases slightly and then decreases with an increasing stress level. The acoustic Barkhausen noise, therefore, cannot distinguish tension from compression. [...]... 6061-T6 41 42 70 7 5- T6 53 32 2024-T4 52 30 Magnesium 46 37 7 0-3 0 brass 62 28 Phosphor bronzes 160 11 Monel 482 3.6 Zirconium 50 0 3.4 Zircaloy-2 720 2.4 Titanium 54 8 3.1 Ti-6Al-4V alloy 172 0 1.0 Type 304 stainless steel 700 2 .5 Inconel 600 980 1.7 Hastelloy X 1 150 1 .5 Waspaloy 1230 1.4 Aluminum alloys Many factors influence the conductivity of a metal, notably, temperature, composition, heat treatment and. .. Phys., Vol 50 , 1979, p 29 85 14 D.C Jiles, Review of Magnetic Methods for Nondestructive Evaluation, NDT Int., Vol 21 (No 5) , 1988, p 31 1-3 19 15 K Ono and M Shibata, Magnetomechanical Acoustic Emission of Iron and Steels, Mater Eval., Vol 38, 1980, p 55 16 M Shibata and K Ono, Magnetomechanical Acoustic Emission A New Method for Nondestructive Stress Measurement, NDT Int., Vol 14, 1981, p 227 17 K Ono,... W Sachse, and H Fukuoka, Acoustoelasticity and Ultrasonic Measurement of Residual Stresses, in Physical Acoustics: Principles and Methods, Vol XVII, W.P Mason and R.M Thurston, Ed., Academic Press, 1984, p 6 1-1 43 D.S Hughes and J.L Kelly, Second-Order Elastic Deformation of Solids, Phys Rev., Vol 92, 1 953 , p 11 45 R.M Bozorth, Ferromagnetism, Van Nostrand, 1 951 G.A Matzkanin, R.E Beissner, and C.M Teller,... Southwest Research Institute References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 M.R James and O Buck, Quantitative Nondestructive Measurements of Residual Stresses, CRC Crit Rev Solid State Mater Sci., Vol 9, 1980, p 61 C.O Ruud, "Review and Evaluation of Nondestructive Methods for Residual Stress Measurement," Final Report, NP-1971, Project 139 5- 5 , Electric Power Research Institute, Sept... Ferromagnetism, Van Nostrand, 1 951 8 G.A Matzkanin, R.E Beissner, and C.M Teller, "The Barkhausen Effect and Its Applications to Nondestructive Evaluation, " State of the Art Report, NTIAC-7 9-2 , Nondestructive Testing Information Analysis Center, Southwest Research Institute, Oct 1979 9 J.C McClure, Jr., and K Schroder, The Magnetic Barkhausen Effect, CRC Crit Rev Solid State Sci., Vol 6, 1976, p 45 10 R.L Pasley,... Wave Velocity Change in Polycrystalline A-36 Steel, J Appl Phys., Vol 54 , 1983, p 4 856 31 H Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Ultrasonic Longitudinal Waves in Steels, J Appl Phys., Vol 57 , 19 85, p 155 5 32 H Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Surface Waves in Steels, J Appl Phys., Vol 58 , 19 85, p 3921 33 H Kwun, Measurement of Stress... (No 5) , 1988, p 31 1-3 19 K Ono and M Shibata, Magnetomechanical Acoustic Emission of Iron and Steels, Mater Eval., Vol 38, 1980, p 55 M Shibata and K Ono, Magnetomechanical Acoustic Emission A New Method for Nondestructive Stress Measurement, NDT Int., Vol 14, 1981, p 227 K Ono, M Shibata, and M.M Kwan, Determination of Residual Stress by Magnetomechanical Acoustic, in Residual Stress for Designers and. .. Shear Wave Velocity Change in Polycrystalline A-36 Steel, J Appl Phys., Vol 54 , 1983, p 4 856 H Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Ultrasonic Longitudinal Waves in Steels, J Appl Phys., Vol 57 , 19 85, p 155 5 H Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Surface Waves in Steels, J Appl Phys., Vol 58 , 19 85, p 3921 H Kwun, Measurement of Stress in Steels... Nondestr Test Commun., Vol 1, 1984, p 1 75 M Namkung and D Utrata, Nondestructive Residual Stress Measurements in Railroad Wheels Using the Low-Field Magnetoacoustic Test Method, in Proceedings of the 1987 Review of Progress in Quantitative Nondestructive Evaluation, Vol 7B, D.O Thompson and D.E Chimenti, Ed., Plenum Press, 1988, p 1429 24 25 26 27 28 29 30 31 32 33 34 35 36 Eddy Current Inspection Revised... current in the coils Solenoid-type coil is applied to cylindrical or tubular parts; pancake-type coil, to a flat surface The electromagnetic field in the region in the part and surrounding the part depends on both the exciting current from the coil and the eddy currents flowing in the part The flow of eddy currents in the part depends on: • • • The electrical characteristics of the part The presence or absence . angle of 50 ° represents a differential applied stress relative to the 45 line of 160 - 55 = 1 05 MPa (23 .5 - 8 = 15. 5 ksi). When several values are taken from several lines both above and below. Laboratory, 1964 5. W.L. Rollwitz and J.P. Classen, "Magnetoabsorption Techniques for Measuring Material Properties," Technical Report AFML-TR- 65 -1 7, USAF Contract No. AF-33( 657 )- 10326,. Measuring Material Properties. Part II. Measurements of Residual and Applied Stress." Technical Report AFML-TR-6 6-7 6 (Part II), USAF Contract No. AF- 33(6 15 ) -5 068, Air Force Materials Laboratory,