Nondestructive Evaluation and Quality Control (1998) Part 6 doc

Magnesium alloy M1A 15 27.5 29 59.5 20 37.5 Monel 16.5 30 33 79 23 44 Titanium 14 26 29 59 20 37 (a) Measured from a direction normal to surface of test material. (b) In water at 4 °C (39 °F). (c) Using angle block (wedge) made of acrylic plastic Beam Intensity. The intensity of an ultrasonic beam is related to the amplitude of particle vibrations. Acoustic pressure (sound pressure) is the term most often used to denote the amplitude of alternating stresses exerted on a material by a propagating ultrasonic wave. Acoustic pressure is directly proportional to the product of acoustic impedance and amplitude of particle motion. The acoustic pressure exerted by a given particle varies in the same direction and with the same frequency as the position of that particle changes with time. Acoustic pressure is the most important property of an ultrasonic wave, and its square determines the amount of energy (acoustic power) in the wave. It should be noted that acoustic pressure is not the intensity of the ultrasonic beam. Intensity, which is the energy transmitted through a unit cross-sectional area of the beam, is proportional to the square of acoustic pressure. Although transducer elements sense acoustic pressure, ultrasonic systems do not measure acoustic pressure directly. However, receiver-amplifier circuits of most ultrasonic instruments are designed to produce an output voltage proportional to the square of the input voltage from the transducer. Therefore, the signal amplitude of sound that is displayed on an oscilloscope or other readout device is a value proportional to the true intensity of the reflected sound. The law of reflection and refraction described in Eq 5 or 6 gives information regarding only the direction of propagation of reflected and refracted waves and says nothing about the acoustic pressure in reflected or refracted waves. When ultrasonic waves are reflected or refracted, the energy in the incident wave is partitioned among the various reflected and refracted waves. The relationship among acoustic energies in the resultant waves is complex and depends both on the angle of incidence and on the acoustic properties of the matter on opposite sides of the interface. Figure 6 shows the variation of acoustic pressure (not energy) with angle of reflection or refraction ( ' l , l , or t , Fig. 5) that results when an incident longitudinal wave in water having an acoustic pressure of 1.0 arbitrary unit impinges on the surface of an aluminum testpiece. At normal incidence ( l = ' l = l = 0°), acoustic energy is partitioned between a reflected longitudinal wave in water and a refracted (transmitted) longitudinal wave in aluminum. Because of different acoustic impedances, this partition induces acoustic pressures of about 0.8 arbitrary unit in the reflected wave in water and about 1.9 units in the transmitted wave in aluminum. Although it may seem anomalous that the transmitted wave has a higher acoustic pressure than the incident wave, it must be recognized that it is acoustic energy, not acoustic pressure, that is partitioned and conserved. Figure 7 illustrates the partition of acoustic energy at a water/steel interface. Fig. 6 Variation of acoustic pressure with angle of r eflection or refraction during immersion ultrasonic inspection of aluminum. The acoustic pressure of the incident wave equals 1.0 arbitrary unit. Points A and A' correspond to the first critical angle, and point B to the second critical angle, for this system. Fig. 7 Partition of acoustic energy at a water/steel interface. The reflection coefficient, R, is equal to 1 - (L + S), where L is the transmission coefficient of the longitudinal wave and S is the transmission coefficient of the transverse (or shear) wave. In Fig. 6, as the incident angle, 1 , is increased, there is a slight drop in the acoustic pressure of the reflected wave, a corresponding slight rise in the acoustic pressure of the refracted longitudinal wave, and a sharper rise in the acoustic pressure of the refracted transverse wave. At the first critical angle for the water/aluminum interface ( 1 = 13.6°, 1 = 90°, and t = 29.2°), the acoustic pressure of the longitudinal waves reaches a peak, and the refracted waves go rapidly to zero (point A', Fig. 6). Between the first and second critical angles, the acoustic pressure in the reflected longitudinal wave in water varies as shown between points A and B in Fig. 6. The refracted longitudinal wave in aluminum meanwhile has disappeared. Beyond the second critical angle ( l = 28.8°), the transverse wave in aluminum disappears, and there is total reflection at the interface with no partition of energy and no variation in acoustic pressure, as shown to right of point B in Fig. 6. Curves similar to those in Fig. 6 can be constructed for the reverse instance of incident longitudinal waves in aluminum impinging on an aluminum/water interface, for incident transverse waves in aluminum, and for other combinations of wave types and materials. Details of this procedure are available in Ref 1. These curves are important because they indicate the angles of incidence at which energy transfer across the boundary is most effective. For example, at an aluminum/water interface, peak transmission of acoustic pressure for a returning transverse wave echo occurs in the sector from about 16 to 22° in the water relative to a line normal to the interface. Consequently, 35 to 51° angle beams in aluminum are the most efficient in transmitting detectable echoes across the front surface during immersion inspection and can therefore resolve smaller discontinuities than beams directed at other angles in the aluminum. Reference cited in this section 1. A.J. Krautkramer and H. Krautkramer, Ultrasonic Testing of Materials, 1st ed, Springer-Verlag, 1969 Ultrasonic Inspection Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los Angeles; and the ASM Committee on Ultrasonic Inspection * Attenuation of Ultrasonic Beams The intensity of an ultrasonic beam that is sensed by a receiving transducer is considerably less than the intensity of the initial transmission. The factors that are primarily responsible for the loss in beam intensity can be classified as transmission losses, interference effects, and beam spreading. Transmission losses include absorption, scattering, and acoustic impedance effects at interfaces. Interference effects include diffraction and other effects that create wave fringes, phase shift, or frequency shift. Beam spreading involves mainly a transition from plane waves to either spherical or cylindrical waves, depending on the shape of the transducer- element face. The wave physics that completely describe these three effects are discussed in Ref 1 and 2. Acoustic impedance effects (see the section "Acoustic Impedance" in this article) can be used to calculate the amount of sound that reflects during the ultrasonic inspection of a testpiece immersed in water. For example, when an ultrasonic wave impinges at normal incidence ( 1 = 0°) to the surface of the flaw-free section of aluminum alloy 1100 plate during straight-beam inspection, the amount of sound that returns to the search unit (known as the back reflection) has only 6% of its original intensity. This reduction in intensity occurs because of energy partition when waves are only partly reflected at the aluminum/water interfaces. (Additional losses would occur because of absorption and scattering of the ultrasonic waves, as discussed in the sections "Absorption" and "Scattering" in this article.) Similarly, an energy loss can be calculated for a discontinuity that constitutes an ideal reflecting surface, such as a lamination that is normal to the beam path and that interposes a metal/air interface larger than the sound beam. For example, in the straight-beam inspection of an aluminum alloy 1100 plate containing a lamination, the final returning beam, after partial reflection at the front surface of the plate and total reflection from the lamination, would have a maximum intensity 8% of that of the incident beam. By comparison, only 6% was found for the returning beam from the plate that did not contain a lamination. Similar calculations of the energy losses caused by impedance effects at metal/water interfaces for the ultrasonic immersion inspection of several of the metals listed in Table 1 yield the following back reflection intensities, which are expressed as a percentage of the intensity of the incident beam: Material Back reflection intensity, % of incident beam intensity Magnesium alloy M1A 11.0 Titanium 3.0 Type 302 stainless steel 1.4 Carbon steel 1.3 Inconel 0.7 Tungsten 0.3 The loss in intensity of returning ultrasonic beams is one basis for characterizing flaws in metal testpieces. As indicated above, acoustic impedance losses can severely diminish the intensity of an ultrasonic beam. Because a small fraction of the area of a sound beam is reflected from small discontinuities, it is obvious that ultrasonic instruments must be extremely sensitive to small variations in intensity if small discontinuities are to be detected. The sound intensity of contact techniques is usually greater than that of immersion techniques; that is, smaller discontinuities will result in higher amplitude signals. Two factors are mainly responsible for this difference, as follows. First, the back surface of the testpiece is a metal/air interface, which can be considered a total reflector. Compared to a metal/water interface, this results in an approximately 30% increase in back reflection intensity at the receiving search unit for an aluminum testpiece coupled to the search unit through a layer of water. Second, if a couplant whose acoustic impedance more nearly matches that of the testpiece is substituted for the water, more energy is transmitted across the interface for both the incident and returning beams. For most applications, any couplant with an acoustic impedance higher than that of water is preferred. Several of these are listed in the nonmetals group in Table 1. In addition to the liquid couplants listed in Table 1, several semisolid or solid couplants (including wallpaper paste, certain greases, and some adhesives) have higher acoustic impedances than water. The absorption of ultrasonic energy occurs mainly by the conversion of mechanical energy into heat. Elastic motion within a substance as a sound wave propagates through it alternately heats the substance during compression and cools it during rare-faction. Because heat flows so much more slowly than an ultrasonic wave, thermal losses are incurred, and this progressively reduces energy in the propagating wave. A related thermal loss occurs in polycrystalline materials; a thermoelastic loss arises from heat flow away from grains that have received more compression or expansion in the course of wave motion than did adjacent grains. For most polycrystalline materials, this effect is most pronounced at the low end of the ultrasonic frequency spectrum. Vibrational stress in ferromagnetic and ferroelectric materials generated by the passage of an acoustic wave can cause motion of domain walls or rotation of domain directions. These effects may cause domains to be strengthened in directions parallel, antiparallel, or perpendicular to the direction of stress. Energy losses in ferromagnetic and ferroelectric materials may also be caused by a microhysteresis effect, in which domain wall motion or domain rotation lags behind the vibrational stress to produce a hysteresis loop. In addition to the types of losses discussed above, other types exist that have not been accounted for quantitatively. For example, it has been suggested that some losses are caused by elastic-hysteresis effects due to cyclic displacements of dislocations in grains or grain boundaries of metals. Absorption can be thought of as a braking action on the motion of oscillating particles. This braking action is more pronounced when oscillations are more rapid, that is, at high frequencies. For most materials, absorption losses increase directly with frequency. Scattering of an ultrasonic wave occurs because most materials are not truly homogeneous. Crystal discontinuities, such as grain boundaries, twin boundaries, and minute nonmetallic inclusions, tend to deflect small amounts of ultrasonic energy out of the main ultrasonic beam. In addition, especially in mixed microstructures or anisotropic materials, mode conversion at crystallite boundaries tends to occur because of slight differences in acoustic velocity and acoustic impedance across the boundaries. Scattering is highly dependent on the relation of crystallite size (mainly grain size) to ultrasonic wavelength. When grain size is less than 0.01 times the wavelength, scatter is negligible. Scattering effects vary approximately with the third power of grain size, and when the grain size is 0.1 times the wavelength or larger, excessive scattering may make it impossible to conduct valid ultrasonic inspections. In some cases, determination of the degree of scattering can be used as a basis for acceptance or rejection of parts. Some cast irons can be inspected for the size and distribution of graphite flakes, as described in the section "Determination of Microstructural Differences" in this article. Similarly, the size and distribution of microscopic voids in some powder metallurgy parts, or of strengtheners in some fiber-reinforced or dispersion-strengthened materials, can be evaluated by measuring attenuation (scattering) of an ultrasonic beam. Diffraction. A sound beam propagating in a homogeneous medium is coherent; that is, all particles that lie along any given plane parallel to the wave front vibrate in identical patterns. When a wave front passes the edge of a reflecting surface, the front bends around the edge in a manner similar to that in which light bends around the edge of an opaque object. When the reflector is very small compared to the sound beam, as is usual for a pore or an inclusion, wave bending (forward scattering) around the edges of the reflector produces an interference pattern in a zone immediately behind the reflector because of phase differences among different portions of the forward-scattered beam. The interference pattern consists of alternate regions of maximum and minimum intensity that correspond to regions where interfering scattered waves are respectively in phase and out of phase. Diffraction phenomena must be taken into account during the development of ultrasonic inspection procedures. Unfortunately, only qualitative guidelines can be provided. Entry-surface roughness, type of machined surface, and machining direction influence inspection procedures. In addition, the roughness of a flaw surface affects its echo pattern and must be considered. A sound beam striking a smooth interface is reflected and refracted; but the sound field maintains phase coherence, and beam behavior can be analytically predicted. A rough interface, however, modifies boundary conditions, and some of the beam energy is diffracted. Beyond the interface, a coherent wave must re-form through phase reinforcement and cancellation; the wave then continues to propagate as a modified wave. The influence on the beam depends on the roughness, size, and contour of the modifying interface. For example, a plane wave striking a diaphragm containing a single hole one wavelength in diameter will propagate as a spherical wave from a point (Huygens) source. The wave from a larger hole will re-form in accordance with the number of wavelengths in the diameter. In ultrasonic inspection, a 2.5 m (100 in.) surface finish may have little influence at one inspection frequency and search-unit diameter, but may completely mask subsurface discontinuities at other inspection frequencies or search-unit diameters. Near-Field and Far-Field Effects. The face of an ultrasonic-transducer crystal does not vibrate uniformly under the influence of an impressed electrical voltage. Rather, the crystal face vibrates in a complex manner that can be most easily described as a mosaic of tiny, individual crystals, each vibrating in the same direction but slightly out of phase with its neighbors. Each element in the mosaic acts like a point (Huygens) source and radiates a spherical wave outward from the plane of the crystal face. Near the face of the crystal, the composite sound beam propagates chiefly as a plane wave, although spherical waves emanating from the periphery of the crystal face produce short-range ultrasonic beams referred to as side lobes. Because of interference effects, as these spherical waves encounter one another in the region near the crystal face, a spatial pattern of acoustic pressure maximums and minimums is set up in the composite sound beam. The region in which these maximums and minimums occur is known as the near field (Fresnel field) of the sound beam. Along the central axis of the composite sound beam, the series of acoustic pressure maximums and minimums becomes broader and more widely spaced as the distance from the crystal face, d, increases. Where d becomes equal to N (with N denoting the length of the near field), the acoustic pressure reaches a final maximum and decreases approximately exponentially with increasing distance, as shown in Fig. 8. The length of the near field is determined by the size of the radiating crystal and the wave-length, , of the ultrasonic wave. For a circular radiator of diameter D, the length of the near field can be calculated from: (Eq 7) When the wavelength is small with respect to the crystal diameter, the near-field length can be approximated by: (Eq 8) where A is the area of the crystal face. Fig. 8 Variation of acoustic pressure with distance ratio for a circular search unit. Distance ratio i s the distance from the crystal face, d, divided by the length of the near field, N. At distances greater than N, known as the far field of the ultrasonic beam, there are no interference effects. At distances from N to about 3N from the face of a circular radiator, there is a gradual transition to a spherical wave front. At distances of more than about 3N, the ultrasonic beam from a rectangular radiator more closely resembles a cylindrical wave, with the wave front being curved about an axis parallel to the long dimension of the rectangle. Near-field and far-field effects also occur when ultrasonic waves are reflected from interfaces. The reasons are similar to those for near-field and far-field effects for transducer crystals; that is, reflecting interfaces do not vibrate uniformly in response to the acoustic pressure of an impinging sound wave. Near-field lengths for circular reflecting interfaces can be calculated from Eq 7 and 8. Table 3 lists near-field lengths corresponding to several combinations of radiator diameter and ultrasonic frequency. The values in Table 3 were calculated from Eq 7 for circular radiators in a material having a sonic velocity of 6 km/s (4 miles/s) and closely approximate actual lengths of near fields for longitudinal waves in steel, aluminum alloys, and certain other materials. Values for radiators with diameters of 25, 13, and 10 mm (1, , and in.) correspond to typical search-unit sizes, and values for radiators with diameters of 3 and 1.5 mm ( and 0.060 in.) correspond to typical hole sizes in standard reference blocks. Table 3 Near-field lengths for circular radiators in a material having a sonic velocity of 6 km/s (4 miles/s) Near-field length for radiator with diameter of: Wavelength 25 mm (1 in.) 13 mm ( in.) 9.5 mm ( in.) 3.2 mm ( in.) 1.5 mm (0.060 in.) Frequency, MHz mm in. cm in. cm in. cm in. cm in. cm in. 1.0 6.0 0.24 2.5 1.0 0.52 0.20 0.23 0.09 . . . . . . . . . . . . 2.0 3.0 0.12 5.3 2.1 1.3 0.50 0.68 0.27 0.009 0.0035 . . . . . . 5.0 1.2 0.04 13.4 5.3 3.3 1.3 1.9 0.75 0.18 0.07 0.02 0.008 10.0 0.6 0.02 27 11 6.7 2.6 3.8 1.5 0.40 0.16 0.08 0.03 15.0 0.4 0.015 40 16 10 4.0 5.7 2.2 0.62 0.24 0.14 0.055 25.0 0.24 0.009 67 26 17 6.7 9.4 3.7 1.04 0.41 0.24 0.095 Beam Spreading. In the far field of an ultrasonic beam, the wave front expands with distance from a radiator. The angle of divergence from the central axis of the beam from a circular radiator is determined from ultrasonic wavelength and radiator size as follows: (Eq 9) where is the angle of divergence in degrees, is the ultrasonic wavelength, and D is the diameter of a circular radiator. Equation 9 is valid only for small values of /D, that is, only when the beam angle is small. When the radiator is not circular, the angle of divergence cannot be assessed accurately by applying Eq 9. For noncircular search units, beam spreading is most accurately found experimentally. Beam diameter also depends on the diameter of the radiator and the ultrasonic wavelength. The theoretical equation for -6 dB pulse-echo beam diameter is: (Eq 10) where S is the focusing factor and is 1. Focusing the transducer (S < 1) produces a smaller beam. For a flat (that is, nonfocused) transducer (S = 1), the beam has a diameter of 0.25 D at the near-field distance N, where N depends on the ultrasonic wavelength as defined in Eq 7. The overall attenuation of an ultrasonic wave in the far field can be expressed as: P = P 0 exp (- L) (Eq 11) where P 0 and P are the acoustic pressures at the beginning and end, respectively, of a section of material having a length L and an attenuation coefficient . Attenuation coefficients are most often expressed in nepers per centimeter or decibels per millimeter. Both nepers and decibels are units based on logarithms nepers on natural logarithms (base e) and decibels on common logarithms (base 10). Numerically, the value of in decibels per millimeter (dB/mm) is equal to 0.868 the value in nepers per centimeter. A table of exact attenuation coefficients for various materials, if such data could be determined, would be of doubtful value. Ultrasonic inspection is a process subject to wide variation in responses, and these variations are highly dependent on structure and properties in each individual testpiece. Attenuation determines mainly the depth to which ultrasonic inspection can be performed as well as the signal amplitude from reflectors with a testpiece. Table 4 lists the types of materials and approximate maximum inspection depth corresponding to low, medium, and high attenuation coefficients. Inspection depth is also influenced by the decibel gain built into the receiver-amplifier of an ultrasonic instrument and by the ability of the instrument to discriminate between low-amplitude echoes and electronic noise at high gain settings. Table 4 Approximate attenuation coefficients and useful depths of inspection for various metallic and nonmetallic materials Using 2-MHz longitudinal waves at room temperature Attenuation coefficient, dB/mm (dB/in.) Useful depth of inspection, m (ft) Type of material inspected Low: 0.001-0.01 (0.025- 0.25) 1-10 (3-30) Cast metals: aluminum (a) , magnesium (a) . Wrought metals: steel, aluminum, magnesium, nickel, titanium, tungsten, uranium Medium: 0.01-0.1 (0.25- 2.5) 0.1-1 (0.3-3) Cast metals (b) : steel (c) , high-strength cast iron, aluminum (d) , magnesium (d) . Wrought metals (b) : copper, lead, zinc. Nonmetals: sintered carbides (b) , some plastics (e) , some rubbers (e) High: >0.1 (>2.5) 0-0.1 (0- 0.3) (f) Cast metals (b) : steel (d) , low-strength cast iron, copper, zinc. Nonmetals (e) : porous ceramics, filled plastics, some rubbers (a) Pure or slightly alloyed. (b) Attenuation mostly by scattering. (c) Plain carbon or slightly alloyed. (d) Highly alloyed. (e) Attenuation mostly by absorption. (f) Excessive attenuation may preclude inspection. References cited in this section 1. A.J. Krautkramer and H. Krautkramer, Ultrasonic Testing of Materials, 1st ed, Springer-Verlag, 1969 2. D. Ensminger, Ultrasonics, Marcel Dekker, 1973 Ultrasonic Inspection Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los Angeles; and the ASM Committee on Ultrasonic Inspection * [...]... following sections, along with corresponding forms of data presentation, interpretation of data, and effects of operating variables Subsequent sections describe various components and systems for ultrasonic inspection, reference standards, and inspection procedures and applications In addition, the article "Boilers and Pressure Vessels" in this Volume contains information on advanced ultrasonic techniques... presentations is their ability to reveal the distribution of flaws in a part on a cross section of that part Although B-scan techniques have been more widely used in medical applications than in industrial applications, B-scans can be used for the rapid screening of parts and for the selection of certain parts, or portions of certain parts, for more thorough inspection with A-scan techniques Optimum results... unit back and forth on the surface of the part being inspected relative to a position centered over the flaw and observing the effect on both flaw echo and back reflection If the search unit can be moved slightly without affecting the height of either the flaw echo or back reflection, it can be assumed that the sound beam is sufficiently larger than the flaw) Control settings on the instrument and physical... interference from standing waves occurs when the ultrasound of a single frequency is introduced into a part Usually, only a small amount of the direct beam is absorbed by the receiver; the remainder is reflected back and forth repeatedly within the testpiece, soon filling the entire volume of material with a spatial field of standing waves These standing waves create interference patterns of nodes and antinodes... the good and bad C-scan zones Figure 26( a) shows the region of poor ultrasonic transmission (zone A, Fig 25) Inadequate consolidation, porosity sites, bunched fibers, and large grain sizes are visible throughout this zone Specimens sectioned through the region of good ultrasonic transmission (zone B) exhibited no porosity (Fig 26b) Fig 26 Photomicrographs of specimens taken from the good and bad C-scan... coherent radiation; in a thin-wall specimen that produces front and back wall echoes, the two reflected pulses show phase differences and can interfere coherently If the pulse contains a wide band of frequencies, interference maxima and minima can occur at particular frequencies, and these can be related to the specimen thickness Sound conduction is utilized in flaw detection by monitoring the intensity... used to analyze the type, size, and location (chiefly depth) of flaws B-scans: This format provides a quantitative display of time-of-flight data obtained along a line of the testpiece The B-scan display shows the relative depth of reflectors and is used mainly to determine size (length in one direction), location (both position and depth), and to a certain degree the shape and orientation of large flaws... search unit on the same spot causes the echo indication to vary randomly; some peaks rise and others fall, and the position of the signal shifts in either direction on the time trace, as indicated by arrows on the oscilloscope screen display in Fig 19(c) Traversing the search unit in an arc would cause the signal to vary randomly in amplitude and to broaden slightly rather than shift in position If the... Pulser circuit, or rate generator, to control frequency, amplitude, and pulse-repetition rate of the voltage pulses that excite the search unit Receiver-amplifier circuit to convert output signals from the search unit into a form suitable for oscilloscope display Sweep circuit to control (a) time delay between search-unit excitation and start of oscilloscope trace and (b) rate at which oscilloscope trace... lay-up, and tensile bar specimens were removed from good (zone B, Fig 25) and poor (zone A) sound-transmission regions The results of tensile testing from good and bad C-scan zones showed no correlation This should be expected because the fiber strength dominates and because the matrix contribution is minimal even with porosity or laminar-matrix defects The broken tensile specimens were then polished and . 0.07 0.02 0.008 10.0 0 .6 0.02 27 11 6. 7 2 .6 3.8 1.5 0.40 0. 16 0.08 0.03 15.0 0.4 0.015 40 16 10 4.0 5.7 2.2 0 .62 0.24 0.14 0.055 25.0 0.24 0.009 67 26 17 6. 7 9.4 3.7 1.04 0.41 0.24. 25, 13, and 10 mm (1, , and in.) correspond to typical search-unit sizes, and values for radiators with diameters of 3 and 1.5 mm ( and 0. 060 in.) correspond to typical hole sizes in standard. acoustic impedance and amplitude of particle motion. The acoustic pressure exerted by a given particle varies in the same direction and with the same frequency as the position of that particle changes