Method for Measuring Biaxial Stress in a Body Subjected to Stress Inducing Loads

1. Field of the Invention

The invention generally relates to a method for nondestructively measuring principal components of complex stress fields in a test article, and more particularly to an improved method for measuring stress ultrasonically.

2. Description of the Prior Art

The use of nondestructive methods based upon ultrasonic birefringence has been suggested for measuring stress level changes in laboratory specimens of anisotropic materials. However, only one component of three dimensional stress fields normally are obtainable through use of such methods. While the birefringence methods tend to work reasonably well in uniform laboratory specimens, where only one principal stress axis exists, it is noted that stress is a complex force field and where two or more of the stress components are near the same magnitude, birefringent differentials may be near zero, particulary when large stress levels actually exist in a specimen.

Therefore, there currently exists a need for a practical nondestructive method for measuring orthogonal components of complex stress fields in test articles subjected to loading in non-laboratory environments.

It is, therefore, a general purpose of the instant invention to provide a nondestructive method of measuring sub-surface biaxial stress fields in engineering structures which is simple and economic to employ and overcomes the aforementioned disadvantages.

It is an object of the instant invention to provide a method for nondestuctively measuring biaxial stress which overcomes the aforementioned difficulties and disadvantages.

It is another object to provide a nondestructive method of measuring biaxial stress levels existing in engineering materials and structures.

It is another object to provide a nondestructive method of measuring sub-surface biaxial stress in a non-laboratory environment.

It is another object to provide a method of measuring biaxial stress in a body subjected to stress loading which includes the steps of measuring time differentials between ultrasonic shear waves and longitudinal waves propagated along parallel paths through the stress materials.

It is another object to provide a method wherein the time differential between the second wave echo for a longitudinal wave and the first wave echo for each of a pair of orthogonally related shear waves propagated through a test article is determined and compared to time differentials established between the second wave echo for a longitudinal wave and the first wave echo for each of a pair of orthogonally related shear waves propagated through a calibrating specimen of the same material, thermal conditioning, and grain orientation as the test specimen.

These and other objects and advantages are achieved through the use of a method wherein time differentials between ultrasonic shear waves and longitudinal waves propagated through a calibrating specimen are established to provide indications of stress in the specimen and thereafter time differentials are established between ultrasonic shear waves and longitudinal shear waves propagated through test articles and compared to the time differentials established to provide indications of stress, for thus determining the magnitude of stress in the test articles, as will hereinafter become more readily apparent by reference to the following description and claims in light of the accompanying drawings.

FIG. 1 is a diagrammatic view illustrating a calibrating specimen subjected to tensile stress to be measured in accordance with the method which embodies the principles of the instant invention employing longitudinal and shear wave transducers attached thereto.

FIG. 2 is a graphic view depicting a series of transit time differentials for first wave echos for ultrasonic shear waves and second wave echos for longitudinal waves propagated through the specimen shown in FIG. 1, with alternate orientations of the shear wave transducer being employed.

FIG. 3 is a table depicting a typical compilation of data prior to plotting on the graph illustrated in FIG. 2.

FIG. 4 is a diagrammatic view illustrating oscilloscope circuitry which may be employed in performing the method of the instant invention.

The characteristics of ultrasonic waves are generally well understood. For example, it is known that the principal effect of stress on ultrasonic waves propagated through metallic materials occurs in the direction of material vibration or particle motion for the particles of the crystalline lattice of metallic materials. It is known, also, that X-cut crystals are suitable for use as transducers for propagating longitudinal waves to initiate vibration in the direction of propagation, while Y-cut crystals are suitable for use as transducers for propagating shear waves orthogonally related to the direction of propagation. For the sake of convenience, transducers employed in propagating longitudinal waves are hereinafter referred to as longitudinal wave transducers while transducers employed for propagating shear waves are hereinafter referred to as shear wave transducers.

It is also recognized by those familiar with the measurement of stress in metallic bodies employing ultrasonic waves that longitudinal waves propagated along paths normally related to planes of stress tend to be unaffected by the stress while the transit time for shear waves propagated along similar paths tends to increase as tensile stress increases and decreases as compressive stress increases. Therefore, since a shear wave transducer can be rotated 360.degree. it is possible to obtain transit time values indicative of sub-surface stress occuring along multiple stress axis by employing the transit time for the longitudinal waves as a reference and the transit time of the shear waves as stress indications.

Hence, the determination of magnitudes of stress ultrasonically is essentially a measurement of ultrasonic velocity through the material of a test article. The following expression depicts this velocity/stress relationship.

In addition to stress, numerous material properties affect velocity of sound in a medium. Fortunately, the effects of these variations can be accounted for by adequate calibration.

As illustrated in FIGS. 1 and 4 of the drawings, a calibration specimen, designated 10, FIG. 4, may be subjected to tensile stress along orthogonally related axes, while a longitudinal wave transducer 12 is attached to the body for propagating longitudinal waves along a path normal to the planes of the stress while a shear wave transducer 14 is attached thereto for propagating shear waves along paths paralleling the paths of the longitudinal waves. Of course, the particle motion induced by the longitudinal wave transducer 12 is in the direction of wave propagation, while the particle motion induced by the shear wave transducer 14, for each orientation thereof, is in a direction normal to the direction of wave propagation, as indicated by the directional arrows D.sub.1, D.sub.2 and D.sub.3 of FIG. 1.

As illustrated in FIG. 4, where desired, the transducers 12 and 14 are connected in separate channels of a suitable dual beam oscilloscope, at diode switches 16. The time base of the oscilloscope is, in operation, expanded until each cycle of the narrow pulses can be observed visually. The distance or time between corresponding peaks of the two signals utilized as indicators is proportional to the stress in the specimen.

Since the circuitry of the oscilloscope forms no part of the instant invention and the design, purpose and modes of operation are well known, a more detailed description of the circuit is omitted in the interest of brevity. However, it should be appreciated that the instant invention embodies the concept of measuring time differentials between ultrasonic shear waves and longitudinal waves propagated through a stressed material along parallel paths, as indicated on the screen of the oscilloscope.

The indication obtained for the transit time of longitudinal waves propagated through a stressed material is suitable for use as a reference indication, while the transit time indications for shear waves are suitable to be employed as indications of stress.

The velocity of longitudinal waves in aluminum is approximately 2.46.times. 10.sup.5 cm/sec. while the velocity for shear waves is 1.22.times. 10.sup.5 cm/sec. Thus the transit time for the second wave echo for the longitudinal wave is of the same order of magnitude as the time required for the initial shear wave echo. This relationship provides a time differential small enough to be measured with high accuracy and substantially eliminates the need for highly accurate material thickness measurements. Of course, since the shear wave transducer 14 is capable of being reoriented to 90.degree. with respect to its initial orientation, it is possible to achieve a transit time measurement for the shear wave echos as particle motion is induced along orthogonally related planes of the stress axes for thus providing biaxial sensing of stress components.

In order to perform the method embodying the instant invention, a calibrating specimen having the same characteristics of the article to be tested is selected. Viz, a specimen is acquired from the same material as the test article, having the same thermal conditioning and the same grain orientation with respect to the accessible surface as the test article is used for calibration purposes.

As depicted in FIG. 3, data is compiled for a first orientation for the shear wave transducer 14, indicated ORIENTATION "A", FIG. 2, as the specimen is incrementally loaded. Similarly, data is compiled for the alternate orientation of the transducer 14, indicated ORIENTATION "B", FIG. 2.

With a zero load on the specimen, a measurement of the time differential between the second wave echo for the longitudinal wave and the first wave echo for the shear wave for each biaxial orientation of the transducer is made. The time differential is then converted to nanoseconds/inch and plotted on the graph of FIG. 2. For example, where the time differential between the first wave echo of the shear waves and the second wave echo for the longitudinal wave is 460.5 and the total length of the path along which the wave is propagated and reflected is four inches, the time differential is divided by the total length of the path to provide the time differential in terms of distance, or 115.1. The measurement is repeated for each load as the test specimen is incrementally loaded, in a uniform manner, through five increments. The data thus obtained, shown in FIG. 3, is plotted as shown in FIG. 2.

The shear wave transducer 12 is then rotated through 90.degree. to ORIENTATION "B" and the calibrating process repeated for obtaining and compiling the data for the alternate orientation, as aforementioned. This data is then plotted, for ORIENTATION "B", as illustrated in FIG. 2.

Where desired, the time differential between the second longitudinal wave echo and the shear echo of each biaxial orientation of the transducer for an annealed calibration specimen is measured in order to indirectly measure the low level of residual stress existing in the calibration specimen and adjusting the stress values of the calibration specimen accordingly.

Measurement of stress in a test article is achieved by employing a longitudinal wave transducer and a shear wave transducer for obtaining time differential values between the second wave echo for longitudinal wave and the first wave echo for shear wave propagated by the transducer 14 at two orientations, namely, ORIENTATION "A" and ORIENTATION "B". The value of the time differentials thus acquired are compared to the data plotted on the graph of FIG. 2 for purposes of determining the magnitude of sub-surface stress along stress axes paralleling the direction of particle motion for the shear wave at each orientation of the transducer 14.

In summary, the method of the instant invention is performed by serially subjecting a calibration specimen to a series of incrementally increased stress loads of known magnitudes; obtaining the time differential between the transit time for the second wave echo for longitudinal wave ultrasonically propagated through the calibration specimen along a given path by a longitudinal wave transducer as the specimen is subjected to each stress load and the transit time for the first wave echo for each of a pair of shear waves propagated in orthogonal planes ultrasonically along a path paralleling said given path by a shear wave transducer vibrating the particles of the crystalline lattice of the body in parallelism with the stress axes extended through the calibration specimen as the specimen is subjected to each stress load; obtaining two series of transit time differential values and dividing the transit time differential values by the total length of the paths traversed by the ultrasonic waves for thus establishing a series of stress values for ORIENTATION "A" and ORIENTATION "B" for the transducer 14; obtaining a first transit time differential value between the second wave echo for longitudinal wave propagated along a first path through a test specimen, such as the wall of a pressure vessel subjected to unknown stress, and a first wave echo for a shear wave propagated, by a shear wave transducer in an orientation corresponding to ORIENTATION "A", along a second path extended through the specimen paralleling the first path for vibrating the crystalline lattice thereof in parallelism with the first stress axis for the test specimen; comparing the transit time differential value obtained for the test specimen to the series of stress values for ORIENTATION "A" for determining the magnitude of stress occuring along the first stress axis of the test specimen; rotating the shear wave transducer 14 through 90.degree. and obtaining a second transit time differential between the second wave echo for a longitudinal wave propagated through the test specimen and a first wave echo for a shear wave propagated through the test specimen by the transducer in an orientation corresponding to ORIENTATION "B" for vibrating the crystalline lattice in parallelism with a second stress axis; and comparing the second transit time differential value thus obtained for the test specimen to the second series of stress values for ORIENTATION "B" for thus determining the magnitude of stress occurring along the second stress axis of the specimen.

In view of the foregoing, it should readily be apparent that the method of the instant invention comprises a practical, nondestructive method for ultrasonically measuring stress in test articles such as engineering structure and the like.

Although the invention has been shown and described in what is conceived to be the most practical and preferred method, it is recognized that departures may be made therefrom within the scope of the invention, which is not to be limited to the illustrative details disclosed.