Method for residual stress measuring

Details Method for residual stress measuring Method for residual stress measuring

2000-01-13

2001-05-22

Williams, Hezron (Department: 2856)

Measuring and testing

Vibration

Resonance, frequency, or amplitude study

C073S599000, C073S600000, C073S602000

Reexamination Certificate

active

06234020

ABSTRACT:

The invention relates to a method for measuring residual stress in a metallic specimen wherein, by means of an ultrasound transducer an ultrasound entry wave having a ground frequency is introduced into a surface of the metallic specimen, the ultrasound entry wave is measured by means of an entry wave detector and the succeeding ultrasound exit wave is measured by means of a exit wave detector and the measured values of the ultrasound entry and exit wave are used to determine residual stress.
This method is in particular applicable and suitable for determining and mapping the values and distribution of residual stress in an entire aluminium plate, in a non-destructive fashion, using ultrasound scanning techniques. Although the invention will in the following be explained with respect to this application, the invention is not restricted thereto, but may also be applied to measure other mechanical or microstructural parameters such as grain size distribution or texture.
Rolled aluminium plates are routinely used for the manufacturing of complex machined parts. The use of rolled aluminium plates is of particular interest in aircraft and aerospace industries due to fairly uniform and predictable internal structure and mechanical properties as compared with those of forged or cast aluminium primary products. The manufacturing of aluminium plates is a multi-step process and includes casting of ingots, hot rolling the ingots to the desired thickness, normally up to 275 mm, several steps of heat treatment, and the application of mechanical work such as stretching or cold compression. During the course of the manufacturing process, stress is formed in the plate which is removed for the most part by heat treatment and the application of mechanical work. Some residual stresses might, however, remain in the plate. Depending on the magnitude of residual stress, the integrity of the plate may be jeopardized. For example, during the process of machining complex parts out of aluminium plates, residual stresses are relieved after every step of the machining operation. This results in some distortion each time. This may sometimes render the machined part completely unsuitable for the application for which it is intended. For many parts machined from rolled aluminium plates, for example those used in aircraft construction, the machining operation is a complex and prolonged process. Distortions due to residual stress in plates are often discovered when a considerable amount of material and production time is already consumed. It is, therefore, of great importance to both, producer and end user of aluminium plates to ascertain that the residual stress in these plates are at their minimum. This is only possible by using a non-destructive method which enables the measurement of residual stresses in an entire plate, and is suitable for being used online.
There are a number of test methods which are currently used by plate manufacturers to obtain an indication of the magnitude of residual stress in each individual plate. In one such test method, known under specification number BMS 7-323 a specimen is removed from a specified area of the plate and clamped in a fixed position. Several layers of specified thickness are removed from the specimen by machining and at each step, the magnitude of deflection is measured by a deflection gage. One deficiency of this method is that it is a destructive test. The specimen can only be taken from the areas near the end of a plate. Furthermore, cutting the specimen will result in releasing some of the stress. As a result, the test does not represent an entire plate with any accuracy.
Another method often used is the “Standard Test Method for Determining Residual stresses by the Hole-Drilling Strain-Gage Method” prescribed by ASTM E 837-92. In this method three strain gages are placed over the area of interest on the plate and a hole is drilled in the geometric centre of the strain gage rosette. The relieved strains are measured with a strain recording instrument and the results are related to the level of residual stress in the area of interest. One disadvantage of this method is its semi-destructive nature; it leaves the plate with shallow holes at the tested areas which may affect the use of the plate for further processing. Furthermore, in order to assess the level of residual stresses in the entire plate, numerous holes must be drilled; this process is not only time consuming, but affects the future use of the plate as well.
There are a number of non-destructive test methods which are known to have been tried as a tool to evaluate residual stresses and yet none of them were found suitable for routine operation. One example is the use of x-ray diffraction techniques as described in ASTM E 915-90. Another example is the use of photo elastic methods which was originally designed for the determination of residual stress in a transparent specimen, using polarizing microscope and optical retardation compensation procedures as described in ASTM C 978-87. Both of the above methods require elaborate laboratory work and the test results have been inconclusive.
Also other parameters of a metallic specimen, in particular parameters relating to the bulk of the specimen such as grain size distribution and texture, are commonly determined by taking a sample of the specimen at the spot of interest and performing an off-line test to establish the value of the parameter of interest. Just like described above in relation to residual stress measurement, also the determination of these parameters suffer from the drawback that the determination is offline, destructive, time consuming and restricted to relative small areas.
EP-A-0 456 028 discloses an inspection apparatus utilizing a pulse compression which apparatus comprises a signal generator, a transmission/reception probe; first and second correlators and an adder. The signal generator generates a composite transmission signal consisting of signals Sap(t), Sbp(t) and Sbq(t) respectively based on a basic unit signal ga(t) and a sequence {p}, the signal ga(t) and a sequence {q}, a basic unit signal gb(t) and the sequence {p}, and the signal gb(t) and the sequence {q}, to the probe to transmit the composite transmission signal to a target. The first correlator performs a correlation operation of echo signals Rap(t), Raq(t), Rbp(t) and Rbq(t) corresponding to the signal Sap(t), Saq(t), Sbp(t) and Sbq(t) by utilizing reference signals Ua(t) and Ub(t) based on the sequences to provide results Caap(t), Caaq(t), Cbbp(t) and Cbbq(t). The second correlator performs a correlation operation of the results Caap(t), Caaq(t), Cbbp(t) and Cbbq(t) by utilizing the sequences {p} and {q} to provide compresses pulses Caapp(t), Caaqq(t), Cbbpp(t) and Cbbqq(t). These pulses are summed up at the adder to provide a composite compressed pulse C having the large amplitude main lobe and small amplitude side lobes. An object of the disclosed invention is to provide an inspection apparatus which is capable of obtaining a compressed pulse having side lobes with a low level, preferably zero, in addition to being inexpensive and capable of attaining a high operational speed.
U.S. Pat. No. 5,474,070 discloses an ultrasonic pulse-echo method and apparatus that has particular application in making precision measurements of compressibility in any backscattering material, in particular organic tissue. The method employs a standard transducer or transducer containing device which is translated transaxially, thereby compressing or displacing a proximal region of a target body in small known increments. At each increment, a pulse is emitted and an echo sequence (A-line) is detected from regions within the target along the sonic travel path or beam of the transducer.
Resulting time shifts in echo segments corresponding to features in the target, corrected for regions of varying sonic speed along the sonic path, provide relative and quantitative information concerning the strain caused by the compressions. The s

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