Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2000-05-16
2003-02-25
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C073S637000
Reexamination Certificate
active
06523418
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an apparatus and method for non-destructive testing of stress within solid materials using the L
CR
ultrasonic technique. Specifically, the present invention relates to an improved technique and apparatus which provides a more accurate stress measurement in curved engineering components such as pressure vessels, tanks and piping.
2. Description of the Related Art
The non-destructive testing for stress in metals has long been recognized as an important method for evaluating metal components to predict both the failure location and rate, to identify stressed components prior to failure, and many other safety related considerations. Non-destructive testing is used extensively in a wide variety of industries including the aviation, automotive, petroleum, and chemical industries, and various construction and structural related fields. The use of non-destructive testing on specific components ranges from the testing of steel turbine blades in jet engines to steel support beams in bridges and other large structures. The benefit of non-destructive testing is self-evident. Components in use can be tested to determine the stress levels in the components without damaging or destroying the components.
Techniques such as x-ray diffraction and Barkhausen noise analysis have been successfully applied for nondestructive stress measurements. While the x-ray techniques are quite reliable in their measurements, they measure stress in only the top few angstroms of the surface, and the results may not be indicative of internal stresses. The Barkhausen method is based on small changes in magnetic permeability due to stress. Application of the Barkhausen method is limited to the surface and to electrical conducting materials. It has been available for a number of years, but has not been shown to give generally reliable results.
The L
CR
ultrasonic technique indicates stress through the acoustoelastic principle where small variations in strain affect the wave speed. By measuring the wave speed (or travel-time between known points) the change in stress can be calculated. Other material variations such as texture and temperature also affect the travel-time. The investigator using the L
CR
method must be aware of these other effects so that the best data indicative of stress variation is obtained.
The relationship of measured L
CR
wave travel-time change and the corresponding uniaxial stress is given by:
Δσ
=
E
Lt
0
(
t
-
t
0
-
Δ
t
T
)
where &Dgr;&sgr; is change in stress, E is Young's modulus, and L is the acoustoelastic constant for longitudinal waves propagating in the direction of the applied stress field, as given in Table 1. Travel-time change (&Dgr;t) is the measured travel-time (t) minus the reference travel-time (t
o
). The reference travel-time t
0
is a property of the probe sensor spacing for an assumed stress free material.
TABLE 1
Typical values for acoustoelastic constant (L)
Material
Load
L
Pearlitic steel
Tension
−2.38
Compression
−2.45
4140 steel
Tension 2.25 MHz
−2.2
Tension 5 MHz
−2.36
Clear acrylic-aircraft grade
Tension
−2.14
Temperature induced speed changes occur both in the material being investigated and in the probe material. The relationship of wave speed and temperature (dc/dT) is given by:
ⅆ
c
ⅆ
T
=
k
T
m
s
-
°
C
.
where k
T
is the constant for a particular material, as given in Table 2.
TABLE 2
Temperature effects on wave speed
Material
k
T
PMMA
−2.3
Pearlitic Steel
−0.55
The effect of temperature on travel-time will be
Δ
t
T
=
d
k
T
Δ
T
where d is the travel distance in the material and &Dgr;T is the measured temperature change. Thus, as shown in Table 2, the temperature effect for PMMA is greater than that of the steel. Where data are collected under moderately uniform temperature conditions, the temperature effect, &Dgr;t, can be ignored. For large temperature variations, a suitable correction in the travel-time can be made.
Texture as typically encountered in cold-rolled plates and other structural shapes can have a significant affect on the wave speed. While the affect of texture on the L
CR
wave speed is less than that encountered by the shear waves often used in acoustic-birefringence stress measurements, there still is concern about the effects.
Special data collection procedures may be used to minimize the effects of texture. In many items where stress is a concern, the texture may be uniform throughout. In these cases, L
CR
travel-times taken with the probe always at the same orientation relative to the geometry of the item may be free of texture variation. In this case, the major effect may be stress. This has been found to be true for plates and welded structures. However, there is a need for more data on additional structures and shapes before this assumption may be more widely made.
Ideally, the L
CR
pulse is a true, nondispersive wave travelling at the longitudinal wave speed of the material. There are shape and material effects, however, that can cause dispersion of the wave. In many of these cases, the wave can still be used for stress measurement by the careful operator, and by choosing the proper probe.
Wave-guide effects are one of the most serious causes of dispersion, although they are easy to eliminate due to knowledge of the geometry of the test specimen. These effects occur in plates and pipes, which act as the waveguide, when the wavelength of the wave approaches some fraction of the thickness. Typically, when the ratio of plate thickness to wavelength is ten or above, there is no risk of any waveguide effect. Satisfactory results have been obtained with ratios of five.
Texture effects, discussed above, and grain boundary scattering also affect the pulse shape. Texture may be evaluated with a contact shear wave acting across the thickness. Grain boundary scattering may be evaluated with attenuation measurements also across the boundary. Data is still being collected to determine acceptable ranges for L
CR
stress measurement in light of these dispersive effects.
Choosing the proper reference location within the L
CR
pulse can enable the collection of reliable data. When the data are collected, a wave form is observed that crosses above and below a reference of zero. Typically, the second positive zero crossing at the first arrival of the pulse is used as this reference. In nondispersive conditions, this location is easy to identify at all pulse arrivals. Under dispersive conditions, however, identification may be more difficult. In difficult circumstances, identification can be aided by sliding a receiver probe along the travel path and observing the change of shape.
Ultrasonic stress measurement techniques have been developed in the past. Some use longitudinal waves, but they have not met with success due to the absence of a method for accurately controlling the coupling state between the probe and the item being inspected. Others use shear (SH) and/or Rayleigh waves which are well known to be less sensitive to stress than is the L
CR
wave.
Accordingly, a need exists for a non-destructive testing method and apparatus to accurately indicate the internal stresses of metal, particularly curved engineering components such as pressure vessels and pipes. The method and apparatus should accurately control the coupling state between the probe and the item being inspected and take into account or avoid various interference factors, thus providing accurate and reliable stress measurements. Further, the method should be useful for both flat materials and curved engineering components, such as pressure vessels and pipes.
SUMMARY OF THE INVENTION
The L
CR
ultrasonic technique is a unique nondestructive method for evaluating stress levels and other mechanical property variations in various engineering components, structures and materials. The inspection is accomplished with a newly designed apparatus inco
Carstens David W.
Carstens Yee & Cahoon LLP
Noori Max
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