Measuring and testing – Specimen stress or strain – or testing by stress or strain...
Reexamination Certificate
2000-06-30
2002-02-12
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
C073S779000
Reexamination Certificate
active
06345534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nondestructive test method for quantitatively determining fatigue of a ferromagnetic construction material, or of a structure comprised of such a material.
2. Description of the Related Art
Conventional nondestructive test methods for determining fatigue of materials are generally based on investigation of generation and growth of cracks in the material, and thus, it is highly important to find out as minute cracks as possible. With such a conventional nondestructive test method, it is practically impossible to evaluate fatigue of the material before cracks are generated.
There is known another type of nondestructive fatigue test method that is applicable to ferromagnetic construction materials or structures comprised of such a construction material. In this test method, the coercive force and magnetic susceptibility of the test material are measured in the range approaching to saturation. In this instance, for precisely determining the coercive force of the test material, it is necessary to provide a magnetizing yoke and a winding coil around it such that the test material can be magnetized to a saturation level and then demagnetized until the internal magnetic flux becomes zero. To this end, a magnetic force has to be applied that is far larger than the coercive force of that material, by using a large magnetizing yoke and allowing a large magnetizing current to flow through the magnetizing coil. A test machine incorporating such a large magnetic yoke and a large capacity magnetizing power source for energizing the magnetic yoke is not only expensive, but also makes the entire system heavy and large in size to require a noticeable installation space.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide an improved test method for nondestructively determining the fatigue of a ferromagnetic construction material, which advantageously eliminates the above-mentioned problems of the prior art.
One aspect of the present invention resides in a method for nondestructively determining fatigue of a test ferromagnetic construction material having a known, initial tensile stress &sgr;
0
, by quantifying a change in effective stress due to aging of the material. The test method according to the present invention comprises the following four steps.
The first step is to measure a magnetic susceptibility &khgr;
c
of the test material in its aged state, under a magnetic field having a predetermined intensity H, according to a relation as expressed by a first equation:
c=&khgr;
c
H
3
(1).
The second step is to determine a susceptibility coefficient c of the test material by putting the magnetic field intensity H and the measured magnetic susceptibility &khgr;
c
of the test material into the first equation.
The third step is to determine a current tensile stress &sgr; of the test material, by putting the value of the susceptibility coefficient c into a second equation:
&sgr;={log (c)−a})b (2)
where a and b are known constants determined by a structure of the test material.
Finally, the fourth step is to determine a change in effective tensile stress of the test material, by comparing the current tensile stress &sgr; of the test material with the initial tensile stress &sgr;
0
of the test material.
Another aspect of the present invention resides in an apparatus nondestructively determining fatigue of a test ferromagnetic construction material having a known, initial tensile stress (&sgr;
0
), by quantifying a change in the effective stress due to aging of the test material. The apparatus according to the present invention comprises:
i) measuring means for measuring the magnetic susceptibility (&khgr;
c
) of the test material in its aged state, under a magnetic field of a specified intensity (H), according to a relation as expressed by a first equation:
c=&khgr;
c
H
3
(1)
ii) stress calculation means for calculating and thereby determining a current tensile stress (&sgr;) of the test material, by determining a susceptibility coefficient (c) of the test material after putting the measured magnetic susceptibility (&khgr;
c
) of the test material and the magnetic field intensity (H) into the first equation, and then putting the susceptibility coefficient (c) into the second equation:
&sgr;={log (c)−a}/b (2)
where a and b are known constants determined by an internal structure of the test material; and
iii) evaluation means for determining a change in effective stress of the test material due to aging thereof, by comparing the current tensile stress (&sgr;) of the test material with its initial tensile stress (&sgr;
0
).
The nondestructive test apparatus according to the present invention, as a whole, may be comprised of a personal computer installed with programs based on the algorithm which enables execution of the above steps.
The principle of the present invention will be described below with reference to experimental test data. To elucidate the interrelationship between the mechanical and magnetic properties of steel materials, test materials were prepared which consist of single crystal pure iron, polycrystalline pure iron, and low-alloy steel A533B, respectively. These test materials were formed into samples having shapes as shown in FIGS.
1
(
a
),
1
(
b
) and
1
(
c
), respectively, which are to be subjected to tensile and hysteresis loop tests. The material formed into a sample as shown in FIG.
1
(
a
) was used for the tensile test, while the material for med into a sample as shown in FIG.
1
(
b
) or
1
(
c
) was used for the hysteresis loop test. As for the hysteresis loop test, the polycrystalline pure iron and low-alloy steel A533B took the shape of FIG.
1
(
b
) while the single crystal pure iron took the shape of FIG.
1
(
c
). Table 1 below shows the composition of the low-alloy steel A533B submitted to the test.
TABLE 1
A533B
C
Si
Mn
P
S
Cu
Ni
Mo
Al
Wt. %
0.18
0.15
1.5
0.004
0.001
0.03
0.66
0.56
0.01
FIGS. 2
to
4
illustrate the stress-strain characteristics of the test samples, obtained from the tensile test.
FIG. 2
represents the results from an Fe since crystal sample, and shows that the strain rate (i.e., extension rate) is 1.5%/min.
FIG. 3
represents the results from an Fe polycrystalline sample, and shows that the strain rate is 1.2%/min.
FIG. 4
represents the results from an alloy steel A533B sample, and shows that the strain rate is 1.2%/min.
FIGS. 5 and 6
illustrate the magnetization curves obtained from the hysteresis loop test under application of stresses.
FIG. 5
shows the hysteresis loop characteristics of an Fe single crystal sample with plastic deformation under a stress (0 MPa, 55 MPa, or 115 MPa), while
FIG. 6
shows the hysteresis loop characteristics of an Fe polycrystalline sample with plastic deformation under a stress (0 MPa, 550 MPa, or 663 MPa). The stresses applied were chosen to be equal to 0 MPa and the stress that develops just before breakage, both of which had been obtained from a preparatory tensile test, and to intermediate values between these two values.
From the gradient of the magnetization curve of a test material as depicted in
FIGS. 5 and 6
, it is possible to determine the magnetization susceptibility &khgr;
c
of the test material at a magnetic field intensity exceeding its coercive force.
FIG. 7
illustrates the relationship of the magnetic susceptibility of the low-alloy steel A533B with the magnetic field intensity H above the coercive force of the material obtained from the magnetization curve under a stress of 663 MPa as depicted in FIG.
6
. Similarly,
FIG. 8
illustrates the relationship of the logarithmic magnetic susceptibility (log &khgr;
c
) of the low-alloy steel A533B with the logarithmic magnetic field intensity H (log H) obtained from the magnetic susceptibility curve &khgr;
c
which changes as a function of the magnetic field intensity H under the stress of 663 MPa as dep
Allen Andre
Fuller Benjamin R.
Iwate University
Knobbe Martens Olson & Bear LLP
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