Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-03-09
2001-06-26
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S597000, C073S599000
Reexamination Certificate
active
06250163
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates, in general, to devices and methods for examining welds, and in particular, to a new and useful spot weld examination apparatus and method using EMATs.
The examination of spot welds using conventional piezoelectric transducers has been established as a viable method of determining spot weld quality. The use of these transducers requires a coupling fluid or gel to be able to generate and receive ultrasonic signals. The need for a liquid couplant makes it impractical to automate this test. A large volume of background material exists for this testing. An overview of this method of testing is given in the ASNT (American Society of Nondestructive Testing)
Nondestructive Testing Handbook
Vol. 7, pages 557-568 “Ultrasonic Testing of Spot Welds in Thin Gage Steel”.
Other non ultrasonic methods of spot weld testing have been publicly disclosed. The following U.S. patents are known to relate to spot weld testing:
U.S. Pat. No. 3,726,130—Method of monitoring a welding operation;
U.S. Pat. No. 4,449,092—Acoustic wave spot welder adaptive control;
U.S. Pat. No. 4,887,025—Method and apparatus for non-destructive checking of spot welds between metal sheets;
U.S. Pat. No. 5,194,709—Method for checking a spot welded portion and spot welding machine; and
U.S. Pat. No. 5,399,827—Method and device for determining the temperature of a spot-welded joint and a method for assessing the quality of a spot welded joint.
In addition, a method of using Lamb waves to monitor spot welds is described in a paper “On-line Ultrasonic Lamb Wave Monitoring of Spot Welds” by S. I. Rokhlin, R. J. Mayhan and L. Adler, published in
Materials Evaluation
Vol. 43, No. 7, pages 879-883, ASNT, 1985.
At present, most spot welds used in manufacturing operations such as automotive chassis assembly are evaluated by destructively testing the welds in a statistical sampling of the assemblies. Due to economics, only a very small sampling of the assemblies are tested. If a welder develops a problem, a large number of assemblies with defective welds can be produced before the problem is detected. The manufacturer may have to scrap a large number of assemblies if a systematic problem in producing the welds is detected during destructive testing. In addition, bad welds can be produced at random, and for reasons other than welder problems. This results in assemblies that are of poorer quality that can fail prematurely in service. To compensate for the likelihood of the presence of bad welds in assemblies, the manufacturer may use a much larger number of welds than would be required if the welds were known to be good, increasing production costs. The majority of these welds receive little or no testing as to the quality of the weld.
An automated, reliable and cost effective method of nondestructively testing spot welds is needed to provide a measure of spot weld quality for each spot weld produced. The present invention satisfies that need.
An ultrasonic testing technique for nondestructive evaluation that does not need a fluid couplant has been developed within approximately the last twenty-five years. The technique depends upon electromagnetic acoustic interaction for elastic wave generation. Radio Frequency (RF) currents flowing in a flat coil placed close to a metal surface induce eddy currents in the metal surface. A static or quasi-static magnetic field is also applied to the metal surface. The eddy current flowing in the surface of the metal in the presence of the magnetic field experience a Lorentz force. This force then generates a mechanical disturbance, coupling to the atomic lattice by a scattering process. In electromagnetic acoustic generation, the electromagnetic conversion takes place directly within the eddy current skin depth. Therefore, no mechanical coupling to the body is needed. The metal surface is its own transducer. Reception takes place in a reciprocal manner.
If an elastic wave strikes the surface of the conductor in the presence of a magnetic field, induced currents are generated in the receiving coil. This is similar to the operation of an electric generator. These transducers are known as EMATs (ElectroMagnetic Acoustic Transducers). EMATs can produce a variety of wave modes including surface waves, longitudinal waves and horizontally polarized shear waves. The absence of a couplant makes it possible to design transducers that operate at elevated temperatures, can be rapidly scanned and can be easily automated. In addition, the operating characteristics of EMATs can be reproduced from one unit to another very easily.
SUMMARY OF THE INVENTION
Two concepts for employing EMATs in spot weld testing have been developed according to the present invention.
In the first approach, an EMAT is placed on a spot weld, generating and receiving shear waves that travel normal to the surface of the plates welded together. The shear wave travels through the weld to the opposite surface. It then reflects back to the surface where the transducer is positioned and is picked up by the transducer operating in a receive mode. The shear wave continues to reverberate between the two surfaces of the welded plates (top and bottom), producing signals each time it strikes the surface where the transducer is resting. If the metal plates are completely unbonded, i.e., no weld, then the shear wave reverberates between the surfaces of the plate the transducer is located on, resulting in echo spacing that is much shorter than for a good weld. If the two plates are only partially fused, then reflections from the partially disbanded areas are seen as well as reflections from the outer surface of the weld. In some cases, the spot welds fuse the two surfaces of the plates together, but the weld does not penetrate throughout the thickness of the plates resulting in a bad weld. This is known as a “cold weld” or “stick weld”. In this case, large ultrasonic reverberations through the total thickness of the two plates are observed, with no reflections from the interface between the two plates, making it difficult to distinguish from the signals obtained in a good weld. However, the ultrasonic reverberations in a good weld decay more rapidly than the reverberations in a “cold” weld because a good weld produces large grains throughout the thickness of the two plates. By processing the received waveforms, the quality of the weld can be determined for the various weld conditions. As an alternative, a separate transmitter EMAT and a receiver EMAT can be used on opposite sides of the weld with similar results.
The second concept for using EMATs to test spot weld quality involves determining the time it takes for the weld to solidify after the welding current has been shut off.
Spot welding is typically used to bond two or more sheets of metal together. This is accomplished by pressing one electrode on the outer surface of the top sheet of metal and another electrode on the outside surface of the lower sheet of metal, and then passing a large current from the electrodes through the metal layers. The metal layers are heated until they become molten, fusing them together.
After the current is shut off, the metal cools and solidifies forming a spot weld. In order to test the quality of the spot welds, a pair of EMATs or ElectroMagnetic Acoustic Transducers, a transmitter EMAT and a receiver EMAT, are incorporated into the spot welder apparatus. The EMATs are connected to an EMAT pulser/receiver in the same manner as in the first concept of the invention. An N=0 Shear Horizontal (SH) plate wave is generated by the transmitter EMAT in the top layer of metal, focused through the spot weld and detected by the receiver EMAT at the bottom layer of metal. Immediately after the welder current is shut off, the transmitter EMAT is fired at a high repetition rate while monitoring the receiver EMAT.
When the metal is molten, no signal is obtained because a liquid cannot support shear waves. As soon as the weld solidifies, the shear wave is able to propagate through the weld, and a signal wi
Latham Wayne M.
MacLauchlan Daniel T.
Baraona R. C.
Edwards R. J.
McDermott Technology Inc.
Miller Rose M.
Williams Hezron
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