1. Field of the Invention
The present invention relates generally to an ultrasonic inspection apparatus for nondestructively inspecting an object such as, for example, a railroad rail. The principles of the present invention may, nevertheless, be incorporated in any testing apparatus in which an object is probed with sequentially generated signals, be they ultrasonic or other types of energy, capable of indicating the presence of a flaw or defect in the test piece.
In recent years, various developments have been made in apparatus and methods for automatically and rapidly inspecting a solid test piece such as a railroad rail. Such developments have been brought on by the need to provide fast, automatic, and reliable inspections that can be conducted economically, yet which permit appropriate maintenance to assure a high degree of safety in operation of rolling stock over such rail.
The developments in ultrasonic and in other nondestructive testing may, of course, be used to inspect objects other than railroad rails with equal advantage.
As further background, it should be noted that some flaws or defects (hereinafter also referred to as "abnormal conditions") are acceptable. For example, abnormal conditions smaller than a certain minimum size may be tolerated in the test piece. Furthermore, certain abnormal conditions result from intentional modification of the test piece and, therefore, are tolerable. For example, a bolt hole in a railroad rail is an abnormality but one that is expected and acceptable. It is desirable that test equipment reliably detect unacceptable abnormal conditions and, within this criterion, not signal a defect when a tolerable abnormal condition is indicated.
2. Description of the Prior Art
Various apparatus have been proposed in the past for performing ultrasonic inspection of test pieces and for analyzing the data generated by the inspection. Such apparatus operate by electrically pulsing an ultrasonic transducer or plurality of transducers to direct a high frequency sound wave or waves into the test piece. Such sound waves are transmitted through the test piece and may be reflected back to the transducer either by an expected or abnormal condition. Upon receipt of the reflected sound wave the transducer is again energized to generate a response signal. Thus, a defect in the test piece may be indicated by receipt of reflected energy from an abnormal condition, that is, receipt of reflected energy at a time after initial propagation of the wave indicating the presence of a condition which should not exist, or by failure to receive reflected energy from an expected condition within an expected time period.
U.S. Pat. Nos. 4,174,636 (Pagano) and 4,165,648 (Pagano) each disclose a two wheel ultrasonic rail testing system. Each wheel carries a plurality of ultrasonic transducers for probing an object such as a railroad rail with energy propagated at ninety degrees to the surface of the rail and forwardly and rearwardly from the wheel. Further ultrasonic transducers are provided for probing opposing gage and field corners of the head of the railroad rail. Because of the multiplicity of the ultrasonic transducers placed in each wheel to propagate energy at various angles in the rail, a wide variety of defects may be detected.
U.S. Pat. No. 3,415,110 (Cowan) discloses an ultrasonic inspection apparatus which includes a plurality of ultrasonic transducers oriented to probe various areas of a rail and which includes apparatus for processing defect signals generated by such transducers to indicate the presence of an unacceptable abnormal condition. More particularly, this system includes circuitry coupled to each differently oriented transducer for pulsing it each time it is moved by a predetermined distance relative to the rail. The response signals or echoes returned to one transducer that indicate a defect are counted and if this count exceeds a predetermined number indicative of an unacceptable abnormal condition an alarm signal is generated. This first counter is coupled to and reset by a second counter coupled to another of the transducers. Accordingly, indication of an unacceptable abnormal condition is dependent upon particular responses from two transducers. It is claimed that the multiplicity of counters prevent the apparatus from generating an alarm from separate acceptable targets or conditions but nevertheless causes generation of an alarm from an unacceptable target or condition that is so shaped as not to reflect ultrasonic energy from all parts.
The apparatus of the Cowan patent is, however, characterized by certain drawbacks. Because each transducer is pulsed at a frequency that is a function of its rate of movement relative to the test piece, non-uniform ultrasonic energy levels may be generated. Further, since the rate at which the ultrasonic transducers are pulsed varies with their speed over the test piece, it may be difficult to correlate pulsing of differently oriented transducers so as to minimize generation of unwanted ultrasonic noise. And, this system is relatively complicated in that responses from more than one transducer which are oriented differently are used to generate an alarm response.
U.S. Pat. No. 4,004,455 (McKee et al) discloses an improved and much simplified flaw detecting apparatus for railroad rails and the like. In this system a speed related pulse generator generates pulses at a frequency that is a function of the speed of the test vehicle which, therefore, is directly related to the distance traveled by the vehicle. This speed related pulse generator is coupled to a counter which counts the speed related pulses. An ultrasonic transducer is pulsed by a free running pulse generator at a fixed rate independent of the speed of the vehicle or the distance traveled by the vehicle. The free running pulse generator operates completely independently of the speed related speed pulse generator. The free running pulse generator is also coupled through a delay to a NAND gate having a second input from the ultrasonic transducer. The delay introduced in the output pulses generated by the free running pulse generator is equal to the time expected from generation of an ultrasonic wave propagated into the rail to return of that wave from a known target such as the bottom of the rail from which the transducer generates an expected response signal. Accordingly, if an ultrasonic response signal is received by the transducer within the expected time, both inputs to the NAND gate are satisfied.
The output from the NAND gate is also connected to the counter. The counter is continuously reset as long as expected response signals are received. However, in the absence of an expected response signal, the inputs to the NAND gate are not satisfied since the input from the delay will be received at a different time than that from the transducer circuitry. Failure of the NAND gate to be satisfied is an indication of a defect. Accordingly, as long as a defect indication is yielded, the counter is permitted to accumulate counts indicative of distance traveled. When the count accumulated by the counter exceeds a predetermined amount, indicative of length of an abnormal condition which is unacceptable, an alarm signal is generated.
The McKee et al apparatus provides advantages over the Cowan apparatus since it is much simplified. Further, since the ultrasonic transducers are pulsed at a fixed rate independent of speed and distance traveled relative to the rail, the level of ultrasonic energy generated may be carefully and precisely controlled. The McKee et al, apparatus is, however, still characterized by certain drawbacks. More particularly, it is specifically designed to indicate defects by absence or untimely receipt of an expected response signal by the transducer. It does not contemplate detection of defects simply by receipt of unexpected response signals. Further, if a defect is unacceptably long yet fails to create loss of a rail bottom response signal during some portion thereof, the counter may be reset to prevent generation of an alarm signal when one otherwise should be generated. Therefore, as disclosed, it is possible that an alarm signal will fail to be generated even from an unacceptable abnormal condition.
Still other ultrasonic inspection apparatus are disclosed in U.S. Pat. Nos. 2,736,193 (Van Valkenburg et al); 3,287,963 (Stanya et al); 3,354,700 (Schindler); 2,875,607 (Boxcer et al); 3,041,872 (Brown et al); 3,135,109 (Werner); 3,233,450 (Fry); 3,251,220 (Joy); 3,260,105 (McNulty); and 3,048,031 (Beaujard et al).