Abstract:
A component which is known to have particular degradation characteristics is instrumented to provide an electrical potential across a section in which a degradation is likely to occur. The potential drop across the component is then monitored to determine when, and the degree to which, the degradation occurs. Predetermined limits are established such that when the degradation level reaches a limit, the component is repaired or replaced.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to non-destructive testing of component materials subject to degradation and, more particularly, to a non-destructive monitoring of aircraft engine components for possible degradation. 
   In the upkeep and maintenance of most mechanical and/or electrical apparatus, repair and/or replacement of parts does not occur until a failure causes the apparatus to be inoperable. At that time, inspection is made to determine the particular failure that has occurred, and a replacement part is installed to bring the apparatus to an operable condition. 
   In contrast, there are certain types of apparatus which, because of safety concerns, are preferably not permitted to have their components reach the level of failure. An aircraft gas turbine engine is such an apparatus. Here, it has become common practice to predict, on the basis of component life histories the operating life of a component and, to periodically repair or replace such a component prior to the time in which it is predicted to fail. In this manner, a useful life of the component is approximated while minimizing the risk of failure. 
   In a turbine engine, component cracking (e.g. creep, low cycle fatigue (LCF), high cycle fatigue (HCF), stress corrosion cracking is usually associated with high stress risers (i.e. radius, bolt holes, flanges, etc) high temperatures, processing defects or combinations thereof. These stress locations can be identified by analysis or by experience from field failures. To mitigate risk from cracking, service life limits are determined for many components such as disks, blades, shafts, air seals, and tubing, and are removed from service before long cracks have a chance to evolve. 
   In the field of fracture mechanics, electrical potential difference is an established laboratory technique for determining crack growth rate characteristics in electrically conducting materials. The electrical field in these specimens is disturbed by the initiation of a crack and varies predictably with increasing crack size. In a case where constant current is imposed through the specimen, the potential drop across the crack plane will increase with increasing crack size. This predictable response to the electrical field is used to relate a change in voltage to crack size and is used as an automating means of continuously monitoring crack size. 
   SUMMARY OF THE INVENTION 
   Briefly, in accordance with one aspect of the invention, a component which is known to have a particular degradation characteristics is instrumented such that an electrical potential is established across a section in which a degradation is likely to occur with time of operation. The potential drop across the component is then monitored to determine when, and the degree to which, the degradation occurs. 
   By yet another aspect of the invention, when the degradation of the component reaches a predetermined level, with the level being predetermined by a review of historical occurrences, the component is repaired or replaced. 
   In accordance with yet another aspect of the invention, a flexible fuel line assembly having a polytetrafluorothylene (PTFE) tube core reinforced on its outer diameter with a stainless steel wire braid is instrumented for monitoring the potential drop thereacross to determine the occurrence of progressive wire strand breakdown which, if allowed to continue would result in failure of the PTFE tube core. When the number of wire strand failures reaches a predetermined level, the fuel line assembly is replaced. 
   In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternative constructions can be made thereto without departing from the true spirit and scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a representative flow chart illustrating a process in accordance with an embodiment of the present invention. 
       FIG. 2  is a sectional view of a fuel line assembly to be monitored in accordance with an embodiment of the present invention. 
       FIG. 3  is a schematic illustration of a component as instrumented in accordance with one embodiment of the present invention. 
       FIG. 4  is a graphical illustration of resultant measurements which are made in accordance with a preferred embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present method can be effectively used with any component that undergoes a cross sectional area change due to the progression of a crack, pitting, corrosion, erosion, wear, etc. The principal involved is that a change in the effective cross sectional area in an electrically conductive component results in a change in its electrical resistance, such that a monitoring of the electrical resistance can be used to effectively monitor the component&#39;s health state. 
   The steps of the process will now be described with reference to  FIG. 1 . Having identified a component that is prone to have a gradual degradation resulting from normal use, it is first necessary to identify the particular failure mode that is likely to occur as set forth in block  11 . This includes not only the particular location on the component but also the particular direction of propagation of the fault. 
   The next step, as shown in block  12 , is to instrument the component to enable the measurement of its electrical resistance or potential drop thereacross. Preferably, the potential drop is measured between two points having a connecting line that is aligned substantially normally to the direction of the fault propagation. That is, to monitor the condition of a crack, the direction of the crack should be substantially normal to the direction of the line connecting the two measurement points such that as the crack increases in width, the potential drop between the two measurement points will be proportionally increased. 
   It is necessary to determine, for the particular fault that is likely to occur to the component, a representative potential drop limit that would indicate the degree to which a degradation, such as how big a crack is allowed to grow, is allowed to occur without any substantial risk of failure. This is accomplished in step  13 . This limit may relate to the size of the crack or the speed of propagation, for example. 
   In order to have a base in which the operational measurements will relate, it is necessary to first measure the potential drop across the component when in the non-degraded condition as shown in step  14 . Any increase in the potential drop that is subsequently measured will be an indication of the degradation that occurs during operational use. 
   The instrumentation is then maintained in its installed position such that the potential drop across the component can be continued to be monitored on a real time basis during operational use of the component as shown in block  16 . Thus, at any time, the specific reading can be considered and compared with historical data to determine whether the condition of component is on track with the predicted performance or whether it may be degrading at a rate that is indicative of a degradation rate that is accelerated from the predicted rate and therefore calls for taking corrective action. The step at block  17  is therefore provided to compare the measured values with the value(s) determined in block  13 , and these values may be indicative of an accumulative limit on the crack size or, alternatively, indicative of an unacceptable propagation rate. In either case, when such limit is reached, indicating that action should be taken, the component should be repaired or replaced as set forth in block  18 . 
   It should be recognized that the above method may be used to monitor any component that has a failure mechanism that evolves from loss of material or change in cross sectional area. For purposes of explanation, the process will now be described in terms of use with a particular application relating to an engine having a fuel line that is subject to degradation. 
   Referring now to  FIGS. 2 and 3 , a flexible fuel line assembly is shown at  21  which consist of a PTFE tube core  22  reinforced on its outer diameter with a stainless steel wire braid  23 . Typical of such an assembly, the structure of the wire braid  23  consists of approximately 175 strands of 0.005 inch conductive, stainless steel wire. The stainless steel wire  23  and the PTFE  22  are held together at each end with a crimped sleeve tube connector indicated at  24  and  26 , respectively. An example of such a flexible hosing is part no. 1A9357 available from Titeflex Corporation, Springfield, Mass. 
   An identified potential failure mode of the flexline fuel line assembly  21  is a corrosion of the stainless steel braid followed by a progressive breakage of the individual stainless steel wires. The progressive failure of the braid  23  could eventually result in the rupture of the inner PTFE core  22 , resulting in fluid leakage. The present method monitors and detects the progression of damage to the wire braid  23  and allows for the replacement of the fuel line assembly  21  in a timely manner before the integrity of the PTFE core  22  can be comprised. 
   In  FIG. 3 , there is shown a schematic illustration of the non-destructive instrumentation arrangement for monitoring the condition of the fuel line assembly  21  in accordance with the method as described hereinabove. Here, a DC current source  27  is connected to provide a voltage drop across a 4 inch strand of the fuel line assembly  21 , with a constant current of 1 amp flowing through the stainless steel wire braid  23 . The voltage drop across the assembly is then continually measured by the volt meter  28  and recorded by a recording device  29 . Such an assembly and instrumentation can then be used during the period in which the fuel line assembly  21  is assembled and operating with an engine in typical use. This allows monitoring of the condition of the fuel line assembly and helps to determine when its condition has degraded to the point that an action of repair or replacement needs to be taken. 
   For purposes of demonstrating the concept, a deterioration of the stainless steel wire braid  23  was simulated by progressively cutting 4 braid wires and recording the voltage drop change that occurred as a result thereof. The results are shown in  FIG. 4  wherein the normalized potential difference (PD) is recorded as a function of time. 
   First, a reference voltage was measured to calibrate the apparatus to show that, in the undegraded condition, the voltage drop, is indicated as zero units as shown at curve a. Subsequently, a first wire was cut and the voltage drop was measured to be 0.2 units as shown at curve b. Similarly, a second, a third and a fourth wire was successively cut and the respective voltage drops measured were as shown at curves c, d and e, respectively. It will be thus seen that an approximate 0.2% change in the voltage was detected for each wire cut. This voltage increase is well above the ambient noise of the system and a clear indication of damage progression. The present invention uses a threshold valve to determine when the fuel line assembly  21  should be replaced, namely the number of mesh wires that would need to break before the PTFE core integrity was compromised. Thus, the instrumentation can be used to automatically measure and detect when the number of broken wires approaches the limit. Alternatively, the rate of breakage of wires can be monitored to determine whether this compares favorably with a normal rate, and if the rate exceeds the normal rate, then one may find it desirable to replace the fuel line assembly at an earlier time than would normally be indicated by the approach of allowing the number to approach the limit, or, alternatively, to use this information to determine the reason for the accelerated rate of failures in the individual wire strands. 
   Although the instrumentation as described hereinabove uses a DC potential system, it should be understood that a similar AC potential system could also be utilized. 
   It should also be recognized that, although the present method has been described in terms of use with a fuel line assembly, it is equally applicable to various other components which may be subject to degradation by any means such as cracking, pitting, corrosion, erosion, wear and so forth. 
   Representative applications of the present method include but are not limited to the following. In bearings, failure due to wear and loss of raceway material can be detected as a measurable change in resistance. A change in resistance across a turbine blade or vane can be used as an indicator of cracking, erosion, creep or foreign object damage. Pressure vessels such as engine cases, undergoing pressure cycles, can be monitored for low cycle fatigue (LCF) cracks in high stress locations. On a larger scale, large aluminum sections of airplane fuselage could be monitored to detect and warn of fatigue cracks at rivet holes. By continuously monitoring these life limiting locations using the potential drop method as described hereinabove, service life can be safely extended and field issues can be monitored as part of a comprehensive maintenance program. 
   While the present invention has been particularly shown and described with reference to a preferred embodiment as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the true spirit and scope of the invention as defined by the claims.