Optics: measuring and testing – Material strain analysis
Reexamination Certificate
2001-02-02
2003-07-01
St. Cyr, Daniel (Department: 2876)
Optics: measuring and testing
Material strain analysis
C356S034000
Reexamination Certificate
active
06587188
ABSTRACT:
PRIORITY CLAIM
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 100 04 384.4, filed on Feb. 2, 2000, the entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to an arrangement and a method for detecting and measuring strain and temperature and variations thereof, of a cover layer applied on a substrate, as well as to a method of making such a sensor arrangement. The sensor arrangement uses only a single sensor for separately determining the strain and the temperature and the variations thereof, of certain portions of surface coating layers, and achieves an application of optical sensors that is protected from outside environmental influences by being integrated into the surface coating layer.
BACKGROUND INFORMATION
It is becoming increasingly important to monitor, observe, control, and regulate various structural characteristics in the fields of conventional transport systems for air travel and space travel, and also in the field of motor vehicle construction. Examples of the above include: systems for determining the load realities, so-called load monitoring or load progression monitoring systems, systems for the early detection of structural damage (so-called health monitoring), systems for predictive maintenance requirements and maintenance support, for example at inaccessible locations or known critical structural locations of vehicles and equipment, and in general various “adaptive systems”, among which the adaptive wing, the adaptive rotor, the adaptive pantograph, the adaptive landing gear, and the adaptive airbag are already known in the art.
In various ways, these systems contribute to satisfying the needs of the customer who has purchased the vehicle or other equipment. For example, in the case of a load progression monitoring system, the consumed operating life and the remaining operating life can be determined from the actual loads to which the vehicle or equipment has been subjected. Thereby, it becomes possible to aircraft, if the actually flown collective or total load is below the nominal collective load at which maintenance or an overhaul or taking the aircraft out of service would be required. On the other hand, such a system provides added safety, for example when the actual collective load is greater than the nominal allowable collective load, whereupon the duration of utilization would be shortened. In view of this, such a monitoring system gives an airline or other operator of aircraft the possibility of an individualized fleet management and achieves a reduction of the operating costs and particularly the inspection, maintenance and service costs.
When an aircraft, or generally any vehicle, approaches the end of its useful operating life, the effort and expense of inspection and upkeep become significantly increased. In this case the next situation arises, whereby the inspection and the like can be automated, whereby the effort and expense thereof can be reduced. Components that were originally designed and manufactured in an error-free or defect-free manner, can now be operated in a fault tolerant manner without any safety losses or limitations, which in turn will lead to an increase of the useful operating life and therewith a decrease in costs.
Furthermore, due to their own specialized adaptive capabilities, adaptive systems predominantly contribute to increases or improvements of various flight characteristics (or driving characteristics in the case of a general vehicle). Depending on the concrete application, such improvements can lead to a reduction of fuel consumption, a reduction of noise generation, an increase in speed of travel as well as safety of travel, and the like.
It is common to all of these systems that they require a very robust and reliable sensor arrangement, which ensures maximum performance with minimum hardware, effort, complexity, and expense. A pertinent quantity or parameter that can be technically measured to provide required information in such systems is especially the strain of a component or material. Conventionally, such strain is especially measured by electrical strain gages such as foil strain gages of present day technology. Alternatively, strain may be measured with various types of piezoelectric or fiber optic sensor arrangements.
Furthermore, various different concepts are known in the art, whereby optical fibers integrated into a component can be used for measuring the strain and the temperature thereof. In this context, the person of ordinary skill in the art makes use of the well known relationship or equation defining the propagation constant or coefficient &bgr; of a light wave in an optical fiber, namely: &bgr;=*L, wherein n is the refractive index of the light wave, i.e. the so-called modal index, and L is the length or measuring length of the fiber. Nearly all presently known measuring concepts for measuring strain and temperature are based on the recognition that “n” and “L” in the above equation are varied as a result of strain and temperature variations. This fact also underlies the basic problem of all known temperature and strain measurements using fiber optic sensors, for all types of structural sensor arrangements, namely that the strain and the temperature have a basic and fundamental influence on the values or parameters that can be determined using available measuring technology. Thus, it is difficult or impossible to separately determine the temperature and the strain, because it is difficult or impossible to separate the influences that the temperature has on the strain, and the combined influences that the temperature and the strain both have on the measured parameters.
Published European Patent Specification EP 0,753,130 B1 discloses a system including a fiber optic Bragg grating sensor (FBGS) integrated into the structure of a fiber-reinforced composite material. A separate determination of strain and temperature is possible by means of the two polarization Bragg resonances, which arise because the optical fiber integrated into the structure becomes doubly refractive, i.e. birefringent, whereby the birefringence itself is temperature dependent. According to this reference, the temperature dependent birefringence effect is so strongly or sharply developed, that one obtains two reflection peaks from the Bragg grating. The spacing between the reflection peaks, i.e. the difference between the Bragg wavelengths of the polarization Bragg resonances, is then used as a measure for the temperature, and the Bragg wavelength of each respective peak is used as a measure for the strain and the temperature. Based on this information, a computer-supported calculation and determination of strain and temperature would be possible.
The above mentioned sensor system, however, suffers the disadvantage that it apparently only applies to fiber optic Bragg grating sensors (FBGS) integrated directly into fiber reinforced composite structural components. Thus, the sensor system must be “built in” to the structural component as it is being fabricated. Also, the optical fiber must be ideally oriented perpendicular to the material fibers of neighboring layers. This fact has significant negative effects on the mechanical characterizing values of the structure under some circumstances, and on the effort and expense of fabrication thereof, and will not be practically acceptable to a person of ordinary skill in the art.
German Patent DE 31 42 392 C2 discloses an arrangement for a rip or crack sensor as well as an embodiment for practically realizing such an arrangement. This arrangement uses optical fibers that are integrated into a “painted-on” coating layer on the surface of a substrate. This German Patent Document discloses the substrate-localized application of non-birefringent fibers on the substrate, but does not suggest the use of a fiber-optic Bragg grating, because the disclosed arrangement simply aims to detect cracks of a surface coating layer by means of the irreversible breaking or disruption o
Gleine Wolfgang
Trutzel Michael
Airbus Deutschland GmbH
Cyr Daniel St.
Fasse W. F.
Fasse W. G.
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