Patent Application
20080107147
Embodiments of the present invention relate generally to non-destructive devices and methods for inspecting structures. More particularly, embodiments of the invention relate to devices and methods for inspecting a skin of a structure for the presence of shallow sub-surface flaws, damages, and trapped foreign matter.
Non-destructive inspection (NDI) of structures involves examining a structure without harming the structure or requiring significant disassembly. NDI methods are often preferred in order to avoid the time and costs associated with the removal of a part and to avoid the potential for causing damages when inspection is needed. In some situations, NDI methods might be the only methods by which inspections may be performed. NDI devices and methods are used in the aircraft industry to inspect aircraft structures such as composite structures and bonded panels. Inspections may identify defects such as cracks, discontinuities, disbonds between layers, voids, and areas having undesirable porosity. Preventive inspections may be performed during manufacturing and at any time during the service life of an aircraft structure to confirm the integrity and fitness of the structure. Inspections may also be prompted by incidents such as collisions and ballistic impacts.
NDI methods that include infrared imaging are available but involve costly equipment and a high degree of training. For example, infrared imaging, or thermography, is used in aircraft industries to inspect composite honeycomb parts for the presence of moisture ingression and to inspect fuselage components for disbonds between layered composite materials. In these applications, infrared imaging provides a graphical display of an inspected structure. Such a display can reveal, to the trained eye, sub-surface flaws in a structure. However, the necessary infrared cameras and technicians trained to interpret thermographic displays represent considerable costs. Thus, while the equipment and experience needed for infrared imaging may be available at specialized repair and maintenance facilities, they are not typically readily available at commercial airports and remote aviation facilities.
Incidents of minor and superficial damages are expected at busy aviation facilities. Commercial aircraft encounter airborne debris in-flight and are subject to inadvertent collisions from luggage loaders and passenger loading ramps. Military aircraft are subject to damages from ballistic weapons and shrapnel. Thus, the skin of an aircraft can have many surface marks. Some surface marks are superficial and are of little or no importance. Other marks overlay sub-surface damages that can threaten the integrity of the marked component. Crews that fuel, load, and generally prepare airplanes for flight face a critical challenge. An airplane with significant component damage must be repaired, despite the inconveniences that might represent, in order to preserve lives. However, needless grounding of planes out of concerns over minor bumps and scratches must be minimized to preserve the commercial viabilities of airline companies, in order to efficiently serve traveling customers, and in order for military aircraft to provide critical support to ground-deployed forces in situations where time may be of critical essence.
Sub-surface damages to aircraft components typically begin at the skins of the components. Damages can propagate inward through a component-skin such that an incident which threatens the structural integrity of a component leaves only a small marking at the surface where an impact occurred. Exterior repair patches can also overlay sub-surface problems. For example, a surface-applied patch can be poorly bonded or can overlay backing material inadvertently sandwiched between the patch and a surface. In many scenarios, maintenance crews would benefit from having devices and methods for inspecting component skins and repair patches for evidence of shallow damages and defects.
It would be advantageous to provide low-cost NDI devices for rapid screening of markings on aircraft-component skins to determine whether shallow sub-surface damages are present. It would be advantageous to provide NDI devices and methods that permit a ground crew, without extensive thermography training, to determine whether a scratched or marked aircraft structure is ready to fly or whether extensive inspections by specialized personnel and equipment are needed. A need exists for low-cost NDI devices that can be feasibly widely distributed at aviation facilities, and that can be used to make rapid fly versus no-fly decisions.
Embodiments of the invention may address at least some of the above needs and achieve other advantages. A first aspect of the invention relates to a method of inspecting a structure. The method includes the steps of measuring the temperature of a first portion of a surface of a structure with a first thermal sensor, measuring the temperature of a second portion of the surface of the structure with a second thermal sensor, and emitting an alert signal if the temperature of the first portion is substantially different from the temperature of the second portion. For example, first and second thermal radiometers having respective fields of view may be used to measure the temperatures of the first and second portions of the surface. A non-alert signal may be emitted if the temperatures of the surface portions are essentially the same. In at least one embodiment of a method according to the invention, an area of the surface is heated, the area including the first and second portions.
A second aspect of the invention relates to an inspection device. An embodiment of the device includes a first radiometer, a second radiometer, an indicator element, and a comparator. The first and second radiometers have respective fields of view along different respective axes. Each radiometer is capable of generating a signal indicative of the temperature of a surface when a surface is disposed in its field of view. The comparator is capable of causing the indicator element to indicate whether the indicated temperatures are substantially different. The indicator element may include first and second indicators, in which case the comparator is capable of causing the first indicator to indicate when the temperatures are substantially different and causing the second indicator to indicate when the temperatures are not substantially different. The first and second indicators may include light emitters capable of emitting different colors of light. The device may include an energy source operable to cause heating of a structure.
A third aspect of the invention relates to a method of inspecting the skin of a structure below the surface of the skin. According to an embodiment of the method, a skin of a structure is heated through an outward surface of the skin to cause heating of the surface. Respective temperatures of proximate portions of the heated surface are concurrently measured using a proximate pair of infrared radiometers. The presence of a fault within the skin below the surface is determined by observing a difference in the respective temperatures. Heating the skin may entail directing thermal energy or ultrasonic energy onto the surface.
A fourth aspect of the invention relates to a method of inspecting an aircraft structure, which may include multiple layers of composite material. According to an embodiment of the invention, a surface of an aircraft structure is heated and a determination is made as to whether a temperature difference is present by concurrently measuring respective temperatures of proximate portions of the heated surface using a pair of thermal radiometers. The aircraft structure is returned to flying service if the temperatures are essentially the same, and further inspection is performed if the temperatures are substantially different. The surface may be visually inspected for impact sites. If an impact site is identified, the thermal radiometers are disposed near the impact site. The radiometers may be disposed at multiple locations near the impact site in order to determine whether temperature differences are present at each of the multiple locations.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
One or more embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
A theoretical approach to understanding the flow of heat into a structure, the production of heat within a structure in response to incident energy such as ultrasonic energy, and the production of temperature gradients on the surface of a structure by which sub-surface defects may be detected are described herein with references to
In
For example, the sub-surface flaw 102 can represent a void or a volume of high porosity. In this example, the flaw 102 insulates against uniform heat flow across its vicinity and causes heat to be temporarily trapped near the surface 110. Thus, the surface portion 122 is expected to temporarily exhibit a higher temperature than that of the proximate surface portion 112. Ultimately, the trapped heat may flow laterally and around the flaw 102 such that, once thermal equilibrium is established, the surface 110 may exhibit a uniform temperature across proximate portions 112 and 122. Nonetheless, a difference in temperature between the proximate surface portions 112 and 122 may temporarily provide evidence of the sub-surface flaw 102.
For further example, the sub-surface flaw can represent a contaminant having a relatively high heat capacity. For example the flaw 102 can represent a bit of contaminant metal embedded in a layered composite structure. In this example, the surface portion 122 may temporarily exhibit a lowered temperature as the contaminant absorbs heat. Subsequently, as the thermal radiant energy 74 is stopped and the structure cools, the surface portion 122 may temporarily exhibit an elevated temperature as the contaminant releases heat. Thus, a sub-surface contaminant or flaw may be detected by a raised or lowered surface temperature. In another example, the flaw 102 can represent moisture ingression in a layered composite structure. The high heat capacity of water can cause temperature differences between moisture contaminated and moisture free portions of a structure. In this example, the surface portion 122 represents a portion of a skin over a honeycomb structure contaminated with water.
In
Thus, if a surface of a structure is briefly bathed with energy that conveys or produces heat, sub-surface flaws may be revealed by transient temperature gradients along the surface. The term “structure” is broadly used in these descriptions in referring to any physical member that has a front surface and a sub-surface composition that is expected to be laterally uniform along a distributed area of the surface. In that context, composition differences under proximate surface portions represent sub-surface flaws. Sub-surface flaws can include, but are not limited to: damages, contaminants; voids; cracks; porosity irregularities; repair artifacts; and other sub-surface conditions. An inspected structure can include any number of materials. An inspected structure can be a uniform and homogenous material, a layered material, a composite material such as graphite-epoxy, or combinations thereof.
However, in the interest of providing particular descriptions of inspection scenarios and embodiments of the present invention, the following descriptions refer to layered structures such as layered composite aircraft components like fuselage and wing members. For example, the layered structures in the following descriptions can be components of a Boeing Dreamliner 787 aircraft. Such an aircraft has an external skin that can be constructed of multiple layers of composite material. Nonetheless, the term “structure” as used herein relates to aircraft structures and other structures as well, including composite structures such as bridges and boats.
Furthermore, a structure can be layered upon its manufacture and can receive additional layers upon repair when a patch is bonded to the outer surface of a damaged area. Thus, an inspection can be related to assurance measures as a structure is manufactured and as repairs are made. For example, an operator can use the inspection device 5 illustrated in
An inspection device 5 according to an embodiment of the invention is illustrated in
The first and second radiometers in
Furthermore, the first and second radiometers 10 and 20 comprise respective similar or essentially identical non-imaging radiometers having four-to-one aspect ratios. The inspection device 5 is disposed in
The comparator 30 receives the output signals 11 and 21 from the first and second radiometers and generates a difference signal 31 received by the sample and hold circuit 40. The difference signal of the comparator 30 conveys information regarding the difference, if any, between the temperatures measured by the first and second radiometers. The comparator may comprise transistor-transistor logic (TTL) circuits. The sample and hold circuit 40 receives the difference signal 31 of the comparator and generates a normalized difference signal 41. The sample and hold circuit 40 is prompted to initiate an inspection session when the trigger 42 is actuated by a user. When the trigger 42 is actuated, the sample and hold circuit samples the value of the difference signal 31 and nulls the normalized difference signal 41 in order to effectively balance the inspection device 5 with regard to the two radiometers at their current signal levels. Thus, minor drifts in the response functions of the two radiometers are nulled at the initiation of an inspection session. Subsequently, for the duration of the inspection session, as the device 5 is scanned over an inspection area, the normalized difference signal 41 conveys any departure of the difference signal 31 from its null value at the initiation of the session.
The level-adjust device 50 receives the normalized difference signal 41 and generates a signal 51 received by the indicator element 60. The level adjust device 50 controls the sensitivity of the inspection device 5, which may be used in various environments for inspecting various materials. For example, it is expected that inspections along the surface of the layered composite skin of an aircraft component may detect the presence of sub-surface flaws by detecting temperature differences of two to six degrees Celsius. Such temperatures would translate to differences of twenty to sixty millivolts in the output signals of balanced radiometers having signals that vary approximately by ten millivolts per degree Celsius. Thus, in this example, a stable twenty millivolt difference between balanced output signals of such radiometers should ultimately alert an operator. However, a mere one millivolt difference may represent an insignificant temperature difference between proximate surface portions. Furthermore, a difference of a few millivolts may merely represent drifts in the response functions of the radiometers or a drift in the balancing of the signals. An adjustment device 52, such as a variable potentiometer or “turn pot” permits an operator of the inspection device or a calibration specialist to adjust the sensitivity of the inspection device by establishing a temperature difference threshold 54 (
The indicator element 60 is illustrated to include a first indicator 62 and a second indicator 64. The first indicator 62 is activated when the temperatures indicated by the first and second radiometers are substantially different. Such a condition represents an alert signal that alerts the operator to the likelihood of a sub-surface flaw in the vicinity of the current disposition of the inspection device 5. The second indicator 64 is activated when temperatures indicated by the first and second radiometers are essentially the same. Such a condition represents a non-alert signal that assures the operator that a sub-surface flaw is not detected at the current disposition of the inspection device. The first and second indicators can comprise respective light sources such as LED devices. For example, the first indicator 62 can comprise an LED device that emits red light when activated, and the second indicator 64 can comprise an LED device that emits green light when activated. The indicator element 60 can additionally include an audible signal emitter 66 that emits an audible alarm as an additional alert signal when the first indicator 62 is activated.
An inspection device according to one or more embodiments of the invention may include, or be used in conjunction with, an energy source that is operable to cause heating of a structure under inspection. For example, the inspection device 5 of
Other inspection devices according to other embodiments of the invention comprise other types of energy sources. For example, an inspection device according to at least one embodiment of the invention includes an energy source that comprises an ultrasonic acoustic device that excites heat production within an inspected structure as shown in
The particular inspection scenarios represented by
In
The temperatures of the surface portions 412 and 422 in the inspection scenario of
It should be understood that the device 5 may not be stationary for long during a typical inspection session. That is,
Because the temperatures of surface portions of a structure are likely to equilibrate in time, the operator of the device 5 will typically apply heat periodically to induce the temperature gradients by which sub-surface flaws may be detected. For example, ten to twenty seconds may typically pass between the heating of a structure and temperature-equilibration among proximate surface portions despite any presence of flaws. Thus the operator of the device 5 should be mindful of typical equilibration times for any given inspected structure and heat application technique so as not to take confidence in the activation of the second indicator 64 when the device is disposed over un-heated or temperature-equilibrated surface portions.
In at least one example of the invention, a heat device provides a shut-down function to assure that inspections provide indicator results only as the device is disposed over a recently heated surface. For example, in
A branch 680 is shown in
As respectively shown in
Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
