Abstract:
A method for fabricating thermal effect standards includes providing an oven, placing at least one temperature sensor at measurement location in the oven, operating the oven, monitoring a temperature output of the at least one temperature sensor, providing at least one composite material specimen, placing the at least one composite material specimen at the measurement location in the oven and heat treating the at least one composite material specimen as at least one thermal effect standard by operating the oven according to the temperature output of the at least one temperature sensor. A method of determining a physical property of a composite material is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending U.S. patent application Ser. No. 12/164,025, and Ser. No. 12/164,026; and Ser. No. 12/164,023, and Ser. No. 12/164,017, all filed concurrently herewith on Jun. 28, 2008, each of which applications is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The disclosure relates to standards for determining thermal effect in composite materials. More particularly, the disclosure relates to a method for fabricating thermal effect standards which are suitable for accurately assessing thermal effect in composite material. 
     BACKGROUND OF THE INVENTION 
     Resin-fiber composite materials are utilized in a variety of applications including the aerospace industry, for example. Structures which are constructed of resin-fiber composite materials may be exposed to heat, which may affect the composite materials in various ways. These heat-induced effects may include chemical degradation in which changes such as oxidation, material loss and the breaking and/or forming of chemical bonds occurs in the polymer chemical structure of the composite materials. Resin decomposition, charring and fiber decomposition of the composite materials may occur at increasing temperatures. 
     Repair or removal of heat-affected resin-fiber composite materials on a structure may involve first determining the degree of harmful heat effect to the composite substrate. Although determining the degree of heat effect to composite materials may be performed by visual inspection, heat effect to resin-fiber composite materials may not be visually apparent. Current methods of determining the presence and extent of heat effect in resin-fiber composite materials includes obtaining a series of infrared spectra of a series of heat-affected composite standards and making a thermal effect multivariate calibration model with the IR spectra and the thermal effect information from the standards. An infrared spectrum obtained from the composite material the heat effect of which is in question can then be predicted by the thermal effect model to determine the presence and assess the degree of thermal effect in the composite material. 
     Calibration of infrared sensors to residue strength in composite materials correlates the resin condition as read from the infrared spectra to the residual strength of the material which degrades as the resin degrades with progressively increasing temperatures. Therefore, the infrared spectroscopy sensors may be calibrated using time-controlled thermal soak standards which are obtained by exposing various composite material controls to various temperatures for a particular time period such as one hour, for example. One method of preparing the standards includes placing the standards in an oven which is calibrated periodically and monitored continuously. Thermal effect to composite materials often does NOT follow a linear course and indeed is usually a combination of overlapping degradation mechanisms in the composite resin material. Moreover, the calibration method may utilize the entire area of the oven cavity rather than the particular area in which the composite standard is confined during heating of the standard. In application, oven temperature readings may be off by 25 F. Therefore, temperature sensors with a calibrated meter may be used to read and verify correct oven temperatures +/−1 degree F. The oven temperature may be set high or low to achieve the CORRECT reading per the temperature sensors. 
     Therefore, a method of making thermal effect standards for composite materials which takes into account the total thermal experience of the composite standard in terms of both time and temperature is needed. 
     SUMMARY OF THE INVENTION 
     The disclosure is generally directed to a method for fabricating thermal effect standards. An illustrative embodiment of the method includes providing an oven, placing at least one temperature sensor at a measurement locations in the oven, operating the oven, monitoring the temperature output of the at least one temperature sensor, providing at least one composite material specimen, placing the at least one composite material specimen at the measurement location in the oven and heat treating the at least one composite material specimen as at least one thermal effect standard by operating the oven according to the temperature output of the at least one temperature sensor. 
     The disclosure is further generally directed to a method of determining a physical property of a composite material. An illustrative embodiment of the method includes providing a series of composite materials with increasing amounts of thermal effect, irradiating the composite materials with broad-spectrum infrared energy, detecting infrared energy reflected from the composite material, obtaining residual mechanical strength data for the thermal effect composite material, performing a multivariate calibration on the spectra of the infrared energy reflected from the composite materials, providing a calibration model of thermal effect using the residual strength of the thermal effect standards as the calibration parameter (Y-block data) for the model, and using the model to predict residual mechanical strength in composite materials in question that may have thermal effect. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a cross-sectional schematic diagram of an oven with multiple temperature sensors placed in the oven to obtain temperature values for composite standards. 
         FIG. 2  is a cross-sectional schematic diagram of the oven, with a composite material specimen placed in the oven for heating in the fabrication of a composite material standard. 
         FIG. 3  is a side view of a heat-treated composite material standard subjected to a mechanical test force in obtaining values for residual strength. 
         FIG. 4  is a flow diagram which illustrates an illustrative embodiment of a method of fabricating thermal effect standards. 
         FIG. 4A  is a flow diagram which illustrates an illustrative embodiment of a method of multivariate calibration for thermal effect in a series thermal effect standards with spectra of infrared energy reflected from the thermal effect standards. 
         FIG. 5  is a flow diagram of an aircraft production and service methodology. 
         FIG. 6  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIGS. 1-4 , an illustrative embodiment of a method of fabricating thermal effect standards is shown. As shown in block  402  of the flow diagram  400  in  FIG. 4 , an oven which is suitable for fabrication of thermal effect standards from composite material specimens is provided. As shown in  FIG. 1 , the oven  1  may include an oven wall  2  which defines an oven interior  3 . A specimen support  4  may be provided in the oven interior  3 . The oven  1  may be any conventional oven which is suitable for heating composite material specimens for the purpose of fabricating thermal effect standards and has 300 to 600 degree F. capability; therefore, various functional elements of the oven  1 , such as heating elements, for example, are not shown in  FIGS. 1 and 2  of the drawings. 
     In block  404  of the flow diagram  400 , at least 5 temperature sensors, such as thermocouples,  12  are placed in the specimen oven  1  at a selected measurement location or locations in the oven interior  3 , as shown in  FIG. 1 . In some applications, a bundle of temperature sensors  12  may be placed in the specimen oven  1  at the selected measurement locations in the oven interior  3 . The temperature sensors  12  may be connected to a temperature sensor meter  16  such as through temperature sensor wiring  13 , for example. The temperature sensor meter  16  may be adapted to monitor the temperature outputs of the temperature sensors  12  during operation of the specimen oven  1 . 
     In block  406  of  FIG. 4 , the specimen oven  1  is operated at predetermined exposure temperatures and exposure times which will subsequently be used to fabricate the thermal effect standards from the composite material specimens. As shown in block  408 , the temperature outputs of the temperature sensors may be measured typically by the temperature sensor meter  16  ( FIG. 1 ). The temperature control settings of the specimen oven  1  may be adjusted to facilitate heating of the measurement location in the specimen oven  1  to the predetermined temperatures as measured by the temperature sensors  12  and indicated by the temperature sensor meter  16 . Therefore, the temperature sensors  12  provide precise temperature measurements of the measurement location in the specimen oven  1  which will correspond to the location at which heating of the composite material specimens will subsequently take place. The temperature control settings of the specimen oven  1  at the predetermined temperatures are noted or recorded to facilitate precision repeatability of the predetermined temperatures at the measurement location in the oven interior  3  for subsequent fabrication of the thermal effect standards. In some applications, the specimen oven  1  may be operated to heat the temperature sensors  12  to predetermined temperatures which lie along a calibration range for fabrication of the thermal effect standards. For example, the specimen oven  1  may be heated to predetermined temperatures of 350° F.; 325° F.; 375° F.; 400° F.; 425° F.; 450° F.; 475° F.; 500° F.; 525° F.; and 550° F. The oven may be pre-heated and temperature stabilized for 15 minutes to the treatment temperature before putting composite panels into the oven. 
     In block  410  of the flow diagram  400 , at least one composite material specimen  20  ( FIGS. 2 and 3 ) is provided. In block  412  of the flow diagram  400  and in  FIG. 2 , the composite material specimen  20  is placed at the measurement location in the oven interior  3 . In block  414  of the flow diagram  400 , the specimen oven  1  is operated to heat the composite material specimen  20  to the predetermined temperatures as measured by the temperature sensors  12  and indicated by the temperature sensor meter  16 . The control settings of the specimen oven  1  are operated at the settings which were noted during operation of the specimen oven  1  at block  406 , for a predetermined time period to fabricate the thermal control standards. Accordingly, the heat generated by the specimen oven  1  induces thermal effect in the composite material specimen  20 . This thermal effect may include, for example, chemical degradation involving changes in the polymer chemical structure; oxidation, material loss and either breaking or forming of additional chemical links; and resin decomposition and charring and fiber decomposition. In some applications, successive composite material specimens  20  may be heated at temperatures which lie along a calibration range for fabrication of the thermal effect standards. For example, the composite material specimens  20  may be heated to predetermined temperatures of 350° F.; 325° F.; 375° F.; 400° F.; 425° F.; 450° F.; 475° F.; 500° F.; 525° F.; and 550° F., respectively. For each temperature setting the oven is preheated and temperature stabilized for 15 minutes before putting the composite panel into the oven. When the heated samples are removed from the oven a slow cooling process is recommended to prevent the samples from cracking and de-laminating. 
     The thermal effect which is induced in the composite material specimens  20  to form the thermal control standards may subsequently be used to determine the presence and extent of thermal effect in actual resin-fiber composite material samples. In block  416 , the thermal control standards are subjected to mechanical forces (reference letter F in  FIG. 3 ) in order to obtain values of residual strength in the standards. Typical mechanical tests that give good results for residual mechanical strength in composite material are un-notched compressions tests and interlaminar shear tests. The values of residual strength obtained for the thermal effect standards may be used to calibrate the infrared spectroscopy sensors (not shown) of an infrared testing device (not shown) to measurement of thermal effect in composite materials, which will be hereinafter described. 
     Referring next to  FIG. 4A , a flow diagram  400   a  which illustrates an illustrative embodiment of a method of multivariate calibration for thermal effect in a series of thermal effect standards with the spectra of infrared energy reflected from the thermal effect standards is shown. In block  402   a , a series thermal effect standards is provided. The thermal effect standards may have been fabricated according to the method which was heretofore described with respect to the flow diagram  400  in  FIG. 4 . In block  404   a , a surface of the thermal effect standard is irradiated with broad-spectrum infrared energy. In the range of 1.6 microns to 2.4 microns (near-IR) or 2.5 to 16.7 microns (mid-IR). In block  406   a , infrared energy reflected from the surface of the thermal effect standards is detected. In block  408   a , the spectra of the infrared energy which is reflected from the surface of the thermal effect standards is used with the residual strength of the thermal effect standards to create a multivariate calibration model for the residual strength of the thermal effect standards. 
     Referring next to  FIGS. 5 and 6 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  78  as shown in  FIG. 5  and an aircraft  94  as shown in  FIG. 6 . During pre-production, exemplary method  78  may include specification and design  80  of the aircraft  94  and material procurement  82 . During production, component and subassembly manufacturing  84  and system integration  86  of the aircraft  94  takes place. Thereafter, the aircraft  94  may go through certification and delivery  88  in order to be placed in service  90 . While in service by a customer, the aircraft  94  may be scheduled for routine maintenance and service  92  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  78  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 6 , the aircraft  94  produced by exemplary method  78  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  78 . For example, components or subassemblies corresponding to production process  84  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  94  is in service. Also, one or more apparatus embodiments may be utilized during the production stages  84  and  86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  94  is in service, for example and without limitation, to maintenance and service  92 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.