Abstract:
An accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens including a fixture connected to a target board including a frame spaced from the target board a predetermined distance by a plurality of stand-offs and having at least one opening defined therein and a transparent element connected to the frame in alignment with each at least one opening in a covering orientation such that the solar radiation from a reflecting solar concentrator impinges upon the test specimens. The predetermined distance is configured such that the frame functions as an extension of a deflector and an operating temperature of the test specimens is shielded from external influences.

Description:
BACKGROUND 
     The present disclosure is directed to an accelerated weathering test apparatus of the type used to concentrate solar radiation on test specimens, and more particularly, to an accelerated weathering test apparatus with a fixture that facilitates a reduction in the operating temperature of the test specimens. 
     Manufacturers of exterior coatings, such as paints and finishes, as well as plastics and other components which tend to degrade under exposure to solar radiation and other weathering effects, often want to know how such products will perform following years of exposure. However, such manufacturers typically require such information in a much shorter time than it would take to expose such materials to weathering effects under normal conditions. Accordingly, accelerated weathering test devices have been developed which accelerate the effects of weathering due to outdoor exposure in a much shorter time so that manufacturers need not actually wait five or ten years in order to determine how their products will hold up after five or ten years of actual outdoor exposure. 
     One known accelerated weathering test device is disclosed in U.S. Pat. No. 2,945,417, issued to Caryl et al. The aforementioned test device includes a Fresnel-reflecting solar concentrator having a series of ten flat mirrors which focus natural sunlight onto a series of test specimens secured to a target board measuring approximately five (5) inches wide by fifty-five (55) inches long. The Fresnel-reflecting solar concentrator directs solar radiation onto the target board area with an intensity of approximately eight suns. Both the bed which supports the mirrors of the solar concentrator, and the target board, are supported by a frame which can be rotated to follow daily movements of the sun. 
     A solar tracking mechanism responsive to the position of the sun, controls the operation of an electric motor that is used to rotate the test apparatus to follow movements of the sun. The axis of rotation of the test machine is oriented in a north-south direction, with the north elevation having altitude adjustment capability to account for variation in the sun&#39;s altitude at various times during the year. 
     Such known testing devices are also provided with an air tunnel mounted above the target board. An air deflector causes air escaping from the air tunnel to be circulated across the test specimens mounted to the target board to prevent the test specimens from overheating due to the concentrated solar radiation to which they are exposed. The amount of air is controlled by the dimension of the gap between the deflector and the specimen. A squirrel cage blower communicates with the air tunnel for blowing cooling ambient air there through. In addition, water spray nozzles are provided proximate to target board for wetting the test samples at periodic intervals to simulate the weathering effects of humidity, dew, rain, etc. 
     Another known accelerated weathering test device is disclosed in U.S. Pat. No. 4,807,247. issued to Robins. The aforementioned test device includes all the structure previously described above with respect to the &#39;417 patent and further includes a system for maintaining a uniform, constant test specimen temperature during daylight hours despite variations in ambient air temperature and variations in solar radiation intensity. 
     The system includes a temperature sensor mounted to the target board for exposure to the concentrated solar radiation and for generating an electrical signal indicative of the temperature of the test specimen mounted to the target board. The system further includes a control mechanism electrically coupled to the temperature sensor and responsive to the electrical signal generated thereby for selectively controlling the application of electrical power to the electrical motor included within the air circulation system. In this manner, the control mechanism serves to vary the speed of the electric motor and thereby control the flow rate of cooling ambient air circulating across the target board so that the temperature of the test specimen remains constant at the desired set point. 
     When the sensed temperature of the test specimen increases, the control mechanism increases the speed of the blower motor to circulate more cooling ambient air across the target board in order to lower the temperature of the test samples back to the desired set point. Similarly, if the sensed temperature of the target samples drops below the desired nominal temperature, the control mechanism decreases the speed of the blower to permit the test samples to warm up back to the desired set point. 
     The temperature control mechanism also includes a user operable adjustment mechanism, in the form of the control knob, for allowing a user to set a static, desired target specimen temperature. A bypass switch is also provided for allowing the user to operate the test device in the controlled temperature-mode as described above, or in an uncontrolled mode wherein the blower motor operates at a constant speed. 
     Standardized testing methods have been developed for operating accelerated weathering test devices of the type described above. The American Society for Testing and Materials (ASTM) has issued standards G90, E838, D4141, D3105, D3841, D5105, E1596 and D4364 covering the testing procedures and the operating parameters for conducting such outdoor accelerated weathering tests. Other standards and appraisals have also been developed and specified by the Society of Automotive Engineers (SAE), Ford, International Standards Organization (ISO), American National Standards Institute (ANSI), Japan Industrial Standard (JIS), namely, SAE J576, SAE J1961, Ford EJB-M1J14-A, Ford EST-M5P11-A, ISO 877, ANSI/NSF 54, JIS Z 2381 and MIL-T-22085D. 
     Apart from outdoor accelerated weathering test devices of the type described above, other test devices are also known which utilize an artificial source of radiation to expose the test specimens. An example of such a test device is disclosed in U.S. Pat. No. 3,664,188 issued to Kockott. While such test devices have the advantage of permitting precise control over radiation intensity, temperature and humidity, such test devices have often failed to duplicate the actual light spectrum of natural sunlight to which the specimens under test will actually be exposed in everyday use. It has been acknowledged and recognized by those of skill in the art that natural sunlight and artificial sunlight test apparatus are distinct from one another and provide different sets of empirical data. 
     Outdoor accelerated weathering test devices of the type described above in regard to U.S. Pat. Nos. 2,945,417 and 4,807,247, have the advantage of using natural sunlight, and hence the specimens under test are exposed to the actual spectrum of sunlight. However, disadvantages of outdoor accelerated weathering test devices have been discovered. 
     One such disadvantage is that the temperature of test specimens cannot be precisely controlled due to exposure to outdoor conditions. For example, exposure to multiples of incident sunlight can raise the temperature of a specimen beyond an acceptable testing range. Furthermore, the temperature of the specimen may rise high enough to damage the test specimen. To solve this problem, accelerated weathering test devices have been devised that attempt to cool the test specimen by artificially blowing air over the test specimen. However, when the outside conditions are windy, the disturbance from the wind degrades the cooling effects of the artificially blown air over the test specimen. 
     Therefore, there exists a need in the art for a device and a method of controlling the temperature and temperature fluctuations of a test specimen in an outdoor accelerated weathering test apparatus regardless of ambient wind disturbances. 
     SUMMARY 
     In accordance with one principle aspect to the present disclosure, an accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens including a fixture connected to a target board including a frame spaced from the target board a predetermined distance by a plurality of stand-offs and having at least one opening defined therein and a transparent element connected to the frame in alignment with each at least one opening in a covering orientation such that the solar radiation from a reflecting solar concentrator impinges upon the test specimens. The predetermined distance is configured such that the frame functions as an extension of a deflector and an operating temperature of the test specimens is shielded from external influences. 
     In another principle aspect of the present disclosure, a fixture is adapted for use in connection with an accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens. The fixture includes a frame having at least one opening and a transparent element connected to the frame in alignment with each at least one opening in a covering orientation such that the solar radiation from the reflecting solar concentrated impinges upon the test specimens. A plurality of stand-offs are operatively contiguous with the frame and target board to space the frame from the target board a predetermined distance such that the frame functions as an extension of the deflector and an operating temperature of the test specimens is shielded from external influences. 
     In another principle aspect of the present disclosure, an accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens includes a fixture connected to a target board including a substantially planar frame spaced from the target board a predetermined distance by a plurality of stand-offs. The frame also has a plurality of openings that are in registration with the test specimens and a plurality of transparent elements formed from an ultraviolet transmissive material connected to the frame, each transparent element in alignment with one of the openings such that the solar radiation from the reflecting solar concentrator impinges upon the test specimens. The predetermined distance is configured such that the frame functions as an extension of the deflector so that the air circulated across the test specimens is compressed between the frame and target board to facilitate increased heat transfer from the test specimens and an operating temperature of the test specimens is maintained in a narrow range close to a desired test specimen temperature, shielded from external influences. 
     In another principle aspect of the present disclosure, an accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens at a first temperature includes a fixture connected to the target board including a frame spaced from the target board a predetermined distance by a plurality of stand-offs. The frame also has at least one opening defined therein, and a transparent element connected to the frame in alignment with each at least one opening in a covering orientation such that the solar radiation from the reflecting solar concentrator impinges upon the test specimens. The predetermined distance is configured such that the frame functions as an extension of the deflector and an operating temperature of the test specimens is less than the first temperature and is shielded from external influences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments are shown in the drawings. However, it is understood that the present disclosure is not limited to the arrangements and instrumentality shown in the attached drawings, wherein: 
         FIG. 1  illustrates a perspective view of an accelerated weathering test apparatus; 
         FIG. 2  illustrates a perspective sectional view of an air tunnel of the accelerated weathering test apparatus of  FIG. 1 ; 
         FIG. 3  illustrates a cross sectional view of the air tunnel of  FIG. 2 ; 
         FIG. 4  illustrates a perspective view of a fixture; 
         FIG. 5  illustrates a detailed sectional view of the air tunnel of  FIG. 3  with the fixture of  FIG. 4  operatively coupled thereto; 
         FIG. 6  illustrates a graphical representation of an operating temperature of various test specimens at different desired test specimen temperatures on the accelerated weathering test apparatus of  FIG. 5  with the fixture of  FIG. 4  sequentially installed and removed; and 
         FIG. 7  illustrates a graphical representation of an operating temperature of a test specimen on the accelerated weathering test apparatus of  FIG. 5  with the fixture of  FIG. 4  alternately installed and removed. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting and understanding the principles disclosed herein, reference will now be made to the preferred embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope is thereby intended. Such alterations and further modifications in the illustrated device and such further applications are the principles disclosed as illustrated therein as being contemplated as would normally occur to one skilled in the art to which this disclosure relates. 
     Referring to  FIG. 1 , a prior art accelerated weathering test apparatus is designated generally by reference  20  and includes a pair of A-frame members  22  and  24  to support the operative portion of the apparatus. The lower ends of the A-frame members  22 ,  24  are interconnected by a base member  26  which is operatively connected to a ground member  28  in order to provide azimuth rotation in the direction indicated by arrow  30  and elevation rotation in the direction indicated by arrow  31 . The elevation direction rotation accounts for periodic variation in the sun&#39;s altitude at solar noon. 
     Rotatively supported from the upper ends of A-frame members  22 ,  24  is a mirror bed frame  32  which supports a plurality of flat mirrors, including those designated by reference numerals  34  and  36 . The plurality of mirrors  34 ,  36  are angled to reflect solar radiation directly impinging upon such mirrors to a target board  38  (see  FIG. 2 ). 
     A pair of standards  40  and  42  extend outwardly from and perpendicular to mirror bed frame  32 . An air tunnel  44  having a generally rectangular cross section is supported by the upper ends of standards  40 ,  42 . Referring to  FIGS. 2 and 3 , target board  38  is supported by the lower wall of air tunnel  44 , and a plurality of test specimens  46  are mounted to the target board  38  for exposure to the concentrated solar radiation, represented in  FIG. 2  by the upwardly extending arrows numbered  39 . The target board  38  may include a single specimen  46  or a plurality of similar or different test specimens  46 , which may be referred to herein as test specimen(s)  46 . 
     A squirrel cage blower assembly  48  communicates with one end of the air tunnel  44 . Squirrel cage blower assembly  48  includes a fan driven by an electric motor to circulate cooling ambient air through air tunnel  44 , represented in  FIGS. 2 and 3  by the outwardly extending arrows numbered  45 . As shown in  FIGS. 2 and 3 , air tunnel  44  includes a deflector  50  which extends for the length of target board  38  and causes cooling ambient air to be circulated across target board  38  for cooling test specimen(s)  46 , represented in  FIGS. 2 and 3  by the arrows numbered  47 . 
     Standards  40 ,  42  are rotatively supported to upper ends of A-frame members  22 ,  24 . A supporting shaft coincident with the axis of rotation in passing through standards  40 ,  42  rotably supports that portion of the test apparatus which tracks daily movements of the sun. In order to properly position the Fresnel-reflecting solar concentrator comprised by mirror assembly  34 ,  36  and reversible electric motor and related gear drive, generally designated by reference number  54 , are provided for periodically rotating the mirror bed and target board assembly to track movements of the sun. The clutch preferably couples standard  40  to a shaft to rotate the mirror assembly  34 ,  36  and target board assembly while permitting manual positioning of the unit at any time to correct for any positioning errors. 
     A solar cell tracking unit  52  controls the application of electrical power to a reversible motor in order to maintain the mirror bed frame  32  perpendicular to incident rays of sunlight. A solar tracker may be of the type which includes two balanced photo cells and a shadowing device mounted above such photo cells for shading them. When an imbalance is detected resulting from one photo cell receiving more sunlight than the other photo cell, an electrical error signal is generated which is amplified and used to apply power to the drive motor  54  for rotating the unit until the photo cells are again balanced, indicating that the unit is properly positioned with respect to the sun. 
     Also shown in  FIG. 1  is a water spray nozzle assembly, designated generally by reference numeral  51 . As shown in  FIG. 1 , spray nozzle assembly  51  is used to periodically spray water at the test specimen(s) to simulate dew, rain, etc. 
     A hinge shield or cover  49  is shown coupled to the air tunnel  44  opposite the air deflector  50 . A door release mechanism  47  is disposed on the air tunnel  44  for engaging and maintaining the shield in a closed position. Upon release, the shield  49  assumes the position shown in  FIG. 1  such that concentrated solar radiation reflected by the plurality of mirrors  34 ,  36  reaches the test specimen(s)  46 . 
     Secured to the target board  38  is a feedback device (not shown) having at least one temperature sensitive component secured in heat conductive relationship therewith. Such component may be a thermistor, thermocouple, resistance temperature device, integrated circuit temperature device or any other suitable device for detecting temperature of the feedback device. The feedback device may be formed from a standardized material having known thermal conductive properties or may be formed from a material similar to that of the test specimen(s)  46 . The temperature sensitive component may be embedded within, attached to a back surface or attached to a front surface of the feedback device. Alternatively, a non-contact optical temperature sensing device may be used in order to determine the temperature of the feedback device. The feedback device is preferably coated with black paint to insure that the feedback device will absorb solar radiation impinging upon the area of the target board  38  to which the feedback device is secured. An appropriate black paint which may be used for this purpose is DUPONT DULUX Super Black High Temperature Enamel. 
     Referring again to  FIG. 1 , a controller box  57  houses the power and controller systems for the apparatus  20 . A power cable  58  supplies electrical power to the apparatus  20  for powering the electric motor  54 , which actuates the fan  48 . A signal cable  60  is connected to the controller system disposed within the control box  57  for communication with remotely disposed devices, such as the feedback devices and input device, as will be discussed below or for communication with a central command for governing the operation of the apparatus  20  in accordance with the present invention. 
     Referring to  FIG. 4 , a temperature reduction fixture  70  for the accelerated weathering test apparatus  20  constructed in accordance with the teachings of the present disclosure is shown. The fixture  70  includes a frame  72  and a plurality of stand-offs  74 . The frame  72  is generally rectangular and includes one or more openings  78 . The frame  72  is spaced from the target board  38  by the plurality of stand-offs  74 . Additionally, the stand-offs  74  provide the mounting of the frame  72  to the target board  38 . The fixture  70  also includes a transparent element  80  that is connected to the frame in alignment with the opening  78  and to cover the opening  78 . 
     The stand-offs  74  can be connected to the frame by methods that are known to those of ordinary skill in the art. For example, as shown in  FIG. 4 , each stand-off  74  can be connected to the frame  72  by a fastener  76 . In another example, the stand-offs  74  and the frame  72  may be co-manufactured as a one-piece component. As shown in  FIG. 4 , the stand-offs  74  may be disposed anywhere along the frame. However, the stand-offs  74  can be disposed at the edges of the frame  72  so as to not block the solar rays  39  passing through the transparent element  80 . To provide additional support for attaching the frame  72  to the target board  38 , additional stand-offs  74  may connect the frame  72  to the target board  38 . Such stand-offs  74  may be disposed along the span of the frame  72 . As shown in  FIG. 4 , three stand-offs  74  can connect the mid-span of the frame  72  to the target board  38 . 
     The frame  72  can support the transparent element  80  by methods that are known to those of ordinary skill in the art. For example, as shown in  FIG. 5 , the frame  72  may include a slot  81  disposed on its inner periphery to accommodate the edges of the transparent element  80 . Accordingly, the thickness of this slot  81  may be slightly larger than the thickness of the transparent element  80  to securely accommodate the transparent element  80 . In another example (not shown), the frame  72  may have a first frame part upon which the transparent element  80  may be placed. The frame can also include a second frame part that can then be placed on top of the transparent element  80 . Accordingly, the transparent element  80  can be sandwiched between the first frame part and the second frame part of the frame  72 . The second frame part can then be fastened to the first frame part to secure the transparent element  80 . 
     Referring to  FIG. 5 , the frame  72  functions as an extension of the deflector  50 . Accordingly, the stand-offs  74  are sized so as to provide the airflow shown by the arrows  147  from the deflector  50  into and through the fixture  70 . The sides of the fixture  70  are enclosed by side walls (not shown) that may be constructed from any material so as to substantially shield the interior of the fixture  70  from the ambient air. The side walls may be constructed from wood, metal, plastic, composite materials, or combinations thereof. Alternately, the side walls may also be constructed from the a transparent material if desired. 
     The fixture  70  substantially shields the airflow  147  from the ambient air while permitting solar rays  39  to pass through the transparent element  80  and strike the test specimen(s)  46 . Accordingly, the transparent element  80  may be chosen so as to permit ultraviolet, infrared, and invisible wave lengths of solar radiation to pass therethrough. In one embodiment of the present disclosure, the transparent panel may be formed from borosilicate glass. Additionally, the transparent element  80  may be chosen so as to filter certain wave lengths of solar radiation if desired. For example, the transparent element may partially block infrared radiation from reaching the specimen(s)  46 . 
     As noted above, the fixture  70  functions as an extension of the deflector  50 . Accordingly, the air that is shown by the arrows  47  is compressed between the frame  72  and the target board  38  to facilitate heat transfer from the test specimen(s)  46 . As will be discussed in the following, the increased heat transfer provides lower operating temperatures for the specimen(s)  46  when such lower operating temperatures are desired. 
     Referring to  FIG. 6 , temperature variations for four types of test specimen mounting configurations with the fixture  70  mounted are graphically shown where the temperature of each test specimen mounting is controlled. The temperature variations shown in  FIG. 6  are for the following test specimen mounting configurations: a 4×12 (inches) backed black panel, a 3×5 backed black panel, a 3×5 unbacked black panel with control feedback, and a 3×5 backed white panel. Accordingly, the noted test specimen mounting configurations will be generally referred to herein as panels. 
     A test specimen  46  is typically mounted between a white panel and a black panel. The white panel can indicate the coolest temperature experienced by the specimen  46 , while the black panel can indicate the hottest temperature experienced by the specimen  46 . A panel may be backed or unbacked. A backed panel is a panel that has a backing, which is typically constructed of wood or metal. An unbacked panel does not have a backing. Accordingly, air can flow under an unbacked panel, thereby resulting in panel temperatures that are typically lower than a similar backed panel. 
     Referring to  FIG. 6 , the temperature of each panel is controlled in a step-down manner to achieve the desired temperature for that panel considering the ambient temperature and conditions. From a time value indicated by the arrow  87  (about  1334 ) to a time value indicated by the arrow  89  (about  1454 ), the temperature of each panel is lowered in a series of steps by setting the desired temperature and controlling the flow of compressed air  49  over each panel. At time value  89 , each panel has substantially achieved its last desired temperature and the lowest possible temperature considering the ambient temperature. As shown in  FIG. 6 , the lowest temperature shown is about 60° C. for the 3×5 unbacked black panel with feedback and the 3×5 backed white panel. The desired temperature of each panel, after the time value  89 , is reset to a higher value and simply allowed to increase. Hence the temperature of each panel is shown to rise in  FIG. 6 . One of ordinary skill in the art will readily appreciate that without the disclosed fixture  70 , the lowest temperatures achieved for each panel as shown in  FIG. 6  may not be possible considering the ambient temperature and conditions shown in  FIG. 6  (i.e., breezy and windy). 
     Referring to  FIG. 7 , temperature variation plot  92  of an operating temperature of the test specimen(s)  46  with the fixture  70  being alternately installed and removed and the temperature of the test specimen(s)  46  not being controlled is graphically shown. As shown by the first time segment  94  of the temperature variation plot  92 , which represents time values between about 30 to 180 minutes of exposure time, the temperature of the test specimen(s)  46  steadily climbs from about 42° C. to 47° C. In this first time segment  94 , the fixture  70  is installed over the test specimen(s)  46  as the test specimen(s)  46  is exposed to the solar rays  39 . The ambient temperature  90  is also shown to steadily rise during this time period. 
     The fixture  70  is removed at the second time segment  96  of the temperature variation plot  92 , which represents time values between about 180 to 275 minutes of exposure time. As shown by the second time segment  96 , the temperature of the test specimen(s) rapidly jumps to over 60° C. As described above, the increased heat transfer provided by a compressed air  149  in the fixture  70  is absent in the second time segment  96 , and as a result, the temperature of the test specimen(s)  46  shows a noticeable movement from the first time segment  94 . 
     In the third through the fifth time segments  98  to  102 , respectively, the fixture  70  is removed and installed to illustrate the effects of the fixture  70  on the temperature of the test specimen(s)  46 . As shown in  FIG. 7 , removal of the fixture  70  rapidly increases the temperature of the test specimen(s)  46 . In addition to providing better heat transfer from the test specimen(s)  46 , the fixture  70  substantially shields the test specimen(s)  46  from ambient wind fluctuations that may disturb the air flow  47 . Accordingly, fluctuations in the temperature of the test specimen(s)  46  can be reduced. 
       FIG. 7  graphically illustrates the noted reduction in the temperature fluctuations of the test specimen(s)  46  when the fixture  70  is being used in the accelerated weathering test apparatus  20 . Comparatively referring to the first time segment  94  and the second time segment  96  of the temperature variation plot  92 , fluctuations in the temperature of the test specimen(s)  46  are more pronounced when the fixture  70  is removed. The fluctuations in the second time segment  96  are due to the disturbance of the air flow  47  over the test specimen(s)  46  by the ambient air. Such disturbances can be substantially blocked by the fixture  70  as shown in the first time segment  94 . Similarly, when the fixture is removed during the fourth time segment  100 , fluctuations in the temperature of the test specimen(s)  46  appear pronounced as compared to any one of the three time segments  94 ,  98 , or  102 , where the fixture  70  is being used. 
     As described in the foregoing, the fixture  70  allows the accelerated weathering test apparatus  20  to maintain the temperature of the test specimen(s) at a lower temperature than possible without the fixture  70 . The lower temperatures of the test specimen(s)  46  are possible due to the fixture  70  providing compressed air  149  to flow over the test specimen(s)  46 . Additionally, the fixture  70  reduces fluctuations in the temperature of the test specimen(s)  46  that may be caused by the ambient wind disturbing the air flow  47  from the air tunnel  44 . 
     Furthermore, while the particular preferred embodiments have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the teaching of the disclosure. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as limitation. The actual scope of the disclosure is intended to be defined in the following claims when viewed in their proper perspective based on the related art.