1、MIL-H DBK-731 1 AUGUST 1984 a MILITARY STANDARDIZATION HANDBOOK I NONDESTRUCTIVE TESTING METHODS OF COMPOSITE MATERIALS = THERMOGRAPHY - /-/ No Deliverable Data Required by This Document THIS DOCUMENT CONTAINS -3q .- PAGES Provided by IHSNot for ResaleNo reproduction or networking permitted without
2、license from IHS-,-,- MIL-HDBK-731 ND 7777970 0038525 T DEPARTMENT OF DEFENSE WASHINGTON D. C. , 20301 MIL-HDBK-731 Military Handbook for Nondestructive Testing Methods of Composite Materiais - Thermography. 1. This standardization handbook was developed by the Department of Defense with the assista
3、nce of the Army Materials and Mechanics Research Center and Drs. Henneke and Reifsnider of Virginia Polytechnic Institute, Blacksburg, Virginia in accordance with established procedure. 2. It is the intent to review this handbook periodically to insure its completeness and currency. Users of this do
4、cument are encouraged to report any errors discovered and any recommendations for changes or inclusions to Amy Materials and Mechanics Research Center, ATTN: DRXMR-SMS, Arsenal St., Watertown, MA 02172-2719. ii Provided by IHSNot for ResaleNo reproduction or networking permitted without license from
5、 IHS-,-,-MIL-HDBK-731 ND m 7777770 0038526 1 m MIL-HDBK-731 FOREWORD 1. This handbook will eventually become a chapter in a larger volume which will probably include the following chapters : Part I: Overview of Characterization Techniques for Part II : Liquid Chromatography, A State-of-The-Art-Revie
6、w Part III : Infrared and Raman Spectroscopy, A State-of-The-Art-Review Part IV : Radiography, A State-of-The-Art-Review Part V: Ultrasonics, A State-of-The-Art-Review Part VI : Ac oust ic Emi s s i on, Part VI1 : Thermography, A State-of-The-Art-Review Part VIII: Annotated Bibliography Part IX: App
7、lications to the Manufacture of Composite Main Rotorblade. Composite Reliability f A Stat e-o f -T he-Ar t -Revi ew 2. Each chapter will be coordinated separately as the amount of materials to review at one time is large. After acceptance of the individual chapters as smaller handbooks, they will be
8、 incorporated into a single volume. 3, It is intended that this vohme serve as a reference in which answers may be found to the more general questions concerning the technical aspects and applications of Thermography. a i ii Provided by IHSNot for ResaleNo reproduction or networking permitted withou
9、t license from IHS-,-,-SECTION 1. 1.1 2. o 2.1 2.1.1 3.0 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.2 3.2.1 3.2.2 3.2.3 3.2.4 4. O 4.1 4.2 4.2.1 4.2.2 4.2.3 4.2.3.1 4.2.3.2 4.2.4 4.2.4.1 4.2.4.2 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3 5.0 5.1 5.2 5.3 6.0 6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.1.5 MIL-HDBK-731 TABLE OF
10、CONTENTS Scope General Background Active and Passive Techniques Contact and Noncontact Methods - General Temperature Measurement for Thermography Contact Methods Coatings - Application Temperature Sensitive Paints Thermal Phosphor Type Thermally Sensitive Papers Liquid Crystals Other Contact Methods
11、 Noncontact Methods Surface Thermal Patterns Infrared Detections Photon - Effect Devices Application of Detectors Application and Thermographic Nondestructive Testing General Procedures Basic Principles of Operation Heat Emission Patterns Active or Passive Heat Emitter Heat Introduction by Passive T
12、hermography Heat Introduction by Radiation and Convection Development of Heat Emission Patterns Passive Methods Hysteresis Energy Heat Patterns Generation of Heat Patterns Active Heat Generation Procedure Vibro t hermograp hy Variations Equipment General Thermal Energy Source Recording and Interpret
13、ation of Images Parameter Selection Conductivity of Materials Effect of Differential Heating Thermal Image Surface Reflectivity Emissivity of the Surface Damping Characteristics of Materials iv PAGE 1 1 1 2 2 3 3 3 4 4 4 5 6 6 7 7 8 8 9 - 9 9 9 10 10 11 11 12 13 13 14 15 15 15 16 16 16 16 17 17 17 1
14、8 18 19 19 = second, measure the areal temperature distribution on the surface of the examined test object; and, third, interpret the results according to well- established and understood physical principles. 1 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from
15、 IHS-,-,-MIL-HDBK-731 2.1 Active and Passive Techniques. For convenience, thermographic NDT techniques may be categorized as active or passive (Ref. 1) when referring to the means by which heat is produced “actively“ in the specimen by trans- formation processes which occur while the test object is
16、being subjected to normal operating, testing, or loading conditions. For example, an electrical component may overheat due to some abnormality when electrical current passes through it, or a structural material might develop hot spots under mechanical loading in regions where damage is occuring. met
17、hods refer to those for which the test object is treated as a path for heat from external heat source to some external heat sink. nonhomogeneities or flaws will cause local differences in thermal conductivi- ties which will evidence themselves in the thermal mapping. zations will be dealt with in mo
18、re detail in section 7.0 on the application to composites. On the other hand, passive In this case, local These categori- 2.1.1 Contact and Noncontact Methods - General. temperatures are measured can also be conveniently categorized, in this case, as contact or noncontact methods (Ref. 2). Contact m
19、ethods of temperature measurement very obviously refer to those by which a physical object is brought into contact with the surface of the test specimen to determine the temperature of the latter, The means by which the Noncontact methods are those which rely upon infrared radiation to determine the
20、 temperature of the radiating source. Both types will be discussed in section 3.0 on Thermographic NDT. 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-731 3. O Temperature Measurement for Thermography. 3.1 Contact Methods. As noted earlie
21、r, the second step in applying thermographic NDT is to measure the surface temperature of the object to be examined. (The first step, exciting the thermal pattern, will be discussed in section 3.2, noncontact methods.) Perhaps the oldest and least expensive methods for doing this fall under the cate
22、gory of contact methods. Such methods require that actual contact be made between the surface of the examined object and the measuring device. There are a large variety of possible tech- niques for making contact surface temperature measurements, most of which rely upon some type of chemical reactio
23、n in an applied coating. Thermal coatings include paints, phosphors, papers, liquid crystals, and a number of other special temperature sensitive compounds. These methods have the distinct advantages of being relatively easy to apply and requiring a low initial investment, particularly if only a sma
24、ll area of material is to be examined. On the other hand, these methods have the disadvantages of generally yielding only a qualitative indication of temperature and of affecting, themselves, the distribution of surface temperature. There are, however, some contact coatings such as liquid crystals,
25、which can be calibrated for temperature measurement. A major disadvantage, common to all of the contact methods, is that one must know, apriori, the approximate value of the temperature that he wishes to me as Ure. 3.1.1 Coatings - Application. To apply a contact thermograpic method, a temperature-s
26、ensitive coating must be placed upon the surface of the examined test object. A change in temperature is evidenced by a change in color or appearance of the coating due to a temperature-sensitive chemical or physical change. This change may be either reversible or permanent, depending upon the type
27、of coating used, As mentioned in the previous paragraph, there are several types of coatings that have been used for thermographic NDT. Addi- tional details may be found in the survey presented in Ref. 2, but some of the highlights of their use are described in sections 3.1.2, 3.1.3, 3.1.4, 3.1.5, a
28、nd 3.1.6. 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-7 31 3.1.2 Temperature Sensitive Paints. Temperature sensitive paints have been developed for use between 40 to 16OO0C. change at a temperature which, under optimum conditions, can
29、be calibrated to within - + 5OC. different color transitions. Most often the color transitions are observable under ordinary light, but certain paints exist which may require the use of ultraviolet. These paints undergo a color Some paints exist which will progress through several 3.1.3 Thermal Phos
30、phor Type. Another coating, which normally requires ultraviolet light for its use, is the thermal phosphor type. as with many such compounds, emit visible light when excited by ultraviolet radiation. varies inversely to its temperature. The rate of decrease of light intensity with temperature is ver
31、y rapid at a critical temperature, as much as 25%/OC within a very limited temperature range around the critical point. Various phosphors exist for use between room temperature and 4OO0c. several ways in which a phosphor may be applied to a surface: stripable coating, in a tape, or as a powder. Thes
32、e phosphors, For thermal phosphor, the intensity of the emitted visible light There are as a paint, a 3.1.4 Thermally Sensitive Papers. At least three different types of thermally sensitive papers exist for thermographic NDT: organic, plastic and infrared copy papers. substance. Upon reaching the me
33、lting temperature, the organic substance melts and is absorbed by the paper, turning its color from white to black. Plastic coating papers are constructed with a plastic coating containing a large density of air bubbles. a pale color. Upon reaching the critical temperature of the paper, the plastic
34、melts, releasing the bubbles and revealing the black color of the paper underneath. to produce a color when melted. Organic papers are coated with a meltable organic These bubbles diffuse incident light giving the paper The infrared copy papers contain dye precursors which react 4 Provided by IHSNot
35、 for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-JJDBK-731 3.1.5 Liquid Crystals. Liquid crystals refer to a special class of materials which at certain temperatures possess the properties of both a liquid and a crystal. These materials will flow as a viscous liqui
36、d while retaining the short range molecular structure of a crystalline solid. One form of liquid crystals, the so-called cholesteric, are dichroic. When unpolarized light is incident upon a dichroic material, it is decomposed into two components, one of which is circularly polarized clockwise while
37、the other is circularly polarized counterclockwise. One of these components is transmit- ted and the other is reflected. The reflected component produces a color when the material is illuminated by white light. As cholesteric liquid crystals are heated, the distances between molecular layers increas
38、es. This distance governs the wavelength of the reflected light. Eventually the liquid crystal melts, and with further increase in temperature, becomes more and more disordered until eventually it achieves the state of an isotropic liquid. One can select liquid crystals from a wide variety of possib
39、lities such that the liquid crystal appears colorless at low temperatures, changes through a succession of colors as it melts and approaches isotropy, and again becomes colorless when the material is totally isotropic. It is possible to obtain many different liquid crystal compounds which exhibit co
40、lor changes at any temperature from -20 to 25OoC. color-change temperature ranges from 10 to 3OoC. relatively fast, 0.1 to 0.2 sec. The sensitivity for the detection of flaws by liquid crystals can be quite good. Depending upon the thermal conductivity and the proximity of the flaw to the surface, f
41、law sizes as small as 118“ square have been detected. The various compounds have varying The response time is a a 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-731 ND H 7779770 0038535 2 = MIL-HDBK-731 3.1.6 Other Contact Methods. Severa
42、l other temperature-sensitive compounds have found some application in thermographic NDT. Thermochromic order-disorder compounds change color with temperature without breaking chemical bonds and hence are reusable. Variable surface-tension compounds have been developed with temperature-sensitive sur
43、face tension (Ref. 2). When sprayed upon a surface, these compounds are repelled from warmer areas and coalesce in the cool areas. Hence, these compounds are more sensitive to temperature differences than they are to the absolute value of the temperature. Finally, frost testing has been applied in s
44、ituations where one wished to find gross flaws or disbonds. To apply this method, one chills the surface of the examined object below the frost point so that a frosted surface is obtained. Differences in heat conductivities in the test object will then evidence them- selves by the patterns formed as
45、 the frost melt. 3.2 Noncontact Methods. Noncontact methods for temperature measurement are based upon the phenomenon of infrared radiation. Infrared radiation is a portion of the electromagnetic spectrum, invisible to the human eye, with frequencies below that of the red portion of the visible spec
46、trum (Figure 1). Early researchers found that this “infrared“ light caused higher temperature readings on thermometers than did visible light. Later workers found that, as a basic law of nature, all matter at temperatures greater than absolute zero spontaneously emits infrared, or thermal, radiation
47、. The radiation is a result of the thermal agitation, or temperature, of the basic components of which all matter is composed: molecules, atoms and subatomic particles. The frequency range covered by infrared radiation is a result of the various energies possessed by the wide range of molecules, ato
48、ms and sub-atomic particles and the various types of motions and quantum jumps available to these particles. Classically, the infrared spectrum was categorized into three subregions-near, intermediate, and far infrared, depending upon the value of the infrared frequency in relation to the red spectr
49、um. The initial reason for this classification was due to the different experimental techniques required to detect the radiation in these different bands. 6 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-731 ND W 7797970 0038536 4 W MIL-HDBK-7 31 3.2.1 Surface Thermal Patterns. One must be aware of the fidelity of the observed surface thermal
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