1、NOT MEASUREMENT SENSITIVE r . MIL-HDBK-7 32A 24 APRIL 1991 SUPERSEDING MIL-HDBK-7 32 1 OCTOBER 1984 MILITARY HANDBOOK NONDESTRUCTIVE TESTING METHODS “ - “ - - - - - -“- ITERIALS ACOUSTIC EMlSSlQN “- -.“ OF COMPOSIIt MA 3 DELIVERABLE DATA REQUIRED BY THIS DOCUMENT AREA ND71 DISTRIBUTION STATEMENT A A
2、pproved for public release; distribution unlimited. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-732A ND m 9999970 OObb888 T m MIL-HDBK-732A DEPARTMENT OF DEFENSE WASHINGTON D.C. 20301 MIL-HDBK-732 Military Handbook for Nondestructive Tes
3、ting Methods of Composite Materials - Acoustic Emission. April 1991 1. This revised standardization handbook was developed by the Department of Defense with the assistance of the US Army Materials Technology Laboratory in accordance with established procedure. It is approved for use by all Departmen
4、ts and Agencies of the Department of Defense. 2. It is the intent to review this handbook periodically to insure its completeness and currency. Users of this document are encouraged to report any errors discovered and any recommendations for changes or inclusions to US Army Materials Technology Labo
5、ratory, ATTN: SLCMT-MEE, Arsenal Street, Watertown, MA 02172-2719. i Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-FOREWORD 1. This handbook is one of four handbooks (listed below) to be used for the nondestructive testing of composites. . MIL-HDBK
6、-731 - Nondestructive Testing Methods of Composite Materials - MIL-HDBK-732 - Nondestructive Testing Methods of Composite Materials - MIL-HDBK-733 - Nondestructive Testing Methods of Composite Materials - MIL-HDBK-787 - Nondestructive Testing Methods of Composite Materials - Thermography Acoustic Em
7、ission Radiography Ultrasonics 2, Each handbook will be coordinated separately as the amount of materials to review at one time is large. After acceptance of the individual handbooks, they should prove useful for general composites testing, with different methods complementing each other. 3. It is i
8、ntended that this handbook serve as a reference in which answers may be found to the more general questions concerning the technical aspects and applications of Acoustic Emission. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-PARAGRAPH 1. 1.1 2. 2.
9、1 2.1.1- 2.1.2 2.1.3 3. 3.1 3.1.1 3.2 4. 4.1 4,2 4-3 4.4 5. 51 5.2 5.2.1 6. 6.1 6.2 6.2.2 6.3 6.4 6.5.1 6e2.1 6.5 7. 7.1 7.2 7.3 7.4 8, 8.1 8.2 8,3 MIL-HDBK-732A ND m 9999970 0066890 8 m MIL-HDBK-732A CONTENTS INTRODUCTION General BASIC SOURCE OF AE General Expression of a Stress Wave Simplification
10、 of Model Magnitude of Acoustic Event Generated Acoustic Emission Event as a Complex Propagating Stress Wave COMPOSITE TEST SPECIMENS OR STRUCTURES Acoustic Emission Testing as a Comparison Controlling Factors for Comparison Acoustic Emission for Basic Studies Technique of Composites TEST INSTALLATI
11、ON OR TEST FIXTURING Fixturing and Interface Characteristics Effect of Size or Volume of Composite Article Use of Artificial AE Sources Other Factors COUPLANTS AND WAVEGUIDES Properties of Couplants Application of Couplants to AE Transducers Special Cases SENSORS TYPES, LOCATION, ATTACHMENT Backgrou
12、nd Resonant and Non-Resonant AE Sensors Response of AE Sensors Choice of Sensors Sensor Locations Propagation Losses Attachment Technique for AE Sensors Basic Design Principles for Attachment of AE Sensors CABLES General Cable Length Choice of Cables Ground Loops PREAMPLIFIERS Primary Function Perfo
13、rmance Dynamic Range PAGE 1 1 2 2 2 2 2 3 3 3 3 8 10 10 10 10 10 11 11 11 11 iii Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-PARAGRAPH 9. 9.1 9.2 9.3 10. 10.1 10.1.1 10.1.2 10.2 10.3 11. 11.1 11.2 12. 12.1 13. 13.1 14. 14.1 14.2 14.3 15 . 15.1 15
14、.2 15.2.1 15.2.2 15.2.3 15.2.4 15.2.5 15.3 15.4 15.5 15.6 16. 16.1 16.2 MIL-HDBK-732A ND W 9999970 OObb87l T m - - MIL-HDBK-732A - CONTENTS Secondary Amplifiers and Filters Functions Various Bandpass Studies Maximum Voltage Limitation TIME DOMAINS AND CHARACTERIZATION OF ANALOG AE SIGNALS Characteri
15、stics of AE Signal Continuous AE Combination of Burst and Continuous Type AE Event Identification Time Measured AE Energy as an Approximation of True Signal Energy AE SOURCES IN COMPOSITES General Idea of AE Generation Categories of Composite Material DISPERSION - STRENGTHENED COMPOSITES General PAR
16、TICLE - REINFORCED COMPOSITES General FIBER - REINFORCED COMPOSITES General Characteristic AE Sources in Fiber-Reinforced Sources in other Composites Composites WAVE PROPAGATION ASPECTS General Aspects of Stress-Wave Propagation Geometric Spreading of the Stress Wave Losses Due to Material Energy Ab
17、sorption of Wave Speeds Dispersion of Stress Waves Scattering of Stress Waves Distinguishing Between Different Types . Effect of Signal Propagation Losses on Reducing Random Flaw Generated Event6 Components of AE Signal Duration SOURCE OR FLAW LOCATION IN COMPOSITES Triangulate Method for Locating A
18、E Area Location Technique - To Determine Stress Wave Energy of AE Sources Peak Amplitude Source Event Most Damage iv PAGE 12 12 12 12 13 13 13 13 13 14 15 15 15 16 16 17 17 18 18 18 18 19 19 19 19 19 19 20 20 20 20 21 21 22 22 22 Provided by IHSNot for ResaleNo reproduction or networking permitted w
19、ithout license from IHS-,-,-MIL-HDBK-732A ND 9999970 0066892 I9 MIL-HDBK-732A CONTENTS PARAGRAPH PAGE 16.3 Area Location Technique - More Sophisticated Summary and RecoFendations Approach 22 22 16.4 17. 17.1 17.1.1 17,2 KAISER EFFECT/FELICITY RATIO Felicity Ratio Effect of Variables on Felicity Rati
20、o Kaiser Effect 23 23 23 23 18. 18.1 18.2 FACTORS OF SIGNIFICANCE IN AE DATA Six Basic Factors Interpretation of Significance in AE Data 24 24 24 19. 19.1 19.1.1 19.2 IN-SITU CALIBRATION OF AE TESTS Advantages Additional Advantage Calibration Using Fracture of Pencil Lead 25 25 25 25 20. 20.1 20.1.1
21、 20.1.2 EXTRANEOUS AE Typical Extraneous Soures of AE Determining if Significant AE is Present Elimination of Extraneous Sources of AE 26 26 26 26 21. 21 1 21.2 21.3 CONTROL CHECKS ON AE TESTING Running Charts AE Electronics Sensor Checks 27 27 27 27 22. 22.1 22.2 ADDITIONAL INSTRUMENTATION General
22、Types of Equipment 28 28 28 23. 23.1 FUTURE USES OF AE FOR NDE General 29 29 REFERENCES 30 FIGURE 1 2 Classical AE Burst Event Schematic of Fiber Composite A); and list of AE Source Mechanisms b) 31 32 V Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-
23、,-MIL-HDBK-732A ND 7997970 00bb893 3 W “ MIL-HDBK-73% 1. INTRODUCTION 1.1 General, Technical principles will be discussed which must be followed in order to insure that a technically sound Acoustic Emission (AE) test is performed on a composite sample. It is relatively simple to attach an AE sensor
24、to a composite and then load the sample. It is a much more complicated process to carry out an AE test that conforms to the known physics relating to AE testing. This handbook will assume that the reader is planning on doing “production testing“ of composites with AE monitoring (“production testing“
25、 in the sense that at least several identical samples will be tested). The organization of this handbook will be to treat first each of the physical entities involved in an AE composite test; then aspects such as AE data and its analysis will be treated. 1 Provided by IHSNot for ResaleNo reproductio
26、n or networking permitted without license from IHS-,-,-2. BASIC SOURCE OF AE 2.1 General ExDression of a Stress Wave. An AE event is a complicated stress wave that is generated at a location in a structure by a rapid change in the local stress state. This can be expressed by the following where is t
27、he change in each of the independent stress components necessary $0 describe the stress state at that point in the structure, is a vector describing the location at which the rapid change in stress state occurs, A t is the time interval over which the stress change ccurs, and aV is the volume (or ar
28、ea for certain AE sources) of the structure which experiences the stresschange. A typical example might be a microscopic failure of a fiber in a composite structure. In this case the stored energy that is rapidly released supplies (among other things) the energy contained in the resulting stress wav
29、e or AE. 2.1.1 Simplification of Model. In reality the model we have adopted is simplified; the AE event is actually generated by the change inCTij as a function of both time and spacial coordinates in the region defined by A V. But since this more complicated model adds nothing to our development h
30、ere, we will use the simplified model, 2.1.2 Magnitude of Acoustic Event Generated. The factors shown in expression (1) can provide some insight into the magnitude of the AE event that is generated. For example, the AE event will be larger for larger ACijIs, for shorter Ats, and for largera Vs. Conv
31、ersely, the AE event will be smaller for smallerdbijt, for longer Ats, and for smaller B Vs. Certain dynamic processes can also generate AE events; for example, sliding friction between two surfaces moving relative to each other. By including “surface“ tractions in theAdij of expression (11, these d
32、ynamic processes can also be included in the general simplified formulation. 2.1.3 Acoustic Emission Event as a Complex ProDaaating Stress Wave. It is important to emphasize that the AE event at the level at which we can currently measure it (or observe it) is a complex propagating stress wave that
33、will follow the physical laws governing stress waves. Hence, the exact theory of wave propagation is intractable for all but very simple structures (for example, an isotropic infinite half space or plate or layer on a half space), Some more complex structures can be approximately modeled by transien
34、t finite element analysis. It will be important to keep these ideas in mind as we discuss AE testing of practical composites, which are normally of complex and finite geometry, as well as anisotropic. 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,
35、-3. COMPOSITE TEST SPECIMENS OR STRUCTURES 3.1 Acoustic Emission Testing as a CornDarison Techniaue. Due to the complexity of stress wave propagation in composites as well as other factors (such as the currently unknown AE source events and the usual commercial AE sensors that respond to surface dis
36、placements and velocities in a complex frequency dependent fashion), it is not generally possible at present to make measurements of a real AE event and then calculate the source function (represented by expression 1). However, using special commercial high fidelity transducers based on the design o
37、f the “NBS conical transducer“ I and the special case of AE source and receiver on opposite surfaces of a plate, the source function can be measured Z . Hence, AE as a nondestructive evaluation technique for composites is, at this time, primarily a comparison technique. This fact means that, to be u
38、seful, baseline AE data must be gathered from a series of “identical“ samples. Then techniques can be established to identify various deviations from the “identical“ samples. It is necessary to establish what is meant by the term “identical samples.“ 3.1.1 Controllin? Factors for Comparison. The fol
39、lowing factors must be controlled for samples to be identical. First, the relevant stress wave propagation characterlstics must be the same. This requirement means that the sample geometry must be the same, the sample material (mechanical properties e.g., modulus and density) must be the same, and t
40、he stress wave observation points and techniques must be the same (i.e., sensors and sensor locations). Second, the stress field throughout the sample must be the same. This fact inherently implies that the sample is loaded or stressed in the same way and that the general flaw structure in the sampl
41、e shall be the same (i.e., same sizes and locations or same sizes and uniformly distributed throughout the structure). Thirdly, the local microscopic AE sources (e.g., microscopic failure mechanisms) must be the same. This requirement means that typical microscopic strengths and deformation properti
42、es must be the same. Now since composites can only be reliably described by statistically based analysis procedures, the sameness that is required here is statistical in nature. It should be noted that we have taken a relatively restricted point of view about the definition of the identical samples
43、needed for comparison. There are currently AE applications for which this level of identity has not been necessary, but in our opinion it. is best to start with the above approach and then prove, if possible, that a less restrictive definition of identicalness is sufficient. 3.2 Acoustic Emission fo
44、r Basic Studies of Composites. In addition to its application for NDE, AE can be used or basic studies of composite materials or structures. For these studies, there are some additional comments which need to be made with respect to test specimens and relevant requirements. For these types of studie
45、s, the first requirement is to test a sufficient number of identical samples so that the AE that occurs in a significant manner for each sample can be characterized relative to the AE that can be considered random for the different samples. The characteristic AE patterns can then be correlated with
46、deformation and micro-failure mechanisms that are known to occur at various loads from other inputs (e.g., mathematical analysis, microstructural studies, microfailure observations, etc.). 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-73
47、2A ND m 9999970 0066896 9 MIL-HDBK-732A 4. TEST INSTALLATION OR TEST FIXTURING 4.1 Fixturina and Interface Characteristics. Since the composite test specimen must be supported in some way and is normally externally loaded, the test fixturing becomes a key part of the test system. Normally, the task
48、of assuring that the load is applied in the same way is not difficult. A more subtle but probably just as important factor concerns the interaction of the test fixture and the test specimen from the point of view of wave propagation characteristics. The stress waves generated by an AE event will pro
49、pagate throughout the test specimen as well as the test support and loading fixturing. The two major variables which are of importance here are first, the wave propagation characteristics of the fixturing and second, the wave propagation characteristics of the interfaces between the composite specimens and the fixturing. The time varying amount of energy wh
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