1、a 3 e- MIL-HDBK-788 ND m 7777770 0039762 4 m- MIL-HDBK-788 25 JULY I989 MILITARY STANDARDIZATION SELECTION OF ACOUSTIC SENSORS NO DELIVERABLE O ATA nEQUInED DY TlIIC DOCUMENT AMCC NIA HANDBOOK EMISION OISTRIRUTION STATEMEHT A Approved for piiblic release; distrihiilion unliniited. - THIS DOCUMENT CO
2、NTAINS CA nor are they readily available in designs suitable for a broad range of applications. Although this document only treats selection of piezoelectric sensors, most of the sensor attributes could apply to the other types of sensors. 3.2 Attributes of a sensor. Manufacturers routinely supply i
3、nformation on some sensor attributes, such as sensitivity, frequency range, operating temperature range, size and weight. Information on other attributes is not always provided and should be requested if sensor selection requires such information. Sensor attributes are listed below in the order of d
4、ecreasing relevance to most applications. However, for some applications, normally insignificant attributes may be of prime relevance. This is especially true for certain environmental conditions. A sketch of a basic AE sensor is given in figure 1. 3.2.1 Sensitivity/frequency response. These charact
5、eristics form one attribute that is generally the most fundamental attribute in selecting a sensor. Based on frequency response, there are two general types of sensors; resonance type and broadband type. 3.2.1.1 Resonance type. This is the most common type of AE sensor. It is intended to be used in
6、a narrow band width that contains its resonant frequency. The sensitivity is very high at the resonant frequency, making this type of sensor appropriate for most of those applications where maximum sensitivity is of primary importance. An AE wave excites the resonance to a peak voltage, resulting in
7、 a ring-down whose duration depends on the peak value. Sensors having resonant requencies in the range 100-200 kHz are most often used. For such sensors, a high amplitude emission can produce ring-down durations of several milliseconds. 3.2.1.1.1 Dual-resonance type. A variant of the resonance type
8、is the dual-resonance type. The sensitivity at the two resonances are normally lower than that of a single resonance type; but the usable frequency band is extended. Sensors are not normally classified by the manufacturers as being 3 Provided by IHSNot for ResaleNo reproduction or networking permitt
9、ed without license from IHS-,-,-MIL-HDBK-788 ND M 7977770 0037770 3 M MIL-HDBK-788 single resonance or dual resonance; but a careful look at their frequency-response plots can reveal this. Figures SA and 2B show examples of frequency-response plots for these types. 3.2.1.2 Broadband type. This type
10、of sensor, also known as a “wideband“ type, may be designed as a multiple-resonance type, resulting in good sensitivity in several narrow regions of the specified broad operating bandwidth. Other designs produce broadband sensors without resonances within the specified operating bandwidth. These are
11、 known as “flat“ response broadband sensors. Broadband sensors are typically more damped and have little “ringing“. They have better fidelity for frequency analysis than the multiple-resonance type. A typical frequency response plot for a “flat“ broadband sensor is shown in figure 2C. 3.2.2 Sensors
12、sensitivity. A sensors sensitivity as a function of frequency is usually determined in one of two ways: surface-wave calibration method or face-to-face ultrasonic calibration method. Although other methods are sometimes used, they are not widely practiced. 3.2.2.1 Determination by surface-wave metho
13、d. The surface-wave (Rayleigh wave) method uses a transient surface wave and a digital signal analysis technique to obtain an absolute calibration of the sensors response to normal displacements. This technique is traceable to methods developed by the National Institute of Standards that is, even th
14、ough the units are different, the dB values are nearly the same, although one is 4 b o Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- MIL-HDBK-7 ND m 7777770 0037773 5 m MIL-HDBK-788 positive and the other negative. Typically, the surface wave meth
15、od results in peak sensitivities that are 3 to 6 dB less than the absolute value of the peak sensitivity obtained by the face-to-face method. This results in confusion when comparing sensitivities of sensors calibrated by different methods. For example, a sensor having a peak sensitivity (ultrasonic
16、 method) of -70 dB re iV/pbar may have a peak sensitivity (Rayleigh wave method) of 65 dB re 1 V/M/S. These correspond to 3.1610-4 V/ubar and 1778 V/M/S, respectively. There is no simple relationship between these numbers. Furthermore, when comparing the frequency-response plots obtained by the two
17、methods, significant differences appear in the shape of the plots at higher frequencies and for broadband censors, in general. This is due to the fact that the sensors response is directly related to the instantaneous average of the particle displacements over the surface of the sensor. For waves tr
18、avelling perpendicular to the sensor surface this averaging effect does not change with frequency. However, for waves travelling parallel to the sensor face (Rayleigh waves) the averaging effect produces decreasing sensitivity as the acoustic wavelength gets smaller than the diameter of the sensor.
19、This means, as the frequency increases, the sensors response to the surface wave method will probably produce a frequency-response plot that is shaped differently than that produced by the face-to-face ultrasonic calibration method. It is therefore advisable to compare broadband frequency responses
20、only for equivalent calibration methods. 3.2.2.5 Temperature range. The operating temperature range of a piezoelectric sensor is determined primarily by the thermal properties of the piezoelectric element, and those of the wearplate and the substance used to bond the sensor to the wearplate. It is n
21、ot necessary to know the details of the design because the manufacturer specifies the operating temperature range. Sensors are available that operate at near cryogenic temperatures as well as up to temperatures of 55OOC. 3.2.2.6 Wearplate. The wearplate, also known as “contact shoe“, “coupling shoe“
22、, “protective shoe“ and “face material“, is typically fabricated of ceramic or epoxy. Other materials occasionally used are anodized aluminum, stainless steel, high nickel-chromium alloys and brass. A ceramic wearplate has an acoustic impedance very similar to that of the most common piezoelectric m
23、aterial, lead-zirconate-titanate (PZT). It is a better impedance match to metals than epoxy. Sensors having epoxy wearplates can be used with better sensitivity on non-metallic surfaces, such as fiberglass reinforced plastics. Although the sensors frequency-response plot is obtained with the wearpla
24、te affixed, only in the surface wave method of calibration is the wearplate necessarily coupled to a metal surface. 3.2.2.7 Differential design. This special design reduces the sensors susceptibility to RFI/EMI, especially to radiated or induced electrical spikes. A differential sensor must be used
25、with a differential preamplifier. It should be noted that some differential sensors exhibit directionality variations as great as 4 dB. 3.2.2.8 Size and shape. The common shape of a sensor is a right-circular cylinder whose diameter is comparable to the width of the piezoelectric element and whose h
26、eight is generally similar to its diameter. Sensor diameters range from a few millimeters to over 50 millimeters. (The very low frequency sensors used in geologic applications may be much larger.) A popular size has a diameter of 20 mm and a height of 16 mm. 5 Provided by IHSNot for ResaleNo reprodu
27、ction or networking permitted without license from IHS-,-,-MIL-HDBK-7 ND 7977770 0037772 7 m MIL-HDBK-788 3.2.2.9 Weight. Sensor weights range from less than a gram to over 500 gramsl with 15 gm being .very common. 3.2.2.10 Housing material. The housing material, also known as the “case material”, i
28、s most often stainless steel or aluminum. Other materials that are used include carbon steel, brass, and high nickel-chromium alloys. 3.2.2.11 Connector, The connector is top or side mounted. Common connector types are BNC and microdot; but other types are used on special sensors. Some sensors have
29、integral cables, usually less than 2 m in length, usually terminated with BNC connectors. 3.2.2.12 Grounding features. Most sensors have housings that are grounded and electrically isolated from the test part by a non-conducting wearplate. If the wearplate is metallic and the test part an electrical
30、 conductor, ground loops can be created unless an intermediate non-conductor is used between the sensor and test part. Note that it would also be important to use a non-conductor between the sensor and any metallic mounting fixture, as well. 3.2.2.13 Directionality. Most sensors have omnidirectional
31、 sensitivity Exceptions to in the plane of contact with variations not exceeding 5 2 dB. this are some differential sensors and sensors having designed directional features. 3.2.2.14 Seal type. Housings are usually epoxy sealed or hermetically sealed. The latter is preferred for hostile environments
32、. 3.2.2.15 Shock resistance. Sensors having integral preamplifiers and some special broadband sensors usually are rated to withstand shocks up to 500 peak g in any direction. Most other sensors typically have shock limits of 10,000 g. 3.2.2.16 Radiation resistance. Sensors can be designed to withsta
33、nd, without degradation, both neutron and gamma radiation. The maximum integrated dose is specified by manufacturers for some sensors. Sensors containing integral preamplifiers are least suited for use in radiation environments. 3.2.2.17 Curie temperature. This is the temperature beyond which the pi
34、ezoelectric material loses its transducing properties. The specified operating temperature range for a sensor is always less than the Curie temperature. However, it must be realized that exceeding this temperature will cause permanent damage to most AE sensors. 3.2.2.18 Capacitance. It is often help
35、ful to know the sensor capacitance when deciding on cable lengths between the sensor and preamplifier. The signal loss due to the cable capacitance is given by: Signal loss = Cc/Cs t Cc, where Cc is cable capacitance and Cs is the sensor capacitance. A meter length of coaxial cable has a capacitance
36、 of approximately 100 pF. Therefore, sensors having capacitance values less than 100 pF would experience signal losses greater than 50%. Sensor capacitance values typically fall in the range 100 pf to 1000 pF. 6 Provided by IHSNot for ResaleNo reproduction or networking permitted without license fro
37、m IHS-,-,- MIL-HDBK-7 ND 7777970 0037773 7 = MIL-HDBK-788 3.2.2.19 Piezoelectric material. The type of piezoelectric material is not always specified by the manufacturer of sensors. The most common material is lead-zirconate-titanate (PZT) and its variants. Other materials used are barium titanate,
38、lithium niobate and lead metaniobate. Knowledge of the piezoelectric material can be used to infer the acoustic impedance and the Curie temperature. 3.3 Integral-preamplifier sensors. Sensors with preamplifiers built into their housings provide high sensitivity and the elimination of the sensor to p
39、reamp connection. When considering selection of such a sensor, additional attributes are investigated. These relate to typical specifications of the preamplifier and include gain, dynamic range and noise. 7 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS
40、-,-,-MIL-HDBK-788 - ND m 7777770 0037774 O MIL-HDBK-788 4. APPLICATION/SENSOR RELATIONSHIP 4.1 General. AE applications are too numerous and various to be easily placed into classes. appropriate sensor specifications for each class, the resulting number of classes would be great and overlapping. The
41、refore, in order to provide guidelines for sensor selection, it is more efficient to describe general “Application Characteristics“ and to discuss how these relate to certain sensor attributes. Some sensor attributes are easily selected, while others can provide a puzzling choice. The most fundament
42、al choice, and often the most important, is the choice of sensitivity/frequency response, This is the essential feature of the sensor and many application characteristics may influence its choice. Often, some measurements must be performed in order to make a confident choice. In discussing the appli
43、cation characteristics, emphasis is placed on selection of sensitivity/frequency response. As discussed in 3.2.1, the two types of calibration methods produce different frequency response plots, especially at high frequencies. In discussing application characteristics, consideration will be given to
44、 whether a particular calibration method is more appropriate in selecting a sensor based on its sensitivity/frequency response. If done with detail sufficient to enable prescription of 4.2 Frequency range. Although AE phenomenon are known to occur over the frequency range 100 Hz to 10 MHz, the most
45、frequently used bandwidth is 100 to 400 kHz. It is highly probable that well over half of all AE measurements to date were made using a resonant sensor having a resonance near 150 kHz. One of the reasons for this is the desire to keep the resonance frequency high enough to be insensitive to most mec
46、hanical background noise, but low enough so that detection of AE at a distance from the source is not significantly reduced by attenuation. AE detection is influenced by attenuation whenever the sensor is not in the immediate proximity of the AE source, that is, it is “remote“ from the source. The t
47、erm “remote detection“ will be used below to signify that the sensor is not to be placed immediately next to or on top of the source of AE to be detected. 4.3 Application characteristics. The application characteristics discussed here are not mutually exclusive and sometimes one characteristic will
48、suggest a sensor attribute that is in conflict with that suggested by another characteristic. Nevertheless, some basic guidelines can be succinctly stated and, following discussion here, are summarized in 5. 4.3.1 Material. Solids and fluids produce and transmit AE. Hard metallic solids have higher
49、amplitude emissions and less attenuation than soft metallics and non-metallicc. High frequency (150-800 kHz) sensors may be used for remote detection in hard metallics, Remote detection in most non-metallics (concrete, rubber, plastics, ceramics, rock, wood, etc.) often requires lower frequency (25 kHz - 150 kHz) sensors. Material characteristics of solids associated with higher amplitude emissions are : High Strength Anisotropy Flawed Material Non Homogeneity 8
