The multiplier becomes a property of the si unit for

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intervals (Table 9). The multiplier becomes a property of the SI unit. For example, a millimeter (mm) is 0.001 meter (m). The volume unit cubic centimeter (cm 3 ) is (0.01 m) 3 or 10 –6 m 3 . Unit submultiples such as the centimeter, decimeter, dekameter and hectometer are avoided in scientific and technical uses of SI because of their variance from the convenient 10 3 or 10 –3 intervals that make equations easy to manipulate. In SI, the distinction between upper and lower case letters is meaningful and 25 Introduction to Acoustic Emission Testing P ART 4. Measurement Units for Acoustic Emission Testing T ABLE 6. SI base units. Quantity Unit Symbol Length meter m Mass kilogram kg Time second s Electric current ampere A Temperature kelvin K Amount of substance mole mol Luminous intensity candela cd T ABLE 7. SI derived units with special names. a Relation to Other Quantity Units Symbol SI Units b Capacitance farad F C·V –1 Catalytic activity katal kat s –1 ·mol Conductance siemens S A·V –1 Energy joule J N·m Frequency (periodic) hertz Hz 1·s –1 Force newton N kg·m·s –2 Inductance henry H Wb·A –1 Illuminance lux lx lm·m –2 Luminous flux lumen lm cd·sr Electric charge coulomb C A·s Electric potential c volt V W·A –1 Electric resistance ohm Ω V·A –1 Magnetic flux weber Wb V·s Magnetic flux density tesla T Wb·m –2 Plane angle radian rad 1 Power watt W J·s –1 Pressure (stress) pascal Pa N·m –2 Radiation absorbed dose gray Gy J·kg –1 Radiation dose equivalent sievert Sv J·kg –1 Radioactivity becquerel Bq 1·s –1 Solid angle steradian sr 1 Temperature, celsius degree celsius °C K Time a hour h 3600 s Volume a liter L dm 3 a. Hour and liter are not SI units but are accepted for use with the SI. b. Number one (1) expresses a dimensionless relationship. c. Electromotive force.
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should be observed. For example, the meanings of the prefix m (milli) and the prefix M (mega) differ by nine orders of magnitude. Units for Acoustics Pressure, Displacement and Related Quantities Acoustic emission is a shock wave inside a stressed material, where a displacement (unit of distance) ripples through the material and moves its surface. A transducer on that surface undergoes this displacement as a pressure. The pressure is measured as force per unit area in pascal (Pa), equivalent to newton per square meter (N · m –2 ). The signal from the transducer is sometimes related to velocity (m · s –1 ), displacement (m) or acceleration (m · s –2 ). Properties of piezoelectric transducers are related to electric charge: a pressure on the element creates a charge (measured in coulomb) on the electrodes. A rapidly changing pressure alters the charge fast enough to allow the use of either voltage or charge amplifiers. After this, signal processing may analyze and store data in terms of distance in meter (m), velocity in meter per second (m·s –1 ), acceleration in meter per second per second (m·s –2 ), signal strength in volt·second (V·S), energy in joule (J), signal in volt (V) or power in watt (W).
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  • Fall '19
  • Nondestructive testing, Acoustic Emission

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