16810(16682) Engineering Design and Rapid Prototyping Lecture 7 a1n Structural Testing Instructor(s) Prof, olivier de weck January 21, 2004 Massachusetts Institute of Technology
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IGa10 Outline Structural Testing Why testing is important Types of Sensors, procedures Mass, Static Displacement, Dynamics Test protocol for 16810 Explain protocol Sign up for time slots 16.810(16682) Massachusetts Institute of Technology
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Data Acquisition and Processing al0 for Structural Testing (1)Sensor Overview Accelerometers, Laser sensors, Strain Gages Force Transducers and load cells gyroscopes (2)Sensor Characteristics Dynamics FRF of sensors, bandwidth resolution placement issues (3)Data Acquistion Process Excitation Sources, Non-linearity, Anti-Alias Filtering, Signal Conditioning (4)Data Post-Processing FFT, DFT, Computing PSD's and amplitude spectra statistical values of a signal such as rms, covariance etc (5)Introduction to System Identification ETFE, DynaMod measurement models 16.810(16682) Massachusetts Institute of Technology
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IGAl0 Why is Structural Testing Important? M Example: Ground vibration Testing Product Qualification Testing Performance assessment a System Identification Design verification Damage assessment Aerodynamic Flutter Testing Operational Monitoring Material Fatigue Testing stimulus A Structural response Ref:http://www.af.mil/photos/may1999/19990518f2235.html System (imAgeistakenfromU.s.AirForceWebsitehttp://www.af.mill) vibration tests at edwards air N DAQ H DSP Force Base, calif. in april 1999 DAQ=data acquisition DSP =digital signal processing 16.810(16682) Massachusetts Institute of Technology
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IGA10 I. Sensor Overview This sensor morphology is useful for classific of typical sensors used in structural dynamics Sensor Morphology Table Type Linear Rotational Bandwidth Low Medium High DerivativePosition Rate Acceleration Reference Absolute Relative Quantity Force/Torque Displacement Impedance Low High Example: uniaxial strain gage Need units of measurement [m], [Nml[)[rad]etc 16.810(16682) Massachusetts Institute of Technology
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a10 Sensor EXamples for Structural Dynamics Example: fixed-fixed Goal: Explain what they measure and how they work beam with center load laser sensors excitation accelerometers gyroscopes strain gages m load cells inductive sensors shaker EI ground First flexible mode frequency: 0=14 F(m+0.375m) 16.810(16682) assachusetts Institute of Technology
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GG.aln Strain Gages Strain △ Strain gages measure strain differential displacement over a finite area via a change in electrical resistance R=lp[Q2 Current Nominal length io: I( With applied strain: I (+△)p implementation Wheatstone bridge iaY ni balance circuit bond to test article strain gages feature polyimide-encapsulated GND constantan grids with V+ copper-coated solder tabs Mfg Ref:http://www.measurementsgroup.com 16.810(16682) assachusetts Institute of Technology Ui
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IG aln Accelerometers Accelerometers measure linear acceleration in one, two or three ￥()→s2X(s-3 axes. We distinguish (generally neglect initial conditions) single Vs. multi axis accelerometers mag +40db/dec DC versus non-DC accelerometers rolloff Recorded voltage response to base motion 1(t)=K(t)+V out Can measure: linear, centrifugal and gravitational acceleration Use caution when double-integrating acceleration to get position(drift) Single-Axis C1 Accelerometer must be aligned with sensing axis C2 Example: Kistler Piezobeam (not responsive at Dc Example: Summit capacitive Manufacturers: Kistler. Vibrometer Summit accelerometer Dc capable) 16.810(16682) Massachusetts Institute of Technology
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in Laser Displacement sensors sensor Records displacement directly x()→>X(s) via slant range measurement TVDC cal Settings 1: 2um-60 ms II: 15um-2ms target l:50m0.15ms0 Resolution tradeoff patlal vs temporal Distance x is recorded via triangulation . 7 between the laser diode(emitter), the target and the receiver range. (position sensitive device -PSD) Advantages Vibrometers include advanced processing contact-free measurement and scanning capabilities Disadvantages Manufacturers: Keyence, MTI Instruments need reflective, flat target limited resolution 1 g 16.810(16682) Massachusetts Institute of Technology Ui
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IGA10 Force Transducers/ Load cells Force Transducers/Load Cells are capable of measuring Up to 6 dOF of force on three orthogonal axes, and the moment (torque)about each axis, to completely define the loading at the sensor's location The high stiffness also results in a high resonant frequency allowing accurate sensor response to rapid force changes Load cells are electro-mechanical transducers that translate force or weight into voltage Mz My They usually contain strain gages internally F Manufacturers: JR3, Transducer techniques Inc 16.810(16682) Massachusetts Institute of Technology 10
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