CEP 726. STRUCTURAL ENGINEERING LABORATORY Updated on 14 Oct 14

Transcription

1 CEP 726 STRUCTURAL ENGINEERING LABORATORY Updated on 14 Oct 14 REINFORCED CONCRETE : (14 sessions) Dr. Supratic Gupta Structural Dynamics : Prof. A. K. Jain (07 sessions) SMART MATERIALS AND : Dr. Suresh Bhalla STRUCTURES (07 sessions)

2 INTRODUCTION TO SENSORS AND DEVICES PIEZOELECTRIC SENSORS ELECTRICAL STRAIN GAUGES ACCELEROMETERS DIGITAL MULTIMETER (DMM) LCR METER (LCRM) OSCILLOSCOPE

3 ELECTRICAL STRAIN GAUGES (ESG) Metal grids Polyimide plastic film ΔR R = S g ε ESG, R R o V o Voltage recording device R o R o V i

4 PIEZOELCTRIC MATERIALS Mechanical Stress T Direct Effect Electrical Charge T Electric Field E Converse Effect Mechanical Strain Elongation

5 PIEZOELCTRIC MATERIALS (Constitutive relations) w E 3 Strain Stress T = + d 31 E Y E S = Electric field h l T 1 Used for sensor applications D = ε T E + d T Charge Electric Stress density field zero

6 STRAIN MEASUREMENT BY PIEZOELCTRIC MATERIALS h 3 D = d T V = Parallel Plate Capacitor 2 C 1 w l T 1 S 1 = d T ε hy E V = K p V Q Voltage α Strain

7 ELECTRO-MECHANICAL MPEDANCE (EMI) TECHNIQUE A PZT patch is surface bonded on a structure using high strength adhesive and excited at high frequency ( khz) by an impedance analyzer. E 3 Structural Impedance A PZT Patch bonded to a structure A simple physical model of system (Liang et. al, 1994) PZT PATCH ACTS AS SENSOR AND ACTUATOR SIMULTANEOUSLY

8 PZT patch E 3 Z h w Z l l Structural Impedance MECHANICAL IMPEDANCE Z F F e jωt o = = j( ωt φ ) u& u& oe = F u& o o e jφ Unique function of structural stiffness, damping and mass Liang et al. (1993, 1994): Based on 1D analysis (for skeletal structures) Electric Admittance = Y = ωj wl h T ε 33 Z a + Z + Z a d 2 31 Y E κl κl tan d 2 31 Y E

9 PZT patch Structure Impedance Analyzer or LCR meter Measurements at multiple frequencies form signature Electromechanical Admittance, Y = G + B j Conductance (S) Conductance Susceptence (S) Susceptance Frequency (khz) Frequency (khz)

10 Strain Strain Time (s) Time (s) PIEZO-ELECTRIC SENSOR ELECTRICAL STRAIN GAUGE 10/14/

11 ACCELEROMETERS m F(t) Seismic mass y(t) m && y + c( y& x& ) + k( y x) = F( t) k c y x = z m b Base mass x(t) For the limiting case of very small frequency rat ω << ω && x zω 2 n 2 n * Very expensive (Typically over Rs 25, 000) * Low bandwidth * Very fragile 10/14/

12 Voltage (V) PIEZO-PATCH Voltage (V) ACCELEROMETER Time (s) PIEZO-PATCH ACCELEROMETER Voltage (V) Frequency (Hz) Frequency (Hz) 10/14/2014 Dr. Suresh Bhalla, Department of Civil Engineering, Indian Institute of Technology Delhi. This material is only for students of CEP 726 at IIT Delhi. Voltag ge (V)

13 SENSING OF FLEXURE Compression Tension - V o D /14/2014 V o

14 Compression Tension - V o D S bott S top V o V = k( S + S o ( top bott ) φ = S + top D S bott 10/14/

15 Curvature φ = S top + D S bott = V o kd Voltage α Curvature TDS 2004B Tecktronix oscilloscope Four channel, realtime, low on resolution IMPORTED 34411A Agilent digital multimeter Single channel, near real-time, high on resolution IMPORTED QDA 1000 Quazar Technologies m Eight channel, real- time, high on resolution, INDIGENOU S

16 CONCRETE VIBRATION SENSOR (CVS) CVS is a ready to use packaged sensor for dynamic response measurement developed especially for reinforced concrete structures such as buildings and bridges. Sensors embedded in RC test structure 10/14/

17 10/14/2014 Dr. Suresh Bhalla, Department of Civil Engineering, Indian Institute of Technology Delhi. This material is only for students of CEP 726 at IIT17 Delhi.

18 COMPARISON-ACCELEROMETER AND CVS ACCELEROMETER CVS

19 DYNAMIC FORCE SENSOR Sensitivity = 25 mv/n In house lab product, cost only a fraction of the imported brand Technology has been transferred to INOV i

20 DIGITAL MULTIMETER (DMM) Model 34411A (Agilent Technologies) Can measure AC and DC voltages, resistance Up U to 50, 000 samples per second Can store 1million readings in the memory Six and half digit display Suitable for both static and dynamic mesurements

21 OSCILLOSCOPE Model TDS 2004B (Tektronix) Can measure voltages as low as 2 mv Up to 1Giga samples per second Four channels Download manual &lc=EN

22 LCR METER (LCRM) Model E4980 Precision LCRM (Agilent Technologies) Can measure the resistive, inductive and capacitive components of any electrical circuit. Electrical Impedance Electrical Admittance Z electrical = R + Xj 1 X = Lω Cω 1 Y = = G + Bj Z Z electrical

23 TEACHING/ LEARNING VIRTUAL SMART STRUCTURES AND DYNAMICS LABORATORY Project funded by MHRD, Govt. of India Phase I completed Experiments part of CEP 726 course

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28 EXPT 1 : EXPT 2 : EXPT 3 : SMART MATERIALSAND STRUCTURES Vibration Characteristics of Structures using Accelerometers and Surface Bonded Piezoelectric Sensors Expt 1 of VSSDL (to be performed in remote mode from V 216) Identification of High Frequency Modes of a Beam in Free- Free Condition using Electro-mechanical Impedance (EMI) Technique Expt 2 of VSSDL (to be performed in remote mode from V 216) Forced Excitation of Beam using Portable Shaker Expt 3 of VSSDL (to be performed physically from V 211) EXPT 4 : Photogrammetry for Displacement Measurement Expt 4 of VSSDL (to be performed in remote mode from V 211) EXPT 5 : EXPT 6 : Vibration Characteristics of RC Beams using Embedded Piezoelectric Sensors (to be performed physically from V 211) Note: This is not Expt 5 of Virtual Lab Modes of Vibration of a simply Supported beam (to be performed physically from V 211) Note: This is not Expt 5 of Virtual Lab

29 EXPT 1 Vibration Characteristics of Structures using Accelerometers and Surface Bonded Piezoelectric Sensors (This is also the expt 1 of the virtual lab. To be performed remotely from SSL V 216 through website of Virtual Smart structures and Dynamics Lab) Excite the aluminium beam into free-damped vibrations (this is automatic) Measure DC voltage across the PZT patch through oscilloscope DMM. Transform the data to frequency domain using FFT (may use MATLAB in Structural Simulation Lab SSL (V 216) Determine first natural frequency and damping ratio using half power band method Repeat the above measurements using accelerometer (smaller beam). Comment on the results of accelerometer and PZT patch. Compare results with theory (frequency) and published data (damping ratio) SUGGESTED READING: Paz (2004); Chopra (2001); Sirohi and Chopra (2000a) Manual of Experiment 1 of Virtual Smart Structures and Dynamics Lab.

30 DAMPING Damping is the energy dissipation i propertiy of a material or system under vibration/ dynamic loading. Half power band width method ξ = ω ω 2 2 ω n 1

31 EXPT 2 IDENTIFICATION OF HIGH FREQUENCY MODES OF BEAM IN FREE-REE CONDITION USING ELECTRO-MECHANICAL IMPEDANCE (EMI) TECHNIQUE (To be performed remotely from SSL V 216 through website of Virtual Smart structures and Dynamics Lab) This is same as experiment 2 of virtual lab PZT patch (on each face) Connect the wires of two PZT patches such that the two are in phase (same electrodes paired), hence axial modes are excited (already connected). Obtain the plots of G and B vs frequency over (5-100 khz) using LCRM. Identify the frequencies of axial modes and compare with theory. Connect the wires of two PZT patches such that the two are in out of phase (opposite electrodes paired), hence flexural modes are excited. Obtain the plots of G and B vs frequency over (5-100 khz) using LCRM. Identify the frequencies of axial modes and compare with theory. SUGGESTED READING: Paz (2004); Chopra (2001); Naidu and Soh (2004a, b), Manual of Experiment 2 of Virtual Smart Structures and Dynamics Lab.

32 EXPT 3 FORCED EXCITATION OF STEEL BEAM USING PORTABLE SHAKER (To be performed remotely physically from V 211through website of Virtual Smart structures and Dynamics Lab) This is Expt 3 of Virtual lab Compute the first three natural frequency of steel RC beam (take measurement of the dimensions) ISJB 150, 3.2m long. Apply a sweep signal (10-40Hz) using the function generator (already set). Measure DC voltage across PZT patch and carry out FFT in MATLAB. Determine the natural frequency and damping ratio, compare and comment results with theory (for frequency) and published literature for damping ratio). SUGGESTED READING: Paz (2004); Chopra (2001), Brownjohn et al. (2003) Manual of Experiment 3 of Virtual Smart Structures and Dynamics Lab.

33 EXPT 4 PHOTOGRAMMETRY FOR DISPLACEMENT MEASUREMENT (To be performed remotely from SSDL V 211 through website of Virtual Smart structures and Dynamics Lab) This is same as Expt 4 of Virtual lab A 100mm B Stationary frame of reference C Reference attached to structure Take picture with IP camera. Apply loads, keep taking measurements with dial gauge as well as recording pictures. Take at least 5 measurements. Compute displacement from the pictures (vertical distance between A and C) using MATLAB or ADOBE photoshop (count pixels) or MS word. Compare the displacement with that measured using dial gauge. Plot load vs displacement from two systems side by side. Comment on the accuracy and resolution of photogrammetry. SUGGESTED READING: Jauregi et al. (2003), Manual of Experiment 4 of Virtual Smart Structures and Dynamics Lab.

34 EXPT 5 VIBRATION CHARACTERISTICS VIBRATION CHARACTERISTICS OF RC BEAMS USING EMBEDDED PIEZOELECTRIC SENSORS (To be performed in direct mode at V 211) Note: This is not the experiment 5 of the virtual lab Excite the beam into free-damped vibrations with a hammer gently Measure DC voltage across the PZT patches (one embedded, other surface bonded), and accelerometer, in time domain through oscilloscope. Transform the data to frequency domain using FFT (may use MATLAB in Structural Simulation Lab SSL (V 216) Determine first natural frequency and damping ratio using half power band method. Comment C t on the results of surface bonded d patch, accelerometer and CVS. Compare results with theory (frequency) and published data (damping ratio) SUGGESTED READING: Paz (2004); Chopra (2001) Manual of Experiment 1 of Virtual Smart Structures and Dynamics Lab.

35 EXPT 6 MODES OF VIBRATION OF A SIMPLY SUPPORTED BEAM PZT sensor support Oscilloscope Impact hammer

36 EXPT 6 PROCEDURE: MODES OF VIBRATION OF A SIMPLY SUPPORTED BEAM (To be performed directly V 211) Compute the first two natural frequencies of the beam theoretically before doing the experiment. Measure all dimensions of the beam Excite the beam into free-damped vibrations with special impact hammer gently. Feed the voltage from impact hammer to first channel of oscilloscopes. Feed sensor responses to other channels of oscilloscopes. Carry out FFT of both force and response, and obtain frequency response function (FRF) as the ratio of response to force. Do this for all sensors. Obtain first two mode using absolute value of the real/ imaginary part of FRF, taking help from the frequencies computed in the first step (browse down to learn the procedure). SUGGESTED READING: Paz (2004); Chopra (2001) Peter Avitable s website:

37 EXPERIMENTAL MODAL ANALYSIS Modal analysis is a process whereby the structure is described in terms of its natural dynamic characteristics, namely the natural frequencies, mode shapes and damping. Accelerometer Voltage measuring device ISMB 150 Shaker Stinger Function Generator Let us apply a sinusoidal signal of constant amplitude, but vary the frequency gradually.

38 TIME AND FREQUENCY DOMAINS Time domain response Fast Fourier Transform (FFT) Natural frequencies Natural frequencies Frequency domain response The frequency domain response is much easier to evaluate as compared to the time domain.

39 MODE SHAPES MODE 2 MODE 1 MODE 3 Mode shape is the deformation pattern of the structure at when excited at a particular natural frequency.

40 FREQUENCY RESPONSE FUNCTION (FRF) It is the ratio of the output response (strain/ displacement/ velocity/ acceleration of a structure (undergoing vibration) at a point to the force applied at the same or other point, at a particular frequency. Both the force and the response are simultaneously measured. FRF (ω) : h( ω) = u F = uoe F e o j( ωt φ ) jωt = u F o o e jφ Unique function of structural stiffness, damping and mass In this experiment, we will measure strain and hence our mode shapes will be strain mode shapes

41 MODE SHAPES FROM FRF h ij = Response at i due to force applied at j. j i, j = 1, 2, 3. h 11 = Response at 1 due to force at 1 h 12 = Response at 1 due to force at 2 h 13

42 MODE SHAPES FROM FRF Similarly, we can experimentally derive: h 21 h 22 h 23 Response at 2, point of impact varied. and also h h 32 h 33 Response at 3, point of impact varied h 31 h 32 h 33 of impact varied. IF THE STRUCTURE IS LINEARLY ELASTIC, WHAT IS THE RELATION BETWEEN: h ij AND h ji MUST BE EQUAL SINCE MAXWELL-BETTI S THEOREM STATES: The displacement at a point due to unit load at the other point is equal to that at the other point due to unit load applied at the first point.

43 MODE SHAPES FROM FRF h 11 h 12 h 13 h 11 h 12 h 13 h 21 h 22 h 23 h 21 h 22 h 23 h 31 h 32 h 33 h 31 h 32 h 33 Real part of FRF Imaginary part of FRF

44 MODE SHAPES FROM FRF In general, e any one row can provide us all the mode shapes. Let us use the imaginary part of the third row i.e h 3j i.e the measurement point is same, excitation point is varied. In all, only one sensor required. MODE 1 f 1 h 31 =h 13 h 32 =h 23 h 33 f 1 f 1 MODE 2 f 2 f 2 f 2 f2 f 2 MODE 3??? Try yourself.

45 Since h ij = h ji MODE SHAPES FROM FRF Any one column can also be used in place of a row. If say the third column is used i.e h j3 i.e three measurement points, one excitation point. Can use three sensors with simultaneous measurements and thus acquire all data in a single go. Or can also repeat three times with one sensor, each time changing position of the sensor. This is called as roving of sensors.

46 Flexural mode shapes MODE SHAPE 1 Voltage (mv) , , , , , 0 200, Distance (cm) ) Voltage (mv) MODE SHAPE , , , 0 100, , , , Distance (cm)

47 REFERENCES Avitable, P. (2001), Experimental Modal Analysis (A Simple Non-mathematical Presentation) Bhalla, S. and Soh C. K. (2004), High Frequency Piezoelectric Signatures for Diagnosis of Seismic/ Blast Induced Structural Damages, NDT &E International, Vol. 37, No. 1 (January), pp Brownjohn, J. M. W., Moyo, P. Omenzettor, P. and Lu, Y. (2003), Assessment of Highway Bridge Upgrading by Dynamic Testing and Finite-Element Model Updating, Journal of Bridge Engineering, ASCE, Vol. 8, No. 3, pp Chopra, A. (2001), Dynamics of Structures, Prentice Hall of India limited, New Delhi. Jauregui, D. V., White, K. R., Woodward, C. B., Leitch, K. R. (2003), Noncontact Photogrammetric Measurement of Vertical Bridge Deflection, Journal of Bridge Engineering, ASCE, Vol. 8, No. 4, pp Naidu, A. S. K. and Soh, C. K. (2004a), Damage Severity and Propagation Characterization with Admittance Signatures of Piezo-transducers, Smart Materials and Structures, Vol 15, Naidu, A. S. K. and Soh, C. K. (2004b), Identifying Damage Location With Admittance Signatures of Smart Piezo- Transducers, Journal of Intelligent Material Systems and Structures, Vol 13, Paz, M. (2004), Structural Dynamics: Theory and Computations, 2 nd ed., CBS Publishers and Distributors, New Delhi. PI Ceramic (2006), Sirohi, J. and Chopra, I. (2000a), Fundamental Behaviour of Piezoceramic Sheet Actuators, Journal of Intelligent Material Systems and Structures, Vol. 11, No. 1, pp Sirohi, J. and Chopra, I. (2000b), Fundamental Understanding of Piezoelectric Strain Sensors, Journal of Intelligent Material Systems and Structures, Vol. 11, No. 4, pp TML (2007), Tokyo Sokki Kenkyujo Co. Ltd., Tokyo.

48 CEL 726 STRUCTURAL ENGINEERING LABORATORY SMART MATERIALS AND STRUCTURES COMPONENT (Conducted by Dr. Suresh Bhalla) GENERAL INSTRUCTIONS 1. After finishing work, all instruments/ tools/ consumables should be back to the designated places. 2. No waste material should be left on the work table after finishing the experiments. All devices, PCs and UPS should be switched off. 3. The laboratory is under video/ audio surveillance. Users found not complying with these instructions will be penalized in the evaluations. 4. A joint report of the experiment should be prepared and submitted in the next laboratory class by each group. The standard format available at i / bh / t d 5. The main contents of the report should be Abstract, Experimental Details, Results and Conclusions. Be very brief; use third person and past tense. Do not show data in the report, only include the plots. For experimental setup, include a well labelled picture of the set up. All pictures should be compressed such that the final size of the document is less than 1MB. 6. Maximum length of report should be TWO pages only, printed on either side of the A4 paper. p 7. Students should save all the original data as well as the electronic copy of the report for future use during the same course.