ABSTRACT INTRODUCTION

Transcription

1 A gauge for measurements of soil-structure interface force due to explosive loading H. Ilstad," T. B0rvik& & K.A. Malo" ^Department of Structural Engineering, The Norwegian Institute of Technology, N-7034 Trondheim, Norway ** Central Staff/Technical Division, Norwegian Defence Construction Service, N-0015 Oslo, Norway ABSTRACT The paper presents some work regarding construction of a soil-structure interface gauge measuring the interface force distribution over much larger areas than possible with conventional pressure gauges. Different backfill/backpack materials surrounding buried protective structures may have different effects regarding the interface forces between the soil and structure when exposed to explosive loading. The motivation of this work is to study these effects. The gauge concept consists of a grid of uncoupled plane areas using PVDF-film sensors between two mating surfaces to measure the pressure. The gauge is described in detail. This concept is attractive because all parts are commercially available and the concept is easy to extend to large scale gauges for use in full scale explosive tests. The interface gauge is designed to measure both uniform pressure and point loads from backfill/-backpack materials. Design, necessary equipment and some experimental evaluations of the demonstration gauge are presented. INTRODUCTION It has been known for many decades that certain crystals, like quartz, generates an electrical charge across the crystal when stressed. This is called a piezoelectrical effect. The amplitude of the signal are directly proportional to the deformation of the piezoelectric material [1 ]. Today appropriately treated polyvinylidene fluoride (PVDF) exhibits piezoelectric properties suitable for a variety of applications. Piezoelectric film is a flexible, lightweight, tough plastic film commercially available in a large variety of thicknesses and areas, and its properties as a transducer includes wide frequencies and vast dynamic range. This properties make the PVDF-film appropriate as impact and shock wave sensors. The Norwegian Defence Construction Service is running a research program regarding backfill/backpack materials surrounding buried protective structures. The

2 358 Structures under Shock and Impact objective of this project is to study the interaction between soil and structure subjected to explosive loading, since different backfill/backpack materials are known to impose quite different loads on a buried structure. The following requirements were essential for development of the interface gauge To measure high pressures up to 50 MPa To measure point loads and their distribution Large scale, gauge size 1 nr Be flexible, give correct results also under structural deformation To be insensitive to tough surface conditions To our knowledge, no commercial gauge is available for this purpose, and hence it was decided to develop a small scale demonstrative gauge to come up with a possible construction. GAUGE DESCRIPTION The gauge concept is shown in Figs. 1 and 2. It consists of a perforated aluminum plate divided by joints into 9 uncoupled plane gauge cells (Fig. la). Since the PVDFfilm is approximately 10 times more sensitive in bending than for lateral pressure it was necessary to avoid bending of the sensors. Exposed to explosive loading, the reduced stiffness of the aluminum plate will ensure that all bending will take place in the joints and not in the brick elements where the PVDF-film is mounted on the surface. The size of the PVDF-film was specified to 20x20x28-10'* mm. A detailed description of the sensor is given in Fig. 2. The PVDF-film was put between two conductors, and protected with a plastic cover on top. The wires were soldered to copper strips with adhesive on one side and the strips were attached to the two sides of the conductors. The connections were isolated by use of glue. The sensors were then glued with LOCTITE 496 to the 9 different load cells. After this the wires were put throw the perforated aluminum plate to the backside and the joints were filled with ARALDITE 2017, which is a flexible adhesive with excellent impact resistance and lap shear strength (Fig. 1 b and Ic). Finally two protective hardened steel plates with a thickness of 0.4 mm were fixed to the top and bottom side of the gauge (Fig d). The most critical part of the circuit is the input resistance. The input resistance affects low frequency measurements as well as signal amplitude since this circuit forms a high-pass filter unit. This implies that the input resistance need to be large so the cut-off frequency is well below the expected operating frequency. Subjected to lateral pressure the signal amplitude generated by the piezo-film is [1]; v- «?* - - w t <D v Azctiv"*"^passiv' ^ ^ where; t = PVDF-film thickness (28 um), A = Conductive electrode area A^= Area subjected to pressure, A^= A-A,^, e = Permittivity ( '~ F/m) q^= Piezoelectric coefficient for the axis of applied stress ( ^ Vm/N)

3 Structures under Shock and Impact 359 Fig.l Demonstration gauge concept 10 Top cover steel plate PVDF-film sensor(20x20x28-10-*) qlued with LOCTITE 496 Wire Aluminium plate Joints filled with ARALDITE 2017 Bottom cover steel plate (7?, = 0.4 mm) Silver conductor PVDF-film Platic cover Copper strips-- Solder Isolation (glue) Wire ^13^5^, 20 ^20 ^ 20 ^5^13^ ^ 106 ^ Fig. 2 Schematic drawing of gauge composition

4 360 Structures under Shock and Impact a = Average pressure (ratio of applied force to the conductive electrode area) The cut-off frequency, f^, is calculated as follows [1] ; fc = where; R^ = Input resistance When a paisa film operates at the cut-off frequency, the output signal is reduced by 3 db. To reduce the signal levels to acceptable values, a resistor with resistance of 6.85 MQ was connected to all the gauge electrodes. During testing the signals were transmitted from the gauge elements to a tape recorder for storage. EXPERIMENTAL SET-UP For calibration and validation purposes of the gauge a small pendulum was build. A schematic drawing of the pendulum is shown in Fig. 3. It consists of a replaceable flat ended nose in aluminum, and a cylindrical load cell and a central rod both of quenched and tempered steel. This choice of material ensured that the pendulum behaved in an elastic way during impact and made it possible to measure the impact force with strain gauges mounted on the load cell. To study the response in the gauge as a function of mass of the pendulum, the pendulum was equipped with interchangeable masses. The different masses of the pendulum are given in Table 1. Table 1 Masses of pendulum Components Central rod Nose (Al) Load cell Nuts Ml M2 M3 M4 M5 Masses (g) The test rig arrangements used during impact testing is shown in Fig. 4. The pendulum was held in position by use of four thin steel wires of equal length fixed to a couple of cantilevers. This way of fixing the pendulum was done to ensure a horizontal impact. The gauge was mounted to a stiff steel plate. It was possible to move the gauge around so that the pendulum could impact any of the 9 different gauge cells. The interface force between the pendulum and the gauge was measured by means of a full bridge strain gauge circuit in the load cell of the pendulum. The strain gauges were mounted normal to each other. The full bridge strain gauge circuit was used to increase sensibility and stability and to avoid possible bending moments introduced by small obliqueness during impact. The electrical signal from the pendulum was connected to an amplifier, and recorded on a THORN EMI SE 3000 FM tape recorder at 60 '7s. After an impact the recorded analog signals was played back to a digital storage oscilloscope, and the force-time

5 Structures under Shock and Impact D=20 D=20 Interchangeable masses Connecting it cable Strain gauges Nose Fig. 3 Schematic drawing of pendulum Central rod Load cell 170 -* *-M 22 0-»- L-. /! : ) cf " / / 1350 ^k / / ^^" ^' 1 Pendulum Fig. 4 /y/ f/ / hp, ^ i ' 7 h =410 Load - ce// T T Side view Test rig arrangement < ^ Stiff steel plate ", ) ^-- Gauge X^ /^ A / L Flnnnp / S S plate...t T ^"" ~~, * Front view

6 362 Structures under Shock and Impact curve was plotted. The same registration equipment was also used for recording and storage of the PVDF gauge cell signals. PRELIMINARY EXPERIMENTAL TESTS In this chapter some of the preliminary experimental tests are presented. A summary of this is given in Table 2. The following identification system was used during impact testing; P - h; - M - Cell A A A A > Impacted gauge cell no. > Total mass of pendulum(kg) ~^ ~^ Drop hight of pendulum (m) Test no. Table 2 Some Experimental Tests Test no. ts (ms) Vpma* (V) v v Gmax (V) PI P P P4 _ P P P P P P Pll P where; t,. VG,^ = Time of impact duration, Vp,^ = Maximum volt signal, pendulum, = Maximum volt signal, gauge Fig. 5 show some typical force-time curves observed during impact. Fig. 5a) shows test no. PI and P3, which is identical experiments, plotted in the same diagram. Both the pendulum and gauge showed a high degree of repeatability during testing. Fig. 5b) shows test no. P6, P7 and P8 plotted in the same diagram. In these tests the mass of the pendulum is held constant, while the impact velocity is increased successively. The output signals from the gauge seems reasonable since:

7 Structures under Shock and Impact 363 Test no. PI and P3 Test no. P6, P7 and P d) Fig. 5 Experimental results increased pendulum impact velocity gives increased V,^, increased pendulum impact velocity gives decreased t, and increased pendulum mass gives increased V,^ and t,. Fig. 5c) shows test no. P. In this plot the gauge cell signals has been multiplied with a scale factor equal to Vp^/V^. The plots indicates a difference in rise time between the pendulum measurements and the PVDF based gauge. The reason for this discrepancy is not yet analyzed. Fig. 5d) shows a comparison between test no. P9 and a direct impact on the PVDFfilm sensor (no glue). This implies that the glue between the gauge layers does not affect the sensors response. There was no signals from the surrounding channels, when one specific channel was impacted. This implies that the 9 gauge cells were completely uncoupled.

8 364 Structures under Shock and Impact A student work carried out [2] have shown that it is possible to use equation (1) to calibrate the PVDF gauge with discrepancies between calculated and measured values less than 5 %. FURTHER WORK A demonstration gauge using PVDF-film as sensor elements has been constructed in order to measure soil-structure interface force. The following topics will be addressed in further work ; The response of the pendulum, which was intended for impact calibration of the gauge will be analyzed. Probably the pendulum needs to be modified in order to give results for calibration purposes. The PVDF gauge will be attached to a flexible foundation allowing bending of the gauge to take place. Similar tests as described above will be performed. The gauge will be verified with respect to uniform pressure. This will be done in a shock tube. Some work regarding the electronic connected to the piezo-films will be done. REFERENCES [1] AMP Incorporated : "Piezo-Film Sensor Technical Manual", Basic Design Kit, May [2] Molle, T. :"Use of Piezo-Film for registration of mechanical stress", Student work, The Norwegian Institute of Technology, Department of Physical Electronics, 1993 (In Norwegian). ACKNOWLEDGEMENT The authors would like to thank the Norwegian Defence Construction Service for their generous support of the research project that forms the basis of the present work.