DEVELOPMENT OF DROP WEIGHT IMPACT TEST MACHINE

Transcription

1 CHAPTER-8 DEVELOPMENT OF DROP WEIGHT IMPACT TEST MACHINE 8.1 Introduction The behavior of materials is different when they are subjected to dynamic loading [9]. The testing of materials under dynamic conditions needs an efficient and reliable equipment to experimentally examine and quantify the dynamic behavior of materials under low velocity impact loads. The range of such impact loads vary from a few m/s to several hundred m/s (such as a bullet impact). It is not possible with any one apparatus to cover a velocity range such as this. Various devices such as explosive or air gun rigs are employed for high velocity impact tests. Drop weight machines are in use to conduct test at low velocity impact loads. This chapter presents the details of a custom-built low velocity drop weight impact test machine which has been designed and fabricated to carry out dynamic tests. 8.2 Design Objectives The drop weight impact test machine was developed to perform low velocity impact tests up to a maximum velocity of 10 m/s. The machine was designed to test specimens of varying geometry which can accommodate a maximum cross sectional area of (150x150) mm 2. The energy of the impactor can also be varied by changing the mass of the impactor. 8.3 Design Principle A machine based on the impacts produced by dropped mass works on the principle of free fall velocity under gravity which is given by where, g = Acceleration due to gravity (m/s 2 ) h = Drop height (m) 100

2 energy which is given by ts potential Potential Energy = m g h. where, m = Drop mass (kg) This energy has to be absorbed by any specimen under test. 8.4 Description of the Machine Figure (8.1) shows the mechanical structure of the developed and commissioned machine. It consists of a frame with two mild steel, channels each 6 m high. The channels were erected vertically on a sound concrete foundation and fixed to a concrete bed. Ground mild steel guide strips were attached to the flat faces of the two channels to guide the fall of the dropped mass. The drop mass (impactor) assembly consists of two flat circular bottom and top masses held between two horizontal flat plates by four bolts. The plates have grooves at their sides. These grooves envelope the guide strips and guide the vertical sliding motion of the drop mass. Several masses can be included between the top and bottom disks to increase the mass of the impactor and thereby the energy of the impactor for testing different materials. The top disk of the impactor is attached with a mass hanger which in turn is suspended by two brackets pinned to a frame that grips the mass hanger and lifts the mass. The metal frame is also guided by the guide strips on the channels. The impactor can be lifted to the required height with the help of a wire rope which runs over a pulley and rope drum assembly. A ratchet mechanism was used to hold the impactor at the desired height to achieve the predetermined impact velocity. The impactor lifting system can be operated manually by hand cranking or automatically by an electric motor. An impactor release mechanism is devised and fitted to the sliding metal frame. This impactor release mechanism is used to release the impactor to so as to effect the sudden drop of the mass. The mass drops on to the test specimen and deforms it while the later dissipates the kinetic energy of the mass. The deformation of the specimen and the force that is transmitted to the base which supports the test specimens have to 101

3 be measured. Care was taken to arrest the accidental drop of the impactor. This was effected by providing two cross bars between the channels. These bars are put in place at all the times until the specimen is completely ready for the test. Steel Channels Load Release Mechanism Mass Carrier Drop Hammer Ratchet Mechanism Platform Figure 8.1 Drop Weight Impact Test Machine 8.5 Instrumentation The drop weight test facility consists of various devices which are used to measure the velocity, force and displacement of the impactor to calculate the energy 102

4 absorbed by the specimen under the test. The details of the instrumentation employed for the purpose is explained below Accelerometer A simple and direct way to measure the velocity is to measure the acceleration of the drop mass. The product of acceleration and mass provides the force of impact. The successive integration of the acceleration signal gives velocity and displacements. An accelerometer (of Bruel and Kjaer make) was fixed onto the top disk of impactor assembly to measure the acceleration of the dropped mass (impactor) and was connected to one of the channels of the data acquisition system. A brief technical specification of the accelerometer used is shown in Table 8.1. The accelerometer is shown in Figure (8.2). Table 8.1 Specifications of accelerometer Piezoelectric Charge Accelerometer B&K 4505 Charge sensitivity 0.3 pc/ms -2 or 3 pc/g ± 2 % Voltage sensitivity 0.28 mv/ms -2 or 2.8 mv/g ± 2 % Piezoelectric material PZ23 Maximum operational shock (± Peak) 20 kms -2 Maximum continuous sinusoidal acceleration 30 kms -2 Mounting stud UNF, 4.5 mm long Frequency range - 5 % : 0.2 Hz to 8000 Hz 0.2 Hz to 8000 Hz Velocity Measuring Device An alternate velocity measuring system was also developed based on the principle of electromagnetic induction to measure the impactor velocity. This velocity measuring system consists of a strip with continuous wire wound coils fixed on it at a distance of 10 cm apart. This coiled strip was then glued to the front web of the left 103

5 mild steel channel. A strong magnet is fitted to the impactor using a bracket. The magnet while moving down with impactor, crosses the wire wound coils and produces electric pulses due to electromagnetic induction. This can be directly captured by the data acquisition system. The known distance between the wire wound coils divided by the time interval between the respective pulses directly indicate the average velocity between them. The velocity measuring device is shown in Figure (8.2). b a c Figure 8.2(a) Accelerometer (b) Velocity measuring device and (c) Moving magnet Design and Fabrication of Load Cell A load cell was devised and built to measure the impact forces generated during the dynamic crushing test on the specimens. The load cell consists of a 200 mm long spring steel tube with an outer and inner diameter of 160 mm and 150 mm respectively. The end faces of the steel tube were ground and welded to flat steel plates of 210 mm diameter and 20 mm thickness at its top and bottom. The assembly was fabricated and extreme care was taken to ensure the squareness of the plates with respect to the tube axis and thus the surface parallelism between the top and bottom plates was assured. Four electrical resistance metal foil type strain gauges were 104

6 mounted centrally on the outer surface of the steel tube at equal intervals at 90 0 to each other. Two diametrically opposite gauges were oriented with their axis along the length of the tube and the other two gauges had their axis in the circumferential direction of the tube. These gauges were inserted on to a four arm wheat stone bridge such that adjacent sides of the bridge had axial and circumferentially oriented gauges. The fabricated load cell with mounted strain gauges is shown in Figure (8.3) Calibration of Load Cell The developed load cell was then calibrated with the help of universal testing machine and necessary instrumentation. The calibration set-up is shown in the Figure 8.4. The load cell with strain gauge sensors was placed between the top and bottom loading platens of UTM and known compressive loads in steps were applied. The corresponding voltage output signal was displayed and recorded on a digital storage oscilloscope. An instrumentation amplifier was used to amplify the lowest possible response of the voltage signal. An amplified voltage signal recorded in the digital storage oscilloscope is shown in Figure 8.5. A series of trails were conducted under loading and unloading conditions. The calibration curve was then plotted for the input load (kn) and out-put voltage (mv) which is shown in Figure (8.6). Figure (8.6) shows the calibration curve for both loading and unloading conditions. The calibration curve indicates the linear relation between the input load and out-put voltage. Therefore the load cell can be used to measure the loads. Figure 8.3 Developed load cell with strain gauges 105

7 Figure 8.4 Calibration set up for the developed load cell Figure 8.5 The output voltage signal in digital storage oscilloscope for loading and unloading conditions. 106

8 Table 8.2 Calibration data for loading and unloading conditions Load (kn) Output(mV) for loading Output ( mv) for unloading Figure 8.6 Typical calibration curves for both loading and unloading conditions 107

9 8.6 Data Acquisition System The accelerometer and the velocity measuring devices were connected to two of the channels independently. The signals sensed by the accelerometer were fed to a charge amplifier that converts the charge signal to a voltage signal. This voltage signal was then sent to a personal computer (or lap top) through a suitable data acquisition card. The data acquired was then converted to a compatible ASCII file that gives the acceleration-time data in digital form. This fundamental data was then mathematically processed in the Hypergraph software to derive velocity, displacement, and force. The force-displacement curve was then plotted and the area under such curve gives the energy absorbed. Figure (8.7) shows the data acquisition system with the associated charge amplifier used in this study. Charge amplifier Data acquisition system Figure 8.7 Elements of data acquisition system 108

10 8.7 Performance Evaluation of Drop Weight Test Machine. The performance of the machine was evaluated by calculating the % error in the velocities measured by the accelerometer and velocity measuring devices with that of the theoretical velocities gained by the falling mass for the known drop heights. Table 8.3 shows the comparative data of the velocities measured by accelerometer and the device. The percentage error of the measured and theoretical velocities are then compared. Table 8.3 Comparative data of the measured and theoretical velocities Drop height (m) H1 = 2 H2 = 3 H3 = 3.5 Velocity measurement by Velocity (ms -1 % Error of Velocity ) (Theory) Trail Accelero- Velocity Accelero- Velocity meter device (ms -1 ) meter device The test results indicate that, the error of velocity indicated by the velocity measuring device is in the range of 4 to 9 percent and that measurement from the accelerometer is in the range of 3 to 7 percent. The error in the measured velocity is probably due to the effect of friction between the guide ways and the impactor assembly. Therefore this error is taken to be allowable and the design of the machine is considered satisfactory. 109