100

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 22500 (150x150) mm2. 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/s2)

h = Drop height (m)

101

ts potential

energy which is given by

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

102

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.

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

Steel Channels

Mass Carrier Load Release

Mechanism

Drop Hammer

Ratchet Mechanism

Platform

103

absorbed by the specimen under the test. The details of the instrumentation employed

for the purpose is explained below.

8.5.1 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 10-32 UNF, 4.5 mm long

Frequency range - 5 % : 0.2 Hz to 8000 Hz 0.2 Hz to 8000 Hz

8.5.2 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

104

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).

Figure 8.2(a) Accelerometer (b) Velocity measuring device and (c) Moving magnet

8.5.3 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

b a

c

105

mounted centrally on the outer surface of the steel tube at equal intervals at 900 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).

8.5.4 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

106

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.

107

Table 8.2 Calibration data for loading and unloading conditions

Load (kN) Output(mV) for loading Output ( mV) for

unloading

0 0 8.5

25 16.8 18.7

50 41.8 28.7

75 54.3 36.8

100 66.2 57.5

150 88.7 70.6

175 122.5 113.7

200 138.75 129.7

225 163.125 167.5

Figure 8.6 Typical calibration curves for both loading and unloading conditions

108

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

109

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) Trail

Velocity measurement by (ms-1)

Velocity (Theory)

(ms-1)

% Error of Velocity

Accelero- meter

Velocity device

Accelero- meter

Velocity device

H1 = 2

1 5.90 5.70 6.26 5.3 8.9

2 5.85 5.75 6.26 6.5 8.1

3 5.90 5.75 6.26 5.3 8.1

H2 = 3

1 5.20 5.20 5.42 4.0 4.0

2 5.18 5.10 5.42 4.4 5.9

3 5.16 5.15 5.42 4.4 5.2

H3 = 3.5

1 8.02 7.80 8.28 3.1 5.7

2 8.00 7.85 8.28 3.3 5.1

3 8.01 7.86 8.28 3.2 5.0

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.