U.S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE • FOREST PRODUCTS LABORATORY • MADISON, WIS. 

U.S. FOREST SERVICE 

RESEARCH NOTE 

FPL-067 

November 1964 

FPL DYNAMIC COMPRESSION TESTING EQUIPMENT 

1 

FOR TESTING PACKAGE CUSHIONING MATERIALS 

By 

R K. STERN, Technologist 

Forest Products Laboratory, Forest Service 

U.S. Department of Agriculture 

Abstract 

A dynamic compression testing system, consisting essentially of a pendulum, 

abutment, and electronic recording equipment, was built by the Laboratory 

for testing package cushioning materials. The tests simulate the loading of 

an item in a cushioned package when the package is impacted. The characteristics, limitations, and theoretical aspects of each component of the instrumentation are presented in detail, particularly in regard to acceleration measurements. 

The recording equipment is also suitable for measurements of dynamic loads 

and strains. 

Introduction 

For the past several years, the U.S. Forest Products Laboratory has conducted 

research on the various characteristics of package cushioning materials that 

are of importance in protecting packaged items from damage during shipment. 

This Note is a revision of Forest Products Lab. Rpt. 2120 of the same title, issued in 1958. 1 
M 126 914 

Figure 1.--Pendulum impact test of cushioning material: (1) Steel-concrete abutment; (2) Loading 

pendulum; (3) Specimen of cushioning material: (4) Accelerometer; (5) Box housing photoelectric tubes; (6) Interrupter arm; (7) Light source: (8) Electronic counter; (9) Electronic counter 

display unit; (10) Thyrotron circuits; (11) Junction andcalibration box for two-channel recording 

system; (12) Cathode-ray oscilloscope; and (13) Land process camera. 

FPL-067 -2-
Much of the earlier work has involved “static,” or slowly applied, compression 

tests of the materials, but more recently dynamic, or rapidly applied, compression tests have been conducted. Two principal facts that have caused this 

transition are: (1) Dynamic compression tests simulate more accurately the 

loading action of an item in a cushioned package when it is dropped, and (2) 

unpublished work has indicated that poor agreement exists frequently between 

static and dynamic compression test data. To facilitate dynamic testing, the 

Laboratory, under the sponsorship of the Air Force Packaging Evaluation 

Agency (MOSPR) Headquarters, Mobile Air Material Area, Brookley Air Force 

Base, Ala., has built the system described in this publication. 

Although the primary purpose for building this equipment has been for 

testing the dynamic compression characteristics of relatively soft cushioning 

material, the basic recording equipment of the system can be used for other 

types of dynamic tests. A brief summary of these is presented in the report. 

Apparatus 

The basic units of the system consist essentially of a loading pendulum, a 

2-ton, steel-concrete abutment, and recording equipment. A test consists of 

drawing the pendulum to a predetermined height and allowing it to swing against 

a specimen of cushioning material which is mounted on a vertical face of the 

abutment. The loading action of the pendulum simulates that of an item in a 

cushioned package when the package is dropped upon a rigid surface. The 

principal components of the equipment are shown in figure 1. 

The various components can be grouped either as testing or recording 

equipment. The remaining portion of this section deals with details of the 

various components. 

Mechanical Testing Components 

Pendulum--Two different pendulums, each capable of being loaded with 

various weights, are employed. One, an especially lightweight, yet stiff, head 

is used to simulate lightweight items. The second head, which is much more 

robust, is used to simulate medium and heavyweight items. 

The lightweight pendulum (fig. 2, left) weighs 2.45 pounds unloaded, is made 

of sandwich material consisting of aluminum facings glued to a resin-impregnated, 

paper-honeycomb core material, can be loaded from 2.45 to 11.0 pounds, and 

can withstand up to 100 pounds per square inch of c o m p r e s s i v e stress. 

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Figure 2.--Loading pendulum used to simulate packaged items. M 126 913 

Z M 109 023 

Figure 3.--A record of acceleration and time for an impact test of bound hair showing the shock 

wave effect on the leading (left) side of the pulse with respect to time. 

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The facings of the 12- by 15- by 2-inch impact panel are 0.020-inch aluminum 

and those of the side panels are of 0.012-inch aluminum. The edges of the impact 

panel are reinforced with balsa wood inserts, and an aluminum mounting block 

for the accelerometer 2-3/4 inches by 1 inch by 1/4 inch in size is glued to 

the rear face of the impact panel. Loading is accomplished by attaching weights 

to the rear face of the impact panel with T-nuts glued to the panel. 

The wood pendulum (fig. 2, right) weighs 13.0 pounds, and it is used for all 

testing at this weight or above. The arms of the pendulum are made of hard 

maple, and the 8- by 8- by 3-inch impact panel is made of laminated birch 

veneer with 1/8-inch steel facings glued to each side. Loading is accomplished 

by attaching steel plates to the arms. 

The fundamental natural frequency of the impact panel of the wooden head 

is about 1,300 cycles per second, and that of the aluminurn head is about 1,000 

cycles per second. 

Loading head design is one of the most important and difficult aspects of 

the development of suitable dynamic compression testing equipment. Ideally, a 

loading head should be quite rigid, yet lightweight. Adequate rigidity will 

prevent data bias caused by cushioning action of the loading head, and provide 

a shock-excited high-frequency characteristic that can be separated rather 

easily from the principal pulse. 2 Light weight, together with loading capacity 

over a wide range, is desirable in order to achieve a high degree of loading 

capability. Adequate robustness, especially in the impact panel, is essential 

to prevent compression failure of the panel under loading. 

No single or combination of materials has yet been tried that will provide 

optimum performance in all the listed requirements. However, sandwich 

construction as described appeared to afford the best compromise between 

the divergent requirements. 

Effects of shock waves.--During the development of a suitable pendulum 

design, certain irregularities of the acceleration-time pulses were recorded. 

This effect is demonstrated in figure 3, which is an acceleration-time pulse 

(time is plotted along the x-axis) obtained by dropping a 5.83-pound pendulum 

on a 2.5-inch rubberized hair specimen with a density of 2.59 pounds per cubic 

foot. The irregularities occurred on the leading edge of the pulses, and the 

The high-frequency component is customarily disregarded in data analysis because it cannot 

be considered as a part of generally applicable data. 

FPL-067 -5-

2 
number of humps varied inversely with specimen thickness. After investigating 

several possible causes for the distortion, Smiley 3 theorized that it was 

caused by shock wave propagation in the materials. To prove this thesis, he 

derived mathematical expressions based upon shock wave theory and then 

found that theoretically expected results agreed closely with actual test results, 

Generally, this effect occurs to some extent in all dynamic compression 

tests of cushioning materials, but it is most pronounced when a lightweight 

loading head is used to test a relatively stiff cushion. 

Supporting wires.--For testing, the pendulum is supported from the ceiling 

by four adjustable, divergent, 10-foot, stranded, steel cables that are 1/32 inch 

in diameter. The wires are attached to the pendulum by clevises, which simplify 

changing of pendulums. The upper ends of the wires are attached to reels, which 

are a part of a ratchet gear assembly. The minimum length adjustment of a 

wire is about 1/20 inch. 

Abutment.--The abutment consists essentially of a cubic yard of concrete 

with a steel plate 23-3/4 by 26 by 2 inches in size embedded in the side that 

is used for testing. The barrier is mounted on two steel I-beams that are 

anchored to a massive concrete slab. The test specimen is attached to the 

steel face of the abutment with pressure-sensitive tape having adhesive on 

both sides. 

Release mechanism-The principal component of the release mechanism is 

a solenoid-activated pair of jaws, remotely controlled by the operator of the 

recording equipment, that are held in a mount which is adjustable vertically 

along a steel upright frame. The frame is movable along the ways of a horizontal 

framework that is anchored to the concrete floor. The vertical motion of the 

release mechanism is parallel to the face of the abutment, and the horizontal 

motion of the upright framework is perpendicular to the face of the abutment. 

Front and side views of the release mechanismand mount are shown in figure 4. 

Ideally, to simulate loading of a cushioned item, the pendulum should strike 

the cushioning material squarely and compress it in a direction perpendicular 

to the face of the abutment. The extent of divergence of the actual loading path 

from the ideal was reduced to a negligible amount by the use of the long wires 

and the placement of the specimen so that its loading occurs at the bottom of 

the swing. The use of the four-wire suspension system eliminated rotation 

of the pendulum during the loading of the specimen. 

Generally, it is desirable to test materials at predetermined rates of loading. 

The final velocity attained by the pendulum just before impact with the specimen 

Smiley, V. N. Investigation of shock waves developed during dynamic tests of cushioning 

materials. WADC Technical Report No. 56-547, U.S, Air Force, Wright Air Development 

Center, Dayton, Ohio. 1957. 

FPL-067 -6-

3 
Z M 126 911 Z M 126 972 

Figure 4.--Adjustable mount for release mechanism: A, front view; B, side view, showing pendulum 

engaged. 

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can be calculated approximately with the use of equations for uniformly accelerated motion. The vertical distance between the position of a reference point 

on the pendulum before release and the position of the same point at impact 

is used as the height of drop in this computation. However, air drag on the 

impact face of the pendulum decelerates it during flight. The magnitude of this 

effect is dependent primarily upon the weight of the pendulum, but is also dependent to some extent upon the angle of incidence of the air on the face of the 

pendulum in flight. 

In practice, therefore, in order to set the point of release to the proper 

height necessary to deliver a pendulum loaded to a specific weight to the 

cushion at a desired velocity, the release mechanism is set at the approximate 

height, indicated by experimentally determined curves. Finer adjustment is 

then made by trial and error until the desired velocity is obtained. The velocity 

at impact is measured for each drop, with a system that is described later in 

this report. 

Elements of the Recording System 

The principal components of the recording system are shown in figure 5. 

Accelerometers.--Some of the types of accelerometers commercially available 

are the piezoelectric crystal, vacuum tube, differential transformer, and strain 

gage. 

The strain-gage type of accelerometer was selected for use in this work 

because its relatively light weight, adequate frequency response, and acceleration 

range characteristics best suited the requirements. Each of the accelerometers 

used weighed 4 ounces. 

Two types of strain-gage accelerometers, both supplied by the same manufacturer, were used in this work. The important characteristics of one type are the 

following: Acceleration range ±100 g; 4 natural frequency 725 cycles per second, 

2,500 ohms; maximum supply voltage, 34 volts; output current (closed circuit), 

8.6 microamperes per g at maximum supply voltage; weight, 4 ounces: and 

response to transverse acceleration, not more than 2.0 percent. 

“g” is herein used to symbolize the dimensionless ratio of a value of acceleration (positive or 

negative in direction) to the constant rate of acceleration of a freely falling body due to the 

earth’s gravitational force (32.2 feet per second per second). 

FPL-067 -8-

4 
M 127 559 

Figure 5.--Block diagram of principal components of the recording system 

M 127 560 

Figure 6.--Schematic diagram of junction box. 

FPL-067 -9-
The characteristics of the second type of accelerometer are the following: 

Acceleration range, ±500 g; natural frequency, 1,550 cycles per second; sensitivity (open circuit), 0.36 millivolts per g at maximum supply voltage; bridge 

resistance, 2,500 ohms; maximum supply voltage, 38 volts; output current (closed 

circuit), 0.14 microamperes per g; weight 4 ounces; and response to transverse 

acceleration, not over 2.0 percent. 

Frequency Response 

Basically, an accelerometer is a damped mass-spring system that will act 

as an accelerometer well below its natural frequency and is known generally to 

be usable only over a certain range of frequencies. The band width of this 

range is determined by the physical parameters of the system and the quantity 

being measured, namely, displacement, velocity, or acceleration. 

In the past, some testing engineers have assumed that an accelerometer 

will accurately reproduce an applied acceleration. if the frequency of the applied 

acceleration is no greater than one-third the natural frequency of the accelerometer and the accelerometer is damped to 0.6 or 0.7 of critical damping. 5 

In this regard, White 6 showed that an accelerometer having a response that can 

be described by a second-order differential equation will reproduce steady 

state oscillatory accelerations with good accuracy if the damping is 0.7 critical. 

and the applied frequency is less than about 0.3 of the natural frequency of the 

accelerometer. 

However, in impact testing such as the dynamic loading of cushioning material, 

the acceleration occurs as a single pulse or transient, which generally resembles 

a half cycle of a sine wave rather than as a steady state oscillation. 

As shown by Levy and Kroll, 7 the transient response of an accelerometer to 

a transient shock is not what one might expect from the steady oscillatory 

response. 

In this work they developed the expected response of accelerometers that 

varied in degree of damping to transient half sinusoid, triangular, and square 

5 

“Critical damping” is defined as the minimum coefficient of damping C that will just prevent a c 

damped of a damped seismic system in response to displacement. Numerically, C 

transient oscillation 

= 2 where k is the spring constant and m is the mass. 

6 c 

White, G. Response characteristics of a simple instrument. Instrument Notes No. 2, 1948. 

Statham Laboratories, Los Angeles, Calif. 7 

Levy, S., and Kroll, W. D. Response of accelerometers to transient accelerations, Research 

Paper 2138, Journal of Resources, National Bureau of Standards, Vol. 45. No. 4. 1950. 

FPL-067 -10-
acceleration pulses of variable duration. From this work they concluded that 

in order to obtain an accuracy of better than 5 percent in measuring peak 

accelerations of transient phenomena, which are nearly half sine waves or 

triangular pulses, the accelerometer must have a natural period of about 

one-third the acceleration pulse duration or smaller and a value for the damping 

constant of 0.4 to 0.7 critical. Expressed in terms of the natural frequency 

f of the accelerometer, f = 

> 3 where f is the natural frequency of the accel- n n t n 

erometer and t is the duration of the acceleration pulse. According to this 

rule, the f of an accelerometer required to reproduce accurately a specific n 

pulse would have to be twice as high as the f indicated by the rule based 

upon oscillatory input. n 

Thus, the type 2 accelerometer, having a natural frequency of 1,550 cycles 

per second, is suitable for measuring, with anaccuracy of better than 5 percent, 

peak accelerations of transient acceleration having a duration between 1.9 

milliseconds and infinity. 

Calibration.--The calibration sensitivity values supplied by the manufacturer 

for the accelerometers have been checked at the Forest Products Laboratory 

by the use of a system described in detail by Godshall. 8 

Junction Box 

The junction box serves as a common connection point for the accelerometer, 

the oscilloscope, and the accelerometer power supply and as a housing for 

precisian calibration resistors, switches for system calibration and power, 

a 0- to 50-volt voltmeter, and controls for voltage adjustment and accelerometer 

bridge balancing. A schematic diagram of the junction box and its component 

parts is shown in figure 6. 

Precision calibrating resistors. --The calibrating resistor bank consists of a 

group of six precision resistors of different values. If one of these resistors 

is connected across an arm of the accelerometer bridge, the output voltage of 

the accelerometer changes as if an acceleration had actually been applied. 

The magnitude of the simulated acceleration can be calculated since it depends 

only on the calibration resistance, the accelerometer resistances, and the 

calibration factor for the accelerometer. The method for calculating the 

simulated acceleration value is given in the appendix, 

8 

Godshall, W. D. The FPL linear deadweight accelerometer calibrator. Forest Products Lab. 

Rpt. 2239. 1962. 

FPL-067 -11-
The calibration voltage is applied to the oscilloscope and appears as a 

horizontal line displaced from the zero or reference line. The vertical distance 

between the two lines represents the acceleration in g’s simulated by the 

particular resistor connected. The six resistors used give different values of 

acceleration from about 6 to 100 g’s for ±100 g accelerometers, and from about 

22 to 355 g” for ±500 g accelerometers. 

The advantages of such a system of calibration are ease of operation and 

freedom from errors, caused by voltage drift in the power supply and the 

sensitivity of the oscilloscope. 

Control of supply voltage.--The supply voltage for the accelerometer is 

varied by means of potentiometer R (fig. 6). The voltage is indicated by the 1 voltmeter. 

Cathode Ray Oscilloscope 

The accelerometer is connected at the junction box to the vertical input of 

a single beam oscilloscope. The principal features of this instrument are 

given in the following paragraphs. 

Amplifier characteristics.--The horizontal and vertical amplifiers are similar 

and provide high sensitivity of 1 millivolt per centimeter or 10 millivolts at 

full-scale deflection. The amplifiers have wide pass bands from d.c. to 300 

kilocycles, and provide balanced input circuits on the six most sensitive 

ranges. The amplifiers also provide single ended input, and they can be either 

a.c. or d.c. coupled. 

Time base.--The oscilloscope is equipped with a calibrated horizontal sweep 

which is linear and accurate within 5 percent. Twenty-one sweep rates, ranging 

from 1 microsecond per centimeter to 5 seconds per centimeter, are available. 

Triggered sweep.--By the use of an externally applied pulse, a single sweep 

can be obtained. 

A photoelectric method is employed to obtain the triggered sweep A small 

aluminum strip attached at the bottom of the rear of the face plate of the 

pendulum (fig. 1, No. 6) serves as an interrupter arm for breaking a light 

beam that is incident on a 0.012-inch slit behind which is it phototube (fig. 1, 

Nos. 5 and 7). Actually, there are two slits spaced 1 inch apart, The light 

and phototube system also serves as a velocity-measuring system. A report 

of the function and circuitry is presented in a subsequent section of this 

publication. 

FPL-067 -12-
Figure 7.--Schematic diagram of pulse-shaping circuit. 
When the pendulum arrives at the bottom of the swing, the interrupter arm 

breaks the first light beam, causing the first phototube to send a voltage pulse 

to a three-stage amplifier (figs. 1, 5, and 7); the amplified signal is then 

applied to the grid of a 2D21 thyratron, causing it to ‘fire” and send a large 

negative voltage pulse to the external sync connection of the oscilloscope. If the 

sweep control is set to driven sweep, a single sweep will be produced when the 

pulse arrives. The use of the thyratron prevents a second sweep from being 

produced when the pendulum rebounds and swings back through the light beam. 

The thyratron must be reset with a pushbutton(fig. 7, SW1) before the next drop. 

Photographic records.--The acceleration-time curves are recorded with a 

Land process camera that is mounted on the oscilloscope (fig. 1, No. 13). It is 

possible to observe the oscilloscope screen visually at the same time that a 

picture is being taken because a half-silvered mirror in the camera assembly 

reflects part of the light from the screen vertically to the camera and allows 

part of the light to be transmitted directly to the observer’s eye. The pictures 

are developed in approximately 10 seconds in the camera. Some examples of 

typical acceleration time records are shown in figure 8. Both pulses were obtained 

from impact tests involving a 13.0-pound pendulum and the equivalent of a 24-

inch drop height. A represents 1.7 pounds per cubic foot wood-fiber felt and 

B represents 5.2 pounds per cubic foot fibrous glass. 

The two calibration lines are photographed with recurrent sweep just before 

the drop. The acceleration pulse is photographed during the impact. The beam 

intensity is set at the optimum rate for the sweep frequency and the amplitudes 

and frequencies contained in the acceleration waveform. Trial drops are helpful 

in determining this setting. 

Figure 8.--Typical acceleration-time records from dynamic compression tests of cushioning 

materials: A, wood-fiber felt; B, fibrous glass. 

FPL-067 -14-
Calculation of acceleration--To calculate the peak value of acceleration from 

the oscilloscope records, the number of divisions between the horizontal 

calibration lines is measured. From this the sensitivity S of the entire recording 

system can be determined from 

(1) 

where A is the number of g’s represented by the particular calibration traces, c 

and D is the number of divisions betweencalibrations lines. The peak accelera- c 

tion in g’s is then 

(2) 

where a is the peak acceleration in g’s and D is the maximum vertical deflection of the trace in divisions. 

The fidelity of the recording system is limited primarily by the accelerometer. 

Therefore, if a type 2 accelerometer, which has a natural frequency of 1,550 

cycles per second, is used, the system described is capable of recording 

within 5 percent the peak amplitude of acceleration-time wave forms having 

a maximum amplitude of 500 g and a duration of 1.9 milliseconds to infinity. 

Velocity-Measuring System 

The velocity of the loading head at the moment of impact with the cushion is 

an important parameter which must be closely controlled. Since the pendulums 

have a large bearing area and must swing through a relatively long arc, air 

resistance decelerates the pendulum considerably during flight. Air drag varies 

with the velocity of the pendulum, and the rate of deceleration varies with the 

loaded weight of the pendulum. To overcome air drag, the pendulum is released 

from a greater height so that the desired impact velocity is obtained. Curves 

of height correction were developed by calculations and empirical tests to 

facilitate rapid determination of release heights, but it was still necessary to 

check the accuracy of the release height settings by measuring the impact 

velocity. To accomplish this, the system described in the following text was 

devised It is noteworthy that the system actually measures tho time required 

FPL-067 -15-
by the pendulum to travel a relatively small known distance near the bottom 

of its swing. With this information, the impact velocity can be computed. Thus, 

the system is based upon the assumption that the velocity is constant during 

the measured time interval. This approximation is valid because the velocity 

of a pendulum is nearly constant near the bottom of its swing. 

Components 

Light-phototube system--As reported previously, the light-phototube system 

consists of two beams of light that are incident on two slits. Each slit is 0.012 

Inch wide and is located in front of a phototube. The slits are spaced 1.008 inches 

apart. In figure 1. the light source is shown as No. 7 and the phototube housing 

is shown as No. 5. 

Pulse-shaping circuit.--In order to utilize the output from the phototubes, 

it was necessary to amplify and reshape the pulses with the circuit shown in 

figure 7. Each pulse goes through three stages of amplification. A 120-cycle 

wave is superimposed upon the pulse because of the fluctuations in light intensity 

as the temperature of the filament changed with 60-cycle alternating current. 

The amplitude of the undesired signal was reduced by adding a 400-micromicrofarad capacity and a 220,008-ohm resistor after the first stage of amplification, 

thus decreasing the low frequency response of the amplifier. 

The amplified pulse from each channel was applied to the grid of a 2D21 

thyratron tube which provided voltage discriminator action. The thyratron tube 

could be set to fire at a specific voltage by means of bias controls. When the 

thyratrons fire, they produce pulses of very short duration, and the time between 

pulses can be measured accurately. To obtain good accuracy the amplitude of 

pulses that are applied to the thyratron tubes must be large so that any drift 

or change in the thyratron firing voltage will cause only a negligible change 

in the firing time. Therefore, the thyratron tubes are set to fire on the leading 

edges of the pulses at a voltage somewhat higher than the noise voltage present. 

The thyratron tubes receive pulses when the pendulum rebounds through the 

beams, but they will not fire again until they are reset. Two neon lights, which 

indicate when the thyratron tubes are fired, are used for adjusting the firing 

voltage of the thyratron tubes. 

FPL-067 -16-
Electronic counter.--An electronic counter (fig. 1, No. 8) is used to measure 

the time between the pulses. The output of the first channel starts the counter; 

that of the second stops it. The time in milliseconds, required by the interrupter 

arm to travel the distance between the slits, is indicated in digital form by 

the counter. 

Check of the System 

The accuracy of the velocity-measuring circuit was checked by dropping a 

20-pound steel plate edgewise through the Bight beams from a height of 30 inches. 

The leading edge of the plate was ground to a knife edge, and a solenoidactivated release mechanism was used to drop the plate. Because of the large 

ratio of weight to area of the plate when falling edgewise through air, the air 

resistance to the motion of the plate was negligible. The recorded time for the 

plate to pass between the slits agreed to within 1 percent of the calculated value 

and the accuracy of the velocity-measurement system was rated to be within 

±1 percent. 

Design Data Derived With the Equipment 

Fundamental Considerations 

Certain considerations must be made by the packaging engineer who wishes 

to select efficient. thicknesses and kinds of cushioning material for protecting 

specific item during shipment. The more fundamental of these are summarized 

in the following paragraphs. 

Fragility of the item--Some system for rating the degree of fragility of 

specific items is a prerequisite for “exact cushion design.” The common rating 

system being used by packaging engineers involves the maximum acceleration 

that specific items can withstand, expressed in multiples of g , the constant 

acceleration exerted by gravity upon a body that is falling freely. 

The relationship between maximum acceleration, gm 

, and maximum force, 

F , is obtained with Newton’s law m 

FPL-067 -17-
(3) 

where F is maximum force in pounds, and W is the loading weight in pounds. m 

The record of the deceleration of a loading device during tests of cushioning 

materials as a function of time has several principal characteristics, namely, 

maximum or peak acceleration, rise time, and period. That the use of peak 

acceleration alone as an index of fragility might be an oversimplification can 

be illustrated by considering an item having one or more fragile elements. 

In addition to magnitude of acceleration, the resistance of the element or 

elements to breakage is related to such factors as their resonant frequency, 

the fatigue resistance of their component material or materials, the period of 

the transient acceleration pulse (or the frequency of the pulses, if repetitive), 

and the rise time of the pulse. Consideration of all of these factors is possible 

by testing individual items under the conditions expected during service, and 

then correlating damage of the item with the characteristics of the accelerationtime pulses obtained from the tests. Such a testing program is ideal, but 

expensive. Thus, until more complete knowledge is gained regarding the relative 

importance of effects of rise time, period, and other factors upon specific items, 

the problem of rating fragility has been simplified by the assumption that peak 

acceleration is a satisfactory comprehensive index of fragility. Nevertheless, 

the apparatus described in this report could be used for investigating the effects 

of rise time and period of transient pulses upon items, if desirable. 

Severity of shock to which container will be subjected.--Frequently this 

factor is expressed in terms of the maximum expected height of drop in inches. 

A 38-inch drop is often used because of its relation to the height of an average 

man’s hands from the floor when he is standing erect. However, the design 

method presented in this report enables one to use any height of drop desired. 

Energy-absorption characteristics of cushioning material. --Cushioning materials differ widely in their mechanical properties, such as stiffness, efficiency, 

and versatility. Generally, no single material is optimum for all cushioning 

applications, and the user must decide which of the different materials is especially suited to his need. The energy-absorption characteristic of a specific 

cushioning material. can be expressed by various curves derived from compression test data for the material, such as force-displacement, stress-strain, and 

static stress-acceleration curves. The dynamic compression testing apparatus 

described in this report was built to facilitate evaluation of this characteristic 

of cushioning materials. 

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Figure 9.--A cushioned pack consists essentially of A , an article; B , a cushion: and C , an outer 

container. 

Figure 10.--An idealized representation of a package during a drop. 

FPL-067 -19-
Idealized Cushioned Package 

A cushioned package is represented in figure 9 wherein A represents the item, 

B the cushioning material, and C the outer container. In order to analyze the 

action of this package when it is dropped, it is necessary to represent the package 

as an idealized mechanical system wherein the cushioning material is represented by a spring. 

Effects of Dropping 

When the package is dropped from a height h (fig. 10, A ), it is given a constant 

acceleration of 1 g by gravity. In general, at the moment of impact with the 

dropping surface (fig. 10, B ), the outer container ceases motion, but the item 

compresses the cushion through a distance S (fig. 10, C ). During compression, 

the cushion absorbs all of the kinetic energy which the item possessed at the 

moment of impact, and at the instant of maximum compression, the maximum 

force is exerted upon the item. Thus, the item also is subjected to maximum 

deceleration at this instant. Ideally, the cushioning would absorb the kinetic 

energy at a uniform rate over a relatively large displacement, thus cushioning 

the item gradually without exerting high stress or causing rapid deceleration. 

Peak Acceleration-Static Stress Curves 

It is desirable, of course, to utilize the most efficient materials for specific 

applications, and peak acceleration-static stress curves are helpful in making 

this selection. 

Several peak acceleration-static stress curves for wood-fiber felt material 

varying only in thickness are shown in figure 11. The material was about 2, 3, 

4, and 5 inches in thickness, as indicated, and 2.4 pounds per cubic foot in 

density. The curves indicate the maximum deceleration which articles that 

differ in load per unit area (static stress) would receive, if they were dropped 

from a height of 30 inches upon the materials represented. The curves were 

derived with the dynamic compression testing system described in this report. 

To derive a single curve, a particular specimen was tested with the pendulum 

loaded to several successive increments of weight. The pendulum simulated 

the loading action of a fragile item. The maximum deceleration recorded for each 

drop was plotted against a value obtained by dividing the corresponding weight 

FPL-067 -20-
M 126 898 

Figure 11.--Peak acceleration static-stress curves for wood fiber felt. 

FPL-067 -21-
M 126 910 

Figure 12.--Making an instrumented drop test of a complete cushioned package. 

FPL-067 -22-
of the pendulum by its bearing area on the specimen. Thus a static stressacceleration curve was derived for that material. Similar curves must be 

derived for different thicknesses of material and heights of drop. 

The packaging engineer, having available curves of this type and knowing the 

weight, bearing area, and fragility of an item, can determine the approximate 

amount of material needed to protect the item. In order to utilize the most 

economical material for the job, he would check curves for all likely materials 

and then select the one that is most satisfactory with respect to costs, amount 

required, availability, tare weight, and related factors. 

Tests of Complete Cushioned Packages 

Precise data representing the performance of the various cushioning materials 

can be obtained by the use of the equipment described in the preceding text, 

and these data would be especially useful for classifying cushioning materials 

according to their efficiency in specifications. However, exploratory tests of 

cushioned items inside containers have shown that vibration of container members 

resulting from impact (especially “flat drops”) frequently will increase the stress 

exerted upon the item by the cushion. The most severe condition exists when 

the item is approaching maximum displacement as it compresses the cushion, 

while the container member on which the cushion is resting is oscillating in a 

direction opposite to that of the item. In this situation, the cushion is compressed 

simultaneously from both sides and a higher stress, and deceleration, is 

produced. In contrast, crushing of container members during impact will often 

aid the action of the cushioning material. The net container effects are determined by such factors as the type and orientation of the container at impact, 

the stiffness of members, moisture content (if the material is cellulosic), 

nature of the dropping surface, and height of drop. Obviously, the cushioning 

protection for a specific item might be inadequate under certain conditions, if 

container effects are ignored in the selection process. 

The transducer-recording system is adaptable for tests of complete cushioned 

packages, as shown in figure 12. For this type of test, the accelerometer is 

mounted on a loading plate, which simulates a fragile item The plate is placed 

on the cushioning material inside the container and the package is dropped. An 

electric hoist and solenoid-activated release mechanism can be used to suspend 

and release the package, and turnbuckles can be used to produce the desired 

container orientation. By testing the various cushioning materials in representative types of containers, static stress-acceleration curves that include 

container effects can be derived. Generally, this approach would be most 

feasible where the number of combinations of material and container is limited. 

FPL -06 7 -23-
Other Container Tests 

The electronic recording equipment has been used for various other container 

tests not involving cushioning materials. 

One test series involved determination of the dynamic tensile loads applied 

during rough handling tests of various tapes being used to suspend aircraft 

members in crates. Modified bonded wire strain gages were used as the 

sensing elements. 

Other work was conducted with large crates to determine the dynamic 

compressive and tensile stress in certain members in order to accomplish 

more efficient design. Bonded wire strain gages or piezoelectric accelerometers 

were used as the sensors in these tests. 

Summary 

A dynamic compression testing system consisting essentially of a pendulum, 

abutment, and electronic recording equipment was built by the Laboratory for 

testing package Cushioning materials. The pendulum test simulates the cushioning 

action of an item in a cushioned package when the package is impacted. The 

equipment can simulate heights of drop up to 52 inches. 

The recording equipment is capable of measuring within ±5 percent accuracy 

peak accelerations up to ±500 g that occur during transient accelerations that 

vary in duration from 1.9 milliseconds to a value limited only by the lowest 

sweep speed of the oscilloscope. The velocity-measuring portion of the system 

is capable of measuring impact velocities of from 0 to at least 20 feet per 

second within ±1 percent. 

In addition to dynamic compression tests of cushioning materials alone, the 

recording equipment can be used for evaluation of the combined performance of 

cushioning materials and the containers in which they are shipped and for 

measurement of dynamic loads and strains in other types of tests. 

Acknowledgment 

The author wishes to acknowledge gratefully the technical assistance rendered 

in this work by present and former Laboratory staff members V. N. Smiley, 

physicist; E. A. F. Anderson, electronics engineer; W. L. James, physicist; and 

W. D. Godshall, electronics engineer. 

FPL-067 -24-
APPENDIX 

Derivation of Simulated Accelerations 

from Precision Calibrating Resistors 

Figure 13.--Schematic diagram of strain-gage accelerometer bridge circuit. 

Figure 13 shows a diagram of the Wheatstone bridge circuit formed by the 

strain gage wires. R is one of the precision calibrating resistors which is c 

connected in parallel with R 4 when calibrating. 

Currents in the two branches are indicated by I 1 and I . 2 It is desirable to 

find E

out 

in terms of the bridge resistances, the calibrator resistor, and 

calibration factor for the accelerometer. 

13: 

or 

(1) 

(2) 

(3) 

From figure 

Adding (1) and (2), 

FPL-067 -25-
The currents are and 

Substituting in (3), we have 

The resistances of each arm of the bridge are specified as being equivalent; 

therefore, 

so 

(4) 

The manufacturer of the strain gage accelerometers specifies a calibration 

factor, F , for each accelerometer which is expressed as the open circuit 

output voltage in microvolts due to a unit input of acceleration with 1 volt 

applied to the input terminals. 

FPL-067 -26-
E

out could then be expressed by E = F N E out in 

-6 x 10 volts where N is the 

number of g’s of acceleration. 

Then 

Solving for R , we have c 

(5) 

The subscripts on R are dropped as the value applies to the input and output 

resistance of the bridge as well as to the individual arms. 

For voltage calibration of the recording equipment, convenient values are 

chosen for R 

c from (5) to give convenient values of N . The values of the calibrating resistors used in the junction box are 2.40, 1.00, 0.680, 0.470, .0.240, 

and 0.158 megohms. 

AS an example of what acceleration is simulated by these resistors, we will 

consider the 2.4 M resistor used with a 100 g accelerometer. The value of F 

given by the manufacturer for the accelerometer is 43.00 microvolts per g 

per volt input and R is 2,586 ohms. Solving 28 for N , the number of g of simulated acceleration, we have 

(6) 

FPL-067 -27- 1.-28 
FOREST PRODUCTS LABORATORY