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Reference

<«BSIR 80-1987

NATL INST. OF STAND & TECH R.I.C.

AlllDS D3bfl7fl

1

athematical Model for Use

luating and Developing

impact Test Methods for

Protective Headgear

Robert E. Beroer

Product Safety Technology Division

Center fee Consumer Product Technoiegy

National Engineering Laboratory

Naticna! Bureau of Stanaards

U.3 Department of Commerce

Washington, D.vC. 20234

October 1979

Issued March 1980

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A MATHEMATICAL MODEL FOR USE IN

EVALUATING AND DEVELOPING

IMPACT TEST METHODS FOR

PROTECTIVE HEADGEAR

Sb /H ] N 'J

Robert E. Berger

Product Safety Technology Division

Center for Consumer Product Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

October 1 979

Issued March 1 980

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

ABSTRACT

A lumped parameter mathematical model was developed to connect injury

parameters in real life head impact environments to output parameters of

test methods for evaluating protective headgear. Analytical/experimental

schemes were developed for mathematically representing the parameters that

characterize each of the three distinct elements of the model: the head or headform, the impact surface, the helmet. A comparison of the model

output to experimental results showed a satisfactory agreement. The model

was shown to be useful in determining test method pass/fail criteria which

correspond to the threshold of injury in the real life situation.

CONTENTS

1. INTRODUCTION 1

2. THE MODEL 2

2.1 Impacting Object

2.2 Impact Surface

2.3 Head or Headform

2.4 Helmet

2.5 Equations

2.6 Output

3. DETERMINING VALUES OF PARAMETERS

3.1 Impact Surface

3.2 Headform

3.3 Helmets

4. EFFECT OF CHANGING TEST PARAMETERS; COMPARISON

OF MODEL OUTPUT TO EXPERIMENTAL DATA 12

5. USING THE MODEL TO DEVELOP PASS/FAIL CRITERIA

IN A TEST METHOD FOR FOOTBALL HELMETS 14

G. SUMMARY AND DISCUSSION 16

REFERENCES 18

CAPTIONS TO FIGURES 19

kDOO-^

CTiCn-fi^CAjtNit'O

1 . INTRODUCTION

This report describes a rational connection between the real life head

injury environment and the method by which the impact attenuation

characteristics of protective headgear are evaluated. The complexity of

this problem is such that we sacrifice some rigor (e.g., we use a rather

unsophisticated mathematical model) in favor of preserving the

aforementioned injury/test connection. Undoubtedly, many refinements and

improvements in approach will be suggested as one follows the rational

development. Nevertheless, given the existing state of test methods for

protective headgear and the present lack of supporting rationales, it is

suggested that the methods described herein can provide a useful tool for

those concerned with rational test method development and promulgation.

A one-dimensional lumped-parameter mathematical model is used to

predict the linear acceleration response history of a helmeted head or headform. The model, which is described in more detail in chapter 2, is

capable of simulating a range of real life situations and a range of test

method configurations. The elements of the model include the head (or

headform), the helmet, and the object which strikes the head. Each of

these elements is composed of several parameters (modelled as masses,

springs, and dash pots). The behavior of each element, and hence of the

full system, depends on the parameter values which are chosen on the basis

of two sets of experimental data:

1 . Data which isolates some particular element so that its

deformation characteristics can be chosen independent of the other

elements. The use of these data in choosing values for the

parameters of the model is shown in chapter 3.

2. Data which represents the response of the system as a whole. This

data was gathered in a previous study J_/, which was concerned with

the effect of changing test parameters on the impact response of

helmeted headforms.

The ability of the model to predict these experimental results serves as measure of its validity. The comparison of the predictions of the model to

the experimental behavior is shown in chapter 4.

Finally, to illustrate how the model might be used in developing test

method criteria, an example for football helmets is given in chapter 5.
2

2. THE MODEL

Except for a few minor changes, the model is the same as that described

in a previous report ^/. In that study, the model was used to illustrate

quantitatively the effect of changing test parameters; however, no attempt

was made to assign realistic values for parameters.

The model is used to calculate the linear acceleration response of a one-dimensional system which consists of three distinct elements (figure

1 ):

1 . head or headform

2 . helmet

3. impact surface

The representation of this system by a one-dimensional model must be

regarded as a major assumption. This basic assumption underlies every

existing headgear test method, all of which measure the linear acceleration

in one-dimensional drop tests. The widespread adoption of linear

acceleration as a head injury indicator has been based on laboratory impact

experiments with cadavers, where the head was observed to move relatively

independent of the body, and on the fact that, in live subjects, the

duration of impact is small compared to the neck muscle reaction time V.

Referring to figure 1 , x^ is the instantaneous position of the

head/helmet interface and x^ is the instantaneous position of the

helmet/impact surface interface . (Note: Xp will be reserved for the

humanoid headform, as discussed below. Alsof x. = x^ will represent the

instantaneous position of the head/impact surface interface when there is no helmet.) In this work, we will adopt the convention that x = 0

represents the initial position of the head/helmet interface. Each

element will now be examined in more detail:

2.1 Impacting Object

The impacting object may be of finite or infinite size. The latter

condition represents a situation where a helmeted head strikes a surface

such as the football field or a road. For the massive surface, the

helmeted head is assumed to be traveling initially at speed V toward the

impact surface. For smaller impacting objects, the object is assumed to be

travelling at speed V toward the stationary helmeted head.

2.2 Impact Surface

The impact surface is considered to be rigid for small impacting

objects, but may be either rigid or resilient for massive impact surfaces,

'/flnen the massive impact surface is rigid, the helraet/impact surface

interface remains fixed at the position = D, where D is the initial

thickness of the helmet liner.
3

When the (massive) impact surface is resilient, a functional

relationship is assumed between the force, F, on the helmet and the

displacement, x = - D, of the helmet/impact surface interface:

F = f(x) (1)

For example, many surfaces can be represented by a power law function,

F = Bx^ (la)

Where B and p are chosen to fit experimental data as described in section

3.1. In particular, if the deformation of the surface is such that

theories of quasi-static, elastic impact apply, the exponent will have the

value p

= 1.5, while B depends upon the geometry and material

characteristics of the impacting objects V. Usually, these theories are valid only if the deformation of the surface is small compared to its

thickness.

In contrast, deformations for some common surfaces (such as artificial

turf) may be significant compared to its thickness. For such surfaces,

alternative formulations of f(x) must be generated. The following

formulation was chosen to represent artificial turf:

f(x) = , (1b)

2 ^d - X

where d is some effective thickness of the surface. In this case, B and d are chosen to fit experimental data. The formulation in (1b) was chosen

because: (1) it possesses such desirable properties as f(x) = 0 when x = 0 and f(x) ^ when x ^ d, (2) it is easily integrable (the value of this

will become evident in section 3.1), and (3) it is suitable for fitting

experimental data (section 3.1).

2.3 Head or Headform

The head is considered to be resilient, but the headform may be either

rigid or resilient. Rigid metal headforms are prescribed in many current

impact tests for protective headgear. In the model, the metal headform is

treated as a rigid body of mass, Mq. The resilient headform model

(sketched in figure 2) represents the human head (in a real life injury

situation) or a humanoid headform. The response of this model was shown to

represent the driving point impedance response of cadaver heads ^/. The

humanoid headform used in the aforementioned experimental study V was also

shown to exhibit a driving point impedance response which was similar to

cadaver heads V

This model’s linear acceleration response to impact also

agrees well with the response of the humanoid headform (see section 3.2).

The values of parameters K, C, M. , depend on the impact location (front,

top, side, back, etc.).

The deformation of the headform model is described by
4

= K [L - (x^ - X2

)] + C(X2 - x^) (2)

where is the force between the masses of the headform element. (The dot

notation above the symbol denotes differentiation with respect to time.) L

is an arbitrary separation of the two masses; the acceleration response of

the headform is independent of L.

2.4 Helmet

The mass of the helmet is assumed to have little or no influence on the

impact response since: (1) only a small portion of the helmet takes part

in the energy absorbing process during the impact, and (2) the kinetic

energy associated with the rest of the helmet has been observed to be

dissipated in flexural vibrations during impact. In figure 1, therefore,

the force is depicted as the same on both sides of the helmet.

In general, the deformation of the helmet liner will be described by

some functional relationship between the force, F, the compressive strain,

e?and the strain rate, e:

f(F, £, e) = 0 (3)

In terms of the notation of figure 1

e and

where D is the initial thickness of the helmet liner.

Two linear spring/dashpot models, the Voigt and Maxwell elements, were

employed in the aforementioned study 2/ and are shown in figure 3. For

each, in turn, equation 3 becomes 2/:

Maxwell: F = E.A e ^ F (3a)

Voigt: F = E^Ae + nAe

where A is the area of contact.

(3b)

The response of these elements was unsatisfactory (see section 3.3), hence

another model was constructed by adding a non-linear spring in parallel

with the Maxwell element (figure 3c). The net force in the element is

therefore given by

F = E

2A/ + (3c)

where F^ is the force in the Maxwell element. Combining with equation

(3a), this new element is described by:
5

F = E^Al - -i F + -1 E^Ae^ + E^Are^'h (3d)

2.5 Equations

The system is completed by adding equations of motion for each of the

masses.

Mix'i

II zc (4)

M2*x*2 (5)

For the case of a resilient helmeted headform, resilient impact surface

and massive impacting object, equations (1), (2), (3), (4) and (5)

represent five ordinary differential equations for the five unknowns , X

2

, x^, F, Fp^. These equations must be supplemented by the initial

conditions

x^(0) = 0

x^(0) = V

X2(0) = -L

*X2(0) = V

F(0) = 0.

If there is no helmet, x^ = x^ and equation (3) is eliminated.

If the impact surface is rigid, x^ is no longer a variable and equation

(1) is not used. For small impacting^objects, which are allowed only when

the impact surface is rigid (this restriction could be easily surmounted),

x^ is again a variable and a new equation is added:

M^’x^ = F (6)

with initial conditions

x^(0) = D

x^iO) = -V

Of course, the initial conditions for x^ and X2

then become

x-,(0) = X2(0) = 0.

For a rigid headform, equation (2) and the variable F„ are eliminated and

is replaced by in equation (4). ”
6

2.6 Output

The system of differential equations was solved numerically on an Interdata 32 mini-computer using a modified Runga-Kutta method £/ which

computes the solution at N equally spaced time steps with interval t. The

accuracy was verified -by: (1) comparing the results to simple cases for

which the exact solution was easily determined, and (2) computing the

results for the same conditions when the time step was reduced by an order

of magnitude. The output variable of most interest was the acceleration of

the headform (the mass Mp when the humanoid headform was used in the model)

as a function of time. Computation runs were terminated when the

acceleration dropped below zero.

Several quantities which have been advocated as being related to the

likelihood of head injury were computed from the acceleration history:

1. Severity Index, SI,

SI a(t)^*^dt

where a(t) is the linear acceleration in g’s (g = acceleration

of gravity) and T is the duration of impact. Severity Index has

been used as a head injury indicator based on data obtained from

studies with cadavers and sub-human primates, and this use has

been widely accepted. For distributed loads to the head, such as occur when wearing protective headgear, a critical value of SI = 1500 (units are seconds) has been suggested as an injury threshold

7/.

^

2.5

a(t)dt (tp - t^

where t^ and tp are the two times within the acceleration pulse

for which the above expression is a maximum. Proponents have

argued that HIC better represents the original data on which SI is

based _8/ but, it has also been shown V that HIC and SI are well

correlated.

2. Head Injury Criterion, HIC,

t,

HIC =

^ (t2 - t^

'1

The integrations required for SI and HIC were computed with a library

subroutine which utilizes a cubic spline fitting scheme 9/. In addition

to the above output parameters, the maximum acceleration, a , the time at

which the maximum acceleration occurs, t , and the ratio SPfinal to

initial velocities, Vp, were also comput®§^ As some headgear test methods

require measurements of dwell times (the time duration over which the

acceleration exceeds specified levels), these were also recorded at the

following g levels: 0, 50, 100, 150, 200, 250,. . .etc.
- 7 -

3. DETERMINING VALUES OF PARAMETERS

3.1 Impact Surface

The parameters of the force vs. displacement relationship for resilient

impact surfaces, equation (1), were chosen from impact data of a bare

(unhelmeted) metal headform against the impact surface. In this case, the

equation of motion of the headform is

Mj^V = -f(x) (7)

which integrates to

^ (x^ - V^) = f(x )dx ( 8 )

where V is the initial velocity. Equation (8) is thus an energy balance.

At the point of maximum displacement, x = x , x = 0 and, from (7),

*x* = a , so that equations (7) and (8) become:

max'

Mo a = -f(x ) R max max

(7a)

2

max

f (x)dx (8a)

0

Therefore, for any integrable functional form, f(x), equation (8a) can be

solved for x^^^ and substituted into equation (7a) to obtain a relationship

between a„ ‘and V. For the functional forms of equations (la) and

(1b), the following relationships result:

For f(x) = Bx^

a = CT

max

where

(9a)

C = 1

(B_) P

and q

= 2p (10a)

In the elastic case, where p= 1.5, q = 1.2. Moreover, for no value of p can q exceed the value 2.0.
- 8 -

For f(x) = Bx/(d^ - x^)^

where

amax

h

C

44V^(h - y^)

(h - 2^^)

h = 4Bdand C = 4dg

(9b)

(10b)

If a vs. V data are collected for an impact surface, the parameters

of the fS^^tional relationship can be determined by fitting a curve of the

functional forms derived in equations (9a) or (9b). This procedure is

illustrated in figure 4 for two surfaces: (1) a cylindrical urethane pad

vdiich is manufactured for impact testing and is specified in an existing

test method (ASTM F429-75) for football helmets 10/, and (2) artificial

turf (5/8 inch backing material) mounted on asphalt. The data for the

former surface is fit to the functional form in (9a), and the data for the

latter surface is fit to the functional form in (9b). Note that the first

surface appears to be well represented by the elastic theory, as q is close

to 1.2.

Characterizations are also required for the hard and soft impact

surfaces used in the aforementioned experimental study where the effect of

changing test parameters was reported V. However, there is evidence

(figure 5) that these surfaces have degraded in the two years since the

experiment and that, therefore parameters obtained from current a vs. V

data would not be useful in predicting the earlier experimental relSlts.

Two data points for each surface were obtained at the time of the

earlier experiment. Only limited confidence can be placed in the results,

but it may be noted that these surfaces are quite similar to the above

mentioned ASTM impact surface (having been developed for the same purpose),

hence these data will be compared to curves which exactly satisfy the

elastic theory (formulation (9a)). Therefore, a value of p

= 1.5 (q = 1.2)

will be assumed, and only the value of B will be chosen by fitting the two

data points to the closest line of the family on B, as shown in figure 6:

Hard: B = 100

Soft: B = 20

3/2 where the units for B are MN/m

3.2 Headform

The metal headform is characterized by the single value Mp. In our

experiment, the mass of the headform and supporting drop apparatus was, Mp

= 4.65 kg.
- 9

-

It is more difficult to determine values for the humanoid headform

parameters K, C, M. and Mp. The following values for front and side

impacts have been verified in the literature:

(kg) M2(kg) C(N-sec/m) K(MN/m)

Front 0.27 4.45 350 8.75

Side 0.18 4.00 420 4.55

There are no citable parametric values for back and top impacts which are

required for comparison of modelling results with previously obtained

experimental data (see Chapter 4). One might argue that a helmeted

headform responds in a similar fashion to impacts in the anterior-posterior

direction, whether the impact is to the front or back: this might justify

modelling back impacts by using values cited for the front. However, no such rationale is available for choosing values to model top impacts.

Acceleration profiles were initially generated by the model using

values for the parameters of the orders of magnitude shown in the table

above. These were compared to acceleration profiles obtained by impacting

the top of the bare humanoid headform on hard and soft surfaces. By trial

and error, the parameters were adjusted to achieve a suitable match,

yielding the following values:

= 0.20

Mp = 4.45

C = 500

K = 7.0

Figure 7 presents comparisons between the predictions of the model (using

the above values) and the bare headform experimental results for (a) soft

and (b) hard surfaces.

The values given above will be used for top impacts, and the values in

the preceding table for front/back and side.

3.3 Helmets

The properties of helmets are difficult to model due to non-linear

behavior. In an earlier report it was shown that a small change in

velocity (from 4.5 to 5.0 m/sec) could lead to large changes in the

acceleration response of the headform. It was therefore decided to develop

the needed data base exhibiting this non-linear behavior over a range of

velocities. The validity of the model and of the values chosen for the

parameters can now be assessed by comparison with the data base.
- 10 -

Twelve helmets were impact tested on the hard surface with a metal

headform at the following velocities: 3.0, 3.5, 4.0, 4.5, 5.0, 5.5 and 6.0

m/sec. The experimental details were exactly the same as in the earlier

study V. The helmets were tested at both the front and back impact sites,

and the results are summarized in tables 1 and 2.

The results of this effect-of-velocity study are shown graphically in

figures 8 and 9 for top and back impacts, respectively. These figures plot

(a) SI and (B) a , and should be regarded as identifying a general range

of behavior of fS8^ball helmets as a function of velocity. It is the

overall description which is considered useful here, and not the

performance of any individual helmet.

Since the math model is intended to be a tool for relating a simulation

of a real life impact situation to a simulation of -a test method

configuration, comparisons are made of these two simulated performances for

a representative collection of mathematically idealized football helmets.

In the present math modelling study, the range is derived from the

spring/dashpot models of figure 3 with parameter values chosen by

comparison with the experimental range in the effect-of-velocity study

(figures 8 and 9). (In contrast, the experimental report V presented

comparisons involving data derived by testing actual helmets under

experimentally simulated conditions.)

Other factors which influenced the selection of values included the

duration of impact and the ratio of rebound velocity to impact velocity.

The former was determined in the earlier experimental study V, and the

latter in the effect-of-velocity study. The impact duration was observed

to be on the order of 10 msec for top impacts and 8 msec for back impacts.

The velocity ratio ranged from 0.35 to 0.70 This ratio reflects the amount

of energy absorption by the helmet liner, and thus guides the values of the

dashpot parameter in the model.

The model was first exercised with the linear spring dashpot elements

described in equations (3a) and (3b), and the results were compared to the

data with top impacts. This comparison is shown for the Maxwell element in

figure 10 for both SI vs. V and a vs. V. The region v/hich is demarked

by the diamond-shaped symbols repPiients the range of performance of actual

football helmets, taken from the data in figures 8 or 9. (This

representation is also used in ensuing figures.) The curvature of the SI

vs. V (figure 10(a)) response of the model is much shallower than for the

experimental data. This follows from the fact that the maximum

acceleration is a linear function of velocity (see figure 10(b)), which

should be expected with a linear model. This is the reason that the nonlinear model of figure (3c) and equation (3d) was used to describe helmet

behavior. All further results pertain to this non-linear model.

The process for determining the values of the helmet parameters , , £

2

* and D was largely one of trial and error. Each set of values can be

regarded as representing a "mathematical helmet." The purpose was to

choose enough sets to represent the range of helmets described by the
- n -

experimental results of figures 8 and 9. The procedure was to fix D at a representative value for the helmet liner thickness (top: D = 2.54 cm,

back: D = 1.90 cm), and then adjust the other parameters until the

response of the "mathematical helmet" appeared to fall within the

experimental range as one of the family. The sets of mathematical helmets

which were used in the remainder of this report are summarized in tables 3

and 4 for top and back impacts, respectively. In figures 11 and 12, the

Severity Index (SI) response of these mathematical helmets, as determined

by exercising the model, are compared to the experimental ranges of figures

8 and 9.
- 12 -

4. EFFECT OF CHANGING TEST PARAMETERS; COMPARISON

OF MODEL OUTPUT TO EXPERIMENTAL DATA

Details of the experiment to determine the effect of changing test

parameters have been described in a previous report V, where the following

test parameters were considered:

Impact Surface: Hard, Soft

Headform: Humanoid, Metal

Velocity: 4.5, 5.0 m/sec

Impact Site: Top, Back

(The data in tables 1 and 2, collected for the separate purposes of the

experiments conducted for this report, are now available for any desired

extension of the analysis of the effect of changing velocity.)

For each impact site, six distinct impact configurations were examined

with a variety of football helmets, as shown in figure 13. The six sets of

experimental data permit construction of seven relationships, each of which

describes the effect of changing a single test parameter (all others held

constant). These seven relationships are indicated in figure 13 by the

double arrows connecting the boxes.

Tne conditions of each box were also simulated with the math model,

using the values obtained in the previous chapter. That is, the simulated

conditions for each of the six boxes were run on the computer for each of

the mathematical helmet representations of tables 3 and 4. The results of

this computer study are summarized in tables 5 and 6.

The results of this math modelling exercise can also be used to examine

the results of changing test parameters, deriving the seven relationships

shown by the double arrov/s in figure 13 from the math model output.

Lastly, the relationships obtained with the math model output can be

compared to the relationships obtained with the experimental data to assess

the performance of the math model in predicting the effect of changing test

parameters.

These comparisons are shown in figures 14 to 20 for the top impact

site. These figures show the effect of changing impact surface (figures

14-16), headform (figures 17 and 18), and velocity (figures 19 and 20). In

each figure, the dashed lines represent the experimental results from the

earlier study. These lines demark the 95% confidence band for a straight

line fit to the experimental data. The results of the model study are represented by the set of symbols, each corresponding to a different

"mathematical helmet." The test of validity of the mathematical modeling

is whether or not the set of symbols suggests a similar relationship to
- 13 -

that established by the confidence band. For the top impact site, the

agreement appears to be suitable.

It was difficult to make comparisons for the back impact site. This

was largely due to the scatter associated with the experimental data. This

was attributed to two sources: (1) Difficulties with the experimental

behavior of the humanoid headform when impacted in the back site have

already been reported J_/. Namely, the headform tends to deform in the

region where its "neck” attaches to the headform support. This "neck

bending" phenomenon absorbs energy and leads to misleadingly low

acceleration and large amounts of scatter in the data. On the other hand,

as discussed in chapter 2, the model is based on the assumption that the

headform exhibits linear acceleration only. (2) Another source of scatter

can be attributed to the small thickness of the helmet liner at the back

site. This leads to more scatter in the experimental data as the impact

becomes more severe.

Of the seven relationships indicated in figure 13, for the back site,

only four could be reasonably characterized by fitting straight lines.

(These had correlation coefficients larger than .75; all seven

relationships for the top impact surface had correlation coefficients

larger than .80 V.) That is, these four have confidence bands which were

narrow, enough to permit a fair comparison of the model predictions with

experimental data (figures 21 - 24). While these comparisons are not quite

as good as for the top site) it still appears as though the results of the

math model would be useful as a first approximation.
- 14 -

5. USING THE MODEL TO DEVELOP PASS/FAIL CRITERIA

IN A TEST METHOD FOR FOOTBALL HELMETS

In this section we will use a hypothetical example to illustrate how

the model may be used. As discussed in the introduction, the model is

intended to relate simulated real life injury situations to the simulation

of a suggested (non-realistic) test method configuration. We will use

football as the activity of interest in order to capitalize on the large

body of experimental data that has already been collected on football

helmets.

Several modes of head impact may be associated with any activity. For

football, we will consider three distinct modes, characterized by the mass

Mo of an impacting object and the relative velocity, V, between the object

and the head.

1. Mo = 00

, V = 5.5 m/sec (Fall against massive impacting object,

such as artificial turf.

2. Mo = 5 kg, V = 7 m/sec (Example: head struck by moderately large

object)

3. Mo = 2 kg, V = 10 m/sec (Example: head struck by small, fastmoving object)

The values shown above were picked arbitrarily to illustrate

differences in the real life/test method relationships between small

mass/high velocity impacts and large mass/low velocity impacts. The three

modes were chosen to resemble several types of head impact situations which

typically occur in football, as illustrated schematically in figure 25.

The first mode represents the head striking the football field surface

after a fall. The formulation for resilient impact surfaces used to

describe artificial turf (as described in sections 2.2, 3.1 and figure 4) were used in the computation. Modes 2 and 3 suggest lesser masses and

higher velocities as may characterize impacts from knees and hands,

respectively. The math model in its present formulation considers only

rigid impacting objects which are non-massive, hence cannot take into

account the energy which is absorbed by the impacting objects themselves.

To compensate for this deficiency, velocities for impact modes 2 and 3 were

intentionally chosen to be less than what might be achievable in real life.

In this example, developing a test method for football helmets, the

real life situation is simulated by exercising the model for each of the

three modes with parameter values that have been used to describe the human

head (section 3.2). These values have only been determined for front and

side impacts, and each set of values will be utilized in these examples.

In addition, for purposes of illustration, the front values will also be

used to describe impacts to the back site. As discussed earlier, this may

be partially justified by the fact that blows to both the front or the back
- 15 -

are in the same anterior-posterior direction (which would be in agreement

with the constraints of the one dimensional model).

For both sets of "head" parameters (front/back and side) we shall use

the "mathematical helmets" which were shown to describe impacts to the back

site. Although side impact data was not collected, visual inspection

suggested that, in many helmets, the impact attenuating features were very

similar in the side and back.

The impact response in the "real life" situation can now be simulated

and computed in all three modes for back and side impacts. The results of

these calculations are shown in table 7. Since this exercise is aimed at

illustrating use of the model to develop a connection between real life

situations and a possible test method, the next step is to simulate a prospective test method.

The ASTM Test Method for Football Helmets F429-75 will be used for this

purpose because it utilizes a metal headform and is readily accessible in a published document. In this test method, a helmeted metal headform is

dropped in guided free fall so that it strikes a specified impact surface

at 5.5 m/sec. The characterization of the impact surface was described in

section 3.1 and figure 4. The results of using the mathematical helmets of

tables 3 and 4 in simulating this test method configuration are also shown

in table 7.

The relationships between the simulated real life situation and the

test method configuration are shown for each of the impact modes in figures

26 - 33 for back site impacts, and figures 34-41 for side impacts. In

each figure, the severity index SI is plotted on the "real life" axis

(abscissa). Figures 26 - 29 and 34 - 36 show the severity index response,

SI, of the test method; figures 30 - 32 and 38 - 40 show the maximum

acceleration response, a , of the test method. In each figure a line has

been fitted to the data Sf^the method of least squares. For each

combination of impact site and test method response parameter, the results

of the three impact modes are summarized on a single graph in figures 29,

33, 37 and 41 for back/SI, back/a , side/SI, and side/a respectively.

msx msx Such graphs are useful in determining test method criteria which

correspond to particular real life injury criteria. Examples of this

procedure are shown in figures 29, 33, 37 and 41 for real life injury

criteria of SI = 1500. It is seen that the associated test method

rejection values would be SI = 1900 and 2000 for back and side,

respectively, and a = 220 g and 230 g for back and side, respectively.

nlaX
- 16 -

6. SUMMARY AND DISCUSSION

This report has presented a mathematical model to connect injury

parameters in real life head impact environments to output parameters in

test methods for evaluating protective headgear. The model may be

particularly useful in determining test method pass/fail criteria which

correspond to the threshold of injury in the real life injury situation.

The validity of the model depends upon the accuracy with which each

element (head or headform, impact surface, range of helmets) is

represented. In chapter 3, plausible parameter values were chosen for each

of the elements. Notwithstanding the assertion that these were reasonable

values, the true test for any model is whether or not it can usefully

predict experimental results. The comparison of the model output to

experimental results was presented in chapter 4 and, in general, the

results were within acceptable limits.

The ability of the model to represent real life head impact situations

is valid only for front and side impacts since the parameter values for the

’’head" element have only been experimentally determined at these sites.

Nevertheless, the close agreement in pass/fail criteria for front and side

impacts (compare figures 33 and 41) indicates that the application of the

model does not depend strongly upon the values of the "head" parameters.

Therefore, until more cadaver data for back impacts is available, the same test method pass/fail criteria should also be used for impacts to the back.

The model should not be applied to represent top impacts in the real

life situation. (They were required in validating the model, however,

because the experimental program contained top impacts.) In the real life

situation, an impact to the top of the head is not accompanied by the

nearly free-body linear acceleration as with other sites. Because such

blows are directed parallel to the neck axis rather than perpendicular to

it. Consequently, different injuries (often to the neck and spine)

manifest themselves. For completeness in evaluating the helmet by the test

method, it is suggested that the top site should still be tested with the

same criteria as for the other sites (helmets generally perform best at the

top site), at least until such time as real life neck injury criteria can be related to the test method.

For other types of headgear, the step-by-step procedure in applying the

model to develop acceptance/rejection criteria for test methods is:

1 . Determine a helmet behavior envelope by collecting injury response

vs. velocity data for representative helmets, as in section 3.3.

2. Determine parameter values for "mathematical helmets" to represent

the range determined in step 1 (section 3.3).

3. Determine modes of impact considered to be significant for this

activity.
17 -

4. Determine parameter values to characterize typical impact surfaces

for each mode, as in sections 2.2 and 3.1.

5. Apply the model, using "mathematical helmets" from step 2 and

impact surfaces from step 4 to each mode of step 3 with the "head"

parameters for front and side impact (chapter 5).

6. Characterize the headform and impact surface of the test method

configuration in like fashion and apply model using the same

"mathematical helmets."

7. Construct curves as in figures 26 to 41 to identify the

relationship between the real life injury situation and the test

method

8. Use these graphs to determine test method pass/fail criteria that

correspond to the onset of injury as in figures 29, 33, 37 and 41.

Finally, the model may have limited application in suggesting design

improvements for protective headgear. If the helmet is characterized

mathematically as in section 2.4, and if material parameters can be related

to particular spring/dashpot components, then a parameter variation

analysis can be performed to indicate which material changes offer the

greatest potential for improved safety.


Table

2.

Effect

of

Velocity

Data

for

Back

Impacts

cvj OO OJ LO CO 00 CO LO o OJ 1 OO r— LO 0^ cn CO • LO OJ cn LO LO 1— r— 1-^

<X) oo LO o CO LO LO CO r— CM LO OJ LO o oo 00 LO CM r— cn CM • r* LO o LO f— LD «?}• cn LO oo OJ OO '=d“ 'Ll- OO CO oo CM

LO oo LO CM CM cn o LO LO CM

O LD ’sd" r— OO CM LO LO LO oo o LO

LO 00 r—• CO LO LO O'* CM cn LO «5d* LO OJ *=d" *“ OJ OO OO oo r“ CM *

CO LO r>*

O o LO cn 00

LO o CO OJ CM o LO CO LO LO CO o • r-^ LO OJ oo r— *?d" o o oo LO CM LO oo 1 OJ ' 1 CM CM CM r— ' CO

cn LO OJ o CO CM LO o LO o o OO o LO cn CM p— CO LD CM LO

CO o o o <d" CO LO LD r— LD r—

OJ 1 1 r~ CM

LO LO oo LO o LO CO OO cn CO CM

O • r— 1

LO cn LO oo r— LO oo LD LO CO LO 00 oo LO *5d“ oo '=d' CO

r*~ o OJ OO o OO LO LO CO LO cn • o o LO r— cn •5d" LO LO r'. LD

CO oo OJ CO CM CM CM CM LD o LO oo CO cn CO LO • o cr> 1 oo oo cn o cn r— CO 11

U3 OJ oo oo =^ i) LO m '=d“ CO o LO CO LO CM CO LO • O'! 1-0 oo cn r~ CM CM LO LO CM

in O') vr CVJ oo oo <=d" •cj- oo OO OO CM o o OJ CTl 00 LO oo LO O LO • OJ CV.I C\i LO cn 00 LO CM CO

LO O-J CVJ OJ oo oo CO CM CM CM CM oo

X

(T3 LO cn CO 0^ LO o cn LO CO 00 CM o P » O'. OJ CTl r—

cn 00 CO cn CM oo LO

oo ' CVJ CM CM CM 1 CM CM CM CO o LO LO CT> LO LO LO LO ccf cn • LO CTi r-^ LO cn OO CM oo o CO

OJ ' CM 1 r— ' CM r-“ CM

LO CO cn r~" o 1^ cn CM CM CO • OJ CT> LO C\J oo LO 1— CO r~ CM CM

CO r— r— 1 r~ ' 1 ' CM o LO CM CM CM cn CO O • o o C\J CO J— cn cn cn LD cn LO

CO fmmm '

-M

O) <c. <c C

r— 1 i 1 1 1 i 1 1 I 1 1 1 OJ Q. Q_ Ll_ (_) CO CO Q_ CL. o nr 1— CO

ZJZ p: CQ UJ

Q CQ o CQ CJ> o C_) < cc
Table: 3.. Parameters of Mathematical Helmets Chosen to

Fit Effect of Velocity Data: Top Impacts

^l(^) n(Kpoise) R P MN

*"2 2 m

Symbols Used

Fig. 14 - 20

5 20 6 20

50

100

200

0

8 20

50

100

200

10 200

500

1000

2000

0

5 10 6 20

50

100

200

0

8 20

50

100

200

10 200

500

1000

2000

0

10 10 6 20

50

100

200 0

8 20

50

100

200

10 200

500

1000

2000

0 -
Table 4. Parameters of Mathematical Helmets Chosen to

Fit Effect of Velocity Data: Back Impacts

-l(^) n(Kpoise)

\ m

R

P MNm

Symbols Used

Fig. 21 - 24

5 20 6 20

100

200

500

1000

o

8 200

500

1000

10 200

500

2000

O

5 10 6 20

100

200

500

1000

o

8 200

500

1000

10 200

500

2000 o

10 10

1

6 20

100

200

500

1000

o

8 200

500

1000

10 200

500

2000 O
Table 5. Results of Simulating Experimental Conditions - Top Impacts

Velocity: \

= 4.5 m/sec

'^H = 5. 0 m/sec

Headform: Humanoid Metal Metal

Impact Surface: Hard Soft Hard Soft Hard Soft

Mathematical Helmet(Ei-n-R-E2) Amax

SI AmaxSI AmaxSI AmaxSI AmaxSI Amax

SI

5-20-6-20 118 619 107 552 123 645 no 569 150 964 132 830

-50 137 777 120 663 145 831 125 693 182 1294 152 1040 -100 156 954 133 779 169 1049 139 826 212 1655 170 1248

-200 177 1178 146 918 197 1341 155 988 246 2109 189 1489

8-20 106 513 98 469 109 526 101 480 128 754 117 678

-50 113 575 103 516 117 596 106 531 146 912 128 786 -100 123 656 110 573 130 692 114 595 167 1106 141 907 -200 138 773 120 652 148 836 125 684 193 1375 156 1061 10-200 114 582 103 519 119 607 106 535 155 978 132 818

-500 129 694 112 594 138 743 117 620 186 1264 149 980 -1000 145 814 123 670 159 897 129 708 213 1552 164 1130

-2000 162 965 133 760 182 1099 141 814 242 1899 179 1300

5-10-6-20 108 507 98 463 112 523 101 475 145 854 127 746

-50 136 725 118 622 145 776 . 123 650 186 1270 153 1016 -100 160 941 134 767 174 1041 141 815 220 1684 174 1258

-200 182 1194 149 927 204 1370 159 1000 255 2174 193 1520 8-20 86 367 81 350 88 371 83 354 119 622 107 570

-50 107 492 97 448 112 510 100 460 152 885 129 752 -100 126 625 no 543 135 667 115 566 181 1159 147 922 -200 147 792 124 655 161 872 130 693 212 1497 165 1114

10-200 119 559 104 492 127 594 108 510 178 1085 143 859

-500 144 742 120 613 158 821 127 648 215 1475 163 1073 -1000 162 906 132 717 183 1036 141 768 242 1822 178 1251

-2000 180 1089 144 828 207 1287 154 898 270 2207 193 1438

10-10-6-20 37 359 81 353 88 356 83 353 103 548 99 536

-50 101 482 95 458 104 486 97 465 135 783 122 722

-100 122 624 no 568 128 644 114 584 166 1051 143 909

-200 146 811 126 700 156 865 132 732 201 1409 163 1132

8-20 85 283 79 281 87 280 81 280 97 414 90 416

-50 36 335 80 332 87 332 81 332 99 527 96 518

-100 87 401 84 390 88 400 85 393 122 665 112 627

-200 105 495 97 466 108 503 99 474 149 859 129 764

10-200 85 354 79 349 87 350 31 350 112 592 104 567

-500 97 447 91 426 100 451 93 431 144 798 124 712

-1000 115 544 103 499 121 562 106 511 170 1007 140 345

1 -2000 133 666 115 583 144 708 120 606 197 1265 155 994
Table 6. Results of Simulating Experimental Conditions - Back Impacts

Velocity: =4.5 m/sec Vh = 5. 0 m/sec

Headform: Humanoid Metal Metal

Impact Surface: Hard Soft Hard Soft Hard Soft

Mathematical Helmet(Ei-n-R-E

2

)

fl max

SI Amax

SI ^max SI AmaxSI AmaxSI amaxSI 1

f 5-20-6-20 152 934 131 774 161 991 136 307 198 1503 153

1 1181 -100 200 1439 160 1069 220 1615 168 1140 273 2503 203 1691 -200 224 1733 173 1228 251 2006 184 1324 308 3083 220 1952

) -500 254 2160 190 1449 293 2605 204 1584 355 3929 242 2306 i< -1000 274 2494 202 1615 323 3096 218 1783 388 4600 257 2567

1 8-200 185 1231 149 933 202 1367 156 988 259 2218 191 1506 1 -500 212 1539 164 1098 239 1776 174 1178 301 2852 212 1786 1

-1000 233 1801 176 1234 267 2141 187 1338 333 3386 227 2009 '

10-200 156 939 131 759 167 1007 136 792 220 1674 169 1219 -500 178 1143 143 372 196 1266 150 920 258 2134 187 1431

1 -2000 216 1548 164 1084 246 1814 174 1165 316 2995 215 1796 1

5-10-5-20 145 818 125 689 153 859 129 712 195 1386 159 1096 1 -100 202 1420 161 1055 224 1599 170 1123 279 2516 205 1689 i| -200 227 1738 175 1231 257 2020 186 1326 314 3120 223 1966 : -500 257 2181 192 1463 298 2639 206 1600 361 3980 245 2329 ji -1000 278 2517 204 1632 328 31 35 220 1801 393 4651 259 2592 ii S-200 193 1263 153 943 214 1418 161 998 274 2343 198 1551 -500 221 1606 169 1131 252 1876 180 1216 316 3014 219 1851 'i -1000 241 1881 181 1276 280 2261 193 1386 346 3557 233 2080 10-200 167 961 135 755 181 1045 141 787 243 1834 179 1272 .1 -500 193 1224 150 904 215 1384 158 957 281 2364 198 1513 |.'

-20C0 229 1675 172 1146 266 2002 183 1238 336 3269 224 1906 li

10-10-6-20 112 564 103 528 115 565 106 534 142 375 129 805

'

-100 161 966 137 310 170 1012 142 833 217 1626 174 1276

[ -200 137 1223 153 967 203 1325 160 1014 255 2111 194 1531 i; -500 221 1622 172 1192 247 1844 182 1273 305 2878 219 1891 -1000 245 1953 186 1367 280 2301 199 1479 343 3528 236 2168 i, 3-20C 143 303 125 695 150 825 128 712 201 1389 162 1117 -500 173 1056 143 852 137 1129 143 886 2i4 1888 185 1379 , -1000 196 1283 156 984 216 1419 164 1036 277 2337 201 1593 10-200 113 587 104 543 114 584 106 547 161 999 138 374 -500 133 743 120 651 143 758 122 663 198 1324 158 1063 -2000

1

177 1069 144 849 193 1155 149 384 256 1990 188 1397 .

1 - J
Table

7.

Results

of

Simulating

Real

Life

Impact

Modes

in

Football

and

ASIM

lest

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CAPTIONS TO FIGURES

1. Elements of mathematical model

2. Humanoid headform model

3. Mathematical helmet models: (a) Maxwell, (b) Voight,

(c) Maxwell plus non-linear spring.

4. Examples ^ of fitting^ a_^„ vs. V data: max

(a) Urethane impact surface of ASTM test method F427-75.

Values of C and q correspond to F = B x P where

p = 1.76 and B = 144.6 MN/m^ -76.

(b) Representation of artificial turf (data from ref. 11).

Values of h and C correspond to F = Bx/(d^ - x^)'/^

where d = .008 m and B = 6539 N.

5. Static force displacement curves showing degradation of

soft impact surface.

6. Determination of parameter B for hard and soft surfaces.

7. Comparison of acceleration profiles of bare humanoid

headform and model.

8. Effect of velocity data for top impacts: (a) SI (b) a^^^

9. Effect of velocity data for back impacts: (a) SI (b) a

^ max

10. Comparison of Maxwell model to elfect-of-velocity

data (a) ^ SI (b) ^ ' a E,=30 MN/m"^, n=15 20 , 30 —

max 1 11. Comparison of mathematical helmet model response to effectof velocity data for top impacts; a) E,=5, n=20, b) E,=5,

n=10, c) E^=10, n=10. ‘ '

'

12. Comparison of mathematical helmet model response to effectof-velocity for back impacts; a) E,=5, n=20, b) E.=5, n=10,

c) E^=10, n=10. ' ^

13. Experimental configurations examined in previous study ]_/

14. Comparison of model (symbols) and experiment (envelope)

for effect of changing impact surfaces (metal headform,

V = 4.5 m/sec, top).

15. Comparison of model (symbols) and experiment (envelope)

for effect of changing impact surface (humanoid headform,

V = 4.5, top)

16. Comparison of model (symbols) and experiment (envelope)

for effect of changing impact surface (metal headform,

V = 5.0, top)
17 . Comparison of model (symbols) and experiment (envelope) for

effect of changing headform (hard surface, v = 4.5, top).

18. Comparison of model (symbols) and experiment (envelope) for

effect of changing headform (soft surface, v = 4.5, top).

19. Comparison of model (symbols) and experiment (envelope) for

effect of changing velocity (hard surface, metal headform, top)

20. Comparison of model (symbols) and experiment (envelope) for

effect of changing velocity (soft surface, metal headform, top)

21. Comparison of model (symbols) and experiment (envelope) for

effect of changing impact surface (metal headform, v = 4.5,

back)

.

22.

Comparison of model (symbols) and experiment (envelope)

for effect of changing impact surface (humanoid headform,

V = 4.5, back).

23.

Comparison of model (symbols) and experiment (envelope)

for effect of changing impact surface (metal headform,

V = 5.0, back).

24. Comparison of model (symbols) and experiment (envelope) for

effect of changing velocity (hard surface, metal headform,

back)

.

25. Modes of impact in football.

26. Relationship between

impact mode 1 , back.

"real life" SI and "test method" SI for

27. Relationship between

impact mode 2, back.

"real life" SI and "test method" SI for

28. Relationship between

impact mode 3, back.

"real life" SI and "test method" SI for

19 . Summary of SI vs. SI for impact modes 1, 2 and 3, back.

30. Relationship between

for mode 1 , back.

"real life" SI and "test method

31

.

Relationship between

for mode 2, back.

"real life" SI and "test method

32. Relationship between "real life" SI and "test method

for mode 3, back.

max

max

max
33. Summary of a^^^ vs. SI for impact modes 1, 2 and 3, back.

34. Relationship between "real life" SI and "test method" SI

for impact mode 1, side.

35. Relationship between "real life" SI and "test method" SI

for impact mode 2, side.

36. Relationship between "real life" SI and "test method" SI

for impact mode 3, side.

37. Summary of SI vs. SI for impact modes 1, 2, and 3, side.

38. Relationship between "real life" SI and "test method" a for mode 1, side.

39. Relationship between "real life" SI and "test method" a for mode 2, side.

40. Relationship between "real life" SI and "test method" a for mode 3, side.

41. Summary of a vs. SI for impact modes 1, 2, and 3, side. max

HEAD OR F

HELMET

F

^ IMPACTING

OBJECT

(SURFACE] HEAD FORM

Xi X3

FIGURE 1

X2 Xi

FIGURE 2

I

^1

‘\JUUU“ il

71

I

(a)

FIGURE 3

V (m/sec)

(a)

|bl

FIGURE 4

FIGURE 5

(S.3I XBUlv

S.D ‘B ‘N0liVd313Q0V

TIME,

t,

m/sec

TIME,

t,

m/sec

|a|

SOFT

IMPACT

SURFACE

|b|

HARD

IMPACT

SURFACE

FIGURE

7

VELOCITY

(H/S)

VELOCITY

(M/S)

FIGURE

8a

FIGURE

8h

CO

VELOCITV

(H/S)

VELOCITY

(H/S)

FIGURE

9b

FIGURE

9a

VELOCITY

(H/S)

VELOCITY

(H/S)

FIGURE

lOa

FIGURE

10b

CO LxJ

A UJX

<s;> CO

CO uu <=& LlJ X


 CO

LO

LT>

UO

LT>

Lr> CO

IS> co’

CO UJ

VELOCITY

(H/S)

FIGURE

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REFERENCES

1. Berger, R.E. and Calvano, N.J., Methodology for Choosing Test

Parameters to Evaluate Protective Headgear, National Bureau of

Standards, NBSIR78-1547, November 1978.

2. Berger, R.E., Considerations in Developing Test Methods for Protective

Headgear, National Bureau of Standards Report, NBSIR76-1 107, August

1976.

3. Hodgson, V.R., National Operating Committee on Standards for Athletic

Equipment Football Helmet Certification Program, Medicine and Science

in Sports, 7 (3) pp. 225-232 (1975).

4. Timoshenko, S. and Goodier, J.N., Theory of Elasticity (2nd Edition)

McGraw Hill, New York (1951).

5. Stalnaker, R.L. , Fogle, J.L., and McElhaney, J.H., Driving Point

Impedance Characteristics of the Head, J. Biomechanics, 4, 127-139

(1971).

6. Gill, S., A Process for the Step-by-Step Integration of Differential

Equations in an Automatic Digital Computing Machine, Proc. Cambridge

Phil. Soc. 47, p. 96 (1951).

7. Hodgson, V.R. and Thomas, L.M., Head Injury Tolerance, in Aircraft

Crashworthiness, K. Saczalski, ed. (1975).

8. Newman, J.A., On the Use of the Head Injury Criterion (HIC) in

Protective Headgear Evaluation, Proceedings of the 19th Stapp Car Crash

Conference, Society of Automotive Engineers, New York (1975).

9. Nilson, E.N., Cubic Splines on Uniform Meshes, Comm. A.C.M., 13, (4)

(1970).

10. Standard Method of Test for Shock Attenuation Characteristics of

Protective Headgear for Football, ASTM F429-75, American Society for

Testing and Materials, Philadelphia (1975).

11. Mahajan, B.M. and Beine, W.B. , Impact Attenuation Performance of

Surfaces Installed Under Playground Equipment, National Bureau of

Standards, NBSIR79-1707, February 1979.
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ACKNOWLEDGMENT

I thank Bill Beine for his essential contributions in data reduction,

statistical analysis, and effective graphical presentations.

NBS>114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR80-19872.Gov*^t' Access|{« No. C-5. R«oi$Jt6rtl' s Accession No.

4. TITLE AND SUBTITLE

A Mathematical Model for Use in Evaluating and Developing

Impact Test Methods for Protective Headgear

5. Publication Date March 1980

S. Performing Organization Code

7. AUTHOR(S)

Robert E. Berger

8. Pertocming Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

l^|^|e<^|^sk/Work Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, st^te, ZIP)

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

13. Type of Report & Period Covered

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Q] Document describes a computer program; SF-185, FIPS Soft/vare Summary, is attached.

16. ABSTRACT (A 200^word or les3 factual summary of most sight ficar^t information. If document includes a significant bibliography or

literature survey, mention it here.)

A lumped parameter mathematical model was developed to connect injury parameters

in real life head impact environments to output parameters of test methods for

evaluating protective headgear. Analytical/experimental schemes were developed

for mathematically representing the parameters that characterize each of the

three distinct elements of the model: the head or headform, the impact surface,

the helmet. A comparison of the model output to experimental results showed a satisfactory agreement. The model was shown to be useful in determining test

method pass/fail criteria which correspond to the threshold of injury in the

real 1 ife situation.

17. KEY WORDS fs/x to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper nam^

separated by semicolons) • Drop test parameter; head injury; helmet; injury criteria; mathematical

model ; test methods

18. AVAILABILITY )TX~> Unlimited

For Official Distribution. Do Not Release to NTIS

r~ Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

rx" Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

13. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

UNCLASSIFIED 74

20. SECURITY CLASS 22. Price (THIS PAGE)

$ 7 .00

UNCLASSIFIED

USCOMM-DC


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