NBSIR 73-262

Some Cutting Experiments

Human Skin and Synthetic

Materials

John R. Sorrells. Robert E. Berger

Consumer Product Systems Section

Measurement Engineering Division

Institute for Applied Technology

October 1973

Interim Report for Period

February -- August 1973

Prepared for Consumer Product Safety Commission

Bethesda. Maryland 20016

NBSIR 73-262

SOME CUTTING EXPERIMENTS ON

HUMAN SKIN AND SYNTHETIC

MATERIALS

John R. Sorrells, Robert E. Berger

Consumer Product Systems Section

Measurement Engineering Division

Institute for Applied Technology

October 1973

Interim Report for Period

February - August 1973

This is a progress report. The work is incomplete and continuing.

Conclusions are not necessarily those that will be included in a final report.

Prepared for Consumer Product Safety Commission

Bethesda, Maryland 20016

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

TABLE OF CONTENTS

Page

I. DISCUSSION OF THE PROBLEM

A. Introduction 1

B. Background Discussion 2

C. Scope of this study 4

II. SKIN CUTTING EXPERIMENT

A. Source and Preparation of Skin

Specimens 8

B. Test Method and Apparatus for

Cutting Skin 9

C. Test Edges 11

D. Cut Measurement Method 11

III. RESULTS AND APPLICATION OF EXPERIMENT

A. Presentation and Discussion of

Results 14

B. Some Limitations of the Experiment 17

C. Discriminating Between Safe and

Hazardous Edges 19

IV. DISCRIMINATION TECHNIQUES USING SYNTHETIC

MATERIALS

A. Inspection Procedure for Unknown .

- Edges 22

B. Testing of Synthetic Materials 23

C. Results of Tests 25

V. CONCLUDING REMARKS

A. Summary and Conclusions 28

B. Recommendations for Future Study 29

/

I. DISCUSSION OF THE PROBLEM

A. Introduction

Under the sponsorship of the Consumer Product Safety Commission

the NBS Measurement Engineering Division is continuing to investigate

hazards associated with certain mechanical features of toys (e.g. points,

edges, and projections). The two main objectives of this investigation

are: 1) to provide quantative data which can be used to establish

and support toy safety regulations; and 2) to develop test methods or

devices which can distinguish between hazardous and non-hazardous

features of toys. The investigations conducted to date under this

program have not attempted to characterize those toy configurations

which might be considered hazardous under high impact or fall-on conditions. Instead, the studies thus far have been concerned primarily

with points, edges and projections which might be potentially dangerous

under "casual handling" conditions.

Three prior progress reports have been submitted on this project.

The first of these, by Moore \J discussed the results of subjective

evaluations of sharp points, and described the design of a test device

for measuring the sharpness and rigidity of points. The second report,

by McGuire and Moore 2^/, included a review of published data, and gave

the results of experimental puncture tests on pigskin and human skin.

That report also gave results of subjective evaluations of point sharpness by physicians. The most recent report, by McGuire et al. _3/,

provided additional data on the puncture resistance of human skin, data

on the measurements of children's hand velocities, and the results of

cutting tests on pigskin with sheared steel edges at various velocities.
- 2 -

This present progress report discusses the results of experimental

cutting tests conducted on excised specimens of human skin with a

variety of typical edges and within a limited range of controlled conditions. This report also describes a method using synthetic materials

as cut indicators to differentiate between hazardous and non-hazardous

edges

.

B . Background Discussion

The establishment of criteria and test methods for categorizing

the hazard potential of exposed edges on toys is a complex problem, for

which there can probably be no simple and universal solution. Certain

edges, such as a broken glass edge or a soft rubber edge, V70uld probably

be judged hazardous and non-hazardous respectively by any criteria.

Between these extremes, however, there may be an infinite number of

possible edge configurations which could occur in real life. Furthermore, in a real play environment, the hazard potential of a given edge

depends not only upon the physical characteristics of the edge, but also

upon many other factors which are involved in the child-toy interaction.

For example, on one toy, a given edge might be perfectly safe if the toy

has very low mass, the edge is supported by compliant material, or the

use of the toy is limited. On another toy, where these factors might

be different, the same edge could be sufficiently hazardous to make it

unacceptable for child use. In addition, many edges which might be

considered safe under casual handling conditions could, if thrown or

fallen on, inflict serious injury to a child due to higher forces which

result from impact. During impact, the force which is developed between

the child's body and an edge may be several orders of magnitude greater

than that experienced during normal handling or casual play.
- 3 -

In addition to those factors which are primarily related to the

physical characteristics of the toy and its use, many factors concerning

the physical attributes of the child must also be considered. Physical

strength of the child, mass and velocities of body parts, and resistance

of the child's body to injury are all significant in determining the

injury potential of toy configurations. This adds greatly to the complexity of the problem since these factors vary considerably with age,

Individual, and body part.

Another complicating aspect of the problem is the complex nature

of skin itself. Human skin is a multi-layered, inhomogeneous , nonuniform material with characteristics ranging widely among different

persons and body sites. The major layers of skin are schematically

illustrated in Figure 1. This is a grossly simplified sketch since skin

is also rich in nerves, pores, hair follicles, etc. "Skin consists of

the relatively thin, cellular epidermis and two dermal layers comprised

predominantly of collagenous and, to a lesser extent, of elastic

fibers. Of these two layers, the deeper one is composed of particularly dense networks of coarse fibers, from which it gets the name

reticular layer." ' 4_/ .

The papillary layer of the dermis is the first layer below the

skin surface which contains any blood vessels - i.e. small capillaries.

Typical thicknesses of the epidermis and dermis are shown in Figure 1,

the larger values occuring on hands and finger tips. According

to Deck's data, the thickness of the reticular layer represents about

80 to 90 percent of the total skin thickness (i.e. epidermis plus dermis).

The subdermis, a relatively thick fatty tissue containing larger blood

vessels and nerve bundles, is not usually considered to be a skin component II and UJ . .
- 4 -

A search of the literature reveals very little information about

low level laceration injuries to human skin. A few studies of skin

trauma have been made in connection vi;ith automotive safety programs.

Gadd et al. Ij conducted a series of tests on unembalmed cadavers vi/herein

metal and glass edges were impacted against several areas of the body,

and measurements v;ere made of the resulting impact forces and lacerations.

Force levels of 90 to 405 pounds were measured during impacts that produced total penetration of scalp tissue.

In connection with studies of automotive glass performance,

Rieser and Chabal 8^/ used stafidard surgical blades to make comparative

cutting tests on human skin, pigskin, chamois skin, and a variety of

synthetic compositions. From these tests, a synthetic skin simulant was

developed and used on headforms to evaluate the performance of automotive

glass during impact. While these studies are probably valuable to the

field of automotive safety, they do not appear to be directly applicable

to injuries associated with child-toy interactions under casual handling

conditions.

C. Scope of this Study

At the start of this investigation, it was emphasized by the sponsor's

representatives that test criteria and procedures applicable to safety

standards for edges on children's toys are urgently needed. Because of

this urgency, this initial study does not undertake the systematic and

theoretical analysis of all possible factors and conditions involved in

laceration injuries to children. Instead, this investigation is based

essentially on empirical methods, and is directed primarily at developing

practical procedures for evaluating the sharpness of edges on toys, under

limited conditions of play. In this report "sharpness" refers to the ability

of an edge to cut human skin.
In this study, only casual play conditions have been considered.

As used here, the terra "casual play" is intended to include the type of

activity usually involved in unpacking, handling, operating and playing

with the toy. The term is also meant to include activities associated

with reasonable misuse of the toy, but not situations where the toy is

fallen on, thrown, or swung, at high velocities. The above definition

of activity level is admittedly imprecise. However, at present, no

quantitative data are available on the full range of force, momentum or

energy that can result from typical child-toy interactions.

The general approach that is used in this study is. based on the

premise that practical methods for judging the sharpness of edges can

be developed without necessarily determining the complex relationship of all parameters involved in cutting human skin. More specifically,

this premise suggests that for casual play conditions, edge safety criteria can be adequately defined in terms of two basic parameters: 1) the

force, F, developed between the skin and the edge in the casual handling

mode; and 2) permissible levels of injury, I, to be tolerated*. Specifications of these two parameters will define a safety criterion,

S.C. = (F,I). In choosing such a safety criterion, the tolerable injury

may depend on the force. Functionally this is written S.C. = (F,I(F)).

Operationally, this means that while only low level injuries (e.g. no

break in epidermis) may be tolerated for force levels in normal use (of

a toy), higher level injuries may be allowed for reasonably forseeable

abuse, and still higher level injuries for abnormal, or unforseen, abuse.

*Initially it was thought that the relative velocity between the edge

and skin might also be an important parameter. However, preliminary

tests, in which other variables were held constant and the velocity was varied, showed no appreciable or definite velocity effect (see section

III. A)

.
- 6 -

The present work seeks to cover a reasonable range of force levels

in the casual handling mode. In the point sharpness study by McGuire

and Moore _2/ it was assumed that the maximum grip strength of pre-school

children defines a reasonable upper limit of casual handling forces.

Based on this assumption, and data from a study by Krogman and McCown _9 /

,

a force of 7.5 kg (16.5 pounds, 74 newtons) was used in evaluating point

sharpness. In a more recent, study by Brovm et al. 10/ , it was found

that pre-school (5-year old) females and males exert average maximum grip

. forces of 19.1 and 20.4 pounds, respectively (85 and 91 newtons). Thus

the present work covers a force range from 0 to 20 pounds (89 newtons).

Also needed is a means of characterizing the injury which would

result from a sharp edge. In this work, it will be assumed that the

depth of cut (or percent depth of cut, D*, see section III. A) is an

adequate measure. Of course, pure laceration is not the only type of

injury which might result from an edge; at high forces, and for sufficiently dull edges, one may expect to find a significant amount of tissue

crushing, whether or not laceration is present. Such crushing leaves a

measurable groove in autopsy specimens (section II. D) and thus a depth

measurement is actually some combination of cutting and crushing. Thus

it is hoped that a depth measurement will be a realistic injury indicator even though part of the injury may be due to crushing. The safety

criterion would then be rewritten:

S.C. = (F, D*(F))

It should be emphasized that many difficulties exist in choosing

the proper force and injury levels that constitute the safety

criterion. As seen above, only maximum child strength data are available
- 7 -

in the literature. However, if these maximum forces are rarely

approached, it seems unrealistic to base safety criteria on them. Also

there seems to be little or no agreement as to what constitutes a tolerable depth of cut. Mothers may prefer that their children never bleed

(epidermis not be broken) . Yet in a limited, informal survey of pediatric

and plastic surgeons in Maryland, Virginia and D.C., the unanimous

opinion was that the dermis must be completely penetrated before the cut

would be considered serious.

Once a safety criterion is established, and edges can be classified

as hazardous or non-hazardous, it remains to devise an inspection procedure to classify unknown edges. As a final objective of this report,

we explore the use of artificial materials as cut indicators to distinguish between safe and dangerous edges. It should be noted that this

concept is not new or unique. At least two independent testing

laboratories and one major manufacturer are known to be using

their own versions of this concept to test the sharpness of edges.

Apparently, however, those test procedures and criteria are based

essentially on subjective judgement rather than experimental measurement and would therefore be unsuitable for regulatory standards. The

test procedures and experimental data presented in this report should

be useful in establishing and supporting such standards.
- 8 -

II. SKIN CUTTING EXPERIMENT

A. Source and Freparation of Skin Specimens

Human skin specimens, ^excised during autopsy, were supplied by

several local morgues and hospitals. In the interest of keeping independent variables to a minimum, it was decided to keep body site

constant throughout the experiment (in section III.B it will be seen

that the variability of the data is quite large without introducing

this additional variable) . Skin from the abdomen was chosen because of

its availability (it was the policy of most autopsy rooms not to excise

large enough specimens from other body sites)

.

In all cases, the specimens were removed before the body was

embalmed, and were kept moist and refrigerated until the cutting tests

were performed. In many cases the specimens were tested within 24 hours

after death, although the average delay was approximately 2 1/2 days.

Most of the specimens were quite small (approximately 1" X 2") and provided space for no more than five or six test cuts. Several, however,

were large enough to accommodate thirty to forty test cuts. General

data on all specimens used in the tests are summarized in Figure 2.

When received for testing, all of the skin specimens still had

most of the sub-dermal tissue intact. Generally, this tissue was mostly

fat, and varied in thickness from one-quarter to three-quarters of an

inch. In preparing the specimens for test, all of this sub-dermal

tissue was carefully removed with a sharp knife. The remaining thickness, consisting of only the dermis and epidermis, was then mounted on

the cutting apparatus.
- 9 -

B. Test Method and Apparatus for Cutting Skin

The basic method used in all of the skin cutting tests is illustrated

schematically in Figure 3. As shown in the illustration, each skin

specimen was mounted directly on a semi-circular, hardwood mandrel, with

the epidermis to the outside. The specimen was secured in position with

several individual loops of fine wire which were wrapped around the

outside of the specinien and tightened by tv/isting the ends of the loop.

The mandrel could be revolved at controlled velocities. With the skin

in motion at a known velocity, a test edge having a specific geometrical

shape, was brought into contact with the skin during a single revolution

of the mandrel. The force exerted by the edge, normal to the surface of

the skin, was controlled by means of a calibrated spring.

To minimize any inertial loading effects between the test edge and

the skin specimen, t^v^o precautions were taken. First, the leading and

trailing edges of the semi-circular mandrel were rounded off so that the

initial contact and final separation between the skin and edge occurred

gradually. Secondly, for each test cut, the release of the edge was

synchronized with the rotary motion of the mandrel so that the edge always

assumed its fon>7ard stop position during the non-cutting part of the

mandrel rotation (flat side of the mandrel facing the edge). The cutting

action then occurred as the curved surface of the mandrel rotated past

the spring loaded edge. In all tests, the forward stop position of the

edge was adjusted so that the edge was capable of cutting slightly into

the mandrel surface, if all other factors permitted. ^ A photograph of the actual apparatus is shown in Figure A. As

shown in the photograph, the hardwood mandrel was held in the chuck of

a small, metal- turning lathe. The drive system for the lathe thus
- 10 -

provided a convenient means of controlling the mandrel speed. Details

of the edge holder and spring assembly can also be seen from the photograph. As shown, the test edge was held in one end of a pivoted lever

arm. An adjustable tension spring, attached to the opposite end of the

lever arm, provided a means for setting the desired test force. This

complete assembly was rigidly mounted to the lathe carriage, and the

manual controls for the carriage provided a convenient means of adjusting

the test edges relative to the mandrel surface, and for locating adjacent

cuts on a given skin specimen. During calibration and operation of the

apparatus, a small set of electrical contacts was used to sense the

motion of the lever arm and to indicate its calibrated position.

No claim is made as to the ability of the loading mechanism, used

in this experiment, to simulate an actual child- toy interaction. In

an actual situation, the reaction time of the body should be an approximate measure of the cutting time of an edge. That is, in a non- impact

Injury situation, one would sense pain and pull away from an edge after

0.27 seconds (a typical reaction time of the body to a pressure stimulus

accompanied by pain, as reported by Woodworth and Schlosberg 11/) , and

the edge would not penetrate further. In the experiment, this time

should be compared to half the total time that a given element of skin

is in contact with the edge; i.e. the time it takes to cut that skin

element to the maximum depth. We have estimated this time to be on the

order of 0.2 seconds, which is the same order of magnitude as the

reaction time. Thus, it may not be entirely invalid to apply the results

of this investigation to injuries associated with other non-impact

loading mechanisms.
- 11 -

C. Test Edges

Throughout the cutting tests, two basic types of test edges were

used. One type was made from 1/8" thick hardened tool steel and had

smooth cutting edges that were ground to specific dimensions. Edges

of this type, having edge angles of from 15" to 105° and edge radii of

from .000" to .008" were used in the tests. The second type of test

edge was made from .059" thick sheet steel and had cutting edges that

were sheared on a good quality, power shear. These edges had an

effective edge angle of 90* but the cutting side of each edge was left

with a rough, irregular burr that is sometimes called a "wire edge" and

is characteristic of most sheared metal edges.

Although a more extensive family of edges was made and used in the

preliminary testing, the final tests were conducted with a selected

group of eleven edges, which are identified and described in more detail

in Figure 5. As indicated, the edges in the final test set were not

selected according to a specific pattern of graduated edge angles or

edge radii. This was not considered necessary, since we were not

attempting to establish safety criteria or testing procedures that are

based strictly on edge geometry. For the purpose of these experiments.

It was considered more important that the test edges have reasonably

typical geometries, and that they range from "sharp" to "dull" in their

ability to cut human skin.

D. Cut Measurement Method

In order to evaluate the results of these experiments, it was

necessary to develop a method for measuring the depth of each test cut,

relative to the thickness of the skin specimen. Optical techniques were

tried but were judged to be unsuitable because of the tim.e required to
- 12 -

make each measurement. A more convenient method was devised using

the apparatus shown in Figure 6.

As shown, the basic measuring device consisted of a mechanical dial

comparator with a special probe tip. The wedge-shaped tip was approximately one-half an inch long and had an edge angle of 15° and an edge

radius of 8 mils. This particular geometry was chosen to facilitate

the insertion of the probe into the test cuts and to minimize the

pressure exerted by the probe during measurements. This tip v/as

attached to the shaft of the dial indicator with its "sharp" edge downward. A piece of 1/2" thick aluminum plate served as a movable specimen block and as a reference surface for the probe. To further minimize

the pressure exerted on the specimen by the probe, the return spring

was removed from the dial indicator mechanism. The lamp shown in the

photograph was used to illuminate the test specimen and facilitate the

positioning of the probe for the measurements.

For each test cut, two measurements were made: Tl, the total

skin thickness immediately adjacent to the cut; and T2, the thickness

of tissue at the bottom of the cut. From these two measurements, the

depth of cut, D, and the percent depth of cut, J)*, were later calculated.

(D = Tl - T2 and D* = D/TI X 100) . This measuring technique was used

because total skin thickness varied from specimen to specimen and within individual specimens as well.

The general procedure used in making the measurements was as

follows. Immediately after cutting, each specimen was placed on the

specimen block, with the epidermis side up, and spread out to its

approximate original size and shape. The probe was then lowered onto
the skin, adjacent to the first cut and allowed to reach equilibrium

before the dial reading (Tl) was recorded. The specimen block was then

repositioned and the probe was lowered into the first cut. Since the

average length of cut was approximately 1 1/4" and the length of the

probe tip was 1/2", it was relatively easy to locate the probe in the

center of the cut where the depth v;as most constant and least affected

by any inertial loading effects. To make certain that the probe was

actually "bottomed" in the cut, the position of the specimen block was

critically adjusted to obtain a minimum reading on the dial indicator.

This reading was then recorded as T2 for that particular cut. The above

procedure was repeated for each cut on the specimen.

In many cases it was found that the duller test edges crushed the

skin rather cutting it. In these instances, the test produced a permanent groove in the specimen but did not cause a break in the epidermis.

When this occurred, the groove was measured in the same way as a cut,

but special note was made that a true cut v;as not produced.
- 14 -

III. RESULTS AND APPLICATION OF EXPERIMENT

A. Presentation and Discussion of Results

A preliminary experiment was run on a particularly large skin

specimen (from a thigh) to determine if there was any appreciable velocity effect. At each of several force and edge combinations, the

tangential velocity was varied in the range of one to ten Inches per

second, and the depth of cut recorded. No appreciable or definite

velocity effect on depth of cut was noted. On the basis of these

observations, all subsequent tests were conducted at a fixed velocity

of one inch per second (the slowest speed was chosen for the ease in

synchronizing the release of the edge)

.

The results of the experimental investigation are shown in Figures

7 to 17. For each edge, the percentage depth of cut, D* (i.e. the depth

of cut non-dimensionalized with respect to the skin thickness adjacent

to the cut) is plotted as a function of the normal force on the edge, -f

The specimen number is recorded beside each data point. Generally, the

lower numbered specimens were cut first.

Qualitatively, the effect of normal force on the percentage depth

of cut is similar for each edge. At some force the edge begins to cut

and the percentage depth of cut increases with force. The rate of

increase of D* appears to decrease with force until the specimen has been

cut all the way through. This observation seems consistent with the

fact that both the density of collagen fibers and the size of individual

fibers increase with depth into the dermis 4^/, i.e. as an edge cuts

increasingly deeper, it becomes increasingly more difficult to cut the

skin.

•tD* was chosen as the dependent variable because it exhibited less variability than the absolute depth of cut, D.
- 15 -

In the appendix, John Mandel describes a technique he used to

extract information about the general shape of the D* vs. F curves.

The nature of the data precluded the use of least squares methods to

fit the data for each edge. Due to the extreme variability in the

data, such a curve would not be very useful in setting safety criteria

anyway. It was found that if a given skin specimen is cut by a given

edge, the data could be fit to equations of the type:

D* = a + Y(l-e~^^)

where 6 is independent of edge and skin specimen. Thus it appears

that this exponential behavior may be related to the physical nature

of the cutting phenomena itself. The coefficients a and y depend upon

both edge and skin specimen. For a given edge, changing the coefficient

serves to raise or lower the entire curve; thus a may be related to

the overall strength of a given skin specimen. The values of the y

coefficient may be related to how the strength of skin specimens vary

with depth. Values for a, 0, and y given in the appendix.

Some of these curves have been plotted in Figures 7 to 17 to

illustrate the general shape of these vs . F curves. Each case is

for a single skin specimen, which is labelled on the curve. Since

specimens 16 and 18 were quite large, it was possible to cut them

through the full force range with many edges. Thus Figures 7, 9, 10

and 13 all contain curves for both specimens. Recall that each curve

was fit to the data for a given specimen and edge. Thus there seems to

be a remarkable consistency (through these figures) in the relative'

shapes of the curves for these two specimens . This observation adds to

the credibility of the mathematical model.
- 16 -

It is very difficult to quantitatively assess how edge angle and

edge radius influence cutting. Generally, the smaller the angle and

radius, the deeper the cut (assuming that all other variables are held

fixed). However, in several instances, the above generalization did

not appear to hold. For example, in Figures 11 and 12 edge E (105°,

no radius) cut deeper than edge F (90", no radius).

Also the two 90° edges, B and F, had drastically different cutting

capabilities (Figures 8 and 12). A profilimeter was used to trace the

geometry of these two edges. It was found that edge F had a significant

edge radius (on the order of .001 inches) in some spots. The photograph

in Figure 18 seems to confirm this point. This evidence indicates that

the microscopic features of an edge may have a very significant effect

on its ability to cut human skin.

Some other qualitative phenomena should be noted. Often, an edge

would leave a permanent measurable groove in the skin, even though the

epidermis was not broken (such data points are indicated by a filled

in circle in Figures 7 to 17). This was more pronounced with the duller

edges. Thus we must suspect that our measurements are a combination of

cutting and crushing of tissue, though we expect that for the sharper

edges, the amount of crush is negligible. For some of the deeper cuts,

it was noticed that the dermis was completely penetrated in spots,

although values of D* = 100% were not measured. This phenomenon occurred

at about the level D* = 80% and is attributed to the hill and valley

structure of the bottom of the dermis (see Figure 1)

.
- 17 -

B. Some Limitations of the Experiment

Perhaps the most obvious difficulty in evaluating the experimental

evidence is the extreme variability in the results from cutting specimens

from different bodies. In order to estimate this variability, measurements were taken on many specimens at 8 lbs. on edge E (Figure 11), and

at 4 and 8 lbs. on edge C (Figure 9). While a clear indication of the

distribution is not available in any of these cases, estimates for the

standard deviations of these points are 1A.6, 10.7, and 14. 1 percent,

respectively. In order to estimate how much of this variability is

attributed to experimental error, repeated cuts were made on the same

specimen with edge and force held constant. This procedure was in turn

repeated on other specimens and for other edges and forces. It was

thus determined that experimental errors are no greater than 5%.

A closer scrutiny of the data at 8 lbs. for edge E reveals another

difficulty. These values were plotted in the order in which the specimens were cut; i.e. as D* vs. N, where N - 1 (the first specimen cut), 2

(the second specimen cut), etc. When a straight line was fitted to

these data by the method of l^ast squares, the slope of the line was

negative and significantly different from zero (using a 5% level of

significance) . This finding lends support to the conclusion that the

edge is dulling from continued use.

This observation points out an inherent difficulty with any cutting

experiment; namely, that the cutting process is likely to dull, and

thus microscopically change the surface geometry of an edge. One method
- 18 -

of avoiding edge dulling might be the use of many edges all ground to

the same specifications. However, to obtain reproducible results, these

edges would have to be macroscopically and microscopically identical.

There are some reasons to suspect that in an actual child-toy

Interaction, a given edge would not cut as deeply as it did in the

experiment. In the experiment, the specimen was mounted on a hardwood

backing with all subdermal tissue removed. Presumably this is a worstcase situation since a softer backing would distribute the force and

increase the resistance to cutting. (However, there are several areas

of the body where skin is backed by bone with little subdermal tissue -

knees, shins, knuckles, forehead, etc.) Also there is some evidence

that live skin is more difficult to cut than the excised autopsy specimens used in the experiment: in preliminary testing, at forces for which

an edge clearly cut the excised specimens, the epidermis of the authors'

skin was not broken. Also, in a penetration experiment Deck 12 /

tested the same skin specimen in vivo and in vitro and found that considerably less force was required to penetrate the specimen after it

was excised. Thus in the absence of any other influences, the use of

the experimental results in supporting safety criteria would be conservative.

However, there are several factors which might lead one to draw

opposite conclusions. For example, the evidence that the edges are

dulling means that an edge may not be cutting as deeply as it would in

the absence of such effects. Another non-conservative factor is that

our contact time estimate (section II. B) was made for a cut of maximum

penetration (D* = 100%). This time estimate would decrease with D*.
- 19 -

Recall that this time was compared with what was assumed to be the

cutting time in an actual child-toy interaction; i.e. the reaction

time of the body. Since many cuts were less than 100%, many contact

times in the experiment were less than this reaction time. Thus in

many instances the edges used in the experiment were not in contact

with the skin as long as they would be in a real situation; and presumably, they would not cut as deeply.

It is, of course, very difficult to quantitatively assess how the

above factors would affect a safety criterion based on the experimental

data. To some extent, the conservative factors will be balanced by the

non-conservative ones, though at this time the net effect cannot be

deduced. In addition, two other factors should be expected to influence

the applicability of the experiment: 1) the difference in resistance

to cutting between abdomen and other more vulnerable body sites,

and 2) the effect of age - the average age of the specimens used was 55

years, much greater than the typical child. Hov/ever, insufficient data

are available to ascertain whether these effects are conservative or nonconservative. (McGuire and Moore Ij summarize some investigations which

bear on this question.)

C. Discriminating Between Safe and Hazardous Edges

For convenience, the data in Figures 7 to 17 has been replotted as

D* vs. edge for a given force. This is shown in Figures 19 to 23 for

the forces A, 8, 12, 16 and 20 lbs., respectively. The position of the

edges on the abcissa has been arbitarily selected, though, generally

they are in the order of decreasing sharpness.
- 20 -

Horizontal lines have been drawn at three representative values

of D*: 80%, 50% and 30%. The D* = 80% level is where serious injuries

may begin to occur (since this is where holes were observed in the bottom

of the dermis) . The D"-^ = 50% level is representative of an injury

which damages the reticular layer of the dermis. The D''- = 30% level

was chosen as an example of a cut which has just entered the reticular

layer. In addition, another injury level representing no break in the

epidermis is discernible in these figures (filled- in circles). We call

this later injury level the "no blood" level, since bleeding does not

occur until the epidermis is totally penetrated. These injury levels

(no blood, 30%, 50%, and 80%) are examples of injury levels 1, 2, 3 and

A, respectively, as described by Mahajan 13/

.

The following convention will be used in determining if an edge

is safe or dangerous at a given force level and for a chosen D* to be

tolerated: if none of the specimens tested for that edge cut as deeply

as the prescribed injury level, the edge is designated "safe"; if only

one specimen cut that deep, we call the edge "possibly safe"; more than

one - "hazardous." Of course the limited number of specimens tested

and the extreme variability in the data will make the labelling of edges

somewhat suspect. It is for this reason that for an edge to be called

safe, no specimens were allowed to cut beyond the prescribed injury

level (as opposed to allowing a certain percentage of weak specimens to

exceed the injury level)

.

In Figure 24a, the safe and possibly safe edges are summarized

for the four injury levels described, and for the five force levels

presented. The Roman numerals in each box refer to discrimination points
between edges, which are suianiarized in Figure 24b. For each of the

discrimination points illustrated, those edges above the line are

hazardous. The discrimination points used in the boxes of Figure 2Aa

were chosen such as to minimize the total number of discriminations

that need to be made. Thus a possibly safe edge may be left in or

taken out for convenience.

As an example, discrimination point II, for v/hich only edges J

and K are considered non-hazardous, will be appropriate for several

safety criteria (defined in section I.C): (8 lb., no blood),

(12 lb, 30%), (16 lb, 50%); see Figure 24a. Thus it is possible to

satisfy multiple safety criteria (as would be needed if the tolerable

injury were chosen to depend on the force level) with a single discrimination. Therefore, the three safety criteria satisfied by

discrimination point II may correspond to levels of normal use, reasonably forseeable abuse, and abnormal abuse.
- 22 -

IV. DISCRIMINATION TECHNIQUES USING SYNTHETIC MATERIALS

A. Inspection Procedure for Unknown Edges

Once a safety criterion has been proposed, it remains to devise a

procedure which will discriminate between safe and dangerous edges on

toys. Unfortunately a simple requirement on edge angle and edge

radius is not adequate; as seen before, microscopic irregularities may

affect an edge's cutting capability. Thus it was decided to seek out a

synthetic material which at some given force would be cut completely

though by only those edges, in our family, deemed hazardous by the safety

criterion. Hopefully, at the same force, those edges deemed non-hazardous

would not completely penetrate the material. The idea is that a field

inspector would place a strip of the material on some constant force

device, run it across an edge associated with a toy, and decide if the

edge is safe or hazardous (depending upon whether or not the material

was cut all the way througli).

For a given safety criterion, we can order our family of edges

from sharp to dull (depending on their ability to cut skin) . We can

also perform a sharp- to-dull ordering of the edges for each synthetic

material (depending on the force required to cut through the material)

.

An ideal test material would be one for which these two orders are

identical. Even so, we can never be sure that an untested edge will

occupy the same position, relative to the members of our family, for

both skin and the synthetic. However, drastic shifts in position are

not expected. Thus, while there always exists the possibility of

wrongly classifying a given edge, the choice of a more conservative

safety criterion should serve to overcome such problems.
- 23 -

B. Testing of Synthetic Materials

For testing the synthetic materials, the apparatus shown in

Figure 25 was used. As shown in the photograph, the force system and

apparatus for holding the test edges was the same as that used in the

skin experiments. The arrangement for holding the test specimens was

modified, however, in order to obtain linear rather than rotary motion

of the specimen, relative to the test edge. This change was made in

order to simulate a proposed field test procedure wherein the test

material would be mounted on a simple, hand-held device and rubbed

along the unknown edge to test its sharpness.

To obtain the required linear motion, the lathe chuck was removed,

and an old, but functional, microtome was mounted on the lathe ways.

The assembly which normally holds the microtome blade was removed, and

a special aluminum mandrel was made and attached to the microtome head.

With this arrangement, one turn of the microtome handle caused the mandrel to reciprocate over a vertical distance of approximately 2 inches.

The procedure used in testing each material was as follows: A

sample of each material was first wrapped tightly around the mandrel

and secured with an adhesive tape. Generally, only a single wrap of the

material was made, directly on the metal mandrel with no intermediate

backing. With the test sample in place, a series of test cuts were

then made to determine the force required for each test edge to cut

completely through the sample material. Prior to each cut, the tension

spring was adjusted to the desired force setting, and the forward stop
- 24 -

position of the test edge was adjusted, relative to the mandrel surface.

The cutting edge was then positioned on the test sample and allowed to

bear against the material with the pre-set force. The microtome handwheel was then turned one complete revolution, causing the test sample

to rub back and forth (down and up) along a two-inch length of the

cutting edge. For some particularly weak materials, only an up or a

dov/n stroke was used (single stroke) . If the material sample was cut

completely through by this action, an electrical circuit was completed

between the edge and the mandrel, lighting a small indicator lamp.

For each test cut, the only data recorded were simple "yes" or

"no" observations as to x>/hether or not total penetration occurred.

For a given test edge, cuts were repeated at various force levels until

the force required to produce total penetration of the particular

material was clearly determined.

In our search for useful cut indicating materials, more than forty

different synthetics have been tested. Approximately half of these

materials were commercially manufactured products, and the remainder were

special formulations made in our own laboratory. Among the commercial

materials tested were various types of paper, PVC foams and sheets , polyethylene foams and sheets, neoprene foams with and v/ithout solid

exterior films, poly tetraf luorethylene (TFE) sheets, and butyl rubber.

The special formulations that were tested included several types of

plastisol sheets, silicone gel sheets, silicone gels reinforced with

rayon fibers, silicone rubber foams, vinyl resin and paraffin films, and

various silicone impregnated papers. In many cases, different thicknesses

of a given material were also tested.
- 25 -

Many of the materials proved to be totally unsuitable as cut

indicators either because the forces required to cut completely through

the material were unreasonably high, or because the difference between

partial and total cuts was not visually distinctive (this difference

could not be appraised electrically for non-metallic edges)

.

C. Results of Tests

Some representative synthetic materials are described in Figure 26.

The results of tests on these materials are shown in Figures 27 to 29.

For any given material and edge, two points were usually plotted. The

upper point (open circle) is a force at which the material was completely

penetrated and at v;hich all higher forces would also cause complete

penetration. The lower point (closed circle) is a force at which penetration V7as not complete; below this force, the material would not be

completely penetrated by the edges. At forces between these two

points both "yes" and "no" (see preceding section) readings were

recorded. That these two points arc not identical is attributed to

variablities in the material and to experimental errors in setting

forces. This latter source of variability should be nearly constant

from one material to the next. Thus the distance between the upper and

lower readings may be interpreted as a relative measure of inhomogeneity

of the synthetic materials. In a given material, we expect that the

variability for any edge is probably as large as for the edge for which

the variability was greatest. The fact that the upper and lower points

were occasionally recorded quite close to one another is considered

fortuitious since only a limited number of cuts were made.

The edges used in the tests on synthetics were the same as in the

skin cutting experiment. Thus, we should expect all of the edges to

be duller (see section III.B) than in the previous investigation. This
- 26 -

means that, for all edges, forces required to completely penetrate the

material will be greater than if the edges had retained their initial

sharpness. Thus the force setting at which the material is used by an

inspector will be greater, and a given edge would be more likely

to completely penetrate the material. So, in this case, the dulling

of edges is a conservative feature.

Recall that our interest is in finding a material which will make

the discriminations of Figure 2Ab, i.e. the material would be cut through

by only those edges above the discrimination point. For a single material

to accomplish this goal, it must order our family of edges (from sharp

to dull) in, generally, the same way that they were 'ordered by skin.

This means that if we view the pairs of points (in Figures 27 to 29) as

being a "function" of the edges, it would be desirable to have this

"function" monotonically increasing. For all of the synthetic materials

In Figures 27 to 29, the relative order of sharpness of the edges is

quite different than for skin. E.g. on most materials, edge E is duller

than edges G and H, while for skin the opposite is true. This difficulty,

in using relatively uniform, synthetic materials to order our edges as

skin did, is probably due to the complex structure of human skin.

Although none of the materials tested can make all of the

discriminations of Figure 24b, some can make one or two. In Figures 27

to 29, a material is said to be capable of making a discrimination if

the open circles, for all edges above the discrimination point, are no

higher than the filled-in circles for all edges below the discrimination

point. This is clearly illustrated in Figure 27 for the SM-2 material.
- 27 -

At 5.5 lbs. (dashed line) only edges J and K do not completely penetrate

the material. This is discrimination II of Figure 2Ab . Other materials

capable of making this same discrimination are SM-5 and SM-8 (Figure 28)

at 2.5 and 3.8 lbs., respectively. Many materials (SM-2 , SM-3, SM-4 , SM-5,

SM-6, SM-7, SM-8, and SM-11) can make discrimination VI. Some materials

come quite close to m.aking other discriminations; except for edge B,

both SM-A and SM-8 can make discrimination V; also Sl-i-1 and SM-7 are

within one edge of making discrimination I. We point out these near

misses because slight variations of these materials might be satisfactory,

and this may be a starting point for future work.

One other property of these synthetic materials should be noted.

Increasing or decreasing the thickness of a given material causes the

"function" of the edges to shift up or down, while generally retaining

the same shape ("relative maxima" and "relative minima" remain fixed).

I.e. the order of the edges, for each material, remains fixed. This is

readily seen for SM-2 and SM-3 in Figure 27 and for SM-9 and SM-10 in

Figure 29. Also a change in thickness does not seem to affect a

material's ability to make a given discrimination. Thus SM-3 can also

make discriminations II and VI as did SM-2.
- 28 -

V. CONCLUDING REMARKS

A. Summary and Conclusions

Non- impact cutting tests were conducted on rigidly supported

specimens of human skin in vitro. Various forces, velocities, and

edge geometries were tested and the depths of the resulting cuts were

measured. Also a variety of synthetic materials were evaluated for

possible use in edge inspection procedures. Several of the more

important points of this study are summarized in the following statements

1. Although the loading mechanism used in the skin cutting

experiment may not simulate a variety of real-life injury

situations, it is believed that the results of the experiment may be applicable to many non-impact injury situations

associated with casual play (see section II. B).

2. Over a range of one to ten inches per second, relative

velocity between the edge and skin did not appear to have

any significant effect on the depth of the resulting cut

(see section III. A).

3. Generally, edges having smaller edge angles and radii

produce the deeper cuts. However, the experimental evidence

indicates that the microscopic features of an edge may have

a very significant effect on its ability to cut skin. Thus

it appears that the sharpness of an edge cannot be defined

quantitatively in terms of gross features such as edge angle

and radius (see section III. A).
- 29 -

4. Empirical curves fitted to the data indicate that the

effect of normal force on the depth of cut is similar

for all edges. The depth of cut increases with force

but the rate of increase appears to diminish exponentially,

(see section III. A and appendix).

5. The wide variations in the experimental data are attributed

primarily to natural differences in the skin specimens (see

section III . B)

.

6. Based on the skin cutting data, useful methods for discriminating between hazardous and non-hazardous edges can be devised,

using permissible level of injury (depth of cut) and force to

define safety criteria (see section IIl.C).

7. Since edge sharpness cannot be defined adequately in terms of

easily measured features, inspection procedures using synthetic materials as cut indicators were considered (see section

IV. A)

.

8. Many synthetic materials were tested but none were found with

the capability of making all of the desired edge discriminations. Some of the materials tested appear to be capable of

making some of the discriminations with reasonable reliability

(see section IV. C).

B. Recommendations for Future Study

In order to establish a safety regulation, which would protect

children against dangerous edges associated with toys used under

casual handling conditions, it is essential that a safety criterion

(section I.C) be chosen. That is, a tolerable injury level (which.
- 30 -

in this report, is measured by depth of cut), and a force level,

representative of children's capabilities in play situations, must be

determined. This latter specification may require studies to determine typical forces that children exert in casual play.

Once a safety criterion is chosen, the next phase would proceed

in a similar manner to the present work. First, more skin cutting

experiments would be conducted in relatively narrow ranges surrounding

the force and injury levels of the safety criterion. Hopefully the

experience gained in the present work would allov; us to specify new

edges with geometries that produce injuries in the correct range.

These edges would be made to stricter tolerances so that "identical"

edges could be obtained - thus the edge dulling problem discussed

in section III.B would be minimized. Then our experience in cutting

synthetic materials should expedite the development of a final inspection procedure.

Recall that the present work was concerned only with injuries

suffered in the casual handling mode. A natural extension of this work

would be to investigate more general loading mechanisms - specifically

impact loading. An apparatus for conducting such studies has been

built in the Consumer Product Systems Section, and some preliminary

tests have already been made. It is expected that many edges, considered safe under a casual handling regulation, would be judged

hazardous under impact conditions.

Because the impact loading mechanism is more general than the

casual handling mode, it may be tempting to delay a "casual handling

regulation" until impact studies are completed. However, in the
- 31 -

relatively long time it would take to conduct such an investigation,

many potential injuries, suffered under casual handling conditions,

could be prevented. Besides, one might not exercise the same degree

of caution under casual handling conditions as he would when impact

conditions are likely. In this sense, the casual handling mode may

deserve special attention in formulating regulations.

Several other studies, which should logically follow or support

the present work, could be suggested. First, this work has pointed

out that insufficient information is available on the nature of the

cutting mechanism. Thus some fundamental investigation of the basic

cutting phenomena, using materials whose properties are well known,

would be very useful. Secondly, there is a need to test the validity

of using in vitro studies to approximate an in vivo situation, as

regards cutting. In vivo testing of animals should prove to be a

valuable tool in obtaining this information. Lastly, arrangements

should be made to conduct future studies in conjunction with facilities

having access to cadavers or large autopsy specimens. The availability

of abundant skin samples would not only reduce the time of the experiment, but would also allow the collection of sufficient data for

statistical analyses.

EPIDERMIS

0.05 to 0.4mm

DERMIS

0.4 to 3.0mm

'

PAPILLARY

LAYER

RETICULAR

LAYER

SUBDERMIS

FIGURE I. SCHEMATIC CROSS SECTION OF HUMAN SKIN
FIGURE 2. SKIN SPECIMEN DATA

Specimen Average Thickness of

Number Age Sex Epidermis and Dermis Test D

1 TTITn 1

1 41 F 1.9 1

3 64 M 1.4 1

4 23 M 1.4 8

5 38 M 1.5 6

6 57 M 1.2 2

7 63 F 1.0 3

8 45 M 0.9 1

9 44 F 1.9 7 10 52 F 2.1 3

11 53 F

" 1.3 1

12 66 M 1.8 1

13 51 M 1.1 1

14 86 M 1.2 2

15 47 M 1.3 2

16 59 M 1.3 3

18 69 F 1.2 8

19 75 M 1.9 2

20 54 M 1.5 1

21 45 M 1.1 1

22 50 F 1.2 1

23 51 M 1.7 3

24 58 F 0.8 1

26 , 43 M 1.7 2

27 41 F 1.4 1

28 20 M 2.2 2

29 48 M 0.9 2

30 51 F 1.6 2

31 1.6

32 62 F 1.1 5

33 M 1.1 4
TOP VIEW

!IZZ>

1 1

1 1

1

1

1

1

1 1

1 1

1 1

FORCE, I to 20

Lbs

END VIEW

VELOCITY, rto lOVSec

HARDWOOD

MANDREL

SKIN SPECIMEN

TEST EDGE

FIGURE 3. SCHEMATIC ILLUSTRATION OF

SKIN CUTTING EXPERIMENT

EDGE ANGLE (a) RADIUS TYPE

A 60* .000" GROUND

B 90* .000' GROUND

C — — SHEARED

D — — SHEARED

E 105* .000' GROUND

F 90* .000" GROUND

6 I5» .002'(.05 mm) GROUND

H 30* .002" GROUND

Z 60* .002" GROUND

J 15* .004'(.l0mm) GROUND

K 90* .002' GROUND

j— .059" (1.5 mm)

TYPICAL SHEARED EDGE

— j-— l/e"(3.2 mm)

TYPICAL GROUND EDGE

FIGURE 5. DESCRIPTION OF TEST EDGES

FIGURE 7. HUMAN SKIN CUT DATA FOR EDGE A
POUNDS 12 4 8 12 16 20

NEWTONS 4.4 8.9 17.8 35.6 53.4 71.2 B9.0

FORCE, F

FIGURE 8. HUMAN SKIN CUT DATA FOR EDGE B
NEWTONS 4.4 8.9 17.8 35.6 53.4 71.2 89.0

FORCE. F

FIGURE 9. HUMAN SKIN CUT DATA FOR EDGE 0
POUNDS 12 4 8 12 16 20

NEWTONS 4.4 8.9 17.8 35.6 53.4 71.2 89.0

FORCE, F

FIGURE 10. HUMAN SKIN CUT DATA FOR EDGE D
FIGURE II, HUMAN SKIN CUT DATA FOR EDGE E
100

7

FIGURE 12. HUMAN SKIN CUT DATA FOR EDGE F
100

90

60

70

60

50

40

30

20

10

POUNDS I 2

NEWTONS 4.4 8.9

O-TRUE CUT • -GROOVE-NO BREAK IN EPIDERMIS

NUMBERS IDENTIFY SPECIMENS

CURVES FITTED TO DATA FOR GIVEN SPECIMEN

SPECIMEN 18

4

17.8

8

35.6

12 53.4

16

71.2

20

39

FORCE. F

FIGURE 13. HUMAN SKIN CUT DATA FOR EDGE G
100

90

70

60

50

30

20

10

O-TRUE CUT

©-GROOVE- NO BREAK IN EPIDERMIS

NUMBERS IDENTIFY SPECIMENS

CURVES FITTED TO DATA FOR GIVEN SPECIMEN

POUNDS I 2 4

NEWTONS 4.4 8.9 17.8 35.6

12

53.4

16

71.2

2C

89

FORCE, F

FIGURE 14. HUMAN SKIN CUT DATA FOR EDGE H
100

90 —

O-TRUE CUT

80 —

O-6R00VE-N0 BREAK IN EPIDERMIS

NUMBERS IDENTIFY SPECIMENS

* 70 —

O CURVES FITTED TO DATA FOR GIVEN SPECIMEN

INEWTONS 4.4 8.9 17.8 35.6 53.4 71.2 89.0

FORCE, F

FIGURE 15. HUMAN SKIN CUT DATA FOR EDGE I
100

90

80

70

60

50

40

30

20

SO

0

O-TRUE CUT

®-6R00VE~N0 BREAK IN EPIDERMIS

NUMBERS IDENTIFY SPECIMENS

CURVE FITTED TO DATA FOR GIVEN SPECIMEN

POUNDS I 2

NEWTONS 4.4 8.9

• 32

17.8

8

35.6

12

53.4

16

71.2

O

33

20

89.0

FORGE, F

FIGURE 16. HUMAN SKIN CUT DATA FOR EDGE J
100

90

80

70

60

50

40

30

20

10 12

10

O-TRUE CUT • -GROOVE- NO BREAK IN EPIDERMIS

NUMBERS IDENTIFY SPECI.MENS

•7

POUNDS I 2 4

NEWTONS 4.4 8.9 17.8

lie

07

• 9

Ol5

16

8 12

36.6 53.4

FORCE. F

Oie • 16

•10

07

• 15

16

71.2

II

I!«

c

I

2(

89

FIGURE 17. HUMAN SKIN CUT DATA FOR EDGE K

s

i

eeuo

elu

<

a

eo

3

U

Iu

> u o 3 O

ec ce H o

I I O •

• >

o o o •

O «D — U. Uio

o

O • o» — bJ

o oo

0006

000

a> o

o

o

o o

CO o o o

10 o o

10 o

M
3

lit e

e

kJa

<«

UJK

O

o

z

1 >o

o

e©

I O ®

o o

CD O O

8

O CDO O §I3> O O CXD O CO

• • —

CO o o H

o o

9 03*

O O

O O 06 o — u. lUo

o

bJ

o o

o

o oo o

o

o

o

0> o

CO o o(0 to o o o o

^ lO a —

'mo dO Hid3Q iN30a3d
«a 'ino do Hid3a iN39u3d
,Q *inO JO Hld3a lN30U3d
'ino JO Hidaa iNaonad
F

4 8 12 16 20

807© in 3z: Xd

50% 3z: xsc HE m X

307® 3E u T

NO

BLOOD UL XL

(0)

EDGE DiSCRIMINATION POINT

A

(b)

X

FIGURE 24. EDGE DISCRIMINATIONS

FOR POSSIBLE SAFETY CRITERIA

FIGURE 26. SOME REPRESENTATIVE SYNrriETIC MTERIALS TESTED

Material General Descr i ption Thickness

(mm)

SM-1 Silicone gel "A" with 1% rayon fibers added 1.78

SM-2 Silicone gel "B" 2.16

SM-3 Silicone gel "B" 0.94

SM-A Paper //I, coated with ^el "A" 0.30

SM-5 A/1 mixture of gels "B" and "A" 0.81

CM (- iri-HSClSOX ri-Lm 7/X 1 17 J. . X/

SM-7 PVC foam* 11 6.35

SM-8 PVC foam * 1.57

SM-9 TFE sheet 0.36

SM-10 TFE sheet 0.25

SM-11 PVC sheet 0.56

*Comiricrcially manufactured materials

\J Single stroke test method
u

o

q:o

26.7 6

22.2 5

17.8 4

13.3 3

8.9 2

4.4 1

40.0 9

35.6 8

31. 1 7

26.7 6

22.2 5

17.8 4

13.3 3

8.9 2

4.4 1

17.8 4

13.3 3

8.9 2

4.4 1 0 0

w <n z o

WTO z

O kJZ

0.

—

SM-I

O o

0

O o • 0

0

• o o

O

ft • —

•

SM-2 — o — o • • —

0

y

O

• o

• o

o

•o

o •

o

o o .

—

SM-3 o

o

Q o o o

O e oo o e

o

• • — o

1

•

I 1 1 1 1 1 1 I 1 1 A 8 0 D E F 6 H I K

EDGE

FIGURE 27. RESULTS OF CUTTING TESTS ON SOME SYNTHETICS.

O- FORCE ABOVE WHICH MATERIAL WAS ALWAYS CUT THROUGH

O- FORCE BELOW WHICH MATERIAL WAS NEVER CUT THROUGH
FIGURE 28. RESULTS OF CUTTING TESTS ON SOME SYNTHETICS.

0~ FORCE ABOVE WHICH MATERIAL V/AS ALV/AYS CUT THROUGH

©-FORCE BELOW WHICH MATERIAL WAS NEVER CUT THROUGH
TI.2 16 — O o

66.7 15 — SM-9

62.3 14 — • o o •

57 8 13 —

53.4 12 — O • O o

48.9 M

44.5 10 — o o 0 •

40.0

35.6

9

8

——

O

o e o O

31.1 7

26.7 6 — e • 0

40.0

35.6

9

8 —

CM— win in1 \J o

• o

31.1 7 o o o

26.7 6 o o • o • e

22.2 5 0 O 9 o o •

17.8 4 o • •

31.1

26.7

7

6

——

SM— Will II1 1 o

o

•

o o

o

o

•

22.2 5 • o

17.8 4 Q • o

13.3 3

o •

8.9 2 •

4.4 1

1

i 1 1 1

1 1 1 1 1 1

NEWTONS POUNDS

A B C D E F G

EDGE

H I J K

FIGURE 29. RESULTS OF CUTTING TESTS ON SOME SYNTHETICS.

O- FORCE ABOVE WHICH MATERIAL WAS ALWAYS CUT THROUGH • -FORCE BELOW WHICH MATERIAL WAS NEVER CUT THROUGH

Appendix. Fitting Empirical Curves to the Relationship

Between Force and Percentage Depth of Cut

An empirical family of curves was fitted to the data, using the

following procedure:

1. From the total data of percentage depth of cut at the

various force levels, two tables were extracted: the first

covering all cases in which data were obtained at the

forces: 1, 2, A, 8 and 12 pounds; the second covering all

cases in wliich data were obtained at the forces A, 8, 12, 16

and 20 pounds. These are referred to as Tables A and B

(which follov/ at the end of this section) . All of the

follov/ing steps were carried out separately on Tables A and B.

2. The column-averages of each table, denoted by the symbol D-^,

were fitted against the force values, using an equation of

the type 14 /

A

A

D* = K - Me

-6F

(1)

A

Where D'^ represents the column- average, and F represents

the corresponding force. It was noted that for both tables,

the parameters K and M were practically identical. Hence

the final equation fitted was

D* = K(l - e"^^) (2)

The values of K and B resulting from these fits were:

Table K 3(lbs '*')

A 101.6 .158

B 76.2 .080

Both fits were very good, as can be seen in Figure Al.
3. For each table, and for each edge/specimen combination,

the D* values were then fitted, by means of straight lines,

A

to the corresponding D* values, using the method of least

^ squares. This yielded, for a given edge and specimen, an

equation of the form:

A

D* = a + 5D* (3)

4. Combining equations (2) and (3), we obtain

' D* = a + 6K(1 - e"^^)

V or: D* = a + y(1 - e

^

)

Such an equation was obtained for each edge/specimen

combination in Tables A and B. The values of a and y

are also given in these tables.

Discussion

The above procedure is, of course, purely empirical. Equation (2)

is merely an intermediate step, allowing us to fit the exponential

type curves with far better precision than if the fits had been

carried out directly in terms of D* vs . F for each edge/specimen combination.

r No claim of theoretical validity is made for the empirical fits,

therefore no attempt should be made to draw conclusions from the

numerical values of the parameters a, S, and y Nevertheless, the

empirical fits can be considered as useful summarizations of the

data and are believed to give reasonably accurate representations

of the type of dependence of percentage depth of cut on force.
TABLE A

D* Values for the Force (lbs.) Coefficients

Edge Specimen 1 2 4 8_ 12 a y_

A 18 23 43 69 96 100 12.0 111

A 16 22 52 63 87 100 15.5 100

B 18 7 22 66 87 100 -9.0 134

C 14 18 39 56 95 100 3.7 118

C 18 10 27 45 73 91 -5.1 111

C 16 20 14 41 62 69 2.3 79

C 30 16 22 34 60 75 0.3 84

C 29 28 44 58 84 100 14.8 99

D 16 0 26 36 63 73 -7.9 98

D 18 2 29 42 70 88 -9.0 113

D 29 11 33 53 89 100 -4.1 126

F 18 11 7 14 32 65 -8.9 71

G 18 13 5 35 51 60 -4.4 76

Averages, D*: 13.9 27.9 47.1 73.0 86.2
TABLE B

Edge Specimen

D*

4

Values

8

for the

12

Force (lbs

16

.)

20

Coefficients

a Y

C 18 45 73 91 93 100 22.9 103

D 16 36 63 73 85 100 7.5 114

E 18 20 66 85 90 100 -9.8 147

F 18 14 32 65 80 71 -20.0 129

G 16 19 24 38 40 40 6.6 47

G 20 28 28 39 40 44 -17.3 46

G 18 35 51 60 77 81 12.75 91

H 23 12 22 34 37 54 -9.1 73

H 27 13 30 44 51 65 -12.1 95

H 28 13 23 35 49 63 -15.5 93

H 26 19 24 38 45 44 3.1 56

I 27 2 22 33 55 52 -25.2 104

I 28 9 15 29 36 42 -10.6 66

I 26 26 27 30 41 46 12.4 39

I 30 20 27 39 48 48 3.1 60

I 29 12 30 23 39 27 7.4 34

A

Averages, D*: 20.3 35.0 47.2 56.6 61.0
FIGURE Al. EMPIRICAL FITS OF AVERAGES (SYMBOL, O) IN TABLES A AND B

ACKNOWLEDGMENT

The authors wish to express their appreciation to

Dr. J. Mandel for his valuable assistance in the

empirical analysis of the data and in preparing

the Appendix to this report.

REFERENCES

1. Moore, J.D., "A Method for Evaluating and Testing the Sharpness

of Points," National Bureau of Standards Report No. 10 673 (1972)

2. McGuire, B.J. and Moore, J.D.: "The Skin Puncture Potential of

Points Associated with Certain Toys," National Bureau of Standards

Report No. 10 839 (April 1972)

3. McGuire, B.J., Sorrells, J.R. and Moore, J.D.: "Resistance of

Human Skin to Puncture and Laceration," National Bureau of Standards

Report NBSIR 73-123 (Feb. 1973)

4. Deck, J.D.: "Skin Histology and the Effects of Injury," unpublished

data, personal communication, University of Virginia School of

Medicine (1972)

5. Whitton, J.T.: "New Values for Epidermal Thickness and Their

Importance," Health Physics, 2A, 1-8 (1973)

6. Southwood, V.F.W.: "The Thickness of the Skin," Plastic and

Reconstructive Surgery, 15, 423-29 (1955)

7. Gadd, C.W., Madeira, R.G., Schneider, D.C., and Nahum, A.M.:

"Tolerance and Properties of Superficial Soft Tissues in Situ,"

Vehicle Research Department, General Motors Corporation (Sept.

1970)

8. Rieser, R.G. and Chabal, J.: "Safety Performance of Laminated

Glass Configurations," Proceedings of the Eleventh Stapp Car

Crash Conference, New York: Society of Automotive Engineers, Inc.

(1967)

9. Krogman, W.M. and McCown, T. : "The Manual and Oral Strengths of

American V/hite and Negro Children, Age 3-5 Years, " Philadelphia

Center for Research in Child Growth (1971)

10. Brown, W.C., Buchanan, C.J. and Mandel, J.: "A Study of the

Strength of Children in the Two Through Six Year Old Age Groups,"

National Bureau of Standards Report NBSIR 73-156 (1973)

11. Woodward, R.S., and Schlosberg, H.: "Experimental Psychology,"

Henry Holt and Co., New York (1955)

12. Deck, J.D.: personal conmunication. University of Virginia School

of Medicine (1972)

13. Mahajan, B.M.: "Systematic Analysis of Mechanical Hazards from Toys,"

National Bureau of Standards Report NBSIR 73-137 (May 1973)

14. Mandel, J.: "The Statistical Analysis of Experimental Data,"

Interscience Wiley, New York (1964)

an

,,| 'r-i ] OF ^ o;-i M. UOGRAPMIC DATA

!-' U i5 !- Iv. , !'.\ .

'

.. . 1 .j .' (. V C C NBSIK 7 3-262

_J,H_E_t:T

Some Cutting Experiments on Human Skin

and Synthetic ^^aterials

7. AU'l HOlUS)

John R. Sorrc.lls; Robert E. Bcr^^er

9. PERFORMING ORGAXIZAl'ION' NAMh AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTNfENT OF CONiSiEKCE

Vi'ASHINGTON, D.C. 2023 4

12. Sponsoring Organization Name acid Address Consumer Product Safety Comnission

Bethesda, Md . 20016

3. Recipient's Accession No.

5. Fublitntioii Djio

October 1973

6. Pi-t foniiing Or/>iii)2ation Code

6, Performing Ofi;afiization

10. Projcc t/Tfirk/ ftork Unit No. 4460141

11. Contract/Grant No.

13. Type of Report & Period Covered

Interim

Feb. - Oct. 1973

Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-v,'ord or less factu:;! summary of most significant information. If document includes a significant ! bibliography or literature survey, men lion it here.)

j

Human skin specimens were cut in vitro with edges of varying geometry

over a normal force range of 0 to 89 newtons. Generally, at a given force,

the smaller the included angle and tip radius of an edge, the deeper the

resulting cut. For a given edge and skin specimen, empirical curves fitted

to the data indicate that the depth of cut increases with force, and that

this rate of increase diminishes exponentially. Based on the experimental

data, a convention was proposed for labelling the edges as safe or hazardous.

Synthetic materials V7ere sought which, v.^hen applied in a specific manner to

the test edges, would be completely penetrated by only those edges labelled

hazardous. Then such materials could be used in an inspection procedure for

unknown edges. Tests were conducted on many synthetics, but only a few showed

possibilities in this regard.

17. KEY WORDS (Alphabetical order, separated bv semicolons)

Cut; Edges, Hazardous; Experimental Studies; Human Tolerances; Injury; Injury

Thresholds; Inspection of Edges; Laceration; Safety; Simulation; Skin; Skin,

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(THIS REPORT)

68

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