NAT L INST. OF STAND & TECH

mill II 111

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II

iilll NIST

NBSIR 74-428

An Inspection Procedure for

Detecting Hazardous Edges

John R. Sorrells, Robert E. Berger

Measurement Engineering Division

Consumer Product Systems Section

National Bureau of Standards

Washington, D. C. 20234

April 1974

Prepared for Consumer Product Safety Commission

. Maryland 20016

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NBSIR 74-428

AN INSPECTION PROCEDURE FOR

DETECTING HAZARDOUS EDGES

John R. Sorrells, Robert E. Berger

Measurement Engineering Division

Consumer Product Systems Section

National Bureau of Standards

Washington, C. C. 20234

April 1974

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. Frwtenck B. Dem. SecretAfv

NATIONAL BUREAU OF STANDAFIOS. Richwd W Robots. Director

I n t roduc t i on

The establishment of safety criteria and test methods for

categorizing the hazard potential of exposed edges on consumer

products is a complex problem. For simplification, this investigation, as previous ones 2_/&2_/, will be limited to the consideration

of a special type of loading mechanism, which has come to be known

as the "casual handling" injury mode. Under casual handling

+ conditions, there is a normal force between the skin and the edge,

a relative velocity in the direction parallel to the edge, but no

relative velocity in the normal direction (except as when cutting

occurs). in particular, impact or fall-on conditions are not

considered. This casua 1 . hand 1 i ng loading mechanism is studied for

convenience in identifying relevant parameters and in measuring

forces. Tn i s is not to imply that other loading mechanisms are

less dangerous or should not be studied.

In an earlier report 1/, the concept of a safety criterion was

discussed. It was indicated that values for two parameters must be

chosen in order to set criteria for edge safety. These are:

1. A force, F, which is considered representative of the

casual handling of products.

2. An injury level, measured by the percent depth of cut

of the skin, which is considered tolerable.

^By "skin" is meant the two outer layers: epidermis and dermis.

Several guidelines were presented for choosing these values.

Together, they constitute the safety criterion:

S(F, 0->)

The Consumer Product Safety Commission has recently chosen two

separate safety criteria 3_/ , depending on the accessibility of an

edge. It was decided than an "exposed""*" edge will be considered

acceptable if it does not break through the epidermis of human

skin when the norma! force between the edge and the skin is 20 pounds

(89.0 newtons). "No break through the epidermis" corresponds to the

"no bleeding" level of the previous report ]_/ This safety criterion can be expressed as:

S| (20 lb., no epidermal break tnrough)

For edges which may be contacted inadvertently, the Commission

has chosen a less conservative safety criterion: such an edge will

be considered acceptable if it does not completely penetrate human

skin when the normal force between the skin and the edge is k pouncs

(17-8 N). The previous report 1/ pointed out that skin could be

completely penetrated in spots although the measured percent depth

of cut was less than 100^. To be conservative, the complete penetration level was taken to be 80-^, leading to the safety criterion:

S ,

, {k lb. , 80^)

The arbitrariness, in deciding whether or not an edge is likely to

be contacted by a user is intentional. Obviously, it will depend

on the particular product considered - the way It is used and for

whose use it is intended.

Break through here means complete penetration of the epidermis

into the papillary layer of the dermis, which contains capillary

b 1 ood vesse I s .

- 3

-

The results of cutting tests on excised human skin, using a

series of eleven test edges, have also been reported \J . For a

given safety criterion, these edges could be divided into "safe"

and "hazardous" categories. For criterion Sj|, only the sharpest

edge of the test family was considered to be hazardous. This is

shown clearly in Figure I (which is Figure 19 in the previous

report). For a given edge, each symbol represents a cut on a

separate skin specimen. The filled-in circles indicate that a

measurable groove remained in the skin, although the epidermis was

not broken. The geometry of the edges is summarized in Figure 2.

For criterion Sj, all of the test edges were considered to be

hazardous. I.e. at 20 pounds (89-0 N) , breaks in the epidermis

were found when even the dullest edges were used.

Although the above safety criteria had not been chosen at the

time, methods of discriminating between safe and hazardous edges

were investigated 1/. It was found that simple definitions of

edge geometry were not adequate because microscopic irregularities

could affect an edge's cutting capability. Thus, synthetic materials were sought which, at some fixed force, would be cut completely

through by only those edges deemed hazardous by a particular safety

criterion. Initially, it was hoped that a single material could be

found that would classify the test edges in the same order of

sharpness as human skin. Then this material could be used as a

cut indicator to discriminate between safe and hazardous edges for

a wide range of safety criteria. Unfortunately, in the scope of this

initial study, this ideal material was not found, although several

materials were found which could be used to satisfy S .

- k

-

The loading mechanism used in the experiments on the synthetic

materials was different than that used in cutting the excised skin

specimens. The skin was mounted on a mandrel that was rotated past

the edge (Figure 3b). When synthetic materials were used, the

mandrel was wiped back and forth in the direction parallel to the

edge (Figure 3c). In order to simulate the loading mechanism of

the skin experiments, the Consumer Product Safety Commission

requested a modification in the discrimination procedure using

synthetic materials y/

.

The purpose of the present report, then, is to describe a

method of discriminating between the safe and hazardous edges of

safety criteria Sj and S||. In addition, this report will describe

a device which can be used to test the hazard potential of exposed

edges

.

Additional Skin Cutting Experiments

In the introduction, it was pointed out that the original test

edges, A through K, (See Figure 2) were unable to provide a discrimination point* for safety criterion Sj. This is because all of the

edges were hazardous; at 20 pounds all edges were capable of

breaking the epidermis of human skin. Thus it became necessary to

supplement the test family with duller edges until edges were found

which would be called safe.

The two additional edges used are L and M, which are also

described in Figure 2. As with edge K, the least sharp of the

original edges, the new edges also have a 90" included angle, but

"'"in the previous report _!_/ , the term "discrimination point" was used

as a means of distinguishing between the safe and hazardous edges

of a given safety criterion (e.g. safe edges were said to fall below

the discrimination point).

- 5

-

with larger edge radii. Cutting tests were made on the same

apparatus and with the same procedure as in the previous report 1/

(see Figure 3b). The force was held constant at 20 pounds (89. 0 N)

The results of these tests are shown in Figure Ten

different specimens were tested on each of the two additional edges

On only one specimen was the epidermis broken.'*" In this case, the

penetration into the dermis was not very significant. None of the

epidermis of the other specimens were broken when edges L and M

were used at 20 pounds (89. 0 N). These data are plotted In graphical form in Figure 5 (which, except for edges L and M, is Figure 23

of the previous report ]_/) •

For a given safety criterion, it was the convention of the

previous report to label an edge "safe" if the prescribed percent

depth of cut (for that criterion) was exceeded on only one skin

specimen. By this convention, edges L and M would be judged safe

for cr i ter i on S

j

.

It remains to find a procedure which will make the following

discriminations among the test edges:

Determination of breaks in the epidermis was by visual inspection.

In a preliminary histological study V, some of these "grooves" were

shown to have made slight penetration into the papillary layer of

the dermis (uppermost dermal layer, containing minute capillaries).

This convention was followed so that a particularly weak specimen

would not form the basis of a safety standard for the bulk of the

population. Besides, there appeared to be some evidence that regulations based on these tests would be conservative 1/.

- 6

-

For S|

:

discriminate edges L and M (safe) from all others; or

discriminate edge M (safe) from all others.

For S,,:

discriminate edge A (hazardous) from all others

Selection of a Synthetic Cut Indicator

With a rotary testing device, such as suggested by the sponsor,

the cutting action of an edge is somewhat different than with a

wiping type of test. With the rotary test, the interaction occurs

between a short segment of the edge and a relatively long segment

of the test material, whereas with a wiping test, the opposite

ho I ds (see F i gure 3)

•

The apparatus for the skin cutting experiments of the previous

report \J was modified so that synthetic materials could be tested

in a rotational mode. The wooden mandrel was replaced by a 3/8

inch (.95 cm) diameter aluminum mandrel which was inserted in the

lathe chuck. Synthetic materials were secured on the mandrel, and

then the test edges were brought into contact with the material at

prescribed forces \J . The mandrel was rotated through one revolution by turning the lathe by hand.

Materials were sought which would make any or all of the three

discriminations listed above. A material was said to make a

particular discrimination, at some given force, if only those edges

^The discrimination of only edge M from the rest would also

satisfy S|, and may be preferred over the above discrimination

because it is more conservative.

- 7

-

labelled hazardous cut completely through the material. Those

materials which appeared promising in the earlier "wiping" experiments j_/ were tested, along with many others. As expected, a

consequence of changing to the rotary motion was that higher forces

were required to completely penetrate a given material with a

given edge. These experiments were considered preliminary in the

sense that they would produce a suitable material for use with an

inspection device. The precise force levels to be used in an

inspection procedure would then be determined by more careful experi

ments on the device itself.

It was decided that, if available, a single material which

could make all three discriminations would be preferred. This

would simplify the inspection procedure - especially for consumer

products having edges which are unlikely to be contacted, as well

as exposed edges. Other criteria which should be met by the chosen

ma te r i a 1 a re :

1. The forces required Tor complete penetration of the

material should not be too high; otherwise the inspection device would not be convenient for hand use. It

was felt that five pounds (22.2 N) is a reasonable upper

limit for the force to be used with the device.

2. The material should be very thin - preferably on the same

order of thickness as the epidermis of human skin VThen any protrusion above an edge, which is capable of

breaking through the epidermis of human skin, will also

be capable of cutting completely through the material.

- 8

-

3. The materia] should be easy to rnount. Thus materials

which are manufactured with adhesive backings will be

preferred over those which are not (provided, of course,

that the adhesive is applied uniformly (with controlled

thickness and does not interfere with a materials ability

to make discriminations).

Of all materials tested, several were found that made the

necessary discriminations: these included some poly(vinyl chloride)

and polyethylene foams and several po I y ( te t raf 1 uoroe thy 1 ene ) tapes

of varying thicknesses. Of these, only the po 1 y ( te t raf 1 uore thy 1 ene

)

tape of .006 inch (.15 mm) total thickness, including a .003 inch

(.075 Tim) adhesive backing, met all of the previous requirements.

Thus, this material was selected for further testing with the

inspection device.

An Inspection Device

In this section, an inspection device, which can be used with

the selected synthetic material to test edges associated with consumer products, will be described. Any such device should satisfy

the following criteria.

1. It should be hand-held. Thus it can be brought to the

product, avoiding awkward and time consuming mounting

procedures

.

2. It should be usable in any direction. This again

avoids positioning the product. It also suggests the

use of springs for setting forces so that the direction

of gravity can be made irrelevant.

3. The diameter of the mandrel, upon which the synthetic

material is mounted, should be sufficiently small so that

tiny openings, which are large enough to accommodate a

child's finger, can also be inspected.

k. It should be capable of making a single rotation (to

prevent more than one pass over the same cut) at a constant

speed of rotation (to avoid a subjective speed set by the

i nspector )

.

5. The force between the edge and the mandrel should be

within tolerable limits, even when the mandrel is spinning.

Any inertial loading which may be induced by the spinning

should be minimized.

6. It should be easy to use.

A device which satisfies these criteria is shown schematically

in Figucje 6. A photograph of the device is shown in Figure 7- The

mechanical subassembly of Figure 6a incorporates a motor driven test

finger to obtain the desired rotary motion. A steel rod, 3/8 inch

(•95 cm) in diameter is used as the test finger. This is coupled

to the output shaft of a small, shaded-pole gear motor. The motor

is mounted to a base plate to which is attached a bearing block

that provides rigid support for the test finger. The three

mutually perpendicular axes, x-x, y-y and z-z (considered attached

to the subassembly) pass through the center of gravity of the

subassemb 1 y

.

The 1/70 HP (0.107 W) motor provides an output shaft speed of

50 RPM (corresponding to a linear velocity of 1 inch per second

(2.5^ cm/s) at the surface of the test finger) and contains an

- 10 -

inteigral brake which automatically stops the output shaft within 5

degrees of rotation after removal of power. This is an important

feature because it becomes possible to maintain a nearly constant

rotational speed of the finger for the entire revolution. To

stop the test finger after one revolution, a single lobed cam is

mounted on the motor shaft. This cam operates an electrical switch

which opens the motor circuit when activated.

The subassembly described above is connected to the main

support unit, shown in Figure 6b, at the points "A"; i.e., the motor

subassembly is allowed to rotate within the main support unit

about the x-x axis. The support unit consists of a reference

plate to which is attached a handle, two bearing support plates,

and a subassembly for adjusting spring tensions.

To provide prescribed contact forces between the test finger

and an edge, two identical springs on either side of the motor

subassembly are connected to the support unit. On the motor

subassembly, the springs are connected at the points marked "C" in

Figure 6a. The line AC is parallel to the y-y axis. The springs

are attached to the points "B" on the support unit. The line B8 is

parallel to the x-x axis.

To operate the assembled device, the selected synthetic

material is applied to the test finger. The device is brought

into contact with an edge at a nominal test position which is

1/4 inch (0.6 cm) from the tip of test finger (point "E" in

Figure 6a). The operator positions the device until the z-z axis

is parallel to the plane containing the reference plate of the

- 1 1 -

support unit, (the shaded surface S in Figure 6b). An indicator is

provided to insure that this lining-up procedure can be effected

within prescribed limits. Stops are provided so that the angle

through which the motor subassembly can rotate is limited to about 5

degrees (0.1 red, corresponding to an arc length of about 1/2 inch

(1.3 cm) for the tip of the finger).

The force between the edge on the test finger can be controlled

by extending or shortening the springs. This is accomplished by

turning a screw (at W in Figure 6b). A scale is provided to note

prescribed force settings. The forces have been calibrated with

an Instron testing machine. If the subassembly is off its stops,

and if the edge is within 1/A inch (0.6 cm) of the nominal test

position, the error in setting the contact force is no greater

than Sconce the force is set, it remains for the operator to depress

a switch (at H in Figure 6b) which closes the motor circuit and

initiates the s i ng 1 e- revo 1 u 1 1 on cutting cycle. Because of the

tolerance allowed in positioning the edge on the test finger,

several parallel cuts can be made without replacing the synthetic

material. The material is then peeled off of the test finger and

visually inspected to see if it has been completely penetrated by

the edge.

Tests with the Inspection Device

A roll of the selected po 1 y ( te t raf 1 uoroe t hy 1 ene ) tape was

tested on the inspection device to find the force settings at

wh I cn the appropriate discriminations (page 6) could be made. For

- 12 -

d given edge, the force required to cut through the material was

sought. Each edge-force combination was tested many times so that

any non- un i f orm i t i es in the material would have been detected.

Generally there was a range of forces in which accurate predictions

of complete penetration could not be made. For example, edge M

would cut through the material at 3-9 lb. (17-3 above, and

would not cut through at 3-5 lb. (15-6 N) and below. Between these

forces, both cuts and no-cuts were recorded. From this data a

force range could be determined for making each of the edge discriminations. This is shown in Figure 8. F and F are the

H I G n L UW

highest and lowest forces at which a particular discrimination could

be made (within 0.1 lb., or 0.4 N). The recommended force settings

are chosen near F'j^iQ^ ' the interest of safety.

It was qualitatively determined that the mechanical strength

of the adhesive backing is negligible compared to that of the

po 1 y ( te t ra f 1 uoro€ thy 1 ene ) film. Thus the accuracy in determining

the forces required to cut through the material is probably limited

by the tolerance in specifying the thickness of the po 1 y ( te traf 1 uoroethylene) film, i.e., .003 ^ .0005 inch (.076 + .013 mm) or + 17^.

Variations in thickness within a single roll are probably reflected

in the force range reported above (on the order of +_ 5^). Variations

from roll to roll may be even larger. This conjecture was confirmed

in some preliminary testing of three samples from separate production batches with average po I y ( te t raf 1 uoro« thy 1 ene ) film thicknesses

of .0028, .0031 and .0033 inches (.071, .079, -OSk mm). Corresponding

shifts in F - and F were indicated, but there was insufficient

I

- 13 -

material for accurate determinations. tven with these relatively

large variations in thickness, the "hazardous" edges for criterion

S| always cut the tape at the recommended force setting.

The manufacture of this po ly(tetraf 1 uoroethy lene) tape to

stricter tolerances may be a worthwhile endeavor for the future.

For the present, the need for a safety standard on edges may

preclude possible problems associated with tape variability -

especially In light of the fact that the variability in skin is

even larger \J . in fact, this latter source of variability may

well overshadow all others.

Summa ry

The device described in section IV can be used to test edges

which are associated with consumer products. The .006 inch (.15 mm)

po 1 y{ te t raf I uoroe thy I ene) tape, which satisfies all criteria for

cut indicators (page/), has been selected as the recommended

synthetic material to be used in the inspection procedure.

At a contact force of 3-8 pounds (16.9 N), this material is

completely penetrated by only those test edges which are capable

of breaking through the epidermis of human skin"*" when the contact

force is 20 pounds (89-0 N). This satisfies safety cri.terion Sj.

for exposed edges on consumer products.

At a contact force of 1.1 pounds (^.9 N) , this material is

completely penetrated by only those test edges which are capable

of completely penetrating human skin when the contact force is

'*"l.e. excised abdomen skin, which is all that was available for

the skin cutting experiments.

- li* -

^4 lbs. (17-8 N). This satisfies safety criterion for edges

that would be contacted inadvertently.

The device described in this report is suggested as a feasible

means of combining all the criteria for rotary device (page 8; into

a workable model. It should be re-emphasized that this is a firsttry prototype model, and it is expected that a finished product

will benefit by many improvements.

It seems appropriate to restate some of the limitations

mentioned in the previous report J_/. First of all, a regulation

based on the prescribed safety criteria will protect against the

stated injuries provided that the loading mechanism is of the

"casual handling" mode. It is possible that such a regulation

will protect against minor impacts, but this must be verified by

expe r i men t

.

Secondly, the difference between cutting excised abdomen

skin and cutting other areas of the body (which are more likely

to be contacted by edges on consumer products) is unknown. It was

suspected \_/ that regulations, based on the results of the skin

cutting experiments, would be conservative. A program is currently

underway to verify this conjecture, and to establish what safety

factors may be in effect.

References

1. Sorrells, J.R. and Berger, R.E., "Some Cutting Experiments on Human Skin and Synthetic Materials," National Bureau of Standards

Report N6SIR73-262 (October 1973)

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

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

Report NBSIR73-123 (February 1973)

3. Armstrong, R.W., Bureau of Engineering Sciences, Consumer Product

Safety Commission, personal communication (1973)

4. Deck, J,D . , ''HI stology of Skin Cuts Made by Selected Edges,"

unpublished, personal communication, university of Virginia School

of Medicine (1973)

5. Whitton, J. T.

, 'New Values for Epidermal Thickness and their

Importance," Health Physics, 24, 1-8 (1973)

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BIBLIOGRAPHIC DATA

SHEET

r; hi II A I li i\ I >H K I I 'I )K 1 NO. HBSiR7k-k2B2. (....v'l Ac , r

rn i.r ami h i in.'

An Inspection Procedure for Detecting

Hazardous Edges

. *1 IllOKi^ John R. Sorre lis; Robert E. Berger

'i.Ki oHMINt. ()K(.ANI/A IK A NAMl ANP AIMIKIS^

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

> , .n ^ .'• I n i; < >r t .in i / al i

. 'n .Nam< .iii,J ' ..fiiph.u Ajjn-sv (Strct-t Cily. State, ZIP)

Consumer Product Safety Commission

5^401 Westbard Avenue

Bethesda, Md. 20016

3. Rcc ipiini's Ai icsMon No.

5. Publ u anon i^ai <

6. Pcrforminfi Organization Code

8. PrrforminK Or^un. Kcpori No. NBSIR7^-A28

10. F^oicLt /Ta.sk, Work Unii No. ^+460502

11. f ontraci Cirani No.

13. Typt- ot Rt pori & Period

f Dvcrt-d Progress

14. Sponsoring Aprnty Code

^.'^ ; K A< 1 A JOO-word or les\ ta> lual summary of mo.-. ! si(^ilii ant inlormation II documeni includes a signiiii am

hi bl lodraphv ur literature survrv mcnliun it hrrr i Previous human skin cutting experiments, using a series of test edges, were

extended to establish the safe and hazardous test edges for each of two safety

criteria chosen by the Consumer Product Safety Commission. A poly (tetraf luoroethylene) tape was chosen as a suitable material which, when applied in a rotary mode to the test edges, would be completely penetrated by only those

edges labelled hazardous. An inspection device, capable of turning the material

through a single revolution while providing prescribed contact forces between

the material and an edge, has been built. The selected synthetic material was

tested with the device to find the force settings for discriminating between

the safe and hazardous test edges of each safety criterion:

1. Ac a contact force of 3.8 pounds (16.9 newtons), this material is

completely penetrated by only those test edges which are capable of

breaking through the epidermis of human skin when the contact force

Is 20 pounds (89. 0 N). This is the safety criterion for "exposed"

edges on consumer products.

2. At a contact force of 1.0 pounds (A.^t N) , this material is completely

penetrated by only those test edges which are capable of completely

penetrating human skin when the contact force is ^.0 pounds (17-8 N)

.

This is the safety criterion for edges which may only be inadvertently

ron ta/: ted

KKY *C)RnS (six to twelve t-nlne^. alphatn-lical order, capitalize only the lirat letter ot the first key word unless a proper

name, separated by semicolon'- ) Consumer product safety; Edges, hazardous; Injury; Inspection of Edges;

Safety; Skin, cutting; Synthet i c ma ter i a 1 s , skin

r or Official Distribution. l)o .Not Krirasf 10 NTIS

' 'rJc- f ruit, Sup "t I'l'C, I liovr rnrrcnt Printing Olticr Washington, [).< . 20iU.\ ^D ' ai. N,.. ' 1 ^

C'rdt-r f rom National Tpchnic.il Inlotmation "service NIlS)

"^r'lngfiria. Virginia \

19. Sl.( I'RnV CLASS

'THIS RF.PORT)

20. SI ( UKI I > ( l.ASS

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21. NO. OF PAGES

23

22. Price

USCOMM-DC 29042.P74