Investigation of plaster adhesion test method

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Investigation of plaster adhesion test method

Hutcheon, N. B.

NATIONAL RESEARCH COUNCIL CANADA

DIVISION OF BUILDING RESEARCH

INVESTIGATION O F P L A S T E R ADHESION TEST METHOD b y N. B . Hutcheon Internal R e p o r t No.

285

of t h e D i v i s i o n of Building R e s e a r c h OTTAWA January

1 9 6 4

P R E F A C E

A s m a l l p a t c h m e t h o d of d e t e r m i n i n g adhesion h a s b e e n adapted by t h e Division f o r s o m e of i t s w o r k on p l a s t e r . S o m e f u r t h e r e x p l o r a t i o n of the method w a s d e s i r a b l e t o a s s i s t i n the i n t e r p r e t a t i o n of r e s u l t s obtained. P l a s t e r itself w a s employed a s a b r i t t l e m a t e r i a l i n a n investigation of t h e effect of c u t t e r p r o f i l e s a n d depth of c u t u s e d i n i s o l a t i n g t e s t p a t c h e s . T h e s e r e s u l t s a r e now r e p o r t e d . T h e w o r k w a s c a r r i e d out u n d e r t h e g e n e r a l d i r e c t i o n of t h e a u t h o r , who is A s s i s t a n t D i r e c t o r of t h e Division. Ottawa J a n u a r y 1964 R o b e r t

F.

Legget D i r e c t o r

INVESTIGATION O F P LASTER ADHESION TEST METHOD

by

N. B. Hutcheon

P r o b l e m s involving bond of p l a s t e r which have been brought t o t h e DiGision h a v e l e d at v a r i o u s t i m e s t o a number of l a b o r a t o r y i n - vestigations in which a t t e m p t s have been m a d e t o m e a s u r e bond strength. R e l a t i v e l y l a r g e s a m p l e s about

9

in. s q u a r e w e r e employed in e a r l i e r work, but t h e s e c a m e t o b e r e g a r d e d a s of doubtful m e r i t . Attention w a s then directed t o a f o r m of t e s t used by Ryder ( 1 ) i n which a s m a l l patch

of p l a s t e r about 2 in. in d i a m e t e r i s i s o l a t e d by cutting, and then pulled f r o m t h e b a s e , t h e r e q u i r e d f o r c e being m e a s u r e d . Subsequently, a number of l a b o r a t o r y t e s t s w e r e c a r r i e d out on v a r i o u s p l a s t e r s y s t e m s on a v a r i e t y of b a s e s .

A s it w a s anticipated that adhesion m e a s u r e m e n t s would

eventually b e m a d e in t h e field a s well as in t h e l a b o r a t o r y , c o n s i d e r a b l e attention w a s given t o t h e development of a p p a r a t u s and techniques which would b e e a s y t o u s e on w a l l s and ceilings as well a s on l a b o r a t o r y t e s t panels. Ryder u s e d a hand-grip s p r i n g device for pulling and m e a s u r i n g adhesion f o r c e s . An a l t e r n a t i v e design employing loading b y c o m p r e s s e d a i r through d i a p h r a g m s w a s designed and c o n s t r u c t e d ( ( F i g u r e 1) ( 2 ) ) . An e l e c t r i c a l l y powered cutter w a s a l s o developed for isolating t h e t e s t patches.

Ryder used t h e s m a l l "patch" method t o study t h e adhesion of p l a s t e r t o c o n c r e t e s o t h a t , p r e s u m a b l y , t h e b a s e m a t e r i a l w a s always v e r y rigid, and t h e p l a s t e r t e s t patch w a s always undercut a s f a r a s t h e concrete. In t h e study of other p l a s t e r s y s t e m s , . the b a s e coat and t h e p l a s t e r b a s e w e r e often weaker than t h e putty coat, and t h e r e w e r e a l s o

questions t o b e r a i s e d about t h e m o s t a p p r o p r i a t e depth and shape of cut. A s p l a s t e r i s a r e l a t i v e l y b r i t t l e m a t e r i a l i t w a s s u s p e c t e d t h a t it would r e s p o n d t o s t r e s s c o n c e n t r a t i o n s a t t h e a b r u p t change i n section f o r m e d by t h e cut, and t h a t t h e a p p a r e n t adhesion s t r e n g t h might be affected by varying t h e depth and p r o f i l e of t h e cut. A number of e x p e r i m e n t s , now t o b e r e p o r t e d , w e r e undertaken t o provide information on t h e s e questions

as a n aid in i n t e r p r e t i n g t h e r e s u l t s of t h e t e s t s and in evaluating t h e t e s t method.

THE PROBLEM

An i s o l a t e d p l a s t e r p a t c h m a y b e r e g a r d e d a s a v e r y s h o r t t e n - s i l e s p e c i m e n with a n a b r u p t change of section a t its b a s e . It is known t h a t abrupt changes in section produce s t r e s s c o n c e n t r a t i o n s i n t e n s i l e s p e c i m e n s i n p r o p o r t i o n t o the s h a r p n e s s o r a b r u p t n e s s of t h e change.

Such localized s t r e s s e s can be m a n y t i m e s higher than t h e a v e r a g e s t r e s s over t h e m i n i m u m section. T h e u s e of f i l l e t s r a t h e r than s h a r p c o r n e r s c a n m a r k e d l y r e d u c e the s t r e s s concentration.

T h e effect of s t r e s s concentrations in ductile m a t e r i a l s being subjected t o a n i n c r e a s i n g load t o f a i l u r e m a y not be s e r i o u s s i n c e t h e m a t e r i a l i s able by i t s ductility t o flow locally, t h u s r e l i e v i n g t h e high l o c a l s t r e s s e s and t h e i r potential effects. Such s t r e s s concentrations, however, a r e of i m p o r t a n c e in ductile m a t e r i a l s under r e p e a t e d loading

s i n c e the); m a y l e a d t o t h e f o r m a t i o n of a c r a c k which i s itself a s t r e s s - r a i s e r , and i s capable of propagating until f a i l u r e o c c u r s . When a m a t e r i a l i s b r i t t l e , i t cannot flow p l a s t i c a l l y t o r e l i e v e t h e l o c a l high s t r e s s e s , and s o m a y f r a c t u r e whenever t h e localized s t r e s s r e a c h e s a limiting value r e p r e s e n t a t i v e of the t e n s i l e s t r e n g t h of t h e m a t e r i a l . S t r e s s concentrations in a b r i t t l e m a t e r i a l m a y t h e r e f o r e induce f a i l u r e a t l o a d s considerably lower than t h o s e which could be c a r r i e d i f the s t r e s s concentrations w e r e not p r e s e n t .

It i s known a l s o t h a t r e a l m a t e r i a l s do not r e s p o n d ideally t o t h e effects of s t r e s s c o n c e ~ l t r a t i o n s , s o that t h e r e d u c t i o n s in b r e a k i n g l o a d s t h a t r e s u l t in t h e c a s e of p r a c t i c a l b r i t t l e m a t e r i a l s a r e usually l e s s than those p r e d i c t e d f r o m theory. P l a s t e r of p a r i s h a s been u s e d a s a m o d e l m a t e r i a l for t h e e x p e r i m e n t a l evaluation of s t r e s s concentrations. T h e b r e a k i n g l o a d s of two s p e c i m e n s m a d e of p l a s t e r , one with a s t r e s s

concentration and one without, can be c o m p a r e d , and t h e s t r e s s concen- t r a t i o n f a c t o r obtained f r o m the i n v e r s e r a t i o of the loads.

F o r t h e s e r e a s o n s i t w a s thought that t h e p l a s t e r adhesion l o a d s found f r o m the s m a l l patch t e s t would v a r y with t h e depth and p r o - file of the cut and that t h e influence of t h e s e f a c t o r s should b e investigated. T h e s t r e s s distributions in t h e isolated p l a s t e r patch a r e undoubtedly

complicated by ( a ) t h e p r o x i m i t y of t h e aluminum d i s c having different

e l a s t i c p r o p e r t i e s than the p l a s t e r , ( b ) t h e e f f e c t of t h e adhesive u s e d which m a y introduce shrinkage s t r e s s e s into t h e p l a s t e r a s it h a r d e n s , and ( c ) t h e v a r i a b l e and complex e l a s t i c - p l a s t i c situation posed by the multiple l a y e r s of t h e r e a l p l a s t e r s y s t e m .

TEST METHOD AND RESULTS

Some p r e l i m i n a r y t e s t s ( S e r i e s A) w e r e m a d e using existing l a r g e panels of a conventional putty coat over a b a s e coat s y s t e m which had been m a d e by a p l a s t e r e r for adhesion t e s t s in t h e l a b o r a t o r y . Two c u t t e r s w e r e u s e d t o i s o l a t e t h e t e s t patches. One w a s square-edged, 1/8 in. wide, a s had been u s e d in a l l p r e v i o u s work. T h e other w a s a l s o 1/8 in. wide but w a s rounded a t i t s end s o t h a t i t f o r m e d a full fillet of 1/16-in. r a d i u s at t h e bottom of t h e cut. Twenty-four p a t c h e s w e r e cut with e a c h c u t t e r , twelve being taken t o t h e depth of the putty coat and twelve being a t varying depths f r o m about 0. 2 t o 0. 4 in. T h e r e s u l t s a r e

shown in T a b l e I, t h o s e for t h e depths t o b a s e coat being a r r a n g e d in o r d e r of load c a r r i e d .

T h e r e was evidence of quite a m a r k e d influence of depth of cut, the l o a d s c a r r i e d being only about h a l f a s g r e a t at depths over 0. 3 in. a s at 0. 2 in. T h e c u t s m a d e t o the b a s e coat w e r e not checked for depth but w e r e about 1/8 i n ; a l l other c u t s w e r e well into t h e b a s e coat. No p a r t i c u l a r benefit f r o m the rounded c u t t e r w a s shown.

F a i l u r e s o c c u r r e d in t h e b a s e coat and not a t the i n t e r f a c e with t h e putty coat.

F i v e homogeneous p l a s t e r of p a r i s s l a b s 12 by 16 by 1 in. thick w e r e c a s t for S e r i e s B t e s t s , and a f t e r 1 day in a damp r o o m w e r e allowed t o d r y t o constant weight in a r o o m a t m o s p h e r e of 50 p e r cent R. H. ; t h i s r e q u i r e d about 2 months. A s e r i e s of p a t c h e s w e r e then cut

for adhesion testing. T h e shapes of t h e c u t t e r s u s e d and t y p i c a l s p e c - i m e n s a r e shown in F i g u r e 2. Depths of cut w e r e m e a s u r e d in t e r m s of the depth of p a r a l l e l portion, which w a s adjusted t o 1/16, 1/8, and 1/4 in. for each shape of cutter. One s p e c i m e n for each cutter shape and each depth w a s p r e p a r e d on each of the five s l a b s using the e l e c t r i c a l l y

o p e r a t e d cutting machine in each c a s e . One s h o r t cyclinder

$

in. long and one 1 in. long w e r e a l s o cut f r o m e a c h slab, and w e r e glued between two d i s c s t o f o r m a t t a c h m e n t s for s p h e r i c a l l y seating tackle in a t e n s i l e t e s t i n g machine. All p a t c h e s w e r e loaded by m e a n s of the a i r - o p e r a t e d m e m b r a n e device, which w a s c a l i b r a t e d in the t e n s i l e t e s t i n g machine (Appendix A).

T h e r e s u l t s of t h e S e r i e s B t e s t s a r e given in T a b l e

I1

_and

F i g u r e 3. R e s u l t s on any one horizontal line in T a b l e I1 apply t o s p e c i m e n s f r o m t h e s a m e slab, t h e s e being p r e s e n t e d in t h e s a m e o r d e r in each group of five r e s u l t s s o t h a t any p o s s i b l e differences between t e s t s l a b s m a y be noted. T h e r e s u l t s a r e not v e r y r e l i a b l e in view of t h e l i m i t e d number of s a m p l e s ( 5 i n each c a s e ) and t h e s u b s t a n t i a l variation shown. T h e

difference between t h e square-ended c u t t e r and t h a t with the l a r g e s t fillet i s not g r e a t , being about 15 p e r cent for 1/8-in. depth of cut. A t r e n d t o w a r d s lower s t r e n g t h s with i n c r e a s i n g depth of cut i s shown. T h e s t r e s s

concentrations shown by the s q u a r e c u t t e r by c o m p a r i s o n with cylinder s p e c i m e n s t r e n g t h s a r e about 1. 5 for 1/8 -in. depth and about 1.75 for 1/4-in. depth of cut.

In view of t h e v a r i a b i l i t y of the r e s u l t s , f u r t h e r t e s t s w e r e m a d e ( S e r i e s C). T h i s t i m e only t h e square-edged c u t t e r t o a depth of

1/4 in. w a s used for c o m p a r i s o n with $-in. and 1 -in. c y l i n d e r s cut f r o m two t e s t s l a b s m a d e and conditioned a s b e f o r e . Both p a t c h e s and

c y l i n d e r s w e r e pulled in the t e n s i l e t e s t i n g machine using suitable s p h e r i c a l l y seating tackle. T h e r e s u l t s f r o m the 1 -in. c y l i n d e r s w e r e spoiled by p r e m a t u r e glue f a i l u r e s and had t o be discarded. A c o m p a r i s o n of patch and cylinder l o a d s for t h e r e m a i n i n g s p e c i m e n s given in T a b l e 111 shows a s t r e s s concentration for the square-end c u t t e r a t 1/4-in. depth of about 2. 2 c o m p a r e d to 1. 75 f r o m S e r i e s B, t h e patch s t r e n g t h s being lower and the cylinder s t r e n g t h s higher than before.

CONCLUSIONS

Although not a c c u r a t e l y e s t a b l i s h e d by t h e s e r e s u l t s , t h e r e does a p p e a r t o b e a s t r e s s c o n c e n t r a t i o n effect v a r y i n g f r o m about 1. 5 t o 2, i n homogeneous m a t e r i a l r e s u l t i n g f r o m t h e u s e of a s q u a r e - edged c u t t e r t o i s o l a t e t e s t p a t c h e s . Modifications t o t h e c u t t e r p r o - file within the l i m i t s s e t by a 1/8 -in. width do not i n d i c a t e sufficient i m p r o v e m e n t t o justify t h e c o m p l i c a t i o n s introduced. T h e effect of t h e s t r e s s c,oncentrations will p r o b a b l y be t o r e d u c e t h e a p p a r e n t bond s t r e n g t h s obtained f o r t h e m o r e b r i t t l e p l a s t e r s m o r e t h a n f o r t h e l e s s b r i t t l e m a t e r i a l .

T h e r e s u l t s obtained a r e f o r a homogeneous m a t e r i a l with n o defined adhesion plane having s t r e n g t h s up t o 10 t i m e s t h o s e expected in a r e a l p l a s t e r s y s t e m . T h a t t h e s t r e s s c o n c e n t r a t i o n s shown will b e found in t h e c a s e of a thin putty coat r e l a t i v e l y weakly bonded t o a b a s e coat of d i f f e r e n t m a t e r i a l m a y b e questioned. T h e r e is r e a s o n , however, t o u s e a l l s u c h r e s u l t s with r e s e r v a t i o n , r e g a r d i n g t h e m not a s a c t u a l s t r e n g t h s but a s v a l u e s which a r e exceeded by the r e a l s y s t e m , b y s o m e i n d e t e r m i n a t e amount. T h e s e r e s u l t s i n d i c a t e t h a t p l a s t e r is

a t b e s t l i k e l y t o be quite v a r i a b l e in s t r e n g t h , s o t h a t a l a r g e n u m b e r of t e s t s a m p l e s should a l w a y s b e used.

T h e a s s i s t a n c e given b y G. Quenneville who m a d e a l l s a m p l e s a n d c a r r i e d out t h e t e s t s is g r a t e f u l l y acknowledged.

R E F E R E N C E S

1 R y d e r ,

J. F.

Methods f o r t e s t i n g t h e adhesion of p l a s t e r t o con- c r e t e . C h e m i s t r y and Industry, August 10, 1957, p. 1090-2. 2 O'Kelly, B. M. P o r t a b l e adhesion t e s t i n g device. M a t e r i a l s

Bulletin No. 250, A m e r i c a n S o c i e t y f o r T e s t i n g and M a t e r i a l s , D e c e m b e r

1960,

p. 3 2 - 3 .

TABLE

I

SERLES A TESTS

- E F F E C T O F DEPTH AND CUTTER SHAJ?E

ON ADHESION TEST STRENGTH

T e s t P a n e l s 1/2-in. B a s e Coat over P l a s t e r B a s e , 1/8-in. P u t t y Coat. C u t t e r s 1/8 in. Wide,

One S q u a r e End, One Rounded End.

Cutter Cut t o b a s e coat, lb 55 61 6 7 67 68 71 7 1 7 3 76 8 6 8 6 9 1 72. 7 Rounded Depth, in. 0 . 1 8 5 0. 19 0. 19 0. 20 0 . 2 1 0 . 2 1 0 . 2 2 5 0. 24 0. 26 0. 28 0. 29 0. 36 Depth, in. 0 . 2 1 0 . 2 2 0. 265 0. 31 0. 31 0 . 3 3 5 0 . 3 4 5 0 . 3 5 0. 35 0. 37 0. 38 0. 38 Avg S q u a r e End Load, lb 6 4 4 9 4 9 5 3 3 9 32 43 3 7 39 3 6 4 5 43 44. 1 End Cutter Load, Ib 7 6 67 53 55 6 4 5 8 5 7 52 5 1 43 43 27 53. 8 (1/16 in. r a d i u s ) Cut to b a s e coat, lb 55 57 62 6 4 66 67 68 7 3 74 76 82 8 2

-

68. 8

TABLE I1

SERIES B TESTS

- LOADS AT FAILURE WITH VARIOUS

DEPTHS AND C U T T E R P R O F I L E S S h o r t C y l i n d e r s 1/2 in. 1 in. 7 60 8 0 4 8 40 8 36 7 28 9 6 7 7 0 6 6 5 6 7 38 4 4 4 7 5 4 7 4 1 C u t t e r A A% B Avg C Avg D Avg A

2-in. Dia. P a t c h e s , Undercut

1/16 in. 48 8 599 512 422 69 1 5 46 49 2 560 5 18 440 563 515 46 4 5 8 2 600 5 30 7 8 6 59 2 47 9 6 0 1 576 570 706 586 Load, lb 1/8 in.

-680 399 48 0 37 1 562 498 46 0 442 376 47 1 40 1 430 560 55 1 49 9 447 49 1 510 56 2 507 56 1 57 3 646 57 0 1/4 in. 460 43 3 37 3 39 2 45 2 42 2 5 4 1 440 4 1 4 398 5 3 5 46 6 6 5 2 316 367 47 1 43 6 448 4 5 0 50 5 509 47 1 529 49 3

TABLE 111

SERIES C

- TESTS WITH SQUARE -CUT PATCHES

AND 1 / 2 -IN. CYLINDERS

Note: Cylinders f r o m Slab 2 failed p r e m a t u r e l y a t glue joints. Slab 1 1/2 -in. Cylinder s B r e a k i n g Load, lb 7 6 2 7 7 0 9 0 0 708 8 6 4 9 0 0 9 18 7 9 0 7 0 7 79 2 8 11 Slab 1 P a t c h e s Breaking Load, Ib 29 6 337 3 2 4 298 3 14 4 58 46

o

42 0 49 0

-

Avg 377 Slab 2 P a t c h e s Breaking Load, l b 2 3

o

2 6 0 2 6 6 422 460 3 50 49 6 236 370

-

343

N E T UPWARD

FORCE

-7

AIR

=I

INLET

Rl NG

I S O L A T E D P L A S T E R

S A M P L E

SECTION THROUGH ADHESION TESTING DEVICE

t

_r

ALUMINUM DISC ADHESIVE ISOLATING CUT

1

7 PUTTY COAT

( 0 ) TESTING OF PATCHES FROM 2-COAT SYSTEM, SERIES A TESTS

r

DEPTH OF CUT BASED ON I

PARALLEL PORTION, X = I/;, I/;,

'1; OR I" PLASTER CYLINDER GLUED

BETWEEN 2 DISCS

(c)

SHORT CYLINDER SPECIMENS EMPLOYED IN SERIES

8 8 C TESTS

FIGURE 2

DETAILS OF TEST SPECIMENS

I

I

I

4

CYLINDERS SERIES C, SLAB

L

-

I"

CYLINDERS SERIES B

I"

-

-

-

-

-

-

SERIES B

-

PATCHES

B

FOR VARIOUS

C

CUTTER PROFILES

A

SEE FIG 2 ( b )

1

1

-

I

-

SERIES C

-

PATCHES

,

SLAB

2 x

'/;

WIDE CUTTER

I/:

DEEP, PROFILE A

-

-

-

-

-

-

I

_.

DEPTH OF PARALLEL CUT, IN

FIGURE

3

RESULTS OF SERIES B AND C TESTS ON PATCHES AND CYLINDERS

Appendix A

CALIBRATION O F P O R T A B L E ADHESION TESTING DEVICE

T h e p o r t a b l e a d h e s i o n t e s t i n g d e v i c e w a s c a l i b r a t e d o r i g i n a l l y in a 1 2 , 0 0 0 - l b u n i v e r s a l t e s t i n g m a c h i n e with low r a n g e s of 120 and 600 Ib, known t o b e ' r e a s o n a b l y a c c u r a t e . T h e p r e s s u r e gauge u s e d t o

m e a s u r e a i r p r e s s u r e w a s u n c a l i b r a t e d s o t h a t t h e c a l i b r a t i o n c u r v e obtained f o r t h e d e v i c e w a s given in t e r m s of t h e gauge p e r f o r m a n c e a t t h e t i m e of c a l i b r a t i o n . It cannot b e j u s t i f i e d t h e r e f o r e t o d r a w c o n - c l u s i o n s f r o m t h e o r i g i n a l c a l i b r a t i o n c u r v e about t h e l i n e a r i t y of r e s p o n s e o r t h e c o r r e s p o n d e n c e with c a l c u l a t e d v a l u e s b a s e d on known m e m b r a n e a r e a s . Subsequent u s e of t h e m e m b r a n e d e v i c e on t w o - c o a t p l a s t e r s y s t e m s e m p l o y e d t h e s a m e p r e s s u r e gauge ( 1 5 p s i r a n g e ) a n d r e s u l t s w e r e c o n v e r t e d t o pounds p e r p a t c h o r t o p s i b a s e d on p a t c h a r e a u s i n g t h e o r i g i n a l c a l i b r a t i o n . S t r e n g t h s obtainc:d did not r e q u i r e t h a t t h e

gauge l i m i t of 15 p s i b e exceeded. In t h e w o r k c o v e r e d in t h i s r e p o r t , h o w e v e r , c o n s i d e r a b l y h i g h e r s t r e n g t h s w e r e e n c o u n t e r e d , r e q u i r i n g a i r p r e s s u r e s up t o 60 p s i t o p r o d u c e f a i l u r e . Unfortunately, o t h e r a i r

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p r e s s u r e g a u g e s of a h i g h e r r a n g e w e r e s u b s t i t u t e d i n i t i a l l y , without r e c a l i b r a t i o n . F u r t h e r , i t had not b e e n e s t a b l i s h e d t h a t t h e m e m b r a n e s in t h e a d h e s i o n device w o i l d s u c c e s s f u l l y w i t h s t a n d t h e s e h i g h e r p r e s s u r e s without slipping o r s t r e t c h i n g t h a t m i g h t i n t r o d u c e v a r i a t i o n s in t h e m e a s u r e d loads. S e v e r a l s t e p s h a d t o b e t a k e n t h e r e f o r e t o r e - e s t a b l i s h valid c a l i b r a t i o n c u r v e s f o r s o m e w o r k t h a t had a l r e a d y b e e n c o m p l e t e d , a s well a s f o r t h e w o r k in hand. A f u r t h e r r e f i n e m e n t had a l s o t o b e i n t r o d u c e d . It w a s s u s - p e c t e d t h a t t h e position of t h e floating p l a t e s c l a m p e d t o t h e m e m b r a n e s r e l a t i v e t o t h e body of t h e a d h e s i o n d e v i c e m i g h t a f f e c t t h e c a l i b r a t i o n , i. e. v a r i a t i o n s in i n i t i a l d i a p h r a g m d i s t o r t i o n m i g h t influence r e s u l t s . T h i s p o s i t i o n i s defined and r e p r o d u c e d b y t h e d e v i c e i t s e l f when it i s u s e d on a flat p l a s t e r s u r f a c e , but not when i t i s r i g g e d f o r c a l i b r a t i o n , u n l e s s t h i s is s p e c i f i c a l l y a r r a n g e d . A jig t o which t h e floating plate position in n o r m a l u s e could b e r e f e r e n c e d w a s t h e r e f o r e c o n s t r u c t e d and r e p r o d u c e d a c c u r a t e l y when t h e d e v i c e w a s p l a c e d in t h e t e s t i n g m a c h i n e f o r c a l i b r a t i o n . It w a s not thought t h a t a n y d i s p l a c e m e n t f r o m t h i s i n i t i a l p o s i t i o n r e s u l t i n g f r o m t h e e l a s t i c i t y of t h e t e s t i n g m a c h i n e weighing s y s t e m a s a i r p r e s s u r e w a s a p p l i e d t o t h e m e m b r a n e s , t h u s i m p o s i n g l o a d on t h e t e s t i n g m a c h i n e d u r i n g c a l i b r a t i o n , would i n t r o d u c e a n y significant e r r o r . T h e a d h e s i o n device u s i n g t h e 60-lb gauge w a s t h e n c a l i b r a t e d in t h e t e s t i n g m a c h i n e without a n y change in t h e m e m b r a n e s . T h i s

c a l i b r a t i o n could not b e c o m p a r e d e x a c t l y with t h e p r e v i o u s one u s i n g t h e 15-lb gauge a s t h e s e g a u g e s t h e m s e l v e s w e r e not c a l i b r a t e d , but gave

approximately the s a m e slope. T h e adhesion device showed a s a t i s - f a c t o r i l y l i n e a r and reproducible c a l i b r a t i o n c u r v e t o provide a s s u r a n c e t h a t the new calibration could be used s a t i s f a c t o r i l y to convert r e s u l t s obtained with the 6 0 _{-1b gauge.}

After t h e work in hand using the 6 0 _{-1b gauge had been c o m -}

pleted, the adhesion device was r e c a l i b r a t e d , using the 15-lb gauge. T h e original calibration c u r v e w a s not reproduced, t h e l o a d s obtained being about 8 p e r cent higher for t h e s a m e gauge p r e s s u r e s a s before. It i s now believed that t h i s c a l i b r a t i o n was faulty. Upon c a r e f u l r e - calibration following r e p l a c e m e n t of the diaphragm, using the original 15 -1b gauge, t h e original calibration c u r v e w a s r e p r o d u c e d t o within 1 p e r cent.

It h a s not yet been established t h a t the m e m b r a n e s can always be used r e l i a b l y a t a i r p r e s s u r e s up t o 6 0 _psi.