' HOGESCHOOL
\ 10 - DELFT14 AÜ6.1956
'^•JnBatSTrbai lo - DELFT
THE COLLEGE OF AERONAUTICS
CRANFIELD
INVESTIGATIONS INTO A PHOTOELASTIC
METHOD FOR DIRECT MEASUREMENT OF
SURFACE STRAINS IN METAL COMPONENTS
by
Kanaalstraat 10 - DELFT REPOST NO. 97 FEBRUARY. 195^ T H E C O L L E G E O F A E R O N A U T I C S
C R A N F I E L D
Investigations into a Photoelastic Method for the Direct Measiorement of Surface Strains in Metal
Components.
by
-John R, Linge, D.C.Ae.., A.P.R.Ae.S.
SUMMARY
This report summarises the work carried out by Scott(1950) and Linge (1951) at The College of Aeronautics, Cranfield, in preparation for their diploma theses.
The technique investigated consists of the examination of surface strains in a metal component vd.th optically sensitive material bonded to the component and the analyses of the photoelastic pattern prodijced under lead by means of polarised light reflected from the surfajce of the metal.
Both theses cover in considerable detail the development of the technique and the results obtained, \ising the photoelastic materials Catalin 800, C,R.39, and the Marco Resins S,B.26 C and S,B,28 C, in
conjunction with Light Alloy, Magnesium and Mild Steel.
The fundamental problem of producing good adhesion betvreen the photoelastic material and the metal surface,coupled with satisfactory
light reflection from the latter, comprises the major part of the initial investigations.
The teira "Metoplastic" is suggested to describe concisely the use of compound specimens for the photoelastic applications considered, Results of the measurement of both elastic and plastic stress concen-trations at holes in plates subjected to uniform tension are presented, together with a qualitiative analysis of the effect on stress distrib-ution of the variation of the pin to hole clearance in lugs.
Owing to the fairly extensive natiire of the theses investigations, it has only been possible to outline the more salient features of the work xandertaken.
This report is based on a thesis subioitted as partial requirement far the avrard of the Diploma of the College of Aeronautics.
1,1 Introduction
1,2. Optical coefficients
1.3 Choice of photoelastic materials 1.4 Photoelastic materials
1.5 Adhesives
1.6 Surface prepeoration
2.1 Test equipment
2.2 Reflection polariscopes
2.3 Universal t a b l e and stand
2.4 Camera2.5 Camera shutter 2.6 Film
3.1 Test specimens
3.2 Metoplastic test specimens 4.1 Test procedure and results
4.2 Bonded stress concentration specimens K,L,M 4.3 Analysis
4.4 Bonded lug specimens R4/l,S4A, and R4B, Si+B 4.5 Direct cast lug specimens R1A and S1A 4.6 Bonded and direct cast Ivig specimens 5»1 General discussion. 5,2 Conclusions Acknowledgements References Appendix Tables 2 to 8 Figures 1 to 24
The principle of the direct measiirement of strains in a ractal stirface using the mediiom of an optically sensitive material bonded to the surface has been suggested by a number of research workers, notably Mesnager^j and Oppel who describes his work in
some detail, although xmfortunately a translation of the latter reference was not obtained until June 1952.
Their v/ork was greatly handicapped by the lack of suitable photoelastic materials and adhesives, and, probably ovdJig to these difficiilties no ftirther development in the technique seems to have beeo^reported since 1937 until quite recently in a paper by Drucker.^ ' •
The Metoplastic Technique
"Metoplastic" is the term which has been coined to concisely describe the method of using a compound assembly of metal and plastic for direct photoelastic strain measurement,
Description of Technique
To the siarface of the metal sheet or coniponent in which it is desired to investigate the strain distribution is bonded a thin sheet or layer of photoelastic material of uniform thickness.
The compound assembly is loaded and the resxiltant isochromatic pattern is viewed by projecting a polarised beam of light onto the photoelastic material; the information contained in the beam reflected from the surface of the metal is analysed by a reflection polariscope in the usual manner and since it passes tv/ice throiogh the plastic the apparent strain-optical sensitivity of the combination is doubled.
Photoelastic material with a nominal thickness of 0.125. in. T/as used almost entirely throiighout the thesis work due to necessity rather than choice, but it appears that the thinnest practicable coating is to be preferred consistent vd.th the desired sensitivity on fringe order required over the range of investigation contemplated.
Clearly the photoelastic material should only be subjected to the strain vAiich is communicated via the layer of adhesive or bonding meditan and not to the direct application of external forces, otherwise fictitious isochromatic patterns vd.ll re suit.
tapered type can be tested in tension to determine the strain-optical coefficient of the combination but in some instances, as in orthodox photoelastic work, it is possible to estimate a fringe, or other coefficient directly from the component itself.
Two methods of bonding have been studied in the present investigations, for convenience the test specimens are here discussed under two separate headings according to the process of manufactiire as;
Bonded Specimens.
Specimens manufactured by cementing sheets of optically sensitive material to the metal surface.
Direct Cast Specimens.
Specimens manufactured by casting and polymerising optically sensitive materials directly into the metal surface.
1.2 Optical Coefficients
The optical coefficients of metoplastic specimens in regions of stress far removed from free or loaded boundaries would appear to approximate to the coefficients determined for the photo-elastic material alone \inder similar conditions. Hoxrever, the latter coefficients are modified by several important factors namely,
(i) The non-viniform deformation occurring across the thickness of a plastic coating under load when bonded by one face to a metal surface.
(ii) The gradual transfer of load from metal to plastic at the boxindarics of the plastic which may not necessarily coincide with those of the metal component, this is of particular interest
in the case of loaded boundaries such as occur in lugs.
(iii) In specimens bonded by means of an adhesive, effects arising from the shear deformation of the adhesive film, the
deformation is thought to be of little consequence in a thin film, i.e. of the order of 0.002.in. or less hence to date has not been studied in detail. The absence of an adhesive film in direct cast specimens precludes the necessity of exeimining this possible source of error.
(iv) The proportion of the total external applied load v/hich is distributed in the plastic material of both bonded and direct cast specimens and v/hich can be of significant magnitude when these methods are used in conjunction v/ith metal sheets of comparable thickness and low Young's modulus.
1.3 Choice of Photoelastic Materials
The choice of phtoelastic materials for the type of application considered is not only governed by the physical and optical properties of such materials but also by those of the material to which it is bonded.
Thus, although in general a high strain-optical sensitivity for the photoelastic material is advantageous due consideration must also be given to properties of the metal, e.g. Young's modulus E, the limit of proportionality, and the general shape of the stress-strain characteristic.
It has been fotind that existing photoelastic materials shovz very little relative advantage over one another when used for metoplastic investigations.
Some of the desirable features of a suitable material can be listed as follows: we require,
(i) A good optical sensitivity, i.e. a low strain-optical coefficient,
(ii) A high stress-strain, amd strain-optical proportional limit, up to fractui*e if possible.
(iii) A lov7 value of Young's modulus E consistent with (i), (iv) Good creep characteristics.
(v) Good machinability,
There are in addition notable advantages in being able to cure casting resins at room temperature.
Requirement (iii) is introduced since of t\To materials with the same strain-optical coefficient that material possessing the loY/er value of E vrould be more acceptable on accoimt of it offering less restraint to the metal surface to which it was bonded
The requirement only assumes some importance when application to thin metal sheets is considered,
1,4 Photoelastic Materials (i) Hard Gelatine Sheet
The potentially attractive optical and physical
properties of hard gelatine sheet were examined briefly by Scott'' • but difficulties of obtaining the material in thicknesses greater
th&n 0,007.ih.forestalled further experiments in this direction, Enquiries revealed that the manufacturers (at the time) were not prcpai'od to specially produce hard sheet, 0.05.in. to 0.100.in. thick.
A Young's modulus E of about 3,25 x lO*' lb,-in . and a strain-optical coefficient better than 0.0001 ins.-in.-N.-in. v/ere suggested by tests on specimens 0.007 in. thick,
(ii) Catalin 800
Preliminary tests only were made to determine the suitability of Cats.lin 800 owing to initial birefringence, poor surface finish esid. lack of flatness of most of the material immediately to hand.
(iii) C.R. 39
Columbia Resin vra.s employed to a large extent on the bonded type of specimen mainly because of its freedom from initial birefringence, clearness, together with good s\irface finish and flatness, rather than from any outstanding advantages afforded by its optical and physical properties,
(iv) Marco Resin
S.B. 26 C. Casting Resin. S.B. 28 C. Surface Coating Resin. The Polyester Resins, S.B. 26C. and S.B, 28C. formed the
main basis of direct cast specimens.
Experiments carried out to ascertain the effect of varying the proportions of the various constituents of S.B. 26C. resulted in the mixtures shoT,vn in TABLE 1. below being chosen as those giving the optimim optical-physical properties * for the application considered,
The proportions for S,B. 28C. vrere chosen by intuitive rather than analytical methods owing to the fact that supplies of this material v/ore not obtained until, toward the closing stages of the investigation, the proportions given therefore do not necessarily represent the optimum mix,
TABTE. 1 . P r o p o r t i o n s of C o n s t i t u e n t s . 1 Marco R e s i n S.B.26C. and S,B.28C. Item Nunbcr 1 . a . 1 . b . 2 .
3.a.
3.b.
4. 1
C o n s t i t u e n t M.R.S.B.26C. rL.R.S.B.28C. MOMOMjiK, C. diT/LYST KJ'TER H.C.H. CPIT/ilST FASTS H. ACCSLERATGR E . P a r t s by Weight S.B.26C. M i x . l . 1 0 0 . 4 0 . 2 . 1 . S,B,28C. j M i x . l . 100. 4 0 .4.
4.
S . B . 2 8 c . , Mr!c.2. 100.5. 1
4. j
4.
Largely confirmed by RSF. 33. and by tests conducted by Sugarman, The B.S.A. Group Research Centre, Sheffield.
Constituents 3.a. and 3.b. are similar, two parts of Catalyst Paste H being equivalent to one part of Catalyst Pov/der H.C.H.
Particular mixes used for metoplastic specimens of which photographs are included in the report are defined by
6
-quoting the material, e.g. M.R.S.B,26C., followed by the proportions of the constituents in the manner, M.R,S.B.26C. 40.-2.-1,
representing items 2, 3a, and 4, respectively.
(v) Other Photoelastic Materials
Kriston, Grade A-2, Monomer, (Catalyst, Benzoyl Peroxide), and also Xylonite vrere briefly examined,
Kriston possessed the disadvantages of brittleness and of requiring a high temperature cure while Xylonite exhibited poor optical sensitivity which discouraged its immediate application.
The sensibly linear stress-strain and stress-optical characteristics of Kriston ^ ° ' up to fracture are worthy of note,
1.5 Adhesives
The production of a s a t i s f a c t o r y bond between p l a s t i c
sheet and a plane surface v/hile f u l f i l l i n g the c o n f l i c t i n g r e q u i r e
-ments of the technique was not i n i t s e l f as straightfor\7ard a s was
a t f i r s t a n t i c i p a t e d .
Some of the d i f f i c \ i l t i e s encountered are discussed i n
Section 4 . 2 .
The main p r o p e r t i e s required i n a p o t e n t i a l l y s u i t a b l e
adhesive a r e :
-( i ) Good p l a s t i c to metal adherence.
( i i ) High sliear s t r e n g t h ,
( i i i ) High shear modulus.
( i v ) Cold c\are, i . e . a t room temperature or a t low oven
temperature,
Q u a l i t a t i v e t e s t s were c a r r i e d out on simple
meto-p l a s t i c smeto-pecimens of the tymeto-pe shown i n PIG. 4 . (see n o t e ) , i n
order to assess the p r o p e r t i e s of the various adhesives and siurfaco
p r e p a r a t i o n s , the occurrence of r e l a t i v e s l i p betvveen the p l a s t i c
s t r i p and the metal p r i o r t o \iltimate f a i l u r e of the assembly a s
a v/hole suggested the a r b i t r a r y c r i t e r i o n of f a i l u r e .
The extent to wliich the p l a s t i c had been separated from the metal
by the shock of fractiore was a l s o noted.
Amongst the several adhesives investigated were:-(i) "/o-aldite" 101 and Hardener 951.
(ii) "Catacol". (iii) "Dtirofix",
Adhesive (i) was used for the bonding of Catalin 800, C,R.39., and (ii) and (iii) for bonding Catalin 800 ajid Xylonite respectively,
Finally, "Araldite" 101 was chosen as being the most satisfactory adhesive for general use.
The Marco Resins employed in the direct casting method formed an integral self-bonding medium and in this respect realised fairly satisfactory characteristics.
As vrLth cemented specimens qualitative tests v/ere performed to assess the .bonding efficiency of various surface treatments using the specimen shown in fig. 4.
Types of siorface treatment v/ere rejected if separation took place between the cast plastic layer and the metal control specimen prior to \iltimate failure.
1.6 Surface Preparation
Careful preparation of the metal and plastic contact surfaces provided the essential basis of good adhesion but equally important in the case of the metal the satisfactory reflection of incident light.
The former requirement together with the impracticability of obtaining an optically flat metal surface, - analagous to the
surface silvered mirror sometimes employed in orthodox doubling polariscopes, suggested that specular reflection was out of the question, in any event distortion of the component or structure under load would have very pixjbably offset the rather doubtfvil advantages to be gained from such a process.
Attention was therefore focussed on the suitability of sandblasted surfaces, a compromise so far as the problems of cement adhesion and light reflection v/ere concerned.
Metal and plastic siorfaces sandblasted with fine grain sand at an air pressvire of about 50 Ib.-in.^ were found in practice
to afford a satisfactory key for the adhesive, the very fine matt surface obtained on both materials also gave an excellent diffused reflection from the surface of the metal.
Specimens in light gauge material shov/ed a tendency to warp as a resxilt of the compressive stresses introduced by sand-blasting, a fa\ilt which had to be corrected prior to final assembly,
Experiments v/ere also carried out for comparative purposes on surfaces prepared with fine-grade emery cloth,
carborundum paste, and others b-urnished with one of the well known makes of commercial metal polish^"», e.g. "Silvo", and "Brasso".
Surfaces prepared with emery cloth alone while bright and reasonably imiform in textiire oould not be described as being highly polished. Polished metal surfaces although possessing efficient light reflection characteristics v/ere found to be
considerably more successful when used in conjunction with direct cast specimens.
The reflection characteristics of sand-blasted surfaces of the metal in direct cast specimens v/as markedly improved by a thin layer-casting consisting of aluminium pov/der in Marco Resin coated on v/ith a fine cloth. The coating v/as allov/cd to cvire in the normal manner prior to the casting of the main layer in the same resin.
The metoplastic specimen so produced consisted there-fore of a sand-blasted finish follov/ed by tv/o separate resin castings self-welded together to produce v/hat might be considered to be virtually a single homogeneous casting,
These experiments follov/ed on unsuccessful attempts to introduce aluminium powder into the adhesive film of bonded specimens (as a filler), to improve reflection characteristics.
2.1 Test Equipment
Transmission and Reflection Polariscopes ! • • • • I • •'••• • • • • • — • • • • • I H ••« •!• • • • • > ' • ' —I
Polarising and analysing elements together v/ith the associated optical equipment fitted to the reflection polariscopes, Types 1 and 2, v/ere adapted from a standard transmission type bench
supplied by The Norwood Engineering Co. Ltd.
The latter apparatus v/as designed for an optical system having a clear field diameter of 4.0 in.
The reflection polariscopes employed a different and necessarily more pov/crful light source, details of which are given below,
Light source "Mazda" coinpact source Mercury Arc Lamp v/ith
transformer.
PCTz/er rating , 250 W,
Maximaim illvmra^.'.ion 18,000 candles /Cm^» Aper-:.v.t'o in lamp housing 2,0 in. dia.
Reflector None Light filter ^ffJ/.TTETf No. 77.
(Protected by v/ater bath) Initial waming-up period Approx, 15 min.
Transmission and reflection polariscopes were arranged to give a dark field and circularly polarised light.
2.2 Reflection Polariscopes. Types 1 and 2.
A self-explanatory diagram of each of the types of reflection polar-iscope used is given in figs 1 and 2.
The majority of the specimens tested were physically larger than the diameter of the polariser and other optical elements hence the presence of a divergent beam of light was unavoidable in both cases a state of affairs which while not entirely satisfactory had to be accepted.
To assist in the maintenance of acciiracy the angle of parallax ex was held to a minimum compatible with limitations imposed by the physical dimensions of the various polariscope elements and by the size of the photographic image required,
During the test a mirror, M 1, was held between the analyser and the camera lens vdien it vra.s desired to examine the development of a photoelastic pattern during loading cycles between photographic exposures. /inother mirror M 2, (fig 1.), used in conjunction v/ith a small disk of "Polaroid" (as an analyser) v/as found e:ctremely useful as a means to survey the pattern in any
metoplastic specimen i-emote from the source of light, e.g. where the specimen formed one half of a double assembly, or where photoelastic material was bonded to both sides of a single metal sheet,
The Norrenberg Doubler, - polariscope Type 2. was designed to eliminate errors arising from the parallax angle :/^, associated v/ith Type 1,
2,3 Universal Table and Stand
An existing table and stand modified to carry the optical systems of both reflection polariscopes permitted the alignment of a polariscope vidth respect to the testing machine by translational and rotational movements of the upper section of the table.
The camera, light so\arce and the associated equijanent of each half of the polariscopes were moixnted on tv/o separate tubular rail assemblies; provision was made for bolting the rails in predetermined positions corresponding to the requirements of each type of polariscope.
2,4 Camera
A standard ex-R.A.P. P.24. aerial survey camera vra.s utilised in conjionction with both types of reflection polariscope to obtain all the photographs taken during the investigations,
The camera originally incorporated an f 2,9 lens (less yellow filter), focal length 8,0 in., designed to focus at infinity.
The camera body was therefore machined internally to take a light alloy extension tube on v/hich v/as remounted the lens and inter-lens iris assembly in order to focus by sliding
adjustments on objects about 3 ft. to 8 ft. distant from the lens. Quick release fittings in the camera cradle and base allowed the direct observation of photoelastic patterns at any time during test v/ithout disturbing the pre-focus position of the camera itself,
2,5 Camera Shutter
The focal plane normally supplied vïith the camera vra.s removed, in lieu a lens-cap plus stop-watch were found to be
s a t i s f a c t o r y for the time exposiires v/hich were g e n e r a l l y never l e s s
than about 10 seconds i n duration, a t f 8.0
2 . 6 . FiHm
Film type , Panchromatic
Film Speed, (;.,. , . , , . , . , .Cnnpr.rable to I l f o r d . H . P . 3 .
( I l f o r d meter 3 2 ° ) .
Format ,.,.,...• ....5f in. x 5f in.Ijpe of cassette ,,.,..standard
Number of exposures loaded, 50
3.1 Test Specimens
Photoelastic Material Control Specimens
The v/ork of reference 2 v/as commenced late in 1950 with tensile tests on the common materials projected for metoplastic application in.order to obtain information on the Young's modiilus E, the proportional limit, and general shape of the optical and stress-strain curves up to fracture.
The detail resiilts of these tests need not be of concern here since several existing handbooks and reports deal comprehensively vd.th this subject.
3.2 Metoplastic Test Specimens
Altogether a total of 24 tapered, (14 bonded and 10 direct cast) and 18 parallel shank specimens (all direct cast)
were majiufactured to the dimensions given in figs. 3 and 4, respectively, covering different combinations of metal, photoelastic material and
surface treatment.
Early vrork on bonded specimens was undertaken with specimens of tapered form chosen to provide a linear stress gradient along the shank and a correspondingly simple photoelastic pattern suitable for p-urposes of calibration v/hen required and acceptable for development work in connection with the reflection polariscopes.
After experiments with the tapered specimens had perfected the method of cementing and had finalised the choice of photoelastic material a further set of specimens were prepared to illustrate a practical application of the technique in the form of the attempted examination of the stress distribution and concentration across the transverse section of symmetry in plates with circular ,and oval holes subjected to uniform tension, (fig. 6 ) .
Later investigations were concerned v/ith qualitative tests to discover the practical adaptability of the technique when applied to loaded holes, in particular the influence of the pin to hole clearance on the stress distribution across the axis of
symmetry,
A series of 4 mild steel lugs with bonded photoelastic material were tested in conjunction with Henv/ood-^» who by control
tests confirmed that preferred grain orientation in the batch of material selected for the lugs v/as non-existent v/ithin the limits of observation. The proportions of the lugs, (fig. 11) v/ere originally chosen to precipitate failure in shear,
Initial vrork on direct cast metoplastic specimens v/as concerned v/ith the parallel shank type shovv'n in fig. 4, and
later with tapered specimens of the type previously described, The investigation v/as concluded v/ith a qualitative assessment of the isochromatic patterns produced in lugs
manufactured by the direct casting method, (fig. 12).
When comparing photographic records due allowance should be made for variations in the negative printing process, variations between sets of prints, and also the report method of reproduction.
Similarly it may be noted that -öic thickness of the photoelastic coatings has not been strictly maintained constant
throughout the investigation, the apparent strain-optical sensitivity exhibited by certain metal-plastic combination is liable to be
4.1 Test Procedures and Results
Tapered and Parallel Shank Specimens
Tapered and parallel shank specimens v/ere loaded in increments of 200 lb. up to about 1,300 lb. generally the maximum for both types.
At the higher loads increments were reduced when desirable to 50 lb. or 25 lb. to offset the rapid increase in
isochromatics due to plastic deformation particularly at the narrov/ neck of the tapered specimen.
Scott investigated the case where sheets of photo-elastic material were bonded to both sides of D.T.D. 6lO B. tapered specimens but concluded that there was no appreciable difference in the photoelastic resiilt v/hen the plastic was bonded to one side or both sides,
Photographs at various stages of loading of a fev/ of the specimens tested in the whole series are given in figs. 15 to 18. together with details of the metal and photoelastic material employed and also their respective thicknesses denoted by, "t",
All the photographs shown v/ith the exception of
fig. 17.(7)., were taken in conjunction with the Type 1. reflection polariscope. (fig.l).
Pig. 15 (l) and (2) show typical results obtained using C.R. 39 in conjunction with D.T.D. 610 B. sheet and tv/o types of s\irface finish at 0.1b. and 1,200 lb. load,
The initial birefringence in the photographs taken at zero load reflects one of the inherent features of the bonding technique, i.e. the unpredictable nature and extent of the stresses arising from the pressures applied during assembly, it is noticeable that these stresses do not generally occur to any great extent at the ends of the plastic adjacent to the pick-ups.
In this respect it is never possible to judge the success or otherwise of a given assembly until after the
irrevocable ciure of the adhesive.
The Catalin 800 layer in fig. 15 (3) zero load, also shov/s initial birefringence but which in this case is very probably
due to stresses in the basic sheet rather than stresses arising from assembly,
The photograph at 1,200 lb. may be compared with (l) and (2) at the same load.
A photograph taken at 1,500 lb, has been included for specimen (3) because it shows "pairing" of the fringes on the tapered shank, tov/ard the neck; this phenomenon was also observed dviring tests on the bonded lug specimens and is commented upon in the appropriate section.
Table 2 shows the tjrpical method used, during the course of work in reference 1 to deteiTiDdne the strain-optical sensitivity in terms of a strain per fringe value, F.
The figiirc numbers, J2, J3 refer to the different load levels at v/liich the corresponding photographs v/cre taken and is the notation originally used.
In determining F. an allov/ance has been made for the proportion of the total applied load vAiich is distributed in the plastic;
Within the elastic range, If E = Young' s modulus
= 2,55 X 105 Ib.-in? for C.R.39. = 10,30 X 10^ Ib.-inf for D.T.D.6IO.B. and t = Thickness of material.
= 0.130 in, for C.R,39 = 0.065 in. for D.T.D.6IOB.
the proportion of the load taken by the plastic
^ (E). C.R.39 (t). C.R.39 X (E). D.T.D.610B, (t). D.T.D.61QB. = 0 ^ , 0 0 3 0 ; 0^050^ 10.30 0.065 say d% Thus P NET. = P - (P X 0,05). lb.
The values of P given in the table are quite consistent over the range of loading recorded.
Photographs at three stages of loading, 0.1b, 600 lb, and 950 lb. are shov/n in fig. 16 (4) and (5), for two types of Marco Resin, S,B,26C, and S.B.28C. cast directly onto sandblasted magnesium alloy sheet, D,T,D, II8A.
Initial birefringence, apparently due to contraction during polymerisation, is marked in both specimens at zero load noticeably again in the shank rather than at the extreme ends of the layers except at the top of specimen (5).
The seemingly high sensitivity of the Marco Resin -D.T,D. 118A. combinations compared with specimens (l), (2), and
(3), is largely the result of the low Young's modulus for the latter material, i,e, about 7,0 x 10°. lb. -in. from fig. 14, The strain-optical sensitivity of the Marco Resins in sheet foira is substantially inferior to that of both Catalin 800 and C,R. 39.
Fig. 17. (6), and (7), (the same specimen) give a comparison betv/een the results obtained using polariscopes, Types 1 and 2 respectively, but employing the same light source in each case. Contraction stresses vAiile of the same order do not appear to be quite as extensive as those in specimen (4)j fig. 16.
The dark xinifoim appearance of specimen (7) partly derives from the residual stresses impressed d\jring the previous loading as specimen (6). Photographs of specimen (6), at 800 lb. are not available since the loading vra.s restricted in order to limit the severity of the stresses referred to above,
The type of results experienced during early vrork on the direct casting method are given in fig. 18 (8); to (ll) inclusive,
To the degree limited by reproduction the photographs
show the different types of definition associated v/ith a particular > type of metal surface finish and also the xiniformity of retardation
throughout the length of the parallel shank,
«
The plastic coatings adhered to the metal sheet up to fracture of the specimen as a whole, in all cases.
4,2 Bonded Stress Concentration Specimens K, L, M.
Dimensions of the stress concentration specimens and details of the types of discontinuity investigated by Scott"" are given in fig. 6.
For simplicity and also to reduce the amount of labour involved in the analysis, values for the strain per fringe coefficient P. were estimated directly from the (p - q) curves, i.e. neglecting
the contribution made by the shear stress (q) across the test section X - X, fig. 7..
The mean fringe vs.lues obtained by direct integration therefoix) tend to overestimate the magnitude of F. irrespective of allowances made for the percentage of the total load taken by the plastic.
The method of analysis applied to all three specimens is described for Type K in particular but is typical of the procedure adopted tlirougiiout,
4,3 Analysis
Part of the total scries of ^^ photographs of isochromatic patterns obtained over a range of loading for the tliree types of specimen are given for tv/o load levels in figs, 19 and 20.
Curves of fringe order, proportional to the strain in the metal and derived from, similar photographs throughout the range of loading have been plotted in fig. 8.
The mean strain per fringe value F was next determined. This was accomplished by finding the mean fringe value of a curve wholly v/itliin the elastic range of the metal and relating it to the external load as shown in Table 4 after an allowance had been made for that proportion of the total applied load which was distributed in the photoelastic material.
The method of obtaining this allov/ance, about 5^> has already been demonstrated for the tapered type of tensile specimen.
Taking F = 0.00127 ins. - in, - N., the fringe order or number from Table 3. can be related to stress magnitude in the metal plate. Table 5. by mea.ns of the stress-strain curve for D.T,D. 610.B. given in fig. Hf.
Hence, by the combined use of Tables 5 and 5 stress distribution curves are obtained for each load level; these are plotted in fig. 9 for specimen K. and clearly show the effect of plasticity at the edge of the hole, fH, with increase of load.
the average stress f A, were estimated. Table 6, The average tensile stress, f A, v/o.s fovind by integration of the stress distribution
curves across section X - X and is in consequence slightly lower than the average stress computed by using the net applied load.
Curves of f^/fE, and f[]/fA, liave been plotted for the three types of specimen in fig. 10. with the appropriate
geometrical factors for elastic conditions given by Hov/land i^- and HeywDod,^"' shov/n in dotted line,
At low loads, when the stress-strain curve of the metal lies entirely v/ithin the elastic range, it is to be expected that the curves of stress concentration v/ould tend to flatten off giving a constant value corresponding to the elastic stress concentration factor, e.g. for k1 irfien
f 4.. ^^*°°° = < 10,000 Ib.-in^. approximately, 2.50
Hov/ever, this trend is not apparent and hence in conflict v/ith established results; Scott considered this to be due to conditions inherent in the test possibly arising in part from the low length/breadth ratio of the plates coupled v/ith the non-uniform distribution of load input at the ends of the plates.
At about the proportional limit of the metal, i.e. 25,000 lb,-in? the stress concentration factors k1 and k2 approx-imate to 1.60 in all cases, above 25,000 lb.-in. the curves are similar in shape and appear to be asymptotic to tmitv at the ultimate stress, from fig. 14. fULT. = 45,000 Ib.-in^*
The strain per fringe values determined for the three specimens v/ere respectively
Type K. " L. " M.
P
II II = 0.00127 ins,-in.-N. = 0.00123 = 0,00150 "The small variations in F were attributed to the different elastic and optical constants and also the thickness of the separate sheets of C.R,39 employed for the manufacture of the specimens,
A generalised form-'-'-'' of an equation originally presented by Stowoll°- connecting the elastic and plastic stress
to the results given in f i g . 10.
K p l a s t i c = 1 + (K e l a s t i c - l ) =—^
v/here,
K.plastic = stress concentration factor in plastic range. K.elastic = stress concentration factor in elastic range. Es. = secant modulus of mü'.terial at point of maximum
stress.
Ee>c.' = secant modulus of material at points far removed from the discontinuity.
The generalised equation, solved by trial and enror methods, is apparently in good agreement with prior investigations-^*^- but gives results 5?o to IC^ti in excess of the curves shown in fig. 10. over the range fE. = fA. = 10,000 lb.-in? to fE.= fA. = 45,000 lb.-in?
4,4 Bonded Lug Specimens. Rl\A, S4/., and R4B, S4B.
No attempt was made to analyse the results obtained on a quantitative basis due to the lack of test data and information concerning the magnitude of certain correction factors (see section 1.3) knov/ledge of vAiich vra.s essential for accurate analysis
particularly in the regions of the coniplex stress systems adjacent to the hole.
The dimensions of the bonded lugs are given in fig. 11 and the assembly'- in which like pairs of lugs were assembled back to back ready for test is shov/n in fig. 13.
Photographs of the isochromatic patterns ob-bained during two separate test irins are presented in figs. 21 and 22.
Dturing the first run, (R2JA and S4A facing the camera), lug Slui. v/as observed to have an ini-tial, asymmetrical birefringent pattern presumably arising from pressure applied during manufacture,
this may be compared with the sensibly stress-free condition in R4A. at zero load.
Attraapts v/ere made to clear the fault in lug SAA since the corresponding type, S4B had deep scratch maj-ks on the outer surface of the plastic, accidentally produced,
Load \vas gradually increased up to a maximum of 12,000 lb. (6,000 lb. per lug) in increments of 2,000 lb., however, as the fault persisted it was decided to interchange ispeoimens S4A and S4B, fortunately, #iile permanent set had occurred in lug R4A (large pin to hole clearance), lugs S4A and S4B (small pin to hole clearance) were relatively unaffected,
The method by which S4B "vms surveyed prior to the inter-change is indicated in fig.l.
During the second test run load was again applied in increments of 2,000 lb. up to the apparent yield, point at the net section as determined by the sudden and disproportionate increase in fringe order, for this reason increments v/ere reduced to 1,000 lb. above the yield point.
The different and changing effect of pin to hole clearance on the stress distribution in each type of lug throughout the loading range up to 18,000 lb. is demonstrated by the isochromatics in fig. 22,
Up to about 15,000 lb. the beneficial influence of a small clearance is seen to be very clearly marked in lug SAB but between 15,000 lb, and 16,000 lb, because of plastic flow the pins in both lugs appear to transmit load through a large proportion of their pixDjected diameter, in consequence above about 15,000 lb. the beneficial effect is largely nullified,
The photograph of the fully developed pattern at 16,000 lb, load is of particular interest since it shows the extensive region of compressive stress produced by the neat fitting pin in S4B vdth a comparatively small region of tensile stress at the outer periphery on the centre-line of the assembly,
The extent of the tensile and cdnpressive stresses in
"RhA appear to be approximately equal,
At 17,000 lb, failure of the C,R,39 and adhesive occurred • in lug R4A just prior to the photographic exposure followed by a similar failure in the plastic on S4B at 19,000 lb.
A photograph of these failures given in fig. 23. shows the symmetrical extent of the adhesive failure about the axis of loading (light region) and the greater area of this failure in S4B.
Above loads of 10,000 lb. ( R V O , and 14,000 lb. ( S 4 B ) diffuse isochromatics can be observed on both sides of the axis of loading wiiich coincide approximately v/ith the location of the fail\ares reported by Henwood3 for lugs of similar type and
Kanaalstraat 10 - DELFT 20
-The apparent "pairing" of the isochromatics at loads of 18,000 lb, and previously noted on tapered tensile specimens cannot at the moment be explained satisfactorily.
4.5 Direct Cast Lug Specimens R1A and S1A
Part of a series of photographs obtained during tests on direct cast lugs are given in fig. 24. The dimensions are given in fig. 12.
Since only two lugs were available for test, R1A (large pin to hole clearance) and S1A (small clearance) it v/as not possible to assess the effect of pin clearance on the stress distribution,
The lugs v/ere assembled baok to back for purposes of test with a loading bar interposed be-tween them, essentially forming one half of -the assembly shov/n in fig. 13.
Considerable initial birefringence can be observed in lug R1A, being most marked rovind the periphery of the hole and at the
sides of the lug, to v/hat degree these stresses were due to contraction during casting and restraint by the me-fcal was not established,
The marked isochromatic pattern produced by local
bearing of the loose fitting pin seems somevdiat surprising when the over-all proportions of the lug and the comparatively low optical sensitivi-by of Marco Resin are considered.
A gradual transition may be observed up to about 20,000 lb, load,(10,000 lb, per lug) as the proportion of the diameter of the pin transmitting load increased due to local yielding at the "point" of contact, however, at 24,000 lb. a high percentage of the bearing area of the pin appears to have become effective.
Diffuse isochromatics, similar to "those observed in the bonded specimens are also visible and as v/ith the latter these
correspond v/ith the location of ultimate failure.
4.6 Bonded and Direct Cast Lug Specimens
Tables 7 and 8 give details of the bonded and direct cast lug specimens and hole dimensions before and after test together with the failing loads and associated stresses reported by Henwood--* for lugs of similar proportions.
5.1 GENERAL DISCUSSION
The investigations have been primarily concerned vd.th some of the practical problems associated with the metoplastic technique, they have in this respect attempted to solve some of these problems vdth the best means available at the time.
The value of the method, although limited in scope by the optical and physical properties of existing photoelastic materials, would appear to lie in its ability to present an over-all stress pictiire in the elasto-plastic and plastic range of a metal component.
Moreover, the deteimination of stress distributions and stress concentrations in components tested \inder representative conditions can be greatly facilitated since the mschanisra of strain measurement is not dependent on the existence of elements of finite gauge length,
Experiments described in reference 10. clearly demonstrate the influence of the gauge length of strain measuring devices on the dimensions of the test specimen, •
In general -üxe lateral constraint offered by conventional photoelastic materials together with a low strain-optical sensitivity prohibits their use (at present) on structures fabricated in very
thin sheet material, on surfaces of compound c\irvature, or v*.ere buckling forms the criterion of failure.
The examination of surfaces of simple curvature appears to bo a feasible proposition and is discussed later.
The somewhat inconclusive test resiiLts obtained to date tend to show that analysis can be carried out on a quantitative basis as exemplified by the stress concentration specimens Scott investigated and by -Khe quite remarkable photographs he obtained during the course of his work,
Apparently the analysis of metoplastic information may be carried out using essentially standard procedures provided these are discretely modified in order to cater for the various factors and errors peculiar to the method.
The influence of the pin to hole clearance on the stress distribution in various types of liig, vd.th increase of load, has demonstrated the sensitivity of the technique and although the results obtained from the direct cast lug components were rather poor the effective strain measuring sensitivity of M.R.S.B.26c.aE appeared to be acceptable in the plastic range of the specimens tested; the tests on both bonded and direct cast lugs show that a s\ifficient n\amber of fringes, accompanied by comparatively small deformation, can be realised for quantitative analysis.
The rather more sensitive "Araldite" casting resins are now being studied at The College of Aeronautics,
Worthy of note is the influence of the pin to hole clearances on the shape and extent of the adhesive failure in the bonded lugs.
Evidently the suitability of a given photoelastic material for metoplastic applications at large strains cannot necessarily be assessed from the magnitude of the strain-optical coefficient estimated v/ithin the elastic range of the material,
however, the optical and physical characteristics of many photoelastic materials are fortunately very nearly line8.r up to a strain of about 0.010 ins.-in. and can be closely represented by a straight line, i.e, up to quite high stresses in the metal sublayer.
Experience suggests that a thickness of about 0.05 in. might be regarded a.s the optimum minimum thickness for photoelastic materials applied as coatings but one which perhaps it will not be possible to achieve in the near future unless plastics having a more accep-fcable strain-optical sensitivity are developed.
Although this figure is entirely arbitrary it is easy to visTjalise difficulties arising from the use of coatings very much less than 0,05 in, because of the practical problems concerned with their manufact\ire, (commercially and by the individual) to the
required thickness and limits of accuracy. There would also appear to be formidable problems associated v/ith the handling and cementing, 0X1 the casting and machining of large sheets of very thin, highly sensitive material to form flat coatings free from initial bire-fringence,
Application of the photoelastic m£?.terial by spraying also offers not inconsiderable problems of thickness control.
Initial birefringence caused by pressxires applied to bonded plastic coatings during assembly and by contraction
(presiimably) of the resin in direct cast coatings, has proved to be troublesome and \inpredictable and not entirely related to the
skill of the operator although manipulative experience is of considerable assistance.
li/hatcver the mode of application it is necessary to remember that the metal, unless of heavy gauge or machined, will infrequently possess a truly flat surface, v/hen casting distortion of thin metal sheets is liable to occur due to contraction of the resin coating during polymerisation.
A thin coating is however desirable and confers certain benefits, mainly in the alleviation of the magnitude of the factors mentioned in section 1,3.
If the metoplastic technique is to be extended to include analysis in the elastic range of a metal structure, v/ithout
resorting to methods of compensation for fractional fringe order determination, it would seem likely that a material having a strain-optical sensitivity of say 5 to 10 times that of C.R.39 is required.
As an interim measure methods of compensation may have to be employed v/hile existing materials are improved or new materials developed; the use of compensatOTB detracts considerably from the directness of the technique and may also involve holding structural components for inconveniently long periods of time while a search procedure is carried out, likewise the examination of dynamic
stresses cannot be londertaken.
Within the elasto-plastic and plastic range of light
alloys and steels the sensitivity of existing photoelastic materials appears to be reasonably adequate. It may therefore be found most
convenient -bo utilise materials of high strain-optical sensitivity in the elastic range of the metals concerned and another v/ith a lov/ strain-optical sensitivity, .i.e. existing materials, in the plastic range.
For all practical purposes existing plastics can be described as leaving the same order of optical sensitivity and hence their merit in indi-vidual cases may have to be decided on
the basis of other desirable properties.
While a sandblasted surface has been found to provide an excellent diffused reflection for isochromatics the same did not hold for isoclinic fringes, the phenomenon has as yet only been briefly studied but may bear some relation ix> the variation of strain across the thickness of the plastic, nevertheless it was observed that increased definition of both isochromatics and
isoclinics resulted from the use of aluminium powder applied with Marco Resin as an extremely thin coat prior to the main casting
of direct cast specimens.
Limited experiments with a closed casting technique in which a siurface casting is formed between the metal component and an auxiliary surface of "Perspex" (for example) coated v/ith a separating agent and spaced 0.125 in. away from the metal surface whilst not conclusive, v/ere indicative of the possibilities of
this extension of direct casting when applied to rigid or flat me-fcal surfaces. Castings made in this v/ay might be further extended to include simple curved surfaces.
The results of some tests on the rapid curing of resins by the passage of an electric current, reported by Evans^7. are
of considerable interest in this connection insofar as they suggest yet another method by which siirface coatings could presumably be manufactured.
It is concluded that the technique may well be
adaptable to the examination of dynamic stresses in coniponents subjected to repeated loading when more sensitive photoelastic materials become available.
5.2 Concluding Remarks
Aspects of further research into problems associated v/ith the technique may be grouped naturally under three main headings,
(i) Accuracy
First it seems essential to assess iiie accuracy of the method by a series of well designed experiments in Vi^ich all
conditions are carefiilly controlled, coupled v/ith a detailed analysis,
(ii) Photoelastic materials.
Existing materials appear to have adequate sensitivity for use in the elasto-plastic and plastic range of light alloys and steels but apparently it will be necessary to develop more sensitive materials for analysis in the elastic range of -Üiese me-fcals.
(iii) Optical equipment.
The apparatus employed for the investigations was in the main adapted from an existing transmission polariscope and was not therefore entirely representative of vrfiat might be accomplished by further financial outlay which at the time
and stage of development vra.s iinjustifiable,
The use of more suitable equipment can in the main be related to cost since anple scope exists for ingenui-fcy in the solution of design problems,
AGKNÖWLEDGEJffiNTS
The author v/ishes to record an appreciation of the great assistance given by members of the staff of The College of
Aeronautics, Mr, S, R. Lev/is, B.Sc., and Mr, C, K, Trotman, B.Sc., members of the staff directly concerned v/ith the project, and
Mr. T. Borthv/ick v/ho supervised the manufacture of most of the test specimens.
Thanks are due to Mr, K. 0. Scott vdio discussed many of the problems previously encountered and Mr. M, J, Hemvood for his co-operation in the testing of the lug specimens.
REFERENCES 1, K. 0. Scott. 2, J, R. Linge, 3, M, J, Henv/ood 4. A. Mesnager. 5. G. Oppel, 6. G. Mabboux. 7. H. Favre. 8. E. Z. Stov/ell,
A photoelastic method for the examination of strains in metal plates. College of Aeronautics Thesis, 1950. (Unpublished) Further investigations in a photoelastic method for the examination of strains in metal plates. College of Aeronautics Thesis, 1951.
The effect of certain parameters upon the ultimate strength of a lug. College of A.eronautics Thesis, 1951. (Unpublished), Sur la determination optiques des tensions interieures dans des solides a trois dimensions, Coraptes Rendios, Paris, V 190, 1930, p, 1249.
Das polarisationsoptische
Schichtverfahren zur Messung der Oberflachenspannung am beanspruchten Bauteil ohne Modell.
Vereines deutcher Ingenieure, Zeitschrift, V 81, 1937, pp. 803 -804.
Applications de la photoelasticimetre a 1'etude des ouvrages en beton, Revue d'Optique. V 11. 1932.
pp. 501 - 507.
Sior \ine methode optique de determination des tensions interieures dans les solides a trois dimensions.
Comptes Rcndus, Paris, V 190, 1930, pp. 1182 - 1184,
Stress and Strain concentration at a circular hole in an infinite plate. N,A,C.A. Tech. Note. 2073. April 1950.
9. G. E . Griffith. 10. H. F. Hardrath and L, Ohman, 11. H, R. Wright. 12. J. C. Oersted. 13. R. C. J. Howland. 14, A. F. C. Brown and V. M, Hickson. 15. L. Lazzarino. 16. J. A. H. Paffet 17, J. D'x\gostino, D. C. Drucker, C. K. Liu and C. Mylonas. 18, R. A. Houston.
Experimental investigation of the effects of plastic flov/ in a tension panel with a circiilar hole.
N.A.CA. Tech, Note. 1705, September 1948.
A study of elastic and plastic stress concentration factors due to notches and fillets in flat plates.
N.A.C.A. Tech, Note 2566. December 1951.
Photometry of the Diffuse Reflection of Light on Matt Surfaces.
Phil. Mag. February 1900, p,199. Entwicklung der Lehre von den Glunze. Pogg. Arm, 1847. p.49.
On the Stresses in the neighbourhood of a circular hole in a strip under
tension.
Phil. Trans. R,S. 1930. p.49. Improvements in Photoelastic
Technique obtained by the use of a Photometric Method,
British Journal of Applied Physics. Vol. 1, 1950, p.39.
New Photoelastic Equipment at -the University of Pisa.
Tecnica Italiana. Vol. 6. No. 4. July-August 1951, pp. 321 - 323.
Summarised Proceedings of a Conference on Stress Analysis,
British Journal of Applied Physics, Vol. 1, 1950. p.22j4.
An iinalysis of Plastic Behaviour of Metals v/ith Bonded Birefringent
Plastic. S.E.S.A. Proceedings, Vol.XII, No.2, pp.115-122.
A Treatise on Light,
19. J. Walker. 20. V. Harrison. 21. J. R. I. Hepburn. 22, M. Hetenyi. 23. M. M. Frocht. 24, H. T. Jessop and F, C. Harris. 25. A. W. Hendry. 26. R, B. Heyv/ood. 27, C. Mylonas, 28, C. E. Taylor. 29. D. J. Coolige.
The /malytical Theory of Light. C.U.P. 1904.
Definition and Measiorement of Gloss,
P.A.T.R.A. 1945,
The Metallization of Plastics. Cleaver-HUme Monograph on Plastics. Handbook of Experimental Stress Analysis.
John Wiley & Sons, Inc. (NEW YORK) 1950. Chapman & Hall Ltd. (LONDON) 1950, Photoelasticity.
John Wiley & Sons, Inc. Vols. I and II.
P h o t o e l a s t i c i t y . ( P r i n c i p l e s and Methods).
Cleaver-Hume P r e s s Ltd. 1949.
An i n t r o d u c t i o n -fco P h o t o e l a s t i c
i m a l y s i s .
BlackLe & Sons Ltd. 1948. Designing by Photoelasticity. Chapman & Hall Ltd. 1952.
The Mechanical and Optical Properties of Catalin 800 and its suitability as a Photoelastic Material,
Proc. VII International Congress of Applied Mechanics, London, September 1949, Vol. IV Paper 14.
A Casting Material for 3 Dimensional Photoelasticity. (iCriston),
Proc. Soc. Experimental Stress Analysis. Vol. VII No. 2.
An Investigation of the Mechanical and Stress-Optical Properties of Columbia Resin, C.R.39.
Proc. Soc. Experimental Stress imalysis. Vol. VI. No. 1,
30. C.R.39 Transparent Thermosetting sheet materials.
Technical Leaflet. Ashdovms Ltd. Eccleston Works, St, Helens, Lanes. 31. Casting vri.th Marco Resin S.B.26 C,
Technical Leaflet No. 31/b. Scott Bader & Co. Ltd.,
109, Kingsvra.y, London, Vf.C.2, 32. Marco Resin S.B.28 C.
Technical Leaflet No. 31).
Scott Bader & Co. Ltd.,
109. Kingsway, London, W,C.2. 33. H. Pessler, Marco Resin as a Photoelastic
Material,
British Journal of Applied Physics. Vol 1., 1950, P.245.
34. N.A. de Bruyne. Some Investigations into the Fundamentals and Applications of Synthetic Adhesives.
Plastics Progress. (Papers and discussions at the British Plastics Convention, 1951)
Reprint. Illiffe & Sons Ltd,, London, 35. Araldite 101 and Hardener 951.
(provisional Instructions)
Technical Leaflet. Aero Research Ltd,, Duxford, Cambridge.
36. Polishing /igents.
Subject of communication from Reckitt & Colman Ltd., Hull,
(Manufacturing Dept.) 22nd July, 1952, 37. C. G. Evans. The curing of resins by the passage
of an electric current,
R,A,E, Tech, Note. No. Chem, 1186. January 1953.
/JDHESIVE
The Araldite Resin and Hardener were mixed in the appropriate quantities recommended by the manufac-fcurers-^^* i.e, 14 : 1 parts by v/eight respectively, who also suggest a minimum curing temperature of 20°C. together vsdth a curing time of 24 hours,
Owing to the high viscosity of the resin it was found extremely difficulty, without recourse to special methods*, to prevent the entry of a considerable number of air bubbles into the mixture, these were not easily removed but could be destroyed to some extent during application of the adhesive.
The application of heat • facilitated mixing with a
consequent reduction in-ftie number of included air bubbles, however, the curing cycle was accelerated to the extent at v/hich satisfactory application of the adhesive became impracticable,
SEECD/IEN ASSE^ffiLY
Assembly of specimens was completed as soon as was possible after surface treatment to forestall the onset of a dull oxide film, which, notably on magnesium sheet, D.T.D, 118 A. formed within a few hours,
The adhesive was spread over both surfaces to be joined and the specimen was then assembled with the photoelastic material uppermost; manipulation of the plastic v/as immediately carried out by hand pressure, with the adhesive still in the liquid state, to force air bubbles remaining in the adhesive film to the nearest and most convenient free edge,
The compound assembly vra.s placed between flat pieces of wood or "Perspex" sheet and subjected to a sensibly uniforrn
pressure of about 6 lb.-in • of adhesive superficial area^by means of clamps on v/eights, until it v/as apparent that initial cure had taken place, pressiore vra.s thence reduced to the order of 1.5 lb,-in' and held at this level for 24 hours.
For example as advocated by Mylonas, Ref. 38. and elsewhere, Except in the case of the bonded lug specimens.
iiPPENDIX
SPECD5EN MiMIPACTURE
Bonded Specimens
Metal and plastic blanks of the -fcapered type of specimen fig. 3 were cut to approximate size by hand and gradually reduced to final size in a tv/o-part filing jig to ensure square edges and in the case of the plastic to prevent birefringence effects at the boundaries.
All material blanks were carefully degreased with
Trichlorethylene before and after the appropriate siorface treatment had been applied particular attention being given during handling to the cleanliness of the metal to plastic con-fcact surfaces.
A similar procedure v/as employed for the manufacture of the stress concentration specimens, K. L. and M, fig. 6, but in addition the edges of the photoelastic material normal to the axis of loading v/erc chamfered -fco reduce the potentially high shearing and tearing stresses in the adhesive and to assist in producing a gradual transfer of load from metal to plastic in this region,
The bonded lug specimens themselves, fig. 11, served as templates for filing the photoelastic materis-l to final size after assembly and cure of the adhesive.
The 0,8750 in. dia. hole in the plastic v/as located by a stepped mandrel, and was enlarged slightly to avoid direct contact between the loading pin and the plastic during test, a s-fcate of affairs which would have undoubtedly produced a fictitious isochromatic pattern.
Four toolmakers clamps were applied at strategic points on each specimen assembly, with small pads of paper interposed, during cure of the adhesive -fco permit observation of the adhesive film and included air bubbles.
A chamfer 0.25 in. v/ide v/as filed on the straight edge of the plastic material for the reasons already mentioned,
On the majority of assemblies (even v/hen clamped) it was found necessary to prevent relative movement between the metal and the plastic during the initial curing period of the adhesive since the latter tended to act as a lubricant betv/een the two surfaces.
THICIglESS OF ADHESIVE FITlt.
Attention v/as focussed primarily on the operation of forming a continuous film of adhesive of reasonably uniform thiclmess v/ithout air bubbles or large voids rather than an acc\irately controlled thickness, in practice the former procedure proved to be the most difficixLt stage in the prodiiction of bonded assemblies,
Adhesive films from 0.0005 in. to 0,001 in. in thickness v/ere noted by Scott and rather greater thicknesses by the author^, i.e. from 0.001 in. to 0.003 in.
DIRECT CAST SPECBiENS
Early experiments with Marco Resin S.B.26 C. showed that fair adhesion resiiLted when layers 0.06 in. thick were cast direct onto degreased but otherwise unprepared light alloy and steel siorfaces v/ithout the use of an intermediate bonding medium..
A further scries of metoplastic specimens in S.B.26 C. and S,B,28 C. figs, 3 and 4 v/ere cast to provide additional information on the self-adhesive properties of the resins when polymerised on sandblasted and polished metal surfaces, and also on sandblasted surfaces which had been treated v/ith an initial coating of resin and altmiinium pov/der in the manner described in section 1,7.
Subsequent to these tests a pair of direct cast lug specimens were prepared to the dimensions given in fig, 12. Previous tests to assess the optical sensitivity of the Marco Resins indicated that a coating of approximately 0.125 in. was
required to give isochromatic patterns of adequate fringe order which in t-um necessitated the exeunination of suitable mould materials in v/hich to cast the resins to this thickness.
Tm CHOICE OF MOUID t-IATERIAL
The only mould material, flexible in application and immediately a-vailable for casting a limited n-umber of specimens appeared to be "Plasticine".
The technique had not been advanced to a stage at which it was considered convenient to manufacture special prefabricated moulds in flexible materials such as "Welvec" or "Plastimold",
Experience showed that the "Plasticine" technique v/as reasonably satisfac-fcciy but possessed three main advantages and two disadvantages.
(i) The same batch of material could be used many times and v/as therefore economical. Intermediate processes, e.g. the remelting of flexible rubbers and hence the problem of exhausting the attendant fumes did not arise, (ii) After being warmed the material was easily shaped -fco
any desired form vath simple tools.
(iii) Problems of sealing could be solved easily and with rapidity should lealcs occur during the pouring of the casting,
(iv) The Monomer C. content of the resin mix -at-fcacked the "Plasticine" locally although not severely; as a result the softened and contaminated skin adjacent to the resin had to be removed in some instances before the same batch of material could be remoulded. (v) The time period required for mould fabrication was
rather excessive until manipvilative experience had been acquired.
SPECIMEN t/IANUFACTURE
Self-explanatory stages in the manijfacture of a cast metoplastic specimen vdtli machining allowance arc shown in fig. 5.
Strips of "Plasticine" bounding the ex-fcreme ends of the surface coating, not of direct interest photoelastically, v/ere
formed in such a manner as to produce the radius shown in fig. 5(4), for the follov/ing reason.
During polymerisation the resin tended to bov; the metal sheet by contraction along the longitudinal axis \vith the resin on the concave side hence a certain amount of flexure was
unavoidable v/hen clamping them to the bed of the horizontal milling machine for reduction to final size,
Because "the strength of the bond in the region of the reduced resin thickness, i.e. at the radius, v/as relatively greater than in the main body of the casting, considerably assistance was obtained by -this expedient in the prevention of failure of the bond at -the extreme edge of the casting,
Separation having once occurred in this region tended to spread vdth the slightest flexure of the specimen and little could be accomplished to correct this fault after it had been initiated,
In lug components a stepped mandrel in mild steel was utilised as a core to form an oversize 0.875 in. dia, hole in the resin castings. The mandrel was lightly coated with a film of machine grease to facilitate removal after final euro of the resin,
MACHINING ALLOWANCES
(i) Edges of castings v/ere splayed upwards and outwards to leave a si:iall allovrance on vdLdth, small chips taken out of the side of the casting during machining operations v/ero thereby removed when the resin v/as filed square to the plane of the metal sheet,
(ii) Castings were invariably made to a depth greater than that finally required to allow for:
(a) Contraction ripples on the surface of the resin, (b) Air bubbles near the surface.
(c) Departure from true level of the table(s) employed for casting operations,
CASTING PROCEDURE
Preparation of the resins, S.B.26 C,-^ and S.B.28C.^ was carried out in -the manner suggested by the manufacturers to
When -fche majority of air bubbles had risen to the surface and had dispersed, the resin mix was poured to an arbitrary thickness of 0,20 in. to 0,25 in. to include the allowances
previously mentioned.
Castings v/ere allowed to gel and cure over a period of about 72 hoijrs at room temperature before further handling was attempted, after this period the mould material v/as completely
stripped from the casting while small regions of a.dhering "Plasticine" were removed with Acetone or Trichlorethylene,
M/.CHINING PROCEDURE
Cast metoplastic of all the -types manofactured v/ere clamped ore at a time to the bed of a Roscher & Eichler vertical milling machine, and were gradually reduced to a nominal -chick-ness of 0,13 in. in increments of 0,03 in. to 0.05 in. by
longitudinal passes of -fche milling cutter. Finishing cuts were made to a depth of about 0,005 in.
Copiovis supplies of soluble oil v/ere pirovlded for purposes of lubrication and to effect adequate cooling of the casting,
Tool marks vfere removed from the surface with fine-grade glass paper and subsequently the coating v/as burnished -fco a mirror-finish with the commercial liquid metal polishes, "Brasso" and "Silvo", Finally, the edges of the plastic were filed square and chamfers v/ere incorporated at the ends of the casting in the manner shown typically in figs. 4 and 12,
DETAILS OF MILLING FiiCHINE
Diameter of Cutter,(Standard End Mill) 2.0 in.
R .P.M. of Cutter 424. Peed along specimen ,. 10 mm. per min,
Lubricant.,,,,... , Soluble oil,
The values quoted above represent the highest speed, -R.P.M,, and slowest feed available on the machine.
F i g u r e Number J 2 J 3 J 4 J 5 J 6 E x t e r n a l Load
ll.
200 400 600 800 1,300 PLASTIC: F r i n g e No. N. 0 . 5 0 0 . 5 0 1.0 1,0 1.50 2 , 0 1.0 1.50 2 . 0 1.50 2 . 0 2.50 C R . 39ESTIMATION OP AVERAGE STRAUS PER FRINGE VALUE y i n . 2 . 4 8 4 . 9 6 2 . 8 3 4 . 2 6 2 , 8 3 1.77 5 . 7 7 4 . 1 5 2 . 9 4 5 , 8 8 4 . 6 8 3.86 t = o . i ; X i n . 0 . 4 9 0 . 9 6 0 . 5 6 0 . 0 8 3 0 . 5 6 0.39 1.12 0.70 0 . 5 7 5 1,16 0,91 0.75 50 i n . Yc C. S. A, i n 2 0,0320 0 . 0 6 2 5 0 . 0 3 i 5 0,0540 0.0365 0.0260 0 , 0 7 3 0 0.0462 0,0375 0,0755 0,0598 0 . 0 4 9 4 Load P n e t l b . 190 380 570 760 1,235 (ung's Modulus E = 2, S t r e s s I b . - m 5 , 9 3 0 6,090 10,400 1 0 , 5 4 0 15,600 2 1 , 9 0 0 1 0 , 4 0 0 16,400 20,300 16,600 21,000 25,400 55 X 10^ LB F Young's Modulias l b - ^ 2 1 0 . . 2 - i n 30x10^ rt 1» N II II II It >• II It II n S t r a i n e i n s . - i n . 0.000575 0,000590 0,001010 0.001023 0.001511 0.002120 0,001010 0,001590 0.001970 0.001610 0.002040 0.002460 AVERAO:
S t r a i n
F?!Sge
i n s - i n - N 0.0011.5 0.001180 0,001010 0,001023 0.001010 , 0,001060 v>i 0,001010 ^ 0,001060 • 0.000990 0.001070 0,001020 0.000980 0.0010506
PHOTOGRAPH NUMBER K 2 K 3 K 4 K 5 K 6 K 7 K 8 K 9 K 10 K 11 APPT.TRD ^ f f i . 1,000 2,000 3 , 0 0 0 4 , 0 0 0 5 , 0 0 0 6,000 7,000 8,000 9,000 1 0 , 0 0 0 * X. i n . F r i n g e No,N X. i n . N X. i n . N X. i n . N X, i n . N X. i n . N X, i n . N X. i n . N X. i n . N X, i n . N FRINGE AMLYSIS -1.40 0 . 5 0 0 - 0 . 96 0 . 5 0 0 - 1 . 0 0 . 9 0 0 - 1 , 0 1,20 0 - 1 , 0 1.50 0 - 1 . 0 1.90 0 - 1 . 0 2 . 2 5
o-a8o
2 . 6 0 0 3 . 2 0 0 . 2 0 4 . 0 1.54 1.0 1.30 1.0 1.40 1.50 1.44 2 . 0 1.25 2 . 0 1.42 3 . 0 1.28 3 . 0 1.18 3 . 5 0 1.0 4 . 5 0BONDED STRESS CONCENTRAl'iON SFECLiilEN TYPE K.
1.62 1.50 1.50 1.50 1.50 2 . 0 1.50 2 . 5 0 1.42 2.50 1.48 3 . 5 0 1.38 3 . 5 0 1.30 4 . 0 1.12 5 . 0 1.60 2 . 0 1.56 2 . 5 0 1.54 3 . 0 1.48 3 . 0 1.50 4 . 0 1.42 4 . 0 1.36 1.50 1.34 6 , 0 1,62 3 . 0 1.60 4 . 0 1.53 3 . 5 0 1.54 4 . 5 0 1.50 5 . 0 1.40 5 . 0 1.43 7 . 0 1.56 5 . 0 1.55 6 . 0 1.46 6 . 0 1,46 8 , 0 1,62 6 , 0 1.60 7 . 0 1*51 7 . 0 1.50 9 . 0 . 1.62 9 . 0 1.62 1 2 , 0 1 1
Photograjih
Number
Area
Under
GvrvG
IN^
Base
Length
Curve
IN'
Mean
N
Net
Load
LB
D ! T ? D . 6 1 C B
