MECHANICAL AND STRESS-OPTICAL PROPERTIES OF PHOTOELAS!IC MATERIALS by YOSHIO TESHIMA A THESIS submitted to OREGON STATE COLLEGE in partial fulfillment of the requirements for the degree or MASTER OF SCIENCE June 1953 l}tSffiDr Redacted for Privacy rrfutrut S;ofr*rur l'f lrrlrut*l h6lnrslrs E ODugr cf lrJou Redacted for Privacy Redacted for Privacy @le.rr Of Eohoot 6rrdu*r Redacted for Privacy Erta Sr;1r lr Dror.aid !tp.C E {rllr f. .hm ,, : ACKNOWLEOOMENT Sincere appreciation is expressed to H. D. Christensen, Assistant Professor of Mechani­ cal Engineering, who sug eated the thesis subject, and under. whose direction this thesis was com­ pleted. TABLE OF CONTENTS Page I. I NTRODUCTION • • • • • • • • • • • • • • • • • l II. THE PROTOBLASTIC METHOD OF STRESS ANALYSIS • • 4 III. BASIC REQUIREMENTS OF PHOTOELASTIC MATERIALS • 9 IV. PHOTOELASTIC MATERIAI,S • • • • • • • • • • • • 1. Gl ass • • • • • • • • • • • • • • • • • • • 15 2. Cellu1oid • • • • • • • • • •. • • • • • • • 16 3. Cata1in 61-893 • • • • • • • • • • • • • • 18 4. Fosterite • • • • • • • • • • • • • • • • • 21 5. Homa1ite CR-39 _ • • • • • • • • • • • • • • 22 6. Y...ris ton • • • • • • • • • • • • • • • • • • 24 7. Pl ex1g1nss and Lucite • • • • • • • • • • • 25 6. l!arb1ette • • • • • • • • • • • • • • • • • 26 9. Viny11te • • • • • • • • • • • • • • • • • 27 10. Summary of the Properties of Photoelastie Materials • • • • • • • • • • • • • • • • ~ 26 V. CASTING ND TESTING SEL: ~CTED POLYESTER RESINS • 31 1 • . Selection of Resins • • • • • • • • • • •. • 34 2. Mixing the Res infl • • • • • • , • • • • • • 35 3. Mol d Prepars. tion • • • • · • • • • • • • • • 38 4. Cas ting and Curing the Resins •••••• ~ 39 5. Machinability of the Resins • • • • • • • • 46 6. Tension Tests • • • • • • • • • • • • • • • 47 7. Strain Creep Tests • • • • • • • • • • • • 55 8. Optical Creep Tests • • • • • • • • • • • • 59 9. Stress Fringe Relationships • • • • • • • • 71 10. Determination of Fringe Constants • • • • • 72 VI. SUMJ~RY OF RESULTS • • • • • • • • • • • • • • 77 VIr. RBCO liDA'riONS FOR FU'IURE INVESTIGATIONS • • 81 VIII. COlWLUSIONS • • • • • • • • • • • • • • • • • 85 IX. BIBIIoGTIA.diY • • • • • • • • • • • • • • • • • 87 APPENDIX •• • • • • • • • • • • • • • • • • • 90 LIST OF FIGURES Figure Page 1. Polariscope Arrangement • • • • • • • • • • • • 5 2. :Mold Assembly •••••• • • • • • • • • • • • 40 3. Lindberg Furnace and Control Panel • • • • • • 42 • 4. ethod of Removing Gaskets • • • • • • • • • • 43 5. Method of Machining Test Specimens • • • • •• 49 6. Test Specimen Templates •••••••••••• 49 7. Arrangement for Tension Tests • • • • • • • • • 51 B. Tension Test of Laminae 4116-4134 (Stress- Strain Curve) ••••••••••••••••• 53 9. Tension Test of Marco MR-2BC (Stress-strain Curve) • • • • • • • • • • • • • •. • • • • • • 54 10. Arrangement for Strain Creep Tests • • • • •• 56 11. Strain Creep Test of Laminae 4116-4134 and Marco MR•2BC (Strain•Creep Curves} •••••• 57 12. Loading Jig for Beam-in-Pure-Bending • • • • • 60 13. Arrangement for Optical Creep Test • • • • •• 61 14. Isochromatic Fringe Patterns for Laminae 4116M4134 CUred for 4 and 6 Hours at 150 F • • 53 15. Isochromatic Fringe Patterns for Laminae 4116-4134 Cured for B Hours at 150 F, and :Marco NR-2BC • • • • • • • • • • • • • • • • • 64 16. Fluorolite Analyzer •••• • • • • • • • • • • 65 17, Optical Creep Test of Laminae 4116-4134 Cured for 4 Hours at 150 F • • • • • • • • • • • • • 66 18. Optical Creep Test for Laminae 4116-4134 Cured tor 6 Hours at 150 F ••••••••••••• 67 19. Optical Creep Teat for Laminae 4116-4134 Cured for 8 Houra at 150 F • • • • • • • • • • • • • 68 LIST OF FIGURES (CONTINUED) Figure Page 20. Optical Creep Test of Marco iffi-280 • • • • • • 69 21. Optical Creep Characteristics of Laminae 4116-4134 and Marco MR-28C • • • • • • • • • • 70 22. Stress Fringe Relationships for Laminae 4116-4134 and Marco MR-280 • • • . • • • • • - 73 LIST OF TABLES Table Page I. Properties of Photoelastic Materials • • • • • 29 II. Cr1tical Temperature Properties of Materials used for Three-Dimensional Photoelasticity • • 30 III. Properties of Liquid Resins • • • • • • • • • 35 IV. Mechanical Properties ot Laminae 4116-4134 and arco MR-280 Compared to Catalin 61-893 • 52 V. Total Strains for Laminae 4116-4134 and Marco MR-280 over the Time Interval of 5 Minutes . to 180 Minutes • • • • • • • • • • • • • • • • 58 VI. Optical Creep Characteristics of Laminae 4116-4134 and Marco !!R-280 • • • • • • • • • • 71 VII. Fringe Constants for Laminae 4116-4134 and Marco MR-280 obtained from Bending and Tension Teats • • • • • • • • • • • • • • • • 76 MECHANICAL AND STRESS-OPTICAL PROPERTIES OF PHOTOELASTIO MATERIALS I, INTRODUCTION The discovez-y of the photoelastic effect resulting. from stressing a transparent, optically isotropic material is credited to Dav·id Brewster, who observed this phenome•, non using glass 1n 1912, and published accounts· of his eltperiment in 1818. Although this discovery marked the birth of photoelasticity, which ha·s .become one of the moat powerful tools in present-day stress analysis, the possibility of applying this method to engineering appli­ cations was not recognized at that time. The basic concept that optical retardations producing color effects were proportional to the principal stresses existing in a stressed ma.terial, the stress-optic law, was formulated independently by F. E. le'Ulttan ttnd Clerk Maxwell in 1841 and 1853 respoet1vely. No practical appl1.cat1ons were made with th1.s me·thod until 1891, when Olarus Wilson published results concerning photoelastie investigations conducted on a s1mply-suppol'"ted beam subjected to a point load. Further applications of this method ere made by Mesnager· in 1901 (ll~ p629; 18; p4). Early workers in this field were seriously hand1• capped by the lack of a suitable .material with which the required models eould be mB.de. Glass, the only material 2 available, was relatively insensitive and was very diffi­ cult to machine. Progress in the development of photo­ elasticity was slow until 1920, when E. G. Coker introduced Celluloid as a model material (10, p325). This plastic was used extensively until the development of Bakelite in the early twenties. Bakelite BT~61•893, which is now manufactured by the Catalin Corporation under the trade name of Oatalin 61-893, bas become the standard material for use in two-dimensional photoelastic studies. In the .. ~· . years since the development of Catal1n 61-893, an entirely new world of synthetic resins has been born. Since all transparent plastics are potential photoelastic materials, it becomes of interest to investigate the suitability of these materials for photoelastic use. A great number of these resins have been used tor stress analysis work, but there are still many which have yet to be investigated. In the field of three-dimensional photoelastioity, a new material, Fosterite, has been ·eveloped by the Westinghouse Research La..boratory1 and it appears that this resin will soon replace Catalin 61•693 for use as a model material 1n this particular type of study. Although the mechanical and str sa-optical properties data of photoelastic materials are available in texts devoted to the subject of photoelasticity, it was believed that any single text did not contain information on many of the materials which have been used and can be used. 3 This paper attempts to summarite all such information, obtained through a survey of available literature, so that it will be available in one work. Also included 1s a report of the preliminary studies for an investigation hose ultimate goal is to find a low cost, easily cast resin which could be substituted for the more expensive Catalin 61-893 for use as a model material in the photoelasticity courses offered at Oregon State College. The prtmary objective of thi& study was to determine the feas1b111.ty of casting several of the com• mercially available polyester resins with the equipment available in the engineering laboratory, and to test the resulting cured resin in an effort to evaluate their worth as photoelastio materials. Recommendations for future 1n• vestigationa on materials which proved successful are also presented. 4 II. THE PROTOBLASTIC ETHOD OF STRESS ANALYSI S So that a clearer understanding of the principles in­ volved in the photoelastic method of stress analysis may be obtained, a short discussion on this subject is included in this paper. Som6 crystals such as mica and calcite exhibit the phenomenon of double refraction, ie, a beam of light fall• ing at normal incidence on the crystals is resolved into two components and transmitted on planes at right angles. Almost all other transparent materials such as glass and the synthetic resins show this effect when the material is subjected to a stress and examined under polarized light. This effect is only temporary since, upon removal of the load, the material once again becomes optically isotropic (11, p8~7, 839). The primary objective of the photoelastic method, therefore, is to measure the double refraction induced throughout the structural model when applied loads deform the material fram which the model is made, and to translate this effect into terms of stress by use of suitable mathematical relationships (18, pl60). The equipment and lens arrangement necessary to produce the required circularly polarized light is shown in figure 1. This polariscope arrangement is available for photoelastic investigations condu.cted at Oregon State College. 5 .. Fig. 1. Polariscope Arrangemen~