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Optical Studies of Crazed Plastic Surfaces 1 Sanford B

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Optical Studies of Crazed Plastic Surfaces 1 Sanford B Journal of Research of the National Bureau of Standards Vol. 58, No.6, June 1957 Research Paper 2767 Optical Studies of Crazed Plastic Surfaces 1 Sanford B. Newman and Irvin Wolock Opt ical studies were .made of crazed surfaces, p rincipall y t hose of cast poly methyl methacrylate. The techl1lques used for t hese st udies we re mult iple-beam inter fe rometry and li ght and electron m icroscop y. The dil~ e lls i o n s of t he craze cracks prod uced b y short-t ime stress crazing and by stress­ so ~ vent crazmg wcre determmed by t he a bove techniques. Stress-crazed specimens con­ tamed t he sma ll est cracks observed during thi s investigation. The dimensions determined in some of t h e~e specimens were : Le ngth, 2 to. 10 mi crons; wi dth, 0.1 to 0.25 m icron; depth, 0.05 to 0.15 mIcron. I n other speellnens and JJl solvent-crazed surfaces t he dimensions were larger. The craze cracks did not appear to aPjJ roach dimensions unresolvable by the electron m lCroscope, b~ t Instead appeared to have a mll1lmUm crack length. AI: elevatIOn of t he surface in t he vicinity of craze cracks was observed, of the order of 0.02 mI cron ~ or c r ack~ pr?duced by stress-crazing and 0.3 mi cron fo r t hose produced by stress­ solvent crazIng .. Th IS n se was observed in crazed specimens while still under load . The tY PI cal surface dIsplacement was observed in specimens crazed in tension produced by fl ex ure as well as jj] pure tenSIo n, and was observed III polvstyrene and in polymethvl alpha-chloro- acrylate as well as in poly methyl methacrylate. " . Some crazed specimens in whi ch t he crazing had apparently "recovered" when viewed with t.he unaided eye still exhi b}ted the surface displacement t'ypical of crazed specimens. Expenments conducted by heatIng stress-crazed specimens and stress-so lvent-crazed speci­ mens to accelerate t he recovery of crazing indi cated t hat surface recovery is essent ially co m­ p lete III t he former case but not 111 the latter. T his d iff C' rence in recovery behavior is prob­ ably due to t he nature of t he derormation associated wit h the two types of crazing. T ~ n s Il e speCIm ens to wh Ich loads had been appli ed t hat were sligh tly too low to produce ImmedI ate crazIn g dId llot exh IbIt surface displacements typical of crazed specimens. 1. Introduction that appear to approl1,ch the limits of resolu tion of the mi croscope, and these require a more powerful When transp aren t thermoplastics such as poly­ tool for the measurement of crack dimension. methyl methacrylate 0 1' polystyrene are exposed Lo a Although t he length and fineness of crazing cr acks suffi cien tly high tensile stress or to solvents, deteri­ can be studied wi th the brigh t-fi eld microscope the oration of their op tical properties occurs. This de­ determination of the dep th of cracks and how 'they terioration resul ts from the formation of cracks tha L aff ec.t surface topography requires other techniques impair light transmission, cause optical aberrations, and mstruments. Both the electron microscope and and l'educe the strength properties of the material. the multiple-beam inLerferometer suggest themselves Commonly lmown as "crazing," Lhis phenomenon is for these purposes. Saunders [8] h as reported ob­ encountered in a variety of produ cts ranging from servations on minu te irregularities in op tical slll'ffLces :=tircraft gl azin~ and. canopies to w a~cl~ crystals, and b v means of multiple-beam interferometry. Because IS ObvlOusly of conSIderable economIC lmpor tance to of the grefLt magnification of minute detail and the manufacturers and users of plasLic items. A number relatively large surface arellS t.hat can be examined of invcsLigators have reported Lhe resul ts of work on simultaneously , mul Liple-beam interferometry lends the mech anism, cllar actel'istics, and elimination or i tself readily to the precise study of the topography prevention of crazing [1 to 7]. 2 H O\\"ever, crazing re­ of plastic sheets. The multiple-befLm or FfLbry­ mains a serious consideration in pll1stics a,pplications Perot-type fringe is a classical interferometric tech­ and many aspects of the problem require intensive nique. A number of applications and refinemen ts of study. the. method h ave appeared in recent years, many of Ph otomicrogr aphs and descriptions of the craze whlCh have been the work of Tolansky and his as­ cracks can be found in the literature [2, 4, 5] . The s?ciates [9, 10]. These techniques yield magnifica­ cracks ViLIy greatly in th eir length and fineness. In tlO.n of detail in one direction only, the direcLion of tensile specimens crazed by the application of stress height or depth . In the mel1S11rement of height this or by stress and solvent together, the cracks are method equals or surpl1sses results obtained with almost invm-iably nor mal to the applied stress. the electron microscope, at the same time eliminating ela~orate Indi viclual cracks can USLI ally be 0 bservcd using a the specimen preparation required by the low-power microscope ; a ] 6-mm objective and lO X latter mstrument. The electron microscope, how­ ocular is generl1 11y more th an adequate. The length ever, yields other informllLion that would be difficult and number of Lhe cracks Cl1n often be determined or impossible to ob tain with available interferometric in this fl1shion. Some Cl'azed surfaces contain cracks techniques. Some indication of how Lhese tools supplemenL one another is evident in the observa­ tions presented. I 'J'his investiga tion was conducted at the National Burean of Standards under the sponsorship and with the financial assistance of t he National Addsory Com· The acrylic and. styrene plastics used in this study mlttee for Aeronantics; the help of this agency and memhers of its staii is grate· are amorphous lugh polymers. In these materials fully acknowledged. 2 Figures in brackets indicate the literature references at the end of this:paper. the total response to an applied stress contains three 339 components. These are the instantaneous elasticity, Tapered tensile specimens which had been stress­ retarded configurational elasticity, and flow . When solvent-crazed for use in another investigation were the stress is removed there results an instantaneous also examined. These specimens tapered in width recovery of the deformation due to the first com­ from 0.500 to 0. 333 in . over a 3-in . reduced sectio~ . ponen t and a slower recovery from the second . De­ The methods of crazing were the same as those formation due to flow is not recovered. These ele­ described in the previous paragraph. ments in the viscoelastic behavior of the polymer Solvent-crazed surfaces were also prepared by can be related to observed microstructural charac­ placing 2-in. squares cut from cast sheets of poly­ teristics of crazed surfaces. It also aids in categoriz­ methyl methacrylate in boiling water for 2 or 3 ing the mechanisms responsible for changes in the min and then exposing them to monomeric methyl cracks and their associated deformations on exposure methacrylate until they were visibly crazed . to elevated temperature or stress. Flexural specimens were stressed statically as cantilevers to produce crazing. 2. Materials 3.2. Preparation of Surfaces for Interferometric Specimens of cast commercial poly methyl meth­ Studies acrylate were the ones chiefly used in this investiga­ tion. Both the general-purpose and heat-resistant Exposed surfaces of the plastic sheets quickly grades were studied . The sheets of cast polystyrene accumulated a film of dust particles from the sur­ were also of commercial origin. The cast polymethyl rounding air. These particles were thoroughly alpha-chloroacrylate was experimental polymer. removed to facilitate the microscopic examination Samples of acrylic laminate were also examined. and permit the deposition of suitable metallic films The two outer layers of this laminate consisted of on those specimens used for multiple-beam inter­ polymethyl meth acrylate and the interlayer was ferometry. Cleansing was canied on with extreme plasticized polyvinyl butyral. The inner ffL ce of the caution, as even gentle wiping with commercial acrylic outerlayer was examined. cellulose tissues or with absorbent cotton was suf­ ficient to produce surface marks that could be de­ tected microscopically. Immersion in a stream of 3. Methods tap water followed by blotting with a good grade of lens tissue caused little apparent surface degradation. 3 .1. Preparation of Crazed Specimens Such a procedure, however, is not adequate for re­ moving all detritus, and foreign bodies were con­ The standard tensile specimens used were type I stantly encountered during the eXfLminations. specimens described in Method 1011 of Federal The glass optical flats and other reference surfaces Specification L- P-406b [11 ]. The specimens were were handled by washing in soap and water and dry­ stress-crazed by tensile loading in a hydraulic uni­ ing with lens tissue or boiled cotton or linen cloths. versal testing machine with the rate of head separa­ Old silver co atings were removed wi th a 30-percent tion at 0.05 in./min. In some cases, the specimens solution of hydrogen peroxide. were loaded until crazing occurred and then the load The cleansed specimens and r efer ence surfaces for was released, whereas in other cases the load was multiple-beam interferometric studies were then applied until failure occurred.
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