The effect of thermal toughening on the impact resistance of simulated safety lenses. E. C. Wigglesworth

Transcription

1 The effect of thermal toughening on the impact resistance of simulated safety E. C. Wigglesworth Flat specimens, simulating eye protector, were generated in seven thicknesses between 1. and 4.0 mm. Half the specimens, randomly selected, were heat treated and all were then dynamically tested to destruction. The results show that the effect of the heat treatment was to increase the impact resistance, measured in terms of fracture velocity. For all thicknesses, the approximate relationship was found to be Vt/V t = V 2, where V t = fracture velocity of the untoughened specimens and V t = fracture velocity of the toughened specimens. The same relationship was found to hold for a group of diopter spherical meniscus of center thickness 2.0 mm. and edge thickness 1.0 mm. and of base curve 6.00 diopter, similarly treated. The relationship was independent of missile diameter in drop-tower tests (simulating blunt trauma from large missiles) and in ballistic experiments (simulating penetrating injuries from small missiles). Key words: impact resistance, ophthalmic, heat treatment, lens thickness, missile size I n an earlier investigation 1 on the impact resistance of eye protector lens materials, it was found that untoughened of thickness mm. and toughened of thickness 2 mm. had identical fracture velocities under the impact of small (.2 mm.) diameter steel balls. As this finding was unexpected, further studies were therefore made to determine the effect of thermal toughening on the fracture velocities From the Australian Defence Scientific Service, Department of Supply, Defence Standards Laboratories, Melbourne, Australia. Manuscript submitted Aug. 2, 71; revised manuscript accepted Oct. 29, 71. Address for reprints: E. C. Wigglesworth, Safety Officer, Defence Standards Laboratories, PO Box 50, Ascot Vale, Victoria, 02, Australia. 992 of specimens, in simulated safety lens form, using a range of ball sizes and thicknesses. Methods Materials and specimen preparation. The specimens were prepared from blocks of various sizes obtained from the Munitions Supply Branch of the Department of Supply. The material, an optical crown, was marked: "B and L. C/2. Melt Refractive Index 1.5, Dispersion 60.5." The was cut, polished, and edged in the laboratories to 50 mm. diameter size. Specimens of 1., 1.7, 2.1, 2.5,.0,.5 and 4.0 mm. thicknesses were produced. To obviate any base curvature effect, all specimens were flat and parallel, the thickness tolerance being ± mm. Specimens of each thickness were randomly sorted into two groups, one being used to simulate untoughened, the other being thermal- Downloaded From: on 11/09/

2 Volume 10 Number Thermal toughening on impact resistance of safety 99 ly toughened to simulate safety. The heat treatment was carried out on a Precision Cosmet Model 702 lens hardening unit, the heating temperature being held within the range 625 C. to 650 C. After heating, the built-in air compressor automatically delivered air through two small-orifice (2.0 mm. diameter) cooling jets for one minute. It was found necessary to vary the heating times with specimen thickness. Insufficient time led to spontaneous fracture during the cooling period; excess time led to warping of the specimens. The heating times finally adopted are shown in Table I. For the thinnest specimens, it was necessary to toughen at the lower end of the temperature range to prevent warping and to produce a clear birefringence pattern. All specimens were checked in a polariscope after heat treatment and again before testing. The birefringence pattern in every case was regular with a trend of decreasing contrast with decreasing thickness. Typical patterns are shown in Fig. 1. During the period of this investigation, the United States Food and Drug Administration published a proposal designed to require the use of safety materials in eye es and sunes. 2 It was thought that tests on a lens conforming to these requirements would be of value. The lens selected was a 50 mm. round, diopter spherical lens, as this could be made with center thickness of 2.0 mm. and edge thickness of 1.0 mm. (thickness tolerances + mm.). Stocks were purchased from a local supplier in finished but untoughened condition: They were then similarly divided into two groups, one of which was thermally toughened in the temperature range 625 to 65 C. in the laboratory. The birefringence pattern of these is shown in Fig. 2. Equipment. In accordance with the recommendation made in a previous report, all specimens were tested dynamically using a 25.4 mm. steel ball in the drop-tower system and a.2 mm. steel ball in the ballistic range. Temperature and humidity were also recorded. Experimental procedure. For direct comparison, toughened and untoughened groups were tested simultaneously, and a standard experimental routine was adopted. In the drop-tower work, ten untoughened specimens, that is one half of the group, were impacted from the specified initial height, any failures being noted as they occurred. Ten toughened were then impacted from the same height, any failures also being recorded. The other half of the untoughened specimens then followed, and finally the other half of the toughened specimens were subjected to the same test. The total number of failures was then checked, the height increased by M., Table I. Heating times as a function of thickness for thermal toughening treatment Specimen thickness (mm.) Heating time (min.) and the same sequence followed with the remaining specimens. This pattern was repeated until all specimens in both groups had been fractured. The height of the initial drop was M. for the 4.0,.5, and.0 mm. specimens, M. for the 2.5 mm. specimens, and M. for the 2.1, 1.7, and 1. mm. specimens. A similar procedure was followed with the ballistic work in which, as with previous work, 1 the specimens were held in welding goggle frames, mounted on a concrete anthropometric head-form. Each group of specimens was divided between the left and right eye-cups to obviate any bias due to asymmetry in the mounting. The test missile was a.2 mm. (Ys inch) steel ball, and the commencing velocities were 15 M. per second for the 4.0,.5, and.0 mm. specimens, and M. per second for all others. The velocity, which was measured for every shot, was increased in increments of M. per second. However, the ballistic procedure had to be varied for one set of experiments. To evaluate specimens of constant thickness against a range of missile sizes, the ballistic equipment was modified to fire 1.6 mm. ( /ic inch) steel balls. In this mode, it proved impossible to shoot with sufficient accuracy to ensure central target strike at a distance of 250 mm., i.e., if the velocity-measuring device remained in position. For this part of the work, therefore, the procedure was varied as follows. The velocity was measured and the corresponding pressure stabilized. The velocity-measuring device was then removed and the head-form positioned so that the specimen was located approximately 25 mm. (one inch) from the gun muzzle. All the specimens were then tested in the standard sequence at this pressure. The velocity was then remeasured and the procedure repeated at the next higher velocity. Errors of up to - 2 M. per second were found at the lower velocities and the results for the 1.6 mm. missile are subject to this error. Downloaded From: on 11/09/

3 994 Wigglesworth Investigative Ophthalmology December 71 Fig. 1. Birefringence patterns of thermally toughened specimens. Thicknesses are: (a),.0 mm.; (b), 2.5 mm.; (c), 2.1 mm.; (d), 1.7 mm.; (e), 1. mm. Results Temperature, relative humidity, and the measured maximum and minimum values of fracture height and fracture velocity, together with the calculated mean values and standard deviation, for the seven specimen thicknesses are given in Tables II and III. As in previous work, there was a wide spread of values between the maximum and minimum. In Table II, the coefficients of variation for the seven thicknesses of toughened varied between and 42 per cent, and for the untoughened between 22 and 4 per cent. In Table III, the corresponding figures were somewhat smaller, varying between 11 and per cent for the toughened and between 11 and 17 per cent for the untoughened ones. Although the highest coefficients of variation were found with the 1. mm. in three of the four series (the fourth being indeterminate), there was no indication of a general relationship with specimen thickness. Fig. 2. Birefringence pattern of thermally toughened diopter, 50 mm. round lens. Center thickness 2.0 mm., edge thickness 1.0 mm. The values of Table II are presented in terms of fracture height but, for comparative purposes, were converted to fracture velocity. These values and those of Table III are shown graphically in Fig.. Mean fracture values for.0 mm. specimens against four sizes of steel balls are given in Table IV: mean fracture velocities of the diopter spherical against Downloaded From: on 11/09/

4 II. Mean fracture heights of toughened and untoughened 50 mm. round, flat simulated safety against the one 25.4 mm.) diameter steel ball Fracture height (M.) ns ness m.) Temperature ( c.) Relative humidity No. of Toughened Range Max. 1 Min. Mean Standard deviation No. of Untoughened Range Max. 1 Min. Mean Standard deviation * of the fractured at M. III. Mean fracture velocities of toughened and untoughened 50 mm. round, flat against the Vs inch (.2 mm.) er steel ball Fracture velocity (M./sec.) ns ness.) Temperature r c.) Relative humidity No. of Max Toughened Range Min. 17 Mean Standard deviation No. of Max Untoughened Range Min Mean Standard deviation Downloaded From: on 11/09/

5 996 Wigglesworth Investigative Ophthalmology December mm ball 25-4mm _ ~" to'l^l toughened untoughened SPECIMEN THICKNESS (mm) 10 M* ' ' 2'5! 0 '5 4-t) Fig.. Fracture velocity as a function of specimen thickness. Table IV. Mean fracture velocities of mm. flat, 50 mm. round toughened and untoughened simulated safety against four sizes of steel balls Ball diameter (mm.) r c) 17 Relative humidity (%) Mean fracture velocity Temperature Toughened (M./sec.) Ten specimens only: all others, specimens. Untoughened (M./sec.) the 25.4 mm. and the.2 mm. steel balls are given in Table V. Discussion Effect of toughening process. As shown in Fig., the effect of thermally toughening simulated safety was to increase the fracture velocity. The approximate relationship found, shown in Fig. 4, was that shown in Formula 1, = V 2 (1) where v' = fracture velocity of the untoughened specimens, and v 2 = fracture velocity of the toughened specimens. Fig. 4 also shows that, for the conditions of these experiments, the relationship was independent both of specimen thickness and of missile size. This result, at first glance, seems to conflict with previous reports on the effect of thermal toughening. Thus, Shand 4 reported that breaking stress could increase by a factor of 2y 2 to 2>Vz, Smith 5 that it may double or even quadruple, the latter figure also being suggested by Jones, 0 while Phillips 7 postulated a possible increase of 660 per cent. All these values, however, were derived from experiments that measured the force required to fracture plates or rods of, supported at their extremities and subjected to gradually increasing loads, centrally applied. This is not the condition against which Table V. Mean fracture velocity of diopter toughened and untoughened Ball diameter (mm.) r c.) Relative humidity Mean fracture velocity Temperature Toughened (M./sec.) 4. 0 Untoughened (M./sec.).1 of safety spectacles are designed to afford protection. Lenses get hit. They are therefore appropriately evaluated, not by measurement of bending stress in a static system, but by measurement of impact strength in a dynamic system. Not only does this introduce a different unit of measurement (velocity instead of force), but comparative performance in this more complex stress system may differ significantly from performance in static tests. An extended discussion of this topic is given by Haward, s who also draws attention to the influence of the failure criterion adopted. The effect of the failure criterion on the results obtained in this study is considered separately below. In general, however, there is no necessary relationship between the results obtained from force measurements in static tests and corresponding results obtained from velocity measurements in dynamic tests. Downloaded From: on 11/09/

6 Volume 10 Number Thermal toughening on impact resistance of safety 997 Comparisons of the relative strengths of toughened and untoughened, obtained in dynamic tests, can be found in reports by Keeney 9 and Stewart. 10 Keeney, using a % inch steel ball, reported fracture heights for several types of. The only comparative figures for thermally toughened and untoughened samples of similar are those for 2.0 mm. thick, crown piano. The values were 54 inches and 27 inches, equivalent to velocities of 5.1 and.6 M. per second, agreeing exactly with the relationship found in this study. Stewart, using a.2 mm. steel ball, reported fracture velocities for thermally toughened and untoughened.0 mm. thick piano of 42 and 25 M. per second, which is in reasonable agreement with the V 2 relationship. Effect of small missiles. Stewart 11 further reported that, for small steel balls of diameter less than 2.0 mm., a reversal, effect took place, i.e., non-heat-treated offered more protection that did heattreated. In the present work, a 1.6 mm. (% 6 inch) missile was used to evaluate.0 mm. flat specimens but this reversal effect was not found. Table IV shows that the fracture velocities obtained with this missile conformed to the general pattern with an increase of approximately V 2 for toughened over untoughened specimens. The reasons for this discrepancy between the two sets of results seem worthy of further investigation, for both authors have stressed the dominant role of these small missiles in perforating eye injuries. Differences in missile size and lens size and shape are here thought to be of little importance. It is thought that the reasons for the discrepancy may be associated with support systems, base curvatures, test methods, and the failure criteria used by the two workers. Stewart worked on "mounted in rubber," and his photographs show them clamped in what is essentially a rubberjawed vertical vice, whereas, in the present work, the specimens were mounted in a CO 5 6- I 4- c n mm &./ ball / / Toughened / / / 2mm ball Thickness 40mm a -5mm b 0mm c 2-5mm. d 2-1mm- e 1-7mm- f Vmm. g j / a b V b Fig. 4. Mean fracture velocities of toughened and untoughened specimens. The straight line is the locus, x = V 2 y. welding cup goggles, as illustrated in a previous paper. 1 This author believes that the stress distributions induced by the two support systems could be markedly different. Additionally, Stewart used of base curvature 9.00 and.00 diopters, whereas this work was carried out with flat. There are also differences in the methods of calculating the fracture velocity and in the failure criteria on which the calculations are based. In their pioneering work, Rose and Stewart plotted partial and complete penetrations and calculated the median value of the overlap range. Stewart 10 later varied this by "averaging from one to ten of the highest velocities resulting in partial penetration with an equal number of the lowest velocities resulting in complete penetration." Stewart defined a complete penetration as one in which "the missile causes to be thrown from the back of the lens or the missile perforates the lens." A partial penetration was one that failed to develop a complete penetration. Throughout the present project, the emphasis has been on the ability of the lens to function as a protective shield. Careful examination of this requirement suggested that the eye would be undamaged if: (1) the rear surface was intact, Downloaded From: on 11/09/

7 998 Wiggiesworth Investigative Ophthalmology December 71 and (2) no piece of the lens had become detached. Accordingly, the failure criterion adopted was as follows: A lens was deemed to have failed if it cracked through its entire thickness or if it physically separated into two or more pieces. A lens that was damaged but did not fail received a further impact at increased velocity. In all drop-tower work, and in earlier ballistic work with the 6.4 mm. ball, no case of nonpropagating front face damage was seen. With some specimens using the.2 mm. ball and with all specimens using the 1.6 mm. ball, however, crater damage was observed at the point of impact on the front face. In Stewart's work, they would have been recorded as partial penetrations, but, in this work, additional impacts at increasing velocity were delivered until through-thickness cracks propagated. Justification for the continuing use of the failure criterion in these circumstances can be found in the work of Silberstein, 1 who has shown that severely damaged can be found in use. Consequently, it was thought that, as the technique differs from reality only in degree and not in concept, its use was appropriate and acceptable. In some cases, specimens had to be examined in the microscope to determine whether the crack had propagated to the back face. Such specimens were deemed to have failed even though, in a few cases, there was no associated spalling, i.e., no discrete particles had been detached from the back face. In the opinion of this author, the reasons for the discrepancy between the results of Stewart and those reported here may be related to one or more of these differences in the experimental conditions and techniques. Effect of lens geometry. The current ophthalmic and safety literature is unanimous in advocating mm. safety because they have greater impact resistance than 2 mm.. However, it is not at all clear whether this is directly attributable to the greater lens thickness or whether it arises because of an increase in the effectiveness of the heat treatment process in the thicker. Since Fig. shows that the strength of both toughened and untoughened specimens varies directly with material thickness, there is therefore no preferential heat treatment component. This finding assists the interpretation of Table V which shows that the impact resistance of the toughened diopter of center thickness 2.0 mm., is substantially higher than that of toughened flat specimens of 2.0 mm. thickness. Comparison of the relative fracture velocities of the two groups of untoughened, taken in conjunction with the conclusion of the previous paragraph, shows that the higher impact resistance of the diopter is a consequence of the different lens geometry and is not a consequence of a differentially preferable stress distribution induced by the thermal toughening process. Limitations of this work. This study has shown that an increase of approximately V 2 in fracture velocity occurs when flat specimens with thicknesses between 1. and 4.0 mm., and of power diopters are heat treated at the temperature stated and in the manner specified. This result should not be assumed to apply for other heat treatments or to of other geometries, i.e., to high plus and to all minus powers. Further work is needed to determine the results in these other cases. The author wishes to thank Dr. B. L. Cole, Head of School, College of Optometry, University of Melbourne, for his continuing interest in this project and for his valuable advice, comments, and discussions. Thanks are also due to Mr. Ken Wood, of Coles and Garrard Pty., Ltd., for arranging the production of the diopter spherical. REFERENCES 1. Wiggiesworth, E. C: A ballistic assessment of eye protector lens materials, INVEST. OPHTHALMOL. 10: 985, Federal Register: 5 FR October 2, 70.. Wiggiesworth, E. C: A comparative assessment of eye protective devices and a pro- Downloaded From: on 11/09/

8 Volume 10 Number Thermal toughening on impact resistance of safety 999 posed system of acceptance testing and grading, Am. J. Optom. To be published. 4. Shand, E. B.: Glass engineering handbook, New York, 58, McGraw-Hill Book Co., Inc., p Smith, G. P.: Glass, in Steere, N. V., editor: Handbook of laboratory safety, Cleveland, Ohio, 67, Chemical Rubber Co., p.. 6. Jones, G. O.: Glass, London, 56, Methuen & Co., p Phillips, C. J.: Glass: The miracle maker, New York, 41, Pitman Publishing Co. p Haward, R. N.: The strength of plastics and, London, 49, Cleaver-Hume Press, Ltd., p Keeney, A. H.: Lens materials in the prevention of eye injuries, Springfield, 111., 57, Charles C Thomas, Publisher, p Stewart, G. M.: Eye protection against small missiles, Am. J. Ophthalmol. 51: 81, Stewart, G. M.: Eye protection against small missiles, Am. J. Ophthalmol. 51: 86, 61.. Rose, H. W., and Stewart, G. M.: Eye protection against small high-speed missiles, Trans. Am. Acad. Ophthalmol. 61: 404, Silberstein, I. W.: The fracture resistance of industrially damaged safety, Am. J. Optom. 9:, 62. Downloaded From: on 11/09/