EFFECTS OF TENSILE PRELOADING AND WATER IMMERSION ON FLEXURAL PROPERTIES OF A POLYESTER LAMINATE June 1956 INFORMATION REVIEWED AND REAFFIRMED No. 1856 1962 LOAN COPY Please return to: Wood Engineering Research Forest Products Laboratory Madison, Wisconsin 53705 This Report is One of a Series Issued in Cooperation with the ANC-17 PANEL ON PLASTICS FOR AIRCRAFT of the Departments of the AIR FORCE, NAVY, AND COMMERCE FOREST PRODUCTS LABORATORY MADISON 5 WISCONSIN UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE n Caoperation with the University of Wisconsin
EFFECTS OF TENSILE PRELOADING AND WATER IMMERSION ON FLEXURAL PROPERTIES OF A POLYESTER LAMINATE1 By ROBERT L. YOUNGS, Technologist Forest Products Laboratory,- 2 - Forest Service U. S. Department of Agriculture Summary This investigation was conducted to determine the effect of tensile preloading followed by water immersion on the flexural properties and water absorption of a typical polyester laminate. Specimens were cut from a parallel-laminated 181 Volan A polyester laminate and subjected to various tensile loads. They were then tested in flexure after being exposed, either while stressed or unstressed, to normal conditions or to various periods of water immersion. Preloading and water immersion appeared to affect flexural properties independently. Preloading tended to lower the modulus of elasticity and modulus of rupture and to raise the proportional limit stress. Although preloading produced statistically noticeable effects on most flexural properties, the effect was so small as to be of little or no practical significance. Immersion in water for 30 days lowered the proportional limit stress and modulus of rupture, but did not significantly affect the modulus of elasticity. Preloading significantly increased the rate of water absorption by immersed specimens, but neither the additional moisture absorbed in 30 days by preloaded specimens nor the additional moisture absorbed by all specimens after 6o or 90 days appeared to have any significant effect on flexural properties. -This progress report is one of a series (ANC-17, Item 55-4) prepared and distributed by the Forest Products Laboratory under U. S. Navy, Bureau of Aeronautics No. NAer 01683 and U. S. Air Force No. DO 33(616)53-20, Amendment A2(55-295). Results here reported are preliminary and may be revised as additional data become available. 2 -Maintained at Madison, Wis., in cooperation with the University of Wisconsin. Report No. 1856-1- Agriculture-Madison
Introduction The effects of water immersion or outdoor weathering on the strength properties of glass-fabric-base plastic laminates are characteristically studied on specimen material that has not been stressed before exposure. Previous studies3,26 have shown that stressing of glass-fabric laminates in tension or flexure, even to levels well below the maximum stress, results in crazing of the resin and perhaps other structural changes that modify the subsequent elastic properties. Since this crazing of the resin would provide paths for the penetration of moisture, it appeared likely that the mechanical properties of laminates that had been exposed to weathering or water immersion might also be modified by previous tensile stresses. It is possible, therefore, that studies of the usual type based on unstressed specimen material may-not be representative of service conditions with respect to deterioration of mechanical properties, total deterioration, or both. This study was designed to be an exploratory investigation of the effects of various levels of tensile preloading and subsequent water immersion on the flexural properties and water absorption of a typical polyester laminate. Description of Material A void-free panel, 1/8 by 36 by 36 inches in size was parallel-laminated of 12 plies of 181 glass fabric with Volan A finish and Selectron 5003 resin, a laminating resin of the polyester (alkyd-styrene) type conforming with the requirements of Military Specification MIL-R-7575A. The resin and fabric used were material that had been checked and found capable of producing laminates that meet the strength requirements of Military Specification MIL-P-8013A. The resin was catalyzed with 0.8 percent of benzoyl 3 Marin, J. Static and Dynamic Creep Properties of Laminated Plastics for Various Types of Stress. NACA Technical Note No. 1105. 1947. Werren, Fred. Effect of Prestressing in Tension or Compression on the Mechanical Properties of Two Glass-Fabric-Base Plastic Laminates. U. S. Forest Products Laboratory Report No. 1811, and Supplement 1811-A, 1950-51. Erickson, E. C. 0., and Norris, C. B. Tensile Properties of Glass-Fabric Laminates with Laminations Oriented in any Way. U. S. Forest Products Laboratory Report No. 1853. 1955. -The term "prestressing" as used in reference (4) is identical in meaning to the term "preloading" used in this report ": The latter term has been adopted to avoid confusion with prestressing of fabric during lamination of panels. Report No. 1856-2-
peroxide by weight. The panel was cured at a pressure of 14 pounds per square inch for 90 minutes in a press at a platen temperature that was gradually increased from 220 to 250 F. Average thickness of the panel was 0.132 inch, resin content 38.4 percent, specific gravity 1.77, and Barcol hardness 67. Preparation of Test Strips The panel was cut along the warp direction of the fabric into strips 1/2 inch wide and 36 inches long by means of a 1/8-inch emery wheel rotated at 1,770 revolutions per minute on the arbor of a variable-speed table saw. The strips were numbered consecutively and assigned to the various tests on the basis of a table of random numbers. At the same time, the distribution of flexural specimens within strips was decided by a modified Latin square arrangement. Test Design Two series of tests were conducted in order to evaluate the effects of preloading and subsequent water immersion on the mechanical properties of a 181 Volan A polyester laminate. In series 1, specimens cut from test strips that had been stressed to a specified level were tested in flexure after normal conditioning or after 30, 6o, or 90 days of water immersion in an unstressed condition. In series 2, test strips were preloaded as in series 1, then subjected to a constant stress for 30 days while at normal conditions or immersed in water. Specimens were then cut from the strips and tested in flexure. In addition to the investigations conducted under series 1 and 2, additional testing designed to give some indication of the effects of similar levels of preloading on resistance to outdoor weathering is in progress. This series of tests, designated as series 3, is being conducted on specimens cut from the preloaded strips of series 1. These specimens will be tested in flexure after outdoor exposure periods of 3 months, 1 year, and 3 years. The tensile stresses to which the material was subjected in preloading were all well below the ultimate tensile strength of the material. The highest stress level was approximately 50 percent of the ultimate strength. The lowest level of tensile stress was close to the initial proportional limit stress of the material, and the highest level was slightly below the secondary proportional limit stress in tension. Report No. 1856-3-
Series 1 - Exposed Unstressed Twenty 1/2- by 36-inch strips were selected and assigned to 1 of 4 groups by a system of randomization. Each of the five strips comprising a single group was subjected to the same level of tensile preload. The 4 groups were each loaded to a different level as follows: (a) no preload, (b) 8,000 pounds per square inch, (c) 16,000 pounds per square inch, and (d) 24,000 pounds per square inch. After the strips were preloaded, 4 flexural specimens 1/2 inch wide and 4 inches long were cut from the center portion of each strip. One specimen from each strip was assigned to each of the following exposure conditions: (a) normal conditions, (b) 30 days' water immersion at 75 F., (c) 60 days' water immersion at 75 F., and (d) 90 days' water immersion at 75 F. Normal conditioning consisted of at least 2 weeks under controlled conditions of 75 F. and 50 percent relative humidity. After the specimens were conditioned, they were tested in flexure. The entire series consisted of 16 plots, each made up of 5 specimens that had been subjected to the same preloading and exposure conditions. Series 2 - Exposed Stressed Eight 1/2- by 36-inch strips were selected and assigned to 1 of 4 groups and 1 of 2 exposure conditions by a process of randomization. Each of the 2 strips comprising a single group was subjected to the same level of tensile preload. The 4 levels of preload were the same as for series 1. After the strips were preloaded, they all were subjected to a constant tensile stress of approximately 5,000 pounds per square inch for a period of 30 days. The stress was applied by gripping both ends of a strip between steel plates and suspending the strip from one end while a dead load sufficient to produce the desired stress was attached to the other end. All strips were suspended in a room in which temperature was controlled at 75 F. and relative humidity at 50 percent. One strip from each preload group was exposed to the room conditions during the exposure period. The other strip from each group was kept immersed in water during the exposure period by means of a length of 1/2-inch diameter rubber tubing that was slipped over the strip, sealed at the lower end just above the grips, and filled with water from the top end. Following the 30-day exposure period, 5 flexural specimens were cut from the center portion of each strip and tested in flexure. Series 2 thus consisted of 8 plots, each made up of 5 specimens that had been subjected to the same preloading and exposure conditions. Series 3 - Exposed Outdoors Unstressed Three additional flexural specimens were cut from each of the 20 preloaded strips of series 1. Of these 3, 1 was assigned to each of 3 weathering times, 3 months, 1 year, and 3 years, thus providing a total of 5 specimens for each of the 4 preload levels at each weathering period. These speci- Report No. 1856-4-
wens were mounted in weathering racks and are being exposed outdoors in an unstressed condition at Madison, Wis. After the designated exposure time, they will be removed, reconditioned under controlled conditions of 75 F. and 50 percent relative humidity, and tested in flexure. Any significant effects of preloading on resistance to outdoor weathering that may become apparent as a result of this series of tests will be reported when the tests are completed. Test Methods Preloading The 1/2-inch-wide strips were preloaded in a testing machine by applying a tensile load of sufficient magnitude to produce the desired stress. Each strip was gripped at the ends in self-alining Templin grips and loaded in tension parallel to the warp direction at a rate of head travel of approximately 0.2 inch per minute. The strip was unloaded and removed from the grips as soon as the desired load was reached. Flexural Tests Flexural specimens 1/2 inch wide and 4 inches long were tested in a testing machine by center loading over a 2-inch span. A small initial load was applied, following which the specimens were loaded to failure at a rate of head travel of approximately 0.036 inch per minute. Load-deflection data were taken by means of a dial that was read to 0.001 inch mounted below the specimen with its stem touching the specimen directly beneath the point of loading. All specimens were weighed and measured for width, thickness, and length both before and after they were conditioned in order to obtain information on water absorption and dimensional change during conditioning. Crazing of Laminate Under Stress Some of the unusual characteristics of glass-fabric-base laminates, such as the dual straight line load-deformation relationships in tension', have been attributed to crazing of the resin under stress. In an effort to obtain information that would shed some light on these characteristics, observations of crazing during and after preloading and after 30 days under constant stress were made at various levels of magnification. 17 Werren, Fred. Mechanical Properties of Plastic Laminates. U. S. Forest Products Laboratory Report No. 1820. 1951. Report No. 1856-5-
Some crazing was visible in the laminate under high magnification (100x) before any stress was applied. The crazing was in the form of fine checks that were oriented both parallel and perpendicular to the warp direction of the fabric, and was probably due to stresses developed in curing the resin. The fact that such stresses were present was indicated by slight crooking of the 1/2-inch-wide strips as they were cut from the panel. Strips were observed during tensile preloading by means of a 6x hand lens with the aid of transmitted light. At a stress level of 3,000 to 4,000 pounds per square inch, the checks perpendicular to the warp direction could be seen to begin elongating slightly, and additional fine checks began to develop in the same direction, apparently following the threads of the fill. The checking was very evident at 8,000 pounds per square inch, and remained in evidence after the strips that were stressed to this level were unloaded. At a stress of about 15,000 pounds per square inch, the crazing had progressed to the point where a definite increase in opacity was evident. This opacity increased steadily up to the highest stress level and remained after the specimens were unloaded. Sudan AX red dye in benzol was applied to a small area on the surface of unstressed material and material that had been stressed to 8,000, 16,000, and 24,000 pounds per square inch in an effort to make the crazing more readily distinguishable. In material treated in this manner, the checks were visible to the naked eye and observation at low magnification brought out quite clearly a distinction between the stressed and the unstressed material and between the various levels of stress. The contrast in number and size of checks associated with the various levels of preloading is shown in figure 1, which consists of photomicrographs (10x) of preloaded material, as well as unstressed control material, to which was applied a Sudan AX red dye, It is apparent that the checks are inclined at a slight angle to the fill direction and follow the threads of the fill yarns. Observation at higher magnification (100x) showed a spiral arrangement of these checks as they followed the weave of the fill yarns. Similar observations and photomicrographs were made of material that had been stressed to 1 of the 4 designated levels and subsequently subjected to a 5,000 pound per square inch dead load for 30 days. As is shown in figure 2, the dead load produced extensive crazing in the material that had not previously been stressed, and increased the number and size of crazing checks in previously stressed material. This difference is most pronounced at the lower preload levels, and is less apparent in material previously subjected to a tensile stress of 16,000 or 24,000 pounds per square inch. Statistical Analysis of Results The majority of the testing was designed so that the results could be analyzed statistically by an analysis of variance. This analysis was carried out for each property under consideration, and afforded a simple and Report No. 1856-6-
efficient means of isolating the main effect of preloading, as well as main effects of water immersion and type of exposure (stressed or unstressed) and the interactions of these effects. Three separate analyses were carried out for each of the flexural properties. The results of series 1 and the results of series 2 were analyzed separately. The results based on comparable conditions of series 1 and series 2 (normal conditioning and 30-day immersion) were then analyzed with the principal aim of isolating the effect of exposure in an unstressed condition versus exposure in a stressed condition. Since the data on water absorption from series 2 were only approximate and were not suitable for rigorous analysis, one analysis of variance, based on data from series 1 only, was carried out for this characteristic. Presentation of Data Average values of flexural properties and water absorption, as determined from tests of series 1 and series 2, are presented in table 1. Also shown are values of standard deviation within plots for each property and each series, where a "plot" in this case refers to the five tests contributing to each average value. The standard deviation figures are based on the error term from an analysis of variance carried out for each property of each series, and serve to indicate the amount of variation of individual values about the reported average. The approximate range of individual values within plots can be determined by multiplying the appropriate standard deviation value by 2.32. Figures 3 through 6 show the relationship of preload to modulus of elasticity, proportional limit stress, modulus of rupture, and water absorption, respectively. Of these, figures 3, 4, and 5 include data from both series 1 and series 2. Figure 6, which shows the relationship between preload and water absorption, includes data from series 1 only, since comparable data from series 2 were based on the weight of entire strips before and after exposure, rather than on weight of individual specimens, and were subject to considerable error, as the data in table 1 indicate. In general, where statistical analysis indicated a significant linear effect of preloading, the relationship was shown by means of a least-squares straight line through the data points. Where significant differences also existed between types of exposure or immersion times, separate lines have been drawn to show the relationship of preloading to the property concerned under these different conditions. Report No. 1856-7-
Analysis of Results General Flexural tests were used to evaluate the effects of tensile preloading in this study because data on strength and elastic properties may be collected easily with this type of test, and it is relatively insensitive to variations caused by slight differences in testing technique. A strong argument could be advanced for the determination of preloading effects by means of compression tests, however, since it would be expected that the crazing developed under tensile stress would have more effect on cam; pressive strength than on tensile strength, as is shown by Marin's data.2- In flexural tests on specimens of the dimensions used in this study, where the span-depth ratio was approximately 16, the failure is generally a combination of compression and tension failures. Although the initial failure in such tests probably is in compression, the failure may be affected by the restraining action of the load point to the extent that effects of preloading on compressive properties are altered appreciably. For this reason, a compression test would be likely to show effects of preloading to a greater degree than the flexural tests used in this study. A statistically significant effect of preloading was evident for most of the properties investigated in these series of tests. This effect was generally slight, however, and probably insufficient to warrant consideration in the establishment of design criteria. On the other hand, if it is true that flexural tests are less sensitive to such effects than are compression tests, the tendencies observed in the present study assume a somewhat greater significance. Modulus of Elasticity The overall effect of preloading was to lower the modulus of elasticity. This is in agreement with previous studies2)4) that have shown the change in elastic properties of glass-fabric-reinforced plastic laminates after they have been subjected to an initial tensile load sufficient to produce crazing of the resin. The effect observed in the present series of tests, however, was also dependent on whether the laminate was exposed in a stressed or an unstressed condition. In series 1, where the specimens were exposed unstressed after an initial stress, a significant linear effect of preloading on modulus of elasticity was observed. Specimens that were not preloaded had a significantly higher modulus of elasticity than those that were preloaded, except that the effect of preloading was not apparent in specimens that had been stressed to 8,000 pounds per square inch. This preloading effect was independent of immersion time, and no significant effect of water immersion on modulus of elasticity was indicated. Report No. 1856-8-
In series 2, where strips were exposed under a constant stress of 5,000 pounds per square inch, there was no significant overall effect of either preloading or immersion on modulus of elasticity. Addition of a constant stress to the preloaded material did not significantly decrease the modulus of elasticity. It is not surprising that the effect of preloading is less apparent in specimens that were exposed under dead load, since the dead load itself was sufficient to produce crazing of the resin, as is evident from figure 2. This crazing produced by the dead load would tend to mask the effect of preloading. The high value for modulus of elasticity at 24,000 pounds per square inch preload stress in series 2 (table 1) also tends to mask the effect of preloading. There are no adequate grounds for rejecting this value, however, so it must be accepted as being due to normal variation. The overall effect of preloading on the combined results at all exposure conditions is shown by the least-squares straight line in figure 3. Proportional Limit Stress The proportional limit stress was raised by preloading and by exposure under stress, and was lowered by soaking. The relationship between amount of preload and proportional limit stress, as indicated by the slope of the straight lines drawn through the data points in figure 4, was very nearly the same under all exposure conditions. The level of the straight line was raised by exposure under stress and lowered by water immersion. The proportional limit stress was significantly decreased after the strips were immersed in water for 30 days. Immersion for 6o or 90 days, however, caused no further significant decrease. The effect of exposure under stress was significant both in normally conditioned material and in material immersed in water for 30 days, as is evident in figure 4. Because of the scatter of data points for immersed material ) no attempt was made to fit lines for all exposure conditions in figure 4. The line drawn for immersed material shows the general relationship between proportional limit stress and preload. Lines showing this relationship for individual conditions would have a similar slope but different mean values. Modulus of Rupture A slight reduction of modulus of rupture with increasing preload was observed in material subjected to normal conditioning. This relationship is indicated by the upper line in figure 5. There was no significant difference in this respect between material exposed to normal conditioning under stress (series 2) and material exposed unstressed (series 1). Report No. 1856-9-
In material subjected to water immersion, no significant reduction of modulus of rupture with increasing preload was observed. A tendency toward reduction of modulus of rupture in this material may have been masked by the greater variability of results when the strips that were immersed in water were tested. Again, no significant difference between the two series was indicated. The modulus of rupture was considerably reduced by water immersion, as is evident from figure 5 and the data in table 1. This reduction was not appreciably changed by immersion for longer than 30 days. Moisture Absorption The rate of moisture absorption showed a strong linear relationship to amount of preload. The solid line in figure 6 shows the overall relationship based on all immersed specimens of series 1. The dashed lines, which represent the individual immersion periods, indicate that the effect of preloading is slightly less after 6o or 90 days of immersion than after 30 days (the relative effect of preloading at each time is related to the slope of the lines). It would be expected that for longer immersion times, similar lines would approach the horizontal, which would indicate little or no effect of preloading. Since the amount of water absorbed did not exceed 1 percent of the original weight of the specimen at any time during the tests, slight differences in weighing technique could have made appreciable differences in the indicated values. This probably accounts for the fact that specimens immersed for 90 days had lower absorption values than those immersed for 6o days, even though all other conditions of exposure were the same. Such differences in technique as the amount of blotting before the strips were weighed, if consistent within each group, would be expected to affect the apparent mean of the group but not the slope of the line expressing the effect of preloading on the group. It should be noted that, although the precentage of moisture absorption continued to increase after 30 days of immersion, this additional absorption had no significant effect on any of the flexural properties of the material studied. This indicates that additional absorption beyond the 30-day period was largely accompanied by movement of moisture from the surfaces into the interior of the specimens without appreciable increase in moisture content near the surface. Since the material at the surface exerts a much greater influence on flexural properties than the interior material of the specimen, an increase in the moisture content of the interior would be expected to have little effect on these properties. Report No. 1856-10-
Conclusions The following conclusions may be drawn on the basis of the results of flexural tests on a 181 Volan A polyester laminate subjected to various exposure conditions while stressed and unstressed after various amounts of tensile preloading: 1. Crazing of the resin was apparent under low magnification at a tensile stress of 3,000 to 4,000 pounds per square inch. This crazing increased as stress was increased, and appeared as checks following the fill yarns of the fabric. 2. Modulus of elasticity was lowered by preloading. This effect was linearly related to the amount of preloading, and was less apparent in material exposed under stress than in material exposed unstressed. Water immersion for periods of up to 90 days had no significant effect on the modulus of elasticity. 3. Proportional limit stress was raised by preloading and by exposure in a stressed condition, and was lowered by immersion in water for 30 days. Longer immersion times caused no further significant decrease in the proportional limit stress. 4. Modulus of rupture was slightly reduced by preloading when the material was not immersed in water. Water immersion for 30 days or longer also reduced the modulus of rupture, but material thus exposed showed no significant effect of preloading. Whether the material was exposed under stress or in the unstressed condition had no effect on the modulus of rupture. 5. Rate of moisture absorption by specimens exposed in the unstressed condition showed a strong linear relationship to the amount of preloading, with a tendency toward accelerated absorption following preload stresses greater than 16,000 pounds per square inch. The rate of absorption increased as the amount of preload increased. Although moisture absorption was significantly greater after immersion for 6o or 90 days than after 30 days, the additional moisture absorbed during the longer immersion periods had no significant effect on flexural properties. 6. The overall effects of preloading on flexural properties appear to be independent of the effects of immersion in water. The results of series 1 and 2 tests do not indicate any significant interaction between water immersion and preloading in their effect on flexural properties. 7. Although preloading produced statistically noticeable effects on most flexural properties, the effect was so small as to be of little or no practical significance. Report No. 1856-11- 1.-19
Table 1.--Flexural properties and water absorption of 181 Volan A polyester laminate after various degrees orpreioading and various conditions and periods of exposure. Each value is the average of five specimens. Preload : stress : (P.s.i.) Series I Exposed unstressed Series 2 Exposed stressed : Normal Water immersion :: Normal : Immersed : condition : :: condition : in water :30 days 60 days : 90 days :: o : 2,810 8,000 : 2,84o 16,000 : 2,740 24,000 : 2,760 : MODULUS OF ELASTICITY (1,000 P.s.i.) 2,86o : 2,85o 2,800 : 2,88o 2,77o : 2,740 2,720 : 2,710 : : : : 2,86o 2,860 2,790 2,760 :: :: :: :: 2,750 2,810 2,710 2,870 Standard deviation) 8o 84 : : : : 2,82o 2,87o 2,760 2,760 PROPORTIONAL LIMIT STRESS (P.s.i.) o : 35,600 : 21,700 : 28,700 : 24,800 38,800 : 28, 300 8,000 : 38,500 : 26,600 : 26,300 : 25,500 :: 39,900 : 26,80o 16,000 : 38,600 : 25,300 : 32,300 : 26,600 :: 40,700 : 28,80o 24,000 : 39,000 : 26,100 : 34,300 : 27,900 :: 43,400 28,800 Standard deviation 3,260 3,87o MODULUS OF RUPTURE (P.s.i.) o : 58,000 : 49,800 : 50,200 : 47,600 6o,800 : 47,400 8,000 : 58,700 : 50,700 : 48,300 : 47,400 :: 59,700 : 47,800 16,000 : 58,200 : 48,700 : 47,900 : 46,800 :: 58,700 : 47,500 24,000 : 57,300 : 47,200 : 50,600 : 46,800 :: 57,600 : 46, 500 Standard deviation 2,600 1,800 8,000 16,000 24,000 Standard deviation 0.081 WATER ABSORPTION (Percent) 2 ID o.45 0.74 0.62-0.53.51.75.68.45.55.78.73.67.88.77 1 -Within-plot standard deviation based on error term of analysis of variance. 2 -These data based on weights of entire strips before and after exposure, rather than on weights of individual specimens. Report No. 1856.41.71
4,--4441 4 411* 40-4- 411// Z M 108 536 Figure 1. --Photomicrographs (10x) showing crazing checks in 181 Volan A polyester laminate after various amounts of tensile preloading without subsequent stressing. A, no preload; B, preload stress of 8,000 pounds per square inch; C, preload stress of 16,000 pounds per square inch; D, preload stress of 24,000 pounds per square inch. The warp of the fabric is oriented in a vertical direction in these photographs. Preloading was in the parallel -to -warp direction.
7. PI 108 537 Figure 2. --Photomicrographs (10x) showing crazing checks in 181 Volan A polyester laminate after various amounts of tensile preloading and subsequent loading under a constant tensile stress of 5,000 pounds per square inch for 30 days. A, no preload; 13, preload stress of 8,000 pounds per square inch C, preload stress of 16,000 pounds per square inch; 0, preload stress of 24,000 pounds per square inch. The warp of the fabric is oriented in a vertical direction in these photographs. Preloading and subsequent loading were in the parallel-to-warp direction.