INFORMAT!fl9 RFV1EWED' AND REAFFiRMED 1965

Transcription

1 TENSILE STRESS-RUPTURE AND CREEP CHARACTERISTICS OF TWO GLASS-FABRIC-BASE PLASTIC LAMINATES No June 1937 INFORMAT!fl9 RFV1EWED' AND REAFFiRMED 1965 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 r 1P [.:.,. FOREST PRODUCTS LABORATORY UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE MAD/SON 5. WISCONSIN In Cooperaboo with the University of Wisconsin

2 TENSILE STRESS-RUPTURE AND CREEP CHARACTERISTICS OF TWO GLASS-FABRIC-BASE PLASTIC LAMINATES 1 By KENNETH H. BOLLER, Engineer Forest Products Laboratory, 2 Forest Service U. S. Department of Agriculture Summary Stress-rupture and strain-time relationships were obtained from tests up to 1, hours on two glass-fabric-base plastic laminates normally used in aircraft design. One was made with a typical polyester resin and 181 Volan A glass fabric, the other with a typical epoxy resin and 181 Volan A glass fabric. Data were obtained to show the effect of tensile loading at to the warp direction under atmospheric conditions of 73 F. and 5 percent relative humidity and at 45 to the warp under the same atmospheric conditions and at 73 F. in water. The data show that the stressrupture curve may be approximated by straight lines on semilogarithm coordinates, r = cro - M log t, and that the strain-time relationship may be shown as a function of the applied stress and time. The strain, e t, at any time, t, may be expressed as E t = e' sinh cr/ cre + to m' sinh o-/ crm. The constants were evaluated and are presented. The data show that the stress-rupture characteristics of the material tested for this report at to the warp are similar to that reported in Forest Products Laboratory Report 239. The data here reported, however, show the lower stresses at which the material fails when loaded at 45 to the warp and it shows that the creep strains at 45 loading are up to 6 times greater than creep strains at to loading. 1 This progress report is one of a series (ANC-17, Item 56-1) prepared and distributed by the U. S. Forest Products Laboratory under U. S. Navy, Bureau of Aero autics No. NAer 1768 and U. S. Air Force Contract No. DO 33(616)56-9. 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. Rept. No

3 Introduction The stress-rupture and strain-time behavior of plastics is important in the design of machine parts and structural members. It is known that plastics, as well as metals and wood, may fail under long-continued stresses that are less than their ultimate short-time strength. It is also known that the deformations caused by long-term loads may become excessive and render materials unsuitable for service. Much research is being done on the creep characteristics of various structural materials at both room and elevated temperatures to obtain design criteria and empirical formulas that represent the data. However, to develop design criteria especially suitable for use in aircraft and missile design, it was essential to obtain the stress-rupture and strain-time relationships of glass-fabric-base plastics in more than one direction. Relationships that result from a tensile stress in the plane of the laminations at and 45 to the warp direction were desired. At to the warp, data were desired for normally dry conditions, 73 F. and 5 percent relative humidity. At 45 to warp, the data were desired for the same dry condition and at 73 F. in water. Since aircraft applications were the primary consideration, the loads were to be applied for periods up to 1, hours. Even though the primary objective of this investigation was to obtain stress-rupture data at 45 to the warp for comparison with those at to the warp, a secondary objective was to obtain some comparative creep data. Tensile strains were measured, and a mathematical evaluation of these data is presented. This work was conducted at the Forest Products Laboratory from January 1956 to May 1957 at the request of and in cooperation with the ANC-17 Panel on Plastics for Aircraft. Laminates Tested Two types of glass-fabric-base plastic laminates were evaluated. One was made with a typical polyester resin, Paraplex P-43, the other with a typical epoxy resin, Epon 828 CL. Both were fabricated with 181 Volan A fabric laid up with adjacent plies parallel. One panel 1/8 inch thick by 36 inches square was made of each type of laminate. Rept. No

4 The polyester panel, 12 plies thick, had ari average total thickness of.131 inch, a specific gravity of 1.79, a resin content of percent, and a Barcol hardness of This panel was cured at a pressure of 1 pounds per square inch at 22 F. for 2 minutes, then at 25 F. for 7 minutes. Quality control specimens were cut from this panel and tested according to Federal Specification L-P-46b. Results showed that the strength parallel to warp was greater than that required by Specification MIL-P-813B. The average properties were as follows: (1) at 73 F. and 5 percent relative humidity, the average maximum stress was 49,98 pounds per square inch as compared with the specified 4, pounds per square inch, the stress at the secondary proportional limit was 29,55 pounds per square inch, and the secondary modulus of elasticity was 2,465, pounds per square inch; (2) after a 2-hour boil the maximum stress was 47,64 pounds per square inch as compared with the specified 38, pounds per square inch, the secondary proportional limit was 26,8 pounds per square inch, and the secondary modulus of elasticity was 2,432, pounds per square inch. The epoxy panel, 13 plies thick, had an average total thickness of.122 inch, a specific gravity of 1.86, a resin content of percent, and a Barcol hardness of 7.4. This panel was cured in a hot press at 215 to 22' F., at contact pressure for 12 minutes then at a pressure of 25 pounds per square inch for 48 minutes. Quality control specimens, cut from this panel and tested according to Federal Specification L-P-46b, showed greater strength than called for in Specification MIL-R-93A and MIL-P The average properties are as follows: (1) at 73' F. and 5 percent relative humidity, the initial proportional limit was 15,69 pounds per square inch, the secondary proportional limit was 44,2 pounds per square inch, and the maximum stress was 6,24 pounds per square inch. For maximum stress, the specification value is 47, pounds per square inch. The initial and secondary moduli of elasticity were 3,56, and 2,592, pounds per square inch respectively; (2) after a 2-hour boil, the initial proportional limit was 15,26 pounds per square inch, the secondary proportional limit was 47,7 pounds per square inch, and maximum stress was 59,45 pounds per square inch. For maximum stress, the specification value is 45, pounds per square inch. The initial and secondary moduli of elasticity were 3,457, pounds per square inch and 2,594, pounds per square inch respectively. Additional control specimens were tested to evaluate the short-time tensile strength of the material at ' to the warp direction and under the same conditions at which the long-time specimens would be tested. The short-time specimens were closely matched with the long-time specimens and tested according to Method 111 of Federal Specification L-P-46b. Rept. No

5 These test results are as follows: (1) The dry maximum stress of the polyester laminate at to the warp was 5,56 pounds per square inch, and that of the epoxy laminate was 58,32 pounds per square inch. These values agree with the quality control sampling. (2) The dry maximum stress of the polyester laminate at 45 to the warp was 21,65 pounds per square inch, and that of the epoxy laminate was 22,8 pounds per square inch. (3) The wet maximum stress of the polyester laminate at 45 to the warp was 14,69 pounds per square inch, and that of the epoxy laminate was 21,81 pounds per square inch. The wet specimens -in these tests were conditioned in water at 73 F. for 6 days before they were tested. Strain data were also observed on these creep-control specimens. The envelopes of tensile stress-strain data for these six conditions are shown in figures 1 to 6. Method of Test Data for the stress-rupture and strain-time relationships were obtained in tension by using the type 2 specimen described in Method 111 of Federal Specification L-P-46b. The specimen was held by 1/4-inch steel plates that were clamped together by four bolts. The load was applied to the specimen through these plates with universal joints, a knife-edged lever system, and weights in creep machines like the one shown in figures 7 and 8. These tensile-creep machines were in a room conditioned at 73 F. and 5 percent relative humidity. Dry tests were made at this condition. Wet tests were made in these machines with the specimens under water at 73 F. The water in the tank of the creep machine was maintained at a constant level. The lever-arm system was calibrated to determine the ratio of the load suspended from the arm to the load on a load cell in the machine. The load cell was a steel tensile specimen 1/8 inch thick that had been previously calibrated for its load-strain characteristics. The lever-arm ratio for each machine was determined and creep tests were made on plastic specimens with the lever arm level. Tensile strain was measured with a creep extensometer (fig. 9). It magnifies the strain in the specimen through a series of mechanical and optical levers (fig. 1). The magnification ratio of each extensometer was determined by comparing strain observations on the load cell with scale or dial readings on the extensometer. A magnification of 1 inch on the dial equaled a strain of.4 inch per inch on the specimen. A magnification of 1 inch on the optical scale equaled a strain of.1 inch per inch on the specimen. A combination of these two ranges was used. For tests of laminates at to the warp, Rept. No

6 the dial was used to observe the elastic and initial deformation. Then creep strains were observed on the optical scale. For tests of laminates at 45 to the warp, the dial was used throughout. An attempt was made to measure the total strains with SR4 gages attached to a spring clip instead of with the lever system shown in figure 9. This was done on specimens tested at 45 to the warp when dry. It was satisfactory for short periods of time for large strains. It was abandoned, however, because the creep in the spring clip and gage were excessive for small strain measurements and could not be tolerated. The load that was applied to the various specimens depended on the quality of the panel, which had been determined from the short-time tensile strength. Stress levels were chosen at 5 percent intervals beginning at 8 percent of the maximum and decreasing until the specimen supported its load for 1, hours. This procedure necessitated the choice of additional intermediate percentages in order to fill out and complete the stress-rupture curve. It also limited the data for stress-time-strain relationship to the high percentages of stress. The loading weights, computed from stress chosen, the cross section of the specimen, and the lever ratio of the creep machine, were supported on a jack while the specimen was being installed in its creep-machine. The weights were then lowered slowly, so that the total load was applied to the specimen in about Z minutes. While the load was being applied, the strains were observed. Sometimes the gears in the extensometer had to be reset because of limitations in travel. The moment at which, in the opinion of the operator, the specimen completely supported the weights marked the beginning of time and of the observed creep strain. The next observation was made 1 minute later, and succeeding ones at intervals that increased approximately geometrically. Timers that were shut off automatically when failure occurred were used to measure the interval of time required to cause failure. Presentation of Data Tables 1 to 4 present the stress-rupture and the strain-time data for the two materials. In each table the stress applied is shown in column 1 and is expressed as a percentage of the short-time ultimate tensile strength in column Z. The time required for each specimen to fail at the constant stress level is shown in column 3. If a specimen had not failed after 1, hours, it was removed and the actual number of test hours was noted. These data are also Rept. No

7 shown in figures 11 to 12, where the steady-stress values are plotted as ordinates on a uniform scale, and the time elapsing until failure occurred is plotted in hours on a logarithm scale as abscissa. Strains were computed from either the dial or the optical-scale readings. The strains noted at the beginning of the test are listed in column 4 of the tables. These initial strains at zero time scattered more than the envelope of strains obtained from the short-time tests (figs. 1 to 6). They scattered on both sides of the average line for the specimens loaded at 45, but for those loaded at they were generally less than the envelope values. The exact cause of this discrepancy is unknown. Several possible causes are known, however, such as a small previous strain history or inability to measure the strain from zero load. Whatever the cause, it is believed that the creep strains observed after zero-time were more accurate than those strains observed during loading. For the analysis of strain due to the constant stress, therefore, the strain values observed after zero-time were added to a mean strain value obtained from the envelope of short-time stress-strain curves. This correction, in reality, shifts the strain-time plot uniformly parallel to the strain axis. Column 5 presents these mean strain values at the respective stress levels at what would have been zero time. In columns 6 to 11 a total strain is presented that is equal to the observed creep strains plus the corrected zerohour strain. The observed creep strains at these periods had been picked from a plot of strain versus time on semilogarithm coordinates. Straintime curves from data of columns 6 to 11 of tables 1 to 4 are presented in figures 13 to 18 for the two materials tested. Discussion of Results Stress - rupture Figures 11 and 12 show the characteristic reduction of stress with increase in time as a stress-rupture curve. Data obtained from each test condition may be represented by a single straight line. A straight line has been drawn through all the data for each test condition from 1/1 to 1, hours to represent the average trend. Equations for these lines are shown in the figures. The equations are of the form y = b - m log x or Tr = o-o - M log t, where o-r is the rupture stress in pounds per square inch at time, t, in hours; cro is the stress or ordinate intercept at 1 hour; and M is the slope of the Rept. No

8 curve. This relationship, with the constants shown on the figures, serves as a guide to maximum stress that can be endured between 1 and 1, hours. The maximum tensile stress values endured in the short-time control tests, a- after 1, hours, cr l (as determined from the curves of figures sl' ' 11 and 12), and the percentage' ratios of o- follow: 1,/crs1 Material Tensile stress o- sl cr1, cr1, 1, p. s. i. 1, p. s. Percent Polyester laminate Tested to warp, Dry jdry Tested 45 to warp, Wet Epoxy laminate Tested to warp, Dry Dry Tested 45 to warp, Net The epoxy laminate has the highest stress in each of the three test conditions from dry strength at to warp to wet strength at 45 to warp. The 1,- hour strength of the polyester laminate, however, is a higher percentage of its short-time strength for two of these test conditions. It should be noted that the 45 wet strength of the polyester laminate after 1, hours is 61.4 percent of its short-time strength, and the 45 wet strength of the epoxy laminate after 1, hours is 5.8 percent of its short-time strength. Even though the strength of the epoxy resin laminate at 45 to the warp is not affected by wet conditions in a short-time test, the slopes of the stress-rupture curves indicate (figs. 11 and 12) that stress and time might affect the epoxy laminate more than the polyester laminate. Exacting analysis of these comparative data is not possible, however, because of the scatter of test data. Creep Many types of engineering and structural materials undergo progressively increasing strains with time while they are at constant stress. The variability and complexity of this strain-time relationship depend on the material, the Rept. No

9 stress or stress orientation, temperature, and other factors. Most materials follow the classic strain-time pattern in three stages after the initial strain that results from the application of the load. The first is a transition stage of decreasing strain rate. The second is a stage of minimum strain rate, and the third is an increasing strain-rate-stage to failure. Numerous attempts have been niade to equate the strain to all of the variables that affect it, including time. Of all the formulas that could represent the strain-time behavior of reinforced plastics at room and elevated temperatures, for short as well as long periods of time, the best equations for total strain, E t, and creep rate, V, that might be usable in aircraft applications appeared to be in the form of y = a + bxn or E t = E + mtn and V = Vin sinh o-/cr. These relationships were suggested for plastics and have been used to some extent by William Findley and his associates. 4' -5- The application of these relationships to the data in the investigation required an evaluation of the constants. At each stress level, the total strain, Et, at time, t, including the initial strain due to application of the load, would be represented by the equation E t = E + mtn, where E t is total strain in inches per inch, and t is time in hours. E m, and n are constants for that stress level. Since there are three unknown constants, a minimum of three points on the curve would be required for their solution. If those three points, E 1 ' 2' and E 3, at t 1, t 2, and t 3 were chosen so that t 2 = (t i t3 ) /Z-, then - E 2 log Z - El t2 log ti or E = 2 E3 (1 E2 E3-4- f ZE 2 3 Kempner, Joseph and Hoff, N. J. Bibliography of Creep for Structural Engineers. WADC TR 56-4, Findley, W. H., Adams, C. H., and Worley, W. J. The Effect of the 2 Laminated Plastics as Interpreted by the Hyperbolic-sine Law and Activation Energy Theory. ASTM Proceedings, 48, p. 1217, 'Findley, W. H. and Khosla, Gautam. An Equation for Tensile Creep of 3 Unfilled Thermo Plastics. Soc. of Plastics Eng. Journal, 17(12), December Rept. No

10 Then the third constant would be computed as follows: t l n Since, however, there were usually more than three points on the strain-time curve from.16 hour to failure for each stress level, the value of c o or n was approximated by the preceeding equations and then ',.he values of e t - c o = mtn were plotted on log-log coordinates, the log (E t - c o) versus log m + n log t. If the resulting points were a straight line, the first approximation was correct. If not, another value of c o was used, and the plotting was repeated. The values of c o, m, and n that best agreed with the data are shown in columns 12, 13, and 14 of tables 1 to 4. Hence, the equation with these constants represents the test data for each stress level. In order to make these strain-time constants useful at stress levels other than those for which they were computed, they must be related to stress. Figures 19 to 24 show the constants at each stress level versus the stress. Smooth curves have been drawn through the points to represent average values. The equations for these curves through values of and m are shown as y = a sinh bx. The value for n was assumed to be a constant, which agrees with Findley's investigations. 4 Using this form for c o and m, E D =E' sinh a- m = ml a- m where e', m', and and o- are constants. These constants were determ e, mined by successive approximations until the curves represented the data as shown in the figures. 6 It is unfortunate that there are not more tests at the lower stress levels, so that these relationships could be more accurately defined. The curves, however, define the relationships in the region for which stress-rupture data are available. With this relationship of the constants to the stress, the total strain equals cr o- E t = e l Binh + tnm" Binh a- em 6 Nadal, A. and Mcyetty, P. G. Hyperbolic Sine Chart for Estimating Working Stresses of Alloys at Elevated Temperatures. ASTM Proceedings, Vol. 43, p Rept. No

11 A summary of the values of the constants that define the relationship of strain to time, and stress and the stress-rupture curves at room temperature is given in table 5. The constants and relationships given in table 5 may be used to compute the total strains of these two materials at stresses and durations of stress, provided those stresses and periods of duration are kept within the limits of the stress-rupture curve. Strains have been computed for 1/1, 1, and 1, hours of loading. They are shown in figures 19 to 24 in conjunction with the short-time stress-strain curve. de Creep rates may also be computed. Since the creep rate, V, equals the at relationship given in table 5 becomes V= (n m t sinh Cr ) t n-1 o- m This is now a basis for comparing creep rates of the two materials at a selected time and stress level. For example, at room temperature, the creep rate of the polyester laminate in tension at to warp at 3, pounds per square inch for 1 hours would be 1.78 x 1-6 inch per inch per hour, compared to 2.36 x 1-6 inch per inch per hour for the epoxy laminate under similar conditions. This shows that there is not too much difference under these conditions between the strain rates of these two materials. The creep rate for the 45 loading in the wet test, however, shows a greater difference. For the polyester laminate, the rate at 8, pounds per square inch at 1 hours is 78 x 1-6 inch per inch per hour, compared to 27 x 1-6 inch per inch per hour for epoxy laminate. The creep rate of the polyester laminate is, therefore, 2.9 times faster than that of the epoxy laminate. Conclusions Stress-rupture and creep characteristics were obtained from tensile tests of typical 181 parallel laminates made of polyester and epoxy resins. Tests were made at and 45 to the warp direction for periods up to 1, hours. Based on the results of these tests, the following conclusions are given: (1) Stress-rupture curves were essentially straight lines when plotted on semilog paper. There was no pronounced endurance limit or leveling off of the curve for periods up to 1, hours. The straight-line relationship denoting the maximum tensile stress, Crr, that the material can endure in time, t, Rept. No

12 can be expressed by crr = cro - M log t where o- is the stress corresponding to rupture in 1 hour and M is the slope of the curve, a constant. (2) Strain-time curves are presented for periods up to 1, hours. At loading, the creep of both polyester and epoxy laminates was very small after the full test load was applied. At 45 loading, however, the strain at failure was usually about 2 or 3 times but as much as 6 times greater than the strain observed immediately after the test load was applied. (3) The stress-rupture strength of the epoxy laminate up to 1, hours under each test condition was greater than the corresponding strength of the polyester laminate. At any stress level between 1/1 and 1, hours, the epoxy laminate withstood the stress longer than the polyester laminate. For two of the three test conditions, however, the rate of strength reduction was greater for the epoxy than for the polyester laminate. (4) The relationship between strain, E t, with time, t, and stress, Cr, may be expressed by the equation cr E t = sinh + to m' sinh cr e cr For the two materials tested, the constants, E t, cre, n, and cr were determined. Thus it is possible to calculate and compare strains at any time and stress within the limits of the stress-rupture curve. (5) Creep rates, V = (n m l sixth ) of the epoxy laminate at to loadam ing were about the same as those for the polyester laminate, but at 45 they were substantially slower for the epoxy than for the polyester laminate. Rept. No

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