COMPRESSIVE AND SHEAR PROPERTIES OF POLYESTER AND POLYIMIDE FILM HONEYCOMB

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1 U. S. FOREST SERVICE RESEARCH PAPER FPL 75 OCTOBER 1967 COMPRESSIVE AND SHEAR PROPERTIES OF POLYESTER AND POLYIMIDE FILM HONEYCOMB CORE FOREST PRODUCTS LABORATORY FOREST SERVICE U. S DEPARTMENT OF AGRICULTURE MADISON, WIS. This Report is One of a Series Issued in Cooperation with the MIL-HDBK -23 WORKING GROUP ON COMPOSITE CONSTRUCTION FOR AEROSPACE VEHICLES of the Departments of the AIR FORCE, NAVY, AND COMMERCE

2 ABSTRACT This report presents the results of compression and shear evaluations of two plastic film honeycomb cores. Properties determined will enable structural engineers to arrive at rational designs of sandwich construction using these cores. The cores were of polyester and polyimide plastic films and of 2-pound-per-cubic-foot density. Cores were evaluated in thicknesses from 0.4 to 1 inch, and it was determined that compressive strengths for cores 1 inch thick were about 65 percent of those for cores 0.4 inch thick. Core shear strengths parallel to the core ribbon direction were twice those perpendicular to that direction. Core shear strengths for 1-inch-thick core were about 80 to 95 percent of those for 0.4-inch-thick core for polyester cores and 60 percent for polyimide cores.

3 COMPRESSIVE AND SHEAR PROPERTIES OF POLYESTER AND POLYIMIDE FILM HONEYCOMB CORE 1 BY PAUL M. JENKINSON, Engineer FOREST PRODUCTS LABORATORY 2 FOREST SERVICE U. S. DEPARTMENT OF AGRICULTURE INTRODUCTION Sandwich constructions comprised of strong thin facings bonded to a thick lightweight core can be used to produce stiff light weight structural panels for use in aircraft and other flight vehicles. Sandwich of polyester and polyimide plastic films may be utilized for special applications where the chemical and physical characteristics of these plastics are advantageous. As an exploratory study, the mechanical properties of honeycomb cores of these plastic were evaluated at 75 F. Honeycomb core shear and compressive strengths tend to decrease as core thickness increases; to determine the magnitude of this effect, specimens were evaluated in thicknesses of 0.40, 0.62, and inch. The Military Handbook 23 Working Group this project and experimental and analytical work was conducted at the Forest Products Laboratory in and CORE MATERIALS USED Cores were produced commercially, The polyester core was received from the manufacturer in sheets of the required thicknesses, and the polyimide core received cut to specimen sizes. Both cures were of inch plastic film formed to 3/8-inch hexagonal cells. Average density of the polyester core was 2.0 pounds per cubic foot. Density of the polyimide shear specimens averaged 2.0 pounds per cubic foot. while the compression specimens averaged 2.3 pounds per cubic foot. PREPARATlON OF SPECIMENS Core received from the manufacturer was conditioned to constant weight in a room maintained at 73 F. and 50 percent relative humidity. Specimens were cut from the sheets of polyester core using a bandsaw. 1 This paper is another report in the series (MIL-HDBK-23)prepared and distributed by the Forest Products Laboratory under U. S. Naval Air Command Order IPR and U. S. Air Force Contract F M Results here are preliminary and may be revised as additional data become available. 2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

4 the resin formed fillets, which bonded the cell walls to the plates. The loading plates were stiff enough to resist bending. A stiffness of at least 600,000 pounds per square inch for each inch of width and core thickness is recommended. 3 On this basis, required loading plate thicknesses were calculated as follows: After the adhesive had cured, specimens were removed from the conditioning room and tested immediately. Samples of each type of core were and the moisture content determined. EVALUATION OF SPECIMENS Compression Specimens Figure 1.--Core shear apparatus showing specimen with attached loading plates, spherical bearing block, and dial gage assembly used for measuring deformations. M Compression specimens were 4 inches square and 0.40, and 1.00 inch thick. Ends of specimens were dipped in epoxy resin to form reinforcing fillets about 1/32 to 1/16 inch deep at each end of the core cells. Shear specimens were 4 inches wide with the length 12 times the core thickness, except fox the 0.40-inch-thick specimens which were 6 inches long (15 times their thickness). Steel loading plates were bonded to the core with a roomtemperature-setting epoxy resin (fig. 1). The core cell ends contacted the loading plates and Compression specimens were mounted between two loading plates, and this assembly was placed between a spherical bearing. block and the upper platen of a testing machine. A specimen and loading plates are shown in figure 2. The loading plates were of magnesium tooling plate 4-1/2 by 6 inches by 1/2 inch thick. These plates were chosen because their flatness and thickness are held to very close tolerances in manufacture. A load of about 50 to 100 pounds was applied while the specimens were alined. Screw jacks were then placed under each corner of the bearing block to prevent further movement of the block. Marten s mirror compressometers were attached to the loading plates to measure deformations, as shown in figure 2. Load was then applied at a rate such that failure occurred in 3 to 6 minutes. Specimens failed by buckling of the cell walls, 3 American Society for Testing and Materials. Shear test in flatwise plane of flat sandwich constructions or sandwich cores. ASTM Standard C FPL 75 2

Figure 2.--Core compression apparatus showing specimen, magnesium loading plates, and Marten's mirror compressometers. M 132 011 followed by crushing and crinkling of the cell corners.

5 Figure 2.--Core compression apparatus showing specimen, magnesium loading plates, and Marten's mirror compressometers. M followed by crushing and crinkling of the cell corners. Most visible evidence of failure disappeared after removal of load. Shear Specimens Shear specimens were evaluated using the apparatus shown in figure 1. The specimens, with attached loading plates, were mounted between notched blocks between the platens of a testing machine, with the tower end supported on a spherical bearing block initial load of 50 to 100 pounds was applied while the loading plates were firmly and evenly seated in the notched blocks by tapping the spherical bearing block with a hammer. Screw jacks were then placed under the four corners of the spherical bearing block (fig. 1) to prevent further movement of the block during application of load. The movable platen of the testing machine was driven at a constant speed such that failure of the specimens occurred in 3 to 6 minutes. Movement of loading plate with respect to the other was measured to inch by a collar and dial gage, as shown in figure 1. The steel collar was attached to one loading plate with set screws, while the dial gage was similarly fastened to the other plate. The spring-loaded dial stern maintained contact with the collar. The collar and dial gage were mounted so that they moved away from each other as the specimen deformed, preventing damage to the dial when failure occurred. Failure occurred by progressive buckling of the core cell walls, followed by crinkling of the cell corners. Most visible evidence of failure would disappear after load was removed. PRESENTATlON OF DATA AND DISCUSSION OF RESULTS Average values and standard deviations of shear and compression properties of the plastic film honeycomb core are summarized in table Properties of individual specimens as well as average values and standard deviations are presented in the appendix in tables A1 and A2. Moisture contents of the cores at 73 F. and 50 percent relative humidity were 0.4 percent for the polyester core and 1.6 percent for the polyimide core. The effect of thickness on flatwise compressive strength of polyester and polyimide honeycomb cores is shown in figure 3. The strength of a core 1.0 inch thick was about 65 percent of that of a 0.4-inch-thick core. Typical compressive stress-strain curves are shown in figure 4. These are drawn for the 5/8-inch-thick cores. The effect of core thickness OR flatwise shear strength is shown in figure 5. The strength of polyester core loaded perpendicular to the core ribbon direction was not markedly affected by core thickness. Strengths of 1-inch-thick polyester core loaded parallel to the core ribbon 3

Table 1--Mechanical properties 1 of polyester and polyimide fiim honeycomb core 2 1 Numbers in parentheses are standard deviations. 2 0.

6 Table 1--Mechanical properties 1 of polyester and polyimide fiim honeycomb core 2 1 Numbers in parentheses are standard deviations inch polyester or polyimide film formed to 3/8-inch hexagonal cells. direction, and for 1-inch-thick polyimide core loaded parallel perpendicular, were about 80 and 60 percent of 0.4-inch-thick core strength, respectively. The shear strength for specimens loaded parallel to the core ribbon direction was about twice that perpendicular to the core ribbon direction. Typical shear stress-strain curves are shown in figure 6 for the polyester core and in figure 7 for the polyimide core. Figure 4.--Typical compressive stressstrain curves for plastic film Figure 3.--Effect of thickness of flatwise compressive strength of piastic honeycomb cores (5/8-inch thick). fiim honeycomb core. M M FPL 75 4

7 For the polyester core, the proportional limit was about half as high and the modulus of rigidity about 40 percent as much for core loaded perpendicular the core ribbon direction as for core loaded parallel to the core ribbon direction. Strain to maximum load twice as high for core loaded perpendicular to the core ribbon direction. The proportional limit stress was about half the maximum shear stress. For the polyimide core, proportional limit was about 40 percent as high and the modulus of rigidity about 20 percent as much for core loaded perpendicular to the core ribbon direction as for core loaded in the parallel direction. to maximum load was about twice as high loaded perpendicular to the core ribbon for core direction. about 25 The proportional stress was only percent of the maximum shear stress, Figure 7 shows the two-stage behavior of the polyimide core, After the initial straight portion of the stress-strain curve, buckling of the cell walls became pronounced shear stiffness dropped markedly. A tension field then developed in the cell walls and a second linear portion of the stress-strain curve was observed. Finally, Figure 5.--Effect of thickness on flat- crinkling of the cell wall corners occurred and wise shear strength of plastic fiim shear stiffness dropped again until the maximum honeycomb core, for loads applied shear stress was reached. Figure 8 shows a failed parallel and perpendicular to core polyimide shear specimen, showing the proribbon direction. M Figure 6.--Typical shear stress-strain curves far polyester honeycomb core (5/8-inch thick) with loads applied parallel and perpendicular to core ribbon directions. M

Figure 7.--Typical shear stress-strain curves for polyimide honeycomb core (5/8-inch thick) with loads applied parallel and perpendicular to core ribbon direction.

8 Figure 7.--Typical shear stress-strain curves for polyimide honeycomb core (5/8-inch thick) with loads applied parallel and perpendicular to core ribbon direction. M nounced buckles which cause the core to carry load a tension action and thus provide shear resistance after initial cell wall buckling. The second proportional limit occurred at about half the maximum shear stress, comparing closely with results for the proportional limit of the polyester core. The slope of the second linear of the polyimide stress-strain curve is about half that of the initial slope which was used to calculate the modulus of rigidity. Because no abrupt failures were observed for the polyimide cores tested in shear and little damage was visible after removal of load, a few specimens were subjected to multiple cycles of loading and unloading. The results are plotted in figure 9. For specimens loaded parallel to the core ribbon direction, the unloading curve for each cycle was roughly parallel to the loading curve. The maximum load for the second loading cycle was somewhat lower and the shear stiffness less than for first cycle, indicating some damage due to initial loading. Practically no residual shear strains remained after load was removed. For specimens loaded perpendicular to the core Figure 8.--Polyimide spear specimen after ribbon direction, results were similar except that failure, showing shear buckles and some plastic deformation occurred, resulting in wrinkles associated with tension field residual shear after load was removed. effects. M FPL 75 6

9 SUMMARY OF RESULTS Compressive strengths polyester and polyimide cores 1 inch thick averaged about 65 percent of those for cores 0.4 inch thick. Care shear strengths parallel to the core ribbon direction were twice those perpendicular to that direction. Shear strengths for 1-inchthick core were about 80 to 95 percent of those of 0.4-inch-thick core for the polyester core and 60 percent the polyimide core. The ratio of the modulus of rigidity for loading perpendicular to the core ribbon direction to that for parallel loading was about 0.4 for the polyester core and 0.2 for the polyimide core. The proportional limit stress was about 50 percent of the maximum stress for the polyester core and 25 percent of maximum stress for the polyimide core. Figure 9.--Typical shear stress-strain curves for polyimide honeycomb core (I-inch thick) showing cycles of loading and unloading. Material is loaded either paraiiei or perpendicular to the core ribbon direction. M

10 APPENDIX Table A1--Compressive properties of polyester and polyimide honeycomb core. FPL

11 1 Denotes direction parallel or perpendicular to core ribbons. Table A2-- Shear properties of polyester and polyimide honeycomb core.

COMPRESSIVE AND SHEAR PROPERTIES OF POLYAMIDE HONEYCOMB CORE

COMPRESSIVE AND SHEAR PROPERTIES OF POLYAMIDE HONEYCOMB CORE U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS. In Cooperation with the University of Wisconsin U.S.D.A. FOREST SERVICE RESEARCH NOTE FPL-0202 JANUARY 1969 COMPRESSIVE