RESULTS Of SOME TESTS ON LOW-DENSITY MATERIALS July 1944 RESTRICTED CLASSIFIED DOCUMENT This document contains classified information affecting the National Defense- of the United States within the meaning of the Espionage Act, USC 50:31 and 32. Its transmission or the revelation of its contents in any manner to an unauthorized person is prohibited by law. Information so classified may be imparted only to per- sons in the military and naval Services of the United States, appropriate civilian officers and employees of the Federal Government who have a legitimate interest therein, and to United States Citizens of known loyalty and discretion who of necessity must be informed thereof. QRY_ This Report is One of a Scries l$$U€d In Cooperation with the ARMy-NAVY-CIVII. COMMITTEE on AIRCRAFT DESI6N CRITERIA Under the Supervision of the AERONAUTICAL BOARD No. 15C9 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with th« University of Wisconsin RESULTS OF SOME TESTS ON LOW-DENSITY MATERIALS^ 3y E. C. 0. ERICKSOi.', Engineer This report presents the results of exploratory tests made at room temperature to determine certain of the mechanical properties of a number of low-density materials examined for possible use as cores for sandwich con- structions. In general, the results are to be regarded only as indicative since the number of tests is small. The study included such tests as edgewise and flatwise compression and tension, flatwise static bending, and shear modulus tests, to provide the strength data currently recognized as the most significant in qualifying ma- terials for core usage. In referring to the application of loads in test, the following defi- nitions apply: Flatwise: Load applied in a direction normal to the plane of the panel as manufactured. Edgewise: Load applied to the edge of the original material, that is, in a direction parallel to the length or width of the panel. Test Materials Test material consisted of two materials received from The Ministry of Air Production, British Air Commission, "Onazote" and "Calcium Alginate," and two domestic materials, "Hycar or Ebonite EX50036," received from The Sponge Rubber Products Company of Derby, Connecticut, and cellular or fibrous cellulose acetate, an experimental material made by the DuPont Company. It is understood that this identical material is no longer, available and that an improved cellular cellulose acetate has since been developed by DuPont. The Onazote was in two nominal thicknesses, l/2 and 3/4 inch. The calcium alginate varied in thickness from 0.4 to 0.8 of an inch. The 18- by "This mimeograph is one of a series of progress reports prepared by the Forest Products Laboratory to further the Nation's war effort. Results here reported are preliminary and may be revised as additional data become available. Mimeo* No* 1509 -1- 12-inch Hycar panel was 0.8 inch thick. The cellulose acetate sample was taken from a sandwich panel received from the Consolidated '\ T ater Power and Paper Company about October 1, 1943. It was composed of fine fibers oriented in one principal direction, parallel to the 3/4-inch thickness of the test sample. Onazote and Hycar are black materials of quite similar appearance. The surfaces of both, as formed during manufacture, are fairly smooth. The material adjacent to these formed surfaces appears to be much denser than the intermediate structure; otherwise both materials appear to be homogeneous. Test Specimens and Test Methods All specimens were tested under ordinary room conditions and were pre- sumably in equilibrium with such conditions. No investigation was made of the effect of moisture on the strength properties or of the behavior of the materials with respect to absorption of water from humid atmospheres or when soaked. No determinations were made of moisture content. Specific gravity as given includes whatever moisture was present. Test specimens of the Onazote and Hycar materials had two original surfaces and four cut edges. The original or formed surface of these mate- rials was not removed prior to test, except as noted in the table of results. Equal numbers of edgewise compression specimens were taken from each of the two principal directions of the samples, lengthwise and crosswise (0° and 90°). The majority of edgewise compression tests were conducted on I- Inch by t (material thickness) by 4-inch specimens as laterally supported columns. A 2-inch gage length Marten's mirror compressometer was used to supply the deformation data. The cellulose acetate specimens were 1 by 3/4 by 3 inches long. Deformation data for these acetate specimens was obtained by averag- ing the readings of two dial gages recording the movement between the fixed and movable heads of the testing machine. A few tests were conducted on l/2- by l/2- by 2-inch specimens of nominal l/2-inch Onazote, using a 1-inch gage length extensometer to obtain deformation data. Flatwise compression tests were conducted on specimens 1 inch or 1-3/4 inches square and with a height equal to the thickness of the material. The elastic properties were obtained from deformation data taken between the fixed and movable heads of the testing machine. Lengthwise tensile specimens of the 3/4-inch Onazote were 1 by (t) by 16 inches long, shaped to have a 2-1/2 inch long central section l/2 In wide. The taper followed a 30-inch radius on each edge. These specimens were tested in self-aligning Tempi in grips. Deformation data was obtained by means of a Tripolitis extensometer equipped with a Last Word (l/lO,000) dial gage. Mimeo. No. 1509 -2- Flatwise tension tests were made on dumbbell- shaped specimens in the standard tensile grips used for molded plastic specimens. These specimens were prepared by gluing wood strips to each face of a 1-inch strip of the test material. Test specimens having a 1- by 1-inch net section were then cut to shape on a band saw. These specimens were tested for maximum load only. Flatwise static bending specimens l/2 to 1-1/2 inches wide were tested as a simple beam with center loading, over spans equal to 16 times the thick- ness of the material. Center deflections were obtained by means of a (1/1,000) dial gage. Modulus of rigidity data were obtained by means of the standard Forest Products Laboratory plate shear test, developed for measuring the shearing moduli in wood (FPL Mimeo. No. 1301); and by torsion-pendulum tests of long thin strips having a width of at least three times the thickness. Test Results Average property values resulting from a specified number of tests, together with the range of values of the four materials, are presented in table 1. All of the materials are believed to be essentially isotropic in the flatwise plane, because of the good agreement between the 0° and 90° data for edgewise compression. The edgewise compression properties reported in- clude the results of tests at both 0° and 90°. Modulus of elasticity for flatwise compression are affected by the inclusion of end conditions in the deformations recorded between heads. Typical edgewise and flatwise compressive stress-strain curves of the four materials are presented in figures 1 and 2, respectively. A typical stress-strain curve for 3/4-inch Onazote in lengthwise tension is shown in figure 3. These curves are based on actual load-deformation data for indi- vidual test specimens whose properties agreed fairly well with those of the average for the group. The end points indicated for each curve represent the ultimate stresses at failure, unless the numerical ultimate is given, in which case, the end points indicate the extent of the deformation data only. Comments Onazote It may be noted that the properties of Onazote in the l/2-inch thick- ness (specific gravity 0.172) were greater than the corresponding properties of the 3/4-inch thickness (specific gravity 0.135). However, on a specific basis, the 3/ 4-inch material is superior to the l/2-inch material in flatwise Mimeo, No. 1509 -3- bending but inferior in edgewise compression. This behavior reflects the presence of denser material adjacent to the surfaces. Further evidence of this was found in testing an edgewise compression specimen taken from tr.e formed edge of the 3/4-inch material. In comparison with similar rectangular specimens having two formed surfaces and four cut edges, this 1- by 3/4- by 4-inch specimen with three formed surfaces and three cut edges differed as follows: 9 percent increase in specific gravity; 90 percent increase in spe- cific modulus, but no change in specific ultimate. Flatwise tensile specimens tested normal to the formed surfaces of the 3/4-inch material failed in a fairly smooth plane parallel to and very close to the original surface of the material. Localization of the failure plane in the denser material near the formed surface would appear to indicate that a plane of weakness exists within the finer structure of the dense surface material. Hycar or Ebonite 3X50036 The characteristics of Hycar with respect to densification at the formed surfaces and types of failure are similar to those of Onazote. Calcium Alginate Calcium alginate machines readily by itself, but is difficult to cut when in combination with harder materials, because it has a tendency to spall and fracture along the edge being worked. Cellular or Fibrous Cellulose Acetate The cellulose acetate is readily compressible perpendicular to the principal fiber direction, but has considerable strength in the fiber direc- tion. The reported crushing strength, flatwise, represents the breakdown strength or first crushing failure. The average ultimate or maximum compres- sive strength flatwise (parallel to fibers) was 290 pounds per square inch, with values ranging from 240 to 320 pounds per square inch. It may be pointed out that the minimum flatwise crushing values of all the reported materials represent the limiting cold press pressures that may be employed in bonding face materials as in sandwich construction. 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