Test Methodology. The three point tests were based on ASTM C393:

Standard Test Method for Core Shear Properties of Sandwich Constructions by Beam Flexure (ASTM C393, 2011). This standard served as a guideline for the tests however not all the details of the standard were strictly followed. Therefore, a detailed description of the specimen preparation, the test setup, and the test procedure is provided in the following sections.

5.1.1.1 Specimen preparation. The specimens for this experiment were produced

by cutting small beams from a larger beam segment using a fine toothed band saw. The specimens were partitioned at random using a ruler and a square. After they were cut, a coarse grit belt sander was used to lightly sand away any imperfections and ensure the sides were adequately straight and orthogonal to the adjacent sides. Four specimens were cut for Type 1 and Type 2 core configurations, and one specimen was cut for the Type 3 core configuration. A picture of one specimen for each core type is presented in Figures 5.1, 5.2, and 5.3. These specimens were approximately 3 in. wide by 8 in. long, and had a depth equal to that of the associated sandwich construction.

Figure 5.1: Three Point Flexural Test Specimen for Type 1

After the specimens were cut to size, strain gauges were applied to the center of the bottom facing of each specimen. The strain gauges were three-wire, 350 ohm, general purpose strain gauges that had a gauge length of 0.125 in., usable strain range of ± 3%, and were applied using the following procedure. The strain gauges were applied more than 24 hours before testing, and they were applied to the specimens in near standard temperature

and humidity conditions. First, the surface of the facings was insufficiently smooth to allow direct installation. As a result, initial preparation involved lightly sanding the facings at each gauge site. Then, two part epoxy (AE-10) was applied to the site to provide a base for the strain gauge. The epoxy was allowed to cure overnight, then it was sanded to a thin smooth surface. The sanded surface was cleaned using an adhesive catalyst. Once the surface had dried, the location of the gauge was marked using a ruler and a felt tip marker.

Then, the gauge was aligned with the markings and affixed to the specimen using clear tape provided by the strain gauge manufacturer. Visual inspection and a ruler were used to ensure the gauge was centered and aligned parallel the sides of the specimen. The tape was then partially removed to expose the bottom side the strain gauge, and the adhesive catalyst was applied to the bottom surface of the strain gauge. Once, the catalyst dried, strain gauge adhesive (M-Bond 200) was applied to the bottom surface of the gauge, and using the tape as a guide, the gauge was pressed onto the specimen. Pressure was applied to the strain gauge by hand for 60 seconds, and the clear tape was removed to ensure the gauge had adhered to the specimen. Next, the gauge wires were protected by securing them to the specimen with tape.

Figure 5.2: Three Point Flexural Test Specimen for Type 2

Figure 5.3: Three Point Flexural Test Specimen for Type 3

5.1.1.2 Test setup. The test setup used for the three point flexural experiment

consisted of a test fixture manufactured by Wyoming Test Fixtures that was modified and installed in an Instron 4469 Universal Testing Machine (UTM). The product name of the fixture is the Long Beam Flexure Test Fixture (Model No. CU-LF), and it was constructed of machined aluminum and carbon steel. For the three point loading experiments, the supports were set up for a beam span of 6 in. and a single loading point positioned at mid-span. The loading pads at the supports and loading point were 1 in. wide flat bars that were free to pivot, and they were considered simple supports that impose no concentrated moment on the specimen. Rubber pads with a Shore A hardness of 60 were inserted at the supports and loading point to help reduce and distribute the pressure concentrations under the loads. Linear potentiometers were mounted to the fixture to measure the deflection of the bottom face at mid-span, and since one was position on each side of the specimen, the average of the two was recorded. The linear potentiometers had a metal spring assisted shaft with a 2 in. stroke length. A linear variable differential transducer (LVDT) with a spring assisted metal shaft and a 4 in. stroke length was mounted to the frame of the Instron

4469 UTM, and this LVDT was used to measure the displacement of the crosshead. The load was measured through the 9 pin output of the Instron 4469 UTM. A photograph of the setup just prior to testing is presented in Figure 5.4.

Figure 5.4: Test Setup Used for the Three Point Flexural Test

5.1.1.3 Test procedure. Specimens for the three point flexural tests were tested on

multiple days under similar temperature and humidity conditions. Before testing, the width of each specimen was measured using digital or dial calipers to the nearest 0.001 in. A minimum of three measurements were taken and the average was reported. The height and facing thickness of the specimens was measured from the original manufactured beams before partitioning any specimens. A minimum of 10 measurements were taken for each

and the average was reported. The specimens for core Types 1 and 2 were not wide enough to interact with the linear potentiometers that measured the deflection of the bottom face at mid-span, therefore wooden extensions were affixed to the bottom of the specimens using double-sided tape so that the deflection could be measured. The lever arm imposed on the extensions was less than ¼ in. and the force applied to them by the linear potentiometers was very small, therefore the use of the extensions was assumed to have a negligible effect on the deflection measurements. Next, the supports were set to a span length of 6 in. and the loading point was set to mid-span using the markings on the test fixture and a ruler was used to verify the positions. The fixture was leveled, and the LVDT and linear potentiometers were then aligned parallel to the loading direction using a bubble level. The specimen was then placed into the fixture and the rubber pads were inserted at the support and loading points. The specimen was positioned with the strain gage at mid-span using a ruler, and the overhang of the specimen was approximately 1 in. from the center of the support to the end of the specimen. Next, the crosshead was lowered until a small preload of 0-20 lb. was applied to the specimen. The deflection and strain readings were then zeroed. A video camera was used to videotape the tests for further review after the tests were complete, and it was turned on at this point. The method of loading the specimens involved displacement control at a rate of 0.1 in/min. The load, the crosshead displacement, bottom face deflection at mid-span, and the strain in the bottom facing at mid-span were recorded at a rate of 1-2 Hz. Finally, the test was ended once the top face deflection reached 30-70% of the depth of the specimen which took 15-25 min. After failure, the specimen was promptly unloaded.