Test Methodology. The four point flexural tests used procedures 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.

5.2.1.1 Specimen preparation. As with the three point tests, 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 using the same procedures as with the three point tests, and a coarse grit belt sander was again used to lightly sand away any imperfections and ensure the sides were adequately straight and orthogonal to the adjacent sides. Three specimens were cut for the Type 1 and Type 2 core configurations, and one specimen was cut for the Type 3 core configuration. One specimen for each core type – Type 1, Type 2, and Type 3 – is presented in Figures 5.19, 5.20, and 5.21, respectively.

These specimens were approximately 3.5-4.5 in. wide by 26 in. long, and had a depth equal to that of the associated sandwich construction.

Once the specimens were the appropriate size, strain gauges were applied to the center of the top and bottom faces of each specimen. The same strain gages were used, three-wire, 350 ohm, general purpose strain gauges that had a gauge length of 0.125 in. and usable strain range of ± 3%. The gauges were applied using the same procedures as those used for the three point test specimens. The gauges were applied at least 24 hours before testing, and they were applied in nearly standard temperature and humidity conditions. The surface was lightly sanded at the gauge site. Then, two part epoxy (AE-10) was applied to

the site to provide a base for the gauge. The epoxy was allowed to cure overnight, then it was sanded to a thin smooth surface. The 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. 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 to the sides of the specimen.

The tape was then partially removed to expose the bottom of 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, then the clear tape was removed to ensure the gauge had adhered to the specimen. Finally, the gauge wires were protected by securing them to the specimens with tape.

Figure 5.19: Four Point Flexural Test Specimen for Type 1

Figure 5.20: Four Point Flexural Test Specimen for Type 2

Figure 5.21: Four Point Flexural Test Specimen for Type 3

5.2.1.2 Test setup. The same fixture manufactured by Wyoming Test Fixtures was

installed in the Instron 4469 UTM, and it was reconfigured for the four point flexural testing. The fixture is presented in Figure 5.4 in Section 5.1.1.2. For the four point loading experiments, the supports were set up for a beam span of 24 in. and the two loading points were positioned at the third points of the span. Therefore, the spacing between the load points and the support points was a constant 8 in. The loading pads at the supports and

loading points were increased to 1.5 in. in width by placing ¼ in. thick plates between the stock loading bars and the specimen, and the bars remained free to pivot. Consequently, the supports and load points were again considered to be simple supports that impose no concentrated moment on the specimen. Rubber pads with a Shore A hardness of 60 were again inserted at the supports and loading points to help reduce and distribute the stress concentrations under the concentrated loads. Initially, the same linear potentiometers were mounted to the fixture to measure the deflection of the bottom face at mid-span, and since one was positioned on each side of the specimen the average of the two was recorded.

Unfortunately, during the four point flexural tests one of the potentiometers was broken, and only one was recorded for the second and third Type 1 specimens as well as all of the Type 2 specimens. Once again, an LVDT was mounted to the frame of the Instron 4469 UTM, and it 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 set up just prior to testing is presented in Figure 5.22.

5.2.1.3 Test procedure. Specimens for the four point flexural tests were tested on

multiple days under similar temperature and humidity conditions, and the same general procedure as the three point flexural tests was used. Before the test was started, 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 cross-sectional dimensions such as the height and facing thickness of the specimens was measured from the original manufactured beams before cutting the specimens. A minimum of 10 measurements were taken for each dimension and the average was reported.

Figure 5.22: Test Setup Used for the Four Point Flexural Test

As in the three point tests, some of specimens 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. This was again assumed to have a negligible effect on the deflection measurements. Next, the supports were set to a span length of 24 in. and the loading points were moved the third points of the span distance 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 along with the plates used to widen the loading area were inserted at the support and loading points. The specimen was positioned with the strain gauges 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. The video camera was turned on to record the results visually. Then, displacement was applied to the specimen at a rate of 0.05-0.1 in/min. The load, the crosshead displacement, the deflection of the bottom face at mid-span, and the strain in the bottom and top facings at mid-span were recorded at a rate of 1-2 Hz. Finally, the test was ended once the crosshead displacement reached 30-60% of the depth of the specimen, which took 10-20 min with the exception of specimen 1-2-L, which was stopped at close to 160% of the specimen depth and took nearly 50 min. Once the test was stopped, the specimen was promptly unloaded.