Results for Type 1. For the three point flexural tests, four specimens were

prepared and successfully tested for the sandwich construction with the Type 1 core. The false nonlinearities and offset were corrected for each of the data curves using the previously mentioned procedures. The bottom face deflection and the strain in the bottom

facing at mid-span were then plotted as the dependent variable versus the load in Figures 5.5 and 5.6, respectively. From these curves, several observations were made about the material behavior, and based on the observations made during the tests, a failure mode for the Type 1 specimens was determined.

Figure 5.5: Three Point Flexural Test Results of Applied Load vs. Mid-span Bottom Face Deflection for Type 1

All of the curves have a similar shape with two distinct regions. In first the region, the response was very linear and this can be attributed to the constituent materials. The sandwich construction consisted of a Type 1 core made of rigid polyurethane foam and glass fiber reinforced polyurethane facings. The initial response of the rigid polyurethane foam is apparently linear elastic, which is evident in the results of the flatwise compression

and tension tests detailed in the previous section. As for the glass reinforced polyurethane facings, polyurethane is not typically linear elastic but can often be approximated as linear elastic, and coupled with glass fibers which are generally considered linear elastic, the composite has a behavior that is relatively linear elastic. As a result, the initial response in the first region is essentially linear elastic.

Figure 5.6: Three Point Flexural Test Results of Applied Load vs. Mid-span Bottom Face Strain for Type 1

In the second region, the response became nonlinear, and this is due to the crushable nature of the rigid polyurethane foam core and its relatively low stiffness. In the flatwise compression tests, the rigid polyurethane foam had nonlinear response that was characterized by an apparent yield point at its usable strength, at which point the foam

could not carry any additional stress. During the three point flexural tests, the stress concentrations under the load became larger than the usable compressive strength in the foam, which lead to yielding of the foam under the load. Once the foam began to yield, the top facing had very little support and the lack of stability coupled with the compressive stress in the top facing due to bending moments caused a buckle wave or wrinkle to form under the load. The buckle wave that formed had a wavelength proportional to the width of the loading bar. At this point, the top face began to deflect much more than the bottom face as foam under the load started to crush, which resulted in a permanent indentation in the top of the specimen. This failure mode is often referred to as local indentation. Then, the applied load continued to increase, but the rate at which it increased began to gradually decrease until it peaked, at which point a large portion of the foam under the load had yielded and the top facing had wrinkled excessively under the load. The load then began to decrease, and excessive deflection of the top facing eventually led to high stress concentrations under the edges of the loading bar that caused a facture in the facing and the core material underneath one edge of the loading bar. From this point on, the load began to decrease in an erratic stepped manor.

This type of failure occurred in Specimens 1-1-S, 1-2-S, and 1-4-S. As for Specimen 1-3-S, local indentation caused nonlinearity in the response, but before excessive local indentation could cause ultimate failure, a sudden facture occurred in the foam which resulted in an abrupt drop in the load. The fracture appeared to originate in the foam near the top facing just under the load and propagated diagonally though the core until it reached the bottom facing, where it propagated through the foam along the interface between the core and the bottom facing, at which point a large portion of the core separated from the

bottom facing. However, the failure occurred so quickly that the exact location where the fracture started is uncertain. This type of fracture is indicative of failure in the foam core due to shear stresses, but it is not entirely known why it only occurred in one of the specimens. One possibility could have been irregularities in the facing as the thickness of Specimen 1-3-S was not as uniform as the other specimens. The thickness of the top facing could have been larger under the point load than the average thickness, causing the facing to achieve a higher resistance to excessive local indentation.

In summary, the initial failure mode of all the Type 1 specimens was local indentation. The primary ultimate failure mode was excessive local indentation leading to fracturing of the facing and core due to high stress concentrations at the edges of the loading bar. However, one specimen ultimately failed due to shear stresses in the core. A picture of the initial crushing of the foam where the response became nonlinear is shown in Figure 5.7.

Figure 5.7: Three Point Flexural Testing Initial Failure by Local Indentation for Type 1

Also, pictures of the ultimate failure due excessive wrinkling of the facing, as well as shear failure in the foam, can be found in Figures 5.8 and 5.9, respectively. A detailed report of the three point flexural test results for each Type 1 specimen is presented in Appendix C.

Figure 5.8: Three Point Flexural Testing Ultimate Failure by Excessive Local Indentation for Type 1

Figure 5.9: Three Point Flexural Testing Ultimate Failure by Shear Fracture in the Core Material for Type 1