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This and the next slide are from:
Isaac M. Daniel, “Influence of core properties on the failure of composite sandwich beams”, Journal of Mechanics of Materials and Structures, Vol. 4, Nos. 7-8, 2009
ABSTRACT: The initiation of failure in composite sandwich beams is heavily dependent on properties of the core material. Several core materials, including PVC foams and balsa wood were characterized. The various failure modes occurring in composite sandwich beams are described and their relationship to the relevant core properties is explained and discussed. Under flexural loading of sandwich beams, plastic yielding or cracking of the core occurs when the critical yield stress or strength (usually shear) of the core is reached. Indentation under localized loading depends principally on the square root of the core yield stress. The critical stress for facesheet wrinkling is related to the core Young’s and shear moduli in the thickness direction. Experimental mechanics methods were used to illustrate the failure modes and verify analytical predictions.
About these figures Professor Daniel writes:
“Sandwich columns were subjected to end compression and strains were measured on both faces. The stress-strain curves for three columns with aluminum honeycomb, Divinycell H100 and Divinycell H250 cores are shown in Figure 13. Photographs of these columns after failure are shown in Figure 14. The wrinkling stress is defined as the stress at which the strain on the convex side of the panel reaches a maximum value. Note that the column with the honeycomb core failed by facesheet compression and not by wrinkling. The measured failure stress of 1,550 MPa is much lower than the critical wrinkling stresses of 2,850 MPa and 2,899 MPa predicted by ([Eq] 11 [Hoff & Mautner]) and ([Eq] 13 [modified Heath]), the former for c = 0.5. The columns with Divinycell H100 and H250 foam cores failed by facesheet wrinkling, as seen in the stress-strain curves of Figure 13. The measured wrinkling stresses at maximum strain for the Divinycell H100 and H250 cores were 627 MPa and 1,034 MPa, respectively, and are close to the values of 667 MPa and 1170 MPa predicted by ([Eq] 13 [modified Heath]. Agreement with the [Hoff and Mautner 1945] prediction would require coefficient values of c = 0.834 and c = 0.662 in ([Eq] 11 [Hoff & Mautner]).”
References:
[Heath 1960] W. G. Heath, “Sandwich construction: correlation and extension of existing theory of flat panels subjected to lengthwise compression”, Aircr. Eng. Aerosp. Technol. 32:8 (1960), 230–235.
[Hoff and Mautner 1945] N. J. Hoff and S. E. Mautner, “The buckling of sandwich-type panels”, J. Aeronaut. Sci. 12:3 (1945), 285–297.
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