Honors Capstone Project
Date of Submission
Dr. Barry D. Davidson
Dr. Alan J. Levy
Mechanical and Aerospace Engineering
Engineering and Computer Science
Capstone Prize Winner
Won Capstone Funding
Sciences and Engineering
Electro-Mechanical Systems | Mechanical Engineering | Other Mechanical Engineering
Sandwich composites utilize a low density core and relatively stiff face sheets. These structures are ideal for applications that require high compressive strength, high bending stiffness, and very low weight such as aerospace vehicles. However, one problem with sandwich composites is their susceptibility to low velocity impact damage. Low velocity impacts result in both external damage, in the form of dents, and internal damage, in the form of core crushing, face sheet delaminations (two adjacent plies separating from one another), fiber fractures and matrix cracks. In general, it is assumed that visibly evident damage will be repaired. Barely visible impact damage (BVID) therefore represents a threshold, such that damage of this size or smaller must be considered to exist in flight structure, and structure must therefore be designed to tolerate this level of damage without a loss in performance. In order to design structures appropriately, it is necessary to understand the type and extent of internal damage present at or near the BVID threshold. Such damage assessments are then used as input for structural performance determinations.
The purpose of this paper is to investigate how structural and impact parameters affect the nature and extent of damage in sandwich composites in the vicinity of BVID. The particular sandwich composites that were studied are comprised of an aluminum honeycomb core and face sheets made from multiple plies of unidirectional graphite fibers in an epoxy matrix. The plies in the face sheets have fibers oriented in the 0°, 90°, 45° and -45° directions. These plies are relatively stiff in the fiber direction and compliant in the perpendicular direction. Plies of different directions are stacked on top of each other to build face sheets that are quasi-isotropic, i.e., that have the same strength and stiffness in their in-plane directions. The parameters that are investigated in this paper are the core thickness, core density, face sheet stacking sequence (the sequence that the plies in various directions are placed on top of one another), load, and indenter diameter. To this end, specimens are indented using a quasi-static indentation test. In this test, load is applied monotonically using a fixed diameter indenter until the permanent dent becomes barely visible. This approach has been shown to produce essentially the same type of damage as low-velocity impact, but allows for more consistent and controllable levels of damage to be created. The damage was then evaluated non-destructively via ultrasonics and destructively via cross sectioning and microscopy. The results obtained by these two methods were then compared and synthesized to obtain an understanding of the internal state of damage as a function of those parameters studied. It was found that the two parameters that are most important are the face sheet stacking sequence and the core density. In terms of stacking sequence, delaminations are most prominent between plies with large differences in their fiber orientations. For adjacent plies with very different fiber directions (i.e., a 90° ply followed by a 0° ply), there is a large mismatch in stiffness and in coefficient of thermal expansion. This causes large shear stresses, which in turn lead to delamination. In addition, stiffer, higher density cores are observed to cause more delamination to occur than lower density, more compliant cores. It is expected that the data and trends collected in this study may be used to provide guidance for choosing structural geometries that optimize weight, cost, and impact resistance for practical structural applications.
Eisenberg, David, "Evaluation of Quasi-Static Indentation Damage in Aluminum Honeycomb Core - Graphite/Epoxy Sandwich Structures" (2010). Honors Capstone Projects - All. 360.
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