Dynamic axial crushing of multi-layer core structures of folded chevron patterns

Basily B. Basily, E. A. Elsayed

Research output: Contribution to conferencePaperpeer-review

Abstract

The objective of this paper is to investigate our recently developed innovative sheet folding theory and manufacturing processes in designing impact energy absorbing structures with superior properties than existing structures such as honeycomb while achieving a volume reduction between 40 to 50%. Initial results indicate that we can mathematically generate three-dimensional patterns and use our folding technology to produce such patterns by simply folding flat sheets of materials, resulting in a significant cost savings. The three-dimensional patterns, folded from different sheet materials, can be used as cores for laminated structures for impact energy absorptions applications, such as, in high speed airdrops of heavy items, and in improving crash worthiness of vehicle body and bumpers. The results of testing samples of the Chevron patterns (the simplest to fold from flat sheets) indicate that core structures made from this pattern will serve as absorbers of high velocity impact energy per unit volume when compared with the well-known and typically used honeycomb structures. Core structures are widely used in many applications ranging from temporary facilities, automotives, floor decks, bridge decks to the most advanced aircraft components. In all cases, core materials address the need for high strength and stiffness at low weight. There are several design criteria for the core structures, the most common ones briefly described blew. High Specific Strengths: for most core applications, it is necessary to maximize the compression and shear strength-to-weight ratio for structural efficiency purposes. This is important for cores primarily subject to compressive loads. In such cases, the design should also have equal shear properties both in the length and width directions. Honeycomb cores do not always have balanced properties, but the high specific properties compensate for that. Damage tolerance is the second important design criterion for core structures. The damage tolerance design assumes that initial defects or flaws are assumed to exist in structures before operation, and that these initial defects will grow during operation. Therefore, the design under this criterion requires that the material resist propagation of damage under both static and fatigue loading. The damage tolerance is usually quantified in a probabilistic form. The method of bonding the face-sheets to the core is critical. Current cores achieve this in two very efficient ways. For solid cores such as balsa and foam cores the bond is over the entire surface area, resulting in very efficient bond. Hence, lack of damage tolerances in these cores is usually due to moisture degradation, especially for balsa, and in core crushing, for foams. Honeycomb cores are bonded on a line at the intersection of the honeycomb and the face-sheet. Non-Catastrophic Failure Modes: The third design criterion is ensuring that failure of the core structure is non-catastrophic. This means the graceful failure in compression and shear loading is required. This is usually achieved by limiting the unstable buckling mode of the core structure. The core designs should not have highly directional properties. A typical example of undesirable directional properties is the conventional hexagonally shaped honeycomb core where there is a factor of two differences in its shear strength in the length versus width direction. Dynamic Impact Energy Absorption: Impact energy absorption is another important criterion for the design of core structures subject to dynamic impact loads such as cushions for high velocity airdrop packages, automobile bumpers and items subject to single or multiple impact strikes. The core should be designed to absorb the total dynamic impact energy in order to minimize the destructive effect of unabsorbed energy on the items protected (cushioned) by the core structure. This paper focuses on the design of new core structures that maximize impact energy absorption at minimum core volume. Honeycomb sandwich structures are typically used in absorbing impact loading, The energy absorption performance of these structures under impact is strongly influenced by both the honeycomb geometrical configuration and the mechanical properties of the honeycomb material. The machine designed and constructed for continuous production of folded patterns, successfully folds patterns of different geometries from different sheet materials. This was achieved by implementing a novel technique, in which, sheet material is pre-folded through a set of sequential and circumferentially grooved rollers, followed by final set of cross folding rollers engraved with specific patterns (Patent is applied for by Rutgers University). Impact tests were conducted on Kraft paper Chevron folded structures samples using four orientations, namely; a) flat orientation, b) side orientation c) vertical orientation and d) axial orientation for cylindrical samples, The orientation of folded structure is related to the direction of folded pattern and lamination with respect to the direction of the impact. Impact testing was conducted at four settings of predetermined different speeds which provided a pre-calculated impact energy of about: 25, 130, 200, 300, 400 and 525 lb.ft The test results of the vertical impacted sample outperform the honeycomb samples as it absorbs the total energy at a uniform load when compared with the honeycomb The total deflection is significantly less than the honeycomb sample, implying that energy absorbed per unit volume for the vertical sample is higher than that of the honeycomb sample. Hence, a significant reduction in packaged volume can be achieved using the vertical sample configuration. Combining these three directional configurations would provide a means of tailoring the cushioning pad to absorb a given impact energy at minimum volume for a specific object fragility.

Original languageEnglish (US)
Pages759
Number of pages1
StatePublished - 2004
EventIIE Annual Conference and Exhibition 2004 - Houston, TX, United States
Duration: May 15 2004May 19 2004

Other

OtherIIE Annual Conference and Exhibition 2004
Country/TerritoryUnited States
CityHouston, TX
Period5/15/045/19/04

All Science Journal Classification (ASJC) codes

  • Engineering(all)

Keywords

  • Folding geometry
  • High speed airdrop
  • Honeycomb
  • Impact energy absorption
  • Kraft paper
  • Lamination
  • Plastic hinge
  • Sandwich structures
  • Sheet material folding
  • Tessellation

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