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honeycomb structures

honeycomb structures

  •  Many honeycomb structures, such as wing spoilers, fairings, flight controls, and landing gear doors, have thin face sheets that have experienced durability problems that could be grouped into three categories: low resistance to impact, liquid ingression, and erosion. 
  • These structures have adequate stiffness and strength but low resistance to a service environment in which parts are crawled over, tools dropped, and service personnel is often unaware of the fragility of thin-skinned sandwich parts.
  •  Damages to these components, such as core crush, impact damages, and disbonds, are quite often easy to detect with a visual inspection due to their thin face sheets.
  •  However, they are sometimes overlooked or damaged by service personnel who do not want to delay aircraft departure or bring attention to their accidents, which might reflect poorly on their performance record. 
  • Therefore, damages are sometimes allowed to go unchecked, often resulting in the growth of the damage due to liquid ingression into the core. Nondurable design details (e.g., improper core edge close-outs) also lead to liquid ingression.
  • The repair of parts due to liquid ingression can vary depending on the liquid, most commonly water or Skydrol (hydraulic fluid). Water tends to create additional damage in repaired parts when cured unless all moisture is removed from the part.
  •  Most repair material systems cure at temperatures above the boiling point of water, which can cause a disband at the skin-to-core interface wherever trapped water resides. For this reason, core drying cycles are typically included prior to performing any repair. 
  • Some operators take the extra step of placing a damaged but unrepaired part in the autoclave to dry to preclude any additional damage from occurring during the cure of the repair. Skydrol presents a different problem. 
  • Once the core of a sandwich part is saturated, the complete removal of Skydrol is almost impossible. The part continues to weep the liquid even in cure until bond lines can become contaminated and full bonding does not occur. Removal of contaminated core and adhesive as part of the repair is highly recommended.

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