Simulation of impact on sandwich structures

While it is true that honeycomb core can trap moisture, it is a myth that widespread moisture ingression is an inevitable outcome when honeycomb sandwich structures are exposed to real world environments. It is also a myth that when moisture ingression occurs, progressive weight gain and strength loss are inevitable outcomes. Using a rebuttal of a 2004 honeycomb-critical paper as the focal point, this paper summarizes multiple sources which indicate both that moisture ingression is preventable and, even if it does occur, that proper material choices can prevent severe consequences. These claims are further supported by results from in-plane compression testing of typical (0.064 g/cm3 aramid-paper/phenolic core with 0.116 cm carbon/epoxy faces) composite honeycomb sandwich specimens after impacting, water submersion, and extensive thermal cycling.

Honeycomb Core and the Myths of Moisture Ingression

The assessment of skin/core bond in honeycomb samples was carried out by a bistatic ultrasonic non-contact system. Lamb waves were generated by a Nd:YAG pulsed laser and detected by an air-coupled capacitance transducer located in pitch-catch configuration. Periodically spaced line sources, generated on the sample surface using a four element lenticular array, allowed generation of narrow-band directional Lamb waves. Changes in amplitude of the transmitted signal permitted detection of skin/core disbonds.

Non-contact ultrasonic inspection of skin/core bond in honeycomb with Lamb waves

In the past two decades, researchers have been developing prepreg materials with matrix properties that can allow the elimination of the additional adhesive traditionally used between the core and the skins of composite sandwich structures. There have been several publications on self-adhesive prepreg used for sandwich structures; but none with a comparative study for primary structural application, from the same fabrication basis.
This research focused on the properties of adhesiveless honeycomb sandwich structure with carbon graphite prepreg, while assessing the structure with adhesive film at the skin-to-core interface simultaneously. In the study, laminate and honeycomb sandwich panels were fabricated and tested with consistent lay-up, curing, and testing processes, all fully documented. Sandwich panels were made with the CYCOM 977-2 prepreg system from Cytec and the AF191 adhesive film from 3M, while the adhesiveless sandwich panel had the MTM45-1 prepreg systems from Advanced Composites Group (ACG). Laminate panels were also fabricated using the two different prepregs. Specimens from the panels where tested for physical and mechanical properties, as well as moisture absorption performance. The results obtained from the non-destructive testing and the experiments confirmed that the self-adhesive prepreg physical properties met the components and void content recommendations for use in primary structures. In addition to analysis of existing published data, mechanical tests were performed in room, hot and cold temperatures, as well as dry and wet conditions. The results suggested that aramid honeycomb sandwich structure with self-adhesive carbon graphite prepreg systems, alongside similar structure using additional adhesive, demonstrates the ability to be used for primary structure.

Adhesiveless honeycomb sandwich structure with carbon graphite prepreg for primary structural application: a comparative study to the use of adhesive film

Moisture ingress in aircraft honeycomb sandwich structures is an ongoing issue that has attracted significant attention from aircraft operators, maintenance depots and the research community. Moisture ingress can lead to skin-to-core bonding degradation, affecting structural integrity. Current procedures used for removal of accumulated moisture found within the composite honeycomb rudders of Canadian Forces’ aircraft impart a significant maintenance burden and excessive aircraft downtime. Moisture removal approaches used for similar structures by other nations are usually complex and invasive. This paper outlines the development of an accelerated, effective and non-invasive approach to removing moisture from the rudder sandwich structure, taking advantage of the original water ingress paths. An experimental study was conducted to evaluate the effects of such drying parameters as temperature, vacuum level, vibration, as well as water removal paths. The moisture removal approach developed was then applied to full-size structures and was proven to be simple and effective.

http://aix2.uottawa.ca/~rueno089/ge_lic.pdf

Investigation of an accelerated moisture removal approach of a composite aircraft control surface

: Many of the fixed and removable panels on the RAAF F-111 aircraft are made up of bonded honeycomb sandwich panels. Experience with the RAAF fleet has shown that a serious problem exists with degradation and damage of these panels. A review of the literature was undertaken to gain an understanding of the extent of this problem. It was found that panels were subject to large areas of adhesive bond separation and corrosion damage. This damage was believed to be caused by the ingress of water in the panel through poor sealing at the edges or after repair of the panels. Moisture in the panel is also believed to cause adhesive degradation that may reduce the strength of the bonds in such panels. At the same time the literature was surveyed to determine the design load cases for such panels. This information was used to develop a simple finite element model of a bonded honeycomb sandwich panel. This model was in turn used to generate data on the loading and failure of such panels. In addition, an understanding ofF-111 Structural Repair Manuals and the RAAF Engineering Standard C5033.

/pdf/loading-degradation-and-repair-of-f-111-bonded-honeycomb-4nan2oq9nf.pdf

R. A. Bartholomeusz

Papers

Loading, Degradation and Repair of F-111 Bonded Honeycomb Sandwich Panels - Preliminary Study