Out-of-autoclave (OoA) prepreg materials and methods have gained acceptance over the past decade because of the ability to produce autoclave-quality components under vacuum-bag-only (VBO) cure. To achieve low porosity and tight dimensional tolerances, VBO prepregs rely on specific microstructural features and processing techniques. Furthermore, successful cure is contingent upon appropriate material property and process parameter selection. In this article, we review the existing literature on VBO prepreg processing to summarize and synthesize knowledge on these issues. First, the context, development, and defining properties of VBO prepregs are presented. The key processing phenomena and the influence on quality are subsequently described. Finally, cost and environmental performance are considered. Throughout, we highlight key considerations for VBO prepreg processing and identify areas where further study is required.

A review of out-of-autoclave prepregs – Material properties, process phenomena, and manufacturing considerations

Effect of MWCNTs added by electrostatic flocking method on adhesion of carbon fiber prepreg/Nomex honeycomb sandwich composites

Aerospace sandwich structures are often manufactured using an autoclave co-cure process, in which prepreg facesheets are cured and bonded to honeycomb core simultaneously. Co-cure involves coupled physical phenomena, including gas migration, prepreg consolidation, resin/adhesive flow and crosslinking, and potential mechanisms of defect formation. Due to the “black box” nature of the process, however, scientific understanding of the complex and interacting phenomena that cause defects remains incomplete. We address this challenge using an in situ visualization method that enables direct observation of the skin/core bond-line in realistic autoclave conditions. Five cases are presented, which span a range of process conditions leading to various defect-formation phenomena, and show the effectiveness of in-bag pressurization for preventing bond-line defects. We demonstrate that in situ diagnostics can eliminate much of the trial-and-error typically involved in process troubleshooting, by providing insights into the physics of co-cure that would otherwise be difficult to obtain.

Process diagnostics for co-cure of sandwich structures using in situ visualization

Miniature pressure sensors were embedded into the honeycomb core of sandwich panels featuring both tool-side and bag-side skins. The pressure response was measured throughout the vacuum hold and elevated temperature processing stages of both oven and autoclave manufacturing. The elevated temperature processing measurements validated the honeycomb core pressure model presented in Part I, confirming that gas flow primarily occurs through the bag-side skin in semi-infinite panels. Aramid core panels showed much higher honeycomb core pressures than aluminum cores during elevated temperature processing. Higher core pressures during processing led to more gas flow and as a result, cured skins with interconnected porosity. The best sealed aramid core honeycomb skins were processed under one atmosphere of positive pressure with a vented vacuum bag, avoiding the continuous extraction of gas through the skins by the vacuum pump while the resin was highly mobile.

Vacuum-bag-only co-bonding prepreg skins to aramid honeycomb core. Part II. In-situ core pressure response using embedded sensors

An experimental investigation on the sandwich composites composed of the carbon fiber face sheets and Nomex honeycomb core has been carried out in this study. Multiwalled carbon nanotubes were adde...

Improvement in skin–core adhesion of multiwalled carbon nanotubes modified carbon fiber prepreg/Nomex honeycomb sandwich composites:

Composite sandwich structures offer several advantages over conventional structural materials such as lightweight, high bending and torsional stiffness, superior thermal insulation and excellent acoustic damping. One failure mechanism in a composite sandwich structure is the debonding of the composite facesheets from the core structure. A well-formed adhesive fillet at the interface of the honeycomb core cell walls and the laminate is a significant factor in preventing bond failure. In the present work, honeycomb composite sandwich panels are manufactured using a low-cost vacuum-bag-pressure-only out-of-autoclave manufacturing process. CYCOM®5320 out-of autoclave prepreg is used for the facesheet laminates and FM® 300-2U film adhesive is used for the facesheet-to-core bond. The manufactured composite sandwich panels are of aerospace quality with a facesheet laminate void content of around 1%. In this study, adhesive fillet formation and adhesive mechanical strength are evaluated as a function of several d...

K. Rupel

Papers

Evaluation of Skin-core Adhesion Bond of Out-of-autoclave Honeycomb Sandwich Structures