Testing Carbon Fibre and Core Materials

Today a client who builds custom automotive parts and body components for race cars was kind enough to drop us off some samples of Carbon Fibre with cores. This client has tons of experience retailing and working with two-part high durability paints for industry and home projects. The incredible value of these samples is they give us direct data of how lay ups will affect strength when laying up carbon fibre, carbon fibre with a foam core, carbon with a balsa core and honey comb core. The information provided by the client as well as the evidence from the samples was surprising and gives incredible into insight into your projects.

To begin with the client laminated 3K Carbon fibre twill (45 degree 2x2 weave) into sheets using vacuum bagging techniques for the most consistent epoxy lamination with the least voids and air bubbles. The carbon fibre was saturated with West System and the Endura in-house Epoxy, and put under peel ply, breather fabric and bagging film and set under vacuum for the cure time of the resin. This process is ideal for the testing because it reduces any variation between the samples without excess resin, voids, de-laminations, compression issues. The quality of the samples is impeccable and they are properly constructed for consistent testing.

Vacuum bagging is the ideal techniques when making high durability consistent parts for the automotive, aircraft, bikes, hockey sticks and carbon fibre tube. Vacuum bagging requires a mold surface, epoxy or infusion resin, composite material (fiberglass, Kevlar, carbon fibre, etc.), peel ply (release bag or stretchlon film) on top of your composite material, breather fabric or infusion mesh/tube, vacuum bag, sealant tape, suction cups and vacuum pump/venturi pump. Some of these materials are single use for each lamination, but there is a quality difference between the final result of the item.

The samples were weighed and flex tested to failure using two pins, a chain and a crane scale to weigh force. This was valuable for his testing because they want to make a strong ultra-light parts for a car they are custom manufacturing and he wanted the ideal strength using the best epoxy resin, carbon fibre layer count and core for this specific job. He was kind enough to bring some unused samples and the results of the testing to BoatCraft because we can use this knowledge to educate our clients when beginning to construct composite materials for trailer/ camper repairs, marine floor and transom repair, automotive panels and outdoor repairs.

The samples are 2 ½ x 7 ½ inches. Vacuum bagged 3K carbon fibre twill with epoxy resin. Samples with a core have two layers of carbon fibre in the same orientatin on both sides of the core.

In our conversations at the shop with many different projects, there is always a client need for low cost of repair and simplest materials to a minimum to complete the repair. There is always a challenge with balancing the best repair (ideal repair, proper materials and resins) and convenience or budget restricted projects, however, BoatCraft will always provide the information for the ideal repair that is factory standard or greater so the client will not have failure of the project or the need to redo a repair because of product quality or technique failure. In the discussion with the experience composite car builder, he stated very clearly that simply reducing on layer of carbon fibre on one side of the core material reduced the strength of the sample by half. What this means is reducing the materials on one side of the composite sandwich construction will reduce its strength in the lesser side by half. This is incredible information, because it is most commonly discussed with new first-time builders not equally laminating the topside and bottom side of a core material like marine plywood or foam core. This is important to understand when a builder is repairing or constructing walls, floors and transoms, where it is common to only fiberglass the topside of the material and not both sides. Once layer of fiberglass on the top side and bottom side is part of the ideal construction process to increase lifespan, durability and reduce the risk of failure. The claim that we hear is that fiberglass on the topside is sufficient for floors and trailer walls, which will lead to humidity and water entering the backside of the plywoods and in a short time begin to lose stability and delaminate.

The other interesting discovery in the information provided was the strength of the core material was not directly correlated to the density of the core material. The higher density core materials like honey comb and plywood ended up being significantly stronger when combined with the carbon fibre than the single material carbon fibre. The material benefits of using a plywood versus a foam or composite core is the weigh savings and resin cost. The most resin absorbed was the honey comb (with the most weight), the plywood was strongest and reasonably light, and the foam core absorbed the least resin and was the lightest. This brings into consideration the value of choosing the best materials, including core materials, for the strength and weight of the final result as well as combining multiple materials in the sandwich constructions. Choosing the core materials and proper sandwich composite construction is the ideal way to build, and generally is also the middle path between the most expensive materials versus home grade bulding materials. (read the blog post about choosing Choosing the Appropriate Wood for a Marine Build or Repair)

The final extremely intriguing thing that this expert mentioned is post curing the epoxy to increase the epoxy strength using basic tools and materials. Post curing is using controlled heat over a longer cure time to cook the resin and increase its strength beyond the regular 21 degree Celsius cure temperature. What the expert mentioned was that using heating blankets wrapped around the object is sufficient to create a post cure effect. There is more complex heat delivery systems like a plywood box with a heat source that can be decreased over time. Not all epoxies can be post cured, but the benefits of post curing are many times more strength without adding more material or resin. The sample using honey comb core required prepreg carbon fibre that required a 48 hour post cure at 40 degrees Celsius. The post cure temperatures are defined on the Technical Data Sheets for the specific prepreg fabrics. This information can be found on post curing and prepreg fabrics that can be used to greatly strengthen hand applied fiberglass with epoxy. West System and Entropy Resins CLR are able to be post cured.

This is the summary of the interaction and discussion with the composite car builder expert on the subject of his testing and techniques. This information is crucial to share because small errors can lead to large scale failure, specifically when choosing the incorrect core material, and techniques that will greatly improve your final result.