Inspection of composites fiberglass and carbon fiber structures may be required for advanced critical applications of composites materials.  Identifying any potential problems with the composites structure is extremely important.

Of course problems can be identified through destructive testing-drilling holes, making cuts, etc.  It also may be necessary to do testing in a non-destructive manner, i.e. not cutting into the laminate that is being tested.

One method of accomplishing non-destructive testing of composites laminates is using ultrasound.  This Youtube video demonstrates the use of this method.

As you can see, the damage is found in this carbon fiber laminate.  This information is a flag that can be used to decide whether to make a repair or replace the structure.

Delamination within the fiberglass or carbon fiber part will result in a much weaker structure than the design intended.  Other areas of the composites part will be further strained by weaknesses in other areas and may also fail.

This ultrasonic test can help to identify otherwise invisible problems with either the original manufacturing process or damage during the life cycle of the composite part.

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An advantage of using Soric as a core is that it flows resin very well for infusion.  It is easy to cut and handles well.

Disadvantages also abound.  One of them is the possibility of print-thru on the surface of the laminate.  Another is the negative effect on the structural properties of the laminate.  This non-woven material does not have much crush resistance such as a balsa or foam material.  A serious issue that I have found is the higher risk of delamination.  Like any core, this material works by separating the two skin layers to create a sort of “I beam” effect.  The problem is that this material is not inherently strong within itself.   Though it does become saturated with resin during a proper infusion, it is not nearly as strong as glass or carbon fiber reinforcement.

As the video demonstrates, a proper resin infusion can look easy.  With proper materials, practice, and knowledge it can be.

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The required stiffness of a section of a composites part depends upon the overall design and service expectations.  Several factors, including the life expectancy of the object, the load rating of the composites surface, the inter-laminar bond strength, will help determine the threshold requirements.  Testing is very important to determine the life expectancy of the part and whether it meets the requirements of its’ job.

Panel stiffness can be modified to meet these requirements using two basic methods.  One way to increase the stiffness of an unsupported composites panel is to reduce the size of the panel through additional support structures.  The other way is to increase the panel thickness across the same area.  Choosing which method to use depends upon the engineering of the part and determining which method is acceptable with the surrounding part layout.  If there is room to add supports, this is likely a quick and easy option.  If there is room to increase the thickness, adding new or additional coring materials may be a simple solution.

Building a strong and stiff composites structure can be accomplished with the extremes of  building a robust “skeleton” with small open areas and a thin skin, or having a basic, limited “skeleton” with a heavy duty cored laminate that supports itself.

A combination of the two usually works out best.

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There are many advantages to vacuum bag molding, a few of which include:

• Improved resin/glass ratio
• More consistency across the laminate and part -to -part as compared to open layup
• Containment of air emissions from the resins

As compared to hand layup and chop layup, there are a few disadvantages, including

• Higher consumable material cost
• Higher capital equipment cost
• Difficulty with superior surface finish

Some parts are more suitable for vacuum bag molding than others.  It also depends upon which process it is being compared with.

Vacuum bag molding requires an extremely tight seal between the mold and the bag.  Molds with multiple pieces or holes for inserts can be difficult to complete a seal.

Parts that are overly large and complex can present challenges with placing resin and reinforcement before the cure cycle starts.  The bag must be completely sealed and under full vacuum before the curing cycle of the resin begins.

The basic premise of vacuum bag molding is that the air is removed from the bag, allowing the atmosphere (air on the outside of the bag) to push the bag onto the part on the mold, compressing the layers of resin and reinforcement.  Many misinterpret the process as “sucking the extra resin out.”  We are merely allowing the laminate to be compressed by the weight of the air above us in the atmosphere to consolidate it before cure.  The excess resin is usually absorbed by extra layers of sacrificial material inside the bag.

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This video covers a quick resin infusion example, repair of the damaged laminate, and other useful information.  Peel strength and impact testing is covered, and demonstrates the superiority of fiberglass laminates over aluminum.

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The objective of gelcoat is to create a uniform thickness across the part, and have it be 18 to 25 mils thick when wet. As it cures, this thickness is reduced when some of the chemicals evaporate off. Spraying gelcoat onto a mold returns the best quality finish, but it may also be brushed in areas that are difficult to spray. Achieving a uniform thickness is difficult at intersecting corners, deep narrow areas, and difficult-to-reach sections of the mold.

The gelcoat is typically applied in three passes and allowed to “gas off” in between coats. This allows for some of the chemical evaporation to start, and can lead to problems if not done correctly. These passes are also usually bi-directional, where the first and third pass are in one direction and the second is in another, again to help achieve uniformity.

Areas of excessive gelcoat thickness can lead to cracking and surface finish problems. Areas of insufficient gelcoat thickness can “alligator” which is a surface flaw requiring extensive repair.

Gelcoat Application is a tricky part of the process and the manufacturer of the application equipment and the raw material is the best resource for best practices. Experience is the best formula for great results.

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