Carbon Fiber Coupon Testing Project - February 2026
A testing program to analyze stiffness and failure modes of composite coupons, based on layup sequence
Overview
This project focuses on the design and execution of a composite coupon testing program, and includes the development of a custom test setup, composite layup sequences, and composite coupons. My primary goal was to determine how carbon fiber layup sequence affects part stiffness and failure, allowing me to design and build more efficient components and assemblies. This project is not associated with a course, and thus, no outside constraints or design requirements exist. I expect this project to take about a semester, as it requires access to ITLL testing equipment that is off-limits to the general public. At the time of this writing, fabrication for most coupons has been finished, and I'm awaiting lab access for performance testing.
Project Objectives
Project objectives included:
- Design a reliable, thought-out testing jig for consistent testing
- Follow ASTM testing standards
- Develop multiple composite layup sequences
- Test composite coupons and analyze test data to draw conclusions
Fixture Design
I had no previous experience using testing equipment, and with limited exposure, I wasn't sure if the ITLL lab would have fixtures available, or whether I'd need to design my own. To err on the side of caution, I developed my own fixture. That way, if the lab does have a fixture, I can use that, and if it doesn't, I can use mine. I knew that I wanted to use a three-point bending configuration with two fixed supports and a centrally applied load. With this in mind, I decided to follow ASTM D7264, Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials (Procedure A – 3-Point Loading).
This method involves several key parameters:
- Span-to-thickness ratio: 32:1
- Three-point bending configuration
- Carbon fiber composite focus
This incorporated a limited number of design features testing method lacks specific coupon dimensions, so I designed the fixture to be adjustable.
With limited fixture-oriented testing experience, I incorporated a limited number of design features. These included:
- A T-Slot integrated into the fixture base for easy fixture adjustment, only requiring tool access from above.
- Removable rollers, allowing for future replacement if rollers become worn or other roller materials or lengths are desired.
- Engraved measurement markings, allowing for fast and easy setup.
- 14 mm loading nose connection for easy mounting to test apparatus.
The fixture base measures 220 mm x 80 mm x 25 mm, and features a 15 mm T-slot.
Base mounted sliding supports span the base's width, and measure 60 mm tall. Each sliding support has a 5 mm counterbored vertical hole, designed for an M5 bolt. This M5 bolt threads into a receiver inside the base's T-slot, and tightening the bolt locks the sliding support in place. Each sliding support has machined cutouts for a 12 mm support pin. Pins and supports are drilled with a counterbore, for 2 M3 screws each, allowing for easy pin removal and replacement.
The loading nose features the same removable 12 mm pin design with 2 M3 bolt mounting interfaces. A 24 mm diameter cylinder protrudes from the top of the loading nose. A 14 mm hole is bored into this cylinder, with a 10 mm cross hole. This serves as the attachment point to the testing apparatus, which has a 14 mm male cylinder.
All fixture parts use 1/2 mm, 45 degree chamfers for a finished look. Overall, the fixture design is relatively simple and intuitive to use.
Coupon Design and Fabrication
There are few coupon-dimension-focused requirements for ASTM D7264, so I was free to pick coupon dimensions. I used a common composite thickness for ASTM D7264 of 4 mm. Starting with thickness makes everything else easier to design. For example, with a 32:1 span-to-thickness ratio and a 4 mm thickness, the testing span will be 128 mm. ASTM guides suggest a total span about 20% longer than the testing span, leaving a total specimen length of 153.6 mm. For consistency, I rounded this up to 154 mm. Generally, ASTM D7264 tests use a specimen width of 13 mm. However, the test specifically states that width can be altered if 13 mm is impractical. With this in mind, I modified my coupon design to be 20 mm wide. This made the layup process less sensitive to variation.
I designed the mold as a 200 mm x 80 mm x 30 mm block with a 154 mm x 20 mm x 4 mm + 2 mm cutout. An extra vertical 2 mm was added to prevent resin overflow, and to account for a pre-vacuum layup thicker than 4 mm. Even when using mold release, a solid mold would complicate part release. Thus, I designed the mold to be disassembled and held together via M8 bolts. With this design, layup would be completed with the mold bolted together, and once cured, the mold would be unbolted and forcibly disassembled. This process can still be difficult, so wedge cutouts were added, allowing for the use of wedges and pry bars to aid mold Disassembly.
Laminate Construction
I used 2x2 twill weave for the wing project, as it has aesthetically pleasing properties. Visuals aren't a priority here, so I opted for plain weave 3k carbon fiber from US Composites. I had leftover 635 thin epoxy, and decided to use that.
Materials:
- US Composites 5.7 oz/yd² Plain Weave 3K Carbon Fiber
- US Composites 635 Thin Epoxy
The sheets are 0.010" thick, so I'll need around 16 sheets to get to a 4 mm total thickness. Because the coupons are relatively small, and carbon fiber is sold by the yard, material shortage was not a concern.
As the project's goal was to determine the effects of stacking sequences, I came up with a series of stacking sequences, each with 16 total layers. To do this, I established a "normal" line to reference, (traveling along the coupon center, lengthwise) and varied layer orientation off this line. In theory, the carbon fiber coupons will stiffest when bent in plane with the carbon fabric, and thus, I focused on orienting the fabric in different ways and sequences.
I developed 5 layup sequences, identified by letters A-E. I focused on variety and the testing envelope. I planned to make 2 coupons of each for testing redundancy. All coupons were manufactured via vacuum bagging, with a 60/40 resin ratio by weight. Coupon edges were lightly sanded after fabrication.
Testing Procedure
The planned testing procedure is straightforward. Coupons would be loaded onto the test fixture, with rollers spaced at 128 mm on center. The testing machine will be run and two data categories will be collected as the load is applied.
Once the loading nose makes contact, applied load and nose displacement data will be collected, all the way through part failure. Once the coupon yields, it will be removed from the fixture, and photographed. This procedure is repeated once, for a total of 2 tests per coupon configuration.
Data Analysis
Applied load and loading nose displacement data will be used to calculate flexural stress, and flexural modulus, which will represent how much stress the coupon can withstand before yielding, and how stiff the material is while loading, respectively. Essentially, this will clarify each coupon's yield strength and deflection under load.
I will calculate flexural stress using the flexural stress formula, which calls for coupon geometry, applied load, and support span. In this case, my applied load will be the final load before the coupon yields.
I will calculate the flexural modulus using the flexural modulus formula. It calls for coupon geometry, support span, and the slope of the load-displacement curve. I'll calculate this slope using data from the testing sequence. Both equations are below:
Next Steps
At the time of this writing, coupon molds have been 3D-printed, coupon materials have been ordered, and coupon fabrication is expected to start soon. I am still awaiting lab access, and I've been told I may have to wait until the spring session is over for space to clear up. Regardless, I plan to take this step-by-step, starting with coupon fabrication. I plan to update this page as the project continues.