Sandwich Composite Testing for Surfboard Design

[S Stevens]

Hello everyone!

I'm Sascha, and my partner is Matt. We're both students at UC San Diego, majoring in Structural Engineering with an emphasis on Aerospace structures. With the support of Rock West Composites and the UCSD community, we've delved into an exhilarating research project on surfboard construction throughout summer. Over the next six weeks, we'll walk you through our project, from setting up our testing rig to the design and testing of our coupons. It's been a journey filled with challenges and breakthroughs, and we're eager to share our experiences.

P.S. Below is a sneak peek of some of our coupons designs from the summer. See you in a week!

 

Edited October 1, 2023 by S Stevens

[John Kimball]

Welcome to the forum. I can’t wait to see what you do and your thought process along the way. 

[S Stevens]

Diving into Our First Batch of Surfboard Coupons at UCSD!

Hello Composites Community! Sascha here, along with Matt. This week, we're thrilled to delve into the specifics of our first batch of surfboard coupons. Let's break it down:

1. Dimensions: The Blueprint

Our coupons are designed to mirror a standard surfboard. They measure 24” x 6” x 2.5”, with the thickness aligning with typical surfboard dimensions. On both the top and bottom layers of the coupon, we apply 4oz E-Glass. Additionally, the top layer receives an extra 6oz E-Glass reinforcement. This consistency ensures our research remains as authentic and applicable as possible.

2. The Theory: Why Multiple Coupons?

In our research, the use of multiple coupons isn't just about quantity—it's about depth and precision. By producing a range of coupons, we're able to create an envelope of expected structural properties for a standard surfboard. This multitude allows us to derive various statistical metrics, offering a more comprehensive understanding of surfboard behavior. When we introduce a new surfboard design, we can compare it to the average surfboard's properties, providing a more meaningful comparison than juxtaposing two individual coupons. This approach accounts for the inherent variations in materials, the fabrication process, and testing conditions, ensuring our analysis is both thorough and accurate.

3. The Layup Process: Crafting the Coupons

Fabricating these coupons is both an art and a science. We start with sheets of EPS foam, each ¾ inch thick, and laminate them together using an EPS spray adhesive. Once laminated, we shape the foam block with a hot wire foam cutter. But that's not all! We then bisect the foam cores and integrate a balsa wood stringer right in the center. The final touch? Laminating the coupons with E-Glass. The top layer even gets an additional reinforcement, mirroring traditional surfboard construction. We use epoxy as our resin of choice and employ a vacuum bagging procedure to ensure uniformity in our coupons.

We're excited to share this journey with all of you and look forward to your feedback and insights. Stay tuned for more updates from our end! For a visual treat, check out the pictures below that detail a few of the 16 coupons we produced, all representing the general surfboard design.

 

EPS Foam Layup on the Hot Wire Cutter

 

4oz E-Glass Preparation Process

 

EPS Foam Core with Balsa Wood Strings

 

Vacuum Bagged Coupons After Wet Layup

 

Final Production Coupons

 

 

 

 

[M Dulansky]

Hi all! My name is Matt and I am going to take over for Sascha this week and delve into the construction of our testing rig and initial failure modes of our first few designs.

Intro

Throughout this project we used an MTS tensile testing machine located on campus at UC San Diego. This machine gave us extremely precise readouts of Force vs. Displacement that we utilized for our data analysis. The following image is a picture of a very similar testing machine to the one we used, we made various mounts for testing our specific coupons which I will discuss here. 

Figure 1: MTS Tensile Testing Machine

1. Four Point Bending Test Rig

Originally Sascha and I hoped to follow ASTM standard 393 for out of plane loading of Composite Sandwich panels, specifically the four point bending option. After machining the complicated geometry required by the specification we had the following result.   

 

Figure 2: Initial Testing Rig 

The bottom of the sandwich panel was then loaded with a similar setup, only with the two 60 durometer rubber contact pads spaced 22 inches apart instead of 6 inches. The goal of this test setup is to achieve compression failure in the top face sheet of the composite, which occurs from an excessive moment.

 

2. Initial Results 

Using this setup we actually found that our coupons, made from 4 and 6 oz laminate fiberglass cloth were failure in shear due to the weak shear strength of the foam. This does not resemble the failure mode of a surfboard from out of plane impact so we needed to change our design. The following image shows the shear failure experienced by a sandwich panel from this test rig. The two parallel lines show the shear failure caused directly from the rubber contact pads.

 Figure 3: Incorrect Failure Mode of First Testing Rig

3. Redesign

At this stage we understood that we needed to distribute the load from the testing machine in order to overcome the shear failure we experienced. The issue was that we were unsure how to overcome this because of the excessive deflection of the sandwich panel before failure. Eventually we decided to create a curved shell by which to press onto the coupon in order to distribute the load along the length of the coupon as it bends. After completing this we were able to get the desire failure mode we wanted, a singular buckle on the top face sheet of the specimen. The following images show both of these things.

 

                       Figure 4: Curved Shell Testing Rig                                                                   Figure 5: Buckling Failure Mode

 

4. Conclusion

Through this process we were able to create an efficient testing rig, as well as determine the most effective testing methodology for the remainder of our designs. Look forward to next week where we start to dig into the various designs that we began implementing, and all the tribulations associated with each design. See you next week!

[lo_0l]

On 10/14/2023 at 6:42 PM, M Dulansky said:

Hi all! My name is Matt and I am going to take over for Sascha this week and delve into the construction of our testing rig and initial failure modes of our first few designs.

Intro

Throughout this project we used an MTS tensile testing machine located on campus at UC San Diego. This machine gave us extremely precise readouts of Force vs. Displacement that we utilized for our data analysis. The following image is a picture of a very similar testing machine to the one we used, we made various mounts for testing our specific coupons which I will discuss here. 

Figure 1: MTS Tensile Testing Machine

1. Four Point Bending Test Rig

Originally Sascha and I hoped to follow ASTM standard 393 for out of plane loading of Composite Sandwich panels, specifically the four point bending option. After machining the complicated geometry required by the specification we had the following result.   

 

Figure 2: Initial Testing Rig 

The bottom of the sandwich panel was then loaded with a similar setup, only with the two 60 durometer rubber contact pads spaced 22 inches apart instead of 6 inches. The goal of this test setup is to achieve compression failure in the top face sheet of the composite, which occurs from an excessive moment.

 

2. Initial Results 

Using this setup we actually found that our coupons, made from 4 and 6 oz laminate fiberglass cloth were failure in shear due to the weak shear strength of the foam. This does not resemble the failure mode of a surfboard from out of plane impact so we needed to change our design. The following image shows the shear failure experienced by a sandwich panel from this test rig. The two parallel lines show the shear failure caused directly from the rubber contact pads.

 Figure 3: Incorrect Failure Mode of First Testing Rig

3. Redesign

At this stage we understood that we needed to distribute the load from the testing machine in order to overcome the shear failure we experienced. The issue was that we were unsure how to overcome this because of the excessive deflection of the sandwich panel before failure. Eventually we decided to create a curved shell by which to press onto the coupon in order to distribute the load along the length of the coupon as it bends. After completing this we were able to get the desire failure mode we wanted, a singular buckle on the top face sheet of the specimen. The following images show both of these things.

 

                       Figure 4: Curved Shell Testing Rig                                                                   Figure 5: Buckling Failure Mode

 

4. Conclusion

Through this process we were able to create an efficient testing rig, as well as determine the most effective testing methodology for the remainder of our designs. Look forward to next week where we start to dig into the various designs that we began implementing, and all the tribulations associated with each design. See you next week!

I'm not familiar with surfboard testing, but is the four-point method more indicative of the way someone's feet would naturally be positioned?

Also, is the goal of the coupons to test the resistance to a failure between the two sides or front and back?

Very cool! Thank you for sharing your project. Looking forward to more 🙂

[MSouther]

Nice work! Proper failure-mode inducing and load-distribution are big challenges with sandwich testing. 

Looking forward to seeing more! 

[S Stevens]

Carbon Fibre Coupons: A Look at Dark Arts Surfboard Design

Hello Composites Community! It's Sascha, with Matt by my side. This week, we're shining the spotlight on our latest creation: carbon fibre coupons. Let's dive right in:

1. Carbon Fibre Coupons: The Details

Our carbon fibre coupons are designed using a 3k 200g twill weave. Each side undergoes a single layer wet hand layup, ensuring precision and quality. Unlike our fiberglass coupons, these don't have a double-layered top skin. The entire process is completed with vacuum bagging, and notably, there are no stringers in this design.

Carbon Fibre Coupons

 

2. Emulating Dark Arts Surfboards

Dark Arts Surfboards utilize the strength and agility of carbon fiber to craft surfboards that promise enhanced performance and durability. In our research, our carbon fibre coupons serve as a tool to delve deeper into these claims, aiming to validate and understand the true benefits and innovations behind the Dark Arts design.

 

Pyzel x Dark Arts Surfboard

 

3. Performance Expectations: Stiffness and Strength Trade-offs

With the introduction of carbon fibre and the absence of a stringer, we're venturing into a realm of trade-offs. We anticipate a balance between the enhanced stiffness provided by the stronger composite skin and the potential flexibility due to the lack of a stringer. Our tests aim to determine if these boards not only exhibit increased stiffness but also possess a higher maximum failure load.

We're delving deeper into our research and will keep you updated on our findings. Your feedback and insights are always appreciated.

[S Stevens]

On 10/16/2023 at 5:48 PM, lo_0l said:

I'm not familiar with surfboard testing, but is the four-point method more indicative of the way someone's feet would naturally be positioned?

Also, is the goal of the coupons to test the resistance to a failure between the two sides or front and back?

Very cool! Thank you for sharing your project. Looking forward to more 🙂

Thank you for your questions, lo_0l!

When it comes to surfboard testing, we opted for the deflected curve design over the four-point (or quarter point) bending method. The reason behind this choice is that the deflected curve more accurately replicates the failure mode surfboards experience after a heavy wave impact. While the quarter point bending method is commonly used in beam bending tests due to its straightforward translation to moment and shear diagrams, it doesn't quite capture the real-world failure mode of surfboards in the way the deflected curve does.

To address your second question, our tests are designed to simulate the failure mode a surfboard undergoes when impacted by a wave, rather than the load when someone is riding the surfboard.

I hope this provides clarity! We're always happy to share insights about our project and appreciate your interest.