Composite Strain Testing for Student Rocket Structures

Student rockets require structures that are both lightweight and strong enough to withstand the demanding loads of launch and flight. To validate the composite materials for the AQUILO IX rocket, the team combined tensile testing, strain gauges, and Dewesoft data acquisition to evaluate their mechanical performance. The results confirmed the suitability of the carbon-fiber design while identifying weaknesses in the hybrid carbon/glass-fiber structure that require further development before it can be used as flight hardware.

Composite materials for student rocket structures

Student rocket structures must be lightweight, strong, and stiff enough to withstand the demanding loads of integration, launch, and flight. Every additional gram affects vehicle performance, while structural failure can jeopardize the entire mission. For this reason, the mechanical properties of composite materials must be carefully evaluated before they are used in critical rocket structures.

The AQUILO Rocket Team, based at Inholland University of Applied Sciences in Delft, designs, manufactures, and tests many of its rocket components in-house. The Structural Department is responsible for developing composite structures and verifying that they can withstand the expected mechanical loads throughout the mission.

Each year, students from more than 25 countries and diverse technical backgrounds work together to design, build, test, and launch a new rocket. This hands-on approach allows students to apply engineering principles to a complete aerospace project while collaborating across structural, propulsion, electronics, and recovery disciplines.

For the AQUILO rockets, major components including the carbon-fiber aeroshell, flight computers, recovery systems, and propulsion systems are developed and manufactured in-house. This gives the team direct control over the design and manufacturing process while making composite material testing and structural validation an essential part of developing reliable flight hardware.

AQUILO rocket team and the AQUILO IX rocket

Founded in 2014, the AQUILO Rocket Team is a student project within the Aeronautical Engineering program at Inholland University of Applied Sciences in Delft. Students work together to design, manufacture, test, and launch sounding rockets for experimental and educational missions.

The Structural Department is responsible for designing and manufacturing key rocket components, including structural tubes, fins, nose cones, and sections housing the motor and flight computer. Alongside manufacturing, the team conducts research and testing to validate the performance of these components before flight.

Designing AQUILO IX for EuRoC 2026

The AQUILO IX rocket was developed to compete in the 3,000-meter solid rocket category at EuRoC 2026. The objective is to reach an apogee as close as possible to 3,000 meters using a solid rocket motor. AQUILO IX was also designed to carry a 100 × 100 × 100 mm payload while maintaining reliable flight performance and structural efficiency under demanding launch conditions.

The rocket is powered by an internally developed solid rocket motor capable of producing approximately 11 kN of peak thrust. AQUILO IX also incorporates an internal structure and custom-designed airbrakes to help control its flight and achieve the targeted apogee. These requirements make understanding and validating the mechanical properties of the rocket’s structural materials particularly important.

Developing a hybrid carbon/glass-fiber structure

A significant change in AQUILO IX is the transition from previous aeroshell designs with a core material to a fiber-reinforced composite aeroshell. The new concept combines carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) to balance structural performance with the requirements of the onboard electronics.

Carbon fiber provides high stiffness and strength but can interfere with radio-frequency transmission. The team therefore incorporated glass fiber in specific areas of the structure, allowing antennas to be positioned within the rocket body while maintaining a lightweight composite design.

This hybrid CFRP/GFRP concept created an important engineering question: could the transition between the two composite materials withstand the required structural loads? The tensile and strain tests described in the following sections were performed to evaluate the mechanical performance of the composite configurations and determine their suitability for future rocket structures.

Composite tensile and strain test setup

Composite material testing objectives

In student rocket applications, structural components must be lightweight, strong, and stiff enough to withstand handling, integration, launch, and flight loads. Composite materials are well suited to these requirements because of their high strength-to-weight ratio. However, their mechanical performance depends on factors such as fiber orientation, layup configuration, manufacturing quality, and loading direction.

Testing is therefore essential to verify the mechanical properties of composite materials before they are used in rocket structures. This was particularly important for the AQUILO Rocket Team because some of the CFRP material supplied by sponsors had expired or been quarantined. Although the material could potentially retain suitable mechanical properties beyond its specified shelf life, its performance needed to be validated before use.

The main objective of the test was to evaluate the mechanical response of composite samples representative of materials used in AQUILO rocket structures. Tensile loads were applied to the samples while strain gauges and Dewesoft data acquisition were used to measure the relationship between applied force and strain.

The recorded data was then used to determine key mechanical properties, including maximum strain, maximum stress, force-extension gradient, and estimated Young’s modulus. In addition to evaluating the individual composite materials, the testing established a repeatable measurement workflow that the team can use for future composite material testing and structural validation.

CFRP, GFRP, and hybrid composite rest aamples

The tensile tests included composite samples with different fiber orientations and material configurations to evaluate how their layup affects mechanical performance. The specimens included carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) layers with 0°, 90°, and 0°/90° fiber orientations.

The test program focused on three representative composite configurations: a unidirectional CFRP [0]₁₀ sample, a CFRP [0,90]₅s laminate, and a hybrid CFRP/GFRP [0,90]₅s sample. The hybrid specimen combined carbon and glass fibers and was included to evaluate the proposed material transition for future AQUILO rocket structures. 

All test samples were manufactured in the Inholland composites laboratory and prepared according to the specimen geometry requirements summarized in Table 1. These requirements define parameters such as specimen dimensions, tolerances, flatness, and tab geometry to ensure suitable samples for tensile testing.

Composite tensile test bench

To perform our tensile test on the samples, the main instrument we will use is the Universal Testing Machine.

A Universal Testing Machine (UTM) is a test machine used to determine the mechanical properties of a sample by applying tensile, compressive, or transverse loads. A universal test machine is designed to meet a wide range of tests by simply switching out different grips and fixtures.

Strain gauge installation and quarter-bridge wiring

For the strain measurements, the team used HBM 3/350ZE LY41 strain gauges in a quarter-bridge configuration.

Before installing each strain gauge, the surface of the composite specimen was lightly sanded to improve adhesion. The prepared area was then cleaned with acetone to remove dust, sanding residue, oil, and other contaminants. A reference line was marked on the specimen to ensure that the strain gauge was correctly aligned with the direction of the applied tensile load.

A small amount of cyanoacrylate adhesive was applied to the prepared surface, and the strain gauge was carefully positioned along the reference line. Teflon tape was then used to maintain pressure on the gauge for approximately 10 minutes while the adhesive cured.

After installation, the strain gauge was connected in a quarter-bridge configuration to the Dewesoft data acquisition system for strain measurement during the tensile tests.

Strain gauge data acquisition with DewesoftX

Strain measurement setup and configuration

The strain gauge channels were configured in DewesoftX according to the technical specifications provided by HBM. The strain gauges had a nominal resistance of 350 Ω and a gauge factor of 2.05. These parameters were entered into the strain gauge sensor settings to convert the measured bridge signal into strain.

A 10 Hz low-pass filter was applied to reduce measurement noise. Before each test, the quarter bridge was balanced in DewesoftX to compensate for the initial offset and establish the unloaded specimen condition as the zero-strain reference.

The complete measurement workflow connected each strain gauge in a Wheatstone quarter-bridge configuration to the Dewesoft data acquisition system. DewesoftX recorded and displayed the strain signals in real time throughout the tensile test.

The strain measurements were synchronized with the applied force data, allowing the team to analyze the relationship between tensile force and strain and evaluate the mechanical response of each composite sample.

Real-Time strain measurement in DewesoftX

The strain gauge channel was displayed as a time-based signal, with strain shown in µm/m, equivalent to microstrain. This allowed the composite sample's response to be observed in real time as the tensile load was applied.

Composite tensile test results and analysis

The data gathered were used to derive material-specific properties such as the maximum stress and the force-strain gradient. To determine the best strength-to-weight ratio, we tested 3 samples: CF [0]10, CF [0.90]5s, and a mix of CF and GF [0, 90]5s.

Calculating Composite Stiffness from Force and Strain

The force–strain gradient was calculated using two points from the linear region of the curve. Using the data, we obtained the force and strain to calculate the gradient using the formula:

Unidirectional CFRP [0]10 test results

The force–strain gradient was calculated using two points from the linear region of the curve. 

The force–strain response shows a nearly linear relationship during tensile loading. This indicates that the measured region of the composite sample behaved elastically and that the strain gauge measurement was stable; the force on the sample was applied in the direction of the fibers. The maximum force reached was approximately 52.4 kN, with a measured strain of about 9300 µε, yielding a force–elongation gradient of 19.34 N/µm, indicating the sample's stiffness during the elastic phase.

The force that the sample sustained was 52 kN. Given that the loads on the aeroshell would be highest near the motor, which produces 11 kN of force, this yields a safety factor of 4.7, which is much greater than the value allotted for our application. 

The CF [0]₁₀ sample fractured in the gripping portion of the testing machine. This suggests that the break might have been induced by a local concentration of stresses in the gripping zone, rather than occurring solely in the gauge section.

CFRP 0,90]5s Test Results 

The force–strain gradient was calculated using two points from the linear region of the curve, as mentioned in the previous section. 

The maximum force reached about 34.94 kN, whereas the highest observed strain was around 14,785 µm/m. To calculate stiffness, two data points from the linear portion of the force-strain graph were used before the sample's ultimate failure.

Compared with the CF [0]₁₀ sample, the CF [0,90]5s sample exhibits a smaller force-extension gradient. This indicates lower axial stiffness due to the 90° fibers, whose contribution to stiffness is relatively small under tensile loads. Nevertheless, the presence of the 90° fibers could still enhance other performance characteristics of the composite, such as transverse stiffness.

Hybrid CFRP/GFRP composite test eesults

AQUILO IX has seen an evolution in how our team uses composites, with the core challenge being that our antennas are in the nosecone of a 3.4-meter rocket. The team decided to manufacture the engine tube of our rocket from a CFRP-GFRP hybrid.

The hybrid sample was manufactured by alternating CFRP and GFRP layers. A gap of approximately 2 cm was maintained between the overlapping regions to represent the intended transition between the two materials. The resin systems will bind to each other. The sample was cured using the same oven cycle as the CFRP samples to ensure consistency and comparability across test specimens.

The image below shows the GFRP section within the rocket.

The failed CF/GF specimen indicated failure near the material transition zone. This means the interface between the CFRP and GFRP sections is the most critical part of the specimen. Moreover, the failure did not occur uniformly along the gauge length but was localized around the interface, where stress is transferred between the materials. Thus, the proposed hybrid material must be enhanced to meet the requirements for use as the structural component of the rocket aeroshell or the engine tube.

The location of the rupture confirms that the issue is not only the strength of individual CFRP or GFRP layers, but also the load transfer between the two materials at the hybrid transition.

This test sample was equipped with two strain gauges, one oriented horizontally and one vertically. This was important as it allowed us to visualize the deformations in two directions.

During loading, the vertical strain rose substantially, reaching approximately 16,700 µm/m, whereas the horizontal strain remained comparatively small. The force–strain curve derived from vertical strain did not exhibit a clear linear pattern, unlike that for the carbon fiber samples. As a result, a reliable stiffness gradient could not be determined for the hybrid sample.

The non-linear relationship suggests that the sample's deformation was not uniform; this was expected because the materials have two distinct characteristics and are infused with two different resin systems, thereby complicating load transfer between them. Several grip adjustments were also recorded during the test, which may have influenced the measured strain response.

Composite material test results and key findings

These tests also enhance team AQUILO’s knowledge base and experience, providing the structural department with insight into complex composite structures. This could greatly improve the design and production of future projects. The table below shows the results of the three tests.

After completing the tests, the recorded force and strain data were transformed into stress–strain curves for each sample. This allowed for a direct comparison of the mechanical responses across different composite configurations. The results aligned with expectations for the carbon fiber samples: the unidirectional CF [0]₁₀ sample exhibited the highest tensile performance, while the CF [0,90]5s laminate showed reduced axial stiffness and strength due to the 90° layers.

However, the hybrid CF/GF sample performed significantly worse than expected. The sample failed at approximately 11.52 kN, corresponding to a maximum stress of about 131.6 MPa. This was considerably lower than that of the carbon fiber samples and did not meet the required safety factor for safe structural operation. Based on this result, the current hybrid configuration is not yet suitable for use in a future load-bearing aeroshell or engine tube.

The failure location also provided an important design insight. Since failure occurred near the transition region between the carbon fiber and glass fiber sections, the connection between the two materials appears to be the most critical weakness in the current configuration. Further development should therefore focus on improving the CF/GF interface, including the intersection length, layup transition, bonding quality, and curing strategy. Additional testing will be required before this hybrid material concept can be considered viable for flight hardware.

The table below shows the mechanical properties provided by the CF manufacturer. The results of the CF [0] are comparable to the datasheet.

Using these properties, we can determine whether the composite structures can be used in manufacturing the next rocket, which will feature a load-bearing aeroshell to reduce weight. In conclusion, based on our analysis, the CF [0] ₁₀ sample is suitable for use in the next rocket in the AQUILO lineup. It is well above the required safety factor; additional work could further reduce the weight, improving performance, but the hybrid sample requires further research to determine what could be improved.

References

1. Smith, G. M. (2026, 04 10): Strain Gauge Applications - Practical Uses of Strain Gage Sensors

2. Instron:. ASTM D3039: Tensile Properties of Polymer Matrix Composite Materials.

3. Dewesoft: Strain and Stress Measurement.

4. HBK: Strain gauges for experimental testing. Retrieved from