Challenge

To advance real-time structural health monitoring (SHM) of braided composites by optimising the integration and placement of embedded sensors, like microwires, and enabling its industrial viability.

Background

Braided composites are widely used in aerospace, automotive and civil engineering for their high strength-to-weight ratio and corrosion resistance. The University of Sheffield Advanced Manufacturing Research Centre (AMRC) has prior experience embedding sensors in composite manufacturing for real-time data collection and SHM, demonstrating the feasibility of integrating distributed fibre optic sensing into automated processes. 

Building on this foundation, this project focused on improving the integration of optical fibres and microwires within braided preforms, specifically addressing sensor survivability and data processing challenges. 

Funded by the High Value Manufacturing (HVM) Catapult, the research aimed to enhance the robustness and industrial viability of embedded sensors in these materials by refining existing tools and leveraging past insights.

The microwires used in the research were developed by Universidad Del Pais Vasco / Euskal Herriko Unibertsitatea (UPV/EHU) and Tamag Iberica SL, and the data acquisition system, facilitated by Rise Research Institutes Of Sweden Ab, enabled the advanced monitoring and analysis throughout the project.

Innovation

The research’s core innovation involved embedding intra-ply continuous optical fibres and microwires directly within braided fibre structures. This created an integrated sensor network capable of real-time stress, strain and damage detection with high sensitivity and minimal structural interference.

Multiple trials were conducted, embedding sixteen microwires evenly into biaxial and triaxial braided preforms with data recorded throughout braiding, resin infusion and mechanical testing. Three configurations of preform stacks, created using six-ply layers with varying microwire depths, were monitored at key stages, including dry preform, bagging, vacuum application, resin infusion and curing. 

To assess the impact of microwire placement, in-plane shear testing was performed on three braided composite panels with microwires embedded in different layers (one, three and five). The testing took place at Element Materials Hitchin laboratory.

The panels, made of T700 12k carbon fibre with an Araldite LY564 resin matrix, were tested at two millimetres per minute. Initial weak signal reception was resolved by directly clamping the microwire reader to the specimens, enabling a more reliable correlation between force and signal power.

Result

The study evaluated the correlation between applied force and microwire signal power in composite test specimens, highlighting the significant influence of microwire placement depth on signal reliability and signal response. 

• Microwires in the top layer (M30800) showed signal decorrelation after about 100 seconds due to early surface damage, matrix cracking and fibre-matrix debonding. 
• Deeply embedded microwires in the fifth layer (M30802) exhibited lower sensitivity to stress changes and increased signal attenuation, limiting their effectiveness. 
• The optimal placement was the third layer (M30801), maintaining a strong correlation with force for approximately 150 seconds. 

The study concluded that embedding microwires at this depth enhances real-time SHM by balancing sensitivity to material deformation, and resistance to early signal degradation and signal clarity. 

This microwire placement minimised interference from surface damage while ensuring reliable signal tracking during both elastic and early plastic deformation phases, consequently enhancing non-destructive evaluation methods for composite materials under mechanical loading.

Impact

This research represents a significant advancement in SHM for braided carbon fibre composites, addressing long-standing limitations in real-time damage detection. By successfully integrating microwires into braided preforms and optimising their placement, the project has demonstrated a viable method for embedding sensor networks without compromising structural integrity.

The ability to monitor internal stress, strain and damage progression with high sensitivity offers transformative benefits for industries relying on long-term composite performance, such as aerospace, automotive and civil engineering.

Traditionally, SHM has relied on external sensors, which provide limited insight into internal failure mechanisms like fibre breakage and delamination. This research bridges that gap, enabling more proactive maintenance strategies, reducing inspection cost and improving the longevity of composite structures. 

The findings contribute to more reliable, non-destructive evaluation techniques, making real-time monitoring a feasible addition to high-performance composite applications. Furthermore, the insights gained on sensor survivability and signal clarity pave the way for more robust SHM systems, influencing future manufacturing processes.

The improved understanding of microwire placement and signal behaviour from this research also supports ongoing advancements, notably in the IDEKO-led, EU-funded INFINITE project which aims to revolutionise aerospace composite materials by integrating sensor networks throughout the manufacturing process and lifecycle.