PSI - Issue 75

Sixin Liu et al. / Procedia Structural Integrity 75 (2025) 200–204 Sixin Liu/ Structural Integrity Procedia (2025)

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ultimately catastrophic fracture. Early and reliable detection of fatigue progression is therefore essential for ensuring structural integrity and preventing unscheduled downtime or catastrophic accidents. Conventional fatigue monitoring technologies — including resistance-based strain gauges, acoustic emission sensors, ultrasonic inspection systems, and electrochemical crack sensors — offer robust measurement capabilities but suffer from practical limitations. These systems typically require external power sources, complex data acquisition hardware, and periodic calibration. Moreover, many of these methods are reactive in nature, detecting fatigue damage only after crack formation or significant degradation has occurred [2]. This reactive approach limits their utility in proactive maintenance regimes, particularly in remote or resource-constrained environments. To address these challenges, this study introduces a novel class of visually readable polymer-based fatigue sensors that provide an intuitive and passive method for real-time fatigue state indication. These sensors are fabricated using cost-effective thermoplastic materials — principally polypropylene (PP) — and are designed to fail in a predictable manner at pre-determined fatigue intervals. Their operation is based on a geometric stress riser mechanism: the inclusion of precisely machined V-shaped notches induces stress concentration, promoting fatigue crack initiation under cyclic loading. The sensors are intended to be surface-mounted on metallic components and require no embedded electronics, external power, or post-processing equipment, making them ideal for scalable deployment in civil, marine, and aerospace structures. This paper presents the full development cycle of the proposed sensor system, encompassing its design rationale, finite element modelling (FEM) strategy, experimental validation, and potential applications. Emphasis is placed on demonstrating the predictability of fatigue-induced failure through both computational simulations and physical testing, thereby validating the sensor’s utility for early -stage damage monitoring in structural health management frameworks. 2. Sensor Design and Operating Principle The proposed fatigue sensor is a monolithic, strip-like component fabricated from polypropylene — a widely available thermoplastic with well-characterised fatigue and viscoelastic properties [3]. The sensor incorporates symmetrically placed double V-shaped notches, each with an opening angle of precisely 89.3°, strategically chosen to induce localised stress amplification under uniaxial tensile cyclic loading. This geometry serves as a crack initiator, ensuring that fatigue damage accumulates in a controlled and spatially defined region [4]. The sensor is intended to be bonded directly onto the surface of a metallic host structure, such as an aluminium alloy substrate (AA1050), using a structural adhesive (e.g., TA4610) applied following standardised surface preparation protocols. This adhesive interface plays a critical role in transferring the mechanical loads from the host structure to the sensor without introducing significant compliance or slippage. Once installed, the sensor undergoes the same stress cycles as the monitored component. Due to the combined effect of the notch geometry and the fatigue prone nature of PP under repeated loading, a crack initiates from the notch root and propagates progressively across the sensor width. The failure of the sensor manifests as a visible fracture, providing an unambiguous indication that the monitored structure has reached a specific fraction of its fatigue life. By adjusting parameters such as notch depth, width, angle, and material thickness, sensors can be tuned to fail at discrete life fractions (e.g., 25%, 50%, 75%), enabling a tiered damage progression approach. These tiers act as visual indicators of cumulative damage, offering operators a simple yet reliable means of estimating remaining life. Importantly, the proposed polypropylene (PP) fatigue sensor is not a simple counter of stress cycles. Its behaviour is physically and mechanically coupled to that of the monitored structure. Once bonded to the host surface, the sensor is subjected to the same cyclic stress history as the metallic substrate. The inclusion of engineered V-notches ensures that fatigue cracks in the PP strip initiate and propagate in a controlled and accelerated manner relative to the substrate,