Hugo R. Williams

Self-healing polymer composites: mimicking nature to enhance performance

Autonomic self-healing materials, where initiation of repair is integral to the material, are being developed for engineering applications. This bio-inspired concept offers the designer an ability to incorporate secondary functional materials capable of counteracting service degradation whilst still achieving the primary, usually structural, requirement. Most materials in nature are themselves self-healing composite materials. This paper reviews the various self-healing technologies currently being developed for fibre reinforced polymeric composite materials, most of which are bioinspired, inspired by observation of nature. The most recent self-healing work has attempted to mimic natural healing through the study of mammalian blood clotting and the design of vascular networks found in biological systems. A perspective on current and future self-healing approaches using this biomimetic technique is offered. The intention is to stimulate debate outside the engineering community and reinforce the importance of a multidisciplinary approach in this exciting field.

Self-healing composite sandwich structures

Impact damage can degrade the flexural strength of composite sandwich structures by over 50% due to a loss of skin support inducing localized skin buckling. Various self-healing methodologies have been applied to laminated composites but the concept of delivering a healing agent from a remote reservoir to a region of damage via a vascular network offers the potential for a robust and replenishable system housed in the core of a sandwich structure. In this pilot study a vascular sandwich structure that appears as a conventional sandwich composite has been developed and tested. The network has been shown to have negligible influence on the innate static mechanical properties of the host panel. Infiltration of the vascular network with a pre-mixed epoxy resin system after impact damage demonstrated a complete recovery of flexural failure mode and load. Infiltration with the same resin system from separate unmixed networks, where self-healing is initiated autonomously via mixing within the damage, has also been shown to fully recover undamaged failure load when both networks are successfully breached.

Biomimetic reliability strategies for self-healing vascular networks in engineering materials

Self-healing via a vascular network is an active research topic, with several recent publications reporting the application and optimization of these systems. This work represents the first consideration of the probable failure modes of a self-healing system as a driver for network design. The critical failure modes of a proposed self-healing system based on a vascular network were identified via a failure modes, effects and criticality analysis and compared to those of the human circulatory system. A range of engineering and biomimetic design concepts to address these critical failure modes is suggested with minimum system mass the overall design driver for high-performance systems. Plant vasculature has been mimicked to propose a segregated network to address the risk of fluid leakage. This approach could allow a network to be segregated into six separate paths with a system mass penalty of only approximately 25%. Fluid flow interconnections that mimic the anastomoses of animal vasculatures can be used within a segregated network to balance the risk of failure by leakage and blockage. These biomimetic approaches define a design space that considers the existing published literature in the context of system reliability.

Minimum mass vascular networks in multifunctional materials

A biomimetic analysis is presented in which an expression for the optimum vessel diameter for the design of minimum mass branching or vascular networks in engineering applications is derived. Agreement with constructal theory is shown. A simple design case is illustrated and application to more complex cases with branching networks of several generations discussed. The analysis is also extended into the turbulent flow regime, giving an optimization tool with considerable utility in the design of fluid distribution systems. The distribution of vessel lengths in different generations was also found to be a useful design variable. Integrating a network into a structure is also discussed. Where it is necessary to adopt a non-optimum vessel diameter for structural integration, it has been shown that small deviations from the minimum mass optimum can be tolerated, but large variations could be expected to produce a punitive and rapidly increasing mass penalty.

A probabilistic approach for design and certification of self-healing advanced composite structures

Design and certification of novel self-healing aerospace structures was explored by reviewing the suitability of conventional deterministic certification approaches. A sandwich structure with a vascular network self-healing system was used as a case study. A novel probabilistic approach using a Monte Carlo method to generate an overall probability of structural failure yields notable new insights into design of self-healing systems, including a drive for a faster healing time of less than two flight hours. In the case study considered, a mature self-healing system could be expected to reduce the probability of structural failure (compared to a conventional damage-tolerant construction) by almost an order of magnitude. In a risk-based framework this could be traded against simplified maintenance activity (to save cost) and/or increased allowable stress (to allow a lighter structure). The first estimate of the increase in design allowable stresses permitted by a self-healing system is around 8 per cent, with a self-healing system much lighter than previously envisaged. It is thought these methods and conclusions could have wider application to self-healing and conventional high-performance composite structures.

A Mars hopping vehicle propelled by a radioisotope thermal rocket: thermofluid design and materials selection

Rocket-propelled vehicles capable of travelling a kilometre or more in a ballistic ‘hop’ with propellants acquired from the Martian atmosphere offer the potential for increased mobility and planetary science return compared with conventional rovers. In concept, a radioisotope heat source heats a core or ‘thermal capacitor’, which in turn heats propellant exhausted through a rocket nozzle to provide thrust. A systematic study of the thermodynamics, heat transfer and selection of core materials for a Mars hopper was undertaken. The aim was to advance the motor design and assess technical risks and feasibility. Analytical and numerical motor models were developed; the former to generate thermodynamic performance limits, an ideal hop distance and plot a materials selection chart using simple explicit relations. The numerical model assessed the effect of core configuration and geometry. A hop coefficient Chop is shown to characterize the effect of core geometry independently of core material and temperature. The target hop distance of 1 km is shown to be robust. A moderate advantage to pebble-bed cores over a core consisting of straight channels was suggested. High-performance engineering ceramics such as boron carbide offer the longest hop providing the core temperature can be increased significantly above 1200 K.

Bioinspired self-healing of advanced composite materials

Lightweight, high strength, high stiffness fiber reinforced polymer composite materials are leading contenders to improve the efficiency and sustainability of many forms of transport. They offer immense scope for incorporating multifunctionality due to their hierarchical internal architecture. One limiting factor in their wider exploitation is relatively poor performance under impact loading, a crucial aspect of any safety critical design, leading to a significant reduction in strength, stiffness and stability. This results in conservative design and higher mass structures. Self-healing has the potential to mitigate damage resulting from impact, thereby improving design allowables or offering other benefits such as reduced maintenance and inspection schedules. The work presented in this paper shows that either compartmentalised hollow-fiber or continuous vascular network self-healing approaches can be used for the repair of advanced composite structures. In the nearer term, the specific placement of self-healing plies or individual fibers to match a critical damage threat has been shown to repair internal matrix cracking and delaminations throughout the thickness of a laminate when assessed in both a flexural and compressive loading state. In the longer term, integration of a pervasive, circulatory vascular network within the foam core of a composite sandwich structure has also been shown to offer a marked benefit. The network has negligible influence on structural performance whilst being able to provide reattachment of the foam core and laminate skin after impact damage. In the case studied, a sizeable recovery in flexural and compression after impact strength, and restoration of primary failure mode was observed. Such systems offer significant potential for restoring structural integrity to a composite component during service and prolonging residual life after a damage event. Copyright