K. Pingkarawat

Self-healing of delamination fatigue cracks in carbon fibre–epoxy laminate using mendable thermoplastic

This article examines the self-healing repair of delamination damage in mendable carbon fibre–epoxy laminates under static or fatigue interlaminar loading. The healing of delamination cracks in laminates containing particles or fibres of the mendable thermoplastic poly[ethylene-co-(methacrylic acid)] (EMAA) was investigated. The results showed that the formation of large-scale bridging zone of EMAA ligaments along the crack upon healing yielded a large increase (~300%) in the static mode I interlaminar fracture toughness, exceeding the requirement of full restoration. The mendable laminates retained high healing efficiency with multiple repair cycles because of the capability of EMAA to reform the bridging zone under static delamination crack growth conditions. Under fatigue loading, healing by the EMAA was found to restore the mode I fatigue crack growth resistance, with the rates of growth being slightly less than that pertinent to the unmodified laminate. The EMAA bridging zone, which generated high toughness under static loading conditions, does not develop under fatigue loading because of rapid fatigue failure of the crack bridging ligaments. Similar to the multiple healing capability of EMAA under static loading, multiple healing of delamination fatigue cracks is confirmed, with the fatigue crack growth rates remaining approximately unchanged. This study shows that EMAA was capable of full recovery of fatigue crack growth resistance and superior healing efficiency for static loading.

Healing of carbon fibre–epoxy composites using thermoplastic additives

The healing efficiency and healing mechanisms of selected insoluble thermoplastics blended into an epoxy resin and its respective carbon fibre–epoxy matrix composite is investigated. The capacity of two reactive thermoplastic additives (polyethylene-co-methacrylic acid (EMAA) and polyethylene-co-glycidyl methacrylate (PEGMA)) and two non-reactive thermoplastics (ethylene vinyl acetate (EVA) and acrylonitrile butadiene styrene (ABS)) to heal cracks in the epoxy resin network and heal delaminations in carbon–epoxy composite is determined. The thermoplastics were able to partially repair the fractured epoxy, although different healing mechanisms were operative. The thermoplastics (except ABS) were partially or completely effective in restoring the mode I interlaminar fracture toughness and fatigue resistance of delaminated composites. The healing efficiency of the thermoplastics, defined by the percentage recovery to the interlaminar fracture toughness of the composites, increased in the order: ABS (lowest), PEGMA, EVA and EMAA (highest). Healing by the reactive EMAA and PEGMA thermoplastics involves a unique pressure delivery mechanism whereas healing by the non-reactive EVA thermoplastic is controlled by its viscosity and adhesion to the fracture surfaces. ABS was ineffective as a healing agent in the composite due to its high viscosity which impeded flow into the delamination crack.

Healing of fatigue delamination cracks in carbon–epoxy composite using mendable polymer stitching

This article presents an experimental investigation into the mode I delamination fatigue properties and fatigue crack healing mechanism of a self-healing carbon fibre–epoxy composite containing mendable thermoplastic stitches. Mode I interlaminar fatigue tests using double cantilever bending specimens show that through-the-thickness reinforcement of the composite with mendable poly(ethylene-co-(methacrylic acid)) stitches is highly effective in healing delamination cracks and restoring the fatigue properties. Aided by a pressure delivery mechanism unique to this type of mendable thermoplastic, the healing agent stored in an interconnected network of stitches is able to flow into narrow delamination cracks. The mode I interlaminar fatigue resistance as well as the fracture toughness of the composite was fully restored by poly(ethylene-co-(methacrylic acid)) stitches. Transverse tension tests were performed to determine the traction law of the healing agent, which controls the healing efficiency and interlaminar toughening mechanism under static and fatigue mode I interlaminar loading.

Effect of modification of cyclic butylene terephthalate on crystallinity and properties after ring-opening polymerisation

Ring-opening polymerisation of the macro-cyclic oligomer of butylene terephthalate at elevated temperatures was found to produce high molecular weight polybutylene terephthalate that was both highly crystalline and brittle. The impact of reactive and non-reactive additives on the polymerisation, crystallisation and final crystal structure was extensively studied using differential scanning calorimetry and rheological methods and correlated with observed improvements in fracture toughness and tensile properties for both the neat resin and mode I and II fracture toughness of the fibre-reinforced composites. The reactive modifiers used were bi-functional epoxy resins, namely diglycidyl ether of bis phenol A, butanediol diglycidyl ether and bis[(glycidyl ether)phenyl)]-m-xylene, while the non-reactive modifiers were selected β nucleants and the ductile thermoplastic, polyethylene glycidyl methacrylate.

Explosive Blast Resistance of Naval Composites: Effects of Fiber, Matrix, and Interfacial Bonding

An experimental investigation is presented into the explosive blast response of fiber-reinforced polymer laminates used in naval ship structures. Blast tests using plastic explosive charges were performed in air on square target plates made of carbon–polyester, glass–polyester, carbon–vinyl ester, or glass–vinyl ester laminates, which are composite materials used in naval ships. The laminates were dynamically loaded by shock waves of increasing pressure and impulse, and the deformation, damage, and residual mechanical properties were determined. The amount of blast-induced damage and the postblast mechanical properties depend on the fiber reinforcement, polymer matrix, and interfacial strength between the fibers and matrix. E-glass laminates have higher resistance to blast-induced delamination cracking and tow rupture than carbon fiber composites. Furthermore, glass or carbon fiber laminates with a vinyl ester matrix have superior blast damage resistance compared to composites with a polyester matrix. The higher damage resistance is attributed to the higher flexural strain energy capacity and interlaminar fracture toughness of laminates containing glass fibers or vinyl ester matrix. The explosive blast damage resistance is also higher in laminates with strong interfacial bonding between the fibers and matrix. The experimental results presented in this chapter reveal that the blast damage resistance of laminates can be improved by the judicious selection of fiber, matrix, and interfacial strength.

Fatigue Properties of Aerospace Z-Pinned Composites

This paper presents an experimental study into the effect of through-thickness z-pin reinforcement on the in-plane and out-of-plane (delamination) fatigue properties of carbon-epoxy composites used in aerospace structures. The in-plane fatigue strength and fatigue life (load cycles-to-failure) of aerospace composite materials are reduced by z-pins. The in-plane compressive fatigue properties decrease when the volume content of z-pins is increased. Reductions to the in-plane fatigue properties are due to microstructural damage caused by the z-pins. However, the out-of-plane (delamination) fatigue properties of composites are increased greatly by z-pins. The mode I, mode II and mixed mode I/II delamination fatigue properties increase rapidly with increasing volume content of z-pins. The improvement is due to the z-pins forming a large-scale bridging zone along the delamination which resists fatigue crack growth. The work clearly reveals that a trade-off exists between the in-plane and out-of-plane fatigue properties of z-pinned composites. Improvements to the delamination fatigue properties come at the expense of lower in-plane fatigue performance, and this is a key consideration for the design of z-pinned aerospace composite structures.

Delamination Fatigue Properties of Z-Pinned Composites

This paper presents an experimental investigation into the mode I interlaminar fatigue resistance of carbon fibre-epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance and crack bridging toughening mechanisms are determined. The delamination growth rate also slowed when the volume content or length of the z-pins was increased or the z-pin diameter was reduced.