Healing of aerospace carbon-epoxy composites using thermoplastic agents.

The general aim of this PhD project is to investigate the self-healing of aerospace carbon-epoxy composites using thermoplastic agents. The project explores key parameters on the healing process, including the type, volume content and shape of the thermoplastic agent, repeatability of the healing process, and healing method (convection heating, ultrasonic welding). The PhD also investigates the influence of thermoplastic agents on the in-plane properties of carbon-epoxy composites. It was found that poly[ethylene-co-(methacrylic acid)] (EMAA) was capable of repairing delamination damage and recovering the mode I interlaminar fracture toughness and fatigue resistance. High recovery (~300%) to the mode I interlaminar fracture toughness was achieved after healing via the formation of a large-scale crack bridging zone consisting of highly ductile EMAA ligaments, which is a delamination toughening mechanism unique to thermoplastic healing agents. The carbon-epoxy composites retained high healing efficiency following multiple repair operations due to the ability of EMAA to reform the crack bridging zone. EMAA was also capable of recovering the mode I fatigue resistance (defined by Paris curves) of the composites following healing. The PhD thesis presents a comparative experimental study into the healing efficiency of EMAA in delaminated carbon-epoxy composites when the repair process is thermally activated by heating induced with ultrasonic waves. It was discovered that bursts of short-duration, high frequency ultrasonic pulses were capable to thermally activating the healing process, and the healing efficiency (defined as recovery of mode I fracture toughness) was up to 130%. Multiple healing operations and full recovery to delamination toughness were achieved with ultrasonics. In addition to EMAA, the PhD project investigates the healing properties of other thermoplastics; namely PEGMA, EVA and ABS. The thermoplastics, except ABS, were capable of partially or completely restoring the delamination toughness and fatigue resistance of the composite. Healing with EMAA and PEGMA involved a pressure delivery mechanism whereas healing with EVA was controlled by its viscosity and adhesion properties. Through-the-thickness stitching with EMAA filaments on the delamination toughness and self-healing properties of carbon–epoxy composites was investigated. The delamination toughness and fatigue resistance of the composites increased with the volume content of EMAA stitches. During the healing these bridging stitches melt and flow into the delamination via the pressure delivery mechanism, and this resulted in full restoration of the toughness and fatigue properties. Multiple healing operations and full recovery to delamination toughness and fatigue resistance were achieved

Improving the mode I delamination fatigue resistance of composites using z-pins

This paper presents an experimental investigation into the mode I interlaminar fracture toughness and 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 crack growth rate under cyclic interlaminar loading slowed rapidly with increasing z-pin content up to a limiting concentration (~2% by volume), above which no further improvement to the fatigue resistance was achieved due to a transition in the fatigue cracking process. The delamination growth rate also slowed when the z-pin diameter was reduced or the z-pin length was increased. The dependence of the fracture toughness and fatigue resistance on the volume content, diameter and length of the z-pins is related to the crack bridging toughening processes induced by the z-pins

Comparative study of metal and composite z-pins for delamination fracture and fatigue strengthening of composites

The influence of the material properties of z-pins on the mode I delamination properties of carbon-epoxy laminates is investigated. Improvements to the delamination fracture toughness and fatigue resistance of laminates reinforced in the through-thickness direction with z-pins made of metal (copper, titanium or stainless steel) or unidirectional carbon fibre composite is determined. Irrespective of the material, the z-pins are highly effective at increasing the fracture toughness and fatigue resistance by forming a large-scale bridging zone along delamination cracks. However, the fracture toughening and fatigue strengthening capacity of the z-pins is strongly dependent on their material properties, and increased in the order: copper (least effective), titanium, stainless steel and carbon fibre (most effective). The fracture and fatigue failure modes of the z-pins are also dependent on their material properties. The study reveals that improvements to the mode I fracture toughness and fatigue resistance can be optimized via a judicious choice of the z-pin material. Z-pins with high stiffness, strength and fatigue resistance (such as carbon fibre) provide the greatest improvement to the delamination toughness and fatigue strength

Stitched mendable composites: Balancing healing performance against mechanical performance

The effect of through-the-thickness stitching with thermoplastic filaments on the healing efficiency and mechanical properties of mendable carbon-epoxy composites is investigated. Stitching with filaments of polyethylene-. co-methacrylic acid (EMAA) provides composite materials with high delamination resistance and self-healing efficiency, but at the expense of reduced mechanical properties. The modes I and II interlaminar fracture toughness properties and healing performance are improved greatly by thermoplastic stitching. However, stitching reduces the tensile and compressive properties of mendable composites. Experimental research reveals that the delamination toughness and healing properties increase whereas the mechanical properties decrease with increasing areal density or size of the mendable stitches. However, the percentage improvements to the interlaminar fracture toughness (in the order of 35-650%) and healing efficiency (50-250%) are much greater than the reductions to the mechanical properties (less than 50%) caused by thermoplastic stitching

Ultrasonic activation of mendable polymer for self-healing carbon-epoxy laminates

Healing of matrix cracks or delamination in fibre-reinforced composites containing mendable polymers requires heating to melt the thermoplastic agents. This paper presents the first investigation into the use of ultrasonic welding to activate a mendable polymer, poly[ethylene-co-(methacrylic acid)] (EMAA), for healing a carbon-epoxy laminate. Mode I interlaminar fracture toughness tests were carried out on specimens containing two different concentration levels of interlaced EMAA fibres to quantify the healing efficiency of ultrasonic vibration. Experimental results reveal that bursts of short-duration, high frequency ultrasonic pulses are able to thermally activate the mendable polymer to repair delamination cracks. Examinations of the fracture surface indicate partial healing of delamination cracks, which were sufficient to completely recover delamination toughness (repair efficiency up to 130%). Furthermore, multiple repairs and recoveries of interlaminar fracture toughness of the composite were achieved with ultrasonic welding. The repair efficiency using ultrasonic welding, however, was found to be less than conventional heating by thermal oven. Nevertheless, the ultrasonic welding technique is portable and can be used for rapid in-field repair of composite structures containing mendable polymers

Controlling the electrical conductivity of fibre-polymer composites using z-pins

Carbon fibre composites have inherently high through-thickness electrical resistivity, which limits their application when high electrical conductivity is required. The use of z-pins to increase the through-thickness electrical conductivity of composite laminates is investigated in this paper. The through-thickness and in-plane electrical properties of a unidirectional carbon-epoxy laminate reinforced with carbon fibre composite or metal (copper, stainless steel, titanium) z-pins are characterised. Experimental tests and analytical model reveal that z-pins can increase the through-thickness electrical conductivity of the composite material by many orders of magnitude (up to 10 6 ). The through-thickness electrical conductivity can be controllably increased via the judicious choice of the material type and volume content of the z-pins. Large improvements to the through-thickness conductivity can be achieved without the z-pins altering significantly the in-plane conductivity of the composite material

Comparative study of the mode I and mode II delamination fatigue properties of z-pinned aircraft composites

This paper compares improvements to the mode I and mode II delamination fatigue resistance of an aerospace composite material achieved by z-pin reinforcement. Mode I (cyclic crack opening) and mode II (cyclic crack sliding) interlaminar fatigue tests were performed on a carbon fibre reinforced epoxy composite reinforced in the through-thickness direction with different volume contents and diameters of z-pins. Paris curves obtained from displacement-controlled fatigue tests reveal that z-pins are more effective at resisting the initiation and growth of delamination cracks under mode I than mode II conditions. Both the mode I and II fatigue resistance increase with the z-pin content due to the formation of a large-scale extrinsic crack bridging toughening zone, although fatigue strengthening is greater for mode I. Improvements to the mode I and mode II delamination fatigue resistance are also dependent on the z-pin diameter. Similarities and differences in the mode I and mode II fatigue properties are related to the fatigue strengthening mechanisms induced by z-pins for the two load conditions

Mixed-mode I/II delamination fatigue strengthening of polymer composites using z-pins

An experimental investigation is presented into the improvement to the delamination fatigue resistant properties of z-pinned carbon fibre-epoxy composite under mixed-mode I/II cyclic interlaminar loading. Delamination fatigue tests are performed on unpinned and z-pinned composites under different mixed-mode ratios spanning mode I to mode II interlaminar cyclic conditions. The fatigue resistance and fatigue strengthening mechanisms induced by the z-pins is dependent on the cyclic mixed-mode ratio. The threshold critical strain energy release rate needed to initiate delamination growth in the z-pinned composite increases with the G I -to-G II ratio. The fatigue crack growth rate slows considerably and the critical strain energy release rate for fast fatigue fracture increases with the G I -to-G II ratio. The delamination fatigue strengthening induced by the z-pins increases with the G I -to-G II ratio due to a transition in the crack bridging toughening process from pin pull-out under mode I dominated loads to combined tensile and shear fracture under mixed-mode loads to pin shear rupture under mode II dominated loads. It is also found that the effect of the G I -to-G II ratio on the fatigue properties is greater for the z-pinned composite compared to the unpinned laminate due to the high fatigue sensitivity of z-pins to the mixed-mode ratio

Self-healing of delamination cracks in mendable epoxy matrix laminates using poly[ethylene-co-(methacrylic acid)] thermoplastic

This paper investigates the self-healing repair of delamination cracks in a carbon fibre-epoxy laminate using the mendable thermoplastic poly[ethylene-co-(methacrylic acid)] (EMAA). The effects of different types (fibres or particles) and concentrations of the mendable EMAA agent on the self-healing efficiency was measured using mode I interlaminar fracture toughness testing and fractographic analysis. The EMAA was effective in healing delamination damage and increasing the fracture toughness compared to the original laminate. High healing efficiency was achieved by the wide area flow of EMAA (increase of ~25 times) through delamination cracks under the pressure delivery mechanism

Numerical analysis of the heat transfer properties of z-pinned composites

The controlled increase of the through-thickness heat transfer properties of fibre reinforced polymer composites using z-pins is demonstrated using finite element (FE) modelling. The modelling reveals that the through-thickness thermal diffusivity of composite materials can be tailored to a specific value via the judicious choice of the volume content and thermal conductivity of z-pins. The effects of the volume content (up to 5%) and material type (carbon fibre composite, steel, titanium, copper) of z-pins on the through-thickness and in-plane thermal diffusivities of a carbon-epoxy composite is analysed. The through-thickness thermal diffusivity of the composite increases linearly with the z-pin content, and this is because the pins act as a thermally conductive pathway. The through-thickness thermal diffusivity value of the composite also increases with the thermal conductivity of the z-pin, however with an accompanying increase to the amount of lateral heat transfer. FE modelling reveals the thermal properties of composites in the in-plane directions are not affected significantly by z-pins. This study reveals that z-pinning is a novel approach to increase the through-thickness heat transfer properties of composites