Stephen C. Galea

Development of structural health monitoring systems for composite bonded repairs on aircraft structures

The application of bonded composite patches to repair or reinforce defective metallic structures is becoming recognized as a very effective versatile repair procedure for many types of problems. Immediate applications of bonded patches are in the fields of repair of cracking, localized reinforcement after removal of corrosion damage and for reduction of fatigue strain. However, bonded repairs to critical components are generally limited due to certification concerns. For certification and management of repairs to critical structure, the Smart Patch approach may be an acceptable solution from the airworthiness prospective and be cost effective for the operator and may even allow some relaxation of the certification requirements. In the most basic form of the Smart Patch in-situ sensors can be used as the nerve system to monitor in service the structural condition (health or well-being) of the patch system and the status of the remaining damage in the parent structure. This application would also allow the operator to move away from current costly time-based maintenance procedures toward real-time health condition monitoring of the bonded repair and the repaired structure. TO this end a stand-alone data logger device, for the real-time health monitoring of bonded repaired systems, which is in close proximity to sensors on a repair is being developed. The instrumentation will measure, process and store sensor measurements during flight and then allow this data to be up-loaded, after the flight, onto a PC, via remote (wireless) data access. This paper describes two in-situ health monitoring systems which will be used on a composite bonded patch applied to an F/A-18. The two systems being developed consists of a piezoelectric (PVDF) film-based and a conventional electrical-resistance foil strain gauge-based sensing system. The latter system uses a primary cell (Lithium- based battery) as the power source, which should enable an operating life of 1-2 years. The patch health data is up- loaded by the operator using an IR link. The piezoelectric film-based sensing system is self-powered and has been designed to operate using the electrical power generated by an array of piezoelectric films, which convert structural dynamic strain to electrical energy. These transducers power the electronics which interrogate the piezoelectric film sensors, and process and store the patch health data on non-volatile memory. In this system the patch health data is up-loaded by the operator using a magnetic transreceiver. This paper describes the development and evaluation of the two systems, including issues such as system design and patch health monitoring techniques.

Characterization of the effects of applied electric fields on fracture toughness and cyclic electric field induced fatigue crack growth for piezoceramic PIC 151

The influence of applied electric field on fracture toughness and cyclic electric field induced fatigue crack growth behaviour was characterized for an actuator piezoelectric ceramic under the combined loading of a high electric field and a mechanical stress. Results show that there exists a strong anisotropic effect on fracture toughness and electric field induced fatigue crack growth in polarized PZT. It is found that the apparent fracture toughness in the orientation parallel to the polarization direction is much higher than that in the transverse orientation. Under a positive electric field, increasing the electric field intensity reduces the fracture toughness in the transverse orientation but enhances that in the parallel orientation. However, the reverse is true under a negative electric field. A sphere cavity model in dielectrics was employed to characterize the effect of the external applied electric field on the evolution of cracking in an indentation. The results also show that low electric field intensity does not result in fatigue crack growth in PZT. For a relatively high applied electric field, the cracks initially grow quickly and then are arrested. This result is very significant for the long-term durability of PZT actuators.

Vibro-impacting power harvester

The certification of retro-fitted structural health monitoring (SHM) systems for use on aircraft raises a number of challenges. One critical issue is determining the optimal means of supplying power to these systems, given that access to the existing aircraft power-system is often problematic. Previously, the DSTO has shown that a structural-strain based energy harvesting approach can be used to power a device for SHM of aircraft structure. Acceleration-based power harvesting from airframes can be more demanding than a strain based approach because the vibration spectrum of an aircraft structure can vary dynamically with flight conditions. A vibration spectrum with varying frequency may severely limit the power harvested by a single-degree-of-freedom resonance-based device, and hence a frequency agile or (relatively) broadband device is often required to maximize the energy harvested. This paper reports on an investigation into the use of a vibro-impact approach to construct an acceleration-based power harvester that can operate in the frequency range 29-41 Hz.

Designing for piezoelectric ceramic wafers bonded on structures using force transfer criteria

Piezoelectric ceramic wafers embedded or surface mounted are able to generate transverse forces to control structural shape and vibration. These actuators are excited by applying an electric field through the thickness of the piezoelectric ceramic wafer. The proper design of their dimensions such as wafer thickness, and applied actuation voltage will enhance the system reliability, and guarantee the optimal control authority. This paper shows that the force transfer from the piezoelectric ceramic wafer to the structure, rather than the shear stress transfer in bond lines, becomes a key issue in the design when the length relative to the thickness of the piezoelectric ceramic wafer is large. It should be noted at this point that the shear stress distribution in the bond lines may influence the durability of the piezoelectric ceramic actuation system and, therefore, should also be considered when designing the system as a whole. This aspect of design is not discussed here. The force transfer is basically determined by the force transfer efficiency and block force in association with the wafer thickness. Therefore, this paper presents a design window for piezoelectric ceramic wafers bonded onto structures that is based on the relations between the force transfer and the wafer thickness, and also accounting for the applied actuation voltage and dielectric breakdown voltage of the piezoelectric material. The design window clearly describes the upper bound of the force transfer and the maximum value that can be achieved. It is also used to find an appropriate wafer thickness and actuation voltage when a minimum value of force transfer is required. In addition, an analytical model is developed in order to express the relation between the force transfer and the wafer thickness. This model is compared to the results from finite-element analysis. Limitations of the analytical model are described.

Finite element modeling to determine thermal residual strain distribution of bonded composite repairs for structural health monitoring design

The economic implication of fleet upgrades, particularly in Australia with military aircraft such as the F-111 and F/A-18, has led to an increasing reliance on composite repair technology to address fatigue and corrosion-affected aircraft components. The increasing use of such repairs has led to a research effort to develop various in-situ health monitoring systems that may be incorporated with a repair. This paper reports on the development of a theoretical methodology that uses finite element analysis (FEA) to model the strain profiles which optical sensors, on or within the patch, will be exposed to under various operational scenarios, including load and disbond. Numerical techniques are then used to predict the fibre Bragg grating (FBG) reflections which occur with these strain profiles. The quality of these reflection are a key consideration when designing FBG based structural health monitoring (SHM) systems. This information can be used to optimise the location of both surface mounted, and embedded sensors, and determine feasibility of SHM system design. Research was conducted into the thermal residual strain (TRS) within the patch. A finite element study revealed the presence of significant thermal residual strain gradients along the surface of the tapered region of the patch. As Bragg gratings are particularly sensitive to strain gradients, (producing a result similar to a chirped grating) the strain gradient on the composite at potential sensor locations both under load, and in the event of disbond was considered. A sufficiently high gradient leads to an altered Bragg reflection. These spurious reflections need to be considered, and theoretically obtained reflections can provide information to allow for load scenarios where the Bragg shift is not a smooth, well defined peak. It can also be shown that embedded fibres offer a higher average thermal residual strain reading, while being subject to a much lower strain gradient. This particularly favors the optical disbond detection system that is being developed. While certification concerns exist with embedding sensors in repairs, this study shows that embedded optical fibre sensors may provide for a health monitoring system with enhanced reliability and sensitivity.

Smart structures approaches for health monitoring of aircraft structures

Due to economic pressures both commercial and military aircraft fleets are operating a greater number of ageing aircraft. It can therefore be expected that the occurrence of structurally significant defects will significantly increase in the future. In the Australian context, a significant portion of the Royal Australian Air Force fleet are being operated well past the designed life. For example the F-111C fleet will be in service till the year 2015 which is about 20 years more than the original design life of the aircraft. As fleets get older a greater share of the operators resources need to be used on through-life-support of the airframe. One way of reducing costs and increasing aircraft availability is through the use of smart materials technology. Smart materials are materials with the ability to respond to changes in the operating environment or to other stimuli in an intelligent way. This ability may be achieved from sensors and actuators embedded in or attached to the structure or, more simply, from an inherent response mechanism in the material. In the context of ageing airframes, smart materials/structures technology has excellent potential to provide improvements in through-life support, including health and usage monitoring (HUMS), with the eventual aim of allowing condition based maintenance procedures to be adopted rather then relying on current expensive time-based maintenance procedures. This paper discusses the development and evaluation in DSTO of smart structure technologies to be applied to structural health monitoring of aircraft structures. Systems are being developed by DSTO with the specific aim of retro-fitting to existing airframe structures (e.g. smart repairs and reinforcements with the ability to self-monitor patch system integrity), where the systems need to be autonomous, distributed, robust and reliable. The paper expands on the health monitoring techniques being developed and evaluated, including the use of electrical-resistance foil strain gauges, piezoelectric elements and optical fibre sensors. Since the emphasis here is on retrofitting systems to existing structures the paper also touches on ftilly-autonomous systems which incorporate self-powering and wireless access techniques.

Use of shape memory alloys for strength and fatigue life enhancement of cracked metallic structures

A number of techniques are available which will enhance residual static strength and fatigue life of metallic components. In the case where a crack exists a simple method is to stop drill the crack tips. More elaborate techniques consist of stop drilling the crack tips, cold expanding the resultant hole and inserting an interference-fit plug. The application of bonded composite repairs to cracked metallic structures has also become an accepted technique to enhance strength and fatigue life of cracked metallic components. This paper looks at the possibility of using smart materials such as shape memory alloys (SMA) for the life enhancement of metallic structures. An investigation into the use of SMAs was completed using finite element modeling of one particular concept known here as the annular SMA repair which was presented as one method of possibly increasing the fatigue life of holes. From the results given in this paper it was shown that the inclusion of a SMA ring, interference fitted into a machined annulus in an aluminum plate, induces a significant compressive stress around the boundary of a hole. Also, discussed are other ideas which use SMAs for repairing cracked metallic components. Although the current studies are only conceptual and qualitative they do illustrate the feasibility and potential of such techniques to enhance strength and fatigue life of metallic components.

Crack-growth detection using a surface-mounted piezotransducer array

This paper reports on an experimental program aimed at assessing the feasibility of monitoring crack growth by means of a surface-mounted piezotransducer array. The host structure is interrogated using lamb-waves generated and sensed by elements in the array. An example is described involving a uniaxial edge-notched specimen subject to cyclic loading where the array is shown to be capable of detecting on a sustained basis crack growth increments of less than 0.1 mm. Although the piezoceramic elements were exposed to cyclic stress there was no measurable indication of element degradation, indicating good fatigue durability properties under the loading regime considered. Experimental results are also shown that demonstrate a strong crack-state dependence, highlighting the need for care when interpreting signals from the piezotransducers.