Zaira P. Marioli-Riga

A Standard Analysis Methodology for the Stress Analysis of Repaired Aircraft Structures with the Method of Composite Patch Repair

The method of composite patch repair is a very modern method of repairing damaged aircraft structures, which presents many advantages over the traditional methods of repair. Many analytical as well as numerical methods have been suggested, from time to time, for the stress analysis of such bonded repairs. The engineer (especially one with not very much experience or theoretical background) who will be asked to design a repair may find it difficult to choose the most appropriate method for the specific problem that he has to deal with, among the large number of available analysis methods. The scope of this paper is to suggest a very specific, standardized, step by step analysis methodology for damaged/repaired aircraft structural components.

Nondestructive evaluation of crack propagation under a composite patch repair using the eddy current method

The eddy current method is used to trace crack propagation under a composite patch repair of a cracked metallic structure, after mechanical testing in fatigue. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thickness are checked for several patch thicknesses. Notched specimens 6 mm thick were fabricated using 2024-T3 Aluminum. Boron Epoxy patches bonded with film adhesive were applied to the one side of the metallic specimens. Initial notches were 10 mm long, while the thickness of the reinforcement was varying from 2 layers (0.25 mm) to 7 layers (0.875 mm) in order to represent actual structural composite patch repairs. Crack propagation from the tip of the notches was achieved by fatigue loads. The estimation of required loads to cause fatigue crack propagation was done by means of three-dimensional finite elements analysis. The eddy current method was then applied to trace the crack tip under the patch after their mechanical testing. Accuracy of the eddy-current method was verified by measuring the crack lengths on both sides of the specimen and comparing the results. The eddy-current method was found to be fully capable of tracing the crack propagation under the composite patch, requiring only proper calibration for the generator. Small differences in the crack lengths between the patched and the unpatched side of the specimen were explained by their non-symmetric configuration, which induced different stress intensity factors at the patched and the unpatched sides, as finite elements analysis has clearly shown.

A study of active thermography approaches for the non-destructive testing and evaluation of aerospace structures

The prerequisite for more competent and cost effective aircraft has led to the evolution of innovative testing and evaluation procedures. Non-destructive testing and evaluation (NDT & E) techniques for assessing the integrity of an aircraft structure are essential to both reduce manufacturing costs and out of service time of aircraft due to maintenance. Nowadays, active - transient thermal NDT & E (i.e. thermography) is commonly used for assessing aircraft composites. This research work evaluates the potential of pulsed thermography (PT) and/or pulsed phase thermography (PPT) for assessing defects (i.e. impact damage and inclusions for delaminations) on GLARE and GLARE type composites. Finally, in the case of the detection of inserts - delaminations C-Scan ultrasonic testing was also used with the intention of providing supplementary results.

Selection of Optical Fibers Paths and Sensor Locations for Monitoring the Integrity of Composite Patching

In order to select appropriate optical fiber paths and locations for Bragg Grating sensors, the research group has studied a classically cracked metallic structure repaired with a ‘smart’ bonded composite patch using finite element analysis. The patch was bonded over a cracked aluminum plate by means of a thin adhesive layer. The primary loading axis of the metal was assumed parallel to the direction of the optical fibers used. A variety of optical fiber paths and sensor positions was considered, along with their ability to measure the developed strain field and to trace the position of the crack tip. It was concluded that a fiber optics network is indeed capable of tracing effectively the critical parameters required for the monitoring of structural integrity of the composite patch-reinforced structures (i.e. strains developed at the patch and at the tip of the crack). It was found that at least two Bragg Grating sensors should be used at each side of the crack per optical fiber, in order to enable adequate monitoring of the strain field and the position of the crack tip. Different locations should be chosen according to the configuration of the patch (one or two-sided).

Thermal Analysis by Numerical Methods of Debonding Effects near the Crack Tip under Composite Repairs

Composite patch repair of metallic structures has become a rapidly grown technology in the aerospace field due to the demand for significant increases in the useful life of both military and civilian aircraft. This has led to significant advances overall in the repair technology of cracked metallic structures. Adhesively bonded composite reinforcements offer remarkable advantages such as mechanical efficiency, repair time, cost reduction, high structural integrity, repair inspectability, damage tolerance to further causes of future strains, anticorrosion and antifretting properties. However, because of the different nature and properties of the materials that form a repair (metals, composites, adhesives), side-effects may occur: debonding due to high stress concentration in the vicinity of the crack, thermal residual stresses because of different thermal expansion coefficients of the adherents, etc. In this paper a three-dimensional finite elements analysis of the area around a patch repaired crack of a typical aircraft fuselage is performed, taking into account both the properties and the geometry of the involved materials. Examined in this case are 2024-T3 aluminum alloy as base material, FM-73 as the adhesive system and F4/5521 boron/epoxy prepreg as the patch material. Through the thickness stresses near the crack tip and along the patch edges with and without temperature effects are calculated and debonding near the crack tip is examined. Finally, the calculated results are compared with existing theories.

Computational Analysis and Optimization for Smart Patching Repairs

A classical cracked metallic structure, repaired with a ‘smart’ bonded composite patch with embedded optical fibers (to detect the strain field variations of the loaded structure), has been studied here-in. Finite element analysis was used, where-in the composite patch was modeled as a layered structure with three-dimensional elements constituting six different laminae. Each lamina is assumed to have different mechanical properties, according to the studied case, in order to simulate different stacking sequence. A resin rich ‘eye’ pocket has also been modeled in order to simulate the exact form of resin area produced during the manufacturing process. The patch is bonded over a cracked aluminum sheet through a small adhesive layer placed in between. External loads were applied only on the metal structure, as in a real repair case. The primary loading axis of the metal was assumed to be parallel to the direction of the optical fibers. Due to the different nature of the materials that form the composite patch, complex mechanical interactions between the fibers and the surrounding material occur, resulting in a complicated strain field along the optical fiber sensor. This affects the structural integrity of both the patch and the repair. Different optical fiber layer positions were considered, to study their effect on the resulting strain field and the structural integrity of the patch. Analysis concluded that the best available embedding position of an optical fiber in a laminated patch coincides to the one predicted as neutral surface, according to Roses analytical equations.

Nondestructive techniques in the investigation of aircraft materials: determination of defects and patches assessment

An experimental investigation was conducted for the determination of defects at aircraft components and for the composite patches evaluation. Cracked aluminum panels, untreated and repaired with carbon and boron composite patches, were inspected. The non-destructive techniques used in the assessment of these aircraft materials were infrared thermography and fiber optics microscopy. Infrared thermography is used for the localization of defects on aluminum panels, as well as on repaired ones with composite patches. Furthermore, the detection of defects on repaired aluminum panels that have undergone to fatigue testing, is attempted. Fiber optics microscopy is employed in order to examine the surface morphology of both carbon and boron composite patches. The results of this laboratory research work can lead to the development of an integrated non- destructive method for in field inspections of aircraft components.

Optimization of Embedded Optical Sensor Location in Composite Repairs

Optical fibers were embedded in a bonded composite patch in order to detect the strain field variations of a load bearing structure. The study concentrated on a classical cracked metallic structure repaired with this ‘smart’ patch and using finite element analysis. Six different laminates constituted the model of the composite patch, a layered structure with three-dimensional elements. Each laminate is assumed to have different mechanical properties, according to the case under any specific study, in order to simulate different stacking sequence or material used. A resin rich ‘eye’ pocket has also been modeled in order to simulate the exact form of the resin area produced during the manufacturing process. The patch is bonded over a cracked aluminum sheet through a small adhesive layer placed in between. External loads were applied only on the metal structure, as in a real repair case. The primary loading axis of the metal was assumed to be parallel to the direction of the optical fibers. The different nature of the materials that form the composite patch generated complex mechanical interactions between the fibers and the surrounding material, resulting in a complicated stress field along the optical fiber sensor, which affects the structural integrity of both the patch and the repair. Different optical fiber positions were considered, moving towards the horizontal and vertical dimensions of the patch, as well as different patch architectures (single and double patch configurations), with the hope of studying their effect on the structural integrity of the patch.

Smart patches: self-monitoring composite patches for the repair of aircraft

Conventional aircraft repair techniques employ bolted or riveted metallic reinforcements, which frequently introduce additional stress concentrations leading to further cracking and creating areas difficult or impossible to inspect. Bonded composite repairs (“patches”) result in the elimination of stress concentrations caused by additional fastener holes, improved strength to weight ratio and present a sealed interface. This reduces even further the danger of corrosion and fretting under the repair, gives greater flexibility in design and lessens application time while lengthening fatigue life. Embedding optical fibres and sensors into the patch, and combining this with advanced data collection and processing systems, creating a so-called “smart patch”, will enable the real-time assessment of aircraft structural integrity resulting in reliable prediction of maintenance requirements for repaired structures. This paper describes the current state of the art in smart patch technology, and includes a detailed description of the measurement problem and of the work being undertaken to solve it, at both the component and system level. An analysis of typical crack behaviour, based on FE modelling is presented and this demonstrates the need for optical strain sensors having a very short gauge length. The paper discusses the advantages and limitations of very short Fibre Bragg Gratings (FBGs) in this context and also provides early experimental data from 1mm and 2mm gratings which have been fabricated for this purpose. The paper also describes the impact of the measurement and environmental constraints on the design of the FBG interrogation system and presents the results of initial trials. The work is being undertaken in the framework of a collaborative project (ACIDS) which is co-funded by the European Commission.

Damage Detectability on Aluminum Alloy Panels Under Composite Patching by Various NDT Techniques

Composite patching method is widely applied for repair cases of metallic aircraft structures due to more efficient performance than conventional repairs. However, the detection of structures integrity under patch, during the service life of aircraft, by non-destructive means is considered of great importance. In the present study, different NDT techniques such as active infrared thermography, eddy currents and electrical impedance spectroscopy, were applied for the detection of simulated artificially introduced damages - notches, on the surface of aluminum aircraft skin panels, Al 2024-T3, under composite patching (carbon reinforced laminates). The detection sensitivity of each technique was investigated based on the relation between thickness of composite patch and specific parameters of each method aiming at the development of a reliable, for this purpose, quality inspection technique.