Development of probability of detection data for structural health monitoring damage detection techniques based on acoustic emission

Structural Health Monitoring (SHM) techniques have been developed as a cost effective alternative to currently adopted Non-Destructive Testing (NDT) methods which have well understood levels of performance. Quantitative performance assessment, as used in NDT, needs to be applied to SHM techniques to establish their performance levels as a basis for technique comparison and also as a requirement for practical aerospace application according to set regulations. One such measurand is Probability of Detection (POD). This paper reports experiments conducted to investigate the location accuracy of the Acoustic Emission (AE) system in monitoring events from HsuNielson and fatigue crack AE sources as a route to establish the POD of AE in SHM. It was found that fatigue crack tips could be located at 90% POD within 10 mm accuracy

An experimental approach to quantify strain transfer efficiency of fibre bragg grating sensors to host structures

This paper developed a method to evaluate the strain transfer efficiency of fibre Bragg grating sensors to host structures. Various coatings were applied to fibre Bragg grating sensors after being fabricated. They were epoxy, silane agent and polypropylene, representing different surface properties. A neat epoxy resin plate was used as the host in which the coated fibre sensors were embedded in the central layer. The tensile strain output from the FBGs was compared with that obtained from electrical strain gauges which were attached on the surface of the specimen. A calculating method based on the measured strains was developed to quantify the strain transfer function of different surface coatings. The strain transfer coefficient obtained from the proposed method provided a direct indicator to evaluate the strain transfer efficiency of different coatings used on the FBG sensors, under either short or long-term loading. The results demonstrated that the fibre sensor without any coating possessed the best strain transfer, whereas, the worst strain transfer was created by polypropylene coating. Coatings play a most influential role in strain measurements using FBG sensors

Fatigue crack growth rates under variable amplitude load spectra containing tensile underloads

An extensive research program was performed to investigate the load interaction effect of the combined action of small amplitude high R ratio cycles and large amplitude low R ratio underloads on the crack growth of large cracks. The study was driven by the needs of the damage tolerance approach in the helicopter structures, which requires robust knowledge on the crack growth behaviour of the advance high strength alloys under the characteristic helicopter spectra loading. The study was conducted on three metallic alloys, Ti-10V-2Fe-3Al, Al8090 T852 and Al7010 T76351 using compact tension specimens (w=70mm, t=17mm). The potential drop technique was used for the measurements of the crack length. The crack opening loads were determined from the applied load versus crack opening mouth displacement curve using a curve fitting technique and crack opening displacement gauge. The experimental results show that cracks can grow faster than the life predictions with no load interaction effects under spectra containing tensile underloads. The acceleration effects are different depending on the number of the small cycles, the Kmax, the R ratio of the small cycles, the underload cycle and the material. Significant closure observations on the underloads and on the small cycles of variable amplitude loading spectra were made. Based on the test finding and on the studies of other researchers, it is suggested that the acceleration effects are mainly due to the reduction of crack opening point of the tensile underloads comparing with the Constant Amplitude Loading (CAL) data. An extensive evaluation of the ability of FASTRAN model to predict the fatigue lives under the tested loading spectra was carried out. The evaluation focuses on the influence of the constraint factor a and the ∆Keff curve inputs on the predictions. The model produces very good and consistent predictions for the three alloys, when the inputs represent adequately the actual fatigue mechanism. The model predicts the measured acceleration effects by reducing the closure level of the underloads.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Damage sensing in CFRP composites using electrical potential techniques

This Thesis investigates the damage sensing capabilities of the electrical potential measurement technique in carbon fibre reinforced polymer composites. Impact damage was introduced in multidirectional laminates and its effect on potential distribution studied. It was found that delaminations and fibre breakages within the laminate can be detected and located by measuring potential changes on the external composite surface. The extent and size of potential changes were significantly affected by the position of the current electrodes in relation to the potential measurement probes. A numerical model was developed investigating the effect of different size delaminations, located in various positions within the lamina, on electrical potential distributions on the external ply, and a quantitative analysis of the numerical results is presented. The numerical simulations demonstrated that the measured potential changes on the external ply were in proportion to the delamination size. The numerical and experimental results were compared and the optimum configuration of current electrodes and potential probes for damage detection selected. The response of electrical potential to mechanical strain, in unidirectional and multidirectional samples was also investigated. It was found that the conductive medium, used for introducing the current, defines the piezo-resistance performance of the composite. A finite element model was developed able to predict the effect of inhomogeneous current introduction in unidirectional specimens on electrical potential and piezo-resistance. The effects of temperature and water absorption on potential measurements were also presented.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Investigation of cyclist and pedestian impacts with motor vehicles using experimentation and simulation

Physical tests were performed with a bicycle and a dummy in a controlled laboratory environment to reproduce cyclist accidents. The kinematics of 13 sled tests were used to identify the cyclist head impact location, understand the interaction between the cyclist and bicycle and to validate a mathematical model. The finite element software code LS-DYNA was used to simulate 70 cyclist and pedestrian accidents with motor vehicles with four different vehicle shapes which supplemented the physical testing. The study has shown that when cyclists and pedestrians were struck by any of the vehicles their whole body kinematics can be distinguished into two phases, initially a rotation followed by a sliding action. The Sports Utility Vehicle (SUV) vehicle produced more of a rotation action rather than sliding, whereas the other vehicles produced a combination of the two. The current pedestrian legislation does not cover all head impact locations for cyclists and therefore needs to be extended to encompass the windscreen and A-Pillar regions of the vehicles. The wrap around distance (WAD) for all the vehicles, apart from the SUV, should be extended to encompass a larger region. For the SUV the current WAD region is adequate in protecting cyclists and pedestrians and does not need to change. The predicted head impactor angle for cyclists is 40 degrees which is lower than the current legislative value of 65 degrees and the predicted pedestrian head impact angle is higher at a value of 80 degrees for the MPV, SM and LFC. For the SUV the proposed impactor angle increased to 100 degrees for cyclists and pedestrians. This research has demonstrated significant differences in terms of input variables and outcomes between cyclist and pedestrian accidents involving vehicles. It has used mathematical models to obtain injury data from a human mathematical model and physical testing to replicate real world cyclist accident scenarios. Recommendations have been proposed for future legislative testing techniques for cyclists, based on existing pedestrian legislation. These recommendations to alter legislation will improve vehicle design and make future vehicles more cyclist-friendly.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Aircraft crash survivability from viscous injury in vertical impacts

This research investigated viscous injury from vertical impact loading to determine if it is critical to survivability of aircraft accidents. A unique database was built from autopsy reports and accident investigations combining injury data with the vehicle impact data. Computer models were created and used to assess injury potential. Common design limits and actual crash data from full scale research experiments were used as inputs. The results were analyzed according to published injury thresholds and compared with real accident autopsies to determine the validity of the hypothesis. Heart and Aortic Injury (HAI) has been considered a critical survivability factor through out the history of mechanized transportation. The mechanisms of HAI in the aircraft environment were never well characterized. Automotive research identified important HAI injury mechanisms related to the forward and lateral impact vectors. This research investigated the vertical impact vector. A model was developed to evaluate the biomechanical response of a simplified visco-elastic system, and incorporated into a system model which included the occupant and aircraft seat. This approach was similar to the development of spine injury criteria and provided the advantage of a macro level evaluation of the injury thresholds and assessment of the criticality in survivable accidents. Evaluations of real accidents sustaining HAI characterized a range of impact severity and approximate boundaries for survivability with HAI and internal organ injury. Viscous injury potential from vertical impact was found to be less critical than potential spine injury. Detailed analysis of HAI documented in autopsy reports and the corresponding accident investigations found that HAI was associated with cockpit environmental factors rather than inertial displacement mechanisms. Vertical displacement of the heart due to inertial loads is not a critical factor in survivable accidents given current aircraft technology. Inertial loading to the heart and aorta is a contributory factor for viscous injuries in aircraft accidents.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Visual inspection reliability for composite aircraft structures

This thesis presents a study of the effects of surface colour, surface finish and dent shape on the visual inspection reliability of 3D surface indentations common in shape to those produced by impact damage to carbon fibre reinforced epoxy laminates. Falling weight (2.5kg) apparatus was used to produce impact damage to non-painted, non-mesh Hexcel AS4/ 8552 carbon fibre reinforced plastic (CFRP) laminates and painted AS4/ 8552 laminates containing bronze mesh and glass fabric lightning strike protection layers. Ø20 mm and Ø87 mm hemispherical tip impacts to painted 17ply and 33ply laminates at varying energy levels typically produced circular shaped, smoothly contoured, rounded sectional profiles with an absence of surface breaking cracks. Sectional profiles through coordinate measuring (CMM) data of the impact dents were described using a set of geometric variables. Identifying relationships between impact energy and the geometric variables allowed the typical sectional profile through impact damage dents from Ø20 mm and Ø87 mm hemispherical tips on 17ply and 33ply painted CFRP laminates to be calculated for energies between 5J to 80J. Calculated sectional profiles typical of impact damage dents to CFRP laminates were reconstructed as simple revolved shapes using 3D computer aided design (CAD) models. The 3D CAD models were computer numerical control (CNC) machined into 3mm Plexiglas panels to produce facsimiles of hemispherical impact damage dents on CFRP laminates. Facsimile specimen sets of sixteen 600 mm x 600 mm panels were produced in gloss and matt grey, white and blue finishes. Each set contained the same 32 different sized machined dents representing Ø20 mm and Ø87 mm hemispherical tip impact damage to 17ply & 33ply painted CFRP laminate. Each facsimile specimen set was combined with similarly finished unflawed (dent free) panels. 64 panels in each colour/ finish were presented for 5 seconds in a randomised order to a minimum of 15 novice participants in a visual inspection task lasting approximately 25 minutes. II A set of corresponding visual inspection experiments were performed in which physical specimens were replaced by digitally projected actual size photorealistic images of the machining CAD data. Comparisons between the results of the physical and virtual specimen trials revealed differences in detectability for similarly sized dents. The detection results obtained from visual inspection of physical specimens demonstrated that the detectability of dents similar to those caused by higher (>40J) energy impacts from a Ø87 mm hemispherical tip was less than that of the dents caused by lower energy (90% detection were observed on grey specimens and the highest number of dents returning 0% detection were observed on matt blue specimens. The difference in detection rates for similarly sized dents on a gloss and matt finish was least on grey coloured specimens and greatest on blue coloured specimens.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

An investigation on the diagnostics and prognostic capabilities of acoustic emission (AE) on a spur gearbox

The aim of this research project is to explore a new technique, Acoustic Emission (AE), on both the diagnostic and prognostic capabilities in monitoring gear teeth degradation (pitting), and compare with the more widely used techniques such as vibration monitoring and Spectrometric Oil Analysis (SOA). Furthermore, by employing the experimental results and past literature, a model in predicting the amount of gear surface pitting wear using AE activity level was proposed. The successful forinulation of this proposed model may be able to predict the remaining life of the gear after pitting has been detected, thereby allowing timely replacement to be carried out without the risk of catastrophic failure. A series of experimental tests which include seeded defect simulations, study on the effect of operating parameters over AE (under isothermal conditions), AE source determination tests and accelerated gear fatigue tests have been performed to investigate the diagnostics and prognostics capabilities of AE via a back-to-back gearbox set up. The experimental results achieved have highlighted some significant findings: (a) The variation in rotating speeds, change the AE levels in a much significant amount as compared to the same variation in applied load. (b) The prime source of AE was postulated to be asperity contact under rolling and sliding of the meshing gear teeth surfaces. (c) AE technique has a far better degradation (pitting) monitoring capability compared to vibration and SOA techniques. These findings have made a vast contribution in condition monitoring of gearbox using AE technique and the proposed model has also offered opportunity to make AE a potentially viable and effective tool in diagnosis and prognosis of gearbox or even other rotating machinery defects.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Implications of substrate geometry and coating thickness on the cracking resistance of polymer-based protective coatings

Welded steel T-sections of different weld fillet geometries coated with water ballast tank protective coatings were subjected to thermal cycling with a temperature range from 60°C to -10°C. Cracks developed in the coatings at the weld line, propagating longitudinally along it. The number of cycles required to create 1 mm cracks was strongly dependent on the weld geometry and the coating Dry Film Thickness (DFT). Finite Element Modelling (FEM) was employed to calculate thermally induced strain fields in the coatings subjected to the same temperature range. FEM predicted that the greatest strain concentrations are present at the coating surface within the weld fillet region. Increased DFT and decreased fillet radius leads to increased maximum principal strains. Numerical analysis predicts that greatest strain ranges promoting the earliest cracking/failure are found in thicker coatings applied to smaller weld radii. Experimental observations confirm this