P.T. Curtis

Modeling the Lofting of Runway Debris by Aircraft Tires

Runway debris lofting by aircraft tires can lead to considerable damage to aircraft structures, yet there is limited understanding of the lofting mechanisms. The aim of this study is to develop accurate physically based models to understand and predict the stone lofting processes. The research entailed both experimental work and finite element modeling of a tire partially rolling over a stone. Parametric studies were conducted to characterize the influence of factors such as stone geometry and tire conditions in the lofting processes. To validate the finite element models, experimental studies were conducted using a modified drop weight impactor covered with rubber to simulate a tire vertically approaching aluminum balls and real stones. A high-speed video camera was used to observe the loft mechanisms and calculate the loft velocities, angles, and spin rates. A finite element model of the impactor demonstrated good agreement with the experimentally observed loft mechanisms.In general, lofting occurred either at high speed and low angles or vice versa, depending on the degree of interaction between the stone and the ground.

Rate dependent multiscale modelling of fibre reinforced composites

Abstract A multiscale modelling technique has been developed to predict the strain rate dependent properties of a fibre reinforced composite system. The full stress–strain behaviour through yield of an amine cured epoxy resin matrix (tetraglycidyl 4,4′-diaminodiphenylmethane) has been predicted using group interaction modelling. These data are used in a three-dimensional finite element model to obtain strain concentration factors of fibres adjacent to a fibre break in a unidirectional composite. A Monte Carlo simulation is performed to predict the tensile failure strain of a single composite layer and by assembling these layers the ultimate failure strain of the composite system is predicted. The technique is repeated over three different strain rates and consistency with experiment is observed.

Improved models for runway debris lofting simulations

Numerical models used to simulate the lofting mechanisms of runwary stones were developed to assess the threat to aircraft structures from runway debris impacts. An inflated aircraft tyre model, which was validated by comparison with experimental indentation tests, showed that over-rolling of stones under typical take-off conditions led to only modest vertical loft velocities of less than 5 m/s. Experiments using a drop weight impactor simulated a section of aircraft tyre descending upon stones. These tests demonstrated that ofting was achieved for impacts with low rubber thickness. However, for impacts with greater rubber thickness, lofting was suppressed. Using more realistic tyre geometries resulted in launches with backspin, but only horizontally along the ground in the direction of the tyre axis. The speed at which launches occurred was proportional to the rate of descent of the tyre section and would consequently determine the loft speeds due to potential asperity ofting.

Self-Sensing Composites: In-Situ Detection of Fibre Fracture

The primary load-bearing component in a composite material is the reinforcing fibres. This paper reports on a technique to study the fracture of individual reinforcing fibres or filaments in real-time. Custom-made small-diameter optical fibres with a diameter of 12 (±2) micrometres were used to detect the fracture of individual filaments during tensile loading of unreinforced bundles and composites. The unimpregnated bundles were end-tabbed and tensile tested to failure. A simple technique based on resin-infusion was developed to manufacture composites with a negligible void content. In both cases, optical fibre connectors were attached to the ends of the small-diameter optical fibre bundles to enable light to be coupled into the bundle via one end whilst the opposite end was photographed using a high-speed camera. The feasibility of detecting the fracture of each of the filaments in the bundle and composite was demonstrated. The in-situ damage detection technique was also applied to E-glass bundles and composites; this will be reported in a subsequent publication.

Characterization of the Mechanical Behavior of a Polymer-Based Laminate and Constituent Fibers at Various Quasi-Static Strain Rates

AbstractPredictive computational modeling of the response of armor systems to dynamic threats such as blast and impact requires understanding and quantification of the behavior of the armor materials. This paper describes the mechanical characterization of Dyneema HB26. The in-plane tensile, compressive, and shear stress-strain behavior and strength of the laminate at low rates has been determined experimentally. The tensile behavior of the Dyneema SK76 fibers, which comprise 83% of the laminate has been determined, including the effect of temperature and rate.

Understanding the Thickness Effect on the Tensile Strength Property of Dyneema®HB26 Laminates

In this study, an experimental and numerical investigation is presented on the effect of thickness and test rate within the pseudo static regime on the tensile properties of Dyneema®HB26 laminates. A detailed experimental presentation on the tensile testing of different thickness is presented and highlights the commonly seen observation that the tensile strength of a laminate reduces as a function of the specimen thickness. To understand these experimental observations, a constitutive material model of the individual macro fibril is developed and applied to modelling the fibre and upscaling to the laminate. The modelling strategy is implemented into ls-dyna and used to perform a parameter study on the specimen geometries used in the experimental study. The model assumes that the fibril strength is a function of the amorphous volume within the fibre and hence fibril. It can be observed that the experimental behaviour can be simulated by modelling the interface between laminate plies and the fibril, and hence fibre failure. The weak interfaces from the fibril to the laminate scale make the testing of fibres and laminates very difficult. Hence, it is proposed that the intrinsic fibril strength should be used as a measure of strength, and the fundamental strength is determined through numerical studies.

Analytical and Numerical Study of Early-Time Response in Pulse-Loaded Visco-Elastic Composites

Hyperbolic partial differential equations with one space variable are used to investigate the longitudinal wave propagation through an elastic composite medium. A high order Lagrangian finite element is used to model the wave propagation and the weak-form Galerkin weighted residual method is adopted for solving the governing differential equations, viz., the one-dimensional wave equation which is extended to include damping and strain-rate effects. The numerical solutions are compared to analytical solutions (where they exist) and excellent temporal and spatial correlation is achieved, within 90-95% accuracy. It is found that damping leads to a decrease in peak stresses and strains by up to 11% for 5% of critical damping, even during the direct loading phase. It is shown that the inclusion of strain-rate did not have an effect on strains but led to an increase in stresses by almost 95%. The inclusion of both damping and strain-rate effects together increased stress values by up to 70% compared to the non-viscous cases.

Improved Aircraft Tire and Stone Models for Runway Debris Lofting Simulations

Numerical models used to simulate the lofting mechanisms of runway stones were developed to assess the threat to aircraft structures from runway debris impacts. An inflated aircraft tire model, which was validated by comparison with experimental indentation tests, showed that over-rolling of stones under typical takeoff conditions led to only modest vertical loft velocities of less than 5 m/s. Experiments using a drop weight impactor to simulate a section of aircraft tire descending upon stones, demonstrated that lofting was achieved with impacts with low rubber thicknesses, but with greater rubber thickness lofting was suppressed. Using more realistic tire geometries resulted in launches with backspin, but only horizontally along the ground in the direction of the tire axis. The speed at which launches occurred was proportional to the rate of descent of the tire section and would consequently determine the loft speeds due to potential asperity lofting.