Gasser Abdelal

Thermomechanical Response of Variable Stiffness Composite Panels

Analysis of non-traditional Variable Stiffness (VS) laminates, obtained by steering the fiber orientation as a spatial function of location, have shown to improve buckling load carrying capacity of flat rectangular panels under axial compressive loads. In some cases the buckling load of simply supported panels doubled compared to the best conventional laminate with straight fibers. Two distinct cases of stiffness variation, one due to fiber orientation variation in the direction of the loading, and the other one perpendicular to the loading direction, were identified as possible contributors to the buckling load improvements. In the first case, the increase was attributed to the favorable distribution of the transverse in-plane stresses over the panel platform. In the second case, a higher degree of improvement was obtained due to the re-distribution of the applied in-plane loads. Experimental results, however, showed substantially higher levels of buckling load improvements compared with theoretical predictions. The additional improvement was determined to be due to residual stresses introduced during curing of the laminates. The present paper provides a simplified thermomechanical analysis of residual stress state of variable stiffness laminates. Systematic parametric analyses of both cases of fiber orientation variations show that, indeed much higher buckling loads could result from the residual stresses present in such laminates.

Numerical and experimental investigation of aircraft panel deformations during riveting process

In collaboration with Airbus-UK, the dimensional growth of aircraft panels while being riveted with stiffeners is investigated. Small panels are used in this investigation. The stiffeners have been fastened to the panels with rivets and it has been observed that during this operation the panels expand in the longitudinal and transverse directions. It has been observed that the growth is variable and the challenge is to control the riveting process to minimize this variability. In this investigation, the assembly of the small panels and longitudinal stiffeners has been simulated using static stress and nonlinear explicit finite element models. The models have been validated against a limited set of experimental measurements; it was found that more accurate predictions of the riveting process are achieved using explicit finite element models. Yet, the static stress finite element model is more time efficient, and more practical to simulate hundreds of rivets and the stochastic nature of the process. Furthermore, through a series of numerical simulations and probabilistic analyses, the manufacturing process control parameters that influence panel growth have been identified. Alternative fastening approaches were examined and it was found that dimensional growth can be controlled by changing the design of the dies used for forming the rivets.

The Collapse Behaviour of Cold-formed Steel Portal Frames at Elevated Temperatures

This paper describes the results of non-linear elasto-plastic implicit dynamic finite element analyses that are used to predict the collapse behaviour of cold-formed steel portal frames at elevated temperatures. The collapse behaviour of a simple rigid-jointed beam idealisation and a more accurate semi-rigid jointed shell element idealisation are compared for two different fire scenarios. For the case of the shell element idealisation, the semi-rigidity of the cold-formed steel joints is explicitly taken into account through modelling of the bolt-hole elongation stiffness. In addition, the shell element idealisation is able to capture buckling of the cold-formed steel sections in the vicinity of the joints. The shell element idealisation is validated at ambient temperature against the results of full-scale tests reported in the literature. The behaviour at elevated temperatures is then considered for both the semi-rigid jointed shell and rigid-jointed beam idealisations. The inclusion of accurate joint rigidity and geometric non-linearity (second order analysis) are shown to affect the collapse behaviour at elevated temperatures. For each fire scenario considered, the importance of base fixity in preventing an undesirable outwards collapse mechanism is demonstrated. The results demonstrate that joint rigidity and varying fire scenarios should be considered in order to allow for conservative design.

Experimental and Numerical Evaluation of Thermal Performance of Steered Fibre Composite Laminates

For Variable Stiffness (VS) composites with steered curvilinear tow paths, the fiber orientation angle varies continuously throughout the laminate, and is not required to be straight, parallel and uniform within each ply as in conventional composite laminates. Hence, the thermal properties (conduction), as well as the structural stiffness and strength, vary as functions of location in the laminate, and the associated composite structure is often called a “variable stiffness” composite structure. The steered fibers lead not only to the alteration of mechanical load paths, but also to the alteration of thermal paths that may result in favorable temperature distributions within the laminate and improve the laminate performance. Evaluation of VS laminate performance under thermal loading is the focus of this chapter. Thermal performance evaluations require experimental and numerical analysis of VS laminates under different processing and loading conditions. One of the advantages of using composite materials in many applications is the tailoring capability of the laminate, not only during the design phase but also for manufacturing. Heat transfer through variable conduction and chemical reaction (degree of cure) occurring during manufacturing (curing) plays an important role in the final thermal and mechanical performance, and shape of composite structures. Three case studies are presented in this chapter to evaluate the thermal performance of VS laminates prior to and after manufacturing. The first case study is a numerical analysis that investigates the effect of variable conductivity within the VS laminate on the temperature and degree of cure distribution during its cure cycle. The second case study is a numerical analysis that investigates the transient thermal performance of a rectangular VS composite laminate. Variable thermal conductivity will affect the temperature profile and results are compared to a unidirectional composite. The effect of fiber steering on transient time and steady state solutions is compared to the effect of unidirectional fibers. The third case study is an experimental one that was conducted to evaluate the thermal performance of two variable stiffness panels fabricated using an Advanced Fiber Placement (AFP) Machine.

A Multiphysics Simulation Approach for Efficient Modeling of Lightning Strike Tests on Aircraft Structures

A numerical approach is proposed and demonstrated for efficient modeling of the thermal plasma behavior present during a lightning strike event. The approach focuses on events with timescales from microseconds to milliseconds and combines the finite element method, magnetohydrodynamics, and similitude theory. Similitude theory is used to scale the problem to require considerably less computing resource. To further reduce the computational burden and to resolve the numerical difficulty of simulating the nearly zero electrical conductivity of air at room temperature, an approach based on cold-field electron emissions is introduced. Simulations considering turbulent flow have been considered, modeling a test configuration from literature designed to inspect composite material performance and applying an aerospace standard test profile (Waveform-B). Predicted peak temperatures (of the order of ~40 000 K) and pressures (of the order of 0.1–0.2 MPa) suggest that the pressure loading during a Waveform-B event will have a minimal effect on composite material damage.

Transient Heat Conduction of Variable Stiffness Composite Laminate

Recent research on Variable Stiffness (VS) laminates, which are constructed by steering the fiber orientation as a spatial function of location, have shown to improve laminate performance under mechanical loads. Two distinct cases of stiffness variation can be achieved either by variation of the fiber orientation in the direction of the global x-axis, or perpendicular to it. In the present paper, thermal analysis of a VS laminate is performed to study the effect of steering fibers on transient heat conduction under uniform heat flux using finite element method. The goal of the present paper is a parametric study of the effect of variable stiffness properties on transient response including time to reach steady state and temperature profile. Also, stress resultants and maximum stress location are investigated under different boundary conditions. A FEM algorithm is applied to exactly incorporate the boundary conditions for stress resultant analysis.

Quantifying the Influence of Lightning Strike Pressure Loading on Composite Specimen Damage

Experimental work has shown that a component of lightning strike damage is caused by a mechanical loading. As the profile of the pressure loading is unknown a number of authors propose different pressure loads, varying in form, application area and magnitude. The objective of this paper is to investigate the potential contribution of pressure loading to composite specimen damage. This is achieved through a simulation study using an established modelling approach for composite damage prediction. The study examines the proposed shockwave loads from the literature. The simulation results are compared with measured test specimen damage examining the form and scale of damage. The results for the first time quantify the significance of pressure loading, demonstrating that although a pressure load can cause damage consistent with that measured experimentally, it has a negligible contribution to the overall scale of damage. Moreover the requirements for a pressure to create the damage behaviours typically witnessed in testing requires that the pressure load be within a very precise window of magnitude and loading area.

Micro-satellite structure fracture investigation techniques

Mounting accuracy of satellite payload and ADCS (attitude determination and control subsystem) seats is one of the requirements to achieve the satellite mission with satisfactory performance. Deviation of the position of the mounting seat for Multi-Band-Earth-Imager (MBEI) is caused by cracks in the plate of the basis unit and bracket for attachment of MBEI. These cracks were detected during inspection of the satellite strength mock-up after vibration testing for air transportation phase. Most probable reason of the cracking is fatigue damage as strength mock-up structure was subjected to prolonged vibration loading during various loading cases. Total vibration duration during testing is about 56 hours. In order to study the cracking reasons, finite element modeling of the structural parts of the basis unit including MBEI bracket and instrument MBEI is subjected to harmonic response to simulate vibration loading for the case of air transportation. Numerical results are compared with the experimental ones, and mechanical design of the basis-plate unit is modified.