Design and numerical modeling of a pressurized airframe bulkhead joint

The structural loading on a conceptual lap joint in the empennage of a civil aircraft has been investigated. The lap joint interfaces the end-pressure part-hemispherical bulkhead to the cylindrical fuselage. The pressure bulkhead is made of carbon fiber reinforced plastic materials. The aim of the study is to present numerical results of the induced structural loading from the fuselage positive internal pressure differential and the localized high stress intensity field at the lap joint location. A methodology for the appropriate numerical approach to analyze the domed pressure bulkhead is presented. The results of the numerical investigation showed that the laminate loading levels calculated by the use of either initial sizing analytical formulas for pressurized domes or by the use of equilibrium nodal loading from finite element models of low fidelity compared to refined finite element analysis can be significantly underestimated. Some of the implications on carbon fiber reinforced plastic structural sizing at the specified location are developed

Micromechanical modelling and interfacial strength prediction of multidirectional laminated fibre reinforced polymers

Delamination initiation and propagation is a common failure mode in laminated composites that must be considered when assessing damage in composite structures. Delamination usually propagates at the interface between laminas. Current approved testing procedures address the inter-laminar strength in fracture modes I and II for interfaces of unidirectional laminas oriented in the same direction. The aim of this study was to investigate the interlaminar fracture initiation strength in multi-layer lamina interfaces by the use of micromechanical numerical analysis. Representative volumetric elements with randomly distributed fibres and the ability of numerically modelling fibre-matrix interfacial debonding were generated with different ply interfacial orientations. Failure initiation and damage sequences were captured and the global stresses where failure initiated were determined for the studied configurations. Insights on the variations in the strength observed due to the different lamina orientations were provided

Hydroelastic analysis of ice shelves under long wave excitation

Abstract. The transient hydroelastic response of an ice shelf under long wave excitation is analysed by means of the finite element method. The simple model, presented in this work, is used for the simulation of the generated kinematic and stress fields in an ice shelf, when the latter interacts with a tsunami wave. The ice shelf, being of large length compared to its thickness, is modelled as an elastic Euler-Bernoulli beam, constrained at the grounding line. The hydrodynamic field is represented by the linearised shallow water equations. The numerical solution is based on the development of a special hydroelastic finite element for the system of governing of equations. Motivated by the 2011 Sulzberger Ice Shelf (SIS) calving event and its correlation with the Honshu Tsunami, the SIS stable configuration is studied. The extreme values of the bending moment distribution in both space and time are examined. Finally, the location of these extrema is investigated for different values of ice shelf thickness and tsunami wave length

Sensitivity of composite scarf joints to manufacturing deviation and disbond under tensile load

Scarf joints are an effective method of bonding thick composite laminates for applications such as the repair of composite aircraft structures. However, concerns remain about their damage tolerance characteristics. Typically composite scarf repairs to aircraft structures require use of hand tools or rudimentary jigs. If the scarf is incorrectly prepared, this may cause a profile deviation to the joint, affecting the bond line stresses and in turn, reducing the residual strength of the joint or repair. The subject of this work examined the sensitivity of composite scarf joints to machining profile deviation and artificial disbond, when subject to static tensile load. Tensile test specimens were prepared with two different configurations of scarf for representing an undercut or imprecise scarf typical of a machining error. In addition, sensitivity of the scarf joints in the presence of an artificial disbond was also tested. Results indicated that for the specimens tested, the scarf is relatively insensitive to minor profile deviation, but highly sensitive to an artificial disbond. Experimental results were also compared with finite element analysis

Liquid hydrogen storage tank loading generation for civil aircraft damage tolerance analysis

The study presented is a preliminary approach and a proposal to the derivation of a loading spectrum for fatigue and damage tolerance analysis for civil aviation Liquid Hydrogen storage tanks. It is anticipated for the first generation of LH2 storage tanks for aviation to utilize metallic lightweight materials. Existing solutions are either too structurally heavy or with a short life span, both constraints making them unsuitable for aircraft vehicles were less mass and longevity is of paramount importance. The objective of the work was to provide suggestions for the generation of representative loading spectra for storage tank fatigue and damage tolerance preliminary design analysis and sizing

Liquid hydrogen storage tank virtual crashworthiness design exploration for civil aircraft

Civil aviation industry is researching for alternative fuel energy sources to substitute current hydrocarbon-based aviation fuels. Carbon free emissions flights could be achieved with fuels like Hydrogen either through combustion or via electricity producing fuel cells. It is of great importance to explore the airframe designs to house Hydrogen in its cryogenic liquified state. The objective of the study herein was to provide a conceptual qualitative analysis related to the crashworthiness behaviour of civil aircraft carrying liquid Hydrogen fuel storage tanks. The design parameters of interest were the storage tank location in the airframe, the structural energy absorption following crash landing scenarios and the structural deformation of the structure surrounding the tanks, penetrating the survival space of the occupants. Several structural design arrangements were proposed and compared. Simulation results indicated that the optimal location for the fuel storage greatly depends on the actual aircraft layout as well as on the future civil aircraft airworthiness requirements that are still under development for that type of fuel energy source

Bird strike virtual testing simulations and results, for preliminary airframe design structural optimization

External airframe structural components facing the aircraft flight direction, are prone to bird collisions. Aircraft manufacturers meet the bird strike airworthiness requirements through physical bird strike testing. Mainly due to the high costs involved in the certification process, recent studies have highlighted the capabilities and benefits of hybrid simulation-experiment techniques that reduce certification costs. The numerical investigation presented herein, studied the bird-strike simulation methodologies implemented to support airframe manufacturers to partially fulfill the current certification airworthiness requirements. The methodology can be also applied during preliminary aircraft parametric design stages. In the current study, the method was applied onto an aircraft wing leading edge preliminary design, which led to design exploration by correlating the leading edge skin materials and thicknesses with the rib pitch positioning. The bird-strike impact model was simulated using the Smoothed Particle Hydrodynamics numerical method using ABAQUS® Explicit finite element package. The materials benchmarked were aluminum alloy 2024-T3, carbon fiber reinforced epoxy IM7/8552 and S2 glass Fiber Metal Laminate GLARE®. The design goal of the case study was to provide with preliminary evidence for impact resistance, quantified as residual permanent structural deformation of the critical structural components for which design charts were drawn and presented herei

Crashworthiness behaviour of a composite fuselage section with cargo door

This paper present the crashworthiness assessment of a composite fuselage section with a cargo door by means of the Finite Element (FE) software ABAQUS/Explicit. In crashworthiness research, no analysis, either experimental or numerical, of a composite fuselage section with a cargo door has ever been carried out. Therefore, the numerical analysis of the vertical drop test of three models of composite fuselage sections, representative of a regional aircraft fuselage, will be performed: a typical fuselage section without cargo door, a section with the cargo door but not the appropriate reinforced structure and a section with the surrounding door structure reinforced as in commercial aircraft sections with such cut-outs. In order to guarantee the integrity of the passengers as well as the structure, the crash kinematics of each model as well as the accelerations experienced by the passengers have been compared and examined in detail. The comparison between the three models allowed to identify the penalty that a duly reinforced cargo door structure induces on the crashworthiness of a composite fuselage

Numerical simulation of bolted joints pull through failure

Finite Element Analysis numerical models were generated to simulate and investigate the pull-through damage of bolted joints on composite laminates. Three-dimensional elements were used along with a user material subroutine incorporating the material failure criterion in Abaqus® software. Simplified Discrete Ply Modelling (DPM) and Cohesive Zone Modelling (CZM) were also employed. The numerical model predictive capability was assessed and the parameters influencing the pullthrough failure process were investigated. The residual bearing strength of bolted joints following pull-through damage suggested a qualitative agreement between the numerical model and the experimental testing results

Experimental testing correlation with numerical meso-scale modelling of CFRP structures and the significance to virtual certification of airframes

The design of structural components has altered fundamentally since laminated composites were proved excellent candidate materials in aerospace applications. The key aspects rendering CFRPs preferable to metals, are mostly their significantly higher specific mechanical properties, and the design flexibility through the stacking sequence selection. However, the currently in use limit and polynomial failure criteria, are inadequate to accurately predict all experimentally observed failure modes and damage specificities of the lamina individual constituents, imposing difficulties in the numerical certification of airframe composites. Thus, component and lamina-level testing are sometimes inevitable, requiring industrial resources which are expensive as well as environmentally costly. For that reason, virtual testing could be more promising in substituting real experimental testing, if conducted under advanced failure criteria which better describe the nature of failure. In this study, the open hole tensile (OHT) test has been simulated under the LaRC05 phenomenological failure criterion, with embedded strain-based progressive damage material behavior. A relatively common composite material in aerospace structures has been selected, IM7 8552 of Hexcel, to compare the numerical strength predictions with its corresponding experimental values. The simulations carried out are based on a standard test method by ASTM international, which address the standardisation of strength tests of polymer matrix composite laminates. The, model was created in ABAQUS/Explicit under the VUMAT user subroutine. The resulted predictions have been found to well – correlate with the testing data, irrespective the specimen stacking sequence