Experiment-Based Explicit Dynamics Analysis for Bird Strike Damage Prediction in Composite Structures

Bird strike analysis is a common type of analysis performed during the design and analysis of rotorcraft. These simulations are carried out in order to predict whether various designs will pass the necessary certification tests. In the past, the only way to determine whether forward-facing aeronautical composite structures could withstand bird strikes was with time-consuming physical tests. In the research of bird striking, the bird impact test is the most effective method. But the existing data of test results are highly disperse, so that they do less help for the design of aeronautical composite structures and also cost more. Tests usually needed to be repeated several times because components often failed and were required for each new design. There is a large variability in numerical bird models, composite modeling approaches and complexity of simulation processes to design the sandwich structures of an aircraft. This paper investigates the composite structures modeling for bird strike phenomenon by using state-of-the-art modeling tools capable of predicting the experiment-based composite structural damage, damage location, failure size and failure mode due to impact and addresses a critical review on analysis techniques. This paper also demonstrates the state-of-the-art bird strike simulation methodology developed, and the accuracy of modeling approaches available in explicit codes is discussed.     

Numerical simulations of low-velocity impact on an aircraft sandwich panel

The potential hazards resulting from a low-velocity impact (bird-strike, tool drop, runway debris, etc.) on aircraft structures, such as engine nacelle or a leading edge, has been a long-term concern to the aircraft industry. Certification authorities require that exposed aircraft components must be tested to prove their capability to withstand low-velocity impact without suffering critical damage.

This paper describes the results from experimental and numerical simulation studies on the impact and penetration damage of a sandwich panel by a solid, round-shaped impactor. The main aim was to prove that a correct mathematical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design an impact-resistant aircraft structure.

Part of this work presented is focused on the recent progress on the materials modelling and numerical simulation of low-velocity impact response onto a composite aircraft sandwich panel. It is based on the application of explicit finite element (FE) analysis codes to study aircraft sandwich structures behaviour under low-velocity impact conditions. Good agreement was obtained between numerical and experimental results, in particular, the numerical simulation was able to predict impact damage and impact energy absorbed by the structure.     

Bird strike damage analysis in aircraft structures using Abaqus/Explicit and coupled Eulerian Lagrangian approach

The subject of this paper is numerical prediction of bird strike induced damage in real aeronautical structures using highly detailed finite element models and modern numerical approaches. Due to the complexity of today’s aeronautical structures, numerical damage prediction methods have to be able to take into account various failure and degradation models of different materials. A continuum damage mechanics approach has been employed to simulate failure initiation and damage evolution in unidirectional composite laminates. Hashin’s failure initiation criteria have been employed in order to be able to distinct between four ply failure modes. The problem of soft body impacts has been tackled by applying the Coupled Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. This improvement in impactor deformation modelling resulted in a more realistic behaviour of bird material during impact. Numerical geometrical and material nonlinear transient dynamic analyses have been performed using Abaqus/Explicit. The main focus of the work presented in this paper is the application of the damage prediction procedure in damage assessment of bird impact on a typical large airliner inboard flap structure. Due to the high cost of gas-gun testing of aircraft components, experimental testing on the real flap structure could not have been performed. In order to evaluate the accuracy of the presented method, the bird and composite damage model have been validated against experimental data available in the literature.     

State-of-the-Art Finite Element Modeling of Rotorcraft Main Rotor Blade for Bird Strike Damage Analysis

This paper demonstrates the state-of-the-art composite modeling methodology to investigate the bird strike phenomenon using available numerical bird models through experimental tests and simulation tools. The present work is based on the application of nonlinear explicit finite element analysis to simulate the response of rotorcraft main rotor blade root end under high velocity impact load. The damage behavior of blade made of composites under soft body impact depends upon bird size, blade size, blade span-wise location of impact and bird orientation with respect to hitting location, blade rotational speed and rotorcraft cruise speed. Bird model is considered as hydrodynamic with length-to-diameter ratio of 2. A bird strike event is characterized by loads of high intensity and short duration. A transient explicit nonlinear finite element-based impact analysis using Autodyn has been carried out to predict bird strike resistance to withstand 1.0 kg bird at critical flight condition. Numerical analysis indicates that blades do not tear which agrees well with the physical test conducted.     

Modelling of impact forces and pressures in Lagrangian bird strike analyses

The paper aims at evaluating and improving the accuracy of bird impact numerical analyses performed with finite element explicit codes, focusing on the modelling of the spatial and temporal pressure distributions exerted on the target by the impacting body. A Lagrangian approach is adopted, interfacing the ESI/Pam-Crash solver code with an automatic trial-and-error procedure for the elimination of the excessively distorted elements. The theoretical formulation relevant to the impact of a cylindrical soft body against a rigid target is reviewed and this idealised case is adopted to validate the presented approach with increasingly refined finite element schemes. A sensitivity study is then carried out, adopting differently shaped bird models and varying the material hydrodynamic and deviatoric responses. A set of models is selected comparing the results with the experimental average values and the scattering reported in literature for the most significant loading parameters in impacts on rigid targets. The model shape and the calibration parameters of the bird material used in these models are subsequently adopted in the analyses of impacts on a deformable polycarbonate plate. The numerical results obtained with increasingly refined bird models are presented and discussed. A range of modelling parameters is finally suggested to perform reliable numerical analyses on aircraft structures and a criterion is proposed to select the models for a reasonably conservative approach to the design of a bird proof structure.     

Modelling fabric reinforced composites under impact loads

The paper describes recent progress on the materials modelling and numerical simulation of the in-plane response of fibre reinforced composite structures. A continuum damage mechanics model for fabric reinforced composites under in-plane loads is presented. It is based on methods developed for UD ply materials (Compos. Sci. Technol., 43 (1992) 257), which are generalised here to fabric reinforcements. The model contains elastic damage in the fibre directions, with an elastic–plastic model for inelastic shear effects. Test data on a glass fabric/epoxy laminate show the importance of inelastic effects in shear. A strategy is described for determining model parameters from the test data. The fabric model is being implemented in an explicit FE code for use in crash and impact studies and preliminary results are presented on a plate impact simulation.     

Aircraft impact analysis of nuclear safety-related concrete structures: A review

Safety-related nuclear structures, including concrete nuclear containment vessels, constructed before September 11, 2001, were not purposely designed to resist any impact loading greater than a light aircraft crash. Since 2001, safety of nuclear facilities against a deliberate or accidental large civilian aircraft impact has drawn much attention worldwide. However, current design guides for nuclear structures provide limited information on analysis methodologies for such aircraft impact. This document presents basic general knowledge required to analyze concrete structures for an aircraft impact and provides a summary of available numerical and experimental investigations that may be used to benchmark an aircraft impact simulation. The methodologies available for an aircraft impact analysis are overviewed with an emphasis on structural damage analysis. Constitutive models used for concrete, reinforcing steel, and composite materials are addressed in this paper.     

Hybrid approach in bird strike damage prediction on aeronautical composite structures

This paper deals with the problem of numerical prediction of bird strike induced damage on aeronautical structures. The problem of soft body impacts has been tackled by applying a hybrid Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. Eulerian modeling of the bird impactor resulted in a more realistic behavior of bird material during impact, which has lead to an enhanced response of the impacted structure. The work presented in this paper is focused on damage modeling in composite items of aeronautical structures. The bird impactor model and damage modeling approaches have been validated by comparison with experimental gas gun results available in the open literature, while the complete damage prediction procedure has been demonstrated on a complex airplane flap structure finite element model.     

Hybrid approach in bird strike damage prediction on aeronautical composite structures

This paper deals with the problem of numerical prediction of bird strike induced damage on aeronautical structures. The problem of soft body impacts has been tackled by applying a hybrid Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. Eulerian modeling of the bird impactor resulted in a more realistic behavior of bird material during impact, which has lead to an enhanced response of the impacted structure. The work presented in this paper is focused on damage modeling in composite items of aeronautical structures. The bird impactor model and damage modeling approaches have been validated by comparison with experimental gas gun results available in the open literature, while the complete damage prediction procedure has been demonstrated on a complex airplane flap structure finite element model.     

Numerical simulation of impact tests on GFRP composite laminates

The design of advanced composite structures or components subjected to dynamic loadings requires a deep understanding of the damage and degradation mechanisms occurring within the composite material. The present paper deals with the numerical simulation of low-velocity impact tests on glass fabric/epoxy laminates through the LS-DYNA Finite Element (FE) code. Two laminates of different thickness were subjected to transverse impact at different energy levels and modeled by FE. Solid finite elements combined with orthotropic failure criteria were used to model the composite failure and stress based contact failure between plies were adopted to model the delamination mechanism. The final simulation results showed a good correlation with experimental data in terms of both force–displacement curves and material damage.     

冰雹冲击复合材料层合板仿真研究

针对飞机空中受冰雹撞击会造成复合材料结构分层或损伤问题,用有限元软件ANSYS/LS-DYNA对复合材料的抗冰雹冲击行为进行分析。采用光滑质点流体动力学方法(Smooth Particle Hydrodynamic,SPH)模拟冰雹冲击刚性平板的高度非线性力学行为。通过对比模拟结果与实验数据知两者吻合较好,验证冰雹模型的准确性。将该模型引入冰雹冲击复合材料结构模型,采用粘聚区模型(Cohesive Zone Model,CZM)预测复合材料结构的分层损伤,获得合理计算结果;并分析冰雹撞击层合板损伤情况及不同参数对层合板损伤影响。     

Numerical modelling of crack growth profiles in integral skin-stringer panels

Integral (monolithic) structures can play a significant role in high efficiency structural design. According to the current technological manufacture methods, integral structures have an impact on fabrication cost and on weight reduction. However, until now, some critical aspects have limited the use of these structures. Conventional structures with mechanical or chemical (by adhesion) joints are advantageous because of their damage tolerant and fail safe behaviour. The presence of two separate parts, skins and stringers, guarantee the structural integrity of the component when propagating defects and cracks are present and are thus the key factor in aircraft structures. Focusing our attention on aircraft related structures, the aim of this paper is to show the application of numerical methodologies to evaluate the behaviour of integrally machined skin-stringer panels in the presence of propagating cracks. The described activity resulted from the “Analytical Round Robin on Crack Growth and Residual Strength Prediction in Integral Structures” proposed by ASTM Task Group E08.04.05 while different FE approaches for a single type integral panel with a propagating crack have been introduced in this paper. The crack growth evaluation based on the numerical models agrees well with the experimental results.     

Numerical simulation of bird strike damage prediction in airplane flap structure

Numerical analyses of bird impact damage in complex aircraft structures have been performed using ABAQUS/Explicit. A Lagrangian formulation was used for the bird model in combination with various material models. Several failure and damage modes have been considered for different material models used in the inboard flap of a typical large transport aircraft. A submodeling approach has been used to reduce computational time. Parametric analyses have been performed using different bird sizes, impact locations and velocity vectors.     

Numerical/experimental impact events on filament wound composite pressure vessel

Impacts on pressure vessels, produced by winding glass fibre with vinyl ester resin over a polyethylene liner, were numerically and experimentally investigated in the current work.Pressure vessels were experimentally tested under low velocity impact loads. Different locations and incident energies were tested in order to evaluate the induced damage and the capability of the developed numerical model.An advanced 3-D FE model was used for simulating the impact events. It is based on the combined use of interlaminar and intralaminar damage models. Puck and Hashin failure theories were used to evaluate the intralaminar damages (matrix cracking and fibre failure). Cohesive zone theory, by mean of cohesive elements, was used for modelling delamination onset and propagation.The experimental impact curves were accurately predicted by the numerical model for the different impact locations and energies. The overall damages, both intralaminar and interlaminar, were instead slightly over predicted for all the configurations.The model capabilities to simulate the low velocity impact events on the full scale composite structures were proved.     

Prediction of impact damage on sandwich composite panels

Sandwich structures are extensively employed in the aerospace and automobile industries. The understanding of their behaviour under impact conditions is extremely important for the design and manufacturing of these engineering structures since impact problems are directly related to structural integrity and safety requirements. This paper investigates the damage behaviour of composite sandwich panels with aramid paper honeycomb (NOMEX) and polyetherimide (PEI) foam cores under transverse impacts at high velocities. A numerical model was developed using the dynamic explicit finite element (FE) structure analysis program PAM-CRASH. For both sandwich structures numerical analysis reproduces physical behaviour observed experimentally in high velocity impact tests.     

Structural collapse analysis of framed structures under impact loads using ASI-Gauss finite element method

The main objective of this study is to devise a technique, which, when implemented into finite-element codes, is efficiently applicable to impact collapse analyses of framed structures. In this study, the formerly developed adaptively shifted integration (ASI) technique for the linear Timoshenko beam element is modified into the ASI-Gauss technique by placing the numerical integration points of the two consecutive elements forming an elastically deformed member in such a way that stresses and strains are evaluated at the Gaussian integration points of the two-element member. On comparison with the ASI technique, the ASI-Gauss technique proves its higher accuracy and efficiency in elastic range. Moreover, instead of applying impact loads in the form of nodal forces, we consider the impact phenomenon by means of contacts between the elements involved and the elemental contact algorithm is verified from the point of conservation of energy. Impact analyses considering member fracture with different sets of parameters are performed using a high-rise framed structure and a small aircraft. From the results obtained, we can observe propagation phenomena of impact loads and shock waves. Also, a proper difference in impact damage is obtained by different sets of parameters. The results also indicate that the mass of the aircraft has a stronger influence on impact damage than its velocity. Moreover, soon after impact, tensile stresses are observed in the columns that were compressed by dead loads before impact.     

Simulation of High Velocity Impact on Composite Structures - Model Implementation and Validation

High velocity impact on composite aircraft structures leads to the formation of flexural waves that can cause severe damage to the structure. Damage and failure can occur within the plies and/or in the resin rich interface layers between adjacent plies. In the present paper a modelling methodology is documented that captures intra- and inter-laminar damage and their interrelations by use of shell element layers representing sub-laminates that are connected with cohesive interface layers to simulate delamination. This approach allows the simulation of large structures while still capturing the governing damage mechanisms and their interactions. The paper describes numerical algorithms for the implementation of a Ladevèze continuum damage model for the ply and methods to derive input parameters for the cohesive zone model. By comparison with experimental results from gas gun impact tests the potential and limitations of the modelling approach are discussed.     

Implication of unequal rivet load distribution in the failures and damage tolerant design of metal and composite civil aircraft riveted lap joints

Riveted lap joints are being used widely in civil aircraft structures. Conventional design procedures assume that the joint can be designed as if all rivets carry load equally. As found in literature associated with fatigue and fracture, forensic studies on structural failures, this assumption is not entirely valid. In this paper, the regulatory codes for civil aircraft as applicable to riveted joints in the form of FAR 25 regulations are briefly reviewed. The regulatory code discusses safety factors in an implied way, but has little specific recommendations for riveted joints. However, studies on the failures of specific aircraft illustrated in this paper add to the argument that both static strength and life are affected by the initial design procedures for the riveted joints. In this paper, finite element models for metal–metal, composite–metal, composite–composite lap joints are studied. A three row lap joint used in commercial aircraft and which was part of failure studies is also examined. Unequal rivet loads and in cases, nearly 50% more than conventional design has been seen in linear finite element analysis. Elasto-plastic analysis using rivet flexibility shows re-distribution of loads. Based on these observations, the effect of rivet loads on life estimation including the use of concepts such as by-pass stresses is discussed. These results have implications for static strength at ultimate load, damage tolerance and fail safety and are discussed in this paper. Next, in a composite–composite lap joint, the influence of ply-angle on the rivet loads is studied. Also, a composite–metal lap joint is studied for the rivet load distribution and life estimation. It is found that the load shared by the rivet rows in a composite–metal lap joint are not symmetric and therefore are more susceptible to cracking and subsequent failure as the unequal distribution can cause some of the rivet loads to be high. In conclusion, the issue of fail safe and damage tolerant design of civil aircraft structures with riveted joints are addressed, especially the implication of unequal load distribution on the failures of such joints and it is suggested that these unequal rivet load distributions be catered for at the early design stage itself via finite element analysis and the possibility of an over-arching safety factor could be considered that incorporates both ultimate load and damage tolerance conditions.     

Development and Validation of a Novel Bird Strike Resistant Composite Leading Edge Structure

A novel design of a fibre-reinforced composite Leading Edge (LE) of a Horizontal Tail Plain (HTP) is proposed. The development and validation approach of the innovative composite LE structure are described. The main design goal is the satisfactory impact resistance of the novel composite LE in the case of bird strike. The design concept is based on the absorption of the major portion of the bird kinetic energy by the composite skins, in order to protect the ribs and the inner LE structure from damaging, thus preserving the tail plane functionality for safe landing. To this purpose, the LE skin is fabricated from specially designed composite panels, so called ‘tensor skin’ panels, comprising folded layers, which unfold under the impact load and increase the energy absorption capability of the LE. A numerical model simulating the bird strike process is developed and bird strike experimental testing is performed, in order to validate the proposed layout and prove the capability of the structure to successfully withstand the impact loading. The numerical modelling issues and the critical parameters of the simulation are discussed. The present work is part of the European Aeronautics Research Project, ‘Crashworthiness of aircraft for high velocity impact – CRAHVI’ [1].