Numerical study of the structural behaviour of impacted composite laminates subjected to compression load

Composite materials allow all the benefits which a high specific strength involves, in a design process their application involves many critical problems. Currently, these problems, such as environmental conditions, notch sensitivity, damaging under low velocity impacts, are taken into account by means of the application of conservative design safety factors regarding the ultimate tensile strength. In order to try to reduce these safety factors, this work aimed to study and to understand the impact damage growing mechanisms due to compression loads. To this purpose, compression tests have been experimentally performed on composite panels, which have been previously subjected to low velocity impact phenomena, considering impact energies of 6 J, 10 J and 13 J respectively. Moreover, numerical model able to simulate Low Velocity Impacts (LVI) and Compression After Impacts (CAI) onto CFRP panels is proposed. A single explicit finite element analysis has been carried out by using the Abaqus^® finite element code; the need to build a numerical model, which allows simulation in only one analysis both LVI and CAI steps, depends on the difficulty to import the impact damage distribution into a separate compression analysis. In fact, in only one analysis the compression step can occur directly onto the impacted plate, which allows to consider the effective impact damage distribution as the starting configuration for quasi static analysis under operating loads.     

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.     

Numerical investigation of onset and evolution of LVI damages in Carbon–Epoxy plates

In order to study the onset and the evolution of low velocity impact damages in Carbon–Epoxy plates, a numerical investigation has been led. A detailed finite element model has been created by using the finite element code Abaqus® which, thanks to the different implemented algorithms, allowed considering both intra-laminar and inter-laminar failure criteria.In particular, the numerical modelling technique of such failure criteria allowed predicting delamination growth, by using special purpose-elements (cohesive elements) and fiber and matrix failure, by using Hashin criteria.Moreover, with the aim to reduce the required CPU time, a global/local finite element modelling approach has been proposed.For validation purpose, numerical results have been compared with data from two sessions of experimental impact tests. The considered impact energy values are 6 J, 10 J and 13 J respectively.     

A robust numerical approach for the simulation of skin–stringer debonding growth in stiffened composite panels under compression

In this paper, a numerical study on skin–stringer debonding growth in stiffened composite panels has been carried out. A novel numerical methodology is proposed here to investigate the compressive behaviour of a stiffened composite panel in the presence of skin–stringer partial separation. The novel numerical methodology, able to overcome the mesh size and time increment dependency of the standard Virtual Crack Closure Technique (VCCT), is an evolution of a previously developed and tested numerical approach for the circular delaminations growth. The enhancements, with respect to the previously developed approach, rely mainly in the capability to deal with the different defect shapes characterising a skin–stringer debonding. The proposed novel methodology has been implemented in a commercial finite element platform and tested over single stiffener composite panels. The effectiveness of the suggested numerical methodology, in predicting the compressive behaviour of stiffened panels with skin stringer debondings, has been preliminary confirmed by comparisons, in terms of load versus applied displacement and debonding size at failure, with literature experimental data and numerical results obtained with the standard VCCT approach.     

A progressive failure model for mesh-size-independent FE analysis of composite laminates subject to low-velocity impact damage

An original, ply-level, computationally efficient, three-dimensional (3D) composite damage model is presented in this paper, which is applicable to predicting the low velocity impact response of unidirectional (UD) PMC laminates. The proposed model is implemented into the Finite Element (FE) code ABAQUS/Explicit for one-integration point solid elements and validated against low velocity impact experimental results.     

Numerical evaluation for debonding failure phenomenon of adhesively bonded joints at cryogenic temperatures

In the present study, material characteristics, such as inelastic constitutive behaviour and debonding failure, of an adhesively bonded joint (ABJ) at cryogenic temperature have been evaluated using a computational approach. The modified Bodner-Partom model (BP model) has been introduced to describe the material nonlinearities of ABJ. The Gurson-Tvergaard model (GT model) has also been implemented into the constitutive model in order to analyse the phenomenon of debonding failure. An ABAQUS user-defined subroutine UMAT is developed using a damage-coupled constitutive model based on an implicit formulation. The numerical results are compared with a series of lap shear tests of ABJ at cryogenic temperature in order to verify the proposed method.     

Analytical and experimental studies on the low-velocity impact response and damage of composite laminates under in-plane loads with structural damping effects

In this paper, low-velocity impact response and damage of composite laminates under in-plane loads are analytically and experimentally investigated. The authors recently proposed a modified displacement field of plate theory, considering the effect of initially loaded in-plane strain, and used a finite element program to analyze the structural behavior of the composite laminate. In this study, the program is upgraded to account for the structural damping effect of the laminate. A pendulum type impact test system and an in-plane loading fixture are constructed for the experimental study. The analytical and experimental impact behaviors are compared at different impact energy levels for cases with an initial in-plane tensile load and a compressive load, as well as cases without the initial in-plane load. The results show good correspondence between the analytical and experimental impact force histories. The effect of the initial in-plane load reduces for higher impact energies. The numerical estimation of the damaged area is in good agreement with the results from C-scanning experiments.     

Behaviour of multiple composite plates subjected to ballistic impact

An experimental and numerical investigation has been carried out to study the behavior of single and multiple laminated panels subjected to ballistic impact. A pressurized air gun is used to shoot the impactor, which can attain sufficient velocity to penetrate all the laminates in a multiple laminated panel. The incidental and residual velocity of the impactor is measured to estimate the energy absorption in the impact process. The commercially available code ABAQUS has been used for the numerical simulation where the impactor has been modeled as a rigid body and the laminates have been modeled with a simple shell element. A user material model based on a continuum damage mechanics concept for failure mechanism of laminated composites has been implemented. Experimental tests showed that the numerical model could satisfactorily predict the energy absorption. Most interestingly, it has clearly demonstrated a feasible phenomenon behind counterintuitive experimental results for the multiple laminated panels.     

Interfacial damage in biopolymer composites reinforced using hemp fibres: Finite element simulation and experimental investigation

The present study aims at considering the effect of interfacial damage on the mechanical performance of a starchy composite reinforced using hemp fibres. Mechanical behaviour is approached experimentally using tensile testing coupled to digital image acquisition. Thermomoulded samples with single fibres are designed to allow sample testing perpendicularly to the direction of fibre alignment. Experimental evidence of localised damage is then highlighted in the elasticity stage. Finite element computation is attempted to explain the observed damage using an adequate mechanical model that considers weak adhesion between phases and dynamic evolution of damage. Predicted results show that the FE model is able to reproduce the observed behaviour suggesting that local damage evolution is a serious mechanism affecting the performance of the studied composite.     

Mitigating performance of elastic graded polymer foam coating subjected to underwater shock

The possible mitigating effect of elastic Density Graded Polymer Foam (DGPF) coating on the marine structure subjected to underwater shock is investigated. A 1-D unified nonlinear finite element model based on the updated Lagrangian frame is built to solve both the transient response of foam coated structure and dynamic cavitation of water near fluid–structure interface. The mitigating effect of DGPF coating with respect to design parameters such as average density, density difference (uneven density), gradient functions and load intensity is explored. It is illustrated that DGPF is superior in underwater shock protection to the equivalent uniform foam if the foam density is properly distributed while load density is not so high. Lower density foam in the water side is helpful to reduce the total impulse transmitted from water. But the total energy absorption capability may be discounted as the coating enters densification phase earlier.     

The low velocity impact response of foam-based sandwich panels

This paper describes the results of a combined experimental/numerical study to investigate the perforation resistance of sandwich structures. The impact response of plain foam samples and their associated sandwich panels was characterised by determining the energy required to perforate the panels. The dynamic response of the panels was predicted using the finite element analysis package ABAQUS/Explicit. The experimental arrangement, as well as the FE model were also used to investigate, for the first time, the effect of oblique loading on sandwich structures and also to study the impact response of sandwich panels on an aqueous support.     

Dynamic response of full-scale sandwich composite structures subject to air-blast loading

Glass-fibre reinforced polymer (GFRP) sandwich structures (1.6 m × 1.3 m) were subject to 30 kg charges of C4 explosive at stand-off distances 8–14 m. Experiments provide detailed data for sandwich panel response, which are often used in civil and military structures, where air-blast loading represents a serious threat. High-speed photography, with digital image correlation (DIC), was employed to monitor the deformation of these structures during the blasts. Failure mechanisms were revealed in the DIC data, confirmed in post-test sectioning. The experimental data provides for the development of analytical and computational models. Moreover, it underlines the importance of support boundary conditions with regards to blast mitigation. These findings were analysed further in finite element simulations, where boundary stiffness was, as expected, shown to strongly influence the panel deformation. In-depth parametric studies are ongoing to establish the hierarchy of the various factors that influence the blast response of sandwich composite structures.