Alastair Johnson

Computational methods for predicting impact damage in composite structures

This paper describes recent progress in materials modelling and numerical simulation of the impact response of fibre-reinforced composite structures. A continuum damage-mechanics (CDM) model for fabric-reinforced composites is developed as a framework within which both in-ply and delamination failure may be modelled during impact loading. Damage-development equations are derived and appropriate materials parameters determined from experiments. The CDM model for in-plane failure has been implemented in a commercial explicit finite element (FE) code, and new techniques are used to model the laminate as a stack of shell elements tied by contact interface conditions. This approach allows the interlaminar layers to be modelled and strength reduction due to delamination to be represented; it also provides a computationally efficient method for the analysis of large-scale structural parts. The code is applied to predict the response of carbon-fabric-reinforced epoxy plates impacted at different velocities by a steel impactor. A comparison of structural response and failure modes from numerical simulations and impact tests is given which shows a good agreement for the prediction of delamination damage at low impact energies and fracture and penetration at higher impact energies.

Modelling Soft Body Impact on Composite Structures

The paper describes recent progress on materials modelling and numerical simulation of soft body impact damage in fibre reinforced composite structures. The work is based on the application of finite element (FE) analysis codes to simulate composite shell structures under impact loads arising, for example, from bird strike on a wing leading edge. A composites ply damage mechanics model and interply delamination model have been implemented in an explicit FE code which is used to predict impact damage in shell structures. Soft body impactors such as gelatine (substitute bird) or ice (hailstones) are highly deformable on impact and flow over the structure spreading the impact load. They are modelled by a particle method in which the FE mesh is replaced by interacting particles. The failure models and code developments are applied to the numerical simulation of gas gun impact tests in which gelatine projectiles are fired at glass fabric/epoxy cylindrical shells.

Modelling fabric reinforced composites under impact loads

Abstract 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.

Experimental Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

This paper presents quasi-static and dynamic modelling of bolted composite structures using the explicit finite element code PAM-CRASH. User controlled point link (PLINK) elements were investigated for modelling the bolted composite joints used in the structures. Simulation results were compared with quasi-static and dynamic structural testing reported previously. Two loading configurations were considered. It was shown that the PLINK element modelling approach agreed well with the experimental results for both loading configurations and for one case offered significant improvements over other simplified bolt modelling methods. A stacked shell modelling approach was used to model the interlaminar delamination damage present in the ball-loaded impact mode. The overall response of the structure was significantly improved by the addition of these energy absorbing interfaces.

Failure mechanisms in energy-absorbing composite structures

Quasi-static tests are described for determination of the energy-absorption properties of composite crash energy-absorbing segment elements under axial loads. Detailed computer tomography scans of failed specimens were used to identify local compression crush failure mechanisms at the crush front. These mechanisms are important for selecting composite materials for energy-absorbing structures, such as helicopter and aircraft sub-floors. Finite element models of the failure processes are described that could be the basis for materials selection and future design procedures for crashworthy structures.

Analysis of Crushing Response of Composite Crashworthy Structures

The paper describes quasi-static and dynamic tests to characterise the energy absorption properties of polymer composite crash energy absorbing segment elements under axial loads. Detailed computer tomography scans of failed specimens are used to identify local compression crush failure mechanisms at the crush front. The varied crushing morphology between the compression strain rates identified in this paper is observed to be due to the differences in the response modes and mechanical properties of the strain dependent epoxy matrix. The importance of understanding the role of strain rate effects in composite crash energy absorbing structures is highlighted in this paper.

FEM/SPH Coupling Technique for High Velocity Impact Simulations

In this work, impact simulations using both meshfree Smooth Particle Hydrodynamics (SPH) and combined FEM/SPH Method were carried out for a sandwich composite panel with carbon fibre fabric/epoxy face skins and polyetherimide (PEI)foam and hybrid (Nomex/PEI foam) core. A numerical model was developed using the dynamic explicit finite element (FE) structure analysis program PAM-CRASH. The carbon fibre/epoxy facings were modelled with layered shell elements, whilst SPH particles replaced solid elements in the core. The efficiency and the advantages of pure meshfree SPH and combined FEM/SPH methods were demonstrated by comparing the core deformation modes and impact force pulses measured in the experiments to predicted structural impact response for a range of impact velocities.

Modelling Impact Damage in Composite Structural Elements

Publisher Summary This chapter reviews recent progress on composites materials modeling and numerical simulation of impact on fiber-reinforced composite structures from both hard and soft projectiles. To reduce certification and development costs, computational methods are required by the aircraft industry to be able to predict structural integrity of composite structures under high-velocity impacts from hard objects such as metal fragments, stone debris and from soft or deformable bodies such as birds, hailstones, and tire rubber. Key issues are the development of suitable constitutive laws for modeling composites in-ply and delamination failures, determination of composites parameters from dynamic materials tests, materials laws for deformable impactors, and the efficient implementation of the materials models into Finite Element [“FE”] codes. These problems involve both multi-scale and multi-physics modeling techniques. The multi-scale aspects arise because impact damage is localized and requires fine-scale modeling of delamination and ply damage at the micromechanics level, while the structural length scale is much larger. The multi-physics aspects arise in fluid-structure interactions when soft bodies such as gelatin or ice flow extensively on impact with the structure and require fluid modeling techniques.

A Stacked-Shell Finite Element Approach for Modelling a Dynamically Loaded Composite Bolted Joint Under in-Plane Bearing Loads

This paper presents the results of a study into a novel application of the “stacked-shell” laminate modelling approach to dynamically loaded bolted composite joints using the explicit finite element code PAM-CRASH. The stacked-shell approach provides medium-high fidelity resolution of the key joint failure modes, but is computationally much more efficient than full 3D modelling. For this work, a countersunk bolt in a composite laminate under in-plane bearing loading was considered. The models were able to predict the onset of damage, failure modes and the ultimate load of the joint. It was determined that improved debris models are required in order to accurately capture the progressive bearing damage after the onset of joint failure.

Failure Mechanisms and Energy Absorption in Composite Elements under Axial Crush

Test methods are presented to determine failure modes and energy absorption properties of composite crash structural elements from quasi-static tests on chamfered carbon fabric/epoxy tube segment specimens under axial compression loads. High speed film and CT scans of failed specimens are used to identify trigger mechanisms, failure mode evolution at the crush front and failure processes during steady crushing. FE models of failure were developed which could be the basis for materials selection and design procedures for crashworthy composite structures. These are based on meso-scale composites ply damage models combined with cohesive interfaces to represent delamination failures, which damage and fail when the interface fracture energy is reached. The models are implemented in an explicit FE code and parameters for the ply damage and delamination models were obtained from related materials test programmes. The FE models were applied to simulate axial crushing in tube segments and C-channels, showing good predictions of measured peak forces at failure initiation, steady crush forces and total energy absorption.