Fibre metal laminates subjected to preload and low velocity impact
Creators: name: Rathnasabapathy, Minoo
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400101
subject: Aerospace materials
classificationCode: 400102
subject: Aerospace structures
classificationCode: 400199
subject: Aerospace engineering not elsewhere classified

description: This PhD thesis presents an original research study into the dynamic response, damage and failure of Fibre Metal Laminates (FMLs) when subject to low velocity impact events. FMLs are currently used in a variety of aerospace structures, most notably fuselage sections to the Airbus 380 aircraft, which may experience point impact loading from bird strike, large hailstones or other localised damaging events. Therefore, understanding the impact behaviour of FMLs is essential for damage-tolerant design and structural integrity assessment.<br><br>This PhD project aims to develop a physics-based finite element model capable of accurately predicting the impact response of FMLs under unloaded and pre-loaded conditions (tension and compression) as well as the post-impact mechanical properties. The model is validated using data obtained from a comprehensive suite of low velocity impact tests performed on several types of FMLs representative of the materials used in aircraft structures. The model is shown capable of analysing all the critical damage modes of FMLs at the intra-ply and ply-based levels. This includes predicting the sequence of damage initiation and progression as well as the interactions between the various damage modes. Such a high fidelity model for analysing the impact response of FMLs has not been previously reported.<br><br>A comprehensive literature review into published research into the impact response of FMLs is presented in the PhD thesis. The review reveals that while numerous studies have assessed the low velocity impact response of FMLs, most studies have been experimental. However, experimental studies are usually limited to quantification of damage after the impact event, and lack the ability to detect in-real-time the interior damage to FMLs. Several numerical models have been developed to analyse the impact damage to FMLs, however due to the layered structure and different material properties of the individual metal and composite constituents, the existing FE models lack the capability to analyse all possible impact damage mechanisms.<br><br>A research study is presented in the PhD thesis into the FE analysis of FMLs subjected to impact while in a stress-free (unloaded) state. A full description of the FE model is presented, including its capability to predict the deformation and damage to the individual metal and composite layers and the interfacial regions between the layers. The FE modelling methodology takes into account three modelling parameters; namely the choice of element, material and mechanical behaviour, and relevant failure criteria for each of the FML constituents as well as the interfaces. Appropriate geometric dimensions and boundary conditions were defined to represent the experimental impact test conditions used for validation. The numerical accuracy of the FE model is assessed using data obtained from low velocity impact tests performed on two types of FML configurations: (i) FML 1 2/1-0.4 and (ii) FML 2 3/2-0.4 consisting of thin layers of aluminium alloy and glass-epoxy composite. The FML configurations were chosen to replicate two commercially available GLARE materials. Impact tests were performed using an instrumented drop-test impact rig fitted with a spherical steel impactor. Different impact energies were achieved by adjusting the impactor drop height. Data obtained from the experimental tests include the dynamic response, damage profile and damage severity to the FMLs for a range of impact energy levels. The FE model is found capable of predicting such impact parameters as the impact force-time response, dent depth, deformation and cracking to the aluminium layers, intraply and interlaminar cracking to the composite layers, and interfacial cracking between the metal and composite layers. The FE model also predicted with good accuracy the increases to the different types of damage with increasing impact energy.<br><br>The literature review revealed that virtually all experimental and numerical studies into FMLs subject to low velocity impact have been conducted under stress-free conditions, which is not representative of real-world aircraft applications when loads are exerted on FML structures. The FE modelling methodology was therefore extended to predict the impact response of FMLs under tension and compression pre-load conditions. Experimental impact-under-load tests were performed on FMLs to validate the FE model.<br><br>The research revealed that tension preloading of FMLs has a significant influence on the impact response due to the increase in rigidity and overall flexural stiffness. The FE model revealed that the contact duration of the impact event was reduced, and this allows impact-induced stress waves to propagate faster through FMLs causing damage to initiate earlier than the stress-free condition. Simulations and analyses performed using the FE model also revealed that the initiation and severity of impact damage modes such as plastic deformation, fibre failure, adhesive failure and delamination increased with the tension preload level exerted on the FMLs. Under tensile preload, intralaminar and interlaminar damage to the composite layers propagated further, thereby extending the visible damage region.<br><br>The application of a compressive preload gave rise to a ‘softening effect’ that reduces the flexural stiffness of the FMLs. The FE model predicted that interlaminar and intralaminar damage would develop in a more localised region directly under the impact site, and this was confirmed by experimental impact tests. The FE model provided new insights into the evolution of some damage modes after the impact event due to an ‘additional buckling effect’ as the FMLs exposed to compressive loads as the impactor rebounds.<br><br>This PhD project also involved a comprehensive experimental investigation into the compression-after-impact (CAI) properties and damage tolerance of FMLs subject to low velocity impact. The research revealed that the impact damaged FMLs experience significant delamination and debonding under axial compression loading, and this created multiple sub-laminates within the materials. As the buckling stress of any sub-laminate is lower compared to the intact material, the FMLs underwent sub-laminate buckling. The combination of local buckling and sub-laminate buckling during CAI testing reduced the residual compressive failure stress of the FMLs. Aluminium cracking and fibre failure were also identified as critical damage mechanisms that reduced the CAI strength of FMLs.<br><br>Significant research outcomes have been produced as a result of this PhD project. The development of a new FE analysis modelling methodology, extensively validated using experimental results, has provided novel insight into all critical damage modes, the sequence in which they occur, and their dynamic interactions under stress-free and stress-loaded conditions. Research into the damage tolerance of FMLs and assessment of the level of degradation due to impact-induced damage modes provides a full picture of the effect of critical damage modes that influence the CAI strength of FMLs.