Finite element simulation of flat nose low velocity impact behaviour of carbon fibre composite laminates.

The work detailed in this thesis includes an extensive finite element simulation of low velocity impact behaviour of carbon fibre reinforced laminated composite panels subjected to flat and round nose impacts. Carbon fibre composites are being widely used in aerospace structures due to their high strength and stiffness ratios to weight and potential to be tailored for structural components. However, wing and fuselage skins are vulnerable to foreign object impacts during manufacturing and service from tools maintenance tools and tool box drops. Such impacts particularly from flat nose tool drops inflict internal damages that are difficult to detect through routine inspections. The internal damage (barely visible) may cause severe degradation of material properties and reduction in compressive strength that might lead in unexpected catastrophic failures. Such failures result in loss of human lives and structural assets. That is a major concern for the aircraft industry. Most of the existing research is based on damage detection and control to improve integrity of structures so that an aircraft could reach nearest safe place to avoid failure after damage is detected. It is very difficult to evaluate overall structural congruity and performance as structural degradation and damage progression occur after impacts. The impact is a dynamic event which causes concurrent loading and re-distributions of stresses once a ply fails or damage occurs. Most of the reported studies are based on physical experiments which are expensive, time consuming, and limited. The efficient way to predict performance evaluation of an impacted structural component is through integrated computer codes that couple composite mechanics with structural damage and failure progressions. Computational models can be very useful in simulating impact events, interpreting results, and making available the fast predictive tools for pre-design analysis and post-impact damage evaluations. This investigation is primarily simulation based that integrates experimentally and numerically evaluated impact response of laminates manufactured by AircelleTM Safran Ltd and HexcelTM Composites. Literature review and basic mathematical formulations relevant to the impact of composite laminates were commented. Pre-assumed damage induced static load-deflections simulation using “PTC Creo SimulateTM” were carried out for eight,sixteen, and twenty four ply laminates subjected point, low, medium, and large nose impacts. The methodology used was based on previous experimental studies that assumed that impact damage provides the same stress concentration effect as crack, regions of degraded materials, or softer inclusions. The same assumptions were incorporated into simulation by inserting pre-assumed damage zones equivalent to impactors’ nose tips with within the volume of the laminates. Damage initiation, growth, and accumulation were investigated in terms of real scenarios with reference to the undamaged specimens. Several internal damage mechanisms were analysed via damage shift in multiple locations throughout the laminates’ volumes. The simulations can be useful to predict and correlate information on: existence, type, location, and extent of the damage in the impacted system to applied load. With this information and the loads applied to the system, measures can be proposed to reduce adverse effects of the impact induced damage. The compressive residual strength after impact was predicted via buckling simulation models. The in-plane buckling analysis was implemented into PTC Creo SimulateTM. Effects of the pre-assumed damage ply, damage zone, and coupled damaged-ply with damage zone were investigated via through-thickness re-locations. Critical buckling load and mode shapes were predicted with reference to the mid-surface of the laminates. Local buckling was simulated by introducing damage in a single ply adjacent to surface of the sub-laminate. Cases of pre-assumed delaminated ply from top to the mid-surface were simulated. Cases of mix-mode buckling analysis were simulated from coincident and combined effects of pseudo damaged ply as well as damage zones. The simulations predicted information can be useful to predict useful life remains (prognosis) of the damaged system. The information could also be useful to predict residual strength of similar cases at the beginning from material level, loading scenarios, damage progression to component and system level at various rates. Material property characterization tests were conducted to verify the industry provided input and used in drop-weight impact simulations. The properties were augmented with micro-macro mechanics formulations to approximate full range of engineering constants from reliably determined Young’s modulus. The drop-weight simulation of eight, sixteen, and twenty four ply specimens of quasi-isotropic lay-ups having different thicknesses and impacted from round and flat nose impactors were implemented in the ABAQUSTM software using explicit dynamic method. Two independent models were implemented in the software. The first model simulates displacement, velocity, and acceleration quantities. The second model computes in-plane stresses required to efficiently evaluate 3D stresses. The in-plane stress quantities were numerically integrated through-the-thickness utilising equilibrium to evaluate ply-by-ply through-thickness stresses. The evaluated stress values were then utilised in the formulation set of advanced failure criteria to predict damage progression and failure modes. Simulation produced results were compared and verified against experiment produced data. Drop-weight impact tests were conducted to verify the selected simulation produced results. Non-destructive techniques and advanced data filtering algorithms available in MATLABTM were utilized to filter the noisy data and predict damage zone and load threshold. The selected simulation results were compared against the experimental data, intra simulation results, and the results available in the literature and have to agree up to 90%. The investigation concludes that the simulation models could efficiently predict low velocity impact response of variety of carbon fibre composite panels that could be useful for design development with reduced testing