Development of a Degradation Model for the Collapse Analysis of Composite Aerospace Structures

For stiffened structures in compression the most critical damage mechanism leading to structural collapse is delamination or adhesive disbonding between the skin and stiffener. This paper presents the development of a numerical approach capable of simulating interlaminar crack growth in composite structures as a representation of this damage mecha-nism. A degradation methodology was proposed using shell layers connected at the nodes by user-defined multiple point constraints (MPCs), and then controlling the properties of these MPCs to simulate the initiation and propagation of delamination and disbonding. A fracture mechanics approach based on the Virtual Crack Closure Technique (VCCT) is used to detect growth at the delamination front. Numerical predictions using the degradation methodology were compared to experimental results for double cantilever beam (DCB) specimens to dem-onstrate the effectiveness of the current approach. Future development will focus on address-ing the apparent conservatism of the VCCT approach, and extending the application of the method to other specimen types and stiffened structures representative of composite fuselage designs. This work is part of the European Commission Project COCOMAT (Improved MA-Terial Exploitation at Safe Design of COmposite Airframe Structures by Accurate Simulation of COllapse), an ongoing four-year project that aims to exploit the large strength reserves of composite aerospace structures through more accurate prediction of collapse

Water impact of rigid wedges in two-dimensional fluid flow

A combined experimental and numerical investigation was conducted into impact of rigid wedges on water in two-dimensional fluid conditions. Drop test experiments were conducted involving symmetric rigid wedges of varying angle and mass impacted onto water. The kinematic behaviour of the wedge and water was characterised using high-speed video. Numerical models were analysed in LS-DYNA® that combined regions of Smoothed Particle Hydrodynamics particles and a Lagrangian element mesh. The analysis captured the majority of experimental results and trends, within the bounds of experimental variance. Further, the combined modelling technique presented a highly attractive combination of computational efficiency and accuracy, making it a suitable candidate for aircraft ditching investigations

Effects of bondline flaws on the damage tolerance of composite scarf joints

Scarf repairs to aircraft structures need to sustain design ultimate load in the presence of flaws due to manufacturing and impact by foreign objects, in order to demonstrate compliance with airworthiness regulations. This paper presents an investigation into the effect of disbonds on the load-carrying capacity of adhesively bonded scarf joints. Experiments were conducted on scarf joints containing disbonds of varying lengths. The results showed that the load-carrying capacity of scarf joints decreases with the size of the bondline flaw at a faster rate than the reduction in the effective bond area. Fractographic analysis showed that the fracture occurred in the composite matrix adjacent to the adhesive-composite interface, at a distance equal to a small fraction of ply thickness. Computational analyses using the virtual crack closure technique (VCCT) and the cohesive zone model (CZM) confirmed these experimental observations: model predictions using composite material properties were in better correlation with experimental results than those using adhesive properties. Furthermore, CZM is capable of predicting the effects of flaws of all sized being considered, while the VCCT model is only applicable to joints containing flaws greater than a certain size

Hierarchical surface features for improved bonding and fracture toughness of metal-metal and metal-composite bonded joints

Structural adhesive joints involving Selective Laser Melting (SLM) titanium bonded to titanium or to a composite material have significant potential for weight and cost saving in aerospace and other industries. However, the bonding potential of as-manufactured SLM titanium is largely unknown, and the use of additional hierarchical surface features has not been explored or characterised. Here we demonstrate with the use of SLM that a hierarchy of two surface features at different length scales can improve the fracture toughness of metal-metal and metal-composite bonded joints. At one length scale (10-15 μm), we established through fracture toughness testing that the intrinsic irregular roughness of the SLM surface maximises the bonding potential for both metal-metal adhesive joints (KIc=1.38 kJ/m2) and hybrid metal-composite co-cured joints (KIc =1.20 kJ/m2). We then combined this with surface features at a larger length scale (200 μm). For metal-composite joints, the use of groove surface features was found to deflect the crack path, which increased the fracture toughness of the joint by up to 50% for outward protruding grooves to a value of KIc=1.65 kJ/m2. We identified the rise in fracture toughness as a combination of an increase in the crack path length and a shift from pure mode I to a mixed-mode crack growth. We found that the relative contributions of these two factors were approximately equal. This work demonstrates that SLM-manufactured titanium can have significant advantages over conventional titanium for bonded joints. In comparison with conventional techniques, SLM surfaces can be used in adhesive bonds without the need for expensive and time-consuming surface preparation, and the design freedom allows for surface features that can significantly improve performance

Effect of disbonds on the fatigue endurance of composite scarf joints

The certification of scarf repairs requires that the repair is capable of handling flight loads in the presence of disbonds. This paper presents a study of the fatigue disbond growth behaviour of scarf joints. By determining the strain energy release rates of a disbond in a scarf joint subjected to a unit load, a predictive model based on linear elastic fracture mechanics is presented, which is shown to correlate well with experimental results. This method offers a promising technique for predicting the fatigue life of composite scarf joints with disbonds

The efficiency of ultra-high molecular weight polyethylene composite against fragment impact

This paper presents an experimental investigation into the ballistic resistance of ultra-high molecular weight polyethylene (UHMW-PE) composite, and compares its performance against a range of common metallic and composite armour materials. An extensive experimental program was conducted to determine the ballistic limit velocity (V<inf>50</inf>) of UHMW-PE composite against 12.7 and 20 mm fragment simulating projectiles (FSPs) for a wide range of thicknesses. For protection against these projectiles, UHMW-PE composite was found to be consistently more mass efficient than rolled homogeneous armour steel (RHA), high hardness armour steel (HHA), aluminium alloy 5059-H131, and polymer composites reinforced with aramid, glass or carbon fibres. In terms of armour space claim, UHMW-PE composite was found to be less efficient than both steel types and glass fibre-reinforced plastic, though it was comparable to aramid fibre-reinforced plastic, and was more efficient than aluminium 5059-H131 and carbon fibre-reinforced plastic. Scaling effects were observed that showed metals were more effective against smaller projectiles in terms of armour mass required to stop a given projectile kinetic energy. These effects were not observed to the same extent for UHMW-PE composite, giving rise to a higher UHMW-PE mass efficiency against larger projectiles

The effect of target thickness on the ballistic performance of ultra high molecular weight polyethylene composite

The ballistic performance of thick ultra-high molecular weight polyethylene (UHMW-PE) composite was experimentally determined for panel thicknesses ranging from 9 mm to 100 mm against 12.7 mm and 20 mm calibre fragment simulating projectiles (FSPs). Thin panels (similar to <10 mm thick) were observed to undergo large deflection and bulging, failing predominantly in fibre tension. With increased thickness the panels demonstrated a two-stage penetration process: shear plugging during the initial penetration followed by the formation of a transition plane and bulging of a separated rear panel. The transition plane between the two penetration stages was found to vary with impact velocity and target thickness. These variables are inter-related in ballistic limit testing as thicker targets are tested at higher velocities. An analytical model was developed to describe the two-stages of perforation, based on energy and momentum conservation. The shear plugging stage is characterised in terms of work required to produce a shear plug in the target material, while the bulging and membrane tension phase is based on momentum and classical yarn theory. The model was found to provide very good agreement with the experimental results for thick targets that displayed the two-stage penetration process. For thin targets, which did not show the initial shear plugging phase, analytical models for membranes were demonstrated as suitable

Application of selective laser melting (SLM) for hybrid aerospace structures

The study investigates the adhesion properties of SLM manufactured Titanium (Ti-6Al-4V) alloy surfaces for metal-metal and metal-composite hybrid joints using Mode I Double Cantilever Beam (DCB) specimens. The results indicate that the inherent micro-topology of the SLM surface is able to maximise the bonding potential of the adhesive. With the introduction of additional macro-surface features, the crack path is deflected from a straight mode I path and follows the design of the surface features. Selected macro-features were found to increase the fracture toughness by up to 50%. Finite element analysis indicates that the rise in fracture toughness is due to two factors: 1) the increase in the effective crack path length and 2) a shift from mode I to mode II crack growth