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Julian Peterson
Julian Peterson

HEXELS 2.5 (Full !EXCLUSIVE! Crack)

The BBC[8] has asserted that their influence decreased from 2000 onwards. Police officers, speaking off-record to British newspapers, have said that the family has been credited with acts that they simply did not carry out and this could be true given the number of alleged key gang members killed or imprisoned. However the Metropolitan Police took the Adams' alleged crimes sufficiently seriously to consider the need to involve a CPS-lead team of detectives and the Security Service (MI5) in order to crack the Adams mafia-like organised crime cartel.[2]

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Carbon fiber sheet moulding compounds (CF-SMC) are a promising class of materials with the potential to replace aluminium and steel in many structural automotive applications. In this paper, we investigate the use of CF-SMC materials for the realization of a lightweight battery case for electric cars. A limiting factor for a wider structural adoption of CF-SMC has been a difficulty in modelling its mechanical behaviour with a computational effective methodology. In this paper, a novel simulation methodology has been developed, with the aim of enabling the use of FE methods based on shell elements. This is practical for the car industry since they can retain a good fidelity and can also represent damage phenomena. A hybrid material modelling approach has been implemented using phenomenological and simulation-based principles. Data from computer tomography scans were used for micro mechanical simulations to determine stiffness and failure behaviour of the material. Data from static three-point bending tests were then used to determine crack energy values needed for the application of hashing damage criteria. The whole simulation methodology was then evaluated against data coming from both static and dynamic (crash) tests. The simulation results were in good accordance with the experimental data.

Carbon fiber sheet molding compounds (CF-SMC) are a class of materials composed of pre-preg chips or bundles of chopped carbon fibers dispersed in a matrix material. The most common matrix materials are epoxy and vinyl-ester resins [5, 6]. CF-SMC material possess a unique combination of properties being lightweight, having high strength values, crack resistance and a competitive price. Using this material to replace the aluminium for a battery case, can lead up to 30% weight reduction, while maintaining excellent mechanical properties [6]. The CF-SMC raw material possesses a certain viscosity level, thus allowing complex geometry to be moulded by using a pressing and curing process. In order for the resin to polymerize and solidify, the pressed parts need to be exposed to heat. The heat is generally provided within the press tool. Curing time is in the order of minutes, [6] allowing for high-output industrial applications. In comparison with traditional carbon fiber products, CF-SMC allows for the decoupling of the product quality from the operator ability. This is due to the quasi-isotropic nature of the material, as well as the adoption of pressing techniques [5,6,7,8,9].

However, failure initiation and damage progression depend on local fiber orientation. The very nature of the fibers creates a modelling challenge for dynamic events [33]. The yield strength of the fiber is almost an order of magnitude higher compared to the matrix. During crash events, the crack propagation exhibits a preferred direction along the fiber chips as can bee seen in Fig. 12. Generally the crack front travels along the path of least resistance even if this means an increase of the crack length.

Shell elements are the workhorse of the car industry for thin walled structures. Thus we developed a model based on them. In order to predict the complex failure behaviour during crack propagation (as seen in Figs. 12, 13) a randomized direction approach was used. In this approach each shell element is assigned an in-plane random material orientation with a value from \(0^\circ \) to \(180^\circ \) respective to the local element coordinate system (Fig. 14).

Damage process of the specimen the three stages of the dynamic test: contact (1), crack initiation (2) and crack propagation (3). Note the material fails along the edges of the fiber chips. The nature of the crack is quite complex due to the local anisotropy of the material

This approach is suitable for simulating large components such as battery cases for electric vehicles. Considering the material damage in the design phase, allows for a reduction of weight. This is an important step for evaluating further introduction of CF-SMC components in the automotive industry. We observed a highly damage tolerant material behaviour, with a large amount of energy absorbed before complete material failure. Crack growth was also hampered by the presence of randomly oriented carbon chips, that resulted in segmented cracks. All of these properties make the CF-SMC an advantageous material for safety critical car components.

In comparison, a combination of SM and SH performances in sandwich structures has been scarcely studied [27]. Although these structures are widely used in many engineering applications, such as space shuttles, aircrafts, ships, cars, wind-turbine blades or buildings, they are subjected to critical problems such as transverse load-induced impact damages. In particular, in composite sandwich structure, the skins are responsible for protecting and shielding the core as well as supporting bending loads, while the core is able to separate and fix the skins, to resist transverse shear and in-plane load, and entering other functions such as absorbing impacts or insulating heat transfer. The ability of the core to host other functionalities made it possible to evaluate the possibility to add self-healing properties into the sandwich core. In this regard Williams et al. designed and tested vascular self-healing systems for foam-cored sandwich panels, which, after damage and self-healing, show a strength comparable to the undamaged ones [28]. Moreover, the core of a composite sandwich structure is very often a polymer foam, and recently SMP foams with integrated self-healing properties were proposed. In particular, Li and Xu and Li et al. suggested a SMP syntactic foam sealant for a compression-sealed expansion joint in a bridge floor or concrete roadway [29,30]. The integration of these SMP-based syntactic foams as cores of sandwich structures was then formulated by Behl et al., who proposed the external confinement of fiber-reinforced polymer skins as an out-of-plane constraint to the SMP core [31]. In detail, Behl et al. fabricated the syntactic foam by dispersing glass hollow spheres and multi-walled carbon nanotubes into a SM polystyrene matrix without the direct use of particles for extrinsic self-healing. Thus, they studied the shape memory effect alone with the partial confinement of the sandwich skin and demonstrated the crack-closing capability by the SM effect of the SMP core. However, a combination of SM with self-healing systems based on healing agents can be interesting and almost crucial for such composite sandwich structures. However, a combination of SM with self-healing systems based on healing agents can be interesting and almost crucial for such composite sandwich structures. Previous conclusions in this regard were made by authors with shape memory composite sandwiches with self-healing properties for marine applications [32]. In particular, this first approach was made to simulate the self-healing function through the fabrication of sandwiches with thin carbon fiber laminates for skins and SMP foams for core. In that case, once ruptured, the failure zone was covered with uncured epoxy resin in liquid state and after was recovered. However, the study was limited to a post-rupture action that simulated the insertion of a healing agent.

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