L. Iannucci

A failure model for the analysis of thin woven glass composite structures under impact loadings

Abstract A progressive damage model for a woven glass fibre fabric reinforced with epoxy composite was developed and implemented into a well known explicit Lagrangian finite element code. The approach uses a novel damage mechanics formulation to predict in-plane damage in a woven fabric composite defined by matrix cracks and fibre fracture in the warp and weft directions. The model uses a stress based energy dissipation approach and an advanced post failure strain softening methodology. Strain-rates effects, which are commonly observed for woven glass composites are modelled using a damage lag formulation within the constitutive model. Results are compared with laboratory experiments and full-scale tests. The comparisons indicate that the damage model can predict with reasonable accuracy the damage modes observed in both the laboratory and full-scale experiments. Conclusions are presented also on the logical extensions to the damage formulation.

Virtual modelling of safety helmets: practical problems

This work is an attempt to clarify some of the delicate aspects encountered in helmet virtual modelling. Firstly, a review of the literature is illustrated and the main aspects of the models proposed in the past are discussed. Subsequently, the development and testing of a finite element model of a commercially available helmet are analysed. The test conditions for the virtual simulations were those prescribed by the ECE 22.05 standards. Particular emphasis is given to the analysis of fibre-reinforced plastic helmets that, owing to their superior performances, are prone to further developments. The results are compared with experimental data, and possible reasons for discrepancies are analysed.

Effects of the presence of the body in helmet oblique impacts

The oblique impact methods of motorcycle helmet standards prescribe using an isolated headform. However, in accidents the presence of the body may influence impact responses of the head and helmet. In this study, the effects of the presence of the body, in helmet oblique impacts, are investigated. Using the Finite Element method, oblique impacts of a commercially available helmet, coupled with a model of the human body, are simulated. A comparison between full-body impacts and those performed with an isolated headform show that the presence of the body modifies the peak head rotational acceleration by up to 40%. In addition, it has a significant effect on head linear acceleration and the crushing distance of the helmets liner. To include the effect of the body on head rotational acceleration in headform impacts, modifying inertial properties of the headform is proposed. The modified inertial properties are determined for a severe and frequent impact configuration. The results of helmet impacts obtained by using the modified headform are in very good agreement with those of full-body impacts; this verifies the accuracy of the proposed method.

Influence of the body on the response of the helmeted head during impact

The most frequent type of injury that causes death or disability in motorcycle accidents is head injury. The only item of personal protective equipment that protects a motorcyclists head in real-world accidents is the safety helmet. The protective capability of a helmet is assessed, according to international standards, through the impact of a headform fitted with the helmet onto an anvil. The purpose of the present work was to study the influence of the presence of the body on the impact response of the helmeted head. Full-body and detached-head impacts were simulated using the finite element (FE) method. As a consequence of the presence of the body, the crushing distance of the helmet liner was drastically increased. This evidence indicated that the effect of the body should be included in impact absorption tests in order to provide conditions that are more realistic and stringent. The solution to an analytical model of the helmeted headform impact revealed that increasing the headform mass has the same influence on impact outputs, particularly the liners crushing distance, as including the whole body in impact tests. The added mass was calculated by using a helmeted Hybrid III dummy for an impact configuration that frequently occurred in real-world accidents.

Modeling the Lofting of Runway Debris by Aircraft Tires

Runway debris lofting by aircraft tires can lead to considerable damage to aircraft structures, yet there is limited understanding of the lofting mechanisms. The aim of this study is to develop accurate physically based models to understand and predict the stone lofting processes. The research entailed both experimental work and finite element modeling of a tire partially rolling over a stone. Parametric studies were conducted to characterize the influence of factors such as stone geometry and tire conditions in the lofting processes. To validate the finite element models, experimental studies were conducted using a modified drop weight impactor covered with rubber to simulate a tire vertically approaching aluminum balls and real stones. A high-speed video camera was used to observe the loft mechanisms and calculate the loft velocities, angles, and spin rates. A finite element model of the impactor demonstrated good agreement with the experimentally observed loft mechanisms.In general, lofting occurred either at high speed and low angles or vice versa, depending on the degree of interaction between the stone and the ground.

A coupled mixed-mode delamination model for laminated composites

Cohesive elements have now become commonplace in many commercial finite element codes. Their use in impact and crash and other complex composite design problems requires the development of more advanced cohesive element formulations capable of predicting mixed-mode behavior during a complex loading. A cohesive zone model is developed for the simulation of delaminations in laminated composites under mixed-mode loadings, based on a coupling between the normal and tangential components of the relative displacement across an interlaminar interface. Featuring a unique characterization of degraded states, this coupling enables a simultaneous loss of loading capacity of the interface in normal and tangential directions, once the criteria on delamination growth are satisfied. Moreover, the variation of mode participation during the delamination process is effectively considered. A comparison between numerical simulations and experimental data on typical delamination tests shows the robustness and the potential of the model.

Three-dimensional static shape control analysis of composite plates using distributed piezoelectric actuators

In this work, based on a linear piezoelectric constitutive model, a three-dimensional finite element code using an eight-node brick element that includes the anisotropic and coupled field effects of piezoelectric actuators has been developed for the static shape control analysis of fibre reinforced composite laminates. The code was used to study voltage sensing and actuation capabilities of piezoelectric actuators on composite laminates. The required input voltages to the actuators in order to achieve a specified structural shape were determined using a weighted shape control method. The code was validated using two test cases obtained from the literature. The results were found to show good correlation for voltage actuation. However, since determining input voltages to achieve the desired structural shape is a type of inverse problem, there are no explicit solutions and hence the results obtained from the present model were not similar to those reported in the literature. The second validation also suggests that the anisotropic and coupled field effects of the piezoelectric actuators cannot be neglected as this has been shown to underestimate the required control voltages. The effects of different lamination angles, boundary conditions, plate length-to-thickness ratios and actuator dimensions on the control voltages have also been reported.

Static and Dynamic Energy Absorption of Aluminum Honeycombs and Polymeric Foams Composites

The use of innovative materials as energy absorbers for motorbike helmets might have a significant impact on the safety of motorbike riders, currently among the most vulnerable road users. In this regard, the effect of the interaction between aluminum honeycombs and polymeric foams in the design of innovative liners is investigated by studying their behavior under compressive loading conditions. Quasi-static and impact compressive loadings were applied to samples of two-layered structures made of aluminum honeycombs and expanded polystyrene foams, in order to observe their deformation modes, determine their energy absorption properties and the influence of the loading speed in their mechanical response. The same tests were performed on the expanded polystyrene foams and on the honeycomb layers separately. The results were compared and it was shown that although aluminum honeycombs alone offer the highest energy absorption levels, their coupling with EPS foams is preferred for the protection of the head.

Development of a Representative Unit Cell Model for Bi-axial NCF Composites

Composite materials fabricated from non-crimp fabric (NCF) are showing potential in primary aerospace components and structures. They can be used in common RFI, RIFT, and RTM manufacturing processes, and are showing promise to replace traditional pre-preg systems. The composite designer hence needs a technique, which can be used to predict the elastic properties of the NCF laminate including manufacturing defects from these infusion processes. The present study develops a representative unit cell (RUC) model of the NCF blanket, which can include potential variability in the NCF structure. Representative unit cells have been developed to predict the two- and three-dimensional mechanical properties in the bi-axial NCF composite. The model is based on the two-dimensional methods of Naik and Scida for woven fiber composites, incorporating the elastic anisotropy theory of Lekhnitskii to extend the predictions to three dimensions. Continuous tow layers with uniform thickness and fiber volume fraction are assumed, and the effects of crimp due to stitching and surrounding resin regions are simulated. The input data are based on measurements and observations of a ±45° carbon-epoxy NCF composite having a 63% fiber volume fraction and less than 1% voids. Output from the model is obtained in terms of the stiffness and compliance matrices as well as traditional engineering properties. For the tensile and compressive moduli, comparisons with data from the carbon/epoxy NCF composite show the values are over-predicted by 11.5—11.6% in tension and 13.5—13.9% in compression. Improved predictions are obtained using a simple stiffness model that allows for different behaviors of the resin as continuous layers or gaps between tows. The resin layers, compared to the resin as gaps, significantly decrease the in-plane moduli, and the model is considered to simulate local through-thickness shear deformation in the resin layers.

A Constitutive Model for Glass Fibre Fabric Composites under Impact

A new composite constitutive failure model for woven glass fibre fabric reinforced with epoxy/polyester was developed, and implemented into an explicit dynamic non-linear finite element code for both the shell and solid element implementation. The approach uses a novel damage mechanics formulation to predict both matrix cracks and fibre fracture in the warp and weft directions with the woven fabric composite, subject to an impact or other dynamic event. Strain-rate effects are implicitly incorporated into the model using a damage lag methodology. Results are compared with laboratory composite beam and plate impact experiments. The comparisons indicate that the damage model can predict with reasonable accuracy the damage modes observed in the laboratory experiments. Conclusions are presented also on the logical extensions to the damage formulation necessary to improve both the accuracy and the scope of composite materials that may be analysed.