LIGHT WEIGHT DESIGN AND VEHICLE SAFETY

SUMMARY Light-weightiness and safety are the two most important requirements that a modern passenger car have to satisfy to stay in the current competitive market. These two targets seem to be in deep contrast one with the other as the high crashworthiness requirements and the consumer expectation for a refined and comfortable cabin, consistently increased both the mass and size of the vehicle over the decades. This trend comes out as quite problematic for the car manufacturers that, at present, are very motivated to pursue the first requirement (the lightweight) that is deeply affecting both fuel consumption and the emission performance of the vehicle. It is becoming more and more evident that by continuing to use the current conventional metallic materials for front crash structures and other vehicle components, it is more and more difficult to get a design solution capable to satisfy the conflicting interests between the vehicle light-weightiness and safety. Besides, some vehicle weight reduction techniques such as Vehicle design changes and Vehicle downsizing have an adverse effect on both customer comfort and safety, since vehicle size and occupant safety are positively related. As a response , today, researchers and car makers development departments are leaned toward finding high performance, advanced materials such as composite, which have higher Specific Energy Absorption (SEA) performance, in order to break the above mentioned conflicting relationship and truly yield to increase safety and, at the same time, reduce weight. Composite materials have the following main advantages for transportation industries • Weight reduction with improved performance ( specific strength 50% less than aluminum and 75% less than steel) • Crash worthiness/safety i.e. structure built with composite can be 6 to 8 times safer than the similar one made by metal due to higher SEA and more favorable failure mode. As a consequence less intrusion into the passenger compartment during a crash event can be achieved. • Larger number of options available to the designer. The reinforcement type and its form produce an infinite variety. Thus stiffness and strength properties can be selected in a range that is extended from mechanical properties comparable with thermoplastic materials to properties, which are greater than high performance steels. However, to implement composite materials for primary and secondary structural applications and substitution of the current traditional material is not straightforward. In fact, although modern composites introduction is dated back 1937, the rate of usage of composites in the automotive industry in all these years has been very slow and the applications have been limited. Some of the reasons are cost i.e. (raw material and manufacturing costs), volume of production (production rate) and insufficient engineering data such as data of material property, sufficient knowledge about failure behaviors, joining, damage inspection and maintenance. The study presented in this thesis is motivated and drawn from the above stated problems. It is aimed to address at some extent the effect of the material change for vehicle structures and to give research and development contributions to some of the challenges. As lack of engineering data and well established knowledge on composite damaging mechanism are major design constraints on the area, E-Glass/Epoxy composite has been selected and tensile experimental program were designed for material characterization and damage analysis. Both destructive and non destructive damage observation techniques had used to understand the damage extent in the laminate and to capture on-set total failure. Vehicles are operated under arduous conditions; therefore all components are experiencing some form of fatigue loading during most of their service life. This type of failure is usually considered as the principal mode of failure of all dynamically loaded mechanical systems and accounts approximately 90 percent service failures. Especially, when such fatigue failure occurs on some safety critical components, it will affect the vehicle integrity and could result in a complete loss of control of the vehicle that is highly risky for the life of occupants and vulnerable road users (VRU) such as pedestrians, motorcyclists and cyclists and persons in personal mobilized devices (e.g.: motorized wheelchairs and scooters). Therefore, when new advanced material is proposed for such an application it has to be accompanied by a detailed fatigue analysis. In this thesis more emphasis had given on the subject and extensive research has been conducted on the fatigue behavior of the selected material. Displacement controlled four point bending fatigue tests with R ratio of 0.1 were conducted on standard and notched specimens and damage development in the composite was continuously monitored through the decrease of bending moment during cycling. The specimens were subjected to different fatigue loading with the maximum loading level up to 75% of the material ultimate flexural strength (UFS). Having obtained an average complete diagram of the fatigue life of the material, in order to understand the type of failure mode and the possible mechanism at different stage of loading, some critical data points have been selected at various stiffness degradation rate and interrupted fatigue tests were conducted to pre-defined number of cycles . Through analyzing the pre cycled specimens using scanning electron microscope (SEM), the failure mode at the three distinctive regions of stiffness degradation regions have been identified. Furthermore, an effort has been made to study effect of notch geometry and notch size on the flexural quasi-static and fatigue performance of the selected material. Automobile safety in general is the study and practice of design, construction, equipment and regulation to minimize the occurrence and consequences of automobile accidents .It is categorized as active safety (Crash Avoidance) and passive safety (Crashworthiness). In the current work, safety is mainly referred passive safety which is measured by conducting impact physical tests or by numerical simulations at component, sled and full-scale level against a deformable or rigid barrier. The component test is aimed to determine the dynamic and/or quasi-static response to loading of an isolated component and crucial in identifying the crush mode and energy absorption capacity. Understanding their performance is also essential to the development of prototype substructures and mathematical models. Therefore, to address the issue of volume of production and production cost, as both are main constraints for transportation industries, an effort had made to identify a vehicle component where pultruded composite products can be used,( as pultrusion manufacturing technologies is relatively cheapest (due to process automation) and suitable for high volume production). After material mechanics and failure study, automotive bumper beam were identified as a suitable candidate component for the desired objective and a finite element simulations are performed using ABAQUS, in order to optimize beam section profile and beam curvature of a bumper for crashworthiness. The research activity is subdivided in two parts and compiled by six chapters. The first part contains four chapters and the second part contains two chapters. As highlighted in the previous paragraph, to understand the material failure behavior is crucial for candidating a novel material for crash absorbing components. The first part of the work is dedicated to material characterization and investigation of the failure mechanism of the selected material and the second part is extended to a component level. The first chapter covers overall introduction about composite material and its application in automotive industries. The chapters briefly discuss about type of materials that constitute composites and their behaviors, composite manufacturing techniques and the overall requirement of maternal for automotive applications. The second chapter is dedicated to material characterization and damage study on a selected composite material with and without the presence of notch. In this activity, primarily, an effort has been made to reduce the manufacturing (drilling) induced damage by closely controlling and selecting appropriate manufacturing parameters through conducting damage observations. Having relatively damage free coupon, the material is characterized and a comprehensive damage study is made through conduction of interrupted tensile tests and the material damage extent at different loading level is studied using different damage observation techniques. The third and fourth chapters cover a four points quasi- static and bending fatigue study of the same material. In chapter three, the bending fatigue behavior of fabric E-Glass/Epoxy composite is investigated. Material stiffness degradation is used as a measure of damage formation and propagation and it is continually monitored using LVDT transducer. Having obtained an average material stiffness degradation trend through conducting test up-to failure for some set of coupons, critical stiffness degradation points are identified. Then an interrupted fatigue test is conducted for a predefined number of cycles to investigate the damage extent at different stage of damage propagation. In chapter four, the effect of notch on the quasi- static and bending fatigue performance of the selected material is discussed. The effect of notch geometry, notch size and loading type are also considered for the study. The used instrumentation and damage monitoring techniques are similar to what presented in chapter three. In chapter five that is belonging to the second part a numerical study is conducted to explore the possibility of substituting the current metallic bumper beam with E-Glass/epoxy pultruded composites and its energy absorbing capability is compared with steel and E-Glass fabric composite. Since optimal design of composite crash absorber cannot be achieved by a simply material substitution, structural optimization of the beam section profile and curvature is also developed to obtain a stable flexural failure of the composite bumper beam. The analysis is done through the investigation of impact event characteristic data, such as force/time, force/deflection, energy/displacement and deflection/time curves. Finally in chapter six the main findings of the activity are briefly summarized.