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

Bending fatigue failure mechanisms of twill fabric E-Glass/Epoxy composite

Composite materials exhibit very complex failure mechanisms because of their heterogeneous property leads the formation of different stress levels within the material and results various combinations of damage. The objective of this research is a contribution to physical understanding of composite failure mechanism under bending fatigue loading ,as a physical understanding of composite failure mechanism is crucial step for developing both theoretical and numerical models. To investigate the fatigue damaging mechanism and the resulting material strength deterioration of a particular composite material, four-point flexural quasi-static and displacement-controlled fatigue tests have been performed. The analysis methodology relies on interrupted flexural fatigue tests. The damage formation and propagation was continuously monitored through the decrement of bending moment during cycling. This leads to a characteristic history curve that can be subdivided into three stage, namely initial, intermediate and final. Each of them has specific trend in stiffness degradation. Through microscope and SEM image analysis of the pre-cycled specimens, four different subsequent failure modes and mechanisms of the targeted material are identified at different stages of component life. A correlation between the trend of the bending moment time history and the sequence of the different failure modes is established

Energy absorbing capability of GMT, GMTex and GMT-UD composite panels for static and dynamic loading - Experimental and numerical study

The impact performance of classical GMT (Glass Matrix reinforced Thermoplastics) and its more recent variants with improved structural characteristics obtained by reinforcement with unidirectional (GMT_UD) and plane fabric (GMTex) lamina has been analyzed both from the experimental and numerical simulation points of view. The results of the experimental tests (quasi-static and impact loading conditions) have been investigated thorough comparing the force–displacement and energy displacement diagrams, paying particular attention to the peak load and to the absorbed energy values. Furthermore, important parameters such as damage index, energy profile and energy absorption efficiency have been used to compare the material impact performance. Visual inspection of the perforated specimens has given an insight of the material damage mechanisms and support explanation of the different energy absorption performance between the three considered materials. Finally some repeated impact tests have been conducted in order to point out the material impact performance also from this relevant point of view. The availability of wide amount of experimental results gives the possibility of an assessment of the ability of numerical models developed by means of ABAQUS FE code to represent the actual physical tests. The obtained results have quite good accordance

Effect of notch on quasi-static and fatigue flexural performance of Twill EGlass/Epoxy composite

An experimental study has been conducted to investigate the effect of notch on flexural (quasi-static and fatigue) performance of Twill E-Glass/Epoxy composite. Standard specimens and specimens with different types of notches configuration such as circular holes, transverse ellipse, longitudinal ellipse and slot geometry, have been prepared from plates and are considered for the study. Displacement-controlled bending fatigue tests with stress ratio R of 0.10 have been conducted on the selected specimens and damage in the composite has been continuously monitored through the decrement of bending moment during cycling. S–N curves are generated for the targeted composite material by cycling the coupons until failure and recording the number of cycles-to-failure. The results of un-notched specimens are then compared with those of coupons which have the selected notch geometries. Besides, the residual mechanical properties (flexural strength) of the material have been measured after loading the specimens to a preset number of cycles for coupon loaded at 30% and 45% of the Average Failure Load (AFL). The results are used to compare the rate of material degradation among the different type of notches. Finally, it is observed that different notched geometry behave differently for quasi-static and fatigue loading. For composite component subjected to quasi-static load, the failure is mainly governed by stress concentration (local failure) i.e. the notch size is not a significant factor. Whereas for composite component subjected to fatigue loading, the notch size becomes a dominant factor for failure and results to be more relevant than the stress concentration

Bending fatigue behavior of twill fabric E-glass/epoxy composite

Twill E-glass/epoxy composite was considered for its bending fatigue behavior. Displacement controlled bending fatigue tests with stress ratio R of 0.1 were conducted on standard 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 loadings with the maximum loading level up to 75% of the material ultimate flexural strength. Early damage was observed after hundreds of loading cycles causing degradation of material stiffness with cycling. The amount of stiffness reduction was observed to be a function of the magnitude of the fatigue loading applied to the specimen. For some selected specimens, after 1 million cycles, fatigue tests were stopped and residual properties were measured. Different levels of reduction on material strength and elastic modulus were found to depend on the level of fatigue loading. Finally detailed discussion is made to correlate the found fatigue data and obtain general description of the material fatigue behavior useful for composite component design

Characterization and damage analysis of notched cross-ply and angle-ply fabric GFRP composite material

One of the main problems related to composite structure joining is use of mechanical joint that damages continuous fibers and leads to reduction of considerable amount of load carrying capacity of composite structure. To underline this problem, damage initiation and mode of fracture around circular hole of cross-ply and angle-ply of Glass Fiber Reinforced Polymer (GFRP) specimens subjected to tensile loading have been studied experimentally and numerically. A comprehensive notch-edge damage analysis was performed with help of recording test video, digital microscope, Polariscope, and layer by layer SEM (scanning electron microscope) analysis. To understand the effect of manufacturing process on the quality of hole and to avoid its influence on the actual result, a number of manufacturing methods and cutting parameters were used and based on the found result appropriate method was selected for further study. From tensile test, results showed that the tensile strength, crack initiations and propagations around hole are influenced by the quality of notch. Experimental observation showed that the effectiveness of finding of damaged zone is dependent on the damage observation techniques. Predicted damaged zone by numerical solution are in good agreement with experimental observations

Geometrical optimization of bumper beam profile made of pultruded composite by numerical simulation

In addition to their high specific strength and specific stiffness composite materials possess high energy absorption capability, that makes them an interesting alternative material for developing automotive safety component, when to combine weight saving and crashworthiness is highly desirable. In this work E-Glass/epoxy pultruded bumper beam is considered and its energy absorption capability is compared with steel and E-Glass/epoxy fabric composite. Furthermore, low velocity impact finite element simulations are performed using ABAQUS, in order to optimize beam section profile and beam curvature for crashworthiness. Results show that, pultruded bumper beam has comparable energy absorption capability with respect to the steel normal production solution. The new composite solution, after appropriate optimization of bumper beam section profile and beam curvature, exhibits yields improved energy absorption characteristics; the development of progressive failure mode leads to lower mean crash load and almost constant force diagram whose value is close to the maximum peak load

Effect of notch on quasi-static and fatigue flexural performance of Twill EGlass/Epoxy composite

An experimental study has been conducted to investigate the effect of notch on flexural (quasi-static and fatigue) performance of Twill E-Glass/Epoxy composite. Standard specimens and specimens with different types of notches configuration such as circular holes, transverse ellipse, longitudinal ellipse and slot geometry, have been prepared from plates and are considered for the study. Displacement-controlled bending fatigue tests with stress ratio R of 0.10 have been conducted on the selected specimens and damage in the composite has been continuously monitored through the decrement of bending moment during cycling. S–N curves are generated for the targeted composite material by cycling the coupons until failure and recording the number of cycles-to-failure. The results of un-notched specimens are then compared with those of coupons which have the selected notch geometries. Besides, the residual mechanical properties (flexural strength) of the material have been measured after loading the specimens to a preset number of cycles for coupon loaded at 30% and 45% of the Average Failure Load (AFL). The results are used to compare the rate of material degradation among the different type of notches. Finally, it is observed that different notched geometry behave differently for quasi-static and fatigue loading. For composite component subjected to quasi-static load, the failure is mainly governed by stress concentration (local failure) i.e. the notch size is not a significant factor. Whereas for composite component subjected to fatigue loading, the notch size becomes a dominant factor for failure and results to be more relevant than the stress concentration

Lightweight Solutions for Vehicle Frontal Bumper: Crash Design and Manufacturing Issues

Vehicle impact on the environment is one of the main concerns in recent years and is encountered in several ways throughout vehicle life cycle. On one hand, fossil fuels are still the main energy source for automobiles and this results in a very large amount of global emissions of Green House Gases (GHG) and in particular of CO2. A large contribution to the noxious gas emission reduction can come from vehicle lightweight design, through the adoption of lighter material solutions. On the other hand, the request of material recycling at the vehicle end of life is growing and it is clearly not sufficient to recycle only its metallic part. Therefore, lightweight design together with end-of-life recyclability is the major challenges for car manufactures and has leaded law makers to set even stricter rules and legislations to contribute to protect the environment. Vehicle fuel consumption and, consequently, noxious gas exhaust are directly depending on the vehicle weight, automakers are developing advanced technologies to tackle the issue. This includes improvements to engines, drive trains, transmissions, and body aerodynamics of the cars but also the utilization of hybrid or full electric power systems or traditional internal combustion engines operated with alternative fuels. However, one of the fundamental and effective means to reduce CO2 emission and end-of-life issues comes through the use of novel lightweight and easily recyclable materials such as composite, particularly composite with thermoplastic matrix as the recyclability of thermoset resin is still relatively complex. In this work, an automobile bumper subsystem is considered for material substitution and innovative design. A number of different composite material types are examined together with two related manufacturing technologies, namely pultrusion and die forming, pointing out the advantages that can come from each alternative. Finally a novel beam-crash box integrated bumper subsystem made from the selected lightweight materials through die forming process is designed and analyzed numerically. The comparison made with the reference steel material solution shows that, through proper geometry optimization, the proposed composite material solution can substitute the current steel solution with a significant weight reduction and comparable or even better performance