FISITA2010-SC-P-02 MYMOSA - MOTORCYCLE ACCIDENT SIMULATION IN LMS VIRTUAL.LAB AUTHORS: Ciubotaru, Leonard (1), Cofelice, Nicola (2), Moreno Giner, David * (2), Manka, Michal (3), Kang, Jian (2) (1) "Transilvania" University, Brasov, Romania ciubotaru_leo@yahoo.com (2) LMS international, Interleuvenlaan 68, Leuven 3001, Belgium nicola.cofelice@lmsintl.com (3) AGH University of Science and Technology, Cracow, Poland mmanka@agh.edu.pl KEYWORDS: multibody simulation, motorcycle crash, MB dummy, MB neck, impact helmet modelling. ABSTRACT Multibody dynamics methodology is an important tool for vehicle accident simulations. In fact, crash test set-ups are often expensive and generally, accident reconstruction reports are not detailed enough to identify the driver behavior after the impact. In the field of Powered Two Wheelers (PTW), the multibody methodology can be very useful to analyze the accident scenario and to understand the rider behavior during the impact. This research presents the simulation results of a crash scenario reproduced by using the multibody software LMS Virtual.Lab Motion and the methodology followed during the research. In the presented paper, the motorcycle and the rider models are defined: the former is composed of 7 rigid bodies and 12 DOF and the latter consists of 24 rigid bodies and 23 joints. A graphic interface, explicitly developed for this purpose, allows the user to easily create a motorcycle model by choosing among several front and rear suspensions and different tire models. By analyzing motorcycle accident statistics and reports, two different crash scenarios have been studied as the most representative: front impact and lateral impact while just the latter one was simulated. The main aim of this work is to compute the Initial Condition for more detailed FE simulation, paying close attention to the head collision. These inputs will be used to further improve the simulation of the helmet crash analysis by introducing different pre-crash factors. This research is being developed within MYMOSA project (MRTN-CT-2006-035965), a research network financed by the Sixth framework Program (Marie Curie Actions) of the European Union. The ambition of this project is to provide a significant contribution to the reduction of motorcycle fatalities and injuries on European roads. 1) INTRODUCTION The objective of this study is to provide an insight of the Powered Two Wheelers (PTW) crash events and to study the biomechanics of the head impact having in mind the improvement of the protection factor of the helmet. The necessity of improving the protection offered by the helmet derives from the fact that although in the last few years the number of motorcycle fatalities has decreased, there are still a large number of victims from the motorcycle accidents.
As it can be seen from the statistics there is continuous increasing of the motorcycle circulating park (69%) between 1994 and 2004 [1]. This increasing number led to the increasing of the motorcycle rider fatalities which in the period of five years (2000-2005) have increased by 5.2% [2], [3]. The European Commission is working towards the reduction of the number of fatalities by improving the protection gear of the rider (like helmet, jacket, hand gloves, etc.), or by introducing innovative active and/or passive protection systems (like sensors for anticipating an obstacle/danger and warning the rider or airbags included in the protection suite). In order to achieve the objective of this research, simulations were carried out by using the multibody software Virtual.Lab Motion. The multibody environment was chosen instead of the FE one mainly because the former can be very helpful to understand the general behavior of the accident set-up (decreasing the overall computational time), while the latter is more suitable to investigate the details of the contact, such as pressure distribution or local deformation. In this research, the multibody systems used for the simulation are: the motorcycle model, the dummy model and the obstacle (Table 1.1). The main parameters of the motorcycle and dummy are based on real models, the specification data can bee seen in Table 1.1. Table 1.1: Specification of the simulation systems Motorcycle Dummy (50% ile) Obstacle Weight: 208 kg (without dummy) Wheelbase: 1420 mm Overall Length: 2080 mm Weight: 73.25 kg Height: 179 cm Car Overall Length: 4025 mm Mass: 1100 kg In the following paragraphs, each multibody system will be described and explain individually. 2) MOTORCYCLE MULTIBODY MODEL In order to easily create the motorcycle multibody model, a GUI (Graphical User Interface) has been developed within Virtual.Lab Motion [4]. The main objective of the motorcycle interface is to let the user create the desired motorcycle by choosing among several suspensions and two different tire models. It is also possible to modify the geometrical characteristics and the dynamic parameters of the suspension in order to make the application flexible and create most of the motorcycle models on the market. In fact the motorcycle classes available are manifold (i.e. sport bikes, touring bikes, cruise bikes, enduro bikes, etc); because of the wide range of design variations in the market, there are many factors that differentiate one motorcycle from the other, such as the geometry (wheelbase, caster angle, trail, etc), the inertia properties, the materials and the different trim. The GUI developed allows the user choose among different models of front and rear suspension and of front-rear tires (as shown in Figure 2.1). Figure 2.1: Motorcycle interface
The GUI was used to create the MBS motorcycle model represented in the motorcycle accident set-up and then adapted in order to be used within the crash simulation. It consists of seven rigid bodies: the front wheel, the lower part of the fork, the middle part of the fork, the upper part of the fork (including the handlebars), the front frame (including the engine and the fuel tank), the swinging arm and the rear wheel (Figure 2.2). The motorcycle model has twelve degrees of freedom: six from the front frame (3 coordinates of the center of mass together with the roll, pitch and yaw angles), one from the steering angle, two corresponding to the rotation of the wheels, two from the suspensions and one for the deformation of the front fork [5], [6]. Figure 2.2: Multibody model of the motorcycle In order to set up the crash, it was necessary to create contact forces between the motorcycle and the other systems that take part in the simulation: contacts between the motorcycle and the obstacle/car, between the motorcycle wheels and the road and contacts between the motorcycle and the human dummy model. LMS Virtual.Lab Motion workbench offers the possibility to create the contact between parts as a CAD contact, i.e. the contact is created between undeformable bodies with arbitrary geometry. When contact occurs, the depth of penetration and velocity are determined and the resultant contact force is calculated [7]. During the impact, the energy absorbed by the deformation of the front fork of the motorcycle is modeled by means of plastic deformation and limitation of the front wheel movement when hitting the body of the motorcycle. A rotational spring damper element is used for simulation of the plastic behavior of the front fork. In case of a crash, the front fork of the motorcycle will deform with respect to the force of the impact. In this case, a threshold limit was considered for the elastic-plastic behavior of the material. That means that if the fork rotates in the wheel plane with an angle smaller then the specified threshold the deformation of the fork will be in the elastic field. In case that the deformation exceeds this limit, the deformation is considered to be in the plastic domain and therefore it will be permanent. In the model it is also taken into account that at some point the front wheel will hit the radiator or the protective mask (depending on the model of the motorcycle) and the deformation will stop. For the angle of the deformation a maximum value was chosen and implemented in the model. Once the maximum value is reached, being in the plastic domain, the deformation will be permanent and the fork will be blocked in that position considering also the elastic restitution.
3) RIDER MULTIBODY MODEL A simplified human body model has been generated using undeformable bodies (described in section 3.1) [8]. Therefore special attention has been paid to model additional details that play an important role during the impact: the neck modeling (described in section 3.2) and the helmet modeling (described in section 3.3). 3.1) Simplified human body modeling The dummy model used in the simulation is composed of 24 rigid bodies: head, helmet, neck (7 vertebras), torso, pelvis, right arm (up, low), left arm (up, low), right hand, left hand, right leg (up, low), left leg (up, low), right foot, left foot. It represents the 50%ile European male, as represented in figure 3.1 (Weight: 73.25 kg; height: 179cm). Figure 3.1: Multibody model of the rider The rigid bodies are connected with 23 joints: - 5 revolute joints (for the elbows and the knees and the C1-C2 joint); - 4 bracket joints (for the wrists and the ankles); - 13 spherical joints (for the hips, the connection between pelvis and torso, the shoulders and the neck) - 1 planar joint in the sagittal plane for the relative movement helmet-head (but neglecting the relative rotation). Under this configuration, the dummy has 55 DOFs (49 internal + 6 absolute). In order to reproduce the anatomic limits of the human body joints [9], for each joint it was introduced a reaction torque which depends on the angular position and velocity of the body parts. In fact, for every internal DOF of the dummy, a rotational spring damper actuator (RSDA) has been considered. For the human joint stiffness, a non-linear behavior for the Torque has been implemented while for the human joint damping, a constant value has been used.
3.2) Neck-head modeling The multibody neck-head model has been developed independently and then implemented into the virtual dummy [10], [11]; it is composed of 7 neck segments, the T1 vertebrae and the head (figure 3.2) and corresponds to a 50 th percentile male. All the neck segments have the same geometry and they represent all 7 neck vertebrae. Each segment was attributed the inertial properties that represent the equivalent mass and moments of inertia of the vertebrae and the attached soft tissues. The inertial properties were defined also for the head. As already mentioned, the bodies are connected with spherical joints. The forcers in the joints are generated with 6DOF nonlinear spring-dampers elements, thus allowing the neck to rotate in three directions according to the human movements: flexion-extension, bending and rotation. The neck is modeled in order to approximate the natural curvature of the human neck (figure 3.2) and the system is connected to the rigid torso by the T1 vertebra. OC R= 190 mm 37 o T1 a) b) Figure 3.2: Head and Neck model. a) Wireframe view; b) Shaded view Validation of the multi-body head-neck model was done by simulating a standard pendulum test, used for calibration of the neck of Hybrid III dummy, and then, compare the results of the simulation with data from the calibration tests. According to FMVSS 572 (Federal Motor Vehicle Safety Standard), there are a set of constrains to respect when choosing the impulse configuration. Neck calibration is performed on a head and neck assembly model. The model is attached to a rigid pendulum, which is allowed to rotate freely. A block of honeycomb material is used to stop the pendulum s rotation and the resulting neck flexion (forward rotation of the head) and extension (rearward rotation of the head) are measured. The calibration test is performed by releasing the pendulum and allowing it to fall freely from a height such that the tangential velocity at the pendulum accelerometer centerline at the instance of contact with the honeycomb is 6.8901m/sec for flexion testing and 5.9520 m/sec for extension testing.
3.3) Helmet modeling In this section, a multibody model for the impact response of motorcycle helmet is described. First, a brief introduction about the helmet structure is reported. Helmet is mainly composed by 3 different layers (as can observe in figure 3.3 [12]): 1- Polystyrene Foam Liner: is the main impact absorbing component, usually made by polystyrene beads (EPS). EPS is chosen because of the low density of the foam, the good impact energy absorption and the economic technology process. 2- Shell: can distribute a relevant portion of the impact energy, especially during impact with convex object. The Shell can be manufactured either by injection molded from ABS (Acrylonitrile Butadiene Styrene copolymer) or rubber-toughened polycarbonate thermoplastics, or molded from polyester thermoset resin reinforced with glass-fiber cloth/mat/rovings (GRP). 3- Comfort Foam: allows a limited range of helmet size to fit a great range of head size. It is made by open-cell foam, capable of deforming to the shape of the head without exerting very high pressure. Figure 3.3: Section of helmet with its components The development of the multibody helmet is based on the 1D model developed by Gilchrist and Mills [12] and the simplification proposed by M. Ghajari (MYMOSA researcher at the Imperial college of London), which allows the prediction of the linear acceleration and impact forces during a standard drop test. Therefore, the 1D analytical model of helmet impact test can be modeled using 3 masses (figure 3.4): M 1 (Shell Liner), M 2 (Headform), M 3 (Anvil-Obstacle). Consequently an analysis of the main deformation mechanism is necessary to create the computational model. The main deformation mechanisms during an impact are: 1- Polystyrene foam crushing (represented by the LINER YIELD block). 2- Shell deformation (represented by the spring-damper k 1a, k 1b and n 1 ). 3- Elastic deformation of polystyrene foam (represented by the spring-damper k 2, n 2 ).
Figure 3.4: Comparison between 1D analytical model of helmet and 3D MBS model After this preliminary study about the behavior of the helmet-headform during the impact, a multibody model has been developed. For the Helmet, the GpTech helmet provided by Dainese has been used for the simulation, while for the Headform, the E headform has been used, as described in the standard ECE 22.05(Uniform Provisions Concerning the Approval of Protective Helmets and of Their Visors for Drivers and Passengers, United Nations 2002 ). The anvil is a fixed body while the inertia properties for both helmet and headform are derived by the available FE model. The only internal joint is a planar joint between the helmet and the headform, which allows the relative translation in the sagittal plane. There is no joint defined between the helmet and the anvil. The dynamic impact response has been simulated with two elements in the MB model: 1) Headform-Helmet contact: it represents the contact between the headform and the helmet. It is modeled by means of a no-linear spring-damper applied between the Headform CG and the replication of the Headform CG belonging to the helmet. 2) Shell-anvil contact and Liner Yield: the former represents the contact between the outer shell of the helmet and the anvil during the impact, the latter represents the yield of the EPS liner during the impact. It is simulated with a Sphere vs. surface contact element, where the parameters have been computed based on [12]. The linear acceleration coming from the drop test simulated in the MBS model match the one computed with the analytical and FE models; consequently the helmet model for impact has been implemented in the multibody rider. 4) ACCIDENT SIMULATION SET-UP From the statistics it can be noticed that the most frequent impact position is the front part of the motorcycle (front wheel plus front fork/suspension) - 65.3%, while just 25.7% have as the first point of contact the center of the side of the engine. The rear impacts are most uncommon, just 6.7% of all accident reports [13]. In order to set up the desired initial conditions we are taking into account the ISO standard 13232-2; in this standard several crash positions and velocities between motorcycle and car are defined. By comparing the MAIDS report and the ISO13232-2 configuration, the lateral impact configuration was chosen as most representative one (figure 4.1). The relative angle between the
vehicles is 90 deg; the linear velocity of the motorcycle is 13.4 m/s, while the other vehicle is stopped. Figure 4.1: Crash Simulation Set-Up (Impact angle = 90 deg, Motorcycle speed = 13.4 m/s, Car speed = 0 m/s) The dummy is attached to the motorcycle by means of five spring-dumper elements that are detached when the impact force exceeds the evaluated limit. These elements are presented in the saddle point: connecting the pelvis with the saddle, left and right foot: connecting the left and right foot with the foot rest element and the right and left hand for griping the handle bar. The grip force is taken from the literature as the maximum of the force that a human can produce when grabbing a cylindrical type object. In order to represent the energy absorbed by the car during the accident, the principle of the conservation of energy has been applied 1 m V 2 1 = mv 2 1 fin + m V 2 1 2 fin + I 2 1 1 1 1 2 1ψ& 1 f + Ed 4.1 2 2 2 2 where: m=mass, V=linear velocity, ψ=angular velocity, I=rotational inertia, E d =dissipated energy, index 1 for motorcycle, index 2 for the car. In this simulation, the car is constrained by a traslational joint to move only in the direction of the motorcycle speed, so its rotational kinetic energy is always equal to zero. In order to represent the lateral stiffness of the car a spring element is used for the energy dissipation. 5) RESULTS, CONCLUSIONS AND FUTURE WORK The research was focused on the development of a methodology for motorcycle impact simulation implementing the realistic behavior of the mechanical components. Close attention was paid to the front side of the motorcycle, which has a big influence over the crash results. The multibody model for crash simulation is feasible to be used in order to evaluate the kinematic magnitudes, such as velocity and acceleration for every part of the body, paying particular attention to the head (Figure 5.1).
a) b) Figure 5.1: Acceleration and velocity profile: a) linear acceleration of the CG of motorcycle frame (green line) and head (red line); b) linear velocity of the CG of motorcycle frame (green line) and head (red line) In Figure 5.2 it can be seen four of the most important points in the time frame of the lateral crash scenario. First frame is the starting position at time zero. After 20 ms the front fork is starting to deform, reaching the maximum deformation at about 30 ms, when the front wheel is hitting the body of the motorcycle. At 60 ms it can be noticed the detachment of the connecting spring elements between the dummy and the motorcycle. When the time reaches 130 ms it can be noticed the starting moment of the peak acceleration. Figure 5.2: The kinematic output of the simulation setup Although the presented research was based on building a methodology to study motorcycle crash scenario in a MBS environment there are a number of improvements that can be made as a future works, in order to increase the usefulness of this simulation up to injury assessment capability: - the obstacle type can be changed to a moving one (e.g. car or van with initial velocity); - since the relative position between the PTW and the other vehicle is very important for a simulation set up this parameter can also be studied;
- improve the energy absorption parameters for the deformation of the car (some complete studies must be done here based on the selected type of car); - for the kinematics of the human body also the contact between the pelvis/upper legs and the tank should be studied in details; - the neck model can be improved by taking into account other parameters then just the vertebrae and the attached soft tissue (e.g. the limits in motion created by the surrounding flesh and muscles or the limit in flexion introduced by the largest cartilage of the larynx, etc) - the helmet-obstacle can be improved by taking into account an deformable obstacle (for this research the obstacle was a rigid one). - in addition, the MBS motorcycle model can be coupled with a MATLAB-SIMULINK controller integrated in the application, in order to simulate the motorcycle behavior during precrash phase (i.e. failed or successful avoidance maneuvers). This controller, called Virtual Rider is already available in VL Motion and it controls the direction and the speed of the motorcycle by means of, respectively, the steering torque and the thrust/braking torques applied on the wheels. 6) REFERENCES [1]. ACEM's view on PTW fatality statistics in Europe: http://www.acem.eu/cms/ptwfatalities.php [2]. EDWIN B. Accident statistics analysis, CIECA Workshop, Nimes, April 2008 [3]. http://www.clutchandchrome.com/articles/safety_2008.htm [4]. Cofelice, N., et al. A Unified Interface For Motorcycle Simulation In Lms Virtual.Lab, COMEC 2009 [5]. David M. et. al. - MYMOSA - A virtual motorcycle rider for closed-loop simulation of motorcycles ISMA conference 2008. [6]. Ciubotaru, L. Et al. Simulation Of Motorcycle Crash Scenario Using Multibody Software LSMVirtual.Lab Motion, 29-30 Octombrie 2009, COMEC2009, The 3rd International Conference on Computational mechanics and Virtual engineering, Braşov, România. [7]. R.S. Sharp, S. Evangelou, D.J.N Limebeer. Advances in the Modelling of Motorcycle Dynamics. Multibody System Dynamics 12:251-283, 2004. [8]. Manka, M. et al., Co-simulation of motorcycle-rider system in road behaviour simulation, ASME 2009. [9]. D.H., Robbins, Anthropometric specifications for mid sized male dummy Volume 1-3, University of Michigan, Michigan, 1983 [10]. S. Himmetoglu, M. Acar, A. J. Taylor, and K. Bouazza-Marouf Wolfson in A multi-body head-and-neck model for simulation of rear impact in cars, School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, UK [11]. Marike J. van der Horst in Human head neck response in frontal, lateral and rear end impact loading: modelling and validation. Eindhoven: Technische Universiteit Eindhoven, 2002. [12]. Willinger, R., et al. Dynamic characterization of motorcycle helmets: modelling and coupling with the human head, Journal of Sound and Vibration (2000) 235(4), pag. 611-625. [13]. In depth investigation of motorcycle accidents, MAIDS 2.0 - http://www.maids-study.eu/ [14]. http://www.mymosa.eu/