Seat belt modelling and testing through virtual prototyping

Road safety requires optimal collaboration and coordination between several factors directly or indirectly involved in order to ensure safety for all public road users within a national transport system. Within this system the tasks of each component must be very clearly defined and regulated so that the whole system functions properly. The paper presents a case study on the operation of the driver restraint system in the driving position. For this purpose, the three-point seat belt was modeled using the LS-Dyna software package. For the proposed analysis, the seat belt with grip on the central pillar of the vehicle was considered. This type of seat belt is the most common passive safety system in series vehicles.


Introduction
Passive safety of vehicles aims to reduce the effects of accidents, even during their occurrence.In the construction of motor vehicles, especially passenger cars, the aim is to ensure minimum safety standards aimed at avoiding serious or fatal injuries to the occupants.
One of the most important roles a vehicle must perform is to ensure the highest possible degree of safety.In recent years, driving safety has taken on a new dimension, with increasingly complex systems designed to decrease the severity of injuries in the event of an accident.Most common road safety engineering deficiencies were identified by the authors in [1], for the South Eastern Europe, and have been presented in the paper as a part of road safety unbreakably connected to the safe System approach.
The group of specialists the study and improvement of security measures is also seen through the numerous published studies.In [2] are treated both research regarding the Active Safety Systems from a theoretical point of view, but the most common road safety engineering deficiencies are also highlighted.Starting from this principle of the safety position, in [3], it is presented a research study regarding the occupant kinematics in the vehicle at the front and rear impact.

Restraint system of occupants of a vehicle-seat belt
The seat belt is so effective that it has retained its features and purpose since 1949, when Volvo presented the first vehicle with such equipment, and to nowadays.The safety belt as standard on all vehicles is the one with three-point grip.The retractable three-point belt operates at pressure and locks when some tension is felt blocking the retractor drum.
The restraint system is actually a safety harness designed to insure the passenger against being accelerated due to a sudden collision or stop.As part of the passive safety system, the belts are designed to reduce the degree of injury, stopping the impact between the passenger and blunt objects in vehicles, or by other passengers.They absorb energy, being designed to stretch slowly at the moment of collision, reducing the speed of inertia of the passenger, for increased protection especially 1303 (2024) 012039 IOP Publishing doi:10.1088/1757-899X/1303/1/012039 2 for passengers with special needs -eg.pregnant women.In [4] it is presented a research facility whose purpose is to analyze the stresses for a pregnant woman's abdomen.It was developed a complex measuring system that registers and processes the movement parameters of the pregnant driver, both independently, but also in relation to the vehicle movement.

Seat belt functioning
During an impact, at a speed of 50 km/h, on an adult of 80 kg acts a force of approx. 2 tons.An impact at 40 km/h subjects the human body to pressures similar to a fall from the third floor of a building.
Seat belts are the most effective means of protection against injury in the event of an accident, being the basic component of the passive safety system of any motor vehicle.The use of the belt reduces the risk of fatal injuries by up to 50% for front seat occupants and up to 75% for rear seat passengers.In the event of an accident, a passenger not wearing a seatbelt is randomly projected, increasing the risk of other occupants being injured or killed by 40%.
To be effective, the belts must be used correctly and adjusted according to the height of the occupant, well-adjusted on the body, without being twisted or passed over blunt objects in pockets, such as pens or glasses.
The Airbag, which is an additional retention system, is effective only under the condition of using the seat belt, and can generate serious injuries in its absence.According to [5], the injuries can be severe in specific passenger positions called out of position or when the airbag receives the inflation command with delay.A preoccupation in this direction is presented in [6], where the study was directed at two types of airbag miss-deployments: late deployment and non-deployment.
The deployment process of the seat belt pretensioners coincides with that of the airbag.In the case of the device in figure 1, the pyrotechnic device (detonator) placed in body 1 forces the fluid from the tube 2 onto the blades on the rotor of a turbine 4. The rotational motion of the rotor with turbine blades causes the axle of the seat belt roller to rotate 3, so that the seat belt is stretched over the body of the occupant.The detonating staple is activated by means of an electrical impulse.The high pressure that occurs in this way forces the piston to move through the tube that is filled with a liquid.This displacement causes the rupture of a membrane that closes the other end of the tube.The liquid, a mixture of water and glycerin (so as not to freeze at the low temperatures that are commonly encountered in winter), is directed at high speed to the turbine rotor blades through the opening that appeared at the end of the tube after the membrane ruptures.The rotation of the turbine drives the roller shaft with the seat belt in the direction of winding, which causes the belt to be tensioned.

The future of restraint systems-seat belt
Large car companies have ongoing concerns to improve the safety features of cars, and the belt plays a particularly important role in this direction and can be radically improved.
In this regard, several partnerships have been initiated between major companies and research institutes to bring to market a new seat belt that will provide increased protection for drivers.The new seat belt that will equip the cars of the future will also have a special cover for the driver's seat, which will know all the time data about his condition.If the driver has health problems that can affect traffic decisions and reactions, he will be alerted by the new technology, which has sensors that provide real-time information on heart rate and breathing.

Seat belt model with three-point grip
For the proposed analysis, the seat belt with grip on the central pillar of the vehicle was considered.This type of seat belt is the most common passive safety system in series vehicles.
The three-point seat belt elements (figure 2) are as follows: lower grip (anchorage), upper grip (anchorage) (on the central pillar of the vehicle), grip (buckle) at the level of the pelvis and retractor.In the first stage, the three seat belt attachment points are defined, respectively: the lower grip, the upper grip and the occupant's pelvis grip.Subsequently, it is necessary to define the connection between the three grips, as seen in figure 4.

Initial data and definition of the motion law of the assembly
The complete model consists of ten parts, comprising in addition to the seat belt and seat, and a part called vehicle.This part is built so that a law of motion can be imposed on the whole assembly.In figure 5 is represented the law of motion of the vehicle, and implicitly of the assembly, being imposed restrictions of connection between the vehicle and the assembly.A vehicle impact speed of 8.2 mm/ms (about 30 km/h) was considered.The simulation was performed for a time interval of 110 ms.
The properties for the materials used to define the parts of the model are shown in the following figures: -material properties for belt sections-seatbelt material -belt load curve (tension vs. force)

Model processing and presentation of the results obtained
Following the processing of the virtual model, results can be extracted for the analysis of the behavior of the seat belt.The interface with LS-Dyna software allows monitoring the behavior of the system both in video format and in the form of numerical results of the parameters of interest.Further, the visibility of the passenger seat has been disabled for the graphical representation of the contour of the displacements, speeds, stresses, deformations and accelerations of the seat belt, in order to be able to observe the results only at the level of the belt system:   The kinematic parameters of both the vehicle and the occupant's seat were also tracked.The two parts are defined as rigid bodies, being possible graphical representation of the variation of displacement, speed and acceleration, after the three directions, with the post processing module for Rigid body (rigid body).
For monitoring the action of seat belts were first defined shell-type elements.Thus, three elements were chosen for the ventral section and three other elements for the thoracic section.These elements were arranged towards the ends of the section, and one was placed in the middle area of the sectionsfigure 10.The elements defined on the thoracic belt are: 166959, 166981 and 167012, and those defined on the ventral belt are: 167116, 167145 and 167160.For the belt elements defined above, both the strain developed by longitudinal direction and the effective strain were plotted.From figure 11 it is noted that the maximum strain value is reached for the element 166981 defined in the midline area of the thoracic belt.The analysis of the belt behavior also involves monitoring the connection zones between the grip and the ventral and thoracic belts, respectively.For this purpose, five seatbelt type elements have been defined, each element being centrally located on the connecting areas.The elements defined and monitored are: 91119, 91165, 91174, 91180 and 91189 -figure 12.
From the graph shown in figure 13 it is noted that the maximum force value is recorded for the element 91119, which was defined on the area connecting the lower grip and the upper grip (on the central pole).According to figure 14, it is found that the element for which the maximum force value was recorded has the smallest length compared to the other four seatbelt elements analyzed.The seat belt assembly also includes two elements that allow the belt to slide, both for tightening and for relaxation.The total slip of the belt at the first point, defined at the level of the upper grip on the central pillar of the vehicle, is plotted in figure 15.The maximum value of the belt slip recorded at this point is 33 mm.The total slip of the belt at the second point, defined at the level of the middle grip of the belt, is plotted in figure 16.It is noted that at this point the slipping of the belt is not allowed, being the area containing the seat belt buckle.

Conclusions
The present study proposes a virtual prototype and testing method of the seatbelt system functioning in order to protect the driver in the event of a frontal collision.For this purpose, the three-point seat belt was modeled using the LS-Dyna software package.LS-Dyna software is a specialized software for the analysis and reconstruction of traffic accidents, using the finite element method.
For the proposed analysis, the seat belt with grip on the central pillar of the vehicle was considered.This type of seat belt is the most common passive safety system in series vehicles.A vehicle impact speed of 8.2 mm/ms (about 30 km/h) was considered.The simulation was performed for a time interval of 110 ms.
Following the processing of the virtual model, results were extracted for the analysis of the behavior of the seat belt.The interface with LS-Dyna software allows monitoring the behavior of the system both in video format and in the form of numerical results of the parameters of interest.They were plotted the contours of the displacements, speeds, stresses, deformations and accelerations of the seat belt.The kinematic parameters of both the vehicle and the occupant's seat were also tracked.

Figure 2 .
Figure 2. Safety belt elements with three-point fastening.

Figure 8 .
Figure 8. Contour of deformations and stresses [kN/mm 2 ] after the longitudinal direction-x axis.

Figure 14 .
Figure 14.Length [mm] of the elements in the connecting areas of the belt.

Figure 15 .
Figure 15.Total belt slip for the first point.

Figure 16 .
Figure 16.Total belt slip for the second point.

Figure 17 .
Figure 17.The force developed by the seat belt retractor.The seat belt retractor is positioned in the lower area of the center pillar of the vehicle.The forces acting it are presented in figure 17.