Safety Analysis of the Five Point Harness

Transcription

1 ENSC Introduction to Biomechanics - Project Report - Safety Analysis of the Five Point Harness Student name & number: Miguel Cruz Student name & number: Luke Gall Student name & number: George Ioannou Student name &number: Vassili Nossov Report due date: July 31, 2013

2 Abstract The following report investigates the effects of a 5-point harness on the cervical spine, mainly C1 and C2 vertebrae. Using Madymo and a class distributed model of a dummy in a car seat we create a 5-point harness system and perform frontal collision testing at 30km/h and 50km/h. Using the results we analyze the torque, displacement, acceleration and velocity of the head and neck to determine if the 5-point harness is more beneficial or detrimental to a typical 3- point harness. We validate our results with previously conducted research and present future recommendations for similar testing. Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 2

3 Contents Abstract... 2 Introduction... 4 Background... 4 Method... 5 Experimental Results and Comparison... 7 Result Validation... 9 Conclusion References Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 3

4 Introduction A 5-point harness is one of the most common seatbelts used in the racing industry and is preferred for its high safety ratings. It consists of 5 straps, 2 located at the shoulders, 2 at the hips and one at the crotch all coming together at a buckle release mechanism (see figure 1). Although this ensures the upper body of the driver is tightly secured at all times, the downfall of this design is the lack of security of the drivers head. In our project we will be focusing on analyzing the accelerations and forces that the human body experiences upon sudden braking with the 5-point harness as the safety mechanism. Using Madymo we will create the 5-point harness over a dummy which is situated inside the default car scenario. The simulation will consist of frontal collisions at both 30 km/hr and 50 km/hr. The data will be compared between the two different speeds as well as the difference between the 5 point harness and a regular seat belt at these speeds. With data to show the benefits/drawbacks of the 5- point harness we will make several suggestions on how it is Figure 1: 5-point Harness possible to improve on any drawbacks we find. Emphasis will be placed on the G-forces that the cervical spine experiences as it is the only part of the body that is not held securely by the harness. Our aim is to ensure that the sudden stopping motion will not cause permanent damage to the cervical spine. Background In 2001, the death of the iconic NASCAR racer Dale Earnhardt proved to be the catalyst that will push the motorsport industry to make major overhauls in the safety of its drivers, especially in harnesses. Dale Earnhardt suffered from Basilar Skull fracture due to having his neck exposed to fatally high loads during the crash. Earnhardt suffered the full whiplash motion of the crash due to stock racing cars having a lack of head restraints as well as having a less than optimal harness restraint. The motion was such that Earnhardt s head was oscillating back and forth without hitting the steering wheel which would have absorbed the impact and undoubtedly saved his life. The aftermath of Earnhardt s death called for more transparency within the motorsport industry with special considerations to safety. Several actions have since been taken to protect drivers, such as improvements on the five-point harnesses and the design of the six-point harness. Furthermore, the use of the HANS device was deemed to be mandatory in order to reduce the whiplash motion of the head. The MADYMO simulation makes several assumptions on the experiment parameters. The first assumption is that the person has a height of two metres. The second assumption is that the Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 4

5 person has a weight of 100 kilograms. The third assumption is that the forces in the neck are simply the joint forces. Furthermore, the time period of interest is between 110 milliseconds and 145 milliseconds. The calculations will refer to the 130 millisecond time frame when the peak torque and forces in the neck occur. Therefore the acceleration used for the calculations is 120 m/s 2 which occurs at 130 milliseconds as presented in Figure 7. The purpose of the calculations is to verify the accuracy of the MADYMO simulation. Method The approach to this project was to minimize the setup time and maximize the time spent on running simulations and acquiring test results. With this in mind we concluded that the simplest approach was to modify the 3-point seatbelt model that was used in lab 4 and optimize the parameters for our project. Our first step was to relocate and reshape the buckle to mimic a real 5-point harness. The buckle was relocated from the right side of the dummy to just above its pelvis which is where the 5 straps of the belt meet. Reshaping of the buckle was also required as the lab 4 model was a rectangle and the 5-point harness utilizes a circular buckle with roughly ½ thickness. Following this we recreated the 2 belt straps that already existed using new bodies, points and joints and placed them on the right side of the dummy (looking from behind the dummy). Additionally we moved the 2 shoulder straps more proximally to the centre of the dummy. Shoulder straps on the 5-point harness are intended to pass over the clavicle and rib cage and converge at the buckle located slightly above the pelvis. The final modification came with the creation of the 5 th belt strap that passes over the pelvis. A similar procedure was used where a new body, point and joint was created and the belt was fitted optimally around the surfaces of the dummy. Our final step was to create all the necessary groups and contacts between the dummy and the new belt straps to ensure that the belt was limiting the motion of the dummy and not passing through it. Upon completion a final belt fitting was performed to optimize the surfaces that the belt interacted with and to ensure they all met at the centre of the buckle. As mentioned in the introduction tests were performed at 30km/h and 50km/h with our main focus centered on the forces on the head and neck while also looking at the displacement of the head and neck and comparing this with the results we attained in lab 4 using the 3-point harness. In Figure 3 you can see the final Madymo representation of our 5-point harness. The belts are located optimally for our tests and the retractor has been removed mimicking a true harness that keeps the driver constrained to the seat at all times, not only when a collision is detected. In Figure 2 you can see a 5-point harness installed in a race car. Notice that it is relatively similar to the model in Madymo. Figure 2: 5-point harness in a race car Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 5

6 Figure 3: Snapshot of 5-point harness Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 6

7 Experimental Results and Comparison Shown below in Figure 4 is a snapshot comparison of a 3-point harness to a 5-point harness. This snapshot was taken at the same time for both simulations for an impact at 30 km/h. It is clear that the 5-point harness does its job at restraining the body of the driver. With a 3-point harness, the head of the driver impacts on the steering wheel even at this low speed. The 5-point harness protects the driver from cranial injury, but this is done at the cost of increased stress on the neck which could potentially cause more harm. Figure 4: Comparison of 3-point and 5-point impact Looking at the position of the driver s head for the two tests in Figure 4 we confirm that the motion is more restricted when using a 5-point harness, reducing the risk of collision with the steering wheel or the cab of the vehicle. This can be seen in Figure 5 where the position of the head is compared for the two scenarios. Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 7

8 Figure 5: Head position comparison The damage to the driver becomes more apparent when the moment applied to the base of the skull is considered. The graph in Figure 6 shows a comparison of the moments applied on the C1 vertebrae when using the two restraint systems (3-point in blue and 5-point in green). Due to the restraint of the body by the 5-point harness, the neck experiences a smoother load with only one peak unlike the 3-point harness, but this peak is greater in magnitude as expected. Figure 6: Neck moment comparison Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 8

9 The second criteria checked was the force applied to the head. One of the main benefits of the 5- point harness is that it prevents the driver from hitting objects in the cab. To compare the net force exerted on the skull of the driver, the acceleration was measured. The results in Figure 7 (3-point in orange, 5-point in blue), show that the acceleration of the skull is actually greater when using the 5-point harness. This does not contradict the design purpose of the restraint, but points out that its presence causes other sources to exert greater stress than the impacts that it prevents. Result Validation Figure 7: Head acceleration comparison To validate our model and the results from it, the 5-point harness results were compared with that of literature. In An investigation into neck injuries in simulated frontal impacts by Naif Al- Shammari and Clive Neal-Sturgess it can be seen that the acceleration of the head in Figure 8 matches our results very closely. Both values can be seen to peak at ~300m/s 2 for the 30km/h case at C1. The difference in magnitude of the acceleration reported is due to their test being a 15g impact compared to our 7g impact. If scaled, the magnitude of our results closely matches theirs. Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 9

10 Figure 8: Head acceleration from Al-Shammari Al-Shammari also reported on the moment that the vertebrae of the cervical spine experience for a series of impacts shown in Figure 9. Our reported moments for C1 are ~40Nm where their tabulated value can be seen at roughly 100Nm. Our results are similar but not identical which is expected due to our different impact force and simulation parameters. Figure 9 Cervical bending moment from Al-Shammari Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 10

11 Conclusion Firstly it must be noted that the analysis conducted was not optimal for a 5-point harness. In a real life scenario the driver is placed in a race seat that eliminates and movement of the body laterally and the 5-point harness limits movement of the body anteriorly. With our results we can conclude that the belt was not optimized to limit the motion of the dummy. Although the motion was greatly reduced in comparison to the 3-point harness it still allowed the dummy to displace to a point where the head made contact with the steering wheel at 50km/h. Secondly, in a race car the steering wheel is placed in a location where the head of the driver cannot impact it at any point during a collision. In regards to our results we find that the 5-point harness aids in restraining the body and preventing the head with making impact with the steering wheel but because of this it sharply increases the forces experienced by the cervical spine. The current solution to this issue is a neck brace that is worn by professional race car drivers. This neck brace limits the movement of the neck and in a collision the neck will not displace more than several millimeters in any direction. In future testing it would be ideal to implement a similar neck brake and also add the helmet as an additional protective measure. The helmet will add weight to the system but the brace should counteract this by limiting the displacement as mentioned earlier. Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 11

12 References I. Al-Shammari, Naif & Neal-Strugess, Clive (April 27 th, 2012): An investigation into neck injuries in simulated frontal impacts II. Ed Hinton, ESPN.com (February 7 th, 2001): Earnhardt s death a watershed moment III. T.J Gibson, K. Thai, Australian Transport Safety Bureau (June, 2007): Helmet protection against Basilar Skull Fracture Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 12

13 Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 13

14 Description of tasks Safety Analysis of the Five Point Harness Miguel Cruz: Madymo simulations, hand calculations, background research, report writing George Ioannou: Madymo simulations, methodology development, report writing Vassili Nossov: Madymo simulations, validation research, report writing Luke Gall: Madymo simulations, theory development, report writing Miguel Cruz, George Ioannou, Luke Gall, Vassili Nossov Page 14