The overall goal of this method for simulating helmet impact is to quantify helmet fit and retention and evaluate the risk of injury of different helmet fit scenarios. This method can help answer key questions in helmet research, such as, the effect of helmet fit on head and neck injury like. Main advantage of this technique is that new insight into the effective helmet fit on injury likelihood maybe quantified using a simple methodology.
Head exposure as well as head and neck injury, maybe studied with this method. Simulate impact to the helmeted head by linearly guiding the head form to hit an impact surface. Assemble a drop tower to consist of an adjustable drop gimbal, an anthropometric test device and a variable impact surface.
Within the head form, arrange nine uniaxial accelerometers in a three, two, two, two configuration to measure linear and angular accelerations at the head form center of gravity. Next, set up a customized velocity gate on the impact tower to measure the impact velocity just before impact. To collect the head acceleration data, set up the data acquisition system.
Use an anti-aliasing low pass filter with a corner frequency of four kilohertz to filter the data prior to sampling. Then, set the sampling rate of each filtered signals to 100 kilohertz. Various scenarios can be tested using the set up, impact speed, impact surface and type of impact, head or torso first, are all variables.
First, raise the head form to the appropriate height. Using drop heights of 0.82 and 1.83 meters, achieve impact velocities of four and six meters per second. The six meters per second impact velocity represents standards in the field.
For an impact surface, set up a flat or a 45 degree angled anvil, cover the anvil's surface with abrasive tape to simulate an asphalt surface. Adjust the anvil's position as needed so that the helmet only impacts the anvil's flat surface. Next, set up the drop tower for a head first or a torso first impact.
Position the block at a height to impact the gimbal when the helmet is 25 millimeters from the anvil. For this test, also use some foam to minimize vibrations from the drop tower. Now, only neck flexion will carry the helmet into the anvil.
Repeat the same impact and fit scenario configuration three times, using a new helmet for each test. Arrange two high speed cameras around the drop tower to capture synchronized images of the helmet and head form movement during impact. Use a macro lens to get a tighter field of view and set the f stop to about eight.
Next, set the image size and frame rate for the recording. Then, synchronize the two cameras using their internal clocks and set up a trigger to start them simultaneously. Place a 250 watt light between the cameras to provide sufficient illumination.
The next step is to calibrate the camera viewed space. Position a calibration cage marked by 17 calibration points in the field of view of both cameras and take a single image from each camera. The cameras must see at least 11 common points.
Follow the steps in the text protocol to calibrate the camera using these images. Position the reference marker between the eyes on the lower forehead and position the other markers uniformly around the head form. Then, take a still reference image of the head form from each camera.
Each camera must see at least three markers including the reference marker. Now repeat this process with the helmet, always remove the detachable visor to improve visibility during tracking and position the markers so that a reference image from each camera views at least four markers. Now, set up and test the simultaneous trigger for data acquisition, camera recording and head form release.
Finally, trigger the start of the data acquisition system with the start of the camera recordings and release the head form. Capture three seconds before the drop and eight seconds after the drop. After the test, manually review and crop the video so they contain just the impact data.
Head linear acceleration, angular acceleration, angular velocity, upper neck force and upper neck moment were computed from the absolute norm of the directional vectors. A neck injury criterion from the neck axial force and moment was also computed. Different events of the impact could be identified.
For instance, head contact with the anvil and a torso first impact is observed as the large peak and the result in linear acceleration. In angular acceleration, two sets of peaks are observed. The first peak occurs as the result of the torso impact, while the second peak occurs as a result of the neck reaching maximum flexion.
In sequence, the events of the impact are torso impact, followed by head contact with the anvil and then, the neck reaching maximum flexion. The magnitude of the vector between the forehead and helmet brim, indicates head helmet displacement. This value represents the helmet's position on the head with large values demonstrating more head exposure.
Changes in head helmet displacement represent helmet movement. The presented methods can be used to examine head protection with helmets, with regard to injury likelihood, helmet retention and stability.
