As mentioned on the Introduction page, the purpose of this project is to design and manufacture an apparatus for testing a helmet according to the Snell M2010 Standard

The team has narrowed down the design to two possibilities: a monorail design and a twin wire design. In both of these designs, the helmet is attached to a head form and then dropped from a specific height. The two differences are primarily in the mechanism used to guide the head form to the anvil. The monorail design uses a steel shaft while the twin wire design uses wire rope.

The results from the analysis can been seen below in the current report section

Some more general information can be seen below:

FOR A COPY OF THE FINAL REPORT CLICK HERE

FOR A COPY OF THE EXPERIMENT REPORT CLICK HERE

For a copy of the infrastructure project click here

For a copy of the Snell 2010 Standards click here

For a copy of the Helmet Standards Comparison click here

For a brief analysis of the different helmet standards in a spreadsheet form click here

The twin wire design takes advantage of two cables to guide the head form to the anvil. This design is lightweight, less expensive than the monorail, and the drop assembly is less likely to absorb impact energy. Some prints of the droptower components can be found in pdf form here

Purpose
The purpose for a wireless data acquisition system for the head impact drop tower is to demonstrate proof of concept for smaller more adaptable systems to be placed inside helmets that are actively in-use to monitor everyday impacts to the head. This project goes a step beyond just a wireless system and aims to give information on the severity of impacts as they occur or in real-time.

Equipment
The wireless data acquisition system used in this project is built from a variety of components. The primary acquisition and wireless communication is done with two Crossbow TelosB Mote Platforms. Each mote is a self-contained unit capable of wireless data transmission and reception and analog to digital conversion of external sensors. The sensor used to measure acceleration during an impact test is a Kistler Picotron Mini Accelerometer Type 8614A1000M1. This accelerometer is capable of measuring ±1000g with a sensitivity of 2.5mV/g. The accelerometer has an onboard charge amplifier making it ideal for use inside the test head form as there is no need for signal conditioning hardware.

Software
The software used to receive the data from the TelosB mote is custom written in Python, an open source programming language. The program continuously plots data as it is received from the mote, providing the user with real-time feedback of the impact test.

Future
The final piece of the data acquisition system is to integrate the accelerometer with the head form and the TelosB mote. There are a few other components that can be added to the system however to make it more complete. First and foremost a high-speed camera is planned to capture the impact of the head with speeds of up to 32,000 frames per second. Second, a load cell can be used to capture the striking force of the head, and give a second estimate of the input energy for each impact, with the first being the acceleration.

The headform model was based on ASME specifications given in polar coordinate form. In order to create a model, the group had to convert these to Cartesian x,y, coordinates. Several layers were created and each layer had its own spline. These section curves were then joined by a lofted surface using SolidWorks. For the solid model or point lists, check the previous links. The plotted layers can be found here

The foam model was created after sections of the SolidWorks model were plotted. Afterwards, the sections were cut out of half inch foam insulation and covered with body filler. The figures below show the rough stages of this development process. Afterwards, the body filler was sanded smooth to provide a consistent, round surface.

Tuffalloy 4270 Series

Tuffalloy 4270 is a series of Liquid Molding Compounds with thermoplastic properties such as high impact, high heat distortion, and low viscosity. This unique chemistry developed by Hapco to meet today’s market demands for prototype and low production parts needs.

Most headforms used by Cadex Inc. are formed out of Magnesium K1a alloy. There are few (if any) thermosets for casting that will come close to matching the material properties of magnesium. The Tuffalloy resin has the highest tensile strength offered by Hapco Inc. and is recommended for the headform application. For a higher strength, fiberglass may be added to the resin mix and experimentation is required for this testing. Tuffalloy 4270 has a hardness shore of 84 D ± 5 which is slightly harder than a hardhat (physical comparison).

Expense

Tuffalloy 4270 is available in quart units or gallon units. A “unit” comes with the appropriate amount of both the resin and hardener. A quart unit contains a total of 4 lbs of material and costs $63.87 plus a $10 re-packaging fee since Hapco Inc. normally sells quart units by the case (6 units per case). A 1-gallon unit contains 16 lbs of material and costs $207.33.

The Hapco Inc. Customer Service Department is contacted via telephone at 877-729-4276.

Mold Information

The mold most likely need to be waxed and sprayed with a mold release, such as Grease-It 4 or 5.

Key Advantages

Thermoplastic type properties
Fast cycle time
Excellent Physical Properties
Prototype use
1:1 Mix ratio

Pouring the Headform

Mix Part A (resin) and Part B (hardener) by hand and pour mixture into mold. Pour from one end so the material pushes air out as it fills the mold. The mix ratio of Tuffalloy 4270 (A:B) is 100:100 by volume and 100:85 by weight. If we do not plan to use the full amount in the containers, we will need to weigh out the correction proportion of each component. This material is very low viscosity and de-airs pretty well on its own, but without vacuum degassing or pressure casting we are likely to get some small air bubbles in the headform since mixing by hand will introduce a lot of air and gas into the material.

Casting Equipment

A pressure chamber would be the best option when casting the Tuffalloy 4270. Curing the headform at 60 – 80 psi would allow for a better headform. The pressure chamber will eliminate voids in the material that would otherwise be seen if the headform cured in an open room at room temperatures and pressure. Tuffalloy 4270 Series can be pressure cast, vacuum cast, or open cast.

Shrinkage

Shrinkage is largely influenced by the following factors:

Mass (total volume and thickness)
Temperature of the material
Maximum temperature reached during the exotherm (reaction)
Temperature of mold
Thermal properties of the mold material (Insulator versus Conductive)