Analysis of Head Impact Testing Apparatus

Transcription

1 Wireless Head Impact Group 16 December 2010 Analysis of Head Impact Testing Apparatus The focus of this document is to quantify the differences between the design options that this group has formulated thus far in the semester. The apparatus must be able to meet the newest of the Snell standards, M2010, since it is the most stringent and meets most other testing standards throughout the world. Since the standard is well defined and the test must meet stringent requirements, the design criteria led this group to two major designs: a drop tower with a monorail guide and a drop tower with a twin-wire guide. Both designs operate on similar principals. The head form is dropped from a specific height onto a stationary anvil and accelerations are recorded inside the head. The height itself would be determined by the friction in the guide mechanism as well as the required impact velocity. Immediate concerns were raised with the need for a free-sliding head support assembly, as well as the need to minimize the amount of energy absorbed by the head support assembly itself. Preliminary sketches were created to facilitate discussion about the pros and cons without spending a significant amount of time worrying about dimensional correctness. These sketches are shown below together as Figure 1. Figure 1: Preliminary sketches of drop apparatus. The left sketch shows a monorail while the right side shows a twin wire design

2 Feasibility In order to determine the feasibility of the two designs, it was important to consider the entire lifecycle through manufacture, testing, and future usage. Preliminary Calculations Common to Both Designs In order to start off the design process, an energy balance was used to provide a ballpark height that the head would have to be dropped from. Friction forces were unknown and would have to be determined experimentally, but the ballpark height would help provide yet another design constraint. Equation 1.1 below shows the equation used to get a baseline height. It is based on a conservation of potential and kinetic energy h= Equation 1.0 h= Equation 1.1 h=. (. =3.1 Equation 1.2 ) Equations 1.0 through 1.2 above do not take into consideration any reduction in velocity due to the drag on the cable or the monorail. The group determined that the blue frame in the Advanced Manufacturing Center would provide adequate height. The frame was already in place and mounting was fairly straightforward. It would also provide approximately 14 feet to work with. Monorail Design In the monorail design, there is a vertical rail which is used to guide the drop assembly towards the anvil at the bottom. As previously mentioned, it was immediately apparent that the rail and drop assembly must slide freely in order to achieve the required velocity in the shortest distance. If the fit was poor then the drop assembly would drag along the rail rather than slide on it. This was further magnified by the fact that the rail also acts as a support to the drop assembly in addition to a guide. Any excessive weight would enhance the dragging issue. Since the drop assembly is rigidly connected to the rail, it was noted that there would be a potential for some of the drop assembly to absorb some of the impact forces. In order to determine just how stiff the arm would have to be, finite element analysis was used to show how rigid the arm would have to be in order for this energy absorption to be negligible. Need for high quality monorail and high quality linear bearings or pillow blocks would be costly. Preliminary estimates for these components alone from MSC or McMaster-Carr would be at least $1000.

3 Twin Wire Design In the twin wire design, there are two vertical cables used to guide the drop assembly toward the anvil. Two wires were chosen for simplicity and ease of construction. More wires could provide more support, but could also cause challenges with alignment. Unlike the monorail design, the twin wire design is more forgiving with regards to apparatus energy absorption. Since the drop assembly is not rigidly fixed to the wires and is not supported by the wires in any way, it provides different energy absorption characteristics. This design was also friendly towards our mounting options. The I-beams would be able to provide support at the top and bottom and no intermediate support structure would be necessary. For a the drop tower prints, consult the website for the latest copy. Selection Matrix In order to quantify the differences between the two designs, it was first necessary to identify the areas of the project that were critical to success such as: 1. Ability to meet the Snell M1010 Standard If the design would not be able to meet the intended standard, those who would use the apparatus in the future would be required to make potentially costly redesigns. It is necessary for the apparatus to meet the standards. 2. Apparatus impact on test results When the apparatus does not change the test results, then it yields similar results to another test apparatus that is already in use in another laboratory. This is important for future users who may need to compare data received from another test. It is also important that the apparatus has a high degree of repeatability because it adds credibility to any data collected later. 3. Cost If the apparatus is too costly to build, then it will not be successful. 4. Ease of wireless implementation Wireless data acquisition is a requirement imposed by the group 5. Design suitable for manufacturing If the design does not take manufacturability into account then the group will only experience larger headaches next semester in the construction and testing phase. 6. Timeline (from analysis through assembly) If a design will meet all other criteria but will take an extended period of time to complete then there must be better options to consider.

4 Criteria Weight (1-4) Monorail (1-10) Twin Wire (1-10) Ability to meet the Snell M1010 Standard Apparatus impact on test results Cost Ease of wireless implementation Design suitable for manufacturing Timeline Weighted Total Table 1: Selection matrix showing the weighted scores for each of the designs

5 FEA Analysis of Both Designs

6 Current Drop Tower Parts List # PART NUMBER SOURCE DESCRIPTION COST QTY TOTAL 6061 Aluminum, 6' long, 1"OD, 0.065" Wall K751 McMaster Carr Thickness K821 McMaster Carr 6061 Aluminum, 12" long, 3" OD, Rod K343 McMaster Carr 6061 Aluminum, 6' long, 1x1" Square Tubing T131 McMaster Carr 304 Stainless, Wire Rope, 1x7 Strand T612 McMaster Carr Jaw and Jaw Turnbuckle, 6.25" Closed Length T34 McMaster Carr 2" OD Mounted Pulley

7 Headform Model 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 website.

8 Headform Foam 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. A figure below shows the rough stages of this development process. Afterwards, the body filler was sanded smooth to provide a consistent, round surface. Headform Composite 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 $ The Hapco Inc. Customer Service Department is contacted via telephone at Mold Information

9 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 Curing the headform at 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 exothermic (reaction) Temperature of mold Thermal properties of the mold material (Insulator versus Conductive)

10 Wireless Data Acquisition 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.

11 Data Acquisition Milestones -Acquire TelosB module to begin preliminary wireless tests: 21 September Complete successful wireless communication: 11 November 2010 Secondary Milestones Completed -PC to Mote communication -Sensor readings from Mote -GUI to capture wireless sensor readings 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.