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Sports Helmets and Impact Testing of Polymers

Grades: 5-8
Author: Sandy Van Natta
Source: Original


Abstract

In sports, participants may be subjected to collisions with balls or other people or even crashes on bikes and skateboards. How is it possible for athletes to tolerate such blows and still "remain in the game"? This activity allows students to use inquiry to investigate the materials used to make sports helmets, a modern form of body armor. The students will perform impact tests on plastic (polymer) samples by dropping a plumb bob from differing heights onto the samples. The tests, modeled after actual industrial testing methods, will measure the brittleness of a material that has been subjected to an intense blow. Both "hard" plastics that may be used in the outer shell of a helmet and foamed plastics that can be used for the inner lining of the helmet will be tested. The students will use data gained from their tests to determine which plastics they feel are most suitable for usage in a helmet.

A discussion of the use of science and technology to aid and protect humans will flow from this lesson. As students examine and determine the properties of various materials, they will be mimicking the scientific and technological processes involved in designing a consumer product for market.

Throughout this lesson, best teaching practices are emphasized. Inquiry and hands on/minds on techniques are used in conjunction with the learning cycle to show the ties between science and technology.


Objectives

What should students know as a result of this lesson?

  • Students will know the importance of safety gear used in sports.
  • Students will be able to explain the purpose of an impact test.
  • Students will explain that impact resistance describes how a material reacts to shock or sudden stress.
  • Students will identify the functions of the hard outer shell and the foamed inner lining of a sports helmet.
  • Students will distinguish between the structure of a helmet made for multiple collisions, such as a football helmet, and a helmet made for a single collision, such as a bike helmet.

What should the students be able to do as a result of this lesson?

  • Students will test the ability of various polymers to resist impact (sudden blows).
  • Students will be able to compute impact in inch-pounds or Newton-meters.
  • Students will work cooperatively.
  • Students will perform tests on materials designed to mimic actual industrial tests and collect data.
  • Students will examine and analyze data collected from the testing of polymers to determine which plastics would be most suitable for use in a helmet.
  • Students will design and make a miniature helmet based upon their data analysis.

Materials

  • A video clip or picture showing a football tackle, a fall from a skateboard or bike, etc.
  • Samples of a variety of hard plastics – cut nearly the same size * - each sample labeled with the correct name
  • Clear tube or stiff cardboard tube from a wrapping paper roll with a viewing slit cut in the side. Tube should be at least 30 inches long. Use a magic marker to mark off the tube at 6 inch intervals.**
  • Plumb bob or other standard weight to be used as a dropping object
  • String - at least 36 inches long
  • Piece of wood to set under the test area – square block
  • Safety goggles
  • Ruler or meter stick
  • Ring stand and clamp – optional,
  • 1 1/2” dia. PVC coupling or small diameter jar lid
  • gloves - optional

*Plastic samples can be cut from the sides of detergent bottles (polyethylene). Lids from plastic food jars are often polypropylene. Pieces of polycarbonate (Lexan) can be obtained from scrap “piles” at glass shops. Plexiglass can be bought at any hardware store. Other plastic samples can be obtained from local stores that specialize in plastics. Sheets of high density polyethylene, ultra high molecular weight polyethylene, Lexan, and polypropylene can be purchased from Universal Plastics. Contact Tim Smathers at 330-645-6873. A 2 ft x 2 ft square of 1/8” plastic ranged from $4.45 to $9.60. E and T Plastics (1-800-383-7753) has ABS as well HDPE, UHWWPE, and others but 4 x 8 foot pieces must be purchased. This company will cut the pieces to any size.

**Clear tube is available at hardware stores. Look for tubing made to cover fluorescent lights. It comes in 4 ft and 8 ft lengths and is inexpensive. Various foam or padding materials such as styrofoam, foam rubber, polyurethane foam, bubble wrap, rubber, etc. Materials should be of similar thickness. Miniature or regular flat chocolate bars such as Hershey’s, Hollow or cream filled chocolate balls, ½ of a plastic Easter egg or sports ball egg

Dow’s Great Stuff™ insulating foam sealant or other similar variety. Ground up polymer samples from an athletic shoe - rubber samples from the outer sole and foam samples from the mid sole. Samples are inexpensive and available through Nike at www.airtoearth.com. A kit of activities is available with a curriculum guide for $24, or without the guide for $19 (the guide can be downloaded for free). However only the grind is needed for this lab. A grind refill kit is available for $11.

The crosslinked polyethylene foam can be purchased from Nantahala Outdoor Center, 1-800-232-7238. It is their minicell foam (Foam-MF-SX-650) and is $3.50 a square foot.

***Impact Testing Kits and refills can be ordered from: Educational Innovations Inc. www.teachersource.com; 888-912-7474

Impact Testing kit #IMP–100 – includes instructions, string, plumb bob, clear tube, wooden block, plastic collar, and 22 testing materials (2 samples of 6 different hard plastics and 5 different foams)

Impact Testing Refill #IMP-125 – 8 pieces of 6 different hard plastics and 5 foams for a total of 88 testing samples


Procedures

Engagement

A short video clip showing a football tackle or a fall from a bike or skateboard can be used to catch students' attention and allow you to present the problem. Ask students how they think athletes can withstand tremendous blows and still "remain in the game". After students have determined that protective body equipment is needed to minimize injury, you can tell them the purpose of these activities is to test materials and to determine the best ones to use in protective headgear (helmets).

To make your initial discussion more interesting the following background information is given for your use:

In sports, body armor is designed to protect the participant from projectiles (mainly balls), from human impact, and from crashes. Many sports, such as football, cycling, and baseball require athletes to wear helmets. The outer shell of the helmet must be impact resistant while the interior must mitigate an impact by slowing the deceleration of the head.

The National Operating Committee on Standards for Athletic Equipment (NOCSAE) has developed voluntary test standards designed to reduce head injuries by establishing requirement of impact attenuation for football helmets/face masks, baseball/softball batting helmets, baseballs and softballs, and lacrosse helmets/face masks. The various regulatory bodies for sports, including the NCAA and the National Federation of State High School Associations have adopted these standards.

In football, on the field concussions are considered one of the most serious of contact sports injuries. Concussions are most likely to happen as the result of a blow to the side of the head, rather than the front or top, according to new research conducted by Biokinetics & Associates. Football is responsible for approximately 100,000 concussions in the United States each year.

The NOCSAE test standard for football helmets involves mounting a helmet on a synthetic head model and dropping it a total of 16 times onto a firm rubber pad from a height of 60 inches. Drops are made onto 6 locations on the helmet. Most of the drops are conducted at ambient temperatures, however, at least 2 of the drops are conducted immediately after the exposure of the helmet to 120° F for four hours. The equivalent of a 60-inch drop test would occur if a player running at 17.9 ft/sec (12.2 mph) ran into a flat surface which stopped his head in less than one inch. Most players run faster than this (average speed of a player running 40 yards in 4.8 seconds is 25 ft/sec) but vary rarely would the head be stopped in such a short distance on the football field.

In baseball and softball, the primary hazard is being struck by a pitched or batted ball. Therefore, baseball helmets are mounted on an instrumented headform that is free to move. An air cannon is used to shoot a baseball from close range into the helmeted headform at 60 mph. Impact accelerations are measured and a Severity index is calculated.

Assessment: Assessment is informal at this stage. Try to involve all students in the discussion. Allow students to cite examples from their own experiences relating to the importance of protective body equipment.

Exploration I

Exploration I allows students to test materials that could be used in the outer shell of a football or baseball helmet.

Procedure:

  • Obtain the labeled plastic samples for testing. Set up the testing apparatus as shown. The plastic sample can be placed on top of a PVC coupling or the rim of a jar lid so that the weight can penetrate through the sample before hitting the wood. The end of the tube should be immediately above the plastic sample.

Note: the tube can be held by hand if a ring stand is unavailable. Younger students may want to hold the tube anyway – for a more secure grip.

Sports Helmets Impact Testing Lesson

  • Unscrew the top of the bob and attach a string so that it is centered on the bob. Lower the bob down through the tube so that it is 6 inches above the plastic sample.
  • Release the string so that the bob falls freely onto the plastic sample. Inspect the sample for cracks and breaks. Failure of a sample can be deformation that is evident on the back side of the sample, crack initiation, or complete breakage.
  • If the sample has not "failed", increase the height from which the bob is dropped by 6 inches. Keep increasing the drop test height by 6 inches each trial until the sample is broken or until you have reached a maximum of 30 inches. Record the height at which the sample breaks or the maximum height used to test it.
  • Multiply the height of the drop by the weight of the bob to obtain impact in inch-pounds.

All Impact Standard testing is done in the English system. Therefore, the English system will be used in this laboratory, too. The unit of measure for the impact test is the inch-pound. If a 10 pound weight is dropped an inch onto the sample, then it equals 10 in-lb. You will be using an 8 oz plumb bob (or 0.5 lb.). The plumb bob will be dropped through a tube from increasing heights onto the plastic sample. If you would prefer to use metrics, you can compute impact in Newton-meters.

Assessment: Make sure students are following proper laboratory procedures. Monitor the groups and make sure they are recording information on their worksheets.

Explanation I

After completion of testing and recording of data, students should report their findings and determine the most suitable materials for use in a particular type of helmet, of the ones tested, based on their results. There are no wrong answers here as long as students can back up their statements using evidence from their data.

Ask students to explain impact resistance and its importance in determining materials to be used in the outer shell of a helmet. Make sure they know that impact resistance describes how a material reacts to a shock or sudden stress. This test measures the brittleness of a material that has been subjected to an intense blow. The property of toughness describes the material's ability to withstand such an impact.

To relate student testing to product development in industry let students know that most product engineers do not use the results from impact tests alone to determine which material will be used in a commercial product. Decisions may also involve information gathered from tests such as stress/strain, tensile strength, ductility, or thermal sensitivity. However, this lab concentrates on the impact resistance of polymers that could be used in constructing sports equipment.

One test used in industry is the Gardner Impact Test. During this test, a weight is dropped from a specified height onto a test sample until it breaks. Although the sample may be any size, industry typically uses a 3 x 5-inch sample with a 1/16 to 1/4 inch thickness. (Since a teacher needs to use whatever materials are readily available, sizes in this lab may vary. It is suggested that samples should be as reasonably close in size as possible.)

Temperature is a major variable during testing. Most industrial tests are conducted at 20° C. These tests will be conducted at room temperature.

The discussion in this section can now lead to the importance of the foam linings placed inside of helmets. This leads you into Exploration 2.

Assessment: Collect and evaluate student worksheets for completeness and accuracy.

Exploration II

Exploration II allows students to investigate the properties of various polymeric materials which could be used in the interior of a helmet. Testing will be used to assess a material's ability to protect an object (such as a head!). Weights will be dropped on various foam materials placed over a "fragile" candy bar. Students will be able to determine which materials offer the best impact attenuation and even be able to design their own materials for testing.

Procedure:

  1. Using one type of foam material, set up the apparatus as shown below.
  2. Hold the plumb bob, narrow end pointed downward at the top of the tube. Drop the bob through the tube. Visually try to see how far the foam is compressed. Measure the indentation.
  3. Remove the foam material from the top of the candy bar. Check the bar for breakage and cracks.
  4. Repeat the drop test from the same height using the other available foam materials.

Variation: The candy bars can be frozen. If so, they will show even more damage resulting from the drop. Allow students to suggest and try their own testing procedures if reasonable.

Sports Helmets Impact Testing Lesson

Extension (Optional)

Design your own protective packing material. Mix various combinations of ground up foam and rubber with a small amount of white glue. Two sample recipes are provided.

  1. Mix 1/4 cup foam with 1/4 cup rubber and 3 tsp of white glue. Press the mixture into two cups of a muffin tin with a spoon. Allow the mixtures to dry 48 hours before testing.
  2. Mix 3/8 cup foam with 1/8 cup rubber and 3 tsp of white glue. Press the mixture into two cups of a muffin tin with a spoon. Allow the mixtures to dry 48 hours before testing.

You can also fill the muffin tin cups with foam sealants from spray cans such as Dow's Great Stuff™ insulating foam sealant. Great Stuff™ can also be sprayed directly onto wax paper.

Repeat the drop tests using the new packing materials and compare results.

Assessment: Make sure students are following proper laboratory procedures. Monitor the groups and make sure they are recording information on their worksheets.

Explanation II

Have students report their findings. This can lead to a discussion on different types of helmets and the use of rigid foam in some and soft foam in others. Again, any student answer is acceptable as long as it can be supported by data.

Helmets work by bringing the head (and brain) to a relatively gradual stop upon impact. When an unprotected head strikes against a hard surface, inertia causes the brain to slam forward against the skull. This causes bruising and bleeding.

If you look inside most helmets you will see foam pads. The foam softens the shock by gradually crushing to absorb impact energy, thus cushioning the blow. This slows the stopping process from about 1 millisecond to 6 milliseconds, thus reducing the spike of energy to the head and brain. As the foam crushes or deforms, it converts some of the crash energy into heat.

According to the Law of Conservation of Energy, energy is not lost but converted to some other form of energy. What a helmet does during a crash is referred to as “energy management”. Some of the energy of the collision is converted to heat.

Current helmets perform energy management with some type of foam. There are two types of foam: one type is stiff and crushable, the other is rubbery or squishy. Their characteristics make it possible to design a helmet for one very hard impact, a number of hard impacts, or a very large number of softer impacts. Denser foams resist very hard impacts. Softer foams compress more easily in lesser impacts giving better protection against milder injuries. Crushable foams are ideal for helmets designed for one hard impact (like bike helmets). When the foam crushes, it does not bounce back like a spring to make the impact worse. Rubbery foams (football and skateboard helmets) provide multiple impact protection but are less protective in very hard impacts.

Assessment: Collect and evaluate student worksheets for completeness and accuracy.

Elaboration

You may choose to present information on the actual plastics used in various helmet types or have students research this information on the Internet. Students might try to hypothesize why one material is favored for football helmets and others for baseball or biking helmets. Encourage students to suggest and try their own testing methods on the plastics.

Assessment: Have students write a short report on their findings. This report can be collected and evaluated or given orally to the class.


Prerequisites

N/A


Best Teaching Practices

  • Hands-on/Minds-on Learning
  • Inquiry
  • Authentic Problem Solving

Alignment with Standards

NGSS Standards:

  • MS-PS1-3 Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.
  • MS-PS2-1 Apply Newton's Third Law to design a solution to a problem involving the motion of two colliding objects.
  • MS-PS2-2 Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.
  • MS-PS3-1 Construct and interpret graphical displays of data to describe the relationships of kinetic energy to the mass of an object and to the speed of an object.
  • MS-PS3-5 Construct, use and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.
  • MS-ETS1-1 Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution,taking into account relevant scientific principles and potenitial impacts on people and the natural environment that may limit possible solutions.
  • MS-ETS1-2 Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.
  • MS-ETS1-3 Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
  • MS-ETS1-4 Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.

Common Core Standards:

  • RST.6-8.1 Cite specific textual evidence to support analysis of science and technical texts.
  • RST.6-8.3 Follow preciesly a multistep procedure when carrying our experiments, taking measurements, or performing technical tasks.
  • WHST.6-8.2 Write informative/explanatory texts, including the narration of historical events, scientific procedures/experiments, or technical processes.

National Standards:

  • Content Standard A: 5-8 Science as Inquiry
  • Content Standard B: 5-8 Physical Science
  • Content Standard E: 5-8 Science and Technology
  • Content Standard G: 5-8 History and Nature of Science
  • National Education Technology Standards 3. Technology productivity tools
  • National Education Technology Standards 5. Technology research tools

Ohio Standards:

  • Grades 6-8 Scientific Inquiry Benchmarks A and B
  • Grades 6-8 Scientific Ways of Knowing Benchmark A
  • Grades 6-8 Physical Sciences Benchmark A
  • Grades 6-8 Science and Technology Benchmarks A and B

Content Knowledge

Information on types of helmets, current testing standards used on athletic equipment, and current industrial tests used on equipment is given in the Explanation sections of this activity. Students need only to be able to read a ruler and perform simple math calculations.

After completing this activity, students should know how important product testing is in determining the use of a material in a consumer product. Even the simplest objects may have hours of testing and design involved before they can be brought to the market.

Resources used:


Safety

Goggles should be worn during experimentation. Perform the tests on the center of your work table or desk to minimize the chance of the weight falling on your foot. Younger students may wish to set the apparatus directly on the floor. Since some plastic samples may break, the person holding the tube may wish to wear gloves. Closed-toed shoes are recommended. If a spray foam sealant is used, use the can in a well-ventilated area and follow all safety precautions on the can.


Applications

Impact testing is a common industrial procedure. Any material which must sustain a blow will be tested from sports equipment, to car parts, to construction materials. A plastic is chosen for use in a particular product by its physical properties as well as cost. Industry will use the product that best suits a manufacturer's specifications while maintaining a reasonable cost.


Assessment

Have the students design a miniature helmet using the knowledge they have gained during their testing. Each student can be given half of a plastic egg shell or half of a plastic sports ball shell (sports ball shells are available and in the same general store location as the eggs at holiday time) to be used as the outer shell of a helmet. Ask each student to design and make a lining for their "helmet". The helmet must protect a round hollow or cream filled chocolate ball during an impact test drop of 30 inches.

Each participant can be given their helmet shell to take home and line with whatever shock absorbing material they choose. Helmets can be brought back to the group for testing at a future date if possible. You may want to give small prizes for any helmet which protects the chocolate ball during the impact test. The "survival" of the chocolate ball can be used as the assessment.


Other Considerations

Grouping Suggestions: Be aware of students' abilities and ethnic backgrounds when choosing groups. Try to place students of varied abilities and backgrounds in each group. For this particular activity, it may be helpful to place a student actively involved in sports, and who regularly wears a helmet, in as many groups as possible.

Pacing/Suggested Time: The Engagement through both Explanation sections can be completed in two 45 minutes periods. Students can complete most of the final assessment (designing their own helmet ling) at home. It will take at least 15 to 30 minutes to test the student helmets in class after completion depending on the amount of discussion generated during the testing.


Printable PDF Worksheets

Sports Helmet Worksheet