This lesson plan can be useful to high school engineering/technology, physics, or environmental science teachers who are teaching sustainability. The lesson revolves around exploring the environmental impacts of different materials (aluminum, stainless steel, and titanium) that could be used for bike handlebars.It is designed to be completed in one to two standard class periods.The Eco-Material Adviser in Inventor is used for comparing different materials.An optional math-heavy engineering section explores how a material property (yield strength) can be used for calculating the thickness required for each material.
Begin a discussion of what “better” might mean in terms of the environment. Ask students which materiasl they think are “better” and why.
Continue the discussion around “better for the environment” for these three materials
Show the “Material Selection: Environmental Properties of Materials” video, which is 3:40 long video:http://www.youtube.com/watch?v=fNwyftI7e5cThe video discusses sustainability of materials -- sources, processing, embodied energy, and health impacts. It also mentions the Eco-Materials Advisor in Inventor, which is the tool that will be used in this lesson.
Discuss what each of these things means in terms of the environment:Carbon Dioxide Impact – global warming is caused by carbon dioxide buildup in the atmosphere, so the more CO2 that is emitted, the worse a material is for the environment.Recyclable – if a material can be turned into another product, it might offset some of its environmental downsidesSustainably Harvested/Extracted – this implies that the rate at which the material is taken from the earth is the same as the rate at which it is replenished. Wood and plants can be a good example.Toxicity – the material itself might not be toxic, but byproducts of manufacturing it might be.
Show the “Material Selection: Physical Properties of Materials” video, which is 3:18 long video: http://www.youtube.com/watch?v=54pQd1Jq4eYThe video discusses life-cycle analysis of materials, and addresses design tradeoffs for materials.
Discuss what each of these things means in terms of bicycle handlebars:Weight – the handlebars, along with the whole bike, shouldn’t be too heavy.Strength – the bike must withstand the forces put upon it during riding.Rust-proof – the handlebars should be able to get wet and not be affected.Manufacturable – if the material isn’t easy to shape, it won’t be easily manufactured.Cost – the cost must match who the bike is for (consumer? professional?)Not brittle – this is a material property that means the handlebars won’t easily break
Embodied energy represents the amount of energy used during the lifetime (i.e. “life cycle”) of the product --including the amount of energy “saved” in future products through recycling the product at the end of its life.The carbon footprint represents the amount of carbon dioxide emitted during the lifetime of the product.These are often two of the most important factors considered in environmental analysis of new products. Calculations of these two factors takes into account the four components shown. Using metal as an example:Extraction – this means digging metal ores out of the ground.Refining/Manufacturing – refining metals means separating them from their ore, and manufacturing usually involves heating up the metal and shaping it.Packaging – any additional materials that house the productsTransportation – getting the ores from the mines, and the metals from the factories.
While embodied energy and other resources required to extract, process, and dispose of materials should be considered, the amount of energy needed to USE a product must also be taken into account.In fact, it might be that most of the environmental impact comes from use, and the other stages become less critical in comparison. This is true of appliances, such as washing machines, that need water and electricity to run.
These two images are taken from the “Environmental Properties of Materials” video.It implies to mine and refine 1 kilogram of steel requires less energy, water, and other resources than 1 kilogram of aluminum. But then much less of those three things is required to recycle the aluminum.NOTE: the aluminum kilogram is shown as bigger than the steel one because steel is more dense than aluminum. Since the mass is 1 kilogram for both, the aluminum has a greater volume (density = mass / volume).
The Eco Material Adviser (EMA) is a tool for determining the environmental impact of material selection. The free version (in Inventor 2012 or later) has around 50 materials that can be assigned to parts and compared.
We will follow the recommended EMA Workflow for students. A downloadable PDF with more detail is available at http://sustainabilityworkshop.autodesk.com/sites/default/files/core-page-files/ema_quickguide.pdfHere is the additional detail from the PDF for each of the points:Establish design requirements and environmental priorities:Understand environmental goals, performancerequirements, and how the product will be usedSet baseline materials and processes: Identify parts to investigate, assign materials and processes, establish a baselineSearch for alternatives:Browse, search, reference material datasheetsWeigh trade-offs of alternative materials and processes:Apply alternate materials, compare results, develop reportsMake and document the material choice:Apply changes, make any resulting design changes, embed data in your model
This is step one of the workflow.Ask students what “design requirements” their handlebars should have. You can write their suggestions on the board, and then compare to the next slide. You can take a general list, and then try to distinguish between whether each suggestion is a “must have” or a “like to have” feature.
Actual handlebars might have different requirements from these, depending on who might buy them (e.g. consumer vs. professional).
This is step two of the workflow.We define the baseline to be handlebars made from a given material (aluminum) and process (forging/rolling), with the dimensions specified.
Engineering/technology teachers may wish to spend some time on what a “process” is, since it will be a choice in the Eco Materials Adviser.For other teachers, the main idea is that embodied energy and carbon footprint are determined in part by the manufacturing process (beyond extraction/refining). The image on the right shows red-hot metal being fed into a processing machine, which is an indicator of the high amount of energy needed to manufacture the metal.NOTE: Handlebars might be extruded, rather than rolled, but extrusion is not an option in the free, limited Eco-Materials Adviser.
“Functional Unit” is a term used in lifecyce analysis. The example used here is for a tire swing that might hang from a tree. Using more, thinner strands of rope can accomplish the same task as a single thicker strand – holding up the tire while someone swings from it. The “functional unit” of the rope is as an tire support that doesn’t break, and all different options must fulfill that requirement.Ask the class what the “functional unit” for bicycle handlebars is. ANSWER: Working handlebars that don’t break.References:http://books.google.com/books?id=Q1VYuV5vc8UC&pg=PA37http://books.google.com/books?id=Mcq-hYQSOwAC&pg=PA92
Another example might be a shopping bag, designed to hold 50 pounds of stuff. Paper bags need more material than plastic, so it’s not so clear which is better for the environment. NOTE: Point out that REUSABLE cotton bags might be better than either paper OR plastic.
This is step threeof the workflow.Alternative materials can be used to make an equivalent “functional unit” to the given aluminum handlebars.
This is an Ashby diagram, named after its inventor, engineer Mike Ashby from the UK. It can be a little intimidating at first. The main idea is that it compares two physical properties of a wide variety of materials. Since there are variations within each type of material, the bubbles show the approximate range of those variables.The three types of materials for this lesson are called out at the top – aluminum alloys, titanium alloys, and steels. They are the tall, skinny red bubbles, set within the larger lighter red bubble for metals.You can ask which of the three is the least desnse, and which can have the highest strength. ANSWER: Aluminum alloys are the least dense, titantium alloys can be the strongest (but note the vertical overlap between bubbles, such that three different metals could all have the same strength).MATH NOTE: This is a log-log plot, which means that the axes go up by a factor of ten, instead of the usual linear scale that students are probably used to.More Ashby charts:http://www.grantadesign.com/download/charts/new_strength_density.pdfhttp://www.rose-hulman.edu/~stienstr/em203/Ashby
Because different materials have different properties, we don’t have to use the same geometry for each one.Some tubes should be thicker, and some should be thinner.
Because different materials have different properties, we don’t have to use the same geometry for each one.Some tubes should be thicker, and some should be thinner.
Engineering/technology and physics teachers: We can “model” the handlebars as a cantilevered beam, which is a well understood engineering structure. This allows us to use the mathematical approximations to calculate the design of the handlebars.The force on the handlebar is from the rider. The moment generated where the beam is cantilevered depends on the force from the rider and the distance of the force from the fixed point, M = r * F.
It is worth pointing out that the cantilevered beam is how we are “modeling” the handlebar. This is another assumption, but a reasonable one given how bike handlebars are used. If the force was pushing in, instead of pushing down, a different model would be required. Learning how to model behaviors is part of learning science and engineering.
It is always good to make any assumptions in the design explicit.
Yield strength is acharacteristic of a material. It is the amount of stress (an engineering term) that the material can take before “bending out of shape.”The yield strengths for our three materials are given, as found in data sheets referenced below. The strongtest material is titanium, the weakest is stainless steel. NOTE: Austinitic is a materials science term that refers to the crystal structure of the material.Aluminum info:http://en.wikipedia.org/wiki/6061_aluminium_alloyDatasheet: http://www.matweb.com/search/DataSheet.aspx?MatGUID=3a2e111b27ef4e5d813bad6044b3f318Stainless Steel info: http://en.wikipedia.org/wiki/Austenitic_stainless_steel Datasheet: http://www.matweb.com/search/DataSheet.aspx?MatGUID=abc4415b0f8b490387e3c922237098daTitanium info: http://cartech.ides.com/datasheet.aspx?i=101&E=269 Datasheet: http://www.matweb.com/search/DataSheet.aspx?MatGUID=a0655d261898456b958e5f825ae85390
Since the strength order is titanium, aluminum, then stainless steel, we should need the least amount of titanium and the most amount of stainless steel to make the same “functional unit” of bike handlebars.
These next few slides are engineering and math heavy. Teachers can choose to skip these slides, or simply skim over them as an example of how engineers think about part design.In order to continue “modeling” our handlebars, we consider what it would take to break them. Another engineering term, the “section modulus,” relates the moment and the yield strength. The section modulus depends on the shape of the part.More info: https://en.wikipedia.org/wiki/Section_modulus
For a hollow tube, the section modulus is the equation given. Regardless of the math, the idea of an Outer Diamter and Inner Diameter is worth pointing out to students.Since the dimensions of the original aluminum tube are given, we can calculate the section modulus for it.
From the section modulus and the yield strength, the moment required for failure can be calculated for the aluminum handlebars.Since we are looking for equivalent functional units, we can assume that this is the same moment that the handlebars from ALL the materials will have to withstand.NOTE: Since we are assuming that the aluminum handlebars were properly designed, we can assume that this is the moment from the bike rider that they were designed to withstand.
This math trick allows the section modulus for different material to be related through their relative yield strengths. That is, the section modulus for stainless steel is 1.19 times greater than the section modulus for aluminum.
The assumption was that all of the materials need to have the same Outer Diameter to fit into the handlebar mount. This is fairly advanced algebra manipulation to calculate the Inner Diameter based on the new section modulus. This is optional to have students do on their own. MATH HINT: To take the fourth root on a scientific calculator, you can raise a number to the 0.25 power.Note that the Inner Diameter for stainless steel, 15.6mm, is smaller than the I.D. for aluminum, 17.8mm. This means that the thickness of the stainless steel will be greater, and fits with the earlier idea that the weaker stainless steel will require more material.
The section modulus for titantium is less than that for aluminum (0.29 times lower), which means the I.D. will be bigger (thinner wall), and less material than aluminum.
This is a summary of the results from the previous few slides. The Inner Diameters will be needed for the Inventor activity. But the main idea is that engineering formulas and math, based on the use of models, allow parts to be designed to meet certain performance requirements.
This is step four of the workflow.Inventor will help identify which materials are better for the environment given the requirements for the functional unit made from different materials.
This Is step five of the workflow – the final step.Stainless Steel is probably the worst choiceAluminum is similar to Titanium, and uses much more water, but is cheaperDoes this match what we know about what handlebars are actually made of?Yes it Does! Most handlebars are made of aluminum, although some are titanium or carbon fiberIn real life there are other considerations that make titanium a less appealing option [additional associated costs make it a much more expensive choice]
