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This is similar to springs that immediately stretch when a force is applied. With the force constantly applied, the material continues to strain or displace over time. Eventually the material reaches equilibrium, which is seen by the horizontal line. When the force is removed, the initial decrease in strain is equal to the amount of strain the material instantaneously experienced when it first had the force applied.
Polymer Engineering Science and Viscoelasticity
Over time, the material returns to its original configuration and its strain becomes zero. Hysteresis : When a weight is applied to a spring, the spring stores energy to be able to return to its original configuration once the weight is removed. The amount of energy stored is equal to the amount of energy it took to displace the spring. When viscoelastic materials have a force applied to them and then removed, it takes more energy to displace the material than it does to return the material to its original configuration.
In other words, it consumes more energy during the loading phase applying a load and stretching the material compared to the unloading phase taking the load away and allowing the material to return to its original state. This energy difference is caused by the material losing energy during the loading phase, due to heat dissipation or molecular rearrangement within the material.
Engineers calculate the amount of energy that was lost by analyzing the stress-strain diagram generated while stretching loading and unstretching unloading the material. The area between the loading and unloading curve represents the energy lost. Figure 5: Stress-strain diagram of a biological material that was loaded and then unloaded. The area between the two lines is equal to the energy lost during this process. This is termed hysteresis.
Figure 5 shows a stress-strain curve that was generated by loading and unloading a biological material. The loading portion is the top curve and the unloading region is the bottom curve. Figure 6. Force vs. The peak force and hysteresis decrease with each cycle.
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This is termed preconditioning. Preconditioning: If the viscoelastic material is continued through this loading and unloading process, then the amount of energy lost in a cycle decreases until it reaches equilibrium close to zero energy lost. The amount of force it takes to displace the material also decreases with more cycles until the equilibrium point is reached. Exposing a viscoelastic material to this type of cyclic loading allows the viscous part of the material response to be dissipated and only the elastic portion remains. This is why the material is able to reach equilibrium.
The viscous part of a material response can be difficult to fully characterize and understand, but the elastic behavior is easily understood and repeatable, making preconditioning useful to engineering researchers who need to compare elastic solids to viscoelastic materials. Many people also usually unknowingly take advantage of this unique characteristic of viscoelastic materials when blowing up balloons. It is common to see people using their hands to repeatedly stretch the balloon material before blowing them up.
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This preconditions the balloon with the purpose to reduce the amount of force required to stretch or inflate it. It is easier to blow up a balloon after preconditioning it. Figure 6 shows a force vs. The area between the lines of each cycle decreases and the peak force of each cycle decreases. Notice how large the difference is between the first and second cycle. This is the largest change observed between cycles. After the second cycle, the peak force and hysteresis continue to decrease but not as dramatically. The material eventually reaches equilibrium at which point it becomes hard to pick out the different cycles because they are overlapped.
Watch this activity on YouTube. We have learned that viscoelastic materials behave very differently than elastic materials. They exhibit time-dependent material behavior, such as strain rate dependence, stress relaxation, creep, hysteresis and preconditioning.
Altmetric – Polymer Engineering Science and Viscoelasticity
What might change how viscoelastic materials respond to force? Listen to student responses. How does this information affect engineers? Listen to student responses? When designing devices, engineers must fully understand these properties if the materials they have selected or the environment that their devices will operate in is viscoelastic.
If the devices are interacting with viscoelastic materials, then engineers must understand how those materials respond to forces so that failure can be prevented, and so that the devices perform successfully as intended. Hands-On In Class: During the lesson, have students get out the silly putty that they made during the associated Creepy Silly Putty activity or use purchased Silly Putty and experiment with it:. Worksheet: After the lesson, have students complete the attached Viscoelasticity Worksheet.
Review their answers to gauge their mastery of the subject matter. Extension Activity: Research the "Big Dig" ceiling collapse online. Write a short paragraph describing what caused the failure and why it is important for engineers to understand viscoelasticity. Things to include:. During the construction of the Fort Point Channel Tunnel in Boston, concrete ceiling panels were hung using bolts embedded in epoxy a type of polymer.
Over time, the bolts pulled out of the epoxy, causing a three-ton panel to crash on the roadway below.
The bolts pulled out of the epoxy because viscoelastic materials deform over time when force applied to them until reaching equilibrium creep. The panels weighed too much, which caused too large a deformation before reaching equilibrium. It is important to understand viscoelasticity so that you understand how the materials in your design will behave under certain conditions.
This helps engineers to safely design devices and structures that do not fail and put people at risk. They could determine which polymers deform over time and which don't to make an informed decision on which polymer would be appropriate for the design. Find more information about polymers at the University of Southern Mississippi's Polymer Science Learning Center What are polymers and why do they act the way they do? Where are polymers? Types of polymers and how they are used. Making stuff with polymers. Polymer games and slimes. DGE However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.
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Grade Level: 11 Lessons in this Unit : 1 2 3 4 5 Time Required: 45 minutes Lesson Dependency Lesson dependency indicates that this lesson relies upon the contents of the TeachEngineering document s listed. Creepy Silly Putty. Print this lesson Toggle Dropdown Print lesson and its associated curriculum.
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