HW7 ENME382 S10 Section0201 solution

# HW7 ENME382 S10 Section0201 solution - ENME382 Section 0201...

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ENME382 Section 0201 Homework 7 1. a. What is meant by the term elastic strain? What is the equation that relates the applied stress to the elastic strain? Elastic strain is recoverable strain. When you remove the stress, the material returns to its original size. The simple equation that relates applied stress to elastic for linear elastic materials is Hooke's Law: g G ±² , where g is the applied stress, ² is the strain and ± is the elastic modulus. b. What is meant the term plastic strain? Is there a simple equation that relates applied stress to plastic strain? Plastic strain is permanent or non-recoverable strain. In general you need experimental data (i.e. a stress strain curve) because plastic behavior depends on the chemistry and the thermo-mechanical history of the material. The thermo-mechanical history refers to prior deformation and any heat treatments done to the material that affect dislocation density and grain size and orientation. Equations that describe plastic behavior like the %RA (eqn 7.12), % EL (eqn 7.11) and the strain hardening equation (eqn 7.19) exist but because they depend on the specific microstructure, history and chemistry of the material, they aren't simple like Hooke's Law. c. When a sample is stressed, how do you know whether it will experience only elastic strain or elastic and plastic strain? As we discussed in class, the yield strength generally refers to the stress at which there is measurable plastic strain. In fact, we shouldn't exceed the proportional limit if we don't want to our material to exhibit any plastic deformation. The yield strength is often used as the 'break point' between elastic and plastic behavior because it is easier to quantify than the proportional limit. Note: For metals that exhibit upper and lower yield points, we call the average value of the lower yield point the yield strength. However, if you don't want any plastic deformation you would need to stay below the upper yield point. d. Briefly describe one application where a material is likely to experience elastic strain. There is no single answer to this question. There are many possible answers. Generally while you form a spring using processes that cause plastic deformation, you don't want to have plastic deformation when you use the spring in an application. Another example might be elastic deformation of the beams support beams in a building during a storm - its ok if they flex a little you just don't want them to bend. e. Briefly describe one situation where a material is likely to experience plastic strain. As above, there are many possible answers. A paper clip experience plastic deformation when it is bent into shape and when you unbend it when you are bored. Another example is when you are in an automobile accident and your car crumbles to absorb impact energy and protect the people inside. f. What is the name of the stress at which noticeable plastic deformation begins?

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## This note was uploaded on 11/28/2010 for the course ENME 250 taught by Professor Shang during the Spring '10 term at Maryland.

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HW7 ENME382 S10 Section0201 solution - ENME382 Section 0201...

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