Quiz_3_Solution__MSE_220

MSE 220 Fall 2018 - Quiz 3 October 18, 2018 Name: _____Solution_____________________________ Discussion Honor Pledge (write out): ______________________________________________________________________________ ______________________________________________________________________________ Signature: ________________________________ This study source was downloaded by from CourseH on :17:00 GMT -05:00 2 1. (6 pts) Describe the 3 strengthening mechanisms for metals discussed in class and how each provides strengthening. Strengthening By Grain Size Reduction – A material with smaller grains has more grain boundary area. The grain boundaries act as barriers to dislocation motion for two reasons. First, two adjacent grains will have different orientations and a dislocation passing from one grain to another grain will have to change direction of motion to continue moving along a slip direction. Due to direction change, the shear stress resolved along the new direction will be different meaning continued dislocation motion may require a higher applied load. Second, the atomic disorder within the grain boundary results in discontinuity of slip planes between the grains. This means a dislocation would not be able to continue moving within the slip plane. Grain boundaries inhibit dislocation motion making metals stronger and less ductile. Solid-Solution Strengthening – Metal solid solutions are created by alloying a metal with “impurity atoms” that go into either substitutional or interstitial solid solution. The impurity atoms distort the lattice and impose a strain field on the surrounding host atoms. These strain fields interact with dislocations and make dislocation motion more difficult. Increasing the amount of impurity atoms will increase the strength. Strain Hardening – Strain hardening is the process by which the strength of a metal increases as the metal is plastically deformed. Dislocation density increases as a metal is deformed during cold working. Each dislocation distorts the surrounding lattice creating a strain field. The strain field of one dislocation will interact with other dislocations making dislocation motion more difficult. Increasing the dislocation density increases the likelihood of interaction between dislocations. 2. (4 pts) A brittle material contains an external crack of 1 mm and is to be loaded to a tensile stress of 500 MPa. What is the minimum fracture toughness required to insure this material will not fracture due to the applied stress? Assu