Plan for Friday, September 25, 2015

Class Activities

9:00-9:10	Introduction, meet your neighbors
9:10-9:15	Questions about syllabus, about class, etc.
9:15-9:25	What is mechanical engineering design? Team up with your three nearest neighbors and come up with a definition of mechanical design in your own words.
9:25-9:30	Discuss answers, write key words on the board
9:30-9:40	Stress/Strength
9:40-9:45	Stress/Strength question
9:45-9:55	Factor of Safety
9:55-10:00	Factor of Safety question
10:00-10:05	Break
10:05-10:10	Project One intro activity
10:10-10:20	Break up into CATME groups
10:20-10:25	Team introductions
10:25-10:45	Reverse engineering
10:45-10:50	Discuss

Intro 9:00

Jason and Matt
Lecturer - teach and improve engineering education - mention that we will try out new things in class and that i want feedback
Group into groups of 4 - Your name - What machines you hope to design in the future
Any syllabus questions? - Lecture topics will change, hw and project dates are fixed, check - Lecture prep guides: you will be tested on these things - Ask questions on piazza
Poll students about devices in class: - Laptop to class? On Fridays? - Laptop or tablet? - Laptop, tablet, or smart phone?

Mechanical Design 9:15

Group into groups of 4.
Goal: definition of mechanical engineering design in 1-2 sentences.
10 minutes, ends at 9:25
Use previous experience and chapter 1

My definition

The process of creating a solution to a problem or need utilizing mechanical systems and principles under constraints such as limited resources, limited knowledge, or codes and standards. The process involves creativity, ideation, ingenuity, and analysis that may need to guarantee safety, maintainability, sustainability, ethical standards, etc.

difference in need (beginning) and problem (specifications required)

In the news

Ethics: Volkswagen scandal: https://en.wikipedia.org/wiki/Volkswagen_emissions_violations

Saftey: Mecca crane collapse: https://en.wikipedia.org/wiki/Mecca_crane_collapse An engineer for the group said that the crane was erected in "an extremely professional way", and the accident was an "act of God" https://thenypost.files.wordpress.com/2015/09/crane.jpg?w=840

Typical process

understand the problem
identify the knowns
id the unknowns and formulate solution strategy
state all assumptions and decisions
analyze the problem
eval solution
present

Standards/Codes

standard: set of specifications to achieve uniformity, efficiency, quality
code: specs to control saftey, efficiency, performance

Strength 9:30

Max stress < strength
strength: property of the component/part/element
\(S\) typically deontes strength
stress: a property of the component's state at a specific point
\(\sigma\) normal stress and \(\tau\) shear stress
Needs margin to ensure failure rare or improbable
Focus on areas of the part that are critical stress areas

Class Question

If you are designing a diving board and you want to guarantee that it will never break during use which of these would likely be the best to do:

Compute the maximum possible stress at every point in the diving board and make sure the yield strength of the material is not exceeded.
Make sure the maximum load applied to diving board when jumping never exceeds the tensile strength load.
Compute the maximum possible stress at points that are likely to have the highest stress and make sure the yield strength of the material is not exceeded.
Draw a moment diagram of the cantilever beam and find the highest stress due to the moment to ensure it never exceeds the yield strength.

Uncertainty 9:45

What's wrong with this?:

The yield strength of hot rolled mild steel is 220 MPa.

Design factor: deterministic based on absolutes

\begin{equation*} n_d = \frac{loss of function paramter}{maximum allowable parameter} \end{equation*}

Class Question

If the load that will cause failure is between 90 and 110 lbs and you'd like a design factor of 2, what is the max allowable load?

\begin{equation*} P_{max} = \frac{P_{fail}}{n_d} = \frac{90 \textrm{ lbs}}{2} = 45 \textrm{ lbs} \end{equation*}

Factor of safety

Factor of Safety Method

Analyze all loss of function modes
Choose mode that leads to smallest design factor to govern decisions

Factor of Safety: The actual design factor after the part is fully designed.

Why would the Factor of Safety be different than the design factor?

This is most typically:

\begin{equation*} n_d = \frac{S}{\sigma \textrm{ or } \tau} \end{equation*}

because stress may not vary linearly with load.

Sample Problem

A square cross section rod is loaded axially with a static load of 1000+/-10 lbs. The strength of the material is 25 kpsi and the desired design factor is 4. Determine the minimum width of the square cross section. Then select a preferred fractional inch size from Table A-17 and report the factor of safety.