Introduction to EngineeringBike Lab #4 – 1Introduction

Agenda Strength & Reliability

Strength And Reliability

Failure, FatigueHow can engineers design a safe

structure?

Answer: Design for Reliability

Engineers must know the loads that will be applied to the structure.

Engineers must know the strength properties of the materials used for the structure.

Answer: Design for Reliability

Engineers must be able to design the structure such that, at every point, the stress due to the maximum predicted loads that will be applied to the structure will not cause the structure to fail

Design for Reliability in other engineering disciplines:

Maximum information flow rate on a computer network (Computer and Electrical Engineers).

Maximum loading rate for water treatment facility (Civil Engineers and FABE).

Other

Two Recent Engineering Failures

Assume: The crash was caused by debris from the tire that punctured the fuel tank.

The crash of the Concorde

Two Recent Engineering Failures

The crash of the Concorde

• Could this kind of loading have been predicted?

• Could the design be such that the fuel tanks could be protected?

• Could a stronger material have been used for the fuel tanks?

11 campers died when a 30-inch line exploded 200 yards from where they had their tents. The pipe was installed in 1950.

Two Recent Engineering Failures

Natural Gas Line Explodes in New Mexico

• What caused the pipe to fracture? • Was it designed correctly? • Was the material manufactured with no

defects? • Did corrosion change the material strength

over the years?

Natural Gas Line Explodes in New Mexico

Two Recent Engineering Failures

From 1941 to 1945, 2710 Liberty ships were built.

By February 1946, 362 ships, over 13% of the fleet, had suffered at least one major fracture.

An Engineering Failure During WWII - The Liberty Ships

Some were lost at sea, but one named “The Schenectady” broke in two 24 hours after launching while tied up near the outfitting dock.

An Engineering Failure During WWII - The Liberty Ships

Approaches for Safe Design

“Perfect world” approach: The applied load is known exactly, the strength is known exactly.

STRUCTURE STRENGTH > MAX APPLIED LOAD

How do engineers know the strength of a specific material? Use data from handbook. Measure the strength in the lab.

Consider an example of 1/8” diameter, ordinary zinc-plated steel screws. Results from tensile testing of 75 screws provide the following data:

Is the “perfect world” approach acceptable?What value would you use based on this data?

700 750 800 850 900

0

5

10

15

20

Failure Load (Pounds)N

umbe

r of

Sam

ples

Approaches for Safe Design

F.S. =Expected Structure Strength

Expected Maximum LoadF.S. > 1

Factor of Safety (F.S.) Approach

The factor of safety accounts for uncertainty in material strength, loads, design flaws, manufacture flows, etc.

Most components are designed with a factor of safety approach. F.S. for a bridge can be 3 - 5.

What are the advantages of this approach?

What are the disadvantages of this approach?

Factor of Safety (F.S.) Approach

Risk in Design

Load

Structuredesignedmaximumstrength

Frequency

of

occ

urr

en

ce

Applied load

Perfect world: NO RISK since the actual load never exceeds the designed strength.

Structuredesignedmaximumstrength

Appliedload

Load

Frequency

of

occ

urr

en

ce

RISK OF FAILURE

Real world: Uncertainties in design and in actual loading conditions introduce risk

Risk in Design

How do things fail ???

Overload: The applied load in a single event is higher than the material strength.

An Example is the Concorde crash. The force applied to the fuel tanks by the flying pieces of rubber was more than required to puncture the tank.

Fracture: A crack exists in the structure. The sharp crack creates a stress concentration that causes the crack to propagate under a load that would otherwise be safe. Cracks grow by a process of Fatigue.

Microscopic cracks exist in engineering materials (typically at the surface). During repetitive low amplitude loading the cracks grow. Once a crack reaches a critical length, the component fractures

How do things fail ???

How do things fail ???

Fatigue is the cause of most mechanical failures.

In a good design, fracture of one component will not cause a catastrophic failure of an entire structure. An example on the next slide

Aloha Flight 243 from Hilo to Honolulu on April 28, 1988

Aloha Flight 243

The 19 year-old aircraft had taken off and landed 89,680 times prior to the accident. Each time the cabin was pressured like a balloon. The repetitive load on the skin caused fatigue in the skin between the rivets.

Due to the good design, the fracture was limited and the rest of the structure was strong enough to keep the airplane flying.

Aloha Flight 243

Fatigue

Crack closed

Crack open

Region of veryhigh stress

Crack closeda crack growth

StressLow stress

Low stress

High stress

Tim

e

Crack origin

Slow crackgrowth

Fast crackgrowth

Beachmarks

Typical Fatigue Fracture Surface

Analysis Of Fatigue

S - N CURVESS-N Curves:Experimental curves that are used by engineers to predict the number of times a component can be loaded to a certain level of stress.

The aluminum alloy hasno endurance limit.

Endurance Limit:The stress amplitude for which a material has infinite life (more than a million cycles).

Part II - Assessment of Fork Design

Your goal in this lab is to observe the stress conditions in the front forks of your bicycle under actual field conditions, and to make judgments about the adequacy of the fork design.

In Lab

Two riders (the lightest and heaviest in your group) will ride the bikes.

1. Set up the data logger as you did for Bike Lab #3, except set the acquisition rate to 1 reading every 2 seconds

2. View the data collected by the datalogger in real time by clicking on "Receive, Real Time".

In Lab

3. Record the voltage signal for the unloaded bike.

Initial signal = volts

4. Record the voltage signal for each of the 2 riders.

Rider 1: weight = lbs., signal = volts

Rider 2: weight = lbs., signal = volts

In Lab

5. Reset the datalogger to have an acquisition rate of 50 Hz.

6. Take the bicycle outside and have both riders ride the bike for about 45 seconds each.

Leave the bike sit unloaded between riders for a few seconds so the data will clearly show when you switched riders.

In Lab

7. Bring the bikes back to the lab. Upload your data from the datalogger to the PC and save it to disk.

8. Import the data into Excel and save it as a spreadsheet.

After Lab

1. In the spreadsheet create a plot of stress vs. time for the experimental data. Label and add a legend to your plot.

2. Find the maximum stress observed for each rider. What percent of the yield stress for your bike frame is this maximum value?

After Lab

3. How many times larger is this dynamic stress than the stress observed when the rider was sitting still?

4. Prepare a paragraph evaluating the fork design of your bicycle. Do you think it is over-designed, under-designed, or just right?

After Lab

5. Consider the weight of the rider, the lifeexpectancy of the bike, and the yield stress of the bike frame material.

6. What rider weight limit would you impose for this fork design?

After Lab

Prepare a team Lab Report using the standard format given and include the following: Plot of stress vs. time. Calculations including answers to questions 2

and 3 above. Answer to question 4. First page of spreadsheet (don’t include all

pages of data)

Assignment

Re-read Bike Lab #4 procedure