Sheet Metal Fatigue Fixture

The goal of the project is to design and fabricate a fatigue fixture that can determine the fatigue-life (S-N) curve for a given sample.

=Problem Definition= The purpose of this project is to create a fully reversible fatigue fixture that can test the fatigue life of a given sample. With this data, we should be able to create an S-N curve for the sample. These given samples can be bent, heat treated or etc. prior to testing.

=Design Value Proposition= Any metal will break if you pull or bending it hard enough. The more force you use, the faster it will break. If you use less force over and over again, it will take longer to wear out and break. An engineer needs to know this durability for materials after it has been bent or pulled many times. We know what this durability is for many metals, but things like heating and bending the metal can change its durability. Commercial machines to measure the durability of a material can cost between $10,000 and $300,000. Our goal is to build a low cost machine that can test pieces of sheet metal and tell us how durable it will be over time. This can be used to measure the strength of sheet metal after it has been worked (heat treated, finished, formed, ect.) or to verify the failure properties that are reported by a manufacturer.

Background
SEL products are exposed to various cyclic stresses during their service life. These stresses are the result of thermal cycling, vibration, and direct loading. Sheetmetal, due to its non-isotropic grain structures, will have unique fatigue properties depending on part geometry. Hence, the S-N curves for specific sheetmetal materials are not commonly available. SEL products are rated for abnormally long life; therefore, it is important to know the S-N curves of given materials to predict the life of a product.

Fatigue testing requires multiple sample runs and each run takes a long time. To allow this, the fixture needs to be able to operate safely unattended. An ideal design will be optimized for quick setup, ease of use, cost of implementation, and repeatability.

Deliverables
We have to deliver a final design fatigue fixture that can accomplish everything on the specifications list, but most importantly it most be able to create a S-N curve with very little human interaction.

Specifications
The final product should have these considerations in mind:
 * Handle up to 40 ksi of Static Stress.
 * Overall Height, Width and Length shouldn't surpass 60 inches, 36 inches and 36 inches respectively.
 * Can find an S-N curve of a sample at a minimum of 1 inch by 0.5 inches and a maximum of 6 inches by inches.
 * Any material should be able to be tested, but specifically aluminum and plastic.
 * Should be able to fit on a desktop.
 * The fixture shouldn't surpass 70 pounds in weight.
 * Eliminate all pinch points on the fixture.
 * Setup Time for the fixture cannot exceed 45 minutes.
 * User interface must be easy to view and interact with.
 * Must be able to configure the operating force and frequency for a run.
 * Must be able to run without continuous human interaction for a long period of time.
 * The software must be able to generate an S-N curve using the results of the test runs.
 * Control System must be able to detect when a failure occurs during the run.
 * Operational noise cannot exceed 80 decibels.
 * The frequency range that we must meet must be 1-100 Hz.
 * Range of error cannot exceed 15%.
 * Peak power load cannot exceed 1500W.
 * Fixture must cost under $2,000.
 * Fixture must be fully and partially reversible.
 * Must be able to run from a wall socket.

=Design Considerations= This section will go through the three design choices we went through; those being Axial, Cantilever and 3-point Bending.

=Axial (Concept #1)=

At first Axial sounded like the most obvious option to go with, however the more we looked into the requirements, the more Axial looked impossible to do.

Axial had the most simple design, but it also introduced a lot of issues. Axial has a lot of potential for buckling forces and that would prevent compressive loads, which we need in order to make the fixture fully reversible. Buckling forces also directly effect the amount of force needed to fracture not only the sample, but the fixture itself. The max force needed to fracture a 6"x6" aluminum sample would be 14400 lbs, which is unobtainable for what we're trying to do, so if we were going with Axial, the longest a sample could get is 1.72".

This showed us that Axial wasn't a good choice for this project.

=Cantilever (Concept #2)= For the Cantilever Design the initial design was with round pipe supports and two base plates. The slots in the base plate were to reduce the weight of the machine. The round spacers between the plates are to model rubber shock absorbers. The tubing would be welded together. The initial design for the power train involves a pulley and a fully adjustable wheel linkage. The clamps are just plates to with bolts to sandwich the ends of the sample in place.

The second design for Cantilever is the same as the first just with some more updates to the clamping on the motor linkage end as we found that with two solid clamps we would over constrain our Free Body Diagram of our sample and create indeterminate loading. We solved this by adding roller clamps to the motor linkage instead of the positive clamping we originally considered.

Our third design involved some major changes to the design. For starters we changed the round pipes out for square tubing and we modeled our motor and a new motor linkage setup. Further more we added a block for to to hold our linkage parts and change our loading from cyclical to pure bending.

The reason we did not pursue this design was the inconsistency in the failure point. In theory it should always be right at the bottom of the top clamp, but after talking with Dr. Bob Stephens a fatigue expert at the University of Idaho we learned that this was not always the case in reality. This would give us inaccurate results and would be outside our required accuracy.

=3-point Bending (Concept #3)= Our first 3-point design was a steel structure using rollers to apply points onto the speicimen, the speicmen was held down using two clamps on each end, the issue with this design was that this created a two fixed end conditions which would not yeild the correct shear moment diagram that we needed.

After some small changes we had our second design which held most of the same components with the biggest change having rollers hold down the end of the specimen which gave us the proper shear moment diagram to ensure that we would have the max moment at the center of the specimen. The issues with this design were that we were concerned that the sample would slide out of the rollers once the object began to vibrate.

Our third design and current design has an added L-bracket to ensure no movement in the horizontal direction, there are also added t-slots which will hold the L-brackets in place. We decided to go with this design because it was the simplest set up that would provide the results that we wanted.

=Project Learning= Decision Matrix's for the Fixture and Motor

Comparison of Failure Detection Methods

Axial Stress Calculations

Comparison of Loading Methods

Motor Linkage Power Required Analysis

Cantilever Design Images (1)

Cantilever Design Images (2)

Cantilever Design Review Concept

Concept Review Design Recommendations

Displacement Driven Cam Idea

Fixed Displacement Idea

Motor Linkage Idea

=Final Design=

=Validation= Link to our Design Validation Plan. It states our requirements, how we're planning to test them and the results of said tests. Design Validation Plan

=Team Members=

=Additional Documentation=

Project Schedule

Gantt Chart

Meeting Minutes

Meeting Minutes Folder

Presentations

Snapshot #1 Presentation

Concept Review Presentation

Snapshot #2 Presentation

Client Interview

Client Interview w/ SEL

Budget

Budget