ii) Design 2

Design 2 attempts to combine tensile fatigue testing and rotating bending fatigue testing capabilities into one platform.  Tensile loads and bending loads would be applied at one end of the specimen.  The bending load would be applied via a concentrated force.  The opposite end of the specimen would be constrained in order to allow no horizontal translation.  This end would be rotated, most likely by a motor.  A diagram of this setup can be seen below.

Disadvantages of Design 2:

-Machine must be constructed.

-Construction would require tight tolerances.

-Funding may not be sufficient for necessary materials and components.

-Data acquisition capabilities must be addressed.

-Tensile Loading and Bending Loading methods must be addressed.

Advantages of Design 2:

-True combined loading tests would be capable of being conducted.

-Machine would likely have no restrictions for accessibility

-Machine can be made to meet all requirements.

-Simple in design.

iii) Design 3

Design 3 is an adapted version of a schematic drawing in the Foundations of Materials Science and Engineering textbook (Smith/Hashemi, 2006, 7). A layout of the design can be viewed in (Figure: 3-4). While the principal is the same as the other designs it is the bending force applied that makes this design unique. While the other designs utilize a concentrated force to induce bending this design utilizes moments. Equal but opposite moments applied at opposite ends of the specimen produce a bending force in the middle of the specimen. 

Disadvantages of Design 3:

-Machine must be constructed.

-Construction would require tight tolerances.

-Funding may not be sufficient for necessary materials and components.

-Data acquisition capabilities must be addressed.

-Tensile Loading method must be addressed.

-Extensive Dampening would be required as bending load would be constantly moving.

-Complexity of moving parts causes concerns.

Advantages of Design 3:

-True combined loading tests would be capable of being conducted.

-Machine would likely have no restrictions for accessibility

-Machine can be made to meet all requirements.

iv) Design 4

Design 4 as seen in (Figure: 3-5), is similar to designs 2 and 3 as it is designed to test the Rotating Bending Fatigue as well as tension fatigue of a specimen simultaneously.   The bending load would be applied via a concentrated force applied in the middle of the specimen.  The tensile load would need to be applied through equal and opposite forces on both ends of the specimen.  This is necessary in order to ensure that the bending force is applied at the same point through out the span of a test.

Disadvantages of Design 4:

-Machine must be constructed.

-Construction would require tight tolerances.

-Funding may not be sufficient for necessary materials and components.

-Data acquisition capabilities must be addressed.

-Tensile Loading and Bending Loading methods must be addressed.

-Applying equal and opposite forces to the specimen presents difficulties.

Advantages of Design 4:

-True combined loading tests would be capable of being conducted.

-Machine would likely have no restrictions for accessibility

-Machine can be made to meet all requirements.

Design Concept Selection: Decision Matrix

Once our designs were completed we needed to determine which one suited our needs the best. The first step was to create a decision matrix (see Figure: 3-6) to help our group choose a design. The categories we rated for the decision matrix were cost, safety, reliability, ease of use, and performance.

Cost (0.2): These scores were based on the expected cost to construct or implement each design concept relative to the other concepts.  This category received a weight of 0.2 due to a fixed budget.  

Safety (0.1):  These scores were based on the imagined risk to a trained operator, using the equipment for its intended purpose.  This category was not heavily weighted as operators are expected to be well versed in using the machine.

Reliability (0.25):  Reliability was based on the projected amount of maintenance required (least amount of maintenance receives highest score).  This category is the second highest weighted category due to the nature of the testing to be performed on the machine (high cycles).  Near-continuous operation is expected.  Any time spent fixing the device is time lost to testing as well as the possibility of failure during testing.

Ease of Use (0.05):  Ease of use is based on the ease of inserting and removing specimens as well as operator access to machine and power supply.  Additionally, the ease of adjusting the magnitude and frequency of the tensile and rotating-bending loads was considered.  A low rating was given to ease of use for similar reasons as safety.

Performance (0.4):  Performance was based on the expected ability of a design to perform repeated valid fatigue tests in a timely manner.  Performance received the highest weight because valid data is crucial to the overall objective of this project.

Decision:

          After analysis of each design it was concluded that design 2 would be pursued.  It was projected that design would provide the best performance of the four designs.  Design 2 also received high scores for safety, reliability, and ease of use.  Cost was the only concern for design 2 however it was determined that the $2000.00 budget was sufficient for the purchasing of all components.   

Decision Matrix:

Design 2: