Name: Westley Gomez

Problem Definition

The purpose of this project is to produce an impact testing machine to simulate blunt force trauma on bone samples. The customer, Dr. Shen, Professor at Ohio Northern University, would like to investigate skeletal injuries associated with varying levels of blunt force trauma. Of particular interest is the pig skull, though long bones may be a future topic. The device is to be placed in the materials lab, where it can be easily accessible and operated by both faculty and students. A handbook will be provided that contains work instructions on safety, setup, operation, components, calibration, and changing impactors.

Several constraints were decided upon during the beginning stages of the design process. The first constraint, as assigned by the customer, is that the machine must generate enough force to break a pig skull. According to The Journal of Forensic Biomechanics, the energy required to break a pig skull in an impact is between 14.1 and 68.5 Joules. As such, the machine must be able to deliver an impact with an energy of 68.5 Joules. Next, it is a constraint that the force generated must not exceed the limit of the force plate. To ensure that the force of the impactor does not break the force plate. A safety factor of 1.5 is required in all calculations.

Subsequently, the bone must not break from the structure by which it is mounted in the machine. The area of interest on the bone is where it is impacted, and damage to other areas of the bone during the impact must be avoided. This constraint will be judged by empirical observation. Also to be judged empirically is the safety of the operators. Research indicates that there is not any previously established safety standards used for similar impacting devices. Because of the lack of safety requirements, the machine must have shields to avoid operator or bystander injury and be safe to operate without experience in impact testing. Another constraint is that the machine itself should not move during operation. The machine as a whole should move less than 1 mm each test.

The criteria considered in the design was used to narrow our focus to the best design solution for the customer. To simulate a wide range of impact scenarios, it is important that the machine is as versatile as possible. The machine will have at least two different impact heads and masses that will create several scenarios depending on the bone being tested. Using the different masses, the machine will be able to achieve a large range of velocities. The machine will have an adjustable mount, to be able to test as many bone shapes and sizes as possible besides skulls.. The machine must be aesthetically pleasing to encourage its use, and should be easy to operate with minimal set up requirements. Cost is also a consideration in choosing the best design, an approximate budget can be seen in Table 1. It is important that the machine is low cost and designed to incorporate as many readily available materials as possible.

Potential Solutions

The functional decomposition for this project yielded quite a few different possible solution combinations. The most important part of the design is the method by which energy is stored and ultimately delivered to the bone.

The first solution uses gravity in a vertical design (Figure 1). The energy transferred to the bone comes from gravitational potential energy. The design has a fixture that moves vertically on a rail guided system. Since this design is vertical, friction between the fixture and guide is not a significant issue. Interchangeable impact heads that contact the bone are attached to this fixture. To vary the velocity and energy of this design, the fixture can be dropped from different heights. The force plate can be easily, but not permanently, integrated into this design.

The second solution makes use of a spring in a horizontal design (Figure 2). The energy transferred to the bone comes from the elastic potential energy contained in the compressed spring. This design still has a fixture that moves along a guide, but now the guide is horizontal. It’s important to note that because the guide is horizontal, friction is going to be more significant than a vertical design. The loaded spring pushes against the fixture and sends it moving down the track towards the target bone. Again, the impact heads on the fixture are interchangeable. How far the spring is compressed determines the speed of the fixture. With a horizontal design like this, the force plate can no longer be used.

The third solution is very similar to the second. The spring, however, is now replaced by a pneumatic air cannon (Figure 3). The fixture will have a cylindrical mount that slides in the end of the air cannon, like a bullet inside a gun. When the cannon fires, the abrupt change in air pressure will accelerate the fixture down the horizontal track. Again, the impact heads on the fixture are easily interchangeable. The speed can be controlled by changing the air pressure released into the cannon. Since this design is horizontal as well, the force plate can no longer be used and friction is again going to be significant. How the design was chosen?

Through the use of a decision matrix, it was determined that the vertical design utilizing gravitational potential energy would be best. Such a design should be least expensive, take up the least floor space, and should give the best control over impact velocity and energy. The decision matrix can be seen in Appendix C, Table 2.

There are several things to consider for the mounting device that physically holds the bone in place. Ideally, it needs to hold a variety of bone types from skulls to long bones, like a tibia. OSU puts the ends of long bones in a plaster mold to hold them in place (Figure 4). A skull could also be mounted in this way. Another possible solution would be to use clamps to hold the bone in place. Memory foam could be used on the end of these clamps to disperse the stresses from clamping. For skull type bones, big pads of memory foam could move in from the sides to secure the bone at any orientation relative to the impactor.

In addition to making it simple enough for anyone to use our customer, Dr. Shen, was looking for something easy to use and maintain, with parts readily available. This design also integrates the use of a force plate which can be used to determine the amount of energy that is transmitted through the bone. It is through our customer that we were able to get a basis for what is needed in her design.

Learning Experience

Through this process there are many ways a product can be chosen. When working with a group it is easy to forget how overcomplicated the process can get due to conflicting personalities or not asking the right questions. With a majority decided to integrate some many data acquisition modules and bells and whistles that were clearly in excess. This blinded the group from true purpose of what the device was meant to do and what our client actually wanted. It was form over substance, and even a dissenting opinion was written off as something that could be done later or just ignored. Group think is a dangerous notion and it very well would have sent the capstone project down in flames.

From this I learned verification is probably the best way to ensure the project is moving in the right direction. Whether it be asking direct questions to the client/customer, following up on a phone call to various manufacturers, or just reviewing the math behind a decision matrix. This provides much more control of the project then force of personality or the title given to an individual at the time.

Appendix A - Solution Drawings

Figure 1 - Vertical Gravitational Potential Energy

Figure 2 - Horizontal Elastic Potential Energy

Figure 3 - Horizontal Potential Energy due Pressure of Air

Figure 4 - Mounting with bondo

Appendix C - Decision MatrixTable 2 - Energy Storage Decision Matrix

Vertical Gravitational Potential

Horizontal Elastic Potential

Horizontal Pneumatic Potential

Cost 0.3 10 6 3

Size 0.1 10 4 4

Look 0.1 6 7 10

Interchang- eable Impactors 0.05 5 5 5

Energy Range 0.15 8 5 5

Velocity Accuracy 0.15 9 6 3

Materials Availability 0.15 10 8 8

8.9 6 4.95