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Requirements
Requirements
1. Client
The gripper needs to hold dog bone samples securely throughout the entire testing process. Minimal effect on the preload and testing of specimens.
2. User
The gripper must be easy to operate without any pinch points. The jaw faces should be able to hold on to the specimen adequately.
3. Safety
The gripper’s capacity must meet or exceed the maximum capacity of the tensile testing machine’s maximum force.
4. Standard
The gripper must meet or exceed the testing parameters outlined in ASTM D638. Specifically, the gripper's compliance should be minimized such that the strain of the entire assembly is not to exceed 1% of the total longitudinal strain between the two gauge marks on the test specimen at any time during the test and at any load up to the rated capacity of the machine.
5. Law
The gripper’s design and manufacture shall not infringe upon any existing intellectual property.
6. Environment / Sustainability
All manufacturing processes in the production of the device shall conform to or exceed
whatever is left of EPA’s standards.
7. Cost Efficiency
It must be cost effective, but without loss in accuracy.
8. Weight
The weight of the gripper is not a critical requirement.
Specifications
Specifications
Maximum Force Capacity [kN]
1
Working Temperature Range [C°]
-10 to +100
Jaw Face Width [mm]
19
Jaw Face Length [mm]
19
Grip Length [mm]
87
Grip Body Width [mm]
49
Grip Width [mm]
49
Weight [kg]
0.2
Jaw faces may be diamond serrated (60O) with a pitch of 1.5mm.
Material and Process Selection
Material and Process Selection
A critical feature the grippers must have is the ability to hold specimens securely with little to zero compliance. The gripper must be stiff, and weight is not a critical parameter. The ultimate strength, or stress at failure, is not critical because the gripper will be designed to operate at a safe stress range. Finally, cost should be optimized in order to maximize profits.
Similar requirements apply to a testing machine in a smaller scale, although specifications for a scaled down testing machine will be different. Cost should be optimized due to limited funds.
The manufacture of the real and scaled versions of the gripper will be made from a monolithic homogeneous material, this is to minimize complexity and potential strain in the gripper assembly. The exact process will depend on the material. However, they will be manufactured through a subtractive manufacturing process, most likely precision CNC machined.
Important Properties:
Stiffness – Young’s Modulus
Fracture toughness
Formulas
Scale the specimen
Scale the specimen
Static analysis
Static analysis
General formula's
General formula's
The static coefficient obtain by using the table
The static coefficient obtain by using the table
Base on equation, the most important maniacal property is Young's modulus E and stiffness
Base on equation, the most important maniacal property is Young's modulus E and stiffness
The main criterion in the selection of materials is the modulus of the material. The modulus needs to be high enough such that the grippers will not experience any strain under normal testing conditions.
From this Ashby chart, metals, ceramic and some hybrid materials are potential candidates.
Similar considerations apply to a scaled down prototype version of its industrial equivalent.
Treating the gripper as a tie in tension, the material index for this Ashby chart is E/C. Again, metals, composites, and ceramics are the primary groups. Following this guideline and maximizing the modulus, cast irons, aluminum alloys, carbon steels, and silicon carbide are all potential candidates.
Similar materials would be considered for a smaller prototype of the gripper.
For a displacement-limited brittle failure the guideline is K1c/E . Following this guideline and maximizing fracture toughness, ceramics are no longer a potential material because of their brittle nature. This leaves the steel family, and other metallic alloys. Of these options, steels, aluminum alloys, and copper alloys have the best combination of fracture toughness and young’s modulus.
Similar considerations apply to a scaled down prototype version of its industrial equivalent.
Summary of Analysis
Given the constrains of limited compliance and fracture toughness above, the three most appropriate materials for the grippers are: Carbon Steels, Aluminum alloys and Copper alloys.
A monolithic construction will require a subtractive manufacturing process in order to obtain the geometry required. Potential subtractive processes include, EDM machining, CNC machining and (jig) grinding.
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