Question

met 14400 lab hardness testing test methods discussion questions: 1. when and why does the amount of time between testing and reading the value matter for determining material hardness? 2. what practical significance does hardness test have in making material selections for plastics products? give at least one example of a product where hardness matters. 3. what do you think happens to most polymers surface hardness as the temperature increases? why? 4. what mechanical property of a material is related to hardness? 5. does the rockwell hardness test performed in this experiment meet astm d785 requirements? if not, what would disqualify this as an astm test? 6. list your rockwell hardness values by material and the published values you found. 7. did the rockwell hardness values correlate with published data? what is the source of your published data? if the values do not agree, discuss what you believe caused the discrepancy. 8. list your shore durometer test values by material. 9. as shown in the next page, two hardness correlation graphs are provided from matweb.com. using the appropriate graph for one of your shore durometer specimens, comment on how well your test results agree with those of matweb.

Explanation:

1. The time - interval matters as some materials may undergo post - indentation relaxation or creep, altering the measured hardness value.
2. Hardness testing helps in choosing plastics for products like plastic gears where appropriate hardness ensures durability and proper functioning.
3. As temperature increases, most polymers' surface hardness decreases because increased thermal energy disrupts intermolecular forces.
4. Elastic modulus is related to hardness as both are measures of a material's resistance to deformation.
5. To determine if the Rockwell hardness test meets ASTM D785, one must check if testing conditions (load, indenter type, etc.) match the standard.
6. This requires actual test data collection and comparison with published values.
7. Comparison of test values with published data helps assess accuracy; discrepancies may be due to sample differences, testing errors, etc.
8. Requires recording of Shore Durometer test values for each material tested.
9. Comparison of test results with MatWeb graphs helps evaluate the reliability of the test results.

Answer:

1. The time between testing and reading matters due to post - indentation changes in the material.
2. Hardness testing is crucial for plastic product material selection, e.g., in plastic gears.
3. Polymer surface hardness generally decreases with increasing temperature due to intermolecular force disruption.
4. Elastic modulus is related to hardness.
5. Check testing conditions against ASTM D785 to determine compliance.
6. Record and compare Rockwell hardness values for each material.
7. Compare with published data and discuss discrepancies if any.
8. List Shore Durometer test values for each material.
9. Comment on the agreement between test results and MatWeb graphs.