Testing Materials
- Eric Gharghouri
- Maciek Pajak (Deactivated)
There are 4 main types of tests necessary to determine the strength of a composite.
Tensile strength
Compression
Shear
Fatigue
Tensile strength
Tensile strength is a material's ability to resist tearing due to tension. To test a material's tensile strength it is placed in a tensile testing system and pulled on until the material is stretched too much to go back to its original shape. The zone in which a material is able to perform its best is within the yield strength of the material (shown in the graph below).
This graph represents the average stress-stain curve for materials. In order to choose a material, something with a high yield strength is best, as it is the force a material can withstand without deformation. Strain/Work hardening is the strengthening of a metal or polymer by plastic deformation. Strain/Work hardening may be desirable, undesirable, or inconsequential, depending on the context. Strain hardening may be useful but for making a car it seems like it would be difficult to make a material and stretch it to the desired size and shape. Moreover, the MidnightSun car is made of carbon fiber which cannot be stretched. Carbon fiber is brittle so it is prone to cracking.
How to test tensile strength?
To perform tensile testing a, universal test machine frame, load cell, controller and/or indicator, and proper grips and fixturing to hold the sample are required.
The universal machine frame is the structure needed to pull the material apart. It provides rigid support.
Load cells measure the amount of force applied to the sample.
A controller indicator controls how the frame behaves during testing. I.E how fast and how much force is applied to the material being tested.
Grips are required to hold the material during testing. These change based on the material being tested.
https://fractory.com/ultimate-tensile-strength/
https://interestingengineering.com/understanding-tensile-strength-its-importance-in-engineering
https://www.admet.com/testing-applications/test-types/tension-testing/
Compression strength
Compressive strength is a material's ability to resist deforming or breaking under compression.
How is compressive strength tested?
Compression tests are typically done by applying pressure to a material using flat platforms. This test is usually done on a block of material rather than a sheet. During the test, various properties of the material are calculated and plotted as a stress-strain diagram which is used to determine qualities such as elastic limit, proportional limit, yield point, yield strength, and, for some materials, compressive strength.
https://www.instron.us/en-us/our-company/library/test-types/compression-test
https://www.mee-inc.com/hamm/tension-and-compression-testing/
Shear strength
The shear strength of a material is its ability to resist forces that cause the material's internal structure to
slide against itself (shown in the picture to the right).
How to test shear strength?
The main difference between shear and tensile and compression is that the forces being applied are parallel to the upper and lower faces of the test subject. Many materials behave much differently in shear compared to tension or compression. One common practice is pictured to the right where the test subject is a bolt (dark grey object), one of the blades ( the white rectangles) is kept stationary while the other is moved in a parallel plane. A double sheer test is where a second stationary blade is placed to the left of the shearing blade as pictured to the right. Another test is lap shear testing which is used to determine the shear strength of an adhesive like various different glues. The adhesive in question is applied to two metal plates and then pulled to failure(pictured below are various forms of lap testing and how it is conducted).
https://www.metengr.com/testing-services/mechanical-testing/shear-testing
https://matmatch.com/learn/property/shear-strength
https://www.corrosionpedia.com/definition/1026/shear-strength
Material Fatigue
Material fatigue is a phenomenon where structures fail when they are subjected to a repetitive force or load. Material Fatigue occurs even when the force or stress exerted on the material is much less than the material’s static strength. Fatigue is the most common source of failure when it comes to mechanical structures.
How to test Material Fatigue
To test material fatigue, tests such as the tensile test, compression test, and shear test are done for long periods of time with less force. The force applied typically resembles the frequency of a sin wave.