Material Strength Testing

ENGL 21007

Member: Ishvar Sitaldin

Date: 4/20/2023

Introduction

According to new developments at Space Travel Inc. new aircraft speeds can be reached which lie in the 30,000 m/s range, in order to determine which materials are suitable for use at these new speeds we have tested many different types of materials to determine their maximum allowed forces. The tests which will be conducted are strength tests, which focus on the tensile strength of the different materials. The tested materials are materials which are currently used in space travel, but we will also include materials which haven’t been considered for these types of applications. The materials we will be testing during this lab are:

– Rebar (Carbon Steel)

– Aluminum

– Carbon Fiber

Procedure

A sample of a material can be tested for its maximum strength by performing a tensile strength test. During a tensile strength test a test piece is subjected to increasing forces until it snaps or breaks. The data which is collected from this test are the applied force and the deformation of the sample. This data is then documented in a Stress – Strain diagram. As the name suggests, this diagram consists of 2 axes, namely the Stress axis and the Strain axis. The stress and strain are calculated using the following formulas:

Stress:		(1)
Strain:		(2)

The first step we need to take is to prepare each sample for the test. We need to make sure to measure the initial length () and initial diameter () of the sample. Afterwards we insert the sample into a Universal Testing Machine and begin the test. This will be done for all of the materials and afterwards the subsequent Stress-Strain diagrams will be created.

Figure 1. The general buildup of a stress – strain diagram.

The reason we use this type of test is ultimately to be able to make a stress-strain diagram, since a lot of information can be gathered from this. In general, a stress-strain diagram consists of 3 main parts. The first part of this curve is called the linear elastic region. In this region the stress () is proportional to the strain () and thus obeys Hooke’s Law.

Hooke’s Law:

(3)

The slope in this region () is known as Young’s Modulus. Within this linear elastic region, the material only deforms elastically. This means that any deformation which occurs in this region is reversible. The end of this region marks the beginning of plastic deformation, and this point is known as Yield Strength. During plastic deformation any deformation which is experienced by the material is permanent. Even when the applied forces are removed, the material maintains its deformed state.

The second region of this curve is called the strain hardening region. In this region the curve reaches its maximum sustainable stress, which is called Ultimate Tensile Strength. In this region the stress on the material increases which causes the material to elongate.

The third region of the curve is called the necking region. In this region the cross section of the material starts to decrease, which will ultimately lead to fracture.

Results

Sample 1:

The first material we will be testing is Construction Steel. We will be testing this material as a reference to show exactly how much stronger the other materials are.

Sample Data:

 = 12.5 mm

 = 75 mm

 = 122.7 mm²

Test Data:

#	∆L (mm)	F (kN)		#	∆L (mm)	F (kN)
1	0.5	24.5		16	8	57.75
2	1	33.5		17	8.5	58.75
3	1.5	43.75		18	9	59.75
4	2	45.5		19	9.5	60.25
5	2.5	45.5		20	10	60.75
6	3	46.5		21	10.5	61.5
7	3.5	46.75		22	11	62.25
8	4	48.5		23	11.5	62.75
9	4.5	49		24	12	63
10	5	50.75		25	12.5	63.5
11	5.5	52.25		26	13	63.75
12	6	53.5		27	13.5	64.25
13	6.5	54.75		28	14	64.5
14	7	55.75		29	14.5	64.75
15	7.5	57		30	15	65
#	∆L (mm)	F (kN)		#	∆L (mm)	F (kN)
31	15.5	65.25		51	31	68.75
32	16	65.5		52	32	68.75
33	16.5	65.75		53	33	68.75
34	17	66.25		54	34	68.75
35	17.5	66.5		55	35	68.75
36	18	66.5		56	36	68.75
37	18.5	66.75		57	37	68.5
38	19	66.75		58	38	68.5
39	19.5	67		59	39	68.25
40	20	67		60	40	68
41	21	67.25		61	41	67.25
42	22	67.5		62	42	66.25
43	23	67.75		63	43	63
44	24	67.75		64	44	61.25
45	25	68		65	45	55
46	26	68		66	46	41.25
47	27	68.25
48	28	68.25
49	29	68.25
50	30	68.25

Table 1: Obtained data for  and F for the Construction Steel Sample. Using these values Stress (σ) and Strain (ε) can be calculated.

Figure 2. Stress-Strain Curve for Carbon Steel using data from Table 1.

Sample 2:

The next material sample is made from Aluminum Alloy.

Figure 3. Stress-Strain Curve for Aluminum Alloy. Obtained from Plotly.com.

Sample 3:

The last sample we will be looking at is made from Carbon Fiber.

	Stress-Strain Curve for Carbon Fiber

Figure 4. Stress-Strain Curve for Carbon Fiber. Obtained from researchgate.net

              Note: 1 MPa = 1*10⁶ N/mm²

Discussion

In order to determine which of the materials is most suitable for our use case we must first decide what results we need to look at. When using a material for a sturdy construction, we should not experience any permanent deformation due to any experienced forces. This means that when deciding what material to use, we must look at the Yield Strength of the material as opposed to the Ultimate Tensile Strength. The Yield Strength determines the maximum force the material can sustain before experiencing permanent deformation.

This means that we should determine the approximate Yield Strength for each of the tested materials. For Rebar the approximate Yield Strength was around 0.375 kN/mm² or 375 N/mm² according to Figure 1.

According to Figure 2, the Yield Strength for the Aluminum Alloy was approximately equal to 5.5*10⁶ N/mm².

And finally, according to Figure 3, the approximate value for the Yield Strength of Carbon Fiber was 163.8 MPa which can be converted to 163.8*10⁶ N/mm².

When comparing these values, we can safely conclude that Carbon Fiber is the strongest material that has been tested during this experiment and thus is the best material to use in this case.