Lab report

Alrashaid 1

Alrashaid 4

Alrashaid 3

Abstract:

The goal of this experiment is to determine the tensile properties of four specific specimens: low density polyethylene, high density polyethylene ,aluminum, and steel. A standard test loads on a tension testing machine with a constant strain rate was used for this experiment. The results will be plotted to form stress-strain diagrams that will be used to utilize the properties and behavior of the tested specimnes.

Backround:

As the engineering field has been developing nowadys, the importance of studying the tensile properties of the numerous avalible materials has increased. To use them in different structural applications as in bridges, pressure vessels, ships, and automobiles. Utilizing the tensile properties of the a certain material helps in the calculations for the safety factor of a certain design. Also, the material chosen in a project is based on the tensile properties of the material and its capacity of holding without fracture.

Theories:

Engineering Stress : σ = P/A0,

True Stress : σ T = P/AT, σ T = σ (1+ ϵ ).

True Stress: ϵ = (lf – l0) / l0.

True train: ϵ t = ln (li/l0) = ln (1+ ϵ).

Precent elongation= ((lf – l0) / l0) *

Precent Error = ((|Experimental Value – Theoretical Value|)/Theoretical Value)*

Modulus of elasticity E = slope of young’s modulus line in stress vs strain curve

Equipments:

1- Four specimens of: low density polyethylene (LDPE), high density polyethylene (HDPE), aluminum, and steel.

2- Dial caliper and ruler

3- Tensile testing device.

Procedure:

Four specimens of different material were obtained, the materials were determain to be High Density Polymers ,low Density Polymers, 2024Aluminum,and 1018 Steel. Using both the calibar and the metal ruler the initial demensions of the test specimens were measured the demensions were the lengths, the widths, and thickness. The demesions were measured twice and the avrage of them were taken. With the parameters recorded the specimnes were put in the Tension testing machine. To garanty accuret results the specimesn have to fastesn and secured tighkty into the teeth of the machine inorder to prevent slipping ,which has a negative income on the results. When testing the materials were devived into two groups metals and polymers. The machine was set to a strain rate of 0.2 inches per minute for the metals and 2.00 inches per minute for the polymers. The maximum load used varied for the different materials, 200 lb for the LDPE, 500 lb for the HDPE, and 4000lb for the Aluminum and steel. Each time the test was done the final lengths were measured. After the test was preformed the results were transferred to an excel spread sheet where the results were plotted to form a stress and strain curve for the different materials.

Discussion:

By observing the stress-strain curves the different tensil properties for the various tested materials can be seen. The material with the largest ultimate tensile strength is steel shown in Figure 1. What can be noticed on the curve of this particular specimen is the the wavey distortions before the curve reaches the ultimate tensile stress. This is due to the properties of steel in which some discontinuities within the grain structure and impurities within the sample are formed which yielded almost constant strain under the increasing tensile load. This happens with the aluminun sample but in a lesser manner. Steel also experiences more necking before failure comparinng it with the Aluminum sample. It can be cunclueded that Aluminum was more brittle than steel. Both polymers LDPE and HDPE have a largees plastic region, which means they deforem a lot more before faluir accures in comparison with thhe steel and aluminum sample. This can be seen in figures 3 and 4 The table below shows all the calculations done that helped in forming the graphs.

Specimen	Width (in)	Thickness (in)	Areao (in^2)	Lo	Lf	%Elongation
Steel 1018	0.503	0.125	0.062875	3.79	4.7050955	24.145
LDPE	0.839	0.129	0.108231	3.75	10.2133125	172.355
Al 2024	0.505	0.127	0.064135	3.75	4.5742125	21.979
HDPE	0.737	0.12	0.08844	3.75	10.3402875	175.741

Fail load (lb)	ENG stress (psi)	Strain (psi)	ENG strain (psi)	True strain	True stress
3985.367	63385.55865	17.14908	0.1714908	0.158277126	74255.59881
152.1301	1405.605603	19.40467	0.1940467	0.177348126	1678.358732
4111.328	64104.28003	20.44067	0.2044067	0.185987081	77207.62437
343.628	3885.436454	12.256	0.12256	0.115611791	4361.635546

Young’s modulus for the matrials was alos calculated. This was done by taking the slope of young’s modulus line in stress vs strain curve shown in figures 1, 2, 3, and 4. The calculated values were campared to the given values and the error was obtained. The below table shows the results :

Material	Calculated Modulus	Given Modulus	Error %
1018 steel	1940785.802	29000000	93.30764
2024 Aluminum	212019.4049	1000000	78.79806
LDPE	30897.88491	80000	61.37764
HDPE	159807.4659	145000	10.21205

Figures:

Figure 1: Stress-strain curve for 1018 Steel

Figure 2: Stress-strain curve for 2024 Aluminum

Figure 3: Stress-strain curve for LDPE

Figure 4: Stress-strain curve for HDPE

Conclusion:

After prefprmimg this experiment and studying the obtianed results it can be concluded that different materials behave in a different manner to tension loads. Some materials are ductile others are brittle. This leads to that not all materials are made to a specific specifications, polymers can withstand tension more than metals, high density polymers have a better percent elongation than lower density polymers. Also, steel is more ductile than alluminum. This data that has been obtained can be utilized in other studies such as calculating the variance in strength parameters of material. Therefore, there are certain applications that some materials are a beter chouce than other.