Tensile Testing

Aluminum 6013

Tensile Testing

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Tensile testing is a process in which a uniaxial force is slowly applied on a specimen in order to measure the material's response to such forces. Stresses develop within a specimen when loads are applied on it. These stresses cause the specimen to deform first elastically and then plastically. If loading continues after plastic deformation occurs, the stresses developed within the specimen lead to failure.
Important information regarding the mechanical properties of the material, such as yield strength, tensile strength, modulus of elasticity and ductility, are obtained by performing a tensile test. In the case of Al6013, a tensile test proves extremely helpful when determining how the mechanical properties of the material change after subjecting Al6013 samples to other processes such as casting, welding and heat-treating.
The 'Instron universal testing machine' is widely used to perform tensile tests. After placing the specimen in the machine, a tensile force is applied. The strain rates used in these tests are usually very small, in the range of 10 -4 to 10 -2 per second. With the help of a strain gage (a.k.a extensometer) and an electronic data collection system, the Instron machine is able to collect (and deliver) detailed information about the amount that the specimen stretches due to the application of the tensile force.
Using the data collected from the tensile test, it is easy to calculate the engineering stress (=Force/x-sectional area) and strain (=Elongation/initial length) experienced by the specimen throughout the tensile test. A plot of strain vs. stress is then used to obtain values for the mechanical properties of the sample:

-The elastic modulus is the slope of the stress-strain curve in the linear region.
-The yield stress is calculated using the 0.2% offset method.
-The ultimate tensile strength is the maximum load the specimen bears, i.e. the highest stress value reached during the test.

Method

We took measurements of the width, thickness and length of the gauges (central area of the machined, dog bone-shaped samples) before performing the tensile test on the three Al6013 samples (casted, welded and heat-treated). To continue, we calibrated the Instron machine and located the samples securely into it. Then we run the test: a static load was applied until fracture. Force and elongation were recorded throughout the test.

We used the collected data to plot strain vs. stress curves for each of the three scenarios (casted, welded, heat-treated) and, based on these plots, we determined the mechanical properties of each sample. Finally, we compared the samples based on their properties to determine whether casting, welding and annealing affect the Al6013 samples in a desirable way, i.e. whether these processes enhance the properties that make this alloy appropriate for the manufacture of fuselage panels.

Results

· Initial dimensions of the tensile tested samples

Al6013 Sample

Length in m

Width in m

Thickness in m

X-sectional area in m

Casted

0.081407

0.0133096

0.0033528

4.46244E-05

Heat-treated

0.0799084

0.0130302

0.0034544

4.50115E-05

Welded

0.080518

0.0130048

0.0033528

4.36025E-05

· Strain vs. Stress plots

Analysis

· Mechanical properties of each sample

o The yield strength for each case was determined graphically, the last point in the elastic deformation area.

o Ultimate tensile strength was also determined graphically: the point in the plot corresponding to the highest stress experienced.

o To find modulus of elasticity, we fitted a trend line to the elastic deformation regions. The slope of such lines is the modulus of elasticity.

As illustrated by the graph above, the heat-treated sample was stronger both in terms of yield strength and ultimate tensile strength. This is explained by the different microstructure of this sample, produced by the heat treatments. Higher strength results from the smaller grains created during heat-treatment. Given that there are more grain boundaries in the microstructure after the hardening processes, dislocations movement is more limited which is analogous to higher strength.

The significantly lower strength of the welded sample (both in terms of yield strength and ultimate tensile strength) can be attributed to the welding process. Due to the material loss that results from welding, the sample’s gauge was thinner than the other samples (especially at the weld.) This explains why the sample fractured exactly at the weld.

Additionally, and as illustrated by the strain vs. stress plots, the welded sample underwent very little (almost none) plastic deformation. Welding makes Al6013 brittle, which is responsible for the absence of plastic deformation. For the manufacture of fuselage panels, brittleness is highly undesirable due to the higher danger imposed by panels that will immediately fail, instead of first deforming, then subjected to too high levels of stress.

Given that strength is a desirable property for the manufacture of fuselage panels, we concluded that heat-treating had a positive effect on Al6013 and welding had a negative effect on it. Hence, Al6013 should be heat treated but not welded prior to the actual manufacture of fuselage panels.