LNNano - Brazilian Nanotechnology National Laboratory

Samples for XTMS station

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Samples for XTMS station

The choice of a sample design for a XTMS experiment is very dependent on the test’s requirements.

The most used sample design is the dog bone / bowtie design, combining a flat surface for X-Ray reflection and a reduced center section for tension testing. The pins used to hold the sample in place can only hold up to 10 kN, which limits the possible applicable tensions in this type of sample.

The cones that hold the samples fit very precisely in the machine, and uneven and crooked samples can affect the thermal and electrical contact of the system. Thus, good quality machining must be done to reach the necessary precision for an ideal test.

When using this type of design, two choices must be made.

1) Cross Section type (size and rounded or flat):

The cross section will determine the Force/Tension ratio, which can limit the total applicable Tension in the sample.

The XTMS station has jaws ready to mount with samples 5 or 2 mm thick, but thin copper plates (0.25 mm or 0.5 mm) can be used to increase the thickness when necessary. However, the thickness must still be machined with the required precision (+0,0/-0,1 mm) for ideal contact.

The 5 mm thick samples are the most common size. They can be fabricated with rounded edges, which allows for higher reproducibility with the non-contact dilatometer, especially in Strain tests. A 5 mm thick sample with 6 mm diameter at the center has ≈26.0 mm² and can be used to apply tensions up to ≈385 MPa. A 5 mm x 5 mm square cross section sample can be used with similar limits (max. 400 MPa).

The 2 mm thick samples are commonly used for tests requiring higher stresses. They cannot be fabricated with rounded edges due to their size, but a 2 mm x 5 mm sample can be used to apply stresses up to 1,000 MPa, while a 1.5 mm x 5 mm sample can be used to apply up to 1,333 MPa.

2) Free Span length:

The Free Span length influences the cooling rate and the thermal gradient of the sample.

Generally a free span of at least 20 mm is desired; it features a gradient compatible with the most used beam size, is easy to assemble in the machine and has a good cooling rate.

Shorter free spans show better cooling rates, but are also harder to assemble and require smaller beam sizes due to the higher thermal gradients.

Longer free spans have smaller thermal gradients that may be useful for specific post-experiment tests, but the cooling rate suffers as a result.

A special short free span can also be used for room temperature strain tests to ensure that the deformation is concentrated in the center section of the sample. This type of free span is shown in the third sample below.

These are the most commonly used sample designs:

5620

Bowtie, rounded, 5 mm x 6 mm diameter cross section, 20 mm free span: Hot tension test with very good strain control. Max. stress allowed: 500 MPa. PDF: Bowtie_RD_5x6_20

5520

Bowtie, flat, 5 x 5 mm cross section, 20 mm free span: Thermal treatment and dilatometry tests. Max. stress allowed: 520 MPa PDF: Bowtie_FT_5x5_20

252204

Bowtie, flat, 2 x 5.2 mm cross section, 4 mm free span: Room temperature tension tests, requiring high stresses. Max. stress allowed: 1300 MPa PDF: Bowtie_FT_2x5.22_04

 

Other designs must be used if even larger tensions are to be applied, using a screw instead of a pin to apply force.

Comparison table of cooling rates (free cooling)

Cooling rates

Carbon steel AISI 1020
Bowtie rounded 5×5 mm

freespan = 15mm

freespan = 20mm

freespan = 25mm

1100°C – 1000°C

-102°C/s

-71°C/s

-53°C/s

900°C – 800°C

-78°C/s

-60°C/s

-45°C/s

700°C – 600°C

-54°C/s

-46°C/s

-33°C/s

500°C – 400°C

-32°C/s

-30°C/s

-22°C/s

300°C – 200°C

-9°C/s

-8°C/s

-6°C/s