Editing Vickers hardness test (section)

==Precautions==
When doing the hardness tests, the minimum distance between indentations and the distance from the indentation to the edge of the specimen must be taken into account to avoid interaction between the work-hardened regions and effects of the edge. These minimum distances are different for ISO 6507-1 and ASTM E384 standards.

{| class="wikitable"
|-
!   Standard   !! Distance between indentations !! Distance from the center of the indentation to the edge of the specimen
|-
| ISO 6507-1 || > 3·''d'' for steel and copper alloys and > 6·''d'' for light metals || 2.5·''d'' for steel and copper alloys and > 3·d for light metals 
|-
| ASTM E384 || 2.5·''d''|| 2.5·''d''
|}
Vickers values are generally independent of the test force: they will come out the same for 500&nbsp;gf and 50&nbsp;kgf, as long as the force is at least 200&nbsp;gf.<ref>[http://www.instron.us/wa/applications/test_types/hardness/vickers.aspx Vickers Test] {{Webarchive|url=https://web.archive.org/web/20141021002940/http://www.instron.us/wa/applications/test_types/hardness/vickers.aspx |date=21 October 2014 }}. [[Instron]] website.</ref> However, lower load indents often display a dependence of hardness on indent depth known as the [[indentation size effect]] (ISE).<ref>{{Cite journal|last=Nix|first=William D.|last2=Gao|first2=Huajian|date=1998-03-01|title=Indentation size effects in crystalline materials: A law for strain gradient plasticity|journal=Journal of the Mechanics and Physics of Solids|volume=46|issue=3|pages=411–425|bibcode=1998JMPSo..46..411N|doi=10.1016/S0022-5096(97)00086-0|issn=0022-5096|doi-access=free}}</ref> Small indent sizes will also have microstructure-dependent hardness values.

For thin samples indentation depth can be an issue due to substrate effects. As a rule of thumb the sample thickness should be kept greater than 2.5 times the indent diameter. Alternatively indent depth, <math>t</math>, can be calculated according to:
:<math>t = \frac{d_{\rm avg}}{2\sqrt{2}\tan{\frac{\theta}{2}}} \approx \frac{d_{\rm avg}}{7.0006},</math>