Explaining the Luders Band in Low Carbon Steel

[SoundFondler]

Hi, first post!

I have a first year exam tomorrow, but I can't find any where which gives a good explanation of the Luders Band displayed in low carbon steel.

Can anyone shed any light?

Thanks

 

[Astronuc]

I don't know why this isn't discussed in depth in undergrad textbooks on materials, but apparently it isn't. One probably has to go to the journals to learn the details. One thing to keep in mind is that most metal alloys are polycrystalline with variable composition at the grain and subgrain level. In addition grains have different sizes and crystallographic orientation.

Luders, Lueders, Lüders, and Luder's, Lueder's, Lüder's lines or bands (also called 'stetcher strain marks', Hartmann lines, Piobert lines) occur when steel and some Al-Mg alloys which experience yield point elongation. It is caused by local yielding in material that otherwise doesn't yield, so there are adjacent regions with variations on local plastic strain. The phenomenon is also described as discontinuous or non-uniform yielding.

See Lüders lines at http://nhml.com/resources_NHML_Definitions.php

Yield Point Elongation - In materials that exhibit a yield point, the Yield Point Elongation (YPE) is the difference between the elongation of the specimen at the start and at the finish of discontinuous yielding (the area in which an increase in strain occurs without an increase in stress).


http://www.steel-trp.org/TRPGreenBook2006/9944factsheet.pdf
Yield point elongation (YPE) is considered undesirable for surfacecritical
applications where the steel is formed, since “strain lines” Luders bands are created during forming.


In iron and low carbon steel, C and N atoms pin dislocations, with the pinning so strong that YPE occurs.

See Yield Point Elongation in Al-Mg Alloys

See also page 5 of - http://udf-cat.tstu.edu.ua/fileserver/Engeneering/Science%20of%20materials/Science%20of%20materials/3%203.6%20solid%20solution%20hardening..pdf use 'save target as' to save pdf

On the Propagation of Lüders Bands in Steel Strips
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JAMCAV000067000004000645000001&idtype=cvips&gifs=yes

Phenomenological theory of Lüders bands
http://www.springerlink.com/content/l9t08n85h4483652/

http://en.wikipedia.org/wiki/Lüder_band
Lüder bands often are a result of strain ageing by discontinuous yielding and can be commercially important for producing wrinkled lined finishes.


Rolling sheet material to 0.5% to 1.5% (~1.0%) reduction of thickness apparently eliminates or reduces yield point elongation.

 

[SoundFondler]

Thanks so much!

The exam went really well, so all is good!

 

[AussieDave]

I have a materials question that's asking me to calculate the "magnitude of Luder's Strain" from a bit of low carbon steel that we ran through the machine to break it apart and produce a load-elongation graph.

I know what the region is, the elongated yield point which can be seen on the graph. I just don't know what it is that they are asking for.

 

[Astronuc]

When loading a material like low carbon steel, there is a point on the stress-strain (load-displacement) curve where the yield point drops as strain continues. The yield stress decreases from the upper yield value to the lower yield value, and the strain (displacement) continues to increase while the stress stays roughly constant (actually there can be a little fluctuation). At some point, the stress increases as the material continues to strain.

The relatively flat portion of that stress strain curve, i.e. the (yield) extension at the slightly lower stress (lower yield stress limit), is the Luder's strain, and that's the "magnitude of Luder's strain". See figure 1 of - http://fcp.mechse.illinois.edu/reports/FCP_Report040.pdf

 

[Urmi Roy]

Hi,

My teacher told me about the Leuder band concept and said that it is the region on the engineering stress-strain curve between the point at which yield starts till the point when strain hardening begins.

However, I'd like to know in a more generalised way as to how dislocation movement plays a role in each stage of a stress-strain diagram.

I found something called the resolved stress-strain diagram , which has three regions (2nd diagram), but how would one superimpose this onto a regular stress-strain diagram (1st diagram)? For example, does the easy-glide region of dislocation (2nd diagram) represent elastic deformation upto proportional limit?

 

[usm125]

sorry i can't find how to post a new thread in this forum