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0005346Dwarf FortressMaterial Propertiespublic2012-02-20 19:072014-07-25 13:48
Toady One 
Toshiba SatelliteWindows VistaHome SP2
0005346: Obsidian swords always blunt, due to low SHEAR_YIELD
Obsidian swords made at the craftdwarf's workshop can't inflict slashing damage, instead acting like blunt clubs. In comparison, even elf wooden swords can cut skin.

The reason this happens is because obsidian uses the default stone template values for combat cutting calculations: [SHEAR_YIELD:15000] used marble
Wood has:[SHEAR_YIELD:40000] used pine
Skin/muscle have:[SHEAR_YIELD:20000] used human skin
Bone/nails/claws/teeth have: [SHEAR_YIELD:115000]

As a result, teeth and wood can cut you, but blades of volcanic glass don't.

I'd recommend setting obsidian to use the shear values similar to glass in order to make swords work as intended.
Relevant forum posts/thread:

http://www.bay12forums.com/smf/index.php?topic=91297.msg2541772#msg2541772 [^]

http://www.bay12forums.com/smf/index.php?topic=91297.msg2541922#msg2541922 [^]

Relevant for determining the intended function of the obsidian swords:

http://en.wikipedia.org/wiki/Macuahuitl [^]
Probable Quick Fix
Issue History
2012-02-20 19:07UristDaVinciNew Issue
2012-02-20 19:10UristDaVinciNote Added: 0020314
2012-05-17 23:11zweiIssue Monitored: zwei
2012-06-29 22:33UristDaVinciNote Added: 0023127
2014-07-23 20:36UristDaVinciNote Added: 0027378
2014-07-23 20:37UristDaVinciTag Attached: Probable Quick Fix
2014-07-23 20:48FootkerchiefSummaryObsidian swords always blunt => Obsidian swords always blunt, due to low SHEAR_YIELD
2014-07-25 13:48Toady OneStatusnew => resolved
2014-07-25 13:48Toady OneFixed in Version => Next Version
2014-07-25 13:48Toady OneResolutionopen => fixed
2014-07-25 13:48Toady OneAssigned To => Toady One

2012-02-20 19:10   
Bug also existed in 31.25
2012-06-29 22:33   
I verifed that this bug still exists in 0.34.11 by adding IS_METAL to obsidian to permit the creation of obsidian swords in the arena. I also verified that adding glass-like SHEAR values to obsidian permits swords to cut up creatures rather than bruising them.

I found a research thesis online in which the material properties of obsidian were tested: http://dc.library.okstate.edu/utils/getfile/collection/theses/id/2594/filename/2595.pdf [^] . Note the biaxial strength (in this case, tensile stress in both directions) of 35 ± 6.7 MPa for black obsidian. This would be equivalent to [TENSILE_FRACTURE:35000]. More importantly, setting the shear values would permit effective weapon usage, i.e:

[SHEAR_YIELD:35000] 35 MPa
[SHEAR_STRAIN_AT_YIELD:114] shear modulus 30.7 GPa
2014-07-23 20:36   
Bug still exists in 0.40.04

Despite obsidian being given a higher MAX_EDGE, it still can't cut as effectively as glass.

The research thesis link in the post above still works.