|This article is about an older version of DF.|
Materials have a number of properties representing real world variables that describe how they respond to inputs. In particular, the game now has a number of variables that describe what happens to a material when it's put under stress.
What is stress?
In the real world, an object is stressed when a force is applied to the object. Depending on the nature of the force applied, this stress can take a number of forms, and the object can respond differently based on its material and how that material handles different stresses.
In the material raws, whenever you see 'yield', 'fracture', or 'strain at yield', that property is a stress-related quality.
When does Dwarf Fortress make stress calculations?
At present, DF seems to only apply forces during combat, and thus only stresses objects (generally armor and various body layers) at that time.
The first thing you'll notice is that the second word in each stress variable is one of Yield, Fracture, or Elasticity. These are mechanical performance terms.
The first set of words are things like Impact, Bending, and so forth. These describe modes of applying force.
The following explanations assumes real world physics sort of apply (since Toady One chose real world properties).
Mechanical Performance Properties
Yield: This is almost certainly 'Yield Strength', which is the amount of stress needed to cause a material to go from elastic deformation to plastic deformation. (That is, if you cease stressing the object, does it revert to its original shape or not). Since most objects only elastically deform over small distances of deformation, high Yield values generally means it takes a lot of force to noticeably 'stretch' them (but see elasticity).
Fracture: The fracture point is the amount of stress or force necessarily to cause the material to fail, or in other words, to break.
Strain at yield (or elasticity): This variable tells you how much deformation occurs to the material while it is deforming elastically. That is, as long as the force is less than the yield strength, stress * elasticity = deformation distance. The smaller the elasticity, the less deformation occurs under stress.
Note: Strain at yield is the inverse of the Elastic Modulus. Thus a highly elastic material has low elastic modulus, and engages in less elastic collisions.
Modes of Applying Force
Impact: Force applied by a sudden strike, like a hammer.
Compressive: Force applied by exerting pressure on an object, like trying to squish something between your hands.
Tensile: Force applied by pulling on something, like suspending one object via another. (e.g., if you suspend an elf from a metal pole, you are applying a tensile force to the pole).
Torsion: Force applied by twisting something. Note that you're twisting some portion of the object relative to itself to cause a torsion stress to be applied to it. (Consider trying to twist a metal rod by grasping at either end and attempting to wring it - yes, you'd have to apply a lot of force to succeed).
Shear: Force applied by pushing part of the material so it tries to slide relative to another part of it. Ie, pushing at the top of an object when the bottom part is fixed to the ground is going to primarily apply a shear stress to it (the top part will try to move in the direction you push, and the lower part will resist this shear stress).
Bending: Force applied by bending a material.
Hypotheses based on Real World
- High strain at yield should reduce the effectiveness of a weapon. (the more it deforms, the longer it takes to break contact with the struck surface and thus the more inelastic the collision is).
- High strain at yield should be good for armor, because it decreases the force transferred by a weapon (to a point - it can't be so inelastic as to render the armor useless!)
Effects on Combat
Dwarf Fortress only features a limited combat system. Item decay does not seem to be simulated properly at all, so the hypotheses are largely incorrect.