|
Simbody
|
|
Simbody
|
Define the physical properties of the material from which a contact surface is made, including properties needed by a variety of contact response techniques that might be applied during contact. More...
#include <ContactSurface.h>
Public Member Functions | |
| ContactMaterial () | |
| Default constructor creates an invalid contact material; you must specify at least stiffness before using this material. | |
| ContactMaterial (Real stiffness, Real dissipation, Real staticFriction, Real dynamicFriction, Real viscousFriction=0) | |
| Create a contact material with a complete set of compliant contact material properties. | |
| bool | isValid () const |
| Return false if material properties have not yet been supplied for this contact material. | |
| Real | getStiffness () const |
| Return the material stiffness k=(force/area)/%strain. | |
| Real | getStiffness23 () const |
| Return precalculated 2/3 power of material stiffness k (k^(2/3)). | |
| Real | getDissipation () const |
| Return the material dissipation coefficient c, in units of 1/velocity. | |
| Real | getStaticFriction () const |
| Return the coefficient of static friction (unitless). | |
| Real | getDynamicFriction () const |
| Return the coefficient of dynamic friction (unitless). | |
| Real | getViscousFriction () const |
| Return the coefficient of viscous friction (1/velocity). | |
| ContactMaterial & | setStiffness (Real stiffness) |
| Supply material stiffness k=(force/area)/%strain. | |
| ContactMaterial & | setDissipation (Real dissipation) |
| Supply material dissipation coefficient c, which is the slope of the coefficient of restitution (e) vs. | |
| ContactMaterial & | setFriction (Real staticFriction, Real dynamicFriction, Real viscousFriction=0) |
| Set the friction coefficients for this material. | |
| void | clear () |
| Restore this contact material to an invalid state containing no stiffness specification and default values for dissipation (none) and friction (none). | |
Static Public Member Functions | |
| static Real | calcPlaneStrainStiffness (Real youngsModulus, Real poissonsRatio) |
| Calculate the contact material stiffness k from its linear elastic material properties Young's modulus E and Poisson's ratio v, assuming that the material is free to move under pressure. | |
| static Real | calcConfinedCompressionStiffness (Real youngsModulus, Real poissonsRatio) |
| Calculate the contact material stiffness k from its linear elastic material properties Young's modulus E and Poisson's ratio v, assuming that the material is confined such that its volume must be reduced under pressure. | |
| static Real | calcDissipationFromObservedRestitution (Real restitution, Real speed) |
| Given an observation of the coefficient of restitution e at a given small impact velocity v, calculate the apparent value of the dissipation coefficient c, which is a material property. | |
Define the physical properties of the material from which a contact surface is made, including properties needed by a variety of contact response techniques that might be applied during contact.
Two basic techniques are supported:
Each of these techniques can respond to both impacts and continuous contact, and provides for both compressive and frictional effects.
For compliant contact we distinguish two cases:
The elastic foundation layer thickness h may be specified at the time this material is used to construct a ContactSurface; the material properties supplied here should be independent of that.
Compliant contact models respond via forces that are generated by finite deformations of the contact surface, using theory of elasticity. Rigid contact models instead enforce zero deformation at the contact surface via the reaction forces of non-penetration contraints, and respond to impacts by generating impulsive changes to velocities rather than forces.
Energy dissipation upon impact is dealt with differently in the two schemes. In compliant contact, energy dissipation occurs via a continuous function of deformation and deformation rate, based on a material property c we call the "dissipation coefficient" (Hunt & Crossley call this "alpha"). In rigid contact, energy dissipation occurs via a "coefficient of restitution" e that defines what fraction of impact velocity is lost on rebound. It is common practice, but blatantly incorrect, to use a constant value for the coefficient of restitution. Empirically, coefficient of restitution e is known to be a linear function of impact velocity for small velocities: e=1-c*v where c is the dissipation coefficient mentioned above. At large velocities plasticity and other effects cause the coefficient of restitution to drop dramatically; we do not attempt to support plasticity models here.
| SimTK::ContactMaterial::ContactMaterial | ( | ) | [inline] |
Default constructor creates an invalid contact material; you must specify at least stiffness before using this material.
Defaults are provided for dissipation (none) and friction (none).
| SimTK::ContactMaterial::ContactMaterial | ( | Real | stiffness, |
| Real | dissipation, | ||
| Real | staticFriction, | ||
| Real | dynamicFriction, | ||
| Real | viscousFriction = 0 |
||
| ) | [inline] |
Create a contact material with a complete set of compliant contact material properties.
This does not set the coefficient of restitution. There are a variety of static methods provided to aid in calculating the stiffness and dissipation from available data.
| bool SimTK::ContactMaterial::isValid | ( | ) | const [inline] |
Return false if material properties have not yet been supplied for this contact material.
| Real SimTK::ContactMaterial::getStiffness | ( | ) | const [inline] |
Return the material stiffness k=(force/area)/%strain.
| Real SimTK::ContactMaterial::getStiffness23 | ( | ) | const [inline] |
Return precalculated 2/3 power of material stiffness k (k^(2/3)).
| Real SimTK::ContactMaterial::getDissipation | ( | ) | const [inline] |
Return the material dissipation coefficient c, in units of 1/velocity.
| Real SimTK::ContactMaterial::getStaticFriction | ( | ) | const [inline] |
Return the coefficient of static friction (unitless).
| Real SimTK::ContactMaterial::getDynamicFriction | ( | ) | const [inline] |
Return the coefficient of dynamic friction (unitless).
| Real SimTK::ContactMaterial::getViscousFriction | ( | ) | const [inline] |
Return the coefficient of viscous friction (1/velocity).
| ContactMaterial& SimTK::ContactMaterial::setStiffness | ( | Real | stiffness | ) | [inline] |
Supply material stiffness k=(force/area)/%strain.
A compliant contact model will then calculate the contact area A and strain fraction s to calculate the resulting force f=k*A*s.
| ContactMaterial& SimTK::ContactMaterial::setDissipation | ( | Real | dissipation | ) | [inline] |
Supply material dissipation coefficient c, which is the slope of the coefficient of restitution (e) vs.
impact speed (v) curve, with e=1-cv at low (non-yielding) impact speeds v.
| ContactMaterial& SimTK::ContactMaterial::setFriction | ( | Real | staticFriction, |
| Real | dynamicFriction, | ||
| Real | viscousFriction = 0 |
||
| ) | [inline] |
Set the friction coefficients for this material.
| static Real SimTK::ContactMaterial::calcPlaneStrainStiffness | ( | Real | youngsModulus, |
| Real | poissonsRatio | ||
| ) | [inline, static] |
Calculate the contact material stiffness k from its linear elastic material properties Young's modulus E and Poisson's ratio v, assuming that the material is free to move under pressure.
In this circumstance the effective stiffness is given by the plane strain modulus k=E/(1-v^2), and the stiffness never exceeds (4/3)E when the material becomes incompressible as v->0.5 as is typical of rubber. If this material is to be used in a circumstance in which its volume must be reduced under contact pressure (because there is no room to move), consider using the confined compression modulus instead.
| static Real SimTK::ContactMaterial::calcConfinedCompressionStiffness | ( | Real | youngsModulus, |
| Real | poissonsRatio | ||
| ) | [inline, static] |
Calculate the contact material stiffness k from its linear elastic material properties Young's modulus E and Poisson's ratio v, assuming that the material is confined such that its volume must be reduced under pressure.
In this case the effective stiffness is given by the confined compression modulus k=E(1-v)/((1+v)(1-2v)). Note that the stiffness becomes infinite when the material becomes incompressible as v->0.5. That means rubber will have a near infinite stiffness if you calculate it this way; make sure that's what you want!
| static Real SimTK::ContactMaterial::calcDissipationFromObservedRestitution | ( | Real | restitution, |
| Real | speed | ||
| ) | [inline, static] |
Given an observation of the coefficient of restitution e at a given small impact velocity v, calculate the apparent value of the dissipation coefficient c, which is a material property.
You should get the same value for c from any observation, provided that v is non-zero but small. The coefficient of restitution as v->0 is 1 for all materials.
Coefficient of restitution at low impact speed is linearly related to impact velocity by the dissipation coefficient c (1/velocity) by e=(1-cv) so we can calculate c if we have observed e at some low speed v as c=(1-e)/v. If the coefficient of restitution is 1 we'll return c=0 and ignore v, otherwise v must be greater than zero.
| void SimTK::ContactMaterial::clear | ( | ) | [inline] |
Restore this contact material to an invalid state containing no stiffness specification and default values for dissipation (none) and friction (none).
1.7.3
