Detecting Contact - Revision history

Nairnj: /* Notes */

2026-03-29T18:41:15Z

Notes

← Older revision		Revision as of 18:41, 29 March 2026
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	# Note that the contact-position offset values of 0.8 and -0.58 apply only to calculations with [[MPM_Methods_and_Simulation_Timing#Input_Commands\|two particle per cell]] in each direction and linear shape functions (e.g., [[MPM Methods and Simulation Timing\|uGIMP and CPDI shape functions]]). The positive cutoff has to be changed for other particle sizes and spline shape functions. For one particle per cell and linear shape functions, a good value is 1.07. For three or more particles per cell, the value can be varied until you get good visual results in simple contact. The value should decrease as the number of particles per cell increases. A negative cutoff corresponds to a specific function that applies only when two particle per cell in each direction and linear shape functions. Use of any other number of particles or shape function should restrict the value to the best possible positive value and not use a negative value.		# Note that the contact-position offset values of 0.8 and -0.58 apply only to calculations with [[MPM_Methods_and_Simulation_Timing#Input_Commands\|two particle per cell]] in each direction and linear shape functions (e.g., [[MPM Methods and Simulation Timing\|uGIMP and CPDI shape functions]]). The positive cutoff has to be changed for other particle sizes and spline shape functions. For one particle per cell and linear shape functions, a good value is 1.07. For three or more particles per cell, the value can be varied until you get good visual results in simple contact. The value should decrease as the number of particles per cell increases. A negative cutoff corresponds to a specific function that applies only when two particle per cell in each direction and linear shape functions. Use of any other number of particles or shape function should restrict the value to the best possible positive value and not use a negative value.
	# The <tt>ContactPosition</tt> command is needed whenever all or some contacting material surfaces are initially separated ``and`` you are not getting normals by a [[Surface Normals#Regression Methods\|regression ~~methods~~]. Note that the command is essential for handling initially-separated surfaces and causes no problems for surfaces that start out in contact. They are still modeled correctly.		# The <tt>ContactPosition</tt> command is needed whenever all or some contacting material surfaces are initially separated ``and`` you are not getting normals by a [[Surface Normals#Regression Methods\|regression method]]. Note that the command is essential for handling initially-separated surfaces and causes no problems for surfaces that start out in contact. They are still modeled correctly.
	# Similarly, the <tt>ContactPositionCracks</tt> command is needed whenever all or some cracks surfaces are initially separated from materials.		# Similarly, the <tt>ContactPositionCracks</tt> command is needed whenever all or some cracks surfaces are initially separated from materials.
	# Although <tt>ContactPosition</tt> and <tt>ContactPositionCracks</tt> methods can handle both initially-separated or initially-contacted interfaces, it is usually more accurate to omit this command when all surfaces are initially in contact. Some exceptions are when the problem has large relative displacements such as sliding of a block or rolling of a disk along a surface; these actually correspond to parts of the surfaces that were separated coming into contact as the simulation evolves.		# Although <tt>ContactPosition</tt> and <tt>ContactPositionCracks</tt> methods can handle both initially-separated or initially-contacted interfaces, it is usually more accurate to omit this command when all surfaces are initially in contact. Some exceptions are when the problem has large relative displacements such as sliding of a block or rolling of a disk along a surface; these actually correspond to parts of the surfaces that were separated coming into contact as the simulation evolves.

Nairnj: /* Notes */

2026-03-29T18:40:54Z

Notes

← Older revision		Revision as of 18:40, 29 March 2026
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	# Note that the contact-position offset values of 0.8 and -0.58 apply only to calculations with [[MPM_Methods_and_Simulation_Timing#Input_Commands\|two particle per cell]] in each direction and linear shape functions (e.g., [[MPM Methods and Simulation Timing\|uGIMP and CPDI shape functions]]). The positive cutoff has to be changed for other particle sizes and spline shape functions. For one particle per cell and linear shape functions, a good value is 1.07. For three or more particles per cell, the value can be varied until you get good visual results in simple contact. The value should decrease as the number of particles per cell increases. A negative cutoff corresponds to a specific function that applies only when two particle per cell in each direction and linear shape functions. Use of any other number of particles or shape function should restrict the value to the best possible positive value and not use a negative value.		# Note that the contact-position offset values of 0.8 and -0.58 apply only to calculations with [[MPM_Methods_and_Simulation_Timing#Input_Commands\|two particle per cell]] in each direction and linear shape functions (e.g., [[MPM Methods and Simulation Timing\|uGIMP and CPDI shape functions]]). The positive cutoff has to be changed for other particle sizes and spline shape functions. For one particle per cell and linear shape functions, a good value is 1.07. For three or more particles per cell, the value can be varied until you get good visual results in simple contact. The value should decrease as the number of particles per cell increases. A negative cutoff corresponds to a specific function that applies only when two particle per cell in each direction and linear shape functions. Use of any other number of particles or shape function should restrict the value to the best possible positive value and not use a negative value.
	# The <tt>ContactPosition</tt> command is needed whenever all or some contacting material surfaces are initially separated ``and`` you are not ~~using~~ normals ~~method other than~~ [[Surface Normals#Regression Methods\|regression]]. Note that the command is essential for handling initially-separated surfaces and causes no problems for surfaces that start out in contact. They are still modeled correctly.		# The <tt>ContactPosition</tt> command is needed whenever all or some contacting material surfaces are initially separated ``and`` you are not getting normals by a [[Surface Normals#Regression Methods\|regression methods]. Note that the command is essential for handling initially-separated surfaces and causes no problems for surfaces that start out in contact. They are still modeled correctly.
	# Similarly, the <tt>ContactPositionCracks</tt> command is needed whenever all or some cracks surfaces are initially separated from materials.		# Similarly, the <tt>ContactPositionCracks</tt> command is needed whenever all or some cracks surfaces are initially separated from materials.
	# Although <tt>ContactPosition</tt> and <tt>ContactPositionCracks</tt> methods can handle both initially-separated or initially-contacted interfaces, it is usually more accurate to omit this command when all surfaces are initially in contact. Some exceptions are when the problem has large relative displacements such as sliding of a block or rolling of a disk along a surface; these actually correspond to parts of the surfaces that were separated coming into contact as the simulation evolves.		# Although <tt>ContactPosition</tt> and <tt>ContactPositionCracks</tt> methods can handle both initially-separated or initially-contacted interfaces, it is usually more accurate to omit this command when all surfaces are initially in contact. Some exceptions are when the problem has large relative displacements such as sliding of a block or rolling of a disk along a surface; these actually correspond to parts of the surfaces that were separated coming into contact as the simulation evolves.

Nairnj: /* Imperfect Interface Extrapolations */

2026-03-29T18:39:43Z

Imperfect Interface Extrapolations

← Older revision		Revision as of 18:39, 29 March 2026
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	A challenge to modeling [[Imperfect Interfaces\|imperfect interfaces]] in MPM is accurately calculating the tangential displacement discontinuity. The contact detection methods above focus on finding normal vector and normal separation ([[Surface Normals\|LogReg regression method]] works best). These [[Surface Normals\|surface normal methods]] look at material points on the two sides of the interface and cannot tell whether or not they got there by prior tangential displacement. Finding the tangential discontinuity is therefore challenging. The solution used in [[NairnMPM]] is to extrapolate displacements to the grid and use them to determine tangential discontinuity (method 2 above).		A challenge to modeling [[Imperfect Interfaces\|imperfect interfaces]] in MPM is accurately calculating the tangential displacement discontinuity. The contact detection methods above focus on finding normal vector and normal separation ([[Surface Normals\|LogReg regression method]] works best). These [[Surface Normals\|surface normal methods]] look at material points on the two sides of the interface and cannot tell whether or not they got there by prior tangential displacement. Finding the tangential discontinuity is therefore challenging. The solution used in [[NairnMPM]] is to extrapolate displacements to the grid and use them to determine tangential discontinuity (method 2 above).

	[[NairnMPM]] automatically uses displacements to find tangential separation for any contact modeled as an imperfect interface. The simulation can use [[Crack Settings\|crack settings]] (or the <tt>ContactPositionCracks</tt> command) for cracks or use [[Multimaterial MPM#Multimaterial Mode Input Commands\|<tt>ContactPosition</tt>]] in [[Multimaterial MPM\|multimaterial mode]] to use position extrapolations. These settings only affect calculation of normal vector and normal separation; imperfect interfaces will ~~alwaysuse~~ displacements to find tangential separation.		[[NairnMPM]] automatically uses displacements to find tangential separation for any contact modeled as an imperfect interface. The simulation can use [[Crack Settings\|crack settings]] (or the <tt>ContactPositionCracks</tt> command) for cracks or use [[Multimaterial MPM#Multimaterial Mode Input Commands\|<tt>ContactPosition</tt>]] in [[Multimaterial MPM\|multimaterial mode]] to use position extrapolations. These settings only affect calculation of normal vector and normal separation; imperfect interfaces will always use displacements to find tangential separation.

	Note that using displacements to find tangential discontinuity implicitly assumes zero tangential discontinuity at the start of the simulation. This situation is normal when modeling imperfect interfaces.		Note that using displacements to find tangential discontinuity implicitly assumes zero tangential discontinuity at the start of the simulation. This situation is normal when modeling imperfect interfaces.

Nairnj: /* Imperfect Interface Extrapolations */

2026-03-29T18:38:59Z

Imperfect Interface Extrapolations

← Older revision		Revision as of 18:38, 29 March 2026
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	A challenge to modeling [[Imperfect Interfaces\|imperfect interfaces]] in MPM is accurately calculating the tangential displacement discontinuity. The contact detection methods above focus on finding normal vector and normal separation ([[Surface Normals\|LogReg regression method]] works best). These [[Surface Normals\|surface normal methods]] look at material points on the two sides of the interface and cannot tell whether or not they got there by prior tangential displacement. Finding the tangential discontinuity is therefore challenging. The solution used in [[NairnMPM]] is to extrapolate displacements to the grid and use them to determine tangential discontinuity (method 2 above).		A challenge to modeling [[Imperfect Interfaces\|imperfect interfaces]] in MPM is accurately calculating the tangential displacement discontinuity. The contact detection methods above focus on finding normal vector and normal separation ([[Surface Normals\|LogReg regression method]] works best). These [[Surface Normals\|surface normal methods]] look at material points on the two sides of the interface and cannot tell whether or not they got there by prior tangential displacement. Finding the tangential discontinuity is therefore challenging. The solution used in [[NairnMPM]] is to extrapolate displacements to the grid and use them to determine tangential discontinuity (method 2 above).

	NairnMPM automatically uses displacements to find tangential separation for any contact modeled as an imperfect interface. The simulation can use [[Crack Settings\|crack settings]] (or the <tt>ContactPositionCracks</tt> command) for cracks or use [[Multimaterial MPM#Multimaterial Mode Input Commands\|<tt>ContactPosition</tt>]] in [[Multimaterial MPM\|multimaterial mode]] to use position extrapolations. These settings only affect calculation of normal vector and normal separation; imperfect interfaces will alwaysuse displacements to find tangential separation.		[[NairnMPM]] automatically uses displacements to find tangential separation for any contact modeled as an imperfect interface. The simulation can use [[Crack Settings\|crack settings]] (or the <tt>ContactPositionCracks</tt> command) for cracks or use [[Multimaterial MPM#Multimaterial Mode Input Commands\|<tt>ContactPosition</tt>]] in [[Multimaterial MPM\|multimaterial mode]] to use position extrapolations. These settings only affect calculation of normal vector and normal separation; imperfect interfaces will alwaysuse displacements to find tangential separation.

	Note that using displacements to find tangential discontinuity implicitly assumes zero tangential discontinuity at the start of the simulation. This situation is normal when modeling imperfect interfaces.		Note that using displacements to find tangential discontinuity implicitly assumes zero tangential discontinuity at the start of the simulation. This situation is normal when modeling imperfect interfaces.

Nairnj: /* Imperfect Interface Extrapolations */

2026-03-29T18:38:44Z

Imperfect Interface Extrapolations

← Older revision		Revision as of 18:38, 29 March 2026
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	=== Imperfect Interface Extrapolations ===		=== Imperfect Interface Extrapolations ===

	A challenge to modeling [[Imperfect Interfaces\|imperfect interfaces]] in MPM is accurately calculating the tangential displacement discontinuity. The contact detection methods above focus on finding normal vector and normal separation ([[Surface Normals\|LogReg regression method]] works best). These [[Surface Normals\|surface normal methods]] look at material points on the two sides of the interface and cannot tell whether or not they got there by prior tangential displacement. Finding the tangential discontinuity is therefore challenging. The solution used in NairnMPM is to extrapolate displacements to the grid and use them to determine tangential discontinuity (method 2 above).		A challenge to modeling [[Imperfect Interfaces\|imperfect interfaces]] in MPM is accurately calculating the tangential displacement discontinuity. The contact detection methods above focus on finding normal vector and normal separation ([[Surface Normals\|LogReg regression method]] works best). These [[Surface Normals\|surface normal methods]] look at material points on the two sides of the interface and cannot tell whether or not they got there by prior tangential displacement. Finding the tangential discontinuity is therefore challenging. The solution used in [[NairnMPM]] is to extrapolate displacements to the grid and use them to determine tangential discontinuity (method 2 above).

	NairnMPM automatically uses displacements to find tangential separation for any contact modeled as an imperfect interface. The simulation can use [[Crack Settings\|crack settings]] (or the <tt>ContactPositionCracks</tt> command) for cracks or use [[Multimaterial MPM#Multimaterial Mode Input Commands\|<tt>ContactPosition</tt>]] in [[Multimaterial MPM\|multimaterial mode]] to use position extrapolations. These settings only affect calculation of normal vector and normal separation; imperfect interfaces will alwaysuse displacements to find tangential separation.		NairnMPM automatically uses displacements to find tangential separation for any contact modeled as an imperfect interface. The simulation can use [[Crack Settings\|crack settings]] (or the <tt>ContactPositionCracks</tt> command) for cracks or use [[Multimaterial MPM#Multimaterial Mode Input Commands\|<tt>ContactPosition</tt>]] in [[Multimaterial MPM\|multimaterial mode]] to use position extrapolations. These settings only affect calculation of normal vector and normal separation; imperfect interfaces will alwaysuse displacements to find tangential separation.

Nairnj: /* ContactPosition Command */

2026-03-29T18:36:18Z

ContactPosition Command

← Older revision		Revision as of 18:36, 29 March 2026
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	<li>Define a contact-position offset, such as δ<sub>con</sub>= 0.8δx, and detect contact when δ<sub>n</sub><δ<sub>con</sub>. The contact-position offset may need to change depending on other simulations settings.[[#Notes\|<sup>1</sup>]]</li>		<li>Define a contact-position offset, such as δ<sub>con</sub>= 0.8δx, and detect contact when δ<sub>n</sub><δ<sub>con</sub>. The contact-position offset may need to change depending on other simulations settings.[[#Notes\|<sup>1</sup>]]</li>

	<li>~~Undertake of~~ more precise analysis of relation between extrapolated position of a material and actual distance to a node. First, we write the separation as:		<li>Use a more precise analysis of relation between extrapolated position of a material and actual distance to a node. First, we write the separation as:
	<br><br>		<br><br>

Nairnj: /* Basic Contact Detection */

2026-03-29T18:34:03Z

Basic Contact Detection

← Older revision		Revision as of 18:34, 29 March 2026
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	<dt>Approach velocity		<dt>Approach velocity
	<dd>The first step is to decide if the two surface are approaching each other in the normal direction. This criterion reduces to		<dd>The first step is to decide if the two surface are approaching each other in the normal direction. This criterion reduces to
	<math>\Delta\vec v \cdot \hat n < 0</math> where <math>\Delta\vec v = \vec v_a - \vec v_b</math>. When this criterion is not satisfied, the surfaces are separating and assumed to not be in contact. In early multimaterial MPM,<ref name="bard"/> this criterion alone was assumed sufficient to be in contact. It is easy to visualize, however, that this assumption detects contact when surfaces are approaching each other but are still separated. For improved contact, a second criterion based on displacements is ~~added~~.		<math>\Delta\vec v \cdot \hat n < 0</math> where <math>\Delta\vec v = \vec v_a - \vec v_b</math>. When this criterion is not satisfied, the surfaces are separating and assumed to not be in contact. In early multimaterial MPM,<ref name="bard"/> this criterion alone was assumed sufficient to be in contact. It is easy to visualize, however, that this assumption detects contact when surfaces are approaching each other but are still separated. For improved contact, a second criterion based on displacements is needed.

	<dt>Displacement check		<dt>Displacement check

Nairnj: /* Basic Contact Detection */

2026-03-29T18:33:27Z

Basic Contact Detection

← Older revision		Revision as of 18:33, 29 March 2026
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	<dt>Approach velocity		<dt>Approach velocity
	<dd>The first step is to decide if the two surface are approaching each other in the normal direction. This criterion reduces to		<dd>The first step is to decide if the two surface are approaching each other in the normal direction. This criterion reduces to
			<math>\Delta\vec v \cdot \hat n < 0</math> where <math>\Delta\vec v = \vec v_a - \vec v_b</math>. When this criterion is not satisfied, the surfaces are separating and assumed to not be in contact. In early multimaterial MPM,<ref name="bard"/> this criterion alone was assumed sufficient to be in contact. It is easy to visualize, however, that this assumption detects contact when surfaces are approaching each other but are still separated. For improved contact, a second criterion based on displacements is added.
	~~     ~~
	<math>\Delta\vec v \cdot \hat n < 0 ~~\qquad {\rm~~ where~~} \qquad~~ \Delta\vec v = \vec v_a - \vec v_b</math>

	When this criterion is not satisfied, the surfaces are separating and assumed to not be in contact. In early multimaterial MPM,<ref name="bard"/> this criterion alone was assumed sufficient to be in contact. It is easy to visualize, however, that this assumption detects contact when surfaces are approaching each other but are still separated. For improved contact, a second criterion based on displacements is added.

	<dt>Displacement check		<dt>Displacement check

Nairnj: /* Basic Contact Detection */

2026-03-29T18:32:19Z

Basic Contact Detection

← Older revision		Revision as of 18:32, 29 March 2026
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	[[File:NormalDef.jpg\|right]]		[[File:NormalDef.jpg\|right]]

	The figure shows a surface between two materials (a and b) or between two sides of a crack (above and below). The arrow shows a vector normal to the surface oriented from side a to side b. For both multimaterial contact and crack contact, the first step is to decide if the two velocity fields at the node are actually in contact. [[NairnMPM]] ~~uses~~ two criteria to detect contact.		The figure shows a surface between two materials (a and b) or between two sides of a crack (above and below). The arrow shows a vector normal to the surface oriented from side a to side b. For both multimaterial contact and crack contact, the first step is to decide if the two velocity fields at the node are actually in contact. [[NairnMPM]] combines two criteria to detect contact.

	<dl>		<dl>

Nairnj: /* Separation Using Regression Methods */

2026-03-29T18:31:06Z

Separation Using Regression Methods

← Older revision		Revision as of 18:31, 29 March 2026
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	== Separation Using Regression Methods ==		== Separation Using Regression Methods ==

	When using [[Surface Normals#Regression Methods\|regression methods]] to find surface normals,<ref name="logreg"/> the code builds lists of point clouds around each multimaterial node. Once the normal is found, it examines distance between the two material along that normal by checking each particle around the node. If the ~~particle~~ are deformed, the calculation accounts for that deformation. The two closest ~~particle~~ determine the interfacial separation.		When using [[Surface Normals#Regression Methods\|regression methods]] to find surface normals,<ref name="logreg"/> the code builds lists of point clouds around each multimaterial node. Once the normal is found, it examines distance between the two material along that normal by checking each particle around the node. If the particles are deformed, the calculation accounts for that deformation. The two closest particles determine the interfacial separation.

	These regression methods are preferred over all [[#ContactPosition Command\|grid extrapolation methods]] because they provide a direct calculation of separation and can account for particle deformation. When not using [[Surface Normals#Regression Methods\|regression methods]], however, the separation calculation has to be based on extrapolated particle positions and displacement on the grid. See [[#ContactPosition Command\|below]] for those methods. Note the [[MPM Input Files#Using Explicit Cracks\|crack modeling]] finds normals from crack surfaces and therefore never uses [[Surface Normals#Regression Methods\|regression methods]]. When modeling cracks, therefore, you must select a [[#ContactPosition Command\|grid extrapolation method]] for calculating separation.		These regression methods are preferred over all [[#ContactPosition Command\|grid extrapolation methods]] because they provide a direct calculation of separation and can account for particle deformation. When not using [[Surface Normals#Regression Methods\|regression methods]], however, the separation calculation has to be based on extrapolated particle positions and displacement on the grid. See [[#ContactPosition Command\|below]] for those methods. Note the [[MPM Input Files#Using Explicit Cracks\|crack modeling]] finds normals from crack surfaces and therefore never uses [[Surface Normals#Regression Methods\|regression methods]]. When modeling cracks, therefore, you must select a [[#ContactPosition Command\|grid extrapolation method]] for calculating separation.