Explicit FEA Cracks - Revision history

Nairnj: /* Crack Tip Command */

2015-06-02T21:06:18Z

Crack Tip Command

← Older revision		Revision as of 21:06, 2 June 2015
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	* <tt>(keyid)</tt> specifies a previously defined [[Keypoint Command\|keypoint]] located at the crack tip.		* <tt>(keyid)</tt> specifies a previously defined [[Keypoint Command\|keypoint]] located at the crack tip.
	* Alternatively, the crack tip can be specified by the (<tt>(x),(y)</tt>) coordinates (in mm). The node closest to that point will be the crack tip node.		* Alternatively, the crack tip can be specified by the (<tt>(x),(y)</tt>) coordinates (in [[ConsistentUnits Command#Legacy and Consistent Units\|length units]]). The node closest to that point will be the crack tip node.

	In theory, quarter point elements represent the singular stress field of a crack tip better than conventional elements. They were first proposed by Barsoum (1974).<ref name='qtr'>R. S. Barsoum, "Application of Quadratic Isoparametric Finite Elements in Linear Fracture Mechanics," <i>Int. J. Fracture</i>, <b>10</b>, 603-605 (1974).</ref> Quarter point element are available in [[NairnFEA]] for research use, but they have drawbacks, and therefore are not recommended. Recent experience with crack-closure calculations shows that they converge more slowly to the correct energy release rate than conventional quadratic elements with midside nodes.<ref name='CC'/> Furthermore, prior literature has errors in quarter-point element crack-closure equations, although [[NairnFEAMPM]] has the new corrected equations from Nairn (2011).<ref name='CC'/> Finally, visualization of strain from FEA results is done by differentiation of the displacement field. Since quarter-point elements have singular strains at the crack tip, the strain in those elements will be undefined. These elements can still plot stresses.		In theory, quarter point elements represent the singular stress field of a crack tip better than conventional elements. They were first proposed by Barsoum (1974).<ref name='qtr'>R. S. Barsoum, "Application of Quadratic Isoparametric Finite Elements in Linear Fracture Mechanics," <i>Int. J. Fracture</i>, <b>10</b>, 603-605 (1974).</ref> Quarter point element are available in [[NairnFEA]] for research use, but they have drawbacks, and therefore are not recommended. Recent experience with crack-closure calculations shows that they converge more slowly to the correct energy release rate than conventional quadratic elements with midside nodes.<ref name='CC'/> Furthermore, prior literature has errors in quarter-point element crack-closure equations, although [[NairnFEAMPM]] has the new corrected equations from Nairn (2011).<ref name='CC'/> Finally, visualization of strain from FEA results is done by differentiation of the displacement field. Since quarter-point elements have singular strains at the crack tip, the strain in those elements will be undefined. These elements can still plot stresses.

Nairnj: /* Crack Tip Command */

2013-09-25T22:29:50Z

Crack Tip Command

← Older revision		Revision as of 22:29, 25 September 2013
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	The <tt>CrackTip</tt> command coverts all elements at the crack tip to be quarter point elements. Quarter point elements are [[FEA Elements\|6 or 8-node quadratic elements]] where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements (see figure on the right). Quarter point elements at the crack tip are created by first creating a mesh with [[FEA Elements\|8-node quadrilaterals or 6-node triangles]] and then adding the scripted command:		The <tt>CrackTip</tt> command coverts all elements at the crack tip to be quarter point elements. Quarter point elements are [[FEA Elements\|6 or 8-node quadratic elements]] where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements (see figure on the right). Quarter point elements at the crack tip are created by first creating a mesh with [[FEA Elements\|8-node quadrilaterals or 6-node triangles]] and then adding the scripted command:

	<~~code~~>   CrackTip (keyid)</~~code~~>		<tt>   CrackTip (keyid)</tt>

	or		or

	<~~code~~>   CrackTip (x),(y)</~~code~~>		<tt>   CrackTip (x),(y)</tt>

	In <tt>XML</tt> files, the corresponding commands are		In <tt>XML</tt> files, the corresponding commands are

	<~~code~~>   <CrackTip keypt='(keyid)'/></~~code~~>		<tt>   <CrackTip keypt='(keyid)'/></tt>

	<~~code~~>   <CrackTip x='(x)' y='(y)'/></~~code~~>		<tt>   <CrackTip x='(x)' y='(y)'/></tt>

	where		where

Nairnj: Nairnj moved page Explict FEA Cracks to Explicit FEA Cracks: Fixed spelling

2013-09-15T16:36:22Z

Nairnj moved page Explict FEA Cracks to Explicit FEA Cracks: Fixed spelling

← Older revision	Revision as of 16:36, 15 September 2013
(No difference)

Nairnj: /* Crack Tip Command */

2013-09-12T17:39:26Z

Crack Tip Command

← Older revision		Revision as of 17:39, 12 September 2013
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	<code>   <CrackTip keypt='(keyid)'/></code>		<code>   <CrackTip keypt='(keyid)'/></code>
	<code>   CrackTip x='(x)' y='(y)'/></code>
			<code>   <CrackTip x='(x)' y='(y)'/></code>

	where		where

Nairnj: /* Crack Tip Command */

2013-09-12T17:39:03Z

Crack Tip Command

← Older revision		Revision as of 17:39, 12 September 2013
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	[[File:qtrpt.jpg\|right]]		[[File:qtrpt.jpg\|right]]
	The <tt>CrackTip</tt> command coverts all elements at the crack tip to be quarter point elements. Quarter point elements are [[FEA Elements\|6 or 8-node quadratic elements]] where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements (see figure on the right). Quarter point elements at the crack tip are created by first creating a mesh with [[FEA Elements\|8-node quadrilaterals or 6-node triangles]] and then adding the command:		The <tt>CrackTip</tt> command coverts all elements at the crack tip to be quarter point elements. Quarter point elements are [[FEA Elements\|6 or 8-node quadratic elements]] where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements (see figure on the right). Quarter point elements at the crack tip are created by first creating a mesh with [[FEA Elements\|8-node quadrilaterals or 6-node triangles]] and then adding the scripted command:

	<code>   CrackTip (keyid)</code>		<code>   CrackTip (keyid)</code>
Line 24:		Line 24:

	<code>   CrackTip (x),(y)</code>		<code>   CrackTip (x),(y)</code>

			In <tt>XML</tt> files, the corresponding commands are

			<code>   <CrackTip keypt='(keyid)'/></code>
			<code>   CrackTip x='(x)' y='(y)'/></code>

	where		where

Nairnj: /* Crack Tip Command */

2013-09-12T17:32:40Z

Crack Tip Command

← Older revision		Revision as of 17:32, 12 September 2013
Line 30:		Line 30:
	* Alternatively, the crack tip can be specified by the (<tt>(x),(y)</tt>) coordinates (in mm). The node closest to that point will be the crack tip node.		* Alternatively, the crack tip can be specified by the (<tt>(x),(y)</tt>) coordinates (in mm). The node closest to that point will be the crack tip node.

	In theory, quarter point elements represent the singular stress field of a crack tip better than conventional elements. They were first proposed by Barsoum (1974).<ref name='qtr'>R. S. Barsoum, "Application of Quadratic Isoparametric Finite Elements in Linear Fracture Mechanics," <i>Int. J. Fracture</i>, <b>10</b>, 603-605 (1974).</ref> Quarter point element are available in [[NairnFEA]] for research use, but they have drawbacks, and therefore are not recommended. Recent experience with crack-closure calculations shows that they converge more slowly to the correct energy release rate than conventional quadratic elements with midside nodes.<ref name='CC'/> Furthermore, prior literature has errors in quarter-point element crack-closure equations, although [[NairnFEAMPM]] has the new corrected equations from Nairn (2011).<ref name='CC'/> Finally, visualization of strain from FEA results is done by differentiation of the displacement field. Since quarter-point elements have singular strains at the crack tip, the strain in those ~~element~~ will be undefined. These elements can still plot stresses.		In theory, quarter point elements represent the singular stress field of a crack tip better than conventional elements. They were first proposed by Barsoum (1974).<ref name='qtr'>R. S. Barsoum, "Application of Quadratic Isoparametric Finite Elements in Linear Fracture Mechanics," <i>Int. J. Fracture</i>, <b>10</b>, 603-605 (1974).</ref> Quarter point element are available in [[NairnFEA]] for research use, but they have drawbacks, and therefore are not recommended. Recent experience with crack-closure calculations shows that they converge more slowly to the correct energy release rate than conventional quadratic elements with midside nodes.<ref name='CC'/> Furthermore, prior literature has errors in quarter-point element crack-closure equations, although [[NairnFEAMPM]] has the new corrected equations from Nairn (2011).<ref name='CC'/> Finally, visualization of strain from FEA results is done by differentiation of the displacement field. Since quarter-point elements have singular strains at the crack tip, the strain in those elements will be undefined. These elements can still plot stresses.

	== References ==		== References ==
	<references/>		<references/>

Nairnj: /* Crack Tip Command */

2013-09-12T17:31:32Z

Crack Tip Command

← Older revision		Revision as of 17:31, 12 September 2013
Line 27:		Line 27:
	where		where

	* (keyid) specifies a previously defined [[Keypoint Command\|keypoint]] located at the crack tip.		* <tt>(keyid)</tt> specifies a previously defined [[Keypoint Command\|keypoint]] located at the crack tip.
	* Alternatively, the crack tip can be specified by the (<tt>(x),(y)</tt>) coordinates (in mm). The node closest to that point will be the crack tip node.		* Alternatively, the crack tip can be specified by the (<tt>(x),(y)</tt>) coordinates (in mm). The node closest to that point will be the crack tip node.

Nairnj: /* Crack Tip Command */

2013-09-12T17:30:49Z

Crack Tip Command

← Older revision		Revision as of 17:30, 12 September 2013
Line 17:		Line 17:

	[[File:qtrpt.jpg\|right]]		[[File:qtrpt.jpg\|right]]
	The <tt>CrackTip</tt> coverts all elements at the crack tip to be quarter point elements. Quarter point elements are [[FEA Elements\|6 or 8-node quadratic elements]] where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements (see figure on the right). Quarter point elements at the crack tip are created by first creating a mesh with [[FEA Elements\|8-node quadrilaterals or 6-node triangles]] and then adding the command:		The <tt>CrackTip</tt> command coverts all elements at the crack tip to be quarter point elements. Quarter point elements are [[FEA Elements\|6 or 8-node quadratic elements]] where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements (see figure on the right). Quarter point elements at the crack tip are created by first creating a mesh with [[FEA Elements\|8-node quadrilaterals or 6-node triangles]] and then adding the command:

	<code>   CrackTip (keyid)</code>		<code>   CrackTip (keyid)</code>

Nairnj: /* Crack Closure Calculations */

2013-09-12T17:30:31Z

Crack Closure Calculations

← Older revision		Revision as of 17:30, 12 September 2013
Line 10:		Line 10:
	== Crack Closure Calculations ==		== Crack Closure Calculations ==

	In crack closure calculations, nodal forces and displacements are used to find mode I and mode II energy release rate for crack growth. The calculations are done after the analysis and [[NairnFEAMPM]] is configured to do the calculations. A recent paper on crack closure calculations for a variety of elements and allowing for crack surface tractions is in Nairn (2011).<ref name='CC'>J.A. Nairn, "Generalized Crack Closure Analysis for Elements with Arbitrarily-Placed Side Nodes and Consistent Nodal Forces," ''Int. J. Fracture'', '''171''', 11-22 (2011). ([http://www.cof.orst.edu/cof/wse/faculty/Nairn/papers/CrackClosure.pdf PDF version])</ref>. This reference was the ~~bases of~~ the [[NairnFEAMPM]] calculations. ~~That pointed~~ pointed out ~~the~~ for ~~the~~ best results, the crack plane edges in front of and behind the crack tip must initially be collinear and of be of equal length (Δa).		In crack closure calculations, nodal forces and displacements are used to find mode I and mode II energy release rate for crack growth. The calculations are done after the analysis and [[NairnFEAMPM]] is configured to do the calculations. A recent paper on crack closure calculations for a variety of elements and allowing for crack surface tractions is in Nairn (2011).<ref name='CC'>J.A. Nairn, "Generalized Crack Closure Analysis for Elements with Arbitrarily-Placed Side Nodes and Consistent Nodal Forces," ''Int. J. Fracture'', '''171''', 11-22 (2011). ([http://www.cof.orst.edu/cof/wse/faculty/Nairn/papers/CrackClosure.pdf PDF version])</ref> This reference was the basis for the [[NairnFEAMPM]] calculations. It pointed out that for best results, the crack plane edges in front of and behind the crack tip must initially be collinear and of be of equal length (Δa).

	Crack closure calculations can be done for any [[FEA Elements\|element type]] allowed in [[NairnFEA]]. In addition, one special crack tip element available and it is ~~selected~~ using the <tt>CrackTip</tt> command.		Crack closure calculations can be done for any [[FEA Elements\|element type]] allowed in [[NairnFEA]]. In addition, one special crack tip element available and it is configured by using the <tt>CrackTip</tt> command.

	== Crack Tip Command ==		== Crack Tip Command ==

Nairnj: /* Crack Closure Calculations */

2013-09-12T17:28:53Z

Crack Closure Calculations

← Older revision		Revision as of 17:28, 12 September 2013
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	== Crack Closure Calculations ==		== Crack Closure Calculations ==

	In crack closure calculations, nodal forces and displacements are used to find mode I and mode II energy release rate for crack growth. The calculations are done after the analysis ~~is done~~ and [[NairnFEAMPM]] is configured to do the calculations. A recent paper on crack closure calculations for a variety of elements and allowing for crack surface tractions is in Nairn (2011).<ref name='CC'>J.A. Nairn, "Generalized Crack Closure Analysis for Elements with Arbitrarily-Placed Side Nodes and Consistent Nodal Forces," ''Int. J. Fracture'', '''171''', 11-22 (2011). ([http://www.cof.orst.edu/cof/wse/faculty/Nairn/papers/CrackClosure.pdf PDF version])</ref>. This reference was the bases of the [[NairnFEAMPM]] calculations. That pointed pointed out the for the best results, the crack plane edges in front of and behind the crack tip must initially be collinear and of be of equal length (Δa).		In crack closure calculations, nodal forces and displacements are used to find mode I and mode II energy release rate for crack growth. The calculations are done after the analysis and [[NairnFEAMPM]] is configured to do the calculations. A recent paper on crack closure calculations for a variety of elements and allowing for crack surface tractions is in Nairn (2011).<ref name='CC'>J.A. Nairn, "Generalized Crack Closure Analysis for Elements with Arbitrarily-Placed Side Nodes and Consistent Nodal Forces," ''Int. J. Fracture'', '''171''', 11-22 (2011). ([http://www.cof.orst.edu/cof/wse/faculty/Nairn/papers/CrackClosure.pdf PDF version])</ref>. This reference was the bases of the [[NairnFEAMPM]] calculations. That pointed pointed out the for the best results, the crack plane edges in front of and behind the crack tip must initially be collinear and of be of equal length (Δa).

	Crack closure calculations can be done for any [[FEA Elements\|element type]] allowed in [[NairnFEA]]. In addition, one special crack tip element available and it is selected using the <tt>CrackTip</tt> command.		Crack closure calculations can be done for any [[FEA Elements\|element type]] allowed in [[NairnFEA]]. In addition, one special crack tip element available and it is selected using the <tt>CrackTip</tt> command.