Analysis of 3D Cracks in an Arbitrary Geometry with
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Analysis of 3D Cracks in an Arbitrary Baidu Nhomakorabeaeometry with Weld Residual Stress
Greg Thorwald, Ph.D. Ted L. Anderson, Ph.D. Structural Reliability Technology, Boulder, CO
define an arbitrary shape volume with six surfaces around the crack location. The definition mesh volume has six surfaces to match the shape of the preliminary 3D crack mesh. The 3D crack mesh ANSYS input file is generated by FEA-Crack within the definition mesh volume and is then inserted back into the larger model. The meshes can easily be connected by bonded contact in ANSYS [reference 1], which permits a different mesh pattern between the crack mesh and larger structure mesh. Welds have regions of tensile residual stresses that increases the crack stress intensity and may adversely affect the critical fracture condition. When the crack is in or near a weld, the weld residual stresses can be included in the crack analysis by mapping all the stress components from the uncracked model residual stress analysis results onto the crack mesh as an initial stress. ANSYS uses an initial stress file [reference 2] to include the residual stresses in a crack analysis along with other boundary conditions. Including both the weld residual stress and other loading in the crack analysis gives a more thorough and accurate calculation of the crack front stress intensity.
Abstract Materials containing flaws like inclusions and lack of weld fusion can cause cracks to form and grow; a critical size crack can cause a catastrophic fracture failure, even at low stress. Fracture mechanics allows cracks to be evaluated as benign or requiring repair. Modeling the actual crack location in a complicated geometry is necessary to obtain accurate crack stress intensity values, crucial in a thorough crack evaluation. When existing stress intensity solutions are not available, FEA of 3D cracks provides a way to compute the stress intensity. A method for quickly generating 3D crack meshes within an arbitrary shape volume is needed to efficiently compute the stress intensity. This method uses a mesh of brick elements to define the arbitrary shape volume around the crack in the structure. The 3D crack mesh is generated within the definition mesh and inserted into the larger model; the meshes are connected by bonded contact. For a crack in a weld, the residual stresses can be included by mapping all stress components from an uncracked model onto the crack mesh as an initial stress. The weld residual stress increases the stress intensity. The stress intensity is computed using ANSYS results during post-processing.
Introduction
Since all engineering materials contain flaws, such as inclusions, porosity, lack of weld fusion, and pitting, these defects can cause cracks to form and grow over time in many types of structures. Crack evaluation is important in petroleum, chemical, power generation, aerospace, mechanical, and civil structures. A critical size crack can cause a catastrophic fracture failure, even at low stresses below the yield strength. Using fracture mechanics methods, a crack can be evaluated using the stress intensity at the crack front to determine if it is benign or requires repair, and to compute how quickly the crack will grow. Computing the crack fracture condition and fatigue life allows for an efficient inspection and repair schedule, reducing risk and cost. Computing the critical crack size also verifies that inspection methods can find the crack while it is still smaller than the critical size to cause fracture. Accurate crack stress intensity values, KI, are crucial for a thorough crack evaluation. Stress intensity solutions are available from handbooks and the literature for many basic geometries and crack locations; however, modeling the actual crack location and orientation in a complicated geometry is an important improvement for obtaining accurate crack stress intensity values. When an existing stress intensity solution that matches the structure geometry and crack location is not readily available, finite element analysis of 3D cracks provides a way to compute the crack front stress intensity. Some of the difficult and timeconsuming tasks to create a 3D crack mesh include generating the collapsed brick elements along the crack front and the concentric rings of elements around the crack front for the “spider-web” mesh pattern, cracks following curved surfaces in more complicated geometries, listing the node sets along the crack front correctly for the J-integral calculation, applying crack plane symmetry constraints, applying crack face loads, and extracting the J-integral and stress intensity values from the results. When a variety of crack sizes and locations are examined, the effort to generate each new crack mesh must be repeated. More complicated geometries with numerous possible crack locations prohibit tables of stress intensity values to be computed for all possible cases; instead the stress intensity needs to be computed for each given crack location and size. These time consuming modeling difficulties led to the development of FEA-Crack to generate the 3D crack meshes quickly and easily, and allows cracks at any location to be routinely analyzed. Having an easy-to-use method for quickly generating 3D crack mesh input files within an arbitrary shape volume is needed to efficiently compute the crack front stress intensity at any location within complicated structures. This method uses a grid mesh of brick elements extracted from the larger structure model to
Greg Thorwald, Ph.D. Ted L. Anderson, Ph.D. Structural Reliability Technology, Boulder, CO
define an arbitrary shape volume with six surfaces around the crack location. The definition mesh volume has six surfaces to match the shape of the preliminary 3D crack mesh. The 3D crack mesh ANSYS input file is generated by FEA-Crack within the definition mesh volume and is then inserted back into the larger model. The meshes can easily be connected by bonded contact in ANSYS [reference 1], which permits a different mesh pattern between the crack mesh and larger structure mesh. Welds have regions of tensile residual stresses that increases the crack stress intensity and may adversely affect the critical fracture condition. When the crack is in or near a weld, the weld residual stresses can be included in the crack analysis by mapping all the stress components from the uncracked model residual stress analysis results onto the crack mesh as an initial stress. ANSYS uses an initial stress file [reference 2] to include the residual stresses in a crack analysis along with other boundary conditions. Including both the weld residual stress and other loading in the crack analysis gives a more thorough and accurate calculation of the crack front stress intensity.
Abstract Materials containing flaws like inclusions and lack of weld fusion can cause cracks to form and grow; a critical size crack can cause a catastrophic fracture failure, even at low stress. Fracture mechanics allows cracks to be evaluated as benign or requiring repair. Modeling the actual crack location in a complicated geometry is necessary to obtain accurate crack stress intensity values, crucial in a thorough crack evaluation. When existing stress intensity solutions are not available, FEA of 3D cracks provides a way to compute the stress intensity. A method for quickly generating 3D crack meshes within an arbitrary shape volume is needed to efficiently compute the stress intensity. This method uses a mesh of brick elements to define the arbitrary shape volume around the crack in the structure. The 3D crack mesh is generated within the definition mesh and inserted into the larger model; the meshes are connected by bonded contact. For a crack in a weld, the residual stresses can be included by mapping all stress components from an uncracked model onto the crack mesh as an initial stress. The weld residual stress increases the stress intensity. The stress intensity is computed using ANSYS results during post-processing.
Introduction
Since all engineering materials contain flaws, such as inclusions, porosity, lack of weld fusion, and pitting, these defects can cause cracks to form and grow over time in many types of structures. Crack evaluation is important in petroleum, chemical, power generation, aerospace, mechanical, and civil structures. A critical size crack can cause a catastrophic fracture failure, even at low stresses below the yield strength. Using fracture mechanics methods, a crack can be evaluated using the stress intensity at the crack front to determine if it is benign or requires repair, and to compute how quickly the crack will grow. Computing the crack fracture condition and fatigue life allows for an efficient inspection and repair schedule, reducing risk and cost. Computing the critical crack size also verifies that inspection methods can find the crack while it is still smaller than the critical size to cause fracture. Accurate crack stress intensity values, KI, are crucial for a thorough crack evaluation. Stress intensity solutions are available from handbooks and the literature for many basic geometries and crack locations; however, modeling the actual crack location and orientation in a complicated geometry is an important improvement for obtaining accurate crack stress intensity values. When an existing stress intensity solution that matches the structure geometry and crack location is not readily available, finite element analysis of 3D cracks provides a way to compute the crack front stress intensity. Some of the difficult and timeconsuming tasks to create a 3D crack mesh include generating the collapsed brick elements along the crack front and the concentric rings of elements around the crack front for the “spider-web” mesh pattern, cracks following curved surfaces in more complicated geometries, listing the node sets along the crack front correctly for the J-integral calculation, applying crack plane symmetry constraints, applying crack face loads, and extracting the J-integral and stress intensity values from the results. When a variety of crack sizes and locations are examined, the effort to generate each new crack mesh must be repeated. More complicated geometries with numerous possible crack locations prohibit tables of stress intensity values to be computed for all possible cases; instead the stress intensity needs to be computed for each given crack location and size. These time consuming modeling difficulties led to the development of FEA-Crack to generate the 3D crack meshes quickly and easily, and allows cracks at any location to be routinely analyzed. Having an easy-to-use method for quickly generating 3D crack mesh input files within an arbitrary shape volume is needed to efficiently compute the crack front stress intensity at any location within complicated structures. This method uses a grid mesh of brick elements extracted from the larger structure model to