MD Nastran Elements 2

MD Nastran Elements

1Copyright© 2010 MSC.Software CorporationCopy For Politecnico of Milano


COORDINATE SYSTEMS IN PATRAN

● Coordinate systems are used in the construction and transformation of geometry


COORDINATE SYSTEMS IN PATRAN (CONT.)

● Coordinate systems are also used to define the direction of loads and boundary conditions


COORDINATE SYSTEMS IN PATRAN (CONT.)

● Coordinate systems can also be used to define the analysis coordinate system of a node


CREATING COORDINATE SYSTEMS

● There are three types of coordinate systems: Rectangular, Cylindrical, and Spherical

● There are many ways to create coordinate systems:


● MD Nastran Coordinate systems are used to● Define locations of grid points in space● Orient each grid point’s displacement vector

● Coordinate systems in MD Nastran:● Basic Coordinate System - Implicitly defined reference rectangular coordinate

system (Coordinate System 0). Orientation of this system is defined by the user through specifying the components of grid point locations.

● Local Coordinate Systems - User-defined coordinate systems. Each local

MD NASTRAN COORDINATE SYSTEMS


● Local Coordinate Systems - User-defined coordinate systems. Each local coordinate system must be related directly or indirectly to the basic coordinate system. The six possible local coordinate systems are:● Rectangular CORD1R● Rectangular CORD2R● Cylindrical CORD1C● Cylindrical CORD2C● Spherical CORD1S● Spherical CORD2S

MD NASTRAN COORDINATE SYSTEMS (Cont.)

● MD Nastran Local Coordinate Systems:● The CORD1R, CORD1C, and CORD1S entries define a local

coordinate system by referencing the IDs of three existing grid points.

● The CORD2R, CORD2C, and CORD2S entries define a local coordinate system by specifying the vector components of three points. This is the format used by Patran.


points. This is the format used by Patran.● All angular coordinates are input in DEGREES. All rotational

displacements associated with these coordinates are output in RADIANS.

Rectangular Local Coordinate System (X, Y, Z)

MD NASTRAN RECTANGULAR COORDINATE

SYSTEM


Point A = local coordinate system originPoint B = reference point for z axis directionPoint C = reference point in the x-z planePoint P = grid point defined in local rectangular system

(ux, uy, uz) = displacement components of P in local system

Cylindrical Local Coordinate System (R, θθθθ, Z)

MD NASTRAN CYLINDRICAL COORDINATE

SYSTEM


Point A = local coordinate system originPoint B = reference point for z axis directionPoint C = reference point in the x-z planePoint P = grid point defined in local cylindrical system

(Ur, Uθ, Uz) = displacement components of P in local system

Spherical Local Coordinate System (R, θθθθ, φφφφ)

MD NASTRAN SPHERICAL COORDINATE

SYSTEM


Point A = local coordinate system originPoint B = reference point for z axis directionPoint C = reference point in the x-z planePoint P = grid point defined in local spherical system

(Ur, Uθ, Uφ) = displacement components of P in local system

MD NASTRAN COORDINATE SYSTEM ENTRIES


The tick mark represents the origin of the coordinate system 0

DISPLAY OF COORDINATE SYSTEM 0


Coordinate system 0 is always displayed at the lower left-hand corner of the viewport

CORD1X VS. CORD2X ENTRIES

● By default, coordinate systems are translated into MD Nastran CORD2X entries

● If Coordinate Frame Coordinates in the Translation Parameters form is set to reference nodes, then CORD1X is


reference nodes, then CORD1X is translated where applicable

NESTED COORDINATE SYSTEMS

● Creating nested coordinate systems● By default, the nested relationship is lost during translation to MD

Nastran● If nested coordinate system is desired, the Coordinate Frame

Coordinates in the Translation Parameters form needs to be set to reference frame.


Point 16

Point 23Create a rectangular coordinate system which will be used later to define the direction of the applied

CREATE A RECTANGULAR COORDINATE SYSTEM


Point 17

the direction of the applied load

GRID POINTS

● Grid points are used to specify:● Structural geometry● Degrees of freedom of the structure● Locations of points at which displacements are constrained or loads

are applied● Locations where output quantities are to be calculated


● Locations where output quantities are to be calculated

DEGREES OF FREEDOM

● Each grid point is capable of moving in six directions. These are called degrees of freedom (DOF).

DOF1 = T1 = u1 = translation in direction 1DOF2 = T = u = translation in direction 2

6


1 1DOF2 = T2 = u2 = translation in direction 2DOF3 = T3 = u3 = translation in direction 3DOF4 = R1 = θθθθ1 = rotation in direction 1DOF5 = R2 = θθθθ2 = rotation in direction 2DOF6 = R3 = θθθθ3 = rotation in direction 3 1 2

3

45

6

DEGREES OF FREEDOM (Cont.)

● For each grid point, all six degrees of freedom must be accounted for:● Think in terms of 3D even if the problem is only 1D or 2D. ● Any un-used DOF must be constrained


1 2

3

45

Field Contents

THE NASTRAN GRID ENTRY

● The NASTRAN GRID entry is show below:


Field Contents

ID Grid point identification number

CP Identification number of coordinate system in which the location of the grid point is defined (integer ≥≥≥≥ 0 orblank; default = basic coordinate system)

X1, X2, X3 Location of grid point in coordinate system CP (real)

CD Identification number of coordinate system in which displacements, degrees of freedom, constraints, and solution vectors are defined at the grid point (integer ≥≥≥≥ 0 or blank; default = basic coordinate system).

PS Permanent single-point constraints associated with grid point (any of the digits 1-6 with no embedded blanks)This method of constraining a structure is not recommended.

SEID Superelement ID

THE NASTRAN GRID ENTRY (Cont.)

● Each GRID entry refers to two coordinate systems● The coordinate system in field 3 is used to locate the grid point. This

is called the positional coordinate system.● The coordinate system in field 7 establishes the grid point

displacement coordinate system which defines for the given grid point the directions of displacements, degrees of freedom, constraints, and solution vectors.


constraints, and solution vectors.

THE GRID POINT DISPLACEMENT COORDINATE SYSTEM

● The grid point displacement coordinate system:● The grid point displacement coordinate system is also known as the

output coordinate system because all grid point results (displacements, grid point forces, etc.) are generated and output in this coordinate system.

● The union of all displacement coordinate systems is called the global coordinate system.


coordinate system.

GRID POINT EXAMPLE

● Grid points 10 and 20 are located on the aircraft fuselage as show below. The GRID entry uses coordinate system 5 to define the location of the two points and uses coordinate system 0 to define the grid point displacements.


Coordinate System 5(cylindrical)

Basic coordinate system 0

GRID POINT EXAMPLE (Cont.)

● Suppose we are interested in displacements and forces in the fuselage radial and tangential directions. We can accomplish this by changing field 7 of the GRID entries from coordinate system 0 to coordinate system 5.


Coordinate System 5(cylindrical)Basic coordinate system 0

CONSTRAINTS

RIGID ELEMENTSCLEARANCE

USING THE GRID POINT DISPLACEMENT COORDINATE SYSTEM

● Examples of how the grid point displacement coordinate system is used


SPRINGS

MATERIAL PROPERTIES


MATERIAL PROPERTIES (Cont.)● Linear analysis material types

● Isotropic (MAT1)

● Two-dimensional anisotropic (MAT2)

● Axisymmetric solid orthotropic (MAT3)

● Two-dimensional orthotropic (MAT8)


● Two-dimensional orthotropic (MAT8)

● Three-dimensional anisotropic (MAT9)

● Temperature-dependent material properties are defined on MATTi entries.

MATERIAL PROPERTIES - MAT1

● For purposes of this seminar, we will only deal with the MAT1 entry

● This material definition is for Isotropic materials● Minimum properties:

● E - Young’s Modulus - Modulus for extension and bending● G - Modulus for torsion and transverse shear


● G - Modulus for torsion and transverse shear● υ - Poisson’s ratio ● If only 2 of the above 3 are provided, the following equation is used

to calculate the value for the third:

● For thermal stress analysis● A - Thermal expansion coefficient

MATERIAL PROPERTIES - MAT1 (Cont.)


MATERIAL PROPERTIES - MAT1 (Cont.)


PART 2: 1D FINITE ELEMENT ENTITIES

● In this section of the workshop, we will learn about:● Types of 1D elements available in Nastran● Selection of appropriate elements for modeling tasks● The Nastran CBAR element● Bar Offsets● Element coordinate systems


● Element coordinate systems● Definition of 1D element properties● Orientation for Bar and Beam elements● Display of element cross section● Manual input of sectional properties

● Considering load paths in the truss assembly● The truss members must carry axial and lateral loads due to the way

they are loaded. Shear and bending moment will develop in the members as they are loaded laterally at locations between the truss joints as shown below. We must select an element type that is capable of resisting the shear forces and moments.

P

LOAD PATH IN TRUSS


P

M

● Following are the most commonly used one-dimensional elements in NASTRAN:● ROD Pin-ended rod (4 DOFs)● BAR Prismatic beam (12 DOFs)● BEAM Straight beam with warping (14 DOFs)

COMMONLY USED 1-D ELEMENTS


● Guidelines on 1-D element selection:● In general, select the simplest element which gives you the correct

load path. More complex elements will still do the job, but may give you a lot of unwanted output.

● If only an axial load or torsional load is to be transmitted in an element, then the CROD or CONROD element is the best choice.

● If shear and moment are to be transmitted in an element, then the

ELEMENT SELECTION


● If shear and moment are to be transmitted in an element, then the CBAR is the easiest element to use.

● Use the CBEAM element instead of the CBAR element for the following reasons:

● Variable cross-section● The neutral axis and shear center are not coincident● The effect of cross-sectional warping on the torsional stiffness is significant● The mass center of gravity and shear center are not coincident● The effect of taper on the transverse shear stiffness (shear relief) is significant

ELEMENT SELECTION (Cont.)

● For this problem we will use the CBAR element due to its ability to transmit shear force and bending moment.

● The CBEAM element has additional capabilities which we don’t need for this problem. The use of CBEAM will be demonstrated in the next section.


THE CBAR ELEMENT

● General Features of the CBAR Element● Connected to two grid points● Formulation derived from classical beam theory (plane sections

remain plane under deformations)● Includes optional transverse shear flexibility● Neutral axis may be offset from the grid points (internally a rigid link


● Neutral axis may be offset from the grid points (internally a rigid link is created)

● Principal moment of inertia axis need not coincide with element axis.● Pin flag capability used to represent slotted joints, hinges, ball joints,

etc.

THE CBAR ELEMENT (Cont.)

● General limitations on CBAR:● Straight, prismatic member (i.e., properties do not vary along the

length).● Shear center and neutral axis must coincide (therefore, not

recommended for modeling channel or angle sections).● The effect of cross-sectional warping is neglected.

● Displacement Components:


● Displacement Components:● Six degrees of freedom at each end.

● Force components:● Axial force P● Torque T● Bending moments about two perpendicular directions M1 and M2

● Shears in two perpendicular directions V1 and V2

● CBAR element entry:









● CBAR element coordinate system ● Defined by the orientation vector V● Orients input cross-sectional properties● Orients output forces and stresses● Orients pin flags



x x

zz



CBAR Element Coordinate System



CBAR Element Coordinate System with Offsets

● Following are two examples of when you might define the CBAR element coordinate system orientation vector V with each of the two available options (G0 or X1, X2, X3).



If you are representing stringers on a fuselage with CBAR elements, your input will be minimized by using the G0 option to define the element coordinate system orientation vector V.

Example 1



Example 2

To specify the orientation of the legs of a tripod modeled with CBAR elements as shown, it would be most efficient to use the components of a vector (X1, X2, X3) to define the orientation vector V since the orientation of each of the legs is unique.

Note the 1 and 2 directions of this Section.



The "Bar Orientation" for these sections will be as shown.

● CBAR Offsets● The ends of the CBAR element can be offset from the Grid Points

(GA, GB) by specifying the components of offset vectors WA and WB on the CBAR entry.

● The offset vector is treated as a rigid link between the grid point and the end of the element.



● The element coordinate system is defined with respect to the offset ends of the bar element.

Stiffeners

Centroid ofStiffener

Offset

● Bar Offset Example



Thin sheet

Grid Points


● The OFFT field● OFFT is a character string code that describes how the offset and

orientation vector components are to be interpreted.● By default (string input is GGG or blank), the offset vectors are

measured in the displacement coordinate systems at grid points A and B and the orientation vector is measured in the displacement coordinate system of grid point A.


coordinate system of grid point A.● At user option, the offset vectors can be measured in an offset

coordinate system relative to grid points A and B, and the orientation vector can be measured in the basic system as indicated in the following table:


● The OFFT field (Cont.)


● CBAR Pin Flags● The user specifies DOF’s at either end of the bar element that are to

transmit zero force or moment. The pin flags PA and PB are specified in the element coordinate system and defined in fields 2 and 3 of the optional CBAR continuation.



Example: Pin flag applied to rotational DOF at this end of CBAR creates a hinged joint.

● CBAR Element Properties entry:



● CBAR Element Properties entry (Cont.)




● CBAR Element Properties entry (Cont.)







● Shear Factor K




● Alternative CBAR Element Properties entry:



● PBARL cross-section types












● BAR element internal forces and moments



● BAR element internal forces and moments



EXAMINE RESULTS

● Examine the .f06 file for element stresses


Minimum Combined

Maximum Combined

EXAMINE RESULTS (Cont.)

● Examine the .f06 file for element forces


Date post:	12-May-2017
Category:	Documents
Upload:	bookbee
View:	276 times
Download:	12 times

MD Nastran Elements 2

Documents