Dear Nikon,
Let`s try to clarify concepts, I see you have a real mess:
A)
Frequency analysis: well, it is better to say "Normal Modes Frequency Analysis" or "Real Eigenvalue Analsys" if what you want is to extract the natural frequencies and mode shapes of the structure where damping is neglected. If you run NX NASTRAN solver, this is a SOL103 analysis, easy!. These results characterize the basic dynamic behavior of the structure and are an indication of how the structure will respond to dynamic loading.
- The "
natural frequencies" of a structure are the frequencies at which the structure naturally tends to vibrate if it is subjected to a disturbance. For example, the strings of a piano are each tuned to vibrate at a specific frequency.
- The "
deformed shape" of the structure at a specific natural frequency of vibration is termed its normal mode of vibration. Some other terms used to describe the normal mode are mode shape, characteristic shape, and fundamental shape. Each mode shape is associated with a specific natural frequency.
Please note an important item here: Natural frequencies and mode shapes are functions of the structural properties and boundary conditions. And what about loading?. Well, if we preload the string of a guitar, then vibration frequency increasy. This is know as stress stiffnening effect, the reversed is know as "spin softening". Normally we neglect loadings in real eigenvalue analysis, unless the preloading effect is important, causing stiffening or softening. For instance, rotation causes prestress that modifies stiffness, then centrifugal inertia force in a turbine blade produces a "stress-stiffening" effect that increases natural frequencies.
NX NASTRAN allows to account for stress-stiffening effect (i.e., Differenctial Stiffness) in a Real Eigenvalue Analysis. To account for differential stiffness (stress stiffening) in a dynamic analysis, two subcases are required. The first subcase is a static subcase that contains the loads that give rise to stress-stiffening. The second subcase is the dynamic subcase.
B)
Free-Free Dynamic Analysis:
Well, free-free states that none of the points are held fixed during the calculation of the dynamic transformation vectors. It is where a structure or a portion of a structure can displace without developing internal loads or stresses if it is not sufficiently tied to ground (constrained). These stress-free displacements are categorized as "rigid-body modes" or "mechanism modes".
-
Rigid-body modes occur in unconstrained structures, such as satellites and aircraft in flight. For a general, unconstrained 3-D structure without mechanisms, there are six rigid-body modes often described as T1, T2, T3, R1, R2, and R3, or combinations thereof. Rigid-body modes can improperly occur if a structure that should be constrained is not fully constrained (for example, in a building model for which the boundary conditions (SPCs) were forgotten).
- A
mechanis mmode occurs when a portion of the structure can displace as a rigid body, which can occur when there is an internal hinge in the structure. An example of a mechanism is a ball and socket joint or a rudder in an airplane. A mechanism mode can also occur when two parts of a structure are improperly joined. A common modeling error resulting in a mechanism is when a bar is cantilevered from a solid element; the bar has rotational stiffness and the solid has no rotational stiffness, resulting in a pinned connection when the two are joined.
The presence of rigid-body and/or mechanism modes is indicated by zero frequency eigenvalues. Due to computer roundoff, the zero frequency eigenvalues are numerical zeroes on the order of 1.0E-4 Hz or less for typical structures. The same unconstrained model may give different values of the rigid-body frequencies when run on different computer types.
Best regards,
Blas.
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Blas Molero Hidalgo
Ingeniero Industrial
Director
IBERISA
48011 BILBAO (SPAIN)
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