1Course in ANSYS

Day 4Lesson 8. Structural analysis

Lesson 8 2

Structural analysisDay 1Lesson 1. Introduction to ANSYSLesson 2. BasicsLesson 3. Solution phasesDay 2Lesson 4. ModelingDay 3Lesson 5. MaterialLesson 6. LoadingLesson 7. SolutionDay 4Lesson 8. Structural analysisLesson 8. Structural analysisLesson 9. PostprocessingLesson 10. Constraint equationsLesson 11. ParametersLesson 12. MacrosDay 5Lesson 13. Vibration/dynamic analysisLesson 14. Thermal

Outline for Course in ANSYS:

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Structural analysis Static analysis

Used to determine displacements, stresses, etc. under static loading conditions. Both linear and nonlinear static analyses. Nonlinearities can include plasticity, stress stiffening, large deflection, large strain, hyperelasticity, contact surfaces, and creep.

Modal analysis Used to calculate the natural frequencies and mode shapes of a structure. Different mode extraction

methods are available. Harmonic analysis

Used to determine the response of a structure to harmonically time-varying loads. Transient dynamic analysis

Used to determine the response of a structure to arbitrarily time-varying loads. All nonlinearities mentioned under Static Analysis above are allowed.

Spectrum analysis An extension of the modal analysis, used to calculate stresses and strains due to a response spectrum or a

PSD input (random vibrations). Buckling analysis

Used to calculate the buckling loads and determine the buckling mode shape. Both linear (eigenvalue) buckling and nonlinear buckling analyses are possible.

Nonlinear analysis Contact problems Fracture mechanics Composites Fatigue

Programme for Lesson:

Lesson 8 4

Structural analysis

Structural analysis is probably the most common application of the finite element method. The term structural (or structure) implies not only civil engineering structures such as bridges and buildings, but also naval, aeronautical, and mechanical structures such as ship hulls, aircraft bodies, and machine housings, as well as mechanical components such as pistons, machine parts, and tools.

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Static analysis

A static analysis calculates the effects of steadyloading conditions on a structure, while ignoring inertia and damping effects, such as those caused by time-varying loads. A static analysis can, however, include steady inertia loads (such as gravity and rotational velocity), and time-varying loads that can be approximated as static equivalent loads (such as the static equivalent wind and seismic loads commonly defined in many building codes).

Lesson 8 6

Static analysis Static analysis is used to determine the displacements, stresses,

strains, and forces in structures or components caused by loads that do not induce significant inertia and damping effects. Steady loading and response conditions are assumed; that is, the loads and the structure's response are assumed to vary slowly with respect to time. The kinds of loading that can be applied in a static analysis include:

Externally applied forces and pressures Steady-state inertial forces (such as gravity or rotational velocity) Imposed (nonzero) displacements Temperatures (for thermal strain) Fluences (for nuclear swelling)

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Modal analysis

You use modal analysis to determine the vibration characteristics (natural frequencies and mode shapes) of a structure or a machine component while it is being designed. It also can be a starting point for another, more detailed, dynamic analysis, such as a transient dynamic analysis, a harmonic response analysis, or a spectrum analysis.

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Harmonic analysis

Any sustained cyclic load will produce a sustained cyclic response (a harmonic response) in a structural system. Harmonic response analysis gives you the ability to predict the sustained dynamic behavior of your structures, thus enabling you to verify whether or not your designs will successfully overcome resonance, fatigue, and other harmful effects of forced vibrations.

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Transient dynamic analysis

Transient dynamic analysis (sometimes called time-history analysis) is a technique used to determine the dynamic response of a structure under the action of any general time-dependent loads. You can use this type of analysis to determine the time-varying displacements, strains, stresses, and forces in a structure as it responds to any combination of static, transient, and harmonic loads. The time scale of the loading is such that the inertia or damping effects are considered to be important. If the inertia and damping effects are not important, you might be able to use a static analysis instead.

Lesson 8 10

Spectrum analysis

A spectrum analysis is one in which the results of a modal analysis are used with a known spectrum to calculate displacements and stresses in the model. It is mainly used in place of a time-history analysis to determine the response of structures to random or time-dependent loading conditions such as earthquakes, wind loads, ocean wave loads, jet engine thrust, rocket motor vibrations, and so on.

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Buckling analysis

Buckling analysis is a technique used to determine buckling loads - critical loads at which a structure becomes unstable - and buckled mode shapes - the characteristic shape associated with a structure's buckled response.

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Nonlinear analysis If a structure experiences large deformations, its

changing geometric configuration can cause the structure to respond nonlinearly. An example would be the fishing rod shown in Figure 8.2: "A Fishing Rod Demonstrates Geometric Nonlinearity". Geometric nonlinearity is characterized by "large" displacements and/or rotations.

Nonlinear stress-strain relationships are a common cause of nonlinear structural behavior. Many factors can influence a material's stress-strain properties, including load history (as in elastoplastic response), environmental conditions (such as temperature), and the amount of time that a load is applied (as in creep response).

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Contact problems Contact problems are highly nonlinear and require significant

computer resources to solve. It is important that you understand the physics of the problem and take the time to set up your model to run as efficiently as possible.

Contact problems present two significant difficulties. First, you generally do not know the regions of contact until you've run the problem. Depending on the loads, material, boundary conditions, and other factors, surfaces can come into and go out of contact with each other in a largely unpredictable and abrupt manner. Second,most contact problems need to account for friction. There are several friction laws and models to choose from, and all are nonlinear. Frictional response can be chaotic, making solution convergence difficult.

In addition to these two difficulties, many contact problems must also address multi-field effects, such as the conductance of heat and electrical currents in the areas of contact.

Lesson 8 14

Fracture mechanics Cracks and flaws occur in many structures and components,

sometimes leading to disastrous results. The engineering field of fracture mechanics was established to develop a basic understanding of such crack propagation problems.

Fracture mechanics deals with the study of how a crack or flaw in a structure propagates under applied loads. It involves correlating analytical predictions of crack propagation and failure with experimental results. The analytical predictions are made by calculating fracture parameters such as stress intensity factors in the crack region, which you can use to estimate crack growth rate. Typically, the crack length increases with each application of some cyclic load, such as cabin pressurization-depressurization in an airplane. Further, environmental conditions such as temperature or extensive exposure to irradiation can affect the fracture propensity of a given material.

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Composites Composite materials have been used in structures for a long time. In

recent times composite parts have been used extensively in aircraft structures, automobiles, sporting goods, and many consumer products.

Composite materials are those containing more than one bonded material, each with different structural properties. The main advantage of composite materials is the potential for a high ratio of stiffness to weight. Composites used for typical engineering applications are advanced fiber or laminated composites, such asfiberglass, glass epoxy, graphite epoxy, and boron epoxy.

ANSYS allows you to model composite materials with specialized elements called layered elements. Once you build your model using these elements, you can do any structural analysis (including nonlinearities such as large deflection and stress stiffening).

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Fatigue Fatigue is the phenomenon in which a repetitively loaded

structure fractures at a load level less than its ultimate static strength. For instance, a steel bar might successfully resist a single static application of a 300 kNtensile load, but might fail after 1,000,000 repetitions of a 200 kN load.

The main factors that contribute to fatigue failures include: Number of load cycles experienced Range of stress experienced in each load cycle Mean stress experienced in each load cycle Presence of local stress concentrations A formal fatigue evaluation accounts for each of these factors as

it calculates how "used up" a certain component will become during its anticipated life cycle.