Solidworks Response Spectrum Analysis
Boyan Atanaschev
Prerequisites: Must have attended the basic SOLIDWORKS Simulation class, or must have an experience with SOLIDWORKS + working basic knowledge of finite elements and of basic mechanical principles. The knowledge of basic principles in Vibrations is strongly recommended, but not required.
Who should attend: Designed for users who would like to become productive fast, the advanced course offers hands-on experience on the use of SOLIDWORKS Simulation Dynamics modules. The one-day course provides an overview on a wide range of dynamic analysis topics.
Description: This course is targeted for the users who would like to become productive in analyzing structures subjected to various types of dynamic loading. The material covered includes the time dependent analysis(force loads as well as motion shock loading examples), harmonic analysis and random vibration analysis (MILS-STD-810F example is included), response spectrum analysis, and introduction to nonlinear dynamics simulation.
With linear dynamic analysis, the loads are applied with respect to time or frequency, they can be defined in a deterministic nature (periodic or non-periodic) or non-deterministic, which means they are not precise but defined statistically. Unlike linear static analysis, where only the stiffness, displacement and load are important, with a full linear dynamic system now the damping and mass matrices are included in the equations of motion.
Example: A reciprocating pump, weighing 150lb, is mounted at the middle of a steel plate of thickness 0.5 in., width 20in., and length 100 in., clamped along two edges. During operation of the pump, the plate is subjected to a harmonic force,
Finally, we setup and run our harmonic analysis. The harmonic force definition gives us the excitation frequency as 62.832 rad/sec, which correlates to 10 Hz. With linear dynamic studies we are required to run a modal analysis first, and the modes and the mass participation of those modes drive the accuracy of the linear dynamic analysis. For a case like where we are driving the model in the y-direction you want enough modes to get 90% of the mass in the driving axis. You can check this when you run your modal analysis by looking at the mass participation table.
The final input is just the force. With harmonic analysis any loads you apply are assumed to be applied harmonically. In this case we have a 50 lbf load applied on mass. The amplitude of displacement at 10 Hz is then 0.15 in:
There are three other types of linear dynamic studies which will be covered in future blogs; linear transient, random vibration and shock response spectrum. The procedures to effectively run those are similar to harmonic, but they all have their little intricacies. If you have any questions reach out to the technical resources at your resellers
It is important to design products that are strong yet light in weight while resisting damage against impact or unanticipated shocks and vibration. The static analysis assumes that loads are constant or applied very slowly, ignoring the effects of inertial and damping forces. For many practical cases, loads are not applied slowly. In fact, they change with time or frequency. To simulate such conditions, a dynamic analysis is required.
SOLIDWORKS Simulation Premium offers linear dynamics and nonlinear dynamics capabilities. These types of analyses can help you design structural or mechanical systems to perform without failure in dynamic environments. Linear dynamics studies can be used to analyze stress on moving components, earthquake simulations, time-varying loads such as the high-speed impact on landing gear and more.
Typical loads include shock (or pulse) loads, time-varying loads (periodic or non-periodic), and uniform base motion. You have the option to define support motions (displacement, velocity or acceleration applied to selected supports uniformly or non-uniformly). Initial conditions, such as a finite displacement, velocity or acceleration can be applied to a part or the whole model at time t =0. Modal, Rayleigh, composite modal and concentrated dampers are available for modal time history analysis.
After running the study, you can view displacements, velocities, acceleration or stress, strain, reaction forces, etc. at different time steps. Using the response plot, you can also graph results at specified locations versus time. The image below shows the acceleration response of certain components housed inside an electronics enclosure and subjected to shock test in accordance with the MIL-STD-810G, Method 516.5 criteria.
Response spectrum analysis can be used instead of a time history analysis to estimate the response of structures to random or time-dependent loading environments such as earthquakes, wind loads, ocean wave loads, jet engine thrust or rocket motor vibrations. This can be used if you need only the peak response of the structure and not the entire time history solution. Using response spectrum analysis can significantly reduce the solution time compared to a full modal time history type of analysis.
Input to a response spectrum analysis is the response spectrum, which is defined as the peak response of a single degree of freedom oscillator plotted versus its natural frequency. The results of the modal analysis are used in terms of a known spectrum to calculate displacements and stresses in the model. For each mode, a response is read from a design spectrum based on the modal frequency and a given damping ratio. All modal responses are then combined to provide an estimate of the total response of the structure.
A harmonic analysis can be used to calculate the peak steady state response due to harmonic loads or base excitation. Applications include determining stresses or displacement of components such as rotating machinery, subjected to oscillatory loading. Although you can create a modal time history study and define loads as functions of time, you may not be interested in the transient variation of the response with time. In such cases, you save time and resources by solving for the steady state peak response at the desired operational frequency range using harmonic analysis.
After running the study, you can view peak amplitudes of response parameters (stresses, displacements, accelerations and velocities), as well as response graphs of phase angles of response parameters over the range of operating frequencies.
The random vibration analysis is used to calculate responses due to nondeterministic loads such as the loads generated on the suspension system of an automobile while moving on a rough road, base accelerations generated by earthquakes or pressure generated on an aircraft by air turbulence, etc.
This study requires the input loads to be described statistically by power spectral density (PSD) functions. For example, units of a PSD curve for acceleration input is g/Hz. The solution of random vibration problems is formulated in the frequency domain.
After running the study, you can plot root-mean-square (RMS) values, or PSD results of stresses, displacements, velocities, etc. at a specific frequency or graph results at specific locations versus frequency values. This allows you to get a better insight about the frequency content of the output.The drop test in SOLIDWORKS Simulation Professional can be used to find out the stresses generated on a body dropped from a certain height or moving at a certain speed. However, if you need to evaluate the damage on an object impacted by another moving object, such as a projectile hitting a protective barrier, a nonlinear dynamics study is the way to go.
Nonlinear dynamics studies can handle large displacement with various advanced material models including hyper-elasticity, von Mises plasticity, etc. This type of analysis will require substantial computational resources. You can define initial conditions for velocity, displacement, acceleration or uniform base excitation or pressure loads.
Learn how to analyze structures subjected to various types of dynamic loading. Learn time-dependent analysis (force loads as well as motion shock loading examples), harmonic analysis and random vibration analysis (MILS-STD-810F example is included), response spectrum analysis, and get an introduction to nonlinear dynamics simulation. Includes an eBook.
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Course Description: The course includes the time dependent analysis (force loads as well as motion shock loading examples), harmonic analysis and random vibration analysis (MILS-STD-810F example is included), response spectrum analysis, and introduction to nonlinear dynamics simulation.
Prerequisites: Must have attended the basic SolidWorks Simulation class, or must have an experience with SolidWorks + working basic knowledge of finite elements and of basic mechanical principles. The knowledge of basic principles in Vibrations is strongly recommended, but not required.
There are some important things one should take care of when starting to work with linear dynamics, e.g. to include enough eigenfrequencies to have a representative model or to define a proper damping:
There are cases where the load is too complex to be captured with a regular transient study. Therefore, the loading is transformed into a so-called PSD-curve (Power Spectral Density) which serves as input for a random vibration analysis.
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