Structural dynamic analysis
Experimental modal analysis, operating deflection shape analysis and finite element modal analysis
The vibrations detected on a machine or structure are caused by internal or external force loads exciting a dynamic response in the structure characterised by mass, stiffness and damping parameters. According to the value of these parameters, the structure will have natural modes which, if excited, will generate deformations with more amplitudes.
This phenomenon is the cause of most detected fatigue degradations, but also makes it possible to apply decoupling solutions or dynamic absorption of vibrations.
Different types of analyses are used to understand the vibration behaviour of a machine or structure following dynamic excitations. Among them are experimental modal analysis, finite element modal analysis and operating deflection shape analysis.
The Goal of modal analysis is to determine the vibration matrices of the parameters characterising the natural modes: resonance frequencies with their associated deformed structures as well as the mass and stiffness of natural modes. It involves using incomplete modal data (natural frequencies, natural deformations) using curve fitting methods exploiting the transfer functions acquired based on an appropriate mathematical model taking into account the type of excitation, mode coupling and size of the structure.
Experimental modal analysis is based on a technique that involves the acquisition of transfer functions (cross spectrum) between loads generated by measured and controlled external excitation and the vibration response of the structure in all 3 directions on the grid points representative of the structure under analysis. The excitation methods are generally impact hammer or impact mass, or shaker using force sensors. This experimental approach is used to characterise an existing structure and also provides information on resetting for a mathematical model.
The aim of finite element modal analysis is to construct a mathematical model of a structure or part by discretization from plans or a 3D model. We then define the materials, calculated masses and most importantly the boundary conditions defined theoretically or from the results of measurements. The solver then constructs the modal basis of the structure representing a multi-degree of freedom system. This mathematical approach can be used in the design phase to determine the dynamic behaviour of a structure or curatively to size changes on an existing structure. In this last case, it is possible to adjust the parameter values of the model in relation to the results of the experimental modal analysis.
Modal analysis therefore makes is possible to characterise the dynamic behaviour of a structure and it has numerous applications:
- Validation of a structure or part design in relation to the excitation frequencies defines during the design phase or for a prototype or seed.
- Definition and sizing of modifications the structural resonance frequency or frequencies in relation to the excitation frequencies for repeated degradations or excessive vibration values.
- Definition and validation of a decoupling solution.
- Evaluation of the vibration response of a structure or part by force injection or structural connection.
- Validation of maintenance work affecting the stiffness of the installation (attaching a bearing, end winding baskets, etc.).
- Studies on the noise radiation of a machine or part.
- Characterisation of structural ageing.
Operating deflection shape analysis
The aim of operating deflection shape analysis is to identify the spatial deformation of a structure subject to natural excitation frequencies. This technique involves measuring the frequency response function (or transfer function) between the vibration responses measured in 3 directions based on a grid representative of the structure and a fixed reference point. It is generally implemented when it is not possible to stop installation to conduct a modal analysis or when a large structure and/or with a big mass cannot be excited by conventional excitation methods (hammer or mass).
Deflection shape analysis of a structure under excitation makes it possible to:
- Spatially view the movements of the installation in 3 directions even for low excitation frequencies.
- Identify the most vibrating points and inflexion points where there are the most stresses.
- Verify by comparison the effectiveness provided by a structural modification or change to the operating conditions of the installation.
- Separate a forced structural response from a natural mode response if we have the results of the modal analysis: action on the source of excitation in the first case, action on the structure in the second.
For each type of analysis, a specific methodological approach suitable for each case needs to be defined. The equipment used to carry out these analyses are sold by dB Vib Instrumentation..