June 2024

News for the SDTools community
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[Conference] Hybrid FEM/test twin building, an electric engine case history

SDTools was present at the CSMA 2024 conference https://csma2024.sciencesconf.org/

The talk has addressed challenges associated to the generation of a Hybrid FEM/test twin model of an automotive electric car engine in partnership with Stellantis:

  • Describing test outputs
  • Choosing model parameters
  • Building a reduced parametric model
  • Building a Hybrid FEM/test twin model

Read post… 📖 Read conference paper…

[Release] SDT 7.5 has been released

Last year’s orientations focused on

  • GUI implementation and customization capabilities to answer customer requests on making numerical and test processes accessible through GUI.
  • Parametric superelement/reduced model handling, that is a key capability of SDT and is under continuous development. It enables industrial scalability (external software interaction, post-treatments, data volume management…).
  • Test handling for experimental vibration applications progress. Now integrates parametric tests and improvements for Siemens TestLab compatibility.
  • Custom solvers various optimization.
  • Piezo capabilities for active control and SHM applications.

Read post… Read release notes…

[Expertise] Solutions for flange contact modelling in structural dynamics

Flange support around fasteners is a commonly overlooked topic in industrial structural dynamics applications. It is due to implementation complexity in already large models, and the difficulty of finding relevant values for input parameters. It is however a very sensitive aspect that can prevent from validating a model when ignored.

Check out our take on the subject for vibration applications, and how a zero thickness (ZT) element based implementation can be ported to other codes for simulation process integration

Read post…

[Project] Identification of material property dependency to temperature (Source project)

🤝 In partnership with the “SOURCE” ANR project, SDT has been used by Nassim Benbara, Marc Rébillat and Nazih Mechbal from the DISCoH team in Laboratoire PIMM to identify a polypropylene plate material property temperature dependency, with some surprises on the Poisson coefficient evolution.

A summary of this work is the object of a news post in our website
Read post…

More detailed explanations (among other results) have been published in the Journal of Sound and Vibration
Connect on ReasearchGate Read paper…
and the “SOURCE” ANR project presentation page is available at
📚 Read project presentation…

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Identification of material property temperature dependency

In partnership with the “SOURCE” ANR project, SDT has been used by the DISCoH researcher’s team from PIMM laboratory (Nassim Benbara, Marc Rebillat and Nazih Mechbal) to identify a polypropylene plate material property temperature dependency. A summary of this work is provided below and more detailed explanations (among other results) have been published in [1].

The plate is fitted with two piezoelectric patches, one used as an actuator, the other as a sensor. A sweep signal (100 – 1500 Hz) is sent to the actuator and the transfer function between this signal and the voltage measured by the sensor is built.

To assess the influence of temperature, the plate is placed in an oven equipped with a temperature sensor. It is drilled at two corners and suspended in the oven using two nylon wires, which effectively decouple the vibratory behavior of the plate itself from its environment (grid, vibration of the oven’s internal walls, etc.).

The measurement protocol is defined as follows:

  • A temperature hold is applied to homogenize the plate temperature.
  • A temperature ramp from 0 to 60°C is defined, slow enough to avoid significant temperature gradient across the plate (approx. 1°C/min).
  • An automatic measurement is triggered every two minutes to obtain a series of transfers at different temperatures.

To assess temperature influence, the strategy is to identify the system’s modes (frequency and damping) and analyze their evolution with temperature. A dedicated strategy has been implemented in the SDT toolbox to avoid manually identifying all the transfers, and to enable automated post-processing of the results:

  • Initialize modes on transfers corresponding to the first temperature
  • Automatic, sequential optimization of poles from one temperature to another
  • Construction of a database containing the evolution of all frequencies and modal damping associated with each measurement temperature.
  • Generate curves to analyze results.

Analysis of the evolution curves shows that the modes are highly sensitive to temperature, with maximum damping of around 4% around 20°C. Modal frequencies evolve by +/- 25% compared to the reference temperature of 25.4°C.

The evolution with temperature of each mode is very similar, which was expected given that the material can be considered isotropic: the evolution of the Young’s modulus E(T) impacts all modes equally. The small differences can be explained by

  • a possible degree of anisotropy
  • a temperature gradient in the plate
  • identification bias

In addition to analyzing the influence of temperature on modes, the second objective was to identify the relation between the Young’s modulus and temperature. To achieve this, the system has been modelled in SDT, including the piezoelectric patches whose geometry and properties are available in SDT libraries.

Classical model updating would have tried to minimize the difference between the model and test mode frequencies. The PIMM’s researchers preferred to work on the comparison of transfers directly. The advantage of this strategy is that modal identification is no longer necessary, provided that the test is of good quality, and that the modes haven been first identified. The simulated transfer is obtained in SDT by entering the actuator voltage as input and the receiver voltage as output. The aim is then to use an optimization loop to find the material parameters that minimize the difference between synthesized and measured transfers, for all temperatures.

To limit the number of unknown parameters in this work, the loss factor was fixed to twice the average identified damping at each temperature. First updating attempts showed that it was difficult to obtain a good superposition by only varying the Young’s modulus. Poisson’s ratio was thus considered as an additional parameter. This greatly improved trtansfers superposition and led to the identification of the following material dependency to temperature:

The study’s strategy implemented in SDT to identify model parameters is finally summarized in the figure below.

To be completely independent from modal identification (and perhaps to further improve the quality of recalibration), material loss factor should also be a model parameter, in the same way as Young’s modulus and Poisson’s ratio. This was kept as an improvement perspective of this work.

It is worth mentioning that the influence of Poisson’s ratio on model updating has been little studied and that the variation, at least for this specific test case, is not negligible.


References

[1] Bending waves focusing in arbitrary shaped plate-like structures: Study of temperature effects, development of a digital twin and of an associated neural-network based compensation procedure. 
N. Bernbara, G. Martin, M. Rébillat, N. Mechbal. Journal of Sound and Vibration,Volume 526, 2022.

Solutions for flange contact modelling in structural dynamics

Flange support around fasteners is a commonly overlooked topic in industrial structural dynamics applications. It is due to implementation complexity and the difficulty of finding relevant values for input parameters. It is however a very sensitive aspect that can prevent from validating a model when ignored.

We frequently need to add these features treating our clients’ case studies.

Implementation of flange support does not need to be complicated. Zero thickness elements are a practical way of modelling surface bounding features. They are implemented in SDT as flat bricks (hexa) or wedges (penta) elements and use 2D anisotropic shell constitutive properties.

This topology is a good basis to model glue or debonding, and to implement practical contact effects in dynamics.

As the pre-loaded contact conditions vary, the relevant constitutive parameters are generally unknown from design. Robust design and digital twin building must therefore assess their variability and provide critical configurations. In other approaches, model updating from test correlation will also be likely to highlight operational values for the model.

Having a distributed contact stiffness density is thus meaningful in many vibrations use cases:

👉 When flanges into contact have stiffness differences of several orders so that asperities compression and associated mean surface interpenetration must be accounted for. This is the case when liners or seals are present, or with coarse roughness.

👉 When the contacting surface is not clearly defined so that the apparent stiffness accounts for unloaded areas. That is the case for fasteners, and is thus critical for assembled housing behavior and for vibration transmission to various equipment (PCB, battery packs…)

👉 When the dynamic response induces local contact surface changes so that an equivalent stiffness can be defined for a linear equivalent model.

Varying equivalent elastic contact properties is thus essential for model validation [4]. The example below based on [2] illustrates how the natural frequencies of an engine cradle vary when considering a support stiffness on potential contacting areas around weld spots. The variation is over 10 % and must be considered as a first order parameter.

The challenge resides in the need for the defined contact parameters to remain easily accessible and portable to other codes. Solutions based on contact, surface constraint definition or direct matrix coupling have many drawbacks. When letting the code interpret contact or surface constraints, matching is required, and each code use frequently undocumented smoothing/weighting tweaks for convergence. In many cases, implementation variation in complex models significantly alters the expected result after export, and elastic behavior is seldom available. Exporting constraint matrices is impractical, generates heavy files, and is mesh dependent, making it an undesirable solution despite providing equivalent results.

Zero thickness elements match the requirements. They use classical topologies making it easy to recover stiffness matrix contributions and to parameterize.

Exports to other codes can be based on cohesive elements. These elements are widely available and suitable to our needs. We successfully implemented it for ABAQUS using COH3D8 and COH3D6 elements with COHESIVE SECTION and TRACTION type of ELASTIC. INTER-elements in ANSYS, CIFHEX and CIFPENT for Nastran, CHEXCZ and CPENTCZ in Simcenter Nastran should provide equivalent solutions for the need. It corresponds to a bilinear formulation with no (or very high) debonding thresholds, shear effects are available for tangential adherence.

SDT-Contact provides conversion functionalities to generate zero thickness implementations from contact formulations or constraint-based surface coupling. As we commonly import models from external codes or from existing industrial processes, these features are critical to provide an effortless integration.

The limitation of zero thickness elements resides in the need for a compatible mesh between surfaces. Such feature can be obtained in meshing software, or by splitting preliminary meshed solids (as our Unjoin functionality https://www.sdtools.com/help/feutil.html#UnJoin). When the mesh is not compatible, it is always possible to use the master surface for the zero thickness elements support and use Multiple Point Constraints (MPC) between the zero thickness and the slave surface. For target use cases, locking risks and implementation variations induced by the MPC will be mitigated by the fact that a softer material is present.


References

[1] Understanding friction induced damping in bolted assemblies through explicit transient simulation.
G. Vermot des Roches, E. Balmes, ISMA 2014. 

[2] Error localization and updating of junction properties for an engine cradle model.
G. Vermot des Roches, E. Balmes, S. Nacivet. ISMA 2016.

[3] Updating and design sensitivity processes applied to drum brake squeal analysis.
Martin, E. Balmes, G. Vermot des Roches, T. Chancelier. Eurobrake 2016.

[4] Numerical design and test on an assembled structure of a bolted joint with viscoelastic damping.
C. Hammami, E. Balmes, M. Guskov. MSSP 2015.



SDT 7.5 posted

For download, demos, … see here

SDT 7.5 is compatible with MATLAB 9.4 (2018b) to 24.1 (2024a). In the base and FEMLink modules, key changes of this release are

  • To answer customer requests on making numerical and test processes accessible through GUI, revision and extension work has continued. In particular
    • for the tabs Ident identification of modal tests, MAC shape correlation, MDRE expansion, Report automated report generation.
    • This is enabled by the continued rewrite of the SDT handle and sdtm allowed implementation of generic utilities transversely used in SDT. urn resolutions for object search urnObj. Callback and string formatting urn_fmt are now more explicitly documented. User readability of parameters is better supported using vhandle.uo objects. Integration of graphical tables is now uniformly supported by vhandle.tab.
    • code was made compatible with the deployment of GUI using the MATLAB Compiler and Runtime license.
  • Parametric superelement/reduced model handling continues to be a key capability that differentiates SDT from other software. The latest developments are
    • uniformisation efforts to render SDT a FEM code neutral import form the major software (NASTRAN, ANSYS, Abaqus, …). This has driven recent FEMLink developments of nasread, ans2sdt, abaqus, and documentation efforts SeImport.
    • to answer the need to efficiently animate models in tens of million of DOF and deal with confidentiality issues, new level-set based model selections for display as plane cuts combinations for feplot, see selcut.
    • GUI interactions between state-space building fe2ss  frequency response computations qbode, and state-animation have been notably extended.
    • ability to use superelements in test-analysis correlation processes
    • parametric superelements deal with reduced models with stiffness, complex modulus, mass, thickness, … aggregated in element groups zCoef or distributed in non-linearities NLdata.
    • Integration of parameter inputs in abaqus for SDT compatible types.
    • for large FEM, DOE, JobH applications, out-of-core functionalities have been thoroughly revised and extended. Revision of sdthdfmethods for HDF5 support with optimized read/write capabilities and improved robustness. Implementation of omat object for out-of-core numerical data handling, supporting several underlying file strategies (extended v6, low level HDF5 or high level MATLAB builtin commands HDF5).
    • job generation and monitoring for abaqus, combined with module SDT/JobH
    • cases reduced using nominal periodicity and extendend to variable properties  [2, 3, 4, 5]
  • In relation to the experimental vibration applications
    • ufread and fe_sens support better channel label translation
    • on the experimental modal analysis side, the handling of parametric tests (dependence on temperature, loading, amplitude, …)
  • To support of the development of efficient custom solvers
    • low level optimization associated with time integration continued with performance optimization in vhandle.matrix, viscoelastic transient models, …
    • Contact support for ans2sdt and abaqus, combined with SDT/Contact, or translated as MPC.
    • Moving contact of surfaces was notably extended for the simulation of rail-wheel contact problems.
  • Piezo capabilities for active control and SHM applications, were continued and the associated documentation (https://www.sdtools.com/help/piezo.pdf) will be updated soon.

CSMA 2024 : Hybrid FEM/test twin building, an electric engine case history

SDTools will be present at the CSMA 2024 conference https://csma2024.sciencesconf.org/

The talk [1] to be given will address challenges associated to the generation of a Hybrid FEM/test twin model of an automotive electric car engine in partnership with Stellantis:

  • Describing test outputs
    The electric engine case study being detailed combines strong dominance of harmonic responses and un-measured inputs. The harmonic balance vector signal model chosen gives a space/time/frequency approximation of the response.
  • Choosing model parameters
    Geometry, contacts in bolted joints and laminated stacks, non-linear viscoelastic bushings have here a notable impact.
  • Building a reduced parametric model
    This provides a 2 to 3 orders of magnitude speedup that is necessary for any practical application.
  • Building a Hybrid FEM/test twin model
    Test and FEM are combined using an expansion-based state/parameter estimation process.

[1] https://csma2024.sciencesconf.org/499609/document

SDT 7.3 posted

The major revision 7.2 was only made available as a beta version, with the stable version skipped due COVID related constraints.

SDT 7.3 is the only version fully compatible with MATLAB 9.4 (2018b) to 9.8 (2020a) mostly due to changes in the representation of complex numbers in MATLAB. Key changes of this release are

  • a major rewrite of the SDT handle object with major impact on GUI performance and compatibility with expect future changes in MATLAB GUI. sdth.urn provides a general mechanism for Uniform Resource Names which can now be used to designate and select many GUI or model components.
  • the introduction of the +vhandle package of classes to upgrade earlier functionality implemented in v_handle. In particular, vhandle.matrix provides a user readable access point to C libraries linked into the mkl_utils mex file corresponding to matrix like operators, but with possible parallel or initialization/repeated call (called inspector/executor by INTEL) optimization. This has provided speedup factors between 2 and 100 for a range of high performance time computations using both implicit or explicit time schemes. The SDT/nlsim token may be required for calls to the nl_solve diagNewmark solver (non-linear time domain transients in modal coordinates) and to the nl_solve expNewmark (explicit time solver). The vhandle.chandle object is used to provide init/call separation for non matrix like objects.
  • on the experimental modal analysis side, the handling of parametric tests (dependence on temperature, loading, amplitude, …)

For femlink the main changes were related to GUI and superelement import. For MATLAB compatibility see section 1.10.2.

SDT 7.4 posted

SDT 7.4 is the only version fully compatible with MATLAB 9.4 (2018b) to 9.12 (2022a) mostly due to changes in the representation of complex numbers in MATLAB. Key changes of this release are

  • the introduction of the +vhandle package of classes to upgrade earlier functionality implemented in v_handle. In particular, vhandle.matrix provides a user readable access point to C libraries linked into the mkl_utils mex file corresponding to matrix like operators, but with possible parallel or initialization/repeated call (called inspector/executor by INTEL) optimization. This has provided speedup factors between 2 and 100 for a range of high performance time computations using both implicit or explicit time schemes. The SDT/nlsim token may be required for calls to the nl_solve diagNewmark solver (non-linear time domain transients in modal coordinates) and to the nl_solve expNewmark (explicit time solver). The vhandle.chandle object is used to provide init/call separation for non matrix like objects.
  • on the experimental modal analysis side, the handling of parametric tests (dependence on temperature, loading, amplitude, …)
  • full rewrite of fluid structure interaction implementation in fsc. Added Transmission Loss and Rayleigh integral computation capabilities.

Tutorial at SURVISHNO

Error characterization in modal analysis and model updating. An overview of tools and procedures using the Structural Dynamics Toolbox.

will be presented by Etienne Balmès at the SURVISHNO conference in Lyon on July 8, 2019. This tutorial will mix theoretical considerations and applications using SDT (animations https://www.youtube.com/channel/UCy5PHLw70NqfDDvg24tXJWg , presentation www.sdtools.com/pdf/Vishno19_Tutorial.pdf )

Experimental modal analysis seeks to extract shape and resonance properties from test data. While identification algorithms have been well documented for a long time, at least under the assumption linear behavior, significant differences continue to exist in the implementation details. In particular, non-linear optimization of poles and work by sub-bands is an often overlooked necessity to avoid bias. Once modal data available, the next step is to obtain test/analysis correlation. This requires topology correlation which brings its share of errors, which despite being usually small should be addressed. Global correlation criteria, such as the MAC, are introduced next but for efficient use should be complemented by a set of procedures to localize measurement, topology and correlation errors. While the simple evaluation of correlation is often an industrial objective, the best exploitation of test results is achieved using hybrid approaches combining test with a model, which does not need to be exact to usefully complement the measurements. In particular, expansion methods estimating the full Finite Element responses from data measured at sensors are particularly useful. Energy based criteria on the model side (Minimum Dynamic Residual Expansion or the various variants of Error in Constitutive Relation) have been long known to provide excellent solutions but deployment has been scarce due to an important numerical cost. Simple model reduction strategies are shown to give excellent results for industrial models and open the way for model error localization and updating.

The tutorial is illustrated using standard procedures implemented in the Structural Dynamics Toolbox which provides experimental modal analysis, finite element modeling, model reduction and correlation tools in the MATLAB environment. A main brake squeal application serves a red line and is complemented by illustrations from other industries.

SDT 7.1 released

SDT 7.1 is the only version fully compatible with MATLAB 9.4 (2018b) to 9.6 (2019a) mostly due to changes in the representation of complex numbers in MATLAB. Key changes of this release are

    • A continued effort in making the experimental modal analysis part of SDT section 2.2 fully accessible without any script is nearly complete. Functions however obviously remain accessible from the command line to users will to learn how to use them. The associated docks Id (for experimental modal analysis see section 2.2), CoTopo (topology correlation see section 3.1) and CoShape (test/FEM correlation see section 3.2) have been extended and tutorials have been introduced.
    • A major effort was put on the documentation. The new structuration of demos into tutorials helps training. You can for example see tutorials in various files with d_mesh(‘tuto’), gartid(‘tuto’), d_cor(‘tuto’), d_cms(‘tuto’), …. Equations are now shown as SVG files which improves readability, but may pose problems on some older versions of MATLAB where the help browser does not support SVG.
    • We are still working with the MathWorks on improving reliability of the help browser. To bypass some bugs, you may have to change default location where the help is shown using sdtdef(‘browser-SetPref’,‘-helpbrowser’) or sdtdef(‘browser-SetPref’,‘-webbrowser’). For clickable areas of SVG figures, use Ctrl-Click to open in a new window or right-click and select Open in a new tab.

Outside improved robustness of the GUI, key changes for FEMLink are

    • ans2sdt extended BDF reading in particular for orthotropic materials and substructure export (to ease superelement import). Job submission integration is now supported as a consulting project feature.
    • nasread compatibility with NX Nastran BGSET and BSURFS cards. Documentation of superelement (see d_cms(‘TutoNasCb’)). Performance of MAT9 and set reading.
    •  abaqus significant .inp reading improvements *distribution,*hyperelastic, set handling, … Performance of large .fil reading. Robustness and performance enhancements of resolve commands. Introduction of a .dat reading framework for customer use, with complex modes output reading support.

You can download SDT 7.1 from www.sdtools.com/faq/Release.html