George Lampeas

On the validation of solid mechanics models using optical measurements and data decomposition

Engineering simulation has a significant role in the process of design and analysis of most engineered products at all scales and is used to provide elegant, light-weight, optimized designs. A major step in achieving high confidence in computational models with good predictive capabilities is model validation. It is normal practice to validate simulation models by comparing their numerical results to experimental data. However, current validation practices tend to focus on identifying hot-spots in the data and checking that the experimental and modeling results have a satisfactory agreement in these critical zones. Often the comparison is restricted to a single or a few points where the maximum stress/strain is predicted by the model. The objective of the present paper is to demonstrate a step-by-step approach for performing model validation by combining full-field optical measurement methodologies with computational simulation techniques. Two important issues of the validation procedure are discussed, i.e. effective techniques to perform data compression using the principles of orthogonal decomposition, as well as methodologies to quantify the quality of simulations and make decisions about model validity. An I-beam with open holes under three-point bending loading is selected as an exemplar of the methodology. Orthogonal decomposition by Zernike shape descriptors is performed to compress large amounts of numerical and experimental data in selected regions of interest (ROI) by reducing its dimensionality while preserving information; and different comparison techniques including traditional error norms, a linear comparison methodology and a concordance coefficient correlation are used in order to make decisions about the validity of the simulation.

A hybrid framework for nonlinear dynamic simulations including full-field optical measurements and image decomposition algorithms

Innovative designs of transport vehicles need to be validated in order to demonstrate reliability and provide confidence. It is normal practice to study the mechanical response of the structural elements by comparing numerical results obtained from finite element simulation models with results obtained from experiment. In this frame, the use of whole-field optical techniques has been proven successful in the validation of deformation, strain, or vibration modes. The strength of full-field optical techniques is that the entire displacement field can be acquired. The objective of this article is to integrate full-field optical measurement methodologies with state-of-the-art computational simulation techniques for nonlinear transient dynamic events. In this frame, composite car bonnet frame structures of dimensions about 1.8 m × 0.8 m are considered. They have been tested in low-velocity mass-drop impact loading with impact energies ranging from 20 to 200 J. In parallel, simulation models of the car bonnet frame have been developed using layered shell elements. The Zernike shape descriptor approach was used to decompose numerical and experimental data into moments for comparison purposes. A very good agreement between numerical and experimental results was observed. Therefore, integration of numerical analysis with full-field optical measurements along with sophisticated comparison techniques can increase design reliability.

Analysis of displacement fields from a high-speed impact using shape descriptors

A composite bonnet liner subject to a high-velocity (70 m/s), low-energy (<300 J) impact by a 50-mm-diameter projectile has been investigated using computational simulation and by experiment. High-speed digital image correlation was employed to generate maps of displacement fields over the 1-m2 bonnet at 0.2 ms increments for 0.1 s, that is, 500 datasets, and the results have been compared to those predicted by finite element analysis. Image decomposition was utilised to reduce the dimensionality of both datasets by representing them using adaptive geometric moment descriptors; these descriptors were used to perform quantitative comparisons of the datasets and to test the validity of the model based on all the available data. The model was found to be a good representation of the physical experiment during the first half of the impact event but a less good representation in the remainder of the test, probably because damping effects were not adequately incorporated into the simulation. The methodologies for data comparison and evaluation of model validity proposed and demonstrated in this study represent a significant advance in procedures for ensuring model fidelity and for creating model credibility in the simulation of dynamic engineering events.

Progress in developing a standard for dynamic strain analysis

Previous work has led to the publication of a standard guide for the calibration of optical instruments for making full-field measurements of in-plane strain in pseudo-static cases. A consortium of international organizations drawn from across the innovation process is engaged in extending this earlier work to allow the calibration of instruments capable of making fullfield measurements of in-plane and out-of-plane deformations resulting from time-varying loading that may or may not be repetitive or cyclic. Preliminary designs for a reference material have been completed and are being tested in a round robin exercise. Calibration of the system employed for performing measurements in experiments allows the associated uncertainty to be defined which is an important step in the validation of computational models. However no recognized procedures exist for performing validations of simulations with full-field data from experiments. The consortium is developing appropriate validation procedures based on using image decomposition to enable comprehensive and quantitative comparisons between data sets for strain from experiments and simulations. Progress will be reported and the direction of planned work will be discussed to allow input from the user-community.

Experimental and numerical investigation of AS4/8552 interlaminar shear strength under impact loading conditions

The interlaminar shear behavior of AS4/8552 laminated short beam shear (SBS) coupons were experimentally and numerically investigated under static and impact loading conditions. Experiments were conducted in a range from quasi-static (1.7 × 10−5 m/s) to 3.9 m/s impact velocity using a testing device (ILSS device) that has been developed and adapted in a universal testing machine and a drop tower apparatus. Regarding the interlaminar shear strength (ILSS) values, the experimental investigation showed a low to medium strain rate sensitivity with a 23% maximum ILSS decrease observed at the samples which were tested with the maximum impact speed. In the finite element framework, the novel “stacked shell” or “2.5D” approach is investigated in the simulation of the SBS impact tests; models comprising four stacked sublaminate arrangements were capable of predicting the respective experimental results, with the maximum deviations from the respective experimental data to appear in the cases of high impact velocity.

Analysis of composite bolted joints by a macro-modelling approach

Purpose – The purpose of this paper is the development and the assessment of detailed and macro-modelling methodology approaches, suitable for the analysis of composite material bolted joints. Design/methodology/approach – A benchmark single-lap, single-bolt composite joint configuration is investigated, in order to demonstrate the different joint analysis approaches which are applicable in advanced riveted/bolted parts of aeronautical structures. In particular, several joint macro-models, i.e. numerical and analytical ones, as well as a detailed three-dimensional FE solid joint representation, were developed and compared in terms of stiffness prediction, while they were validated using respective experimental results. In addition, the numerical macro-model is implemented in a full scale, multi-bolt fuselage panel in order to demonstrate its capability to efficiently predict the panel’s response under compressive loads. Findings – Good correlation was observed between the majority of the models’ predictio...

Assessment of impact damage in CFRP by combined thermal and speckle methods

In this contribution we show experimental results for combined thermal and DSPI measurements on several fiber reinforced polymer test samples that have been impacted in a drop tower. Active thermography can give complementary information with regard to the type and size of the damage, but DSPI is used for assessing the effect of the damage, i.e. the difference in the displacement or strain field of a damaged and undamaged specimen.

An evaluation of a protocol for the validation of computational solid mechanics models

The terminology of validation of computational models and the framework within which it should be performed has been well-defined in a series of standards documents developed in the United States. However, there is no universally accepted protocol for performing validation, although a number of approaches have been proposed. The objective of the reported study was to assess the effectiveness and usefulness of a recently published approach that provides an efficient method for comparing data fields from the simulations and experiments, and which incorporates the uncertainty arising in the experimental data in the assessment of the model validity. An international comparison exercise was designed based on the standard for Inter-Laboratory Studies. Fifteen organisations participated in the Inter-Laboratory Study and the results demonstrated the efficacy of the validation protocol and provided feedback on a number of issues, including the definition of regions of interest, the need for a measure of the quality of the simulation results and importance of designing experiments specifically for validation exercises. The refined validation protocol has been incorporated into CEN Workshop Agreement CWA 16799:2014.

Steps towards Industrial Validation Experiments

Imaging systems for measuring surface displacement and strain fields such as stereoscopic Digital Image Correlation (DIC) are increasingly used in industry to validate model simulations. Recently, CEN has published a guideline for validation that is based on image decomposition to compare predicted and measured data fields. The CEN guideline was evaluated in an inter-laboratory study that demonstrated its usefulness in laboratory environments. This paper addresses the incorporation of the CEN methodology into an industrial environment and reports progress of the H2020 Clean Sky 2 project MOTIVATE. First, while DIC is a well-established technique, the estimation of its measurement uncertainty in an industrial environment is still being discussed, as the current approach to rely on the calibration uncertainty is insufficient. Second, in view of the push towards virtual testing it is important to harvest existing data in the course of the V&V activities before requesting a dedicated validation experiment, specifically at higher levels of the test pyramid. Finally, it is of uttermost importance to ensure compatibility and comparability of the simulation and measurement data so as to optimize the test matrix for maximum reliability and credibility of the simulations and a quantification of the model quality.

Delamination Identification in Stiffened Composite Panels Using Surface Strain Data

The structural integrity of a composite structure can be greatly compromised by damage inside the component. Invisible damage, e.g. caused by low-speed impact, can significantly reduces composite components capability to efficiently carry loads. In the present study, an innovative approach of strain-based delamination identification and localization is investigated, based on the efficient processing of full-field surface strain measurements. Surface strain data, potentially derived by full-field optical methodologies are used in the assessment of the delamination pattern, through strain field perturbations caused due to damage evolution. Relations between delamination damage and surface strain field disturbances are established by exploiting data decomposition methods using Zernike polynomial moments. The methodology is successfully demonstrated in the case of a stiffened composite panel.