DIC Experiment - Digital Image Correlation Testing of Lattice Structure

Digital image correlation technologyApplication in lattice structure testing in the rapid prototyping manufacturing industry

Especially with the development of aerospace components, interest in lattice structures is growing day by day. In addition to the significant mass benefits they allow for consideration, they also provide the possibility to optimize their behavior based on their composition patterns. The strong development of metal additive manufacturing technology has made it possible to manufacture parts composed of lattice structures, with materials known for their mechanical properties, such as nickel-based alloys, titanium alloys, or new generation aluminum alloys. These promising building materials have not yet been widely applied in the design of structural components because they require new development methods (simulation, characterization, manufacturing) on a commercial scale. In this case, the Saint-Exupéry Institute (IRT) is conducting a research and development project lastingmonths, calledLASER24, which brings together major aerospace manufacturers to provide them with understanding and design tools from their research departments.EikoSimis a young company specializing in the connection between testing and simulation, providing expertise and software for field measurements using digital image correlation for laser projects. It focuses on testing and simulation methods.Compression test on a BCCZ lattice structure – IRT Saint-ExuperyAs part of the project, the tests conducted on the lattice structure aim to highlight strain patterns in compression or shear configurations, applying different boundary conditions and scale effects when the number of unit cells constituting the specimen varies. The diversity of these configurations should allow for the development and validation of numerical simulation methods. The research at that time was limited to using quasi-static stress at room temperature as the starting point for determining methods. The tested structure consisted of microbeams with a maximum diameter of

1 mm

composed according to theEikoSimBCClattice (body-centered cubic) orBCCZlattice (body-centered cubic and vertical beam) organization.BCCZ lattice structure after a compression test, characteristic shear belt – IRTSaint-ExupéryAdopting more effective measurement methods is an initiative to address the challenges posed by these new geometric structures. Field measurements, especially digital image correlation, are natural candidates for capturing the non-uniformity of strain and local instability phenomena.Image correlation-based compression testing

The implementation of digital image correlation involves applying speckles (contrast patterns) on the specimen, which should be appropriately sized and adapted to the camera resolution. On the lattice structure, speckles of a few microns in lens size are sprayed using a spray gun.

During the testing process, two pairs of high-resolution cameras observe both sides of the sample. This allows us to accurately capture the characteristic strain patterns of the shear band and all complex kinematics. From a measurement perspective, this complicates the processing. According to the method proposed in 

EikoTwin DIC

software, the measurement results are directly related to a point in the three-dimensional space of the model part. There is no limit to the number of cameras during data analysis, which helps measure the displacement of points within their respective fields of view.Instrumentation by 4 cameras of two sides of a lattice structure for a compression test – IRT Saint-Exupéry

is a technique that tracks the movement of points in the structure by assuming that they maintain the same gray level in the images. Therefore, lighting conditions must be controlled to limit measurement uncertainty and ensure tracking of the structure even in cases of large displacements.Verticaldisplacement

field

software, the measurement results are directly related to a point in the three-dimensional space of the model part. There is no limit to the number of cameras during data analysis, which helps measure the displacement of points within their respective fields of view.measured

by image correlation on the simulation mesh, EikoTwin DIC software – IRT

Saint-Exupéry Modeling and computation Several modeling methods for the mechanical behavior of lattice structures can be considered. The most accurate in terms of geometric representation is to describe each microbeam using volume. By discretizing the volume elements, it is possible to get as close as possible to the originally designed geometry. From a physical perspective, it is very important to describe the elastoplastic behavior of structural components, considering the expected strain levels during the compression test. One difficulty at this point is the adaptation laws for describing local mechanical behavior. There are several possible approaches for the R&D department: l The first method can consider the characteristics of the constituent materials from standard tests. In this case, it is recommended to conduct standard tests on samples made from raw materials obtained through additive manufacturing processes identical to those of the lattice structure.

The second method proposed and implemented in the project is to describe the constituent elements of the lattice structure, namely the microbeams. Samples' geometric structures were designed and manufactured on trays identical to those of the lattice structure to obtain their inherent characteristics based on the orientation of the microbeams (vertical, inclined). The advantage of this method is that it allows for observing effects, particularly those related to the fineness of the geometry, which is relatively well-known in additive manufacturing: roughness, shape errors (average diameter, cylindricity), thermal history, and specific metallurgy.

“Microbeam” test pieces for the characterization of the constituent elements 

of  a 

of  latticeEikoSimstructure

“Microbeam” test pieces for the characterization of the constituent elements of

a lattice structure – IRT Saint-Exupéry

The application of loads and boundary conditions is an important part of modeling. Just as the differences between the geometric structures of design and manufacturing can significantly affect the response, the differences between the 'ideal' loads and the actual loads that the structure bears can also have a major impact. However, research departments rarely have access to measurement data for these gaps. Therefore, the initially chosen models largely ignore them.

Simulation of DIC compression test EikoTwin a 5x5x5 Adopting more effective measurement methods is an initiative to address the challenges posed by these new geometric structures. Field measurements, especially digital image correlation, are natural candidates for capturing the non-uniformity of strain and local instability phenomena. lattice structure, solidelement – IRT Abaqus Several modeling methods for the mechanical behavior of lattice structures can be considered. The most accurate in terms of geometric representation is to describe each microbeam using volume. By discretizing the volume elements, it is possible to get as close as possible to the originally designed geometry. From a physical perspective, it is very important to describe the elastoplastic behavior of structural components, considering the expected strain levels during the compression test. One difficulty at this point is the adaptation laws for describing local mechanical behavior. There are several possible approaches for the R&D department: – IRT The first method can consider the characteristics of the constituent materials from standard tests. In this case, it is recommended to conduct standard tests on samples made from raw materials obtained through additive manufacturing processes identical to those of the lattice structure.

For this nonlinear model composed of millions of elements, the issue of computation time arises quickly.EikoSimThe project is currently investigating a simplified method based on linear finite element approximations of geometry with beam kinematics, aiming not to reduce the accuracy of the simulation.

Comparison of test and simulation

The comparison between measurement and simulation results is a challenge in itself. The method of measuring displacements on point clouds is interesting, but it raises new issues when compared to simulations. This method is implemented in the software, based on finite elements, measuring the displacement field directly on the numerical simulation mesh in the same three-dimensional coordinate system. This avoids errors related to the projection of the measurement field onto the mesh, and vice versa. It also saves time for the user. For example, the differences between the ideal loading path and the loading path applied to the part can be visualized directly, allowing for corresponding corrections to the simulation model.EikoTwin-DICDi

erence between and DIC on simulated vertical (left), image correlation evolution simulated the deviation of at point a mesh of DIC as function a time of (right) Outlook

Next step: Modify the initial simulation using measurement results. These tests have taught us a lot about lattice structures, so we want to use these results to improve our simulations and better understand the sources of error. Initially, the displacements measured at the edges can serve as boundary conditions for the simulation.

Then, combining the comparison of experimental and simulated stress displacement curves, this method will allow the implementation of a reverse identification strategy to re-determine model parameters and reduce the gap between experimental and simulated results.

EikoTwin Digital Twin-The software suggests integrating these two basic steps into one environment. This will enable us to consider transferring these methods to the research department in the short term and develop the potential of lattice structures for industrial applications.CreditThe Saint-Exupéry Institute of Technology is a driver of scientific and technological research and transfer in the aerospace, aeronautics, and embedded systems industries, continuously developing safe, reliable, certifiable, and sustainable innovative solutions. The Saint-Exupéry Institute conducts research projects and R&D services supported by technological platforms in four key areas: high-performance multifunctional materials, more electric aircraft, intelligent systems and communications, and systems engineering and modeling.

*

The Saint-Exupéry Institute is a technology research organization recognized by the French government as part of the "Avenir Investment Program" (PIA).*EikoSimis a software company specializing in testing simulation dialogue, dedicated to innovative measurement tools related to digital imaging.

The project is led by the Saint-Exupéry Institute, of which50%“Avenir投资计划”（PIA）的一部分。

*EikoSim是一家专门从事测试模拟对话的软件公司，它致力于数字图像相关的创新测量工具。

EikoSim项目由圣埃克苏普研究所，其中50%由其工业成员出资，50%由法国政府的Avenir投资计划（PIA）出资。它是IRT“晶格”倡议的一部分，该倡议定义了与IRT SystemX领导的DSL（Durabilite des Structures LATTICES）项目以及国家空间研究中心（CNES）的密切合作。