DIC Experiment - Measuring the Mechanical Properties of Ceramic Matrix Composites Under High Temperature Conditions - In Collaboration with Safran Ceramics

Keywords:Digital image related, high-temperature strain measurement, non-contact strain measurement, material performance analysis

AsEikoSima new member of the team, I am very pleased to have the opportunity to outline my new paper: mainly completingSiC/SiChigh-temperature experiments on composite materials, this experiment was conducted under the supervision ofF. HildatLMTlaboratory, while the calibration work of the experiment was completed under the supervision of V. Herb and B. Lacombe.Responsible person:Myriam Berny, R&D engineer, PhD student.Safran Ceramics is working hard to improve its technical skills to analyze and understand the thermomechanical properties of coated and uncoated ceramic matrix composites (

CMC) used in gas turbine engine applications. To achieve this goal, testing needs to be conducted under more representative engine operating conditions. Therefore, it requires high-temperature testing of samples under high-intensity three-dimensional thermal loads (i.e., greater than℃), which is conducted with multi-instrument detection using infrared and visible light cameras (Fig.

). This is to control thermal boundary conditions and motion boundary conditions and understand material behavior. Therefore, the purpose of this paper is to develop different programs to evaluatethe thermomechanical behavior of materials at high temperatures, using thermal and motion full-field measurements and correlations between experimental and finite element (FE1300) calculations.1Fig.1 – Multi-instrumented experiment of a SiC/SiC composite under thermal gradients obtained with a COthe thermomechanical behavior of materials at high temperatures, using thermal and motion full-field measurements and correlations between experimental and finite element (laser beam (collaboration with T. Archer, P. Beauchêne and A. Mavel (ONERA)).High-temperature experiments on composite materials present quite a few challenges. First, the materials exhibit low strain levels (i.e., reversible domain less than0.1%

Fig.1 – Multi-instrumented experiment of a SiC/SiC composite under thermal gradients obtained with a CO₂laser beam (collaboration with T. Archer, P. Beauchêne and A. Mavel (ONERA)).

BNMI-SiC/SiC复合材料的高温实验具有相当多的挑战。首先，材料表现出较小的应变水平（即可逆域小于0.1%）和不均匀行为，这意味着必须使用能够监测热机械响应和测量小幅度位移的全场技术。这些要求导致了基于红外和可见光摄像机的红外热成像和DIC/立体DIC方法的使用。然而，温度升高引起了一些额外的挑战，如黑体辐射和对流，这对DIC测量尤其不利。因此，高温DIC分析需要专门的策略来解释灰度变化，并减轻热雾效应，该效应会扭曲图像并损害位移测量。

基于在这种具有挑战性的环境中获取的图像来识别the thermomechanical behavior of materials at high temperatures, using thermal and motion full-field measurements and correlations between experimental and finite element (属性，要求我们必须遵循以下几个步骤。首先，建立了二维图像测量值与三维有限元模拟热机械计算场之间的关系。提出了两种标定不同摄像机的方法（即投影矩阵法）。一方面，基于自标定方法实现了单台红外相机的标定。另一方面，由于现场斑点标定目标（为实验装置量身定做），其表面有限元模型是众所周知的，并且是一对图像，因此要在全局立体视觉框架中对立体钻机进行标定。EikoSim在EikoTwin DIC中针对可见光相机和红外相机的热附加组件中提出的不同校准程序允许热机械场的拉格朗日测量。此外，它还能够在室温下对小振幅位移（例如，纵向分量为2µm）进行潜在评估，这是分析复合材料力学性能的主要要求。

在校准摄像机后，讨论了高温下的热机械场，以评估测量可靠温度和位移的能力，尽管存在对流效应。由热霾引起的时空波动被证明破坏了所寻求的测量。对温度场进行模态分析，以评估不同现象（即加热过程、对流效应和采集噪声）的贡献，并建立本研究所用红外相机的热不确定度水平和协方差矩阵。此外，还进行了定量分析，以表征热雾效应及其对位移测量质量波动（即显著增加）的影响。

基于这些观察结果，我们得出结论，专用的时空正则化策略是必要的，分别用二维-DICMyriam Berny有限元为基础的立体（FE-stereoDIC）以处理高水平的波动的二维和三维表面位移。在the thermomechanical behavior of materials at high temperatures, using thermal and motion full-field measurements and correlations between experimental and finite element (试样上进行了若干高温实验。已发展的全局时空和立体算法的共同特点是用分离的时空变量（即时间和空间形状函数）参数化运动场的模态分解。对于时空二维DIC，根据所选择的时间参数化，研究了两种实现方法，即先验参数化和通过适当的广义分解框架动态构造模态基。后者还扩展到处理时空亮度和对比度校正，这是校正黑体辐射在高温下产生的影响所必需的。在此基础上，提出了一种参考图像去噪方法。提前设定的时基、模态分解和非侵入性算法方案使得时空正则化也被开发应用于有限元立体DIC，用以测量微应变。这些策略被应用于大型图像序列的分析，并被证明在减轻热雾效应方面是有效的（图2，图3）。

Fig.2 – Effect of spacetime stereoDIC on the measurement of nodal out-of-plane displacements, through the comparison of (a) instantaneous and (b) spatiotemporal approaches.

Fig.3 – Nodal out-of-plane displacement field at 1,300°C (i.e., image no. 1,200) measured with (a) instantaneous and (b) spatiotemporal FE-stereoDIC.

Therefore, at temperatures above1300°Cthe surface measured a consistent and reliablethe thermomechanical behavior of materials at high temperatures, using thermal and motion full-field measurements and correlations between experimental and finite element (displacement field, reducing temporal and spatial fluctuations, achieving a level far below what could be achieved by physically adding fans in the experimental setup. Thus, a unified spacetime framework is beneficial for the analysis of2DMyriam Berny3DCMCs.The final step of this research work is to develop a universal identification algorithm based on weighted temperature and kinematic functions (referred to as weightedDIC). This algorithm uses a framework similar to the kinematic and load data proposed in

EikoTwin Digital TwinFEMU-TU. The weightedis based on full-field temperature and displacement (surface) and considers measurement uncertainty (through covariance matrix) to identify severeFEMU-TUthermal loads on2D/3Dcomposite materials' thermal boundary conditions and thermomechanical properties. The measurement space and time bases are chosen as references, and the calculated fields and sensitivities are projected onto these bases, particularly using calibrated projection matrices. Therefore, the measured and calculated thermomechanical fields are compared "appropriately". Additionally, due to the introduction of spacetime strategies, a regularized displacement field is used to minimize the effects of thermal haze. On the other hand, since a field speckle calibration target (customized for the experimental setup) was used, the calibration of the stereo test bench was conducted within a global stereo framework. By combining all these procedures, the identification of the sought parameters achieved very satisfactory results in four analytical experiments. In particular, compared to measurement fluctuations and noise floors, the levels of thermal residuals (Fig3D) and motion residuals (i.e., the differences between measured and computed fields) were low, highlighting the success of the identification in actual experiments.SiC/SiCFig.4 – (a) Temperature field for the steady state and (b) associated thermal residual field at the end of a FEMU-TU procedure. (c) Change in measured and computed temperatures for five points of the sample whose locations are indicated in (a).4Conclusion

In summary, through the identification from measurement to thermomechanical properties, we have developed a complete and reliable procedure to handle

high-temperature tests. These improvements aim to reduce the impact of temperature rise and enhance the reliability of thermal and motion measurements, allowing for parameter identification under minimal measurement fluctuation effects. These precautions make the assessment of the thermomechanical response of

composite materials tested at high temperatures more reliable..的高温试验。这些改进是为了减少温度升高的影响，提高热测量和运动测量的可靠性，以便在测量波动影响最小的情况下进行参数识别。这些预防措施使得在高温下测试的SiC/SiC复合材料的热机械响应的评估更加可靠。