3D braided composites have been widely applied in aerospace, automotive manufacturing, and wind power generation due to their excellent mechanical properties, impact resistance, and design flexibility. Compared with traditional laminated composites, 3D braided composites incorporate reinforcing fibers in the thickness direction, effectively suppressing delamination failure and enhancing the overall structural strength and durability.

In practical applications, 3D braided composites are frequently subjected to complex multi-axis stress states, such as biaxial tension and shear. These complex stress conditions may induce intricate damage mechanisms within the material, ultimately affecting its overall performance. Therefore, investigating the failure behavior of 3D braided composites under biaxial tension loads is crucial for optimizing material design and enhancing structural reliability.

In 2022, a research team from Harbin Institute of Technology published a paper titled "A Combined Experimental and Numerical Approach to Investigate the Failure Behaviors of 3D Woven Composites under Biaxial Tensile Loading" in the journal *Composite Science and Technology*. The researchers employed a combined experimental and numerical simulation approach to systematically study the failure behavior of 3D woven composites under biaxial tensile loading. By observing the damage evolution process through experiments and combining it with numerical simulations, they revealed the failure mechanisms, providing theoretical support for material design and engineering applications.

Design and Machining of Test Piece

The researchers first summarized the advantages and disadvantages of existing biaxial cross-shaped tensile test specimens, and optimized the specimens based on the characteristics of 3DWC materials, as shown in Figure 1.

Figure 1 Schematic dimensions of the three-dimensional braided composite biaxial cross-shaped test specimen

The machining of the central thinning region in biaxial test specimens has long been a challenging issue in the industry, with accuracy often difficult to ensure. Researchers have employed numerical control (NC) technology to achieve multi-axis milling control for complex structures, demonstrating strong adaptability and high-quality processing. The three-dimensional coordinate analysis of the milling process is illustrated in Figure 2(b).

Figure 2 Center milling area of the biaxial cross-shaped test specimen

The researchers employed reinforcement plates to further enhance the load-bearing capacity of the loading arm. Additionally, a layer of high-adhesion red adhesive film was bonded between the loading arm and the reinforcement plates to prevent delamination during testing. The 3DWC biaxial cross-shaped test specimen with aluminum reinforcement plates is shown in Figure 3.

Figure 3 Cross-shaped test specimen of 3D braided composite with reinforcement sheet

Loading equipment 

The biaxial tensile test was performed on a servo-hydraulic MTS planar biaxial testing machine, with a maximum load of 100kN and DIC technology for three-dimensional strain field capture.

Figure 4 MTS planar biaxial testing machine and 3D DIC equipment

Multi-scale Modeling of 3D Weave Composite

The researchers conducted a detailed analysis based on the RVE, incorporating the actual geometric structure and material parameters of the test specimen, and established a three-dimensional finite element model. Using the meso-mechanics approach, the fiber bundles and matrix were modeled separately to more accurately reflect the microstructure of the material, as shown in Figure 5.

Figure 5 Representative volume element at microscale

The structural scale modeling primarily addresses the material and structural differences between fibers and the matrix. The internal braided structure exhibits periodicity along both X and Y directions, with a total of 8 warp layers and 9 weft layers across the thickness. These layers are sequentially stacked to enhance structural integrity and out-of-plane performance, as illustrated in Figure 6.

Figure 6 Representative volume element at mesoscale

The researchers developed a novel full-scale geometric modeling method capable of characterizing the intricate geometric features of yarns, automatically defining local material orientations, and accommodating irregular structures. This method was applied to reconstruct a 3DWC biaxial cross-shaped test specimen, with detailed explanations provided for meshing and boundary conditions as illustrated in Figure 7.

Figure 7 Macroscopic finite element model and local material orientation of the biaxial cross-shaped specimen

The verification of the double axis test by finite element modeling

The authors proposed a multi-scale damage constitutive model based on the stress transfer relationship between the meso-stress and the micro-stress and the micro-mechanical failure theory. The results show that the shape of the test specimens can meet the requirements of the biaxial test, and the simulation model can simulate the damage morphology well, as shown in Figure 8.

Figure 8 Comparison of Surface Crack Morphology Between Experimental and Simulation Samples

The mechanical response of the test and simulation is in good agreement, as shown in Figure 9. The main damage modes of the test specimen are fiber fracture, matrix cracking, and pure matrix failure.

Figure 9 Comparison of Strain-Tensile Load Curve in Central Region of Test Pieces between Experimental and Simulation

Practical Significance of This Study

1. This study develops a novel full-scale geometric modeling method capable of characterizing mesoscopic geometric features of fiber yarns, automatically defining local material orientation, and applicable to macroscopic irregular structures, for reconstructing 3DWC biaxial cross-shaped test specimens. 2. Integrating MMF theory, this research proposes a multiscale damage model combining micro-, meso-, and macro-scale numerical computations to predict 3DWC biaxial tensile failure behavior. By establishing a stress transfer relationship between meso-stress and micro-stress through stress amplification factors, the damage states of matrix and fibers in yarns can be determined independently without assuming the fiber bundle as transversely isotropic homogeneous material.

Theoretical contribution of the whole text

1. According to the analysis of the strain distribution and crack morphology, the biaxial cross-shaped specimen designed in this study can meet the requirements of various loading ratios and loading forms. The three main failure modes of 3DWC under biaxial tensile loading are longitudinal fiber fracture, transverse fiber cracking and matrix failure.

2. Through comparative analysis of experimental and predictive results, this study demonstrates that the damage evolution process of 3DWC under biaxial tensile loading progresses as follows: Matrix failure and pure matrix failure within the yarn first occur in the biaxial central region, circular chamfer, and loading arms. As the load increases, initial cracks accumulate further, leading to fiber fracture in the weft yarn that extends to multiple fiber bundles, resulting in loss of load-bearing capacity in the weft direction. Fiber fracture in the warp yarn occurs rapidly, ultimately causing complete structural failure.

Article source:

Zheng T, Huang J, Guo L, Sun R, Huang T, Zhou J, Jia F, Hong C. A combined experimental and numerical approach to investigate the failure behaviors of 3D woven composites under biaxial tensile loading. Compos Sci Technol 2023; 236: 109974.

Original link:

https://doi.org/10.1016/j.compscitech.2023.109974.