Research background

Ultra-thin composite materials have been extensively studied and applied in aerospace and high-performance structural manufacturing due to their excellent mechanical properties. Research institutions such as the U.S. Air Force Research Laboratory and Japan's Aerospace Exploration Agency have utilized these materials in the development of cryogenic pressure vessels. The mechanical performance of fiber-reinforced composites under biaxial stress states is crucial for pressure vessel research. Investigating the mechanical behavior of ultra-thin composites under complex stress conditions holds significant engineering and theoretical value. Standard composite materials typically have a single-layer thickness exceeding 100 μm. With advancements in fiber bundle deployment technology, the thinnest single-layer thickness of composites can now reach 15 μm. Reducing the single-layer thickness enhances material microstructure uniformity, improves matrix fluidity during manufacturing, and minimizes resin-rich zones and pore formation within layers. When the single-layer thickness of composites drops below 30 μm, they are classified as ultra-thin composites. In these materials, interfacial boundaries become less distinct, and fiber distribution within layers becomes more uniform. Thinner single layers delay the initiation and propagation of interlayer and intralayer damage, thereby further improving mechanical performance.

In 2024, a research team from Dalian University of Technology published a study titled "Study on Biaxial Tensile Failure Behavior of Ultra-Thin-Ply Carbon Fiber Reinforced Composites" in *Composites Science and Technology*. The researchers fabricated quasi-isotropic laminated cross-shaped biaxial test specimens with layer thicknesses of 24 μm and 100 μm, respectively. They conducted biaxial tensile tests with stress ratios of 0:1, 1:1, and 2:1, establishing failure envelope curves for ultra-thin-layer composites under biaxial tension conditions. These results were compared with those of standard-layer composites. Finally, by analyzing fracture surfaces and acoustic emission signals, the failure mechanisms of ultra-thin-layer carbon fiber-reinforced composites under biaxial stress states were elucidated.

Specimen Design

The cross-shaped biaxial test specimen mainly includes three parts: the central biaxial region, the transition region and the loading arm region. The main parameters to be considered in the design of the biaxial test specimen include the thickness ratio (the thickness ratio between the central region and the loading region), the geometric size of the central region, the transition angle of the transition region and the transition radius of the intersection of the loading arm.

An 1/8-scale finite element model of a cross-shaped biaxial test specimen was developed using ABAQUS. Symmetric boundary conditions were imposed on the left, bottom, and rear boundaries of the model, employing the Continuously Shell (SC8R) element type. The central region was configured with a thickness of 1.6 mm, a transition radius of 5.4 mm, and a necking angle of 8°. Biaxial loads were applied at a 1:1 displacement-to-length ratio, and the results with varying geometric parameters were compared and analyzed.

Figure 1 Finite Element Model of Cross-shaped Sample

The maximum principal strain and Hashin failure coefficient of the nodes along the central axis of the specimen are extracted to evaluate the strain distribution in the central biaxial region of the specimen and predict the failure location.

Figure 2 demonstrates the variation of maximum principal strain along the axis and Hashin failure coefficient in the layer thickness ratio specimen. The results indicate that the layer thickness ratio significantly influences the strain distribution and failure location in the central region of the specimen. The optimized layer thickness ratio design effectively improves strain uniformity and reduces local stress concentration.

Effect of the thickness ratio on the axial strain and the Kirschner failure coefficient of the specimens with the chamfer of 20° in the transition area

The thickness ratio of the transition zone is further designed and studied to meet the requirements of the biaxial test specimens. The optimized design can effectively improve the strain uniformity and reduce local stress concentration.

Figure 3 Cross-shaped sample dimensions

Experimental Method

The experimental materials consisted of quasi-isotropic laminates with single-layer thicknesses of 24 μm (ultra-thin layer) and 100 μm (standard layer). The composite matrix was epoxy resin (CAS No.: 25085-99-8), with Toray T700s fibers at a 50% volume fraction. The composite laminates were fabricated through a hot pressing process: prepregs were manually laid according to a predefined lay-up method, then transferred to a hot press. The temperature was raised to 145°C at a rate of 2°C per minute, with a pressure of 2 MPa applied. After maintaining pressure and temperature for 35 minutes, the laminate was cooled for 20 minutes before demolding. The cross-shaped laminates were cut using mechanical processing methods, with test areas milled. GFRP reinforcement sheets were bonded to the ends using epoxy resin adhesive. Biaxial tensile tests were conducted using the German Zwick-Z150 digital biaxial loading system, as shown in Figure 4. Three different biaxial X-Y load ratios (0:1, 2:1, and 1:1) were applied, with each ratio tested three times.

Figure 4 Test Platform

Interpretation of result 

Figure 5 presents the uniaxial tensile strength and failure modes of ultra-thin-layer and standard-layer quasi-isotropic laminates. The results indicate that the tensile strength of ultra-thin-layer laminates is 1.56 times higher than that of standard-layer laminates. Due to in-plane shear fracture at ±45° layers, the fracture surface of the standard-layer thickness composite specimen is at 45°. In contrast, the fracture surface of the ultra-thin-layer composite specimen is relatively smooth, with all fibers completely ruptured.

Figure 5 Failure strength and failure mode of uniaxial tensile specimens

The experimental data points of ultra-thin laminated plates lie outside the maximum stress criterion envelope, distributed on both sides of the biaxial envelope, while those of standard laminated plates are within the biaxial envelope, distributed on both sides of the maximum stress criterion envelope. Therefore, when predicting the mechanical properties of ultra-thin laminated plates under multi-axial stress conditions, the failure criterion considering biaxial stress states provides more accurate predictions.

Figure 6: Biaxial Tensile Failure Envelope

Under biaxial loading, both standard laminated plates and ultra-thin laminated plates exhibit crack orientation aligned with the X-axis. At a 0:1 load ratio, the cracks in the specimen form a 90-degree angle with the X-axis. Under this condition, stress concentration occurs at the central region's edge, with the Y-axis loading arm partially transferring load to the central area, resulting in fracture at the edge. As the load ratio decreases, stress distribution becomes more uniform in the central region, reducing the crack angle relative to the X-axis and bringing it closer to the specimen center. The crack angle measures 75° at 2:1 load ratio and 45° at 1:1 load ratio. Notably, the fracture surfaces of ultra-thin laminated plates appear relatively flat, indicating a transition from matrix failure caused by in-plane shear to ±45° fiber fracture, with delamination being suppressed. In contrast, standard laminated plates exhibit distinct sawtooth crack paths and delamination. For standard thickness laminated composites, ±45° layers typically demonstrate in-plane shear failure due to matrix failure, where fibers remain unbroken, thus their load-bearing capacity remains underutilized. Additionally, ±45° layers in laminated plates usually initiate delamination after in-plane shear failure. Constrained by adjacent layers, ±45° layers in ultra-thin laminated composites undergo fiber fracture, and the flat failure surface effectively prevents delamination, significantly enhancing load-bearing capacity. Under uniaxial loading, the principal stress direction of fibers in ±45° layers forms a 45-degree angle with the fiber orientation, while biaxial loading further reduces this angle, thereby further improving load-bearing capacity.

Figure 7 Failure mode of biaxial tensile specimen

Under biaxial loading, ultra-thin laminates exhibit signal amplitudes exceeding 80 dB at significantly higher stress levels, while energy signals in low-stress regions decrease markedly, resulting in slower cumulative energy growth. Standard laminates demonstrate stress levels and cumulative energy growth rates within the 70-100 dB range under biaxial loading similar to those under uniaxial loading. When amplitude exceeds 70 dB, the damage mode in carbon fiber reinforced composites is delamination, and when amplitude surpasses 80 dB, it becomes fiber fracture. Under biaxial stress conditions, ultra-thin laminates show distinct advantages in suppressing delamination and delaying fiber fracture. At the same load ratio, 100 μm standard laminates exhibit a steeper cumulative energy growth gradient, indicating sudden macroscopic damage within the laminate. Additionally, under identical stress ratios, ultra-thin laminates demonstrate higher cumulative energy compared to standard laminates. This is attributed to the composite thin layers' suppression of delamination, which transforms the damage mode from matrix failure caused by in-plane shear to fiber fracture constrained by adjacent layers.

Figure 8 Relationship between acoustic emission and stress in the X direction

Full text summary

1. The interaction effect of biaxial stresses in ultra-thin-layer composites is more pronounced than in standard laminates. When predicting the mechanical properties of ultra-thin-layer composite structures under multi-axial stress conditions, failure criteria incorporating biaxial stress states should be adopted. 2. Under biaxial loading, ultra-thin-layer composites demonstrate enhanced delamination suppression and fiber failure delay capabilities. The reduction in single-layer thickness causes the failure mode of ±45 layers to transition from matrix failure to fiber fracture, resulting in higher accumulated acoustic emission energy under the same load ratio.

Article source

Liu Y, Ren Z, Han Y, et al. Study on biaxial tensile failure behavior of ultra-thin-ply carbon fiber reinforced composites[J]. Composites Science and Technology, 2024, 251: 110544.

Original Link

https://doi.org/10.1016/j.compscitech.2024.110544