Research Background

The research on the failure mechanism of long fiber reinforced composite materials is the premise of relevant analysis and prediction, and also provides a theoretical basis for the application of such materials in the industry. Interfiber transverse failure, which can occur in-plane, can also occur in multidirectional laminates subjected to impact loading.

In 2022, the journal "Composite Structures" published the research work of the University of Seville related to the transverse biaxial test of long fiber reinforced composite materials. The researchers optimized the design and fabrication process of the cross-shaped test piece, explored the biaxial stress failure mechanism through biaxial tests, studied the influence of the biaxial loading ratio on the failure load, and obtained the transverse strain under biaxial loading Failure criterion.

Test piece design and preparation

Test piece geometry

The design of the test piece is a major technical innovation of this research work. The research team explained this by combining numerical simulation and experiment in the previous article "Design for a cruciform coupon used for tensile biaxial transverse tests on composite materials". The rationality of the test piece design. Based on this, the researchers first describe the cross-shaped specimen design used in the transverse biaxial tests, and then describe the specific fabrication procedures employed.

Test piece preparation process

 The researchers designed a special mold for the manufacture of a cross-shaped embryo body through prepreg, and then cut and CNC-machined it on this basis, and finally obtained a cross-shaped test piece with a thinned center. The specific steps are as follows: (a) Step 1: compaction process. (b) Step 2: Cutting to size. (c) Step 3: Horizontal vertical assembly. (d) Step 4: Curing is in progress. (e) Step 5: After curing. (f) Step 6: Diamond circular saw cutting. (g) Step 7: Optical inspection. (h) Step 8: CNC machining of fixtures. (i) Step 9: final test piece and reinforcement sheet. Among them, the brand-new CNC machining technology and the design of auxiliary machine tools are used in step 8, which provides a reference for the machining process in similar research work.

Test Loading Equipment

The relationship between vertical stress σv and horizontal stress σh.

The researchers performed tension-tension (T-nT) and tension-compression (T-nC) loading perpendicular to the fiber direction, and obtained the law of the influence of the biaxial loading ratio on the failure load. Analysis of the test results shows that in all cases the resulting failure mechanism is determined by the main tensile force, so that the direction of the failure plane is always perpendicular to this tensile force. On the other hand, the presence of a secondary tensile load (T-nT case) does not appear to have a significant effect on the value of the external tensile load (primary tensile force) leading to failure, whereas in the T-nC case the presence of a secondary compressive load resulting in a lower value for the principal tensile load.

The position and direction of the failure surface

Through the tests, the researchers were also able to evaluate the performance of the designed specimens for different loading cases. From the perspective of biaxial test piece design, the final failure occurs in the expected area, which is in line with the original design expectations. Therefore, in the case of T-nT and T-nC, the external compression value is moderate, which is a feasible design. For such high values of secondary compression, some modification of the geometry or another type of load application system is necessary.

Damage starting point

By using high-speed cameras and analysis of post-failure test pieces, the researchers were able to determine the onset of macroscopic failure, finding specific patterns depending on the load cases used. These results also justify the fabrication process and provide experimental evidence, which may be of great interest for future numerical analyzes evaluating the stress state of the fault zone.

Transverse Strain Failure Relation Under Biaxial Loading

 In addition, the researchers obtained strain gauge measurements at the center of the test section. First, the researchers estimated the corresponding stress by comparing the biaxiality ratio affecting the central region of the specimen with the biaxiality ratio associated with the external load at failure. Secondly, it is verified again that the test results are consistent with the characterization of the material. Finally, a linear trend was identified in the failure strain, which allowed a preliminary derivation of the transverse strain failure criterion under biaxial loading.

Summary

 1.The direction of the failure surface of the test piece is always perpendicular to the direction of the main tension, which verifies that the failure mechanism is controlled by the main tension. The presence of a secondary tensile load had no significant effect on the value of the primary tensile force leading to failure, and the presence of a secondary compressive load resulted in a decrease in the value of the primary tensile load. The failure initiation point in the central region of the test piece was found. The biaxial stress failure mechanism was explored, and the effect of the biaxial loading ratio on the failure load was studied.

2.The test results are consistent with the transverse tensile strain and strength in material characterization. Based on estimates of the corresponding stresses, the biaxial ratio affecting the central region of the specimen was compared with that associated with the external load at the moment of fracture. The failure criterion of transverse strain under biaxial loading is obtained.