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Gradient three‐dimensional (3D) orthogonal preforms with low‐ and high‐density regions were manufactured through the flexible oriented 3D woven process (FO3DWP) at different fiber volume fractions and z‐yarn contents. The layer porosity distribution of preforms was regulated by modifying fiber tension and measured using micro‐computed tomography (micro‐CT) technology. After impregnation with epoxy resin, the out‐of‐plane compression properties of gradient 3D woven composites were evaluated. The results indicated that the out‐of‐plane compression strength could be improved by the gradient design. In particular, the gradient composite with a fiber volume fraction of 0.46 and z‐yarn volume fraction of 0.05 exhibited the highest compression strength at 470 MPa. Furthermore, failure morphologies were analyzed using micro‐fracture photographs and scanning electron microscopy (SEM). The gradient design efficiently protected the interface between z‐directional fibers and the matrix by minimizing z‐yarn bending. However, the compression strength of 3D woven composites decreased as the z‐yarn content increased from 0.05 to 0.15, due to the increased bending of the z‐directional fibers during the compression test.

In order to reduce the wear and tear of Z-yarn implantation into the preform, the frictional behavior of Z-yarn during implantation into the preform was studied. According to the actual implantation conditions of Z-yarns, we designed a guiding array clamping device and a tensile force sensor to study the changes of tensile strength of Z-yarns at different implantation positions and implantation times, results show that the tensile strength of Z-yarns decreases significantly after 25 implantations at the position of 4 N friction force, and it's necessary to replace Z-yarns in time to avoid their fracture in the preform. The use of unadulterated solvent dimethyl silicone oil applied to the Z-yarns, can significantly reduce their wear, the tensile strength of Z-yarns after 60 implantations is only 5.6% lower than that of Z-yarns after 10 implantations. Design the tension control system, and when the tension is 0.46 N, the tensile strength of Z-yarns during implantation is 34.2% higher than without tension. Meanwhile, the relationship between the normal force applied to Zyarn and the implantation length and fiber bundle width are established by combining Hertzian theory and experimental data, and the friction coefficient algorithm during Z-yarn implantation is derived by combining Howell's equation, which solves the problem that the fiber friction coefficient cannot be measured due to the complicated working condition of Z-yarn implantation.The frictional wear mechanism of yarn during Z-yarn implantation into the preform is revealed, and a method to reduce yarn wear and increase its continuous implantation number is proposed.

Abstract：In order to solve the problem of engine cylinder head casting production process of the bottom of the cylinder head surface flatness error detection problem， the design of a laser non-contact cylinder head surface flatness on-line detection system, a method of flatness error detection based on diagonal line is proposed. According to the field flatness detection demand design using laser arm axis and cylinder head horizontal transfer reciprocation engine cylinder head surface flatness error detection system; Through the engine cylinder head surface detection feature point four corners diagonal midline build a mathematical model of the engine cylinder head flatness testing, regression equation to determine the method of least squares plane is ideal plane, calculated flatness error; The laser displacement sensor calibration, given the sensor error calibration regression equation and application of the detection system on different types of engine cylinder head detection. The results showed that：The system has a maximum detection area of 400 mm×2 000 mm, the measurement range of 160-450 mm, the measurement accuracy of 0.03 mm, and simple structure, speed, and is fully capable of reality testing requirements.

The reinforced fibers of three-dimensional (3D) composites are interwoven in space and have better inter-laminar properties than two-dimensional (2D) laminates, and have great application prospects in high-tech fields.Flexible-oriented 3D woven technology is an emerging technology for weaving 3D composite preforms, especially suitable for manufacturing large-thickness and complex preform architectures, and the compaction behavior of preforms has an important influence on its performance. In this paper, using this technology, different types of 3D preforms were woven and subjected to systematic compaction experiments, to explore the effects of different fiber hybrid modes and process parameters on the compaction behavior. It is found that, increasing the number of cycles, wetting and sizing the fibers are all beneficial to improve the compaction ability, but reduce the stress relaxation and recovery ratios. The stress relaxation of the hybrid fiber preform is mainly affected by the carbon fiber. Reducing the dispersion of different fiber bundles is beneficial to decrease the recovery ratio. The research results provide experimental and theoretical reference for the rapid and efficient weaving of hybrid fiber composite preforms in the future.

To ensure the smooth insertion of the side rod/fiber bundle during the three-dimensional woven preform contour locking process, the influence of the diameter, speed, and other parameters on the free end vibration displacement response of the side rod and the fiber-carrying needle is analyzed. By calculating the kinetic energy and potential energy of the two separately, the dynamic equations of the side rod and the fiber-carrying needle are established based on Hamilton's principle and discretized. The state equations are solved using the Runge–Kutta method, and the free end vibration displacement responses of the side rod and the fiber-carrying needle for different parameters are obtained. The results show that the equilibrium position and the maximum amplitude of the free end of the side rod and the fiber-carrying needle decrease with the increase of the section diameter and the decrease of the axial speed. And the circular tube wall thickness has less impact on the free end vibration displacement of the fiber-carrying needle. The overall analysis shows that the fiber-carrying needle vibration response is smaller than that of the side rod under the same parameters.

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