Interlaminar Shear Strength Of Laminates Research Articles

Improved interlaminar property of carbon fiber/epoxy composites with polyurethane/RGO core-shell structure fibrous mat

Interlaminar delamination is the most common failure mode of carbon fiber reinforced epoxy (CF/EP) composites, which severely limits their practical application. In this work, reduced graphene oxide (RGO) was decorated on electrospun polyurethane (PU) fibrous veil to produce a hybrid core-shell structure fibrous mat, which was integrated into CF/EP to thoroughly evaluate their influence on the interlaminar fracture toughness and interlaminar shear strength of laminates. The results show that, compared to those of plain composite, the mode I and II interlaminar fracture toughness of the PU/RGO interleaved composites increases by 170 % and 21 %, respectively, while their interlaminar shear strength synchronously increases by 14 %. The improved fracture toughness could be attributed to the plastic deformation of PU and epoxy mixture, crack deflection caused by RGO and pull out and separation behavior of RGO, etc. Henceforth, our hybrid interleaf can be considered as a promising candidate for designing high delamination resistance composites for structural applications.

Effect of voids and crystallinity on the interlaminar shear strength of in-situ manufactured CF/PEEK laminates using repass treatment

Voids and crystallinity, both affected by the temperature history, have an effect on the interlaminar shear strength of thermoplastic composites. In this paper, CF/PEEK laminates with varied porosity and crystallinity are manufactured by repass treatment (based on the laser-assisted forming) under different interlaminar temperature and times of treatment. The porosity and microscopic features of voids are characterized via the water displacement method and optical microscope, respectively. And further squeeze flow of fiber-resin composite and percolation flow of resin occur during repass treatment, which account for the inhabitation of voids. Crystallinity is analyzed through differential scanning calorimetry, which is affected by both cooling rate and recrystallization of the amorphous phase during repass treatment. The interlaminar shear strength of laminates with varied voids and crystallinity is tested, and the failure modes are studied by optical microscope and scanning electron microscope. As a result, the interlaminar shear strength of laminates, with less voids and higher degree of crystallinity, is improved 32.87% than that of the laminates without repass treatment.

Z‐pin effect on interlaminar mechanical and ablation performance of quartz‐phenolic composites

AbstractWhen a spacecraft flies at a high speed through a high‐heat flow, the ablative composites suffer from severe delamination and bulging due to their weak interlaminar performance. This will interfere normal operations of spacecraft. To improve the interlaminar performance of ablative composites is of great significance to the ablative structures. This study investigated Z‐pin reinforced quartz‐phenolic composites and reinforcement mechanism. First, a thermogravimetry (TG) test was used to analyze the thermal decomposition process of the phenolic. Second, a series of tests, including a single Z‐pin pull‐out, a single‐lap joint tensile and an oxyacetylene ablation test, were conducted to investigate composites' ablation performance. We also deployed a 3D morphometer and an infrared thermography to reveal the function of Z‐pins. The results demonstrated that Z‐pins can improve composites' ablation performance in both structure and function. In the structure, Z‐pins suppress the interlaminar delamination of composites by enhancing the interlaminar strength. As the ablation time increases, interface bridging between the Z‐pin and laminates contributes increasingly to the interlaminar shear strength of laminates. After 60 s of ablation in a flame of 920°C, the interlaminar shear strength of Z‐pin specimens is 84.66% stronger than that of the normal composites. In the function, the Z‐pins suppress large areas of bulges by providing escape channels for pyrolysis gas, and reduce bulging areas by 60%. Z‐pin technology can effectively suppress mechanical erosion of the ablative surface due to the delamination and bulging. It is an important way to improve ablation performance of ablative structures.

孔隙对复合材料层压板湿热性能的影响

针对某飞机上应用的典型的层压板[(±45)/(0,90)2/(±45)]S和[(±45)/04/(±45)/(0,90)]S，研究了孔隙对碳纤维增强环氧树脂基复合材料层压板的吸湿性能、脱湿性能以及室温、湿热和脱湿层间剪切性能的影响。研究结果表明： 两种层压板的吸湿率和最大吸湿量随着孔隙率的增加而上升，层压板吸湿后出现了膨胀，表面出现了裂纹，大多数裂纹都位于层间树脂富集区，层压板内的孔隙在湿热作用下而产生了裂纹并且裂纹在孔隙之间扩展，复合材料层压板在脱湿后会有一部分水分残留，孔隙率大的试样残留的水分稍微多一些。含不同孔隙率层压板的层间剪切强度随着吸湿量的变化比较复杂。 The influence of void on the moisture absorption, desorption and interlaminar shear strength (ILSS) of [(±45)/(0,90) 2/(±45)]S and [(±45)/04/(±45)/(0,90)]S composite laminates used in a aircraft was investigated. The results show that the rate of water uptake and the maximum level of moisture absorption in the two laminates increase with the increasing void content. Expand occurs in the two laminates after moisture absorption, and cracks were found on the surface of laminates. Most of cracks locate at the rich resin between the plies. Cracks can also be found around voids in the laminates and propagate between voids. Some water retained in the composite laminates after desorption and more water was kept in the laminates with larger void content. The ILSS of laminates with different void content varied complexly with the mass gain of moisture absorption.

Environmental Effects in Composite Laminates with Voids

This work presents an experimental program aimed at investigating some aspects of environmental effects on carbon/epoxy laminates with voids. The objective is to obtain experimental data to subsidize acceptance criteria for ultrasonic inspection of composite parts for aerospace applications. A fabrication process that produces laminates with reasonably uniform distribution of voids and nearly constant fibre fraction is described. The interlaminar shear strength of laminates with three different levels of void content was measured at four different conditions: RTD (room temperature, dry), RTW (room temperature, wet), ETW (elevated temperature, wet), and LTW (low temperature, wet). Also, the possibility of extensive degradation of the laminate for a small number of thermal cycles was investigated. Since aerospace applications are considered, a group of specimens was submitted to four cycles of thermal shock between -54°C and 65°C. The experimental results are consistent with the fracture criterion for laminates with voids proposed by Almeida and Nogueira Neto. They postulated mat the presence of voids does not have detrimental effects on the laminate strength unless the void content is above a certain critical level. For laminates not submitted to thermal shock, it was concluded that humidity increases the laminate toughness but reduces its critical void content level. The effect of temperature is to decrease laminate toughness but increase the critical void content level. The four cycles of thermal shock between -54°C and 65°C only have a significant effect on the laminates in RTD condition. For this test condition, it has an effect similar to that of humidity, increasing the laminate toughness but reducing its critical void content level. After thermal shock, laminates under conditions RTD, RTW, and LTW had approximately the same interlaminar shear strength.

A single-lap shear specimen for determining the effect of strain rate on the interlaminar shear strength of carbon fibre-reinforced laminates

A new design of single-lap shear specimen for determining the effect of loading rate on the interlaminar shear strength of laminated composites is described. Finite element analyses are used to optimize the specimen geometry and minimize the variation in the shear stress and the magnitude of the normal stress along the interlaminar failure plane. Experimental results are obtained at a quasi-static and an impact rate of loading for the interlaminar shear strength parallel to the fibres in both unidirectional carbon/epoxy and unidirectional carbon/polyetheretherketone ( peek) laminates and at interfaces across which the fibre orientation is 0°/90° and ±45°. Results for carbon/epoxy laminates are compared with those from an earlier investigation using a double-lap specimen geometry and show a similar small dependence on loading rate. No significant effect of loading rate was observed for the carbon/ peek laminates.