Finite element modelling of bridging micro-mechanics in through-thickness reinforced composite laminates

Delamination and debonding are the major failure modes in laminated composites, which significantly reduce the performance of a structure under compressive or bending loads. To overcome this problem, new composites with through-thickness reinforcement (TTR) have been used to improve the interlaminar strength and damage tolerance of laminated composites [Freitas G, Fusco T, Campbell T, Harris J, Rosenberg S. Z-fiber^TM technology and products for enhancing composite design. In: 83rd Meeting of AGARD SMP, 1996, CP-590; Farley GL, Dickinson LC. Mechanical response of composite materials with through-the-thickness reinforcement. NASA CR-14753, 1993. p. 123–43; Cartié DDR, Partridge IK. Delamination behaviour of Z-pinned laminates In: Williams JG, Pavan A. editors, Proceedings of second ESIS TC4 conference, Les Diablerets, Switzerland, 13–15 September 1999, ESIS Publication, 2000. ISBN 008 043710-9; Greenhalgh E, Hiley M. The assessment of novel materials and processes for the impact tolerant design of stiffened composite aerospace structures. Comp Part A: Appl Sci Manuf 2003;34(2):151–61. [1], [2], [3] and [4]]. Although the TTR can change the structural elastic response of a composite laminate [Stringer LG, Hiley MJ. Through-thickness reinforcement of composites: Z-pinning, Stitching, and 3D weaving. In: 14th International conference for composite materials, ICCM14, 11–14 July, San Diego, CA, 2003; Mouritz AP, Leong KH, Herszberg I. A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Composites Part A 1999;28A:979–91; Grassi M, Zhang X, Meo M. Prediction of stiffness and stresses in z-fibre reinforced composite laminates. Comp Part A: Appl Sci Manuf 2002;33(12):1653–64; Patridge IK, Cartié DDR, Troulis M, grassi, M, Zhang X. Evaluating the mechanical effectiveness of Z-pinning. In: Proceedings of SAMPE/Dayton technical conference, 2003. [5], [6], [7] and [8]] they start working only when delamination propagates in their field, which provides non-linear bridging closure forces that shield the delamination crack from the full delaminating force and moment of the applied loads [Grassi M, Zhang X. Finite element analyses of mode I interlaminar delamination in z-fibre reinforced composite laminates. Comp Sci Technol 2003;63(12):1815–32; Robinson P, Das S. Mode I DCB testing of composite laminates reinforced with z-direction pins: a simple model for the investigation of data reduction strategies. Eng Fract Mech 2004;71(3):345–64. [9] and [10]].Fiber pull-out test has been developed in order to study the micro-mechanics of the TTR bridging a crack under Mode I loading conditions [Cartié DDR, Cox BN, Fleck NA. Mechanisms of crack bridging by composite and metallic rods. Comp Part A: Appl Sci Manuf 2004;35(11):1325–36. [11]], and several analytical models have been developed to analyze these phenomena [Cox B. A constitutive model for through-thickness reinforcement bridging a delamination crack. Adv Comp Lett 1999;8(5):249–56; Jain LK, Mai YW. On the effect of the stitching on Mode I delamination toughness of laminated composites. Comp Sci Technol 1994;51:331–45; Allegri G, Xiang Z. Private communications, 2004. [12], [13] and [14]]. In terms of energy the TTR fiber pull-out can be accompanied by significant amount of energy dissipation due to the frictional work at interface; this process absorbs part of the energy that would otherwise be placed at the delamination front of the structure.The basic objective of this research project was to develop an efficient and accurate finite-element-based numerical tool to simulate the spontaneous propagation of a single TTR pull-out under quasi-static conditions and in the presence of frictional contact between the fiber/matrix interface. The pull-out phenomenon was studied assuming a constant friction coefficient at the interface along the TTR axial direction and using improved contact-elements to solve the frictional contact problem. Load/displacement bridging curves of the fiber pull-out process, which includes elastic deformation with a fully/partially bonded interface plus frictional sliding, were calculated. Moreover, the effect of friction model was also investigated. Numerical results were validated by experimental observations of debonding and frictional sliding of a fiber in steady-state pull-out tests under pure Mode I loading conditions.     

羊栈岭隧道维修加固方案的力学计算分析

羊栈岭隧道围岩软弱破碎，采用钢支撑进行补强加固。根据隧道的断面建立了相应的计算模型，采用荷载结构法对衬砌内的应力进行计算，并基于该计算结果对加固后隧道的稳定性进行了校核。     

Fabrication and characterization of <Emphasis Type="Italic">S. cilliare</Emphasis> fibre reinforced polymer composites

In the recent times, there has been an ever-increasing interest in green composite materials for its applications in the field of industries, aerospace, sports, household etc and in many other fields. In this paper, fabrication of Saccharum cilliare fibre reinforced green polymer composites using resorcinol formaldehyde (RF) as a novel matrix has been reported. A systematic approach for processing of polymer is presented. Effect of fibre loading on mechanical properties like flexural, tensile, compressive and wear resistances has also been determined. Reinforcing of the RF resin with Saccharum cilliare (SC) fibre was done in the form of particle size (200 micron). Present work reveals that mechanical properties of the RF resin have been found to increase up to 30% fibre loading and then decreases. Morphological and thermal studies of the resin, fibre and particle reinforced (P-Rnf) green composites have also been studied.     

Effect of geosynthetic inclusion on the bearing ratio of two-layered soil

In this article, the load-settlement characteristics of unreinforced and reinforced two-layered soil during the loading process are investigated. A series of bearing ratio tests was performed on a granular soil as the base layer overlaying a cohesive soil as the subgrade layer. Three reinforcing conditions (unreinforced, reinforced with nonwoven geotextile, and reinforced with geogrid) at the interface of layers, with four compaction moisture contents (CMCs) of the subgrade layer and three thicknesses of the base layer for both soaked and non-soaked conditions are considered. The results show that the CMC of the subgrade layer has a significant effect on the behavior of two-layered soil, such as swelling amount and the efficiency of the reinforcements. Reinforcing with geogrid resulted in a considerable increase in strength of the soaked samples due to adhesion between geogrids and clayey subgrade layer. For nonwoven geotextiles, strength of the two-layered soil decreased at shallow penetration depths due to reinforcements; and as the penetration increased in depth, the strength also increased. Also, it was found that with decreases in base layer thickness, the test variable's value (i.e., CMC), and the type of geosynthetic reinforcement have significant effects on the behavior of two-layered soil.     

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension-tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412-450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect.     

水电工程三维配筋的应用与思考

阐述了水工建筑物三维配筋的重要意义,介绍了大体积混凝土结构面配筋的基本原理,总结了大体积结构三维配筋的特点,提出了结构模型优化处理的基本方法,并对目前应用中存在的不足提出改进建议。通过三维配筋的应用探索与总结,扩展了三维配筋的适用范围,提高了工作效率。     

用于耐气候花岗岩填充土壤加固的复合土工合成材料的拉伸特性和排水作用

本项研究的最终目的是使加强土墙体系系统化 ,该体系在包含大量低渗透性细小颗粒的风化花岗岩回填土中使用复合土工合成材料。作为达到此目的的阶段性努力 ,进行了试验室的拉伸试验并建立了有限元模型 ,以便分析由抗拉强度较好的土工格栅和排水性能良好的土工布组成的复合土工合成材料的摩擦性能和相互作用。基于有限差分数字模型的分析 ,测试了放在土工格栅下面的土工布的排水性能。从目前的研究可以推断 ,风化花岗岩回填土中的复合土工合成材料或许可用于减少变形和增加抗拉强度 ,并同时具有良好的排水性能     

高阻尼金属基复合材料的发展途径

在综述传统的高阻尼金属材料和金属基复合材料阻尼的特性的基础上，探讨获得高阻尼性能金属基复合材料的途径。     

Effect of silver nanoparticles on the microstructure and mechanical properties of alumina ceramics

In this work, the effect of additions of silver nanoparticles on the microstructure and mechanical properties of alumina-based ceramic was studied. The processing method for the manufacturing of alumina/silver composites imply high energy mechanical milling in a planetary type mill, cold uniaxial compaction and pressureless sintering in an inert atmosphere at 1500°C for 2?h. From the results after the milling step, approximately 45% of the powders have a particle size less than 1?μm. The microstructure observed by SEM is very fine and homogenous; the presence of silver apparently inhibits the grain growth of the alumina. With respect to mechanical properties, increases of the silver content in the matrix causes decrease of Young’s modulus and flexural strength of the composite obtained in 1.5 and 4.6%, respectively. On the other hand, the presence of silver greatly enhances the fracture toughness of alumina, from an increase of 4.2?MPa?m^?0.5 for monolithic alumina to 10?MPa?m^?0.5 for alumina with silver additions of 2?wt- %, representing an improvement of 138% in toughness of alumina. Lastly, hardness for this same sample was incremented in 12%.