Dynamic stress analysis of the interface in a particle-reinforced composite

ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｅｉｎｔｅｒｆａｃｉａｌｄｅｂｏｎｄｉｎｇｍａｙｃａｕｓｅｔｈｅｍｉｃｒｏｖｏｉｄｓ’ｎｕｃｌｅａｔｉｏｎｉｎａｐａｒｔｉｃｌｅ＿ｒｅｉｎｆｏｒｃｅｄｃｏｍｐｏｓｉｔｅ .Ｔｈｅｉｎｔｅｒｆａｃｉａｌｄｅｂｏｎｄｉｎｇｉｓｕｓｕａｌｌｙｇｏｖｅｒｎｅｄｂｙｔｈｅｔｅｎｓｉｏｎｓｔｒｅｓｓｃｒｉｔｅｒｉｏｎ[1,2 ]ｏｒｅｎｅｒｇｙｃｒｉｔｅｒｉｏｎ[3,4].Ｉｎｔｈｅｒｅｃｅｎｔｒｅｓｅａｒｃｈｗｏｒｋｓ,ｔｈｅｐｒｅｓｅｎｔａｕｔｈｏｒｓｐｏｉｎｔｅｄｏｕｔｔｈａｔｔｈｅａｎ…     

3D FINITE ELEMENT ANALYSIS OF THE DAMAGE EFFECTS ON THE DENTAL COMPOSITE SUBJECT TO WATER SORPTION

The damage effects of water sorption on the mechanical properties of the hydroxya-patite particle reinforced Bis-GMA/TEGDMA copolymer (HA/Bis-GMA/TEGDMA) have been predicted using 3D finite cell models. The plasticizer effect on the polymer matrix was considered as a variation of its Young's modulus. Three different cell models were used to determine the influence of varying particle contents, interphase strength and moisture concentration on the debonding damage. The stress distribution pattern has been examined and the stress transfer mode clarified. The Young's modulus and fracture strength of the Bis-GMA/TEGDMA composite were also predicted using the model with and without consideration of the damage. The former results with consideration of the debonding damage are in good agreement with existing literature experimental data. The shielding effect of our proposed model and an alternative approach were discussed. The FCC cell model has also been extended to predict the critical load for the damaged and the undamaged composite subject to the 3-point flexural test.     

复杂条件下FRP-混凝土界面剥离机理研究分析

从理论模型、试验研究和数值模拟讨论了影响FRP-混凝土界面粘结性能的关键因素, 分析了环境因素和加载速率对FRP-混凝土界面粘结性能以及极限承载力的影响, 指出了复杂条件下FRP-混凝土界面分析亟待解决的关键问题.     

FRP-混凝土界面剥离破坏过程并行数值模拟

FRP-混凝土界面粘结性能和抗拉裂能力是外贴FRP片材加固混凝土结构技术的关键问题。基于FRP与混凝土界面面内剪切试验的结果,采用材料真实破裂过程三维并行分析(RFPA3D-Parallel)系统,对FRP-混凝土界面的粘结性能进行了三维并行数值模拟研究。数值试验再现了FRP-混凝土构件的三维破裂过程演化过程,清晰地反映了拉伸载荷作用下FRP-混凝土构件界面剥离破坏的规律,FRP-混凝土界面剥离破坏是一个细观损伤不断产生和宏观裂缝形成的渐进过程,可通过监测FRP-混凝土结构损伤演化过程的声发射来揭示FRP-混凝土结构在外载荷作用下的损伤程度。FRP片材在加载过程中的变形剥离破坏过程大致可以划分为四个阶段：(1)弹性变形阶段;(2)弹性软化阶段;(3)弹性软化剥离阶段;(4)软化剥离阶段。本文的数值计算表明RFPA3D-Parallel并行数值模拟方法为FRP片材-混凝土界面剥离破坏过程和机理研究提供了一个很好的途径,同时也为研究FRP-混凝土工程结构等的损伤断裂机理提供了一个新的分析手段,这对于土木建筑工程中FRP-混凝土结构的工程设计施工、损伤断裂控制及混凝土结构加固等研究无疑具有重要的理论指导和实践意义。     

微尺度金属薄膜屈曲稳定性的实验研究

微尺度金属薄膜的脱粘和屈曲严重影响着膜基结构的性能和使用寿命。本文对微尺度的金属铜薄膜在残余应力和外部压力共同作用下的脱粘屈曲和后屈曲模式进行了研究,用自行设计的单轴对称加载装置进行压力加载,用一台光学显微镜观察薄膜表面的屈曲形貌。在外力作用下薄膜会出现垂直于加载方向的直线型屈曲,但在外力卸载过程中该屈曲并不稳定,会演化成电话线型屈曲,完全卸载后形成泡状屈曲。再次加载后,恢复到直线型屈曲。研究表明:直线型屈曲的不稳定现象主要与薄膜的残余应力、基底的泊松比以及薄膜沿纵向与横向的应力比有关。     

单纤维段裂试验评述

单纤维段裂试验作为复合材料界面剪切强度的一种测试方法被沿用至今.但是, 这种方法的可信度已受到一些研究者的质疑.为了明确单纤维段裂试验的问题, 本文首先对试验技术、试验结果分析等方面作了概述, 并指出: 纤维段裂的饱和状态是单纤维段裂试验的终点标志,以及临界长度是由试验得到的唯一数据, 而这二点是这种试验方法独具的特点, 同时也是这种试验方法难以克服的缺陷.在单纤维段裂试验中, 按照纤维段界面端处的局部损伤模式, 有3种界面端应力奇异性分析的问题需要予以考虑:(1)纤维断裂, 基体没有开裂, 和界面没有脱粘;(2)纤维断裂, 基体开裂, 但界面没有脱粘;(3)纤维断裂, 界面脱粘, 基体已开裂或基体未开裂.在单纤维段裂试验的界面端应力奇异性分析的基础上, 本文对单纤维段裂试验的可靠性进行了研究.结论是: 任何纤维和基体组成的复合材料的单纤维段裂试验都存在界面端应力奇异性, 这就排除了用单纤维段裂试验测定界面剪切强度的可能性.      

Numerical Investigation of the Optimum Placement Locations of Optical Fiber Bragg Grating Sensors for the Health Monitoring of Bonded Composite Repairs

the use of optical Fiber Bragg Grating sensors (FBG) embedded into composite laminates has proven to be an efficient way for the monitoring of the strain field inside composite laminates. The current work focuses on the use of such sensors for structural health monitoring of bonded composite repairs, often used in the aerospace as well as in the maritime industry. In such applications, the embedding location of the FBG sensors is of great importance, since apart from general strain measurements of the composite, they provide the ability of detecting whether a crack has propagated in the metal or a debonding has occurred between the metal and the composite. Within the current work, a representative case of bonded composite repair of a cracked metallic structure is examined and a series of 3D elasticity linear static finite element models are developed. Various crack lengths and patch disbond sizes have been studied while varying strain distributions were recorded along numerous paths inside the structure. Having recorded and analyzed these strain distributions, certain guidelines have been derived concerning the optimum FBG placement inside the patch, in order to ensure efficient structural health monitoring.     

土遗址楠竹锚固界面传力过程研究

竹木锚固技术近年来被广泛应用于土遗址文物加固保护中,但其锚固界面传力机理尚不明确,严重制约着锚固技术的科学化、规模化应用.相关研究成果已经证实了合理的锚固界面粘结-滑移模型对锚固系统性能预测的重要性,本文在此基础上以楠竹-改性泥浆-夯土锚固系统为例,基于考虑完全脱粘现象的三线型粘结-滑移模型开展了锚固界面传力全过程研究.首先将锚固界面传力全过程分为6个连续阶段,分别对各阶段对应的界面应力、应变分布与演化过程进行理论解析,推导了锚杆轴向变形、界面滑移量、界面剪应力、界面剪应变等参数的封闭解,同时给出了极限锚固力与有效锚固长度的计算方法.在此基础上,通过识别载荷-位移曲线特征点对粘结-滑移模型参数进行了标定.最后采用两个土遗址原位拉拔试验对理论解析模型的合理性进行了对比验证,同时着重分析了锚固长度与锚杆轴向刚度两个因素对锚固性能的影响规律.本文提出的解析模型对存在完全脱粘现象的锚固界面传力过程分析具有广泛适用性,能够为土遗址锚固工程设计提供参考与指导.      

Nondestructive examination of polymethacrylimide composite structures with terahertz time-domain spectroscopy

THz reflective time domain spectroscopy (THz-RTDS) has been considered as an effective method to detect hidden objects with potential for supplementing other NDE technologies for foam composite adhesive structure debonding defects. PMI (Polymethacrylimide) is a heat-resistant foam material, with the highest strength and stiffness to weight ratio. It is widely used in various parts of airplanes, especially the wing leading edge and rudder, landing gear doors, wing-body/wingtip fairings and so on. We analyzed the features of adhesive debonding defect based mainly on the variation of the time-domain wave form and compared with the inclusion defect. The quantification of degrees of adhesive debonding can be readily achieved with THz-RTDS images based on the delay of the wave front and the main reflection time-domain waveforms. Typical accuracy of about 100 μm was achieved.     

Dynamic fiber debonding and frictional push-out in mode composite systems: Experimental observations

In the present work a modified Split Hopkinson Pressure Bar (SHPB) system is adopted to perform dynamic fiber push-out experiments on model single fiber composite systems. A tapered punch and a support connect a monofilament composite with the incident and transmitted bars of the SHPB. The tapered punch is used to apply compressive loading to a single fiber (either steel or aluminum) embedded in a surrounding matrix material (EPON 862). The SHPB allows real time measurement of relative fiber/matrix displacement and push-out force, as the debonding and push-out event progresses. Using this technique we have studied the effect of loading rate, material mismatch, fiber length, and surface roughness on the push-out event. It was seen that maximum push-out force increases with increasing loading rate. In addition dynamic interfacial strength and toughness is highly dependent on fiber surface roughness. Results from a finite element analysis incorporating a cohesive failure model were used to extract interface strength and toughness values. It was found that the particular aluminum/EPON interface used is characterized by a dynamic shear failure strength of 48±8 MPa, a mode II fracture toughness of 160±40 N/m, and a friction coefficient of 0.2 at a sliding rate of 6 m/s. For the rates tested here these quantities were found to be approximately constant.