树脂基复合材料蠕变性能研究进展

树脂基复合材料在室温条件下的蠕变被认为是制约其更为广泛应用的主要瓶颈之一。本文综述了复合材料蠕变性能的研究进展,讨论了纤维、基体的蠕变性能以及温度、外部应力和材料性质对复合材料蠕变性能的影响规律。探讨了复合材料的蠕变机理,分析了目前常用的模拟和预测蠕变行为的模型,并就其适用范围进行了讨论,展望了该领域研究的发展态势。     

木塑复合材料蠕变性能的研究进展

简要介绍了材料的蠕变特性以及木塑复合材料蠕变性能较差的原因,并从影响木塑复合材料蠕变性能的温度、应力及加载方式、湿度等因素以及抗蠕变性研究两方面对木塑复合材料蠕变性能的研究进展进行了综述,展望了今后木塑复合材料蠕变性能研究的重点及方向.     

纤维增强塑料蠕变机理的初步探讨

对玻璃钢蠕变试验的研究,国内已有20年历史,国外也有近40年历史。但是,直到目前还未见到进行比较长时间的蠕变试验资料,对蠕变机理也没有见到比较深入研究的文章。本文着重讨论玻璃纤维增强塑料(玻璃钢)的蠕变机理。 一、纤维的蠕变性能 纤维增强塑料的蠕变性能与组份材料的蠕变性能、组份比、成型工艺、应力状态、应力方向等因素有关。目前使用的纤维,主要是玻璃、碳、硼、Kevlar等纤维。除Kevlar有机纤维有较明显的蠕变外,其它纤维在常温下没有或很少有蠕变。例如,为了观察玻璃纤维的蠕     

一种基于吸水树脂的多孔混凝土蠕变性能试验研究

介绍了一种基于吸水树脂的多孔混凝土(SFIC)材料及其制备原理,迚行抗压性能试验和90d蠕变性能试验,幵与普通混凝土迚行对比,分析在不同荷载作用下的蠕变行为。结果表明:SFIC弹性模量约为普通混凝土的1/3,应力变形曲线有明显的残余应力平台;SFIC和普通混凝土在的蠕变变形在90 d基本收敛,SFIC蠕变变形更显著且与荷载水平、弹性模量密切相关。采用蠕变度、蠕变系数对SFIC的蠕变行为迚行分析,幵对长期蠕变系数迚行了拟合预测。     

Al2O3和Cr2O3对镁质材料高温蠕变性能的影响

以电熔镁砂为主要原料,研究了添加剂α-Al2O3和Cr2O3的加入量及加入形式对镁质材料高温蠕变性能的影响,还研究了在加入3%Cr2O3的基础上添加不同形式的Al2O3对镁质材料高温蠕变性能的影响。研究结果表明:随着Al2O3含量的增加,镁质材料的抗高温蠕变性能增强;添加3%Cr2O3的镁质材料其抗蠕变性能优于热风炉上使用的低蠕变高铝制品。     

聚苯乙烯蠕变失效的研究

利用蠕变性能测试及DSC、SEM等手段考察了不同条件(温度、应力、紫外光)下聚苯乙烯(PS)的蠕变行为及其失效机理。结果表明:蠕变温度越高,PS的蠕变失效速度越快,遵循时温等效的原理。但在不同的温度区间,PS蠕变的机理并不同;随着蠕变应力的增加,PS的蠕变失效加快,抗蠕变性能下降;紫外光使PS蠕变的速率明显加快,蠕变失效时间明显缩短。     

聚碳酸酯归—化应力弛豫函数和蠕变函数

对聚碳酸酯的应力弛豫性能和应力蠕变性能进行了实验研究,获得了应力弛豫和应力蠕变曲线,得到了聚碳酸酯的归一化应力弛豫函数和蠕变函数。     

聚碳酸酯归一化应力弛豫函数和蠕变函数

对聚碳酸酯的应力弛豫用应力蠕变性能进行了实验研究,获得了应力弛豫和应力蠕变曲线,得到了聚碳酸酯的归一化应力弛豫函数和蠕变函数。     

用C形试样研究HK——40炉管蠕变裂纹扩展

高温下工作的炉管因蠕变裂纹的扩展导致了炉管的失效.本文采用C形试样进蠕变裂纹扩展试验,并用弹性应力强度因子K,净截面应力σ_(net)以及蠕变条件下的能量积分C~*,对获得的不同温度和不同载荷下的裂纹扩展数据进行处理,结果表明:C~*能够较好地描述蠕变裂纹扩展的整个过程.把材料的蠕变性能与裂纹扩展性能联系在一起,进一步讨论了温度的影响.对炉管受力情况进行简化,把断裂力学的方法应用到炉管的残余寿命估计中去,获得了初步的结果.     

Al2O3f/Mg-6Al-0.5Nd-1Gd复合材料的高温压缩蠕变行为研究

研究了Al_2O_(3f)/Mg-6Al-0.5Nd-1Gd复合材料的高温蠕变行为。结果表明:随着蠕变温度由483K升高到513K,外加应力从60MPa增大到90MPa,复合材料的稳态蠕变速率增加,压缩蠕变性能变差;复合材料的蠕变行为主要受到位错攀移控制;复合材料中由于Al_2O_3短纤维增强相的添加,提高了材料的蠕变表观激活能Q_c和蠕变表观应力指数n;复合材料在载荷恒定时,随着蠕变温度由483K提高到513K,复合材料材料的蠕变门槛应力由57.2MPa降低为32.3MPa。     

FRP应力松弛及徐变性能的研究近展

本文阐述了FRP应力松弛、徐变性能的研究意义,总结了国内外关于应力松弛、徐变性能的最新研究成果及主要影响因素,并探讨了应力松弛和徐变的计算模型,对未来FRP长期性能研究的发展方向做出了展望。     

FRP筋长期力学性能研究进展

 FRP筋具有优异的耐腐蚀性能,是替代普通钢筋和预应力钢筋用于腐蚀环境和特殊工程的最佳选择之一。同 时,FRP筋又具有抗拉强度高、疲劳性能优、徐变松弛性能好等优良力学性能。本文在系统地查阅国内外代表性研 究文献的基础上,对FRP筋的长期力学性能进行了研究和总结。     

Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

Analysis of the nonlinear creep behavior of concrete/FRP-bonded assemblies

This paper investigates the creep behavior of adhesively bonded concrete/fiber-reinforced polymer (FRP) joints, through experimental and modeling approaches. The first part proposes a methodology for predicting the long-term creep response of the bulk epoxy adhesive; such a procedure consists of (1) performing short-term tensile creep experiments at various temperatures and stress levels, (2) building the creep compliance master curves according to the time–temperature superposition principle in order to assess the long-term evolution for each stress level, and (3) developing a rheological model whose parameters are identified by fitting the previous master curves. In our case, it was found that master curves (and, consequently, parameters of the rheological model) are dependent on the applied stress level, highlighting the nonlinear creep behavior of the bulk epoxy adhesive. Therefore, evolution laws of the model parameters were established to account for this stress dependence. The second part focuses on the creep response of the concrete/FRP assembly in the framework of a double lap joint shear test configuration. Experiments showed that creep of the adhesive layer leads to a progressive evolution of the strain profile along the lap joint, after only one month of sustained load at 30% of the ultimate strength. Besides, a finite element approach involving the abovementioned rheological model was used to predict the nonlinear creep behavior of the bonded assembly. It confirmed that creep modifies the stress distribution along the lap joint, especially the stress value at the loaded end, and leads to a slight increase in the effective load transfer length. This result is of paramount interest since the transfer length is a key parameter in the design of FRP-bonded strengthening systems. Moreover, instantaneous and long-term calculated strain profiles were found in fair agreement with experimental data, validating the modeling approach.     

Limiting shear creep of epoxy adhesive at the FRP–concrete interface using multi-walled carbon nanotubes

Fiber reinforced polymer (FRP) composites are widely used in structural strengthening and retrofitting due to their high strength-to-weight ratio and non-corrosive properties. However, one of the recently recognized drawbacks of common FRP strengthening systems is the relatively high shear creep deformation of epoxy adhesives when FRP sheets are used to strengthen concrete structures against sustained loads. On the other hand, carbon nanotubes (CNTs) are reported to provide significant enhancement to various mechanical properties when used in epoxy adhesives. This enhancement is attributed to the extraordinary mechanical properties of the CNTs and their ability to bond to epoxy. In this article, we report the results of experimental and analytical investigations conducted to examine shear creep behavior of multi-walled carbon nanotubes (MWCNTs) reinforced epoxy nanocomposite used at the FRP–concrete interface. Double shear tests were performed on FRP sheets bonded to concrete blocks with MWCNTs reinforced epoxy nanocomposite. Various levels of pristine and functionalized MWCNTs by weight were examined including 0.1%, 0.5%, 1.0% and 1.5%. The viscoelastic behavior of MWCNTs reinforced epoxy nanocomposite was simulated with rheological models and the models' parameters were extracted and discussed. The results show the ability of MWCNTs to significantly reduce creep compliance of epoxy at the FRP–concrete interface making it a viable solution if FRP is used to strengthen concrete structures subjected to sustained stress.     

Tensile creep of a structural epoxy adhesive: Experimental and analytical characterization

Epoxy adhesives are nowadays being extensively used in Civil Engineering applications, mostly in the scope of the rehabilitation of reinforced concrete (RC) structures. In this context, epoxy adhesives are used to provide adequate stress transference from fibre reinforced polymers (FRP) to the surrounding concrete substrate. Most recently, the possibility of using prestressed FRPs bonded with these epoxy adhesives is also being explored in order to maximize the potentialities of this strengthening approach. In this context, the understanding of the long term behaviour of the involved materials becomes essential. Even when non-prestressed FRPs are used a certain amount of stress is permanently applied on the adhesive interface during the serviceability conditions of the strengthened structure, and the creep of the adhesive may cause a continuous variation in the deformational response of the element. In this context, this paper presents a study aiming to experimentally characterize the tensile creep behaviour of an epoxy-based adhesive currently used in the strengthening of concrete structures with carbon FRP (CFRP) systems. To analytically describe the tensile creep behaviour, the modified Burgers model was fitted to the experimental creep curves, and the obtained results revealed that this model is capable of predicting with very good accuracy the long term behaviour of this material up to a sustained stress level of 60% of the adhesive׳s tensile strength.     

BFRP约束方形混凝土柱轴心受压强度模型

目前,FRP(纤维增强复合材料)约束混凝土强度模型大部分都是建立在FRP约束混凝土圆形柱基础之上.而FRP约束混凝土方形柱的侧向约束力受截面形状的影响较大,与圆形柱相比,由于拐角的应力集中导致的侧向约束应力不均匀等,使得FRP包裹方形柱的侧向约束效果明显降低,强度也随之下降.本文基于FRP约束圆柱体的强度公式,考虑方形柱的有效核心区、拐角效应的影响,引入对应的影响系数,建立了BFRP(玄武岩纤维复合材料)约束方形混凝土柱轴心抗压强度模型并与试验数据对比分析,结果表明,该模型具有良好的吻合度.     

带有大开口的船用玻璃钢夹层板结构力学性能分析

玻璃钢复合材料作为一种新型的工业材料,被广泛应用于包括造船行业在内的诸多领域当中。在船体设计时,为了避免因碰撞等外力因素造成船体损伤,需要事先对其相关结构的力学性能进行数值分析。以带有大开口的玻璃钢游艇舷侧夹层板架结构作为研究对象,对该结构的相关力学性能进行了有限元分析,并对其位移变形量和应力分布情况进行了校核。按照此种方式,分别计算了不同大小的静压力载荷、不同大小的冲击载荷、不同的开口形状和尺寸以及不同的载荷作用位置等诸多工况下的仿真算例,分析、比较其计算结果,最终得出结论。     

纤维增强复合材料疲劳性能的温度效应

纤维增强复合材料(FRP)因其轻质高强、耐腐蚀等突出优势受到广泛的关注,但其疲劳性能受材料特性、环境条件和载荷条件影响较大。基于唯象学刚度退化理论,研究了FRP材料的疲劳性能在不同温度和应力水平下的变化规律,推导了FRP材料基于温度变化的刚度退化和疲劳寿命预测等效模型,并在已有试验数据基础上对该模型进行了验证,并将之应用于E型玻璃纤维平纹编织层状材料的疲劳性能预测。结果表明:该模型能有效预测FRP材料的刚度退化规律和等效剩余疲劳寿命;FRP材料疲劳性能的温度效应明显,其影响程度甚至可能超过应力幅的影响。