Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.     

Fatigue properties of hemp and glass fiber composites

The fatigue properties of nonwoven randomly oriented short hemp fiber mat and chopped strand mat (CSM) glass fiber reinforced polyester composites have been studied, mainly in tension–tension mode. Despite having poorer absolute fatigue strength, the hemp fiber composites exhibited less fatigue sensitivity as compared with the CSM glass fiber composites in tension–tension fatigue. This could be correlated with the lower stiffness degradation observed during fatigue of the hemp fiber composites as compared with the glass fiber composites at the same normalized peak stress levels. Also, images recorded during fatigue loading showed that the hemp fiber composites were better at resisting crack formation and growth than the glass fiber composites. These results suggest that hemp fiber composites have the potential to replace glass fiber composites in applications where components are subjected to fatigue loads but the stress levels are of moderate value. POLYM. COMPOS., 35:1926–1934, 2014. © 2014 Society of Plastics Engineers     

Effect of notch size on the fatigue damage behaviour of toughened epoxy adhesive specimens

ABSTRACT

In the present work the influence of notch size on the fatigue damage behaviour of toughened epoxy adhesive specimens is investigated. Notched and un-notched bulk adhesive specimens were fatigue tested at room temperature under tension-tension cyclic loading at a stress ratio of 0.1. The investigation was based on the analysis of fatigue life (SN) and stiffness degradation curves, which were correlated with notch size and applied stress. Finite element analysis (FEA) was carried out in order to evaluate the notch-dependent stress concentrations. Fatigue results evidenced a reduction of lifespan with increased applied stress amplitude and a possible relationship between the inverse slope of SN curves and notch size. Most notched samples exhibited lower fatigue strength in comparison to un-notched, except in the low cycle fatigue range where un-notched and notched samples had similar fatigue strength. Stiffness degradation showed a correlation with applied stress, i.e. an increase in applied stress was accompanied by faster and stronger degradation. For higher loads, un-notched and 0.2 mm notch samples presented greater stiffness degradation prior to failure than other notched samples.     

Interfacial damage on fatigue‐loaded textile–rubber composites

To reach admissible lifetime expectancy, unidirectional textile–rubber composites must show a strong interface. Usually, it is achieved by coating the textile with Resorcinol–Formaldehyde–Latex (RFL). To evaluate fatigue impact on interfacial properties, composites with or without RFL are tested at different numbers of loading cycles and characterized through peel tests, dynamic mechanical analysis (DMA), scanning electron microscopy and energy‐dispersive X‐rays, and nanoindentation. For composites with RFL, the results indicate two main mechanisms for damaging: propagation of pre‐existing fibrillar microcracks at the rubber/RFL interface completed by adhesive debondings at the RFL/textile interface. At first, the propagation of fibrillar microcracks is correlated with decrease of global composite peeling resistance and contribution of interphase to DMA damping. Then, RFL/textile debondings become critical. They are highlighted with a change of peeling failure surface and could be explained by RFL hardening, highlighted by nanoindentation. This questions the choice for RFL as a sustainable adhesive for composites under fatigue loading. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41346.     

反复荷载下GFRP筋与混凝土黏结性能

对直径为16 mm,埋深分别为4d、5d、8d的玻璃纤维增强(GFRP)筋标准立方体拉拔试件进行静载和反复荷载作用下的拉拔试验,研究了2种不同应力水平（60 %Fm、80 %Fm）的反复荷载作用下GFRP筋与混凝土之间的黏结滑移关系,埋深与黏结强度关系,黏结刚度、加载端滑移量随循环次数的演变规律,得到了反复荷载下黏结滑移滞回曲线变化规律。结果表明,反复荷载下较少的循环次数对黏结强度和滑移量的影响不大;当反复荷载应力水平不高、循环次数较少时,黏结强度没有显著的退化,反而在一定程度上有所增加;较高应力水平反复荷载下,GFRP筋与混凝土之间的黏结强度退化较显著。     

The effect of loading rate on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present work, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) under the loading rate of 1, 10, and 100 mm/min are studied by experimental methods. The interfacial mechanical properties of hybrid composites are determined by monotonic and cycle loading-unloading single-lap-shear tests. The experimental results reveal that interfacial shear strength increases with loading rate, while the shear stiffness shows insensitive to loading rate. A regression function is built to describe the variation of interfacial shear strength with loading rate. The cyclic loading-unloading curves of hybrid composites samples indicate that loading rate effects on the interfacial nonlinear behavior of hybrid composites are considered by affecting plastic deformation. In addition, scanning electron microscopy and digital image correlation observations reveal the failure mechanisms of overmolded hybrid composites. The failure behavior of overmolded hybrid composites is mainly CFRT laminate failure for all cases. The evolution of non-uniform strain fields indicates that the fracture of overmolded thermoplastic composites may initiate at the edges and spread out to the far fields.     

Flexural fatigue behavior and residual strength evolution of SiCnws-C/C composites

The flexural fatigue behavior and residual flexural strength evolution of SiC nanowires reinforced carbon/carbon (SiCnws-C/C) composites were investigated. Specimens were loaded at a stress level of 65% of their static flexural strength for 10⁵, 5?×?10⁵ and 10?×?10⁵ cycles, and their residual flexural strength was increased by 4.87%, 13.73% and 62.45% respectively after cyclic loading. Results indicate that the residual strength after cyclic load is affected by the formation and propagation of cracks, interfacial degradation, as well as the relief of residual thermal stress. An appropriate interfacial debonding and releasing of residual thermal stress are responsible for the large improvement of residual strength of SiCnws-C/C composites after 10?×?10⁵ fatigue cycles. Compared with carbon/carbon composites, SiCnws-C/C composites demonstrate higher mechanical strength and stronger resistance to crack propagation, which are ascribed to the strengthening effect brought by the SiC nanowires, including their pull-out, breaking and bridging.     

2.5维机织复合材料疲劳寿命预测

对2.5维机织复合材料进行了静力分析,在受拉伸载荷的情况下将其视为经纱层和纬纱层组合而成的层合板。以单向板疲劳寿命预测为基础,考虑到疲劳加载过程中刚度退化导致的应力重新分配,采用Miner理论对其拉-拉疲劳寿命进行预测,且对该方法进行了试验验证,为预测2.5维机织复合材料的疲劳寿命提供了一种途径。     

Mechanical behavior and fracture toughness characterization of high strength fiber reinforced polymer textile composites

The mechanical and fracture behavior of polymer composites are the subject of great interest from many years and still interesting among the researchers. Composites are extremely used for their superior mechanical, thermal and fracture toughness properties in various sectors such as automobile, aerospace and defense applications. In this article, unidirectional and woven high strength glass, carbon and Kevlar fiber reinforced polymer textile composites are taken into consideration for the comprehensive review of mechanical behavior and fracture toughness characterization. Current review work began with the introduction to polymer textile composites with its manufacturing stages, processing techniques and factors affecting the performance under mechanical loading. The mechanical behavior of high strength fiber reinforced polymer (HSFRP) textile composites was discussed in tension, compression, flexural, low velocity and high velocity impact loading with the recent numerical and experimental characterization studies. Textile geometrical modeling and CAE tools are also described for numerical characterization. Under the influence of mechanical loading on composites, failure occurs actually due to the crack initiation and propagation, so it is also required to characterize. Significant elements of fracture mechanics are well described for the better understanding of fracture toughness characterization. Mode-I, Mode-II, Mode-III interlaminar and Mode-I intralaminar fracture toughness characterization are widely explained by considering the effect of filler content, fiber orientation and fiber volume fraction. Fracture toughness characterization techniques and research summery are uniquely presented by considering various factors under one umbrella for better understanding of fracture behavior. Statistical Weibull distribution is also presented for the failure prediction of composites.

     

Effects of low-energy impact and thermal cycling loadings on fatigue behavior of the quasi-isotropic carbon/epoxy composites

Effects of low-energy impact loading and thermal cycling on fatigue behavior of carbon fiber reinforced epoxy (carbon/epoxy) laminates are examined. A low-energy of 0.62 Joules was adopted to impact carbon/epoxy laminates prior to thermal cycling exposure and fatigue test. The temperature ranged between 60 and −60 °C for thermal cycling and the stress ratio of 0.1 with a frequency of 3 Hz for fatigue loading were used. Impact performances were tested on the virgin specimens and the thermal-cycling exposure specimens. Residual tensile strength and fatigue tests were performed on the laminate composites after being subjected to thermal cycling. The relationship between tensile strength reduction and fatigue performance after thermal cycling was investigated. Stiffness degradation during fatigue testing was monitored; the differences in stiffness for these three composites (virgin specimens, low-energy impacted specimens, low-energy impacted and thermal-cycling exposure specimens) were compared and the coupling effects of low-energy impact and thermal fatigue were studied. Furthermore, the S-N curves were also plotted and the variation was compared on the aforementioned three composites. SEM was used to examine the difference in fracture morphologies on the composites with and without suffering low-energy impact and thermal fatigue.     

Effects of debonding and fiber strength distribution on fatigue‐damage propagation in carbon fiber‐reinforced epoxy

In order to design new fatigue‐resistant composites, the underlying fatigue damage mechanisms must be characterized and the controlling microstructural properties should be identified. The fatigue‐damage mechanisms of a unidirectional carbon fiber–reinforced epoxy has been studied under tension–tension loading. A ubiquitous form of damage was one or a few planar fiber breaks from which debonds or shear yield zones grew in the longitudinal direction during fatigue cycling. This leads to a change in stress profile of the neighboring fibers, and an increase in failure probability of these fibers. The breakage of fibers in the composite is controlled by the fiber strength distribution. The interaction between the fiber strength distribution and debond propagation leading to further fiber breakage was investigated by a numerical simulation. It was found that a wider distribution of fiber strength and a higher debond rate lead to more distributed damage and a higher fracture toughness. Implications to fatigue life behavior are discussed, with reference to constituent microstructure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 457–474, 2000     

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。     

Long‐term hygrothermal response of perforated GFRP plates with/without application of constant external loading

The use of glass fiber‐reinforced polymer (GFRP) composites is increasingly being considered in various applications where the composite is subjected to harsh hot and humid conditions. Although information on the performance of GFRP under hot and humid conditions is available, the characteristics and the response of perforated GFRP under such conditions have not been fully explored. In this article, the response of perforated GFRP plates subject to hot and humid environment is examined. The applicability and accuracy of Fick's model for establishing the amount of moisture absorption by such composites is examined, and an improved model is proposed. The article also demonstrates the influence of constant external loading on such perforated GFRP while undergoing conditioning in a hot and humid environment. Moreover, since the strength and stiffness of composites can be significantly affected by harsh environments, the degradation in the strength and stiffness of the perforated GFRP as a function of time is established. A new model is proposed wherein degradation of the strength of such perforated composites may be established as a function of time and geometric entities. The model can also account for the influence of the applied loading. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

硅橡胶复合材料的多轴疲劳寿命计算及优化设计

基于裂纹扩展理论,采用Python语言开发了橡胶的多轴疲劳寿命算法并进行了试验验证,并计算了硅橡胶哑铃形单轴拉伸试样、平面拉伸试样及十字形等双轴拉伸试样的疲劳寿命;为兼顾硅橡胶复合材料的发电性能和疲劳寿命,采用总能量密度(PI)来评估复合材料的综合性能.结果表明,通过PI评估发现,等双轴拉伸下应变能密度(W)最大,开裂...     

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiC_f/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.     

Enhanced fatigue life of carbon nanotube‐reinforced epoxy composites

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the residual mechanical properties of epoxy composites after different degrees of fatigue have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (25–50 MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The residual strength and modulus were measured after loading at 30 MPa for 5000, 15,000, and 25,000 cycles. The incorporation of a small amount of CNTs increased the fatigue life of epoxy in the high‐cycle, low‐stress‐amplitude regime by 1550%. Micrographs indicate the key mechanisms for enhancement in fatigue life such as CNT crack‐bridging and pullout. POLYM. ENG. SCI., 52:1882–1887, 2012. © 2012 Society of Plastics Engineers     

Damage modeling of oxide/oxide ceramic matrix composites under cyclic loading conditions

Ceramic matrix composites exhibit optimal high temperature property and complex nonlinear behaviors including irreversible deformations and stiffness degradation under different mechanical loading conditions. In the present work, the damage evolution of the material under multi-axial loads considering effects of loading-unloading cyclic was studied and a novel continuum damage constitutive model was proposed. The material degradation was decomposed into monotonic damages and cyclic damages. The anisotropic hardening behavior of the material was considered by taking orientation dependence into account. Compared to the experimental results, the constitutive model could accurately predict the stress-strain response and stiffness degradations of the oxide/oxide ceramic matrix composites for monotonic loading and cyclic loading.     

Model for Cyclic Fatigue of Quasi-Plastic Ceramics in Contact with Spheres

A model of contact damage accumulation from cyclic loading with spheres and ensuing strength degradation in relatively tough, heterogeneous ceramics is developed. The damage takes the form of a quasi-plastic zone beneath the contact, consisting of an array of closed frictional shear faults with attendant "wing" microcracks at their ends. Contact fatigue takes place by attrition of the frictional resistance at the sliding fault interfaces, in accordance with an empirical degradation law, allowing the microcracks to extend. At large numbers of cycles or loads the microcracks coalesce, ultimately into radial cracks. Fracture mechanics relations for the strength degradation as a function of number of cycles and contact load are derived. Indentation–strength data from two well-studied coarse-grain quasi-plastic ceramics, a micaceous glass-ceramic and a silicon nitride, are used to evaluate the model. Comparative tests in static and cyclic contact loading confirm a dominant mechanical component in the fatigue. At the same time, the presence of water is shown to enhance the fatigue. The model accounts for the broader trends in the strength degradation data, and paves the way for consideration of key variables in microstructural design for optimum fatigue resistance.     

The mechanical performance of cross-plied composites

The mechanical performance of fiber glass epoxy, cross-plied laminates having either ductile or brittle matrices, was evaluated in tension. The laminates with ductile matrices have higher initial strength, slower crack propagation in the transverse layers and higher ultimate stress and strain. In both the ductile and brittle systems, the laminae have higher strength, stiffness and toughness than equivalent unidirectional composites. This improved performance results from the interaction between the perpendicular layers which gives them additional stiffness due to shear and transverse coupling effects and also increases the resistance of each individual layer to crack propagation and plastic flow.     

不同结构纺织复合材料准静态侵彻实验分析及有限元模拟

本文研究了两种不同结构三维结构纺织复合材料——三维正交机织玻璃纤维,不饱和树脂复合材料和双轴向纬编针织玻璃纤维／不饱和树脂复合材料在MTS材料试验机上的准静态侵彻测试。以纯铝MTS实验数据为标定,分析了准静态侵彻载荷一位移曲线及其破坏机理,比较了不同结构纺织复合材料以及纯铝的位移一载荷曲线,由此计算得到位移与吸能关系曲线。同时根据复合材料各自织物中纤维束排列及织物成型特点,分别建立了复合材料的细观结构模型和单胞模型。编写用户子程序（VUMAT）,用ABAQUS软件进行了有限元模拟。结果表明：三维纺织复合材料各自损伤结果和载荷-位移曲线与实验结果吻合较好,证明有限元的有效性。三维正交结构复合材料抗侵彻能力优于双轴向纬编针织复合材料,但是破坏过程中其抗侵彻能力幅值变化差异大,没有针织复合材料抗侵彻能力稳定。