Parametric study on contact sensors for MASW measurement-based interfacial debonding detection for SCCS

Steel-concrete composite structures(SCCS)have been widely used as primary load-bearing components in large-scale civil infrastructures.As the basis of the co-working ability of steel plate and concrete,the bonding status plays an essential role in guaranteeing the structural performance of SCCS.Accordingly,efficient non-destructive testing(NDT)on interfacial debondings in SCCS has become a prominent research area.Multi-channel analysis of surface waves(MASW)has been validated as an effective NDT technique for interfacial debonding detection for SCCS.However,the feasibility of MASW must be validated using experimental measurements.This study establishes a high-frequency data synchronous acquisition system with 32 channels to perform comparative verification experiments in depth.First,the current sensing approaches for high-frequency vibration and stress waves are summarized.Secondly,three types of contact sensors,namely,piezoelectric lead-zirconate-titanate(PZT)patches,accelerometers,and ultrasonic transducers,are selected for MASW measurement.Then,the selection and optimization of the force hammer head are performed.Comparative experiments are carried out for the optimal selection of ultrasonic transducers,PZT patches,and accelerometers for MASW measurement.In addition,the influence of different pasting methods on the output signal of the sensor array is discussed.Experimental results indicate that optimized PZT patches,acceleration sensors,and ultrasonic transducers can provide efficient data acquisition for MASW-based non-destructive experiments.The research findings in this study lay a solid foundation for analyzing the recognition accuracy of contact MASW measurement using different sensor arrays.

Sandstone-concrete interface transition zone (ITZ) damage and debonding micromechanisms under freeze-thaw

The sufficient bond between concrete and rock is an important prerequisite to ensure the effect of shotcrete support. However, in cold regions engineering protection system, the bond condition of rock and concrete surface is easily affected by freeze-thaw cycles, resulting in interface damage, debonding and even supporting failure. Understanding the micromechanisms of the damage and debonding of the rock-concrete interface is essential for improving the interface protection.Therefore, the micromorphology, micromechanical properties, and microdebonding evolution of the sandstone-concrete interface transition zone(ITZ) under varying freeze-thaw cycles(0, 5, 10, 15, 20) were studied using scanning electron microscope, stereoscopic microscope, and nano-indentation. Furthermore, the distribution range and evolution process of ITZ affected by freeze-thaw cycles were defined. Major findings of this study are as follows:(1) The microdamage evolution law of the ITZ under increasing freeze-thaw cycles is clarified, and the relationship between the number of cracks in the ITZ and freeze-thaw cycles is established;(2) As the number of freeze-thaw cycles increases, the ITZ's micromechanical strength decreases, and its development width tends to increase;(3) The damage and debonding evolution mechanisms of sandstone-concrete ITZ under freeze-thaw cycles is revealed, and its micromechanical evolution model induced by freeze-thaw cycles is proposed.

Numerical Simulation of Particle/Matrix Interface Failure in Composite Propellant

Interface debonding between particle and matrix in composite propellant influences its macroscopic mechanical properties greatly. For this, the laws of interface cohesive damage and failure were analyzed. Then, its microscopic computational model was established. The interface mechanical response was modeled by the bilinear cohesive zone model. The effects of interface properties and particle sizes on the macroscopic mechanical behavior were investigated. Numerical simulation of debonding damage evolution of composite propellant under finite deformation was carried out. The debonding damage nucleation, propagation mechanism and non-uniform distribution of microscopic stress-strain fields were discussed. The results show that the finite element simulation method based on microstructure model can effectively predict the trend of macroscopic mechanical behavior and particle/matrix debonding evolution process. It can be used for damage simulation and failure assessment of composite propellants.

Mechanical Characterisation of Interface for Steel/Polymer Composite Using Pull-out Test: Shear-Lag and Frictional Analysis

Fibre-matrix interface is known to have contribution to the mechanical performance of fibre-reinforced composite by its potential for load transfer between the fibre and the matrix. Such load transfer is of great importance in dentistry when a post is used for fixing a ceramic crown on the tooth. In this study, a pull-out test was carried out to analyse the interfacial properties of a steel fibre embedded in a polyester and epoxy matrices. It was found that the fibre-matrix interface is debonded on the whole embedded length when the fibre stress reached the debonding stress. Then, the fibre stress fell down to the initial extraction stress required to pulling out the debonded fibre from the matrix. Both debonding stress and initial extraction stress initiated a linear increase with the implantation length after the debonding stress reached horizontal asymptotes. To analyse the fibre-matrix load transfer before debonding, an analytical shear-lag model was adopted to in this test conditions. Fitting the experimental results with the analytical model provided the interfacial shear strength. By considering the Coulomb friction at the fibre-matrix interface during the fibre extraction process, an analytical model which considers Poisson＇s effects on both fibre and matrix, was developed. In this model, knowledge of the initial extraction stress of the fibre provides the residual normal stress at the fibre-matrix interface.

Effects of interface adhesive and resin cohesive strength on tensile strength of resin sand based on numerical analysis

A plane-strain unit-cell finite element model was proposed to study the effects of resin/sand interface adhesive and resin cohesive strength on the overall tensile strength of resin sand,as well as the fracture modes.The main micro-scale characteristics of the numerical model were extracted from the micrograph of resin sand specimens by three-dimensional X-ray microscopy(3 D-XRM).The extended finite element method(XFEM)and cohesive behavior method were employed to explicitly describe the resin fracture and sand/resin interface debonding,separately.The corresponding experimental observation of micro-scale failure behavior based on the scanning electron microscopy(SEM)was presented for a comparison.The numerical results show that the initial failure of the model occurs at the sand/resin interface,followed by consequent resin failure.Dependent on the resin cohesive strength,the location of resin failure varies from the central zones to resin neck arc zones.A typical mixed mode fracture is observed,which is consistent with the corresponding micro-scale experimental observation.When the resin cohesive strength ranges between 8 and 12 MPa,the resin cracks occur at the central zone of resin bridges and propagate perpendicularly to the tensile direction until through cracks happen.At a higher range(between 12 and 16 MPa),interface cracks cross with resin cracks,bonding bridges of resin sand are broken.The interface adhesive strength has a more significant effect on the overall tensile strength of resin sand than the resin cohesive strength.The overall tensile strength of resin sand increases first then keeps stable with the increase of the resin cohesive strength.This work attempts to establish a numerical model which accurately describes the complicated mixed mode fracture of resin sand,which is beneficial to understand deeply the fracture mechanism of resin sand.

DYNAMIC STRESS ANALYSIS OF THE INTERFACE IN A PARTICLE-REINFORCED COMPOSITE

The analysis of the dynamic stress on the particle-matrix interface in particle-reinforced composite for the reason that this stress may lead to the microvoids' nucleation due to the interfacial debonding were studied. For simplification, a sphere containing a concentric rigid spherical particle was taken as the representative volume element (RVE). The Laplace transformation was used to derive the basic equations, and the analytical solutions were obtained by means of Hankel transformation. Moreover, the influences of the inertia and viscosity on the debonding damage were also discussed.

Mechanical Response of the Composite Material—Concrete Interface in FRP-Strengthened Concrete Elements:Finite Element Simulation

In structural elements strengthened with Fiber Reinforced Polymer(FRP),debonding failure modes should be taken into consideration.Under specific circumstances,they may provoke a global,premature failure of the structural element.In other cases,they should be accounted for in the modeling in order to obtain more accurate results.Despite the large amount of research work carried out in this field in the last few decades,debonding failure modes are still not fully understood.This contribution is focused on a numerical procedure designed to model the progressive loss of bond action between FRP and concrete.The two-stage procedure is integrated into incremental,finite element analysis.The proposed algorithm uses experimentally obtained slip-stress relationship.Predefined failure criteria are used to predict the local bond failure.In the reported case study,an experimental set-up widely employed to investigate debonding is modeled.Results obtained by finite element analysis are discussed.

Elastic Wave Scattering From a Partially Debonded Elastic Cylindrical Inclusion

The problem of elastic wave scattering from a partially debonded elastic cylindrical inclusion is investigated by using the wave function expansion method and singular integral equation technique. The debonding regions are modeled as interface cracks with non-contacting faces. The mixed boundary conditions of the problem lead to a set of simultanious dual series equations which, by introducing the dislocation density functions as unknowns, can be further converted to a set of singular integral equations of the second type. Solving these equations numerically, we obtain the dynamic stress intensity factor(DSIF), the scattered far-field pattern and the scattering cross section(SCS). The numerical results for an inclusion with one debond are presented to show the distinguishing feature of the problem-the low frequency resonances in both DSIF and SCS. Finally, as a special example, we discuss the scattering of elastic waves by a circular arc-shaped crack in a homogeneous medium. The presented method is valid when stresses have oscillatory behavior.

DYNAMIC ANALYSIS OF A BURIED RIGID ELLIPTIC CYLINDER PARTIALLY DEBONDED FROM SURROUNDING MATRIX UNDER SHEAR WAVES

This paper investigates the dynamic behavior of a buried rigid elliptic cylinder partially debonded from surrounding matrix under the action of anti-plane shear waves (SH waves). The debonding region is modeled as an elliptic arc-shaped interface crack with non-contacting faces. By using the wave function (Mathieu function) expansion method and introducing the dislocation density function as an unknown variable, the problem is reduced to a singular integral equation which is solved numerically to calculate the near and far fields of the problem. The resonance of the structure and the effects of various parameters on the resonance are discussed.

Effect of interface debonding on matrix multicracking evolution of fiber-reinfroced ceramic-matrix composites

An analytical methodology was developed to investigate the effect of fiber/matrix interface debonding on matrix multicracking evolution of fiber-reinforced CMCs(ceramic-matrix composites).The Budiansky-Hutchinson-Evans shear-lag model was adopted to analyse the micro-stress field of the damaged composites.The critical matrix strain energy criterion,which presupposes the existence of an ultimate or critical matrix strain energy with matrix,was obtained to simulate the matrix multicracking evolution of CMCs.With the increase of the applied stress,the matrix multicracking and fiber/matrix interface debonding occurred to dissipate the additional energy entered into the composites.The fiber/matrix interface debonded length under matrix multicracking evolution was obtained by treating the interface debonding as a particular crack propagation problem.The conditions for no-debonding and debonding during the evolution of matrix multicracking were discussed in terms of two interfacial properties,i.e.,the interface shear stress and interface debonded toughness.When the fiber/matrix interface was bonded,the matrix multicracking evolution was much more intense compared with the interface debonding;when the fiber/matrix interface was debonded,the matrix crack density increased with the increasing of interface shear stress and interface debonded energy.The theoretical results were compared with experimental data of unidirectional SiC/CAS(calcium alumina silicate),SiC/CAS-Ⅱand SiC/borosilicate composites.

Meso-mechanical Interfacial Behavior of Elbow Steel Fiber Reinforced Concrete

The strain distributions near the interface when the elbow steel fiber is pulled out from the half-mould concrete matrix are directly measured using a combined method of single fiber pull-out test and digital image correlation. Meanwhile, the real-time processes of the bonding, debonding and sliding at the interface are observed. The micro-mechanism of the strain localization in the failure process of interface when debonding occurs and the strengthening mechanism at the imbedded fiber are discussed. The experimental results show that the meso-scale strain localization gives rise to the localization of shear damage near the fiber interface. This strain localization characterized by the debonding process near the interface occurs, develops and moves gradually at an apparently regular interval. At the elbow part of the imbedded fiber, the peak value of the shearing stress occurs. But the primary debonding does not occur at this place because the strength of the shear damage is increased at the local area of the elbow part in the concrete, displaying an apparent reinforced effect at the end of the fiber.

填充颗粒形貌对复合固体推进剂力学性能影响的有限元数值分析

填充颗粒的形貌对复合固体推进剂细观力学性能具有重要影响。考虑相界面和颗粒形貌的影响,在细观层次上建立了代表性体积单元(RVE)计算模型,通过引入内聚力模型(CZM),研究了混合颗粒形状、混合颗粒粒度级配以及不规则填充颗粒等细观颗粒形貌因素对复合固体推进剂力学性能的影响。结果表明:当模型填充颗粒均为粒径150μm的小颗粒时,填充形貌对其力学性能影响很小,而填充颗粒为粒径290~420μm的大颗粒时,颗粒越圆润,其界面处更容易发生“脱湿”损伤,导致其力学性能明显下降;在具有相同大颗粒圆形填充物时,小颗粒形貌对初始模量影响较大,若小颗粒长径比由1增大到2,其初始弹性模量减小,而最大抗拉强度变化较小;而在具有相同椭圆形大颗粒填充物时,具有圆形小颗粒填充物的最大抗拉强度更大;多边形填充颗粒模型与圆形填充颗粒模型界面失效模式有所不同,前者力学性能明显高于后者。因此,当细观填充颗粒形貌差异较大时,推进剂力学性能较好。

钢板-UHPC组合加固损伤RC梁的受弯性能试验研究

为研究钢板-超高性能混凝土组合加固损伤钢筋混凝土梁的受弯性能,对1根RC梁及2根加固梁开展了试验研究,并对其承载力、挠度、应变、裂缝发展情况等进行了分析。结果表明:相较于未加固RC梁,加固梁在抗裂性能、刚度及极限承载力方面有了大幅提升,其中开裂荷载分别提高2.71倍、3.19倍,承载力分别提高了54%、65%,刚度提升近1倍。由于破坏模式均为新老混凝土界面剥离破坏,极限状态下钢板及钢筋均未达到屈服强度,未能充分发挥材料性能,在加固设计中应避免此类破坏的发生。此外,基于桁架模型理论对钢板-UHPC组合加固梁剥离承载力进行了理论推导,建立了理论计算公式。结果表明:基于桁架模型理论建立的剥离承载力公式计算值与试验结果吻合,相对差值在10%以内,此理论模型可靠。

界面缺陷对CFRP加固钢筋混凝土梁抗弯性能的影响研究

为研究界面缺陷对CFRP加固钢筋混凝土梁抗弯性能的影响,制作了3根钢筋混凝土梁对比试件,并进行了四点弯曲试验。同时,采用ABAQUS分析软件建立了非线性有限元模型,并进行了数值模拟分析。结果表明:当存在多个不连续缺陷时,试件上的CFRP提前发生剥离,剥离荷载下降了20.77%,导致试件的破坏模式发生改变,虽然极限荷载仅降低2.46%,但挠度降幅高达15.37%,延性下降显著;界面缺陷附近易产生应力集中,促使多裂缝均衡发展,导致裂缝间距减小;界面缺陷影响CFRP剥离起始发生位置,更易引起CFRP沿试件宽度方向贯穿剥离。

钢制金属I型夹芯板界面疲劳脱粘特性分析

[目的]针对金属I型夹芯板界面的脱粘失效破坏,开展金属夹芯板界面疲劳脱粘特性分析。[方法]基于内聚力模型理论,考虑时间历程、三维结构节点多自由度的损伤演化方程与收敛判定准则,开发适用于三维复杂结构的界面疲劳脱粘模拟程序;与焊接接头脱粘实验结果进行对比,验证所开发程序的准确性,并进一步对钢制金属I型夹芯板的界面疲劳脱粘行为进行数值模拟。[结果]结果显示,所开发的三维界面疲劳脱粘数值模拟程序与焊接接头脱粘实验间最大的模拟误差仅为14.05%;I型夹芯板承受面外载荷时,夹芯板界面处的脱粘破坏主要表现为沿焊缝方向的表面裂纹扩展,当裂纹扩展至夹芯板长度的70%左右时,裂纹开始贯穿腹板形成贯穿裂纹。[结论]所开发的三维复杂结构界面疲劳脱粘程序可以实现对金属夹芯板界面疲劳脱粘寿命的有效评估,对实际工程应用具有一定的指导意义。

基于超声能量特征的复合结构材料脱粘检测方法

针对受限于复合结构的厚度、层数、材料种类等因素导致超声回波信号混叠,难以实现界面脱粘检测的问题,提出一种直接利用超声纵波反射混叠信号的能量特征进行复合结构脱粘缺陷检测的方法。首先建立单层与多层复合结构的超声信号反射与透射特性模型;然后提取并分析不同界面粘接状态下结构的超声回波能量特征,并基于该模型进行钢-橡胶-橡胶复合结构的脱粘检测仿真,通过引入能量差异值进行脱粘界面识别;最后,利用超声波探伤设备对钢-橡胶-橡胶复合结构试样进行实际脱粘缺陷检测。理论分析和实验验证均表明,该方法可以实现多层复合结构脱粘缺陷的有效检测。

新型GAP/RDX/TEGDN推进剂宽应变率下的力学性能及模型参数标定

高能低易损固体推进剂力学模型参数的准确性对其宏观力学响应预测具有重要的意义,为解耦标定固体推进剂黏弹塑性损伤模型参数,提出一种基于试验的模型参数标定方法。采用万能试验机和分离式霍普金森压杆装置开展准静态和动态试验,标定GAP/RDX/TEGDN(GRT)固体推进剂黏弹塑性损伤参数,分析其宽应变率下的力学性能和损伤机理。试验结果表明:GRT推进剂的力学行为表现出明显的应变率依赖性,随拉伸速率(1~1000 mm/min)的增加,抗拉强度和延伸率分别提高75%和43.33%;随准静态压缩应变率(0.001~1 s^(-1))的增加,屈服强度提高16.67%;随动态压缩应变率(2100~4100 s^(-1))的增加,屈服强度提高72.98%;材料的断裂应变与应力状态相关,断裂应变随应力三轴度(η取值为0.33~0.74)增大而减小了36.36%。根据GRT推进剂细观表征得出准静态拉伸下以界面脱粘为主的破坏机制,准静态压缩和动态压缩下以颗粒破碎为主的破坏机制。在此基础上,通过准静态和动态压缩试验标定Plastic-Kinematic本构模型参数,应力松弛试验标定Prony级数参数,缺口试验和准静态拉伸试验标定断裂损伤模型参数,所提方法能够更精准地标定模型参数,为推进剂宏观力学性能预测分析提供支撑。

CFRP复合材料板材的耐水酸碱性能研究

腐蚀环境下碳纤维增强环氧树脂(CFRP)复合材料的性能退化是影响材料与结构安全服役的关键.为获得CFRP板材在腐蚀溶液中的耐久性能,将CFRP板材浸泡在水、酸与碱溶液中长达150 d.通过水吸收、热力学测试与微观结构分析获得CFRP板材的长期性能演化.结果表明:树脂板与CFRP板材在水、酸与碱溶液中的水吸收行为类似.在酸溶液中,树脂板与CFRP板材均获得较高的水吸收量,这是因为酸溶液促进了树脂的松弛作用,为水分子的进一步侵入提供了更多的扩散空间.此外,树脂的水解作用降低了树脂板与CFRP板材水吸收量.长期的加速老化导致CFRP板材拉伸强度退化高达29.7%,玻璃化转变温度退化高达12.4%.CFRP板材拉伸强度的退化主要由树脂的塑化与水解、纤维与树脂界面的脱粘所导致.碱溶液加速纤维与树脂界面脱粘、树脂水解,这导致碱溶液浸泡下的CFRP板材拉伸强度退化远大于水与酸溶液,其纤维与树脂界面完整性最差,纤维表面没有树脂残留.CFRP板材玻璃化转变温度的退化主要受控于树脂的塑化.酸溶液下玻璃化转变温度的退化程度远大于水与碱溶液,这是因为酸溶液下CFRP板材拥有更高的吸水率,导致树脂发生更严重的塑化作用.

Effect of neutral polymeric bonding agent on tensile mechanical properties and damage evolution of NEPE propellant

Introducing Neutral Polymeric bonding agents(NPBA) into the Nitrate Ester Plasticized Polyether(NEPE)propellant could improve the adhesion between filler/matrix interface, thereby contributing to the development of new generations of the NEPE propellant with better mechanical properties. Therefore,understanding the effects of NPBA on the deformation and damage evolution of the NEPE propellant is fundamental to material design and applications. This paper studies the uniaxial tensile and stress relaxation responses of the NEPE propellant with different amounts of NPBA. The damage evolution in terms of interface debonding is further investigated using a cohesive-zone model(CZM). Experimental results show that the initial modulus and strength of the NEPE propellant increase with the increasing amount of NPBA while the elongation decreases. Meanwhile, the relaxation rate slows down and a higher long-term equilibrium modulus is reached. Experimental and numerical analyses indicate that interface debonding and crack propagation along filler-matrix interface are the dominant damage mechanism for the samples with a low amount of NPBA, while damage localization and crack advancement through the matrix are predominant for the ones with a high amount of NPBA. Finally, crosslinking density tests and simulation results also show that the effect of the bonding agent is interfacial rather than due to the overall crosslinking density change of the binder.

发动机黏接界面损伤破坏过程细观数值仿真

目的揭示拉伸加载下发动机黏接界面的损伤破坏规律,以及典型参数对该损伤破坏过程的影响规律。方法以建立的含预制宏观裂纹的硝酸酯增塑聚醚(NEPE)推进剂装药的黏接界面结构的细观有限元模型为基础,开展发生推进剂内聚损伤破坏、推进剂/衬层黏接界面损伤破坏和混合型损伤破坏的数值仿真计算,讨论不同损伤破坏形式下的裂纹扩展规律,以及推进剂基体强度、颗粒/基体黏接界面模量和强度、推进剂/衬层黏接界面模量和强度对损伤位置和损伤程度等的影响规律。结果颗粒/基体黏接界面的“脱湿”是发生推进剂内聚损伤破坏时的主要损伤形式,损伤临界应变阈值约为30%。推进剂/衬层黏接界面损伤破坏时,裂纹扩展路径与预制裂纹方向一致。混合型损伤破坏包括颗粒/基体黏接界面“脱湿”、推进剂/衬层黏接界面脱黏和推进剂基体撕裂,裂纹在推进剂/衬层黏接界面发生扩展的临界应变阈值约为20%,颗粒/基体黏接界面发生“脱湿”损伤及裂纹扩展的临界应变阈值约为60%。推进剂基体强度、颗粒/基体黏接界面强度和推进剂/衬层黏接界面强度对装药黏接界面结构损伤破坏的影响更为显著,前2个参数的增大均能导致发生“脱湿”损伤的位置向推进剂/衬层黏接界面移动,黏接界面结构损伤破坏模式发生转变的临界推进剂/衬层黏接界面强度阈值处于0.80~1.00MPa。结论NEPE推进剂装药的黏接界面结构的损伤破坏形式和损伤程度随细观结构模型材料参数的不同而发生改变。