Ultrasonic Ply-by-Ply Detection of Matrix Cracks in Laminated Composites

In the design of cryogenic fuel tanks for the next generation Reusable Launch Vehicles (RLVs), permeability of cryogenic fuels across the thickness of the tank is a critical issue. In this context, the detection of matrix cracks and their connectivity takes on an unprecedented significance. In this work we develop an ultrasonic backscattering technique for the detection of matrix cracks in each of the plies of a damaged graphite/epoxy laminated composite.     

复合材料多轴联动超声C扫描检测技术评述

随着航空、航天装备研制中非规则曲面复合材料制件比重的增加,传统的超声C扫描检测手段已难以满足实际应用需求。多轴联动超声C扫描检测技术为非规则曲面复合材料制件的检测提供了重要技术支撑。本文分析和评述了多轴联动超声C扫描检测的技术原理、典型设备形式和主要功能,分析了存在的问题,并指出发展方向。     

Use of Acoustic Emission to Evaluate Residual Strength in FRP Pipes after Impact Damage

Acoustic Emission (AE) has proven to be a useful tool in the monitoring of fiberglass pressure vessels. The ASME Piping and Pressure Vessel Code (subsection X) implements AE as an approved method for quality control and in service testing of fiberglass pressure vessels, making it an accepted technique for the evaluation of fiber composite structures. This article presents the result of AE monitoring of fiber reinforced plastic (FRP) pipes after impact damage has incurred. The results from these tests indicate that AE can be used as a tool to predict the level of damage incurred in the pipes after impact and during pressurization. The use of the Felicity Ratio as the benchmark for evaluation is presented as the key evaluating value.     

Evaluation of FRP and hybrid FRP cables for super long-span cable-stayed bridges

This paper evaluates the safety factors, the applicable lengths, and relative cost of FRP (fiber reinforced polymer) and hybrid FRP cables that are potentially suitable for cable-stayed bridges with a super long-span of between 1000 m and 10,000 m. Following previous studies on 1000-m scale cable-stayed bridges with FRP cables, two kinds of hybrid FRP cables – the previously discussed hybrid basalt and carbon FRP (B/CFRP) cable and the newly-developed basalt and steel-wire FRP (B/SFRP) cable – as well as conventional steel cable, CFRP cable, and BFRP cable are further investigated focusing on their promise in meeting potential requirements for super long-span bridges. Some major results are as follows: (1) a three-stage model for determining safety factors of cables with different kinds and lengths is proposed; (2) a threshold of λ² is suggested to achieve both high material and stiffness utilization efficiency, based on which the applicable lengths for different kinds of cables were evaluated; and (3) hybrid B/SFRP cables and BFRP cables are comparable in cost to steel cables within a 3000 m span, while hybrid B/CFRP cables and CFRP cables demonstrate a superior performance/cost ratio over a longer span.     

Effect of interphase on fibre-bridging toughness of a unidirectional FRP composite thin plate

A parametric analysis of the toughening mechanisms in a uniaxially fibre reinforced polymer (FRP) thin plate with a power-law hardening shear interphase is presented. An interfacial shear-lag model is used to analyse the relationship between the crack surface traction exerted by the intact fibres and the crack opening displacement (COD). Numerical solutions of the equations governing bridge-toughening are given. Two special kinds of interphase, i.e. linearly elastic and perfect plastic, are discussed. The results demonstrate that the toughening ratio of the composite thin plate is sensitive to several parameters, e.g. the thickness of the interphase between fibre and matrix, the hardening parameter of the interphase, the interfacial shear properties (stiffness and strength), the fibre radius and the far-field load. The results of this investigation will be beneficial to the selection of constitutive materials, the improvement of mechanical behaviour and the fabrication process of FRP composites.     

Laser-Ultrasonics: From the Laboratory to the Shop Floor

Ultrasonics is a powerful technique for inspecting and characterizing industrial materials. It not only can detect bulk and surface flaws, but also obtain information on material microstructure, which determines engineering properties, such as elastic moduli and ultimate strength. However, traditional ultrasound requires liquid or contact coupling for its generation and detection, making it difficult or impossible to apply in many industrial situations. This occurs, in particular, on curved parts and on parts at elevated temperature, a situation widely found in industrial products and during the processing of industrial materials.Through a continuing effort that started more than 10 years ago, the Industrial Materials Institute of the National Research Council of Canada working in collaboration with UltraOptec Inc. has developed a technique called laser-ultrasonics, that circumvents the limitations of the conventional techniques. This novel technique is based on the generation and detection of ultrasound with lasers. The technology we have developed has been demonstrated to be applicable to real industrial conditions. In particular, a system was brought to a steel mill to measure on-line the wall thickness of tubes at 1000°C moving at 4 m/s. The capability of our technology to inspect advanced aircrafts made of composite materials was also demonstrated by inspecting a CF-18 in the hangar of a maintenance facility. UltraOptec Inc. is now in the process of commercializing this technology, in particular, for these two demonstrated industrial applications.     

拉挤成型复合材料板桩抗弯刚度简化计算

将折线型截面复合材料板桩整体刚度表示成翼缘和腹板刚度之和,基于层合板理论分层计算翼缘和腹板的局部刚度,然后提出整体结构抗弯刚度和抗剪刚度的简化计算公式,因此结构在弯曲荷载作用下的位移理论值可采用Timoshenko方程获得。开展拉挤复合材料板桩四点弯试验,将测量的荷载-位移曲线与理论值进行对比,表明本文推导的刚度计算公式在弹性范围内具有较好的精度,其中跨高比更大的试件理论与试验值更接近。该理论公式也可用于计算Z型、槽型、工字型等截面复合材料结构的刚度和位移。     

Parametric Studies of Laser Generated Ultrasonic Signals in Ablative Regime: Time and Frequency Domains

A laser pulse incident on a material may generate ultrasound by means of two different phenomena: thermoelastic effect at low power density and ablation effect at high power density. Ablative generation of ultrasound is necessary for some critical applications such as on-line weld quality monitoring in which strong signals are required to compensate the elevated temperature and the long path length. While the waveform in time domain has been discussed extensively in the literature, there is little knowledge about the frequency components of laser ultrasound, although this information is necessary for practical applications. In this paper, analytical results from both thermoelastic and ablative regimes are reviewed. Laser ultrasonic signals (longitudinal waves and surfaces waves) generated by laser ablation are measured in a number of metal samples (2024 A1, 6061 A1, 7075 A1, mild steel, and copper) with a broadband laser interferometer. The frequency spectra are analyzed and compared for different thicknesses (50.8 mm, 25.4 mm, 12.7 mm, and 6.4 mm) and for different power densities. Hanning windowing is applied to the longitudinal pulses in time domain before frequency analysis is performed. The experimental data match the theoretical predictions very well. The results show that the frequency spectrum extends from 0 to 15 MHz, while the center frequency occurs near 2 MHz. The detailed distribution of the spectrum is dependent on the material, thickness, and laser power density.     

Mechanical response of anchored FRP bonded joints: A nonlinear analytical approach

This article presents a nonlinear analytical solution for the prediction of the full-range debonding response of mechanically anchored, fiber-reinforced polymer (FRP) composites from the substrate. The nonlinear analytical approach predicts, for any monotonic loading history or bonded length, the relative displacements (or slips) between materials, the strains in the FRP composite, the bond stresses within the interface, and the stresses developed in the substrate. The load-slip responses of FRP-to-substrate interfaces with short and long bonded lengths are motives of analysis and discussion. The solutions obtained from the proposed approach are also compared with other experimental results found in the literature.     

Ultrasonic characteristics of graphite/epoxy composite material subjected to fatigue and impacts

Fiber-reinforced composites, because of their superior specific strengths and stiffnesses, are used in many aircraft components. However, in this application these composites are subjected not only to fatigue loading, but to occasionally high velocity impact due to the bird injection, hail, dust, and rain. Thus, it is important to evaluate the residual life and degradation due to combined fatigue and impact loadings. Unidirectional graphite epoxy composites (MA8276-Tiger) which are used in the aerospace industry were impacted by a free falling weight at energy levels of 0.567j, 1.134j, and 1.571j [impact energy toughness (j/cm³); 0.12, 0.24, 0.34], respectively. The subsequent changes/degradation in elastic moduli, strength, toughness, and fatigue properties were measured after different number of impacts. It was found that for all energy levels these properties vary linearly with the number of impacts. Furthermore, attenuation changes is not a good ultrasonic parameter for degradation estimation, since it does not incorporate the micro- and macrocracks beyond the impact point. However, these micro- and macrocracks have significant effect on the mechanical properties. In contrast to the attenuation, the stress wave factor, which indicates the efficiency of wave propagation along the specimen, correlates very well with degradation, and it can be used effectively to measure the residual strength after impact. Ultrasonic characteristic on specimens subjected to combined fatigue and impact were also studied. Based on these experiments, it is concluded that the loss in fatigue residual life due to impact loads may be predicted by measuring the effects of the impact load on attenuation and stress wave factor. It was found that the reduction in fatigue life is proportional to sudden changes in attenuation and stress wave factor. Damage accumulation models based on Coffin-Manson equation, was suggested for impact and combined fatigue and impact. It was found that residual properties and fatigue life can be estimated from these models.     

聚苯酯基复合材料在超声电机中的磨损性能

作为行波超声电机的接触层摩擦材料,聚苯酯基复合材料的磨损性能对电机的驱动特性和寿命均会产生重要的影响。将聚苯酯基复合材料粘贴在40型圆盘形行波超声电机定子齿面,在不同的电机驱动方式和摩擦组合下测试接触层磨损性能,并使用扫描电镜观察磨损表面形貌。结果表明,超声驱动下,电机接触层以疲劳磨损为主要磨损机制。并且当电机采用聚苯酯基复合材料和铜转子的摩擦组合时,其性能磨损较好。接触界面应力模拟分析表明,行波超声电机接触界面在短时间内即可完成108周次以上的疲劳载荷作用,因此接触层主要磨损机制为超声疲劳。     

A Continuum Based Thick Shell Element for Large Deformation Analysis of Layered Composites

A new continuum based thick shell model is presented for modeling orthotropic laminated shell structures undergoing large elastic deformations. An equivalent single-layer model involving seven nodal degrees of freedom is used. In that layered model, there are no restrictions on the number of layers, their thickness and their stacking sequence. The shell model accounts explicitly for the thickness change in the shell, as well as the normal stress and strain states through its thickness. Shear locking is avoided using an assumed natural strain formulation, while thickness locking is avoided using modified displacement interpolation functions. The performance of the layered shell element is tested using several linear and non-linear composite plate and shell problems involving anisotropic, angle and cross-ply laminates, cylindrical and spherical shells.     

Highly Impact-Resistant Silk Fiber Thermoplastic Composites

Silk fibers combine good stiffness and strength with a very high strain to failure and are as such highly promising to realize composites with high impact resistance. It is shown that to realize this potential it is quite beneficial to employ matrix materials of high strain to failure, particularly thermoplastic matrices. High impact resistance is thus achieved, well above the values for the pure matrices. Below the glass-transition temperature of the thermoplastic matrix, the impact energy absorption decreases. The adhesion between fiber and matrix also plays a significant role; lower adhesion typically increases the low-velocity penetration impact resistance, due to the spread of damage. Finally, the fiber architecture is pivotal; when a woven fabric is used which is unbalanced in strength, the impact resistance reduces in correspondence with the weakest material direction. A quasi-isotropic layup has a lower capacity for deformation than a balanced woven configuration which likely explains the observed lower penetration impact resistance.     

GBT formulation to analyse the buckling behaviour of FRP composite open-section thin-walled columns

This paper presents a Generalised Beam Theory (GBT) formulation to analyse the local and global buckling behaviour of FRP (fibre-reinforced polymer) composite thin-walled columns with arbitrary open cross-sections, which takes into account both shear deformation and cross-section deformation effects. After describing the steps and procedures involved in performing the GBT cross-section analysis of an arbitrarily branched composite (laminate plate) thin-walled member, the paper addresses the numerical implementation of the proposed GBT formulation, carried out by means of the finite element method (GBT-based beam element) – particular attention is devoted to the derivation of the element linear and geometric stiffness matrices, which incorporate all the material coupling effects. In order to illustrate the application and capabilities of the proposed formulation and implementation, several numerical results are presented and discussed, dealing with the local and global buckling behaviour of FRP composite I-section columns with different ply orientations and stacking sequences. Taking advantage of the GBT modal features, deep insight is acquired on the complex composite member buckling mechanics, namely those involving bending–torsion or global–local coupling effects. In particular, one investigates the influence of (i) the constitutive assumption regarding the transverse extension occurring in the cross-section composite walls and (ii) the distribution of pre-buckling normal stresses (due to axial compression) on the buckling behaviour of I-section columns. For validation purposes, the above results are compared with values recently reported in the literature and estimates obtained from shell finite element analyses.     

MoSi2基复合材料的制备技术

本文总结了采用粉末冶金、机械合金化、等离子体溅射、注射成型、固态置换反应、原住反应弥散相粒子生成技术等方法制备MoSi2基复合材料工艺的主要特点及原理，并对MoSi2基复合材料的研究进行了展望。     

碳微球表面包覆氧化锌的制备与表征

利用化学气相沉积法,制备了直径约370nm的碳微球（CMSs）;然后用浓HNO3和H2O2对其进行氧化修饰,使其表面接上羟基等含氧官能团;最后再与醋酸锌和三乙醇胺反应,在CMSs表面包覆ZnO层形成ZnO/CMSs复合物,在此过程中考察了醋酸锌和三乙醇胺物质的量比和反应时间对复合物的影响。采用场发射扫描电子显微镜、红外光谱、热重分析仪和X射线衍射仪等对各阶段产物进行了形貌和结构的表征与分析。结果表明：醋酸锌和三乙醇胺的物质的量比为12,反应时间为2h时,得到的ZnO/CMSs复合物外观形貌光滑,包覆均匀且不易脱落。     

Nonlinear Ultrasonic Damage Characterization of Limestone

Characterization of dolomitic limestone rock samples with increasing levels of damage is presented using a nonlinear ultrasonic approach. Limestone test samples with increasing levels of damage were created artificially by exposing virgin samples to increasing temperature levels of 100, 200, 300, 400, 500, 600, and 700°C for a ninety minute period of time. These samples were first characterized using ultrasonic dilatational and shear phase velocity measurements and corresponding attenuations. Then, a nonlinear approach based upon noncollinear wave mixing of two dilatational waves was used. Criteria were used to assure that the detected scattered wave originated via wave interaction in the limestone and not from nonlinearities in the testing equipment. It was observed that both the dilatational velocity and the noncollinear wave mixing approach are able to characterize the level of damage in limestone rock. However, the nonlinear approach is more sensitive to damage accumulation by about two orders of magnitude.     

Experimental study of FRP tube encased concrete cylinders exposed to fire

In recent years, fiber-reinforced polymer (FRP) tube encased concrete cylinders have been widely studied for their use in civil infrastructure. However, the performance of these FRP/concrete systems exposed to fire is a serious concern due to the combustibility of FRPs. This paper presents test results of temperature distributions and residual strength of three groups of FRP tube encased concrete cylinders (Groups 1, 2, and 3) exposed to a jet fire of 982 °C for a time period of 4 min, 8 min, and 12 min, respectively. The temperature distribution of the confined concrete cylinders under fire was recorded by the embedded thermocouples. After fire test, uniaxial compression tests were conducted to evaluate the residual strength. The results were compared with that from a group of FRP tube encased concrete cylinders without fire exposure (Group 4). To improve the fire resistance of the confined concrete cylinders, a fireproof additive was also applied to three other groups of specimens (Groups 5, 6, and 7) and these specimens were also fire tested for 4 min, 8 min, and 12 min, respectively. SEM was used to examine the burn-out of resin by fire. The test results show that the fire exposure has a significant effect on reducing the residual strength of the confined concrete cylinders. The resin has been fully burned-out and carbonized after 8 min of fire exposure. After 12 min of burning, the FRP tube basically loses its capacity to provide any meaningful confinement. The fireproof additive used is effective in minimizing the effect of the fire hazard.     

Modeling the effect of high temperature on the bond of FRP reinforcing bars to concrete

The effect of high temperature on the bond between fiber reinforced polymers (FRP) reinforcing bars (rebars) and concrete was studied. The bond strength exhibited a severe reduction of 80–90% at relatively low temperature (up to 200°C), accompanied by changes in the pullout load-slip behavior. A semi-empirical model was developed in order to describe the extent of reduction in the bond strength as the temperature rises. The model is based on the following parameters: glass transition temperature of the polymer layer at the surface of the rod; polymer's degree of crosslinking; the residual bond strength at high temperature after the polymer of the external layer of the rebar ceased to contribute to the bond. The parameters of the rods that were tested for pullout at various temperatures were introduced into the model. The output curves of bond–temperature relationships showed good agreement with the test results.