钨合金空心弹体垂直侵彻过程研究

进行了钨合金空心弹体垂直侵彻钢靶板和混凝土靶板的实验。通过对回收空心弹体的分析，指出钨合金空心弹体破坏的典型宏观特征和主要微观机制分别为：空心弹体危险区域的塑性大变形及断裂和危险截面处材料的绝热剪切失效。通过自行设计的弹载存储测试系统，成功地获得钨合金空心弹体危险截面处应变的时间历程。应变历程曲线真实地反映出侵彻目标过程中应力波效应致使弹体受到动态压缩和动态拉伸载荷的反复作用，压应力远高于拉应力，而且应力水平与目标靶板的材料性能密切相关。研究结果不仅有助于空心弹体侵彻模型的建立，而且为空心弹体的结构设计与选材提供理论依据。     

Adiabatic shear localization in titanium and Ti-6 pct Al-4 pct V alloy

Ballistic impact experiments were conducted on 12.5 mm thick commercial purity titanium and Ti-6 pct Al-4 pct V alloy plates using steel “stepped” projectiles with 10.5 mm diameter. The impact velocities varied between 578 m per second and 846 m per second, and a flash X-ray technique was used to determine projectile velocity and to assure the normality of impact. The microstructural damage mechanisms associated with impact (shear band formation, shock wave propagation, and dynamic fracture) were analyzed by optical, and scanning and transmission electron microscopy. Elliptical and spherical cavities were observed along the bands. Microindentation hardness differences between the bands and adjacent regions were slight for the targets; for the projectiles, the hardness in the band was significantly lower than that of surrounding regions. Observation of the fractured regions along the bands showed unique features indicating possible melting. Transmission-electron microscopy of a shear band in titanium revealed microcrystalline features (∼0.3 μm diameter) with poorly defined grain boundaries. Formerly Graduate Student, Department of Metallurgical and Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801     

Flexural Behavior of Hybrid FRP-Concrete-Steel Double-Skin Tubular Members

This paper presents the results of an experimental study on the flexural behavior of a new type of hybrid FRP-concrete-steel member as well as results from a corresponding theoretical model based on the plane section assumption and the fiber element approach. This new type of hybrid member is in the form of a double-skin tube, composed of a steel inner tube and an FRP outer tube with a concrete infill between the two tubes, and may be employed as columns or beams. The parameters examined in this study include the section configuration, the concrete strength, and the thicknesses of the steel tube and the FRP tube, respectively. The results presented in this paper show that these hybrid beams have a very ductile response because the compressive concrete is confined by the FRP tube and the steel tube provides ductile longitudinal reinforcement. The beams' flexural response, including the flexural stiffness, ultimate load, and cracking, can be substantially improved by shifting the inner steel tube toward the tension zone or by providing FRP bars as additional longitudinal reinforcement. The predictions from the theoretical model are in reasonably close agreement with the test results. Differences between the test and predicted results arise from factors not considered in the theoretical model, including the existence of a strain gradient in the confined concrete, concentrations of cracks and the slips between the concrete and the two tubes; these are issues to be accounted for in the development of a more accurate model in the future.     

高强度钢板中的两种类型绝热剪切带

 利用穿甲燃烧弹撞击不同抗拉强度的40CrNi2Mo钢板,观察了弹坑周围绝热剪切带出现的位置和扩展方向,测量了弹坑剖面上绝热剪切带的总长度,分析了绝热剪切带与冲塞破坏的关系。按照扩展方向和出现位置的不同可以将钢板中的绝热剪切带划分为E型和P型,前者主要分布在合格损伤弹坑的剖面,基本沿着金属塑性变形流线向钢板正面扩展,不会直接引发冲塞破坏。后者主要分布在即将发生冲塞的钢板中,基本垂直于金属塑性变形流线向钢板背面扩展,它的出现表明钢板处于冲塞破坏的临界状态。     

弹丸高速冲击条件下高强度钢板抗弹性能

摘要：为了研究弹丸高速冲击条件下不同洁净度的40CrNi2Mo钢板的抗弹性能，利用12.7mm穿甲燃烧弹对抗拉强度分别为800和1200MPa级的钢板进行抗弹性能测试。通过观察不同强度钢板出现的损伤形貌，评定背面强度极限，分析了穿甲机制。结果表明：抗拉强度为800MPa级的钢板在弹丸冲击过程中以塑性扩孔方式侵彻，抗弹性能随着强度升高而提高，与洁净度关系不大。抗拉强度为1200MPa级的钢板，弹丸冲击过程中因钢板较低的绝热剪切临界失稳应变而出现绝热剪切；由于塑韧性较低，低洁净度钢板阻止绝热剪切引发裂纹扩展的能力较弱，因此形成与绝热剪切相关的裂纹，导致抗弹性能降低；高洁净度钢板抗弹性能相对较高，因背面出现剪切裂纹而失效，此裂纹与绝热剪切无关。     

Assessing the Efficiency of CFRP Discrete Confinement Systems for Concrete Cylinders

Concrete columns requiring strengthening intervention always contain a certain percentage of steel hoops. Applying strips of wet layup carbon fiber-reinforced polymer (CFRP) sheets inbetween the existent steel hoops might, therefore, be an appropriate confinement technique with both technical and economic advantages, when full wrapping of a concrete column is taken as a basis of comparison. To assess the effectiveness of this discrete confinement strategy, circular cross-sectional concrete elements confined by distinct arrangements of strips of CFRP sheet are submitted to a direct compression load up to the failure point. The influence of the width of the strip, distance between strips, number of CFRP layers per strip, CFRP stiffness, and concrete strength class on the increase of the load carrying capacity and ductility of concrete columns, is evaluated. An analytical model is developed to predict the compressive stress-strain relationship of concrete columns confined by discrete and continuous CFRP arrangements. The main results of the experimental program are presented and analyzed and used to assess the model performance.     

Residual Strength of Impact-Damaged CFRP Used to Strengthen Concrete Structures

Although previous research has demonstrated the improvement in performance of reinforced concrete structures enhanced with externally applied carbon fiber reinforced polymers (CFRP), the effect of transverse impact damage on the strength of the CFRP enhancements is unknown, and no guidelines have been provided that describe which impact events warrant CFRP repair or replacement. The impact events, such as dropped tools, collisions, and low-speed projectiles may cause critical damage to the epoxy matrix and fibers that is undetectable through visual inspection. The purpose of this research is to provide insight into the level of transverse impact needed to initiate critical damage in wet layup CFRP enhancements, which will serve as a guideline for inexpensive and immediate damage assessments. To simulate a variety of impact events, impactors (tups) of different sizes and shapes were dropped from several heights. The impacts were performed with a guided drop-weight apparatus, designed to achieve free-fall behavior. The results show that impacts that only cause indention of the FRP surface do not significantly affect the tensile strength, but impacts that cause crushing of the epoxy (seen as whitish areas) can indicate as much as a 63% residual tensile strength. Furthermore, for the test conditions considered, tests showed that impacts with a peak impact pressure greater than 21?MPa (3,000?psi) reduced the tensile strength of the CFRP.     

Analytical Model for FRP-Confined Circular Reinforced Concrete Columns

One disadvantage of most available stress–strain models for concrete confined with fiber-reinforced polymer (FRP) composites is that they do not take into consideration the interaction between the internal lateral steel reinforcement and the external FRP sheets. According to most structural concrete design codes, concrete columns must contain minimum amounts of longitudinal and transverse reinforcement. Therefore, concrete columns that have to be retrofitted (and therefore confined) with FRP sheets usually contain lateral steel. Hence, the retrofitted concrete column is under two actions of confinement: the action due to the FRP and that due to the steel ties. This paper presents a new designed-oriented confinement model for the axial and lateral behavior of circular concrete columns confined with steel ties, FRP composites, and both steel ties and FRP composites. Comparison with experimental results of confined concrete stress–strain curves shows good agreement between the test and predicted results.     

Normal- and High-Strength Concrete Circular Elements Wrapped with FRP Composites

Reinforced concrete columns usually have a minimum amount of transverse steel reinforcement this transverse reinforcement can have non negligible effects on the response of columns retrofitted with fiber-reinforced polymers (FRP). This paper presents a test program that was designed to study the behavior of small- and large-scale normal- and high-strength concrete circular columns confined with transverse steel reinforcement, FRP, and both transverse steel reinforcement and FRP under concentric loading. The effect of the main variables—such as the unconfined concrete strength, the volumetric ratio, the type and the yield strength of the transverse steel reinforcement, the concrete cover, and the number of FRP layers—are studied in this research program. The test results show that the enhancement of the confined concrete strength and strain is more pronounced in specimens with normal-strength concrete. It is also shown that the rupture of the FRP in the specimens with higher volumetric transverse steel reinforcement ratios corresponds to larger axial compressive strength and strain and that the postpeak behavior of these specimens is more ductile.     

3D Finite-Element Analysis of Substandard RC Columns Strengthened by Fiber-Reinforced Polymer Sheets

Numerical analyses are performed to predict the stress–strain behavior of square reinforced concrete columns strengthened by fiber-reinforced polymer (FRP) sheet confinement. The research focuses on the contribution of FRP sheets to the prevention of elastic buckling of longitudinal steel bars under compression, in cases of inadequate stirrup spacing. A new Drucker–Prager-type plasticity model is proposed for confined concrete and is used in constructed finite-element model. Suitable plasticity and elasticity models are used for steel reinforcing bars and fiber-reinforced polymers correspondingly. The finite-element analyses results are compared against published experimental results of columns subjected to axial compression, to validate the proposed finite-element model. Stress concentrations in concrete core and on FRP jacket are investigated considering circular or square sectioned, plain or reinforced concrete columns. Geometry of the section as well as the presence of steel bars and stirrups affect remarkably the variation and magnitude of stress on FRP as percentage of its tensile strength.     

Analytical Model for FRP Confinement of Concrete Columns with and without Internal Steel Reinforcement

The paper aims to contribute to a better understanding of the behavior of reinforced concrete columns confined with fiber-reinforced polymer (FRP) sheets. In particular, some new insights on interaction mechanisms between internal steel reinforcement and external FRP strengthening and their influence on efficiency of FRP confinement technique are given. In this context a procedure to generate the complete stress-strain response including new analytical proposals for (1) effective confinement pressure at failure; (2) peak stress; (3) ultimate stress; (4) ultimate axial strain; and (5) axial strain corresponding to peak stress for FRP confined elements with circular and rectangular cross sections, with and without internal steel reinforcement, is presented. Interaction mechanisms between internal steel reinforcement and external FRP strengthening, shown by some experimental results obtained at the University of Padova with accurate measurements, are taken into account in the analytical model. Four experimental databases regarding FRP confined concrete columns, with circular and rectangular cross section with and without steel reinforcement, are gathered for the assessment of some of the confinement models shown in literature and the new proposed model. The proposed model shows a good performance and analytical stress-strain curves approximate some available test results quite well.     

Feasibility of Thermoplastic Composite Jackets for Bridge Impact Protection

The research focused on the effects of low velocity impact loading on high-strength concrete confined by a prefabricated polypropylene jacket and comparing the results with similar specimens confined by carbon fiber-reinforced polymer (CFRP) composites. In order to accomplish this, both static and dynamic load tests were performed. Concrete cylinders were used for static loading. Twelve concrete cylinders were prepared for static load testing: three were plain concrete and used as control specimens, three were wrapped with one layer of unidirectional CFRP composites, and six were confined by the polypropylene jacket. The thickness of the polypropylene wrap was machined to different thicknesses; three 3?mm and three 6?mm. The cylinders were standard (D×H) 152?mm×305?mm. Cylinders were loaded to failure in uniaxial compression using a Tinius-Olsen Universal Testing Machine. Impact testing was performed using four (D×H) 152?mm×914?mm columns. The columns consisted of one control sample; one CFRP composites wrapped, and two (one of each thickness) wrapped with polypropylene. Impact testing was conducted using an Instron drop-tower testing machine.     

Theoretical Model for Fiber-Reinforced Polymer-Confined Concrete

Fiber-reinforced polymer (FRP) composites have found increasingly wide applications in civil engineering due to their high strength-to-weight ratio and high corrosion resistance. One important application of FRP composites is as a confining material for concrete, particularly in the strengthening or seismic retrofit of existing reinforced concrete columns by the provision of a FRP jacket. FRP confinement can enhance both the compressive strength and the ultimate strain of concrete significantly. This paper presents a new stress–strain model for FRP-confined concrete in which the responses of the concrete core and the FRP jacket as well as their interaction are explicitly considered. Such a model is often referred to as an analysis-oriented model. The key novel feature of the proposed analysis-oriented model, compared to existing models of the same kind, is a more accurate and more widely applicable lateral strain equation based on a careful interpretation of the lateral deformation characteristics of unconfined, actively confined, and FRP-confined concrete. Through comparisons with independent test data, the proposed model is shown to be accurate not only for FRP-confined concrete but also for concrete confined with a steel tube, demonstrating the wide applicability of the model to concrete confined with different confining materials. The accuracy of the proposed model is also shown to be superior to existing analysis-oriented stress-strain models through comparisons with test data.     

FRP-Confined Concrete Model

A uniaxial model for concrete confined with fiber-reinforced polymers (FRP), but also with steel jackets or conventional transverse reinforcement, is presented. The model, which is suitable to be inserted into fiber-type beam-column models, explicitly accounts for the continuous interaction with the confining device due to the lateral strain of concrete, through an incremental-iterative approach. The relation between the axial and lateral strains is implicitly derived through equilibrium between the (dilating) confined concrete and the confining device. This relation allows one to trace the state of strain in the jacket and to detect its failure. The model is compared with a set of experimental tests and shows very good agreement in both the stress-strain and the stress-lateral strain response. Evidence of the main aspects of the behavior of FRP-confined concrete is given that points out differences in the response when using fiberglass or carbonfiber. Predictive equations to determine the ultimate strength and strain of concrete confined with FRP are derived and tested on a number of experimental data.     

侧向冲击荷载下钢筋混凝土墩柱的性能

采用数值仿真技术建立了足尺钢筋混凝土墩柱精细有限元模型, 分析了侧向冲击荷载下墩柱的动态响应和抗冲击性能, 提出了一种基于截面损伤因子的损伤评估方法, 讨论了不同碰撞参数对钢筋混凝土墩柱破坏模式和损伤机理的影响.结果表明: 冲击荷载下钢筋混凝土墩柱的耗能主要分为接触区域局部耗能和构件整体耗能; 当冲击体的初始动能恒定时, 冲击质量和冲击速度的不同组合会导致钢筋混凝土墩柱损伤破坏机理的显著差异; 基于截面损伤因子的损伤评估方法可以比较准确地描述墩柱的破坏状态.轴压力对墩柱抗撞能力的有利贡献比较有限, 且墩柱随着轴力的增大更易发生剪切破坏; 冲头刚度对碰撞力和墩柱动态响应的影响十分显著.      

Torsional Design of Hybrid Concrete Box Girders

Hybrid concrete box-girder bridges that include prestressed slabs and corrugated steel webs provide a major improvement over traditional prestressed concrete box-girder bridges. To reduce the self-weight, high strength concrete is used for the top and bottom slabs and corrugated steel webs are employed for the webs. Because the weight of the girders has been reduced, the span length can be increased for more cost-effective design. A series of systematic tests on hybrid concrete box girders subjected to torsion has been performed. According to the test results, an analytical model was developed. Using the developed analytical model, a step-by-step procedure for torsional design of such bridges is presented in this article. Based on the design procedure proposed, a girder is designed by the analytical model and checked to satisfy structural codes.     

Three-Dimensional Damage Model for Concrete. II: Verification

A three-dimensional damage model for concrete has been proposed in the preceding paper, Part I: Theory. This paper focuses on the application of the damage model for quasi-brittle materials such as concrete. The determination of model parameters for the evolution rule of damage is discussed. The model parameters to consider the frictional stress and the stiffness reduction under hydrostatic compression are also studied. For verification, the proposed model is applied to concrete subjected to confined compression, triaxial, loading, and cyclic loading. Consistent results, as compared with other researchers’ experimental data, were obtained, and the proposed model is considered worthy of further research work.     

Performance Enhancement of Steel Columns Using Concrete-Filled Composite Jackets

This paper studies the cross-sectional behavior of steel columns strengthened with fiber-reinforced polymers (FRPs). The composite column is constructed by wrapping the steel I-section column with epoxy-saturated glass- and carbon-FRPs (GFRP and CFRP) sheets in the transverse direction and subsequently filling the voids between the FRP and the steel with concrete. Experimental tests were performed on stub columns under axial compression including one to three CFRP wraps. A corner treatment technique, to avoid stress concentration at the corners and to improve confinement efficiency, was also investigated. A simplified analytical model was developed to predict the axial behavior of the composite columns. Experimental results showed significant enhancement in the behavior of the composite columns primarily attributable to the confinement mechanism imposed by the FRP jacket and concrete. Increasing the corner radius resulted in higher compressive strength of the confined concrete and ultimate axial strain of the composite columns. Good agreement between the analytically developed axial load-displacement relationships and the test data indicates that the model can closely simulate the cross-sectional behavior of the composite columns.     

Damage Detection in Concrete by Fourier and Wavelet Analyses

The effectiveness of vibration-based methods in damage detection of a typical highway structure is investigated. Two types of full-scale concrete structures subjected to fatigue loads are studied: (1) Portland cement concrete pavements on grade; and (2) a simply supported prestressed concrete beams. Fast Fourier transform (FFT) and continuous wavelet transform (CWT) are used in the analysis of the structures’ dynamic response to impact, and results from both techniques are compared. Both FFT and CWT can identify which frequency components exist in a signal. However, only the wavelet transform can show when a particular frequency occurs. Results of this research are such that FFT can detect the progression of damage in the beam but not in the slab. In contrast, the CWT analysis yielded a clear difference between the initial and damaged states for both structures. These findings confirm the conclusions of previous studies conducted on small-scale specimens that wavelet analysis has a great potential in the damage detection of concrete. The study also demonstrates that the approach is applicable to full-scale components of sizes similar or close to actual in-service structures.     

Influence of Initial Microstructure on Warm Deformation Processability and Microstructure of an Ultrahigh Carbon Steel

Various isothermal compression tests are carried out on an ultrahigh carbon steel(1.2% C in mass percent), initially quenched or spheroidized,using a Gleeble-3500system.The true stress is observed to decrease with increasing temperature and decreasing strain rate.The true stress of the initially quenched steel is lower than that of the initially spheroidized steel at high deformation temperature(700℃)and low deformation strain rate(0.001s-1).The value of the deformation activation energy(Q)of the initially quenched steel(331.56kJ/mol)is higher than that of the initially spheroidized steel(297.94kJ/mol).The initially quenched steel has lower efficiency of power dissipation and better processability than the initially spheroidized steel.The warm compression promotes the fragmentation and the spheroidization of lamellar cementites in the initially quenched steel.The fragmentation of lamellar cementites is the spheroidizing mechanism of the cementites in the initially quenched steel.Results of transmission electron microscope investigation showed that fine grains with high angle boundaries are obtained by deformation of the initially quenched steel.