Assessment of the residual fatigue strength in RC beams

In conventional analysis and design procedures of reinforced concrete structures, the ability of concrete to resist tension is neglected. Under cyclic loading, the tension-softening behavior of concrete influences its residual strength and subsequent crack propagation. The stability and the residual strength of a cracked reinforced concrete member under fatigue loading, depends on a number of factors such as, reinforcement ratio, specimen size, grade of concrete, and the fracture properties, and also on the tension-softening behavior of concrete. In the present work, a method is proposed to assess the residual strength of a reinforced concrete member subjected to cyclic loading. The crack extension resistance based approach is used for determining the condition for unstable crack propagation. Three different idealization of tension softening models are considered to study the effect of post-peak response of concrete. The effect of reinforcement is modeled as a closing force counteracting the effect of crack opening produced by the external moment. The effect of reinforcement percentage and specimen size on the failure of reinforced beams is studied. Finally, the residual strength of the beams are computed by including the softening behavior of concrete.

Enhancement of fatigue life of Ni-Ti-Fe shape memory alloys by thermal cycling

The effect of thermal cycling on the load-controlled tension-tension fatigue behavior of a Ni-Ti-Fe shape memory alloy (SMA) at room temperature was studied. Considerable strain accumulation was observed to occur in this alloy under both quasi-static and cyclic loading conditions. Though, in all cases, steady-state is reached within the first 50-100 cycles, the accumulated steady-state strain, epsilon(p.ss), is much smaller in thermally cycled alloy. As a result, the fatigue performance of them was found to be significantly enhanced vis-a-vis the as-solutionized alloy. Furthermore, under load-controlled conditions, the fatigue life of Ni-Ti-Fe alloys was found to be exclusively dependent on epsilon(p.ss). Observations made by profilometry and differential scanning calorimetry (DSC) indicate that the 200-500% enhancement in fatigue life of thermally cycled alloy is due to the homogeneous distribution of the accumulated fatigue strain. (C) 2010 Elsevier B.V. All rights reserved.

Fatigue in Ti-6Al-4V-B alloys

The addition of small amounts of B to Ti-6Al-4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress-strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-beta grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti-6Al-4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress-strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening. (c) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Alloy design for fracture resistance

Several methods for improving the strength of metallic materials are available and correlations between strength and various microstructural features have been established. The purpose of this paper is to review parallel developments favouring improved fracture resistance. Resistance to fracture in monotonie loading, cyclic loading and when fracture is environment-aided have been considered in steels, aluminium alloys and anisotropic materials. Finally, the question of optimising alloy behaviour is discussed.

High-temperature low-cycle fatigue behavior of a NIMONIC PE-16 superalloy—Correlation with deformation and fracture

Low-cycle fatigue (LCF) responses of NIMONIC PE-16 for various prior microstructures and strain amplitudes have been evaluated and the fatigue behavior has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed at 923 K on samples possessing three different prior microstructures: alloy A in solution-annealed condition (free of γ′ and carbides), alloy B with double aging treatment (spherical γ′ of 18-nm diameter and M23C6), and alloy C with another double aging treatment (γ′ of size 35 nm, MC and M23C6). All three microstructures exhibited an intial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudevs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behavior in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testign. Fresh precipitation of fine γ′ was confirmed by the development of “mottled contrast” in alloy C. Evidence for the shearing of the ordered γ′ precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to the reduction in the size of the γ′ and consequent softening. Coarser γ′ precipitates were associated with Orowan loops. The observed fatigue behavior has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples.

Characterization of Clean and Fouled Rail Track Ballast Subsurface Using Seismic Surface Survey Method: Model and Field Studies

The efficiency of track foundation material gradually decreases due to insufficient lateral confinement, ballast fouling, and loss of shear strength of the subsurface soil under cyclic loading. This paper presents characterization of rail track subsurface to identify ballast fouling and subsurface layers shear wave velocity using seismic survey. Seismic surface wave method of multi-channel analysis of surface wave (MASW) has been carried out in the model track and field track for finding out shear wave velocity of the clean and fouled ballast and track subsurface. The shear wave velocity (SWV) of fouled ballast increases with increase in fouling percentage, and reaches a maximum value and then decreases. This character is similar to typical compaction curve of soil, which is used to define optimum and critical fouling percentage (OFP and CFP). Critical fouling percentage of 15 % is noticed for Coal fouled ballast and 25 % is noticed for clayey sand fouled ballast. Coal fouled ballast reaches the OFP and CFP before clayey sand fouled ballast. Fouling of ballast reduces voids in ballast and there by decreases the drainage. Combined plot of permeability and SWV with percentage of fouling shows that after critical fouling point drainage condition of fouled ballast goes below acceptable limit. Shear wave velocities are measured in the selected location in the Wollongong field track by carrying out similar seismic survey. In-situ samples were collected and degrees of fouling were measured. Field SWV values are more than that of the model track SWV values for the same degree of fouling, which might be due to sleeper's confinement. This article also highlights the ballast gradation widely followed in different countries and presents the comparison of Indian ballast gradation with international gradation standards. Indian ballast contains a coarser particle size when compared to other countries. The upper limit of Indian gradation curve matches with lower limit of ballast gradation curves of America and Australia. The ballast gradation followed by Indian railways is poorly graded and more favorable for the drainage conditions. Indian ballast engineering needs extensive research to improve presents track conditions.

Evaluation of damage in reinforced concrete bridge beams using acoustic emission technique

Acoustic emission (AE) testing is a well-known method for damage identification of various concrete structures including bridges. This article presents a method to assess damage in reinforced concrete (RC) bridge beams subjected to incremental cyclic loading. The specifications in the standard NDIS-2421 were used to classify the damage in RC bridge beams. Earlier researchers classified the damage occurring in bridge beams by using crack mouth opening displacement (CMOD) and AE released and proposed a standard (NDIS-2421: the Japanese Society for NonDestructive Inspection). In general, multiple cracks take place in RC beams under bending; therefore, utilisation of CMOD for crack detection may not be appropriate. In the present study, the damage in RC beams is classified by using the AE released, deflection, strains in steel and concrete, because the measurement of the strains in steel and concrete is easy and the codes of practice are specified for different limit states (IS-456:2000). The observations made in the present experimental study have some important practical applications in assessing the state of damage of concrete structural members.

Analysis of fatigue crack growth in reinforced concrete beams

Mechanical behavior of reinforced concrete members is influenced by the action of unknown crack bridging reactions of rebars. Under cyclic loading, due to progressive growth of cracks, this bridging action contributes to the overall strength, stiffness and hysteretic behavior of the member. In this work, fatigue behavior of reinforced concrete beams are studied using a crack propagation law, developed using dimensional analysis for plain concrete with the effect of reinforcement being simulated through constraint exerted on the crack opening. The parameters considered in the model are fracture toughness, crack length, loading ratio and structural size. A numerical procedure is followed to compute fatigue life of RC beams and the dissipated energy in the steel reinforcement due to the shake down phenomenon under cyclic loading. Through a sensitivity study, it is concluded that the structural size is the most sensitive parameter in the fatigue crack propagation phenomenon. Furthermore, the residual moment carrying capacity of an RC member is determined as a function of crack extension by including the bond-slip mechanism.

Kaiser effect observation in reinforced concrete structures and its use for damage assessment

Reinforced concrete (RC) beams of span 3 m were tested under incremental cyclic load at different loading rates and simultaneously during the fracture process in the RC beams acoustic emissions (AE) were recorded. An attempt has been made to study the Kaiser effect as a measure of damage in RC beams. It was observed that RC beams made with high strength concrete under incremental cyclic loading showed an obvious Kaiser effect before the failure load. The results may be useful to study the damage in concrete structures and provide a reference for the application of Kaiser effect in engineering practice. (C) 2014 Politechnika Wroclawska. Published by Elsevier Urban & Partner Sp. z.o.o. All rights reserved.

Optimization of clamped beam geometry for fracture toughness testing of micron-scale samples

Fracture toughness measurements at the small scale have gained prominence over the years due to the continuing miniaturization of structural systems. Measurements carried out on bulk materials cannot be extrapolated to smaller length scales either due to the complexity of the microstructure or due to the size and geometric effect. Many new geometries have been proposed for fracture property measurements at small-length scales depending on the material behaviour and the type of device used in service. In situ testing provides the necessary environment to observe fracture at these length scales so as to determine the actual failure mechanism in these systems. In this paper, several improvements are incorporated to a previously proposed geometry of bending a doubly clamped beam for fracture toughness measurements. Both monotonic and cyclic loading conditions have been imposed on the beam to study R-curve and fatigue effects. In addition to the advantages that in situ SEM-based testing offers in such tests, FEM has been used as a simulation tool to replace cumbersome and expensive experiments to optimize the geometry. A description of all the improvements made to this specific geometry of clamped beam bending to make a variety of fracture property measurements is given in this paper.

离心机中动冰荷载的模拟

前言吸力式基础的应用起源于欧洲,最初用作浮动结构的稳固基础，1994年欧洲Europipe16/-11E第一次将海洋采油平台建在吸力式基础上[1].……

颗粒增强铝基复合材料冲击疲劳裂纹扩展的行为和机理

本文针对发展新一代步兵战车复合材料履带板所面临的关键问题，结合其实际受载特点，设计制备了冲击疲劳实验加载装置，并着重从实验设计及机理分析上进行细致深入的探索，揭示了Al_2O_3/LC_4复合材料冲击疲劳破坏的微观过程和机理。首先分别对SiC_P/LC_4、Al_2O_(3P)/LC_4 及基体 LC_4 进行了显微组织的观察与定量分析，并对其拉伸、三点弯曲破坏过程进行了在位观察，结合其断裂形貌的观察与分析，揭示出颗粒增强铝基复合材料断裂破坏的根本原因是颗粒的聚集及脆性相在晶界的严重偏聚。针对这一结论，给材料制备单位提出工艺改进意见。对工艺改进后制备的复合材料进行常规力学性能的测试，结果表明，其拉伸性能明显优于改进前制备的相应材料。为了进行冲击疲劳的实验研究，在分析步兵战车履带板实际受载特点的基础上，自行设计制备了冲击疲劳实验的加载装置。主要包括主体框架和测量系统，前者与小型振动系统配合使用可以实现冲击能量为 0.3J、冲击频率为 1Hz、冲击速度为 0.6m/s 的多次冲击实验；后者可以准确记录下任意时刻的冲击载荷波形及冲击疲劳载荷的循环数。为了考察颗粒与加载速率对复合材料疲劳机理的影响，实验研究了 Al_2O_3/LC_4 复合材料和 LC_4 纯基体材料在冲击疲劳和常规疲劳过程中裂纹的扩展过程及扩展速率。综合结果发现：与LC_4纯基体材料相比，Al_2O_3/LC_4复合材料疲劳裂纹扩展得更为迅速。复合材料中，由于颗粒的加入，两种疲劳方式下袭纹都发生严重偏转；裂纹经过颗粒时，多数是绕过，少数是切过颗粒；冲击疲劳裂纹扩展速率明显高于常规疲劳裂纹扩展速率。纯基体材料中，两种加载方式下，裂纹基本都以穿晶的方式扩展，裂纹常常表现为小锯齿状；冲击疲劳裂纹尖端的塑性变形程度比常规疲劳更大；冲击疲劳裂纹比常规疲劳裂纹更曲折，表现出多尺度的锯齿状（Zig-Zag）特征；冲击疲劳裂纹扩展速率高于常规疲劳的裂纹扩展速率。在基本实验的基础上，进一步对断口及裂纹扩展途径进行了微观观察和定量分析，最后综合全文的实验和统计结果，讨论了颗粒增强铝基复合材料的冲击疲劳机理。复合材料疲劳裂纹扩展速率的提高主要与裂纹的偏转有关，裂纹更倾向于沿着颗粒与基体的界面扩展；两种材料的疲劳裂纹扩展速率均随加载速率的增加而增加，呈现加载速率的反作用。加载方式的改变，一方面，由于冲击情况下载荷持续时间降低，使裂纹扩展速率降低；另一方面，加载速率的提高使得断裂韧性值降低，材料变脆，裂纹扩展速率升高。这两个方面相互影响，相互竞争，决定实际的裂纹扩展速率。两种材料中，不同加载速率下的疲劳裂纹扩展的微观机制基本一致，没有明显的本质区别。

粉质黏土周期荷载后的不排水强度衰化特性

在3种不同围压下进行了一系列重塑粉质黏土的静三轴和动-静三轴不排水剪切试验,得到了不同动应力水平条件下粉质黏土的孔压、动应变和不排水剪切强度值.通过对土样的不排水强度、孔压和动应变的无量纲化处理,确定了周期荷载作用后粉质黏土的不排水强度比与动载引起的孔压比和动应变比之间的相关关系.试验结果表明:粉质黏土在周期荷载作用后的不排水强度衰减程度取决于动载引起的动应变比值和孔压比值.当周期荷载引起的动应变比值小于0.1时,孔压比增长较快,土样的不排水强度几乎没有衰减;当动应变比大于0.1时,孔压比增长变慢,土样的不排水强度明显衰减;当动应变比值接近1时,孔压比值达到0.9,土样的不排水强度衰减程度约达到55 %.

Extracting material response from simple mechanical tests on hardening-softening-hardening viscoplastic solids

Compliant foams are usually characterized by a wide range of desirable mechanical properties. These properties include viscoelasticity at different temperatures, energy absorption, recoverability under cyclic loading, impact resistance, and thermal, electrical, acoustic and radiation-resistance. Some foams contain nano-sized features and are used in small-scale devices. This implies that the characteristic dimensions of foams span multiple length scales, rendering modeling their mechanical properties difficult. Continuum mechanics-based models capture some salient experimental features like the linear elastic regime, followed by non-linear plateau stress regime. However, they lack mesostructural physical details. This makes them incapable of accurately predicting local peaks in stress and strain distributions, which significantly affect the deformation paths. Atomistic methods are capable of capturing the physical origins of deformation at smaller scales, but suffer from impractical computational intensity. Capturing deformation at the so-called meso-scale, which is capable of describing the phenomenon at a continuum level, but with some physical insights, requires developing new theoretical approaches.

A fundamental question that motivates the modeling of foams is ‘how to extract the intrinsic material response from simple mechanical test data, such as stress vs. strain response?’ A 3D model was developed to simulate the mechanical response of foam-type materials. The novelty of this model includes unique features such as the hardening-softening-hardening material response, strain rate-dependence, and plastically compressible solids with plastic non-normality. Suggestive links from atomistic simulations of foams were borrowed to formulate a physically informed hardening material input function. Motivated by a model that qualitatively captured the response of foam-type vertically aligned carbon nanotube (VACNT) pillars under uniaxial compression [2011,“Analysis of Uniaxial Compression of Vertically Aligned Carbon Nanotubes,” J. Mech.Phys. Solids, 59, pp. 2227–2237, Erratum 60, 1753–1756 (2012)], the property space exploration was advanced to three types of simple mechanical tests: 1) uniaxial compression, 2) uniaxial tension, and 3) nanoindentation with a conical and a flat-punch tip. The simulations attempt to explain some of the salient features in experimental data, like
1) The initial linear elastic response.
2) One or more nonlinear instabilities, yielding, and hardening.

The model-inherent relationships between the material properties and the overall stress-strain behavior were validated against the available experimental data. The material properties include the gradient in stiffness along the height, plastic and elastic compressibility, and hardening. Each of these tests was evaluated in terms of their efficiency in extracting material properties. The uniaxial simulation results proved to be a combination of structural and material influences. Out of all deformation paths, flat-punch indentation proved to be superior since it is the most sensitive in capturing the material properties.

A discrete dislocation analysis of fatigue crack growth

Cyclic loading of a plane strain mode I crack under small scale yielding is analyzed using discrete dislocation dynamics. The dislocations are all of edge character, and are modeled as line singularities in an elastic solid. At each stage of loading, superposition is used to represent the solution in terms of solutions for edge dislocations in a half-space and a non-singular complementary solution that enforces the boundary conditions, which is obtained from a linear elastic, finite element solution. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated into the formulation through a set of constitutive rules. An irreversible relation between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip is also specified, which permits crack growth to emerge naturally. It is found that crack growth can occur under cyclic loading conditions even when the peak stress intensity factor is smaller than the stress intensity required for crack growth under monotonic loading conditions; however below a certain threshold value of ΔKI no crack growth was seen.