Fracture Mechanics Based Model for Joints under Cyclic Loading

A generalized interface model for joints and cracks in quasi-brittle materials is formulated. The proposed model marries an existing fracture mechanics based one developed for monotonic loading of concrete with another frictional based model developed for the cyclic response of rock joints to address the (reverse) cyclic response of rough surfaces in the presence of cohesive stresses. The properties of the model and its capability to capture several experimentally observed behaviors are shown by the numerical simulations performed. This joint constitutive model is particularly suitable to simulate the seismic response of dam/rock joints subjected to seismic excitation, or of concrete joints under reverse cyclic loading.     

动载荷下边坡的失稳分析

根据公路和铁路承载结构以及载荷性质的不同,利用LUSAS有限元分别对两种动载的属性进行了分析,得出了两种载荷的位移、应力的时程曲线和应力变化轮廓,以及两种不同结构体的本征值与振动频率的关系,利用这些结果得出了两种动载荷的\     

关于岩石的动静态侵入断裂

对岩石、玻璃和陶瓷等脆性材料的动静态侵入断裂实验及理论研究现状进行了总结和评述。以实验结果为依据,描述了岩石在动静态侵入载荷作用下所产生的裂纹类型及其形成与扩展过程;讨论了用半经验,半理论方法建立的裂纹长度与侵入载荷(或压头冲击速度)之间的关系。最后指出了上述研究结果在岩石力学参数测定及凿岩机具设计与研制中的一些应用。     

Interface Behavior of a Bimaterial Plate under Dynamic Loading

The paper deals with interface behavior of bimaterial ceramic-metal composites under dynamic time-harmonic load. The first plate is precracked with a normal crack touching the interface between the plates. It is assumed that the respective restriction for the ratio of energy release rates of the plates allowing the occurrence of an interface single delamination before the initiation of the normal crack in the second plate is satisfied. The growth of interface delamination is not considered. The used approximate shear-lag dynamic approach gives a possibility to obtain solutions in a closed form for axial and shear stresses of the structure. At an elastic-brittle interface behavior theoretical predictions for single debond length of two bimaterial structures are calculated. The parametric analysis reveals the sensitivity of the interface single debond length and shear stress to the type of bimaterial structure and to the characteristics of the dynamic load—in particular its frequency and amplitude. All results are illustrated in figures and tables and are discussed.     

Deformation and Fracture Behavior of Alumina Particle-Reinforced Al 6061-T6 Composite during Dynamic Mechanical Loading

Alumina particle-reinforced aluminum 6061-T6 composites are subjected to deformation at high strain rates in torsion and in compression using Hopkinson bars. The volume content of alumina particles in the aluminum alloy are 10 and 20 pct. While occurrence of adiabatic shear bands is evident under compression, the specimens fractured rapidly in torsion at high strain rates. Fracture occurs in both testing modes by ductile shear failure with elongated dimples originating mostly from the particle matrix interface. Most of the reinforcing particles along the crack propagation path fractured in torsion. However, excessive thermal softening inside the shear bands in the specimens under compression loading caused debonding rather than fragmentation of most particles as the crack propagates along the shear bands. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

裂隙矿岩的动载特性

用SHPB高速冲击试验机,对4种不同裂隙类型的矿岩进行高速冲击试验,测定矿岩在高速冲击作用下应力波形、应变历史、应变率历史、应力历史、应力-应变曲线、应力-应变-应变率的关系、应力-应变-裂隙类型的关系,算出动态破坏强度和动态弹模.定量进行动静态矿岩力学特性比较,得出裂隙矿岩与完整矿岩在不同力学状态下的数值关系.     

Assessment of Subballast Filtration under Cyclic Loading

This paper presents an investigation into the seepage hydraulics of a layer of subballast filter subjected to cyclic loading in a fully saturated environment. A multilayer mathematical approach was used to predict the time-dependent permeability of this filter, with a reduction in porosity as a function of compression under cyclic loading, and the amount of base soil (<150?μm) trapped within the filter voids being the two main aspects of this proposed model. Laboratory test results conducted on a novel cyclic loading permeameter apparatus were used to validate the proposed model. The family of equations that are an integral part of the proposed model are then presented in the form of compact visual guidelines anticipated to provide a more practical tool for railway design practitioners.     

Behavior of One-Dimensional Consolidation under Time-Dependent Loading

This paper extends Olson’s solution for one-dimensional consolidation under time-dependent loading with the assumption of constant coefficient of consolidation, to those with varying loading-dependent coefficients of consolidation. The investigation first presents the experimental study on a series of one-dimensional consolidation tests with various loading rate. Subsequently, the viscoelastic theory was applied to estimate the consolidation settlement and compared with the experimental results. Four consolidation tests with different loading rates were performed on three types of remolded clay soils with various plasticity indices to evaluate loading rate effects on the consolidation settlement. After explicitly performing the falling head permeability test at various stages during the conventional consolidation tests, the variation of coefficient of consolidation with external pressure was established and converted to the relationship between the coefficient of consolidation and the time of the corresponding pressure applied on the specimens for each loading rate test. The hereditary integral of viscoelastic theory was then applied to predict the settlement curve with time for each loading rate test. The findings indicate that the theoretical predictions using the loading-dependent coefficients of consolidation and experimental results in term of consolidation settlement are in good agreement.     

Simulation of Soil Behavior under Blast Loading

A viscoplastic cap model was previously developed to address the high strain rate effect on soil behaviors. Although the model is an improvement over the inviscid cap model, it does not update soil density and bulk modulus as the shock wave propagates through the soil. Further, soil should be modeled as a three-phase porous media to accommodate various degrees of water saturation. This is especially true for the soil mass surrounding the source of energy release because each of the three phases responds differently to shock loading. A revised cap model comprising a Gruneisen equation of state for each of the three phases has been developed. These equations of state for solid, water, and air have been integrated with the viscoplastic cap model to simulate behaviors of soil with different degrees of water saturation. Numerical results from this revised soil cap model compared closely with experimental data from explosive tests in both dry and saturated soil.     

Behavior of Railroad Ballast under Monotonic and Cyclic Loading

A relatively new method for mechanized maintenance of railroad ballast (stoneblowing) puts a layer of single size stone between the ballast and each tie creating a two-layer gravel support. To get a better understanding of the behavior of this arrangement series of large diameter, triaxial tests have been carried out on single size and layered specimens. A new method of quantifying particle breakage during testing has been developed, and a conceptual model used to explain the combined effects of shearing and breakage on observed specimen behavior.     

Dynamic Response of Retrofitted Masonry Walls for Blast Loading

A full-scale blast test was conducted on eight masonry walls reinforced with two and four layers of carbon fibers and two types of polymer matrices. The walls were then subjected to a 0.45-kg pentolite booster suspended from the ceiling of a test structure. The pressure-time history caused by the blast and the resulting displacement response were measured during the test. This paper presents a summary of the test program and the corresponding results from a nonlinear single degree of freedom analysis. The results provide a basis for determining effective means of retrofitting existing masonry walls and designing new structures to withstand blast loads. The paper also outlines a fiber-reinforced polymer retrofit design procedure for walls subjected to blast loading.     

Interlaminar Fracture of Composites under Tension and Compression

Interlaminar fracture of AS4∕3502 graphite∕epoxy material system is investigated using a double cracked‐lap‐shear (DCLS) specimen and a single cracked‐lap‐shear (SCLS) specimen. A fundamental feature of the designed specimens is their ability to be tested under net tensile and compressive loadings. The specimens exhibit mixed‐mode or mode II behavior depending on the loading direction. The specimens are designed to precipitate crack growth at a designed‐in site in a gage section. In the specimen design process, overall dimensions of the specimens are selected so that local disturbances in the stress field will not interact, there is adequate length to permit crack growth, and overall buckling will not occur under compressive loading. The experimental results confirm that the specimens and tests perform as designed, It is observed that: (1) There is an increasing resistance to crack growth under tensile loading; (2) interlaminar fracture under compression is a totally unstable process; and (3) tension and compression behaviors are considerably different. Fracture surfaces in the unstable regions from short beam shear and DCLS specimen tests exhibit similar characteristics.     

模具钢P20的动态断裂韧性

利用示波冲击技术、透射电镜、相分析和Ｘ射线衍射等试验方法研究了塑料模具钢Ｐ２０的碳含量波动、不同热处理工艺和不同使用温度对其动态断裂韧性行为的影响。并探讨了在相应条件下材料的组织、结构特征及变化规律。该研究结果为优化Ｐ２０钢的生产工艺、延长模具的使用寿命提供了理论依据。     

Dynamic Fracture of a Zr-based Bulk Metallic Glass

In the present study, dynamic fracture experiments are performed on fully amorphous Liquidmetal-1 (LM-1), a Zr-based BMG, to better understand fracture initiation and propagation in notched specimens. Experiments are conducted on notched (110 μm notch radius) four-point bend specimens using an instrumented modified split-Hopkinson pressure bar apparatus. The results of these experiments suggest that the critical dynamic stress intensity factor achieved by the notched LM-1 specimens is ~110 MPa m^1/2, which is similar to the fracture toughness determined from previous quasi-static fracture experiments. This insensitivity of the fracture toughness to crack tip loading rate suggests negligible loading-rate sensitivity on the dynamic fracture initiation toughness in LM-1. In situ high-speed camera images of the notched sample during the dynamic loading process show multiple fracture initiation attempts and subsequent arrests prior to catastrophic fracture initiation. Controlled stress wave loading experiments designed to induce sub-critical levels of damage in the notched specimens show extensive deformation banding extending 150 to 200 μm outward from the notch. The deformation bands, nominally perpendicular to each other, run along the direction of the notch and perpendicular to it. They are consistent with slip-line fields in notched samples of elastic perfectly plastic materials. Subsequent loading of the damaged specimen again shows several attempts at crack initiation followed by blunting; the initial sub-critical damage in the region around the notch is understood to increase the energy required for catastrophic specimen failure and is consistent with an increase in the effective notch radius due to preexisting damage.     

Undrained Capacity of Plate Anchors under General Loading

Under general conditions of loading, a plate anchor is subjected to six degrees of freedom of loading, three force components and three moment components. Prediction of the anchor performance under general conditions of loading requires realistic estimates of the anchor pullout capacity for each individual load component as well as the interaction effects when these loads are applied in combination. This paper presents an analysis of plate anchor capacity under these general conditions of loading. The study considers a range of plate width-to-length ratios ranging from 1:1 to 2:1. The anchor capacity estimates and interaction relationships were developed based on finite-element studies and upper bound plastic limit analyses. Interaction relationships developed from the numerical and analytical studies were fitted to a simple six degrees-of-freedom yield locus equation.     

Analysis of Angle Shaped Footing under Eccentric Loading

Footings are often subjected to eccentric loading due to (1) moments with or without axial forces; (2) the oblique loading; and (3) their location near the property line. Due to eccentric loading, the footing tilts and the pressure below the footing does not remain uniform. The tilt of footing increases with an increase in the eccentricity and the bearing capacity reduces considerably. Therefore, footing sizes increase and make the design uneconomical. The finite-element analysis of an angle shaped footing under eccentric loading has been carried out in the present study. One side vertical projection of footing confines the soil and prevents its lateral movement. It was concluded that footing subjected to uniaxial eccentric loads can be designed for no or negligible tilt by giving the footing an angle shape. The depth of footing projection will depend upon the eccentricity width ratio.