Leaching effect on mechanical properties of cement-aggregate interface

This study concerns the characterization of the mechanical properties of the cement paste and of the cement-aggregate interface in the degraded state by a chemical leaching mechanism. This study is carried out within the framework of the MIST laboratory. The experimental tests were carried out on cement paste and composites consisting of aggregates bound by the same cement paste prepared with a water/cement ratio of 0.5. The particular experimental devices, designed and produced for the purposes of this study, made it possible to directy solicit the composites at the cement-aggregate interface. As the process of leaching with the deionized water occurs very slowly, the experimental study is accelerated in the laboratory by replacing water by ammonium nitrate solution. To quantify the development and kinetics of chemical degradation at the cemented bond, the concrete leaching fronts are characterized at different times of degradation by using phenolphthalein. The local mechanical tests (tensile and shear) are performed on cement paste and composite at different degradation rate. The experimental results show a fast drop in shear or tensile strength of about 45% at the beginning of degradation. These results confirm the effect of the cement paste-aggregate interface degradation on the mechanical properties.     

Shear band formation in polycarbonate-glass bead composites

The shear band formation at glass beads embedded in a polycarbonate matrix subjected to a uniaxial tension has been investigated by microscopic in situ observation. The degree of interfacial adhesion was varied by different glass surface treatments. To gain insight into the three-dimensional stress field requirement for shear band formation, the distributions of several elastic failure criteria around an isolated adhering glass sphere in a polycarbonate matrix have been computed with the aid of finite element analysis. It was found that the mechanism for shear band formation is fundamentally different for adhering and non-adhering glass beads. In the case of excellent interfacial adhesion, the shear bands form near the surface of the bead in regions of maximum principal shear stress and of maximum distortion strain energy. In the case of poor interfacial adhesion, shear band formation is preceded by dewetting along the interface between bead and matrix.     

WC/铁基表面复合材料的热疲劳裂纹形成过程

为了对高热疲劳性能的表面复合材料的设计提供理论依据 , 采用热震试验方法对通过真空实型铸渗(V2 EPC) 方法制备的 WC/铁基表面复合材料的热疲劳性能进行了研究 , 重点讨论了热疲劳裂纹的形成机制。研究结果表明 , 随着 WC颗粒体积分数的增加 , 表面复合材料的热疲劳性能有所改善 , 当 WC体积分数达到 52 %时 , 复合层表面在经过 10次热循环后能保持完好。热疲劳裂纹扩展机制研究表明 , 陶瓷 WC颗粒增强铁基表面复合材料的热疲劳裂纹的产生和扩展是由 WC和基体本身的热应力和二者界面交变循环应力共同交互作用的结果 , 可以通过选用高热导率的陶瓷颗粒作为增强体、 改善陶瓷颗粒本身的微观质量和采用与陶瓷颗粒热膨胀系数相接近的基体等方法提高复合材料的热疲劳性能。      

Normal zone stationary distribution in superconducting composites with contact resistance

Heat and electrical contact resistances exist in composite superconductors between the normal metal and the superconductor in most cases. Frequently by some reasons these resistances are high enough. It is shown in this paper that in composite superconductors with noticeable contact resistance the existence of the resistive regions of finite size is possible. Such regions (resistive domains) are stable in the regime of fixed current and the temperature and the current density are non-uniform in the cross-section of the composite superconductor in the vicinity of the resistive domain. We find the I-V characteristic of a sample with a resistive domain and the minimum domain existence current ir. It is shown that the current iris much less than the minimum normal zone propagation current ip. We investigate the process of the resistive domain formation due to the heat pulse. Note that the resistive domains are unstable in the regime of fixed current in uniform composite superconductors without the contact resistance and are stable in non-uniform ones. The existence of the stable resistive domains largely regulates the process of the superconductivity destruction and the following recovery of the superconductivity in the presence of the transport current. Moreover the resistive domains existence is responsible for some hysteresis phenomena accompanying the superconductivity destruction and recovery. Some previous results were published in the short note.     

Multiple shear band formation in metallic glasses in composites

Multiple shear banding is observed in metallic glasses during tensile deformation of laminated composites containing such glasses. The phenomenon is related to (1) the local stress concentration that develops as a result of the formation of the first shear band, (2) the distribution in stress required to initiate shear banding in tensile loading, and (3) the properties of the surrounding matrix. The tendency for localized and uniform multiple shear banding has been determined. This was done by utilizing a finite element method (FEM) to simulate the local stress state in the vicinity of the shear band first formed, and by determining the distribution in shear band initiation strengths. The experimental data were combined with the FEM analysis to predict locations of secondary shear band initiation. Localized secondary banding is predicted for large initial slip displacements, whereas uniform banding is expected when the initial slip displacement is small.     

Macrokinetics of intermetallic compound formation in the diffusion zone

A macrokinetic model is developed for reaction diffusion in binary metallic systems for situations when the phase composition of the diffusion zone does not correspond to the equilibrium phase diagram.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 3, pp. 957–965, December, 1988.     

Plastic zone formation around an arresting crack

We have followed the evolution of strain near the tip of an arresting cleavage crack in a pressure vessel steel using a strain gage technique. The measurements were made using a linear array of strain gages located along the intended crack path and 0.65B (B is the specimen thickness) above this plane. the gages were read using instrumentation capable of resolving strain in time intervals of between 2 and 4 s. The results were obtained in a thick plate, large enough that reflected elastic waves did not contribute significantly to the strain records or crack motion during the time interval of interest.To within the resolution of this technique, the gages did not detect plasticity near the rapidly propagating cleavage crack tip. At the instant of arrest, however, the strain gages detected a plastic zone which increased in intensity over a period of several milliseconds. We have interpreted this increasing intensity as an elastic-plastic boundary emanating from the arresting crack tip and eventually reaching an equilibrium size, i.e., that predicted from the applied stress intensity factor and the static plastic properties. A simple analysis is presented to estimate the velocity of this elastic-plastic boundary from the strain data and results are given for several experiments.

---

Résumé En utilisant des jauges de contraintes, on a suivi, dans un acier pour capacités sous pression, l'évolution des déformations près de l'extrémité d'une fissure de clivage en cours d'arrêt. Les mesures ont été faites en recourant à une disposition de jauges en ligne, située le long due parcours obligé de la fissure et à une distance de 0.65 × l'épaisseur de l'éprouvette au-dessus du plan de fissuration. Une instrumentation de lecture des indications des jauges permettait de déceler les variations de déformations toutes les 2 à 4 micro-secondes. Les résultats ont été collationnés pour une tôle forte suffisamment grande pour que les contributions des ondes élastiques à l'enregistrement des déformations ou à la croissance de la fissure durant cet intervalle de temps soient peu significatives.Dans les limites de résolution propres à cette technique, il est apparu que les jauges de contraintes ne décelaient pas de plastification près de l'extrémité de la fissure de clivage en cours de propagation rapide. Cependant, au moment de l'arrêt, les jauges détectent la formation d'une zone plastique, dont la taille s'accroît sur un intervalle de plusieurs millisecondes.On a interprété cette croissance en termes d'une frontière élasto-plastique qui émane de l'extrémité de la fissure lorsqu'elle s'arrête, et qui atteint ensuite un état d'équilibre, tel que celui prédit par le facteur d'intensité de contraintes appliqué, et par les propriétés plastiques du matériau en conditions statiques.On présente une analyse simple en vue d'estimer la vitesse de cette frontière élasto-plastique à partir de données de déformations, et des résultats relatifs à plusieurs essais sont fournis.

---

---

National Institute of Standards and Techonology

---

National Institute of Standards and Techonology     

The formation of residual stresses in laminated thermoplastic composites

The means by which residual stresses are generated in continuous carbon-fibre-reinforced thermoplastic composites are reviewed, with specific attention to the macroscopic stresses which form on a ply-to-ply level. Different means for their estimation are assessed, and approximate stress levels are defined for a variety of materials. The capacity for stress relaxation to occur during processing is examined, and it is shown that little useful stress-relieving ability can be promoted in such systems.     

Peptide-mediated formation of single-wall carbon nanotube composites

The formation of silica- and titania-coated single-wall carbon nanotubes (SWNTs) using a mutlifunctional peptide to both suspend SWNTs and direct the precipitation of silica and titania at room temperature is demonstrated.     

Machining of UD-GFRP composites chip formation mechanism

The chip formation mechanism in orthogonal machining of unidirectional glass fiber reinforced polymer (UD-GFRP) composites is simulated using quasi-static analysis. Dynamic explicit finite element method with mass scaling is used for analysis to speed up the solution. A two-dimensional, two-phase micromechanical model with elastic fiber, elasto-plastic matrix and a cohesive zone is used to simulate the debonding interface between the fiber and the matrix. The elements of the fiber are failed once the maximum principal stress reaches the tensile strength and the matrix elastic modulus is degraded once the ultimate strength is reached. The effect of fiber orientation, tool parameters and operating conditions on fiber and matrix failure and chip size is also investigated. The degradation of the matrix adjacent to the fiber occurs first, followed by failure of the fiber at its rear side. The extent of sub-surface damage due to matrix cracking and interfacial debonding is also determined.     

Process zone in fracture toughness testing of HSLA steels

Materials Science - The method of statistic fractography used after the fracture toughness tests provided the following information:     

Process zone analysis for the single-edge notched specimen: Part II. Process zone growth and crack propagation

Process zone growth and crack propagation in the single-edge notched (SEN) specimen are studied using the relations among applied load, notional crack and process zone lengths, and crack opening displacement derived in the first part of this work [1]. Process zone growth is simulated by increasing the notional crack length while keeping the traction-free crack length constant. A model for crack propagation based on either critical crack tip opening displacement (CTOD) or critical process zone length, as criteria for traction-free crack extension is proposed. The influence of closing pressure distribution, initial traction-free crack length, and crack extension criterion on the behavior of load vs. CMOD curves is discussed. The present model can be used to model load-deformation behavior from initial loading through softening to failure of nonlinear materials, as is verified by comparing the theoretical and experimentally determined load vs. crack mouth opening (CMOD) curves for concrete beams.     

Process zone analysis for the single-edge notched specimen: Part I. Process zone size and crack profile

A systematic analysis of the process zone is presented. Relations among the process zone size, load and crack opening displacement (COD) are derived using the weight function method and a power function closing pressure distribution in the process zone. These relations are specialized and used to study the behaviour of the single-edge notched (SEN) specimen loaded in tension and in bending. This study shows that the method presented here may be successfully used to analyze the fracture of finite-size specimens.     

Formation of a graphite-rich zone in centrifugally cast copper alloy graphite composites

The formation of the graphite-rich zone in centrifugally cast copper alloy containing originally 7 and 13 vol % graphite particles, cast at 800 and 1900 r.p.m. was studied as a function of the rotational speed of the mould and the volume fraction of graphite particles. The calculated recovery of graphite particles in copper alloys was found to be around 99% and 95%, for 7 and 13 vol% graphite particles originally added to the melts, respectively. The ratio of the thickness of graphite-rich zone to the casting thickness was observed to increase with increasing volume fraction of graphite particles and with decreasing rotational speed of the mould. These ratios were found to be between 0.56 and 0.76 for centrifugal casting of copper alloy containing 7 and 13 vol % graphite particles, respectively, cast at 800 r.p.m.; the corresponding ratios were between 0.42 and 0.71, respectively, for the alloys cast at 1900 r.p.m. The ratio of the thickness of the graphite-rich zone to the casting thickness has been discussed in terms of the velocity of the particle and the solid/liquid interface. © 1998 Chapman and Hall