Investigation on Aging-induced Softening of Eutectic Microstructurein SnBi/Cu Interconnect by Nanoindentation

Nanoindentation was used to probe softening of the eutectic microstructure in a SnBi/Cu interconnect following thermal aging. Tests were conducted at constant loading rates and under constant loads to characterize the flow stress and creep behavior of the eutectic microstructure. Aging was found to reduce the flow stress but to increase strain-rate sensitivity of the eutectic SnBi microstructure. While the reduction in the flow stress was related to the coarsening of the eutectic microstructure, the increased creep rate sensitivity was resulted from additional contribution of grain boundary movement to indentation creep.     

裂解炉管的蠕变应力计算模型

通过对乙烯裂解炉管运行工况的分析 ,在金属扩散理论与渗碳失效机理的基础上提出了渗碳产生的应力计算方法。对HP Nb材料制成的乙烯裂解炉管在 832～ 90 2℃下的应力场进行了模拟 ,分析了炉管在温度、内压、渗碳和蠕变等交互作用下炉管管壁应力的分布情况。结果表明 ,渗碳是引起炉管失效的主要因素。     

A steady-state model for diffusion-controlled fracture

Elevated temperature cracking associated with embrittlement from intergranular diffusion of impurities is studied using cohesive zone model for small-scale creep, diffusion and damage. The constitutive equation for the cohesive zone is coupled with stress-assisted diffusion of impurities into the grain boundary. A Kachanov-type damage model is used to describe the effect of impurity concentration on grain boundary strength. Numerical studies reveal the influence of various material parameters and loading conditions on the cracking process.     

Long-term behaviour of beams prestressed with aramid fibre cables: Part 1: a general method

This paper deals with the analysis of the time evolution of stresses in statically determinate beams with compact cross-sections prestressed or partially prestressed by aramid fibre reinforced polymer cables. The long-term behaviour of both concrete and the cables is taken into account to suggest a general method that solves the problem.

A constitutive law is needed to verity the reliability of the numerical solution by means of comparisons between experimental tests and numerical analyses and to evaluate the effect exerted by relaxation of the aramid cables on the cross-section behaviour. A long-term constitutive law of an aramid cable (based on the experimental tests performed by two research teams) is presented.

A comparison with the only well-related experimental test that was found in the scientific literature justifies the hypotheses set to take into account the phase between pouring of concrete and release of the cables from the bulkheads in the pretensioning systems.

This general method will be adopted as the reference to verify the reliability of an approximate solution that will be presented in another paper.     

Failure prediction for multi-material creep test specimens using a steady-state creep rupture stress

The steady-state stress distributions in single- and multi-material notched and waisted specimens were investigated for practical creep test specimens using material properties obtained for materials from a service-aged CrMoV pipe weldment. The tri-axial stress conditions existing in notched and waisted specimens machined from welded pipes were identified. By using a steady-state effective failure stress, it has been shown that an approximate method, based on steady-state stress, for the prediction of rupture life and failure position can produce reasonably accurate results. The applicability of the approximate method was confirmed by comparing the results obtained using it with those obtained from corresponding creep continuum damage modelling. These results indicate that use of steady-state stress analysis, as an approximate technique, may be useful for assessing creep failure behaviour, for determining the effect of specimen size and for generating material properties for welded components.     

Characterization of Multi-layer Weld Metal and Creep–Rupture Behavior of Modified 10Cr–1Mo Welded Joint

The rupture behavior of the modified 10Cr–1Mo steel multi-layer welded joint is determined by the fine-grain zones of the weld metal adjacent to the fusion line during the long-term creep test at 620 °C. The microstructures of multi-layer weld metal before and after the creep tests were characterized in detail, and its role in creep behavior was systematically investigated. Most grain boundaries of subgrains represented the low-angle boundaries in the weld metal adjacent to the fusion line both before and after the creep test. The widths of grains in the fine-grain zones were about 0.5–1 μm. The fracture morphology appeared as "wave" structure due to the cracking initiating from multi-layer grain boundaries in the fine-grain zones. Some W elements that melted into weld metal adjacent to the fusion line altered the thermodynamic and kinetic conditions of the Laves phase formation during long-term creep exposure. Laves phase particles mainly distributed along the grain boundaries due to the faster diffusion and segregation of Mo, W, and Si elements. Moreover, higher-density grain boundaries in the fine-grain zones led to easier nucleation and growth of Laves phase particles. Compared with other areas in the welded joint, the size of Laves phase particles in the fine-grain zones of the weld metal adjacent to the fusion line was the largest ones. The interface between Laves phase particles and the matrix acted as the nucleation site of creep micro-cavities. The creep micro-cavities grew up at the expense of fine-grain boundaries and even grew across the grain boundary deeply into adjacent grains, and then developed to cracks in the fine-grain zones.     

Superplastic compressive flow in MgB2

Macroplasticity in the brittle, superconducting ceramic MgB₂ would allow for the mechanical drawing of thin, dense superconducting wires, as done for metallic superconductors. Here, we report very large uniaxial compressive deformation (engineering strain of 67% or true strain of −1.1) without fracture at 1000 °C for specimens densified from commercially available MgB₂ powders with MgO and MgB₄ second phases. Plastic flow occurs under a diffusion-controlled mechanism with activation energies of 255–447 kJ mol⁻¹ and stress exponents of 1.4–2.0, indicative of superplastic behavior.     

Fatigue Modeling of Concrete-Filled Fiber-Reinforced Polymer Tubes

Field applications and laboratory research have shown the feasibility of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) in bridges. Yet, their widespread applications require developing appropriate design and analysis tools for different types of loading, particularly fatigue loads. An analytical tool is developed to trace the response of CFFTs under fatigue loading. The FRP material models are calibrated against fatigue and creep coupon tests. Material models are cast into a fiber element analysis, with an algorithm to simulate strain profile, moment-curvature and residual bending strength at any given time or after any number of fatigue cycles in a single or multiple stages of loading. Comparisons with available test data show good agreement with model predictions. A detailed parametric study shows that fatigue response of CFFT beams can improve by either increasing the reinforcement index or the effective modulus of FRP tube in the longitudinal direction. Higher load ranges may drastically reduce fatigue life. Therefore, it is important to limit the load level on CFFTs for a reliable and predictable member performance. The study also recommends reducing fiber orientation in angle plies with respect to the axis of the beam to improve fatigue performance of the CFFT member.     

A multi-scale model for coupled heat conduction and deformations of viscoelastic functionally graded materials

An integrated micromechanical-structural framework is presented to analyze coupled heat conduction and deformations of functionally graded materials (FGM) having temperature and stress dependent viscoelastic constituents. A through-thickness continuous variation of the thermal and mechanical properties of the FGM is approximated as an assembly of homogeneous layers. Average thermo-mechanical properties in each homogeneous medium are computed using a simplified micromechanical model for particle reinforced composites. This micromechanical model consists of two isotropic constituents. The mechanical properties of each constituent are time–stress–temperature dependent. The thermal properties (coefficient of thermal expansion and thermal conductivity) of each constituent are allowed to vary with temperature. Sequentially coupled heat transfer and displacement analyses are performed, which allow analyzing stress/strain behaviors of FGM having time and temperature dependent material properties. The thermo-mechanical responses of the homogenized FGM obtained from micromechanical model are compared with experimental data and the results obtained from finite element (FE) analysis of FGMs having microstructural details. The present micromechanical-modeling approach is computationally efficient and shows good agreement with experiments in predicting time-dependent responses of FGMs. Our analysis forecasts a better design for creep resistant materials using particulate FGM composites.     

Evaluation of C* integral for interacting cracks in plates under tension

A closed form solution for C* integral of two interacting cracks in plates under tension is developed on the basis of reference stress method. Comprehensive finite element (FE) creep analyses are carried out to provide the benchmark of the interaction evaluation of multiple cracks. Results indicate that more pronounced interaction is observed between the C* of double cracks and that of a single crack compared to that denoted by stress intensity factor (SIF). Overall good agreement is achieved between the proposed method for C* of multiple crack interaction and the FE results which provides confidence in practical application.