Study on the effect of corrosion on the tensile behavior of an AZ91 alloy

The effect of corrosion on the tensile behavior of an as‐cast AZ91 alloy was investigated via systematical experiments. The corrosion residual strength of the AZ91 magnesium alloy is approximately negative linear dependent on the extreme depth of corrosion pit, which was quantitatively obtained by the corrosion morphology analysis and the statistic analysis. Experimental results reveal that the corrosion residual strength of the AZ91 magnesium alloy dropped quickly in the first 40 h due to the initiation and development of the corrosion pit and subsequently decreased slowly as a result of the development of the corrosion pit encountered the β phase. The β phase at the grain boundary of the primary α phase acts as an effective barrier to prevent the development of the corrosion pit into the alloy matrix and thus remarkably prevents the drop of the corrosion residual strength. In general, the variation of the corrosion residual strength exhibits an exponential decay regulation within the testing time. Consequently, the empirical prediction of the corrosion residual strength was given with regard to the variation of the corrosion residual strength varied with the immersion time.     

Effect of repetitive processing on the mechanical properties and fracture toughness of dynamically vulcanized iPP/EPDM blends

The effect of repetitive processing on the mechanical properties and fracture toughness of dynamically vulcanized isotactic polypropylene/ethylene‐propylene‐diene rubber blends (TPVs) with and without addition of β‐nucleating agent (β‐NA) was studied. The results showed that the repetitive processing did not cause much loss in the mechanical properties of TPVs, especially for TPVs with β‐NA, and TPVs with β‐NA showed better performance stability than TPVs without β‐NA. Essential work of fracture (EWF) approach was used to study the fracture behavior, and the results showed that the value of w_e (the specific essential work of fracture) of TPVs without β‐NA showed a significant decrease while that of TPVs with β‐NA almost kept constant after repetitive processing. Differential scanning calorimetry and wide‐angle X‐ray diffraction were used to study the variation of crystalline structures, and the results indicated that the repetitive processing showed no significant influence on the crystalline structures of TPVs, and the β‐NA maintained high‐nucleating efficiency after repetitive processing. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Comparison of CTOD standards: BS 7448-Part 1 and revised ASTM E1290

Crack tip opening displacement (CTOD) has been calculated using the plastic hinge model with an assumed rotational center since the British Standards Institution (BS) standardized BS5762 in 1979. The American Society for Testing and Materials (ASTM) accepted the plastic hinge model and standardized E1290 in 1989. However, ASTM revised E1290 in 2002, and has proposed a conversion from J to CTOD. CTOD-based fracture toughness evaluation has been widely used for the defect assessment of many welded structural components, and two different CTOD calculations could lead to confusion for Fitness-for-Service. In this study, the effects of CTOD testing methodologies on CTOD values were investigated according to round robin tests conducted by the Japan Welding Engineering Society (WES), and the concept of CTOD as a fracture parameter is discussed.     

A simplified FE model for pull-out failure of spot welds

A simplified model is developed to simulate the pull-out failure behavior of spot welds in full vehicle crash simulations. The failure is only considered in the base steel which is characterized with the Gurson model. The parameters of the Gurson model are optimized from modeling several different test types of base steel. The weld region is represented by a single solid element, and its properties are optimized from modeling the coach-peel and lap-shear joints. The simplified model is validated by modeling the spot-welded coupon tests. It is indicated that an acceptable time step size for full vehicle crash simulations can be achieved with the model.     

Evaluation of different modelling conditions in the cohesive zone analysis of single-lap bonded joints

Cohesive Zone Models (CZM) are widely used for the strength prediction of adhesive joints. Different simulation conditions, such as damage initiation and growth criteria, are available for use in CZM analyses to provide the mixed-mode behaviour. Thus, it is highly relevant to understand in detail their influence on the simulations’ outcome. This work studies the influence of different conditions used in CZM simulations to model a thin adhesive layer in single-lap joints (SLJ) under a tensile loading, for an estimation of their influence on the strength prediction under diverse geometrical and material conditions. Validation with experimental data is considered. Adhesives ranging from brittle to highly ductile and overlap lengths (L_O) between 12.5 and 50 mm were considered. Different studies were considered: Variation of the elastic stiffness of the cohesive laws, different mesh refinements, study of the element type, and evaluation of several damage initiation and growth criteria. The analysis carried out in this work confirmed the known suitability of CZM for static strength prediction of bonded joints and pointed out the best set of numerical conditions for this purpose. Inaccurate results can be obtained if the choice of the modelling conditions is not the most suitable for the problem.     

基于多孔介质热弹性理论的井壁诱导缝成因

常规纯弹性理论通常把钻井过程中直井井壁诱导缝的形成归结为钻井液密度过高或者水平主应力差值过大,然而该理论却并不能完全合理地解释深井井壁诱导缝的成因。为揭示井壁诱导拉伸缝形成机理,考虑钻井液循环期间低温钻井液与高温井壁围岩的热交换,基于多孔介质热弹性力学理论,建立了适于深部地层的井壁稳定分析模型,研究了流固热耦合作用下的井周周向有效应力、孔隙压力与温度分布计算模型。计算结果表明:(1)在最大水平地应力方位,井眼钻开初期多孔弹性作用居于主导,孔隙压力在近井壁地带出现谷值,对诱导缝的形成起到了抑制作用;(2)然而随着钻井液与地层热交换的进行,钻井液的冷却作用愈加重要,井周周向有效应力由挤压状态逐渐转变为拉伸状态,从而导致近井壁地带诱导缝的形成。结论认为,为避免诱导缝的形成,在工程上一方面需加强钻井液的封堵性,控制液柱压力向井周的扩散;另一方面也可以通过调整优化钻井液密度与流变性能,控制井底钻井液当量密度(ECD)。     

Experimental investigation on sealing efficiency of chemical grouting in rock fracture with flowing water

This paper presents an experimental investigation of the main factors that influence the sealing efficiency of chemical grouting in a permeated fracture replica. Chemical grouting has been widely used to control groundwater inrush in underground mining. However, the relationship between the sealing efficiency of grouting and its influencing factors is one of the many critical issues that still remain unsolved. The four factors tested in the orthogonal experiment include the fracture aperture width, initial water flow speed, gel time, and grout take. The sealing efficiency is evaluated and graded by the reduction of water drainage through the fracture after grouting. The experimental results of the orthogonal arrays show that two most influencing factors on sealing efficiency are the initial water flow speed and the aperture width, where the former has a higher impact than the latter. In a comparison with the two other factors, it is found that the grout take and gel time have less influence. The patterns of grout propagation are classified into four types: (1) total sealing off; (2) partial sealing off with erosion along boundaries; (3) partial sealing off with internal erosion; and (4) total erosion. The interaction between the injected grout and flowing water slows down the water flow and the grout starts to solidify at a downstream location where the retention time for grout is longer than its gel time.     

Heterogeneous materials in compression: Correlations between absorbed,released and acoustic emission energies

Compression tests on concrete and rocks – limestone, marble and granite – specimens have been carried out to investigate on the correlation between absorbed, released and acoustic emission energies. To this aim, the complete load vs. displacement curves have been obtained up to complete failure, evidencing very different post-peak responses – variable from normal softening to catastrophic snap-back – by varying the material, even for the same size and slenderness. Due to the expected brittle behaviour of some of the considered specimens, the loading process has been controlled by means of the circumferential strain with a linked chain placed around the cylindrical specimen at mid-height. The absorbed energy per unit surface is then computed through the overlapping constitutive law. Such a parameter, that results to be almost scale-invariant, is compared to the elastic energy accumulated in the body at the point of instability to define a structural brittleness index. Furthermore, all the tests have been monitored by means of the acoustic emission technique. The obtained data have been analysed in details, in order to draw a connection with the released energy in the post-peak regime, and to discriminate the different failure mechanisms.     

岩石、混凝土类材料复合裂纹断裂准则的探讨

本文对岩石与混凝土类脆性材料的强度和断裂机理进行了简要分析。在最大分离位移(MAXSD)屈服准则的基础上,建立了三维复合型裂纹的断裂准则。并与混凝土材料的三维复合裂纹和Ⅰ、Ⅱ型复合裂纹的断裂试验结果进行了比较。     

抗热冲击性陶瓷

对材料进行急热急冷处理所施加的温度变化—热冲击导致材料的表面与内部产生温度梯度(温差)。高温部分与低温部分之间因热膨胀量不等而产生应力—热应力,当热应力达到或超越材料的破坏应力(强度)时,材料发生破坏—热冲击破坏。陶瓷材料机械性质中表现最复杂举动的热冲击性不仅与材料本身性质不关,特别是热膨胀系数,还与其它条件,如热传导系数等有关。在加热过程中,如热传导速率满足不了升温