On crack twinning

The general formulation of the crack branching problem is given in terms of perturbation theory. The Wiener-Hopf method has been applied to derive an exact analytical solution of a singular plane problem of elasticity theory for a semi-infinite straight crack with two symmetric rectilinear cuts of finite length issuing from the crack tip. The solution of this problem can be applied to construct a theory for corrosion crack twinning, as well as for cracks having similar geometries, but other physical mechanisms. However, this problem is of considerable interest in studying the retardation in growth of a main crack crossed by a small slit.     

Coherent twinning phenomena: towards twinning superlattices in III-V semiconducting nanowires

We report evidence in GaP and InP nanowires for a coherent modulation of the structure along the wire axis. By using electron diffraction, we have observed an additional series of diffraction peaks consistent with a quasiperiodic placement of twinning boundaries along the wire. This observation is indeed unexpected, as the vapor-liquid-solid growth conditions used to produce the nanowires were not modulated. The averaged repeat distance of the structure, i.e., the distance between twin boundaries, has been found to depend on the temperature gradient imposed in the growth zone. Future control of the twinning superlattice period should allow significant design possibilities for electronic, thermoelectric, thermal and electro-optic applications of semiconducting nanowires.     

玻璃纤维筋锚具研制及其力学性能试验研究

将纤维增强筋(FRP筋)混凝土梁裂缝的开展过程视作FRP筋由两侧混凝土中拔出的过程,建立了基于粘结-滑移的FRP筋轻骨料混凝土梁裂缝宽度微分方程。根据规范给出的裂缝宽度限值,合理确定滑移量上限,提出并引入了适用于梁正常使用阶段的FRP筋钢纤维轻骨料混凝土“低滑移”阶段粘结-滑移本构模型。进而,在明确最大裂缝间距l_max、裂缝宽度放大系数h₂/h₁与裂缝截面纤维混凝土残余应力σ_fib等特征参数的基础上,利用迭代算法建立了FRP筋钢纤维轻骨料混凝土梁最大裂缝宽度计算模型。基于正常使用阶段裂缝宽度实测数据对建议模型的适用性进行评估,结果表明：裂缝宽度限值0.5 mm内,建议模型能够准确预测FRP筋钢纤维轻骨料混凝土梁的最大裂缝宽度。     

Deformation twinning in zircaloy 2

Abstract

Fully recrystallised zircaloy 2 samples were subjected to different degrees of uniaxial compression. Grains of high Taylor factors showed {1012}〈1011〉 deformation twins, noticeable up to 13–16% compression. Twinning strongly affected the crystallographic texture and also brought in clear differences in stored energy and residual stress between the suspected parent and product grains/orientations of twinning. At later stages of deformation, where presence of twinning was insignificant, aforementioned heterogeneity was further supplemented by heterogeneity in microstructure – clear presence of fragmenting and non-fragmenting grains. Direct observations on twin fraction, twin deviation and twin continuity had shown an apparent peak in twinning by ～7·5% compression, an observation explainable through a simple model of twin decay by in grain misorientation development.     

Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

The effect of twinning and detwinning on the mechanical property of AZ31 extruded magnesium alloy during strain-path changes

In order to investigate the effect of twinning–detwinning on the mechanical properties of AZ31 extruded magnesium alloy pre-compression and pre-stretch deformation were conducted along extrusion direction (ED) at 1%, 3%, 5% strain levels. After pre-strain, the strain-path was inverted by performing tensile or compressive tests at room temperature. Results showed that the detwinning behavior occurred during the inverse tension after the pre-compression. Although due to the aforementioned effect the tensile yield strength decreased, by increasing the pre-compressive levels both fracture elongation and peak strength improved. In the inverse compressive tests after pre-stretch the {1 0 −1 2} twinning was restrained and the volume fraction of twins decreased, leading to the improvement of yield strength by increasing in pre-stretching levels.     

New split mechanical seal launched

Splitex is a fully split mechanical seal being introduced by Burgmann.This is a short news story only. Visit www.worldpumps.com for the latest pump industry news     

Deformation twinning in ordered CuPt

Ordered 50 atomic percent copper-platinum has been deformed and examined by transmission electron microscopy. Both slip and {111} deformation twinning were observed. A model is offered to explain how the twinning is compatable with the L1₁ superlattice.     

The nanostructured origin of deformation twinning

We have revealed the fundamental embryonic structure of deformation twins using in situ mechanical testing of magnesium single crystals in a transmission electron microscope. This structure consists of an array of twin-related laths on the scale of several nanometers. A computational model demonstrates that this structure should be a generic feature at the incipient stage of deformation twinning when there are correlated nucleation events. Our results shed light on the origin of twinning-induced plasticity and transformation toughening, critical to the development of advanced structural alloys with high strength, ductility, and toughness.     

A study of twinning in polyethylene

Both {1 1 0} and {3 1 0} deformation twinning modes have been predicted for polyethylene. The existence of the former twinning mode has been confirmed in this work by non-photographic X-ray methods. When comparing and explaining bulk and single crystal twinning behaviour in terms of the influence of fold surface geometry, certain discrepancies arise. These may be resolved by noting that {1 1 0} twins occur at relatively low stresses and that {3 1 0} twins are formed under conditions of high strain during which the crystalline morphology is changed from lamellar to microfibrillar.     

Bismuth crystal twinning in thermal cycling

It was found that thermal cycling of monocrystalline bismuth in a regime of (77 373°K) leads to the appearance of a network of system {110} thermal twins in the surface layer of the specimens, accompanied by a decrease in microhardness at low loading.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 1, pp. 148–149, July, 1980.     

316L不锈钢加工硬化机制及孪生行为

对固溶处理后的316L不锈钢试样进行了拉伸实验,根据Ludwik真应力应变模型对拉伸实验曲线进行了非线性拟合,并用Crussard-Jaoul法计算和分析了Ludwik模型中的加工硬化指数（n）,同时通过对拉伸试样的微观组织观察,分析了316L不锈钢的加工硬化机制.实验结果表明：316L不锈钢在拉伸变形过程中加工硬化非...     

New CMOS-compatible mechanical shear stress sensor

A new approach to the measurement of mechanical stresses is presented. The new sensor design utilizes the fact that shear stresses in the silicon lattice generate an electric field perpendicular to an electric current. The sensor effect is characterized by a new piezo-bridge-coefficient, which relates the sensitivity of the sensor structure to its crystallographic orientation. The sensor is based on a CMOS-compatible structure. It offers the possibility to realize highly sensitive single-element stress sensors for use in MEMS or in smart force measurement strips, as well. An example of a signal conditioning circuit is shown. Special designs with improved sensitivity and low noise are presented. The response to parasitic magnetic fields is measured and strongly reduced. Furthermore, the temperature behavior was analyzed and finally optimized