Transverse crack growth in glass/epoxy composites with exactly positioned long fibres. Part II: numerical

Using finite element as the basis, transverse crack growth in long fibre composites have been investigated. Three criteria concerning pure matrix growth, fibre/matrix interface growth and crack kinking out of a fibre/matrix interface have been implemented to form a Crack Propagation Module for a software package. Numerically obtained crack paths have been compared to the ones obtained experimentally in Part I, and finally the Crack Propagation Module has been used on constructed examples in order to obtain information about the influence of various fibre distributions on the crack growth process.     

基于亲水性可控棉线有效分离苏丹I和罗丹明B

近年来,食品安全问题引起了人们的广泛关注.非食用色素因价格低廉、着色能力强而被非法用于食品加工过程,严重威胁消费者生命健康.苏丹I和罗丹明B是食品加工中最常被滥用的两种典型非食用有机着色剂,进入人体后会在相应酶的作用下产生致癌物.高效液相色谱法是用于违禁色素分离检测的主要方法,但该方法所需仪器昂贵,专业程度高且操作繁琐,限制了其在色素鉴定中的应用.棉线作为一种低成本固定相已被用于有机色素的色谱分离,但是对于其亲疏水性在色素分离中的作用目前仍不清楚,也未见将该系统直接用于色素分离收集的报道.针对上述问题,本研究采用等离子体可控预处理的方式实现对棉线亲疏水性能的调控,并进一步探究了棉线亲疏水性在棉线基色谱分离和收集过程中的作用.在等离子体处理过程中,调整了棉线的缠绕方式、等离子体功率和暴露时间,并通过扫描电镜、接触角测试和线上液体流动实验对棉线内部结构以及棉线亲疏水性进行了表征.结果表明,等离子体处理并不影响棉线的加捻结构和纤维间空隙大小;棉线以"之"字形悬空缠绕在支架上可增大暴露面积,从而获得更高的亲水性能;棉线的亲水性高度依赖于等离子处理时间和仪器功率,在10.15 W的等离子机中处理8 min被证明是最佳处理参数,可使棉线具有最高的亲水性能.X射线光电子能谱分析(XPS)表明,经等离子体处理后棉线的含氧基团显著增多,这可能是棉线亲水性提高的原因.以乙醇/水溶液为展开剂,探究了棉线亲水性对色素分离的影响,发现棉线亲水性提高可以显著增强色素的分离效果.色素与棉线的结合力以及流动相在棉线上的流动能力可能是色素分离效果的决定性因素.在色素分离收集实验中,分别以乙醇/水溶液和甲酸/甲醇溶液作为展开剂对罗丹明B和苏丹I进行分离收集,并对不同阶段的洗脱液进行了紫外吸收光谱分析.在乙醇/水溶液体系中,两种色素虽然可先后通过棉线固定相,但在苏丹I洗脱液中仍然含有微量的罗丹明B;而在甲酸/甲醇溶液体系中,两种色素可以更有效地分离,且分离速度更快.综上,本研究揭示了亲水性在线基色谱系统中的关键作用,开发了一种简便、低廉和易于操作的线基色素分离方法,并将其用于非食用色素苏丹I和罗丹明B的有效分离和收集.本研究有望为高性能线基色谱分离系统的设计和构建提供理论指导,并进一步拓展线基流控分离方法的应用领域.     

The influence of roller burnishing on the fatigue crack propagation in notched round bars—Experimental observations under three-point bending

Roller burnishing involves a local plastic deformation on the surface that permits the fatigue strength of structures to be increased. Crack propagation is delayed by the introduction of compressive residual stresses. In this way, the process is particularly useful in the presence of stress concentrators, for example in the fillets of crankshafts or in notched shafts. Crack propagation in round bars has been widely investigated, experimentally and numerically. However, the aim of the present work is to study roller burnished notched shafts (whose groove approaches the fillets of crankshafts) loaded in three‐point bending. Experimentally, a sequence of destructive interrupted tests has been performed to obtain the crack kinetics and shapes during fatigue life. Thus, the beneficial influence of roller burnishing has been confirmed on both crack initiation and propagation. Some cracks were even stopped by compressive residual stresses. Otherwise, the crack shape observed was very original : cracks propagated first at the edge of the coupon and then, into the depth of the shaft. Multicracking and crack closure have been pointed out and appear to be of great importance.     

A CRITICAL STUDY OF THE CRACK CLOSURE EFFECT ON NEAR-THRESHOLD FATIGUE CRACK GROWTH

Abstract— Crack closure in the near-crack-tip region has been considered to be an important contribution to the development of a crack-growth threshold for macroscopic cracks. Recent analytical work, however, has suggested that closure well back of the tip may be the controlling factor. In order to check on this possibility, material has been machined away far behind the crack tip in order to eliminate long-range closure. Removal of this material did not eliminate the threshold, but did lower the threshold level by approx. 10% for tests conducted on 6.3 mm-thick X7090-T6 powder metallurgy aluminum alloy.     

A study of COD and crack initiation by a replication technique

A silicone rubber has been used to infiltrate cracks created during slow bend tests on notched HY 100 and HY 130 steel specimens at room temperature. Crack opening displacements have been measured directly from the hardened rubber replicas of the crack. The growth of the stretch zone is in agreement with that predicted by slip line analysis. Crack initiation is seen to be a nebulous process occurring irregularly along the slot front. The initial growth of a fibrous crack has also been studied.     

304不锈钢管滚压成形圆锥外螺纹组织及性能研究

滚压加工螺纹可以获得更高的螺纹强度、精度,其在加工效率和环保性方面的优越性不容忽视.同时,滚压加工可以解决某些工艺方法不能解决的问题,如对特大型缸体的加工,可以达到减少机床和夹具、提高工件加工精度、缩短加工周期和节约作业面积的效果.为此,本文采用一种四滚丝轮车床滚压加工得到了304不锈钢管圆锥外螺纹.通过金相分析、扫描电镜、显微硬度计和万能试验机等手段,研究了304不锈钢管滚压成形圆锥外螺纹的组织及性能,并与传统的套丝管螺纹进行对比.研究结果表明,套丝加工的管螺纹,齿表层出现孔洞、金属组织被切断,其内部显微组织仍是原有的等轴晶,齿形不完整;而滚压加工使得螺纹表层组织细化,形成了沿螺纹齿形连续分布的纤维组织.套丝加工后304钢管螺纹硬度有所提高,但幅度不大;与套丝加工相比,滚压加工后304钢管螺纹的抗剪拉力和硬度均有大幅提高.     

THE BEHAVIOUR OF FATIGUE CRACKS SUBJECT TO APPLIED BIAXIAL STRESS: A REVIEW OF EXPERIMENTAL EVIDENCE

Abstract— Experimental studies aimed at understanding the fatigue process in metals and polymers have usually been performed under uniaxial stress. Only in the last two decades or so has much experimentation been carried out on fatigue crack propagation under biaxial stress. This paper reviews the available published data. Crack propagation behaviour under biaxial stress is dictated by 3 parameters: stress biaxiality itself, which is defined here as the ratio of the in-plane principal stresses, crack angle with respect to the applied principal stress directions and stress intensity factor range. Depending on the first two parameters, cracks may grow in Mode I, Mode II or Mixed-Mode. Crack growth data have been presented using these three divisions. Two short sections have been included on initiation and cyclic stress/strain behaviour under biaxial stress to emphasise the fact that crack growth cannot be fully understood without knowing something of them. The accumulated data do not lead to adequate conclusions on either the qualitative or quantitative behaviour of cracks subject to cyclic in-plane biaxial stress. Reasons for the confusion and even contradiction of independent results are put forward and some discussion given to the possible directions of future experimental work.     

Multi-axial fatigue life prediction of drill collar thread in gas drilling

With its advantages of lost circulation prevention, drilling speed improvement and reservoir protection, gas drilling technology has been widely applied in _Sichuan and _Xinjiang oilfields in _China. However, drill collar failures have often occurred under high weight on bit (WOB) during gas drilling. These incidents have not only caused serious economic loss but hampered the development and application of gas drilling. Finite element analysis is used to determine the drill collar thread stress distribution and, using simulation of drill string dynamics, multi-axial fatigue life theory is used to calculate the life of a standard API drill collar connection. The computed results reveal the early drill collar fatigue failures in gas drilling. Then, the multi-axial fatigue life of drill collar with double shoulder thread is analyzed under the same loads. Analysis shows that the fatigue life of a double shoulder thread is some thirty times that of the API thread. So drill collar with double shoulder thread could be an efficient way to solve the fatigue failure problems of drill collar in gas drilling. The work presented in this paper can provide theoretical foundations for safe and efficient drilling with gas.     

基于裂纹扩展尺寸概率密度演化法的疲劳可靠性预测方法研究

疲劳破坏是金属构件的一种主要破坏形式。该文提出一种新的疲劳可靠性预测方法—裂纹扩展尺寸概率密度演化法,该方法通过求解裂纹扩展尺寸时变概率密度函数来预测疲劳寿命可靠性。首先建立随机裂纹扩展模型,然后根据此模型建立概率密度演化方程,并运用数值方法进行求解。最后的算例证明了裂纹扩展尺寸的概率密度函数具有演化特征,新方法计算结果与蒙特卡罗法的结果符合较好。     

Tri-dimensional numerical modelling of plasticity induced fatigue crack closure

Fatigue crack closure has been studied numerically with 2D finite element models as alternative to experimental methods. In a few cases fatigue crack closure behaviour has been analyzed with tri-dimensional models. In this work, a compact C(T) aluminium specimen has been modelled tri-dimensionally. The minimum element size is smaller than common accepted recommendation. This parameter limits the mesh size along the thickness. The number of divisions of the thickness is a special issue studied in this work. Different thicknesses, range of loads and stress relations have been calculated. The results show the crack behaviour through the thickness. The plastic zone is visualized and quantified. An abrupt transition is observed in a thin external slice of the specimen; only captured with a fine meshing of the thickness. Crack closure/opening along the thickness have been computed. Results show that closure is only relevant in this small external area and its size is not affected by the thickness. Results match experimental observation and have been correlated with experimental tests.     

Highly efficient fibrous dye-sensitized solar cells based on TiO2 nanotube arrays

The structure of fibrous dye-sensitized solar cells, which were constructed by a TiO(2) nanotube array on Ti wire as the photoanode twisted by a Pt wire counter electrode, has been first systematically investigated by accurately controlling the thread pitch distance of screwed Pt wire. It has been revealed that the thread pitch will strongly influence the photovoltaic performance and kinetic processes in fibrous solar cells. The effect of the length of the TiO(2) nanotube on cell performance has also been discussed. After optimization, a relatively universal optimized thread pitch value of 1 mm for fibrous DSCs has been proved and the light-to-electricity conversion efficiency has been remarkably improved to 5.84%.     

基于涡流方法的智能螺纹检测仪的研制

针对目前存在的内螺纹人工检测工作量大的问题,采用基于单片机技术的涡流方法研制了一种智能螺纹检测仪。先设计了一个简单的阻抗测量电路,建立了探头对不同螺纹状态试样和试验频率的阻抗图,然后通过分析该阻抗图找出了适合于用幅度法检测的频率范围,最后通过对探头施加合适的正弦激励信号,用测量探头两端电压的方法完成螺纹检测。对给定的螺纹试样检验结果表明,该仪器可以准确地判断出试样的半牙、无牙和合格状态,并能给出相应的输出信号。     

Effect of intermetallic particles and grain boundaries on short fatigue crack growth behaviour in a cast Al–4Cu–3Ni–0.7Si piston alloy

The short fatigue crack growth behaviour in a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post‐mortem analysis of crack profiles. Crack–microstructure interactions have been clearly delineated using a combination of optical microscopy, scanning electron microscopy and electron backscatter diffraction. Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.     

Rißbeginn und Rißfortschritt in unlegierten Kohlenstoffstählen. Untersuchungen mit Hilfe der Schallemission

Investigations of Crack Initiation and Crack Propagation in Plain Carbon Steels by Means of Acoustic-Emission Crack initiation and propagation has been studied during fatigue tests with ferritic, ferritic-pearlitic and pearlitic specimens by means of acoustic emission (A.E.) instrumentation. The ferrite and the ferrite-pearlite showed acoustic emission only in connection with fast yielding of large volumes. Early steps of crack initiation showed no detectable acoustic emission because of very small yielded volumes and small amounts of crack surface friction. Gapping of the cracks inhibited A.E. due to friction during the early steps of crack propagation. Pearlitic specimens showed strong A.E. prior to microscopic detection of cracks, ascribed to fast fracture modes. Cracks in ferritic and ferritic-pearlitic CT-specimens are only detectable by crack surface friction. The increase of yielded volume with the propagating crack is too small to be distinguished from friction noise. Pearlitic specimens showed strong burst-signal activity ascribed to cleavage fracture and fast fracture modes. Great differences in A.E. of specimens with the same microstructure are in most cases the result of differing crack surface topography and therefore very differing friction areas. Friction noise depends clearly upon loading conditions and crack surface geometry.     

弯矩载荷作用下偏梯形套管连接螺纹参量敏感性分析

井眼弯曲段的套管在承受常规载荷的同时还要承受弯矩载荷作用, 因此导致弯曲段套管连接螺纹失效率较直井段要高, 然而目前对套管连接螺纹在弯矩载荷作用下的力学行为认识尚不明确。针对该问题, 该文基于虚功原理、接触非线性理论及Von Mises 屈服准则, 运用有限元分析软件建立并计算了弯矩载荷作用下套管连接螺纹的三维力学模型, 获得了套管连接螺纹的应力分布规律, 并用实验结果与数值模拟结果对比验证了模型的有效性;在此基础上对螺纹锥度、承载面角度、导向面角度、齿高及螺距五个螺纹参量进行了敏感性分析, 最后总结了弯矩载荷作用下各螺纹参量对其应力分布的影响规律。该文的研究工作为套管偏梯形连接螺纹的优化设计提供了科学依据, 对减少套管连接螺纹失效具有重要的工程价值。     

Stitched glass/PP composite. Part I: Tensile and impact properties

Stitching has proven to be an effective way to increase the through-the-thickness mechanical properties of fibre-reinforced polymer composites. However, there are rare investigations which concentrate on the stitching effect on fibre-reinforced thermoplastic polymer composite, particularly under different temperature environments. Here, we investigate the tensile and impact behaviours of stitched glass/polypropylene woven composites. The effect of various sewing threads, stitch row orientations, and spacing are evaluated. Our data indicate that the stitching in through-the-thickness direction considerably increases the impact damage tolerance especially at low temperature. In addition, glass sewing threads does not deteriorate the tensile performance of the stitched composite. The study of ductile ratio (D.R.) shows that suitable sewing thread can reduce the sensitivity of ductile behaviour of composite to the variation of temperature. A strong correlation of energy absorption with respect to sewing thread fracture work in relation to its fibre volume fraction was found.     

ELECTRICAL RESISTANCE VARIATIONS ACROSS A GROWING FATIGUE CRACK AS A FUNCTION OF THE LOAD CYCLE

Greater understanding of fatigue crack growth requires detailed measurements of crack tip plastic flow. A high resolution crack length measurement system based on the electrical potential method has been used to measure the variations in electrical resistance of specimens containing growing fatigue cracks. Crack growth increments caused by individual load cycles could be resolved down to a size of 0.1 μm. The electrical resistance has been found to vary cyclically as a function of the applied load. These variations are consistent with Bowles' observations that the crack initially grows with a very sharp crack tip followed by plastic blunting which is resharpened during unloading. Crack closure effects could be observed in the results generated from tests conducted with stress ratios of 0 and 0.1, but not for a test with a stress ratio of 0.5.     

芯轴螺纹升角对钛合金内螺纹管挤压变形均匀性的影响研究

目的 为了促进金属周向的流动、提高金属的流动均匀性以及钛合金内螺纹管的成形质量,确定合适的螺纹升角大小。方法 通过数值模拟技术对内螺纹芯轴的螺纹升角在10°～40°范围内进行适当调整,并用相对均差值衡量挤压稳态内螺纹管的流动均匀性,探究螺纹升角的变化对内螺纹管成形的影响。结果 适当增大螺纹升角至30°,可以促进金属周向的流动,提高金属流动均匀性。结论 当内螺纹芯轴螺纹升角为30°时,金属流动更为均匀。对优化后的内螺纹芯轴进行挤压试验,获得了成形质量良好的TA1钛合金内螺纹管。     

A numerical study of fatigue crack closure induced by plasticity

Crack closure delays the intrinsic mechanisms responsible for crack growth, therefore, it must be considered in fatigue crack growth modelling. The objective of this work is to develop a numerical procedure to predict crack closure induced by plasticity. First the crack closure was experimentally measured on M(T) 6082‐T6 aluminium alloy specimens of 3 mm thickness. A pin microgauge was used with the compliance technique. Then different parameters of the numerical procedure were analysed, namely the finite element mesh and the crack propagation scheme. The size of crack‐tip elements has an important influence and it is recommended to be of the same order of cyclic plastic zone. Crack‐opening levels only 10% lower than experimental results were obtained considering kinematic hardening and two load cycles in each increment.     

Fracture of Polymethylmethacrylate under Pulse Loading Conditions

Crack propagation in polymethylmethacrylate has been investigated experimentally under shock loading conditions generated by a pulse magnetic field. Using microstructural investigations, dynamic fracture surfaces of polymethylmethacrylate were analyzed and compared depending on the distance from the notch tip. A diagram of various fracture types depending on the pulse loading duration has been constructed.