PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE CRACK GROWTH RATE AND FATIGUE THRESHOLD

The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete asperities model. A wide range of load ratios, both positive and negative, are examined. Particular emphasis is placed on compressive excursions, i.e. negative R loadings. The inelastic discrete asperities model is a micro-mechanical analysis based on the plastic crushing of a single asperity (or multiple asperities) located on the crack face close to the crack tip and under dominantly plane strain conditions. Experimental data have indicated that the primary crack face contacts which obstruct closure are immediately adjacent to the crack tip, although segments of the crack face more distant from the crack tip are not neglected. However, the more distant asperities are a part of the past crack advance history which does not influence current behaviour. By use of this model, it is shown that the effect of the load ratio can be adequately predicted once some baseline information on mechanical material properties and surface roughness is provided. The model also provides useful trend information and explains many of the observed phenomena, e.g. the ‘saturation’ of the compressive underload effects. For a constant applied nominal stress intensity factor range, ΔK_nom , it is shown that the effective stress intensity factor range, ΔK_eff , initially decreases as the positive R decreases (corresponding to the increasing influence of closure), reaches a minimum around R = 0, and then starts increasing with negative R (corresponding to the plastic crushing of the asperities which reduces closure), eventually reaching a saturation level below ΔK_nom . Conversely, for an assumption of a constant ΔK_eff , the applied ΔK_nom increases as the positive load ratio decreases, reaching a maximum around R = 0, and then decreases with more negative R values, eventually reaching again a saturation level (above ΔK_eff ). It is also shown that the effect of material hardness can be directly analysed based on this model.     

Singular stress fields in interfacial notches of hybrid metal matrix composites

This paper presents results from a study on the singular stress fields in metal matrix composites in which dual matrices exist. The adjoining metallic matrices flanking the interface can deform plastically with powerlaw strain hardening. These matrices may have both different hardening exponents and different yield strengths. An asymptotic analysis coupled with numerical eigen-analysis solved the spatial structure of the singular stress field at radial- and angular-dependent parts: . The dependence of the strength of the singular stresses on the matrix properties is discussed. The effects of local geometry on the nature of singular stresses are addressed. Highlights for interfacial notches are reported here. The drivers for this study are interfacial notches and free-edges in hybrid metal matrix composites (Fig. 1). However, the results can also be applied to other advanced structures which are composed of two or more distinct components or phases such as bone-implant interfaces and surface mounts in electronic packages     

Strength of epoxy-resin-based insulation systems in transverse tension and shear using two novel test pieces

A novel test specimen type has been developed to generate engineering data on bonded aluminium surfaces. The test is designed to simulate conditions in a superconducting coil, operating at 4.2 K, in a region where a glass/epoxy composite laminate is loaded primarily in through-thickness tension and in shear. We include the effects of thermal contraction from the cure temperature to the operating temperature of 4.2 K. The test specimen avoids the large stresses associated with the edges of the composite in other through-thickness tension test pieces and results calculated from the measured failure loads of specimens are the peak stresses in the centre of the specimen.     

Analysis of Forced Flow of Granular Materials in Vertical Pipes without and with Air Permeation

The problems associated with grain elevation and conveying under forced flow in vertical pipes are discussed. Based on experimental results, a theory is presented to describe forced flow with varying degrees of air permeation up to and just beyond the fluidization point. The theory takes into account the boundary and internal frictional properties, the degree of consolidation of the bulk granular material, and the stress fields that occur during forced flow. The force to elevate grain in a vertical tube is shown to be composed of two components, one to overcome Coulomb friction and initiate motion, and the other a time-dependent component that depends on the stiffness and damping characteristics of the granular material. The Coulomb friction component increases approximately exponentially with column height due to the positive feedback effect of the shear stresses at the pipe wall opposing the motion. Air permeation is shown to significantly reduce this component of the conveying force by reducing both the internal friction and the apparent bulk specific weight, the latter being the actual bulk specific weight less the air pressure gradient. Air permeation has a very significant effect on reducing both the bulk stiffness and, particularly, the damping characteristics, thereby reducing the time-dependent component of the conveying force.     

On size and technological effects in fatigue analysis and prediction of engineering materials and components

Analytical approaches concerning size, stress gradient and technological effects such as surface roughness and residual stresses induced during manufacturing processes are presented and discussed in this paper. Their implementation into the Short-Crack-Model for fatigue-life (lifetime to initiation of cracks of a size of 0.5–1 mm) prediction of engineering components subjected to cyclic loading is explained in detail. The procedures to consider the aforementioned effects are demonstrated by using an example of a forged and tempered steering shaft made of low-alloyed steel subjected to variable amplitude bend loading. The corresponding experimental results are used to check the accuracy of the analytical fatigue-life prediction. The comparison between analytically calculated and experimentally determined fatigue-life values emphasises the significance of technological effects (surface roughness, residual stresses) on fatigue-life estimation and the usefulness of the Short-Crack-Model for fatigue-resistant design of engineering components.     

On the local elastic–plastic behaviour of the interface in titanium/silicon carbide composites

The purpose of this study is to conduct a high-resolution nonlinear finite element analysis of the elastic–plastic behaviour of titanium/silicon carbide composites subject to transverse loading. This class of metal matrix composites is designed for the next generation of supersonic jet engines and deserves careful assessment of its behaviour under thermo mechanical loads. Three aspects of the work are accordingly examined. The first is concerned with the development of a representative unit cell capable of accurately describing the local elastic–plastic behaviour of the interface in metal matrix composites under thermal and mechanical loads. The second is concerned with the determination of the influence of mismatch in the mechanical properties between the inhomogeneity and the matrix upon the induced stress fields and the plastic zone development and its growth. The third is concerned with unloading and the role played by the interface upon residual stresses. It is found that the maximum interfacial stress in the matrix appears in the case involving cooling from the relieving temperature with subsequent applied compressive loading. It is also found that the mismatch in mechanical properties between the matrix and the inhomogeneity introduces significant changes in the stress distribution in the matrix. Specifically, it is observed that the maximum radial and tangential stresses in the matrix take place at the interface. The plastic deformation of the matrix leads to a relaxation of these stresses and assists in developing a more uniform interfacial stress distribution. However, the matrix stresses and the resulting equivalent plastic strains still reach their maximum values at that interface. The results show similarities in the patterns of the interfacial stress distribution and plastic zone development for all ranges of fibre volume fractions and loading levels examined. However, they also show marked differences in both the magnitude and patterns of matrix stress distribution between the adjacent inhomogeneities as a result of interaction effects between the fibres.     

Recognition and Importance of Restrictions on Randomization in Industrial Experimentation

Statisticians typically recommend completely randomized experimental designs. The reasoning behind this advice is theoretically sound. Unfortunately, engineers who typically run industrial experiments frequently fail to recognize restrictions on randomization, i.e., split-plot experiments, and are often unaware of the risks associated with analyzing split-plot experiments as if they were randomized. Similarly, issues concerning the inference space of the experiment frequently are not given adequate consideration. Conversely, statisticians frequently are unaware that a restriction on randomization does not necessarily translate into less information than a completely randomized design.

In this paper, we discuss a proactive methodology for identifying and incorporating information concerning restrictions on randomization and inference space in industrial experiments. We also present the factor relationship diagram (FRD), a tool that assists engineers in the recognition of restrictions of randomization and guides the development of questions that encourage the experimenter to understand those sources of variation that may contribute to a lack of precision in a split-plot experiment or lack of repeatability in inference space different from that studied in the experiment. Examples that illustrate the use of the methodology and the FRD are included.     

用EWB进行自动控制原理实验的探索

将虚拟电子平台软件EWB用于自动控制原理实验，补充了其它几种实验方法的不足，既实用、灵活又简单直观，使自控原理实验的质和量都得到了很大提高，用EWB模拟的自动控制实验系统、所用的信号源以及各种测试仪器都与现场实物相仿，但功能更强，使用更方便。     

哌啶对MO/MCM-41催化剂上二苯并噻吩加氢脱硫反应的影响

在固定床反应器上考察了哌啶对二苯并噻吩(DBT)的加氢脱硫(HDS)反应活性及反应路径的影响。反应所用催化剂为MCM-41分子筛担载的Co-Mo或Ni-Mo硫化物,MoO3的负载量为20%,Co(Ni)与Mo的摩尔比为0.75。反应前,催化剂用10% H2S和90% H2的混合气进行硫化,硫化温度为400℃,硫化时间为3 h。HDS反应压力为5.0 MPa,温度为260-340℃,催化剂用量为0.2 g。反应原料为含哌啶和DBT分别为0-0.3%和0.8%的十氢萘溶液,液时空速为27 h-1。研究结果表明,无论是在Co-Mo／MCM-41 催化剂上还是在Ni-Mo／MCM-41催化剂上进行DBT的HDS反应,少量哌啶的存在都会大幅度降低催化剂的活性。由DBT的HDS反应产物的选择性分析发现,哌啶对HDS的抑制作用主要体现在对加氢反应路径的毒害作用。随着反应温度的升高,哌啶的加氢脱氮活性提高,HDS的反应活性接近于原料中没有哌啶时的活性,说明哌啶的毒害作用可能是因为它与含硫化合物竞争吸附而低温下加氢脱氮活性较低所致。     

Stress relaxation and creep in an impression test

A localized stress relaxation technique is described suitable for testing miniature structures and components. The actual size of the area that can be tested depends on the size of the indenter and the sensitivity of the load measurement device. However, the indenter should not be so small that macroscopic theory of deformation cannot be used. The technique described includes a quantitative treatment of the data which can be handled by a computer. Extensive data collection can be made so that many deformation parameters can be examined by using this simple test.