无缝紫铜管冷弯开裂的原因分析

在废品分析的基础上,通过一系列工艺试验表明:无缝紫铜管冷弯开裂分为韧性和脆性2种,热处理温度及预先冷变形量不当是造成冷弯开裂的主要原因.     

The cold compaction of several mineral powders

Four mineral powders with widely different mechanical properties have been studied in cold compaction. The effect of final compacting pressure and particle size have been investigated. An analysis of the experimental data indicates that the same basic empirical compaction equation can be applied to the compaction of all four powders. A theoretical model for the compaction process is developed and the empirical parameters are correlated with the mechanical constants of the materials.     

The cold compaction of several mineral powders

Four mineral powders with widely different mechanical properties have been studied in prolonged cold compaction. An analysis of the time-dependent deformation has been attempted by splitting the finite strain into two components, one a linear function of time. A single compaction equation has been developed and applied to all four powders. The empirical constants of this equation have been examined, proposals concerning the mechanism of compaction under the prolonged application of pressure have been advanced and a correlation between the empirical parameters and the mechanical constants of the materials has been attempted. Suggestions for future work have been made.Dr P. Lawrence, BSc, PhD, is now a member of the ACI Technical Centre, Sydney, NSW, Australia.     

Powder compaction law for cold die pressing

We develop a new theoretical model for metal powder compaction that describes the relationship between the green porosity of the compacts and the applied external pressure. This model, applicable to ductile powders both with uniform and non-uniform particle sizes and regular and irregular particle shapes, contains only empirical constants that are physically explained. Among these constants, values related to the plastic behaviour of the material constituting the powder particles as well as the friction coefficient between the powder and die walls are included. The interparticle friction is also considered as a kind of constraint that diminishes the local pressure borne by the fully dense material. Also the tap porosity, an extremely useful parameter that contains the morphometric information of the powder, is considered. The proposed model has been experimentally validated with six metal powders of different types. The agreement between the model and the experimental data is very satisfactory over the tested pressure range. For comparison, the classic model by Fischmeister and Artz has also been fit to the experimental curves.     

Quality assurance and quality control of subgrade compaction using the dynamic cone penetrometer

The Dynamic Cone Penetrometer (DCP) is a device that is used in the construction industry for the assessment of in situ soil compaction quality. Over the past few decades, numerous correlations have been developed between the DCP test results and soil strength and stiffness parameters. This paper proposes a comprehensive set of criteria and recommendations for quality control (QC) of compacted subgrade that take into account the inherent statistical variability of DCP test results. For the development of the QC criteria, a new statistical methodology is used to extract representative test values from the raw field DCP test data. In order to use the proposed QC criteria, soils are first classified into two broad categories (fine-grained and coarse-grained) depending on their fabric and response to compaction efforts. Test results indicate that (i) for fine-grained soils, the DCP test values have good correlation with the plasticity index (PI), which is indicative of the type and amount of clay content of the soil and (ii) for coarse-grained soils, the DCP test values have good correlation with the optimum water content of the soil, which is directly related to its in situ density. DCP blow count correlation equations are presented for both soil categories. Recommendations for field DCP testing and data analysis are also provided to highlight the significance of the statistical distribution of the DCP test results in QC testing of compacted subgrade.     

Uniaxial strain rate effects in pharmaceutical powders during cold compaction

The results of uniaxial compression tests on some pharmaceutical powders subjected to strain rates of between 10^–3 and 10⁵ s^–1 are given. The tests fall into three main categories: low-strain-rate tests (10^–3–10 s^–1) performed on a servohydraulic variable speed-compression machine at constant compression rate; medium-strain-rate tests (10²–10³ s^–1) carried out on a drop hammer; and high strain rate tests (10³–10⁵ s^–1) performed on a high-pressure air projectile launcher compaction apparatus. Axial and radial pressures, as well as displacement-time measurements, are made. Powders tested include: Dipac sugar; sodium chloride; potassium bromide; lactose; paracetamol d.c.; avicel; calcium phosphate; and copper sulphate. The influence of compression rate on the form of the characteristic pressure-density and radial-axial pressure relationships during uniaxial straining is presented. The investigation showed that the general tendency for all powders tested, except for paracetamol d.c., is to exhibit increased compaction pressure with strain rate up to 10⁵ s^–1. Due to morphological and compositional effects, paracetamol d.c. softens with the rate of straining up to about 10² s^–1 and at higher rates it behaves like other powders. Also the mean radial pressure at the die wall (obtained by a pin-type transducer) shows that the friction conditions are variable during the process, and their effect tends to decrease as the speed of compaction increases, resulting in more uniform density compacts. Finally, by observing the decay of both axial and radial pressures with time under constant volume conditions, a reasonably linear behaviour is obtained for all materials tested, particularly the axial relaxation curves, over the period recorded.     

Cracking,damage and fracture in four dimensions

Concrete and cracking are nearly synonymous despite our best efforts and intentions. Relationships between cracking and the stress states that lead to cracking can be instructive. In an effort to better understand these relationships, X-ray microtomography was used to make high-resolution three-dimensional digital images of small concrete specimens under load. Using 3D image analysis, quantitative measurements of internal crack growth were made that include effects of crack tortuosity, branching and microcracking. Successive images at different levels of cracking and damage provide us with a detailed picture of internal crack progression. When coupled with load-deformation response, bulk material properties such as fracture toughness or damage variables can be quantitatively linked with cracking. Measurements to date have shown distinct fracture regimes linked to crack formation and propagation. In addition, the crack measurements offer a way to provide a physical basis for a scalar damage variable.     

Stochastic analysis on coordination number distribution of particles during powder compaction

This paper deals with the change in the coordination number distribution in spherical mono-sized silica powder compacts to discuss its particle-packing process during Cyclic CIP (Cold Isostatic Pressing) with increasing the number of cycles at the maximum CIP pressure of 100 MPa. By measuring the distance between the first closest particles (viz. contact particles) to a center particle, the coordination number of the center particle is determined. The resultant coordination number distributions are analyzed with stationary Markov process. The transition probability matrix obtained by the data at the number of cycles from 1 to 10 gives sufficient predictions for the experimental data at the number of cycles of 100 and 1000.     

泡沫沥青冷再生混合料成型方法

采用振动成型设备以压实度为指标确定了泡沫沥青冷再生混合料振动成型的振动时间。振动成型、静压成型、马歇尔成型及旋转压实成型试件在总空隙率相同的条件下,对比了不同成型方法混合料的力学强度,分析了强度差异产生的原因。基于CT检测手段,分析了不同成型方法混合料内部空隙分布状态、空隙分形维数、最可几孔径及平均孔径。结果表明:旋转压实的"揉搓作用"显著增大了冷再生混合料的间接拉伸强度,振动成型次之,静压成型与马歇尔成型混合料间接拉伸强度(ITS)较接近,而四种成型方法混合料的无侧限抗压强度差别不大;旋转压实及振动成型会影响泡沫沥青冷再生混合料内部细颗粒的分布,进而影响混合料内部空隙的分布状态,这两种成型方式混合料内部空隙数目多且等效半径小。不同压实方法的压实机理的差异导致混合料内部空隙平均孔径、空隙维数及最可几孔径存在差异。空隙特征的不同是引起冷再生混合料力学强度差异的本质原因。     

Cracking node method for dynamic fracture with finite elements

A new method for modeling discrete cracks based on the extended finite element method is described. In the method, the growth of the actual crack is tracked and approximated with contiguous discrete crack segments that lie on finite element nodes and span only two adjacent elements. The method can deal with complicated fracture patterns because it needs no explicit representation of the topology of the actual crack path. A set of effective rules for injection of crack segments is presented so that fracture behavior beginning from arbitrary crack nucleations to macroscopic crack propagation is seamlessly modeled. The effectiveness of the method is demonstrated with several dynamic fracture problems that involve complicated crack patterns such as fragmentation and crack branching. Copyright © 2008 John Wiley & Sons, Ltd.     

A study of compaction of composite particles by multi-particle finite element method

We study the compaction of composite mixture of soft and hard micro-/nano-size particles using multi-particle finite element method. In this method, each particle is fully discretized into finite element mesh. Local non-uniform contact deformation and non-uniformity stress chains transmitted through the composite are well illustrated. In this work, we focus on the effect of inter-particle friction and volume fraction of hard particles on compaction pressure. Results of closed die compaction of 400 two-dimensional monosize spherical particles of zero and 40% volume fraction hard particles are presented. The results show that compaction pressure increases with inter-particle friction and volume fraction of hard particles. The predicted compaction pressure curves are in good agreement with experimental data and other models.     

Cracking particles: a simplified meshfree method for arbitrary evolving cracks

A new approach for modelling discrete cracks in meshfree methods is described. In this method, the crack can be arbitrarily oriented, but its growth is represented discretely by activation of crack surfaces at individual particles, so no representation of the crack's topology is needed. The crack is modelled by a local enrichment of the test and trial functions with a sign function (a variant of the Heaviside step function), so that the discontinuities are along the direction of the crack. The discontinuity consists of cylindrical planes centred at the particles in three dimensions, lines centred at the particles in two dimensions. The model is applied to several 2D problems and compared to experimental data. Copyright © 2004 John Wiley & Sons, Ltd.     

Densification behavior of aluminum alloy powder mixed with zirconia powder inclusion under cold compaction

Densification behavior of composite powders was investigated under cold compaction. Experimental data were obtained for aluminum alloy powder mixed with zirconia powder inclusion under triaxial compression. The Cap model with constraint factors was implemented into a finite element program (ABAQUS) to simulate compaction responses of composite powders during cold compaction. Finite element results were compared with experimental data for densification behavior of composite powders under cold isostatic pressing and die compaction. The agreement between experimental data and finite element calculations from the Cap model with the constraint factors was good for composite powders with low volume fractions of inclusions.     

Crystalline rice husk silica reinforced AA7075 aluminium chips by cold compaction method

Aluminium matrix composite is highly demanded in various industries due to its low density and good mechanical properties as most commonly studied for metal matrix composite. The properties of the composite be improved with the addition of reinforcement significantly such as silicon carbide, aluminium oxide, and boron carbide that can be mixed easily to metal matrix composite. The study of crystalline rice husk silica reinforced AA7075 aluminium chips on mechanical properties were investigated. The rice husk ash was burned at 1200 °C and it was characterized in the crystalline phase by conducting x-ray diffraction test. The mechanical properties of aluminium matrix composite were obtained by microhardness and compression tests. Results of mechanical properties for the addition of rice husk silica up to 7.5 wt.% composition of crystalline rice husk silica showed increase value of microhardness and compression strength which are the highest value of 75.94 HV 0.1 and 443 MPa, respectively compared to another aluminium matrix composite. Hence, based on investigation to crystalline rice husk silica reinforced aluminium, it has good potential to improve the mechanical properties of aluminium matrix composite which were dependent to the composition of crystalline rice husk silica reinforcement in aluminium matrix composite.     

Fatigue and fracture paths in cold drawn pearlitic steel

This paper analyses the influence of microstructural anisotropy of a progressively drawn pearlitic steel (orientation of pearlitic lamellae in the drawing direction) on the microscopic and macroscopic evolution of cracking paths produced by fatigue and fracture. The fatigue crack path is always contained in the transverse section of the wires, i.e., the subcritical propagation develops under a global mode I, so that the main crack path is associated with mode I and some very local deflections take place to produce a roughness in the fatigue crack path depending on the drawing level. The fracture crack path evolves from a global mode I propagation following the transverse plane in slightly drawn steels (including the hot rolled bar that is not cold drawn at all) to a global mixed-mode propagation associated with crack deflection in intermediate and heavily drawn steels (the latter with a strong mode II component), the deviation angle being an increasing function of the drawing degree in the steel.     

Cracking processes in Barre granite: fracture process zones and crack coalescence

This paper presents a comprehensive study of the cracking and coalescence behavior of granite specimens with pre-existing flaw pairs. Uniaxial compressions tests were conducted on Barre granite with pre-existing flaw pairs of varying inclination angles $(\upbeta )$ , bridging angles $(\alpha )$ and ligament lengths (L). The cracking processes were recorded using a high speed camera to capture crack initiation and determine the mode (tensile or shear) of cracking. Visible fracture process zones of grain lightening, referred to as “white patching”, were also observed. White patching corresponded to fracture process zones that developed before visible cracks appeared. Cracks were typically preceded by a corresponding linear white patching. Diffusive area white patching was also observed near locations where surface spalling eventually occurred. Shear cracks occurred less often when compared to other brittle materials such as gypsum and marble and tensile cracks were typically much more jagged in shape (saw-toothed) due to the larger size and higher strength mineral grains of granite. Crack coalescence behavior trended from indirect to direct shear and combined shear-tensile to direct tensile coalescence as the flaw pair bridging angle $(\alpha )$ or flaw angle $(\upbeta )$ increased. As the ligament length (L) between flaws increased, more indirect coalescence was observed. As expected, due to the increased occurrence of tensile cracking in granite, more indirect tensile coalescence was observed in granite compared to other materials previously studied.