On the origin of strain hardening in glassy polymers

The influence of network density on the strain hardening behaviour of amorphous polymers is studied. The network density of polystyrene is altered by blending with poly(2,6-dimethyl-1,4-phenylene-oxide) and by cross-linking during polymerisation. The network density is derived from the rubber-plateau modulus determined by dynamic mechanical thermal analysis. Subsequently uniaxial compression tests are performed to obtain the intrinsic deformation behaviour and, in particular, the strain hardening modulus. At room temperature, the strain hardening modulus proves to be proportional to the network density, irrespective of the nature of the network, i.e. physical entanglements or chemical cross-links. With increasing temperature, the strain hardening modulus is observed to decrease. This decrease appears to be related to the influence of thermal mobility of the chains, determined by the distance to the glass-transition temperature (T−T_g).     

滚动式冷床在高钢级油管生产线中的应用

根据石油开采业对石油机械产品的需求 ,利用机械理论基础知识 ,通过实践设计 ,提出了对几何尺寸长的金属管类进行调质处理后的均匀冷却方法 ,以满足其性能要求。     

25Cr3Mo3NiNb二次硬化钢中的碳化物

利用TEM和萃取相分析方法，研究了25Cr3Mo3NiNb二次硬化钢淬火回火组织中的碳化物。结果表明，随淬火奥氏体化温度的升高，M6C型碳化物逐渐溶解。于1050℃奥氏体化时M6C型碳化物全部溶解，淬火态试样中只有少量的Nb(C，N)颗粒和自回火M3C型碳化物。随回火温度的升高，先后析出ε、M3C、M2C和M7C3等类型的碳化物。Nb(C，N)颗粒可以阻止淬火奥氏体晶粒的异常长大，而高温回火析出的M2C碳化物有二次硬化作用，从而提高回火稳定性和高温强韧性。     

Analytical Calculation of Large-Scale Deflection of Pulse-Loaded Elliptic Plates

Large deflections of pulse-loaded elliptic and round plates made of hardenable elastoplastic materials are investigated analytically using an energy-based approach. Formulas that are convenient for engineering design have been derived.     

Sixty years of dislocations

It is well-known that with the appearance of three independent papers by Taylor, Polanyi and Orowan in the year 1934, the concept of crystal dislocations was born. Since then, dislocation theory has had many spectacular successes. It is quite appropriate therefore to be aware of the state of development of this exciting subject, sixty years after its discovery. A flavour for the vast subject of the applications of dislocation mechanisms to real materials is presented by choosing three examples, one each, drawn from metallurgy, physics and electronics. The topic of ‘Strength of metals and alloys’ is the first one, as this is also the author’s area of research. The phenomenon of solidification and crystal growth forms the next topic, especially in view of the seminal contributions made by A R Verma and his school from India. Dislocations play a useful role in the strengthening of solids, but how influential are they in affecting the performance of modern semiconductor devices? In the third example, the interesting and painstaking work done to settle this question is reviewed. Can we regard carbon fibre as thetransistor of dislocation theory? How shall we understand the long-established effects such as corrosion-fatigue, superplasticity and shape memory as well as the electrochemical and electro-mechanical properties of dislocations in semiconductor and non-metallic crystals? Answers to these questions belong to the realms of the future developments in dislocations. The talk is concluded with a discussion of these topics.     

薄板坯连铸连轧工艺对铌微合金化高强度钢组织和性能的影响

首先建立薄板坯连铸连轧工艺的试验室模拟技术，并运用该模拟技术，研究薄板坯连铸连轧工艺(CSP)和传统板坯再加热工艺(TRP)两种工艺对铌微合金化高强度钢的显微组织和力学性能的影响。研究结果表明：CSP钢的晶粒细化效果不如TRP钢，两者的平均铁素体晶粒尺寸分别为8．17μm和6．30μm。在CSP试验钢板中铌的析出量较大，特别是在铁素体中细小颗粒的铌的析出物较多，沉淀强化效果较强。CSP试验钢的σ0.5和σb分别较TRP工艺低约40MPa和约25MPa，同时其低温冲击韧性较好，FATT温度较低。     

Prediction of the energy dissipation rate in ductile crack propagation

In this paper, energy dissipation rate D vs. Δa curves in ductile fracture are predicted using a ‘conversion’ between loads, load‐point displacements and crack lengths predicted by NLEFM and those found in real ELPL propagation. The NLEFM/ELPL link was recently discovered for the DCB testpiece, and we believe it applies to other cracked geometries. The predictions for D agree with experimental results. The model permits a crack tip toughness R(Δa) which rises from J_c and saturates out when (if) steady state propagation is reached after a transient stage in which all tunnelling, crack tip necking and shear lip formation is established. J_R is always greater than the crack tip R(Δa) and continues to rise even after R(Δa) levels off. The analysis is capable of predicting the usual D vs. Δa curves in the literature which have high initial values and fall monotonically to a plateau at large Δa. It also predicts that D curves for CCT testpieces should be higher than those for SENB/CT, as found in practice. The possibility that D curves at some intermediate Δa may dip to a minimum below the levelled‐off value at large Δa is predicted and confirmed by experiment. Recently reported D curves that have smaller initial D than the D‐values after extensive propagation can also be predicted. The testpiece geometry and crack tip R(Δa) conditions required to produce these different‐shaped D vs. Δa curves are established and confirmed by comparison with experiment. The energy dissipation rate D vs. Δa is not a transferable property as it depends on geometry. The material characteristic R(Δa) may be the ‘transferable property’ for scaling problems in ELPL fracture. How it can be deduced from D vs. Δa curves (and by implication, J_R vs. Δa curves) is established.     

Calculation of fatigue limits of case-hardened specimens with consideration of mean stresses and residual stresses

 The strength of case-hardened parts under cyclic loading can be determined in an experimental manner, but this can produce a large amount of costs due to the expenditure of material and testing time. Therefore, a calculation method is developed, that strongly reduces the number of experiments. The presented model is based on Weibull's weakest-link concept and allows to compute the survival probability of a case-hardened part which is loaded close to the fatigue limit. The essential quantities in this model are the distribution of Vicker's hardness of the material, the exponents of the Weibull distribution of the volume and the surface, the residual stress state, the surface roughness and the surface oxidation depth. By integrating the survival probabilities of the surface and the volume, the survival probability of the entire specimen or part can be calculated, which allows to compute the fatigue limit. The necessary parameters have to be determined from reference specimens. The model is successfully examined by comparing experimental and calculated results of smooth and notched case-hardened specimens under alternating torsion, rotating bending, repeated tension and tension-compression. Received 20 August 2001 / Accepted 11 November 2001     

Procedure for the evaluation of the accumulation of defects in metallic structural materials under complex elastoplastic loading

Within framework of the continual fracture mechanics, we describe the engineering approach to the assessment of scattered microdamage accumulation kinetics in metallic materials under elastoplastic loading conditions in case of plane stressed state. Automatized experimental stand and the respective investigation technique are discussed. The stand has been developed based on modification of the UMé-10T electromechanic test machine. State-of-the-art computer technologies and microprocessing hardware are incorporated in the stand automation. We present the technique of experimental assessment of damage accumulation kinetics in metallic structural materials under complex elastoplastic loading conditions with account of two different fracture (cleavage and shear) processes, which technique is based on measuring the specific electric resistance of the specimen. __________ Translated from Problemy Prochnosti, No. 1, pp. 128–137, January–February, 2006.     

On the microhardness testing of electrically stressed poly(phenylene oxide): Polystyrene blends

Microhardness measurements have been performed on untreated (virgin) and electrically stressed, solvent‐cast laboratory‐prepared samples of pure poly(phenylene oxide) (PPO), pure polystyrene (PS), and PPO : PS polyblends with different weight proportions. Results of such measurement on untreated polyblend sample show that microhardness (H_v) increases with increase in the content of PS up to 10 wt %, which attributed to the existence of homogeneous phase morphology. However, this feature is not observable in samples containing higher content of PS. Electrical stress is found to modify considerably the mechanical property of polymer. The effect of electric field on the microhardness of such samples (PPO : PS :: 90 : 10) has been characterized by the existence of a peak. Trapping of charge carriers in electrically stressed samples imparts hardening to the polyblend up to an applied step field of 190 kV/cm. However, the excessive charging beyond this step field value destroys this characteristic. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008