Enhancement in micro-fatigue resistance of UHMWPE and HDPE processed by SCORIM

The tribological performance of UHMWPE and HDPE polymers, processed by a novel polymer process technology SCORIM (Shear-Controlled ORientation TEChnology) in injection moulding, shear controlled orientation in injection moulding (SCORIM) was evaluated against a through-hardened steel under different test conditions and compared with those processed by conventional injection moulding. Results indicate a significant improvement in the wear resistance using the SCORIM technology as a result of an increase in the mechanical properties. SEM microscopy shows a change of wear mechanisms dominated by micro-fatigue when using conventional injection moulding, compared to mild abrasive wear when using the SCORIM technology under the same test conditions. Examination of microstructure reveals the random aggregation of polymer molecules in the samples processed by a conventional injection moulding. Using the SCORIM technology, the molecules were sheared and orientated and a fibril microstructure formed as an in situ fibre reinforced composite. DSC analysis shows an increase in a second phase shish kebab structure in the samples processed by SCORIM technology. The fibril microstructure with an increase in shish kebab structure results in a significant improvement in wear resistance. Using the surface normal to the direction of the orientated molecular fibril microstructure as a contact surface, the possibility of the initiation and development of micro-cracks was reduced, especially the micro-cracks parallel to the contact surfaces.     

Non-conventional injection molding of poly(lactide) and poly(ε-caprolactone) intended for orthopedic applications

Biodegradable polymers such as poly(lactide) (PLA) and poly(epsilon-caprolactone) (PCL) are increasingly used in biomedical applications as temporary implants. However, melt processing of these materials in particular of PLA is difficult due to the temperature sensitivity. Within this study, PLA and PCL were injection molded conventionally and by using the process shear controled orientation in injection molding (SCORIM) in order to investigate the effect of processing parameters on the physical properties of the moldings. Therefore, flexural testing, differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), molecular weight (MW) and orientation measurements were performed. PLA showed high sensitivity to melt temperature. In the case of amorphous poly(DL-lactide), the molecular weight and subsequently the ductility is substantially reduced by processing at higher melt temperatures. In the case of crystallizable poly(L-lactide), higher melt temperatures and shear induced by the SCORIM process resulted in enhanced crystallinity, which compromised the mechanical properties. Generally, SCORIM processing improved the mechanical properties, in particular the ductility, by orientating the molecular structure. PCL was shown to be less sensitive to shear and temperature than PLA. Stress at yield and stiffness are more improved by SCORIM processing. However, the processing temperature in combination with the grade used proved to be influential for the mechanical properties of resulting moldings.     

Flow-induced fiber orientation in gas-assisted injection molded part

Polyamide 66 with 33 wt.% glass fiber (DuPont, Zytel 70G33) was molded by gas-assisted injection molding (GAIM). Scanning electron microscope (SEM) micrographs indicated that fibers orientated notably in the core layer and slightly in the region near the mold wall, but aligned disorderly in the region near the gas channel. However, fibers orientated remarkably in the center of the thickness of the GAIM part, which was greatly different from the fiber orientation behavior in the samples molded by the conventional injection molding (CIM) and the water-assisted injection molding (WAIM) as reported in the literatures. Combining with a previous simulation dealing with gas penetration, the mechanisms for fiber orientation in the GAIM part are also discussed.     

Influence of applied stress on the γ′ directional coarsening in a single crystal superalloy

It was found in a commercial single crystal superalloy CMSX-4 that preferential orientation of the γ′ rafting in dendrite core was not dependent on the sign of the applied stress σ_A, but on the sign of the sum of the applied stress σ_A and a critical equivalent stress σ_c. This critical equivalent stress σ_c caused by material inhomogeneity has been predicted previously; however it was first determined experimentally to be in the range 39.8–47.2 MPa in the present study. Moreover, it qualitatively demonstrated that the critical equivalent stress σ_c played a significant role in the creep behavior of superalloys at high temperature and low stress.     

Longitudinal shear properties of human compact bone and its constituents,and the associated failure mechanisms

Human compact bone specimens were tested in longitudinal shear at two different strain rates. The maximum stress and energy absorption capacities were 50.40±14.08 MN m^–2 and 20720±9310J m^–2 respectively for 14 embalmed specimens tested at a cross head speed of 2.1×10^–6 m sec^–1. The maximum stress was found to be 75% of the transverse shearing strength. Bone specimens were also tested after selectively dissolving the organic and mineral components. The results showed that the composite strength of bone was much higher than the summation of strengths of its organic and mineral phases. Fractographic examination of the fracture surfaces showed that debonding of the interfaces between the osteons and the surrounding bone matrix and between the osteonal lamellae were the main mechanisms of longitudinal shear failure.     

蜘蛛丝/聚-L-乳酸静电纺纤维束的形貌和力学性能

以1%(质量分数,下同)蜘蛛丝和9%聚-L-乳酸(PLLA)的混合液为纺丝溶液,采用静电纺丝方法制备了蜘蛛丝/PLLA复合纤维构成的连续纱线。探讨了纺丝温度、卷绕速度、后拉伸倍数等对纤维和纱线的形态结构及力学性能的影响。研究发现,加入蜘蛛丝蛋白后,PLLA复合纱线的强度提高了13%,纤维直径从1.1μm减小到了550nm;加热区温度为150℃时纱线强度最大;卷绕速度为(105±5)r/min时可以实现稳定纺丝;拉伸1.5倍时,纱线断裂强度和初始模量分别提高了66%和92%,断裂伸长率下降。     

High-strain-rate superplastic deformation behavior of a powder metallurgy-processed 2124 Al alloy

High-strain-rate superplastic behavior of a powder-metallurgy processed 2124 alloy prepared through extrusion at a high ratio of 70 : 1 was investigated. A maximum tensile elongation of 700% was obtained at 823 K and at a strain rate of 10^–2 s^–1. Deformation behavior of this alloy was similar to those reported for other many HSR superplastic materials. Incorporation of threshold stress into the constitutive equation reveals that the true stress exponent is 2 and true activation energy for plastic flow is comparable to that for lattice diffusion in pure aluminum. Comparison of the present alloy with the 2124 Al composite indicates that the composite is weaker than the unreinforced alloy in the temperature range where grain boundary sliding is rate-controlled.     

工艺参数对聚甲醛微注塑制品力学性能的影响机理EI北大核心CSCD

基于单因素实验,研究工艺参数对不同厚度聚甲醛(POM)微注塑制品屈服应力、弹性模量、断裂强度和断裂伸长率等力学性能指标的影响,并基于制品形态结构分析工艺参数对制品力学性能的影响机理。实验结果表明,随着注射速度的增大,1.0mm厚微制品的皮层厚度减小,过渡层厚度增加,结晶度增大,综合效应使得屈服应力、断裂强度和弹性模量增大,断裂伸长率减小;0.2mm厚微制品的皮层厚度占主导地位,其力学性能是由皮层的力学性能决定,皮层厚度先增大后减小使得屈服应力、断裂强度和弹性模量先增大后减小,断裂伸长率先减小后增大。随着熔体温度的升高,1.0mm厚微制品的分子链取向度减小,皮层厚度减小,收缩量增大,使得屈服应力、断裂强度和弹性模量减小,断裂伸长率增大;而0.2mm厚微制品的皮层减小,但过渡层增加,结晶度增大,且补料更充分,综合作用使得屈服应力、断裂强度和弹性模量增大,断裂伸长率减小。随着模具温度的升高,1.0mm厚微制品的皮层比例减小,结晶度增大,结晶度影响占主导,使得屈服应力、断裂强度和弹性模量逐渐增大,断裂伸长率减小;而0.2mm厚微制品的皮层厚度占主导,皮层厚度明显减小使得屈服应力、断裂强度和弹性模量减小,断裂伸长率增大。     

Anomalous creep behaviour of a Ni-22 at % Cu alloy and the miscibility gap

The purpose of this study is to understand the anomalous creep behaviour of Ni-22 at % Cu alloy at the suggested critical miscibility gap temperature, below 598 K (0.36T _m). The Cu-Ni system is classified as a class II solid solution at temperatures above 0.4T _m, and it is also experimentally verified by the authors that the characteristic creep behaviour of the alloy used for this work is that for a class II solid solution. However, at low temperatures, this particular alloy shows different creep behaviours, with small stress increment in the steady state, sigmodial creep deformation is observed while with large stress increases normal primary creep occurs. When unloading the stress during creep and ageing at the test temperature, no softening due to recovery is observed but the same creep rate is achieved. The activation energy of the creep for the quenched and aged specimen is anomalously high, 326 kJ mol^–1, however, for the annealed specimen it was 167 kJ mol^–1 which is the same for that of pipe diffusion. On the basis of the observed experimental results and proper analysis, it is hypothesized that, at the test temperature, the possible formation of the solute clustering is responsible for the high activation energy and stress exponent for the creep deformation. Using the mechanical testing, creep test, it is experimentally verified that Cu-Ni system has a miscibility gap at low temperature.     

Application of contact element method in the numerical simulation of thermal stress

The interaction relationship between casting and mold (core) decides the stress level of casting and mold (core). In this article, the stress field during the casting process of stress frame casting was simulated by contact element method provided of the professional casting simulation software, ProCAST, compared its results with the simulation results of sand mold with full rigidity. Meanwhile, the influence of shake-out temperature on residual stress was also in study. It showed that the stress result of full rigidity mechanical model is bigger than that of the contact element method. The casting residual stress first increases and then decreases along with the elevation of the shake-out temperature, and the residual stress reaches the maximum when the shake-out temperature is 600°C. The lower is the shake-out temperature, the smaller is the casting deformation.     

Application of contact element method in the numerical simulation of thermal stress

The interaction relationship between casting and mold (core) decides the stress level of casting and mold (core). In this article, the stress field during the casting process of stress frame casting was simulated by contact element method provided of the professional casting simulation software, ProCAST, compared its results with the simulation results of sand mold with full rigidity. Meanwhile, the influence of shake-out temperature on residual stress was also in study. It showed that the stress result of full rigidity mechanical model is bigger than that of the contact element method. The casting residual stress first increases and then decreases along with the elevation of the shake-out temperature, and the residual stress reaches the maximum when the shake-out temperature is 600°C. The lower is the shake-out temperature, the smaller is the casting deformation.     

The effect of the skin thickness and spherulite size on the mechanical properties of injection mouldings

In this work are studied the relationships between the microstructure and the mechanical properties of an injection moulded propylene-ethylene copolymer. Distinct microstructures were obtained by processing, through a moulding programme that includes the variation of the injection and the mould temperatures and the injection flow rate. They were characterized by the skin ratio (measured by polarised light microscopy) and the spherulite size (evaluated by small angle light scattering system). Tensile tests were carried out at two different constant loading velocities: 2 mm/min (3.33 × 10^–5 m/s) and 3 m/s, in order to assess the initial modulus, the yield stress, the strain and the energy at break. The results are presented in terms of the relationships between the chosen microstructural parameters and the selected tensile properties. The skin thickness is evidenced as an important microstructural feature. The role of the core spherulite size is secondary or even negligible. The results also show that other microstructural parameters must be considered to establish more general microstructure-properties relationships.     

Mechanical properties and fracture morphologies of poly(phenylene sulfide)/nylon66 blends—effect of nylon66 content and testing temperature

Poly(phenylene sulfide) (PPS) was melt blended with Nylon66 and the mechanical properties and corresponding fracture morphologies were investigated. The thermal distortion temperature (HDT) of PPS/Nylon 66 blend showed that the inherent thermal stability of pure PPS can be maintained up to 30 wt% Nylon66, but then it started to decrease linearly thereafter to that of pure Nylon66 based on the rule of mixtures relationship. Tensile tests of PPS/Nylon66 blends at testing temperatures of –30, 25, 75, and 150°C showed that the maximum stress decreased up to 30 wt% Nylon66, and started to increase thereafter. Strain at break showed little change at low nylon content regardless of testing temperature, however, a large strain at break increase could be observed at more than 30 wt% Nylon66 and at 150°C testing temperature. At the same testing temperatures, the impact strength of PPS/Nylon66 blends was investigated, and it was found that an impact strength increase at all testing temperatures could be observed at more than 30 wt% Nylon66.     

Electrical Resistance of W/Si–Ge (0–75 wt % Ge) Contacts

The electrical resistance of the contact between tungsten and silicon–germanium alloys of different compositions was measured as a function of temperature. The results indicate that the room-temperature contact resistance increases with increasing Ge content, annealing temperature, and annealing time. With increasing measuring temperature, the contact resistance drops to a level of the experimental error. Annealing at 1070 K tends to break down both W/p-Si–Ge and W/n-Si–Ge contacts.     

Elevated-temperature strengthening in carbide (Al4C3)-dispersed aluminium

The effect of strain rate on the compressive flow behaviour of DISPAL 2 is investigated in the temperature range 473–823 K. The stress exponent, n, was 28 in the temperature range 473–623 K, while it increased to 59 above 673 K. The activation volume and energy for deformation were 70 b ³ and 100–200 kJ mol^–1, respectively, in the temperature range 473–623 K. In the higher range, 673–823 K, the observed activation volume of 300–500 b ³ and the activation energy of ~ 1086 kJ mol^–1 cannot be reconciled with any of the deformation mechanisms. A new model-based creep equation for dispersion-strengthened materials proposed by Rosler and Arzt has been applied to the flow data from 673–823 K. Its predictions are in agreement with the experimental data in the temperature range 673–723 K. The predictions of the model, however, differ from the experimental data at 773 and 823 K.     

Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (σ<30 MPa), both the apparent stress exponent (n_app≈2) and the apparent activation energy (Q_app≈48 kJ mol⁻¹) are low, while at high stresses (σ>34 MPa), these parameters are much higher (n_app=9–15 and Q_app=230–325 kJ mol⁻¹) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y₂O₃ interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.     

Filler-Enhanced Piezoelectricity of Poly-L-Lactide and Its Use as a Functional Ultrasound-Activated Biomaterial

Poly-L-lactide (PLLA) offers a unique possibility for processing into biocompatible, biodegradable, and implantable piezoelectric structures. With such properties, PLLA has potential to be used as an advanced tool for mimicking biophysical processes that naturally occur during the self-repair of wounds and damaged tissues, including electrostimulated regeneration. The piezoelectricity of PLLA strongly depends on the possibility of controlling its crystallinity and molecular orientation. Here, it is shown that modifying PLLA with a small amount (1 wt%) of crystalline filler particles with a high aspect ratio, which act as nucleating agents during drawing-induced crystallization, promotes the formation of highly crystalline and oriented PLLA structures. This increases their piezoelectricity, and the filler-modified PLLA films provide a 20-fold larger voltage output than nonmodified PLLA during ultrasound (US)-assisted activation. With 99% PLLA content, the ability of the films to produce reactive oxygen species (ROS) and increase the local temperature during interactions with US is shown to be very low. US-assisted piezostimulation of adherent cells directly attach to their surface (such as skin keratinocytes), stimulate cytoskeleton formation, and as a result cells elongate and orient themselves in a specific direction that align with the direction of PLLA film drawing and PLLA dipole orientation.     

气体辅助注射成型聚丙烯的多层次结构

用扫描电镜(SEM)观察了气体辅助注射成型(GAIM)和常规注射成型(CIM)等规聚丙烯(iPP)在不同部位的结晶形态。发现CIM试样的"皮-芯"结构不明显,而GAIM试样在不同部位则形成了包括球晶、串晶和取向片晶,进而表现出明显的多层次结构。在结晶形态分析的基础上,初步探讨了GAIM制品多层次结构的形成机理。     

Effects of stress ratio and temperature on fatigue crack growth in a Ti–6Al–4V alloy

Fatigue thresholds and fatigue crack growth (FCG) rates in corner notched specimens of a forged Ti–6Al–4V aero-engine disk material were investigated at room temperature and 350 °C. The threshold stress intensity range, ΔK_th, was determined by a method involving a step change in stress ratio (the ‘jump in’ method). It was found that for three high stress ratios (R=0.7–0.9), where crack closure effects are widely accepted to be negligible, there were similar ΔK_th values at room temperature and 350 °C under the same R. For a given temperature, ΔK_th was observed to decrease from 3.1 to 2.1 MPam with R increasing from 0.7 to 0.9. The fatigue crack growth rate was influenced by increasing temperature. For high stress ratios, FCG rate at 350 °C was higher than that at room temperature under the same ΔK. For a low stress ratio (R=0.01), higher temperature led to higher FCG rates in the near-threshold regime, but showed almost no effect at higher ΔK. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a K_max effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the K_max and fatigue threshold, is proposed to describe FCG rates in the near-threshold and Paris regimes for both temperatures. The predictions compare favourably with experimental data.     

Fracture mirrors in Columbia Resin CR-39

The relationship of fracture stress to mirror size for Columbia Resin, CR-39, is investigated. The fracture mirror constant is influenced by strain rate and test temperature, and gives values of 3.28 and 2.87 MN m^–3/2 for static and impact tests at room temperature, and 1.36 MN m^–3/2 for static tests at –150° C. The fracture surface energy for the initiation of unstable fracture decreases at high loading rate and at low temperature. Fractographic observations made on the fracture surface show some characteristic features reflecting the microstructure of this thermosetting plastic. The changes in the mirror constant and the fracture surface energy are discussed from the viewpoint of fracture mechanics.