Fatigue propagation behaviour of polystyrene/polyethylene blends

Fatigue crack propagation (FCP) of injection-moulded polystyrene (PS) and 95/5, 85/15 and 70/30 PS/high-density polyethylene (HDPE) blends at loading frequencies of 2 and 20 Hz was studied. The FCP results showed that increasing the HDPE content caused a progressive reduction of the fatigue crack growth rates, especially when a styrene/ethylene– butylene/styrene (SEBS) terpolymer was added as a compatibilizer. Increasing the loading frequency also led to a fatigue crack growth rate reduction. Moreover, the fatigue crack growth rates were lower at a given cyclic stress intensity factor range, K, when the crack propagated normal, instead of parallel, to the melt-flow direction during injection moulding. Fractographic observations indicated that discontinuous growth bands (DGBs), associated with the fracture of crazes in the plastic zone, were present through most or all of the fracture surfaces of the PS/HDPE specimens. In the presence of sufficient HDPE, these DGBs were formed by the initiation, growth and coalescence of large dimples initiated at HDPE particles ahead of the microscopic crack front, similar to a multiple crazing effect. The loading frequency effect on the FCP behaviour of these blends is attributed to a time-dependent deformation process. It is concluded that the FCP behaviour of these blends is strongly affected by the loading direction with respect to the matrix and minor phase orientation, by the presence of a compatibilizer, by the composition of the blend and by the testing conditions. © Chapman & Hall.     

HIPS/HDPE共混物的动态黏弹行为与相形态

采用动态流变测试和扫描电子显微镜技术,考察高抗冲聚苯乙烯(HIPS)/高密度聚乙烯(HDPE)共混物的动态黏弹行为与相形态,对比1%(质量分数,下同)的纳米和微米CaCO_3对HIPS/HDPE(30/70)不相容共混物的增容效果。结果表明:当HDPE小于30%时,HIPS/HDPE共混物在低频区的复数黏度和储存模量均显示出明显的正偏差,而当HDPE大于30%时,则呈现负偏差;前者与HDPE和PB粒子间的相互作用相关,而后者归因于HDPE基体与PS分散相之间较弱的界面相互作用。当HIPS为基体时,HDPE分散相粒子呈现较宽的尺寸分布;而当HDPE为基体时,PS分散相呈现双模尺寸分布,对应于两种不同类型的PS分散相粒子的存在。1%的纳米CaCO_3对HIPS/HDPE(30/70)不相容共混体系起到了一定的增容效果,CaCO_3纳米粒子主要位于HIPS/HDPE相界面以及HDPE连续相内;而微米CaCO_3对该共混体系仅起到了增黏而非增容作用,CaCO_3微米粒子仅位于HDPE连续相内。     

聚氯乙烯/聚苯乙烯共混物在过氧化物作用下的交联和降解问题

通过流变实验和共混物中聚苯乙烯的分子量测定发现，在用过氧化二异丙苯（DCP）对聚氯乙烯（PVC）/聚苯乙烯（PS）/丁腈橡胶（NBR）（质量比(46/46/8）共混体系实施原位交联时，PS在自由基引发下发生降解，且随着DCP含量的增加，其降解程度随之增加，在适中的DCP含量(0.15%）时，共混物的冲击强度最佳，在上述体系中加入苯乙烯时，苯乙烯单体在均聚的同时还与NBR和PVC发生接枝反应，从而起增容作用，同时在一定程度上抑制了共混物中PS的降解。     

Modes of failure under creep/fatigue loading of a nickel-based superalloy

Crack growth experiments have been carried out under combined creep and fatigue loading at 700° C on a hot isostatically pressed powder nickel alloy. A fractographic investigation has been undertaken of the modes of failure over a frequency range of 0.001 to 10 Hz. The observations indicate that under static loading and at low frequencies failure is intergranular and controlled by creep processes, whereas at high frequencies a transgranular fatigue fracture is obtained. The transition from creep to fatigue behaviour is found to be progressive, and to begin at a lower frequency the higher the ratio of cyclic to mean load. In the transition region a mixed intergranular and transgranular fracture surface is observed, which correlates well with the recorded proportion of creep to fatigue crack growth.     

ASSISTED CRACK GROWTH IN MARAGING STEEL WELDS BY GASEOUS HYDROGEN

Abstract— The effects of environmental hydrogen content on fatigue crack growth rates (FCGRs) in T-250 maraging steel plates and laser welds were investigated. The influence of ageing treatments on fatigue characteristics of the alloy was also studied. Experimental results revealed that the accelerated FCGRs in the presence of hydrogen were always associated with changes in fracture modes that appear in compact-tension specimens. Even for overaged specimens with excellent resistance to gaseous hydrogen embrittlement, such an acceleration of crack growth in hydrogen could not be avoided. The crack path of underaged specimens in hydrogen was found mainly along prior austenite boundaries for steel plates and along coarse columnar boundaries for welds. In gaseous hydrogen, peak-aged welds exhibited intergranular and quasi-cleavage mixed fracture modes, compared to mainly quasi-clevage for similar aged steel plates. Hence, the enhancement of crack growth in hydrogen was more pronounced for the welds. Overaged welds showed higher FCGRs than the same aged steel plates only in hydrogen and for Δ K values greater than 20MPa√m.     

Fractographic study of high-density polyethylene pipe

High-density polyethylene (HDPE) pipe is now being used as an alternative to medium-density polyethylene (MDPE) for gas, water, sewage and waste-water distribution systems. Laboratory tests appear to show that HDPE is more able to suppress rapid crack propagation (RCP), whilst remaining sufficient resistance under the operational circumstances that lead to the type of slow crack growth observed in service failures. There have been many fractographic studies on MDPE pipe materials, actual pipe and fittings, but little on HDPE. A fractographic study of the type of HDPE pipe in current production has been undertaken. For these tests, whole pipe sections were subjected to either static or dynamic internal (water) pressurization fatigue loading. Failure mechanisms are discussed based on the fracture morphologies resulting from these tests. A further argument for good resistance of HDPE pipe to RCP is suggested. © 1998 Chapman & Hall     

The effects of phase morphology and adhesion between phases on fracture of ethylene-vinylalcohol copolymer/nylon 6/12 copolymer blends

The effects of phase morphology and the adhesion between phases of ethylene-vinylalcohol copolymer(EVOH)/nylon 6/12 copolymer blends on the fracture properties were estimated. Films of the blends which were obtained by extrusion processing showed different phase morphologies depending on the composition of the nylon 6/12 copolymer. The morphology of the partially miscible blend (EVOH and nylon 6f-nylon121-f where f=0.8) was needle-like in appearance. On the other hand the immiscible blend (EVOH and nylon 6f-nylon121-f where f=0.5) had equiaxed particles of nylon 6/12. The plastic deformation of films of the blends was observed using transmission electron microscopy. Deformation zones were observed for both blends but extensive debonding of particle interfaces was observed in the immiscible blend system. These observations are reinforced by our measurements of the interfacial fracture energy, Gc, between EVOH and nylon 6f-nylon121-f made using a double cantilever beam test. Gc decreases monotonically as 1–f increases. The fracture toughness of the partially miscible blend film measured at low temperature (–80°C) was higher than that of EVOH alone and there was fractographic evidence of a larger crack tip plastic deformation zone. In contrast, the fracture toughness of the immiscible blend was lower than that of EVOH and there was fractographic evidence of extensive debonding of the second phase nylon particles. This result suggests that it is important to have good adhesion between phases to achieve the optimum fracture toughness of these polymer blends. © 1998 Chapman & Hall     

Influence of a fine talc on the properties of composites with high density polyethylene and polyethylene/polystyrene blends

Fine talc filled high density polyethylene (HDPE) and HDPE/polystyrene (PS) blends were extruded, injection moulded and characterized. Some of the mechanical properties of the talc filled HDPE and talc filled 75/25 HDPE/PS blend were deduced from stress–strain measurements. A comparison between the effect of the talc on the properties of the filled HDPE and filled 75/25 HDPE/PS blend showed that the mineral filler had the same effect on both systems provided that its array in the organic matrix is almost the same in both cases. In fact, the rheological results proved that the dispersion of talc in the HDPE matrix was not really affected by the presence of PS. The study particularly focused on the effect of talc on the ultimate tensile strength of the filled HDPE and that of the filled blend. It has been noted that the brittle nature of PS neutralizes, to a certain extent, the degrading effect of talc on this property. Furthermore, both PS and talc have a complementary effect on the stiffness and the resilience of HDPE/PS/talc blend composites.     

HDPE/PA6共混阻隔材料的制备及研究

采用熔融挤出法制备了高密度聚乙烯(HDPE)与马来酸酐(MAH)的接枝物(HDPE-g-MAH),并用红外光谱证实了接枝反应。以此接枝物为相容剂制备HDPE/PA6共混材料。研究了PA6和相容剂用量对共混材料形态结构、力学性能及阻气性能的影响。SEM表明相容剂可以明显改善HDPE与PA6的相容性。PA6的加入显著提高了HDPE对氧气的阻隔性能。     

Insight into the molecular arrangement of high-density polyethylene polymer chains in blends of polystyrene/highdensity polyethylene from differential scanning calorimetry and Raman techniques

Polystyrene/high-density polyethylene (PS/HDPE) blends were synthesized by melt blending in a single screw extruder. Co-continuity measurements using solvent extraction and scanning electron (SEM) micrographs showed that co-continuity was obtained around 35% PS. Thermal analyses measurements revealed a reduction in crystallinity of the HDPE phase around the co-continuous composition. Raman analyses across the entire composition range of these blends showed an increase in the normalized integral intensities of the 1128 cm(-1) and 1061 cm(-1) stretching vibrations of the HDPE molecules. The presence of all-trans HDPE chains that are not packed into an orthorhombic structure is used to explain the simultaneous occurrence of reduced crystallinity and increased intensity of all-trans HDPE stretch vibrations.     

An innovative method to improve the electrical conductivity of multi-walled carbon nanotubes filled polyamide 6/polystyrene blends

An innovative method has been successfully developed to improve the electrical conductivity of polyamide 6/polystyrene (PA6/PS) blends in this paper. PA6/PS blends containing multi-walled carbon nanotubes (MWCNTs) are prepared by the radical polymerization of styrene in the presence of ?-caprolactam (CL) and MWCNTs, followed by the in-situ anionic ring-opening polymerization of CL. In the resulted PA6/PS blends, MWCNTs are selectively located at the interface of PA6 and PS. Because the interface of 0.5 and 1.0 wt.% MWCNTs filled blends with PA6/PS weight ratio of 70/30 is continuous, a MWCNTs conductive pathway is formed in these two blends, which results in a decrease of volume resistivity by about 9 orders of magnitude.     

Fatigue crack propagation in cast duplex stainless steels: thermal ageing and microstructural effects

The ferrite phase of cast duplex stainless steels becomes embrittled after thermal ageing, leading to a significant decrease in fracture properties. In the present paper, the influence of ageing and solidification structure on the fatigue crack growth rates (FCGRs) and on the fatigue crack growth mechanisms in a cast duplex stainless steel is studied. FCGRs measured at room temperature increase slightly after ageing at 400 °C, due to ferrite cleavage and to the resulting irregular shape of the crack front. The crack propagates without any preferential path by successive ruptures of ferrite and austenite phases. The macroscopic crack propagation plane depends on the crystallographic orientation of the ferrite grain. Secondary cracks can appear due to the complex solidification structure. This in turn influences the FCGR. The fatigue crack closure level decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening of these materials.     

Fatigue fracture processes in polystyrene

Fatigue crack growth characteristics in polystyrene were studied as a function of stress intensity factor range and cyclic frequency. Precracked single edge notched and compact-tension type specimens made from commercially available polystyrene sheet (mol.wt. =2.7×10⁵) were cycled under constant load at frequencies of 0.1, 1, 10 and 100 Hz, producing growth rates ranging from 4×10^–7 to 4×10^–3 cm/cycle. For a given stress intensity level, fatigue crack growth rates were found to decrease with increasing frequency, the effect being strongest at high stress intensity values. The variable frequency sensitivity of this polymer over the test range studied was explained in terms of a variable creep component. The macroscopic appearance of the fracture surface showed two distinct regions. At low stress intensity values, a highly reflective, mirror-like surface was observed which transformed to a rougher, cloudy surface structure with increasing stress intensity level. Raising the test frequency shifted the transition between these areas to higher values of stress intensity. The microscopic appearance of the mirror region revealed evidence of crack propagation through a single craze while the appearance of the rough region indicated crack growth through many crazes, all nominally normal to the applied stress axis. Electron fractographic examination of the mirror region revealed many parallel bands perpendicular to the direction of crack growth, each formed by a discontinuous crack growth process as a result of many fatigue cycles. The size of these bands was found to be consistent with the dimension of the crack tip plastic zone as computed by the Dugdale model. At high stress intensity levels a new set of parallel markings was found in the cloudy region which corresponded to the incremental crack extension for an individual loading cycle.     

EFFECT OF LASER BEAM RADIATION ON FATIGUE CRACK PROPAGATION IN AISI 4150 STEEL

The effect of laser beam radiation on fatigue crack growth in AISI 4150 steel was performed on compact-tension (CT) specimens, in which a composite region (CR) comprised of the hardened zones (HZs) on the top and bottom surfaces and the base metal (BM) in the interior, was aligned either along or normal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 °C preheated laser-hardened specimens. When the crack propagated along the laser tracks (LTs), the fatigue crack growth rates (FCGRs) of the laser-hardened specimen were lower than those of the base plate, particularly at low ΔK ranges. On the other hand, for a crack propagating normally to the LTs, decelerated FCGRs in the regions preceding the CR and accelerated FCGRs within the CR itself were found. However, enhanced FCGRs in the CR were not found in preheated specimens with a bainite structure in the HZs as the crack grew normal to the LTs. The enhancement of FCGRs in the CR, which became more accentuated at high ΔK values, was closely related to an embrittled microstructure (martensite) in the HZs.     

EPDM/PS交替多层复合材料的力学性能分析

用自行设计的微纳多层共挤出体系制备了三元乙丙橡胶/聚苯乙烯(EPDM/PS)交替多层复合材料。偏光显微镜和扫描电镜分析表明, 所制备的复合材料具有规则层状交替结构。与同组分的一般共混样品相比, 64层EPDM/PS交替复合材料表现出不同的拉伸断裂行为: 初期的PS断裂行为和后期的EPDM拉伸行为。EPDM和PS层保持了较好的连续性, 且EPDM层阻止了PS层裂纹向相邻PS层的发展, 使64层样品与同组分的一般共混样品相比具有较高的拉伸强度和杨氏模量。讨论了在PS相中加入相容剂苯乙烯-乙烯-丁烯-苯乙烯嵌段共聚物(SEBS)对64层EPDM/PS交替复合材料力学性能的影响, 发现SEBS的加入提高了PS相的韧性, 并且改善了EPDM层与PS层界面的相互作用。      

高密度聚乙烯/尼龙6共混物的形态结构对其性能的影响

本文利用微层共挤方法制备了具有层状交替结构的HDPE/PA6共混物,利用常规熔融共混挤出方法制备了与层状共混物具有相同组成比的海岛结构共混物。通过DSC,FT-IR及力学性能测试等方法研究了共混物的形态结构对其界面化学反应、结晶行为和力学性能的影响。研究结果表明:在共混物中引入少量马来酸酐接枝高密度聚乙烯时,化学反应在界面进行,与海岛结构的共混物界面面积相比,层状共混物的界面接触面积小,界面化学反应相对较弱,但层状共混物的屈服强度和断裂伸长率有大幅度提高。层状结构对HDPE和PA6的结晶行为影响很小。     

Crack tip deformation fields and fatigue crack growth rates in Ti–6Al–4V

In this paper we present an overview of experimental and modelling studies of fatigue crack growth rates in aerospace titanium alloy Ti–6Al–4V. We review work done on the subject since the 1980s to the present day, identifying test programmes and procedures and their results, as well as predictive approaches developed over this period. We then present the results of some of our recent experiments and simulations. Fatigue crack growth rates (FCGRs) under constant applied load were evaluated as a function of crack length, and the effect of overload (retardation) was considered. Crack opening was measured during cycling using digital image correlation, and residual stress intensity factor was determined using synchrotron X-ray diffraction mapping. Modelling techniques used for the prediction of FCGRs are then reviewed, and an approach based on the analysis of energy dissipation at the crack tip is proposed. Finally, directions for further research are identified.     

Effect of nanoclay and SEBS-g-MA on the morphology and properties of immiscible poly(methyl methacrylate)/polystyrene blend

In recent years, nanoclays are being used as compatibilizer for various immiscible polymer blends. However, little work has been done on the morphology of immiscible polymer blends in presence of both the nanoclay and a reactive compatibilizer. Here, we report the synergistic effect of nanoclay and SEBS-g-MA on the morphology and properties of (70/30 w/w) PMMA/PS blend. Scanning electron microscopy study of the blend with various amount of nanoclay and SEBS-g-MA indicated a reduction in the average domain sizes (D) of dispersed PS phase in PMMA matrix compared to that in the pure blend. Addition of both SEBS-g-MA and nanoclay significantly lowered the D of PS in the blend compared to that with only SEBS-g-MA or clay. X-ray diffraction study and transmission electron microscopy revealed the presence of intercalated clay platelets in PMMA matrix, as well as, at the interface of the (70/30 w/w) PMMA/PS blend-clay nanocomposites. Addition of SEBS-g-MA in the blend-clay nanocomposites promoted the exfoliation of clays in PMMA matrix. Thus, exfoliated clay platelets in PMMA matrix effectively restricted the coalescence of dispersed PS domains while SEBS-g-MA improved the adhesion between the phases at the interface. At certain loading (phr), storage modulus, elongation at break and thermal stability of the blend were greatly improved when both the nanoclay and SEBS-g-MA were present in the blend. The use of reactive compatibilizer and nanoclay in polymer blends may lead to a high performance material which combines the advantages of compatibilized polymer blends and the merits of polymer nanocomposites.     

Fracture mechanisms in polystyrene/laponite nanocomposites prepared by emulsion polymerization

Transmission electron microscopy (TEM) has been used to investigate the effect of laponite clay on microdeformation in thin latex-based polystyrene (PS) films, in which the laponite was concentrated at the original interfaces between the PS particles. At room temperature, a transition was observed from crazing in pure PS to a coarser fibrillar deformation mode as the laponite content increased. Moreover, whereas pure PS showed increasingly homogeneous deformation as T approached T_g, the fibrillar deformation zones observed in the nanocomposites persisted up to T just below T_g, and there was some evidence for yielding behaviour at even higher T in the presence of laponite. The macroscopic fracture resistance of the films, as assessed from double edge-notched tensile specimens, initially increased with laponite content, but decreased for laponite contents greater than 5 wt% with respect to the styrene monomer. This was attributed to a decrease in local ductility, consistent with the observation of reduced deformation ratios in the deformation zones by TEM, and to the intrinsic weakness of the laponite stacks and/or the PS-laponite interface. Thus, specimens with laponite contents comparable with the estimated threshold for percolation of contacts between the laponite stacks showed extremely brittle behaviour, associated with crack propagation along the interfaces between the latex particles.     

用MBO(口恶)唑啉合成LDPE/PS合金相容剂及其对合金的增容作用

采用１,３－双－（恶唑啉基）－苯为偶联剂通过反应性加工实现ＰＳ－ｇＭＡｎ和氯化聚乙烯之间反应方法来合成ＬＤＰＥ／ＰＳ共混体系相容剂,在ＬＤＰＥ／ＰＳ共混体系中加入１０％此相容剂,其冲击强度提高２．３倍,拉伸强度和弯曲强度也有所提高。通过ＳＥＭ、ＤＭＡ和ＤＳＣ分析表征表明加入此相容剂后,ＬＤＰＥ／ＰＳ共混物的相容性有显著改善。