Electron microscopic investigations of initiation and growth of crazes in polystyrene

The crazes in polystyrene (PS) were investigated by using a high voltage electron microscope (HVEM, accelerating voltage of 1000 kV). The early stages of the formation and the growth of the crazes were studied in detail.The smallest deformation structures visible are weak domains or microvoids with diameters of 10–15 nm and distances of a few 10 nm between them. They act as craze nuclei and are located in narrow, long pre-craze zones. Conclusions are drawn on the processes of initiation and propagation of the crazes; both are based on molecular heterogeneities and on an increasing heterogeneity of deformation. In particular, the transformation of the closed cell structure of the voids into the open cell structure of the craze fibrils is described.Growth of crazes in thickness definitely occurs by drawing new polymeric material from the craze boundaries into the craze.     

Measurement of the dynamic fracture toughness with notched PMMA specimen under impact loading

In the present study three-point-bend impact experiments were conducted using an instrumented Charpy pendulum with a laser displacement measurement to better understand the correlation between impact velocity and the dynamic effects observed on the load-time curves. The experiments were performed at impact velocities ranging from 1 to 4 m/s.The aim of this work is to measure the dynamic fracture toughness at high impact velocities where the classical method is limited by the inertial effects. The direct measurements of the specimen deflection are successfully used for the toughness evaluation. The results obtained with this method, which are compared to other studies, indicate that this approach seems promising for brittle materials such as PMMA.     

ABS的MAH/St多单体熔融接枝及其对PA6/ABS共混体系相形态和力学性能的影响

通过多单体熔融接枝的方法制备出了具有较高接枝率的ABS接枝物 (ABS g (MAH co St) ) ,并对其接枝机理进行了初步探讨 .研究表明 ,MAH、St接枝ABS时 ,反应主要发生在ABS中聚丁二烯的双键部位 .同时 ,当MAH与St的用量比约为 1:1时接枝率达到最高 .ABS g (MAH co St)作为尼龙 6 (PA6 ) ABS共混体系相容剂起到了良好的增容效果 .实验证明 ,相容剂使用前后 ,共混物的相区尺寸由几十 μm减小到 1μm以下 ,且分布更加均匀 ;共混物的拉伸强度和冲击强度等力学性能也同时得到均衡改善 .     

Ultrafast electrokinetics

The influence of a high electric field applied on both fluid flow and particle velocities is quantified at large Peclet numbers. The experiments involved simultaneous particle image velocimetry and flow rate measurements. These are conducted in polydimethylsiloxane channels with spherical nonconducting polystyrene particles and DI water as the background flow. The high electric field tests produced up to three orders of magnitude higher electrokinetic velocities than any previous reports. The maximum electroosmotic velocity and electrophoretic velocity measured were 3.55 and 2.3 m/s. Electrophoretic velocities are measured over the range of 100 V/cm < E < 250 000 V/cm. The results are separated according to the different nonlinear theoretical models, including low and high Peclet numbers, and weak and strong concentration polarization. They show good agreement with the models. Such fast velocities could be used for flow separation, mixing, transport, control, and manipulation of suspended particles as well as microthrust generation among other applications.     

Investigation of polycarbonate/acrylonitrile butadiene styrene/multiwall carbon nanotube nanocomposites under impact loading

In this study, the effect of polycarbonate (PC)/acrylonitrile butadiene styrene (ABS)‐reinforced multiwall carbon nanotube (MWCNT) nanocomposites under a high‐velocity impact was investigated. PC/ABS (70/30 w/w)/MWCNT nanocomposites containing 1, 2, and 4 wt% were used to manufacture samples for this study. The samples were fabricated in sheet form with 100 × 100 mm dimensions and tested by gas gun for high‐velocity impact tests. The experimental results indicate that the energy absorption, limit velocity, and tensile modulus of the nanocomposite samples increased by approximately 121%, 52%, and 103% for the PC/ABS (70/30 w/w)/2 wt% MWCNT samples respectively. These results were confirmed by a transmission electron microscopy analysis test that was conducted for the state of dispersion of MWCNTs in the nanocomposite samples. The transmission electron microscopy results show that the best morphological structure of carbon nanotube at the interface of PC and ABS is that for the nanocomposite containing 2 wt% MWCNTs, which led to improved interface of the nanocomposites and higher mechanical properties. Copyright © 2016 John Wiley & Sons, Ltd.     

Ar atom emission as a probe of craze formation and craze growth in polystyrene

A report of measurements of Ar emission during the loading of polystyrene and high impact polystyrene in vacuum is presented. Argon was introduced into the material prior to the experiment by storing the samples in an Ar atmosphere. The development of crazes during loading was monitored by videotaped visual observations and scattered light measurements. Increased Ar emission is observed at the onset of crazing, provided that the crazes intersect the surface. The strength of the Ar signal depends upon the extent of crazing; especially intense signals are observed from samples which display significant crazing prior to fracture. High-impact polystyrene shows intense emissions at yield which soon decay due to the depletion of Ar from the near surface material. The emission intensity rises again prior to fracture, when surface crazes become connected to crazes in the bulk. Thus the emission of volatile species during deformation reflects the growth of crazes intersecting the surface, as well as changes in the “connectivity” of the craze network. © 1993 John Wiley & Sons, Inc.     

Damage assessment of nanofiller-reinforced woven kevlar KM2plus/Epoxy resin laminated composites

The resiliency of advanced laminated nanocomposite materials to mitigate impact load is an essential characteristic for material selection and product design. This paper investigates the effect of nanofillers and its effect on the damage resistance performance of a newly developed woven Kevlar fabric. Three types of nanofillers were investigated: (1) Silicon carbide (SiC), (2) aluminum oxide (Al₂O₃), and (3) multiwalled carbon nanotube (MWCNT). The nanofillers were dispersed using shear mixing and sonication into the epoxy to reinforce Kevlar fabric. Moreover, the effect of the nanofiller's concentration on the damage resistance performance was analyzed. All specimens had 10 layers of Kevlar fabric (KM2plus) stacked with a 0° angle. To evaluate the damage resistance performance a drop-weight impact test was conducted using a maximum drop height of 100 cm. X-ray diffraction was used to evaluate the level of material damage caused by the impact load. The addition of nanofillers enhanced the flexural properties of the composite and as well as its resiliency towards impact loads. In particular, the 0.5 wt% MWCNT laminated Kevlar/epoxy composite possessed the highest impact damage resistance capacity. Furthermore, the damage evolution was not observed within the impact area and in the surrounding areas for specimens with 0.5 wt% MWCNT. Therefore, the results indicate that the optimal nanofiller content for Kevlar KM2plus/epoxy nanocomposites is 0.5 wt% MWCNTs.     

The influence of interphases on properties of epoxy resin composites

Water sorption was determined and dynamic-mechanical measurements made on dry and water-containing systems. Different types of surface treatments of the glass fiber were studied. Immobilization of polymer chains in the interphase is determined by the nature of the curing system, annealing conditions, and surface treatment of glass fibers. Penetrating water can be found at three kinds of locations in the composite; water in the interphase has different properties than water in the polymer matrix and in microvoids. This fact can be used as a microscopic probe in epoxy-containing composites. Water content depends on the density of polar groups and the density of the network. At higher temperatures water causes crazes, at lower it mainly acts as a plasticizer. Water in crazes does not affect the glass transition temperature Tg, but it decreases (tan δ) and weakens the material. As long as water mainly goes into swelling, energy transfer between the resin and the matrix is not affected. The reinforcement then works as it should. The results demonstrate the importance of interphase properties on the behavior of the composite.     

Experimental investigation and modeling of the mechanical behavior of PC/ABS during monotonic and cyclic loading

The purpose of this work is to characterize the mechanical behavior of blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) during monotonic and cyclic loading. Compression experiments were performed using a SHIMADZU universal testing machine (10⁻⁴ to 10⁻² s⁻¹) and a split Hopkinson pressure bar (1600–5000 s⁻¹), with, the test temperatures ranging from 293 to 353 K. The influence of the rate and temperature on the deformation of PC/ABS is discussed in detail. Based on the investigation of numerous constitutive models, a phenomenological model called DSGZ was chosen to describe the compression behavior of PC/ABS. This model could not accurately reproduce the deformation of polymers at high strain rates when utilizing the same material coefficients for the low and high strain–rate deformations. In addition, this model was unable to capture the deformation features during unloading and subsequent reloading when adopting the original stress–strain updating algorithm. Hence, some improvements to the model have been implemented to better predict the deformation. Finally, the model predictions are shown to be consistent with the experimental results.     

Effects of polybutadiene-g-SAN impact modifiers on the morphology and mechanical behaviors of ABS blends

A series of PB-g-SAN impact modifiers with different ratio of PB to SAN ranging from 20.6/79.4 to 91.9/8.1 were synthesized by seeded emulsion polymerization. ABS blends were prepared by blending these PB-g-SAN impact modifiers and SAN resin. The rubber concentration of these ABS blends was kept at a constant value of 15 wt%. The influences of different impact modifier on the mechanical behavior and morphology of ABS blends have been investigated. The dynamic mechanical analysis on ABS blends shows that T_g of the rubbery phase shifts to a lower temperature, (tan δ)_max of the rubbery phase increases and then decreases with the increase of PB concentration in PB-g-SAN impact modifier. A uniform dispersion of rubber particles in the matrix can be observed when PB/SAN ratio in PB-g-SAN impact modifier is in the range from 20.6/79.4 to 71.7/28.3. When it exceeds 71.7/28.3, an agglomeration of rubber particles occurs. The mechanical tests indicate that the ABS blend, in which PB/SAN ratio in the impact modifier is 71.7/28.3, has the maximum impact strength and yield strength.     

Impact testing of polymeric foam using Hopkinson bars and digital image analysis

The present investigation aims at testing polymeric foam under impact loading using large diameter nylon Hopkinson bars and optical field measurements. Accurate average stress-strain relations can be obtained when soft large diameter polymeric pressure bars and the appropriate data processing are used. However, as there are generally no homogeneous strain and stress fields for polymeric foams, an optical field observation is needed. In contrast to quasi-static tests where the digital image correlation (DIC) measurement is commonly used, technical difficulties still remain for the reliable use of DIC under impact conditions. In this paper, an accurate synchronization method based on the displacement measurement of the end of pressure bars (calculated by a robust DIC algorithm) is preferred to conventional MCDL box time synchronization. Also, the bar end displacement measurement offers a complementary calibration method for the tension/strain conversion coefficient. Strain fields are obtained for tests on foam sample at impact velocities up to 20 m/s. The localized strain fields permit better understanding of the observed stress plateau from SHPB results. The relevance of the present method for establishing mechanical response of polymeric foam is then demonstrated.     

STUDIES ON FLAME-RETARDANT PC/ABS MULTIPLE-ELEMENT ALLOY

The preparation technology of flame-retardant PC/ABS alloys was studied in this paper. Using a high-efficiencyflame retardant system and by means of multiple-ingredient compatibilizing, PC/ABS alloys with excellent impact strengthand flame retardant property were prepared. The experimental results showed that by using PS-g-MAH and SMA assynergistic compatibilizers, the notched Izod impact strength and flammability of PC/ABS alloy obtained in the present work can be up to 175 J/m and UL-94 V0, respectively.     

Effective waste heat recovery from engine exhaust using fin prolonged heat exchanger with graphene oxide nanoparticles

Waste heat recovery is an important alternative to reduce the energy consumption in industrial processes. Heat Exchangers are used effectively for heat recovery. Thus, the role of heat exchangers for waste heat recovery system is crucial. The exclusive of heat transmission of a heat exchanger can be improved by many methods such as by modifying the geometries and using nano-additives of different concentration. In this continuation, a modified geometry of finned heat exchanger is developed with CFD analysis. Modified heat exchanger includes the fins in the internal pipe to improve heat transfer. Nanoparticles of graphene oxide with various concentrations are introduced in working fluid. A steady numerical study is performed by using ANSYS Fluent with k-omega turbulence model for exhaust flow. Variation at inlet velocities of exhaust gas and water, particles concentration and internal fin geometry are considered. The reduction in hot fluid temperature from 6 m/s to 2 m/s enhanced the effectiveness by approximately 33.3%. The decrease in hot fluid velocity to 2 m/s and 6 m/s can reduce its outlet temperature by 100 K and 14 K at 0.03 m/s cold fluid temperature. The inclusion of nanoparticles at 0.1% can enhance the effectiveness by maximum of 7%.     

Non-linear material characterization and numerical modeling of cross-ply basalt/epoxy laminate under low velocity impact

The low velocity impact behavior of basalt/epoxy composites, seen as an eco-friendly replacement of glass-epoxy composites, has not been studied systematically so far. Here, the elastic elasto-plastic properties, strengths, intralaminar and interlaminar fracture energies were determined. The intralaminar energies were determined using compact tension and compression tests. The elasto-plastic properties needed in the plastic potential were determined using off-axis test. These properties are used in Finite Element (FE) code with an elasto-plastic damage model developed earlier to simulate the impact response of cross-ply laminates basalt/epoxy laminates. Low velocity impact (LVI) experiments at 10 J, 20 J and 30 J are performed on these composites. The FE simulation is successful in capturing force, energy, deflection histories and damage zones showing a close match to the experiments. A comparison of impact force history and damage area (ultrasonic C-scan) of basalt-epoxy laminates with glass epoxy laminates having same volume fraction shows nearly similar peak forces but the major axis of the ellipsoidal damage zone was bigger in glass/epoxy laminates.     

Surface modification of boron nitride via poly (dopamine) coating and preparation of acrylonitrile‐butadiene‐styrene copolymer/boron nitride composites with enhanced thermal conductivity

A biology‐inspired approach was utilized to functionalize hexagonal boron nitride (h‐BN), to enhance the interfacial interactions in acrylonitrile‐butadiene‐styrene copolymer/boron nitride (ABS/BN) composites. The poly (dopamine), poly (DOPA) layer, was formed on the surface of BN platelets via spontaneously oxidative self‐polymerization of DOPA in aqueous solution. The modified BN (named as mBN) coated with poly (DOPA) was mixed with ABS resin by melting. The strong interfacial interactions via π‐π stacking plus Van der Waals, both derived from by poly (DOPA), significantly promoted not only the homogeneous dispersion of h‐BN in the matrix, but also the effective interfacial stress transfer, leading to improve the impact strength of ABS/mBN even at slight mBN loadings. A high thermal conductivity of 0.501 W/(m·K) was obtained at 20 wt% mBN content, reaching 2.63 times of the value for pure ABS (0.176 W/(m·K)). Meanwhile, the ABS/mBN composites also exhibited an excellent electrical insulation property, which can be expected to be applied in the fields of thermal management and electrical enclosure.     

PC/ABS blends: Compatibilization and mechanical behaviour

PC/ABS(M) blends, encompassing the whole composition range between pure PC and ABS(M), were prepared by melt-mixing in a Brabender-like apparatus. Thermal, mechanical and impact tests were performed on compression moulded specimens. Inward Tg shifts were detected for PC and ABS(M) in the blends with respect to pure PC and ABS(M) values, indicating an interaction between the component domains. This finding was confirmed by the comparison of the experimental tensile moduli with the Kernels model predictions, showing an evidence of a good adhesion between the phases. A synergistic effect was observed for the impact strength as well as for the maximum stress at an ABS(M) blend content of about 25 weight %. All the results are interpreted on the basis of an interlayer existing at the boundary between the PC and ABS phases. A preliminary investigation on the influence of the ABS internal composition, keeping constant all the other conditions (mixing, processing, specimen preparation), was carried out as well. Differences in the properties of PC/ABS(M) and PC/ABS(B) blends are thoroughly discussed. The compatibility between PC and ABS domains seems to be scarcely influenced by such a parameter in these blends.     

Impact behaviour of Dyneema® fabric-reinforced composites with different resin matrices

This paper presents an experimental study on the impact behaviour of composite laminates made of a Dyneema^® woven fabric and four different resin matrices. Three thicknesses of each kind of resin laminate were subjected to impact by a spherical steel projectile in a velocity regime ranging from 100 to 200 m/s. The results revealed that the laminates having flexible matrices performed much better in perforation resistance and energy absorption, but had a greater extent of deformation and damage than the counterparts with rigid matrices. It was found that the matrix rigidity played a crucial role in controlling the propagation of transverse deformation, and thereby the local strain and perforation resistance of laminates. The more rigid matrix restrained the laminate's transverse deformation to a smaller area at a given time, which led to higher local strain and lower perforation resistance. Fibre failure in tension was identified as the dominant failure mechanism for the tested laminates.