Impact damage resistance of graphite/epoxy laminated composites

Damage in graphite/epoxy laminated composites resulting from low-velocity impact was studied. The impact damage mechanisms and mechanics were first investigated by adopting the impactors with a line-nosed head from which the damage was simplified from a complicated three-dimensional pattern to a two-dimensional one. Based on the results of the line-loading impact study, a model was developed for predicting the impact damage in the materials resulting from point-loading impact. The model consists of a stress analysis for calculating transient dynamic stresses during impact and a failure analysis for predicting the extent of delaminations resulting from the impact. A computer code, “3DIMPACT”, was developed based on the model during the investigation. The predictions from the code agreed fairly well with the test data.     

Analysis on low velocity impact damage of laminated composites toughened by structural toughening layer

The intralaminar and interlaminar damages of U3160/3266 laminated composites toughened by polyamide nonwoven fabric (PNF) under low velocity impact are investigated through a numerical model which considers both the three‐dimensional continuum damage mechanics (CDM) and the bilinear cohesive zone model (CZM). The analysis of the intralaminar damage is implemented by the ABAQUS/Explicit finite element code coupled with a user‐defined subroutine VUMAT where the longitudinal failure, transverse matrix cracking, and nonlinear shear of the material are taken into account. Then the effects of the thickness and strength of PNF/3266 interlayer on the damage of composites are numerically analyzed. The results reveal that damage morphology can be simulated qualitatively compared to the experimental counterparts. With the decreasing interlayer thickness or the increasing interlayer strength, the damage area is effectively reduced. This work provides an effective model to predict the low velocity impact damage of composites, and is helpful for the optimization of interlayer toughened composites. POLYM. COMPOS., 38:1280–1291, 2017. © 2015 Society of Plastics Engineers     

Effect of vulcanizing system on the crosslink density of nitrile rubber compounds

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Binary accelerator systems are being widely used in industry and are becoming increasingly popular because of the fact that such mixed systems can produce a vulcanizate with superior mechanical properties compared to those of stock cured with a single accelerator. The authors have studied the performance of a binary accelerator system based on cyclohexyl benzothiazole sulfenamide (CBS), tetramethyl thiuram disulphide (TMTD) in the sulfur vulcanization of nitrile rubber. The amount of sulfur and accelerator was varied to change the network crosslink density of vulcanizates. The observed mutual activity has been discussed based on the mechanical properties and crosslink density. The physical crosslink density of the various nitrile rubber mixes was estimated using the Kinetic Theory of Elasticity. The mechanical properties of the various rubber compounds were related to the corresponding crosslink density estimated for each compound. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2440–2445, 2005     

高抗冲性聚氯乙烯树脂简述

采取原位聚合将抗冲改性剂ACR与VCM单体接枝聚合,改善了PVC树脂的抗冲击性能。介绍了ACR及高抗冲PVC树脂的制备,对高抗冲PVC树脂的质量进行了测试分析。     

Effect of cavity pressure on crosslink density of injection moulded silicone rubber

Abstract

The effects of injection moulding conditions (temperatures 120 and 160°C, pressures from 5 to 50 MPa) on the crosslink density of the resulting parts (determined from equilibrium solvent swelling experiments) have been studied for an elastomer of the liquid silicone rubber type in which crosslinking results from platinum catalysed vinyl–silane addition. It was shown, unexpectedly, that a pressure increase leads to an increase in the cure rate, despite the unfavourable effect of pressure on viscosity.     

New insights into the damage assessment and energy dissipation weight mechanisms of ceramic/fiber laminated composites under ballistic impact

Ceramics/composite laminated armor structures have become the mainstream in the design of bulletproof materials. To obtain systematic improvements in composite targets, it is essential to design the structure of each ceramic/fiber to ensure synergistic energy matching between them. However, the challenges of heavy workload and high-costs limit the experimental testing of these composites. In this report, B₄C ceramics/ultra-high molecular weight polyethylene (UHMWPE) composite targets are studied and the impact of ceramic splicing size, impact position and size on the anti-elastic performance is predicted. Finite element analysis is used to comprehensively analyze critical obstacles of the failure damage, energy dissipation weight and ballistic mechanism of each material. Prediction results indicate that ceramics damage, fiber damage, and fiber delamination account for about 65%, 21%, and 14% of the total energy consumption, respectively. Square splicing and large size ceramics/fiber composites are found to have the best anti-elastic properties when the center position is impacted. This is ascribed to the transversal rapid stress wave propagation on the ceramic surface and the beneficial energy dissipation of the composite-backing arising from the longitudinal stress wave transfer of homogeneous panels. The results provide insights for identifying viable design methods for each structure, optimizing the matching of panels and backplanes, which reduces the number of experimental tests need to validate a given structure.     

Environmental effects on the strength and impact damage resistance of alumina based oxide/oxide ceramic matrix composites

In this study the effects of high temperature and moisture on the impact damage resistance and mechanical strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates were exposed to either a 1050°C isothermal furnace-based environment for 30 consecutive days at 6 h a day, or 95% relative humidity environment for 13 consecutive days at 67°C. Low velocity impact, tensile and short beam strength tests were performed on both ambient and environmentally conditioned laminates and damage was characterized using a combination of non-destructive and destructive techniques. High temperature and humidity environmental exposure adversely affected the impact resistance of the composite laminates. For all the environments, planar internal damage area was greater than the back side dent area, which in turn was greater than the impactor side dent area. Evidence of environmental embrittlement through a stiffer tensile response was noted for the high temperature exposed laminates while the short beam strength tests showed greater propensity for interlaminar shear failure in the moisture exposed laminates. Destructive evaluations exposed larger, more pronounced delaminations in the environmentally conditioned laminates in comparison to the ambient ones. External damage metrics of the impactor side dent depth and area directly influenced the post-impact tensile strength of the laminates while no such trend between internal damage area and residual strength could be ascertained.     

Effect of crosslink density on transport of industrial solvents through polyether based polyurethanes

Polyurethanes based on PPG 2000 with variable concentrations of TDI and TMP were prepared and used for sorption studies, employing homologous series of hydrocarbons such as benzene, toluene and xylene. The sorption was observed to be non-Fickian in nature. The solubility parameter of the polyurethane series was observed to be 9·7(calcm^-3)^1/2. The polymer solvent interaction parameter χ was found to be lowest in benzene, suggesting higher interaction with it. The sorption and diffusion coefficients were observed to increase with a decrease in the degree of crosslinking. Molecular weights between crosslinks were calculated using the Flory–Rehner equation and compared with those obtained theoretically. © 1998 Society of Chemical Industry     

Effect of crosslink density on thermal conductivity of epoxy/carbon nanotube nanocomposites

The effect of the polymeric crosslink density on the thermal conductivity of an epoxy nanocomposite was investigated by adding two different diamine‐functionalized multiwalled carbon nanotubes (diamine‐MWNTs) to the epoxy resin as co‐curing agents and conducting fillers. Tetramethylenediamine (TMDA)‐MWNTs resulted in an epoxy nanocomposite with a higher crosslink density than octamethylenediamine (OMDA)‐MWNTs. Interestingly, epoxy/TMDA‐MWNT nanocomposites under 1.5 wt % nanotube concentration, showed a higher thermal conductivity than an epoxy/OMDA‐MWNT nanocomposite with the same concentration of nanotubes. In contrast, for higher diamine‐MWNT concentrations (over 2.0 wt %), the thermal conductivity of the epoxy/OMDA‐MWNT nanocomposite was higher than that with TMDA‐MWNTs. We observed that for low MWNT concentrations, where a percolating network was not formed, a high crosslink density enhanced the thermal conductivity via phonon transport. However, for high MWNT concentrations, a high crosslink density hinders the formation of a percolating network and lowers the thermal conductivity. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44253.     

A new numerical model for the analysis on low‐velocity impact damage evolution of carbon fiber reinforced resin composites

A new numerical simulation method was proposed to predict the mechanical behavior of carbon fiber reinforced resin composites under low‐velocity impact load. The impact damage evolution can be characterized in the form of energy dissipation which can be calculated through the new numerical model. The evolution mechanism of delamination was analyzed through distinguishing between the normal induced delamination and tangential slip induced delamination. The drop weight tests were conducted on composite laminates with five kinds of stacking sequence. Experimental analysis was also presented in this article. The damage area and distribution was investigated through ultrasonic C‐scan. The prediction had a good agreement with the experimental results through the comparison of impact response. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44374.     

Carbon nanofibers for strain and impact damage sensing in glass fiber reinforced composites based on an unsaturated polyester resin

In this article, a successful employment of carbon nanofibers (CNFs) as a filler in the unsaturated polyester (UP) matrix of a glass fiber reinforced polymer (GFRP) is reported. Because of the high aspect ratio of carbon nanofibers, very small amount of these particles were sufficient to significantly modify the electrical properties of the obtained glass fiber composites: for this reason, nanocomposite matrices were produced using no more than 1 wt% of these nanoparticles. The goal of this work was to investigate the possibility to correlate the presence of a mechanical stress, or the onset of damage, in the composite produced, with the variation of electrical resistance. Following this goal, the electrical resistance of the samples was constantly measured during their mechanical testing. Two different kinds of load were applied: flexural and impact. It was possible to show that a systematic variation in the electrical resistance of the composite takes place in correspondence of both the growth of a flexural state of stress, and the creation of an impact damage. In the case of the flexural load, the electrical resistance versus strain curves provides information on the growth of damage well before such damage affects the stress–strain curve. In the case of the impact damage, electric resistance measurements were able to monitor the loss of mechanical integrity before the complete failure. SEM pictures of the crack surface have confirmed the role of the carbon nanofibers in the sensing process. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Mode I fracture resistance characteristics of graphite/epoxy laminated composites

Mode I crack resistance behavior of fiber‐reinforced (graphite/epoxy) composites laminated unidirectionally and anti‐symmetrically was studied. Double cantilever beam (DCB) specimens of stacking sequences, [O₁₂//O₁₂] and [(O/90)_3s//(90/O)_3s] were used where // represents the initial crack location. Resistance curves (R‐curves) were constructed for three initial crack lengths in order to determine the effects of initial crack length on the resistance behavior. The resistance force, G_R, for a crack increment was determined from the compliance calibration method. The results show that for the case of [(O/90)_3s//(90/O)_3s], the initial crack deviated from the midplane and propagated in a zigzag fashion within 13^th(90‐deg), while the crack propagated along the midplane for a [O₁₂//O₁₂] case. The results also show that for both cases, G_R was affected by the initial crack length before G_R was stabilized. However, G_R was not affected by initial crack length when G_R was stabilized for each case.     

Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles

Model diglycidyl ether of bisphenol-A based epoxy resins containing well-dispersed 15 nm block copolymer (BCP) nanoparticles were prepared to study the effect of matrix crosslink density on their fracture behavior. The crosslink density of the model epoxies was varied via the controlled epoxy thermoset technology and estimated experimentally. As expected, it was found that the fracture toughness of the BCP-toughened epoxy is strongly influenced by the crosslink density of the epoxy matrix, with higher toughenability for lower crosslink density epoxies. Key operative toughening mechanisms of the above model BCP-toughened epoxies were found to be nanoparticle cavitation-induced matrix shear banding for the low crosslink density epoxies. The toughening effect from BCP nanoparticles was also compared with core-shell rubber-toughened epoxies having different levels of crosslink density. The usefulness of the present findings for designing toughened thermosetting materials with desirable properties is discussed.     

Influence of the filler and monomer quantities in the rheometrical behaviour and crosslink density of the NBR-cellulose II composites

Cellulose II (the structural component) was incorporated into latex of butadiene-acrylonitrile copolymer (the matrixial component) by coprecipitation of nitrilic latexcellulose xanthate mixture, according procedure developed in this Institute. According to different types of elastomeric compositions with 26%, 33% and 45% of acrylonitrile (monomers) and cellulose II up to 25 phr were evaluated. This study tries to correlate the rheometrical behaviour, which was estimated by cure parameters, with crosslink density values, which were obtained by swelling method using organic solvent. It was found that the amounts of both acrylonitrile and cellulose present in these several composites affect the cure results, and this fact suggests the presence of a rubber-filler interaction.     

Tailoring engineered cementitious composites for impact resistance

This paper presents results of deliberate tailoring of engineered cementitious composites (ECC) for impact resistance. Microstructure control involving fiber, matrix and fiber/matrix interface was based on steady-state dynamic crack growth analyses accounting for rate dependence of composite phases. Uniaxial tensile stress–strain curves of the resulting impact resistant ECC were experimentally determined for strain rates ranging from 10^? 5 s^? 1 to 10^? 1 s^? 1. Low speed drop weight tower test on ECC panels and beams was also conducted. Damage characteristics, load and energy dissipation capacities, and response to repeated impacts, were studied.     

Effect of organic acids on the mechanical properties of phenolic resin composites

The effect of salicylic acid and its derivatives on the properties of phenolic resin composites was evaluated. The composites were reinforced with aluminum oxide particles in both solid and hollow forms. Differential scanning calorimetry studies have shown that the reaction rate of phenolic resin was accelerated by salicylic acid, but was not affected by the other compounds. Salicylic acid also reduced the flexural strengths of the phenolic resin composites. The strength was decreased by more than 30% in comparison to that with no acid added. In contrast, two derivatives of this acid—sodium salicylate and 4‐hydroxybenzoic acid—have minimal impact on the flexural strengths of the composites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 642–647, 2000     

Effect of crosslink density and internetwork grafting on the transparency of polyurethane/polystyrene interpenetrating polymer networks

Because of their incompatibility and the different refractive indices of the homopolymer components, polyurethane/polystyrene interpenetrating polymer networks are turbid by nature. Different parameters likely to enhance their transparency are examined: the crosslink density of each network and the level of internetwork grafting. The results prove that the latter factor is the most effective, as in some cases, very clear and transparent samples are obtained. Correspondingly, preliminary investigations of the dynamic mechanical properties show an inward shift of the glass transition temperatures for such systems. It is concluded that parameters able to cause a higher degree of phase dispersion can yield transparent materials.     

Effect of morphology,crosslink density,and miscibility on interpenetrating polymer network damping effectiveness

The areas under the linear loss modulus versus temperature curves (loss area, LA) and tan δ versus temperature curves (TA) were evaluated for a number of acrylic, methacrylic, styrenic and butadiene based copolymers and interpenetrating polymer networks, IPNs, as a function of crosslink density and comliosition, and were compared with values predicted by group contribution analysis. The LAs of the sequential IPNs, cross-poly(n-butyl methacrylate)-inter-crosspolystyrene, were found to exhibit up to 30% larger LAs than the poly(n-butyl metacrylate-stat-styrene) copolymers, which had LAs slightly less than the values predicted from the group contribution analysis. At constant chemical composition (50% n-butyl methacrylate, 50% styrene), LA equals 14.4 GPa K for the IPN, attributed to a synergistic effect resulting from the IPN's microheterogeneous morphology, as compared with 10.7 GPa K for the single phase, miscible copolymer. Increases in the LA with increased concentration of polymer, network II were also observed for cross-poly(ethyl acrylate)-inter-crosspolystyrene and cross-polybutadiene-inter-cross-polystyrene IPNs. On the other hand, cross-polybutadiene-inter-cross-poly(methyl methacrylate) IPNs had LAs much lower than were predicted by the group contribution analysis, which were attributed to lower miscibility in this system relative to the other systems evaluated. In general, decreased crosslink densities and lower concentrations of network II increased TA. These findings demonstrate how the morphology of a multiphase polymeric material can affect LA and TA, with significant increases In damping capability over the average of the component polymer values.     

Creep damage resistance of ceramic–matrix composites

The tensile creep and creep fracture properties in air at 1300 °C are documented for two ceramic fibre-reinforced ceramic–matrix composites (CFCMCs). These recently developed materials were produced with woven bundles of Hi-Nicalon™ fibres reinforcing either A1₂O₃ or enhanced SiBC matrices, allowing data comparisons to be made with similar CFCMCs having different fibre–matrix combinations. The results confirm that the longitudinal fibres govern the rates of strain accumulation and crack growth, but the fracture characteristics are determined by fibre failure caused by oxygen penetration as matrix cracks develop. The analysis then suggests that carbon fibre-reinforced doloma–matrix composites could offer a combination of creep-resistant fibres and creep damage-resistant matrices suitable for long-term load-bearing service in high-temperature oxidizing environments.     

交联密度对HNBR/聚甲基丙烯酸正丁酯IPN阻尼性能的影响

用序列法制备氢化丁腈橡胶(HNBR)/聚甲基丙烯酸正丁酯(PnBMA)互穿聚合物网络(IPN)材料,通过改变两网络的硫化剂用量得到不同交联密度的HNBR/PnBMA IPN。采用动态粘弹谱仪测试IPN的阻尼性能,结果表明HNBR/PnBMA IPN材料的阻尼特性与交联密度密切相关。随着第一网络交联密度的增加,阻尼曲线逐渐由双峰转变为单峰,阻尼峰宽降低。第二网络交联密度的增加使阻尼曲线整体下降,双峰形态不变。通过改变两网络的交联密度可调整阻尼峰的位置、宽度及阻尼峰值。