Preparation and properties of polyamide-6-boehmite nanocomposites

Colloidal boehmite particles have been included into a polyamide-6 matrix by in situ polymerization. The particles have been used without any surface modification. Characterization of the nanocomposites has been carried out using transmission electron microscopy (TEM), dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC). TEM images indicate that the particles have been homogeneously dispersed in the polymer. DSC results show that the presence of boehmite affects the form of crystallization of polyamide-6, in which case formation of γ-structure is favored over the α-structure and an additional α′-phase is formed. Some mechanical reinforcement of the matrix has been accomplished as indicated by DMA results. The modulus level off at high boehmite concentrations can be explained by the reduction of the crystallinity, which cancels the effect of the filler.     

Fatigue crack propagation in mica-filled polyolefins

Edge notched samples of polypropylene (PP) and high-density polyethylene (HDPE) containing different mica concentrations were tested in mode I tensile loading. Crack growth was approximated by a non-linear regression of exponential form using statistical software (SAS). Characterization of fatigue crack propagation (FCP) was made using the Paris-Erdogan law. The crack front in PP was preceded by a wide plastic zone in which craze developed, leading to a discontinuous crack growth. Using spline functions, a margin between maximum and minimum FCP rates, recorded during the crack progression, is presented along with the average FCP rates. It is shown that mica-reinforced PP samples exhibit higher FCP rates than unfilled PP. In HDPE, mica reduces FCP rates resulting in a higher resistance to fatigue crack propagation. Effect of test frequency is presented for unfilled polymers and 10 percent mica concentration by weight in both matrices. An increase in the test frequency has no significant effect on FCP rates for both raw and mica-reinforced PP. Unfilled and mica-filled HDPE show noticeable decrease in FCP rates with increasing frequency.     

Nematic phase formation of Boehmite in polyamide-6 nanocomposites

Nematic phase behavior of titanate-treated Boehmite rods in a polyamide-6 matrix is reported. Optical polarization microscopy (OPM) and wide-angle X-ray scattering (WAXS) performed during heating and cooling cycles, are used to provide information on the level of orientation of Boehmite rods and polyamide-6 crystallites. The nematic orientation of Boehmite is clearly indicated by the permanent birefringence in OPM, which persists above the melting point of the polymer. The WAXS data show that the projection of Boehmite peaks on the 2D detector transform from isotropic to anisotropic as the Ti-Boehmite concentration is increased, regardless of temperature or the physical state of the polymer. Nematic order parameters are obtained by fitting a Maier-Saupe type function into the WAXS intensity curves. According to that, nematic order of the Boehmite peak increases gradually with the Ti-Boehmite content and it is unaffected by the heating-cooling cycles. As for the polyamide-6, nematic order of the γ-phase and one of the α-phase peaks decrease while that of the other α-phase peak increases with the number of cycles. Based on these observations, a structure for the colloidal liquid crystalline nanocomposite samples is proposed.     

Effect of sample thickness on the mechanical properties of injection-molded polyamide-6 and polyamide-6 clay nanocomposites

The effect of the thickness on the mechanical properties of injection-molded specimens of pure polyamide-6 (PA6) and polyamide-6 clay nanocomposites (PA6-NC) with 5 wt% of layered silicates was investigated. Plates of 0.5, 0.75, 1 and 2 mm thickness were characterized in the injection direction using Dynamic Mechanical Analysis under torsion and tension respectively, and tensile tests. The fracture surfaces were analyzed by Scanning Electron Microscopy. In contrast with PA6, PA6-NC showed thickness effect and clear differences in the mechanical and thermomechanical properties between skin and core, especially in the 2 mm thick samples. Increasing thickness in PA6-NC led to a reduction of tensile modulus and yield stress. In the fracture surface of the thicker tensile specimens the formation of a sheet-like structure was observed. Multiple voiding in the core causing initial failure in this region and a stiffer skin with a better orientation of the layered silicates in the injection direction are two important elements of a micromechanical model proposed in this paper to explain the fracture mechanism in PA6-NC.     

Fatigue crack propagation of rubber-toughened epoxies

Epoxies containing epoxy-terminated butadiene acrylonitrile rubber (ETBN) or amino-terminated butadiene acrylonitrile rubber (ATBN) were prepared and studied in terms of fatigue crack propagation (FCP) resistance and toughening mechanisms. Rubber incorporation improves both impact and FCP resistance, but results in slightly lower Young's modulus and T_g As T_g increases, the degree of toughening decreases. Rubber-induced shear yielding of the epoxy matrix is believed to be the dominant toughening mechanism. Decreasing fatigue resistance with increasing cyclic frequency is observed for both neat and rubber-toughened epoxies. This result may be explained by the inability of these materials to undergo possible beneficial effects of hysteretic heating. FCP resistance is linearly proportional to M_c^1/2, where M_c is the apparent molecular weight between crosslinks determined on the rubber-toughened material. FCP resistance also increases with increasing static fracture toughness K_IC. ATBN-toughened epoxies demonstrated better fatigue resistance than ETBN-toughened systems.     

Investigation on polypropylene and polyamide-6 alloys/montmorillonite nanocomposites

Propropylene (PP) and polyamide-6 (PA6) alloys nanocomposites were prepared using melt intercalation technique by blending PP and PA6 while used organophilic montmorillonite (OMT). The melt intercalation of PP and PA6 alloys was carried out in the presence of a compatibilizer such as maleic anhydride-g-polypropylene (MAPP). Their structures were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and High Resolution Electronic Microscope (HREM). It was found different blend sequences have influence on the dispersibility of OMT and self-assembled structures of OMT appeared in PP and PA6 alloys. The crystallization behavior and crystal structure of PP and PA6 alloys/montmorillonite nanocomposites were investigated by X-ray diffraction. It showed that the blend sequences have influence on crystal structure and a higher cooling rate results in increasing of γ-crystalline phase. Flammability properties are characterized by Cone Calorimeter, which show an unusual phenomenon.     

Microstructure and mechanical properties of microcellular injection molded polyamide-6 nanocomposites

The microstructure and mechanical properties of microcellular injection molded polyamide-6 (PA6) nanocomposites were studied. Cell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, were based on the material system and molding conditions. The correlation between cell density and cell size of the materials studied followed an exponential relationship. Supercritical fluid (SCF) facilitated the intercalation and exfoliation of nanoclays in the microcellular injection molding process. The orientation of nanoclays near the surface of microcells and between microcells was examined and a preferential orientation around the microcells was observed. Nanoclays in the microcellular injection molding process promoted the γ-form and suppressed the α-form crystalline structure of PA6. Both nanoclays and SCF lowered the crystallinity of the parts. Microcells improved the normalized toughness of the nanocomposites. Both microcells and nanoclay had a significant influence on the mechanical properties of parts depending on the molding conditions.     

Fatigue crack propagation behavior of cellulose esters

The effect of plasticizer concentration on fatigue crack propagation (FCP) rate in cellulose acetate-propionate (CAP) was determined. Compact tension specimens were machined from 6.2 mm-thick injection molded plaques and tested on an MTS servohydraulic testing machine using a sinusoidal waveform with a frequency of 1 Hz. Two FCP mechanisms were identified: a crazing mechanism, which dominated at low values of stress intensity factor range, ΔK, and a shear yielding mechanism, which dominated at high values of ΔK. The value of ΔK at the onset of the transition from the crazing mechanism to the shear yielding mechanism was a function of plasticizer concentration, and therefore yield strength of the CAP. The transition in crack propagation mechanism created a V-shaped feature on the fracture surface, which could be used to weight the contributions from the two crack propagation mechanisms to the overall FCP rate.     

Fatigue crack propagation and damage evolution in PBT-GF and SAN-GF

Our earlier investigations of fatigue behavior in PBT-GF and SAN-GF with different fiber lengths have shown that fatigue crack propagation (FCP) can be described in terms of elastic-plastic fracture mechanics. In this work it is shown that the influence of structural material parameters on the resistance to FCP correlates with the extent of energy dissipation at the crack tip. With increasing fiber length, the zone of energy dissipation is increased. By means of microscopic investigations, the prevailing damage in the zone of energy dissipation is identified as micro cracks in the matrix.     

Moisture absorption in polyamide-6 silicate nanocomposites and its influence on the mechanical properties

The influence of the amount of silicate and the amount of absorbed moisture on the mechanical properties of PA6 nanocomposites is discussed. Diffusion coefficients have been determined from moisture absorption experiments and similar amounts of water were absorbed by nanocomposites with different silicate concentrations. The modulus of the nanocomposites increases with increasing amount of silicate and decreases with increasing amount of absorbed moisture. However, the ductility of the nanocomposites decreases with increasing amount of silicate and increases with increasing moisture content. A more hydrophobic modification of the particles results in a reduction of the degree of exfoliation in PA6, and consequently in a lower modulus, higher ductility and an increased diffusion coefficient compared to particles that are better exfoliated. PA6 nanocomposites can compensate for the decrease of the modulus of PA6 when water is absorbed from the environment.     

增压锅炉耐火砖疲劳裂纹扩展特性研究

基于陶瓷材料断裂力学理论,将影响疲劳裂纹应力强度因子的裂纹尺寸作为评价耐火砖安全使用的依据,通过研究陶瓷材料裂纹扩展的一般规律,建立了耐火材料的裂纹扩展模型,逐一计算两个影响因素对于裂纹应力强度因子的影响,得到裂纹尺寸与应力强度因子的关系曲线,可得出在一定范围内,裂纹应力强度因子随裂纹尺寸的增加而增加,但随后会逐渐降低,由此可预估含裂纹缺陷耐火砖的剩余使用寿命。     

Fatigue crack propagation of sheet molding compounds in various evironments

The effect of the absorption of water and isooctane on the rate of fatigue crack propagation of sheet molding compounds (SMC-R30 and SMC-R65) was investigated. A crack extension gage was used to measure the crack length. Results show that the absorption of water decreases the rate of fatigue crack propagation in the initial cycles but increases the rate of propagation in the final cycle. The absorption of isooctane into SMC-R65 tends to decrease the rate of fatigue crack propagation. Microscopic observation shows considerable swelling of the polyester matrix due to the absorption of water and no significant apparent effect due to the absorption of isooctane.     

Fatigue crack propagation in,graphite fiber reinforced nylon 66

The rate of fatigue crack propagation in graphite fiber reinforced nylon 66 was measured. A model of the form å = β [K_max^1-γ ΔK^γ]^r was used to correlate the rate of crack propagation å with the maximum stress intensity K_max and the amplitude of the stress intensity ΔK experienced by the notched specimen during the fatigue test. The quantities β, γ and r were constant at fixed temperature and frequency of the test. It was also found that there exists both an upper and a lower threshold of stress intensity for the slow ropagation of damage during fatigue. The mechanism of crack propagation in the short graphite fiber reinforced nylon was found to be similar to the growth and fracture of crazes in thermoplastics. The propagation of damage at the crack tip is controlled by matrix deformation, cavitation, fiber breakage and fiber pullout. Damage can propagate in the absence of crack growth until a critical point is reached at which time the material fractures catastrophically.     

压力容器焊接接头疲劳裂纹萌生寿命的探讨

对 1 6Mn R焊接接头进行了控制应变幅条件下的疲劳试验 ,获得其疲劳特性参数以及恒应变幅载荷下的疲劳—寿命关系 ,建立了焊接接头低周疲劳寿命曲线方程 ,并将试验结果同局部应力应变法计算的结果进行了比较 ,为该焊接接头应变疲劳研究提供了理论依据     

Preparation and rheology of polyamide-6/attapulgite nanocomposites and studies on their percolated structure

The polyamide-6/attapulgite nanocomposites were prepared via an in situ polymerization route with attapulgites pre-modified with cetyltrimethylammonium bromide (CTAB) and toluene-2,4-diisocyanate (TDI). Morphology observation showed that the exfoliated attapulgite fibers were well dispersed in the polyamide-6 matrix on a nanometer scale and formed a percolation network structure. The rheological behaviors of such polymer/fibrous clay nanocompostie samples were investigated by an ARES rheometer with parallel plate geometry. The storage moduli (G′), loss moduli (G″), and dynamic viscosities of these samples increased monotonically with attapulgite content at low frequencies. The presence of attapulgites caused these nanocomposite melts to have solid-like behaviors and slower relaxation. This behavior can be explained in terms of the development of a grafting-percolated fibrous-silicate network structure. Monte Carlo simulations were performed to determine the critical threshold for attapulgites fibers in 3D. The calculated critical threshold from simulations fitted the results of our rheological experiments very well.     

Mechanical and rheological properties,and morphology of polyamide-6/organoclay/elastomer nanocomposites

The effects of ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) terpolymer and three types of organoclays (Cloisite^® 15A, 25A, and 30B) on mechanical and rheological properties, and morphology of impact modified polyamide-6/montmorillonite ternary nanocomposites were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), parallel disk rheometry, melt flow index measurements, and tensile and impact tests. The materials were prepared by melt blending using a co-rotating twin-screw extruder. XRD and TEM analyses showed that exfoliated-intercalated nanocomposites were formed in both polyamide-6/Cloisite^® 25A and Cloisite^® 30B binary nanocomposites and in ternary systems. SEM micrographs showed that rubber domain sizes were larger in the nanocomposites than in their corresponding polyamide-6/elastomer blends. Generally, tensile strength, Young's modulus, and elongation at break decreased with the addition of elastomer to polyamide-6/organoclay binary nanocomposites. In the melt state, liquid-like behavior of polyamide-6 slightly turned to pseudo solid-like in the binary and ternary nanocomposites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

Fatigue crack growth and fracture in glass sphere-filled nylon 6

The common degrading effect of glass beads on the static fracture energy and the fatigue crack propagation response in nylon 6 materials is examined by conducting fracture mechanics tests and by considering the progress of cracks through the composites. The scanning electron micrographs indicate that the cracks travel through regions of polymer matrix and also along the interfaces between polymer and glass beads. It is demonstrated that, although fracture of the polymer regions requires considerable energy, cracking of the interfaces usually absorbs very little. Thus, the crack propagation is preferably concentrated on these microstructural regions, which is the cause of the decrease in fracture energy and increase in fatigue crack growth rate with increasing amount of glass spheres in the composite. Partial properties of the matrix and the interface are introduced in order to describe the fracture behavior and to improve the understanding of the gross fracture processes. The combination of these partial properties with the volume fraction of filler and certain geometrical factors by a modified rule of mixture leads to critical values for the failure of the composites, which are in reasonable accord with the measured fatigue and fracture data.