Effects of cross section design and testing conditions on the flexural properties of wood/PVC composite beams

Highly filled wood/poly(vinyl chloride) (WPVC) composites were manufactured in five different cross sections by using an industrial‐scale twin‐screw extruder. The flexural properties of such composites were evaluated, with the effects of sample direction, span length, and rate of loading also being taken into account. The experimental results suggested that the cross section design of WPVC composite products had a significant effect on the flexural properties. Better flexural properties were obtained when testing the WPVC composites in an edgewise loading direction. The findings of this work suggested that a low number of hollow cores with thick flanges and webs should be used to obtain a composite with better flexural properties. The WPVC composite with low density was observed to exhibit lower flexural strength, and the rate of loading had a marginal effect on the flexural properties of the composites. The minimum L/d ratios of the WPVC beam to be used for steady flexural properties were 10 in the edgewise direction and 16 in the flatwise direction. J. VINYL ADDIT. TECHNOL, 2010. © 2009 Society of Plastics Engineers     

Foam sandwich composites with cyanate ester based syntactic foam as core and carbon‐cyanate ester as skin: Processing and properties

Foam sandwich composites were processed using cyanate ester‐based syntactic foam as core and carbon fabric‐cyanate ester composite as skin. They were processed by a one‐step compression‐molding technique. The mechanical performance of the sandwich composites was evaluated in terms of flatwise tensile strength (FTS), flatwise compressive strength, and edgewise compressive strength. The dependency of these properties on the core composition was investigated. FTS initially increased with the increase in resin content of the syntactic foam core. However, higher resin content in the core led to a diminution in FTS due to high void content. The flatwise compressive strength and edgewise compressive strength and the corresponding moduli values showed an increasing trend with increase in resin content of the core despite the presence of voids at high resin content. The failure modes of the composites under different loading conditions have been examined. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Stabilizations of molecular structures and mechanical properties of PVC and wood/PVC composites by Tinuvin and TiO2 stabilizers

Three different UV stabilizers, 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐ditertpentylphenol (Tinuvin XT833), 2‐(2H‐benzotriazol‐2‐yl)‐p‐cresol (Tinuvin P), or rutile–titanium dioxide (TiO₂) were incorporated into poly(vinyl chloride) (PVC) and wood/PVC (WPVC) composite, and mechanical and physical properties and photostabilities were monitored. The polyene and carbonyl sequences of PVC increased with UV weathering time and with presence of wood flour. The yellowness index increased because of polyene and carbonyl productions, whereas the brightness increased because of the photobleaching of lignin in wood. The photostabilities of PVC and WPVC could be improved through the use of UV stabilizers. Tinuvin P was recommended in this work as the most effective stabilizer for PVC and WPVC composites. The stabilization effect was interfered by presence of wood particles. The mechanical property changes corresponded well to the structural changes under UV for neat PVC. For WPVC composites, the presence of wood particles played more significant effect on the mechanical properties during UV aging than the UV stabilizer. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

开孔聚酰亚胺泡沫粘接性能研究

探讨了BHPI-J-2010胶粘剂的固化度、耐热性、固化工艺、粘接铝的拉剪强度及粘接聚酰亚胺泡沫的可行性;研究了BHPI—J-2010胶粘剂粘接聚酰亚胺泡沫／45号钢的拉伸性能及固化工艺;考查了开孔聚酰亚胺泡沫／45号钢粘接试样高温拉伸和蠕变性能及胶粘剂的使用寿命。结果表明,BHPI—J-2010胶粘剂对开孔聚酰亚胺泡沫具有良好的粘接性能。     

Crystalline phase transformation of polytetrafluoroethylene in a fatigue test

In this study, we aimed to characterize the mechanical response of polytetrafluoroethylene (PTFE) laminates under a tension–tension load‐control fatigue test (frequency = 5 Hz, load ratio = 0) and provided an analysis of the failure patterns of the PTFE material with consideration of crystalline phase transformation. In the final results, the evolution of the cyclic creep strain and stress–number of cycles to failure (S–N) curves presented duplex properties accompanying the fatigue life increasing to high cycles (cycle fatigue > 10⁵). A simple phenomenological damage index was defined in this study to describe the cyclic creep process. Additionally, the scanning electronic machine investigation suggested that local fibrosis caused by crystalline phase transformation to phase I led to the initiation of fatigue crack, and the fiber formation and orientation was found to be beneficial to a higher tensile strength and better resistance to crack propagation. The aspect of cyclic‐load‐induced crystallization was observed by the microfocus hard X‐ray diffraction beamline from a new insight. The crystalline phase transformation led to a gradient distribution of crystallinity and lateral crystallite size along the crack propagation direction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41113.     

Fatigue characterization and reliability analysis of wood flour filled polypropylene composites

In this article, the flexural fatigue behavior of wood flour filled polypropylene (PP) composite samples was investigated, and a probabilistic approach is presented to evaluate the fatigue reliability. A nondimensional analysis was adopted to establish a prediction equation for the fatigue life of the composites, and the S‐N curves and survivability diagrams are then constructed to evaluate the fatigue reliability for the wood plastic composites. The predicted results in the nondimensional fatigue model are better related to the testing data than the general S‐N curve, and they could be used as the predictive model for fatigue life analysis of the wood plastic composites. The best‐fit S‐N curve and its corresponding bounds based on 95% confidence are provided, and the fatigue data are well distributed within a 95% confidence range. A two‐parameter Weibull distribution function is used to analyze statistically the fatigue life test results of composite samples based on the probability of survival concept which is proposed to incorporate the probability of survival and the fatigue life time of the composites at a specified lifetime. Weibull graphics are plotted for the composite samples used in the fatigue test, and the S‐N curves are then drawn for different reliability index (RI) of 0.01, 0.1, 0.5, 0.9, 0.99, and 0.368 using these data. These S‐N curves are introduced for the identification of the first failure time as reliability and safety limits. It is shown that the nondimensional fatigue model and two‐parameter probability distribution model can be used successfully for reliability evaluation and lifetime estimation of wood plastic composites. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

疲劳/蠕变交互作用下塑木复合材料的断裂损伤

塑木复合材料在动态载荷作用下，其断裂损伤并非纯疲劳或纯蠕变作用的结果。利用交变载荷的试验方法，研究了在疲劳／蠕变交互作用下塑木复合材料的断裂损伤行为。结果表明，在交变载荷为破坏载荷的80％和60％时，其疲劳／蠕变断裂曲线为三段式曲线，即瞬时弹性变形阶段、延迟弹性变形阶段和加速断裂阶段；在交变载荷为破坏载荷的40％时，38h内其疲劳／蠕变曲线为两段式曲线。随着最大载荷保持时间的增加，塑木复合材料进入延迟弹性变形阶段越晚，弯曲挠度增加越快，断裂寿命降低。     

Effect of different fiber orientations on compressive creep behavior of siC fiber-reinforced mullite matrix composites

The compressive creep behavior of monolithic mullite and a composite made of mullite reinforced by 40 vol% SiC fiber were investigated at temperatures from 1100 to 1200°C and under stresses from 5 to 55 MPa in air with a loading direction parallel and perpendicular to the fiber direction. For both situations the composite exhibits better creep resistance than monolithic mullite, although there is a creep anisotropy. The improvement in creep resistance when the fibers are parallel to the loading directions is due to the shedding of the applied stress on the SiC fibers, and the improvement in creep resistance when the fibers are perpendicular to the loading direction occurs because the fibers inhibit the lateral deformation of the mullite matrix along the fibers. The improvement mechanisms of the composites were confirmed further by their creep-recovery study, which indicated that the two types of composite specimens exhibit both an apparent creep-recovery behavior on load removal, due to the relaxation of the residual stress state between the mullite matrix and the SiC fibers after unloading. ©     

Creep fatigue in engineered wood fiber and plastic compositions

Creep behaviour of unmodified and functionally modified thermoplastic‐wood fibre composites was studied. For PVC, PE and PP‐based composites creep is strongly dependent on the amount of load, time and temperature. A small rise in the temperature above ambient temperature increased creep significantly for PVC‐woodfiber composites. Instantaneous creep resistance of woodfibre‐filled PP is higher than that of PE‐based composites. PP and PE‐based wood composites were modified with maleic and maleimide compounds. Maleic or maleimide modification of woodfibre improved transient creep behaviour of PP‐woodfibre composite but it did not show practically any effect on instantaneous creep. A mathematical model has been proposed to predict creep behaviour of PVC, PP an PE‐based wood fiber composites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 260–268, 2000     

Antifungal properties and material characteristics of PVC and wood/PVC composites doped with carbamate‐based fungicides

Antifungal properties of polyvinylchloride (PVC) and wood/PVC composites (WPVC) were studied with two different fungicides through disk diffusion and dry weight techniques against Aspergillus niger as a testing fungus. Three different types of woods, including Xylia kerrii Craib & Hutch.(XK), Hevea brasiliensis Muell.(HB), and Mangifera indica Linn.(MI), were used. The disk diffusion results revealed that the addition of 3‐iodopropinyl‐N‐butylcarbamate (IPBC) in the PVC and WPVC markedly reduced the diameter of fungi, whereas the addition of methylbenzimidazole‐2‐ylcarbamate(Carbendazim) did not. The results from IPBC samples by the dry weight technique corresponded to those by the disk diffusion method. Adding fungicides tended to slightly increase the flexural properties of PVC, whereas in the case of WPVC those properties were deteriorated by IPBC and carbendazim additions. The total color change was less when IPBC only was introduced into PVC. FTIR spectra suggested that no chemical structure changes were observed after the addition of IPBC and carbendazim. The higher surface contact angle and fungicide release values for IPBC samples could imply greater diffusibility of IPBC in PVC and WPVC matrices, accompanied by increased antifungal activity. WPVC with HB wood showed the best antifungal performance, at an optimal concentration of 10,000 ppm, when compared with XK and MI woods. POLYM. ENG. SCI., 54:1248–1259, 2014. © 2013 Society of Plastics Engineers     

Fatigue damage in composite cylinders

In this study, optimal angle‐ply orientation of symmetric composite cylinders under fatigue loading is investigated. The fiber‐reinforced plastic cylinders were manufactured from E‐glass/epoxy. The layers were manufactured symmetrically in [±75°]₂, [±60°]₂, [±55°]₂, and [±45°]₂ orientations. Burst pressure of filament‐wound composite cylinders under alternating pure internal pressure was measured experimentally. Internal fatigue pressure testing method was applied to the composite cylinders in close‐ended condition. For this study, a PLC controlled hydraulic pressure testing machine has been employed. The static burst pressure values of specimens were measured; subsequently, fatigue test pressure was applied in 70, 60, and 50% stress levels of burst pressure for each orientation. Damage propagations of the composite cylinders on these stress levels were observed as whitening, leakage, and final failure for [±60°]₂, [±55°]₂, and orientations. When the damage propagation of [±75°]₂ angle‐ply cylinder was observed, whitening and leakage did not occur and final failure occurred suddenly. Stress‐cycle curves obtained from the tests are given in graphics. Experimental results reveal that variation in stress levels and the winding angles have considerable effects on final failure cycles, which is also presented graphically. The optimum winding angle for the composite pressure cylinders or vessels under internal fatigue pressure load was obtained as [±45°]₂ orientation. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

An investigation of the mechanical behavior of carbon epoxy cross ply cruciform specimens under biaxial loading

In this study, carbon epoxy cruciform type specimens with a cross‐ply layup were biaxially and uniaxially in‐plane loaded using four independent servo‐hydraulic actuators. Four different biaxial loading ratios were investigated when the applied load was quasi‐static. A comparison between experimental observations (for the damage and strain evolution of the biaxially loaded central section of the specimen) coming from digital image correlation measurements and a 3D finite element damage model will be shown. The symmetry and the uniformity of the strain field on the biaxially loaded zone were by the use of digital image correlation measurements investigated. Furthermore, the failure loads coming from the load cells of the machine were straightforward compared with the output of the finite element damage model. POLYM. COMPOS., 31:1554–1561, 2010. © 2009 Society of Plastics Engineers     

Effect of frequency upon fatigue strength of a short glass fiber reinforced polyamide 6: A superposition method based on cyclic creep parameters

The fatigue behavior of a conditioned short glass‐fiber reinforced polyamide 6 was studied and the effect of the cyclic frequency investigated. Load controlled fatigue tests were performed, and the strains and surface temperature of specimens were recorded continuously. The number of cycles to failure was found to be dependent upon cyclic creep rate, as is typical for short glass fiber reinforced polyamides in the conditioned state. A strong reduction of fatigue strength was observed for increasing cyclic load frequency. This was mainly related to the specimen temperature increase due to hysteretic self heating and its effect on the cyclic creep speed. A frequency superposition method is proposed, expressing the relationship between temperature rise, applied stress, and cyclic creep speed in terms of a parameter derived from the Larson–Miller steady creep parameter. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Fatigue behavior of medium-density polyethylene pipes for gas distribution

This paper illustrates the factors that control brittle failure under fatigue loading for test specimens cut from medium-density polyethylene pipes for gas distribution. A square bar specimen cut from a pipe with a notch was made and a fatigue test was conducted to cause a brittle failure. To obtain the correlation among stress range, frequency, temperature, and cycles to failure in this fatigue test, Coffin-Manson's frequency-modified fatigue life equation was adopted and the material constants were determined. By gradually lowering the frequency, the resistance to creep can be estimated because cycles to failure—indicating the fatigue damage—decreased, and the actual loading time—indicating the creep damage—increased.     

Hygrothermal degradation of the composite laminates from woven carbon/SC‐15 epoxy resin and woven glass/SC‐15 epoxy resin

The degradation mechanism for hygrothermal aging of woven carbon‐epoxy and woven glass‐epoxy composite laminates was investigated in the micro‐scale. Interlaminar shear and cross laminar flexural tests were performed on notched and unnotched specimens to know the mechanical performance of the composite laminates. The Interlaminar Shear Stress (ISS) for both the composites was also evaluated and correlated with the number of hygrothermal cycles. Four‐point bending and tensile or compression shear loading configurations were also used. The stress at the onset of delamination (Delamination Damage Tolerance, DDT) was identified from the load‐deflection curve of the flexural specimens and correlated with the number of hygrothermal cycles. It was found that both the ISS and DDT decrease with the exposure time. Dimensional stability was almost unchanged throughout the aging process, although there was a very little moisture absorption (∼1.3%) in glass‐epoxy and carbon‐epoxy composite laminates. SEM photomicrographs of the delaminated surface show that failure occurs suddenly in a macroscopically brittle mode by crack initiation and propagation method. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.     

Modeling deformation of a melt-infiltrated SiC/SiC composite under fatigue loading

Results from fatigue experiments done on a SiC/SiC composite are presented. A micromechanics-based model is used to study the observed behavior under cyclic loading. The model includes consideration of progressive damage, creep and oxidation of the fiber and matrix. Comparison of model predictions with test data showed that the deformation during fatigue in this material is explained primarily by damage in the form of matrix microcracking and interface debonding, in combination with creep under the cyclic load. Stiffness of the material was observed to not change significantly during fatigue indicating that the contribution of fiber fracture to deformation is limited. Fiber fracture however was found to determine final failure of the composite. Failure under cyclic fatigue loading was found to be affected by load transfer from the matrix to the fiber due to damage and creep, and by progressive degradation of the load-carrying fibers due to the combined effect of oxidation and load cycling.     

Effects of fiber size on short‐term creep behavior of wood fiber/HDPE composites

When natural fiber‐thermoplastic composites are used in long‐term loading applications, investigating creep behavior is essential. The creep behavior of high‐density polyethylene (HDPE)‐based composites reinforced with four sizes of wood fibers (WFs) (120–80, 80–40, 40–20, and 20–10 mesh) was investigated. The instantaneous deformation and creep strain of all WF/HDPE composites increased at a fixed loading level when the temperature was increased incrementally from 25 to 85°C. At a constant loading level, composites containing the larger‐sized WFs had better creep resistance than those containing smaller‐sized fibers at all measured temperatures. The creep properties of composites with smaller‐sized WFs were more temperature‐dependent than those with larger‐sized WFs. Two creep models (Burger's model and Findley's power law model) were used to fit the measured creep data. A time–temperature superposition principle calculation was attempted for long‐term creep prediction. The Findley model fitted the composite creep curves better than the four‐element Burger's model. From the predicted creep response of the WF/HDPE composites, two groups of small fibers (120–80 and 80–40 mesh) had the lowest creep resistance over long periods of time at the reference temperature of 25°C. The largest WFs (10–20 mesh) provided the best composite creep resistance. POLYM. ENG. SCI., 55:693–700, 2015. © 2014 Society of Plastics Engineers     

Impact behavior of glass–epoxy composites containing foam material

The comparative performance under repeated low mass pendulum impacts of glass–epoxy (G–E) composites, without and with the inclusion in the form of individual flexible foam sheet layer of either two numbers or three numbers at predetermined positions in the lay-up, is reported. Employing square cross-section test coupons, the orientation of the test specimen was changed with respect to the impact direction such that, in one case, the G–E part and the foam layers constituting the system was lying along the direction of impact in an edgewise manner; in the second case, the change was achieved by turning the specimen by 90° i.e., perpendicular (flatwise). The number of impacts causing specimen failure was noted in all the cases. While foam-free samples sustained a greater number of hits in the first set of experiments, foam-bearing ones performed better in the flatwise configuration. To interpret these observations, light macroscopic examination was conducted on the impacted samples. A correlation could be established between the macroscopic features and the impact results. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1565–1571, 1998     

Influence of Stress Ratio on the Elevated-Temperature Fatigue of a Silicon Carbide Fiber-Reinforced Silicon Nitride Composite

The influence of stress ratio on the tensile fatigue behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C. Tensile stress ratios of 0.1, 0.3, and 0.5 were examined. Fatigue testing was conducted in air, at a sinusoidal loading frequency of 10 Hz. For peak fatigue stresses below the proportional limit of the composite (approximately 195 MPa at 1200°C) specimens survived 5 × 10⁶ cycles, independent of stress ratio. At peak stresses above the proportional limit, fatigue failures were observed; fatigue life decreased significantly as the stress ratio was lowered from 0.5 to 0.1. Creep appears to be the predominant damage mechanism which occurs during fatigue below the proportional limit. Both mechanical cycle-by-cycle fatigue damage and creep contribute to specimen failure at peak stresses above the proportional limit.     

Fatigue of teflon bladder materials

The fatigue properties of Teflon laminates employed in the fabrication of liquid propellant expulsion bladders were studied by cyclically stretching specimens to constant load in order to achieve a correlation of cycles to failure versus the maximum stress amplitude. This approach provides a useful technique for evaluating the resistance of bladder materials to fatigue failure which could be caused by vibration-induced cyclic loadings during ground testing and launch. Further, the data provides for a direct comparison of the relative fatigue properties of Teflon laminates and, in particular for those which were studied, the relative fatigue properties correlated with their ultimate breaking stresses. This observation, which suggests a simple and rapid method for evaluating relative fatigue behavior of candidate Teflon laminates, is discussed along with the effect of delamination on the fatigue properties observed for one laminate.