Open hole flexural and izod impact strength of unidirectional flax yarn reinforced polypropylene composites as a function of laminate lay‐up

An open hole flexural strength and impact energy of flax yarn‐reinforced polypropylene (PP) composites were studied in this work. Highest flexural strength and strength retention were observed for axial (0₆) and cross‐ply (0/90/0)_s laminates, respectively, while also examining the influence of laminate lay‐up and open hole size on flexural strength. It was found that maleic anhydride‐grafted polypropylene (MAPP)‐treated composite laminates achieved marginal improvement on flexural strength for all kinds of laminate lay‐up. Off‐axial laminates (±45₆) showed a good strength retention for open hole laminates after MAPP treatment. The fractography study confirmed microbuckling and matrix crack propagation over the compressive and tensile side of the laminate, respectively. Furthermore, severe surface damage was detected over the tensile side of 8‐mm hole size laminates. Impact test of the flax/PP laminates showed slight improvement by MAPP treatment. High‐ and low‐impact energy was experienced for axial and off‐axial laminates. The damaged impact sample shows evidence of fiber pull‐out for untreated flax yarn reinforced laminates. POLYM. COMPOS., 34:1912–1920, 2013. © 2013 Society of Plastics Engineers     

Compression behavior of woven glass/epoxy specimens using a new end‐loaded hybrid specimen

A new compression specimen was applied to woven glass/epoxy laminates. The specimen consists of epoxy layers cast on the sides of the laminate to prevent buckling. Thin‐sheet aluminum ends enable alignment and avoid crushing under end loading, which does not require any special fixture. The compression stress–strain behavior of the laminate was obtained from the specimens by discounting the previously measured stress–strain curve of the epoxy backings. Despite the higher scatter in compression tests, the average modulus was practically identical to the tensile modulus. Moreover, failure occurred away from the ends in nearly all of the specimens tested. The average compressive strength was 84% of the tensile strength and consistent with the flexural strength measured in four‐point bending tests. The present compression specimen could, therefore, become an interesting alternative to the more elaborate standard test methods available. Nevertheless, this new compression testing approach needs further evaluation involving application to other materials. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Processing and properties of tape-cast alumina/zirconia laminates composites

In the present work, laminar ceramic structures formed by layers of alumina and partially stabilized zirconia were fabricated by water-based tape casting. Rheological, physical and mechanical properties of slurries and laminates were evaluated. The laminates consisted of stacked alumina and zirconia green tapes produced by thermopressing. Pyrolysis was carried out at 450 °C and sintering at 1500 °C. The alumina/zirconia laminates were studied for a better understanding of the formation behavior and crack propagation at the laminate interface. The flexural strength values of laminates depend on the stress state on their surface. The laminates with the highest amount of zirconia layers presented low strength values (6.7 MPa), while the laminates with more alumina layers had a higher strength level (57.7 MPa). This is because these laminates have alumina layers on the surface which are in a state of residual compressive stress.     

连续玻纤增强聚丙烯混纤纱织物层压成型工艺研究

采用两种不同形式的混纤纱机织物为原料,利用层压成型的方法制备了连续玻璃纤维(GF)增强的聚丙烯(PP)板材。研究了层压温度、压力、保压时间和混纤纱机织物形式对层压板材的弯曲性能和层间剪切强度(ILSS)的影响。结果表明,当层压温度为230℃,层压压力为8.5 MPa,保压时间为30 min,降温过程冷却速度为0.5℃/min时,层压板材的力学性能最佳。弯曲强度和模量分别达到352.58 MPa、23.09 GPa,ILSS达到27.37 MPa。此时,纤维含量和空隙率分别为72.25%、2.03%。在最优工艺条件下制备的两种不同织物形式层压板材弯曲强度和弯曲模量以及ILSS:2/2斜纹织物平纹织物。两种织物层压板材的空隙率:2/2斜纹织物平纹织物。     

铺放成型工艺参数对复合材料板材弯曲强度和层间剪切强度的影响

为得出铺放成型过程中热风枪温度、铺放速度、压力辊压力、压力辊温度和底板温度五个工艺参数对复合材料板材弯曲强度和层间剪切强度(ILSS)的影响,用自制连续玻璃纤维增强聚苯硫醚预浸带进行铺放成型实验制备复合材料板材,分别用正交试验和单因素实验研究其规律。结果表明,压力辊压力对板材ILSS影响最大,底板温度对ILSS影响最小,铺放速度对弯曲强度影响最大,压力辊温度对弯曲强度影响最小。在合适的范围内,降低铺放速度,升高压力辊压力,选择适中的热风枪温度、底板温度和压力辊温度,有利于材料弯曲性能和ILSS的提升。在铺放小车速度为40 cm/min、压力辊压力为0.4 MPa、压力辊温度为240℃、热风枪温度为340℃和底板温度为240℃的条件下,GF/PPS复合材料板材的层间剪切强度和弯曲强度达到最优值,分别为79.94 MPa和1097.37 MPa。     

Ion-Exchanged Glass Laminates that Exhibit a Threshold Strength

Glass laminates, fabricated to include periodic thin layers containing biaxial compressive stresses, exhibit a threshold strength, i.e., a stress below which failure will not occur. Ion-exchange treatments in KNO₃ at 350°–450°C for periods of 3–72 h were used to create residual compressive stresses at the surface of soda lime silicate glass sheets. Wafer direct bonding of the ion-exchanged glass sheets resulted in glass laminates with thin layers of compressive stress adjacent to the glass interface and perpendicular to the laminate top surface. Critical strain energy release measurements of the bonded interface were used to optimize the bonding temperature/time to avoid significant relaxation of the stress produced by ion exchange. Stress profiles, determined via the wafer curvature measurement method, showed a residual compressive stress maximum of 328 MPa for an ion exchange temperature of 450°C. The threshold flexural strength of the ion exchanged glass laminates was determined to be 112 MPa after the introduction of indentation cracks with indent loads ranging from 1 to 5 kg. In contrast to similar ceramic laminates, where cracks either propagate across the compressive layer or bifurcate within the compressive layer, the cracks in the glass laminates were deflected along the interface between the bonded sheets.     

Determination of prepreg tack

Prepreg materials — fibre-reinforcement preimpregnated with uncured resin — are widely used for the manufacture of large composite material components. Current commercially available prepreg materials often show significant variations in tack strength, from point to point within a sheet and from sheet to sheet. Such inconsistencies can lead to void formation in the final composites laminate. This paper describes the techniques and apparatus developed for the investigation of the surface tack strength of adhesives in general and of prepreg materials in particular, as a function of contact time and pressure and of rate of separation. It is hoped that the more detailed knowledge of prepreg tack will enable the production of more consistent material and hence the manufacture of improved quality composite laminates.     

高沸醇木质素环氧树脂改性水泥砂浆的力学性能研究

用高沸醇竹子木质素合成了木质素环氧树脂和木质素环氧树脂亲水衍生物。用红外光谱对产物进行了表征,并研究了不同聚/灰比和养护条件对高沸醇竹子木质素环氧树脂及其亲水衍生物改性水泥砂浆的力学性能的影响。结果表明:随着聚/灰比的增加,试样的抗折强度提高,而抗压强度降低,试样的韧性增强,当聚/灰比大于0.12时,抗折强度明显提高;混合养护条件所得试样的综合性能优于水养护条件。     

纤维层合板梯度化后对弯曲刚度与层间应力的影响

本文介绍了纤维层合板力学梯度设计的新概念,以平纹正交织物复合材料为例,用板壳弯曲的有限元法,探讨了层合板的最佳弯曲刚度.     

Preliminary Evaluation of Hybrid Titanium Composite Laminates

In this study, the mechanical response of hybrid titanium composite laminates (HTCL) was evaluated at room and elevated temperatures. Also, the use of an elastic-plastic laminate analysis program for predicting the tensile response from constituent properties was verified. The improvement in mechanical properties achieved by the laminates was assessed by comparing the results of static strength and constant amplitude fatigue tests with those for monolithic titanium sheet. Two HTCL were fabricated with different fiber volume fractions, resin layer thicknesses and resins. One panel was thicker and was poorly bonded in comparison with the other. Consequently, the former had a lower tensile strength, while fewer cracks grew in this panel and at a slower rate. Both panels showed an improvement in fatigue life of almost two orders of magnitude. The model predictions were also in good agreement with the experimental results for both HTCL panels.     

Effectiveness of cement and plaster layers in protection of FRP confined concrete exposed to high temperatures

In recent years, fiber-reinforced polymers (FRPs) materials have shown great potential as materials for repair and reinforced concrete structures such as beams or columns by externally bonding FRP sheet(s) onto the surface of substrate concrete structures. However, the performance of FRP systems exposed to fire is a serious concern due to the combustibility of FRPs. This study introduces the results of an experimental investigation on the behavior of the circular columns of concrete under a load of axial compression, confined by an envelope of composite materials (carbon fiber and glass fiber) and protected by a layer of mortar cement or plaster coating, after they have been subjected at various temperature (23, 120, and 350 °C). The specific objectives of this study are verifying the applicability and the effectiveness of the proposed technique to improve the behavior of concrete in fire resistance and evaluate the effect of composite materials and the layer coating type used. The results indicated that protecting heat circular confined columns, with a layer of mortar cement or plaster has a significant effect on the axial strength and the ductility. It was shown that the ultimate load and axial strain of heated columns can be restored up to the original level or greater than those of unheated columns. However, the effect of a layer of plaster is more significant than a layer of mortar cement. So this coating system would enhance fire resistance of the FRP, safety and reliability of FRP reinforced concrete structures.     

Preliminary Evaluation of Hybrid Titanium Composite Laminates

In this study, the mechanical response of hybrid titanium composite laminates (HTCL) was evaluated at room and elevated temperatures. Also, the use of an elastic-plastic laminate analysis program for predicting the tensile response from constituent properties was verified. The improvement in mechanical properties achieved by the laminates was assessed by comparing the results of static strength and constant amplitude fatigue tests with those for monolithic titanium sheet. Two HTCL were fabricated with different fiber volume fractions, resin layer thicknesses and resins. One panel was thicker and was poorly bonded in comparison with the other. Consequently, the former had a lower tensile strength, while fewer cracks grew in this panel and at a slower rate. Both panels showed an improvement in fatigue life of almost two orders of magnitude. The model predictions were also in good agreement with the experimental results for both HTCL panels.     

Flexural failure of unidirectional carbon/epoxy composites: Effects of interleaving and flexural rate

The effects of a low-modulus high-strain interleaf layer, introduced at the midplane, on unidirectional carbon/epoxy laminates under flexural loading were investigated. The apparent energy-to-fail of the laminates increased significantly whereas the maximum load decreased slightly upon interleaving. Real-time microscopic observations during static flexure tests at the lowest deflection rate indicated a dramatic change of failure mode from the dominantly compressive fracture of the baseline laminate to the dominantly tensile fracture process upon interleaving. Failure modes of interleaved and baseline laminates showed no significant changes with test rate in the deflection rate range of 10⁻⁶ to 10⁰ m/sec.     

Effect of carbon nanotube on cured shape of cross-ply polymer matrix nanocomposite laminates: analytical and experimental study

Cured shape of cross-ply [0₂/90₂]_T fiber-reinforced composite laminates and the effect of multi-wall carbon nanotubes (MWCNTs) on the mechanical and thermal properties of laminates have been investigated. The nanocomposite laminate used in this study is composed of three phases: carbon fiber, polymer matrix and MWCNT. The volume fractions of 0%, 1%, 2% and 3% MWCNT were used to prepare nanocomposites. The mechanical and thermal properties of nanocomposites and fiber-reinforced nanocomposite laminates have been calculated by using analytical micromechanical models. Analytical micromechanical models were applied to determine the mechanical and thermal properties for two-phase nanocomposite composed of MWCNT and polymer matrix. The obtained mechanical and thermal data were considered as matrix properties and used in properties calculation of three-phase fiber-reinforced nanocomposite laminates. A developed model was used to determine curvature and the effect of MWCNT on the curvature in different specimens. Nonlinear relations have been considered for non-mid-plane strain equations. The addition of 1% volume fraction of MWCNT led to decreasing curvature and increasing critical size almost 14% and 9%, respectively, in different directions. The results were used to develop model and to compare with those calculated by Hyer model. Finally, unsymmetrical cross-ply [0₂/90₂]_T laminates with different weight fractions of 0, 0.1%, 0.25% and 0.5% MWCNT were fabricated. The curvature of cured composite laminates, obtained by experimental study, was compared with that developed as Hyer models and good agreements were observed between the predicted model and experimental data. The experimentally predicted and developed model for the curvature of cross-ply [0₂/90₂]_T fiber-reinforced nanocomposite laminates is better than the Hyer model.     

Properties of carbon fiber composite laminates fabricated by coresin film infusion process for different prepreg materials

A new cocured process called coresin film infusion (co‐RFI) process, which combines RFI process and prepreg/autoclave process, was introduced and four kinds of commercial carbon fiber prepreg material systems and a kind of resin film were applied to fabricate co‐RFI laminates. The compatibility between the resin film and the prepreg matrix and the application of co‐RFI process were investigated based on the resin flowability, glass transition temperature of cured resin, processing quality of laminate, and variation in resin modulus on cocured interphase region measured by nanoindentation. Furthermore, mode I (G_IC), mode II (G_IIC) delamination fracture toughness, and flexural strength and modulus were measured to evaluate the mechanical properties of cocured laminates with different prepreg materials. The experimental results show that thickness and fiber volume fraction of co‐RFI laminates with the four kinds of prepreg materials are similar to those of prepreg laminates and RFI laminate with acceptable differences. In addition, there are no obvious defects in co‐RFI laminates. Moreover, the reduced modulus of resin at cocured interface and glass transition temperature values of the mixed resin reflect good compatibility between prepreg matrix resin and RFI resin. The G_IC, G_IIC values, and flexural performances of cocured laminates lie between and even exceed those of prepreg laminates and RFI laminates, indicating no weakening effect in the cocured interface. Therefore, the co‐RFI process is believed to effectively fabricate composite with low cost and it can be applied using various prepreg systems. POLYM. COMPOS., 34:2008–2018, 2013. © 2013 Society of Plastics Engineers     

Effect of BN or B4C content on physical and tribological properties of copper-clad laminates reinforced with carbon fiber and epoxy resin

Plasma treatment was used to improve the surface roughness of copper foil. The copper-clad laminates reinforced with carbon fiber, boron nitride (BN), or boron carbide (B₄C), and epoxy resin were prepared by hot pressing. The effect of BN or B₄C content on the physical properties and tribological properties of copper-clad laminates reinforced with carbon fiber and epoxy resin were studied. The resulting copper-clad laminate exhibited desirable properties, such as dielectric constant, peel strength, oxygen index, and arc resistance, which were influenced by the concentration of BN or B₄C particles. Additionally, the wear and friction properties of the laminate were evaluated, revealing the effects of load, sliding speed, and particle content on weight loss, specific wear rate, and coefficient of friction. SEM analysis of worn surfaces provided insight into the stages of wear, highlighting the importance of an oxide layer in reducing wear and protecting the copper surface.     

湿热环境对碳纤维/环氧复合材料的冲击破坏特性影响

为了研究湿热环境对碳纤维/环氧树脂(CFRP)复合材料抗冲击性能的影响,对碳纤维/环氧复合材料层合板进行70℃水浴处理,采用锥头圆柱形弹体对湿热饱和试样和干燥室温试样进行速度分别为45 m/s、68 m/s、86 m/s的冲击,采用激光测速仪测量冲击前后的速度,然后采用超声C扫描检测系统、超景深三维显微系统、扫描电镜(SEM)等方法对试样的冲击破坏进行检测。实验结果表明:随着冲击速度的增加,试样的破坏投影面积增加;在速度较低时,湿热环境对碳纤维/环氧树脂层合板的损伤孔洞面积影响更大;湿热处理之后的碳纤维/环氧树脂层合板层间性能明显降低。     

The production and properties of oriented polypropylene laminates

The mechanical properties of isotactic polypropylene can be enhanced in one direction by a combination of rolling and drawing in a rolling mill. The longitudinal strength and modulus of the oriented polypropylene (OPP) increases with an increase in draw ratio; however, the transverse properties remain relatively unchanged. In this study, multidirectional OPP laminates [with (0)₂, (0/90)_s and (0/±45)_s lay-ups] were made to obtain sheet materials with improved properties in more than one direction in the plane. A hot-plate welding technique was used to produce these translucent and recyclable laminates. The in-plane properties of the laminates were successfully predicted with classical laminate theory (CLT), which is commonly used to predict the properties of fiber reinforced materials. These laminates can be quasi-isotropic and were found to have improved modulus [up to 6 GPa for (0/90)_s laminates] and strength [up to 150 MPa for (0/90)_s laminates] as well as exceptionally good impact toughness.     

The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics

The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotehed specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength, The propagation energy and thus the total energy for polyester laminates displays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and-thus the total impact energy increases with Increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.     

涂层对速凝3D打印水泥砂浆力学性能的影响

3D打印砂浆普遍存在层间界面粘结较弱、抗折强度较低的问题,而膨胀水泥浆涂层涂覆于砂浆表面可产生表层压应力,进而提升其抗折强度,作为界面剂涂覆于层间可同时增强其界面粘结强度。本文通过将硫铝酸盐水泥与膨胀剂混合而成的涂层涂覆于速凝3D打印砂浆的表面与层间,研究了该涂层对不掺纤维及掺0.5%(体积分数)玄武岩纤维3D打印砂浆试件力学性能的影响规律。结果表明,涂覆层间涂层对无纤维与掺纤维速凝3D打印砂浆的层间界面粘结强度提升率分别为21.4%、12.2%,同时对其抗压强度也有一定提升作用。仅涂覆表面涂层与仅涂覆层间涂层对3D打印砂浆的抗折强度提升效果相当;同时涂覆表面及层间涂层对3D打印砂浆的抗折强度提升效果最显著,与无纤维、掺纤维的无涂层试件相比,提升率最高分别可达44.2%、23.2%。涂层对于无纤维3D打印试件层间粘结强度及抗折强度的提升效果优于掺纤维试件。