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复合材料黄麻环氧层合树脂纤维取向的对拉伸性能的影响M. R. Hossain1,2,* , M. A. Islam1, A. V. Vuurea2, and I. Verpoest2材料与冶金工程，孟加拉科技工程技术大学, 达卡-1000, 孟加拉国冶金与材料工程学院，鲁汶大学，Arenberg的总线2450，44，3001赫维，比利时接收在2012年5月4日，最后修订时在2012年10月25。摘要黄麻，孟加拉国的骄傲，与那些人工人造纤维如玻璃，芳纶等，其优越特定属性在复合材料领域获得很大的关注。在这个研究中，对黄麻复合材料制成的真空辅助树脂浸润（VARI）技术进行了调查。黄麻纤维预成型件堆叠的序列（0/0/0/0），0/45/ -45/ 0和0/90/ 90/ 0。对于所有的情况下，总共有25%出多黄麻纤维的体积分数成立。开发复合材料，其特征在于通过拉伸试验和实验结果由此得到与理论值进行比较。经过拉伸试验，断裂表面被切断，高分辨率FEG SEM下观察。在的情况下0/0/0/0和0/45/ -45/0薄层的复合材料，已被发现具有较高的纵向拉伸强度比横向方向。然而，对于0/90/ 90/0椎板复合材料，拉伸强度在两个方向上彼此是非常接近的。对于所有开发的复合材料，实验结果表明，开发的复合材料的拉伸性能强烈地依赖于黄麻纤维的拉伸强度，拉伸性能的影响黄麻纤维是非常敏感的缺陷。最后，复合材料一个具有争论的的拉伸表现显示在电子显微镜下观察的断口形貌。关键词：黄麻层压; UD；接口; VARI。2013 JSR。 ISSN：2070-0237（打印）2070-0245（在线）。保留所有权利。DOI：J.科学/10.3329/jsr.v5i1.10519。 住宅5（1），43-54（2013）。1 介绍黄麻在孟加拉国越来越多的部门已经在复合材料领域占据了一席之地，相当十年前。在许多复合应用，其成本低，在织领域的多功能性、环保性和适度机械性能难以匹敌一些人工纤维，如应用玻璃、凯夫拉等。然而，生物降解性和黄麻环保行为只是中断的亲水性，这反过来又影响了复合材料的力学性能以及黄麻纤维增强复合材料1，2的应用。虽然其拉伸实力是非常敏感的缺陷和跨度，黄麻跟天然纤维一样具有良好的特定的机械性能。其中影响黄麻纤维的拉伸强度一个最敏感的缺陷是其管腔或中空空间。目前流明可以作为在BWB黄麻纤维在复合材料中的缺陷的来源，并启动失败。这些对拉伸强度的影响的严重程度取决于几何形状和体积小部分的管腔。在同一时间，管腔或可用的体积分数管腔的临界尺寸和形状也取决于黄麻纤维的跨度尺寸。作为一个因此，拉伸性能得到他们的平均值通常是纠正3-6。黄麻纤维束有很多的纠缠。因此，它是非常困难的，以单向（UD）预制件黄麻纤维徒手手动干燥条件下7。在另一方面，梳理干燥或潮湿的条件下，在纤维中引入了更多的缺陷。在同一时间，黄麻纤维变得逐渐变薄8。出于这个原因，黄麻织物的编织通常是优选的。然而，在这种情况下，各向异性的性质也可能到达8，9。由于自然的扭曲和黄麻天然纤维一样的纠缠，他们都塞满了亚麻籽油。这些毛绒黄麻纤维梳理机，纱线特殊类型的前的无纺布的制备8。但是，亲水性的性质，黄麻油的存在干涉。此外，油的存在提供了非常逊色热塑性和热固性聚合物的强化过程中的接口。因此，额外的清洗和干燥步骤变得非常必要复合前的准备10，11。其结果是， UD黄麻预型体或粗纱制备已成为一个非常有用的步骤，获得时下重视。要实现多向各向同性的行为，不同角上适当的纤维取向是必要的，而这只能通过乘法层叠体的制备12来实现。UD堆叠在不同的角度给出了复合材料各向异性的物理和力学性能13。不过，多样的复合材料的优越和适度优越的机械性能，在体积分数上，具有高达50的增强纤维可以通过传统程序完成，像天然纤维这样的黄麻的压缩成型和手糊成型14。材料预浸，树脂传递模塑（RTM），真空辅助树脂渗透（VARI，RTM类似，渗透压力上有区别）来制作热固性聚合物基复合材料14，15。虽然，这些工序在人工纤维增强复合材料方面过时了十年，但它的多功能性，仍然吸引了天然纤维复合材料的研究人员利用这些技术16。因此，UD黄麻纤维的粗加工成品技术和和恰当的复合材料制备技术的结合比起制作连续的热固黄麻预浸料坯或者完成产品的各种应用要重要的多2 实验。 2.1.材料与方法在这项研究工作中，沤，水洗，晒干的孟加拉白色B级（BWB）黄麻采自孟加拉国黄麻研究所（BJRI）。在一堆采集来的黄麻中，进行单黄麻纤维的分离和拉伸试验。该从单一黄麻纤维拉伸试验得到的强度值是不相同的，从纤维到的纤维。因此，分散带很宽。为了避免这个问题，许多研究者在此字段中纠正一些数学关系式6的实验值。在这项研究工作，他们在获得单纤维拉伸测试结果后，又对结果进行了纠正。为了黄麻纤维增强复合材料的制造，4层大小400mmX400mm的黄麻纤维束层叠体型坯分别堆在以下尺寸序列0/0/0/0，0/45/ -45/ 0和0/90/ 90/ 0，如图所示。有必要指出这里用到的黄麻纤维被水浸湿做的预成型体。之后以复合材料制造在60下干燥整夜。 Ccba图1。BWB黄麻预成型体的层顺序的a）0/0/0/0；b）0/45/ -45/0和c）0/90/ 90/ 0。在复合加工之前，环氧树脂（埃皮科特828Lvel，双酚A和epichlorehydrin）和二氨基环己烷固化剂混合在一起。黄麻VARI技术制作黄麻环氧基复合材料，如图所示。 图2。在黄麻环氧树脂复合材料制造工艺签VARI设置和树脂a）VARI设置和b）树脂（由箭头表示）前面。但是应当指出的是，VARI对于复合材料制造是一个很好的技术。在这项研究工作中，预制棒放在真空袋内并保持固定在模具表面。然后真空式用来出去室内空气以及空气中夹杂的自由水分。为了加速形成真空，预成型体加热至40并持续半个多小时。树脂在真空下渗透。渗透一完成，真空包装袋的两侧夹紧并且温度以每分5 - 10速率升至135。在此温度下，复合物完全固化。在该技术中，体积分数占25的BWB黄麻纤维复合材料有不同的纤维取向。图3。堆叠顺序和黄麻环氧树脂复合材料的拉伸载荷方向（ASTM D3039）;）0/0/0/0，二）0/45/ -45/0和c）0/90/ 90/0。拉伸试样制备按照ASTM D3039标准（尺寸标本：长250mm，厚度为40.5毫米，宽151.5mm的量具长度100mm）。标本切成小齿台锯和整理完成了1200级砂纸并储存在烘箱中过夜，在50C前测试。图3示出了装载方向的拉伸试样。所有的拉伸试验，进行与帮助英斯特朗万能试验机（型号4467），细胞附着在30kN的负载和引伸计长度50mm。提及的是，所有的拉伸试验执行上面的横头速度为0.85mm/min。对于所有的情况下，至少5个标本测试。拉伸试验后，复合材料的断裂表面被切断，他们观察在一个非常高的分辨率FEG（场发射枪）SEM模型PHILIPS XL30FEG。3。结果与讨论BWB黄麻纤维环氧树脂复合材料的拉伸试验，在计算机中进行控制（的英斯特朗数据采集软件）万能试验机。应力应变因此，在拉伸试验过程中产生的曲线表示在图4中。 图4。典型的应力应变曲线）纵向和b）横向BWB黄麻环氧树脂复合材料;向。表1示出的纵向拉伸试验结果的总结。一个常见的说法是失败的强度和应变在主体装载方向（0）有一个下降趋势，增加叶片角度。达的拉伸性能在横向方向上的复合材料示于表2。常见的说法从表2中，在横向方向上的强度值有增加的趋势的增加黄麻纤维角度。表1中。积层板纵向拉伸行为。表2中。横向拉伸行为的层压板。UD和0-90复合材料的力学性能的UD和0-90复合材料在深入讨论之前，我们必须知道BWB黄麻纤维和环氧树脂基体分开，示于拉伸性能的影响表3中。EPIKOTE828 LVEL环氧树脂的和BWB黄麻纤维的平均拉伸性能的影响。黄麻纤维的强度取决于纤维的结构，它的缺陷密度，在抱怨压力和拉力测试过程中的延误和应变率。其结果是，黄麻纤维示出了广泛分布带像其他天然纤维的抗拉强度。为了取得最大可能的拉伸强度的纤维，平均拉伸强度值的值首先绘制各种纤维跨度。从这个情节，最大可能的拉伸强度值通过外推法得到在Y轴。因此，在测试过程中，得到的范围内的刚度值的为BWB黄麻纤维。但是，对于一个单一材料的刚度值应该是一个独特的价值，而不是一个范围。为了黄麻纤维的刚度值，以消除缺陷和其他因素的影响，这些随后的修正程序开发等5，6。在复合材料的情况下，混合两种或多种材料的不同的属性在一起，以得到所需的性能，这通常是不同的构成材料。对于复合材料力学性能的测定，混合法则为研究人员提供了非常有用的想法。其中一个重要的数学关系对于这下面给出：s =s V +s V 1 stands for stress (1)其中下标c，f和m分别代表的复合材料，纤维和基体、V是体积级分。按照混合法则计算UD复合材料的复合强度272.47兆帕25体积百分比BWB黄麻增强环氧树脂。但实验值是112.69兆帕，这是只有41的理论值。此类型的纤维加强效率低，也提到了17。该复合材料的拉伸强度的降低值的原因是存在的各种浓度和几何形状的基体和纤维的缺陷。图5示类型的缺陷BWB黄麻纤维的研究过程中，观察在工作。 图5。SEM照片显示使用在BWB黄麻纤维的缺陷;一）横向缺陷和b）xsectional缺陷。图6。拉伸强度变化跨度相对BWB黄麻。图6代表满量程的长度为一个函数的BWB黄麻纤维的强度。从这个数字，是非常清楚的是，黄麻纤维的拉伸强度随增加在跨距长度。 defoirdt等。 6也观察到了这种类型的效果的情况下不同的天然纤维。从这个图中，另一个观察是，为每个分散跨度比较高的跨距长度相比较低的跨距长度。在图6，强度值的范围绘制每个跨度长。从这个趋势线（也可从图6中给出的趋势方程），很明显的平均强度为5mm的跨度为800MPa左右，但外推的最大值为844.72（当跨度尺寸是非常小的，即接近于零）。在这里，重要的是要提到，这是很困难的，在许多情况下，不可能开发工程产品无缺陷，具有较高的可能性在大跨度的试验片的比例或尺寸较大的缺陷也很高。此外，它是黄麻纤维的表面状态也很明显，并不总是相同的。其结果是，相容和密合性黄麻纤维与基体发生变化，这也有助于降低开发的复合材料的拉伸强度类似的纵向拉伸强度，在横向方向上的拉伸强度也低于理论值。从表2中，很明显的是，在横向方向的拉伸强度显着低于长度方向的。这背后的原因是，黄麻纤维内部的纤维基体界面和缺陷大多是占主导地位的复合材料的拉伸强度。在这种类型的复合材料中，尤其是不均匀的纤维含量，缺乏矩阵/纤维之间的粘接接口，空洞，黄麻纤维的固有缺陷，严重降低拉伸强度的复合材料18，19。这些缺陷主要的制造过程中产生过程中，大多是围绕纤维基体界面累计20，21。其结果是合并的降解效果，实验复合材料在横向方向上的拉伸强度显着低于纵向方向。所以，在任何方向上的最大纤维强度效率并未实现22，23。也可以是在纵向方向上的拉伸强度值越高解释其断口形貌。对于0/0椎板复合材料，不同的两个步骤断口形貌已被观察到。起初，剥离在矩阵/光纤接口发生。矩阵打破，因为其相对较低的拉伸强度。在最后，黄麻纤维具有较高的抗拉强度值被打破。这现象，如图所示。 7。由于黄麻纤维具有高的拉伸强度，所以复合材料表现出较高的拉伸强度在纵向方向上。图7。纵向荷载作用下的综合故障。在横向方向上的情况下，0/0椎板黄麻纤维复合材料的拉伸破坏纤维切片（由于图8a）和剥离（表示由箭头图8b），在纤维基体界面已被发现是占主导地位的断裂模式，图。 8。在这里，一个显着比例的承载部分是由弱的纤维/基体覆盖接口。其结果是，为0/0黄麻纤维复合材料的片材，大幅减少在拉伸强度进行了观察。拉伸破坏的步骤的概要，是示意性地示出在图9。 图8。纤维与基体横向拉伸负荷下失败的）切片和纤维B）剥离图9。BWB的黄麻环氧复合材料的连续失败事件的示意图。3.2。0/+45/ -45/0，复合材料的力学性能的影响在0/+45/-45/ 0的复合材料的情况下，纵向拉伸强度劣于UD复合材料。这背后的原因是，在UD较高的强度黄麻纤维和环氧树脂基体的弱控制的拉伸强度。但是，在的情况下0 /+45/-45/ 0光纤矩阵接口主要是占主导地位的综合实力，在这些接口缺陷浓度较高。其结果是，为0 /+45/-45/ 0复合，拉伸强度是穷人在纵向方向。另一方面，横向强度的0 /+45/ -45/ 0的复合材料，显示比UD复合材料的较高的值。这背后的原因是，在大部分UD上面的纤维基体的界面缺陷。但是，在的情况下，0 /+45/ -45/ 0的复合45/-45层作为一个在45和-45的方向的阻力源。其结果是，对于0/+45/ -45/ 0复合材料，横向拉伸强度为稍高于在UD横向方向。52纤维取向的影响对于层压板，简单的混合法则不适用。情况下，0 /+45/-45/ 0复合材料，内层有两个（分别为45和-45），其中内部在压力下不同层的行为。他们的反应也是不同的和复杂的。在为了在复合材料中的增强纤维，以避免复杂的行为纤维的物理形态而言，实验结果已经说明和复合材料的断裂面。图10。0/+45/ -45/ 0复合材料层合板的断裂表面）纤维碎片和b）股份唇波状表面。图10表示典型的0 /+45/ -45/ 0复合材料的断裂面。由此图中，很明显，失败主要是由纤维矩阵和矩阵 - 矩阵剪切基体和纤维的失败，和纤维基体界面失败。光纤矩阵接口故障指示的剪切唇形波浪断裂面（用三角形表示）24。一些纤维在45的方向（由箭头标记）中也观察到的拔出。既然有纤维基质剪切，使纤维碎片也观察断裂面（由方表示）。图图10还表示矩阵的一个大岛（用圆圈表示），这表明纤维基质的分布是不均匀的。这种非均匀光纤矩阵分布还负责为黄麻环氧树脂复合材料的拉伸性能较低。周围纤维基质故障指示的球晶型的存在下压缩强制如图。 11A（用圆圈表示）。这受压区更脆比周围的基质。当施加拉伸应力受压区显示矩阵的倾向周围纤维开裂（黑色箭头表示由图11A）。该存在压缩力也证实了裂纹区周围纤维，（由在图的矩形。 11b）的。此外，一些脆性光纤故障也观察到三角形图表示。 11C。图11。裂缝横向拉伸下黄麻环氧复合加载）球状的失败，B）裂纹和三）光纤衰竭。4.结论。在这项研究工作中，黄麻纤维增强环氧树脂基复合材料的开发真空辅助树脂渗透（VARI）技术与预制堆叠序列（0/0/0/0），0/45/ -45/ 0和0/90/ 90/0。这些复合材料的特点是拉伸测试观察断裂面高分辨率FEGSEM下。由此研究工作，得出如下结论-一。在的情况下0/0/0/0和0/45/ -45/ 0椎板复合材料，纵向拉伸强度已被发现高于横向方向。然而，对于0/90/ 90/ 0椎板复合材料，在拉伸的方向的差异实力，没有观察到。二。对于所有发达国家的复合材料，实验结果显示，发达国家的复合材料的拉伸性能强烈地依赖于材料的拉伸强度的纤维，黄麻纤维的拉伸性能的影响是非常大的缺陷敏感。三。关于复合材料的拉伸性能，所得到的理论值从规则的混合物偏离实验值，而这偏差是在横向方向上的情况下更加显着。四。压缩骨折的模式是由于球状型外观和裂缝黄麻纤维周围。对于UD黄麻环氧树脂复合故障的序列，基体开裂，矩阵裂纹光纤矩阵接口，部分纤维断裂，纤维切片撤出矩阵。但是，在横向方向上，它是由纤维构成的切片和形成的纤维碎片鸣谢作者真诚感谢比利时鲁汶大学材料科学与工程冶金系赫维的支持。参考文献1。A. 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Chate JACQUEMIN，复合材料：A部分41，108（2010）。24。 /mssm/KML_BIO.htm，2010/12/1。 Effect of Fiber Orientation on the Tensile Properties of Jute Epoxy Laminated Composite M. R. Hossain1,2,* , M. A. Islam1, A. V. Vuurea2, and I. Verpoest2 1Department of Materials & Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh 2Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark, Arenberg 44, bus 2450, 3001 Heverlee, Belgium Received 4 May 2012, accepted in final revised form 25 October 2012 Abstract Jute, the pride of Bangladesh, has gained interest in the composite field due to its superior specific properties compared to artificial manmade fibers like glass, kevlar, etc. In this study, jute composites made with the vacuum assisted resin infiltration (VARI) techniques were investigated. Jute fiber preform stacking sequences were (0/0/0/0), 0/+45/-45/0 and 0/90/90/0. For all cases, a total of 25% volume fraction of jute fiber was incorporated. The developed composites were characterized by tensile tests and the experimental results thus obtained were compared with that of the theoretical values. After tensile tests, fracture surfaces were cut and observed under high resolution FEG SEM. In the case of 0/0/0/0 and 0/+45/-45/0 lamina composites, longitudinal tensile strength has been found to be higher than that of the transverse direction. However, for 0/90/90/0 lamina composites, tensile strengths in both directions were very close to each other. For all developed composites, experimental results revealed that the tensile properties of the developed composites strongly depend on the tensile strength of jute fiber and that the tensile properties of jute fiber are very much defect-sensitive. Finally, a discussion of the tensile behaviors of the composites is initiated in terms of the fracture morphologies observed under the SEM. Keywords: Jute laminate; UD; Interface; VARI. 2013 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. doi: /10.3329/jsr.v5i1.10519 J. Sci. Res. 5 (1), 43-54 (2013) 1. Introduction Jute, a growing sector in Bangladesh, has occupied a place in composite field quite a decade ago. Its low cost, versatility in textile field, eco-friendly nature and moderate mechanical properties have outnumbered the applications of some artificial fibers like glass, kevlar, etc. in many composite applications. However, biodegradability and environment friendly behaviors of jute are just interrupted with the hydrophilic nature, * Corresponding author: hrashnal Available Online Publications J. Sci. Res. 5 (1), 43-54 (2013) JOURNAL OF SCIENTIFIC RESEARCH /index.php/JSR 44 Effect of Fiber Orientation which in turn affects the composite mechanical properties as well as the applications of jute fiber reinforced composites 1, 2. Jute like natural fiber has good specific mechanical properties, although its tensile strength is extremely defect and span sensitive. One of the most sensitive defects that affect the tensile strength of the jute fiber is its lumen or hollow space in it. Lumen present in the BWB jute fiber can act as a source of defect in the composite and initiates failure. The severity of these effects on tensile strength depends on the geometry and volume fraction of the lumen. At the same time, the volume fraction of lumen or availability of lumen of critical size and shape also depends on the span size of the jute fibers. As a result, tensile properties are usually corrected for getting their average values 3-6. Jute fiber bundle has a lot of entanglement. So, it is very difficult to make unidirectional (UD) preform of the jute fiber manually with bare hand under dry condition 7. On the other hand, hackling under dry or wet condition introduces more defects in the fiber. At the same time, jute fiber becomes gradually thinner 8. For this reason, woven jute fabric is usually preferred. However, in this case, anisotropic properties might also arrive 8, 9. Due to natural twist and entanglement in jute like natural fibers, they are stuffed with linseed oil. These stuffed jute fibers are then hackled by special type of machine and yarns are made prior to woven fabric preparation 8. But, the hydrophilic nature of jute is interfered in the presence of oil. Moreover, the presence of oil gives very inferior interfaces during the reinforcement of both thermoplastic and thermoset polymers. So, additional washing and drying steps become very essential before composite preparation 10, 11. As a result, UD jute preform or roving preparation has become a valuable step, which is gaining a great importance nowadays. To achieve multidirectional isotropic behaviors, proper fiber orientation in different angle is necessary, which can only be done by multiply laminate preparation 12. Stacking the UD ply in different angles gives composite with anisotropic physical and mechanical properties 13. However, multiply composites of superior and moderately superior mechanical properties, with up to 50% volume fraction of fiber reinforcement, are possible to fabricate through conventional procedures like compression molding and handlayup for jute like natural fiber 14. Prepegging, resin transfer molding (RTM) and vacuum assisted resin infiltration (VARI, similar to RTM, but differs in infiltration pressure) for making thermoset polymer based composites 14, 15. Although, these processes are quite a decade old for artificial fiber reinforced composite, but its versatility still attracted the natural fiber composite researchers to utilize these techniques 16. Therefore, a combination of techniques to make UD jute fiber preform along with suitable composite fabrication is necessary for making continuous jutethermoset prepreg or finished product for various applications. 2. Experimental 2.1. Materials and methods In this research work retted, water washed and sun dried Bangla White Grade B (BWB) jute was collected from Bangladesh Jute Research Institute (BJRI). From the bunch of the M. R. Hossain et al. J. Sci. Res. 5 (1), 43-54 (2013) 45 collected jute, single jute fibers were separated and tensile tests were carried out. The strength values obtained from single jute fiber tensile test are not identical from fiber to fiber. As a result, the scatter band is very wide. To avoid this problem, many researchers in this field corrected the experimental values by some mathematical relationships 6. In this research work, single fiber tensile test results were also corrected following them. For the fabrication of the jute fiber reinforced composites, four layer laminate preforms of size 400mmX400mm were made with jute fiber bunch and stacked them in the following sequence 0/0/0/0, 0/+45/-45/0 and 0/90/90/0 as shown in Fig. 1. It is to be mentioned here that the jute fibers were wetted with water to make the preforms. After making the preforms, they were dried at 60C overnight prior to composite fabrication. Fig. 1. Stacking sequence of BWB jute preform; a) 0/0/0/0, b) 0/+45/-45/0 and c) 0/90/90/0. Before the composite fabrication, epoxy resin (Epikote 828Lvel, Bisphenol A and Epichlorehydrin) and diaminocyclohexane hardener were mixed together. Then jute epoxy based composite was made with VARI technique as shown in Fig. 2. Fig. 2. VARI setup and resin front during jute epoxy composite fabrication process; a) VARI setup and b) resin front (indicated by arrow). It is to be noted that VARI is a well accepted technique for composite fabrication. In this research work, the preform was put inside the vacuum bag that was kept fixed with the mold surface. Then vacuum was applied to remove inside air along with free moisture. In order to accelerate the vacuum process, the preform was heated to 40C and the process was run for half an hour. Then resin was infiltrated under vacuum. As soon as the infiltration was completed, both sides of the vacuum bag were clamped and b c b a 46 Effect of Fiber Orientation temperature was increased to 135C at a heating rate of 5 10C/ min for necessary curing. At this temperature, the composite was fully cured. Following this technique, 25% volume fraction of BWB jute fiber composites were made for different fiber orientations. Fig. 3. Stacking sequence and tensile loading direction (ASTM D3039) of jute epoxy composite; a) 0/0/0/0, b) 0/+45/-45/0 and c) 0/90/90/0. Tensile specimens were prepared following ASTM D3039 standard (dimensions of specimen: length 250mm, thickness 40.5mm, width 151.5mm, gage length 100mm). The specimens were cut using small toothed table saw and finishing was done with 1200 grade emery paper and stored over night in an oven at 50C prior to test. Fig. 3 shows the loading direction of the tensile test specimens. All tensile tests were carried out with the help of Instron universal testing machine (model 4467) having 30kN load cell attached in it and extensometer gage length 50mm. It is to be mentioned that all tensile tests were performed at a cross-head speed of 0.85mm/min. For all cases at least 5 specimens were tested. After tensile tests, composite fracture surfaces were cut off and they were observed under a very high resolution FEG (field emission gun) SEM of model PHILIPS XL30 FEG. 3. Results and Discussion Tensile tests of BWB jute fiber epoxy composite were carried out in the computer controlled (Instron data acquisition software) universal testing machine. The stress strain curves thus generated during the tensile tests are represented in Fig. 4. 0204060801001201401600.00%0.20%0.40%0.60%0.80%1.00%1.20%Strength MPaStrain %0/0/0/0 0/+45/-45/00/90/90/0051015202530350.00%0.20%0.40%0.60%0.80%1.00%Strength MPaStrain %0/0/0/00/+45/-45/00/90/90/0 Fig. 4. Typical stress strain curve of BWB jute epoxy composite; a) longitudinal and b) transverse direction. a b c M. R. Hossain et al. J. Sci. Res. 5 (1), 43-54 (2013) 47 Table 1 shows the summary of the longitudinal tensile test results. A common remark is that the strength and strain to failure in the principal (0) loading direction has a decreasing trend with increasing lamina angle. The tensile properties of the developed composites in the transverse directions are presented in Table 2. The common remark from Table 2 is that the strength values in the transverse direction have an increasing trend with the increasing jute fiber angle. Table 1. Longitudinal tensile behavior of laminates. Table 2. Transverse tensile behavior of laminates. Lamina type Strength MPa SD Strain to failure SD Youngs modulus GPa SD 0-0 11.06 3.30 0.35% 0.04% 3.25 0.62 0-45 21.33 2.08 0.80% 0.38% 4.46 0.64 0-90 39.10 10.85 0.53% 0.19% 8.97 0.74 3.1. Mechanical properties of UD and 090 composites Before going to in-depth discussion for the UD and 090 composites we must know the tensile properties of BWB jute fiber and the epoxy matrix separately, which are shown in Table 3. Table 3. Average tensile properties of epikote 828 Lvel epoxy resin and BWB jute fiber. Lamina type Strength MPa SD Strain to failure SD Youngs modulus GPa SD 0-0 112.69 18.31 0.82% 0.17% 14.59 2.28 0-45 64.31 13.18 0.64% 0.15% 10.46 0.56 0-90 42.54 6.42 0.43% 0.05% 11.13 1.47 Materials Strength MPa Youngs modulus GPa Strain to failure % Epoxy (Epikote 828Lvel) 81.7213.16 3.890.53 2.230.50 BWB jute 844.72142.47 (extrapolated) 55.662.11 (corrected for span length) 1.670.31 48 Effect of Fiber Orientation The strength of jute fiber is dependent on the fiber structure, its flaw density, griping pressure and slippage during tension test and strain rate. As a result, jute fiber shows a wide scatter band in tensile strength as like as other natural fibers. In order to obtain the maximum possible tensile strength value of the fiber, average tensile strength values of various fiber spans were plotted first. From this plot, the maximum possible tensile strength value was obtained by means of extrapolation on to the Y-axis. Consequently, during the test a range of stiffness values for BWB jute fiber were obtained. But, for a single material the stiffness value should be one unique value rather than a range. In order to eradicate the effect of these flaws and additional factors on stiffness values of jute fiber a correction procedure developed by other 5, 6 was followed. In the case of composites two or more materials of different properties are mixed together to get required properties, which are usually different from that of the constituent materials. For the determination of mechanical properties of composites, rule of mixture provides very useful idea for the researchers. One of the important mathematical relations for this is given below: mmffcVV+=1 stands for stress (1) where subscript c, f, and m stand for composite, fiber and the matrix and V is the volume fraction. As per the rule of mixture the calculated composite strength for UD composite should be 272.47 MPa for 25 volume percentage BWB jute reinforced epoxy. But the experimental value is 112.69 MPa, which is only 41% of the theoretical value. This type of low efficiency of fiber strengthening has also been mentioned by other 17. The reasons behind the decreased value of tensile strength of the composite are the presence of defects in both the matrix and fiber of various concentrations and geometries. Fig. 5 indicates the types of defects that were observed in BWB jute fiber during the research work. Fig. 5. SEM micrographs showing the defects in BWB jute fiber used; a) lateral defect and b) x-sectional defect. M. R. Hossain et al. J. Sci. Res. 5 (1), 43-54 (2013) 49 Fig. 6. Tensile strength variations of BWB jute relative to span lengths. Fig. 6 represents the strength of BWB jute fiber as a function of span lengths. From this figure, it is very clear that the tensile strength of the jute fiber decreases with increase in the span length. Defoirdt et al. 6 also observed this type of effect in the case of different natural fibers. From this figure, another observation is that the scatter for each span is relatively higher for lower span length compared to that of the higher span length. In Fig. 6, the range of strength values were plotted for each span length. From this trend line (also from the trend equation given in Fig. 6), it is clear that the average strength for 5mm span is around 800MPa, but the extrapolated maximum value is 844.72 (when the span size is very small, i.e. close to zero). Here, it is important to mention that it is very difficult and in many cases, impossible to develop engineering product to be defect free and that the possibility of having higher proportion or larger size defects in long span test specimen is also high. Moreover, it is also obvious that the surface conditions of jute fibers are not always identical. As a result, compatibility and adhesion between jute fiber and the matrix vary, which also contributes to lower tensile strength of the developed composites. Similar to the longitudinal tensile strength, tensile strengths in the transverse direction is also lower than that of the theoretical values. From Table 2, it is clear that, in transverse direction the tensile strength is significantly lower than that of the longitudinal direction. The reason behind this is that the fiber-matrix interfaces and defects inside the jute fiber mostly dominate the tensile strength of the composites. In this type of composites, especially with inhomogeneous fiber content, lack of bonding between matrix/fiber interfaces, voids, inherent defects of the jute fiber, etc. seriously degrade the tensile strength of the composite 18, 19. These defects mainly generate during the fabrication process and are accumulated mostly around the fiber-matrix interface 20, 21. As a result of the combined degrading effects, the experimental tensile strength of the composites in y = -8.775x + 844.7R = 0.66402004006008001000120014001600180020000510152025303540Strength MPaSpan length mm5mm10mm20mm35mmAverageLinear (Average)50 Effect of Fiber Orientation the transverse direction becomes significantly lower than that of the longitudinal direction. So, in any direction, the maximum fiber strength efficiency has not been achieved 22, 23. The higher values of tensile strengths in the longitudinal direction can also be explained by its fracture morphologies. For 0/0 lamina composites, two step type of fracture morphology has been observed. At first, debonding at the matrix/fiber interfaces took place. Then matrix was broken because of its relatively lower tensile strength. At last, jute fiber having relatively higher tensile strength value was broken. This phenomenon is shown in Fig. 7. As the jute fiber has a high tensile strength, so the composite showed higher tensile strength in the longitudinal direction. Fig. 7. Composite failure under longitudinal loading. In the case of transverse direction, tensile failure of 0/0 lamina jute fiber composites fiber slicing (indicated by circle Fig. 8a) and debonding (indicated by arrow Fig. 8b) at fiber-matrix interfaces have been found to be dominant modes of fracture, Fig. 8. Here, a significant proportion of the load bearing section is covered by the weak fiber/matrix interface. As a result, for 0/0 lamina of jute fiber composites, a drastic decrease in tensile strength was observed. The summary of tensile failure steps are shown schematically in Fig. 9. Fig. 8. Fiber and matrix failure under transverse tensile load; a) fiber slicing and b) debonding. Pullout Fiber splitting Debonding M. R. Hossain et al. J. Sci. Res. 5 (1), 43-54 (2013) 51 Fig. 9. Schematics of sequential failure events of BWB jute epoxy composite. 3.2. Mechanical properties of 0/+45/-45/0 composite In the case of 0/+45/-45/0 composites, the longitudinal tensile strength are inferior to that of UD composite. The reason behind this is that in UD both the relatively high strength jute fiber and weaker epoxy matrix control the tensile strength. However, in the case of 0/+45/-45/0 fiber-matrix interfaces mainly dominate the composite strength and that the concentration of defects are higher on these interfaces. As a result, for 0/+45/-45/0 composite, the tensile strengths are poor in longitudinal directions. On the other hand, the transverse strength of the 0/+45/-45/0 composite, showed higher value than the UD composite. The reason behind this is that in UD most of the defects are at the fiber matrix interface. But, in the case of 0/+45/-45/0 composite the +45/-45 ply acts as a source of resistance in +45 and -45 directions. As a result, for 0/+45/-45/0 composite, the transverse tensile strength is slightly higher than UD in the transverse direction. 52 Effect of Fiber Orientation For laminates, the simple rule of mixture is not applicable. In case of 0/+45/-45/0 composite, there are two interior layers (respectively, +45 and -45), where the interior layer behaves differently under stress. Their responses are also different and complex. In order to avoid the complex behavior of reinforcing fibers in the composite, the experimental results have been explained in terms of physical morphologies of the fibers and fracture surfaces of the composites. Fig. 10. Fracture surface of 0/+45/-45/0 composite laminate; a) fiber debris and b) share-lip type wavy surface. Fig. 10 indicates the typical fracture surface of 0/+45/-45/0 composite. From this figure, it is clear that failure is dominated by fiber-matrix and matrix-matrix shearing, matrix and fiber failure, and fiber-matrix interface failure. Fiber-matrix interface failure is indicated by shear-lip type wavy fracture surface (indicated by triangle) 24. Some fiber pullout in 45 direction is also observed (marked by arrows). Since there is fiber matrix shearing, so fiber debris is also observed on the fracture surface (indicated by square). Fig. 10 also indicates a large island of matrix (indicated by circle), which indicates that the fiber matrix distribution is non uniform. This non uniform fiber matrix distribution is also responsible for lower tensile property of the jute epoxy composite. Spherulitic type of matrix failure around fiber indicates the presence of compressive force as shown in Fig. 11a (indicated by circle). This compressive zone is more brittle than the surrounding matrix. When tensile stress is applied this compressive zone shows the tendency of matrix cracking around the fiber (indicate by black arrow Fig. 11a). The presence of compressive force is confirmed by crazing zone around fiber, (indicated by rectangle in Fig. 11b). Additionally some brittle fiber failure was also observed as indicated with triangle in Fig. 11c. M. R. Hossain et al. J. Sci. Res. 5 (1), 43-54 (2013) 53 Fig. 11. Crazing of jute epoxy composite under transverse tensile loading a) spherulitic failure, b) crazing and c) Fiber failure. 4. Conclusions In this research work, jute fiber reinforced epoxy matrix composites were developed by vacuum assisted resin infiltration (VARI) techniques with preformed stacking sequences (0/0/0/0), 0/+45/-45/0 and 0/90/90/0. These composites were characterized by tensile tests and observation of fracture surfaces under high resolution FEGSEM. From this research work, the following conclusions are made. a. In the case of 0/0/0/0 and 0/+45/-45/0 lamina composites, longitudinal tensile strength have been found to be higher than that of the transverse direction. However, for 0/90/90/0 lamina composites, directional difference in tensile strength was not observed. b. For all developed composites, experimental results revealed that the tensile properties of the developed composites are strongly dependent on the tensile strength of fiber and that the tensile properties of jute fiber are very much defect sensitive. c. Concerning the tensile properties of composites, the theoretical values obtained from the rule of mixture deviate from that of the experimental values and that this deviation is more significant in the case of transverse direction. d. Compressive fracture mode is attributed to spherulitic type appearance and crazing around jute fiber. e. For UD jute epoxy composite the sequences of failure that were matrix cracking, matrix crazing at fiber-matrix interface, partial fiber breaking, fiber slicing and pullout from matrix. However, in transverse direction, it is composed of fiber slicing and formation of fiber debris. Acknowledgemen
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