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復合材料黃麻環(huán)氧層合樹脂纖維取向的對拉伸性能的影響 M. R. Hossain1,2,* , M. A. Islam1, A. V. Vuurea2, and I. Verpoest2 材料與冶金工程，孟加拉科技工程技術大學, 達卡-1000, 孟加拉國 冶金與材料工程學院，魯汶大學，Arenberg的總線2450，44，3001赫維，比利時 接收在2012年5月4日，最后修訂時在2012年10月25。 摘要 黃麻，孟加拉國的驕傲，與那些人工人造纖維如玻璃，芳綸等，其優(yōu)越特定屬性在復合材料領域獲得很大的關注。在這個研究中，對黃麻復合材料制成的真空輔助樹脂浸潤（VARI）技術進行了調查。黃麻纖維預成型件堆疊的序列（0/0/0/0），0/45°/ -45°/ 0和0/90°/ 90°/ 0。對于所有的情況下，總共有25%出多黃麻纖維的體積分數(shù)成立。開發(fā)復合材料，其特征在于通過拉伸試驗和實驗結果由此得到與理論值進行比較。經(jīng)過拉伸試驗，斷裂表面被切斷，高分辨率FEG SEM下觀察。在的情況下0/0/0/0和0/45°/ -45°/0薄層的復合材料，已被發(fā)現(xiàn)具有較高的縱向拉伸強度比橫向方向。然而，對于0/90°/ 90°/0椎板復合材料，拉伸強度在兩個方向上彼此是非常接近的。對于所有開發(fā)的復合材料，實驗結果表明，開發(fā)的復合材料的拉伸性能強烈地依賴于黃麻纖維的拉伸強度，拉伸性能的影響黃麻纖維是非常敏感的缺陷。最后，復合材料一個具有爭論的的拉伸表現(xiàn)顯示在電子顯微鏡下觀察的斷口形貌。 關鍵詞：黃麻層壓; UD；接口; VARI。 ?2013 JSR。 ISSN：2070-0237（打印）2070-0245（在線）。保留所有權利。 DOI：J.科學http://dx.doi.org/10.3329/jsr.v5i1.10519。 住宅5（1），43-54（2013）。 1 介紹 黃麻在孟加拉國越來越多的部門已經(jīng)在復合材料領域占據(jù)了一席之地，相當十年前。在許多復合應用，其成本低，在織領域的多功能性、環(huán)保性和適度機械性能難以匹敵一些人工纖維，如應用玻璃、凱夫拉等。然而，生物降解性和黃麻環(huán)保行為只是中斷的親水性，這反過來又影響了復合材料的力學性能以及黃麻纖維增強復合材料[1，2]的應用。 雖然其拉伸實力是非常敏感的缺陷和跨度，黃麻跟天然纖維一樣具有良好的特定的機械性能。其中影響黃麻纖維的拉伸強度一個最敏感的缺陷是其管腔或中空空間。目前流明可以作為在BWB黃麻纖維在復合材料中的缺陷的來源，并啟動失敗。這些對拉伸強度的影響的嚴重程度取決于幾何形狀和體積 小部分的管腔。在同一時間，管腔或可用的體積分數(shù)管腔的臨界尺寸和形狀也取決于黃麻纖維的跨度尺寸。作為一個因此，拉伸性能得到他們的平均值通常是糾正[3-6]。黃麻纖維束有很多的糾纏。因此，它是非常困難的，以單向（UD）預制件黃麻纖維徒手手動干燥條件下[7]。在另一方面，梳理干燥或潮濕的條件下，在纖維中引入了更多的缺陷。在同一時間，黃麻纖維變得逐漸變薄[8]。出于這個原因，黃麻織物的編織 通常是優(yōu)選的。然而，在這種情況下，各向異性的性質也可能到達[8，9]。由于自然的扭曲和黃麻天然纖維一樣的糾纏，他們都塞滿了亞麻籽油。這些毛絨黃麻纖維梳理機，紗線特殊類型的前的無紡布的制備[8]。但是，親水性的性質，黃麻油的存在干涉。此外，油的存在提供了非常遜色熱塑性和熱固性聚合物的強化過程中的接口。因此，額外的清洗和干燥步驟變得非常必要復合前的準備[10，11]。其結果是， UD黃麻預型體或粗紗制備已成為一個非常有用的步驟，獲得時下重視。 要實現(xiàn)多向各向同性的行為，不同角上適當?shù)睦w維取向是必要的，而這只能通過乘法層疊體的制備[12]來實現(xiàn)。UD堆疊在不同的角度給出了復合材料各向異性的物理和力學性能[13]。不過，多樣的復合材料的優(yōu)越和適度優(yōu)越的機械性能，在體積分數(shù)上，具有高達50％的增強纖維可以通過傳統(tǒng)程序完成，像天然纖維這樣的黃麻的壓縮成型和手糊成型[14]。 材料預浸，樹脂傳遞模塑（RTM），真空輔助樹脂滲透（VARI，RTM類似，滲透壓力上有區(qū)別）來制作熱固性聚合物基復合材料[14，15]。雖然，這些工序在人工纖維增強復合材料方面過時了十年，但它的多功能性，仍然吸引了天然纖維復合材料的研究人員利用這些技術[16]。因此，UD黃麻纖維的粗加工成品技術和和恰當?shù)膹秃喜牧现苽浼夹g的結合比起制作連續(xù)的熱固黃麻預浸料坯或者完成產(chǎn)品的各種應用要重要的多 2 實驗。 2.1.?材料與方法 在這項研究工作中，漚，水洗，曬干的孟加拉白色B級（BWB）黃麻采自孟加拉國黃麻研究所（BJRI）。在一堆采集來的黃麻中，進行單黃麻纖維的分離和拉伸試驗。該從單一黃麻纖維拉伸試驗得到的強度值是不相同的，從纖維到的纖維。因此，分散帶很寬。為了避免這個問題，許多研究者在此字段中糾正一些數(shù)學關系式[6]的實驗值。在這項研究工作，他們在獲得單纖維拉伸測試結果后，又對結果進行了糾正。為了 黃麻纖維增強復合材料的制造，4層大小400mmX400mm的黃麻纖維束層疊體型坯分別堆在以下尺寸 序列0/0/0/0，0/45°/ -45°/ 0和0/90°/ 90°/ 0，如圖所示。有必要指出這里用到的黃麻纖維被水浸濕做的預成型體。之后以復合材料制造在60℃下干燥整夜。 C c b a 圖1。BWB黃麻預成型體的層順序的a）0/0/0/0；b）0/45°/ -45°/0和c）0/90°/ 90°/ 0。 在復合加工之前，環(huán)氧樹脂（埃皮科特828Lvel，雙酚A和epichlorehydrin）和二氨基環(huán)己烷固化劑混合在一起。黃麻VARI技術制作黃麻環(huán)氧基復合材料，如圖所示。 圖2。在黃麻環(huán)氧樹脂復合材料制造工藝簽VARI設置和樹脂a）VARI設置和b）樹脂（由箭頭表示）前面。 但是應當指出的是，VARI對于復合材料制造是一個很好的技術。在這項研究工作中，預制棒放在真空袋內并保持固定在模具表面。然后真空式用來出去室內空氣以及空氣中夾雜的自由水分。為了加速形成真空，預成型體加熱至40℃并持續(xù)半個多小時。樹脂在真空下滲透。滲透一完成，真空包裝袋的兩側夾緊并且溫度以每分5 - 10℃速率升至135℃。在此溫度下，復合物完全固化。在該技術中，體積分數(shù)占25％的 BWB黃麻纖維復合材料有不同的纖維取向。 圖3。堆疊順序和黃麻環(huán)氧樹脂復合材料的拉伸載荷方向（ASTM D3039）;）0/0/0/0，二）0/45°/ -45°/0和c）0/90°/ 90°/0。拉伸試樣制備按照ASTM D3039標準（尺寸標本：長250mm，厚度為4±0.5毫米，寬15±1.5mm的量具長度100mm）。標本切成小齒臺鋸和整理完成了1200級砂紙并儲存在烘箱中過夜，在50°C前測試。圖3示出了裝載方向的拉伸試樣。所有的拉伸試驗，進行與幫助英斯特朗萬能試驗機（型號4467），細胞附著在30kN的負載和引伸計長度50mm。提及的是，所有的拉伸試驗執(zhí)行上面的橫頭速度為0.85mm/min。對于所有的情況下，至少5個標本測試。拉伸試驗后，復合材料的斷裂表面被切斷，他們觀察在一個非常高的分辨率FEG（場發(fā)射槍）SEM模型PHILIPS XL30FEG。3。結果與討論BWB黃麻纖維環(huán)氧樹脂復合材料的拉伸試驗，在計算機中進行控制（的英斯特朗數(shù)據(jù)采集軟件）萬能試驗機。應力應變因此，在拉伸試驗過程中產(chǎn)生的曲線表示在圖4中。 圖4。典型的應力應變曲線）縱向和b）橫向BWB黃麻環(huán)氧樹脂復合材料;向。表1示出的縱向拉伸試驗結果的總結。一個常見的說法是失敗的強度和應變在主體裝載方向（0°）有一個下降趨勢，增加葉片角度。達的拉伸性能在橫向方向上的復合材料示于表2。常見的說法從表2中，在橫向方向上的強度值有增加的 趨勢的增加黃麻纖維角度。 表1中。積層板縱向拉伸行為。 表2中。橫向拉伸行為的層壓板。 UD和0-90復合材料的力學性能的UD和0-90復合材料在深入討論之前，我們必須知道 BWB黃麻纖維和環(huán)氧樹脂基體分開，示于拉伸性能的影響 表3中。EPIKOTE828 LVEL環(huán)氧樹脂的和BWB黃麻纖維的平均拉伸性能的影響。 黃麻纖維的強度取決于纖維的結構，它的缺陷密度，在抱怨壓力和拉力測試過程中的延誤和應變率。其結果是，黃麻纖維示出了廣泛分布帶像其他天然纖維的抗拉強度。為了取得最大可能的拉伸強度的纖維，平均拉伸強度值的值首先繪制各種纖維跨度。從這個情節(jié)，最大可能的拉伸強度值通過外推法得到在Y軸。因此，在測試過程中，得到的范圍內的剛度值的為BWB黃麻纖維。但是，對于一個單一材料的剛度值應該是一個獨特的價值，而不是一個范圍。為了黃麻纖維的剛度值，以消除缺陷和其他因素的影響，這些隨后的修正程序開發(fā)等[5，6]。在復合材料的情況下，混合兩種或多種材料的不同的屬性在一起，以得到所需的性能，這通常是不同的構成材料。對于復合材料力學性能的測定，混合法則為研究人員提供了非常有用的想法。其中一個重要的數(shù)學關系對于這下面給出： s =s V +s V 1 [σ stands for stress] (1) 其中下標c，f和m分別代表的復合材料，纖維和基體、V是體積級分。按照混合法則計算UD復合材料的復合強度272.47兆帕25體積百分比BWB黃麻增強環(huán)氧樹脂。但實驗值是112.69兆帕，這是只有41％的理論值。此類型的纖維加強效率低，也提到了[17]。該復合材料的拉伸強度的降低值的原因是存在的各種濃度和幾何形狀的基體和纖維的缺陷。圖5示類型的缺陷BWB黃麻纖維的研究過程中，觀察在工作。 圖5。SEM照片顯示使用在BWB黃麻纖維的缺陷;一）橫向缺陷和b）xsectional缺陷。 圖6。拉伸強度變化跨度相對BWB黃麻。 圖6代表滿量程的長度為一個函數(shù)的BWB黃麻纖維的強度。從這個數(shù)字，是非常清楚的是，黃麻纖維的拉伸強度隨增加在跨距長度。 defoirdt等。 [6]也觀察到了這種類型的效果的情況下不同的天然纖維。從這個圖中，另一個觀察是，為每個分散跨度比較高的跨距長度相比較低的跨距長度。在圖6，強度值的范圍繪制每個跨度長。從這個趨勢線（也可從圖6中給出的趨勢方程），很明顯的平均強度為5mm的跨度為800MPa左右，但外推的最大值為844.72（當跨度尺寸是非常小的，即接近于零）。 在這里，重要的是要提到，這是很困難的，在許多情況下，不可能開發(fā)工程產(chǎn)品無缺陷，具有較高的可能性在大跨度的試驗片的比例或尺寸較大的缺陷也很高。此外，它是黃麻纖維的表面狀態(tài)也很明顯，并不總是相同的。其結果是，相容和密合性黃麻纖維與基體發(fā)生變化，這也有助于降低開發(fā)的復合材料的拉伸強度類似的縱向拉伸強度，在橫向方向上的拉伸強度也低于理論值。從表2中，很明顯的是，在橫向方向的拉伸強度顯著低于長度方向的。 這背后的原因是，黃麻纖維內部的纖維基體界面和缺陷大多是占主導地位的復合材料的拉伸強度。在這種類型的復合材料中，尤其是不均勻的纖維含量，缺乏矩陣/纖維之間的粘接接口，空洞，黃麻纖維的固有缺陷，嚴重降低拉伸強度的復合材料[18，19]。這些缺陷主要的制造過程中產(chǎn)生過程中，大多是圍繞纖維基體界面累計[20，21]。其結果是合并的降解效果，實驗復合材料在橫向方向上的拉伸強度顯著低于縱向方向。所以，在任何方向上的最大纖維強度效率并未實現(xiàn)[22，23]。 也可以是在縱向方向上的拉伸強度值越高解釋其斷口形貌。對于0/0椎板復合材料，不同的兩個步驟斷口形貌已被觀察到。起初，剝離在矩陣/光纖接口發(fā)生。矩陣打破，因為其相對較低的拉伸強度。在 最后，黃麻纖維具有較高的抗拉強度值被打破。這現(xiàn)象，如圖所示。 7。由于黃麻纖維具有高的拉伸強度，所以復合材料表現(xiàn)出較高的拉伸強度在縱向方向上。 圖7?？v向荷載作用下的綜合故障。 在橫向方向上的情況下，0/0椎板黃麻纖維復合材料的拉伸破壞纖維切片（由于圖8a）和剝離（表示由箭頭圖8b），在纖維基體界面已被發(fā)現(xiàn)是占主導地位的斷裂模式，圖。 8。在這里，一個顯著比例的承載部分是由弱的纖維/基體覆蓋接口。其結果是，為0/0黃麻纖維復合材料的片材，大幅減少在拉伸強度進行了觀察。拉伸破壞的步驟的概要，是示意性地示出在圖9。 圖8。纖維與基體橫向拉伸負荷下失敗的）切片和纖維B）剝離 圖9。BWB的黃麻環(huán)氧復合材料的連續(xù)失敗事件的示意圖。 3.2。0/+45/ -45/0，復合材料的力學性能的影響在0/+45/-45/ 0的復合材料的情況下，縱向拉伸強度劣于 UD復合材料。這背后的原因是，在UD較高的強度黃麻纖維和環(huán)氧樹脂基體的弱控制的拉伸強度。但是，在的情況下0 /+45/-45/ 0光纖矩陣接口主要是占主導地位的綜合實力，在這些接口缺陷濃度較高。其結果是，為0 /+45/-45/ 0復合，拉伸強度是窮人在縱向方向。另一方面，橫向強度的0 /+45/ -45/ 0的復合材料，顯示 比UD復合材料的較高的值。這背后的原因是，在大部分UD上面的纖維基體的界面缺陷。但是，在的情況下，0 /+45/ -45/ 0的復合45/-45層作為一個在45°和-45°的方向的阻力源。其結果是，對于0/+45/ -45/ 0復合材料，橫向拉伸強度為稍高于在UD橫向方向。 52纖維取向的影響 對于層壓板，簡單的混合法則不適用。情況下，0 /+45/-45/ 0復合材料，內層有兩個（分別為45°和-45°），其中內部在壓力下不同層的行為。他們的反應也是不同的和復雜的。在為了在復合材料中的增強纖維，以避免復雜的行為纖維的物理形態(tài)而言，實驗結果已經(jīng)說明和復合材料的斷裂面。 圖10。0/+45/ -45/ 0復合材料層合板的斷裂表面）纖維碎片和b）股份唇波狀表面。 圖10表示典型的0 /+45/ -45/ 0復合材料的斷裂面。由此圖中，很明顯，失敗主要是由纖維矩陣和矩陣 - 矩陣剪切基體和纖維的失敗，和纖維基體界面失敗。光纖矩陣接口故障指示的剪切唇形波浪斷裂面（用三角形表示）[24]。一些纖維在±45°的方向（由箭頭標記）中也觀察到的拔出。既然有纖維基質剪切，使纖維碎片也觀察斷裂面（由方表示）。圖圖10還表示矩陣的一個大島（用圓圈表示），這表明纖維基質的分布是不均勻的。這種非均勻光纖矩陣分布還負責為黃麻環(huán)氧樹脂復合材料的拉伸性能較低。周圍纖維基質故障指示的球晶型的存在下壓縮強制如圖。 11A（用圓圈表示）。這受壓區(qū)更脆比周圍的基質。當施加拉伸應力受壓區(qū)顯示矩陣的傾向周圍纖維開裂（黑色箭頭表示由圖11A）。該存在壓縮力也證實了裂紋區(qū)周圍纖維，（由在圖的矩形。 11b）的。此外，一些脆性光纖故障也觀察到三角形圖表示。 11C。 圖11。裂縫橫向拉伸下黃麻環(huán)氧復合加載）球狀的失敗，B）裂紋和三）光纖衰竭。 4.結論。 在這項研究工作中，黃麻纖維增強環(huán)氧樹脂基復合材料的開發(fā)真空輔助樹脂滲透（VARI）技術與預制堆疊序列（0/0/0/0），0/45°/ -45°/ 0和0/90°/ 90°/0。這些復合材料的特點是拉伸測試觀察斷裂面高分辨率FEGSEM下。由此研究工作，得出如下結論- 一。在的情況下0/0/0/0和0/45°/ -45°/ 0椎板復合材料，縱向拉伸強度已被發(fā)現(xiàn)高于橫向方向。 然而，對于0/90°/ 90°/ 0椎板復合材料，在拉伸的方向的差異實力，沒有觀察到。 二。對于所有發(fā)達國家的復合材料，實驗結果顯示，發(fā)達國家的復合材料的拉伸性能強烈地依賴于材料的拉伸強度的纖維，黃麻纖維的拉伸性能的影響是非常大的缺陷敏感。 三。關于復合材料的拉伸性能，所得到的理論值從規(guī)則的混合物偏離實驗值，而這偏差是在橫向方向上的情況下更加顯著。 四。壓縮骨折的模式是由于球狀型外觀和裂縫黃麻纖維周圍。對于UD黃麻環(huán)氧樹脂復合故障的序列，基體開裂，矩陣裂紋光纖矩陣接口，部分纖維斷裂，纖維切片撤出矩陣。但是，在橫向方向上，它是由纖維構成的切片和形成的纖維碎片 鳴謝 作者真誠感謝比利時魯汶大學材料科學與工程冶金系赫維的支持。 參考文獻 1。A. 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A，巴卡爾，R. Dungani，J.復合母校。 0（0）， 1（2012年）。 20。 M. Boopalan MJ Umapathy，P. Jenyfer4，硅，145（2012）。 http://dx.doi.org/10.1007/s12633-012-9110-6 21。 Z.香港炎，L.迪紅，Z，W.保昌，東興和C.俞勇，TRANSAC。有色金屬 金屬SOC。中國19，470（2009）。 http://dx.doi.org/10.1016/S1003-6326（10）60091-X 22。 M.北條M.美津濃，田中，M. T.安達T. Hobbiebrunken，，SK哈，復合材料科學。 技術69，1726（2009）。 http://dx.doi.org/10.1016/j.compscitech.2008.08.032 23。樓Y. Abou Msallem的，N.布瓦亞爾，A.普瓦圖，D.德勞和S. Chate JACQUEMIN， 復合材料：A部分41，108（2010）。 24。 http://research.ae.utexas.edu/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 UD; Interface; VARI. ? 2013 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. doi: http://dx.doi.org/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 www.banglajol.info/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 hand–lay–up 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 jute–thermoset 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 60°C 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 40°C 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 135°C at a heating rate of 5 – 10°C/ 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 4±0.5mm, width 15±1.5mm, 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 50°C 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. 0 20 40 60 80 100 120 140 160 0.00% 0.20% 0.40% 0.60% 0.80% 1.00% 1.20% St re ng th M Pa Strain % 0/0/0/0 0/+45/-45/0 0/90/90/0 0 5 10 15 20 25 30 35 0.00% 0.20% 0.40% 0.60% 0.80% 1.00% St re ng th M Pa Strain % 0/0/0/0 0/+45/-45/0 0/90/90/0 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 Young’s 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 0–90 composites Before going to in-depth discussion for the UD and 0–90 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 Young’s 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 Young’s modulus GPa Strain to failure % Epoxy (Epikote 828Lvel) 81.72±13.16 3.89±0.53 2.23±0.50 BWB jute 844.72±142.47 (extrapolated) 55.66±2.11 (corrected for span length) 1.67±0.31 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: mmffc VV σσσ +=1 [σ 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 + 844.7 R2 = 0.664 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 5 10 15 20 25 30 35 40 Str en gth M Pa Span length mm 5mm 10mm 20mm 35mm Average Linear (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°/