Elastic properties, notched strength and fracture criterion in untreated woven jute–glass fabric reinforced polyester hybrid composites

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Abstract

The elastic properties and notch sensitivity of untreated woven jute and jute–glass fabric reinforced polyester hybrid composites has been investigated analytically and experimentally. Elastic properties were predicted by using the rule of hybrid mixture and classical lamination theory (CLT). Analytical investigation on notch sensitivity was carried out by using well known point stress criterion (PSC) and modified PSC criterion. Experimentally, elastic properties and notch sensitivity were determined by conducting tension test on specimens. The objective of the investigation is to determine the extent of applicability of stress fracture criteria that were developed for synthetic fibre composites, to natural fibre and hybrid composites. The characteristic dimension of PSC criteria was found to increase slightly with the increase in the hole size for all the types of laminates considered. However, modified PSC model has resulted in excellent agreement between the experimental and predicted values. The effect of hybridization of glass fibre with jute fibre as well the effect of hole size on the notch sensitivity of the composites under consideration is discussed.

Introduction

Natural fibres such as jute, sisal, banana, bamboo, etc. offer several advantages [1], [2] over conventional fibres such as glass, carbon, graphite, kevlar, etc. The advantages are low density, low cost, good thermal and insulating properties, renewability, bio-degradability, high specific strength, etc. These advantages have kindled the interest to the examination of natural fibres as reinforcement in polymer matrix composites by number of researchers in the recent years. Among the natural fibres, jute fibre is considered to be most promising material because of its wide commercial availability in the required form and at a low cost. Jute based composites have already been proved to be a potential material for various structural and non structural low load bearing applications. Structural applications of composites require that holes be drilled into the laminates to facilitate joining of structural parts or to provide access to the interior of the structure. These holes induce stress concentrations which significantly reduces the load carrying capacity of the composites. Literature on elastic properties of synthetic fibre composites is well published. Also, considerable literature [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] is available both analytical and experimental, which addresses the problem of notch sensitivity and fracture criterion in synthetic fibre composites with circular holes and cracks. Awerbuch and Madhukar [3] presented a comprehensive review of various fracture models for predicting notched strength of the laminates. Whitney and Nuismer [4] proposed the point stress criterion (PSC) and average stress criterion (ASC) based on the normal stress distribution adjacent to the notch edge. These criteria involve the parameters; unnotched strength and a characteristic dimension. They assume that the characteristic dimension is a material constant and is independent of the notch size. Pipes et al. [5] consider that the characteristic dimension of PSC model is not material constant and proposed a modified PSC model which assumes an exponential relationship between the characteristic dimension and the size of the discontinuity. Notch sensitivity of graphite/epoxy fabric laminates was experimentally investigated by Lagace [6] by conducting tension test on specimens made from woven fabric prepreg and those of similar construction made from unidirectional tape prepreg. They showed that the notch sensitivity of fabric laminates under tension is generally the same as tape laminates of similar configuration. Ingvar Erikkson and Aronsson [7] developed a new criterion called damage zone criterion (DZC) based on the assumption that a damage zone is present in the maximum stress region of the laminate, when the tensile strength of the notched laminate reaches the tensile strength of the unnotched laminate. Taking into account the stress redistribution by damage development, an analytical expression was derived to predict the notched strength on the basis of fundamental parameters unnotched strength, σ0 and critical damage zone size, d0. They used the criterion for predicting the notched strength of Graphite/epoxy laminates with holes and cracks and showed that it provides an improved accuracy when compared with PSC. Naik et al. [8] addressed the effect of the stacking sequence on the notch sensitivity of unbalanced E-glass/epoxy fabric laminates. Kim and Kim [9] reported the effects of hole size and specimen width on the fracture behavior of woven glass and woven carbon fabric composites. Akbar Afaghi Khatibi et al. [10] developed a new Effective Crack Growth Model (ECGM) to evaluate the tensile residual strength of composite laminates with a circular hole, based on stress redistribution and damage growth. de Azevedo Soriano and de Almeida [11] evaluated notch sensitivity of carbon/epoxy fabric laminates with circular holes. They studied the effect of ±45° layers on the notch sensitivity by testing six different lay ups with increasing number of ±45° layers ranging from (0)5 to (45)5. Belmonte et al. [12] conducted studies on damage growth and fracture at circular holes in quasi-isotropic laminates made from woven glass fabric in epoxy resin. In addition to PSC and ASC models, they predicted the notched strength by using critical damage growth model (CDG), which is developed based on the observation that an intense damage zone near the notch edge propagates stably initially and then catastrophically to failure. Fracture toughness Kc and unnotched laminate strength required for prediction of notched strength by CDG were evaluated. The predicted notched strength by PSC, ASC and CDG models were found to agree well with the experimental observations.

The literature reveals that all the fracture models developed till date were applied to synthetic fibre composites only. Despite the recent increased attention to natural fibre composites, there has been virtually no work on analytical prediction of elastic properties and notch sensitivity of natural and natural/synthetic fibre hybrid composites. In order to evaluate the suitability of jute based composites for various structural applications, it is particularly important to determine the extent of applicability of the fracture theories and correlations developed for laminates made from synthetic fibre, to the composite under consideration. This is the subject of the current paper, wherein an attempt is made to evaluate the elastic properties and notch sensitivity of jute and jute–glass fibre reinforced polyester hybrid composites.

Section snippets

Evaluation of elastic properties by rule of hybrid mixture

The elastic properties of unidirectional jute and glass fibre lamina were predicted from fibre and matrix properties (Table 1) using simple rule of mixture relationships from the mechanics of materials approach. From these values, the elastic properties of 0/90° woven jute fabric ply and glass fabric ply, were predicted by using the following relations [13]:E11=KEl+(1-K)EtE22=(1-K)El+KEtυ12=υltK+(1-K)ElEtG12=Gltwhere E1 and Et are Young’s modulus in longitudinal and transverse direction

Experimental procedure

Woven jute fabric 22 × 12 (22 yarns of Tex 310 in warp direction and 12 yarns of Tex 280 in weft direction, per inch) having an average weight of 367 g/m2 and average thickness of 0.8 mm is directly procured from Kolkota, West Bengal, India. The plain and balanced woven E-glass fabric of weight of 360 g/m2 is supplied by Binani Industries Limited, Mumbai, India. The resin system consists of Isothalic polyester NRC-200-220 supplied by Naphtha Resins and Chemicals Pvt. Ltd., Bangalore, India, cobalt

Results and discussion

The Young’s modulus in warp and weft direction and shear modulus are obtained from the slope of the initial portion of stress–strain plots shown in Fig. 3, Fig. 4, Fig. 5, respectively. The predicted and experimental values of elastic properties are shown in Table 4. The results indicate a deviation in the range of 0–15%. This deviation can always be expected in jute based composites, due to non-uniformity of jute yarns. The elastic modulii predicted by CLT and hybrid rule showed good

Conclusion

The elastic properties of woven jute and jute–glass fabric hybrid composites have been evaluated. The effects of hybridization of glass fibres and hole size on notched sensitivity of woven jute fabric composites were investigated analytically and experimentally. Based on the results of this study, the following important conclusions are drawn:

  • 1.

    Rule of hybrid mixture can be conveniently used for predicting the elastic properties of bidirectional [0/90] jute and glass fabric reinforced hybrid

Acknowledgements

The authors are thankful to Dr. Aravamudhan, Director and S. Sadasivan, Group Director, Aeronautical Development Establishment, Bangalore (India), for providing us the testing facilities. We are thankful to Mrs. Chhaya Rajput, Scientist ‘E’ of Aeronautical Development Establishment for her help during experimentation.

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