Experimental Tensile Strength Analysis of Woven-Glass/Epoxy Composite Plates with Central Circular Hole

The use of composite materials in aerospace engineering, as well as in maritime structure has increased significantly during the recent years. The extensive use of composite materials in industrial applications should make composite structural engineers and scientists more aware of the advantage and disadvantage of this material and provide them with necessary data and certification process. One of the problems in composite structures is the existence of hole. Hole can not be avoided in actual structures, since it may be the necessity of providing access for maintenance or due to impact damage. The presence of hole will weaken the structures. Therefore, in this paper, the effect of hole on the strength of glass-woven/epoxy composite will be discussed. Extensive tests have been carried out to study the effect of hole-diameter on the tensile strengths of these specimens. The results showed that the bigger the hole-diameter compared to the width of the specimens has weakened the structures further, as expected. Further study should be carried in the future to model it with the finite element and theoretical analysis precisely.


Introduction
The use of composite materials in aircraft structures dates back in 1970s, mostly for military aircrafts. During those years, boron-epoxy, carbon-epoxy were developed to meet the needs for military aircrafts. The application of these materials began with secondary structures in 1980s. Since then, the use of composite materials has been flourished to primary structures, such as wing and recently fuselage. The latest Boeing aircraft, B-787 Dreamliner, uses 50% of the total structural weight in composite materials [1]. Their design was based on Boeing's long experience on using composite materials since B-737 programs. On the other hand, Airbus A-350 XWB uses 52% of the total weight with composite materials [2]. In the marine structures, composite materials was used various applications, such as small crafts, fishing boats, naval vessels, underwater vehicles and others [3]. The continuous development in composite materials in both aerospace and marine structures gives opportunities and challenges to engineers and scientists working with this material [4] and [5]. One of the challenges is in the field of airworthiness process. Extensive tests should be carried out in order to fulfil the airworthiness requirements.
The presence of hole in actual structures usually cannot be avoided, in order to give access for maintenance or due to impact damage. The hole will weaken the structures, since it gives high concentration factor in the adjacent to the hole. Therefore, extensive tests should be carried out to study this phenomenon.
Kim et al [6] investigated failure strength of CFRP composite structure with open hole under tensile loading. The method developed was in a good agreement with the failure criterion within an error rate of 10%. Tino and de Aquino [7] studied the fracture characteristics of notched glass fiber reinforced plastic under tensile load. The effect of stress concentration on the failure mode was investigated. Meanwhile, Soutis and Fleck [8] investigated the failure of carbon fibre T800/924C composite plate with hole under compression load. Both experimental and theoretical analyses were done. They concluded that the agreement between the theory and experiment data was acceptable.
Arslan, et al [9] studied the effect of circular hole on the failure of composite plate. Tsai-Hill failure criterion was used, together with first-ply-failure assumption. Finally, Toubal, et al [10] studied stress concentration in a circular hole in composite plate experimentally, using a non-contact measurement method namely electronic speckle pattern interferometer (ESPI). The data was then compared to the theoretical solution of Lekhnitskii, as well as finite element study. The results are mixed, depending on the lay-up configuration of the composite plate.
In this paper, experimental study on the tensile strength of woven-glass/epoxy composite plate with circular hole was presented. The effect of lay-up configuration on the tensile strength was studied,

Woven-glass/epoxy Specimens
The specimens were made using woven-glass type WR200, and the epoxy was Bisphenol Aepichlorohydrin with hardener type Versamid 140. The fibre volume fraction was 50%.

Testing Method
All testing has been done using universal testing machine Tensilon RTF-1310 with crosshead speed of 2.0 mm/second. Fig.2 shows tensile testing arrangement.

Data Analysis
Two-parameters Weibull distribution was used in the data analysis [11]. The Weibull distribution is given by Eq (1) where n is the number of data observation and x(i) denotes the ith smallest observations (i = 1 corresponding to the smallest data observation and i = n corresponds to the largest). By arranging the data and use linear regression in Eq.3, the parameters b and c can be calculated, and hence the probability and reliability of the materials can be computed using Eq.1. In this paper, reliability of 50% was taken as the specimen strength.

Results and Discussion 4.1 Tensile Test Results
The results are given in Fig.3, for the hole diameter of 3 mm, for different lay-up configuration. Results for different hole diameter are also available. Fig.3 shows that most specimens failed in elastic mode, except specimens configurations of (±45 o )s, which failed in elastic-plastic mode; where plastic failure was the dominant one. It is a well-known fact that specimens having (±45 o )s lay-up configuration has matrix-dominated properties. Therefore, it is expected that these specimens will produce large plasticity mode.  Fig 3(a) produces a kink in their load-displacement curves. These indicate the occurrence of first-ply and last-ply failure within the specimens. When the (0 o /90 o )s specimens are given tensile loads, the 90 o ply will fail first, which is called first-ply failure; leaving the 0 o ply to resist tensile loading until it all fails, that is called last-ply failure.
The diagram of maximum tensile loads for all lay-up configuration and hole-diameter is given in Fig.4. It shows that lay-up configuration of (0 o /90 o )s have the highest tensile strength, while the (±45 o )s are the weakest. The (±45 o )s lay-up is the strongest in shear loads, but the weakest in tensile loads. The strength of (0 o /90 o /±45 o )s lay-up configuration is between the other two configurations. Fig. 4 also shows that the bigger hole-diameter will significantly reduce the plate strength. The reduction of tensile strength is more prominent in (0 o /90 o )s compared to other lay-up configuration. This coincides with the maximum stresses occurred in the adjacent to the hole. Therefore in structural application where hole is necessary, it is recommended to use more (0 o /90 o )s lay-up configuration. This is the first step in predicting the strength of composite plate with central circular hole. Further study will be carried out regarding to modeling of this plate using finite element method or theoretical analysis.