**1. Introduction**

Woven fabric reinforced composites are the most important and widely used forms among textile structural composites. In recent years, fracture mechanics has found an extensive applications in damage analysis of composite laminates, especially in delamination analysis. Delaminations may occur during manufacture because of incomplete curing or may result from impact damage; or they may result from the interlaminar stresses that develop at stress-free edges or discontinuities. It has been mentioned that delamination in a composite laminate usually occurs at the interface of different oriented plies and tends to grow and also it can be a major problem for laminated composite structures. Sometimes, delaminations may also be result of contamination of the pre-preg, the poor ply adhesion, or they may form locally in regions of high void content. The problems of interlaminar performance

are discussed along with the technique used to measure them and the fracture mechanics principles applied to improve them, because laminated composite materials have been increasingly used in large transport aircraft structures due to their low density compared to other materials. The delamination resistance of laminated composites can be measured by critical SERR (strain energy release rates). The DCB (Double Cantilever Beam) is the most popular specimen configurations in the experimental determination of Mode I interlaminar fracture toughness. Aliyu and Daniel [1], used conventional double cantilever beam specimens with different configurations to measure the Mode I fracture toughness in a graphite/epoxy composite. The effect of stacking sequence on energy release rate distribution across the specimen width the multidirectional DCB specimens are determined by Davidson et al. [2]. They investigated eight different stacking sequences, with the delamination growth between 30°/−30° interface.

Sun and Zheng [3], analyzed the distribution of strain energy release rate, *G* at the crack fronts of double cantilever beam specimens by means of the plate finite element. They found a boundary layer phenomenon in the distribution of *G* at the crack front, which causes the strain energy release rate to vary along the straight crack front. Toygar et al. [4] measured the fracture toughness value of carbon/ epoxy composite materials by using the CMOD (crack mouth opening displacement) method experimentally using SENT (Single Edge Notch Tension) specimens. The finite element study was carried out by using 2-D model to obtain the fracture toughness value of woven carbon/epoxy composite numerically with the ABAQUS finite element software package. Mode I DCB tests were performed on carbon/epoxy woven laminates in weave style, which means as plain weave such as [0°/90°]16, where 16 means the number of layers, lay-up specimens by Morais et al. [5]. The starter crack was created at mid-thickness, between the 0° and 90°, stand for the directions of fill and warp strips, respectively. The test results were apparently consistent with the assumptions of the CBT (Corrected Beam Theory) that was used to obtain the interlaminar critical SERR, *G*IC. The measured values were higher than those of unidirectional [0°]24 specimens, especially the final propagation values. A finite-element analysis confirmed the applicability of the CBT for interlaminar propagation along the two [0°/90°] interfaces. They have found that the intralaminar *G*IC is significantly smaller than the interlaminar *G*IC. This will prevent pure interlaminar propagation in multi-directional specimens with high interlaminar fracture toughness. The interlaminar fracture is a common mode of failure for composite materials, especially in laminated architectures [6–8]. Interlaminar fracture mechanics has proven useful for characterizing the onset and growth of delaminations. Delamination onset and debonding in fracture toughness specimens (such as Double Cantilever Beam Specimen) and laboratory size coupon type specimens (such as the skin/stringer debond specimen) has been investigated [9–11].

In this study, the influence of test parameters on Mode I delamination resistance is examined. A detailed discussion on test method and data reduction schemes for Mode I interlaminar fracture toughness characterization of composite laminates have been done by considering the test parameters such as width, different fiber orientation and lay-up and ext. In the case of laminated composites, the Griffith criterion is applied to calculate the critical amount of energy required to propagate a crack and DCB test is used to obtain this material property. In a result of this, DCB tests on woven-fabric-reinforced glass/epoxy specimens were performed in accordance with ASTM Standard D5528–01 and ISO 15024, DIN EN ISO 75-1 and DIN EN ISO 75-3 Standards [12–16] to measure the delamination resistance under Mode I loading. Therefore, the present research has been focused on the characterization of delamination growth in laminated structures and has been done to determine Mode I interlaminar fracture toughness, *K*IC of woven laminates [0°/90°]16 and [±45°]16 lay-up

*DOI: http://dx.doi.org/10.5772/intechopen.99268 Failure Modes in Fiber Reinforced Composites and Fracture Toughness Testing of FRP*

composites such as in weave style as plain weave and with 16 number of layers. The obtained experimental data were reduced by using data reduction technique CCM (Compliance calibration method) to determine the critical SERR, *G*IC. CCM generates a least squares plot of log (*δi*/*Pi*) versus log (*ai*), where *i* represents the number of specimen and changes from 1 to minimum 5 and *P* and *δ* represent the measured values of load and load point displacement, respectively during the test, and *a* represents the crack length, to use the visually observed delamination onset values and all the propagation values [17]. Additionally, fracture mechanics based finite element models of described DCB tests were developed to confirm the experimental results. The finite element analysis has been carried out to accomplish the delamination analysis of the specimen. Critical load levels, the geometrical and material properties of the test specimens were used as input data for the analysis to evaluate the Mode I energy release rate at the onset of delamination crack propagation.
