**2. Materials, sample preparation and measurement of mechanical properties**

Delamination growth has typically been related to the cyclic SERR, *G*, for composite materials, using a power law expression [18–20], known as the Paris Law. Fracture toughness may be described as the critical energy that a material may absorb before failure. Independent of specimen geometry, this property describes the general resistance of a material to delaminate [21].

In this study, DCB specimens having density 1,72 g/cm3 were manufactured in Dokuz Eylul University laboratory from the woven-fabric-reinforced glass/epoxy materials and the detail of sample cutting [0°/90°]16 and [±45°]16 woven DCB specimens are given in **Figure 1a** and **b**, respectively. All were prepared from manufactured woven composite plate. The prepared composite plate has sixteen laminas and has a volume fraction of glass fiber of approximately 60%. During manufacturing, a thin non-adhesive Teflon film with 0.07 mm thickness were placed at the midplane (between 8th and 9th plies) at one end to simulate an initial delamination by considering ASTM Standard D5528–01 and British test method ISO 15024.

The mechanical tests to obtain the mechanical properties were carried out by using the prepared specimen which is given in **Figure 2**. The universal mechanical test machine, which is used, is available with different clamping techniques and accessories such as digital click-on extensometer from Zwick Roell. The width and thickness of each specimen were measured using a micrometer, at the center and each end. The tensile test was conducted following ASTM Standard E-681 [22].

### **2.1 Video extensometer**

Video extensometer ME-46 is a high resolution and non-contact strain measurement system which is capable of measuring both longitudinal and lateral strain, modulus and Poisson ratio of polymer composites (see in **Figure 3a**). The advantage of the non-contacting method is that the strain is directly measured and end-effects from the gripping system need not to be considered [24] and for non-contact strain measurement the detail is given in **Figure 3b**. The measurement was carried out optically and there is no requirement for physical contact between the extensometer and the test specimen. Therefore, it can be applied to all sample without and slippage and related test specimen breakage problems [23]. The mechanical properties of this woven-fabric-reinforced glass/epoxy composite were obtained experimentally by using video extensometer under the consideration of ASTM Standards and were given in **Table 1**.

#### **Figure 1.**

*The detail of cutting of sample from manufactured woven-fabric-reinforced glass/epoxy composite plate a) (0°/90°) fiber orientation, b) (±45°) fiber orientation.*

**Figure 2.** *a) Geometries of test specimen for mechanical properties testing, b) view of tension test.*

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

**Figure 3.**

*a) Schematic detail of video extensometer [23], b) schematic drawing of the non-contact varying temperature deformation measuring system [24].*

## **2.2 Optical deformation and strain measurement system**

Aramis [21] is an optical technique from GOM (Gesellschaft fur Opticsche Mestechnik), Germany. It can be used to non-destructively measure the deformation and strain profiles of an object surface under loading. It has advantages of simple specimen preparation, large measuring area, non-contact and full field measurement, no laser illumination, material independent determination, full field and graphical results, three-dimensional results and good mobility. Its capabilities


**Table 1.**

*Physical and mechanical properties of woven specimen in the principal material direction.*

include materials testing, stability estimating, components dimensioning, nonlinear behavior examination, creep and aging processes characterization. Because the deformation and strain of an object under mechanical loading is associated with its structure integrity, the abnormalities and irregularities in deformation and strain profiles indicate the damage presence. It can be adapted to assess the structural integrity and non-destructive testing of polymer composites.
