**2.3 DCB specimen for interlaminar fracture toughness**

The average specimen width, *b*, was 25 mm and/or 40 mm for DCB specimen and an illustration of it was given in **Figure 4**. The hinges were mounted on the top and bottom surfaces of the end of DCB specimen arms by using an epoxy adhesive, which was cured. The average thickness *h*, was 2.5 mm, the initial delamination length *a*0, was produced by using Teflon has a distance 47 mm from the line of load application to the crack tip. The DCB specimens were tested in a tensile testing machine where a tensile load was applied to the specimens through hinges (see **Figure 5**). Immediately before testing, a thin layer of black paint and marked in 1 mm increments were mounted on the specimen to observe crack propagation, starting from the tip of the insert to a length of 47 mm as shown in **Figure 6**. Tests were carried out in a AG-50kNG Shimadzu universal testing machine at 2 mm/ min crosshead speed. Following the procedures of ISO 15024:2001, a pre-cracking cycle was performed and given in **Figures 7**–**10** for different orientation and width of specimen. Initiation values from the insert were then recorded. Further measurements were made in an additional loading cycle, where the crack was allowed to propagate. During the DCB tests, load, displacement, and temperature

**Figure 4.** *The dimensions of DCB test specimen.*

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

**Figure 5.** *Experimental setup for DCB specimen and fracture in DCB test.*

**Figure 6.**

*Overview of loading device and test setup of DCB specimen and delamination a) initially b) progress.*

measurements were recorded and video images of the delamination growth were also recorded.

The specimen was subjected to displacement controlled loading and usually experiences stable delamination growth allowing several values of interlaminar fracture toughness to be determined along the specimen's length. As the delamination grows, fiber bridging usually occurs increasing the energy required to propagate further delamination ISO 15025:2001. Interlaminar fracture mechanics was used for characterizing the onset and growth of delaminations and it's calculation is based on experimental CCM as described by ASTM D5528–01. The compliance method is an effective method for determining the fracture characteristics of brittle materials. Fracture toughness is related to the amount of energy required to create fracture surfaces. There are several ways in which initiation and propagation values of *G*IC can be derived from the recorded load–displacement data. Initiation of delamination is determined by deviation from linearity. *G*IC can be calculated using the load and displacement at the point of non-linearity of the load–displacement curve. The load–displacement data were then recorded. The unloading curve was also registered, as in case of significant permanent deformations and/or non-linearity. The camera was positioned at a distance 1000 mm away from the specimen surface. Images are captured during the test using two CCD-cameras to get CMOD.

#### **Figure 8.**

*Load–displacement characteristics for sample 4, woven [0°/90°]16 with 40 mm width.*

In DCB tests, there are four test groups including: *Type 1–1*, *Type 1–2*, *Type 2–1*, *Type 2–2* and they are given in **Table 2**. In *Type X-X* notation, first one denotes direction of woven and second one denotes the width of the DCB specimen. Therefore, *Type 1–1* represents the woven [0°/90°]16 with 25 mm width specimen, *Type 1–2* represents the woven [0°/90°]16 with 40 mm width specimen, *Type 2–1* represents the woven [±45°]16 with 25 mm width specimen, *Type 2–2* represents the woven [±45°]16 with 40 mm width specimen. The tests were performed at constant room conditions of 22°C and humidity ratio was 50% in respect of the conditions were needed for standards.

To observe the (delamination) crack growth and to identify crack extension *Δa* in mm during the loading, the gripped hinges were pulled apart with a crosshead speed of 2 mm/min in displacement control until satisfactory crack

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

#### **Figure 9.**

*Load–displacement characteristics for sample 3, woven [±45°]16 with 25 mm width.*

#### **Figure 10.**

*Load–displacement characteristics for sample 4, woven [±45°]16 with 40 mm width.*

growth was occurred in the specimen. During the test, the characteristic load– displacement curves were obtained for all test groups and the mean characteristic behavior for each group has been given in **Figures 7**–**10**. One issue with this test is the slope of the compliance changes as the crack propagates along the specimen. To get the value n, the ratio between the slope of the log c, which is the logarithm of compliance values that is obtained for the system, and log a is the logarithm of the crack length is obtained. Both testing and post processing were conducted similarly for each specimen. First the loading has been obtained with a crosshead speed of 2 mm/min in displacement control up to pre-cracking occurs. Then specimen was unloaded at a constant cross head rate of up to 25 mm/ min, the position of the tip of the pre-crack on both edges of the specimen was marked and the procedure was repeated to extent the crack further. Subsequent


**Table 2.**

*Fiber directions and width of DCB test specimens used in experiments and fracture energies in pure mode I obtained by DCB under consideration of ASTM standard D5528–01.*

measurements were made in an additional loading cycle, where the delamination was allowed to propagate. The unloading curve was also registered, as in case of significant permanent deformations and/or non-linearity. Data reduction was performed according to CCM.
