Carbon fiber reinforced plastic (CFRP) composites made from long and continuous fibers demonstrate excellent properties such as low weight and also lifetime maintenance costs owing to their fatigue and corrosion resistance. Those features make composite laminates widely used in various industry sectors. On the other hand, CFRP laminates are often subjected to different loading conditions, which may cause damage of those materials. The most dangerous form of failure in composites is delamination. The fracture toughness represented by the strain energy release rate (SERR) can be determined in accordance with the American Society for Testing and Materials (ASTM) Standards, namely: for the opening mode I, the double cantilever beam (DCB) [
1] test is commonly used, whereas the shearing fracture toughness can be obtained by employing the end-notched flexure (ENF) test [
2]. The scope of the DCB test is measurement of the mode I fracture toughness (
GIC) of unidirectional [0°]
n laminates according to available standards. However, a multidirectional (MD) laminates with different fibers orientation at the delamination plane are commonly used in most composite structures applications. Many experiments were prepared on the mode I fracture of multidirectional laminates with different delamination interfaces [
3,
4]. Apart from experimental investigation several authors conducted numerical analysis on delamination problem in the continuous fiber-reinforced laminates [
5,
6,
7,
8,
9,
10,
11]. When delamination propagates between different oriented layers, various undesirable phenomena such as mode-mixity and crack jumping to neighbor interface may occur [
12]. Other parameters that may influence delamination resistance are: specimen geometry, stacking sequence and intralaminar damage [
13]. Although the initiation toughness is often considered the most important in design point of view, there is significant interest in the measurements of propagation toughness values which are represented by the
R-curve effects [
14]. Schön et al. [
15] prepared an experimental and numerical investigations of fracture toughness prepared on DCB specimens with different delamination interfaces as well as for different materials. Experimental results revealed, that non-zero interfaces had a plateau value approximately four times higher than the initial value, which was almost the same as for the zero interface. Higher toughness values could be probably caused by initial crack deviation from the original symmetric plane. In reality, anticlastic curvature in DCB specimens caused the highest values of the mode I SERR at the center and the lowest at the specimen edges. This non-uniformity was correlated with a non-dimensional ratios
Dc and
Bt introduced by Davidson et al [
16]. Hence, in order to correlate the difference in strain energy release rate at a specimen’s center versus that at its edges a ratio
Dc =
D12/(
D11D12) was used, whereas a stiffness ratio
Bt =
D16/
D11, was found to correlate with the asymmetry in the SERR distribution about the center of the specimen’s width. In order to neglect the non-uniformity of
GI in determination of
GIC it was proposed that the
Dc parameter should be less than 0.25. Nevertheless, Olsson [
17] showed that the specimen was in a state of plane strain for small values of
a0/
b and globally was in a state of a plane stress for large aspect ratios. It was also revealed, that when specimens transferred from plane stress to plane strain the average value of the mode I fracture toughness decreased. Hence, at parameters of DCB specimen such as thickness (
h), initial delamination length (
a0) or width (
b) significantly affected the
GIC value. Therefore, Shokrieh [
18] introduced a non-uniform ratio
β = (
GImax −
GIavg)/
GIavg%, which take into account the influence of abovementioned factors on the SERR distribution along width of the DCB specimen. In general
β was a function of non-dimensional ratio
Dc and geometrical ratios
a0/
b and
a0/
h,
β =
f(
Dc,
a0/
b,
a0/
h). Results showed that the mode I fracture toughness of MD double cantilever beam specimens with 0°//0° fiber angle at delamination plane and
β < 20% could be predictably measuring the
GIC of the unidirectional UD plies with an error lest than 10%. Moreover, it was proved that
β ratio had significant influence on the initiation delamination resistance and it was necessary to be considered in calculation of
GIC by analytical relations which based on the beam theories. In practice, determination of the fracture toughness in composites may be difficult when elastic couplings occur in laminate [
19]. For example, this is the case for laminates where the bending-twisting or the bending-extension couplings take place. Previous research prepared by the author of this paper [
20] conducted on the DCB specimens exhibited elastic couplings under the mode I DCB test revealed, that non-zero components in the bending stiffness matrix [B] caused couplings between the bending moment and the twisting curvature as well as between the twisting moment and the mid-plane normal strains. Presence of shearing deformation and twisting curvature caused non-uniform delamination front which in results could produce an inaccuracies in the fracture toughness determination. In order to investigate the influence of irregular crack front on delamination initiation and the mode I strain energy release rate distributions Jiang et al. [
21] prepared a modified DCB specimen. They concluded, that when the effects of visual deviation
λ and the non-uniformity coefficient
β were not taken into account the fracture toughness values obtained by using classical methods based on the beam theory were underestimated. Moreover, when the effect of curved delamination front during experimental measurement was neglected the errors in the mode I SERR values obtained for multidirectional DCB specimens could reached level up to 47.6% [
22]. Another problem that can generate errors in measurements of the initiation and propagation values of SERR is visual observation of crack length through the test. Actually, the fracture process zone (FPZ) which develops ahead of the crack tip in result of micro-cracking and plasticity can cause unreliable monitoring of crack position during the mode I tests [
23]. Abovementioned factors are not included in classical calculation schemes of the c-SERR recommended by the ASTM Standard. Moreover, visual detection of crack length are incredibly difficult to perform experimentally. To overcome this difficulties, an author of this paper proposed the compliance-based beam method (CBBM) [
24]. This methodology is based on the crack equivalent concept and depends only on the specimens compliance during the test. In addition, this method do not require measurements of actual crack length during propagation which can produces non-negligible errors in the fracture toughness measurements [
25,
26]. Moreover, this methodology can be a great tool to create an artificial
R-curve, which was proved in [
27] where an excellent agreement between the experimental and numerical
R-curves was achieved demonstrating adequacy of the proposed method.
The main goal of this paper is to evaluate the influence of elastic couplings on behavior of the carbon/epoxy composite laminates subjected to the DCB tests. Specimens with different delamination interfaces and stacking sequences exhibited the bending-twisting and the bending-extension couplings were examined. Values of the critical strain energy release rate obtained by using standardized methods were compared with results obtained by using the CBBM method. In addition, experimental investigations of initiation and propagation of delamination were executed by using the acoustic emission technique and the scanning electron microscopy.