The experimental methods selected within this section aim at the evaluation of the fiber-matrix interface of the different material combinations. The transverse bending test and the mode I interlaminar fracture toughness test were chosen as they have been proven to be sufficiently sensitive to changes of the fiber-matrix interface in CFRP [60, 61]. Besides macro-mechanical tests such as the transverse four-point bend test and the mode I interlaminar fracture toughness test, micro-mechanical nano-indentation tests were conducted. After micro-mechanical testing, visual inspection of the tested specimens was carried out via scanning electron microscopy (SEM).
2.2.1 Four-point bend test
To determine the flexural properties of composites made of different material com-binations, the four-point bend test setup was selected due to the constant bending moment without introduction of shear between the loading noses. According to DIN EN ISO 14125 B [62], the flexural strength and the flexural modulus transverse to the fiber direction were determined. The used test setup is presented in Figure 2-3.
The test specimens were 15 mm in widthwand 60 mm in lengthlwith a thicknesst of 2 mm. The support and loading noses were 2 mm in radius. The support span L was adjusted to nominally 45 mm and the load span L to 15 mm. All tests were carried out with a crosshead speed of 2.00 mm/min in a displacement-controlled mode on a Hegewald & Peschke 100 kN universal testing machine at room temper-ature. The mid-point deflection was determined by means of a video-extensometer with telecentric lenses.
l L L’
t
Figure 2-3 Four-point bend test setup with support span L and a load span L’=L/3.
The flexural strength transverse to the fiber directionσf2 is determined by dividing the maximum load Pmax times the support span L by the cross-sectional area of the specimen times thickness, according to Equation 2-1:
σf2 = PmaxL
w t2 . (2-1)
The flexural modulus Ef2 is calculated as follows Ef2 = 0.21L3
w t3 ΔP
Δs. (2-2)
Δs denotes the difference in deflection s −s where s and s correspond to the strain of the outer fiber of f= 0.0005 and f= 0.0025. The difference in force ΔP corresponds to Δs.
2.2.2 Double-cantilever beam test
The double-cantilever beam (DCB) test method based on ASTM D 5528 [63] was applied to determine the mode I interlaminar fracture toughness. For all DCB tests, a test fixture according to the side clamped beam (SCB) hinge system developed by Renart et al. [64] was used. In this case, the specimen is clamped from the top and the bottom by a specifically designed grip zone. By using this test setup, adhesive bonding of loading blocks or piano hinges to the test specimen is eliminated [64].
Figure 2-4 shows the specimen geometry (t= 3 mm, w= 25 mm, initial delamina-tion length a0 of 63 mm and l= 125 mm) and a DCB test specimen clamped by the SCB fixture that is installed in a Hegewald & Peschke 100 kN universal testing machine.
a0 l t
Nonadhesive insert Load line
a) b)
Figure 2-4 a) DCB test specimen with initial delamination length a0 from load line to end of insert and b) test specimen clamped to the SCB test fixture mounted to a Hegewald
& Peschke 100 kN universal testing machine.
At first, a pre-crack with a delamination length between 3 to 5 mm was created by loading and unloading of each specimen with a crosshead speed of 3 mm/min. Sub-sequently, each specimen was reloaded until fracture at the same crosshead speed as used for the pre-crack. The two loading cycles were recorded with a camera
at 4 frames per second. The opening mode I interlaminar fracture toughness GIc and the propagation values GI resulting in the delamination resistance curve (R curve) were detected by visual observation and calculated by using the modified compliance calibration (MCC) method. According to the MCC method the mode I interlaminar fracture toughness GI is calculated by taking into account the com-pliance C, force P and geometric dimensions such as specimen width w as well as thickness t:
GI = 3P2C2/3
2A1w t . (2-3)
Here, the delamination length is first normalized by the specimen thickness (a/t) and plotted against the cube root of compliance including visually observed de-lamination onset values and propagation values. Subsequently, a least square plot is generated where the slope of the line represents A1 [63]. The opening mode I interlaminar fracture toughness GIcwas determined based on the first visible crack extension from the reloading curve after the pre-crack has generated a defined crack front.
2.2.3 Statistics
All results from mechanical testing were analyzed by means of a confidence interval according to the test standard ISO 2602 [65]. A two-sided confidence interval was used for the population mean m at a confidence level of 95 % as the following equation shows:
¯
x−t0.975
√n s < m <x¯+t0.975
√n , (2-4)
where ¯x is the arithmetic mean of n results and s is the standard deviation. Forn independent measurements, t is calculated as follows:
t= x¯−m s/√
n. (2-5)
The ratio t0.975
√n can be directly gathered from the Student’s t distribution given in [65].
2.2.4 Nano-indentation
The nano-indentation technique is commonly employed to measure the modulus and hardness of various materials at the nanoscale. In this work, nano-indentation is used to measure the modulus and hardness of the region between matrix and carbon fiber that is influenced by the use of different sizings. This region is often
re-ferred to as interphase. The interphase is assumed to start somewhere in the fiber until the fiber properties convert from bulk to surface properties. At this point the actual interface between fiber and matrix is present. The interface connects the surface properties from fiber and matrix. Coming from the interface into the matrix, the polymer behavior changes due to the transition from surface to bulk properties [66].
In previous studies, nano-indentation was applied to determine the transverse prop-erties of carbon fibers [67], to characterize the interphase between carbon fibers to epoxy resins [68] or to vinyl ester matrix [69]. In addition, nano-indentation is used to determine the hardness of polymers induced by crystallinity changes after processing with different cooling rates as present for AFP and autoclave consoli-dation [70].
Within this work, the nano-indentation technique serves to investigate if an in-terphase establishes between different carbon fiber sizings and polyamides. The influence of the investigated carbon fiber sizings can be determined by analyzing the width and hardness of the developed interphase.
The so-called Berkovich indenter with a tip in the shape of a three-sided pyramid with a face angle of 65.27◦ [71] was used. The principle of this technique is presented in Figure 2-5 along with a SEM image of the Berkovich indenter.
Carbon fiber
Matrix 7 μm
Nano-indenter
m
a b
Figure 2-5 a) Principle of nano-indentation on carbon fibers surrounded by matrix; b) SEM image of Berkovich indenter tip [72].
The Berkovich indenter is ideally suited for polymers as a sharper point results from the three-sided pyramid than from other indenter geometries such as the four-sided Vickers tip. This enables a better control of the indentation process [71].
Assuming a completely developed plastic area under the indenter tip, the mean contact pressure is defined by dividing the indenter loadP by the project areaAas Equation 2-6 presents. The mean contact pressure is also described as indentation hardness H [71]:
H = P
A = P
24.5h2p. (2-6)
The projected area Ais expressed by Equation 2-7 along with the plastic depth of penetration hp:
A= 3√
3h2ptan2θ. (2-7)
The elastic modulus E of the examined materials is calculated from the slope of the tangent to the initial unloading section of the load-displacement curve as follows [71]:
E = 1