The FNCT tf* in air, water and a 2wt% aqueous solution of the detergent Arkopal are illustrated for
different PE-HD types in figure 46.
Figure 46: FNCT tf* of different PE-HD types tested in air, water and the detergent Arkopal.
For all PE-HD types, the tf* obtained in air are always larger than that in water and the detergent
solution, which is smaller than that in water (tf, air* > tf, water* > tf, detergent*, Fig. 46). Therefore, air is
regarded as an inert fluid and tf* results obtained in other fluids are referred to the tf* of air. The
failure mechanism in air is assumed to be pure SCG.
Water leads to a moderate reduction of FNCT tf*, which is similar for all PE-HD types with a factor
of approx. 0.5 compared to air. Hence, water is considered as a surface-active fluid, which accelerates crack growth neutrally concerning the ranking of different PE-HD types. Consequently, water leads to an accelerated SCG. The acceleration of SCG induced by a liquid is based on a reduction of interfacial tension that significantly facilitates the creation of additional internal surfaces during the craze-crack mechanism (cf. sections 2.3.2, 2.3.6 and 2.3.7). A detailed consideration of the damage mechanisms in this respect is given in section 5.1.3. Therein, the influences of all different fluids on SCG in PE-HD are discussed thoroughly.
The tf* reduction by the detergent solution Arkopal is significantly higher and different for different
PE-HD types (irregular). Although the PE-HD type ranking is qualitatively unchanged compared to air and water, the tf* reduction is different. For instance, the tf* reduction factor for AGUV is 1/8,
whereas it is 1/14 for AGBD compared to air. Hence, the detergent has a distinct influence on the damage mechanism and is thus classified as a surface-active fluid, which accelerated crack growth irregularly in terms of the differences of PE-HD types in the ranking. According to definition (sections 2.3.6 and 2.3.7), such a surface-active fluid leads to ESC. How far a detergent can be considered to be ‘neutral’ in the sense of swelling and cohesion will be discussed below. Furthermore, the influences on SCG of physical and structural properties of the PE-HD types, such as crystallinity, density and molecular mass, that might also be relevant for their ranking are dealt with in section 5.2.
Fluid-dependent differences in fracture behavior are also apparent in LM and LSM fracture surfaces depicted exemplary for AGUV in figure 47.
AGUV AGBD AQ149 5021DX 5831D
0 100 200 300 400 500 1400 1600 Arkopal air water time to f ailur e tf * / h PE-HD type 0 10 20 30 40 50 60 time to f ailur e tf * / d
a) Air, σL = 9.0 MPa
b) Water, σL = 8.9 MPa
c) Arkopal, σL = 8.9 MPa
Figure 47: LM and LSM (2D and 3D) fracture surfaces for air (a), water (b) and Arkopal (c) of AGUV FNCT specimens tested at 50°C.
Fracture surfaces in air and water are almost equal and exhibit typical SCG signature (section 2.3.4): a brittle and smooth area beginning at the notch surrounds a high and rough ligament, which peaks in the center of the fracture surface (Fig. 47 a, b). The central ligament results from the increased true mechanical stress due to the decreasing residual, non-fractured area at constant force in the specimen center during FNCT and represents a globally ductile fracture behavior (sections 2.3.4 and 5.4). Especially from 2D and 3D LSM height data, differences of fracture surfaces are clearly indicated by the color codes. The color code is selected in a small height range of -0.10 mm to +0.17 mm. Despite the differences in tf*, which have the same trend for different PE-
HD types, the FNCT fracture surfaces in air and water are equal (Fig. 47 a, b). Both exhibit flat surface areas ascribed to brittle fracture behavior surrounding rougher areas with central ligaments
attributed to ductile failure in equal shares. Due to this similarity of fracture surfaces, the assignment of the inert fluid air and the surface-active, neutrally crack growth accelerating fluid water to an SCG mechanism seems to be plausible.
Fracture surfaces of FNCT in Arkopal differ to those obtained in air and water. Although also showing central ligaments typical for FNCT, brittle fracture surfaces in Arkopal exhibit a rather uniformly distributed roughness in contrast to air and water (Fig. 47 c). Additionally, the maximum height and the base area of the central ligament are smaller. These differences in fracture surface appearance and the significant tf* reduction lead to the assignment of such detergent solutions to an
ESC-type mechanism. Because the detergent is surface-active, the acceleration of crack growth and the irregular PE-HD type ranking can be explained. Therefore, the Arkopal solution is denoted as a
surface-active, irregularly crack growth accelerating liquid which leads to ESC in PE-HD.
Due to the same origins of SCG and ESC (craze-crack mechanisms, cf. sections 2.3.5 and 2.3.6), a qualitative rating in terms of the resistance of different PE-HD types against SCG/ESC (SCGR/ESCR) is commonly accepted, as far as predominantly brittle fracture surfaces are obtained in FNCT (section 2.3.4) [4, 9, 13]. Therefore, a detergent (Arkopal) is usually accepted to be used in FNCT to rank different PE-HD types. However, distinct peculiarities of their SCG and ESC behavior (quantitative evaluation) may be disregarded. Thus, an FNCT in fluids such as air and water is needed to test the intrinsic properties of PE-HD types in terms of their SCG resistance. Likewise, ESC inducing test fluids (e.g. surface-active detergents) have to be selected to distinctly evaluate ESC behavior. In both cases, a fracture surface analysis is expedient [157, 181].