Macrostructural characterisation

Macrostructural characterisation at the coupon level aimed to define tensile, compressive and flexural properties of the discontinuous glass fibre, continuous carbon fibre and continuous-discontinuous glass/carbon fibre SMC composites. The highly heterogeneous mi-crostructure of discontinuous SMC composites, combined with a macroscopically important heterogeneity of the hybrid material, gen-erally complicated the prediction of mechanical properties in terms of an appropriate displacement measurement technique.

It is not recommended to measure the modulus with strain gauges or an extensometer, since these techniques do not account for the variability of material properties resulting from the anisotropic and heterogeneous microstructure (Feraboli et al., 2009). Digital image correlation (DIC) has been used extensively to define material prop-erties and damage evolution of composite materials (e.g. by Hild and Roux, 2006; Laurin et al., 2012; Roux et al., 2008 or Johanson

et al., 2015) and is considered to measure displacements to define mechanical properties of the SMC composites investigated in this dissertation.

Another challenge in characterising long fibre reinforced compos-ites, such as SMC, is the important scatter of material properties, especially considering strength that arises from the heterogeneous microstructure (Shirrell, 1985). Hence, specimens with different ori-entation with respect to manufacturing and flow direction are con-sidered for mechanical material characterisation.

4.2.2.1 Tensile tests

Tensile tests were carried out in a conditioned laboratory at 23^◦C on a ZMART.PRO universal testing machine by ZwickRoell, with a load cell capacity of 200 kN according to DIN EN ISO 527-4, 1997 and DIN EN ISO 527-5 (2009).

Figure 4.3:Tensile test setup with stereo digital image correlation (DIC).

4.2 Characterisation at the coupon level

The specimens for tensile testing are described in section 3.2.3.1. Be-fore testing, specimens were hydraulically clamped at a clamping distance of 100 mm (unless otherwise noted) and preloaded to level out any compressive loads resulting due to clamping. The preload was defined according to the recommendation in DIN EN ISO 527-1 (2012) and adjusted for the different materials (Dico GF SMC, Co CF SMC and CoDico GF/CF SMC). It featured a load corresponding to 10 MPa for the continuous carbon fibre SMC tested in the fibre direction and 2 MPa for all other specimen types. Digital image correlation was used to measure the displacement field resulting from uniaxial tension (Figure 4.3). A 4M GOM ARAMIS three-dimensionel (3D) DIC system featured an adjustable base with two 4MP Teledyne Dalsa cameras with 50 mm Schneider Kreuznach objectives. Lighting was realised by two light-emitting diodes (LED) lights (16 W each) with a polarisation filter, and thermal changes to the specimen were negli-gible. A gauge section of approximately 70×10 mm²was considered, and technical strains were calculated. Uniaxial tensile tests were con-ducted with a nominal loading rate of 1.8 mm min⁻¹ ( ˙ε ≈2×10⁻⁴) until fracture. The frame rate of image acquisition was 5 Hz. Tensile modulus of elasticity (Et) and Poisson’s ratio (νt) were determined with a least squares method according to ASTM E111 (2010) (Equa-tion 4.10 in subsec(Equa-tion 4.5) in a strain range of 0.05 % to 0.25 %. The arithmetic mean value of the technical strains was taken into account.

Tensile modulus of elasticity and Poisson’s ratio of continuous carbon fibre SMC specimens loaded perpendicular to fibre direction were determined in the strain range of 0.05 % to 0.15 %, due to low failure strains. For all specimen types, tensile modulus of elasticity and Poisson’s ratio were evaluated for all specimens which did not fail in the clamping region and if the regression coefficient r²was higher than 0.9. Tensile strength was evaluated for all specimens which did not fail in the clamping region. Tensile strength was defined as the maximum load sustained by the specimen before failure (Rt). Strain

at tensile strength (ε_R,t) and failure strain (ε_max,t) were evaluated for all specimens which failed within the gauge section. Failure corres-ponded to a load drop to 0.8·Fmax with Fmax describing the preceded maximum load.

4.2.2.2 Compression tests

Compression tests were performed in a conditioned laboratory at 23^◦C on a ZMART.PRO universal testing machine by ZwickRoell with a load cell capacity of 100 kN according to DIN EN ISO 14126 (1999). The machine was equipped with a hydraulic composite com-pression fixture (Figure 4.4a). The preload was defined according to the recommendation in DIN EN ISO 527-1 (2012) and adjusted for the different specimen types. It featured a load corresponding to 45 MPa for the continuous carbon fibre SMC loaded in fibre direction (0°) and 2 MPa for all other specimen types. The specimens for compression testing were described in section 3.2.3.2. Clamping distance was set to 15 mm and uniaxial compression tests were carried out with a nominal crosshead displacement of 0.8 mm min⁻¹( ˙ε ≈ 2×10⁻⁴) until fracture. Displacement of the specimen was measured with a clip-on extensometer (type: MINI MFA2 Hand clamped extensometer by Mess-& Feinwerktechnik GmbH, accuracy class 0.2 according to EN ISO 9513) on both sides of the specimen (Figure 4.4a). This tool enabled control of the bending of the specimen through calculation of a bending factor (bf) according to DIN EN ISO 14126, 1999. The compressive modulus of elasticity (Ec) was determined with a least squares method according to ASTM E111 (2010) (Equation 4.10 in subsection 4.5) in the strain range of 0.05 % to 0.25 % with averaged strains resulting from displacement measurement of two sides of the specimen. It was evaluated for all specimens which did not fail in the clamping region, if the regression coefficient r² was higher than 0.9 and the bending factor, b_f, did not exceed 0.1. Compressive strength

4.2 Characterisation at the coupon level

(Rc) was evaluated, if the specimen did not fail in the clamping region and if the bending factor (b_f) was below 0.1 until fracture. Strain at compressive strength (ε_R,c) and failure strain (ε_max,c) corresponding to a force value equal to 0.8·Fmax were evaluated for all discontinu-ous glass and continudiscontinu-ous carbon fibre SMC specimens which failed within the gauge section.

A slightly modified test setup (Figure 4.4b) enabled the capture of damage evolution and resulting strain fields. For this purpose, se-lected tests were run in combination with a 4M GOM ARAMIS 3D digital image correlation system featuring an adjustable base with two 4MP Teledyne Dalsa cameras with 50 mm Schneider Kreuznach objectives. Lighting was realised by two LED lights (16 W each) with polarisation filter, and thermal changes to the specimen, were neg-ligible. The digital image correlation system captured displacement fields of the front or side face of the specimen, and an OLYMPUS E-M5 Mark II 16MP digital camera with a 50 mm objective additionally captured the damage evolution of some specimens (Figure 4.4b).

(a) (b)

Figure 4.4: Compression test setup with HCCF clamping system and (a) two sided clip-on extensometer, (b) digital image correlation system and digital camera.

4.2.2.3 Bending tests

Flexural properties were determined by three-point bending tests.

The methodology for bending testing of fibre reinforced polymers is detailed, for example, in ASTM D7264/D7264M (2015) and DIN EN ISO 14125 (1998). However, depending on type of reinforcement, slightly different test setups, most importantly in terms of the dis-tance of lower supports, are recommended. Difficulties arise mainly due to possible shear effects in the material leading to a falsified flexural modulus. To accommodate these effects, preliminary tests aimed to define an appropriate test setup in terms of the distance of lower supports. In this matter, three-point bending tests were performed in a conditioned laboratory at 23^◦C on a ZMART.PRO universal testing machine by ZwickRoell with a load cell capacity of 20 kN with rectangular specimens featuring a width of 15 mm.

The length of the specimens varied from 15 mm to 150 mm, realising different length to thickness ratios. The distance of the lower support was set to a nominal distance of 12 mm, 18 mm, 24 mm or 48 mm (Figure 4.5). Specimens were loaded with a constant crosshead dis-placement of 1 mm min⁻¹according to ASTM D7264/D7264M (2015).

Three-point bending tests with a nominal distance of 96 mm and 120 mm (span-to-thickness ratio of 1:32 and 1:40) were carried out with a loading speed to achieve a nominal strain rate ˙ε = 0.01 min⁻¹ at the lower surface of the specimen according to DIN EN ISO 14125 (1998). Table 4.1 lists the different test setups and testing parameters considered for three-point bending tests.

Every specimen was preloaded with 2 MPa. Deflection was measured with a laser measurement system type opto NCDT 2300 by MICRO-EPSILON.

4.2 Characterisation at the coupon level

Table 4.1:Three-point bending test parameters.

Nom. distance Diameter of Diameter of Loading of lower supports lower supports loading nose speed

12 mm 4 mm 7.5 mm 1 mm min⁻¹

Maximum strain at the outer surface (ε_f) and stress at the outer surface (σ_f) were determined according to ASTM D7264/D7264M (2015) and DIN EN ISO 14125, 1998. Stress (σ_f) and strain (ε_f) are

with the applied force F, support span L, mid-span deflection s, thick-ness h and width b of the specimen. However, Equation 4.1 and Equation 4.2 are valid only for small deflections. If maximum deflec-tion exceeds 0.1·L, DIN EN ISO 14125, 1998 recommends to calculate flexural stresses and strains according to

σf = ^3FL

Flexural modulus of elasticity (E_f) was determined with a least squares method according to ASTM E111 (2010) in the strain range

of 0.05 % to 0.25 % (according to Equation 4.10). Strain at flexural strength (ε_{R, f}) and flexural failure strain (ε_{max, f}), defined as the resulting strain corresponding to the point in stress-strain evolution at which load has dropped to a 0.8 ·Fmax were also evaluated. Ad-ditionally, damage evolution was captured for a number of selected specimens with an OLYMPUS E-M5 Mark II 16MP digital camera with a 50 mm objective. All specimens were mechanically loaded with the same side up with respect to placement of the semi-finished sheet in the mould.

(a)

10 mm

(b)

Figure 4.5:Three-point bending test setup combined with DIC (a) or OLYMPUS-M5 Mark E (b).