6.1 Evaluation of testing methods and preliminary studies
6.1.3 Flexural properties of SMC composites
Determination of flexural properties of fibre reinforced polymers requires consideration of the heterogeneous microstructure of the composite and a non-uniform stress distribution over the specimen’s thickness (Zweben et al., 1978). Depending on test setup and the distance of lower the supports, a more or less significant shear stress results due to an out-of-plane force acting on the specimen, which strongly depends on reinforcement architecture and is more pro-nounced for continuous fibre reinforced materials than for fibrous reinforcement based on chopped fibres. Different standards recom-mend different test setups in terms of minimum support span to handle the aforementioned challenges and to minimise shear ef-fects during bending testing of fibre reinforced polymers (ASTM D7264/D7264M, 2015; DIN EN ISO 14125, 1998).
A fourth preliminary study involved three-point bending tests with different support distances to define an appropriate support distance for the different SMC composites. It aimed to characterise the evo-lution of the apparent flexural modulus, which theoretically reaches a value in the same range as the tensile modulus of elasticity if a sufficiently large span distance is considered (Zweben et al., 1978).
Figure 6.5 depicts the evolution of the relative modulus of elasticity of Co CF SMC, Dico GF SMC and CoDico GF/CF SMC as a function of the span-to-thickness ratio. The experimentally determined tensile modulus of elasticity (arithmetic average) was considered to calculate the relative modulus. As clearly seen, the continuous carbon fibre SMC is most sensitive to evolving shear stresses if an out-of-plane load is applied.
0 5 10 15 20 25 30 35 40 45 50 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Span (L) to thickness (h) ratio (L/h) Relativemodulusofelasticity(Ef/Et)
Dico GF SMC Co CF SMC
CoDico GF/CF SMC
Figure 6.5:Relative flexural modulus of elasticity of continuous carbon fibre SMC, discontinuous glass fibre SMC and continuous-discontinuous glass/carbon fibre SMC as a function of span-to-thickness ratio.
In DIN EN ISO 14125 (1998), a span-to-thickness ratio of 1:40 is rec-ommended to evaluate flexural properties of continuous carbon fibre reinforced composites in fibre direction. The American Society for Testing and Materials (ASTM D7264/D7264M, 2015) recommends a span-to-thickness ratio of 1:32. In order to define a suitable test setup to deduce the flexural properties of continuous carbon fibre SMC and continuous-discontinuous glass/carbon fibre SMC, bending tests were carried out with a span-to-thickness ratio of 1:32 and of 1:40.
Flexural modulus of elasticity Ef and flexural strength σf were deter-mined based on stress and strain calculations according to Equations 4.2 and 4.1.
6.1 Evaluation of testing methods and preliminary studies
Figure 6.6 depicts flexural properties of continuous carbon fibre SMC, with fibres aligned parallel the longitudinal axis of the specimen, determined for a span-to-thickness ratio of 1:32 and of 1:40. Flex-ural modulus of elasticity slightly increased with an increasing span-to-thickness ratio. However, considering the variations of stiffness within the testing campaign, this increase was not significant. No variation in terms of flexural strength was observed.
90 100 110 120
800 900 1000 1100
Flexural modulus of elasticity in GPa
FlexuralstrengthinMPa
0◦(L/h = 1:32) 0◦(L/h = 1:40)
Figure 6.6:Flexural modulus of elasticity and flexural strength of continuous carbon fibre SMC for a span-to-thickness ratio of 1:32 and 1:40 (• = median, += mean, box indicates 25th to 75th percentile, lines indicate minimum and maximum or 1.5 interquartile range, respectively).
In addition, flexural properties of continuous-discontinuous glass/carbon fibre SMC in terms of flexural modulus of elasticity and flexural strength did not show considerable difference if the span-to-thickness ratio was increased from 1:32 to 1:40 (Figure 6.7). Based on the aforementioned results, a span-to-thickness ratio of L/h = 1:32 was chosen to characterise flexural properties.
50 60 70
400 500 600 700 800
Flexural modulus of elasticity in GPa
FlexuralstrengthinMPa
0◦(L/h = 1:32) 0◦(L/h = 1:40)
Figure 6.7: Flexural modulus of elasticity and flexural strength of continuous-discontinuous glass/carbon fibre SMC for a span-to-thickness ratio of 1:32 and 1:40 (•= median,+= mean, box indicates 25thto 75thpercentile, lines indicate minimum and maximum or 1.5 interquartile range, respectively).
6.1 Evaluation of testing methods and preliminary studies
Three-point bending leads to a highly localised stress and strain field.
A fifth preliminary study was conducted, aiming to define the vari-ation of stiffness within one discontinuous glass fibre SMC specimen to account for the highly heterogeneous microstructure. For this purpose, flexural modulus of elasticity was determined three times for the same specimen. First, the specimen was placed symmetrically in the middle on the two lower supports. Two further measurements were carried out with the specimen shifted 10 mm to the left and right, respectively. This preliminary study was carried out with a span-to-thickness ratio of 1:20 to allow a sufficient overlap of the specimen at the lower supports when shifting the specimen between the measurements. As depicted in Figure 6.5, a span-to-thickness ratio of 1:20 already enables neglect of the influence of shear during an out-of plane loading of Dico GF SMC.
Figure 6.8 shows, that intra-specimen variations considering flexural modulus of elasticity were marginal for specimens aligned in the flow direction, with variations in the range of 1.3 % < CV < 4.5 %.
Variations were slightly more important for specimens extracted perpendicular to flow (2.1 % < CV < 7.3 %).
1 2 3 4 5 6 6
8 10 12 14
Specimen number EfinGPa
0◦
1 2 3 4 5 6 6
8 10 12 14
Specimen number EfinGPa
90◦
Figure 6.8:Flexural modulus of elasticity (Ef) determined for three different positions of six discontinuous glass fibre SMC specimens in 0° and 90°, with a span-to-thickness ratio (L/h) of 1:20,•= individual stiffness value for a distinct specimen.
A sixth preliminary study aimed to determine the coefficient of fric-tion (µf) between SMC material and support of the test setup. The ex-perimentally determined value was µf ≈0.24. Considering Equation 3b in DIN EN ISO 14125 (1998) and Equation 4.3, resulting frictional forces lead to an overestimation of flexural stresses of≈0.1 MPa for a deflection of 20 mm (L/h = 32:1, nominal specimen thickness of 3 mm). This deflection equals a flexural strain εf ≈ 3.9 % and is hence significantly higher than that of the observed failure strains for the considered SMC materials. Hence, frictional effects have no relevance.