PSI - Issue 50

Alexander Eremin et al. / Procedia Structural Integrity 50 (2023) 65–72 Alexander Eremin / Structural Integrity Procedia 00 (2019) 000 – 000

68 4

(5)

t l     

where Δε t and Δε l are differences in transverse and longitudinal strains. The range of the longitudinal strain is the same as above. For Poisson ratio evaluation the strain values were obtained as the mean of the strain values in the central part of the corresponding strain field. 3.3. Digital image correlation technique Digital image correlation was utilized within all tests in order to obtain full-field strain data. Prior to testing the speckle pattern was painted on the laminate surface using UV-printing. The speckle pattern has a white background and black randomly distributed dots. Also, it contains special calibration markers allowing a processing of the raw data in a semi-automatic manner: image dimension scale and DIC mask are automatically derived from a known marker pattern reducing manual work. The imaging was performed via DSLR Canon 700d with a frequency of 1 Hz. Frame resolution is 5184x3456 px while the speckle-covered area of the specimen has a size of 4708x1100 px. The size of this area is 107x25 mm 2 resulting in a scale of 44 px/mm. After the testing raw image data is collected and processed in a Vic-2D software by Correlated solutions. 4. Results and discussion 4.1. Tension of aramid and carbon fiber reinforced composites with 45/-45 layup Aramid fibers comparing to carbon fibers are less stiff, have lower strength but demonstrate much higher strains to failure, so could better absorb mechanical energy. Laminated composites inherit fiber properties. Table 1 presents mechanical properties of composites with ±45 layups subjected to tensil e loading. It is seen that shear elastic modulus and ultimate tensile stress for AFRP are much lower than for CFRP. Both composites have failed at values of the engineering shear strain exceeding 5%, thus the results of the tests are limited by this value. However AFRPs reach the ultimate stress at very high values of the engineering shear strain – about 30-35%. This is due to the type of the reinforcement. Woven fabrics can withstand very high strain due to interlocked fiber bundles and scissor-like motion while shear along the fiber axis as CFRP demonstrate is reduced. Offset shear strength and maximum in plane shear of AFRP are lower than CFRP by ~32%, as well as AFRP has lower maximum shear stress (at γ 12 =5%) by ~30% and lower shear modulus by ~59%. 4.20 ±0.08 The stress-strain diagrams obtained during tensile tests for 3 (of 5) specimens are presented in Fig. 1a for AFRP and Fig. 2a for CFRP. Dashed horizontal line marks the offset shear strength, representing the point after which a nonlinear behavior of the material begins. illustrate strain fields Deformation process has been characterized by digital image correlation technique. Strain fields are presented in Fig. 1b and Fig. 2b where longitudinal and transverse fields were placed together. Nonlinear behavior of the loading curves of CFRP starts at lower strains than for AFRP and is related to the higher stiffness of the carbon fibers. This nonlinear loading trend continues up to the maximum shear stress at ~3% of the engineering shear strain. From this point the load starts to drop to the 5% of the engineering shear strain. This load drop continues (not shown in the figures) up to the final breakage of the specimen in two pieces. The breakage is accompanied by high shear slippage of fibers. Aramid woven fabric has a higher ability to absorb deformations without fracture thus the load increases steadily up to the 5% and it is continued up to the breakage of the specimen in two pieces occurring at 45-50% of the engineering shear strain. The deformation of AFRP specimens occurs with Table 1. Mechanical properties of aramid and carbon fiber polymers with ±45 layups. Maximum in-plane shear stress τ m , MPa Offset shear strength τ 0.2 , MPa In-plane shear modulus of elasticity G, GPa AFRP CFRP 46.07 ± 2.09 65. 57±3.31 32.32 ± 3.37 47.25 ± 3.42 1.73 ±0.05