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This study investigates the impact of high through-thickness compressive stress on the fibre direction tensile strength of carbon/epoxy composite laminates. Experimental and numerical analyses were conducted, proposing two fibre failure criteria. The findings suggest that high through-thickness compressive stresses can significantly reduce fibre-direction tensile strength, leading to primary failure by fibre tensile mode.
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Journal Club Effect of high through-thickness compressive stress on fibre directiontensile strength of carbon/epoxy composite laminates Composites Science and Technology 90 (2014) 1–8 Khong Wui Gan, Michael R. Wisnom & Stephen R. Hallett Advanced Composites Centre for Innovation and Science University of Bristol UK Deng'an Cai 15 July 2015
Contents • Background • Experiment design • Numerical analysis and failure criterion • Results and discussion • Conclusion
Background TTc & T : Through-thickness compression & in-plane tension Design requirements Paucity of test data
Experiment design • Material Hexcel's carbon epoxy prepreg system: IM7/8552 Stacking sequence: [(0/90)7/0/(90/0)7] • Dimensions of specimen
Experiment design • Load cases • TTc & T with indenters of R = 10, 20, 40, 80mm Load control mode • TTc & T with indenters of R = 20mm Displacement control mode • Pure TTc with indenters of R = 10mm
Numerical analysis and failure criterion • Finite element (FE) model • Linear elastic material properties • A penalty-based frictionless surface-to-surface contact • Thermal residual stresses (ΔT = -160 ℃, cool down from the curing process) • Stress analysis • Out-of-plane shear stress is zero (on the symmetry plane) • Only three principal stresses: —— Longitudinal tension —— Transverse constraint from the boundary conditions —— Through-thickness compression
Numerical analysis and failure criterion • Failure criterion • Tresca stress criterion Fibre failure criterion A: • Modification of Tresca stress criterion Fibre failure criterion B: A is a material constantwhich can be measured from a simplemechanical test such as a uniaxial longitudinal tensile test.
Results and discussion • Experimental results
Results and discussion • Failure surface Specimen failed on symmetry plane Load control Unidirectional specimen Splitting fracture Cross-ply specimen flat fracture Crushing damage TTc at 17.6kN TTc at 50.3kN Displacement control
Results and discussion • Failure surface Indenter R=10mm Tensile fibre fracture mode SEM image of the fracture surface (pure through-thickness compression loading)
Results and discussion Indenter R=10mm • FE results TTc=40kN, T=2.3kN TTc=15kN, T=28.4kN
Results and discussion • FE results Criterion B Criterion A The maximum combined stresses of all biaxial load cases
Results and discussion • The maximum values of stress combination followingboth criteria mostly fall above the manufacturer’s longitudinal tensilestrength. • In an isotropic material, Tresca stress criterion implicitly suggests that failure should occur at an angle of 45°. But the fibres did not fail at a shear angle from the fracture surfaces. • Both the failure criteria are conservative and useful for fiber failure stress prediction, with Criterion B performing slightly better for the present tests. • More test data is needed for better understanding of the biaxial or triaxial stress interaction.
Conclusion • This study aimedto quantify the interaction between fibredirection tensile stress and through-thickness compressive stressin composite laminates. • A full range of biaxial loads (TTc &T)with indenters of differentradiihas been investigated . • High through-thickness compressive stresses cansignificantly decrease the fibre-direction tensile strength. • All specimens failed primarily by a fibre tensile mode. • Two fibre failure criteria have been proposed, which can be used satisfactorily as conservative criteria for triaxial loading.
