Bending fatigue of hybrid composite rods
The flexural fatigue behavior of hybrid composite rods comprised of unidirectional carbon and glass fibers was investigated. Damage was evaluated by monitoring stiffness loss as a function of cycles, and bending fatigue failure was defined in terms of strength retention. The acoustic emission technique and microscopic examination were used to characterize damage progression and failure mechanisms. The number of cycles to failure depended on applied stress level, and a two-parameter Weibull analysis was used to incorporate probability of failure to the S–N curve. Damage initiated and propagated as a result of matrix cracking and fiber bundle failures within the GF shell. Bending fatigue damage only initiated when the hybrid was exposed to a deflection in excess of 42% of flexural strength, which does not occur in actual conductor field use. Damage reached a saturation point along the GF/CF interface because of the stress concentration that existed between the two material systems, resulting in asymptotic behavior of the stiffness loss. Because damage did not extend into the CF core, static mechanical properties were retained to ∼85% or more.
Fatique and damage-tolerance analysis of composite laminates: Stiffness loss, damage-modelling, and life prediction
Abstract The prediction of fatigue life and evaluation of progressive damage for general composite laminates are studied analytically. From theories of damage tolerance, residual-modulus degradation, and residual-strength d degradation, a simple and approximate approach is proposed for the prediction of progressive stiffness loss, matrix-crack density, and delamination area in terms of tension-tension fatigue load and number of cycles for general laminates containing 0° plies. The proposed approach provides four choices for predicting tension-tension allowable fatigue life and for assessing fail-safety for structures made of composite laminates, namely, a residual-modulus criterion, a matrix-cracking criterion, a delamination-size criterion, and a residual-strength criterion. The approximate analytical relationship between S-N curves for general laminates containing 0° plies and for a unidirectional 0°-ply laminate is proposed. Three glass/epoxy laminates of [0/90] s , [±45/0/90] s , and [0/±45] s lay-ups, and one [0/90±45] s T300/5208 graphite/epoxy laminate under tension-tension fatigue are used to illustrate and verify the proposed approach. The analytical results are in good agreement with experimental data.
Experimental study on bond behavior between high-strength grout and deformed steel bars
Abstract Engineers use high-strength grout to connect precast concrete members for its low shrinkage and good fluidity. This paper designed 87 central pull-out specimens using the control variable method to investigate the influence of factors on the rebar-grout bond property such as compressive strength, steel grade, bar diameter, transverse stirrups, and bond length. The test result showed that improving grout compressive strength (85.8 MPa–122.7 MPa) cannot improve the bond strength, which was different from the previous conclusions of concrete pull-out tests. Simplified formulas were presented for calculating the characteristic values of bond strength, residual strength, peak slip, and relative slip of load-holding platform based on parameter analysis. Then, a constitutive model was proposed to predict the full-range bond stress-slip curve of high-strength grout and steel bar specimens from bearing to failure. Finally, FEA models were established to evaluate the applicability and accuracy of the constitutive model.