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.
Damage development in pultruded composites for optical telecommunications cables under tensile and flexural fatigue
This paper investigates the long-term behaviour of pultruded glass-fibre composites, which are used as load-bearing components in optical telecommunications cables. Both flexural and tensile fatigue tests were conducted using the materials in their commercial product forms. Fatigue damage progression was monitored on-line by the acoustic emission technique. Damage accumulation was quantified by the stiffness loss evaluated from the change of resonance frequency in a free vibration test. The resonance measurement is able to determine modulus degradation to an accuracy of 0.01%. A definition of fatigue failure under tensile and flexural fatigue is proposed. Fatigue damage development is discussed and is found to be dependent on stress level and stress history.
Increasing pulp yield in kraft cooking of softwoods by high initial effective alkali concentration (HIEAC) during impregnation leading to decreasing secondary peeling of cellulose
Abstract Pulp yield can be improved by a more homogeneous delignification of the chips, achieved by improved impregnation prior to the cooking stage. Complete and efficient impregnation is obtained by increasing the diffusion rate by means of an impregnation liquor with a high initial effective alkali concentration (HIEAC). In the present study, the effect of HIEAC in the impregnation was evaluated and compared to a reference impregnation procedure and a prolonged impregnation. After the various impregnation scenarios, the alkali concentration was always adjusted to the same level in the beginning of the cooking stage. Impregnation with a HIEAC resulted in yield improvements by 1–1.5% units, due to a higher cellulose yield and possibly also to higher yield of glucomannan. The HIEAC with an even alkali distribution within the chips prior to the cooking stage resulted in a more uniform delignification carbohydrate degradation. Yield increase obtained by uniform delignification is due to both decreased shives content as well as less secondary peeling.