The analytical solution are derived for a general 3-D helical curved beam. The equilibrium equations are listed as twelve ordinary differential equations. All force, moment, rotation and displacement components form a set of differential equations of the same pattern. Once the curvature and torsion are specified, the analytical solutions can be derived, if the pattern of differential equations can be solved. Helical curved is found to be solvable. The analytical solutions of 2-D curves of circular, elliptical, cycloid, cantenry, parabolic curves are demonstrated here. The analytical solution of 3-D helical curve with variable curvature is also demonstrated.

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The purpose of this study is to elucidate factors affecting the statistical variations typically observed in the tensile strength of natural plant fibers such as kenaf bast fibers (KBFs) which recently have been looked upon as promising reinforcing fillers of environmentally-preferable polymeric composite materials. A statistical finite element modeling framework by combining beam and solid elements was proposed with both the variable cross-sectional geometries and the meso-scopic internal organic structures stochastically considered. The present modeling strategy was then effectively applied to the stress analysis and strength simulations of natural plant fibers under axial tension. From numerical examples in the case of KBFs, it was shown that the meso-scopic internal organic structures, which consist of elementary fibrous cells (EFC) and inter-cell material (ICM), gave rise to axial stress concentrations in EFC and free-edge shear stresses in ICM, both of which should initiate the fiber fracture. Furthermore, by applying two-parameter Weibull analysis to the simulation data, the dependency of fiber strength and its initiating failure modes upon gauge length was also examined, which shows fairly good similarity with the actually-observed experimental results of monofilament tensile strength of natural plant fibers.

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Fatigue life evaluations in low dissolved oxygen conditions have been found to be conservative regarding the prediction of the fatigue strength of biomaterials. However, dissolved oxygen is considered to increase crack propagation rates. It is necessary to observe whether the strength in low dissolved oxygen conditions is higher. In this paper, the authors observed the effects of dissolved gas on the corrosion fatigue characteristics of austenitic stainless steels. Fatigue strength was higher with low dissolved O₂ NaCl aq at a stress ratio of R= 0.1. However, fatigue strength did not change with low dissolved O₂ NaCl aq at a stress ratio of R= 0.5 because of the effect of work hardening. Cycles to crack initiation were longer and crack growth rates were lower in the low dissolved O₂ NaCl aq. The mechanism of decreasing fatigue strength can be explained by the oxidation process during slipping. In conclusion, dissolved oxygen has the effect of accelerating crack propagation processes.

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In order to clarify the loading frequency effect on the Fatigue Crack Growth Rate in a low pressure hydrogen gas environment, fatigue crack growth tests were carried out on an A6061-T6 aluminum alloy at several loading frequency levels in pure dry hydrogen gas, in air and in pure dry nitrogen gas. Hydrogen enhances the FCGR. Moreover, the growth rate increases as the loading frequency decreases not only in hydrogen gas but also in nitrogen gas. The main cause of the loading frequency effect is related to thermal activation in the plastic zone at the tip of a crack growing across the grains by slip-off. The loading frequency effect concerning hydrogen itself cannot be revealed. A transition loading frequency exists below which the FCGR is practically independent of the loading frequency.

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This paper presents a finite element method to investigate the effects of delamination on free vibration characteristics of graphite-epoxy pretwisted shallow angle-ply composite conical shells. The generalized dynamic equilibrium equation is derived from Lagrange's equation of motion neglecting Coriolis effect for moderate rotational speeds. The theoretical formulation is exercised by using an eight noded isoparametric plate bending element based on Mindlin's theory. Multi-point constraint algorithm is utilized to ensure the compatibility of deformation and equilibrium of resultant forces and moments at the delamination crack front. The standard eigen value problem is solved by applying the QR iteration algorithm. Finite element codes are developed to obtain numerical results concerning the effect of twist angle, location of delamination and rotational speed on natural frequencies of delaminated angle-ply composite conical shells. The mode shapes are also depicted for a typical laminate configuration. Parametric studies of symmetric and anti-symmetric angle-ply laminates provide the non-dimensional natural frequencies which are the first known results for the type of analyses carried out here.

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The mechanism for a bolt and nut self loosening under repeated bolt axial tensile load has yet to be clarified, despite much investigation into this phenomena. In this paper, the self loosening mechanism is derived from basic strength of materials equations, which results in the following conclusions. (1) When load is applied, a slip occurs on the screw thread surface, and the bolt shank twists clockwise, that is it descends on the lead angle of the screw thread. At the end of this process, counterclockwise restitution torque T_S1- is generated by the twisted angle of the bolt shank. However, there is no rotation of the nut. (2) When the load is released, if T_S1- exceeds the friction torque T_W0 on the nut bearing surface generated by the decreased bolt axial tension, a slip occurs on the nut bearing surface, and the bolt and the mating nut rotate counterclockwise as one unit. To satisfy the relationship T_S1->T_W0, the maximum bolt axial tension F₁ must be larger than cF₀. Here F₀ is the minimum bolt axial tension and the coefficient c depends on the bolt shape and the friction coefficient. The rotational behavior of the bolt and the nut derived from this analysis concurs with experimental and FEM calculation results of other researchers. For steel joints, it is believed that rotational loosening rarely occurs when there is no separation between joined parts.

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The fatigue and fracture behavior of C/C composites fabricated using fine-woven carbon fiber laminates were investigated in several notched and smooth specimens. Slits and notches were cut in some specimens, and the effect of fiber direction and notch configuration on the fatigue behavior was examined. Axial load or bending moment was applied, and the fracture strength and the fracture mechanism were investigated. The fatigue limit was dependent on stress ratio, fiber direction and notch configuration. It was found that the fatigue limit and tensile strength of the notched specimens were higher than those of the smooth specimens. So, the fracture behavior of the material was different from metals and plastics. The local interlaminar debonding of fiber sheets and local shear deformations are important factors in the evaluation of static and fatigue strength.

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