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The fatigue behavior of welded thin-walled T-joints made up of both circular hollow section (CHS) braces and chords, subjected to cyclic in-plane bending, is described in this paper. CHS chords and braces are of thicknesses less than 4 mm. Current fatigue design guidelines show that the design of welded tubular nodal joints is restricted to thicknesses greater than or equal to 4 mm. The increased availability and use of thin-walled (t<4 mm) tubes of high-strength steels in recent years, in structures subjected to cyclic loading, means that it is important to study the fatigue behavior of welded thin-walled tubular nodal joints. In this paper, welded thin-walled CHS-CHS T-joints subjected to constant-stress-amplitude cyclic in-plane bending range are studied. The stress concentration factors (SCFs) determined experimentally at the brace and chord crown positions are shown to be about 30% and 40% respectively of the SCFs determined using parametric equations in existing fatigue design guidelines. The fatigue tests showed that in welded thin-walled CHS-CHS T-joints, a through-thickness crack occurs when the surface crack length along the weld toes in the chord has grown to a length equal to about 40% of the circumference of the brace member. An end of test failure criterion was proposed as an alternative to the through-thickness failure criterion, in obtaining data that is suitable for determining fatigue design S-N curves.

A prosthetic knee is predominantly subjected to cyclic action leading to fatigue failure during its operation. However, cyclic failure is more severe as it occurs below the yield strength and may happen even without prior indication. This study presents a cyclic structural analysis of an existing polycentric mechanical prosthetic knee by finite element simulation and its experimental validation as per the ISO 10328:2016 standard. The three-dimensional (3D) model is imported to the ANSYS 20.1 workbench to study stress distribution and fatigue life in the knee for ensuring its safety performance. The maximum von-Mises stresses developed at the front and back joint bars made of A390.0-T5 and A390.0-T6 cast aluminium are 100 and 89MPa, respectively, in both loading conditions compared to their mechanical fatigue strength of 89MPa. The maximum deformation and average number of cycles for fatigue failure are 2.66 and 2.53mm and $2.95\times 10^6$ and $2.81\times 10^6$ for simulation and experimental results, respectively. These results suggest that this polycentric knee unit has poor fatigue strength and thereby does not meet the ISO structural standard. A validation test has been performed with an average error of 4.75% between the simulation and experiments results showing higher reliability.

Due to continuously varying loads, Cruciform welded joints suffer fatigue. Limitations of the conventional 3-piece cruciform fillet welded joint (CFWJ), used to construct all present, metallic complex structures, are uprooted here. A broad literature survey and physical examination of many welded structures are carried out to find a research gap from the exploration available on CFWJ. How the use of this CFWJ is questionable for suitability and prone to structural failure is shown. To overcome the limitations of CFWJ, a novel 2-piece coplanar Cruci-Trap Welded Joint (CTWJ) is developed especially for complex welded structures subjected to a fluctuating loading. A novel groove shape is formed in base metal. CTWJ increases the overall load-carrying capacity of the structure in all nature. This paper also points to the possibility of fatigue failure of cruciform welded joints and the various methods for fatigue life assessment of welded structures. As a result, the proposed novel CTWJ promotes 28.5% more ultimate tensile strength and 25.5% more elongation than the conventional CFWJ method.

The intervertebral disc is organized with a concentrated proteoglycan solution, the central nucleus pulposus, held within the strong collagen network of the outer annulus fibrosus. The disc exhibits a viscoelastic response when subjected to loads and deformations. Disc degeneration, and/or spondylotic changes that are generally considered to be associated with aging, result in a spinal segment with decreased stiffness. However, in the cervical spine of cerebral palsy patients suffering from athetotic movements of the neck, there is a very early onset of disc degeneration and spondylotic change. Acceleration of disc degeneration has been shown to take place in the spines of animals subjected to excessive extension-flexion of the head and neck. Repetitive torsion of the disc has led to structural regression in in vitro studies using animal spines. Delaminated lamellae and/or disruption of the annulus fibrosus are always recognized in the early stages of the destructive process of the intervertebral disc structure. Disruption of the collagen network may be a result of fatigue failure by repetitive loading, which in turn causes the high tensile stresses in the annulus fibrosus from the development of large hydrostatic pressures within the nucleus pulposus. Loosening of the collagen network may be a key factor leading to the loss of proteoglycans and water, finally inducing the development of disc degeneration. A “degenerated disc” can be induced through pure mechanical fatigue failure of the tissue, as an age-independent degradation of the cartilaginous tissue.

In fatigue testing of railway vehicles' structure, stress is usually increased to accelerate the testing process, so that the fatigue failure mechanism can be manifested in a shorter time. For determining the stress, the accelerated life model is key. In this paper, the fatigue life test of typical welded joints of railway vehicles is first carried out. The Weibull distribution of accelerated life model of welded joints is then used to estimate the parameters. Accelerated life parameters under 50%, 95% and 99% reliability are obtained, and the results are compared with IIW standard. It was found that the dynamic stress amplitude of vehicle welded joints is generally lower than that corresponding to 1E+07 cycles, and that the slope parameter of the model is 3.70. In addition, the constant term under 95% is 8.402E+011. The parameters of the welded joints meet the IIW standard very well in terms of the fatigue performance of the welded structures in China. This study provides a reference for future designing and testing of railway vehicles.