PSI - Issue 76

Filip Likavčan et al. / Procedia Structural Integrity 76 (2026) 145– 150

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computational assessment of fatigue strength, it is necessary to join the cyclic properties of the material used and the parameters of the expected operating load [1]. There are several materials used in the construction of bicycle and electric bicycle frames, whether metal alloys or composites. Aluminum alloys, steels, and, in exceptional cases, titanium alloys are used from metal materials. Composites are mainly carbon fiber-based composite materials in an epoxy matrix. To a lesser extent, polyamides containing short carbon fibers are also used. Steel is currently used to a much lesser extent in the construction of bicycle frames than in the past. However, it still has its place in cycling, for example in BMX bikes (BMX bikes are small bikes designed for jumps and acrobatic tricks). Steel is very suitable for this type of bike due to its good fatigue properties, good ability to absorb strong impacts and, thanks to its high strength, it should also withstand impacts with obstacles. Chromium and molybdenum alloy steels (e.g. AISI 4130 steel) are very widely used in the bicycle industry. High-strength steels from Reynolds are a speciality. For example, Reynolds 953 is a stainless steel with a special martensitic structure that can have a tensile strength exceeding 2000 MPa. This material easily rivals composite materials and even surpasses them in some properties. Composite materials can be designed to achieve an exceptional combination of stiffness, strength, weight, durability, etc. On the other hand, they lose the advantage of anisotropic mechanical properties compared to metallic materials. A typical composite material used in the manufacture of bicycle frames is a long-fiber composite material with carbon fibers and an epoxy matrix. Carbon composites with dispersedly distributed microfibers are also used to a lesser extent; these are polymers with an admixture of microscopic carbon fibers. Fiber carbon composites have a very good weight-to-strength ratio, which makes them suitable for the construction of reliable and lightweight bicycle frames. In general, these materials also have very good fatigue properties due to their structure, in which the individual fibers in the matrix slow down the propagation of cracks. The disadvantage of these materials is their relatively high brittleness under concentrated impacts. When designing a bicycle frame, their anisotropy can be exploited so that the designer optimize stiffness by specifically positioning the fibers in the frame to dampen vibrations from the road while maintaining good torsional stiffness when pedaling, which increases pedaling efficiency. Aluminum alloys are very suitable for use in the bicycle industry due to their mechanical properties. They are lightweight, strong, and easily processed using various technologies. Aluminum alloys with the designations EN AW 6061 T6 and EN AW 7005 T6 are most commonly used for the production of bicycle frames. This article will analyze in more detail the specifics of estimating the fatigue strength of bicycle frames: • Use of multiple manufacturing technologies in frame production • Fatigue life assessment of welds • Effect of geometric inaccuracy on fatigue life • Multi-axial stress state

Nomenclature YS yield stress TS tensile strenght

2. One material with multiple levels of mechanical properties The technologies used in the production of aluminum alloy bicycle frames include gravity casting, die casting, liquid die casting, low pressure casting, forging, hydroforming, welding, etc. Individual material processing technologies result in different microstructures and different densities and morphologies of defects. This results in different properties of the final bicycle frame component despite the identical chemical composition of the material [2-6] (see Fig. 1.). In terms of fatigue strength, the negative consequences are · During casting, internal cavities and defects due to insufficient flow into the mold or decreasing viscosity of the