Ρυθμός εξέλιξης ρωγμής σε κόπωση χάλυβα TRIP700 σε υψηλό λόγο τάσεων

The significant technological advancements of recent decades have led to an increasing demand for new materials with superior mechanical properties, capable of meeting increasingly stringent requirements.In particular, the automotive industry has been experiencing constant development, with continuous innovations and progress, creating the need for materials that combine enhanced strength, stiffness, and weight reduction.Such mechanical properties are required to ensure passenger safety, while also contributing to the vehicle's weight reduction, which is crucial for improving fuel efficiency and, consequently, reducing the environmental impact.Advanced high-strength steels (AHSS), a new class of steel that first emerged in the 1970s, have drawn significant attention, especially in the automotive industry, due to the outstanding combination of mechanical properties that they exhibit.The great strength, ductility and low density, as well as the energy absorption capacity that they present, have rendered them increasingly appealing to the industry.A particularly noteworthy category of steels among AHSS is TRIP Steels, Transformation Induced Plasticity Steels.Their properties arise from their complex microstructure, which consists of ferrite, bainite, retained metastable austenite and small percentages of martensite.The key mechanism that contributes to their great performance is the martensitic transformation that occurs under mechanical loading, during which the retained austenite -which imparts ductility to the material-transforms into martensite, a harder but more brittle phase.This phenomenon enhances the material's strength and the fracture resistance.In this thesis, the TRIP700 Steel was studied, and fatigue experiments were conducted to determine the influence of prior plastic deformation on fatigue crack growth.Specimens with varying levels of pre-strain were tested and compared with undeformed reference specimens.The fatigue tests were conducted under a stress ratio of R=0.5 and the results were compared with data obtained under R=0.1 from a Institutional Repository -Library & Information Centre -University of Thessaly 06/03/2026 18:13:41 EET -139.180.4.69 vi previous diploma thesis, in order to evaluate the influence of different stress ratios on the material's behavior.