Numerical and experimental analyses of the plasticity-induced fatigue crack growth in high-strength steels

Numerical analyses of plasticity-induced fatigue crack growth (FCG) were performed for high-strength steels. FCG proceeds by large crack tip deformations promoting transfer of material from the tip apex to the crack flanks, thus rendering plastic crack growth based on plastic deformations without bond breaking or material splitting at the crack tip (Laird–Smith mechanism of FCG by blunting and re-sharpening). Plastic FCG rates (da/dN)_p were obtained and numerical results were fitted with a Paris-like equation (da/dN)_p = CΔK^m. An experimental validation of the simulations was performed by comparing the numerical Paris-like equation with real (experimental) Paris equations in the steels.     

Micromechanics of fracture in notched samples of heavily drawn steel

This paper analyzes the consequences on fracture of the combined effects of triaxial stress states generated by notches of very different geometries and microstructural evolution produced by a heavy cold drawing when eutectoid high-strength prestressing steels are manufactured. The anisotropic fracture behaviour of these materials with high level of strain hardening is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.     

Failure analysis of a lifting platform for tree pruning

The paper presents an analysis of the failure in service of a lifting platform used for tree pruning. Different fracture mechanics techniques were used to reveal the causes of failure such as analysis of the fracture surface, mechanical and microstructural characterization of the material and fracture mechanics tests to determine the critical value of the stress intensity factor and to characterize the subcritical crack growth under fatigue. Results yield do conclude that the platform failure cause was the subcritical crack growth in the welded joint where the welding was discontinuous, thereby producing a stress concentration effect similar to that of a crack.     

Fatigue behaviour of bolted joints

This paper analyzes the tensile fatigue behaviour of bolted joints constituted by commercial steel bolts. They were tested under both monotonic and fatigue tensile loading, with different R-ratio. Results show that under increasing monotonic tensile loading the bolted joint is not the failure zone of the bolt, whereas such a bolted joint is the failure region under cyclic loading. The fatigue life decreases with the increase of the stress range and with the maximum stress, and pre-loading enlarges the fatigue life. Fatigue fracture surface shows a geometry of crescent moon in the case of short cracks and such a shape evolves towards a quasi-straight crack front in the case of long cracks. Fatigue fracture usually happens at the root of the first notch inside the bolted joint, although fracture initiation may happen in several consecutive notch roots, increasing the initiation angle of the fatigue crack as the applied stress diminishes.     

Fracture Performance of Progressively Drawn Pearlitic Steel under Triaxial Stress States

We study fracture processes in specimens with axially symmetric notches made of prestressed steels with various degrees of cold drawing of the wire. For this purpose, we use steel samples drawn to the required degrees. The intensity of drawing or the level of strains (characterized by the yield limit) is used as the main parameter of the technological treatment of the material. In specimens with notches of various shapes, we analyze the role of triaxiality of stresses. Special attention is given to the behavior of the sensitivity of the wire to notches as a function of triaxiality of stresses and the yield limit. The changes caused by the anisotropy of strength formed as a result of cold drawing are studied on the microscopic level.     

The effect of increasing soaking time on the properties of premixing starch–glycerol–water suspension before melt-blending process: comparative study on the behavior of wheat and corn starches

Polymer Bulletin - The reduction in the torque consumed during the preparation of thermoplastic starch to the minimum value was achieved by reaching equilibrium state of the premixing suspension of...     

Critical stress intensity factors in steel cracked wires

In the conceptual framework of fracture mechanics analyses, the study of cracked wires axially loaded has the highest interest since numerous structural elements (e.g. wires, cables, cordons or tendons) work under such a type of loading during their service lives. So, a method that allows the determination of stress states at the crack front should be welcome as a useful way for ensuring the structural integrity of those components for different environmental conditions (air, stress corrosion cracking, hydrogen embrittlement,…). To fill this gap, an engineering estimation of the critical stress intensity factor (SIF) is proposed in this paper for eutectoid steel cracked wires under axial loading. The critical SIF is calculated by considering, apart from the fatigue precrack, the subcritical crack propagation before final fracture. Such a subcritical crack propagation is the process zone (by micro-void coalescence MVC) in the case of fracture in air, the subcritical cracking by localized anodic dissolution (LAD) in stress corrosion cracking (SCC) and the tearing topography surface (TTS) in hydrogen assisted cracking (HAC). In addition, different SIF solutions are used in the analysis so as to have a more complete picture of the different phenomena leading to failure and to provide the designer with sound scientific tools. This method allows the engineer to design in the framework of structural integrity and damage tolerance.