Key Engineering Materials Vols. 592-593

Abstract: Flat specimen of 316L steel was cyclically pre-deformed with constant plastic strain amplitude to early stage of fatigue life relevant to the period of cyclic strain localization and fatigue crack initiation. To document slip activity and reversibility/irreversibility of persistent slip bands (PSBs) in situ experiments in the high-resolution SEMFEG under special imaging conditions were performed. The half-and full-cycle slip activity and distribution of plastic strain within PSBs in individual grains were investigated via slip steps generated in half-and full-cycle deformation after intermediate vibration polishing. After completion of in situ tests the surface topography in identical locations was quantitatively documented using atomic force microscopy (AFM).

Abstract: The influence of single overloads on the fatigue crack initiation and propagation of an EN AW 7475-T761 alloy was investigated using SEN-specimens. Into a base load with an amplitude of 80 MPa and an Rvalue of R=-1 tension compression overloads with an amplitude of 240 MPa have been introduced every 10,000 cycles. The crack length was measured with a DC-Potential drop method. The experiments have shown that cracks are initiated in the first 5% of the total lifetime and, therefore, the cyclic lifetime is dominantly determined by crack propagation. The investigation of the crack surfaces exhibit that a different number of individual cracks propagates within the specimens. The differences in the cyclic lifetimes indicate that the interaction between these cracks directly influences the cyclic lifetime. This shows that the differences in the crack propagation behaviour are responsible for the scatter of the determined cyclic lifetimes.

Abstract: Analysis of microcrack and mesocrack formation in austenitic steel thin filmsprepared after low-cycle fatigue (LCF) testsfrom bulk samples is presented using TEM techniques. Location, orientation and interaction of microcracks with microstructure components of the steel were determined. Plastic zone ahead of mesocrack tip and the structure changes in it were analyzed. Crystallography of slip bands and deformation twins and their relation with the microcrack propagation direction were also determined. The impact of grain anisotropy and inhomogeneous distribution of stress relaxation ahead of mesocrack tip in plastic zone were considered. Influence of sizes of mesocracks [ and microcracks and their relation with the trajectory and crystallography of propagation are also discussed.

Abstract: The study is focused on finding and describing microscopic paths of modes II and III cracks in commercially pure α-titanium (hcp structure). Experiments were done on cylindrical bars with a circumferential cracked notch loaded in shear (modes II, III and II + III) and torsion (mode III). Fractography analysis showed that the crack growth was crystallography-controlled and the deflection and twisting angles of crack propagation direction (with respect to the shear plane) were relatively high. This behavior is a mixture of previously observed morphology in ARMCO iron (bcc) and austenitic steel (fcc). It can be understood in terms of a number of available slip systems in bcc, fcc and hcp lattices.

Abstract: The aim of this paper is to summarize basic fatigue parameter values obtained for plain C30/37 and C45/55 class concrete specimens during dynamic tests. Selected approximation curves for relative mechanical-fracture parameter values over time − compressive cube strength, modulus of elasticity, effective fracture toughness and specific fracture energy − were used to determine the most accurate fatigue parameter values corresponding to the age of specimens at the time dynamic tests were performed. Three models are used for standard description of the S−N curve: the Weibull and Gumbel models developed by Castillo et al. as well as the power law equation. Fatigue properties were obtained from three point bending tests conducted on beam specimens (100×100×400 mm nominal size) with a central edge notch (depth 10 mm).

Abstract: Miniaturized Disc-Bend Test (MDBT), also called the Small Punch Test (SPT) is used for characterizing the mechanical properties of metals, when only a small volume of material is available. This study was dedicated to investigating the mechanical properties of AZ91 magnesium cast alloy. The casts were prepared via gravity sand casting and have sections with different wall thickness. The examined samples were cut out of 30 mm and 10 mm thick walls. The correlation between results obtained from the tensile tests and MDBT was determined.

Abstract: This paper presents the results of durability tests of AW7075 aluminium alloy, which is widely used in producing high-durable and particularly reliable constructions in aeronautical and automotive industries. The plain-strain fracture toughness, depending on the orientation of fracture direction with respect to the rolling direction of the material, and the fatigue crack growth rate were determined. Based on scanning electron microscope (SEM) studies the two stages of fatigue fracture are shown: the first is plastic striations and the second is phenomenon of connecting micropores that were formed around intermetallic second phase particles. As a results of research a model of fatigue crack for the second stage is presented.