Effects of Pre-Tensile Deformation on the Fatigue Fracture Behavior of Annealed 7005 Aluminum Alloy Plate

In the present study, the fatigue life and fatigue fracture characteristics of annealed 7005 aluminum alloy plates subjected to different pre-tensile deformations were investigated. The results obtained upon increasing the pre-tensile deformation of the alloy plate to 20% revealed that the second-phase particles did not show any obvious changes, and that the thickness of the thin strip grain slightly decreased. The dislocation distribution in the alloy matrix varied significantly among the grains or within each grain as the dislocation density gradually increased with increasing pre-tensile deformation. Moreover, the fatigue performance of the annealed 7005 aluminum alloy plate was significantly improved by the pre-tensile deformation, and the alloy plate subjected to 20% pre-tensile deformation exhibited an optimal fatigue life of ~1.06 × 106 cycles, which was 5.7 times and 5.3 times that of the undeformed and 3% pre-stretched alloy plates, respectively. Two fatigue life plateaus were observed in the pre-tensile deformation ranges of 3–5% and 8–12%, which corresponded to heterogeneous dislocation distribution among various grains and within each grain, respectively. Moreover, two large leaps in the plot of the fatigue-life–pre-tensile-deformation curve were observed, corresponding to the pre-tensile deformation ranges of 5–8% and 16–20%, respectively.

Effects of ECAE process on the fatigue crack growth rate of copper metal

In this study, the effects of the number of passes performed by the Equal Channel Angular Extrusion as a severe plastic deformation process on copper metal's microstructure and mechanical properties, especially its resistance to fatigue crack growth, have been investigated. The experimental results show that as the number of processes passes increases, the copper metal grains become finer and as a result less stress is concentrated at the starting points of the fatigue fracture, which delays the fracture. For example, after performing 8 ECAE process passes, the threshold values of fatigue crack growth increases by 113.2% relative to the base metal. Moreover, as the grains become smaller, the number of grains and consequently the number of grain boundaries will increase and thus more obstacles will be placed in the way of crack growth. Also, the SEM images indicate that many fine and equiaxed dimples in processed copper become smaller as the number of passes increases. This shows that finer and more equiaxed grains will be obtained by repeating the ECAE process and thus repeating the occurrence of recrystallization. It was cleared that this process improves the mechanical properties of the copper other than the failure strain. However, by increasing the number of process passes, this problem can be significantly reduced. Highlights The fine grains considerably delay the fatigue fracture By ECAE process, the threshold value of fatigue crack growth increases by 113.2% All zones resulting from fatigue fracture are recognizable in fractured ECAE sample The SEM images indicate that a ductile failure has occurred in the tensile samples

MICRO-CRACK ANALYSIS IN THE STUDY OF FATIGUE FRACTURE IN THE HAZ IN THE HIGH YIELD STRENGTH MICRO-ALLOYED STEELS

Various applications have been described in the literature for the High-Strength and Low-Alloy steels (HSLA) industry, analysing their use both in industrial and marine equipment and machines and in structures that require appropriate resilience values and toughness at low temperatures. For successful operation under conditions as large structures under extreme service conditions, it is essential to ensure the proper toughness both in base metal (USITEN 355 0.5 Ni Grade I steel) and in the heat-affected area of the weld. (ZAC). This research carries out Crack Tip Opening Displacement (CTOD) tests, showing, in this article, the first part of the test corresponding to fatigue pre-cracking and a summary table of the results of fracture toughness, to guarantee that, under the conditions which exist in welding, both the fatigue fracture values and the fracture toughness are acceptable by the applicable standards. Keywords: SMAW, weld line, CTOD, stress intensity factor, input heat energy, crack growth rate, fatigue fracture, fracture toughness

Evaluation of the threshold for the development of fatigue cracks in a railway steel under harmonic and operational loading

The results of experimental studies of the fatigue crack development in 20GFL steel specimens cut from a cast bolster of a freight car are presented. The ratio of the threshold stress intensity coefficient Kth determined from the kinetic diagram of fatigue fracture and from the average parameters of the operational loading process is considered using the experimental results with a simulation of operational loading. Tests were carried out upon the development of permanent blocks of crack opening in the specimen (in a rigid loading mode). The operational process is presented in the form of a block of consecutive loading cycles recorded during the test of the car frame in conditions typical for a straight section of the railway track. The threshold operational level is determined by the algorithm of gradual reduction of the loading similar to the original process. The regularities in a decrease of the rate of crack development and corresponding decrease in the load were determined. Subsequent extrapolation of the obtained experimental regularities to zero value of the crack propagation rate provided estimation of the threshold loading level, similar to the initially specified value. It is shown that the value of the threshold level of the fatigue crack development in low-alloy steel 20GFL obtained from the fatigue fracture diagram (i.e., under harmonic loading) is significantly higher than that obtained from the estimate based on the average values of the operational loading process. The considered model of operational loading gives greater damage compared to harmonic loading, on the basis of which the survivability of structural elements is usually assessed.

Post-Fatigue Fracture and Marginal Behavior of Endodontically Treated Teeth: Partial Crown vs. Full Crown vs. Endocrown vs. Fiber-Reinforced Resin Composite

Objectives: To investigate in vitro post-fatigue fracture behavior of endodontically treated molars having been differently restored. Methods: A total of 120 extracted human molars were used. A total of 120 specimens in 14 test groups and one control group (n = 8) were root canal treated. After postendodontic sealing and build-up (AdheSE Universal, SDR), additional MOD preparations were cut. Postendodontic restorations were: Direct restorations (Tetric EvoCeram BulkFill bonded with AdheSE Universal and EverX Posterior/Essentia bonded with G-Premio Bond; as filling (F) or direct partial crown (DPC) after reducing the cusps 2 mm; indirect adhesive restorations (partial crown/PC vs. full crown/FC): e.max CAD, Celtra Duo, both luted with Variolink Esthetic; indirect zirconia restorations (partial crown/PC vs. full crown/FC), luted with RelyX Unicem 2; indirect non-bonded cast gold restorations (partial crown/PC vs. full crown/FC; Degunorm), luted with Ketac Cem. Before and after 300,000 thermocycles (5/55 °C) and 1.2 Mio. A total of 100 N load cycles, replicas were analyzed under a SEM for marginal quality in enamel and dentin (where applicable) and finally, specimens were loaded until fracture. Results: In direct groups, there was no difference between RC and FRC in fracture strength (p > 0.05); however, direct partial crowns showed higher post-fatigue fracture resistance. Regarding marginal quality, intracoronal FRC restorations exhibited more gap-free margins in enamel than RC. In the indirect groups, there was no significant difference between partial and full crowns in any of the adhesively luted ceramic groups regarding post-fatigue fracture resistance. Zirconia partial crowns exhibited significantly lower marginal quality in enamel. Indirect groups performed significantly better than direct groups in fracture resistance. Within the indirect restorations, both cast gold groups and zirconia full crowns exhibited the highest fracture resistance being superior to control teeth. Significances: Within the limits of this in vitro investigation, it can be concluded that any kind of indirect restoration with cusp replacement is suitable for ETT restoration when a certain cavity extension is exceeded. All indirect restorations, i.e., endocrowns, partial crowns, and full crowns showed a promising performance after in vitro fatigue-loading.