Papers by Keyword: Fractography

Abstract: Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L1₂ γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

Abstract: The aim of the work was to find out the causes of cracking of the A120 crane track rail. Although the track was regularly inspected by defectoscopic techniques, after five years of operation, the rail was broken at the weldment. The elemental composition of the welded joint materials was verified and the mechanical properties were determined. A fractographic description of the fracture surface was performed and the structure of the material was evaluated. Several factors contributed to the formation of crack, the combination of which led to a rail failure. Defects of the welded joint, acting as notches, from which they initiated fatigue cracks, unfavourable microstructure of the weld metal, low toughness of the base material and operating environment and conditions were dominant.

Abstract: The aim of the current study is to perform a failure analysis on specimens extracted from 4 mm sheets of AZ31B, according to ASTM 4377G and 3 mm sheets of Cu99 alloys according to SR EN 1652:2000, that were joined together using FSW and FSW-IG processes. In order to promote the suitability and advantages of implementing FSW-IG processes in the automotive and aerospace industry, the papers authors performed destructive testing on the samples extracted, namely tensile strength test, following with topography analysis using scanning electron microscopy – SEM, combined with energy dispersive spectroscopy EDS, micro-hardness tests and fractography investigations. Results highlighted a considerable improvement in the ultimate tensile strength of the welded joint, a higher degree of deformability of the welded joint in the case of FSW – IG, compared to classical FSW; while maintaining the same process conditions and parameters. Experimental data is in accordant with automotive and aerospace imposed compliances, thus presented and discussed in the following scientific article.

Abstract: The metastable beta titanium alloy Ti-3Al-5Mo-7V-3Cr (Ti-3573) was used as experimental material in this paper. Different cold rolling reduction was conducted in this titanium alloy. The results show that the total elongation (A₅₀) reached maximum value of 16% after 30% cold rolling. The tensile strength reached maximum value of 910 MPa when the cold reduction increases to 50%. Microstructural evaluation suggested that the precipitation of α phases, deformation twin and stress-induced martensite is responsible for the enhanced tensile properties. Moreover, the TRIP/ TWIP deformation mode contributes to the improvement of the ductility of the titanium alloys.

Abstract: Problems lived with fatigue fracture for the safe design of the members and structures. In this study, rotating bending tests were performed to investigate the fatigue crack propagation behavior of induction quenched and tempered JIS S45C low carbon steel. Hardness distribution was checked by the Vickers hardness test machine and the microstructure in cross section and fracture surface were observed with an optical microscope and a scanning electron microscope. The depth of the hardened boundary was approximately 1 mm from the surface and formation into martensite occurred at the surface of the specimen. It was ascertained that fracture surface of notched specimens consisted of the five fracture types. In addition, the maximum stress intensity factor of fatigue cracks increased during rotating bending test on notched specimen. The relation between SIF and the fracture surface is discussed.

Abstract: Beta titanium alloys have several attractive features; this has resulted in this group of alloys receiving much attention since 1980’s. Among the attributes which distinguish them for their superiority over other structural materials are (i) high strength to which they can be heat treated, resulting in high strength to weight ratio (ii) high degree of hardenability which enables heat treatment in large section sizes to high strength levels (iii) excellent hot and cold workability, making them as competitive sheet materials etc. The standard heat treatment consists of solution treatment in beta or alpha plus beta phase field followed by aging. However, certain aging treatments can render the materials in a state of little or no ductility; the designer has to be aware of this behaviour and has to keep away from such treatments while working with the materials. Such unfavourable aging treatments may adversely affect not only the static properties such as reduction in area and elongation in a tensile test, but also dynamic properties such as impact toughness. Results of fractographic studies are in line with those of mechanical testing. The authors would present the foregoing analysis, based primarily on the wide-ranging researches they carried out on beta titanium alloy Ti15-3 and to some extent data published by researchers on other grades of beta titanium alloys. An attempt is made to explain the mechanisms underlying the embrittlement reactions that take place in beta titanium alloys under non-optimal aging treatments.

Abstract: Bending behavior of a new thermomechanically processed low-alloy steel containing 0.40 wt.% carbon has been investigated. The processing included laboratory hot rolling to 10 mm thick strips followed by direct quenching to different quench-stop temperatures followed by slow furnace cooling to room temperature stimulating hot strip mill processing. The final microstructures were upper and lower bainite with yield strengths of a ~700 and ~1200 MPa, respectively. Local microstructures were characterized using a field-emission scanning electron microscope, microhardness profiles were measured, and bendability was determined using three-point brake press bending. The minimum applicable punch radius for a defect-free bend was 28 mm (2.8 times thickness) for the high-strength lower bainitic microstructure, while it was much smaller, i.e. 20 mm (2.0 times thickness) for the lower strength upper bainitic microstructure. Fractographic examination of the cracked surfaces revealed a more ductile fracture behavior for the upper bainitic microstructure.

Abstract: In this study three-layered materials composed of one zinc layer between two magnesium layers were prepared. Diffusion at the Mg-Zn boundary leads to the formation of thermodynamically more stable, yet mechanically very brittle intermetallic phase. Homogenous distribution of the fine-grained MgZn₂ intermetallic phase in magnesium or zinc alloys has a positive effect on strength of these alloys. In a form of continuous thick layer stretching throughout the whole material, the phase may leads to deterioration of mechanical properties. However, the mechanism of fracture has not yet been sufficiently described. The Mg based materials with one layer of Zn were investigated in terms of chemical composition and mechanical properties and fractographic evaluation. The materials with 0.25 mm, 0.5 mm, 1 mm and 2 mm thick layer of Zn were processed via bidirectional hot pressing method at 300 °C and 500 MPa. The phase and chemical composition of prepared materials was characterized by XRD and SEM-EDS methods. The mechanical properties were evaluated based on the results of three-point bend test and fractographic analysis of fracture surface. The results showed formation of MgZn₂ intermetallic phase on the interface of Mg and Zn layers and solid solution of Zn in Mg. The results showed that the presence of Zn layer leads to improvement of mechanical properties when compared to pure Mg prepared at the same condition. The strengthening effect of solid solution and intermetallic phase may be the reason of the increase of flexural strength.

Abstract: For the corrosion resistance analysis of the heat exchanger plates (made of AISI 316L steel) the samples with visible damage was delivered. The major part of the surface damages was located at the place of "close proximity" (or surface contact) of individual plates. Some of delivered samples showed an unequal layers of sediments, which indicating a different flow velocities of operating fluid through the plates. At locations of the upper part of the plates with no sedimentation (high velocity flow), the most surface damage was detected in "near contact" areas as well as outside. On the other hand, the area of the lower part of the plates, where the sediment deposition was massive (lowest velocity flow), was observed the smallest surface damage. The results of the chemical composition analyses showed a lower amount of molybdenum and a higher amount of phosphorus in case of all samples. The contents of the key elements necessary for the corrosion resistance (chromium and nickel) were only just above the lower limit of the prescribed chemical composition interval. For detailed study of surface damage, selected defects were observed and documented using scanning electron microscopy. Localized damage showed intercrystalline failure of material with typical surface morphology degraded as a result of cavitation damage under hydrodynamic stress.

Abstract: This paper deals with applications of biomaterial in the human body. Each biomaterial is characterized by biofunctionality and biocompatibility [1]. The choice of biomaterial for medical applications is established on mechanical properties. Therefor the Ti6Al4V alloys, which properties are relatively low modulus, good fatigue strength, formability, machinability, corrosion resistance [2], are most widely used in biomedical replacements, implants, and prosthesis. Despite the excellent properties of the titanium alloy, endoprosthesis often fails and the hip replacement is necessary. Common causes are overloading and cracking, static or dynamic. Other causes of failure include injury, implantation failure, manufacturing inaccuracies, and non-compliance with the manufacturing process.