Papers by Keyword: Fractographic Analysis

Abstract: Aircraft accident investigations are paramount for the continuous improvement of aviation safety. As such, maximizing the amount of evidence that can be gathered from an accident site is critical. However, with composites being increasingly used for the construction of airframes, additional challenges are introduced into the handling of debris. Specifically, composites under combustion are known to release potentially noxious fibers and gases, posing a threat to individuals in the surrounding area. In response thereto, so-called fixant solutions (also referred to as hold-down solutions) can be used to minimize the release of inhalable fibers. Nonetheless, researchers have highlighted the risk for these solutions to interfere with the accident investigation process by masking, altering or even destroying fractographic features needed to determine the source or sequence of composite failures. Consequently, literature calls for more research focused on characterizing the influence of fixant solutions on the failure analysis of aeronautical composites. In this study, the impact of one form of fixant solution – namely, wetted water (i.e., water with a surfactant) – is evaluated. A [(0/90)]₈ woven carbon fiber/epoxy composite sample is damaged in tension (representing an aircraft accident-causing failure), burnt (simulating an accident site fire) and doused in a wetted water fixant solution. The fracture surface is subsequently evaluated via a scanning electron microscope (SEM) and the condition of fractographic features relevant to the failure analysis is qualitatively evaluated. Through the findings of the study, a better understanding of the impact of a fixant solution on the failure analysis conducted during an aircraft accident investigation can be obtained. Moreover, the results can be used to develop protocols related to the handling of burning composites aiming to maximize both, the safety of individuals involved as well as the evidence needed to conduct a thorough investigation into the causal factors of an accident.

Abstract: Polyethylene (PE) has an exclusive set of properties, such as good toughness and mechanical resistance as well as high flexibility, and ease conformation. However, when exposed to degrading agents, such as heat, humidity and radiation, macromolecular changes can be observed and consequently affect the PE. This work evaluated for the first time the thermomechanical and fracture behavior of low-density polyethylene (LDPE) subjected to degradation at different times of exposure to ultraviolet (UV) radiation. The changes induced in the chemical structure and the highlighted behavior were investigated through thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier-transformed infrared spectroscopy (FTIR), reticulation content, tensile test, and scanning electron microscopy (SEM). The results suggest that UV radiation modify the failure of the polymer by reducing its crystallinity and dramatically increasing its degree of crosslinking. These modifications impair the LDPE mechanical performance as well as its thermal stability.

Abstract: In this study, the effect of the β-Al₅FeSi phases on fracture surfaces in secondary AlSi7Mg0.3 cast alloys with common and higher amount of iron was investigated. Iron addition caused the formation of different Fe-rich intermetallic phases in aluminium alloys. Components made of secondary aluminium alloys commonly have a higher amount of such phases. Sharp needles as β-Al₅FeSi phase lead to initiate stress tension, thereby contributing to increased risk of micro-cracks formation on the fracture surfaces. To determine the effect of β-Al₅FeSi to fracture surfaces of AlSi7Mg0.3 cast alloy, SEM microscopy with energy-dispersive X-ray spectroscopy (EDX) was used to study the amount of needles phases, their morphology and violation wave. It was found that increasing Fe content increased the size and the number of Al₅FeSi phases. The fractographic analysis of fracture surfaces shows an increasing amount of cleavage fracture in materials with a higher amount of iron, too.

Abstract: Additive Friction Stir (AFS) has the potential for extensive future application in metal based additive manufacturing. Powder based AFS is specifically useful for fabricating functionally graded structures. But, the consolidation of powder inside the hollow tool used in this operation hinders the powder based AFS process. This problem could be resolved by Additive Friction Stir Processing (AFSP) while maintaining the key advantages of AFS. A 3D deposit structure of height 5 mm and width 64 mm was made from Al6061 alloy powder by AFSP. Mechanical properties like ultimate tensile strength, yield strength and micro-hardness of the deposit were evaluated in both longitudinal and transverse directions. The ultimate tensile strength and micro-hardness of the deposit were comparable to Al6061-O and there was a significant increment in tensile yield strength. Also, the isotropic nature of the deposit could be inferred from similar mechanical properties in the longitudinal and transverse direction. Dimple ruptures seen in fractographic analysis gave evidence to the ductile nature of the deposit.

Abstract: The paper shows that in the conditions of a deficit of high-quality metal charge in steels melting production, the effective use of waste from abrasive grinding in the composition of the metal charge is a promising method for reducing the cost of steel. The most rational solution for the disposal of metallurgical wastes may be the agglomeration of fine-grained and fine-dispersed materials using briquetting technology widely used in many countries around the world to produce multi-purpose briquettes.

Abstract: The tendency of the aluminum alloy D16 system Al-Cu-Mg to natural (NA) and artificial aging (AA) after hardening and high pressure torsion (HPT) under a pressure of 6 GPa at room temperature was studied using optical metallography, scanning electron microscopy, electrical conductivity tests. The dependences of microhardness and electrical conductivity were constructed depending on the exposure time. It is shown that heat treatment (HT) (hardening + HPT + NA) leads to an increase of hardening of the alloy D16 compared to standard HT (T4) in 2,3 times, at preservation of level of conductivity.

Abstract: There is a need to determine the stresses value for responsible designs in the oil and gas refining, petrochemical and chemical industries, at which the defect reaches its maximum size. This stress value can be taken as the limit state criterion for strength calculations of vessels and apparatus. The article presents a technique for determining the limit state criterion for carbonaceous and low-alloy steels using the example of steels C1020 and A 516-55 in the state of delivery. As a limit state criterion it is proposed to use the stress value corresponding to the microcrack formation moment. The studies are based on the uniaxial tension tests results at room temperature of flat samples with V-shaped stress concentrators of 2 mm deep, accompanied by an acoustic emission nondestructive testing method. Based on the obtained pulse amplitude and pulse energy dependences on time in the process of testing and fractographic studies, a technique for estimating acoustic emission signals was developed to determine the microcrack formation moment for test steels. Stress values that can be taken as the limit state criterion for steels C1020 and A 516-55 in the delivery state for strength calculations vessels and apparatus are determined using the developed experimentally technique based on the high-amplitude acoustic emission to low-amplitude signals ratio. With the help of specimen fractures fractographic analysis the legitimacy of using the developed technique is justified. Thus, at room temperature for steel C1020, the stress at which a microcrack begins to form is 420 MPa, for steel A 516-55 it is 455 MPa.

Abstract: The austenitic cladding of the WWER pressure vessel is made from two different layers with different fracture toughness values. Based on the fractographic analysis of the tested specimens in the initial, as well as in the irradiated conditions, it was found that individual failure micromechanisms take place during the crack propagation. The obtained results were used to find the relationship between the failure micromechanism changes and the fracture toughness values, as well as to assess the effect of neutron irradiation on the failure micromechanisms.

Abstract: The specific desired properties for structures and components working in critical environments (e.g. different structure parts of power plants) require current information about degradation processes coming out in materials. Obtaining of this information by the help of the classical tests of mechanical properties (tensile test, Charpy test, fracture toughness test, creep test etc.) is very limited namely in the case of nuclear power plants pressure vessel. Hence, the new innovative techniques based on miniaturized specimens have been developed for evaluation of mechanical properties and their changes. One of very promising techniques is Small Punch Test. Present paper deals with characterization of three different steels (15Ch2MFA, 10GN2MFA with different heat treatment and steel O8Ch18NT10 with various degree of deformation).

Abstract: This article deals with testing of fatigue properties of hardened gears at bending. These tests are defined by the standard ISO 6336 - method B, when testing is carried out under reference conditions close to the real operation. This type of testing may be helpful to evaluate the influence of new materials, design parameters, production technology, or the effects of surface finishing. The first part of this article deals with the theory of fatigue testing methodology using modified stair-case method and characteristics of the testing machine. The second part of the paper is devoted to the characteristics of the experimental material and the results achieved. The case hardened steel gears alloyed by Cr-Ni-Mo were selected for the experiment. Hardened gears were subjected to local hardness measurement and subsequently the bending fatigue tests were performed. Ruptured samples were analyzed using a scanning electron microscope. The local bending stress was determined using the modified computational model based on the ISO 6336 standard methodology. The focus of this work consisted in evaluation of the influence of the filler root surface treatment on fatigue limit at bending on the basis of the realized experiments and their confrontation with the equivalents specified in the standards ISO 6336 and ČSN 01 4686.