Papers by Keyword: Failure

Abstract: As part of the project “Thermostat for universal use in (electro) mobility”, an analysis of three currently used thermostats failed during operation was performed. In all three cases, cracks occurred in the brass outer casings. The cracks were open and fractographical observation was performed. The microstructure of the material was evaluated using both the light and scanning electron microscopy. The local composition of the material was determined by EDS microanalysis. Furthermore, hardness profiles were measured. The cracks were predominantly intergranular with a smaller portion of transgranular cleavage. The microstructure was formed by a mixture of α- and β-phase grains and lead particles. In addition to the stress caused by the overpressure of the molten wax, a higher level of residual stresses caused by deformations can be expected. The failure was caused by the mechanism of stress-corrosion cracking. Metal induced embrittlement or/and corrosion fatigue could interact too.

Abstract: One of the principal failures of the supporting and working rolls on the Quarto rolling system is the spalling of their surfaces due to the gradual accumulation of defects in contact volumes of a material. Nowadays to prevent such kind of degradation failures it is necessary to check out the stationary condition of the contact strength, which doesn’t answer to the question of the duration of the roll staying in the working state, i.e., its resource (lifetime). In the article a new physical-analytical model of the roll’s failure is suggested by the criterion of the surface layer fracture and the corresponding methodology of the expected resource calculation. The approach is based on the kinetic concept of damage of solid bodies, the energy (thermodynamic) condition of fracture of solid bodies and the fundamentals of reliability forecasting for technical objects. The feature of the suggested methodology is that the definition of the failure moment for the supporting or working rolls of the Quarto system derived from the condition of the current density of defects in surface layer of a material reaching the critical value, which is the function of the enthalpy of a material melting..

Abstract: This research analyzed the causes of failure in the leaf spring of a cold diesel Dump Truck. The test was conducted in the form of hardness using the Rockwell method and a hardness value of 104.8 HRB while microstructure analysis was used to view and identify fracture surfaces through SEM and optical microscope. Furthermore, the distribution of each spring load was determined by analytical analysis and found to be 28983 N while the chemical composition was analyzed by comparing the spring microstructure with the AISI 5150 standard. The finite element analysis conducted with the FEMAP software showed the maximum stress and strain values in the leaf spring were recorded on the left and right sides. The stress intensity factor (K_I) around the end of the initial crack was also found to be greater than the fracture toughness of the material (K_IC), thereby, causing the initiation and propagation of cracks in the leaf springs. It was, therefore, concluded that the failure in the spring was due to an initial defect caused by the dynamic load experienced by the spring, thereby, leading to the initiation and propagation of cracks up to the final fracture.

Abstract: This study presents a numerical analysis of the tube expansion process by conventional tube-end forming versus single point incremental forming (SPIF) using DEFORM. The work includes the assessment of the strain paths within the principal strain space of these processes with respect to the formability limits as well as their evaluation within the equivalent strain versus stress triaxiality space. The results obtained demonstrated that the mechanics of tube flaring process in conventional and incremental forming are substantially different. This analysis of formability in the light of the accumulated equivalent strain and the average stress triaxiality allowed a better understanding of the differences between both processes in terms of the fracture limit strains.

Abstract: Warm deep-drawing of pre-aged (under-aged) blanks of 7xxx series aluminum alloys (Al-Zn-Mg) at moderate temperatures of roughly 120–230°C is a promising route for producing parts with considerable geometrical complexity, good paint bake hardening response, and, thus, excellent final mechanical properties. Furthermore, oil-based lubricants can be used, eliminating the need for elaborate cleaning routines. However, finite element (FE) simulation of the process is challenging: time-temperature regimes during coupon testing for material cards should closely follow the real conditions in the press because the material undergoes significant changes at warm-forming temperatures, such as recovery and precipitation/coarsening/reversion of hardening phases. When convective heating is used for Nakajima or tensile testing, heating rates are usually too low to adequately represent real process conditions (where inductive or contact heating may be used). Here we present a method for establishing FE material cards and calibrating the GISSMO damage model using miniaturized tensile specimens for a dilatometer with inductive heating. The simulations are compared with warm deep-drawing experiments of pre-aged 7xxx and good agreement of minimum draw temperature for two alloys is achieved. The findings are discussed with regards to transmission electron microscopy investigations and final mechanical properties published earlier. It was found that warm-forming is suitable to produce complex 7xxx parts with high final strength. Conditions in the press can be represented by using miniaturized tensile specimens and inductive heating for calibration of material cards/damage models.

Abstract: With the goal to define a cost-effective and efficient process to identify adequate materials for sheet metal forming processes, it is crucial to evaluate the formability of materials. Forming limit curves (FLC) are used to analyze the forming and failure limits of sheet metals and dependence of the major (φ₁) and minor strain (φ₂) from the uniaxial stress-strain area through the plane-strain point to the biaxial strain area. According to ISO 12004-2, the FLC is performed by Nakajima or Marciniak tests. Due to the experimental setup and the preconditions, pre-stretching occurs in the specimens and bending and friction effect are the result. The determination of the onset of necking (FLC) results mathematically from a “best-fit inverse parabola” on section lines. In addition, the failure point, i.e. the maximum strain value one frame before failure, is also analyzed. In contrast, tensile, notched tensile and hydraulic bulge tests, which together have a potential to map an alternative FLC, exhibits a linear strain path evolution. The behavior of the various strain paths of Nakajima and the alternative methods are examined for necking and cracking. Furthermore, the fracture surfaces are investigated by confocal laser scanning microscopy to identify influences of the different FLC methods on the fracture mechanics. FLCs were conducted with the Nakajima and the alternative FLC characterization method for a ductile steel (DX54D). To ensure transferability, the tensile tests are also performed with a high-strength steel (DP800). The FLC of the ductile steel, generated through the alternative method, exhibits a similar shape to the Nakajima generated FLC with the advantage of a constant strain rate leading to linear strain paths and a lower number of tests. The same results are achieved for the uniaxial strain tests with DP800.

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: This paper aims to review the methodologies used to conduct microstructure evaluation of the photovoltaic (PV) interconnection. This analysis is important to identify the microstructural properties of the interconnection for failure analysis purposes. The interconnection becomes a major concern towards the efficiency and reliability of PV technology. In this paper, the common techniques used for the interconnection technology such as soldering, conductive adhesive and ultrasonic were presented with the assessment method to identify the failure mode and failure mechanism at the bonding interface. The identification of the failure mode and failure mechanism through visual analysis and conformation of failure phenomenon was important to highlight the risks and develop the countermeasures. The evaluation of microstructure characterization techniques in the electronics and PV industry has been presented by identifying the outcomes of each technique with different reliability tests. The discoveries of failure analysis in the electronics industry were more matured and becomes the reference to the PV development. The outcomes from this review could be beneficial to improve the interconnection bond in the PV industry by eliminating or minimizing the failure through design modification at the earliest point in the development process.

Abstract: Aiming at the reliability of thin-film thermocouples applied to turbine blades at high temperatures, combined with high-temperature tests and finite element analysis, this paper studies its failure mechanism and thermal stress under thermal load. Multi-layer thin-film thermocouple samples were prepared on ceramic substrate, and high-temperature tests were carried out under different temperature loads, and the phenomenon of film shedding and cracking was observed using electron microscope. This paper analyzes the failure mechanism of the film sensor based on the function and structure, and uses ANSYS to analyze the thermal stress distribution of the film under high temperature load. Combining several existing theoretical models, this paper analyzes the factors affecting the thermal stress of the film and conducts simulation verification.

Abstract: The subject of the article is the analysis of the design of concrete members for shear from the point of view of theory and code regulations. These give formulae for verification of shear resistance and recommended limits of individual parameters, respectively. The authors explain the relevant formulae using a Strut and Tie analogy, especially the effect of the inclination of struts and the inclination of stirrups on the shear capacity of beams as well as the effect of a single concentrated force acting at distance up to 2d from the edge of support.