Post-failure Analysis Research Articles

Failure Analysis of a Centrifugal Compressor Impeller Made of 17-4PH Steel in the Moist Hydrogen Sulfide Environment

The impeller under consideration is a rotor wheel of a centrifugal compressor, made of 17-4PH martensitic precipitated hardening stainless steel. Its total operating lifetime was estimated to 80,000 h, but it fractured beforehand, 4 days after the scheduled overhaul without any observed pre-symptoms. The important causes of the failure were its working conditions: a moist H2S environment, the applied shaft fitting stress, and the martensitic structure of its material. In the article the post-failure analysis of an impeller is described, explaining the root cause of the impeller failure by means of both experimental and Finite Element Methods (FEM). Sulfide Stress Cracking (SSC) was found to be the primary failure mechanism for the impeller under investigation. Details of the investigations and possible corrective measures to improve performance impellers used under harsh environment conditions are discussed.

Investigation of SiC Trench MOSFETs' Reliability under Short-Circuit Conditions.

In this paper, the short-circuit robustness of 1200 V silicon carbide (SiC) trench MOSFETs with different gate structures has been investigated. The MOSFETs exhibited different failure modes under different DC bus voltages. For double trench SiC MOSFETs, failure modes are gate failure at lower dc bus voltages and thermal runaway at higher dc bus voltages, while failure modes for asymmetric trench SiC MOSFETs are soft failure and thermal runaway, respectively. The shortcircuit withstanding time (SCWT) of the asymmetric trench MOSFET is higher than that of the double trench MOSFETs. The thermal and mechanical stresses inside the devices during the short-circuit tests have been simulated to probe into the failure mechanisms and reveal the impact of the device structures on the device reliability. Finally, post-failure analysis has been carried out to verify the root causes of the device failure.

Methodology for simulation of the post-failure behavior of flexible catenary risers

New discoveries have been constantly pushing the use of marine risers to harsher environments, moving to ultra-deep waters. However, even with recent advances in the analysis and design of risers, these structures still fail. The rupture of a riser often implies high environmental and economic costs and can compromise the lifespan of neighboring structures, such as other risers, anchoring systems, and submarine equipment. This work aims at performing post-failure analysis of flexible catenary risers to understand their behavior after the rupture occurs. The dynamic buckling phenomenon is discussed and the influence of important parameters such as element length, time step, tangential drag coefficient, soil stiffness, and slenderness is assessed and discussed. This paper also shows that the hysteretic nonlinear bending stiffness has a great influence on the falling riser dynamics. Results are shown in terms of the fall time, and the riser accommodation on marine soil. A statistical analysis using random waves is also performed to assist in understanding the problem and in determining a safe region around the riser to avoid impact with other elements of the submarine layout. The proposed methodology provides important results for damage mitigation in case of accidents in future riser designs.

Identification of uranium hexavalent compounds using X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) has applications in many fields ranging from development of thin films for semi-conductors to post failure analysis of organic coatings and structural adhesives. The current work expands on that versatility by applying XPS to the growing field of nuclear forensics. This was achieved by the synthesis and characterisation of several uranium compounds, predominantly in the hexavalent state associated with the nuclear fuel cycle, and by X-ray diffraction and Raman spectroscopy analysis prior to XPS. Spectral characteristics for each compound are discussed, and interpretations made through observations in the binding energy of the U4f region as well as secondary energy loss features such as shake up satellites. The interpretation of such features is related to the stoichiometry, oxidation state and bonding structure of a range of uranium compounds. As XPS is typically insensitive to structural (crystallographic) variations, a rationale is provided for the relationship between structural variations, as measured by Raman and X-ray diffraction and compared to the open literature, and the XPS satellite to parent peak intensity of uranium compounds, providing a novel and useful approach for uranium compound characterisation. In addition to the novel approach described, Wagner chemical state plots have also been generated to provide another comparison tool.

An experimental study on the mechanical behaviour of bonded and hybrid bonded-bolted composite joints using digital image correlation (DIC) technique

Most studies usually use load-displacement curve or post-failure analysis to understand the mechanical behaviour of hybrid bonded-bolted (HBB) joints. While load sharing in HBB joints is the crucial design parameter and it is important to capture strain field changes or failures under real-time loading conditions. Therefore, two-dimensional digital image correlation (2D-DIC) technique is applied to experimentally investigate the behaviour of bonded and HBB composite joints under quasi-static tensile loading. A flexible epoxy paste adhesive is used. The bonded joint had a hole in the centre to determine the effect of the difference between the stress concentration due to the existence of the hole and the additional stress around a hole due to installation of the bolt. The strain fields around the bolt in a HBB joint and around the open hole in a bonded joint were compared accurately. The DIC technique was able to effectively and rapidly measure the strain field and identify the onset of failure around the fastener during the tests. Finally, the advantages and disadvantages of the DIC technique compared to the classical similar techniques for characterizing the composite joints are briefly explained.

Effects of core splice joint width on the performance of composite sandwich structures with honeycomb core: An experimental study

Composite sandwich structures are commonly considered in large-scale aerospace applications due to their performance on a per mass basis. The nature of a large-scale sandwich structure generally necessitates the use of multiple sections of core to fill out the structural form. These core sections must be spliced together to ensure that shear loads are appropriately transmitted throughout the core. Because core installation in a large-scale component is a challenging operation, core splice joint width can be difficult to control in manufacturing. As such, the effects of core splice joint width on sandwich structure performance should be well understood. This study examines the effects of core splice joint width in honeycomb sandwich structures via mechanical testing and post-failure analysis. A threshold core splice joint width is shown to exist with respect to core shear, while the integrity of the facesheet-to-core interface is shown to degrade with increasing core splice joint width.

Bearing Strength and Failure Mechanisms of Riveted Woven Carbon Composite Joints

This research aimed to determine riveted carbon/epoxy composites’ mechanical performance when fabricated by resin transfer molding (RTM). As this manufacturing process is gaining importance in the aeronautics and automotive industries, assembly methods and their reliability must be studied in terms of their airworthiness and transportation implementation. The study case resumes the determination of the bearing strength of RTM-woven carbon composites for different rivet joint diameters (1/8, 5/32 and 3/16 in). The joint shear strength was obtained following the ASTM D5961 instructions, and post-failure analysis was carried out by a computerized tomography scan. A residual strength curve is provided with the results to infer the bearing strength for the riveted composites as a function of the rivet width-to-diameter ratio. A discussion of the fracture mechanism and tensile strength is carried out to assess the understanding of the riveted woven composites.

Quantitative assessment of the hydrogen induced cracking for 2.25Cr-1Mo-0.25V steel under electrochemical charging conditions

Hydrogen induced cracking (HIC) sensitivity of a vanadium-modified high strength steel at room temperature was studied by hydrogen concentration measurement, post-failure analysis, and interface characterization. It was found that the HIC sensitivity was relatively low as indicated by the small number of surface blisters and the random occurrence of internal cracks. This is partially attributed to the low hydrogen pressure in the defects, resulting from the high hydrogen solubility of the material. The immune inclusions with no cracks were identified after severe charging condition featuring more intact interface, indicating that the integrity of the interface and the size of voids may control the crack initiation.

Postfailure Analysis of Slopes by Random Generalized Interpolation Material Point Method

Abstract In this paper, the generalized interpolation material point (GIMP) method will be utilized to simulate the postfailure behavior of slopes that accounts for spatial variability. Because the...

Multimaterial laminated composites: An assessment of effect of stacking sequence on flexural response

Abstract Various industries are aiming to develop low weight-high performance materials especially in structural and automotive applications. Fiber metal laminated composite materials are the one such material which meets the global requirement and are in a position of replacing current material. Metallic and composite material combinations owe to enhance the performance. In present research work, two types of tin sheet/glass fiber reinforced laminated composites were prepared such as Sn-G4-Sn and Sn-G2-Sn-G2-Sn. The current research work reveals the effect of stacking sequence on the mechanical behaviour of tin sheet/glass fiber laminated composites. The flexural properties of the laminated composites have been evaluated. The results revealed that there is no significant variation in flexural strength; however, Sn-G2-Sn-G2-Sn laminated composite exhibited superior performance in flexural modulus with an improvement of 52.97%. Additionally, to understand the various failure modes in the laminated composites post failure analysis was carried out using stereo microscope. Macrographic/Post failure analysis revealed that delamination between the interface of the Sn and composite layer was dominant in experimented laminated composites.

Post-failure analysis of the Maierato landslide using the material point method

The 15 February 2010 Maierato landslide has become the symbol of the hydrogeological instability of the Calabria region (Southern Italy). It occurred after a long period of rain in an area prone to landslides. Some videos filmed during the event documented the high velocity and the flow-like character of this landslide after the slope failure. Several studies focused on the pre-failure and failure stages of the Maierato landslide in order to interpret its failure mechanism and to establish the main factors of triggering. However, none of these studies analysed the post-failure stage of this landslide. In the present study, the Material Point Method (MPM) is used to simulate the run-out process of the Maierato landslide and provide a possible explanation of this process. Different scenarios are considered and for each of them the simulated post-failure configuration of the displaced material is compared to that detected after the event. Numerical results match fairly well the observed ones when a severe reduction in the shear strength of the involved soil is accounted for in the analysis. This strength reduction is attributed herein to the high excess pore water pressures generated by the large distortions of the moving soil mass. Moreover, the back-calculated post-failure strength is consistent with that found in several case histories of slope failures involving liquefied materials.

Electro-Thermo-Mechanical Reliability of Recessed Barrier AlGaN/GaN Schottky Diodes Under Pulse Switching Conditions

Pulse reliability of AlGaN/GaN recessed Schottky diode is studied under transient overstress conditions, typically encountered in power converters. Degradation of the Schottky diode during both free-wheeling operation (high forward current injection) and reverse blocking state (high-voltage stress) is studied. Defect generation and the associated degradation were found to be uncorrelated with the nature of interface formed due to various surface treatments (at metal/GaN Schottky interface). During forward conduction, trap-assisted Schottky interface degradation is studied using on-the-fly ${I}$ – ${V}$ and ${C}$ – ${V}$ characterization under high-current stress. Under high-voltage stress, in the reverse blocking mode, mechanical strain evolution and defects generation were found to be dominant degradation modes, which are studied in detail using on-the-fly micro-Raman spectroscopy. Post-failure analysis was performed using SEM, TEM, and EDX, which reveals distinct failure signatures at the safe operating area (SOA) boundary. TCAD simulations are used to gain deeper physical insights into the observed degradation mechanism. Finally, a qualitative failure model, explaining the distinct failure physics, is presented based on observations and findings, from various electrical, optical, Raman spectroscopy, and electron microscopy investigations.

Dry sliding wear behavior of Inconel 718 additively manufactured by DMLS technique

PurposeAerospace and defence industries use the materials having better properties at elevated temperatures, and Inconel 718 is one of that. The complexity in realizing complex and intricate shapes necessitate the product realization through additive manufacturing. This paper aims to investigate the wear behaviour of additive manufactured material.Design/methodology/approachThe wear behaviour of additively manufactured Inconel 718 samples through direct metal laser sintering process at three different build orientations was experimentally investigated using a standard pin-on-disc wear tester.FindingsAmong the varied wear parameters, the load was identified as the most influencing parameter on the wear rate. In addition, the post-failure analysis of the worn surface of the pins under the scanning electron microscopy revealed the presence of various wear mechanisms.Originality/valueAlmost, the industries are now focussed on their production through additive manufacturing owing to its advantages. The present work displays the wear behaviour of the additive manufactured Inconel 718 and its associated wear mechanisms.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2019-0322.

Surface topography analysis based on fatigue fractures obtained with bending of the 2017A-T4 alloy

This work deals with the post-failure analysis of the fatigued fracture in the context of surface topography parameters. The influence of material isotropy on fatigue fractures using metrological methods is presented. The study was carried out on fracture surfaces of different maximum stress at the notch root σmax and stress ratio R. Surface topography and fracture measurements were made using the Infinite Focus technique with the use of the changing the focus, which allowed the collection of data sets. The analysis of the geometric product specification was performed by means of height parameters according to ISO 25178, fractal dimensions as well as surface isotropy. Paper presents that higher values of skewness were received for the stress ratios R = −1, −0.5 than for R = 0, while lower kurtosis values for the stress ratios R = −1 and −0.5 than for R = 0 were observed.

Modular application relevant stress testing for next generation power semiconductors

New applications such as renewable energy sources demand reliable operating lifetimes beyond 15 years. To validate the robustness of power semiconductors, it is paramount to investigate the effects and interactions of various failure and degradation mechanisms. In particular, the degradation of power semiconductors under application relevant stress test conditions until their end-of-life is of interest. The difficulties with such application relevant end-of-life testing is that test systems are very complex and costly. In addition, it is difficult to protect the devices under test sufficiently for post failure analysis without interfering too much with the operation itself. This paper presents a modular test system that can be configured to apply different operating scenarios and apply application relevant stress to the devices under test. Within the presented system, numerous safety features have been implemented to minimize the damage to a failed device under test so that it may undergo further analysis. By making applications relevant stress testing feasible and capable of end-of-life testing, we hope to contribute to making future power electronic system more reliable.

Bimodulus-plastic model for pre-failure analysis of fiber reinforced polymer composites

Fiber-reinforced polymer composites, such as carbon-epoxy composites, are found to exhibit non-linear behavior when mechanically loaded in the transverse and shear directions. Experimental studies suggest that the non-linear behavior is due to two mechanisms: (i) damage in the matrix in the form of cracks and (ii) yielding of the matrix followed by plastic deformation. In this study, a bimodulus-plastic model that includes these two different damage mechanisms to simulate the non-linearity prior to failure is proposed. The pre-failure, onset of failure and post-failure analysis with the proposed model is discussed in detail with emphasis put on the transverse and shear components. The process of determining the material properties and parameters required for defining the new model is discussed and demonstrated. The newly developed model is then validated against the experimental results from three-point flexure tests on the composites. The correlation was good showing that the proposed model was able to simulate accurately the non-linear behavior of the composites and thus predict the failure. Finally, the model is applied to a case study on the failure of a composite riser under internal pressure loads.

Influence of long-term operation of supercritical carbon dioxide based enhanced geothermal system on mineralogical and microstructurally-induced mechanical alteration of surrounding rock mass

Chemico-microstructural and mineralogically-induced time-dependent mechanical alterations of the reservoir rock found in the peripheral zone of a CO2-based EGS is experimentally investigated. A series of uniaxial compressive and triaxial (under 30 MPa constant confining pressure with different temperatures; 25-300 °C) strength tests on granite specimens reacted in ScCO2+water medium for different time periods was performed by coupling acoustic emission (AE) technique. The results were compared with those for intact and water-reacted rock specimens in order to better understand this weakening phenomenon. Scanning electron microscopy (SEM) was also incorporated to evaluate the microstructural and mineralogical alterations.The results indicated that the mineralogical structure of Harcourt granite rock specimens was mainly altered by the dissolution of silicate minerals and plagioclase phase feldspar minerals. In addition, enhanced precipitation of secondary clay minerals including kaolinite, smectite, and illite was observed in the ScCO2+water-reacted rock specimens. The average compressive strength of ScCO2/water-reacted granite specimens found to be reduced with the increase of reaction period. Furthermore, 3.81%, 5.08%, 5.66% and 0.52% reductions in failure strength resulted under 30 MPa confining pressure in 6 months ScCO2/water-reacted granite specimens at 25 °C, 100 °C, 200 °C and 300 °C temperatures, respectively, compared to intact rock values at these temperatures. The post-failure analysis also revealed that the ScCO2/water reaction significantly alters the post-failure behaviour due to micro-mineralogically-induced weakening inside the rock pore structure. It was concluded that enhanced mineral precipitation and dissolution influence in time-dependent degradation of the surrounding rock mass which influence in long-term operation of the geothermal system.

<italic>In situ</italic> Condition Monitoring of IGBTs Based on the Miller Plateau Duration

Aging precursor identification is crucial to achieving online condition monitoring and estimating the remaining lifetime of insulated-gate bipolar transistors (IGBTs). However, existing failure precursors are limited with respect to in situ monitoring. In this paper, the duration of the Miller plateau during the IGBT turn- on transition is proposed as an online precursor indicating two dominant types of failures. Based on the theoretical closed-form expressions, the adverse effects of package-related bond wire fatigue and chip-related gate oxide degradation on the Miller plateau duration are explained. By using a dedicated online measurement system that is implemented on the gate driver side, the proposed precursor is extracted during operation. Discrete IGBTs of two gate structures are employed during the accelerated aging tests. Negative and positive degradation trends of the Miller plateau duration under power cycling and high electric field stress, respectively, are revealed, thus illustrating the validity of the proposed method. Finally, the main differences in physics-of-failure between two types of aged devices are investigated using post-failure analysis. The innovation of applying the Miller plateau duration as a failure precursor involves its ability to accomplish real-time field monitoring and its sensitivity to multiple failure mechanisms.

Effects of subcritical and supercritical CO2 sorption on deformation and failure of high-rank coals

This paper presents the results of an extensive experimental analysis aimed at establishing the effects of subcritical and supercritical CO2 sorption on deformation and failure of coals. Two high-rank anthracitic coals from the South Wales coalfield, obtained from different locations and depths of 150 m and 550 m, are employed for that purpose. The investigations include i) determination of unconfined compressive strengths and elastic moduli of the cores both non-saturated and saturated with CO2 at 2.1 MPa, 4.3 MPa and 8.5 MPa, ii) assessing the dependence of the parameters obtained on CO2 pressure, iii) analysing the effect of CO2 saturation on failure patterns of the samples tested and iv) determination of the particle size distribution after the failure of the samples. Based on the results of twenty coal specimens tested, it is demonstrated that CO2 sorption reduces the uniaxial compressive strengths and elastic moduli by between 29% and 83% for the range of pressures studied. The reductions observed increase gradually up to 4.3 MPa and then reach a plateau. By accommodating the effect of effective stress on compressive strength values, it is shown that chemical weakening of high rank coals is mostly associated with sorption of subcritical CO2, with negligible impact of supercritical CO2 on further parameter reduction. Inspection of failure patterns during uniaxial compression suggests that non-saturated coal specimens fail through axial splitting with rapid crack propagation and high outburst of coal pieces while the failure of cores subjected to CO2 injection occurs through multiple fractures with negligible material outburst. The post-failure analysis demonstrates that CO2 treated samples disintegrate on smaller particles than non-saturated specimens, as up to 5.6 more CO2 saturated coal pieces passed through the sieves considered in this study than non-saturated pieces. It is claimed that this study presents novel insights into the geomechanical response of high rank anthracitic coals to high pressure CO2 injection.

Failure analysis of 650 V enhancement mode GaN HEMT after short circuit tests

The paper presents the results of a post failure analysis performed on commercial 650 V GaN power HEMT after short circuit destructive tests. The used experiment set up includes a protection circuit able to avoid the explosion of the sample during the test. Moreover, it limits the energy involved in the failure and facilitates the identification of the areas where the failure is initiated. The post failure analysis confirms that DUTs exhibit two kinds of failures. In the first failure mode, for which large energies are dissipated in the device before the failure, the damaged area of the chip is quite large and is located close to external drain contacts. In this area, very likely, the temperature exceeds the melting temperature of the metallization. The second failure mode is observed for higher values of the drain voltage and involves lower energies dissipated in the DUT during SC before the failure. In this case, the damaged area is very small and is located below the source field plate at gate edge on the drain side. 2D finite element simulations show that in this region the dissipated power density becomes very high and can cause the local temperature to exceed the temperature limit of GaN/AlGaN structure.