The research on failure analysis of fluid cylinder and fatigue life prediction

To analyze the reasons of fluid cylinders’ rupture, macro-analysis, SEM, composition inspection, metallographic analysis, hardness test and mechanics performance test of fluid cylinders materials were implemented. Two different kinds of fatigue life prediction methods have been proposed which are based on total life analysis and strain–life methodology. The results indicate that: the failure cylinders’ material quality is satisfactory. Fatigue damage caused by high working, stress and corrosion is the main reason of cracking. The fatigue life prediction illustrates that strain–life methodology is well adapted to fluid cylinders.     

A framework for effective management of condition based maintenance programs in the context of industrial development of E-Maintenance strategies

CBM (Condition Based Maintenance) solutions are increasingly present in industrial systems due to two main circumstances: rapid evolution, without precedents, in the capture and analysis of data and significant cost reduction of supporting technologies. CBM programs in industrial systems can become extremely complex, especially when considering the effective introduction of new capabilities provided by PHM (Prognostics and Health Management) and E-maintenance disciplines. In this scenario, any CBM solution involves the management of numerous technical aspects, that the maintenance manager needs to understand, in order to be implemented properly and effectively, according to the company’s strategy. This paper provides a comprehensive representation of the key components of a generic CBM solution, this is presented using a framework or supporting structure for an effective management of the CBM programs. The concept “symptom of failure”, its corresponding analysis techniques (introduced by ISO 13379-1 and linked with RCM/FMEA analysis), and other international standard for CBM open-software application development (for instance, ISO 13374 and OSA-CBM), are used in the paper for the development of the framework. An original template has been developed, adopting the formal structure of RCM analysis templates, to integrate the information of the PHM techniques used to capture the failure mode behaviour and to manage maintenance. Finally, a case study describes the framework using the referred template.     

Experimental and numerical studies on failure modes of riveted joints under tensile load

Though engineers are not advised to utilize riveted joints in tension, rivets will inevitably bear tensile load in realities. Hence, it is interesting to investigate the failure modes of riveted joints when they are under tensile load. Following previous studies (Chen et al., 2011), in this work, the authors selected three sizes of riveted joints to conduct the riveting and tension processes experimentally and numerically. Three kinds of failure modes including pull through, shank breaking, and head breaking were observed. Simulations are able to give almost the same results as those from experiments. Furthermore, three formulas were proposed to calculate the maximum tensile strength of riveted joints. Though the values calculated from these three formulas are approximate, they have the same order of magnitude as real ones. Moreover, they could be utilized to estimate which kind of failure mode may take place when riveted joints were under tensile load.     

Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part I: Corrosion and wear

Titanium tubes used in condensers in a nuclear power plant in China encountered abnormal wall thinning, and was thus forced to temporarily stop operation or it could bring about catastrophic safety problems. Most of the wall thinning happened at quite regular positions on the tubes and these failure tubes were located similarly in the condensers, indicating some common problems. To find out the root cause and mechanism of the thinning failure, we conducted surface deposit analysis, appearance inspection, microstructure analysis and composition analysis of the samples by means of X-ray diffraction (XRD), stereo microscope, scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). The results revealed that the wall thinning was primarily caused by eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. Finally, countermeasures were proposed for repair and prevention.     

Scanning spreading resistance microscopy for failure analysis of nLDMOS devices with decreased breakdown voltage

Scanning Spreading Resistance Microscopy (SSRM) is successfully applied to investigate failing nLDMOS test devices that exhibit a lowered break down voltage (BVDSS) in electrical test. Cross-sectional, two-dimensional maps of the local sample resistivity from fail and reference (pass) devices reveal significant differences of the dopant concentration in individual, specific regions. This important information enables unambiguous identification of the root cause of the device failure to be dopant related. Furthermore, from a set of hypothesis, which explains the failed electrical test, SSRM results confirm exactly one and rule out the other. These are two important steps towards root cause identification. Since a relative comparison of fail and pass SSRM scans is sufficient for this failure analysis, an extensive data calibration for the absolute dopant concentration by means of additional SSRM measurements on test samples with known dopant concentration is not required. The ability of SSRM to prove or disprove miscellaneous fail hypothesis even without data calibration makes this method a very powerful tool for analysis of dopant related failure types.     

Localization of temperature sensitive areas on analog circuits

We introduce a novel easy to apply method to detect critical temperature sensitive areas on analog circuits. Our method is based on heat diffusion on a silicon micro-chip: the corners of a temperature sensitive micro-chip are heated up directly by ESD diodes or infrared laser light. This heat stimulus at the corners results in an inhomogeneous temperature distribution. Thus, the temperature is a function in time and space. The elapsed time to change the chip status from “fail” to “pass” as a reaction to the heat stimulus correlates with the distance to the heat source. This correlation is extracted from COMSOL simulations and experimental results. A numerical program based on that correlation succeeded in localization of the temperature sensitive chip module.Micro-chips affected by corner MOSFETs in the subthreshold regime are used to demonstrate our method.     

Micro-stress analysis and identification of lightweight aggregate’s failure strength by micromechanical modeling

This work is based on a dual approach of experiments and micromechanical modeling in order to characterize the failure behaviors of lightweight aggregate concretes (LWAC). Many classes of LWAC were tested, based on five families of lightweight aggregates (LWA) and three types of mortar matrices: normal, high performance (HP) and very high performance (VHP). Micromechanical modeling is based on an iterative homogenization approach and associated localization: local stress distributions during the uniaxial compression tests can be predicted in LWAC’s components and at their interface. Experimental compressive strengths were measured on manufactured 16 × 32 cm cylindrical specimens. The confrontations between micromechanical modeling and experiments were used to identify LWA’s failure strengths which are difficult to measure, and to quantify the inaccuracies related to conventional methods. These corrected values of LWA’s failure strength were introduced into a failure criterion modeling: associated predictions of LWAC’s compressive strength are in good agreement with the experimental measurements.     

Investigate the causes of cracks in welded 310 stainless steel used in the Flare tip

The present study examines the causes of the cracks in welded 310 stainless steel that has been used in the Flare tip. According to the tests, including metallographic examination, macroscopic hardness test and scanning electron microscopic analysis, the reasons for the nucleation and growth of the cracks in the weld zone have been discussed. The results show that, because of the service temperature of Flare tip between 500 and 900 °C, and hydrocarbon gases such as methane, ethane, sour gas and carbon dioxide that are the combustion products, the component surface has been oxidized and carburized. Thus, the surface carburized oxide layer and also the subsurface damage can be fertile field for the nucleation of cracks. Likewise, the presence of sigma phases, austenite dendrites and interdendritic delta ferrite can cause a drop in toughness in the weld zone and provide fields for the crack growth in the weld zone.     

Computational micromechanics of fatigue of microstructures in the HCF–VHCF regimes

Advances in higher resolution experimental techniques have shown that metallic materials can develop fatigue cracks under cyclic loading levels significantly below the yield stress. Indeed, the traditional notion of a fatigue limit can be recast in terms of limits associated with nucleation and arrest of fatigue cracks at the microstructural scale. Although fatigue damage characteristically emerges from irreversible dislocation processes at sub-grain scales, the specific microstructure attributes, environment, and loading conditions can strongly affect the apparent failure mode and surface to subsurface transitions. In this paper we discuss multiple mechanisms that occur during fatigue loading in the high cycle fatigue (HCF) to very high cycle fatigue (VHCF) regimes. We compare these regimes, focusing on strategies to bridge experimental and modeling approaches exercised at multiple length scales and discussing particular challenges to modeling and simulation regarding microstructure-sensitive fatigue driving forces and thresholds. We conclude by discussing some of the challenges in predicting the transition of failure mechanisms at different stress and strain amplitudes.     

Experimental study of the performance of geosynthetics-reinforced soil walls under differential settlements

The deformation performance and settlement failure mechanism of geosynthetics-reinforced soil (GRS) walls are the two key points of engineering design under the differential settlement. This paper presents model tests of deformation performance and failure mechanism of the GRS wall with and without lateral restriction under differential settlement conditions. The observation and measurement results, including force and vertical displacement of geosynthetics and lateral deformation of facing panels, indicate good settlement control performance of GRS wall during construction and under differential settlement. Results indicate that the influence of the stress state of facing panels on the settlement control performance of GRS wall cannot be ignored. And the differential settlement failure of GRS wall is likely to occur in the joint of facing panels and geosynthetics. For good illustrations, two analytical approaches about deformation and stress of geosynthetics were proposed based on elastic cable theory, in GRS wall with and without lateral restriction. The expressions exclude the necessity to carry out sophisticated numerical analyses to stress and deformation and may help to develop the design guidelines for such GRS wall.