Failure Analysis Procedure Research Articles

Analysis Clarifies Failures of Gas Lift Valves in Subsea Wells

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 215010, “Failure Analyses of Gas Lift Valves That Caused Improper Tubing/Annular Communication in Subsea Wells,” by Sebastião T.X. Bastos, Galileu P.H. Oliveira, and Tatiana Sanomya, Petrobras, et al. The paper has not been peer reviewed. _ This work presents the results of several failure analyses of gas lift valves (GLVs) retrieved from the operator’s subsea wells. These valves were unable to prevent backflow from tubing to annulus, thereby compromising well integrity. These investigations allow a more-precise diagnosis of GLV failures that tends to reduce the risk of problem recurrence and can guide the correction of fragilities in current designs. They also can contribute to better planning and definitions in future well completions, resulting in increased reliability for this method of artificial lift. Failure-Analysis Procedure Usually, GLV failures are detected in tubing-integrity tests. Every time a leak rate above the limits for surface-controlled subsurface safety valves is registered in such an integrity test, a procedure is performed to try to restore the sealing capacity of the GLV by flushing the valve with diesel. Then, another tubing integrity test is performed. When removing a supposedly failed GLV from a well, special care must be taken at the rig to preserve the equipment, retaining its condition inside the well as much as possible. For this reason, the following recommendations are prescribed: - The GLV, and any material adhered to any part of its body, must not be washed. - If parts of the valve, debris, or any material adhered to the valve detach from its body, they shall be collected, placed inside clean plastic bags, and shipped along with the valve for failure analysis. - Equipment packaging requires placement in a proper box for transportation after bubble-wrapping the valve’s body and detached parts. - Equipment storage and transportation shall preserve its original conditions. - Mechanical shocks must be avoided. The failure analysis itself occurs at the valve’s manufacturing shop and must be witnessed by at least two specialists from the operator. A database on GLV failures is kept and provides statistical data and general information on these incidents. This procedure constantly has been subject to changes as new lessons learned point to the incorporation of improvements. For instance, the requisite for not cleaning the GLV at the rig came after observing that deposits could have been removed in the process. Also, more recently, some questions have been raised regarding the execution of the leakage test at the beginning of the inspection at the shop, because such tests could interfere with the position of the GLV’s internals or displace or disarray any deposited material. Thus, this step has been discontinued unless a good reason exists to suppose that the valve internal seals are functional. Failure-Analysis Excerpts The complete paper presents seven cases of failure analysis (four are included in this synopsis) for GLVs retrieved from subsea wells in recent years, accompanied by a short description of the facts of the case. Ordinary actions and events are omitted for brevity.

Enhanced Raman spectroscopy analysis for contamination detection on microelectronic devices using gold nanoclusters grown by DC magnetron sputtering

Surface-enhanced Raman spectroscopy (SERS) is one of the most powerful analytical techniques for the identification of molecules in microelectronics industry for failure analysis protocols. Surface-enhanced Raman spectroscopy (SERS) is one of the most powerful analytical techniques for the identification of molecules in the microelectronics industry for failure analysis protocols. In this work, dry-processed gold nanoclusters were prepared by magnetron sputtering deposition to promote the enhancement of the Raman signal from selected common polymers found in the hard disk drive as surface contamination. The optimized sputtering conditions were applied for SERS on poly-carbonate (PC), polyethylene terephthalate (PET), polypropylene (PP), and high-density polyethylene (HDPE). The Raman spectrum showed the average Raman signal intensity gain at about 114%, 78%, 254%, and 226%, respectively. The SERS with gold nanoclusters, prepared by magnetron sputtering, demonstrates that this method is a clean, simple, highly performing analytical method for failure analysis and can be an alternative method over the use of colloidal gold nanoparticles for contamination investigation in industrial failure analysis procedures, where the sample cleanness during the analysis is critical, as in the microelectronic industry.

Systematic risk management for aplanned gas shutdown in the high-care facility of auniversity hospital

In the course of building extension works at Dresden University Hospital, it was necessary to shut down the central medical gas supply in abuilding with 3 intensive care wards with 22beds, an operating theater tract with 5 operating rooms and 6 normal wards each with 28beds during ongoing services. Thus, for the construction phase there was aneed to establish an interim decentralized gas supply with zero failure tolerance for the affected functional units . Following established procedures for possible risk and failure analysis, aproject group was set up by the hospital's emergency and disaster management officer to develop aproject plan, aneeds assessment and acommunication plan. Avariety of risk factors were systematically identified for which appropriate countermeasures needed to be designed. The needs assessment over 4 h based on physiological parameters for the maximum available 22ventilator beds resulted in 26,000 l of oxygen and 26,000 l of compressed air. A total of 7 supply points were each equipped with two 50l cylinders for both oxygen and compressed air, with atotal availability of 175,000 l of each of the 2 gases. Another eight cylinders each were held in reserve. The project was carried out on aSaturday without an elective surgery program, so that the operating rooms concerned could be closed. The timing was chosen so that double staffing of intensive care personnel was available during the afternoon shift change. In advance, as many of the patients on mechanical ventilation as possible were transferred within the hospital; however, nine of the mechanically ventilated patients had to remain. The technical intervention in the gas supply lasted only 2 h without affecting the patient's condition. During the 2‑h interim supply, 16,500 l of compressed air and 8000 l of oxygen were consumed on the high-care wards. The calculated hourly consumption per ventilated patient was 917 l of air (15 l/min) and 444 l of oxygen (7 l/min). The quantity framework based on empirical values from intensive care medicine was significantly lower. This was more than compensated for by the 10-fold stocking of gas and the predictably lower number of ventilated patients than the maximum occupancy used as abasis. For technical interventions in high-risk areas, careful planning and execution in an effective team is required. Established procedures of project management and risk assessment help to avoid errors.

Review of failure risk and outage prediction in power system under wind hazards✰

Given that wind hazards pose a considerable damage to power system in various countries, researchers have developed methods for disaster prevention to diminish the vulnerabilities. This paper attempts to review the failure analysis procedure under the impact of wind hazards, considering the following attributes: wind field generation method, wind-induced electrical component failure model in transmission system, and building outage prediction model from data-driven view in distribution system. First, we compare the calculation of circulation and translation wind speed, with geographical, temporal and uncertainty factors of wind speed considered. Then, we introduce the wind-induced component failure models derived by wind load calculation and fragility curves. The application of the series reliability model in multiple cluster failures is briefly described. Finally, we summarize the applications of data analysis in building outage prediction models. The reviewed literature contains the statistical learning methods and machine learning methods. It is concluded that the frameworks for component-level failure model and user-level outage prediction model are well established. However, most of the studies still develop two models, separately. Future researchers may target at the hybrid application of both models to better provide fault analysis, prediction and emergency in power system under wind hazards.

Failure analysis of a wear ring impeller

The paper aims to analyze the cause of failure of a wear ring impeller and to analyze the quality of the wear ring material. The analysis was conducted by common failure analysis procedure including chemical composition test, visual examination, microstructure examination, microhardness, and macrohardness. The results show that the sliding wear, softened material, and adhesion wear are found on the outer surface of the wear ring, and the deflection occurs on the fracture surface. The fractography analysis indicates that the initial crack occurs due to the valley that acts as a stress concentrator geometry, and the failure is caused by the fatigue stress. The chemical composition test shows that the overlay material is not in accordance with technical data. The suggestion for the corrective action is to ensure the eccentricity of the overall rotary part, and it must use the proper material to prevent sudden failure.

Artificial intelligence‐assisted fatigue fracture recognition based on morphing and fully convolutional networks

AbstractFatigue fracture is one of the most common metallic component failure cases in manufacturing industries. The observation on fractography can provide direct evidence for failure analysis. In this study, an image semantic segmentation method based on fully convolutional networks (FCNs) was proposed to figure out the boundary between fatigue crack propagation and fast fracture regions from optical microscope (OM) fractography images. Furthermore, a novel morphing‐based data augmentation method was adopted to enable few‐shot learning of sample images. The proposed framework can successfully segment two categories, namely, the crack propagation and fast fracture regions, thus differentiating the boundary of two regions in one image. This artificial intelligence (AI)‐assisted fatigue analysis architecture can complete the failure analysis procedure in 0.5 s and prove the feasibility of fatigue failure analysis. The segmentation accuracy of self‐developed network achieves 95.4% for the fatigue crack propagation region, as well as 97.2% for the fast fracture region. Not only for semantic segmentation DNN, we also prove that our novel data augmentation method can applied at the instance segmentation DNN, such as mask regional convolutional neural network (mask R‐CNN), one state‐of‐the‐art deep learning network for instance segmentation, to achieve similar accuracy.

A multi-model approach for wooden furniture failure under mechanical load

Furniture failure generally appears at junctions between wooden parts. Failure prediction is a challenging problem considering the various technologies used for the assembly, the geometric dimensions of the wooden assembled parts and of the assembly components as well as the material properties of the wooden parts. Being able to provide a procedure for failure analysis is of great interest to the furniture industry. This paper proposes a multi-model approach in 3 steps: (i) a simplified global modeling of the whole structure (high loft bed) taking into account the specific geometry of each wooden part (beams or plates), (ii) a three-dimensional local numerical analysis of a through-bolt junction subjected to the mean critical load identified during a series of experimental compression tests to determine the local stresses in such a corner-type junction, and (iii) the application of an ad hoc failure criterion adapted to the anisotropic behavior of wood for failure prediction in through-bolt junctions.

MULTISCALE MODEL WITH EMBEDDED DISCONTINUITY DISCRETE APPROXIMATION CAPABLE OF REPRESENTING FULL SET OF 3D FAILURE MODES FOR HETEROGENEOUS MATERIALS WITH NO SCALE SEPARATION

In this paper we present a 3D strongly coupled multiscale computational procedure for failure analysis of heterogeneous materials capable of representing a full set of different failure modes under various stress states. The proposed approach targets structural failure modeling by a multiscale method with no need for scale separation where a homogenized structural response is described at the macroscale, while microscale is utilized for representing a full set of 3D damage mechanisms inside the domain. The main novelty in terms of finite element method (FEM) implementation concerns macroscale elements that provide the adequate discrete approximation with embedded discontinuity for capturing localized failure with softening. Computation of element tangent stiffness matrices and residual vectors is performed by assembly of microscale elements whose contributions are statically condensed at the coarse level. A refined mesh built with corresponding microscale element discretization entirely fits in each macroscale element which acts as a container for microscale computational results. The compatibility between both scales is enforced by imposing constraint on the displacement field over the macroelement boundary producing a displacement based coupling in the spirit of localized Lagrange multipliers. Dealing with localized failure mechanisms is enabled through enlargement of such scale coupling through implementation of embedded strong discontinuity in macroscale elements. The computation is performed by using the operator split iterative solution procedure treating sequentially micro- and macroscales. Through several numerical simulations, it is demonstrated that the proposed methodology can reproduce a full set of 3D failure modes with very satisfying performance in dealing with localized failure problems.

Successful approach to mitigate the asphaltenes precipitation problems in ESP oil wells

This study introduces a comprehensive and cost-effective approach to diagnose and treat the asphaltenes precipitation problems in different downhole conditions. The proposed approach has been successfully applied in two oil wells (Well-I, and Well-II) located in the Western Desert of Egypt. The two wells produce oil of moderate to high oil gravity with low asphaltenes content using Electrical Submersible Pumps (ESP). In such operating conditions, solid deposits caused blockage at the pump intake and within the pumping stages in the two wells. This blockage led to a sharp decrease in oil production rate and a significant increase in the operating cost. The existing failure analysis procedure was not able to accurately identify the reasons for the blockage; accordingly, the treatment operations were unsuccessful. On the contrary, applying the proposed approach accurately (1) identified the type of the solid deposits, (2) solved the problem using proper treatment option with minimum cost, and (3) improved the oil production rate. The laboratory tests of the fluid and solid deposits showed that (1) the asphaltenes were unstable in the crude oil and acted as a glue for other minerals, and (2) the blockage was successfully diagnosed to confirm that the two wells had asphaltenes precipitation problem. The laboratory tests were extended to select optimum asphaltene dispersant for wells treatments. The field application results showed a significant increase in the oil production rate from 700 to 1600 STB/D in Well-I and from 470 to 1500 STB/D in Well-II. Moreover, the operating cost decreased considerably from 2.01 to 0.43 $/STB in Well-I, and from 4.37 to 0.52 $/STB in Well-II after applying the proposed approach.

Catastrophic failure of urea prill-tower fan

This work presents failure investigation of a prill-tower fan that was in service for about 20 years in a urea plant. All the fan-blades and the plummer blocks fractured catastrophically during the plant-operation, shocked the plant personnel, and caused a huge shutdown loss. Standard failure analysis procedure involving: site visit, visual inspection, microstructural analysis, fracture analysis, analytical stress calculation and FEM analysis was followed. The investigation confirms that the fan-blades having numerous dents and nicks were operating in a mild corrosive atmosphere of ammonium hydroxide. The fracture analysis suggests that a pre-existing dent on the trailing edge of a blade corroded to form a pre-crack that aligned perpendicular to the long axis of the blade. When the pre-crack grew to a critical size, a small chunk of the blade fractured from the vicinity and ejected towards the blade-tip by centrifugal force. Because of confined space around the fan and limited blade-tip clearance, this triggered a chain reaction of blade fracture and rattled the entire array of fan-blades. The analysis establishes that the plummer blocks failed due to mechanical imbalance of the fan-shaft caused by the blade failure. The investigation concludes that presence of dents and nicks on the fan-blades is the root-cause for this catastrophic failure. Grinding-off of the surface defects to eliminate stress raisers is recommended for averting such a failure in future.

Failure Analysis of Super Hard End Mill HSS-Co

Abstract The failure of tools will make a large impact to the productivity, so it must be investigated to avoid the next failure. In this case, the super hard end mill HSS-Co list 4SE code 6210 was broken when it was used for side milling processing of mild steel AISI A36 with rotation speed, cutting speed and cutting depth of 540 rpm, 0.10 m/min (4 ipm) and 16 mm respectively. Standard procedure of failure analysis was performed including macro-micro investigation using Scanning Electron Microscopy (SEM) with energy-dispersive spectrometry (EDS) attachment, micro hardness test, and Finite Element Methods (FEM) simulation. The results of failure analysis showed that fracture occurred due to stress concentration and micro defects of the super hard end mill. Two parts of fracture surface, rough and fine surface were found. Based on SEM-EDS investigation, it was known that the content of tungsten (W) and cobalt (Co) elements on the rough and fine surface was inhomogeneous. Excessive Co and W elements appeared on the fine surface while they disappeared on the rough surface. Excessive Co will diffuse with tungsten and carbon and lead to the separation of tungsten and carbon elements, so it greatly destroyed the alloys and lead to form the non-stoichiometry carbide points. Hence, the defective manufacturing processes which made the elements distribute inhomogeneous is concluded as the reason of the super hard end mill failure.

Structural failure analysis of a river-crossing transmission line impacted by the super typhoon Rammasun

Steel lattice towers are commonly used as supporting structures for overhead transmission lines. Extreme wind conditions like typhoons can cause significant damage on those steel towers and consequently jeopardize the safe operation of transmission lines. In the line sections where a river crossing is unavoidable, the steel lattice towers are designed to be much taller and stronger than regular towers owing to safety and operation considerations. In previous typhoon events in Hainan Province of China, none of the installed river-crossing towers have collapsed or experienced significant damages, except in the aftermath of super typhoon Rammasun in 2014, when two 95-m tall river-crossing suspension towers collapsed. This paper presents the structural failure analysis of these two collapsed lattice steel transmission towers: The improved genetic algorithm (IGA) was applied to solve the typhoon wind field model for improved resolution of wind records measured during the Rammasun event and based on of 10-min average wind speeds. The IGA used the field measurement data to generate time-varying typhoon key parameters that lead to a more accurate prediction of the wind field than the conventional method. Then a systematic procedure was developed to investigate the failure mechanism of steel lattice towers under fluctuating wind loads combined with rain loads with consideration of the aerodynamic characteristics of the transmission tower-line system. The results show that the root-cause of the structural failure of the two transmission towers was overstressing of the tower main leg members in the second panel above the ground. Based on the numerical simulation of typhoon wind loads and the structural failure analysis procedure applied in this study, a general reinforcement method of river-crossing towers is proposed and the implementation of a typhoon warning method is recommended to identify the potential risks in transmission lines and formulate corresponding typhoon emergency plans to reduce the losses caused by typhoons.

A multiscale modelling procedure for predicting failure in composite textiles using an enhancement approach

Composite materials have largely been analysed for failure and structural performance from the perspective of an anisotropic homogeneous material despite their evident hierarchical nature. Although such an assumption does substantially facilitate the analysis procedure in a computationally efficient manner, predictions on the occurrence of failure differ from reality as the failure mechanisms within composite materials are different from a constituent perspective. This paper demonstrates the ability of a multiscale dehomogenisation procedure that provides failure information at different length scales, namely: the macro- (continuum), meso- (textile) and micro-scale (constituent). This is achieved by adopting an enhancement approach performing simple matrix operations to study the strain distribution. The hierarchical dehomogenisation procedure using Strain Invariant Failure Theory is implemented in Abaqus UMAT subroutine and verified using simple sanity checks and a ‘manual’ sub-modelling technique utilising an open hole tension coupon as an example. A comparison of the two modelling techniques indicate the similar failure predictions at the meso and micro levels with the hierarchical approach presented in this paper being far more computationally efficient. The failure analysis procedure presented in this paper is subsequently demonstrated on a composite I-beam component allowing failure in composite structures to be observed from a constituent perspective where fibre and matrix modes of failure can be identified and examined from an engineering point of view.

Analysis of cracks propagated in a tee pipe fitting

The work investigates the cracks propagated in a tee pipe fitting, part of desulphurization units of an oil refinery company. The part was extracted from an assembly that was manufactured and installed by the refinery maintenance department. The research covers the typical experimental procedure of failure analysis and focused on the cracks examination. Historical data were recorded. The chemical composition of the material was verified. The component was examined and photographed as received. A superheated inlet steam meets a lower temperature inlet steam, mixes on specific areas and comes out of a common outlet. A network of cracks was detected on the mixing areas. Samples were selected and prepared according to the analysis procedure. Both outer and inner surfaces of the fragmented tee pipe were inspected with penetrant liquid. The crack origin and propagation were detected and fracture mode was identified. Selected specimens were macro etched and inspected visually, revealing numerous welding defects. Tee pipe fitting failed on the mixing area. It cracked by thermal fatigue due to simultaneous effect of stress and temperature. Furthermore, poor welding facilitated the crack process.

Minimum Requirements for a Successful Automotive Qualification With Focus on Packaging

Requirements off Automotive Tier1s and OEMs Klaus Pietrczak JCET Group, 46429 Landing Parkway, Fremont, CA 94538 Klaus.Pietrczak@statschippac.com The automotive market size for power and discrete devices is growing at a rate of greater than 7 percent. Market drivers for this growth include ADAS, Infotainment and Powertrain. Getting an automotive device from initial specification to high volume manufacturing (HVM) can take four years compared to the typical three to six months it takes a consumer device. The requirements for automotive devices are much more complex. Increasing electronic content and zero failure over 15 years of lifetime are the major drivers for the ever increasing reliability requirements. The major document for any automotive device is the Production Part Approval Process (PPAP). This presentation will show the required content for the PPAP and which roles in an organization will contribute to it. There will also be a deeper dive into the three major automotive standards: IATF16949, AIAG and AEC. For each standard covered in-depth looking at the purpose, what the goal of the standard is, and a few examples for better understanding. Also covered will be the fact that even with all of these standards, not everything is covered. The device manufacturer having specific requirement on top of the standard requirements are more the standard rather than the exception. We will go into detail regarding who has to deliver what to which section and how they are all linked and interacting. The content on the standards will be completed by a look at the upcoming ISO26262-11, its intent, scope and expected influence on Semiconductor and OSATs. Finally, a look to some of the less defined but still important pieces of the puzzle. Examples of these include failure analysis procedure, product change/cancellation notification and data retention. While not an exhausting list, more often than not each is an important part of the business. An explanation of the purpose and typical requirements for these will be presented. This comprehensive review will give anyone targeting automotive business an overview of all that is required for automotive devices and the timeline to expect to get to HVM.

Failure Analysis and Remedy Procedure for Cracking of Fuel Nozzles in a Gas Turbine

A common failure in a certain type of gas turbine, observed during the first periodic inspection, is radial cracks in the tip plate of gas fuel nozzles. Here, each gas turbine has 18 nozzles. In all nozzles and in all similar units, these cracks of lengths ranging from 1 mm to a maximum of 14.5 mm are observed. As prescribed by the manufacturer, the defective part must be removed and replaced by welding and machining of a new one. But this problem is repeated and observed in the next periodic visits, and in all units. Depending on the number of nozzles in each gas turbine unit and the number of units in total, these repairs are very expensive and time-consuming. In this paper, the failure is analyzed and the causes of the cracks in the nozzles are investigated. Studies show that the main causes of nozzle failure are residual stresses caused by welding and thermal stresses caused by the start-up and shutdown processes. According to results, a solution has been proposed to release these residual and thermal stresses. After the implementation of this method in 1998, no more failure has been reported by the repair team, which proves the effectiveness of this solution. Since this paper has been prepared based on technical reports from the years between 1996 and 1998, the cited references of this paper are these technical reports.

Forensic Engineering Metallurgical Analysis of PTO Air Compressor Rupture and Fire

The coalescer of an air compressor mounted on a utility truck ruptured, resulting in the expulsion of burning oil onto a nearby employee. An investigation ensued to determine the root cause of the injuries. Many potentialcontributing factors were examined, including system and component manufacture, design, installation, maintenance, and use. Metallurgical and failure analysis procedures were used to determine root cause of the system failure and related injuries. A power take-off (PTO)-driven compressor operates at high temperature (200°F) and pressure (110 psig), creating opportunities for dangerous conditions. The system has a safety shutdown control to prevent the system from going over temperature and pressure limits. The exploded coalescer and fire in this case indicated the temperature and/or pressure systems were compromised as well as the control system. Compressor failures are not uncommon; however, violent failures that cause fire and injury are much less common. PTO compressors are relatively simple machines with only about 25 components. However, the proper function of most components is essential to the safe operation of the unit. In this investigation, it was necessary to look at each component relative to its fitness for service and potential contribution to the system failure.

Failure analysis of a carbon steel roller shaft of continuous pad steam machine

This paper presents the failure analysis of carbon steel roller shaft of continuous pad steam machine used in textile industry. The fracture position was located at a stepped diameter. The failed component was the shaft made of carbon steel AISI 1040. Standard procedure for failure analysis was employed in this investigation. Visual examination, chemical analysis, hardness and tensile strength measurements, microstructural characterization, fractography analysis by Scanning Electron Microscopy (SEM) and Finite Element Analysis (FEA) were used for the failure analysis. Using this failure analysis approach, we pinpointed the root cause of failure and developed a means of solving this type of failure in the future. Firstly, the chemical composition of the shaft is done by an Optical Emission Spectroscopy (OES) method, the found chemical composition was matching with required standard value. Mechanical testing consists of two test i.e. tensile test and hardness test and it was found out that the strength and hardness of specimens were within the required capacity. For metallurgical analysis, the microstructure of the shaft was developed by using an optical microstructure. Equal distribution of ferrite perlite shows that heat treatment was performed well and carbon percentage in a material is satisfying the standard values. Thus, it proves that the material used was of good quality and indicates that failure is not due to material property. Further for the fractography, the fractured surface was examined by SEM. The cross-section was taken in a quarter segments and divided into four regions i.e. A, B, C, and D. Fractography morphology mainly showed that the failure of roller shaft was caused due to fatigue. To examine the stress distribution at the fractured surface the Finite Element Method (FEM) was also carried out. Based on the shaft size, a precise ANSYS model was developed. The result of FEM shows that stress concentration was significant at roller shaft step which could reduce the material reliability to some extent. Based on the failure analysis it could be concluded that due to stress concentration a micro crack is initiated along the weak interface and further it converted into the major fatigue failure. Fractography morphology of failed roller shaft also confirms the fatigue failure.

A Case Study on Fatigue Failure of a Transmission Gearbox Input Shaft

In this paper, a root cause analysis of premature failure of a gearbox input shaft, manufactured of AISI 1045-H, was performed through standard procedures for failure analysis. Shaft failed on cross oil hole through a helical fracture and therefore did not meet bogie 100,000 cycles during the verification with 10 Hz frequency cyclic testing. The fracture in the oil hole implied evidence of fatigue (i.e., beach marks on the fracture surface were clearly visible). Prior to improving the fatigue life and suggesting required remedial actions, mechanism of failure has to be understood, especially the initiating point of cracking. To this end, chemical analysis, microstructural characterization, fractography, hardness measurements, and finite element simulation were used to assess the nature of fracture in detail. The fractography analysis showed that fatigue beach marks originate from transition zone of the case on the cross oil hole. This is possibly due to the fact that torsional strength in this area is lower than torsional fatigue strength which leads to fatigue crack initiation, crack growth, and final fracture. At the end of this paper, proper remedial actions have been proposed.

Quasi-static buckling and first-ply failure loads of shear web reinforced glass-fabric composite wind blades

A procedure is proposed to analyze the structural failure of a composite wind blade subjected to quasi-static loads. The structural failure of a wind blade under extreme wind loads which can be treated as quasi-static loads is an important issue to be tackled when assessing the reliability of a wind turbine. In the proposed structural failure analysis procedure, the geometrically nonlinear finite element (FE) method together with several phenomenological failure criteria is used to investigate the possible failure modes such as incipient buckling, interfacial debonding, and first-ply failure of the blade via an incremental loading approach. The failure loads and locations associated with the possible failure modes for a number of small wind blades with different structural configurations are determined to study the appropriateness of the failure criteria. Static tests of several small glass-fabric composite wind blades were conducted to validate the suitability of the proposed procedure. It has been shown that the experimentally and theoretically predicted failure processes of the wind blades are in good agreement. The applications of the proposed procedure are demonstrated by means of a numerical example in which the failure process of a wind blade under quasi-static wind loads is simulated.