Vibration-induced Fatigue Research Articles

Turbulent flow in an I–L junction: Impacts of the pipe diameter ratio

Pipeline junction plays a pivotal role in fluid mixing for biomedical, chemical, and industrial processes. This study introduces an I–L junction for pipeline systems, fostering concurrent flow between branch-pipe injection and the main pipe bulk flow. In contrast to the conventional T-junction with perpendicular injection, the I–L design demonstrates high potential in mitigating vibration-induced fatigue risks, given an optimal branch-to-main pipe diameter ratio, rd. Using unsteady Reynolds-averaged Navier–Stokes equations, the study assesses fluid mixing across a broad range of rd (1/12–1/2.5). The streamline geometry undergoes a transition from well-defined symmetric vortices to unsteady oscillations when the pipe diameters diverge beyond 1/4, arising from vortex shedding in the wake of the branch pipe. Despite the conventional T-junction showing a more homogeneous velocity distribution in the streamwise direction, its turbulent kinetic energy (TKE, both modeled and calculated from the resolved-scale velocities) near the junction is an order of magnitude larger, implying high overall inhomogeneity in the flow. The TKE decays rapidly to an equivalent level compared to the proposed I–L junction approaching discharge, indicating that the peaking of TKE in the T-junction does not significantly contribute to enhanced fluid mixing. Conversely, it can likely result in harmful vibrations inside the pipeline. While the turbulence statistics remain qualitatively unchanged for rd<1/4, an enlarged discrepancy in pipe diameters beyond rd<1/6 yields more favorable mean surface pressure coefficient, CP¯. The results provide insights into pipeline design, recommending an optimal pipe diameter ratio for enhanced mixing of successively collected fluids while retaining improved system reliability.

Predicting Fatigue Life of A Mount of A Device with Shock Absorber

All aircraft operate under vibration loads and these loads cause vibration-induced fatigue damage to the parts. Fatigue damages are dangerous because they occur suddenly without giving a warning before, for this reason they need to be carefully analyzed. In the analysis, especially the natural frequencies and dampings must be determined correctly, because when the vibration loads match with the natural frequencies, resonance is observed and this reduces the life of the part. In this study, the fatigue life of a mount of a device with shock absorber is investigated. Flight tests were carried out in real flight scenarios to provide an input for the analysis. A modal test was performed to model the shock absorbers correctly. Vibration test was performed to prove the accuracy of the prepared finite element model and to determine the damping ratios. Fatigue analyses were performed in frequency domain with a commercial software, nCode, and the accuracy of the analysis was proven by the experiment.

An investigation into vibration-induced fatigue failure of metro vehicle cowcatcher and its structural improvement design

Cowcatchers are used in metro vehicles to remove obstacles on the track. The failure of the cowcatcher will seriously affect the safety and reliability of the train. This paper investigated the fatigue failure of cowcatchers of metro vehicles induced by the high frequency vibration through both field tests and numerical simulations. The dynamic stress, acceleration, and eigenmode of a cowcatcher as well as the wheel out-of-roundness (OOR) were measured to identify the primary factor of the fatigue failure of cowcatchers. The investigation results indicate that the main cause of the premature fatigue fracture of cowcatchers is the first-order bending eigenmode of the cowcatcher (95.4 Hz) excited by rail corrugation with the passing frequency of approximately 95 Hz. A modal differential frequency design method was proposed to improve the design of cowcatcher. The first-order natural mode frequency of the re-designed cowcatcher increases from 95.4 Hz to 160 Hz, which can effectively avoid the serious vibration excited by rail corrugation. Subsequently, a random vibration model was proposed to evaluate the effects of the improved cowcatcher. The acceleration measured in the field was used as the input of the model to reproduce the vibration environment of the cowcatcher. The model was validated by the measurement result of dynamic stress of the original cowcatcher. The fatigue life of the improved cowcatcher was calculated by the model. Compared with the original cowcatcher, the service life of the improved cowcatcher is greatly improved.

Detection of Vibration Induced Fatigue Crack Propagation Using a Hybrid Method

This paper demonstrates a simple technique to detect vibration-induced fatigue cracks using a hybrid method by vibration and acoustic emission techniques. A thin aluminum plate of 6082-T6 was excited using a vibration shaker to achieve a bending mode where the maximum stress exhibited at the plate mid-span. To simulate crack formation, a sharp notch was created. This systematic setup allows mode I crack propagation through plate thickness. The development of cracks over time changed the natural frequency of the plate which leads to the reduction of vibration amplitudes. This experimental technique facilitates the identification of acoustic emission waves during the onset of damage in the presence of noise due to dynamic motion. The effect of crack development on Lamb waves was investigated. The acoustic emission signals were cross-correlated with a Gaussian window of a central frequency of 250kHz. The results show a reduction in the fundamental wave A0, whilst an increase in S0 wave amplitudes at some stages during crack extension. The current experimental work can be an alternative technique for vibration-induced fatigue test evaluation.

On the Suitability of Vibration Acceptance Criteria of Process Pipework

The risk of vibration-induced fatigue in process pipework is usually assessed through vibration measurements. For small-bore pipework, integrity personnel would measure the vibration of the pipework and refer to widely used charts to quantify the risk of vibration-induced fatigue. If the vibration levels are classified as OK, no action is required on behalf of the operators. However, if it is a CONCERN or PROBLEM vibration level, strain measurements are required to adequately quantify the risk through a fatigue life assessment. In this paper, we examine the suitability of a widely used vibration acceptance criteria through finite element models. A total of 4,800 models are used to study the suitability of this vibration acceptance criteria by monitoring both the vibration and dynamic stress. The model comprises a small-bore pipe (2″ SCH 40) that is fitted on a mainline size 5″ SCH 40 using a weldolet; the length of the mainline takes three values resulting in three models. The mainline supporting conditions will be varied using translational and rotational springs. The finite element models will be excited using a point load resembling flow-induced forces (with varying flow velocity and fluid composition). These excitations are obtained from the literature and are based on experimental studies as power spectral density functions. The results show that the studied vibration acceptance criterion is suitable in 99.73% of all the studied models with 68.27% confidence level. For the models with a shorter mainline pipe, the criterial is suitable in 76.5% of the time with 68.27% confidence level.

Study on high frequency vibration-induced fatigue failure of antenna beam in a metro bogie

This paper investigated on the high frequency vibration-induced fatigue failure of antenna beam through both field tests and numerical simulation. In the field tests, the dynamic stress, acceleration, and operational modal of bogie system as well as the short pitch irregularities of rail were measured to identify the primary contributor to the fatigue failure of antenna beam. The result suggested that the rail corrugation- induced impact with the passing frequency of 78 Hz serves as the main driving force of the modal resonance for bogie frame and antenna beam in the bogie system, which can emphasis the bending of antenna beam and further contribute to the reduced fatigue lifetime. To better understanding the failure mechanism and the associated mitigation method, a rigid/flexible coupled dynamic model of rail vehicle, considering the flexibility of bogie frame, wheelset and antenna beam as well as coil spring, was developed by using the modal synthesis method. Based on the developed model, the influence of elastic rubber mounts located on both ends of antenna beam was investigated, and a new design was proposed to relief the bending movement- induced stress concentration in the antenna beam. This is expected to mitigate the vibration fatigue problem of antenna beam arising from the rail corrugation-induced high frequency wheel/rail impact.

Investigation of fatigue damage growth and self-heating behaviour of cross-ply laminates using simulation-driven dynamic test

Structural integrity of aerospace assets is paramount for both the safety and economy of aviation industry. The introduction of composites into the design of aero-structures generated several economic benefits but also led to several challenges, including fatigue damage growth and self-heating behaviour. Fatigue of metals is widely managed by calculations of damage accumulation and prediction of residual life. These techniques do not always apply to the fatigue of composites, where the onset and propagation of damage are still under investigation. Furthermore, vibration-induced fatigue is even less understood because of a handful of failure criteria available and, also, because it is biased by the self-heating conditions of the material itself. The authors have underpinned one failure criterion for vibration fatigue and mapped that against self-heating and environmental temperatures. Despite the advances, several research questions were left open because of the complex multiphysics behaviour of fatigue which outreached the experimental capacity. Therefore, this research suggests a Simulation-Driven Dynamic Test (SDDT) framework that deconstructs vibration fatigue experiments into step-wise steady-state analyses. This novel approach will enable (a) investigating the failure mode mixity of the underlying failure criterion, and (b) simulating the surface temperature during the delamination growth under vibration conditions.

Impact of Ball Burnished Regular Reliefs on Fatigue Life of AISI 304 and 316L Austenitic Stainless Steels.

The present work describes an experimental investigation of the fatigue durability of AISI 304 and AISI 316L austenitic stainless steels, which have regular reliefs (RR) of the IV-th type, formed by ball burnishing (BB) on flat surfaces, using a computer numerical control (CNC) milling center. The methodology and the equipment used for obtaining regular reliefs, along with a vibration-induced fatigue test setup, are presented and described. The results from the BB process and the fatigue life experiments of the tested austenitic stainless steels are gathered, using the approach of factorial design experiments. It was found that the presence of RR of the IV-th type do not worsen the fatigue strength of the studied steels. The Pareto, t-test and Bayesian rule techniques are used to determine the main effects and the interactions of significance between ball burnishing regime parameters. A stochastic model is derived and is used to find when the probability of obtaining the maximum fatigue life of parts made of AISI 304 or 316L reaches its maximum value. It was found that when the deforming force, the amplitude of the sinewaves and their wavenumber are set at high values, and the feed rate is set at its low value, the probability to reach maximum fatigue life for the parts made of AISI 304 or 316L is equal to 97%.

Application of Spectral Method for Vibration-Induced High-Cycle Fatigue Evaluation of an High-Pressure Turbine Blade

Abstract A method of fluid–structure interaction coupling is implemented for a forced-response, vibration-induced fatigue life estimation of a high-pressure turbine blade. Two simulations approaches; a two-way (fully coupled) and one-way (uncoupled) methods are implemented to investigate the influence of fluid–solid coupling on a turbine blade structural response. The fatigue analysis is performed using the frequency domain spectral moments estimated from the response power spectral density (PSD) of the two simulation cases. The method is demonstrated in relation to the time-domain method of the rainflow cycle counting method with mean stress correction. Correspondingly, the mean stress and multi-axiality effects are also accounted for in the frequency domain spectral approach. In this case, a multiplication coefficient is derived based on the Morrow equation, while the case of multi-axiality is based on a criterion which reduces the triaxial stress state to an equivalent uniaxial stress using the critical plane assumption. The analyses show that while the vibration-induced stress histories of both simulation approaches are stationary, they violate the assumption of normality of the frequency domain approaches. The stress history profile of both processes can be described as platykurtic with the distributions having less mass near its mean and in the tail region, as compared to a Gaussian distribution with an equal standard deviation. The fully coupled method is right leaning with positive skewness while the uncoupled approach is left leaning with negative skewness. The directional orientation of the principal axes was also analyzed based on the Euler angle estimation. Although noticeable differences were found in the peak distribution of the normal stresses for both methods, the predicted Euler angle orientations were consistent in both cases, depicting a similar orientation of the critical plane during a crack initiation and propagation process. Finally, the fatigue life estimation was shown to be conservative in the fully coupled solution approach.

Fitness-for-service assessment and failure analysis of AISI 304 demineralized-water (DM) pipeline weld crack

Fitness-for-service assessment (FFS) is employed in oil and gas refiners to assess the structural integrity of an in-service pressure vessels and storage tanks that may contain a flaw or damage. API 579/ASME FFS-1 standard provides guidance for conducting FFS assessments using methodologies specifically prepared for pressurized equipment. In the present investigation, FFS assessment (level 3, based on the Failure Assessment Diagram (FAD)) carried out on a 304 demineralized water stainless steel pipeline containing a crack at weldment eleven months before the failure is explained in detail. Despite the FFS assessment and acceptance of the pipeline, failure occurred eleven months after the FFS assessment. The distribution of the residual thermal stress introduced during welding was estimated by means of Finite Element Analysis (FEA) using Abaqus software. Further investigation revealed that the operating condition of the DM pipeline deviated from the date of FFS assessment and the failed pipeline had been under vibration. Defects such as inclusions and porosities and irregular shape of weld bead were observed in weld joint showing lack of proper quality control (QC) during welding and installation of the pipeline. The heat affected zone (HAZ) region was found to be sensitized (perhaps due to the improper welding procedure during installation) and severe precipitation of chromium carbides was observed on the grain boundaries. Further investigation showed the transgranular nature of the crack propagated perpendicular to the weld metal as a result of the dynamic loading caused by steam hammering effect leading to the vibration-induced fatigue. The Scanning Electron Microscopy (SEM) of the fracture surfaces showed that the fracture surfaces consist mostly of fatigue striations and quasi-cleavage fracture.

Technology Focus: Production and Facilities (December 2018)

Technology Focus Creativity and innovation have long characterized production and facilities, and this year is no exception. Much of the work reported this past year was conducted during the recent period of low oil prices. The economic challenges of the oil industry clearly have provided a strong stimulus for even more creativity and innovation. The use of big data and analytics appeared in a number of papers with an emphasis on the use of artificial intelligence (AI) for building databases used to monitor the health of equipment and structure risk-based-inspection (RBI) strategies. Monitoring data inputs from thousands of sensors (paper OTC 28990) allows an AI application to predict an impending failure and notify operators by text or email when the incipient problem is detected so that proactive maintenance can be scheduled to avoid an unplanned shutdown or catastrophic failure. This strategy is being successfully applied to compressors but no doubt will be used to monitor other high-cost, critical service equipment as well (paper SPE 188803). Progress continues on the design and application of inflow-control devices (ICDs). Introduced only a few years ago, these devices are still in a rapid development stage for both design and application. Now, ICDs are applied successfully to improve the fluid-injection patterns for both steamfloods and waterfloods, the latter being described as a successful field application (paper SPE 189824). For steamfloods, passive and autonomous ICD designs were evaluated and their performance modeled using computational fluid dynamics (paper SPE 189721). The integration of subsurface modeling and surface-facility design by the development of a data-driven stochastic work flow (paper SPE 187462) demonstrated a means to reduce both subsurface and facility costs by reducing the biases that inevitably come into play during the generation of a field-development plan. Other innovative work was reported on field optimization, the prediction of asphaltene precipitation, and the integration of RBI with vibration-induced fatigue failure of installed piping systems. Interesting work not discussed here includes evaluating corrosion under severe conditions and the development of an oil-droplet-coalescing pump for use in water treatment. Recommended additional reading at OnePetro: www.onepetro.org. SPE 188803 Machines Performance Algorithmic Modeling for Anticipating Machines Health Using Real-Time Condition-Monitoring Data by W. Almadhoun, ADMA-OPCO, et al. SPE 189721 Evaluation of Inflow-Control-Device Performance Using Computational Fluid Dynamics by M. Miersma, University of Alberta, et al. SPE 187256 Production Optimization of Shenzi Field in the Deepwater Gulf of Mexico by P. Ashton, BHP, et al. SPE 190149 A Diagnostic Approach To Predict Asphaltene Deposition in Reservoir and Wellbore by Davud Davudov, University of Oklahoma, et al. OTC 28352 Integrating an RBI Approach for Vibration-Induced Fatigue Into a Mechanical-Integrity Program by Paul Crowther, Wood, et al.

$${\mathcal {H}}_{2}$$ H 2 optimization and numerical study of inerter-based vibration isolation system helical spring fatigue life

This paper presents an optimization and numerical analysis of vibration-induced fatigue in a two degree-of-freedom inerter-based vibration isolation system. The system is comprised of a primary, e.g. source body, and a secondary, e.g. receiving body, mutually connected through an isolator. The isolator includes a spring, a dashpot and an inerter. Inerter is a mechanical device which produces a force proportional to relative acceleration between its terminals. A broadband frequency force excitation of the primary body is imposed throughout the study. The goal of the proposed optimization is to prolong the fatigue life of the ground connecting helical spring of the secondary body. The optimization is based on minimizing separately the displacement and velocity amplitudes. Both optimization criteria are compared with regard to spring fatigue life improvement for fair benchmark comparison. The inerter-based optimized systems, in which the $${\mathcal {H}}_{2}$$ index of the receiving body is minimized, are also compared with the optimized systems without inerter. Notable improvements are observed in inerter-based systems due to the inclusion of an optimally tuned inerter in the isolator. The proposed analytical vibration fatigue method optimization results are compared with the finite element method results, and a very good agreement is observed. Most accurate helical spring deflection and stress correction factors are discussed and determined. Furthermore, the inerter concept is successfully implemented into finite element-based dynamic solution.

Optimization of an inerter-based vibration isolation system and helical spring fatigue life assessment

This paper presents an analytical analysis and optimization of vibration-induced fatigue in a generalized, linear two-degree-of-freedom inerter-based vibration isolation system. The system consists of a source body and a receiving body, coupled through an isolator. The isolator consists of a spring, a damper, and an inerter. A broadband frequency force excitation of the source body is assumed throughout the investigation. Optimized system, in which the kinetic energy of the receiving body is minimized, is compared with sub-optimal systems by contrasting the fatigue life of a receiving body helical spring with several alternative isolator setup cases. The optimization is based on minimizing specific kinetic energy, but it also increases the number of cycles to fatigue failure of the considered helical spring. A significant portion of this improvement is due to the inclusion of an optimally tuned inerter in the isolator. Various helical spring deflection and stress correction factors from referent literature are discussed. Most convenient spring stress and deflection correction factors are adopted and employed in conjunction with pure shear governed proportional stress in the context of high-cycle fatigue.

Analysis of fatigue damage of aluminium alloy under multiaxial random vibration

Many studies have investigated the fatigue damage of materials under vibration. However, the mechanism of vibration-induced fatigue damage remains unclear, and no method has been established for evaluating the vibration fatigue strength. Therefore, it is important to establish an evaluation method for the vibration fatigue strength of materials to ensure appropriate strength in their design. In this study, we performed experiments to investigate the fracture mechanism of a material under multi-axial random vibration. We selected aluminium alloy A5056 as the test material and employed button-head-type specimens with a notch. The multi-axial random vibration experiments were performed at different acceleration inputs (10, 20, 30, 40, 50, 60, and 70 Grms) within a frequency band of 10–5000 Hz. During the vibration tests, we conducted observations of vibration behaviour. After the vibration tests, we observed the fracture surfaces of the specimens using a scanning electron microscope. The results show that the fatigue fracture was due to the bending resonance mode for the given shape and dimensions of the specimens used in this study. In addition, cracks initiated at different areas on the fracture surface and later propagated; subsequently, the cracks coalesced. Finally, we discussed whether the fatigue life of materials subjected to vibration can be predicted using finite element analysis.

Generic Approach for Risk Assessment of Offshore Piping Subjected to Vibration Induced Fatigue

Abstract Vibration induced fatigue (VIF) failure of topside piping is one of the most common causes of the hydrocarbon release on offshore oil and gas platforms operating in the North Sea region. An effective inspection plan for the identification of fatigue critical piping locations has the potential to minimize the hydrocarbon release. One of the primary challenges in preparation of inspection program for offshore piping is to identify the fatigue critical piping locations. At present, the three-staged risk assessment process (RAP) given in the Energy Institute (EI) guidelines is used by inspection engineers to determine the likelihood of failure (LoF) of process piping due to VIF. Since the RAP is afflicted by certain drawbacks, this paper presents an alternative risk assessment approach (RAA) to RAP for identification and prioritization of fatigue critical piping locations. The proposed RAA consists of two stages. The first stage involves a qualitative risk assessment using fuzzy-analytical hierarchy process (FAHP) methodology to identify fatigue critical systems (and the most dominant excitation mechanism) and is briefly discussed in the paper. The fatigue critical system identified during stage 1 of RAA undergoes further assessment in the second stage of the RAA. This stage employs a fuzzy-logic method to determine the LoF of the mainline piping. The outcome of the proposed RAA is the categorization of mainline piping, into high, medium, or low risk grouping. The mainline piping in the high-risk category is thereby prioritized for inspection. An illustrative case study demonstrating the usability of the proposed RAA is presented.

Development of Measurement Methods for Vibration-Induced Stress of Small-Bore Piping

The vibration-induced fatigue failure of small-bore piping is one of the common causes of failure trouble at nuclear power plants (NPPs). Therefore, the purpose of this study is to develop the measurement methods of vibration-induced stress for the screening to prevent from fatigue failure mechanism of small-bore piping. First, a measurement method using a single-mass model was introduced, and then, a measurement method using a two-mass model developed as an improved calculation model was proposed. These two kinds of models were validated by vibration tests using mock-up with small-bore branch piping. The results showed that the single-mass model could be used as the coarse screening. Additionally, the two-mass model was found to be suitable to the fine screening due to more accurate measurement of vibration-induced stress. Next, for small-bore piping with typical pattern configurations consisting of several masses and supports, the model considering the supports and the center of gravity being out of pipe centerline was developed and put into practical use. Finally, for the more complex small-bore piping with general piping configurations consisting of many bends, branches, or joints, the method based on the finite element analysis and using the measured values was developed. In the developed method, the differences between the natural frequency and the response acceleration obtained by the measurement and those values calculated using the analysis model are optimized to be enough small, and then, the vibration-induced stress is estimated by superposing the vibration modes of the small-bore piping with the static deformation representing the main piping vibration. In this study, the usability of the developed method was confirmed by the comparison with the numerical results without the measurement error, which were assumed to be the true values. The peak stress induced by vibration frequently occurs at the filet weld part between the small-bore piping and the main piping. The developed methods can be used for various weld geometries although the measurement method using strain gauges cannot be used for such weld parts. The failure possibility by vibration-induced fatigue can be evaluated by comparing the nominal stress measured by the methods in this study with the fatigue threshold stress divided by the stress concentration factor appropriate for the weld geometry.

Minimizing hydrocarbon release from offshore piping by performing probabilistic fatigue life assessment

Abstract Topside piping is the major source of hydrocarbon release (HCR) on offshore oil and gas (OOG) platforms in the North Sea region. Since 21% of piping failures are caused by vibration induced fatigue (VIF), an accurate remnant fatigue life (RFL) assessment has the potential to minimize the chances of HCR from an operating piping system. BS-7910 gives two possible approaches for performing a RFL assessment: the S–N curve approach and the fracture mechanics (FM) approach. Since there are large number of uncertainties (such as uncertainty due to the crack growth model, future loading, material and geometric properties, etc.) involved in the RFL calculation process, therefore it is vital to consider the aforementioned sources of uncertainty in order to arrive at an accurate RFL estimate. Nevertheless, BS-7910 provides limited guidance on how to handle uncertainty in RFL assessment. The most common way of dealing with the aforementioned uncertainty is to evaluate RFL probabilistically. This manuscript thus explains the procedure of the probabilistic RFL assessment of offshore topside piping, with an emphasis on uncertainty quantification, propagation and management. Uncertainty quantification handles the identification and characterization of the different sources of uncertainty that may influence the future behavior of the piping component and, in turn, the RFL estimate. Thereafter, uncertainty propagation employs the formerly quantified uncertainties and utilizes the aforementioned information to estimate the RFL. Finally, uncertainty management deals with performing sensitivity analysis to find the individual contributors to uncertainty in the estimated RFL. A numerical case study illustrating the deterministic and probabilistic RFL assessment of topside piping is presented. Afterwards, probabilistically predicted RFL is used to demonstrate the calculation of an inspection interval. Finally, the implications of probabilistically estimated RFL on HCR from process piping is discussed.

A comparative study of finite element methodologies for the prediction of torsional response of bladed rotors

The prevention of torsional vibration-induced fatigue damage to turbo-generators requires determining natural frequencies by either field testing or mathematical modelling. Torsional excitation methods, measurement techniques and mathematical modelling are active fields of research. However, these aspects are mostly considered in isolation and often without experimental verification. The objective of this work is to compare one dimensional (1D), full three dimensional (3D) and 3D cyclic symmetric (3DCS) Finite element (FE) methodologies for torsional vibration response. Results are compared to experimental results for a small-scale test rotor. It is concluded that 3D approaches are feasible given the current computing technology and require less simplification with potentially increased accuracy. Accuracy of 1D models may be reduced due to simplifications but faster solution times are obtained. For high levels of accuracy model updating using field test results is recommended.

Removal of CrMo alloy steel components from investment casting gating system using vibration-excited fatigue failure

In investment casting, the separation of cast components composed of strong, hard materials from the gating system is a delicate operation, which can incur considerable costs in terms of labor and tools. In this study, we substituted conventional grinder cutting with vibration-induced fatigue failure to facilitate the separation of chromium-molybdenum (CrMo) alloy steel components from an investment casting tree. This process involves the creation of V-shape notches on the ingate structure as well as modifications to component layout within the pattern tree. Vibration-excited dynamic experiments were performed to examine the effects of notch designs on stress concentration and cutting off at the ingates. Mold flow analysis was used to optimize the design of the pouring system to ensure casts of high quality. Finite element method (FEM) and experimental modal analysis (EMA) were conducted to predict the efficacy of the model and vibration characteristics. The application of harmonic response analysis to a casting tree model determined the maximum and minimum principal stresses at ingate notches. Once the stress values at the ingate notches were sufficient to ensure failure conditions, the same parameters were used to perform a final experiment for verification. Experiment results revealed that the breaks indeed occurred at the notches of the ingate. The proposed approach could be used as an alternative to conventional cutting using a grinding wheel as a means of reducing labor costs, increasing safety, and enhancing production efficiency.

An effective method for separating casting components from the runner system using vibration-induced fatigue damage

In the final step of the investment casting process, the casting components have to be cutoff at the ingate from the main sprue or the runner system. Cutting the ingates is normally performed using a grinding wheel cutting machine, which is dangerous and consumes a great deal of labor and time. This work thus introduces a vibration method for separating casting parts from the casting tree. First, a V-shaped notch near the ingate of the runner is designed in order to generate the stress concentration effect during the induced vibration process by a pneumatic hammer. Then, the structural modal properties of the casting tree are analyzed to derive the natural frequencies and mode shapes. The flexible modal vibration of the casting parts due to resonance effect during excited vibration will cause fatigue damage at the notch, allowing the castings to be removed from the tree. Mold flow analyses are conducted to design the gating system for optimizing the casting quality. Harmonic response analysis is performed to derive the vibration response incurred by the pneumatic hammer and to predict the maximum principal stress on the notch. The fatigue damage at the notch is evaluated using the modified Goodman theory. The suitability of the proposed scheme is demonstrated via processing experiments and numerical simulations. The proposed method is quicker, safer, and cleaner than traditional grinding cutoff processes and is thus a promising cost-effective method for application in the investment casting industry.