Several image processing methods have been implemented over recent years to assist and partially replace on-site technician visual inspection of both manufactured parts and operational equipments. Convolutional neural networks (CNNs) have seen great success in their ability to both identify and classify anomalies within images, in some cases they do this to a higher degree of accuracy than an expert human. Several parts that are manufactured for various aspects of turbomachinery operation must undergo a visual inspection prior to qualification. Machine learning techniques can streamline these visual inspection processes and increase both efficiency and accuracy of defect detection and classification. The adoption of CNNs to manufactured part inspection can also help to improve manufacturing methods by rapidly retrieving data for overall system improvement. In this work a dataset of images with a variety of surface defects and some without defects will be fed through varying CNN set-ups for the rapid identification and classification of the flaws within the images. This work will examine the techniques used to create CNNs and how they can best be applied to part surface image data, and determine the most accurate and efficient techniques that should be implemented. By combining machine learning with non-destructive evaluation methods component health can be rapidly determined and create a more robust system for manufactured parts and operational equipment evaluation.

Additive manufacturing (AM), also known as 3D printing, is a rapidly developing technology with tremendous potential in both developmental and production applications. Solar Turbines Incorporated is committed to AM technology for gas turbine applications. The ability to metal 3D print novel designs of turbine blades capable of actual turbine engine operation would effectively reduce design validation cycle time, and allow acquisition of key performance data early in a design campaign. In support of Solar’s advanced manufacturing development and ongoing engine efficiency improvement goals, Solar initiated a project to print a full set of Mercury™ 50 stage 2 turbine blades to be run in a development engine. Solar leveraged years of experience with design and serial production of AM components in support of this project. A significant challenge faced when printing turbine blades is producing metal with mechanical properties sufficient to withstand the rigors of engine operation. As a rotating component within the hot section of the engine, turbine blades experience high centrifugal and pressure loads at elevated temperatures. After investigation of possible alloys capable of meeting the requirements of the Mercury™ 50 design envelope, the gamma prime (γ’) strengthened nickel superalloy Inconel™ 738LC was selected to provide the best opportunity for successful development engine testing. Solar partnered with Oak Ridge National Laboratory (ORNL) to produce the Inconel™ 738LC blades with Electron Beam Melting (EBM) powder bed fusion process. Once a rough blade shape was printed, the fir-tree attachment, blade tip shroud, and air flow path surfaces were finished using both conventional and non-conventional machining processes. In-process monitoring, metallurgical evaluations, mechanical testing, and non-destructive inspection techniques were used to validate the printed blade material integrity and conformance to geometric design intent. Planned future activities include assembly of the AM blades onto a disk for spin pit testing to validate the mechanical integrity and design margin of the blades. The final phase of the project will be to install the bladed disk assembly into a Mercury™ 50 engine at Solar Turbines to conduct a series of hot-fired engine performance tests.

While metal additive manufacturing (AM) promises substantial efficiency gains to the gas turbine manufacturing sector, uncertainty about the quality of parts produced via AM has been a significant hindrance to widespread implementation. Although high fidelity inspection techniques involving computed tomography (CT) and destructive testing have been effective for low volume development activities, new quality assurance solutions are needed that enable rapid, low-cost inspection of serial production AM components. Solar Turbines Incorporated is actively engaged in the development of inspection processes for high production volume AM part acceptance capability of combustion and turbine hot section components. Eddy current inspection (ECI) was identified as a potential non-destructive evaluation (NDE) solution. Based on the principles of electromagnetism, ECI has been successful on conventional materials for surface and near-surface crack detection. However, limited industry data is available regarding the effectiveness of ECI on AM material. The nature of AM-induced discontinuities, specifically for metal laser powder bed fusion (L-PBF) processing, demands high measurement resolution to detect fine features such as bulk porosity, lack of fusion and interlayer discontinuities. Development activities were thus executed to determine the suitability of ECI for detection of AM discontinuities. NDE training sets were printed with intentional variations in key L-PBF processing parameters to simulate the conditions which produce relevant AM material discontinuities. The training sets were then evaluated with a custom ECI system to determine the inspection capability and sensitivity. Inspections were conducted as a function of multiple input frequencies to determine the optimal tradeoff between measurement resolution and depth of penetration. Additional characterization of the training sets was conducted via metallographic analysis to establish correlations between the ECI results and AM material quality. An optimized multi-frequency inspection setting was identified to provide suitable measurement resolution for near surface AM material inspection. Correlations developed between ECI scan data and materials characterization results have enabled the ability to rapidly discriminate between varying discontinuity levels in AM components. Based on these efforts, ECI is considered a suitable inspection technique for materials produced via the L-PBF AM process.

Analysis of the mechanics of an acoustic thermographic non-destructive evaluation (NDE) and its extension to an advanced acoustic wave generation is the topic of this paper. NDE is among the most effective and powerful materials evaluation and prognosis tool available for high performance turbomachines which increasingly incorporate advanced materials and extreme operating conditions. In the current work we investigate the mechanism of heat generation in acoustic thermography in detail using a combination of computation and experiments. We use the validated model to carry out a model based evaluation of an advanced thermographic based acoustic NDE method which if successful this technique could revolutionize the ease of data gathering and data monitoring. We address many of the challenges typical of this type of system, principal among them are crack detection threshold, signature quality and the effect of defect interactions. We use experiments and finite element (FE) based numerical simulation to evaluate the proposed method and draw conclusions on the viability for future extension and integration with other digital technologies for health monitoring. Our FE model is capable of capturing various distinct sources of emission signatures from defects and we validate it with a traditional acoustic method. We also use this method to determine the magnitude of the different sources of heat generation during an acoustic excitation. Defects formed through industrial operation as well as defects formed through artificial manufacturing methods were analyzed and compared.

The method of direct current potential drop (DCPD) can be utilized as an effective, and convenient approach for in-situ damage detection, and as a non-destructive evaluation technique. We present the results from use of a multiprobe DCPD technique for in-situ damage detection in loading of a SiC/SiC composite. It is shown that in three different modes of loading (monotonic, fatigue, and cyclic load-unload), the sensing capabilities of DCPD technique compares well to the techniques of modal acoustic emission (AE) and digital image correlation (DIC). It was also found that DCPD technique provides a far earlier warning of failure under fatigue loading than the other two methods. In addition, we show that strategically placed multiple voltage leads on the specimen surface provides a promising way of qualitatively determining the crack initiation site. Therefore, the use of multiple lead DCPD method, together with other techniques, provides a viable option for sensing damage in ceramic matrix composites (CMCs) with complex geometries, and for applications at higher temperatures.

Unlike more mature E-Class heavy duty gas turbine rotors, F-Class gas turbine rotors have exhibited a variety of failure mechanisms over the past 20 years. From the liberation of nickel turbine posts to large (600 mm) cracks in marriage components, F-Class rotors have failed to achieve the reliability of older units. Now as the F-Class units are approaching the OEM-recommended end of life (EOL), operators are struggling to repair and/or replace as operations and maintenance (O&M) budgets are dwindling. As such, end users are routinely forced to turn to other service providers to provide targeted (limited) inspections aimed at extending the life of these capital parts. While suitable for more mature rotor systems, recent EOL investigations into multiple OEM F-Class rotors have revealed significant issues with limited inspections. Utilizing comprehensive non-destructive testing (NDT), forging defects and surface cracks have been discovered throughout compressor and turbine rotors. However, inspection alone cannot determine if adequate life remains when an indication is found. In addition to the inspections, recommended analytical modeling and requisite material test data for CrMoV, NiCrMoV, and IN706 rotor materials will be overviewed. In some cases, the NDT indications have resulted in the retirement of individual components, as analytical predictions could not provide a suitable extension for those particular components. The concern is highlighted that a significant amount of these findings were in the cold end of the compressor, which would have been missed with more traditional, limited inspections. The goal of this paper is to provide the end user the information to reliably and safely extend the life of their rotor beyond the original OEM recommendation.

Additive manufacturing processes produce components that may introduce material anomalies at any location within a component. NDE inspection can be used to find and remove anomalies that could grow to failure during the service life, but two key inputs are required for an effective inspection: (1) the critical locations to search for anomalies, and (2) the minimum sizes of the anomalies that must be found. A new methodology is presented for probabilistic damage tolerance assessment of additively manufactured components. It consists of a new link between the DARWIN ® probabilistic damage tolerance software and the XRSim X-ray NDE simulation software. DARWIN computes fracture risk and critical initial crack sizes throughout a component, providing the two key inputs that are necessary for effective NDE inspections. XRSim computes location-specific POD curves everywhere in a component, providing the key information needed to assess the influence of inspection on fracture risk reduction. The methodology is illustrated for a representative gas turbine engine component manufactured via the direct metal laser sintering process. The results can be used to optimize the effectiveness of NDE inspection of additively manufactured components.

We developed and successfully applied a direct simulation Monte-Carlo scheme to quantify the risk of fracture for heavy duty rotors commonly used in the energy sector. The developed Probabilistic Fracture Mechanics high-performance computing methodology and code ProbFM routinely assesses relevant modes of operation for a component by performing billions of individual fracture mechanics simulations. The methodology can be used for new design and life-optimization of components, as well as for the risk of failure quantification of in service rotors and their re-qualifications in conjunction with non-destructive examination techniques, such as ultrasonic testing. The developed probabilistic scheme integrates material data, ultra-sonic testing information, duty-cycle data, and finite element analysis in order to determine the risk of failure. The methodology provides an integrative and robust measure of the fitness for service and allows for a save and reliable operation management of heavy duty rotating equipment.

Under a Phase II Navy SBIR program, Materials Research & Design (MR&D) performed an integrated analytical, fabrication and experimental program in order to develop foreign object damage (FOD) resistant silicon carbide (SiC) fiber-based ceramic matrix composites (CMCs), specifically 2D tape SiC/SiC. MR&D believes strongly that the most efficient approach towards the development of FOD-resistant CMCs is to develop a mathematical simulation of the impact event which can be correlated to measured responses and observed failures. Once correlated with available test data, the developed math model was used as part of an Integrated Computational Materials Engineering (ICME) approach to analytically investigate modifications to baseline CMC material in an attempt to identify which properties best influenced FOD resistance. The best candidate material modifications were then selected for fabrication and experimental validation. When considering all of the numerical and empirical data (visible damage, rebound velocity (energy absorption), strain data, NDE inspection and residual strength testing), all of the MR&D-proposed material modifications demonstrated improved FOD resistance when compared to the baseline CMC material.

Foreign object damage (FOD) behavior in 3-D woven SiC/SiC composites of three different fiber architectures were characterized at ambient (21 °C) and high temperatures (1100 °C) using tungsten carbide projectiles impacted at approximately 350 m/s. Non-destructive evaluation (NDE) techniques such as electrical resistance (ER) and micro-computed tomography (micro-CT) were used to evaluate and compare the damage morphologies of the different architectures. Post-impact retained strength was also conducted using a four point flexural test while utilizing acoustic emission (AE) and ER to monitor the damage accumulation.

SiC-based ceramic matrix composites (CMC) in turbine engine applications must sustain fatigue residual life after foreign object impacts that might occur in services. Experiments, nondestructive evaluations (NDE), and simulations have illustrated good correlations between impact energy and foreign object damage (FOD) and fatigue life after impact at room and 1200°C temperatures. Flat and curved five-harness satin (5HS) woven CMCs specimens, consisting of Hi-Nicalon Type S (Goodrich) and Hi-Nicalon (Rolls Royce) in MI SiC matrix, were tested and simulated. Tests measured electrical resistivity (ER), acoustic emission (AE), and microscopy. Simulations used a building block validation strategy and the Multi-Scale Progressive Failure Analysis (MS-PFA) method. Simulations complemented experiments in understanding and predicting the damage states, of impact, and fatigue residual strength after impact of CMCs to form a more complete understanding of the damage mechanisms involved in such events. The GENOA software developed by Alpha STAR Corporation [1, 2, 3] is capable of Durability and Damage Tolerance (D&DT), life, and reliability predictions by means of multi-scale progressive failure analysis (damage and fracture evolution). In general, CMCs are modeled using effective fiber, matrix, and interface constitutive behaviors, from which the lamina stiffness, strengths, and the strain rate effect can be derived. Similarly, the fatigue strength and stiffness degradation, and the effect of defects in a matrix micro crack density, voids, as well as fibers waviness, and damages after impact can be characterized. The final simulation is static loading and impact on a generic CMC SiC/SiC (Sylramic MI 5HS) blade which is to be used in future blade optimization based on minimizing damage incurred. The GENOA software platform supports FAA recommended ASTM standard Building-Block Validation Strategy with reduced tests conducting: 1) Material Calibration and Qualification, and 2) FEM Verification, Validation, and 3) Blind Predictions (Accreditation). The simulation and test comparisons performed included the damage size for both the CMC (fracture) and the steel impactor (plastic deformation), rebound velocities, SN curves for fatigue of pristine and impacted specimens at room and high temperatures. All simulations showed good correlation. The MS-PFA tool demonstrated a great potential for CMC post FOD fatigue life for part certification supported with reduced tests.

The limits of gas turbine technology are heavily influenced by materials and manufacturing capabilities. Inconel alloys remain the material of choice for most hot gas path components in gas turbines, however recent increases in turbine inlet temperature (TIT) are associated with the development of advanced convective cooling methods and ceramic thermal barrier coatings (TBC). Increasing cycle efficiency and cycle specific work are the primary drivers for increasing TIT. Lately, incremental performance gains responsible for increasing the allowable TIT have been made mainly through innovations in cooling technology, specifically convective cooling schemes. An emerging manufacturing technology may further facilitate the increase of allowable maximum TIT, thereby impacting cycle efficiency capabilities. Laser Additive Manufacturing (LAM) is a promising manufacturing technology that uses lasers to selectively melt powders of metal in a layer-by-layer process to directly manufacture components, paving the way to manufacture designs that are not possible with conventional casting methods. This study investigates manufacturing qualities seen in LAM methods and its ability to successfully produce complex features found in turbine blades. A leading edge segment of a turbine blade, containing both internal and external cooling features, along with an engineered-porous structure is fabricated by laser additive manufacturing of superalloy powders. Various cooling features were incorporated in the design, consisting of internal impingement cooling, internal lattice structures, and external showerhead or transpiration cooling. The internal structure was designed as a lattice of intersecting cylinders in order to mimic that of a porous material. Variance distribution between the design and manufactured leading edge segment are carried out for both internal impingement and external transpiration hole diameters. Through a non-destructive approach, the presented geometry is further analyzed against the departure of the design by utilizing x-ray computed tomography (CT). Employing this non-destructive evaluation (NDE) method, a more thorough analysis of the quality of manufacture is established by revealing the internal structures of the porous region and internal impingement array. Flow testing was performed in order to characterize the uniformity of porous regions and flow characteristics across the entire article for various pressure ratios (PR). Discharge coefficient of internal impingement arrays and porous structure are quantified. The analysis yields quantitative data on the build quality of the LAM process, providing insight as to whether or not it is a viable option for manufacture of micro-features in current turbine blade production.

The today’s energy market requires highly efficient power plants under flexible operating conditions. Especially, the fluctuating availability of renewables demands higher cycling of fossil fired power plants. The need for highly efficient steam turbines is driven by CO 2 reduction programs and depletion of fossil resources. Increased efficiency requires higher steam temperatures up to 630°C in today’s units or even more for future steam power plants. The gap between material properties in the hot and cold running parts of a steam turbine rotor is widened by increased live steam temperatures and the increased demand for flexibility. These technical challenges are accompanied by economic aspects, i.e. the market requirements have to be met at reasonable costs. The welding of steam turbine rotors is one measure to balance required material properties and economical solutions. The rotor is a core component of the steam turbine and its long-term integrity is a key factor for reliable and safe operation of the power plant. An important aspect of weld quality is the determination of permissible size of weld imperfections assessed by fracture mechanics methods. The integrity of rotor weld joints is assured by ultrasonic inspection after the final post weld heat treatment with respect to fracture mechanics allowable flaw sizes. This procedure usually does not take credit from the quality measures applied during monitoring of the welding process. This paper provides an overview of an holistic design approach for steam turbine rotor weld joints comprising the welding process and its improved online monitoring, non-destructive evaluation, material technology, and its fracture mechanics assessment. The corresponding quality measures and their interaction with fracture mechanics design of the weld joint are described. The application of this concept allows to exploit the potentials of weld joints and to assure a safe turbine operation over life time.

It was surprisingly reported that a generator rotor could not be balanced to an acceptable vibration level by weights at two balance planes at the drive end (DE) and the non-drive end (NDE) fan rings. Both real measured vibration data and rotordynamic calculations indicate that the rotor at rated speed of 3600 rpm appears to run just above the 2 nd critical speed (couple or conical mode). However, couple weights (same weights placed at both DE and NDE with 180-degree-out-of-phase) have little effect on 1X vibration response. A third balance plane had to be utilized to effectively reduce vibration. This paper uses measured data and rotordynamic modelling to explain these findings. It is found that the 4 th mode could affect synchronous vibration response at rated speed significantly besides the 3 rd mode. The two fan ring balance planes appear to be near the nodal points of the 4 th mode, which explains ineffectiveness of the couple weights to vibration response at rated speed in the field. Measured data from real machines including influence vectors are presented from third balance planes such as the coupling and the exciter ends, besides the fan ring wheels. The 3 rd and 4 th rotordynamic modes are also given along with unbalance response studies.

Increased reliability of industrial gas turbine rotating blades in the hot section of turbines has been achieved by utilization of advanced metallurgical techniques. These techniques include vacuum melting master alloy heats, minimizing residual stresses in blades after machining, and increasing the quality of nondestructive inspection of blades during and after fabrication. In addition, long time stress-rupture tests, corrosion evaluation, and microstructural studies on advanced alloys have provided necessary information for selection of turbine blade alloys for future generations of turbines.

Nondestructive inspection of jet engine turbine blades during field and depot maintenance is accomplished normally with fluorescent penetrant and/or visual techniques. In spite of the widespread use of the penetrant process, it has many disadvantages as a maintenance inspection method. The reliability of penetrant inspection depends upon the preparation of the blades prior to actual penetrant processing and the inspector’s skill in detecting and evaluating defect indications. The process can miss cracks if they are filled with material that blocks the penetrant from entering the defect. An eddy current technique which can detect leading and trailing edge cracks in turbine blades and an instrument based upon this technique were developed under Air Force sponsorship. The instrument has been tested in a rework facility and field maintenance. Results of the evaluation and photomicrographs of typical cracks detected in turbine blades are presented. Results are compared, in some instances, to results obtained with the fluorescent penetrant and/or visual examinations. A field penetrant inspection of leading trailing edges detect cracks at an inspection rate of one stage in 1 hr and 50 min.

This paper reviews the fabrication of technical ceramic components. The basic functions of the various steps in the fabrication process are outlined, including materials preparation, forming, sintering, and final machining. The advantages and limitations of various forming methods are discussed, as is the application of quality control and non-destructive evaluation. Since the use of ceramic components for high temperature gas turbine engines is a promising development, this paper provides an overview of fabrication processes which may come into widespread use for manufacturing such components.

Historically, gas turbine engine disks are retired when they accrue an analytically determined lifetime where the first fatigue crack per 1000 disks could be expected. By definition then, 99.9 percent of these components are being retired prematurely. Retirement-for-Cause (RFC) is a procedure, based on Fracture Mechanics, which would allow safe utilization of the full life capacities of each individual disk. Since gas turbine disks are among the most costly of engine components, adopting a RFC philosophy could result in substantial systems life cycle cost savings. These would accrue from reduced replacement costs, conservation of strategic materials such as cobalt, and energy savings.

Detroit Diesel Allison is actively applying advanced ceramic materials to components in gas turbine engines. Silicon carbide, silicon nitride, aluminum silicate, lithium aluminum silicate, and mullite are materials being used in various components in both the DDA GT 404-4 and AGT 100 engines. Approximately 9400 hr of ceramic component operating time in the GT 404 engine has been accumulated, and design, component processing, proof testing, and engine testing experience have begun to show the applicability of ceramic materials in production engines. Material variability, processing procedures, strength characterization, and nondestructive evaluations are emerging as critical but controllable factors. Ceramic components offer the potential of significant fuel consumption improvements in gas turbine engines for vehicles and other applications.

Development of ceramic components for the Garrett/Ford AGT101, Advanced Gas Turbine Powertrain System Development Project, authorized under NASA Contract DEN3-167 is an iterative process involving current and advanced NDE inspection techniques, visual/dimensional inspection, and mechanical and thermal screening tests. Analytically predicted peak thermal and/or mechanical component stresses have been defined for engine operation and are closely simulated during rig testing. As development testing continues steady state strain and transient thermal data is continually fed back to the analytical models to aid in the design/development process. As part of the ceramic development task for the AGT101 a set of ceramic parts (49), constituting a complete assembly of engine structural hardware has been successfully rig tested to 1144K (1600°F) without incident, qualifying this assembly for subsequent engine testing. During the course of testing approximately 7.5 hours of 1144K (1600°F) operation has been accumulated on the assembly.