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Aircraft Structural Integrity Program (ASIP) Perspective on Accounting for Engineered Residual Stress In Damage Tolerance Analysis
Chuck Babish, U.S. Air Force Life Cycle Management Center
The USAF has significant experience with utilizing and relying upon Engineered Residual Stress (ERS) methods to increase damage tolerance inspection intervals in safety-of-flight structural locations. These includes: interference fit fasteners, cold expanded holes, and laser shock peening. The initial USAF damage tolerance requirements limited the beneficial effects to be used in design and current USAF practices include maintaining these limits during the aircraft sustainment phase. The presentation will describe 3 technical needs that must be satisfied to take “full credit” during the aircraft sustainment phase, namely: (1) validated damage tolerance analysis method, (2) validated quality assurance method, and (3) validated NDI method. The presentation will describe these 3 technical needs in detail along with the rationale for each.
Regulatory Considerations for Residual Stresses in Aircraft and Engine Components
Michael Gorelik, Federal Aviation Administration
Residual stresses in aircraft components can be divided into two broad categories: (a) engineered residual stresses that are deliberately introduced, e.g., shot peening, laser shock peening, low plasticity burnishing and cold working of holes, and (b) residual stresses as a byproduct of manufacturing processes, e.g. bulk residual stresses resulting from the forging or heat treatment process, or near-surface residual stresses caused by machining operations such as drilling, broaching, single-point turning or grinding. These residual stresses can play a significant role in the structural integrity of a variety of high criticality components, including principal structural elements (PSEs) of the airframes, high energy rotating components (disks) in turbine engines, engine propellers etc. This presentation discusses regulatory considerations for both engineered and manufacturing process induced residual stresses in airframes and propulsion engine components of high criticality from the “system” perspective, including measurement or modeling of residual stresses, process controls, NDE, effect of in-service conditions, and incorporation of residual stresses into fatigue and damage tolerance assessment framework.
The Impact of Bulk Residual Stress on the Qualification of Large Aluminum Forgings
Dale Ball, Lockheed Martin Aeronautics Co.
When new materials and processes are adopted for use in primary structural components of man-rated military aircraft, the aircraft manufacturer must demonstrate to the procuring agency that five process factors have been met; these are: stability, producibility, characterized mechanical and physical properties, predictability of performance, and supportability. The adoption of aluminum forgings for large, monolithic, primary airframe components of a current military aircraft has prompted the development of new analytical and experimental procedures which address forging process induced bulk residual stress explicitly. While the presence of these residual stresses has played a role in each of the five process factors cited above, significant effort has been directed toward understanding their impact on mechanical properties and predictability of performance in particular. In this presentation, a brief overview of the steps that were taken in order to qualify the forgings and the explicit residual stress methods will be given. In addition, one of the aforementioned R&D projects leading to these qualifications will be described.
The Engineered Residual Stress Implementation (ERSI) Working Group
Bob Pilarczyk, Hill Engineering, LLC.
The application of engineered residual stresses on new and legacy USAF weapon systems provides an opportunity to significantly extend the fatigue life at critical locations. In order to reach required service life goals and recurring inspection intervals within budgetary constraints, the ability to implement engineered residual stress into analyses is essential. However, it has been repeatedly demonstrated that in order to properly measure, apply, and analyze engineered residual stresses, sophisticated analytical tools and advanced technical knowledge and training are required. The ERSI working group provides the opportunity for collaborative discussion and development of best practices for government, contractor, and academic engineers supporting engineered residual stress implementation. The ultimate goal of the working group is a framework of tools and processes for the general application to aircraft structures, avoiding expensive point design and offering benefits to all stakeholders.
The Inadequacy of Uncertainty Estimation in Residual Stress Measurements
Michael Prime, Los Alamos National Laboratory
Using measurement results for structural design or certification should requite realistic uncertainty estimates because of the critical nature of those objectives. Unfortunately, most uncertainty estimates for residual stress measurements are lower bounds on actual uncertainty, precisely the inverse of what one would prefer for critical data. Underestimated uncertainty pervades scientific measurements because you can only estimate the uncertainties you know about and then know how to estimate, both challenging problems, so this is hardly unique to residual stress. This talk presents examples of demonstrably underestimated uncertainties in residual stress with two contrasting examples: neutron diffraction and incremental slitting. In each case, ideas are presented for improving the uncertainty estimate in ways that are broadly applicable to diffraction and relaxation methods, respectively.
Weber Metals’ Experiences on Residual Stress with Al and Ti Alloy Forgings
Tony Yao, Weber Metals Inc.
Stories will be told about Weber Metals’ overall experience on residual stress in the production of Al and Ti forgings, including examples of warpage and cracking during quenching, as well as canning and distortion phenomena after machining. Possible reason on how residual stress is introduced will be discussed, along with methods to alleviate high stress from the beginning in practice. Stress relieve methods, including cold compressive stress for Al 7050 forgings and thermal stress relieve for Ti forgings, will be addressed. Simulation efforts on quenching, stress relieve and machining processes have been tried and examples will be given on this topic. Experimental measurement of residual stresses using hole-drilling and other non-destructive and destructive methods will be introduced as well with Weber Metals’ own experience.
Comparison of Residual Stress Measurements from Multiple Techniques in Die-forged 7085-T7452
T. J. Spradlin, AFRL
Two parent forgings for monolithic-unitized aerospace components were used for the comparison of multiple residual stress measurement techniques. The specimens were nominally identical (die-forged and quenched) with one having an additional strain-relief step via tuned-die cold working. Multi-component residual stress measurements were made in each component along an identical plane using three different techniques: energy dispersive X-ray diffraction (EDXRD); neutron diffraction (ND); and primary slice removal (PSR) biaxial mapping. Good agreement was found between the EDXRD and PSR measurements in common stress components: in-plane stresses tended to be compressive at the surface and tensile at the center line for both, with stress magnitudes being significantly reduced in the strain relieved component. Out-of-plane were measured for the ND experiment and calculated for the EDXRD experiment using a plane-stress assumption. These stresses were confirmed to be nominally zero for the measurement points common to both experiments.
Residual Stress Production Quality Control
Adrian T. DeWald, Hill Engineering, LLC.
Aircraft engine and structural components are being produced from forgings with increasingly complex geometries in a wide range of aerospace alloys. The forging process involves a number of steps required to attain favorable material properties (e.g., heat treatment, rapid quench, cold work stress relieving, and artificial aging). These processing steps, however, also result in the introduction of residual stress. Excessive bulk residual stresses can have negative consequences including: part distortion during machining and/or during service, reduced crack initiation life, increased crack growth rates, and an overall reduction in part life. This presentation will describe an approach for quality management of residual stresses in aerospace forgings. The quality management system relies upon computational process modeling, residual stress measurement, and the integration of these concepts within the framework of a standard production quality system.
Efficient Shaping and Reshaping of Complex 3D Parts Using Engineered Residual Stress
Matt Watkins, Engineering Software Research & Development, Inc.
Increasing emphasis on affordability of military systems has led to a number of advances in airframe production. For example, high speed machining (HSM) has made it possible to fabricate thin structures that provide improved performance. However, near-surface plastic strains induced by HSM or other surface treatments can lead to excessive component distortion such that the component requires reshaping, resulting in additional cost. In other cases surface treatments are applied specifically to reach a target shape. In this presentation, a new analytical procedure using engineered residual stress is discussed for the optimization of machining and surface treatment parameters such that distortion can be minimized or controlled. The technology is applicable to high speed machining, shot peening, laser peening, and other treatments inducing sub-surface residual stresses (SRS) for both distortion mitigation and forming operations. This procedure is expected to lead to significant cost savings in the manufacturing of metallic aerospace parts by replacing inefficient trial and error experiments with a predictive model. The results of a reshaping validation experiment using a representative aerospace structural component will be presented.
Tailoring Performance of Nanoelectronics Using Strain Engineering
Conal Murray, I.B.M. T.J. Watson Research Center
As the demands of high-performance computing have dictated greater device density and smaller dimensions, new types of designs and materials must also be incorporated. For example, the piezoresistive response of strained semiconductors allows one to change carrier mobility within the current-carrying regions of a device by applying the appropriate state of strain. Understanding the mechanical response within these semiconductor structures requires a determination of the driving force imparted by the adjacent stressor layers. Methods based on wafer curvature are routinely used to assess the corresponding eigenstrain within blanket stressors. However, novel crystallographic orientations of semiconductor substrates can exhibit anisotropic curvature even when the in-plane residual stress of the stressor layer is isotropic. We demonstrate this effect and present an analytical model that can be used to extract the corresponding residual stress in blanket films deposited on substrates with arbitrary orientations.
Applications of Multiple Residual Stress Measurement Methods
Detailed quantification of the residual stress state in a body remains a challenge even with the development of increasingly powerful diffraction and strain relief measurement techniques and sophisticated prediction methods. Multiple application of the same measurement technique can give an estimate of stress measurement uncertainty but its rarely feasible or economic to do so outside of round robin exercises, such as those performed on the VAMAS shrink-fit ring and plug round robin sample and the European NeT consortium on weldment benchmarks. However the application of multiple residual stress measurement methods can give a more complete tensorial, spatial and length-scale characterisation of residual stress in an engineered structure, and reduce uncertainty regarding the true stress state present, especially if measurement techniques based upon diverse principles (e.g. physics and mechanics-based) are chosen. A multiple measurement method approach is required to ‘validate’ weld computational mechanics residual stress simulations for nuclear structural integrity applications. This presentation demonstrates how application of multiple residual stress measurement methods can reduce the uncertainty in quantifying the stress state, as well as revealing the limitations of individual approaches and giving a more complete picture of the stress state in the component of interest.
Some Reflections on Living with Residual Stress
Lyndon Edwards, Australian Nuclear Science & Technology Organisation
There have been many advances in the measurement, modelling and above all, understanding of residual stress since my personal first involvement with residual stress research just over 30 years ago. This presentation will describe the author’s personal view of the developments in the art and science of residual stress and how they have affected scientific and industrial practice and will be illustrated using examples from the Nuclear, Aerospace and Motor Vehicle Industries.
Large-Scale Facilities for Strain Measurement: An Overview
Michael Fitzpatrick, Coventry University, UK
In the last twenty years the number of neutron and synchrotron X-ray facilities offering (semi-)dedicated facilities for engineering strain measurement has increased significantly. Most large-scale facilities now have a beam line that is suitable for engineering strain measurements; and many have dedicated instruments with the facility to handle samples of large physical dimensions and with mass of hundreds of kilogrammes. This talk will provide a brief (and almost certainly incomplete) overview of the engineering strain measurement facilities currently available at neutron and synchrotron X-ray beam lines around the world, along with an assessment of ease of access for international researchers. It will highlight recent developments, and look at gaps in current provision for certain experiments. Finally, we will look at the opportunities offered by the new IMAT instrument at ISIS in the UK, for neutron transmission strain mapping with high spatial resolution.
Strain/stress Determination Near Material and/or Geometric Discontinuities
I. Cevdet Noyan, Columbia University, New York
Most strain/ stress determination techniques assume simplified stress/strain states within the measurement volume from which relevant data is acquired. Such assumptions enable saving precious measurement time, and/or or make any subsequent analysis more tractable. They are usually based on the macroscopic geometry of the sample. These assumptions are also incorporated in numerical models of the systems under examination. Despite widespread usage, such assumptions are not usually justified a-posteriori. Such omissions can lead to serious errors in the stress/strain analysis results. In this presentation we will discuss the effect of local heterogeneities on the residual and applied elastic stress/strain states within macroscopically symmetric samples. Experimental data and modelling results from axial and radial mechanical loading will be presented and compared.
Residual Stresses in Polymer Matrix Composites – Characterization and Modeling
Brent Volk, AFRL/RXCCM
This presentation provides an overview of residual stresses in polymer matrix composites (PMCs) from both an experimental characterization and computational modeling perspective. Residual stresses in PMCs are a result of anisotropy and volumetric effects (thermal expansion, cure shrinkage) in the composite laminate, which can be amplified due to thermal gradients. A careful understanding of the material property evolution during processing facilitates the ability to predict the residual stress state; however, the calibration of the material property evolution during curing is nontrivial. The state-of-the-art in experimental calibration techniques and process model development in addition to efforts to characterize the final residual stress state using Moire interferometry will be presented. A series of case studies that demonstrate the predictive ability of the process models and impact of the residual stress state on part performance will be described.
Hole-Drilling Stress Measurements on a Structural Scale
Gary Schajer, University of British Columbia, Canada
Evaluation of stresses in structures such as bridges, buildings, pipelines and railways is challenging because the loads cannot easily be manipulated to allow direct measurements. Although typically not of the classical type, these stresses are residual stresses because they are self-equlibrating and locked-in. This talk describes an application of the hole-drilling method with Digital Image Correlation (DIC) to evaluate large scale structural stresses. The use of DIC provides a robust means to measure full-field displacements that can easily be scaled to different hole sizes and corrected for measurement artifacts. Experimental measurements are described that demonstrate the measurement method on different structure types including the example practical problem of measuring thermally induced stresses in railroad tracks.
Measurement of Residual Stress Using High-Energy Synchrotron Sources
Armand Beaudoin, Cornell High Energy Synchrotron Source
The high-energy radiation from synchrotron sources, along with developments in detector technology and software for the analysis of diffraction data, provides a means of mapping lattice strains in structural alloys. Broadly speaking, there are two techniques. In energy dispersive diffraction (EDD), a “hardened” polychromatic x-ray beam probes the specimen with slits before and after the sample defining the size of the diffraction volume interrogated. On a modern synchrotron beamline, where an insertion device provides high flux, it is possible to refine the beam to a measurement volume of ~200x200x2000 µm3, yet make measurements in metallic samples a few cm thick. With individual EDD measurements taking on the order of a minute, it becomes possible to develop detailed maps of elastic strain components. In the monochromatic technique, a single wavelength x-ray beam is used. In practice, the thickness through which the measurement may be made is less than that of the EDD method. On the other hand, advances in detectors provide for very detailed and rapid measurements of lattice strain, since multiple lattice strain measurements can be made simultaneously. In this presentation, examples will be provided of both the EDD and monochromatic techniques in use. Emphasis is placed on the interplay between the measurement of residual stress and model validation and on the need to make measurements over a relevant volume of material (enough to enforce mechanical equilibrium) using the number of lattice strain measurements necessary to quantify the full stress state.
MIDAS: Material Informed Digital Design Demonstration for Additive Structures
Paul Shade, AFRL/RXCM
Additive manufacturing presents both extreme potential and concern for component design. The ability to spatially tailor processing conditions opens the door to sophisticated designs with heterogeneous materials. Accounting for this heterogeneity, before exploiting it, requires the ability to link local processing state to properties/performance of local material. A concern with current geometry-based design approaches, such as topology optimization, is not directly accounting for material property changes as geometry updates are made. The MIDAS program addresses this challenge by generating highly-pedigreed data sets including detailed process descriptions (beam path, location specific processing conditions), post-build characterization (X-ray CT, RUS, 3D Optical, SEM) and mechanical testing (milli-tensile, in situ HEDM, notch, torsion testing). These data sets will be made available to the AM community in process-structure and structure-property material modeling challenge problems. Finally, challenge results will be used to feed novel model aggregation akin to weather forecasting strategies.
The Impact of Residual Stress on Hybrid Additive/Subtractive Manufacturing Processes
Jared Heigel, National Institute of Standards and Technology
Metal additive manufacturing (AM) processes, such as powder bed fusion or directed energy deposition, generate a significant amount of stress in the part due to the localized plastic deformation that occurs around the melt pool as the material rapidly cools. The resulting stresses can cause the build process to fail when the part distorts or breaks off the plate. When the build is successfully completed, the remaining stresses will cause the part to distort as it is removed from the build plate or while it is undergoing post-process machining. Although heat treatment can alleviate stress and distortion, it cannot be implemented during hybrid processes that alternate between AM and traditional material removal processes, such as machining. This talk presents the ongoing research at the National Institute of Standards and Technology to understand the process and measurement challenges associated AM and hybrid processes.
Residual Stress Measurements on Additively Manufactured Ti6Al4V ‘Bridge’ Shaped Components
Maria Strantza, Los Alamos National Laboratory
Additive manufacturing (AM) of metals brings new possibilities on the production of low cost industrial applications. However, the residual stresses that develop during the AM process can compromise the performance of the AM components. Previous investigations have indicated that the magnitude of the residual stresses can be affected by various parameters such as the material deposition pattern. In this study, we focus on the investigation of the residual stress state of AM metallic parts. Ti6Al4V ‘bridge’ shaped components were built using island and continues scanning patterns under two different angles (0o and 45o). Measurements were performed by means of energy dispersive x-ray diffraction at the Cornell High Energy Synchrotron Source. Results indicate that the magnitude of the residual stresses is affected by the scanning patterns. These measurements were completed explicitly to validate process models being developed at Lawrence Livermore National Lab and the results will be critically compared to the simulations.
Remote Manipulation Residual Stress Measurement System for Extreme Environments
Ann Marie Phillips, Idaho National Laboratory
The US High Performance Research Reactor (USHPRR) Project is working to develop and qualify a new high-density monolithic fuel to facilitate conversion of five U.S. research reactors from high enriched uranium (HEU) to low enriched uranium (LEU). The fuel system consists of U-10Mo alloy fuel foils with a thin Zr diffusion barrier interlayer, clad in 6061 Al alloy by hot isostatic pressing (HIP). Due to differing mechanical and thermal properties and constrained interfaces, residual stresses can be induced during fabrication and/or developed during irradiation. The residual stress state may have a significant influence on the fuel plate’s ability to resist delamination, one of the primary requirements for fuel qualification. The Idaho National Laboratory and the Los Alamos National Laboratory collaborated to identify potential methods of measuring residual stress of such irradiated fuel plates. The incremental slitting or crack compliance technique, demonstrated on surrogate depleted uranium (DU) plate fuels at LANL, was down-selected due to adaptability for a hot cell. The hot cell requirements, including remote manipulation and an argon environment led to the design of a slitting system with eddy current sensors to measure deflection and a fine, high-speed cutting tool to introduce the slit. Of particular interest is the extent of the stresses that develop at the interfaces between the cladding, Zr barrier layer and the U-Mo fuel, and how these stresses relate to interfacial bond strength, both before and after irradiation, as well as irradiation conditions. Residual stress measurement systems for pre- and post-irradiation measurements have been developed and tested on surrogate materials and compared with traditional slitting using wire EDM and strain gauges.
Full Thermal Simulation of an Arbitrary, Plane, Axisymmetric Residual Stress Field
Tony Parker, Defence Academy of UK
These formulations permit full thermal simulation of an arbitrary plane axisymmetric residual stress field encompassing hoop, radial and axial stresses. Earlier formulations were based upon the determination (analytically or numerically) of a steady state temperature profile within the tube, but this temperature profile can only replicate radial and hoop stresses; in general axial stresses are incorrect. This new thermal simulation provides all three stresses and is achieved by incorporating orthotropic coefficients of thermal expansion that themselves vary with radius. Results are generally highly accurate. These formulations permit straightforward determination of stress intensity factors for arbitrarily-orientated cracks within cylindrical pressure vessels using well-tried FE methods.
Detection of Grinding Stresses Nondestructively with Magnetic Barkhausen Noise
Theo Rickert, American Stress Technologies, Inc.
The detection and prevention of grinding burn is critical to the production of ground components of all types and sizes. Overheating during grinding, commonly known as grinding burn or grind temper, results in undesirable microstructure, hardness, and stress characteristics both on and below the surface. Magnetic Barkhausen Noise (MBN) is quantitative, repeatable and non-destructive for detecting grinding burn. The method is easily automated and thus removes operator influence as a variable. Most importantly, the MBN method is sensitive to the residual stresses which are induced by grinding burns. Using a sample set of carburized spur gears, ground to varying conditions of grinding burn, the MBN method is demonstrated to effectively detect grinding burn and show sensitivity to residual stress. Detection of various intensities of grinding burn, including re-hardening burn, is demonstrated using fully automated MBN instrumentation. Residual Stress depth distributions measured with Hole Drilling and ESPI along with X-Ray Diffraction and electrochemical layer removal are utilized as a quantitative verification method.
Tools for Management of Residual Stresses in Advanced Manufacturing
Michael Stender, Sandia National Laboratory
Tools for lifecycle engineering that adequately incorporate elements of advanced manufacturing processes, mechanical property prediction, environmental service and failure models, as well as uncertainty analysis must also provide access to residual stresses associated with manufacturing and assembly. For long-term aging processes that cannot be accelerated, predictive models are requisite and the evolution of properties during aging amplifies the importance of incorporating accurate residual stress predictions in performance models. Sandia has been developing materials models that can spatially track mechanical properties as well as evolve residual stresses during advanced manufacturing processes with the goal toward predicting component lifetime performance. With growing interest in additive manufacturing where large residual stresses are intrinsic aspects of the process, materials modeling tools are being expanded to capture the differentiating physics of these processes. The presentation will describe the life cycle tools being employed at Sandia with the example of residual stress forecasting in forged product. Additionally, new efforts to incorporate the characteristics of directed energy deposition into residual stress and materials property forecasting will be discussed.
Residual Stress Determination of DMLS Samples by Diffraction
Tom Watkins, Oak Ridge National Laboratory
Direct Metal Laser Sintered (DMLS) Inconel 718Plus samples were examined with x-ray and neutron diffraction and compared to curvature measurements. The microstructure of select samples was examined using SEM+EDS, EBSD and TEM. The residual stresses will be compared to two build parameters: laser power and speed. The measured residual stresses in the bars will be used to validate and refine modelling performed at Honeywell. Understanding and controlling the residual stress is a big component of the modelling effort for additive manufacturing at Honeywell and the additive manufacturing community. Progress to date will be presented.
Residual Stresses in Multiple Passes of Wire Arc Additively Manufactured Stainless Steel
Bjørn Clausen, Los Alamos National Laboratory
The last decade has seen tremendous advances in the ability of X-rays and neutrons at large scale facilities to probe microstructure at unprecedented length and time scales under unique environments that simulate manufacturing conditions. Concurrently, manufacturing is undergoing a revolution as investments are made in advanced manufacturing techniques, such as additive manufacture. It is natural that advanced manufacturing techniques should couple with advanced characterization techniques in order to accelerate the process of qualification of products for critical applications. This talk will present our efforts to utilize high energy x-ray diffraction to measure residual stresses in wire arc additive manufacturing (WAAM) stainless steel after each layer was deposited. Residual stresses and phase fractions were measured in the sample after one layer deposition, re-measured after another layer was deposited on top, and finally again after a third layer was deposited. In addition to the residual stress measurements, in-situ diffraction and imaging measurements were used to determine the temperature, phase fraction (liquid, ferrite and austenite), texture, etc. during deposition and during the rapid cooldown following deposition.
An Overview of Residual Stress Characterization of AM Parts at the ORNL Neutron User Facilities
Andrew Payzant, Oak Ridge National Laboratory
Direct printing of three-dimensional metal parts, or additive manufacturing (AM), has rapidly grown to become one of the major materials categories in our neutron beamline user community at ORNL. Neutron diffraction based mapping of residual stresses is a well-established non-destructive technique for the study of polycrystalline metal components, and ORNL has two dedicated beamlines for this research. The first such experiment at ORNL on an AM object was carried out and published in 1998, on an H13 steel part made by the LENS process at Sandia National Lab. Inherent in the existing metal AM processes is the gereation of stresses, texture, microstructural variations, and defects, all of which impact the mechanical properties and hence the application of these materials. This presentation will highlight the scope of work carried out at ORNL over the past several years on AM parts, illustrating the range of materials (including steel alloys, superalloys, aluminum alloys, and titanium alloys), sample geometries, and the scope of applications (including automotive, aerospace, and others), and the neutron scattering techniques employed.
Quantitative Assessment of Residual Stress Evolution During SLM Additive Manufacturing
Nathan Levkulich, Wright State University
During additive manufacture via selective laser melting (SLM), large thermal gradients and rapid cooling rates can lead to the development of residual stresses that may result in build distortion and thermal cracking. In the present work, a number of nondestructive and destructive techniques were applied to establish the effect of process parameters on residual stress development during SLM of Ti-6Al-4V. The process parameters included substrate overhang, laser power, stripe width, scan speed, build volume, and substrate condition. X-ray diffraction and hole-drilling measurements were performed on several surfaces of each deposit. In addition, the contour and layer removal methods were utilized to measure the residual stress evolution through the thickness of SLM deposits. The results from this work provide a quantitative foundation for future simulations of residual stress evolution during additive manufacture.
Design for Residual Stress: A Concept to Take Full Advantage of Material Potentials by Optimizing the Corresponding Manufacturing Process Chain
Horst Bruennet, Saarland University, Germany
This poster presents a concept for the residual stress optimized process chain design, i.e., design for residual stress. Industrial-related applications are presented and validations are shown with different residual stress measurement techniques: neutron diffraction, X-ray diffraction and optical hole-drilling. In contrast to conventional design approaches, technologies, production scenarios and process chain layout have to be arranged according to the manufacturing process step, that selectively induces the beneficial residual stresses. This is especially effective when applied at an early stage of the product engineering process, where the degree of determination is small enough to allow changes in the process chain layout.
Residual Stress Measurement of PEEK Components
Jason Wild, University of Manchester, UK
Polyaryletheretherketone (PEEK) thermoplastic polymers are commonly used in the Oil and Gas energy sector for a variety of applications. Residual stress is typically generated in such components by changes of temperature, crystallisation, thermal treatments, or a combination of all three. The aim of this study is to evaluate the residual stress state of as-received Victrex 450G PEEK injection moulded plaques. Near-surface residual stresses were measured using a combination of incremental centre hole drilling and micro-Raman spectroscopy. Both techniques indicate the presence of a characteristic “skin-core” short range residual stress field distribution.
Quench-induced Residual Stress Evolution and Relief in Aluminum Alloy Plate and Component
Chenxing Li, Harbin Institute of Technology, China
The aim of the research is to establish a reliable model to predict the evolution of residual stress in aluminum alloy components during the process of quenching and molding. The quench model is based on the hyperbolic sine constitutive relation determined by isothermal compression tests and the HTC calculated by DEFORM-HT as an inverse problem. Firstly, the quench-induced residual stress of aluminum plate is studied, and verified by slitting method, with water and polyalkylene glycol as quenchant in different temperature and concentration, separately. Then, we simulate the quenching-molding chain of a designed component to study the influence of reduction and friction coefficient to quench-induced residual stress relief, which is verified by contour method. The simulation results are considerably consistent with experiment results, which confirms the reliability of our model and indicates a prospect of residual stress control in actual production.
Residual Stress Redistribution in Machining: Measurement and Prediction
Renan Ribeiro, University of California, Davis
The objective of this work is to determine the residual stress distribution in prismatic parts machined from quenched aluminum bars. Three different extraction locations from the interior cross-section are considered. First, residual stress is measured using the contour method. The distribution obtained is typical of quenched bars, exhibiting high tensile stress on the center of the bar (185 MPa), and high compressive stress away from the center (-180 MPa). The measurement data on the bar are then used to develop eigenstrain fields using an inverse analysis. Following, prismatic parts are machined from three different locations on the bar cross section and measurements are carried out to obtain the redistributed residual stress. The magnitudes of residual stress on the prismatic parts vary depending on the location of extraction, but are generally lower than the magnitudes on the quenched bar. Finally, elastic finite element models including the eigenstrain provide predictions of residual stress on the prismatic parts. The eigenstrain model results agree with the measurement data within 20% of the peak stress over most of the cross section of the prismatic parts for all three geometry conditions.
Residual Stress Measurements in Additively-Manufactured Parts
Chris D'Elia, University of California, Davis
Integrating additively-manufactured parts into product supply chains requires an understanding of residual stress effects on typical part quality: dimensional accuracy, mechanical strength, and fatigue performance. Begin by evaluating as-built stainless steel hollow cylinders produced with powder bed fusion (PBF) and bosses deposited with laser engineered net shaping (LENS) for dimensional accuracy. Residual stress measurements on PBF hollow cylinders are performed to quantify axial and hoop stress fields while making corrections for the EDM cutting process. Exploratory measurements on LENS cylindrical and rectangular bosses are also performed using a self-constraint to mitigate plasticity effects during cutting. Consistently across the PBF parts, higher than expected stress fields with limits approaching the material yield strength (450MPa) are reported. These stresses are generally compressive near the inner diameters and tensile near the outer diameters. Exploratory measurements in the LENS parts reveal greater distortions and lower residual stress fields. Future work is planned to incorporate multiple mechanical methods and further verify the residual stresses measured.
Evaluating Residual Stress Directionality by Instrumented Indentation Testing with an Anisotropic Indenter
Jong-hyoung Kim, Seoul National University, Korea
In structures and components, not only is the magnitude of the residual stress important but also its directionality. Instrumented Indentation Testing (IIT) is investigated here. Generally, a Vickers indenter is used for such measurements, but when evaluating the directionality of residual stress an anisotropic indenter such as the Knoop type is needed. Such an indenter yields different indentation loads according to the direction of the longer axis when indenting to the same indentation depth within the given residual stress state. In this study we propose a parameter that matches the indentation load difference due to the residual stress to the residual stresses in each principal stress direction. We also propose a method to obtain the magnitude of residual stress of each principal stress direction component. In addition, since nanoscale Knoop indenters are difficult to manufacture, a new indenter is suggested based on a conventional Berkovich indenter and the stresses in the principal stress directions are evaluated according to the same methodology.
Estimating Indentation Curve of Stress-free State from Stressed State Using Stress Invariant Parameters
Sung Ki Choi, Seoul National University, Korea
Instrumented Indentation Iesting (IIT) is an attractive in-field testing method because it is simple and non-destructive. The basic principle of the method is that if a material has compressive stress, a greater indentation load is needed to reach the same indentation depth compared to a stress-free state. The reverse is true for indentations in a material with tensile stress. To evaluate the residual stress using IIT, we need both indentation curves of stressed state and stress-free state because we have to obtain the indentation load difference at given indentation depth. However, since it is difficult to obtain a stress-free state in weldments and thin films, we need to estimate the stress-free state of the target. After indentation testing, we can obtain not only the indentation load-depth curve but also several other indentation parameters that reflect material properties. We have found some indentation parameters that are invariant to residual stress. In this study, we estimated the indentation curve of stress-free state using these parameters which are invariant to residual stress and compared the estimated one with the real experimental result for the stress-free state.
Stress Calculations Made by ESPI in Near-Yield Residual Stressed Specimens
Niall O’Dowd, Los Alamos National Laboratory
Residual stress measurements made by the hole-drilling technique and strain rosettes are industry standard due to their reliability and relatively low cost. As residual stress approaches the yield strength of the material, however, the act of hole drilling can cause plastic yielding to occur, which must be corrected for accurate results to be achieved. In this paper, we discuss the fabrication of a bi-metallic ring and plug specimen with a known distribution of residual stress that includes values near yield of the material. Hole drilling RS measurements were made using Electronic Speckle Pattern Interferometry (ESPI) to measure the deformation. ESPI deformation measurements are compared to finite element models.
Residual Stress Mapping Using Primary Slice Removal (PSR) Biaxial Mapping
Mitchell Olson, Hill Engineering, LLC.
This poster describes the primary slice removal (PSR) biaxial mapping measurement technique, which determines maps of two residual stress components. Primary slice removal (PSR) biaxial mapping is a superposition of a contour method measurement and a series of slitting measurements in slices removed adjacent to the contour method measurement plane. The measurement begins with a standard contour method measurement to measure a two-dimensional map of the residual stress normal to a plane of interest. After the contour method measurement, several slices are removed adjacent to the contour plane. Subsequently, a series of slitting method measurements are made to form a two-dimensional map one of the in-plane residual stress components. The PSR biaxial mapping technique uses a computation to determine the stress change when removing slices and is able to take advantage of the high precision of slitting measurements. Relevant PSR biaxial mapping measurement will be shown.
Near-surface Residual Stress Measurement Using Incremental Slotting
Mitchell Olson, Hill Engineering, LLC.
This poster will describe a near surface measurement technique that determines one component of residual stress using an incrementally milled slot. The technique consists of machining a slot into a specimen, measuring strain change at a nearby location, then calculating the released residual stress using an elastic inverse. Slotting measurements offers higher measurement precision than standard incremental hole drilling measurement, since a machined slot allows greater strain release for a given amount of stress compared to a circular hole. Measurement repeatability (an indicator of precision) and relevant examples will be shown.
Development and Status of the Laser Shock Peening Station at HiLASE Facility
Sanin Zulic, Institute of Physics ASCR, Czech Republic
This poster will describe the current status of the Laser Shock Peening (LSP) station in HiLASE facility. As a recognized technique for improvement of fatigue life of metal components, Laser Shock Peening (LSP) process will be described. As a major advantage, HiLASE's "Bivoj" laser, with its exceptional parameters, which are used for LSP treatment, will be presented. The design of the LSP station and future plans for development, as well as preliminary residual stress curves on LSP treated samples, will be presented too.
Portable XRD Residual Stress Analyzer
Toshikazu Suzuki, Pulstec USA
Pulstec will exhibit a non-destructive X-ray (XRD) diffraction-based residual stress analyzer. This small, light-weight, low-cost, low-radiation-dose, fast-cycle time analyzer can measure residual stress, FWHM and retained-autenite by detecting the full Debye ring profile from a single incident X-ray angle. The equipment is well suited for lab or on-site use.
Prism - Residual Stress Measurement Based on Hole-drilling and ESPI
Theo Rickert, American Stress Technologies, Inc.
The residual stress measurement system Prism is manufactured by Stresstech Group. Instead of strain-gages, the instrument uses ESPI (Electronic Speckle Pattern Interferometry) to measure surface displacements around the hole as it is being drilled incrementally. Stress depth profiles are then calculated using the Integral Method. Completely new software was released this year. New tools improve measurement setup (and hence measurement quality) and data analysis.
MTS3000 System for Hole Drilling Tests
Alessio Benincasa, SINT Technology, Italy
A demo unit of the complete MTS3000 system will be demonstrated, including mechanical, optical, electronic device and software. Some typical results of residual stress measurements by hole-drilling will also be displayed for different application fields (shot peening, stress relieving, welding, machining). Moreover, some recent upgrades and improvements of the EVAL calculation software will be available, along with the possibility to perform a real-time evaluation and comparison of different tests.
Hill Engineering Residual Stress Measurements
Adrian DeWald, Hill Engineering, LLC.
Hill Engineering works to address issues arising in materials, manufacturing, and design engineering, and has unique capabilities in residual stress measurement, material testing, service life assessment, and mechanical design. The company combines capabilities in residual stress and fatigue engineering to develop and deliver innovative methods for material testing and analysis. Such work enables the use of compressive residual stress treatments to extend the life of aging structures and solve sustainment challenges. Customers come from a broad range of industry, including aerospace, power generation, vehicles, turbo-machinery, and petrochemicals. On display are example residual stress program applications, including a study predicting and measuring the residual stress state of a forged component and a fatigue assessment of parts containing cold expanded holes.
X-Ray Diffraction Residual Stress Measurement in Failure Analysis
Mike Brauss, Proto Manufacturing, Inc.
PROTO will be available to discuss the broad range of X-ray Diffraction (XRD) residual stress instruments and options available, to solve fatigue and failure issues in the laboratory, factory and field. This includes the mXRD ultra-portable system, the iXRD platform for large parts and the high-power LXRD cabinet system. Each product line is available in many different configurations to ensure the most accurate and compatible system is available for your specific measurement needs.
Using AFGROW to Perform Crack Growth Life Predictions
James Harter, LexTech, Inc.
The AFGROW system for crack growth life prediction will be displayed. The new Version 5.3 has the ability to model crack growth rate in two orthogonal directions, tag spectrum load levels for environmental effects and track the amount of damage related to any tagged level.