AIAA SciTech 2015 Kissimmee, Florida The Impact of Forging Residual Stress on Fatigue in Aluminum 05 January, 2015 Aeronautics Company Dale L. Ball Carl F. Popelar Vikram Bhamidipati R. Craig McClung Southwest Research Institute Mark A. James Robert J. Bucci John D. Watton Adrian T. DeWald Michael R. Hill Hill Engineering, LLC Engineering structural integrity DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited; case no. 88ABW 2014 6009. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
The Impact of Forging Residual Stress on Fatigue in Aluminum ABSTRACT Large aluminum forgings are seeing increased application in aerospace structures, particularly as an enabler for structural unitization. These applications, however, demand an improved understanding of the forging process induced bulk residual stresses and their impact on both design mechanical properties and structural performance. In recent years, significant advances in both computational and experimental methods have led to vastly improved characterization of residual stresses. As a result, new design approaches which require the extraction of residual stress effects from material property data and the formal inclusion of residual stresses in the design analysis, have been enabled. In particular, the impact of residual stresses on durability and damage tolerance can now be assessed, and more importantly, accounted for at the beginning of the design cycle. In an effort to support the development of this next-generation design capability, the AFRL sponsored Metals Affordability Initiative (MAI) consortium has conducted a detailed experimental and analytical study of fatigue crack initiation and fatigue crack growth in aluminum coupons with known, quench induced residual stresses. In this study, coupons were designed and manufactured such that simple design features, such as holes and machined pockets, were installed in locations with varying levels of bulk residual stress. The residual stresses at the critical locations in the coupons were measured using multiple techniques and modeled using detailed finite element analysis. Fatigue crack initiation (FCI) and fatigue crack growth (FCG) tests were performed using both constant amplitude and spectrum loading and the results were compared against computed FCI and FCG lives. 2
The Impact of Forging Residual Stress on Fatigue in Aluminum INTRODUCTION FORGING PROCESS MODELING EXPERIMENTAL PROGRAM FATIGUE LIFE ANALYSIS SUMMARY 3
Introduction The use of large forgings is being driven by move toward structural unitization, which can lead to SIGNIFICANT MANUFACTURING COST SAVINGS However, use of unitized structure in general, and large forgings specifically, raises numerous CONCERNS: durability / repair, replace capability damage tolerance / crack arrest capability presence of detrimental (tensile) residual stresses can confound material property data (esp. fatigue crack growth rate data) if not accounted for may result in premature cracking if not accounted for in design The use of large forgings without attention to these concerns can lead to SUSTAINMENT COST INCREASES 4
Introduction Material / structures R&D projects have been conducted in an effort to mitigate these concerns: generate intrinsic, residual stress free, material property data fatigue crack initiation (e vs. N) fatigue crack growth rate (da/dn vs. delta-k) fracture toughness characterize residual stresses / deformations in advanced structures prior to first article analysis test predict the effects of complex geometry and residual stress on structural fatigue performance 5
Introduction Successful utilization of large aluminum forgings, requires management of residual stress effects during both manufacture (machining distortion), and operation (fatigue life) New paradigm for manufacture, sustainment of unitized structure calls for explicit representation of residual stresses during design: location within part spatial distribution at each location stress magnitude at each location This information is determined by analysis (FEA) test (contour, slitting, hole drilling, XRD, ND) observed behavior (deformation, life) 6
The Impact of Forging Residual Stress on Fatigue in Aluminum INTRODUCTION FORGING PROCESS MODELING EXPERIMENTAL PROGRAM FATIGUE LIFE ANALYSIS SUMMARY 7
Forging Process Modeling Forging-specific computational tool for residual stress and machining distortion modeling developed by Alcoa Technical Center Capable of predicting: post-quench residual stress in forging post-cold-work residual stress in forging residual stress in final machined part Forging process modeling used extensively to understand / mitigate bulk residual stress by: modification of quench practices development of Alcoa s Signature Stress Relief (SSR TM ) cold work process Alcoa is using forging process modeling to design and optimize for consistent low residual stresses 8
Forging Process Modeling The process model simulates four important processing steps: 1) Heat treatment: involves elevating the meshed forging geometry to the solution heat treatment temperature 2) Rapid quench: quenching causes high tensile stresses in the core of the forging (shown in red) and high compressive stresses on the surface (shown in blue) 3) Cold work stress relief: involves a specially designed die set that squeezes the forging at room temp. Alcoa s process is trade marked as Signature Stress Relief (SSR TM ) 4) Machining: simulates the extraction of the finished part from the forging through the removal of all elements from the FE model that are not within the machined part profile. While this does not simulate the machining process per se, it does capture the bulk residual stresses and distortions that are left in the finished part due to the forging process 9
Forging Process Modeling Ref: Watton, et. al., Residual Stress Summit, September 2010 2014 Lockheed Martin Corporation, All Rights Reserved. 10
Forging Process Modeling Computed residual stresses cannot be used in design or sustainment related structural analyses without sufficient verification and validation (V&V) AFRL sponsored Metals Affordability Initiative (MAI) program is being conducted in which detailed experimental studies have been performed Comparisons between calculated and measured residual stress in forgings Comparisons between calculated and measured residual stress in machined parts Comparisons between calculated and measured residual stress in fatigue test coupons 11
The Impact of Forging Residual Stress on Fatigue in Aluminum INTRODUCTION FORGING PROCESS MODELING EXPERIMENTAL PROGRAM FATIGUE LIFE ANALYSIS SUMMARY 12
Experimental Program Detailed, coupon level test program conducted in order to quantify the impact that bulk residual stresses have on: Mechanical properties (strength, toughness and fatigue crack growth rate) Constant amplitude and spectrum fatigue crack initiation Constant amplitude and spectrum fatigue crack growth Large set of flat, rectangular coupons were manufactured from a 7085-T74 billet that was produced by Alcoa Billet cut into logs Logs quenched and aged individually Each log left in as-quenched state (i.e. no post-quench cold work) in order to ensure that the level of residual stress in the test coupons was large enough to measurably affect performance 13
Experimental Program Coupon blanks extracted from three longitudinal positions and six transverse positions (total of eighteen unique positions) within each log 14
Experimental Program Using forging process model, Alcoa computed full field residual stresses and strains in a typical log resulting from quench and age processes These results were then used to calculate the residual stresses in coupon blanks 15
Experimental Program Test specimens were designed to allow explicit examination of the interaction of bulk residual stresses with simple, yet typical structural details (like holes and pockets) that introduce stress concentrations Four types of design feature (DF) coupons, as well as simple flat panel coupons were manufactured: DF0 flat panel DF1 centered open hole DF2 eccentric open hole DF3 centered machined pocket DF4 pair of eccentric machined pockets 16
Experimental Program Residual stress (RS) measurements were performed on specimen blanks from multiple log positions (TS, TM, BM, BS) using the slitting method (measured stress component is longitudinal wrt logs and coupons) Blanks were shown to have a RS gradient across width that ranged from compression at the edges to tension near the centerline, with nearly uniform variation through the thickness Each profile represents average of slitting measurements from three separate coupons, and averaging from left to right. 17
Experimental Program RS distributions were calculated and measured for each of the five DF coupon configurations at four of the log positions: BS, BM, TM, and TS. Alcoa computed RS profiles in coupons by mapping RS profile from specimen blank at the appropriate log position onto finite element model of the coupon and then calculating the relaxation due to the removal of the material occupied by the design feature (hole or pocket(s)) A A A A DF1 DF2 DF3 DF4 18
Experimental Program Hill Engineering, LLC performed RS measurements using the contour method on one cross-section (the anticipated critical cracking plane) for each of the five coupon types for each of the four log positions; this resulted in a total of 20 measurements TS TM BM BS contour measurement results for DF1 coupons comparison of measured and modeled results for DF1 coupons (contour data are avg. through thickness 19
Experimental Program Hill Engineering contour data (cont d) TS TM BM BS contour measurement results for DF2 coupons comparison of measured and modeled results for DF2 coupons (contour data are avg. through thickness 20
Experimental Program Hill Engineering contour data (cont d) TS TM BM BS contour measurement results for DF3 coupons comparison of measured and modeled results for DF3 coupons (contour data are avg. through thickness 21
Experimental Program Hill Engineering contour data (cont d) TS TM BM BS contour measurement results for DF4 coupons comparison of measured and modeled results for DF4 coupons (contour data are avg. through thickness 22
Experimental Program Both fatigue crack initiation (FCI) and fatigue crack growth (FCG) tests were conducted by Southwest Research Institute (SwRI) DF specimens were designed specifically to address potential interaction between a design feature and the bulk residual stress DF1 crack initiation and growth at edge of the hole in tensile RS field DF2 crack initiation and growth at edge of the hole in transition or compressive field DF3 crack initiation and growth in fillet radius at top (or bottom) of pocket in tensile RS field DF4 crack initiation and growth in fillet radius at top (or bottom) of pocket in transition or compressive field The test program included two types of loading constant amplitude (CA) with R=0.1 variable amplitude (VA) 23
Experimental Program Fatigue testing was performed using servohydraulic test frames with MTS 647 hydraulic grips, FTA (Fracture Technology Associates) control systems, and closed-loop feedback control. Load cycles applied at approx. 5 to 10 Hz. Strain surveys were completed to ensure the bending strains for each test set-up were less than 5% (per ASTM E647), actual measured bending was nominally 1.5%. Surface crack length measured visually using traveling microscopes with digital scales mounted on mechanical slides. Hole bore crack length measured visually using mirror placed in hole at 45 upper grip specimen microscope lower grip 24
Experimental Program FCI test summary DF1 4 log positions, 2 ea. DF2 4 log positions, 2 ea. DF3 4 log positions, 1 ea. DF4 4 log positions, 1 ea. total of 24 tests During execution of tests, expected crack initiation sites were visually inspected at 1000 cycle intervals. This process was generally successful at detecting cracks with total surface lengths between 0.015 and 0.02 in. Load cycling halted once crack formation detected. Unable to get cracks to initiate in fillet radii in DF3 and DF4 specimens (initiation occurred at pocket corners instead) 25
Experimental Program FCG test summary DF0 (edge crk) 4 log positions, 1 ea. DF0 (center crk) 4 log positions, 1 ea. DF1 (corner crk at hole) 4 positions, 2 ea. DF2 (corner crk at hole) 4 positions, 2 ea. DF3 (surface crk in radius) 4 positions, 1 ea. DF4 (surface crk in radius) 4 positions, 1 ea. total of 32 tests Approx. 0.02 inch pre-crack installed by CA fatigue cycling from EDM starter notch Crack growth life defined as time from 0.03 inch crack to failure fracture surface of failed DF3 specimen 26
The Impact of Forging Residual Stress on Fatigue in Aluminum INTRODUCTION FORGING PROCESS MODELING EXPERIMENTAL PROGRAM FATIGUE LIFE ANALYSIS SUMMARY 27
Fatigue Life Analysis Finite element analysis used to determine local stress at anticipated crack initiation site stress distribution on anticipated crack growth plane FE mesh in vicinity of crack plane for DF3 coupon bivariant, normalized stress distribution on crack plane for DF3 coupon FE mesh in vicinity of crack plane for DF4 coupon bivariant, normalized stress distribution on crack plane for DF3 coupon 28
Fatigue Life Analysis FATIGUE CRACK INITIATION ANALYSIS WITH RESIDUAL STRESS: Traditional strain-life analysis Track cyclic stress-strain hysteresis behavior at control point Calculate damage based on strain amplitude of closed hysteresis loops Incorporate RS effects by using residual stress and strain at crack initiation site to define initial condition from which hysteresis loop tracking is conducted Change in closed loop R e and R s changes calculated damage (mean stress effect) 29
FCI ANALYSIS WITH RESIDUAL STRESS (cont d): Calculated life defined as number of cycles required for damage index to reach unity (1.0) Incremental damage calculations are based on material data curves of strain amplitude vs. number of cycles to crack initiation (typically 0.01 in. flaw) Cumulative damage index vs. time (cycles): inclusion of tensile RS for given applied spectrum causes slight increase in damage rate and slight decrease in crack initiation life (compared to baseline, i.e. no RS) minimal apparent variability among TS, TM, BM and BS RS profiles The Impact of Forging Residual Stress: Fatigue Life Analysis 30
FCI ANALYSIS WITH RESIDUAL STRESS (cont d): The Impact of Forging Residual Stress: Fatigue Life Analysis Cumulative damage index vs. time (cycles): for fixed tensile residual stress and variable applied stress, damage rates increase (and calculated lives decrease) with increase in applied stress, as expected Variation of FCI life with both applied stress and residual stress can be shown in FCI-based stress vs. life curves 31
Fatigue Life Analysis FCI ANALYSIS WITH RESIDUAL STRESS (cont d): Calculated FCI lives compared with experimental results All calculated lives are either within USN life scatter factor of 2.67, or are conservative FCI life only moderately sensitive to inclusion of tensile RS 32
Fatigue Life Analysis FATIGUE CRACK GROWTH ANALYSIS WITH RESIDUAL STRESS: Use LEFM approach Quantify residual stress profile (experiment / analysis / both) on critical plane at control point Calculate residual SIF due to residual stress at control point using weight function or Green s function solution Calculate applied SIFs at control point for bypass, bending and bearing loads using both hand-book and Green s function solutions Superimpose K-residual with applied K due to spectrum loading Standard load interaction model (generalized Willenborg) used Change in max SIF, R, r y, etc. causes increase in crack growth rate and corresponding decrease in predicted crack growth life Note that this method does not address potential changes in the residual stress distribution due to cyclic plasticity or crack advance 33
FCG ANALYSIS WITH RESIDUAL STRESS (cont d): Calculated life defined as number of cycles required for crack to grow from ci to failure Crack length vs. time (cycles): inclusion of tensile RS for given applied spectrum causes significant increase in crack growth rate and decrease in crack growth life (compared to baseline, i.e. no RS) minimal apparent variability among TS, TM, BM and BS RS profiles The Impact of Forging Residual Stress: Fatigue Life Analysis 34
FCG ANALYSIS WITH RESIDUAL STRESS (cont d): Crack length vs. time (cycles): The Impact of Forging Residual Stress: Fatigue Life Analysis for fixed tensile residual stress and variable applied stress, crack growth rates increase (and calculated lives decrease) with increase in applied stress, as expected Variation of FCG life with both applied stress and residual stress can be shown in FCG-based stress vs. life curves 35
Fatigue Life Analysis FCG ANALYSIS WITH RESIDUAL STRESS (cont d): FCG models for cracks at holes (DF1 and DF2) with RS correlate reasonbly well with test data 36
Fatigue Life Analysis FCG ANALYSIS WITH RESIDUAL STRESS (cont d): FCG models for surface cracks in radii do not capture all aspects of observed behavior 37
Fatigue Life Analysis FCG ANALYSIS WITH RESIDUAL STRESS (cont d): Calculated FCG lives compared with experimental results All calculated lives are either within USAF life scatter factor of 2, or are conservative Inclusion of tensile RS significantly improves correlation 38
The Impact of Forging Residual Stress on Fatigue in Aluminum INTRODUCTION FORGING PROCESS MODELING EXPERIMENTAL PROGRAM FATIGUE LIFE ANALYSIS SUMMARY 39
Summary Recent advances in the computational simulation of the forge, quench, cold-work and machining processes for large aluminum forgings are opening the way for a new paradigm in the design, manufacture and sustainment of aircraft structures. This new paradigm requires the explicit inclusion of forging process induced bulk residual stresses in the fatigue analyses used to design and certify the airframe; however, it will not see widespread acceptance until both the residual stress and fatigue life models have been thoroughly validated. The current program shows that the effects of residual stress on fatigue can be simulated with reasonable accuracy for the conditions studied. This in turn suggests that the next-generation design/structural analyses which will rely on these models may be achievable. 40
Evaluation of Residual Stress Effects: Summary The long-term objective of this and related MAI residual stress projects is to advance RS technology to the point that it can influence structural design criteria in the same way that fracture mechanics enabled the development and use of damage tolerant design criteria in the 1960s and 70s specifically: The detrimental effects of tensile RS can be mitigated and/or managed during design by establishing and imposing appropriate requirements for their location, spatial distribution and magnitude, and for the inclusion of their effects during design structural analyses. 41