In recent years a number of studies have been performed in which the impact of residual stress on structural performance, especially fatigue performance, has been evaluated both experimentally and analytically. These efforts are leading to an emerging paradigm in which residual stresses are represented explicitly in structural design, analysis, manufacturing and sustainment calculations. In order for this new approach to be become minimally viable, it is necessary to be able to quantify both the residual stresses in the structural component in question, and the impact that those residual stresses have on component strength and life. However, to achieve general acceptance, especially for critical applications in which the presence of the residual stress directly impacts whether or not the component will meet its design requirements, deterministic quantification alone may not be sufficient. Thus, it may be necessary to quantify the uncertainties in both the residual stresses and the resulting fatigue lives.

Abstract Forging processes include various steps to attain favorable material properties such as heat treatment, rapid quench, cold work stress relieving, and artificial aging. These steps, however, also contribute to bulk residual stress. Excessive bulk residual stresses cause a wide of problems, including part distortion during machining and in use, reduced crack initiation life, increased crack growth rates, and an overall reduction in part life. This paper summarizes recent work aimed at measurement-based assessment of bulk residual stresses in cold-compressed aluminum die forgings. The results show that forging process induced residual stress is a repeatable phenomenon with RMS repeatability less than 5% of yield.

It is widely recognized that the accuracy of notch fatigue calculations can be improved significantly when those calculations are based on the elastic-plastic response strain at the notch root, as opposed to the remotely applied loads or stresses. Two of the most widely used approximations for this response are Neuber's rule and Glinka's equivalent strain energy density method. In the present work, a survey of some of the many published evaluations of these methods was first conducted, and then, additional detailed comparisons with elastic-plastic finite element analyses for a series of semicircular and V-shaped notch configurations were performed. Based on the observed limitations of both the Neuber and Glinka approaches, and with the guidance of the elastic-plastic finite element results, a new (and more robust) approach for the estimation of notch response strains is proposed. This approach calls for the definition of ageneralized notch response curve (GNRC), which is dependent on both the material stress-strain curve and the notch geometry. Once defined, the GNRC allows the determination of the response strain for any applied stress.

In a recently concluded study, the effects of forging process–induced bulk residual stresses on fatigue life were evaluated at both the coupon and large component level. During this program, it was demonstrated that the extraction of confounding residual stress effects from material property data (especially fatigue crack growth rate data), coupled with the explicit inclusion of forging residual stresses in subsequent fatigue analyses, resulted in a significant improvement in the fidelity of those analyses. In the first phase of the program, coupon-level tests were carefully designed, executed, and analyzed, and it was shown that the newly developed methods resulted in analysis versus test life correlation ratios that were either within the United States Air Force (USAF) required scatter factor of 2, or were conservative. This is in contrast to a much broader scatter band (5x) for calculations made using traditional (non–residual-stress-informed) methods. The explicit residual stress method was incorporated into an integrated structural design/analysis tool suite that allows zoning of parts into residual stress–specific regions, automated generation of location-specific fatigue spectra, automated execution of fatigue crack initiation, fatigue crack growth, and residual strength analyses, and automated generation of fatigue-based design, allowable stresses, and margins of safety. In the second phase of the program, the residual stress design procedure was applied (to the furthest extent possible) to the design and manufacture of a large, fighter aircraft bulkhead. The objective of this phase of the program was to demonstrate, by way of two large component fatigue tests, that the technology would scale to the structural level, and that its use would result in components that are either lighter or more durable (or both) than their traditionally designed counterparts. In this article, we compare and contrast the traditional versus residual stress–informed design procedures, and we describe in detail the resulting baseline and “optimized” test articles. A full description of the fatigue tests, along with comparisons between detailed fatigue calculations and test findings, are given.

This paper described an improved fatigue crack growth rate (FCGR) material characterization process that partitioned residual stress effects from the material “true” FCGR behavior, leading to FCGR design curves that are free of residual stress bias. The material used in the program had high residual stress intentionally introduced. Two test methods from the literature were used to characterize the material: the adjusted compliance ratio (ACR) test method for closure correction, and the crack compliance test method for measuring the stress-intensity factor due to residual stress (Kres). Both test methods operated independently on compliance data collected during FCGR tests. Two independent data reduction methods were used to analyze the FCGR data. In the first, the closure corrected ACR data were combined with the Kres data using a power law relationship to normalize the curves into a residual stress free “master” curve, which was then transformed into a more traditional closure free ΔKeff curve. In the second, the Kres data were used in a superposition approach along with a closure model to reduce the data directly to the closure free ΔKeff curve. The two ΔKeff curves were shown to be in good agreement. The closure model was also used to reintroduce the stress ratio effect to generate the familiar da/dN-ΔK family of design curves that were free of residual stress bias. Validation examples were included where curves for the material with high residual stress were compared with data from similar material that had minimal residual stress, and those results were in good agreement. A summary of fatigue crack growth life predictions was also included to show that when residual stress effects are removed from FCGR characterization data and reintroduced in the fatigue life analysis, fatigue life is predictable within the usual 2x scatter factor for damage tolerance analysis.

Recent advances in the simulation of the 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. The use of large forgings permits the unitization of smaller parts (brackets, fittings, lugs, etc.) with primary structural components like spars and bulkheads. This is being done in order to reduce part count, which in turn leads to significant reductions in manufacturing cost. Unitization can also translate into weight reduction / avoidance when comparing against built-up structure, but it raises a number of issues for structural durability and damage tolerance, notably reduced repair / replace capability and reduced crack arrest capability. The viability of the unitization concept is dependent not only on the availability of material systems that retain their mechanical properties in very thick sections, but also on the designer’s ability to retain durability and damage tolerance, and to understand and mitigate the effects of residual stresses.

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 1 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.

The fully effective utilization of large aluminum forgings in aerospace structures has been hampered in the past by inadequate understanding of, and sometimes inaccurate representation of, 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.

The F-35 Joint Strike Fighter program includes three aircraft variants, one of which has been designed and built according to US Air Force requirements, and the other two of which have been designed and built according to US Navy requirements. For all three variants, a system design and development (SDD) configuration aircraft is being subjected to a full-scale durability (FSD) test. In each case, the complete airframe is being subjected to two lifetimes of severe design spectrum loading, with maneuver, catapults/arrestments (carrier variant only) and buffet loads applied as separate, alternating 1000 flight hour blocks during the major test sequence. For the airframe tests, the buffet loads are applied quasi-statically; for the separate vertical tail component tests, they are applied dynamically. In addition, tests of doors and attachments (local tests) are conducted when the full airframe test is down for inspections (as required, for example, between the first and second lifetimes). In this paper, we describe the manner in which the airframe tests were designed, including fatigue spectrum development and test adequacy analyses. In addition, we provide a summary of the test findings to date, along with a description of the analytical simulation for a typical finding. The paper includes an analysis vs test correlation summary that provides an indication of the validity of the fatigue crack initiation (FCI) and fatigue crack growth (FCG) analysis methods used to design the aircraft.

With production run sizes decreasing and unit costs increasing, airframe manufacturers are aggressively pursuing the development of advanced materials, structures and manufacturing processes that will reduce cost without compromising structural integrity. In recent years the use of computational simulation for material discovery, design and development and for structural optimization has become increasingly widespread. This emerging discipline of Integrated Computational Materials Engineering (ICME) is enabling design trade studies with improved fidelity and has shown the potential to reduce the amount of physical testing required for structures certification. Recently, studies have been performed in which computed residual stresses in large aluminum forgings were included in an advanced optimization of several bulkheads. These results were then used to inform a cost model that addressed sustainment, as well as manufacturing and performance costs. In this paper, using the forging studies cited above, we will describe the increasingly important link between ICME and viable affordability assessments.

A “next-generation” design approach that directly addresses some of the issues associated with unitized structure has been developed and deployed. Among other things, this approach requires the extraction of residual stress effects from material property data and the explicit re-introduction of those effects in selected, critical regions of the structure during design analysis. The new capability has been used to inform a cost benefit analysis model that addresses not only the positive impact that structural unitization can have on manufacturing costs, but also the potential negative impacts associated with increased repair / retrofit costs. This approach is an example of the successful application of the integrated computational materials engineering (ICME) process and demonstrates how that process can facilitate the design of structures that meet both performance and cost requirements.

As part of a larger residual stress modeling and standardization program, a computational assessment of the variability in fatigue crack growth life that can result from variability in residual stresses has been conducted. A detailed finite element analysis of the forge/quench/coldwork/machine process was performed in order to predict the bulk residual stresses in a fictitious aluminum bulkhead. The residual stress profiles on ten critical planes were used to calculate residual stress intensity factors for a range of crack types that are typical for aircraft structural details. These stress intensity factors were used in a standard, linear-elastic-fracture-mechanics-based fatigue crack growth algorithm in order to predict fatigue life for typical fighter aircraft spectrum loading. Calculations were made for residual stress profiles which reflected variability due to machined part placement within the parent forging. For the conditions considered in this study (forging process, machined part placement, critical location / crack geometry, material and fatigue spectrum) we have found that variability in fatigue crack growth life due to part placement is on the order of +/- 20% about the mean. Furthermore, we have found from scaling the residual stress distributions for these conditions, that fatigue crack growth life sensitivity is on the order of 15% to 50% change in crack gowth life with every 5ksi change in peak residual stress.

Aggressive performance and weight objectives are driving aircraft manufacturers toward the use of advanced materials and structural concepts that may have inherent, process induced residual stresses in localized, but critical areas. Certification of these structures will require that the influence of these residual stresses be properly accounted for during design. One example of this circumstance is the unitization of lugs and fittings with primary spars and bulkheads. This is being done in order to reduce part count, which, in turn, reduces the necessity for large numbers of fasteners and the associated hole preparation/mating requirements. Such unitization can be achieved through the use of large forgings, which experience has shown may have significant residual stresses in localized areas, even after final machining. For man-rated flight vehicles, primary structural elements are typically designed based on damage tolerance concepts. This requires that fatigue crack growth analysis and testing be used for certification of the structure. Thus, for advanced design concepts based on unitized structure, the influence of residual stress on fatigue crack growth must be addressed. A substantial body of work has been developed over the past three decades by numerous researchers in the field of fracture mechanics with regard to residual stress. In what has become the standard approach to the problem, the residual stress field is used to estimate a residual stress intensity factor (SIF) using weight function or Green’s function techniques. The residual SIF is superimposed with the applied SIF due to service loading and the total is then used in an otherwise unmodified, LEFM-based fatigue crack growth analysis. In this paper, we describe current research directed toward the formal inclusion of residual stress effects in the design of aircraft primary structure. This effort has three focus areas. The first is the extraction of confounding residual stress effects during the characterization of the fundamental fatigue crack growth rate behavior of a critical aluminum alloy. The second is the quantification, both by analysis and experiment, of the location, spatial magnitude, and stress magnitude of the residual stress fields in a candidate forged/machined part. The third is the development of improved fatigue crack growth analysis methods that selectively account for the presence of residual stresses. Each of the three focus areas provides a critical ingredient to a proposed design analysis method in which components are analyzed using intrinsic (residual stress free) material data, with residual stresses then explicitly introduced only in those areas where they are known to exist. The discussion includes the results of a trade study on a wing spar showing potential optimization, both in terms of weight savings in over-designed areas, and service life/damage tolerance enhancement in under-designed areas.

The F-35 Joint Strike Fighter is a stealthy combat aircraft that will serve as the next generation strike fighter. Certification of the airworthiness of this aircraft will be achieved through the demonstration, both by analysis and by test, that a number of structural capability requirements have been met. The testing required for certification is planned and executed according to a building block approach, a time phased test program that parallels and supports the aircraft development. This paper will describe the durability and damage tolerance (DaDT) aspects of this testing and the subsequent application of the results for development and verification of metallic DaDT analytical and computational tools as well as demonstrating overall aircraft structural integrity.

The combined effects of four variables on calculated spectrum fatigue crack growth (FCG) in 7075-T73 aluminum were compared using an experimental design approach (NASGRO 3.0 adaptation). The four parameters considered were: (1) load interaction, (2) load sequence (rainflow counted low-to-high, rainflow counted and randomized, and rainflow counted high-to-low), (3) crack tip stress state (plane stress versus plane strain) and (4) spectrum type (FELIX-28 helicopter rotor blade spectrum versus F-16 wing-root-bending moment spectrum), Each of the three load-interaction models available in NASGRO 3.0 was used to compute load-interaction effects. The investigation concluded that the order of cycles and crack tip constraint play relatively minor roles in computed FCG for problems involving repeated application of either of the given spectra, while the effects of the spectrum type itself, inclusion of load-interaction effects and the synergy of spectrum with load interaction are highly significant.