Contour method repeatability

The contour method provides a spatially resolved two-dimensional map of the component of residual stress acting normal to a plane through a part. A typical contour method residual stress measurement involves three primary steps: 1) Cutting a part in half on the plane where residual stress is to be measured (typically using a wire EDM); 2) Measuring the resulting deformation on the cut surface caused by residual stress release; and 3) Performing an analysis to relate measured deformation to residual stress.

For this case study, the repeatability of the contour method under a variety of relevant conditions was determined. Repeatability is a measure of the precision of a measurement technique, but does not address measurement accuracy. The test specimens evaluated include: an aluminum T section, a stainless steel plate with a dissimilar metal slot-filled weld, a stainless steel forging, a titanium plate with an electron beam slot-filled weld, a nickel disk forging, and an aluminum plate. These specimens were selected to encompass a range of typical materials and residual stress distributions. Each repeatability study included contour method measurements on 5 to 12 similar specimens. Following completion of the contour method measurements, an analysis was performed to determine the repeatability standard deviation of each population. In general, the results of the various contour method residual stress measurement repeatability studies are similar. The repeatability standard deviation tends to be relatively small throughout the part interior, with localized regions of higher repeatability along the part perimeter. These results provide expected precision data for the contour method over a broad range of specimen geometries, materials, and stress profiles.

Aluminum T section specimens were fabricated from bars cut from a thick 7050-T7451 aluminum plate. Prior to cutting, the aluminum plate had been stress relieved by stretching. The bars were then heat treated, including a quench, to induce high residual stress indicative of the -T74 temper. After heat treatment, T-sections were machined from the bars to represent an airframe structural member. Each T-section had a length of 254 mm (10.0 in), a height of 50.8 mm (2.0 in), a width of 82.55 mm (3.25 in), and a leg thicknesses of 6.35 mm (0.25 in).

Contour method measurements were performed at the mid-length of 10 specimens. The mean longitudinal residual stress is shown in Figure 1a. There is compressive residual stress at the left, right, and top edges with tensile stress at the center. The measured residual stress is similar between the 10 measurements and is quantified by the low repeatability standard deviation (Figure 1b). The repeatability standard deviation has a low distribution at most points (average of 5 MPa), with localized regions at the edges of the bottom and center flanges where the repeatability standard deviations is larger (95th percentile at 13 MPa).

Figure 1 – (a) Mean and (b) repeatability standard deviation for the aluminum T-section specimens

Five contour method measurements were performed on the stainless steel dissimilar metal weld specimen. The results (Figure 2a) show the mean longitudinal residual stress is tensile in the weld area and heat-affected zone (maximum = 380 MPa), and near the left and right edges of the plate (maximum = 400 MPa). There is compensating compressive residual stress toward the top of the plate at the left and right edges (minimum = -260 MPa). Most points have low repeatability standard deviations (average of 17 MPa), but there are localized regions near the part boundary where the repeatability standard deviation is larger (95th percentile at 36 MPa) as shown in Figure 2b.

Figure 2 – (a) Mean and (b) repeatability standard deviation for the stainless steel DM welded specimens

Six contour method measurements were performed on the titanium EB welded plate. The mean longitudinal stress (Figure 3a) has tensile stress in the weld area (maximum = 350 MPa) and compensating compressive stress in the heat-affected zone (minimum = -200 MPa). The repeatability standard deviation is low at most points (average of 8 MPa), with localized regions near the part boundary having higher repeatability standard deviations (95th percentile at 17 MPa) as shown in Figure 3b.

Figure 3 – (a) Mean and (b) repeatability standard deviation for the titanium EB welded plate specimens

Measurements were also performed on stainless steel forgings and a nickel disk forging, which are discussed in a more in-depth technical publication.

The results of the repeatability studies show consistent trends among samples, with low repeatability standard deviations over most of the specimen interior and localized regions of higher variability (typically along the part perimeter). The mean repeatability standard deviation ranged from 5 MPa for the aluminum T section to 35 MPa for the stainless steel forging, which represent the minimum and maximum values of the population.

The magnitude of the repeatability standard deviation increases with elastic modulus of the material, as shown in Figure 4a. The materials with the largest elastic moduli also have the largest repeatability standard deviations. Furthermore, when the repeatability standard deviation is normalized by elastic modulus (Figure 4b), the normalized repeatability standard deviation becomes consistent across all specimens, ranging from 70 x 10-6 MPa/MPa to 125 x 10-6 MPa/MPa, with an average value of approximately 100 x 10-6 MPa/MPa. Similarly, the 95th percentile of the normalized repeatability standard deviation is also relatively consistent, but covers a significantly larger range from 150 x 10-6 MPa/MPa to 275 x 10-6 MPa/MPa, with an average value of 220 x 10-6 MPa/MPa.

Figure 4 – Repeatability standard deviation (a) statistics and (b) statistics normalized by elastic modulus.

Reference information:

Repeatability of Contour Method Residual Stress Measurements for a Range of Material, Process, and Geometry, M. D. Olson, A. T. DeWald, and M. R. Hill, Residual Stress, Thermomechanics & Infrared Imaging, Hybrid Techniques and Inverse Problems, Volume 8, Springer, Cham, 2018, pp. 101–113.

Tensile test

A tensile test is a standard method used by material scientists and engineers to determine important material strength and ductility properties. For example, tensile tests can be used to measure the yield strength of a material, which is defined as the stress at which a material begins to deform plastically. Hill Engineering has ample experience performing tensile tests, and other mechanical property tests, in pursuit of our passion to maximize the potential of materials.

Continue reading Tensile test

Cold expansion

Fatigue is one important failure mode that guides the design and engineering of aircraft structure. As we have discussed previously aircraft are often manufactured using rivets and fasteners, which require drilling many holes in the structure during assembly. The holes act as stress concentrations, which tend to be locations where fatigue cracks are found. Compressive residual stresses act to hold cracks shut and result in improved fatigue performance. This residual compressive stress can provide substantial benefits in terms of performance, safety, cost, and inspection requirements. To take advantage of the benefits of compressive residual stress, cold expansion is often applied to aircraft fastener holes. Continue reading Cold expansion

Fracture surfaces evaluation

Aircraft undergo complex loading during their operation and lifecycle. For example, take off, landing, turbulence, and flight/ground maneuvers are all instances where significant loading occurs. The cyclic loading and unloading activates a failure mechanism called fatigue, which is most prevalent at the highest stressed regions. Continue reading Fracture surfaces evaluation

Failure analysis of high strength nuts

Fracture and fatigue are important material performance issues that Hill Engineering examines on a regular basis. Hill Engineering recently contributed to a publication titled “Investigating and interpreting failure analysis of high strength nuts made from nickel-base superalloy.” The publication includes a detailed review of work performed to understand the failure of these fracture critical nuts. The abstract text is copied below. A temporary link to download a pdf of the publication is provided at the bottom. Continue reading Failure analysis of high strength nuts

Opening Up with the Interns

Every summer, Hill Engineering hires university students as interns; aiming to give them insight into industry jobs and provide them with the experience they’ll need to develop their careers post-graduation. We pride ourselves on involving interns in projects which utilize and further explore the concepts they have learned in school. In the past, we’ve had many positive outcomes from our internship program. The interns have delivered fresh perspective on our projects and a few have even transferred to full-time employment at Hill Engineering. This year, we welcome three students into our summer internship program. Continue reading Opening Up with the Interns

We love planes

The dawn of the airplane changed the way we humans viewed the world. In a relatively short amount of time, travel across oceans was reduced from a month or more onboard a sea vessel to a few hours in the air. Getting from one side of the country to the other no longer meant spending days on a train, but a quick flight across state borders. It’s easy to see why planes were welcomed so eagerly into modern society.

But beyond convenience, have you ever taken the time to just consider the technology behind airplanes and what they are capable of? Continue reading We love planes

Extending Life of Fighter Airframes

Fighter aircraft are developed at the cutting edge of technology. Aggressive engineering and reduced margins mean that early production aircraft can have performance shortfalls that are addressed by updates to manufacturing, materials, or software. Because it takes time to reveal shortfalls, there is a significant economic payoff in retrofits that bring the performance of early production aircraft in line with original performance objectives.

The F-22 Raptor is a tactical fighter aircraft developed for the United States Air Force (USAF) with a highly optimized titanium airframe. Full-scale structural testing of the F-22 airframe was carried out in parallel with initial production and identified early fatigue cracking in a number of structural details. A series of improvements were made in later airframes, and structural repairs and retrofits were deployed to improve performance of early production aircraft.

USAF F-22 Raptor tactical fighter aircraft

Hill Engineering had a role in repairing the F-22 wing-attach lugs that carry wing loads into the fuselage. The lug repairs were difficult because the lugs are an integral part of a single-piece, welded section of the titanium fuselage that includes attach points for the wing, engine, and horizontal tail. Working closely with Boeing, Lockheed Martin, and the USAF, Hill Engineering supported design, development, and certification of laser shock peening (LSP) treatments for structural repair. LSP creates a layer of compressive residual stress that improves resistance to fatigue crack initiation and slows fatigue crack growth, and initial tests showed that LSP could extend service life of the F-22.

Illustration of F-22 structure showing wing-attach lugs where laser shock peening was applied

Hill Engineering worked closely with Boeing during design and planning of the engineering program and performed residual stress measurements (slitting and contour methods), residual stress predictions (using ERS-Toolbox®), and assessments of fatigue crack growth to quantify service life improvements. The wing-attach repairs provided significant cost savings for the F-22 program as a result of increased airframe service life and reduced depot maintenance.

High-speed image of a plasma burst during laser shock peening on a test coupon

The F-22 wing-attach repair required Hill Engineering to further develop methods and tools to address challenging problems with structural materials. On-going efforts support complex, safety-critical applications in aircraft structures, petrochemical processing equipment, power generation systems, welded structures, and turbine engines.

Reference information:

Electron Beam Welding of F-22 Structures, R. Zenas, 41st Structures, Structural Dynamics, and Materials Conference, Atlanta, GA, Apr 2000.

Status of F/A-22 Full Scale Fatigue Test, S. Welsh, USAF ASIP Conference, Memphis, TN, Nov 2004.

F-22 Laser Shock Peening Depot Transition and Risk Reduction, K. MacGillivray, et al, USAF ASIP Conference, San Antonio, TX, Dec 2010.

Design and Analysis of Engineered Residual Stress Surface Treatments for Enhancement of Aircraft Structure, M. Hill, et al, USAF ASIP Conference, San Antonio, TX, Nov 2012.

Verification of Analytical Methodology to Minimize Inspection Burdens and to Utilize Full Benefits of Residual Stress Life Enhancement Technique, H. Cai, et al, USAF ASIP Conference, San Antonio, TX, Dec 2013.

Forging Residual Stress

Metallic materials inherit their mechanical properties through various processing steps that are optimized to provide useful spatial distributions of constituents, mechanical deformation, and orientation. Among forming methods for metals, the forging process typically provides parts with superior static strength and fatigue performance. For this reason, forgings are well-suited for use in key structural components requiring strength and durability.

In many aluminum alloys, quenching from high temperature enables high strength, but it also leaves significant levels of residual stress. Residual stress fields require specific attention, especially with respect to managing distortion of parts machined from forgings and in anticipating the effects of residual stress on part performance. When residual stress is not managed appropriately, excessive distortion drives part rejection in manufacturing, and residual stresses shorten service life by accelerating corrosion and fatigue cracking.

Hill Engineering works in the metallic materials supply chain to deliver residual stress technologies for measurement and modeling that benefit suppliers, system integrators, and end users across industry. Some of our most advanced work is in aircraft materials, where our measurements are being used to improve and validate advanced modeling aimed to revolutionize the design of forgings, with specific attention to residual stress fields.

Supported by the US Air Force, and working closely with the F-35 aircraft manufacturer and a key material supplier, Hill Engineering developed and applied residual stress measurement technology for very large aluminum forgings used to produce F-35 bulkheads, frames, and spars.

Cut-away of F-35 showing large single-piece bulkheads, frames, and spars made from forgings

Forging for a single piece bulkhead on F-35 variant with short takeoff and vertical landing capability

Contour method measurements confirmed what was predicted by modeling, that high residual stresses in quenched material are relieved to low levels by cold compression with specially designed dies. The measurement capabilities provided the F-35 team with data that supported their use of airframe materials.

By developing technology to serve the metallic materials supply chain, Hill Engineering delivers capabilities needed to solve challenging problems with structural materials. Contact us to see what we can do together.

Residual stress field in a quenched forging (top) and a cold-compressed forging (bottom) – model forecast (left) and measurement data (right) with tension in red and compression in blue

Reference information:

Forgings – What we Make, 2016, Alcoa Defense.

An Integrated R&D Roadmap for Residual Stress Management in Large Structural Forgings, M. James, et al, 4th Residual Stress Summit, Tahoe City, CA, Sep 2010.

The Impact of Forging Residual Stress on Fatigue in Aluminum, D. Ball, et al, AIAA SciTech, Kissimmee, FL, Jan 2015.