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Prime M.B.,Los Alamos National Laboratory | DeWald A.T.,Hill Engineering, LLC | Hill M.R.,University of California at Davis | Clausen B.,Los Alamos National Laboratory | Tran M.,University of California at Davis
Engineering Fracture Mechanics | Year: 2014

Residual stresses can be a main cause of fractures, but forensic failure analysis is difficult because the residual stresses are relaxed after fracture because of the new free surface. In this paper, a method is presented for a posteriori determination of the residual stresses by measuring the geometric mismatch between the mating fracture surfaces. Provided the fracture is not overly ductile, so that plasticity may be neglected, a simple, elastic calculation based on Bueckner's principle gives the original residual stresses normal to the fracture plane. The method was demonstrated on a large 7000 series aluminum alloy forging that fractured during an attempt to cut a section into two pieces. Neutron diffraction measurements on another section of the same forging convincingly validated the residual stresses determined from the fracture surface mismatch. After accounting for closure, an analysis of the residual stress intensity factor based on the measured residual stress agreed with the material's fracture toughness and fractographic evidence of the failure initiation site. The practicality of the fracture surface method to investigate various failures is discussed in light of the required assumptions. © 2013 Elsevier Ltd.

Woo W.,Korea Atomic Energy Research Institute | An G.B.,POSCO | Kingston E.J.,University of Bristol | Dewald A.T.,Hill Engineering, LLC | And 2 more authors.
Acta Materialia | Year: 2013

Spatial variations of residual stresses were determined through the thickness of 70 mm thick ferritic steel welds created using low (1.7 kJ mm -1) and high (56 kJ mm-1) heat inputs. Two-dimensional maps of the longitudinal residual stress were obtained by using the contour method. The results were compared to neutron diffraction measurements through the thickness at different locations from the weld centerline. The deep hole drilling technique was utilized to confirm the maximum stress locations and magnitudes. The results show that significant tensile stresses (∼90% of yield strength) occur along the weld centerline near the top surface (within 10% of the depth) in the low heat-input specimen. Meanwhile, in the high heat-input weld, the peak stress moved towards the heat-affected zone at a depth of ∼40% of the thickness. Finally, the influence of residual stresses on potential fracture behavior of the welded joints is discussed. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Wagner J.T.,Hill Engineering, LLC
SAE Technical Papers | Year: 2013

Link-X Stability System ("Link-X") is a control arm geometry that forces the control arms to both locate the tire and control the orientation of the chassis. This is achieved via crossing the control arms in the elevation view without having the control arms touch one another. Link-X inverts the overturning moment at each tire and applies the inverted moments to the chassis. In terms of traditional suspension analysis, this geometry creates a high yet completely stable roll center. Changing the vertical distances between the attachment points on the chassis or the spindle changes the amount of anti-roll generated. Anti-pitch is created by shifting the line of intersection of the control arm planes toward the vehicle's center of gravity. Copyright © 2003 SAE International and Copyright © 2003 Society of Automotive Engineers of Japan, Inc.

Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 99.78K | Year: 2008

Hill Engineering is committed to the development and application of engineered residual stress, which is the intentional use of residual stress treatments coupled with sound engineering analysis to improve the performance of metallic structure. Hill Engineering’s experience with recent aerospace programs has highlighted the need and opportunity to develop analytical engineering approaches (and tools) that can robustly and efficiently take advantage of the potential benefits of residual stress treatments. The goal of the present work is to perform a proof of concept demonstration of a design tool for fatigue assessment of surface treated airframe structural components. Key tasks include the prediction of residual stress and fatigue performance (durability and damage tolerance) in surface treated fatigue coupons, which represent the geometry of an important F-22 structural member. Experiments (residual stress measurements and fatigue tests) will be performed to validate the predictions. This proof of concept work will leverage existing Hill Engineering design tools and on-site experimental capabilities in residual stress measurement and fatigue testing.

Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 149.94K | Year: 2014

ABSTRACT: Due to their excellent strength-to-weight characteristics, integral components (i.e., thin-walled components machined from a single piece of material, which typically consist of a series of pockets, ribs, and stiffeners, have become commonplace on modern aircraft structure. The fabrication of integral components is a machining-intensive process that employs non-conventional machining at high material removal rates. One of the biggest limitations of high speed milling of integral structures is distortion, which results from changes in the residual stress state within the machined component. Excessive distortion can lead to the introduction of excessive fit-up stresses during assembly, can result in improper joints/connections, and can result in parts being scrapped. In certain instances, shops are allowed to use mechanical means (e.g., plastic bending over a fixture) to rectify some of the distortion. This can be effective, but is limited to use on simple geometry and this approach is lacking in quality and traceability. The proposed work plan will develop improved technology for correcting distortion (i.e., reshaping back within drawing tolerance) in complex aerospace parts. BENEFIT: The proposed shape correction technology would provide significant improvements to the efficiency of high speed machining processes. Currently, significant losses result from machined parts that are scrapped due to excessive distortion. Generally, significant machining has been performed on these parts prior to scrapping so the scrapped parts have considerable value. A process to efficiently and effectively correct the shape of these distorted parts and return them into the production supply offers the potential for significant cost savings relative to the current approach (scrapping these distorted parts). This technology is important to the aerospace community and is applicable to many other industries as well. For example, ships and space vehicles use similar integral components that have distortion related issues. Large welded structures like pressure vessels and industrial facilities are often adversely affected by distortion due to residual stresses from welding. As this technology becomes more mature there is an opportunity to apply it for benefit in other industries.

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