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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. Source

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. Source

DeWald A.T.,Hill Engineering, LLC | Hill M.R.,University of California at Davis
Conference Proceedings of the Society for Experimental Mechanics Series | Year: 2014

Residual stresses are of interest from an engineering perspective because they can have a significant influence on material performance. For example, fatigue initiation, fatigue crack growth rate, stress corrosion cracking, and fracture are all influenced by the presence of residual stress. The commonly used methods for residual stress measurement include incremental hole drilling and slitting. This paper establishes the repeatability of these techniques through replicate measurements on nominally similar specimens. Each technique was used to measure the residual stress in peened aluminum plates at multiple locations. A statistical analysis of the data was performed to compare the level of repeatability obtained using the two techniques. The standard deviation of the slitting method data was about 2% of the peak stress magnitude, compared with 5% for the hole drilling measurements. © The Society for Experimental Mechanics Inc. 2014. Source

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. Source

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.

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