What is the reason behind D6AC steel being listed as a restricted material?
The Defense Systems Information Analysis Center (DSIAC) received a technical inquiry requesting information regarding the field experience that caused D6AC steel to be listed as a restricted material in NAVAIR EC-434-000-003. We performed literature searches using the Defense Technical Information Center, open sources, and university libraries to determine that specifically, the F-111 failure of 22 December 1969 was the primary reason for D6AC steel becoming a restricted material.
The Defense Systems Information Analysis Center (DSIAC) received support from Mr. Doyle T. Motes, P.E., who is a subject matter expert (SME) in materials science and engineering from Texas Research Institute Austin (TRI Austin). Mr. Motes performed a literature search using a combination of open sources, the Defense Technical Information Center (DTIC), and university library resources to produce an answer to the technical inquiry.
2.0 D6AC Steel
D6AC is considered a medium-carbon (0.4−0.5 wt.%), ultra-high-strength, aerospace steel alloy. It is designed for high-strength structural applications requiring tensile strengths up to 280 ksi and provides a high yield-strength to tensile-strength ratio, combined with good ductility. A tough and fibrous fracture is exhibited at as low as -210 °F (134 °C) for impact testing, and the notch toughness is noted to be excellent. This material has been selected for fracture toughness critical applications at a variety of strength levels. The deep hardening characteristics of D6AC steel make it applicable for production of fairly large sections of material . In addition, the material is weldable, even in heavy sections. However, D6AC steel is susceptible to fatigue and stress corrosion cracking. A selection of mechanical material properties for D6AC is listed in Table 1.
Table 1: Mechanical Properties of D6AC Steel 
|Tempering Temp.||Tensile Strength||0.2% Yield Strength||Elongation||Reduction of Area||Hardness|
|°F||°C||ksi||MPa||ksi||MPa||(%)||(%)||Rockwell C Hardness (HRC)|
3.0 NAVAIR EC-434-000-003
NAVAIR EC-434-000-003 is a specification/standard relating to Material Selection Restrictions for Navy and Marine Corps Aviation Weapon Systems. D6AC steel is employed in a number of weapon systems (especially legacy ones), including the F-111, F-14, H-53E, and E-2C, and is listed as a restricted material in the NAVAIR document.
4.0 Rationale Behind Restrictions on D6AC Steel
The main rationale behind the D6AC use restrictions is associated with the F-111 program. Cracks in the F-111’s D6AC wing attach points were first discovered in 1968 during ground fatigue testing. Afterwards, on 22 December 1969, the left wing pivot fitting of U.S. Air Force F-111 No. 94 aircraft failed during a 4.0 Gee pull-up maneuver out of a practice bombing run at Nellis Air Force Base in Nevada, causing the loss of the left wing. The cause of the failure, resulting in the loss of the aircraft and crew of two, was attributed to the presence of a manufacturing defect (a forging lap) in the D6AC steel pivot fitting [2, 3].
Flight testing associated with this issue continued through 1973. Investigations conducted in the 1970s revealed the cause of failure of the F-111 to be fracture of D6AC steel in the wing pivot fitting due to manufacturing defects missed by conventional nondestructive inspection (NDI) methods available at the time . Critical F-111 structural components fabricated from D6AC steel included the wing pivot fitting, the wing carry through box, major fuselage frames and longerons, as well as the more conventional nose and main landing gear. Mitigation measures to address these problems are denoted by Watters et al., who state that “…the investigations and research into D6AC steel characteristics developed a new discipline called ‘Fracture Mechanics’”. These concerns continued through 1980s and 1990s for United States operators of the aircraft, and into the 2000s after the United States retired the aircraft and during the period where it was exclusively operated by the Australian Air Force. Lax stated that “aside from the SCC [stress corrosion cracking] problem, the main corrosion concern for the F-111 is for pitting corrosion to initiate fatigue cracks, and D6AC steel is the material of most concern” .
In addition to these findings, and complicating factors, the type of defect that led to the 1969 crash of the F-111 was one that was repeatedly missed by NDI during fabrication, as discussed by Forney: “The resulting investigation during 1970 revealed the cause to be the catastrophic fracture of the D6ac high strength steel outer wing pivot fitting due to the presence of a manufacturing-introduced one inch surface crack that had been missed repeatedly by NDI during fabrication…” .
Additionally, the 2015 version of MIL-STD-1568D, which establishes the requirements for materials, processes, and techniques, and identifies the tasks required to implement an effective corrosion prevention and control program throughout the conceptual, validation, development, production, and sustainment phases of the aerospace weapons systems life cycles, also lists D6AC, H-11, 4340M, and 300M steels as materials that are not to be used . An example of the change from the use of D6AC steel to Ti 64 on aircraft is shown in “The Development of the F-14A Wing Center Section” , in which the wing pivot structure material has been changed from D6AC steel on the F-111 to Ti 64 on the F-14.
D6AC steel is susceptible to fatigue cracking and stress corrosion cracking, despite the impressive tensile and yield strengths that can be achieved for different heat treatments. It is for these reasons that it is listed as a restricted material in EC-434-000-003.
 Latrobe Specialty Steel Company. “Lescalloy® D6AC VAC-ARC® High Strength Alloy Steel.” Data sheet, September 2008.
 Tiffany, C. F., J. P. Gallagher, and C. A. Babish, IV. “Threats to Aircraft Structural Safety, Including a Compendium of Selected Structural Accidents/Incidents.” ASC-TR-2010-5002, March 2010.
 Watters, K., F. Rose, M. Heller, P. Callus, B. Madley, and P. Connor. “F-111 Sole Operator Program: Maintaining the Structural Integrity of an Ageing Fighter Aircraft.” RTO AVT Specialists’ Meeting in “Life Management Techniques for Ageing Air Vehicles,” Manchester, UK, 8−11 October 2001.
 Lax, M. From Controversy to Cutting Edge, A History of the F-111 in Australian Service, ISBN 9781920800543, 2010.
 Forney, D. “Nondestructive Evaluation Technology Initiatives Program II (NTIP II), Delivery Order 10, Task 010-015: In Search of Excellence – An Historical Review.” AFRL-ML-WP-TR-2006-4145, May 2006.
 U.S. Department of Defense. Department of Defense Design Criteria Standard. “Materials and Processes for Corrosion Prevention and Control in Aerospace Weapons Systems,” MIL-STD-1568D, 31 August 2015.
 Paez, C. A. “The Development of the F-14A Wing Center Section.” Grumman Aerospace Corporation, https://www.slideshare.net/CarlosPaez25/the-development-of-the-f14a-wcs80108, 2008.