Low plasticity burnishing tested on 300M aircraft landing gear steel

Citation

(2006), "Low plasticity burnishing tested on 300M aircraft landing gear steel", Anti-Corrosion Methods and Materials, Vol. 53 No. 6. https://doi.org/10.1108/acmm.2006.12853fab.003

Low plasticity burnishing tested on 300M aircraft landing gear steel

Low plasticity burnishing tested on 300M aircraft landing gear steel

Keywords: Mechanical behaviour of materials, Steel

AFRL engineers, working with industry, have made significant progress in the study and application of low plasticity burnishing (LPB) for the mitigation of stress corrosion cracking (SCC) and fatigue damage in high strength steels. LPB, a process originated and patented by Lambda Technologies, has been previously demonstrated as an approach to substantially increase the foreign object damage (FOD) tolerance of turbine engine components. In the most recent investigation, researchers at AFRL's Materials and Manufacturing Directorate (AFRL/ML) investigated the use of LPB on 300M steel, widely used for aircraft landing gear. They compared the results to those obtained using conventional shot peening (SP), the current state of the art. The tests indicated that LPB imparts persistent compressive residual stresses in 300M steel surfaces and that LPB-treated specimens more effectively withstand fatigue, FOD-related damage, and SCC. Continuing research efforts could result in more durable steel components for military and commercial aircraft landing gear.

Payoff

Low plasticity burnishing offers greater depth and stability, and higher performance than conventional SP. In addition, LPB can be performed during manufacturing using standard computer numeric control (CNC) machine tools, eliminating the need for shipping components to other facilities for surface treatments.

Background

4340 MODIFIED steel, also known as 300M, is used extensively for aircraft landing gear because of its high strength and other properties. Unfortunately, like other high-strength steels, 300M is vulnerable to corrosion fatigue and SCC, which, if left unresolved, can lead to catastrophic consequences in the landing gear. Plating and SP, used in conventional gear designs, are only partly effective. ML's Systems Support Division studied the effect of LPB treatment on several 300M steel specimens to determine whether the durability of the steel could be improved under adverse conditions. They used LPB to mechanically suppress stress sensitive corrosion failure mechanisms in a 3.5 percent salt solution, studied simulated FOD damage conditions; then compared their findings with SP and low stress ground (LSG) conditions.

LPB produced residual compression to a depth of 1.27mm or 0.050in. and SP only 0.127mm (0.005in.), an order of magnitude less. LPB treatment dramatically improved the fatigue performance and resistance to SCC with and without simulated FOD. The fatigue life of LSG and SP surfaces was significantly lower with respect to the LPB baseline. The fatigue behavior was worse with FOD, simulated with a 0.5mm (0.020in.) – deep electrical discharge machining (EDM) notch, both in air and exposed to salt. (EDM is a metal removal machining method that uses an electrode under carefully controlled conditions to achieve a desired shape.) The researchers terminated the SCC testing of LPB- treated landing gear sections at 150- 180ksi static loads after 1,500h without failure, noting that failure had occurred in as little as 13h without treatment.

Mechanistically, the effect of FOD resulted in early crack initiation and growth, resulting in a decrease in fatigue performance. Despite the existence of similar corrosion conditions, the deep compressive surface residual stresses from LPB treatment mitigated the individual and synergistic effects of corrosion and FOD. The deep compressive layer produced by LPB reduced surface stress, even under high tensile applied loads, suppressing the SCC failure mechanism. The research also demonstrated that LPB provides a deep layer of compressive residual stress that is stable throughout the range of stresses and thermal excursions typically encountered during landing gear manufacture and service. This allows a substantial increase in the fatigue performance of component features, without altering the alloy or design.

The LPB process was originally developed by Lambda Technologies in Cincinnati, Ohio. The basic LPB tool is comprised of a ball supported in a spherical hydrostatic bearing. The ball does not contact the bearing seat, even under load, and is loaded normal to the surface of a component with a hydraulic cylinder in the body of the tool. The ball rolls across the surface of the component in a pattern defined in the CNC code, as in the case of any machining operation. The pressure from the ball causes plastic deformation in the material underneath the ball. The deformed region is constrained by surrounding, undeformed material leaving the treated region in a state of compressive residual stress. No material is removed in the process, and the surface is displaced inward by only a few ten-thousandths of an inch. The tool path and normal pressure applied are designed to create a carefully engineered distribution of compressive residual stress, and the distribution is designed to counter applied stresses and optimize fatigue performance.

The process design for test samples and actual hardware is critical for optimized performance. In addition to ensuring that the design has sufficient compression to meet performance requirements, the design process must take into account the potential for redistribution of the induced compression through thermal and mechanical means, as well as the location and magnitude of compensatory tensile stresses. Efforts are underway to expand the study to hardware containing more complex designs. The Directorate's researchers included: Neal Ontko, Jack Coate, Robert Ware, and Michael Shepard.