AIRCRAFT exhaust systems must be designed to withstand service conditions that include severe vibrations, high temperatures from the hot exhaust gases passing along the inside of the pipes at relatively high velocities, while the outside of the pipe system may be subjected to rain and sea atmosphere. These extreme conditions of operation led to difficulties in the application of stainless steel to the exhaust system. Failures caused by intergranular corrosion from carbide precipitation need no longer be feared when properly stabilized 18-8 stainless steel is used. Stainless steel can be stabilized with either titanium or columbium, the cheaper and more easily worked titanium-type steel being satisfactory for most installations, although there are applications where the columbium type must be used because of its higher yield and tensile strengths. This improved stainless, however has not solved the problem of corrosion at high temperatures in the presence of zinc.
With the ever increasing competition among companies to reduce manufacturing costs without sacrificing product reliability, Bendix-Pacific has engaged the use of Value Engineering in establishing contamination control on manufactured components. The process of establishing contamination control begins before the contract has been awarded. The program proceeds along the following lines: First, a foundation of sound comprehensive methods and procedures fully documented internally. Second, the application of Value Engineering to the Customer design and performance specifications. Third, the alternate proposal showing the logic and reasoning supporting the proposal. Fourth, the implementation of the proposal to the contract. And fifth, the documentation of records for the program. The important factor to remember is that, “contamination control is mandatory for system reliability, however, unrealistic levels of contamination control can only result in a gold plated product.”
The high performance, efficiency, and integrity demanded in advanced aerospace materials applications frequently require the matallurgical union of aluminum alloys to stainless steels. Salt-bath dip brazing and various plating techniques are currently used, but continuing research and development programs will undoubtedly produce other reliable methods of joining the two intrinsically dissimilar metals. The limitations of this bimetallic couple have been overcome, and satisfactory bonding has been achieved.
The surface condition and type of interface which is formed determines the adhesion which can be obtained in a film-substrate couple. Ways to control the surface condition and allow the formation of desirable types of interfaces before, during, and after the deposition of a coating are briefly discussed. The use of sputter cleaning as a surface preparation technique, and “ion plating” as a means of increasing adhesion by increasing the energetics of the deposition process are stressed. In the ion plating process, the surface to be coated is subjected to a high energy ion bombardment before and during the actual deposition.
Recent developments in the field of electroless nickel plating are outlined. Metal ion reduction by boron-containing reductants is a significant advance and gives deposits which are phosphorus-free, metal-boron alloys. Properties of these deposits such as composition, structure, density, hardness, etc. are reviewed. Two types of reducing agents can be employed in aqueous electroless nickel systems. This paper stresses the preference for dimethylamine borane (DMAB) in the plating bath. The special properties of nickel-boron alloys and the versatility of DMAB plating baths suggest a number of applications such as plating of electronic components, high temperature applications, and applications where uniformity and thickness of deposits is a basic requirement. The primary limitation of DMAB is the high cost of the chemical which might make certain applications not economically practical.
Multi-fastener lap joints are vulnerable to fretting fatigue when they are subjected to repeated loading. In general the fretting fatigue condition leads to degraded properties of metallic structures due to the presence of the surface stress concentration resulting at the sites of fretting pits. In many cases, fretting can result in premature structure failures therefore a series of counter measures are frequently taken to minimize fretting especially at the fastener holes. One of major factors that affect the fretting phenomena between the fastener and fastener hole is the surface condition of the fasteners. In this study, the influences of the surface texture and the surface plating of the fastener on the joint life were investigated by conducting double lap shear fatigue testing. It has been found that the joint fretting fatigue resistance is very sensitive to the surface texture of the fastener and as the surface roughness of the fastener is reduced the joint life increases.
Joints represent potential weak points in the structure, the design of the overall structure tends to follow from the design of the joint. To date, metal/composite joints are general in current aircraft structure. Some of the reasons for lower joint efficiency in composites are: brittleness which means little stress relief around the highest loaded holes, anisotropy which leads to higher stress concentration factors, low transverse strength, susceptibility to delamination, and sensitivity to environmental conditions. All of these factors together with the complexity of composite failure modes make the analysis and design of composite joints far more complex than that of metallic joints. Fatigue failure is the root cause which makes aircraft unserviceable. 75-80% of fatigue failures occur in the joints of structure, therefore researching joints is very important for improving aircraft life. Much effort has been put into solving the technical challenge for composite bolted joints.
300M steel is often used in landing gear because of its high strength and high fracture toughness. Conversely, 300M steel is highly susceptible to corrosion fatigue and stress corrosion cracking (SCC), which can lead to catastrophic consequences for aircraft landing gear. Shot peening and plating of the landing gear are used to suppress corrosion fatigue and SCC with limited success. A method that will produce deeper compression in critical regions of landing gear will provide a dramatic improvement in foreign object damage (FOD) tolerance, corrosion fatigue strength and SCC susceptibility. This paper discusses the use of low plasticity burnishing (LPB) to provide a deep layer of residual compression to improve damage tolerance and mitigate SCC of 300M steel. The fatigue performance of LPB processed 300M steel test samples were compared to those in a shot peened or low-stress-ground (LSG) condition.
In a 2-year program sponsored by SJAC, an aqueous electroplating process using alkaline Zn-Ni with trivalent chromium post treatment is under evaluation for high strength steel for aircraft application as an alternative to cadmium. Commercial Zn-15%Ni rack/barrel plating solutions are basis for plating aircraft parts or fasteners. Brightener was reduced from the original formula to form porous plating that enables bake-out of hydrogen to avoid hydrogen embrittlement condition. Properties of the deposit, such as appearance, adhesion, un-scribed corrosion resistance, and galvanic corrosion resistance in contact with Al alloy, were evaluated. Coefficient of friction was compared with Cd plating by torque-tension measurements. Evaluation of the plating for scribed corrosion resistance, primer adhesion, etc. will continue in FY2007.
Cadmium (Cd) plating offers good corrosion resistance, lubricity, solderability, adhesion, and ductility, as well as consistent torque-tension and uniform thickness on components with complex geometries. However, the intrinsic Environmental, Health, and Safety issues associated with Cd have driven many users to seek alternatives. Currently, various Cd replacement teams/programs such as the Joint Group on Pollution Prevention (JG-PP)/Boeing, the Joint Cadmium Alternatives Team (JCAT), the Canadian Cadmium Replacement Program (CCRP), and the REFOCUS Program (AEA Technology, European) have been investigating alternatives to Cd plating. Some of the current coatings being considered in the aerospace industry include Zn-Ni alloy plating, electrodeposited aluminum, electroless nickel, nickel composite, and molten salt aluminum manganese (Al-Mn). Each option has its own particular characteristics; however, most experiments have been conducted on aerospace structural panels or components.
Since 2005, Alcoa Fastening Systems (AFS) and Lockheed Martin have been partnering to identify a Cadmium (Cd) plating replacement for threaded fasteners. Previously reported Phase I, II and III studies resulted in alternative coatings that indicated promise as suitable plating materials. Phase IV and V studies continued the program by testing two different fasteners (NAS1580 and NAS4452) manufactured in AFS facilities. Testing included plating material characterization such as coating thickness, torque-tension relationships, locking and breakaway torque measurements, salt spray (fog) corrosion, stress corrosion, and push-in and interference properties. Additionally, mechanical properties of the plated fasteners were tested (tensile, double shear, durability, and fatigue). Candidates included two electroplated zinc-nickel coating systems (Zn-Ni and Zn-Ni2) and an electrodeposited aluminum coating (AI).
Cadmium electroplating is coming under increasing pressure due to both environmental and worker safety issues. Since 2005, Alcoa Fastening Systems (AFS) and Lockheed Martin have been conducting a collaborative research program to identify the most appropriate fastener coating materials for a Cadmium (Cd) plating replacement. Four candidate coatings were selected for the initial Phase I evaluation: electroless nickel (EN), electroless nickel composite (EN-PTFE), electrodeposited surface mineralization based zinc-nickel (Zn-Ni), and electroplated aluminum (Al). The Phase I testing results indicated that the Zn-Ni and Al coatings were the best of the four candidates for Cd replacement. However, it is hard to conduct direct comparisons with different coating thicknesses, surface treatments, and lubrication among various Cd alternatives. Thus, further evaluation with more careful control of these parameters would be necessary.
The Crew Exploration Vehicle (CEV), also known as Orion, will ferry a crew of up to six astronauts to the International Space Station (ISS), or a crew of up to four astronauts to the moon. The first launch of CEV is scheduled for approximately 2014. A stored water system on the CEV will supply the crew with potable water for various purposes: drinking and food rehydration, hygiene, medical needs, sublimation, and various contingency situations. The current baseline biocide for the stored water system is ionic silver, similar in composition to the biocide used to maintain quality of the water transferred from the Orbiter to the ISS and stored in Contingency Water Containers (CWCs). In the CEV water system, the ionic silver biocide is expected to be depleted from solution due to ionic silver plating onto the surfaces of the materials within the CEV water system, thus negating its effectiveness as a biocide.
As part of preparing for the Crew Exploration Vehicle (CEV), the National Aeronautics and Space Administration (NASA) worked on developing the requirements to manage the fire risk. The new CEV poses unique challenges to current fire protection systems. The size and configuration of the vehicle resembles the Apollo capsule instead of the current Space Shuttle or the International Space Station. The smaller free air volume and fully cold plated avionic bays of the CEV requires a different approach in fire protection than the ones currently utilized. The fire protection approach discussed in this paper incorporates historical lessons learned and fire detection and suppression system design philosophy spanning from Apollo to the International Space Station.
The International Space Station's primary external heat transport system is a single phase ammonia loop called the Active Thermal Control System (ATCS). ATCS loop heat is acquired from the station modules through interface heat exchangers (Internal Thermal Control System water to ATCS ammonia) and from external truss mounted electronics through cold plates. The heat exchangers are compact plate/fin counterflow type and the cold plates are a brazed and bonded construction using a radiation heat transfer interface to the electronics.
The numerical/experimental method for, and results of, evaluating insulation solar radiation transmission effects on in-flight temperature field of a space vehicle are presented. The experiment subsection describes a new experimental method for evaluating materials solar-radiation transmittance, reflectance, and absorptance, as well as demonstrates test results for silica samples with various thicknesses. The calculation subsection includes (1) interpolation of the experimental data into a function of the insulation layer thickness and (2) the numerical solution of the transient heat conduction equation for a thermal protection fragment consisting of an insulation layer cemented to a metal plate. Fragment temperature calculation results for various near-Earth orbits and fibrous silica insulation layer thicknesses are presented. The results obtained show the essential influence of insulation solar-radiation transmission on space vehicle structure temperature in space flight.
Shot peening has been used as a process to improve fatigue life and fatigue strength of metal for centuries. Modern applications began with General Motors Corporation in the 1920's, when they began shot peening valve springs to improve fatigue life. Over the years this process has been applied to a variety of metals to improve fatigue characteristics and with the advent of SEM (Scanning Electron Microscope) technology, failure mechanisms such as SCC (Stress Corrosion Cracking) and IGA (Intergranular Attack) have been identified. Shot Peening has been proven to retard SCC and eliminate IGA. Detailed shot peening specifications have been developed by major aircraft manufacturing companies and many of these practices have been incorporated in the repair and overhaul of operational aircraft. The integration of automated, controlled shot peening into these applications has placed shot peening in the REQUIRED SERVICES category.