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Previous fatigue tests on mechanically fastened aerospace joints showed fatigue cracks often initiated in the countersink of the fastener hole where the fastener head was in contact with and caused fretting on, the hole bore. The work presented here evaluated the potential of a number of possible fastener coatings to reduce fretting and increase the fatigue life of the joint. The coatings were tested in a fretting fatigue test and in a ‘zero load’ fatigue test. The results showed that the best fretting resistance and fatigue life was obtained when aluminum pigmented coating (in accordance with NAS 4006) was used. The results also suggest that both test methods provide a similar ranking of performance. This means that the simpler fretting fatigue test may be useful as an initial screening method. However, more testing is needed to confirm this relationship.

Significant system efficiency gains can be achieved in high-performance aircraft via a unitized structure that reduces parts count. For instance, reduced parts count leads to substantial engineering logistic cost savings through higher levels of subsystem and mounting hardware integration. It also creates performance benefits by eliminating structural connections. Residual stress management, however, remains a major obstacle to capturing full benefits and broadening the application of unitized structure solutions. This paper describes how Alcoa and others are developing tools to overcome limitations in current testing, evaluation, and design practices attributed to residual stress effects. The authors present recent advancements in fracture toughness and fatigue crack growth characterization, along with a new, integrated approach for improved accounting of residual stress effects during fracture critical component design, manufacturing planning, and life management.

In aerospace fastener industry, all materials used or being considered for fastener applications must meet specified minimum shear strength values. It is widely known that shear strength is dependant on the type of shear test. However despite the suspected test dependencies, no detailed open research literature on the fastener shear test, especially shear strength variation via single and double shear approaches, and factors including surface coatings that affect tested shear values, has been conducted. Thus, the objective of this program is to systematically evaluate the effect of coating tribology on the threaded fasteners shear property tested via single shear and double shear testing methods. Five most common fastener finishes were selected, including uncoated, Aluminum CVD, Hi-KOTE, MoS2 coated, and Anodized.

Several significant process and product developments have enabled single-sided projection welding of 6XXX-T4 automotive aluminum sheet (patent pending). Hem welding was performed on geometries typically encountered on door and hood applications using an enhanced version of the HY-PAK® technology. This process is unique because both electrodes approach from the same side of the component, enabling no visible weld mark on the opposing surface. An experimental design was developed to understand the influence of the major process variables (weld current, weld force, projection height) on the tensile shear and button peel performance. The DOE was performed on single-sided projection welds consisting of 0.8mm 6111-T4 aluminum sheet. The results indicate a range of acceptable process parameters that produced tensile shear strengths on the order of the Aluminum Association's T-10 recommended values for resistance welds of that gauge.

This paper presents the results of an investigation into the material variables that influence the weldbonding of aluminum. The four major variables that were considered in this study were the aluminum alloy, type of adhesive, the presence of a forming lubricant, and the metal surface treatment. To maximize the amount of information gained from this study, a statistical design of experiments was used. The particular design used here is an example of a screening design, in which a relatively small number of variable combinations is investigated to identify those variables that have a strong impact on the measured responses. The responses in this experiment consist of both quantitative measurements and qualitative judgments that must be taken together to interpret the experimental results. The “quantifiable” responses included weld parameters (current and percent heat); nugget diameter; weld quality ratings (based on a subjective rating system); and tensile strength.