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An aging aircraft accumulates fatigue cracks commonly referred to as multiple site damage (MSD). A simplified engineering fracture mechanics model, generally referred to as the linkup model (or plastic zone touch model), has been used with some success to describe the MSD fracture phenomenon in 2024-T3 aluminum panels. A disadvantage of the linkup model is that it gives excessively inaccurate results for some configurations. A modified linkup model has been developed through empirical analysis of test data taken from unstiffened panels with MSD cracks at open holes. The modified linkup model was then validated with test data from stiffened panels including single-bay panels with the lead crack centered between stiffeners and two-bay panels with the lead crack centered beneath a severed stiffener. Further validation of the modified linkup model was done with test data from panels with bolted lap joints. Test results were obtained from 112 different panels.

This paper presents the experimental study of hole quality parameters in the drilling of titanium alloy (6Al-4V). Titanium alloy plates were drilled dry using three types of solid carbide drills i.e. 2-flute helical twist drill, straight flute and three-flute drill. The objective was to study the effects of process parameters like feed rate, speed and drill bit geometry on the hole quality features. Typical hole quality features in a drilling process are the hole quality measures such as surface roughness, hole diameter, hole roundness and burr height. The results indicate that proper selection of speed, feed rate, and drill geometry can optimize metal removal rate and hole quality.

The competitive market has forced the industry to develop methodologies to reduce lead-time of the products without sacrificing quality. One of the major metal removal operations in the aerospace industries is drilling. Over 100,000 holes are made for a small single engine aircraft. Naturally, demand for faster production rate results in the demand for high-speed drilling. But the cost of hole-making operations becomes a significant portion of the total manufacturing cost. This paper discusses the high speed drilling of Al-2024-T3 alloy, the effect of feed and speed on hole quality features like oversize, roundness error, burr height and surface roughness.

The work presented here illustrates the wear behavior of CVD diamond coated carbide tools during the machining of carbon fiber-reinforced composites. Cutting experiments were conducted on a CNC milling machine for edge trimming of a 9-mm thick multi-layered carbon fiber-reinforced epoxy laminate in a climb cutting configuration. The effects of feed speed and diamond film thickness on the wear behavior of the coated tools were determined. In addition, characteristics of the worn cutting edge were studied using optical and scanning electron microscopes. It was shown that diamond coated tools generally performed better than the uncoated tools under all conditions. Uniform wear by abrasion of the diamond film, without exposing the substrate, was obtained when cutting at low feed speeds with thicker coatings. At higher feed speeds the wear of the coated tools was characterized by abrasion through the diamond film and exposure and wear of the substrate.

The Refill Friction Spot Joining (RFSJ) is an emerging solid-state spot welding technology that thermo-mechanically creates a molecular-level bond between the work-pieces. RFSJ does not consume any filler or foreign materials so that no additional weight is introduced to the assembly. As the solid-to-liquid phase transition is not involved in RFSJ in general, there is no lack of fusion or material deterioration caused by liquefaction and solidification. Unlike the conventional friction stir spot welding, RFSJ produces a spot joint with a perfectly flush surface finish without a key or exit hole. Currently, the aerospace industry employs solid rivets for fastening the primary structures as they meet the baseline requirements and have well-established standards and specifications.

In the present work we studied the edge trimming process of CFRP with a diamond interlocking “burr” tool. Measurements of tool wear, surface roughness, spindle power and delamination depth were performed for different combinations of spindle speed and feed rate and were subsequently used to characterize machining quality. It was found that direct wear measurement for this type of cutting tool is not conclusive and thus not suitable for assessing tool life and machining quality. Instead, indirect indicators of tool wear were found more suitable for this purpose. Using these indirect methods an equation for tool life was defined and parameters for optimum machining quality were determined.

Vehicle electronics and sensor systems have become indispensable parts in providing safety, comfort, personal communication mobility and many other advanced functions in today's vehicles. As a result, reliability requirements for these critical parts have become extremely important. To meet these requirements, more advanced technologies and tools for degradation monitoring and failure prevention are needed. Currently, the development of diagnostics and prognostics techniques, which employ accurate degradation quantification by appropriate sensor selection, location decision, and feature selection and feature fusion, still remains a vital and unsolved issue. This paper addresses several realistic concerns of failure prevention in vehicle electronics and sensor systems. A unified monitoring and prognostics approach that prevents failures by analyzing degradation features, driven by physics-of-failure, is suggested as a general framework to overcome the unsolved challenge.

Laminated metal-metal composites can have attractive fracture toughness properties; they also offer potentially good fatigue performance. These attributes are reviewed and prospects for improvement discussed. Weak interlaminar bonds are seen to be important, while quite thin layers seem to be most promising for laminates of higher strength materials. The experimental program utilized 0.033 in (0.84 mm) thick laminae of 7075-T6 aluminum alloy, adhesively bonded. Eight-layer composites were compared with solid sheets of nearly the same total metal thickness. Both fracture toughness and fatigue properties were determined. Kc values of more than double the KIc for this alloy were observed in the laminates, while fatigue performance as indicated by comparative S-N curves was found to be slightly improved.

The residual strength of an aluminum panel with a centric hole and one cracked ligament was investigated experimentally. Each of the 7075-T6 aluminum panels which were tested included a cracked ligament of varying length on one side of the centric hole and an uncracked ligament on the other side of the hole. The failure of such a panel subjected to uniform tensile loading normally occurs according to the lower of two modes: brittle fracture or a net section type of yielding. On the other hand, the question of whether one or both ligaments fail is not easily answered. Results show that one or two ligament failure depends upon test conditions such as crack length and loading method. For short crack lengths, the uncracked ligament will fail almost simultaneously with the failure of the cracked ligament.

Multiple site damage (MSD) on aging aircraft accumulates from fatigue loading over a period of time. For ductile materials such as 2024-T3 aluminum, MSD may lower the strength below that which is predicted by conventional fracture mechanics. An analytical model referred to as the linkup (or plastic zone touch) model has previously been used to describe this phenomenon. However, the linkup model has been shown to produce inaccurate results for many configurations. This paper describes several modifications of the linkup model developed from empirical analyses. These modified linkup models have been shown to produce accurate results over a wide range of configurations for both unstiffened and stiffened flat 2024-T3 panels with MSD at open holes. These modified models are easy to use and give quick and accurate results over a large range of parameters.

An experimental study was conducted to investigate ice-adhesion on clean and coated aluminum surfaces. A test apparatus using the parallel plate linear shear technique was designed along with a data acquisition system for conducting the tests and recording the experimental data. A low pulling rate was applied to specially prepared test specimens for measuring the strength of ice adhesion for a range of test conditions. The effects of surface roughness, surface contamination, and water impurity on ice adhesion were investigated. In addition, tests were conducted to evaluate the effectiveness of a low ice-adhesion coating applied to aluminum test specimens. The results obtained showed that the bond between ice and metal was considerably lower for tap water than for distilled water. For the clean and coated aluminum surfaces the strength of ice adhesion varied with specimen roughness. However, no clear trend was established between ice adhesion strength and surface roughness.

Drilling is one of the most widely applied manufacturing operations. Millions of holes are drilled today in manufacturing industries especially in aerospace industry where high quality holes are essential. Rejection and rework rate of the products because of the bad hole is quite high. In this research graphite/honeycomb composite material and aluminum sheet metal has been used. The results show that drill geometry, speed and feed rate have substantial effects on the hole quality and also there was gradual variation of the thrust and lateral forces with feed rates.

Composites are finding more and more applications in the aircraft industry. Drilling good quality holes is a major challenge for the manufacturing industry. The major factors which have an effect on hole quality are cutting parameters like speed and feedrate, machine rigidity, tool material, workpiece material, and tool geometry. The hole quality was studied by measuring the hole diameter and visually observing other parameters like shape and fiber breakout. Force analysis indicates that thrust increases with an increase in feedrate. Speed does not seem to have a very significant effect on thrust. The tool geometry plays a very important role in fiber pullout.