Rj Bucci

Effect of Residual Stress on Fatigue Crack Growth Rate Measurement

Examples are given where influence of residual stress leads to erroneous interpretation of fatigue crack growth rate measurements made in accordance with ASTM Method E 647-78T. The experimental data presented form a basisfor modification of applicable ASTM documents to give recognition to problems caused by residual stress, and to suggest guidelines for minimization of their effect on fracture property measurement.

Analysis and prediction of microstructural effects on long-term fatigue performance of an aluminum aerospace alloy

Abstract A program of experimental and analytical tasks has been conducted to define the linkage(s) between microstructural characteristics and fatigue performance in an aluminum alloy typically used for airframe structural applications. The first goal was to develop data for quantitatively linking measurable characteristics of material microstructure with long-term fatigue performance. The second goal was to develop models to predict fatigue performance based on the microstructural characteristics. The work focused on several process variants of aluminum alloy 7050-T7451 plate. This material was chosen because of its widespread use for flight-critical airframe structural components, and the particular characteristics associated with the manufacturing, service and maintenance of thick section components. Within the framework of this objective, life-limiting microstructural features have been identified and ranked by severity, and models to quantitatively describe the evolution and growth of macrostructural cracks from those features have been developed. The modeling framework has been applied to predict the cyclic lifetime of the 7050 alloy process variants based on the populations of life-limiting microstructural features. In addition, the models have been used to show how changes in the material characteristics may affect the fatigue performance. This includes predictions of the effect of changing the life-limiting microfeature size and shape distributions, and the effect of changing material strength properties. The use of this modeling approach to probabilistically describe the implications of changes in the microstructure has been demonstrated, thereby allowing the effects of material pedigree to be predictively linked with the structural integrity of end components. The modeling framework has potential applications in airframe design support processes, and as a tool for use in material and product form selection processes.

Development of a Proposed ASTM Standard Test Method for Near-Threshold Fatigue Crack Growth Rate Measurement

Results are summarized which provide the basis for development of the proposed ASTM standard test method for measuring and presenting very slow cyclic rates of fatigue crack propagation. The technique for obtaining very slow rate data as K decreases with crack extension is described. Data are reviewed that show the individual and combined effects of various precracking and testing procedures, loading rates, and other testing parameters. The data are used to demonstrate the utility of the method and its limitations. Guidelines are given for the minimization of transient growth rate processes which can confound interpretation of the data. Analytical procedures for fitting near-threshold data are also discussed.

Effect of Prior Corrosion on the S/N Fatigue Performance of Aluminum Sheet Alloys 2024-T3 and 2524-T3

Aviation industry demand for continuous safety improvement in the face of trends toward increasing service life of aircraft and cost control necessitates stronger prevention and control measures to avoid the likelihood ofstructural failures linked to widespread damage involving corrosion and fatigue. New materials with improved damage tolerance attributes can improve the margin of safety in the presence of widespread damage. An excellent example of one such material is new aluminum alloy 2524 (formerly C188) which has improved fracture toughness and fatigue crack growth resistance relative to incumbent alloy 2024. In this study, the effect of prior corrosion on the S/N fatigue performance of 1.60 and 3.17-mm thick 2524-T3 and 2024-T3 bare sheet was evaluated. The fatigue strength of 2524 was approximately 10% greater and the lifetime to failure 30 to 45% longer than that of the 2024. Two main factors are believed to have contributed to the better performance of 2524: a less damaging configuration of corrosion pits and its better fatigue crack growth resistance.

Notch-Yield Ratio as a Quality Control Index for Plane-Strain Fracture Toughness

The ratio of notch-tensile strength to tensile-yield strength (the notch-yield ratio) is correlatable with plane-strain fracture toughness, K I c . As a result of that fact and relative simplicity of notch-tension testing,it is suitable for plant quality control testing for fracture toughness. The test procedures and quality control testing practices are described, and data showing the correlation between K I c and notch-yield ratio from both 1 / 2 and 1 1 / 1 6 -in.-diameter notched tension specimens for several aluminum alloys are presented. The 1 1 / 1 6 -in.-diameter specimen provides a more discriminating correlation at high toughness levels than does the 1 / 2 -in.-diameter specimen. It is emphasized that this is not a procedure for estimating K I c directly, but for providing assurance that K I c equals or exceeds a stated value.

Ranking 7XXX Aluminum Alloy Fatigue Crack Growth Resistance Under Constant Amplitude and Spectrum Loading

Fatigue crack growth (FCG) experiments were conducted on controlled variations of Type 7075 and 7050 aluminum alloys. Alloy FCG resistance was ranked under constant amplitude and simple variable amplitude load spectra. Fracture mechanics and fractographic approaches were used to interpret causes for variation in ranking of 7XXX aluminum alloy FCG resistance with loading conditions. The interpretation is built around clarification of a controlling FCG mechanism that is dependent upon interaction of microstructure and load history. This clarification represents a necessary first step toward knowing which microstructure or which design (test) procedure is optimum for a particular class of application, for example, fighter as opposed to bomber or transport aircraft.

Spectrum Loading—A Useful Tool to Screen Effects of Microstructure on Fatigue Crack-Growth Resistance

The purpose of this paper is to promote consideration of variable amplitude or spectrum fatigue loading as a useful tool for screening effects of alloy microstructure on fatigue crack-propagation resistance. Increased sensitivity to microstructure, practical interpretation, and economy of testing are offered as rationale for advantageous use of this type of test.

The Impact of Forging Residual Stress on Fatigue in Aluminum

Large aluminum forgings are seeing increased application in aerospace structures, particularly as an enabler for structural unitization. These applications, however, demand an improved understanding of the forging process induced bulk residual stresses and their impact on both design mechanical properties and structural performance. In recent years, significant advances in both computational and experimental methods have led to vastly improved characterization of residual stresses. As a result, new design approaches which require the extraction of residual stress effects from material property data and the formal inclusion of residual stresses in the design analysis, have been enabled. In particular, the impact of residual stresses on durability and damage tolerance can now be assessed, and more importantly, accounted for at the beginning of the design cycle. In an effort to support the development of this next-generation design capability, the AFRL sponsored Metals Affordability Initiative (MAI) consortium 1 has conducted a detailed experimental and analytical study of fatigue crack initiation and fatigue crack growth in aluminum coupons with known, quench induced residual stresses. In this study, coupons were designed and manufactured such that simple ‘design features,’ such as holes and machined pockets, were installed in locations with varying levels of bulk residual stress. The residual stresses at the critical locations in the coupons were measured using multiple techniques and modeled using detailed finite element analysis. Fatigue crack initiation (FCI) and fatigue crack growth (FCG) tests were performed using both constant amplitude and spectrum loading and the results were compared against computed FCI and FCG lives.

Failure Analysis of Aramid Fiber Reinforced Aluminum Laminates with Surface and Through Thickness Fatigue Cracks

Fiber reinforced aluminum laminates are able to retard fatigue crack growth by transfer of load from cracked aluminum layers to intact fibers. The resulting crack bridging effect leads to superior laminate fatigue crack growth response compared to that of monolithic aluminum. In real laminate structures, fatigue cracks emanating from an open hole, as well as surface cracks initiated in an outer laminate aluminum layer, can be present. In this investigation, fatigue crack growth data on surface notch and open hole (through thickness notch) initiated fatigue cracks are presented. The failure analysis reveals that the surface initiated cracks propagate only in the outer aluminum layer, with fibers remaining intact. The open hole initiated cracks, on the other hand, propagate through the laminate thickness in all aluminum layers. In this case, depending on the laminate residual stress state (controlled by a post stretch procedure), a complete fiber failure may occur. When the fiber integrity is preserved, however, the laminates fatigue response in all cases is superior to that of monolithic aluminum.

The influence of material quality on airframe structural durability

Previous work on Al-Zn-Cu-Mg alloy 7050-T7451 has shown that improved processing to reduce the size of microporosity results in longer fatigue lifetimes. To differentiate this improvement, Alcoa has implemented smooth specimen fatigue testing on a lot release basis to warranty 7050-T7451 thick (5–6 in., 127–153 mm) plate initial fatigue quality. The present study employs a probabilistic fracture mechanics analysis to demonstrate that the material quality improvement verified in the smooth specimen fatigue tests translates to durability performance in actual structures. Microflaw size distributions measured from failed smooth coupon tests serve as the starting point for probabilistic crack growth analysis. Using crack exceedance probability as a basis for durability performance comparison, a hypothetical calculation shows that a fighter aircraft wing fabricated from improved quality metal outperforms the same wing fabricated from unimproved metal.