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In support of the Federal Aviation Administration's Catastrophic Failure Prevention Program, SRI International has identified advanced materials and is developing shielding concepts to protect critical aircraft components from uncontained engine debris. Full-scale fragment impact tests on a commercial aircraft fuselage confirmed that barriers made from high-strength polymer fabrics in the fuselage wall could prevent penetration into the cabin. A computational capability is now being developed to enable efficient design of fabric fragment barriers. A mathematical model of woven fabric made from Zylon polybenzazoles (PBO), Kevlar, or Spectra was constructed using the data and observations from laboratory tests to measure yarn tensile and friction properties, quasi-static penetration tests to measure the evolution and phenomenology of fabric deformation and failure, and projectile impact tests to measure effects of fabric material, mesh density, boundary conditions (how a fabric is gripped), and projectile sharpness. The model was implemented in the LS-DYNA3D finite element code and used to simulate the failure behavior of yarns and fabrics under impact scenarios. The resulting insights are assisting barrier design. A simplified version of the computational model is being developed to enhance its usefulness to the commercial aircraft industry in designing engine fragment barriers., Original contains color plates: All DTIC reproductions will be in black and white.

New methods were developed and applied to extract and interpret information from the fracture surfaces of corrosion-fatigue specimens of clad and bare 2024-T3 aluminum sheet. We developed two fracture surface topography analysis methods that characterize the effects of environment and a fracture surface analysis and finite element modeling approach to detect and quantify overloads that occurred during fatigue crack propagation. We used these methods to perform a detailed analysis of the fracture surface topography produced in the early stages of corrosion fatigue, and developed an understanding of the effect of environment on the crack nucleation mechanism based on fracture surface features. In addition, cyclic load experiments were conducted on bare and clad 2024-T3 sheet under vacuum and in air, salt water, and arsenated salt water environments to assess the effects of environment, stress, and material condition on nucleation and early propagation kinetics of corrosion-fatigue cracks. For bare 2024-T3 sheet, substantial reductions in fatigue life were observed when the environment produced elongated pits associated with constituent particle corrosion. For clad 2024-T3 sheet, reductions in fatigue life were observed in environments that did not produce pitting (e.g., laboratory air and salt water). A likely mechanism for enhanced crack nucleation in 2024-T3 sheet with pure aluminum clad is the weakening effect of hydrogen on pure aluminum and the concomitant enhancement of cracking kinetics. The analytical methods developed and the analyses performed bear directly on the understanding of corrosion-fatigue in commercial aircraft through the relevancy of the materials, loading conditions, and test environments.

SRI International is performing a program of research to (1) determine the microstructural mechanisms underlying failure of metal matrix composites (MMCs), (2) measure the deformation and failure properties of MMCs under combined loading conditions, and (3) develop computational models for deformation and failure. In addition, we are helping the Wright Laboratory determine the microfailure sequence in a brittle matrix composite (BMC). This report describes the results obtained under Phase 1 of the research effort. The work will be completed in Phase 2 and reported in detail in a more comprehensive technical report. We quantified and analyzed the topography of fracture surfaces to reconstruct the fracture process and delineate the sequence of microfailure events occurring in SCS-6/Timetal 21S MMC under tensile load at room and elevated temperatures and under thermomechanical fatigue conditions. We performed a similar analysis on a SiC-fiber-reinforced borosilicate glass BMC tested at elevated temperature at the Wright Laboratory.

SRI International conducted a three-year program of experiments, microstructural examinations, and analysis to determine the rate sensitivity of deformation and failure of metal-matrix composites (MMC) and to understand and model the role of interfaces and constituents on dynamic failure. Silicon-carbide fiber reinforced Timetal 21S and Ti-15-3 matrices in three heat treatment conditions were studied. The work included dynamic fiber pushout experiments to measure dynamic interface strength, notched three-point-bend experiments with acoustic emission sensors to determine the static fracture resistance, and notched one-point-bend experiments to determine dynamic fracture resistance. Microstructural examinations were performed on transverse and longitudinal cross sections. The nature of interface failure, fiber failure, and matrix failure was established. A key finding was that toughening from fiber bridging is not obtained under bending loads. In bending, fibers fail well in advance of the crack tip and are thereby prevented from bridging matrix cracks. A two-dimensional finite element model of the composite was developed in which fibers, matrix, and interfaces in the vicinity of the notch root were modeled explicitly. The model can be used in parameter studies to investigate changes in interface and constituent properties, and loading rate, and hence assist in designing MMCs having tailored combinations of strength, toughness, and, particularly, greater resistance to bend failure.

SRI International is conducting research to characterize and quantitatively describe the early stages of corrosion fatigue in the fuselage skin of commercial aircraft. Specific objectives are to gain an improved deterministic understanding of the transition from corrosion pit to short crack to long crack and to delineate the effects of environment, skin surface condition, and loading conditions on crack nucleation and propagation rates. This Phase I report summarizes the effort in the first two years of this five-year program. The results suggest that corrosion fatigue in Alclad 2024-T3 involves two competing crack nucleation mechanisms-hydrogen effects in the cladding and pitting at constituent particles in the core alloy. In a given situation, the mechanism which dominates depends on environment (particularly pH) and (weakly) on specimen orientation. Cracks do not necessarily nucleate at the largest corrosion pit, suggesting that the main effect of a pit is not to raise the local stress. Rather, a high local hydrogen concentration associated with accelerated corrosion at a pit could cause cracking in a nearby favorably oriented grain. Propagation rates of short cracks were slightly higher in acidic environments and in specimens with painted surfaces, but were unaffected by material orientation and surface roughness. Corrosion fatigue observations and data from the laboratory experiments are being compared with those from field components to check whether research results represent in-service experience.

We extended classical dynamic fracture mechanics to a class of new commercially useful titanium microstructures and demonstrated that their dynamic fracture behavior differs significantly from that of previously studied model materials. By combining continuum measurements (obtained using the torsion split Hopkinson bar and one-point-bend fracture test methods) and fractogaphic measurements (obtained using fracture surface topography analysis, FRASTA), we generated a complete data base on static and dynamic strength and fracture toughness for various microstructures of the alloy Ti-IOV-2Fe-3AI. We determined effects of microstructural features on microfailure behavior and we modeled some of the observed microfailure processes using finite element analysis. Whereas the dynamic initiation toughness was only moderately higher than the static initiation toughness (at most 20%), a very strong rise was found in the crack propagation toughness with crack extension (as much as a 100% increase) for velocities as low as 100 m/s. This rate dependent resistance curve effect is an intrinsic material property (in contrast to a structural effect) and a strong function of microstructure. We demonstrated that the formation of shear lips is also a strongly rate dependent phenomenon and that shear lips, when they develop, make only a modest contribution to the propagation toughness (on the order of 20% for the case studied here). Our results also showed that, in general, there is not a direct correlation between the dynamic initiation and propagation toughnesses.