Advanced technology composite aircraft structures

Work performed during the 25th month on NAS1-18889, Advanced Technology Composite Aircraft Structures, is summarized. The main objective of this program is to develop an integrated technology and demonstrate a confidence level that permits the cost- and weight-effective use of advanced composite materials in primary structures of future aircraft with the emphasis on pressurized fuselages. The period from 1-31 May 1991 is covered

Effects of intra- and inter-laminar resin content on the mechanical properties of toughened composite materials

Composite materials having multiphase toughened matrix systems and laminate architectures characterized by resin-rich interlaminar layers (RIL) have been the subject of much recent attention. Such materials are likely to find applications in thick compressively loaded structures such as the keel area of commercial aircraft fuselages. The effects of resin content and its interlaminar and intralaminar distribution on mechanical properties were investigated with test and analysis of two carbon-epoxy systems. The RIL was found to reduce the in situ strengthening effect for matrix cracking in laminates. Mode 2 fracture toughness was found to increase with increasing RIL thickness over the range investigated, and Mode 1 interlaminar toughness was negligibly affected. Compressive failure strains were found to increase with increasing resin content for specimens having no damage, holes, and impact damage. Analytical tools for predicting matrix cracking of off-axis plies and damage tolerance in compression after impact (CAI) were successfully applied to materials with RIL

Multi-parameter optimization tool for low-cost commercial fuselage crown designs

The work in progress for developing a methodology and software tool to aid in the optimal design of composite structures is discussed. The methodology is being developed to take advantage of the ability to tailor the composite material in conjunction with the design of the structure. The composites optimization design software UWCODA was found to be very successful in preliminary testing and early experience. UWCODA is a composites design code that uses a number of plies and fiber angles as design variables, employs maximum strain failure criteria for objective function and additional constraints, includes Boeing design tools for stiffened panels, and includes stiffener geometry in the design variables

Advanced composite fuselage technology

Boeing's ATCAS program has completed its third year and continues to progress towards a goal to demonstrate composite fuselage technology with cost and weight advantages over aluminum. Work on this program is performed by an integrated team that includes several groups within The Boeing Company, industrial and university subcontractors, and technical support from NASA. During the course of the program, the ATCAS team has continued to perform a critical review of composite developments by recognizing advances in metal fuselage technology. Despite recent material, structural design, and manufacturing advancements for metals, polymeric matrix composite designs studied in ATCAS still project significant cost and weight advantages for future applications. A critical path to demonstrating technology readiness for composite transport fuselage structures was created to summarize ATCAS tasks for Phases A, B, and C. This includes a global schedule and list of technical issues which will be addressed throughout the course of studies. Work performed in ATCAS since the last ACT conference is also summarized. Most activities relate to crown quadrant manufacturing scaleup and performance verification. The former was highlighted by fabricating a curved, 7 ft. by 10 ft. panel, with cocured hat-stiffeners and cobonded J-frames. In building to this scale, process developments were achieved for tow-placed skins, drape formed stiffeners, braided/RTM frames, and panel cure tooling. Over 700 tests and supporting analyses have been performed for crown material and design evaluation, including structural tests that demonstrated limit load requirements for severed stiffener/skin failsafe damage conditions. Analysis of tests for tow-placed hybrid laminates with large damage indicates a tensile fracture toughness that is higher than that observed for advanced aluminum alloys. Additional recent ATCAS achievements include crown supporting technology, keel quadrant design evaluation, and sandwich process development

Tension fracture of laminates for transport fuselage. Part 2: Large notches

Tests were conducted on over 200 center-crack specimens to evaluate: (a) the tension-fracture performance of candidate materials and laminates for commercial fuselage applications; and (b) the accuracy of several failure criteria in predicting response. Crack lengths of up to 12 inches were considered. Other variables included fiber/matrix combination, layup, lamination manufacturing process, and intraply hybridization. Laminates fabricated using the automated tow-placement process provided significantly higher tension-fracture strengths than nominally identical tape laminates. This confirmed earlier findings for other layups, and possibly relates to a reduced stress concentration resulting from a larger scale of repeatable material inhomogeneity in the tow-placed laminates. Changes in material and layup result in a trade-off between small-notch and large-notch strengths. Toughened resins and 0 deg-dominate layups result in higher small-notch strengths but lower large-notch strengths than brittle resins, 90 deg and 45 deg dominated layups, and intraply S2-glass hybrid material forms. Test results indicate that strength-prediction methods that allow for a reduced order singularity of the crack-tip stress field are more successful at predicting failure over a range of notch sizes than those relying on the classical square-root singularity. The order of singularity required to accurately predict large-notch strength from small-notch data was affected by both material and layup. Measured crack-tip strain distributions were generally higher than those predicted using classical methods. Traditional methods of correcting for finite specimen width were found to be lacking, confirming earlier findings with other specimen geometries. Fracture tests of two stiffened panels, identical except for differing materials, with severed central stiffeners resulted in nearly identical damage progression and failure sequences. Strain-softening laws implemented within finite element models appear attractive to account for load redistribution in configured structure due to damage-induced crack tip softenin

On the phenomena of fracture in particle-board

Nonlocal theories of continuum mechanics are used to model the fracture behavior of particleboard in tension perpendicular to the plane of the board. Classical theories of fracture are examined in comparison to a nonlocal crack solution which has removed the stress singularity at the crack tip. With the removal of the classical stress singularity a fracture criterion based on a maximum stress hypothesis results. The nonlocal fracture equations are in terms of a material's internal characteristics such as the dimensions of internal structures (characteristic length) and the bonding between them (intrinsic strength). Examination of the fracture phenomena using the nonlocal equations provides an insight of the actual mechanism of failure. Many materials, including wood, consist of internal structures which are, themselves, composites of smaller units of structure. The level of internal structure that is important to the fracture process is a function of the relative size of internal structures in comparison to the flaw where fracture initiates and the bond strength at the various levels of internal structure. In order to test the nonlocal fracture model it was essential to work with a composite material for Which the dimensions of its internal structures may be varied. This Was possible with particleboard, a composite formed of wood particles and resin. If the nonlocal model is able to accurately predict the fracture for particleboard, it should be able to describe the manner in which a change in particle Size affects fracture resistance. Experimental results in this study give good correlation with the nonlocal prediction. For the particleboard formed in this study the characteristic length of the nonlocal relationship is shown to correspond to particle thickness at low resin contents (5% for a high efficiency laboratory blender). Fracture toughness increases with the square root of particle thickness as predicted by the nonlocal solution. There should be a limit in which an increase in particle thickness can no longer increase fracture resistance. This limit will depend on the relative intrinsic bond strength between particles and within particles. By increasing resin content, which increases the number of bonds between particles, the limiting process was shown to exist. This study showed that a particleboard formed from relatively long thin particles has an average intrinsic flaw size of approximately 0.34 inch. Because this intrinsic flaw size changed only slightly with change in resin content it was postulated that the resin-particle bond causes stress concentration due to a dissimilar response to load. Hence, the compatibility of the resin and wood particles found in particleboard could be as important to the final board properties as was the amount of resin used. If the orientation of substructures in a material can be expected to change the properties of the material, the material is best described as a micropolar continuum. The micropolar crack solution predicts couple stress (rotational stress) in the vicinity of a flaw. Conceivably, particles in the vicinity of a flaw could rotate to align their strongest axis with the direction of maximum stress. This interpretation may explain the initiation of the crazing phenomena in amorphous glassy polymers. For particleboard a flexible resin and short particles may allow such rotation and a large increase in strength could be expected. Caution should be taken in using the approach until the combined nonlocal micropolar crack problem is solved

A relationship between wave dispersion and fracture strength for a composite material

Wave dispersion and fracture are phenomena that involve. mechanisms triggered at the scale of a material's internal structure. Experiments were performed with a candidate material to judge the hypothesis that wave dispersion measurements can be used to nondestructively predict fracture strength. Wood-based particleboard was chosen as the candidate material because the entire acoustical branch of the longitudinal wave dispersion curve was accessible with ultrasonics. Wave dispersion and fracture were studied in detail for the weak direction of particleboard, through the panel thickness. The prediction of a relationship between wave dispersion and fracture strength is inherent to nonlocal theory. Particleboard has a complex internal structure, rendering it a difficult material to model. Therefore, it is a good test of the generality of nonlocal theory. Wave dispersion data from particleboard was converted into nonlocal moduli using a fast Fourier transform calculation. The variable frequency/variable path-length, continuous wave method was used to measure dispersion. Dispersion in particleboard occurred at frequencies between 400 and 900 kHz. The dominating mechanism was that of micro-geometric dispersion (i.e. decreasing phase velocity with increasing frequency). Frequencies above 900 kHz were effectively "stopped" by particleboard. The dispersion response of particleboard was found to depend on board specific gravity, resin content and particle size. Mode I load-displacement curves of cracked samples of particleboard exhibited initial linear behavior, followed by nonlinear response. Compliance calibration and the J[subscript I]-Integral were used for linear and nonlinear characterization, respectively. The nonlinear behavior was due to the growth of a local damage zone at the tip of the crack. The damage zone contained permanent deformation. As a result, the strict validity of the J[subscript I]-integral approach may have been violated. The failure energy of particleboard appears to be load history dependent. The nonlocal moduli were related to stress concentration or redistribution for relatively weak or strong materials, respectively. Wave dispersion was found to have a very high correlation with the critical strain energy release rate at the onset of nonlinear behavior