Karen E. Jackson

Scale effects in the response and failure of fiber reinforced composite laminates loaded in tension and in flexure

The feasibility of using scale model testing for predicting the full-scale behavior of flat composite coupons loaded in tension and beam-columns loaded in flexure is examined. Classical laws of similitude are applied to fabricate and test replica model specimens to identify scaling effects in the load response, strength, and mode of failure. Experiments were performed on graphite-epoxy composite specimens having different laminate stacking sequences and a range of scaled sizes. From the experiments it was deduced that the elastic response of scaled composite specimens was independent of size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled specimens of certain laminate stacking se quences. A Weibull statistical model and a fracture mechanics based model were applied to predict the strength scale effect since standard failure criteria cannot account for the in fluence of absolute specimen size on strength.

Water Impact Test and Simulation of a Composite Energy Absorbing Fuselage Section

In March 2002, a 25-ft/s vertical drop test of a composite fuselage section was conducted onto water. The purpose of the test was to obtain experimental data characterizing the structural response of the fuselage section during water impact for comparison with two previous drop tests that were performed onto a rigid surface and soft soil. For the drop test, the fuselage section was configured with ten 100-lb. lead masses, five per side, that were attached to seat rails mounted to the floor. The fuselage section was raised to a height of 10-ft. and dropped vertically into a 15-ft. diameter pool filled to a depth of 3.5-ft. with water. Approximately 70 channels of data were collected during the drop test at a 10-kHz sampling rate. The test data were used to validate crash simulations of the water impact that were developed using the nonlinear, explicit transient dynamic codes, MSC.Dytran and LS-DYNA. The fuselage structure was modeled using shell and solid elements with a Lagrangian mesh, and the water was modeled with both Eulerian and Lagrangian techniques. The fluid-structure interactions were executed using the fast general coupling in MSC.Dytran and the Arbitrary Lagrange-Euler (ALE) coupling in LS-DYNA. Additionally, the smooth particle hydrodynamics (SPH) meshless Lagrangian technique was used in LS-DYNA to represent the fluid. The simulation results were correlated with the test data to validate the modeling approach. Additional simulation studies were performed to determine how changes in mesh density, mesh uniformity, fluid viscosity, and failure strain influence the test-analysis correlation.

Development of a Scale Model Composite Fuselage Concept for Improved Crashworthiness

A composite fuselage concept for light aircraft has been developed to provide improved crashworthiness. The fuselage consists of a relatively rigid upper section, or passenger cabin, including a stiff structural e oor and a frangible lower section that encloses the crash energy management structure. The crashworthy performance of the fuselage concept was evaluated through impact testing of a one-e fth-scale model fuselage section. The impact design requirement for the scale model fuselage is to achieve a 125- g average e oor-level acceleration for a 31ft/s vertical impact onto a rigid surface. The energy absorption behavior of two different sube oor cone gurations was determined through quasi-static crushing tests. For the dynamic evaluation, each sube oor cone guration was incorporated into a one-e fth-scale model fuselage section, which was dropped from a height of 15 ft to achieve a 31-ft/s vertical velocity at impact. The experimental data demonstrate that the fuselage section with a foam-block sube oor cone guration satise ed the impact design requirement. A second drop test was performed to evaluate the energy absorption performanceofthefuselageconceptfor an off-axis impactcondition. The experimental data are correlated with analytical predictions from a e nite element model developed using the nonlinear, explicit transient dynamic code MSC/DYTRAN.

Full-Scale Crash Test and Simulation of a Composite Helicopter

Abstract A finite element model of the Sikorsky Advanced Composite Airframe Program (ACAP) helicopter was developed using the non-linear, explicit transient dynamic code, MSC.Dytran. Analytical predictions were correlated with experimental data obtained from a full-scale crash test of the Sikorsky ACAP helicopter flight test article that was conducted in June 1999 at the Impact Dynamics Research Facility of NASA Langley Research Centre, Hampton, Virginia, USA. The helicopter was impacted at 11.58 m/s vertical and 9.9-m/s forward velocity with an attitude of 6.25° pitch (nose up) and 3.5° left roll. Due to the relatively long crash pulse duration, a rigid-body helicopter model with an energy absorbing landing gear model was executed initially. Prior to fuselage contact, a deformable structural model was executed with the rigid-body nodal displacements and velocities used as initial conditions.

Impact Testing and Simulation of a Crashworthy Composite Fuselage Section with Energy-Absorbing Seats and Dummies

AbstractA 25-ft/s vertical drop test of a composite fuselage section was conducted with two energy-absorbing seats occupied by an-thropomorphic dummies to evaluate the crashworthy features of the fuselage section and to determine its interaction with theseats and dummies. The 5-ft. diameter fuselage section consists of a stiff structural floor and an energy-absorbing subfloorconstructed of Rohacel foam blocks. The experimental data from this test were analyzed and correlated with predictions froma crash simulation developed using the nonlinear, explicit transient dynamic computer code, MSC.Dytran. The anthropo-morphic dummies were simulated using the Articulated Total Body (ATB) code, which is integrated into MSC.Dytran.IntroductionA research program was conducted at NASA Langley Re-search Center to develop an innovative and cost-effectivecrashworthy fuselage concept for light aircraft and rotor-craft [1-3]. The composite fuselage concept was designedto meet structural and flight-load requirements and toprovide improved crash protection. The two primary de-sign goals for crashworthiness are to limit the impactforces transmitted to the occupants, and to maintain thestructural integrity of the fuselage to ensure a minimumsafe occupant volume. To meet these objectives, an air-craft or rotorcraft fuselage must be designed for high stiff-ness and strength to prevent structural collapse during acrash. Yet, the fuselage design must not be so stiff that ittransmits or amplifies high impact loads to the occupants.ldeally, the design should contain some crushable ele-ments to help limit the loads transmitted to the occupantto survivable or non-injurious levels.The fuselage concept, shown in Figure 1, consists of astiff upper fuselage, a structural floor, and an energy-absorbing subfloor. The upper section of the fuselagecabin is fabricated using a composite sandwich construc-tion and is designed to provide a protective shell thatencloses the occupants in the event of a crash. The en-ergy-absorbing subfloor is designed to dissipate kineticenergy through stable crushing. Finally, a key feature ofthe fuselage concept is the stiff structural floor. Thestructural floor is designed to react the loads generated bycrushing of the subfloor, and to provide a stable platformfor seat and restraint attachment.During the first year of the research program, a 12-in.diameter, 1/5-scale model composite fuselage was de-signed, fabricated, and tested to verify structural andflight-load requirements [3]. During the second year ofthe research program, energy-absorbing subfloor configu-rations were evaluated using quasi-static testing and finiteelement simulation to determine the best design for use inthe 1/5-scale model fuselage concept [4, 5]. During thethird year of the program, a full-scale version of the fuse-lage concept was fabricated, and a vertical drop test wasconducted to validate the scaling process [6]. Test, analy-sis, and correlation with finite element models were per-formed for each test in the series. For the 1/5- and earlyfull-scale drop tests, the inertial loading that normallywould be provided by seats and occupants was representedwith lead weights. In April 2001, a full-scale fuselagesection was tested with two energy-absorbing seats, eachwith an anthropomorphic dummy occupant. The objec-tive of the drop test was to demonstrate the crashworthi-ness of the fuselage concept for a more realistic loadingenvironment using seats and dummies. The data from thedrop test and the development of an integrated crashsimulation are the focus of this paper.Since the completion of the initial research, the compositefuselage section has been used as a test bed for conductingother crash-related experiments. In 2000, two drop testsof a composite fuselage section were performed for thespecific goal of examining test and analysis correlationapproaches for detailed finite element crash simulations[7]. One test was performed from a drop height of 1.75inches to excite the linear frequency response, and testdata were correlated with an MSC.Nastran analysis. Thesecond test was performed for an impact velocity of 25ft/s, and the test data were correlated with a nonlinear,transient dynamic crash simulation. For both tests, thefuselage section was loaded symmetrically using leadmasses that were attached to the floor through seat tracks.The total floor mass was approximately 1000 lbs. The 25ft/s impact test described in Ref. 7 is of particular interestbecause it was performed at the same initial vertical veloc-ity as the fuselage test with seats and dummies describedin this paper. In addition, the fuselage section describedin Ref. 7 had nearly the same floor loading; however,only lead masses were attached to the floor in that test.

Simulation of Aircraft Landing Gears with a Nonlinear Dynamic Finite Element Code

Recent advances in computational speed have made aircraft and spacecraft crash simulations using an explicit, nonlinear, transient-dynamic,e niteelement analysiscodemore feasible.This paperdescribes thedevelopment of a simplelanding-gearmodel, which accurately simulatestheenergy absorbed by thegearwithoutaddingsubstantial complexity to the model. For a crash model the landing gear response is approximated with a spring where the force applied to the fuselage is computed in a user-written subroutine. Helicopter crash simulations using this approach are compared with previously acquired experimental data from a full-scale crash test of a composite helicopter.

Overview of the NASA Subsonic Rotary Wing Aeronautics Research Program in Rotorcraft Crashworthiness

This paper provides an overview of rotorcraft crashworthiness research being conducted at NASA Langley Research Center under sponsorship of the Subsonic Rotary Wing (SRW) Aeronautics Program. The research is focused in two areas: development of an externally deployable energy attenuating concept and improved prediction of rotorcraft crashworthiness. The deployable energy absorber (DEA) is a composite honeycomb structure, with a unique flexible hinge design that allows the honeycomb to be packaged and remain flat until needed for deployment. The capabilities of the DEA have been demonstrated through component crush tests and vertical drop tests of a retrofitted fuselage section onto different surfaces or terrain. The research on improved prediction of rotorcraft crashworthiness is focused in several areas including simulating occupant responses and injury risk assessment, predicting multi-terrain impact, and utilizing probabilistic analysis methods. A final task is to perform a system-integrated simulation of a full-scale helicopter crash test onto a rigid surface. A brief description of each research task is provided along with a summary of recent accomplishments.

Crash Simulation of a Vertical Drop Test of a Commuter-Class Aircraft

Abstract A finite element model of an ATR42-300 commuter-class aircraft was developed and a crash simulation was executed. Analytical predictions were correlated with data obtained from a 30 ft/s (9.14 m/s) vertical drop test of the aircraft. The purpose of the test was to evaluate the structural response of the aircraft when subjected to a severe, but survivable, impact. The aircraft was configured with seats, dummies, luggage, and other ballast. The wings were filled with 8,700 lb. (3,946 kg) of water to represent the fuel. The finite element model, which consisted of 57,643 nodes and 62,979 elements, was developed from direct measurements of the airframe geometry. The seats, dummies, luggage, fuel, and other ballast were represented using concentrated masses. The model was executed in LS-DYNA, a commercial code for performing explicit transient dynamic simulations. Predictions of structural deformation and selected time-history responses were generated. The simulation was successfully validated through extensive test-analysis correlation.

Size Effects in Scaled Fiber Composites Under Four-Point Flexure Loading

The effect of specimen size on the four-point flexural response of ply-level and sublaminate-level scaled composites was investigated. Two laminates were studied, namely, [+45/-45/+45/-45] s and [+45/-45/0/90] s . The material system used was AS4/3502 graphite/epoxy. Enhanced x-ray radiography and edge photomicroscopy were used to examine damage development in specimens loaded to various fractions of their ultimate load. This nondestructive examination was coupled with observations of the load/deflection response to try to correlate scaling effects with the damage development in the specimens. Results were compared to previous studies involving tensile scaling effects. It was found that the strength of ply-level scaled laminates decreased as specimen size increased, and that this effect was modeled well using the typical fracture mechanics scaling law. Sublaminate-level scaled specimens did not show a pronounced scaling effect. Although there seemed to be a slight decrease in strength with increased specimen size, the effect was small and may not be statistically significant.

A Summary of DOD-Sponsored Research Performed at NASA Langley's Impact Dynamics Research Facility

The Impact Dynamics Research Facility (IDRF) is a 240-ft.-high gantry structure located at NASA Langley Research Center in Hampton, Virginia. The IDRF was originally built in the early 1960s for use as a Lunar Landing Research Facility. As such, the facility was configured to simulate the reduced gravitational environment of the Moon, allowing the Apollo astronauts to practice lunar landings under realistic conditions. In 1985, the IDRF was designated a National Historic Landmark based on its significant contributions to the Apollo Moon Landing Program. In the early 1970s the facility was converted into its current configuration as a full-scale crash test facility for light aircraft and rotorcraft. Since that time, the IDRF has been used to perform a wide variety of impact tests on full-scale aircraft, airframe components, and space vehicles in support of the General Aviation (GA) aircraft industry, the U.S. Department of Defense (DOD), the rotorcraft industry, and the NASA Space program. The objectives of this paper are twofold: to describe the IDRF facility and its unique capabilities for conducting structural impact testing, and to summarize the impact tests performed at the IDRF in support of the DOD. These tests cover a time period of roughly 2 1/2 decades, beginning in 1975 with the full-scale crash test of a CH-47 Chinook helicopter, and ending in 1999 with the external fuel system qualification test of a UH-60 Black Hawk helicopter. NASA officially closed the IDRF in September 2003; consequently, it is important to document the past contributions made in improved human survivability and impact tolerance through DOD-sponsored research performed at the IDRF.