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At least 1,603 records · Page 89

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NASA NTRS1991

Comparison of hand laid-up tape and filament wound composite cylinders and panels with and without impact damage

The results of this experimental comparison of filament wound control (unimpacted) cylinders loaded to failure in axial compression indicates that one fiber cross-over location has no effect on the failure mode or strain in thick walled filament wound graphite-epoxy specimens with stacking sequence (plus or minus 45/90) sub 3s. A comparison between filament wound and hand laid-up tape control cylinders indicates that there is little or no difference in the response of cylinders constructed by using two different fabrication methods, however, unimpacted panels with many fiber cross-overs fail at up to 15 percent lower strains than panels with no fiber cross-overs. A comparison of samples subjected to low speed impact damage prior to compressive loading indicates that impact damage reduces the strain at failure by over 60 percent in tape and filament wound graphite-epoxy cylinders and in tape flat panels. The presence of fiber cross-overs was observed to reduce the strength of filament wound impact-damaged panels, but to have no significant effect on the strength of filament wound impact-damaged cylinders.

Jegley, Dawn C.↗

NASA NTRS1992

Application of composite materials to impact-insensitive munitions

An approach is outlined for developing bullet-impact-insensitive munitions based on composite materials that provide rapid venting of the rocket-motor case. Impact experiments are conducted with test specimens of hybrid laminates of graphite/epoxy and epoxy reinforcing with woven glass fibers. The dynamic strain response and initial impact force are measured with strain gauges, and perforation damage is examined in the plates. The results show that impact damage can be designed by means of parametric variations of the fiber, matrix, and ply orientations. It is suggested that rocket-motor cases can be designed with composite materials to provide rapid venting during the failure mode. The experimental ballistic testing performed provides data that can be used comparatively with analytical data on composite materials.

Neradka, Vincent F.↗

NASA NTRS1992

Aluminum 2219-T87 and 5456-H116 - A comparative study of spacecraft wall materials in dual-wall structures under hypervelocity impact

All earth-orbiting spacecraft are susceptible to high-speed impacts by pieces of orbital debris. To prevent mission failure and possibly loss of life, protection against perforation by high-speed orbital debris particles must be included in the spacecraft design. Although any number of materials can be used to manufacture perforation-resistant structures, aluminum is often used in such systems because of its relatively high strength-to-weight ratio. This paper presents the results of a study in which the high speed impact response characteristics of dual-wall structures made from two different aluminum alloys were analyzed to determine which alloy would be more suitable for use in a perforation-resistant dual-wall structural system that is to be exposed to the orbital debris environment. Impact response characteristics were obtained numerically and experimentally. At impact speeds below 7 km/s, it was found that the two aluminum alloys considered contributed similar levels of perforation resistance; at speeds in excess of 7 km/s, aluminum 2219-T87 was superior to aluminum 5546-H116 in preventing perforation of dual-wall structural systems.

Schonberg, William P.↗

NASA NTRS1992

The Martian impact cratering record

A detailed analysis of the Martian impact cratering record is presented. The major differences in impact crater morphology and morphometry between Mars and the moon and Mercury are argued to be largely the result of subsurface volatiles on Mars. In general, the depth to these volatiles may decrease with increasing latitude in the southern hemisphere, but the base of this layer may be at a more or less constant depth. The Martial crustal dichotomy could have been the result of a very large impact near the end of the accretion of Mars. Monte Carlo computer simulations suggest that such an impact was not only possible, but likely. The Martian highland cratering record shows a marked paucity of craters less than about 30 km in diameter relative to the lunar highlands. This paucity of craters was probably the result of the obliteration of craters by an early period of intense erosion and deposition by aeolian, fluvial, and glacial processes.

Strom, Robert G.↗

NASA NTRS1993

Impact damage tolerance of thin wall composite struts

An experimental investigation was made to study the impact damage tolerance of thin wall composite struts made of both brittle epoxy and toughened epoxy based composite materials. Damage parameters such as barely visible surface damage and internal damage represented by the ultrasonic C-scan, and residual compressive strengths were evaluated against impact energy for two impactor sizes. From both a damage resistance (internal damage vs. impact energy) and a damage tolerance (residual compressive strength vs. internal damage) point of view, the toughened IM7/977-2 struts exhibited better performance than the brittle epoxy based T50/934 struts. This is attributed to the toughening mechanism in 977-2 which impedes delamination initiation from impact, and delamination growth and subsequent buckling under a compression loading. At barely visible damage thresholds, regardless of the impactor sizes, a maximum strength reduction of 45-55 percent was observed for the T50/934 struts, and approximately 10 percent for IM7/977-2 struts. This is of great interest for developing a damage tolerance design approach and risk assessment methodology in which the design allowable would be defined by the residual strength at the threshold of barely visible damage.

Chen, G.-S.↗

NASA NTRS1993

Response of laminated composite plates to low-speed impact by airgun-propelled and dropped-weight impactors

An analytical procedure has been developed to determine the transient response of simply supported, rectangular laminated composite plates subjected to impact loads from airgun-propelled or dropped-weight impactors. A first-order shear-deformation theory has been included in the analysis to represent properly any local short-wavelength transient bending response. The impact force has been modeled as a locally distributed load with a cosine-cosine distribution. A double Fourier series expansion and the Timoshenko small increment method have been used to determine the contact force, out-of-plane deflections, and inplane strains and stresses at any plate location due to an impact force at any plate location. The results of experimental and analytical studies are compared for quasi-isotropic laminates. The results indicate the importance of including transverse shear deformation effects in the analysis for predicting the response of laminated plates subjected to both airgun-propelled and dropped-weight impactors. The results also indicate that plate boundary conditions influence the axial strains more significantly than the contact force for a dropped-weight impactor. The results of parametric studies identify a scaling approach based on impactor momentum that suggests an explanation for the differences in the responses of plates impacted by airgun-propelled or dropped-weight impactors.

Prasad, Chunchu B.↗

NASA NTRS1993

Xenological constraints on the impact erosion of the early Martian atmosphere

This paper considers impact erosion as the agent of the early Martian atmophere escape, by examining xenon-related constraints imposed on the impact erosion. It is found that, if impact erosion of a planetary atmosphere is important, the remnant atmosphere is likely to fall into one of two qualitatively distinct categories: (1) cometary xenon has an unexpected isotopic composition or (2) factors other than impacts, such as outgassing or recycling, are needed. Neither of these categories can readily account for the present Martian atmosphere.

Zahnle, Kevin J.↗

NASA NTRS1993

Impact and volcanism - A momentum scaling law for erosion

The controversy that raged through the 1950s to 1970s over similarities and differences between meteorite impact and volcanic processes is revisited. We propose that there are quantitative similarities in erosion caused by high-speed ejecta produced by either impacts or volcanic processes. Field and petrographic data from the Manicouagan impact crater, Canada, are used to demonstrate that, during the emplacement of the impact melt sheet, erosion occurred at a rate of 2562 kg/sq m per s. Field data for the Mount St. Helens lateral blast of May 18, 1980, suggest an erosion rate of 21 kg/sq m per s, and field data for a small pyroclastic flow on August 7, 1980, suggest an erosion rate of about 14 kg /sq m per s. It is proposed that these three rates were determined dominantly by the momentum of the ejecta, and a quantitative formulation based on lofting theory is given. A new application of the Monte Carlo approach to analysis provides minimum, most likely, and maximum estimates for both the field and the theoretical analyses. The substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales.

Simonds, Charles H.↗

NASA NTRS1991

Jet ejecta mass upon oblique impact

Theoretical models in the jetting regime for symmetric and asymmetric impact of thin plates predict the mass and velocity of jetted material upon oblique impact. However, experimental constraints on the amount of material which form jets upon oblique impact are not known. A series of preliminary experiments were conducted in which tungsten (W) flyer plates at speeds of 1.5 to 2.0 km/s were obliquely impacted into carbon targets at 30 deg in the regime of jetting, yielding radiation temperatures in the about 3200 K range. Both framing-camera and flash X-ray imaging were conducted. Broad cm-sized craters induced by jet ejecta on 2024 Al witness plates were used to infer jet mass. We infer, from measured witness plate crater volumes, that jet masses in the range of 0.01 to 0.06 g are produced by a 32 mm diameter, 6 mm thick W impactor. This is about one to two orders of magnitude less than those calculated from present theoretical models. In contrast, in refractory material experiments, the mass of gabbro ejecta trapped in styrofoam is 0.52 g, which is similar to that calculated.

Yang, W.↗

NASA NTRS1939

Resistance of Transparent Plastics to Impact

The problem of developing a windshield for aircraft which will withstand the effect of bird impacts during flight is a difficult one, as an estimate of the striking energy will indicate. If the average speed of the airplane is considered to be about 200 miles per hour and that of the bird about 70 miles per hour, the speed of the bird relative to the airplane may be as great as 400 feet per second. If a 4-pound bird is involved, a maximum impact energy of approximately 10,000 foot-pounds must be dissipated. To obtain this energy in a drop test in the Washington Monument, it would be necessary to drop a 20-pound weight down the 500-foot shaft. For both theoretical and practical reasons, it is necessary to keep the mass and speed more nearly like those to be encountered. However, to get an impact of about 10,000 foot-pounds with a 4-pound falling body, it would be necessary to drop it from a height of approximately one-half mile, neglecting air resistance. These facts will indicate some of the experimental obstacles in the way of simulating bird impacts against aircraft windshields.

Axilrod, Benjamin M↗

NASA NTRS1952

Landing-gear impact

Report deals with the impact forces in landing gears. Both the landing impact and the taxiing impact have been considered, but drag forces have so far been excluded. The differential equations are developed and their numerical integration is shown, considering the nonlinear properties of the oleo shock strut. A way is shown for determining the dimensions of the metering pin from a given load-time diagram. A review of German literature on landing-gear impact is also presented.

Flugge, W↗

NASA NTRS1945

Analysis and modification of theory for impact of seaplanes on water

An analysis of available theory on seaplane impact and a proposed modification thereto are presented. In previous methods the overall momentum of the float and virtual mass has been assumed to remain constant during the impact but the present analysis shows that this assumption is rigorously correct only when the resultant velocity of the float is normal to the keel. The proposed modification chiefly involves consideration of the fact that forward velocity of the seaplane float causes momentum to be passed into the hydrodynamic downwash (an action that is the entire consideration in the case of the planing float) and consideration of the fact that, for an impact with trim, the rate of penetration is determined not only by the velocity component normal to the keel but also by the velocity component parallel to the keel, which tends to reduce the penetration. Experimental data for planing, oblique impact, and vertical drop are used to show that the accuracy of the proposed theory is good.

Mayo, Wilbur L↗

NASA NTRS1947

A Theoretical Investigation of Hydrodynamic Impact Loads on Scalloped-Bottom Seaplanes and Comparisons with Experiment

An analytical method is presented for calculating the hydrodynamic impact loads and motions experienced by seaplane floats and hulls with scalloped (fluted) bottoms. The analysis treats vertical impact at zero trim in addition to the more general problem of the step impact of a seaplane at positive trim where the flight path is oblique to the keel and to the water surface. Also considered are the transformations required to represent impacts into waves.

Milwitzky, Benjamin↗

NASA NTRS1993

Age of Popigai impact event using the Ar-40 - Ar-39 method

The Popigai impact structure of central Siberia is the largest known impact crater in the Commonwealth of Independent States with an original diameter of some 1OO km. The age of the crater is constrained by the existing stratigraphy to a period between 5-65 Ma. Attempts to date the impact event using conventional K-Ar on whole rock samples and fission track dating on glasses yield a spread of ages between 30 and 45 Ma. Argon step-heating analyses of several whole-rock samples performed with the Argon Laserprobe at the University of Toronto indicated an age of impact of about 36 Ma. However, a more recently reported Ar-40 - Ar-49 result on glass separated from a suevite sample gave a 65 Ma age and raised the possibility that Popigai was involved with the K/T boundary event. We have pursued further analyses at the University of Toronto on a broader spectrum of Popigai samples. These results confirm an age of about 36 Ma for the formation of this crater, and indicate that Popigai was not associated with the K/T boundary event.

Bottomley, R. J.↗

NASA NTRS1993

Modelling hypervelocity impacts into aluminum structures based on LDEF data

Realizing and understanding the effects of the near-Earth space environment on a spacecraft during its mission lifetime is becoming more important with the regeneration of America's space program. Included among these potential effects are the following: erosion and surface degradation due to atomic oxygen impingement; ultraviolet exposure embrittlement; and delamination, pitting, cratering, and ring formation due to micrometeoroid and debris impacts. These effects may occur synergistically and may alter the spacecraft materials enough to modify the resultant crater, star crack, and/or perforation. This study concentrates on modelling the effects of micrometeoroid and debris hypervelocity impacts into aluminum materials (6061-T6). Space debris exists in all sizes, and has the possibility of growing into a potentially catastrophic problem, particularly since self-collisions between particles can rapidly escalate the number of small impactors. We have examined the morphologies of the Long Duration Exposure Facility (LDEF) impact craters and the relationship between the observed impact damage on LDEF versus the existing models for both the natural (micrometeoroid) and manmade (debris) environments in order to better define these environments.

Coombs, C. R.↗

NASA NTRS1993

Do large impact basins in the southern hemisphere of Mars control the distribution of polar structures and deposits?

Among the outstanding problems in Martian geology are the cause of the off-axis and asymmetric distribution of the southern polar layered terrain and residual ice deposits and the cause of the orientation of scarps, valleys, and re-entrant canyons which occur there. A perhaps related problem region is the apparently small number of large (D greater than 500 km) impact basins seen in the relatively well-preserved cratered terrain of the south polar region. Previously only the 850 km wide South Polar Basin was easily recognized. The south polar region was mapped in detail, searching for evidence of ancient, highly degraded impact basins that may have escaped earlier notice, for two reasons: (1) to determine whether the apparent absence of large impact basins is due to incomplete mapping and recognition or a fundamental characteristic of the Martian crust related to the origin of the Martian crustal dichotomy, and (2) to determine whether ancient impact basins, if they exist, exert some control on the distribution of volcanic and polar deposits in the southern hemisphere and on the topography on which these deposits lie. Several promising candidates, including a large pre-Hellas basin in the Malea Planum region and an older but comparably sized basin overlapping South Polar, were previously described. Concentration is on the possible influence of the candidate basins in localizing the asymmetric distribution of polar deposits and in controlling the orientation of structures found within these deposits.

Frey, Herbert↗

NASA NTRS1993

Impact melting and vaporization in planetary regoliths

The thermal phenomena attending high-velocity impact have profound effects on virtually every aspect of the physical, chemical, and optical evolution of planetary regoliths. Not only do impacts pulverize, redistribute, and mix the various components of a regolith, but they also fuse and vaporize them - changing crystalline material to glass, releasing trapped or implanted gases, and spreading vaporized products across the planet's surface, among many other things. Those wishing to understand the details of regolith evolution must incorporate the effects of impact into their approach to the problem. Derived from a more extensive contribution, a short summary of the thermal processes accompanying impact into planetary regoliths, with the immediate acknowledgment that it is neither exhaustive in its consideration of the existing literature nor exact in any of its treatments, is presented. The reader desiring more information is directed to the relevant papers cited at the end; should they fail to provide satisfaction, he or she is then heartily encouraged to attack the problem immediately.

Cintala, Mark J.↗

NASA NTRS1993

Manson impact structure, Iowa: First geochemical results for drill core M-1

The Manson Impact Structure is a large complex impact crater centered ca. S km north of the town of Manson, Iowa. It is the largest intact impact structure recognized in the United States (35 km in diameter). Its Ar-40/Ar-39 age is indistinguishable from that of the Cretaceous-Tertiary (K-T) boundary. The Manson structure may be one element of the events at the K-T boundary. The crater is completely covered by Quaternary glacial sedimentary deposits that are normally underlain by Cretaceous clastic sediments and flat-lying carbonate sediments of Phanerozoic age, as well as Proterozoic red clastic, metamorphic, volcanic, and plutonic rock sequences. The study of a reflection seismic profile, provided by Amoco, was critical in interpreting the structure. In the 35 km diameter zone that marks the extension of the crater the normal rock sequence is disturbed due to the impact, and at the center of the structure granitic basement rocks are present that have been uplifted from about 4 km depth. Our studies consist of detailed petrological and geochemical characterization of all cores, with emphasis on a detailed description of all rock types found in the core samples and their relationship to target rocks. Geochemical data on samples from the Manson M-1 core are presented.

Koeberl, Christian↗