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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records

All sources NASA NTRS DOE OSTI

NASA NTRS2011

Comparison of Open-Hole Compression Strength and Compression After Impact Strength on Carbon Fiber/Epoxy Laminates for the Ares I Composite Interstage

Notched (open hole) composite laminates were tested in compression. The effect on strength of various sizes of through holes was examined. Results were compared to the average stress criterion model. Additionally, laminated sandwich structures were damaged from low-velocity impact with various impact energy levels and different impactor geometries. The compression strength relative to damage size was compared to the notched compression result strength. Open-hole compression strength was found to provide a reasonable bound on compression after impact.

Hodge, Andrew J.↗

NASA NTRS1973

Factors affecting miniature Izod impact strength of tungsten-fiber-metal-matrix

The miniature Izod and Charpy impact strengths of copper, copper-nickel, and nickel-base superalloy uniaxially reinforced with continuous tungsten fibers were studied. In most cases, impact strength was increased by increasing fiber or matrix toughness, decreasing fibermatrix reaction, increasing test temperature, hot working, or heat treating. Notch sensitivity was reduced by increasing fiber content or matrix toughness. An equation relating impact strength to fiber and matrix properties and fiber content was developed. Program results imply that tungsten alloy-fiber/superalloy matrix composites can be made with adequate impact resistance for turbine blade or vane applications.

Winsa, E. A.↗

NASA NTRS1975

Composite impact strength improvement through a fiber/matrix interphase

Research was conducted to improve the impact strength and toughness of fiber/resin composites by means of a fiber coating interphase. Graphite fiber/epoxy resin composites were fabricated with four different fiber coating systems introduced in a matrix-fiber interphase. Two graphite fibers, a high strength and a high modulus type, were studied with the following coating systems: chemical vapor deposited boron, electroless nickel, a polyamide-imide resin and a thermoplastic polysulfone resin. Evaluation methods included the following tests: Izod, flexure, shear fracture toughness, longitudinal and transverse tensile, and transverse and longitudinal compression. No desirable changes could be effected with the high strength fiber, but significant improvements in impact performance were observed with the polyamide-imide resin coated high modulus fiber with no loss in composite modulus.

Cavano, P. J.↗

NASA NTRS1989

A model for compression after impact strength evaluation

One key property commonly used for evaluating composite material performance is compression after impact strength (CAI). Standarad CAI tests typically use a specific laminate stacking sequence, coupon geometry, and impact level. In order to understand what material factors affect CAI, evaluation of test results should include more than comparisons of the measured strength for different materials. This study considers the effects of characteristic impact damage state, specimen geometry, material toughness, ply group thickness, undamaged strength, and failure mode. The results of parametric studies, using an analysis model developed to predict CAI, are discussed. Experimental results used to verify the model are also presented. Finally, recommended pre- and post-test CAI evaluation schemes which help link material behavior to structural performance are summarized.

Ilcewicz, Larry B.↗

NASA NTRS1982

Graphite fiber surface treatment to improve impact strength and fracture resistance in subsequent composites

Graphite (or carbon) fiber composite impact strength improvement was attempted by modifying the fiber surface. Elastomeric particles were made into lattices and deposited ionically on surface treated graphite fiber in an attempt to prepare a surface containing discrete rubber particles. With hard, nonelastomeric polystyrene discrete particle coverage was achieved. All the elastomeric containing lattices resulted in elastomer flow and filament agglomeration during drying.

Paul, J. T., Jr.↗

NASA NTRS2017

The Influence of GI and GII on the Compression After Impact Strength of Carbon Fiber/Epoxy Laminates and Sandwich Structure

This study measured the compression after impact strength of IM7 carbon fiber laminates made from epoxy resins with various mode I and mode II toughness values to observe the effects of these toughness values on the resistance to damage formation and subsequent residual compression strength-carrying capabilities. Both monolithic laminates and sandwich structure were evaluated. A total of seven different epoxy resin systems were used ranging in approximate GI values of 245-665 J/sq m and approximate GII values of 840-2275 J/sq m. The results for resistance to impact damage formation showed that there was a direct correlation between GII and the planar size of damage, as measured by thermography. Subsequent residual compression strength testing suggested that GI had no influence on the measured values and most of the difference in compression strength was directly related to the size of damage. Thus, delamination growth assumed as an opening type of failure mechanism does not appear to be responsible for loss of compression strength in the specimens examined in this study.

Nettles, A. T.↗

DOE OSTI2018

The Selection of Heat Treatment Parameters to Obtain Austempered Ductile Iron with the Required Impact Strength

The effect of austempering parameters on impact energy of non-alloyed cast iron with nodular graphite was examined. The analysis involved the austenitizing temperature in the range of 830-950 °C for the time of 60 min and the austempering temperature in the range of 300-400 °C for the time of 8, 16, 32 and 64 min. The value of the impact energy KV was determined. It has been found that the austenitizing temperature and austempering conditions affect the impact energy of ADI. The conducted analytical studies helped to develop a method for the selection of heat treatment parameters to obtain the ausferritic ductile iron with appropriate impact energy KV. The applied machine learning methods included support vector machine, decision trees, artificial neural networks and the method of variational component analysis.

36 MATERIALS SCIENCE↗

NASA NTRS1992

Removal of surface loop from stitched composites can improve compression and compression-after-impact strengths

Stitching through-the-thickness (TTT) of composite materials produces a surface loop of yarn between successive penetrations. The surface loop is pressed into the surface layers of the composite material during the curing of the laminate, kinking the in-plane fibers near the surface of the material. The compression strength and compression-after-impact (CAI) strengths of carbon-epoxy specimens were measured with and without the surface loop. Removal of the surface loop had no influence on failure mode or failure mechanism, but did significantly increase the compression and CAI strengths.

Farley, Gary L.↗

NASA NTRS2023

The Effects of Impactor Shape on the Compression After Impact Strength of Carbon/Epoxy Face Sheet Foam Core Sandwich Structure

This study presents experimental results of compression after impact (CAI) strength testing of foam core sandwich structure with carbon/epoxy face sheets impacted at three different energies with either a blunt or sharp tip impactor. The impact energies used were chosen to span the barely visible impact damage (BVID) thresholds for the sharp (4.1 J) and blunt (12.9 J) impactors with an impact energy approximately halfway between these (8.1 J) also used. While most impact testing on composites utilize a hemispherical (blunt) impactor, actual damage to a part may be due to an object impacting the part that is not blunt, but sharp and the impact response and resulting CAI strength values may be different for a given impact energy level. In this study, with regards to the impact response, the sandwich specimens showed larger transverse displacements (by about a factor of two) during the impact event when impacted by a sharp impactor versus a blunt impactor. The maximum load of impact was larger for the blunt impactor by about a factor of three. The barely visible impact damage (BVID) threshold energy was lower by about a factor of three for the sharp impactor. The absorbed energy of impact was higher for the sharp impactor. The CAI strength results showed that the sharp impactor gave lower average CAI strength values at the lowest impact energy level used, slightly lower average CAI strength values for the medium impact energy used and about the same average CAI strength values for the highest impact energy used.

sandwich structure↗

NASA NTRS2024

The Effects of Impactor Shape on the Compression After Impact Strength of Carbon/Epoxy Face Sheet Aluminum Honeycomb Core Sandwich Structure

This study presents experimental results of compression after impact (CAI) strength testing of aluminum core sandwich structure with carbon/epoxy face sheets impacted at three different energies with either a 12.7 mm hemispherical, 24.5 mm hemispherical or sharp tip impactor. This study was undertaken to verify whether design allowables typically generated by blunt (hemispherical) impactors may be valid in ‘real world” applications where an impact may not be by a blunt object of a specific size. Results of practical interest in this study were that the visual damage was much easier to see for the sharp impactor, the damage size as detected by thermography was smaller for the sharp impactor for a given impact energy and dent depth was greater for the sharp impactor. The CAI strength results showed that the sharp impactor gave about the same CAI strength values at all three impact energy levels used. The larger hemispherical impactor gave slightly higher CAI results than the smaller blunt impactor. Overall it was deemed difficult to develop a CAI allowable that would cover all impactor shapes with any fidelity and thus using a lower bound strain allowable is suggested.

compression after impact (CAI)↗

NASA NTRS1994

The Effects of Compressive Preloads on the Compression-After-Impact Strength of Carbon/Epoxy

A fixture to apply compressive loads to composite specimens during an impact event was used to assess the effect of prestresses on the compression-after-impact (CAI) strength of 16 ply quasi-isotropic carbon/epoxy test coupons. Advanced design of experiments techniques were used to evaluate a range of prestresses and impact energies on two material systems, T300/934 and IM7/8551-7. An instrumented drop tower supplied impact energies between 1 and 9 Joules for the T300/934 material and between 4 and 16 Joules for the IM7/8551-7 material. The prestress values varied between a low of 5.7 Wa and a high of 287 NDa. Results showed some change in CAI strength that could be attributed to the prestresses on the specimens.

Nettles, Alan T.↗

NASA NTRS2010

Effect of Microscopic Damage Events on Static and Ballistic Impact Strength of Triaxial Braid Composites

The reliability of impact simulations for aircraft components made with triaxial-braided carbon-fiber composites is currently limited by inadequate material property data and lack of validated material models for analysis. Methods to characterize the material properties used in the analytical models from a systematically obtained set of test data are also lacking. A macroscopic finite element based analytical model to analyze the impact response of these materials has been developed. The stiffness and strength properties utilized in the material model are obtained from a set of quasi-static in-plane tension, compression and shear coupon level tests. Full-field optical strain measurement techniques are applied in the testing, and the results are used to help in characterizing the model. The unit cell of the braided composite is modeled as a series of shell elements, where each element is modeled as a laminated composite. The braided architecture can thus be approximated within the analytical model. The transient dynamic finite element code LS-DYNA is utilized to conduct the finite element simulations, and an internal LS-DYNA constitutive model is utilized in the analysis. Methods to obtain the stiffness and strength properties required by the constitutive model from the available test data are developed. Simulations of quasi-static coupon tests and impact tests of a represented braided composite are conducted. Overall, the developed method shows promise, but improvements that are needed in test and analysis methods for better predictive capability are examined.

Littell, Justin D.↗

NASA NTRS1992

The effects of compressive preloads on the compression-after-impact strength of carbon/epoxy

A preloading device was used to examine the effects of compressive prestress on the compression-after-impact (CAI) strength of 16-ply, quasi-isotropic carbon epoxy test coupons. T300/934 material was evaluated at preloads from 200 to 4000 lb at impact energies from 1 to 9 joules. IM7/8551-7 material was evaluated at preloads from 4000 to 10,000 lb at impact energies from 4 to 16 joules. Advanced design of experiments methodology was used to design and evaluate the test matrices. The results showed that no statistically significant change in CAI strength could be contributed to the amount of compressive preload applied to the specimen.

Nettles, A. T.↗

NASA NTRS2021

Compression After Impact Strength Comparison of Rohacell Foam and Aluminum Honeycomb Core Sandwich Structure

This Technical Memorandum is a follow-on study to a previous one1 that examined the compression after impact (CAI) strength of carbon fiber sandwich structure with aluminum honeycomb core and Rohacell foam core with barely visible impact damage (BVID). In this current study a wider range of impact energies were utilized and the CAI strength assessed with these different damage severity levels. This was done to provide a more complete database for the two types of sandwich structure tested in these studies. In addition, the honeycomb sandwich specimens were tested with both the ‘L’ and ‘W’ directions aligned with the axis of compression loading. As in the study examining CAI strength with BVID1, the results were normalized by the mass of the sandwich structure. The results showed that the CAI strength did differ depending on the orientation of the core during testing and that when tested in the W-direction, the honeycomb core sandwich structure had about the same CAI strength as the foam core sandwich structure when normalized by the mass of the sandwich structures.

Sandwich Structure↗

NASA NTRS2008

Effect of Microscopic Damage Events on Static and Ballistic Impact Strength of Triaxial Braid Composites

In previous work, the ballistic impact resistance of triaxial braided carbon/epoxy composites made with large flat tows (12k and 24k) was examined by impacting 2 X2 X0.125" composite panels with gelatin projectiles. Several high strength, intermediate modulus carbon fibers were used in combination with both untoughened and toughened matrix materials. A wide range of penetration thresholds were measured for the various fiber/matrix combinations. However, there was no clear relationship between the penetration threshold and the properties of the constituents. During some of these experiments high speed cameras were used to view the failure process, and full-field strain measurements were made to determine the strain at the onset of failure. However, these experiments provided only limited insight into the microscopic failure processes responsible for the wide range of impact resistance observed. In order to investigate potential microscopic failure processes in more detail, quasi-static tests were performed in tension, compression, and shear. Full-field strain measurement techniques were used to identify local regions of high strain resulting from microscopic failures. Microscopic failure events near the specimen surface, such as splitting of fiber bundles in surface plies, were easily identified. Subsurface damage, such as fiber fracture or fiber bundle splitting, could be identified by its effect on in-plane surface strains. Subsurface delamination could be detected as an out-of-plane deflection at the surface. Using this data, failure criteria could be established at the fiber tow level for use in analysis. An analytical formulation was developed to allow the microscopic failure criteria to be used in place of macroscopic properties as input to simulations performed using the commercial explicit finite element code, LS-DYNA. The test methods developed to investigate microscopic failure will be presented along with methods for determining local failure criteria that can be used in analysis. Results of simulations performed using LS-DYNA will be presented to illustrate the capabilities and limitations for simulating failure during quasi-static deformation and during ballistic impact of large unit cell size triaxial braid composites.

Littell, Justin D.↗

NASA NTRS2022

The Effects of Co-cured Versus Precured Carbon/Epoxy Face Sheets on the Modulus and Compression After Impact Strength of Honeycomb Sandwich Structure

This study presents experimental results of compression after impact (CAI) and stiffness testing of aluminum honeycomb core sandwich structure with quasi-isotropic face sheets that have either been co-cured or precured (and then bonded to the core in a secondary operation). The co-cured face sheets contain some ply waviness whereas the precured face sheets did not. The effect of this waviness was ascertained for two important design parameters for the Space Launch System (SLS) Payload Adapter Fitting (PAF); compressive modulus and CAI strength. It was found that there was no effect on the modulus between co-cured and precured face sheets using the material and processes in this study. In addition, there was no difference in CAI strength behavior between the co-cured and precured face sheet.

sandwich structure↗

NASA NTRS2023

The Effects of Off-Axis Loading on the Compression After Impact Strength of Quasi-Isotropic Face Sheet Honeycomb Core Sandwich Structure

This study presents experimental results of compression after impact (CAI) testing of aluminum honeycomb core sandwich structure with face sheets made of co-cured T1100/3960 quasi-isotropic carbon/epoxy when tested at +22.5⁰ and -22.5⁰ with respect to the 0⁰ fibers. In a previous study examining the CAI strengths of honeycomb sandwich structure, it was found that specimens had different CAI strengths, based on a [-45/90/+45/0]s layup, depending on whether they were tested in the 0⁰ direction (face sheet layup of [-45/90/+45/0]s) or 90⁰ direction (face sheet layup of [+45/0/-45/90]s). The CAI strength results showed that the specimens tested in the 90⁰ direction had a 19% drop in CAI strength compared to specimens tested in the 0⁰ direction. This was attributed to the 0⁰ load bearing plies in the 0⁰ direction specimens being “tucked in” at the center of the specimen thus providing more stability against microbuckling. This raised the question as to what CAI strength would specimens tested at +22.5⁰ (face sheet layup of [-22.5/- 67.5/+67.6/+22.5]s) and -22.5⁰ (face sheet layup of [-67.5/+67.5/+22.5/-22.5]s) have compared to specimens tested in the 0⁰ and 90⁰ direction. Results presented in this study show that the specimens loaded at +22.5⁰ and -22.5⁰ have a similar average CAI strength compared to the specimens loaded in the 0⁰ direction. The specimens loaded in the 90⁰ direction exhibit 16% lower average CAI strength. Additional specimens were tested in the +45⁰ direction to put the 0⁰ load bearing fibers on the outside of the specimen to see if this would decrease the strength as has been documented for undamaged strength. These specimens have average CAI strength values between the 0⁰ direction average CAI strength values and the 90⁰ direction average CAI strength values.

Sandwich structure↗

NASA NTRS1999

Parametric Study of Three-Stringer-Panel Compression-After-Impact Strength

Damage tolerance requirements for integrally-stiffened composite wing skins are typically met using design allowables generated by testing impact-damaged subcomponents, such as three-stringer stiffened panels. To improve these structures, it is necessary to evaluate the critical design parameters associated with three-stringer stiffened-panel compressive behavior. During recent research and development programs, four structural parameters were identified as sources for strength variation: (a) material system, (b) stringer configuration, (c) skin layup, and (d) form of axial reinforcement (tape versus pultruded carbon rods). Relative effects of these parameters on damage resistance and damage tolerance were evaluated numerically and experimentally. Material system and geometric configuration had the largest influence on damage resistance; location and extent of the damage zone influenced the sublaminate buckling behavior, failure initiation site, and compressive ultimate strength. A practical global-local modeling technique captured observed experimental behavior and has the potential to identify critical damage sites and estimate failure loads prior to testing. More careful consideration should be given to accurate simulation of boundary conditions in numerical and experimental studies.

Rousseau, Carl Q.↗