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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Evaluation of the Edge Crack Torsion (ECT) Test for Mode 3 Interlaminar Fracture Toughness of Laminated Composites

An analytical and experimental investigation was carried out on G40-800/R6376 graphite epoxy laminates to evaluate the Edge Crack Torsion (ECT) test as a candidate for a standard Mode 3 interlaminar fracture toughness test for laminated composites. The ECT test consists of a (90/(+/- 45)(sub 3)/(+/- 45)(sub 3)/90))(sub s) laminate with a delamination introduced by a non-adhesive film at the mid-plane along one edge and loaded in a special fixture to create torsion along the length of the laminate. Dye penetrate enhanced X-radiograph of failed specimens revealed that the delamination initiated at the middle of the specimen length and propagated in a self similar manner along the laminate mid-plane. A three-dimensional finite element analysis was performed that indicated that a pure Mode 3 delamination exists at the middle of specimen length away from both ends. At the ends near the loading point a small Mode 2 component exists. However, the magnitude of this Mode 2 strain energy release rate at the loading point is small compared to the magnitude of Mode 3 component in the mid-section of the specimen. Hence, the ECT test yielded the desired Mode 3 delamination. The Mode 3 fracture toughness was obtained from a compliance calibration method and was in good agreement with the finite element results. Mode 2 End-Notched Flexure (ENF) tests and Mode 1 Double Cantilever Beam (DCB) tests were also performed for the same composite material. The Mode 1 fracture toughness was much smaller than both the Mode 2 and Mode 3 fracture toughness. The Mode 2 fracture toughness was found to be 75% of the Mode 3 fracture toughness.

Li, Jian↗
NASA NTRS2002

Strength and Fracture Toughness of Solid Oxide Fuel Cell Electrolyte Material Improved

Solid oxide fuel cells (SOFC) are being developed for various applications in the automobile, power-generation, and aeronautics industries. Recently, the NASA Glenn Research Center has been exploring the possibility of using SOFC's for aeropropulsion under its Zero Carbon Dioxide Emission Technology (ZCET) Program. 10-mol% yttriastabilized zirconia (10YSZ) is a very good anionic conductor at high temperatures and is, therefore, used as an oxygen solid electrolyte in SOFC. However, it has a high thermal expansion coefficient, low thermal shock resistance, low fracture toughness, and poor mechanical strength. For aeronautic applications, the thin ceramic electrolyte membrane of the SOFC needs to be strong and tough. Therefore, we have been investigating the possibility of enhancing the strength and fracture toughness of the 10YSZ electrolyte without degrading its electrical conductivity to an appreciable extent. We recently demonstrated that the addition of alumina to zirconia electrolyte increases its strength as well as its fracture toughness. Zirconia-alumina composites containing 0 to 30 mol% of alumina were fabricated by hot pressing. The hot pressing procedure was developed and various hot pressing parameters were optimized, resulting in dense, crackfree panels of composite materials. Cubic zirconia and a-alumina were the only phases detected, indicating that there was no chemical reaction between the constituents during hot pressing at elevated temperatures. Flexure strength sf and fracture toughness K(sub IC) of the various zirconia-alumina composites were measured at room temperature as well as at 1000 C in air. Both properties showed systematic improvement with increased alumina addition at room temperature and at 1000 C. Use of these modified electrolytes with improved strength and fracture toughness should prolong the life and enhance the performance of SOFC in aeronautics and other applications.

Bansal, Narottam P.↗
DOE OSTI2024

On plasticity-enhanced interfacial toughness in bonded joints

Here, the performance and reliability of many structures and components depend on the integrity of interfaces between dissimilar materials. Interfacial toughness Γ is the key material parameter that characterizes resistance to interfacial crack growth, and Γ is known to depend on many factors including temperature. For example, previous work showed that the toughness of an epoxy/aluminum interface decreased 40 % as the test temperature was increased from -60 °C to room temperature (RT). Interfacial integrity at elevated temperatures is of considerable practical importance. Recent measurements show that instead of continuing to decrease with increasing temperature, Γ increases when test temperature is above RT. Cohesive zone finite element calculations of an adhesively bonded, asymmetric double cantilever beam specimen of the type used to measure Γ suggest that this increase in toughness may be a result of R-curve behavior generated by plasticity-enhanced toughening during stable subcritical crack growth with interfacial toughness defined as the critical steady-state limit value. In these calculations, which used an elastic-perfectly plastic epoxy model with a temperature-dependent yield strength, the plasticity-enhanced increase in Γ above its intrinsic value Γ o depended on the ratio of interfacial strength σ* to the yield strength σyb of the bond material. There is a nonlinear relationship between Γ/Γ o and σ*/σ yb with the value Γ/Γ o increasing rapidly above a threshold value of σ*/σ yb . The predicted increase in toughness can be significant. For example, there is nearly a factor of two predicted increase in Γ/Γ o during micrometer-scale crack-growth when σ*/σ yb = 2 (a reasonable choice for σ*/σ yb ). Furthermore, contrary to other reported results, plasticity-enhanced toughening can occur prior to crack advance as the cohesive zone forms and the peak stress at the tip of the original crack tip translates to the tip of the fully formed cohesive zone. These results suggest that plasticity-enhanced toughening should be considered when modeling interfaces at elevated temperatures.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Fiber reinforced solids possessing great fracture toughness: The role of interfacial strength

The high tensile strength characteristic of strong interfacial filament/matrix bonding can be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibers. An analysis is presented for toughness and strength which demonstrates, in broad terms, the effects of varying the coating parameters of concern. Results show that the toughness of interfaces is an important parameter, differences in which may not be shown up in terms of interfacial strength. Some observations are made upon methods of measuring the components of toughness in composites.

Atkins, A. G.↗
NASA NTRS1974

Theoretical and experimental comparisons of the nonlinear energy method to the J-integral, R-curve and COD methods in fracture toughness testing

Because of the current high degree of interest in the development of a standard nonlinear test method, analytical and experimental comparisons have been made between the R-curve, COD, J-integral and nonlinear energy methods. A general definition of fracture toughness is proposed and the fundamental definitions of each method are compared to it. Experimental comparisons between the COD, J-integral, nonlinear energy and standard ASTM methods have been made for a series of compact tension tests on several aluminum alloys. Some of the tests were conducted according to the ASTM standard method E399-72, while the specimen thickness was reduced below the minimum requirement for plane strain fracture toughness testing for several other test series. The fracture toughness values obtained by the COD method were significantly higher than the toughness values obtained by the other three methods. All of the methods displayed a tendency to yield higher toughness values as the thickness was decreased below the ASTM plane strain requirement.

Liebowitz, H.↗
NASA NTRS1976

Determination of nonlinear energy toughness values for cyclic loading applications

For several years the nonlinear energy method proposed by Liebowitz and Eftis has been examined as a failure criterion for static testing of center-cracked and compact tension specimens. Since the method appears to be valid under conditions of crack-tip plasticity, subcritical crack growth and load relaxation, tests have been conducted to ascertain the merit of this method as a failure criterion under cyclic loading conditions. The nonlinear energy toughness for cyclic loading is obtained from an envelope of the cyclic load-displacement record, which naturally imposes some restrictions on the loading program. The cyclic toughness parameter has been evaluated for thin center-cracked sheets of 2024-T3 and 7075-T6 aluminum alloys. Both alloys exhibited a significant reduction of the cyclic toughness parameter with increasing cyclic life in a manner similar to the classical S-N diagram. It is suggested that this method may serve the design process by allowing the establishment of a fracture toughness parameter capable of including the effects of the entire loading history of a structure into the fracture toughness requirements.

Poulose, P. K.↗
NASA NTRS1988

Microstructural effects on fracture toughness of polycrystalline ceramics in combined mode I and mode II loading

Fracture toughness of polycrystalline alumina and ceria partially-stabilized tetragonal zirconia (CeO2-TZP) ceramics were assessed in combined mode I and mode II loading using precracked disk specimens in diametral compression. Stress states ranging from pure mode I, combined mode I and mode II, and pure mode II were obtained by aligning the center crack at specific angles relative to the loading diameter. The resulting mixed-mode fracture toughness envelope showed significant deviation to higher fracture toughness in mode II relative to the predictions of the linear elastic fracture mechanics theory. Critical comparison with corresponding results on soda-lime glass and fracture surface observations showed that crack surface resistance arising from grain interlocking and abrasion was the main source of the increased fracture toughness in mode II loading of the polycrystalline ceramics. The normalized fracture toughness for pure mode II loading, (KII/KIc), increased with increasing grain size for the CeO2-TZP ceramics. Quantitative fractography confirmed an increased percentage of transgranular fracture of the grains in mode II loading.

Singh, D.↗
NASA NTRS1989

Indentation fracture toughness of sintered silicon carbide in the Palmqvist crack regime

The fracture toughness of a sintered dense alpha-SiC was estimated by the Vickers indentation microfracture method in the low-load Palmqvist crack regime. It was observed that the use of simultaneously obtained Vickers hardnesses does not yield reliable fracture toughness values, nor does application of the median-crack-derived equations. It is necessary to utilize a load-independent, crack-free hardness value with this toughness estimation method. Although several of the curve-fitting equations yield similar toughnesses, it is concluded for the Palmqvist crack system in this alpha-SiC that the Niihara-Morena-Hasselman (1982) equation is the only one which yields fracture toughness values in agreement with conventional measurement techniques.

Li, Zhuang↗
NASA NTRS1989

Factors influencing the Mode I interlaminar fracture toughness of a rubber toughened thermoplastic matrix composite

The use of a rubber modified thermoplastic resin has been investigated as a method to improve the Mode I interlaminar fracture toughness of a unidirectional continuous carbon fiber composite. Test results show that the improvement in the fracture toughness is less than expected due to rubber particle agglomeration, solvent and molding induced crystallization of the matrix and poor fiber/matrix adhesion. The plastic zone in composites utilizing tough matrices can extend well beyond a single interfibrillar spacing. However, the development of the plastic zone is limited due to the failure of the fiber/matrix interface. In order to fully evaluate the potential of tough composites using toughened matrices, any improvement made in the matrix toughness must be coupled with improvements in the fiber/matrix adhesion.

Parker, D. S.↗
NASA NTRS1989

Toughness properties of a three-dimensional carbon-epoxy composite

The three-dimensional (3D) orthogonal interlocked fabric contains through-the-thickness reinforcement in order to enhance the interlaminar fracture toughness of the composite. The interlaminar fracture toughness of a carbon-epoxy orthogonal interlocked fabric composite was experimentally determined by use of the recently developed tabbed double cantilever beam specimen. The data reduction methods applicable to these tests and materials and the interpretation of the results were discussed. The results of critical strain energy release rate, G(IC), were compared to those of a two-dimensional (2D) laminate having the same in-plane structure. The energy-dissipating crack propagation processes were described. The in-plane fracture toughness of the three-dimensional fabric was experimentally measured and compared to that of the two-dimensional laminate. The through-the-thickness fibers were found to create a tenfold increase in interlaminar toughness, and a 25-percent improvement in the in-plane fracture toughness.

Guenon, Valerie↗
NASA NTRS1989

Tough poly(arylene ether) thermoplastics as modifiers for bismaleimides

Several aspects of research on thermoplastics as toughness modifiers are discussed, including the contribution of the backbone chemistry and the concentration of the poly(arylene ether) thermoplastic to fracture toughness, influence of the molecular weight of the poly(arylene ether) thermoplastic on neat resin fracture toughness, and the morphology of the thermoplastic modified networks. The results show that fracture toughness of brittle bismaleimide resins can be improved significantly with poly(arylene ether) thermoplastic levels of 20 percent by weight, and that high molecular weight poly(arylene ether) based on bisphenol A provides the highest degree of toughening. Preliminary composite evaluation shows that improvements in neat resin toughness translate into carbon fabric composite.

Stenzenberger, H. D.↗
NASA NTRS1993

Extrusion process optimization for toughness in balloon films

An experimental optimization process for blown film extrusion is described and examined in terms of the effects of the technique on the toughness of balloon films. The optimization technique by Cantor (1990) is employed which involves the identification of key process variables including screw speed, nip speed, bubble diameter, and frost-line height for analysis to optimize the merit function. The procedure is employed in the extrusion of a low-density polyethylene polymer, and the resulting optimized materials are toughness- and puncture-tested. Balloon toughness is optimized in the analytical relationship, and the process parameters are modified to attain optimal toughness. The film produced is shown to have an average toughness of 24.5 MPa which is a good value for this key property of balloon materials for high-altitude flights.

Cantor, K. M.↗
NASA NTRS2005

Fracture Toughness in Advanced Monolithic Ceramics - SEPB Versus SEVENB Methods

Fracture toughness of a total of 13 advanced monolithic ceramics including silicon nitrides, silicon carbide, aluminas, and glass ceramic was determined at ambient temperature by using both single edge precracked beam (SEPB) and single edge v-notched beam (SEVNB) methods. Relatively good agreement in fracture toughness between the two methods was observed for advanced ceramics with flat R-curves; whereas, poor agreement in fracture toughness was seen for materials with rising R-curves. The discrepancy in fracture toughness between the two methods was due to stable crack growth with crack closure forces acting in the wake region of cracks even in SEVNB test specimens. The effect of discrepancy in fracture toughness was analyzed in terms of microstructural feature (grain size and shape), toughening exponent, and stable crack growth determined using back-face strain gaging.

Choi, S. R.↗
NASA NTRS2006

Assessments of Fracture Toughness of Monolithic Ceramics-SEPB Versus SEVNB Methods

Fracture toughness of a total of 13 advanced monolithic ceramics including silicon nitrides, silicon carbide, aluminas, and glass ceramic was determined at ambient temperature by using both single edge precracked beam (SEPB) and single edge v-notched beam (SEVNB) methods. Relatively good agreement in fracture toughness between the two methods was observed for advanced ceramics with flat R-curves; whereas, poor agreement in fracture toughness was seen for materials with rising R-curves. The discrepancy in fracture toughness between the two methods was due to stable crack growth with crack closure forces acting in the wake region of cracks even in SEVNB test specimens. The effect of discrepancy in fracture toughness was analyzed in terms of microstructural feature (grain size and shape), toughening exponent in R-curve, and stable crack growth determined using back-face strain gaging.

Choi, Sung R.↗
NASA NTRS2012

Comparison of Intralaminar and Interlaminar Mode-I Fracture Toughness of Unidirectional IM7/8552 Graphite/Epoxy Composite

The intralaminar and interlaminar mode-I fracture-toughness of a unidirectional IM7/8552 graphite/epoxy composite were measured using compact tension (CT) and double cantilever beam (DCB) test specimens, respectively. Two starter crack geometries were considered for both the CT and DCB specimen configurations. In the first case, starter cracks were produced by 12.5 micron thick, Teflon film inserts. In the second case, considerably sharper starter cracks were produced by fatigue precracking. For each specimen configuration, use of the Teflon film starter cracks resulted in initially unstable crack growth and artificially high initiation fracture-toughness values. Conversely, specimens with fatigue precracks exhibited stable growth onset and lower initiation fracture toughness. For CT and DCB specimens with fatigue precracks, the intralaminar and interlaminar initiation fracture toughnesses were approximately equal. However, during propagation, the CT specimens exhibited more extensive fiber bridging, and rapidly increasing R-curve behavior as compared to the DCB specimens. Observations of initiation and propagation of intralaminar and interlaminar fracture, and the measurements of fracture toughness, were supported by fractographic analysis using scanning electron microscopy.

Czabaj, Michael W.↗
DOE OSTI2025

Scalable fabrication of a tough and recyclable spore-bearing biocomposite thermoplastic polyurethane

Thermoplastic polyurethanes (TPUs) are a class of versatile thermoplastic elastomers, but most of their products lack a proper recycling strategy or have no end-of-life solutions. To pursue a sustainable end-of-life solution for TPU-based products, self-disintegrating biocomposite TPUs have recently been developed by embedding spores of TPU-degrading bacteria into TPUs via melt extrusion. Herein, we improve upon spore-bearing biocomposites and demonstrate industrially relevant manufacturing conditions for fabricating biocomposite TPUs. To minimize the coloration of biocomposite TPUs, spore production was modified. The innate brown color of the resulting materials was diminished by reducing FeSO 4 in sporulation media, generating white spores without compromising spore productivity, viability, morphology or heat-shock tolerance. Biocomposite TPUs containing white spores displayed a 45 % increase in toughness compared to TPUs without spores, while retaining ∼ 90 % spore viability post processing. Furthermore, biocomposite TPU fabrication was demonstrated using a scalable continuous extruder followed by injection molding. Biocomposite TPUs generated by these industry-relevant processes exhibited comparable toughness improvement and spore viability to biocomposite TPU prepared using a lab scale microcompounder, while enhancing productivity by 30-fold. Finally, spore addition significantly improved the recyclability of biocomposite TPUs, enabling 80 % toughness retention after 5 rounds of iterative melt processing. Additionally, no negative effect on the lifespan of the generated TPUs was observed over 1 year of storage. Overall, this study confirms that spore-bearing biocomposite TPUs are promising for practical applications, offering an accessible method to enhance toughness and sustainability of commercial TPUs through the incorporation of spore-based living fillers.

36 MATERIALS SCIENCE↗
NASA NTRS2018

Effect of Control Mode and Test Rate on the Measured Fracture Toughness of Advanced Ceramics

The effects of control mode and test rate on the measured fracture toughness of ceramics were evaluated by using chevron-notched flexure specimens in accordance with ASTM C1421. The use of stroke control gave consistent results with about 2% (statistically insignificant) variation in measured fracture toughness for a very wide range of rates (0.005 to 0.5 mm/min). Use of strain or crack mouth opening displacement (CMOD) control gave approx. 5% (statistically significant) variation over a very wide range of rates (1 to 80 μm/m/s), with the measurements being a function of rate. However, the rate effect was eliminated by use of dry nitrogen, implying a stress corrosion effect rather than a stability effect. With the use of a nitrogen environment during strain controlled tests, fracture toughness values were within about 1% over a wide range of rates (1 to 80 micons/m/s). CMOD or strain control did allow stable crack extension well past maximum force, and thus is preferred for energy calculations. The effort is being used to confirm recommendations in ASTM Test Method C1421 on fracture toughness measurement.

silicon nitride↗
NASA NTRS2018

Effect of Control Mode and Load Rate on Fracture Toughness of Brittle Ceramics

The effects of control mode and rate on the fracture toughness of ceramics were measured by using chevron-notched flexure specimen in accordance with ASTM C1421. The use of stroke control gave consistent results with about 2% variation in measured fracture toughness for a very wide range of rates (0.005 to 0.5 mm/min). Use of strain or CMOD control gave an approximately 5% variation over a very wide range of rates, with the measurements being a function of rate. However, the effect was eliminated by use of dry nitrogen, implying a stress corrosion effect rather than a stability effect. With the use of nitrogen for strain control, fracture toughness values were within about 1% over a wide range of rates (1 to 80 micro-epsilon/sec.). CMOD or strain control did allow stable crack extension well past maximum load, and thus is preferred for energy calculations. The effort is being used to confirm recommendations in ASTM International standard C1421 on fracture toughness measurement.

control mode↗