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Results for “FRACTURE TOUGHNESS”

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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Effects of stitching on fracture toughness of uniweave textile graphite/epoxy laminates

The effects of through-the-thickness stitching on impact damage resistance, impact damage tolerance, and Mode 1 and Mode 2 fracture toughness of textile graphite/epoxy laminates were studied experimentally. Graphite/epoxy laminates were fabricated from AS4 graphite uniweave textiles and 3501-6 epoxy using Resin Transfer Molding. The cloths were stitched with Kevlar(tm) and glass yarns before resin infusion. Delamination was implanted during processing to simulate impact damage. Sublaminate buckling tests were performed in a novel fixture to measure Compression After Impact (CAI) strength of stitched laminates. The results show that CAI strength can be improved up to 400% by through-the-thickness stitching. Double Cantilever Beam tests were performed to study the effect of stitching on Mode 1 fracture toughness G(sub 1c). It was found that G(sub 1c) increased 30 times for a low stitching density of 16 stitches/sq in. Mode 2 fracture toughness was measured by testing the stitched beams in End Notch Flexure tests. Unlike in the unstitiched beams, crack propagation in the stitched beams was steady. The current formulas for ENF tests were not found suitable for determining G(sub 2C) for stitched beams. Hence two new methods were developed - one based on crack area measured from ultrasonic C-scanning and the other based on equivalent crack area measured from the residual stiffness of the specimen. The G(sub 2c) was found to be at least 5-15 times higher for the stitched laminates. The mechanisms by which stitching increases the CAI strength and fracture toughness are discussed.

Sankar, Bhavani V.↗
NASA NTRS1982

Interrelation of material microstructure, ultrasonic factors, and fracture toughness of two phase titanium alloy

The pivotal role of an alpha-beta phase microstructure in governing fracture toughness in a titanium alloy, Ti-662, is demonstrated. The interrelation of microstructure and fracture toughness is demonstrated using ultrasonic measurement techniques originally developed for nondestructive evaluation and material property characterization. It is shown that the findings determined from ultrasonic measurements agree with conclusions based on metallurgical, metallographic, and fractographic observations concerning the importance of alpha-beta morphology in controlling fracture toughness in two phase titanium alloys.

Vary, A.↗
NASA NTRS1986

Concepts for interrelating ultrasnic attenuation, microstrucutre and fracture toughness in polycrystalline solids

Conceptual models are advanced for explaining and predicting empirical correlations found between ultrasonic measurements and fracture toughness of polycrystalline solids. The models lead to insights concerning microstructural factors governing fracture processes and associated stress wave interactions. Analysis of the empirical correlations suggested by the models indicate that, in addition to grain size and shape, grain boundary reflections, elastic anisotropy, and dislocation damping are factors that underly both fracture toughness and ultrasonic attenuation. One outcome is that ultrasonic attenuation can predict the size of crack blunting or process zones that develop in the vicinity active cracks in metals. This forms a basis for ultrasonic ranking according to variations in fracture toughness.

Vary, A.↗
NASA NTRS1988

Concepts for interrelating ultrasonic attenuation, microstructure, and fracture toughness in polycrystalline solids

Conceptual models are advanced for explaining and predicting empirical correlations found between ultrasonic measurements and fracture toughness of polycrystalline solids. The models lead to insights concerning microstructural factors governing fracture processes and associated stress wave interactions. Analysis of the empirical correlations suggested by the models indicate that, in addition to grain size and shape, grain boundary reflections, elastic anisotropy, and dislocation damping are factors that underlie both fracture toughness and ultrasonic attenuation. One outcome is that ultrasonic attenuation can predict the size of crack blunting or process zones that develop in the vicinity of active cracks in metals. This forms a basis for ultrasonic ranking according to variations in fracture toughness.

Vary, A.↗
NASA NTRS1980

Fracture toughness of boron/aluminum laminates with various proportions of 0 deg and plus or minus 45 deg

The fracture toughness of boron/aluminum laminates was measured on sheet specimens containing central slits of various lengths that represent cracks. The specimens were loaded axially and had various widths. The sheets were made with five laminate orientation. Fracture toughness was calculated for each laminate orientation. Specimens began failing at the ends of the slit with what appeared to be tensile failures of fibers in the primary load carrying laminae. A general fracture toughness parameter independent of laminate orientation was derived on the basis of fiber failure in the principal load carrying laminae. The value of this parameter was proportional to the critical value of the stress intensity factor. The constant of proportionality depended only on the elastic constants of the laminates.

Poe, C. C., Jr.↗
DOE OSTI2025

Assessment of irradiation embrittlement effect on fatigue life of a pressurized-water reactor pressure vessel using the fracture toughness master curve approach

The reactor pressure vessel (RPV) is a critical structural component in pressurized-water reactors, and it is designed to withstand extreme conditions, such as high pressures, elevated temperatures, and prolonged radiation exposure. Ensuring RPV integrity is essential for the safe and reliable long-term operation of nuclear power plants, especially as aging mechanisms such as fatigue and irradiation embrittlement pose increased risks. Fatigue, caused by cyclic thermal and mechanical loading, can lead to crack initiation in localized high-stress regions. Simultaneously, neutron irradiation, particularly in the beltline region, progressively reduces fracture toughness, increasing susceptibility to brittle fracture. These combined effects of fatigue and irradiation embrittlement potentially impact the RPV structural integrity, necessitating fitness-for-service assessments. This study applies the fracture toughness master curve approach to evaluate the impact of irradiation embrittlement on RPV fatigue life. A coupled thermo-mechanical stress analysis identifies critical stress locations under normal service transients, pinpointing regions most vulnerable to fatigue crack initiation and growth. Stress intensity factors for postulated flaws at these locations are calculated, enabling an assessment of fatigue life under irradiated and unirradiated conditions. The results indicate that neutron irradiation embrittlement accelerates the conditions in which a critical crack can form and lead to failure, particularly at lower temperatures. The failure occurs where reduced fracture toughness limits the material’s resistance to crack growth. Axial cracks at nozzle corners are the most life limiting without irradiation, while circumferential cracks demonstrate longer fatigue lives. The findings highlight the importance of incorporating irradiation effects into fatigue life predictions to ensure the long-term structural integrity of RPVs.

22 - GENERAL STUDIES OF NUCLEAR REACTORS↗
NASA NTRS1979

A new basis for the determination of fracture toughness

A study is presented which shows that the growth of the plastic zone and the constraint in a compact tension specimen depends significantly on specimen width. The analysis permits the estimation of the contribution of the growth of plastic zone to the deviation from linearity. The contribution of the crack growth to the deviation from linearity is evaluated from the analysis of a typical R-curve data. A combination of these two analyses enables one to define a very simple procedure for the determination of fracture toughness. The fracture toughness is defined as the stress intensity value at which the crack extension starts. The good agreement between analytical results and experimental KQ and KIC values determined over a wide range of thicknesses, widths, and materials justifies the proposed procedures. The KIC determined according to this procedure is independent of specimen width and such a procedure enhances the range of applicability of the K concept to a wider combination of configurations and materials.

Banerjee, S.↗
NASA NTRS1987

Rate effects on delamination fracture toughness of graphite/epoxy composites

The effects of loading rate on interlaminar fracture toughness were investigated for two graphite/epoxy materials, one with a brittle matrix and the other with a toughened one. Mode I delamination fracture toughness was studied by means of uniform-width, width-tapered, and height-tapered double cantilever beam specimens. The specimens were loaded at various crosshead rates corresponding to crack extension rates up to 26 m/s. It was found that for the brittle-matrix composite the energy release rate increases up to a certain crack velocity and thereafter it decreases. In the case of the toughened-matrix composite the energy release rate decreases monotonically with increasing crack velocity.

Daniel, I. M.↗
NASA NTRS1983

Development of plane strain fracture toughness test for ceramics using Chevron notched specimens

Chevron-notched four-point-bend and short-bar specimens have been used to determine the fracture toughness of sintered aluminum oxide and hot-pressed silicon nitride ceramics. The fracture toughness for Si3N4 is found to be essentially independent of the specimen size and chevron notch configuration, with values ranging from 4.6 to 4.9 MNm exp -3/2. In contrast, significant specimen size and notch geometry effects have been observed for Al2O3, with the fracture toughness ranging from 3.1 to 4.7 MNm exp -3/2. These effects are attributed to a rising crack growth resistance curve for the Al2O3 tested.

Bubsey, R. T.↗
NASA NTRS1999

Fracture Toughness and Reliability in High-Temperature Structural Ceramics and Composites: Prospects and Challenges for the 21st Century

The importance of high fracture toughness and reliability in Si3N4, and SiC-based structural ceramics and ceramic matrix composites is reviewed. The potential of these ceramics and ceramic matrix composites for high temperature applications in defense and aerospace applications such as gas turbine engines, radomes, and other energy conversion hardware have been well recognized. Numerous investigations were pursued to improve fracture toughness and reliability by incorporating various reinforcements such as particulate-, whisker-, and continuous fiber into Si3N4 and SiC matrices. All toughening mechanisms, e.g. crack deflection, crack branching, crack bridging, etc., essentially redistribute stresses at the crack tip and increase the energy needed to propagate a crack through the composite material, thereby resulting in improved fracture toughness and reliability. Because of flaw insensitivity, continuous fiber reinforced ceramic composite (CFCC) was found to have the highest potential for higher operating temperature and longer service conditions. However, the ceramic fibers should display sufficient high temperature strength and creep resistance at service temperatures above 1000 'C. The greatest challenge to date is the development of high quality ceramic fibers with associate coatings able to maintain their high strength in oxidizing environment at high temperature. In the area of processing, critical issues are, preparation of optimum matrix precursors, precursor infiltration into fiber array, and matrix densification at a temperature, where grain crystallization and fiber degradation do not occur. A broad scope of effort is required for improved processing and properties with a better understanding of all candidate composite systems.

Dutta, Sunil↗
DOE OSTI2025

Fracture toughness measurement of ceramics with fuel pellet geometry

The crack initiation of nuclear fuel pellets, driven by extreme temperature gradients, is a critical factor impacting reactor performance. Although this fact is widely acknowledged, there is limited fracture toughness data for specimens with representative geometry and fabricated using relevant processing methods. Here, to address this, a new chevron-notched short-rod specimen configuration with diameter and length both equal to 10 mm, representative of a conventional fuel pellet and tested in tension, was developed in an attempt to validly measure Mode I fracture toughness, K Ic . Test calibration was carried out by testing polycrystalline ZrO 2 , SiC, and Si 3 N 4 ceramics fabricated with conventional fuel pellet geometry and known fracture toughnesses. The results suggest that the method described in this paper produces valid K Ic measurements using conventional ceramic fuel pellets.

Ceramics↗
NASA NTRS1992

Fracture toughness of advanced ceramics at room temperature

Results of round-robin fracture toughness tests on advanced ceramics are reported. A gas-pressure silicon nitride and a zirconia-toughened alumina were tested using three test methods: indentation fracture, indentation strength, and single-edge precracked beam. The latter two methods have produced consistent results. The interpretation of fracture toughness test results for the zirconia alumina composite is shown to be complicated by R-curve and environmentally assisted crack growth phenomena.

Quinn, George D.↗
NASA NTRS1977

Estimating plane strain fracture toughness of high strength aluminum alloys from crack arrest toughness

A comparison is made between fracture toughness KIc as measured by recommended ASTM procedures and crack arrest toughness KIa as measured on more than 100 bolt-loaded double-cantilever beam (DCB) specimens from 7075, 7050, and 7049 alloy plates. Close agreement was found between the two values, KIa being on the average less than KIc over a specified range. This indicates that a simplified test based on a bolt-loaded DCB specimen could be used for quality control, lot release, and screening purposes. Measurements of crack length and specimen deflection are all that are required. The specimens do not have to be fatigue precracked, nor is a tensile machine needed.

Dorward, R. C.↗
NASA NTRS1988

Dynamic fracture toughness of ceramic composites

The dynamic fracture toughness vs crack velocity relationships of TiB2 particulate-reinforced SiC-matrix and SiC whisker-reinforced Al2O3-matrix composites were determined at both room temperature and 1200 C with impacted, single-edge notched three-point bend specimens. Rapid crack initiation and propagation were monitored by a laser interferometric-displacement gage system. A FEM model that transmitted the measured outside-impact load to the specimen within the furnace was used to characterize the entire loading system-specimen unit. Small differences were measured between the room temperature and 1200 C dynamic responses.

Yang, K. H.↗
NASA NTRS2008

Effects of Cryogenic Temperature on Fracture Toughness of Core-Shell Rubber (CSR) Toughened Epoxy Nanocomposites

This study investigated the effects of core-shell rubber (CSR) nanoparticles on the mechanical properties and fracture toughness of an epoxy resin at ambient and liquid nitrogen (LN2) temperatures. Varying amounts of Kane Ace MX130 toughening agent were added to a commercially available EPON 862/Epikure W epoxy resin. Elastic modulus was calculated using quasi-static tensile data. Fracture toughness was evaluated by the resulting breaking energy measured in Charpy impact tests conducted on an instrumented drop tower. The size and distribution of the CSR nanoparticles were characterized using Transmission Electron Microscopy (TEM) and Small Angle X-ray Scattering (SAXS). Scanning Electron Microscopy (SEM) was used to study the fracture surface morphology. The addition of the CSR nanoparticles increased the breaking energy with negligible change in elastic modulus and ultimate tensile stress (UTS). At ambient temperature the breaking energy increased with increasing additions of the CSR nanoparticles, while at LN2 temperatures, it reached a maximum at 5 wt% CSR concentration. KEY WORDS: liquid nitrogen (LN2) properties, fracture toughness, core-shell rubber (CSR).

Wang, J.↗
NASA NTRS1983

Dynamic fracture toughnesses of reaction-bonded silicon nitride

The room-temperature dynamic fracture response of reaction-bonded silicon nitride is investigated using a hybrid experimental-numerical procedure. In this procedure, experimentally determined crack velocities are utilized to drive a dynamic finite-element code or dynamic finite-difference code in its generation mode in order to extract numerically the dynamic stress intensity factor of the fracturing specimen. Results show that the dynamic fracture toughness vs crack velocity relations of the two reaction-bonded silicon nitrides do not follow the general trend in those relations of brittle polymers and steel. A definite slow crack velocity during the initial phase of dynamic crack propagation is observed in reaction-bonded silicon nitride, which results in a nonunique dynamic fracture toughness vs crack velocity relation. In addition, it is found that a propagating crack will continue to propagate under a static stress intensity factor substantially lower than K(IC).

Kobayashi, A. S.↗
NASA NTRS1986

The effects of radiation on the interlaminar fracture toughness of a graphite/epoxy composite

An experimental study is made of the effect of electron irradiation (10 to the 10th rad), simulating a 30-year geosynchronous orbit exposure, on the fracture toughness of a graphite/epoxy composite, T300/934. The double cantilever beam (DBC) test is used to determine Mode I (peel) critical strain energy release rate and the edge delamination tension (EDT) test is used to determine mixed Mode I and II (peel and shear) critical strain energy release rate. It is found that the electron interaction of the epoxy matrix material enhances the fracture toughness properties of the composite and that the test temperature has a significant effect on the fracture toughness of both baseline and irradiated material.

Funk, J. G.↗
NASA NTRS1980

Performance of Chevron-notch short bar specimen in determining the fracture toughness of silicon nitride and aluminum oxide

Ease of preparation and testing are advantages unique to the chevron-notch specimen used for the determination of the plane strain fracture toughness of extremely brittle materials. During testing, a crack develops at the notch tip and extends stably as the load is increased. For a given specimen and notch configuration, maximum load always occurs at the same relative crack length independent of the material. Fracture toughness is determined from the maximum load with no need for crack length measurement. Chevron notch acuity is relatively unimportant since a crack is produced during specimen loading. In this paper, the authors use their previously determined stress intensity factor relationship for the chevron-notch short bar specimen to examine the performance of that specimen in determining the plane strain fracture toughness of silicon nitride and aluminum oxide.

Munz, D.↗