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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Increased fracture toughness of graphite-epoxy composites through intermittent interlaminar bonding

Intermittent interlaminar bonding, which can lead to a large increase in the fracture surface area, was achieved through the introduction of thin perforated Mylar between the layers of a multi-layer continuous-filament graphite-epoxy composite. For the best optimum condition included in this study, fracture toughness was increased from about 100 kJ/sq m for untreated specimens to an average of about 500 kJ/sq m, while tensile strength dropped from 500 MPa to 400 MPa, and elastic modulus remained the same at about 75 GPa. An approximate analysis is presented to explain the observed improvement in toughness.

Felbeck, D. K.↗
NASA NTRS1975

Fracture toughness testing data. A bibliography

This bibliography is comprised of approximately 800 reference citations related to the mechanics of failure in aerospace structures. Most of the references are for documents that include fracture toughness testing data and its application or documents on the availability and usefulness of fracture mechanics analysis methodology. The bibliography represents a search of the literature published in the period April 1962 through April 1974 and is largely limited to documents published in the United States.

Carpenter, J. L., Jr.↗
NASA NTRS1975

Monitoring crack extension in fracture toughness tests by ultrasonics

An ultrasonic method was used to observe the onset of crack extension and to monitor continued crack growth in fracture toughness specimens during three point bend tests. A 20 MHz transducer was used with commercially available equipment to detect average crack extension less than 0.09 mm. The material tested was a 300-grade maraging steel in the annealed condition. A crack extension resistance curve was developed to demonstrate the usefulness of the ultrasonic method for minimizing the number of tests required to generate such curves.

Klima, S. J.↗
NASA NTRS1976

Monitoring crack extension in fracture toughness tests by ultrasonics

An ultrasonic method was used to observe the onset of crack extension and to monitor continued crack growth in fracture toughness specimens during three-point bend tests. A 20-MHz transducer was used with commercially available equipment to detect average crack extension less than 0.09 mm. The material tested was a 300-grade maraging steel in the annealed condition. A crack extension resistance curve was developed to demonstrate the usefulness of the ultrasonic method for minimizing the number of tests required to generate such curves.

Klima, S. J.↗
NASA NTRS1985

Fractured toughness of Si3N4 measured with short bar chevron-notched specimens

The short bar chevron-notched specimen is used to measure the plane strain fracture toughness of hot pressed Si3N4. Specimen proportions and chevron-notch angle are varied, thereby varying the amount of crack extension to maximum load (upon which K sub IC is based). The measured toughness (4.68 + or - 0.19 MNm to the 3/2 power) is independent of these variations, inferring that the material has a flat crack growth resistance curve.

Salem, J. A.↗
NASA NTRS1987

Fracture toughness of Si3N4 measured with short bar chevron-notched specimens

The short bar chevron-notched specimen is used to measure the plane strain fracture toughness of hot pressed Si3N4. Specimen proportions and chevron-notch angle are varied, thereby varying the amount of crack extension to maximum load (upon which K sub IC is based). The measured toughness (4.68 + or 0.19 MNm to the 3/2 power) is independent of these variations, inferring that the material has a flat crack growth resistance curve.

Salem, Jonathan A.↗
NASA NTRS1983

Dynamic fracture toughness of glass

Experimentally determined dynamic crack propagation histories in wedge-loaded, modified tapered, and rectangular double cantilever beam specimens were used to drive a dynamic finite element code in its generation phase. The resultant dynamic fracture toughness versus crack velocity relation, during the initial crack acceleration phase of these dynamic fracture specimens, was erratic but followed the standard Gamma-shaped curves of brittle polymers and metals during subsequent crack propagation at terminal velocity and crack deceleration phases. The distinct initiation phase of dynamic crack propagation, which was not observed in dynamic fracture of brittle polymer and metal specimens, is attributed to the lower stored energy in the glass specimen.

Kobayashi, A. S.↗
NASA NTRS2016

A Modified Edge Crack Torsion Test for Measurement of Mode III Fracture Toughness of Laminated Tape Composites

Modifications to the edge crack torsion (ECT) test are studied to improve the reliability of this test for measuring the mode-III fracture toughness, G (sub IIIc), of laminated tape fiber-reinforced polymeric (FRP) composites. First, the data reduction methods currently used in the ECT test are evaluated and deficiencies in their accuracy are discussed. An alternative data reduction technique, which uses a polynomial form to represent ECT specimen compliance solution, is evaluated and compared to FEA (finite element analysis) results. Second, seven batches of ECT specimens are tested, each batch containing specimens with a preimplanted midplane edge delamination and midplane plies with orientations of plus theta divided by minus theta, with theta ranging from 0 degrees to 90 degrees in 15-degree increments. Tests on these specimens show that intralaminar cracking occurs in specimens from all batches except for which theta = 15 degrees and 30 degrees. Tests on specimens of these two batches are shown to result in mode-III delamination growth at the intended ply interface. The findings from this study are encouraging steps towards the use of the ECT test as a standardized method for measuring G (sub IIIc), although further modification to the data reduction method is required to make it suitable for use as part of a standardized test method.

Czabaj, Michael W.↗
NASA NTRS2017

A Review of the Proposed KIsi Offset-Secant Method for Size-Insensitive Linear-Elastic Fracture Toughness Evaluation

Recently proposed modifications to ASTM E399 would provide a new size-insensitive approach to analyzing the force-displacement test record. The proposed size-insensitive linear-elastic fracture toughness, KIsi, targets a consistent 0.5mm crack extension for all specimen sizes by using an offset secant that is a function of the specimen ligament length. The KIsi evaluation also removes the Pmax/PQ criterion and increases the allowable specimen deformation. These latter two changes allow more plasticity at the crack tip, prompting the review undertaken in this work to ensure the validity of this new interpretation of the force-displacement curve. This paper provides a brief review of the proposed KIsi methodology and summarizes a finite element study into the effects of increased crack tip plasticity on the method given the allowance for additional specimen deformation. The study has two primary points of investigation: the effect of crack tip plasticity on compliance change in the force-displacement record and the continued validity of linear-elastic fracture mechanics to describe the crack front conditions. The analytical study illustrates that linear-elastic fracture mechanics assumptions remain valid at the increased deformation limit; however, the influence of plasticity on the compliance change in the test record is problematic. A proposed revision to the validity criteria for the KIsi test method is briefly discussed.

James, Mark↗
NASA NTRS2017

A Review of the Proposed KIsi Offset-Secant Method for Size-Insensitive Linear-Elastic Fracture Toughness Evaluation

Recently proposed modifications to ASTM E399 would provide a new size-insensitive approach to analyzing the force-displacement test record. The proposed size-insensitive linear-elastic fracture toughness, KIsi, targets a consistent 0.5mm crack extension for all specimen sizes by using an offset secant that is a function of the specimen ligament length. The KIsi evaluation also removes the Pmax/PQ criterion and increases the allowable specimen deformation. These latter two changes allow more plasticity at the crack tip, prompting the review undertaken in this work to ensure the validity of this new interpretation of the force-displacement curve. This paper provides a brief review of the proposed KIsi methodology and summarizes a finite element study into the effects of increased crack tip plasticity on the method given the allowance for additional specimen deformation. The study has two primary points of investigation: the effect of crack tip plasticity on compliance change in the force-displacement record and the continued validity of linear-elastic fracture mechanics to describe the crack front conditions. The analytical study illustrates that linear-elastic fracture mechanics assumptions remain valid at the increased deformation limit; however, the influence of plasticity on the compliance change in the test record is problematic. A proposed revision to the validity criteria for the KIsi test method is briefly discussed.

James, Mark↗
NASA NTRS1974

The circumferentially notched cylindrical bar as a fracture toughness test specimen

The present work reviews the use of the notched cylinder as a screening specimen for use in sorting materials with respect to their relative levels of plane strain fracture toughness. Irwin's work in developing an estimate of stress intensity factors for the circumferentially cracked cylinder is reviewed. Some discrepancies between stress intensity factors determined with fatigue cracked notched cylinders and those determined with plate specimens are discussed. Problems in the technical realization of fatigue cracking a specimen are briefly discussed, and the problem of ensuring low eccentricity of loading when testing notch cylindrical specimens is considered. Finally, specimen size requirements for testing are examined.

Brown, W. F., Jr.↗
NASA NTRS1982

Increasing Metal Fracture Toughness

In technique developed at Langley Research Center several thin sheets of metal are diffusion-brazed together in vacuum furnace to create thick piece of metal that retains much of fracture toughness of its thin components. Technique is expected to make many of high-strength stainless steels, not currently suitable, usable at cryogenic temperatures.

Lawing, P. L.↗
NASA NTRS1987

Interlaminar shear fracture toughness and fatigue thresholds for composite materials

Static and cyclic end notched flexure tests were conducted on a graphite epoxy, a glass epoxy, and graphite thermoplastic to determine their interlaminar shear fracture toughness and fatigue thresholds for delamination in terms of limiting values of the mode II strain energy release rate, G-II, for delamination growth. The influence of precracking and data reduction schemes are discussed. Finite element analysis indicated that the beam theory calculation for G-II with the transverse shear contribution included was reasonably accurate over the entire range of crack lengths. Cyclic loading significantly reduced the critical G-II for delamination. A threshold value of the maximum cyclic G-II below which no delamination occurred after one million cycles was identified for each material. Also, residual static toughness tests were conducted on glass epoxy specimens that had undergone one million cycles without delamination. A linear mixed-mode delamination criteria was used to characterize the static toughness of several composite materials; however, a total G threshold criterion appears to characterize the fatigue delamination durability of composite materials with a wide range of static toughness.

Obrien, T. Kevin↗
NASA NTRS1989

Interlaminar shear fracture toughness and fatigue thresholds for composite materials

Static and cyclic end notched flexure tests were conducted on a graphite epoxy, a glass epoxy, and graphite thermoplastic to determine their interlaminar shear fracture toughness and fatigue thresholds for delamination in terms of limiting values of the mode II strain energy release rate, G-II, for delamination growth. The influence of precracking and data reduction schemes are discussed. Finite element analysis indicated that the beam theory calculation for G-II with the transverse shear contribution included was reasonably accurate over the entire range of crack lengths. Cyclic loading significantly reduced the critical G-II for delamination. A threshold value of the maximum cyclic G-II below which no delamination occurred after one million cycles was identified for each material. Also, residual static toughness tests were conducted on glass epoxy specimens that had undergone one million cycles without delamination. A linear mixed-mode delamination criteria was used to characterize the static toughness of several composite materials; however, a total G threshold criterion appears to characterize the fatigue delamination durability of composite materials with a wide range of static toughness.

O'Brien, T. Kevin↗
NASA NTRS1986

Improved fracture toughness corrosion-resistant bearing material

A development program was performed to establish whether a corrosion-resistant bearing material, such as a 14Cr steel, could be modified to allow carburization, thereby providing the excellent fracture toughness characteristics feasible with this process. The alloy selected for investigation was AMS 5749. Several modifications were made including the addition of a small amount of nickel for austenite stabilization. While some promising results were achieved, the primary objective of an acceptable combination of case hardness and microstructure was not attained. Because the high chromium content presents a serious problem in achieving a viable carburizing cycle, a number of experimental steels having lower chromium contents (8 to 12%) were produced in laboratory quantities and evaluated. The results were basically the same as those initially obtained with the modified AMS 5749. Corrosion tests were performed on AMS 5749, AISI M50, and 52100 bearing steels as well as some of the lower chromium steels. These tests showed that a reduced chromium level (10 to 12%) provided essentially the same corrosion protection as the 14Cr steels.

Bamberger, E. N.↗
NASA NTRS1987

Comparisons of various configurations of edge delamination test for interlaminar fracture toughness

Tests were performed on brittle (T300/5208) and toughened-matrix (T300/BP907) graphite/epoxy laminates using various configurations of edge delamination tension (EDT) test specimens in order to assess the usefulness of each configuraion for measuring interlaminar fracture toughness. All configurations of the EDT test are found to be useful for ranking the delamination resistance of composites with different matrix systems. An analysis of test results suggests that delamination is governed by a linear failure criterion relating strain energy release components GI and GII.

O'Brien, T. Kevin↗