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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Development of fracture mechanics data for two hydrazine APU turbine wheel materials

The effects of high temperature, high pressure ammonia were measured on the fracture mechanics and fatigue properties of Astroloy and Rene' 41 turbine wheel materials. Also, the influence of protective coatings on these properties was investigated. Specimens of forged bar stock were subjected to LCF and HCF tests at 950 K (1250 F) and 3.4 MN/sq m (500 psig) pressure, in ammonia containing about 1.5 percent H2O. Aluminized samples (Chromizing Company's Al-870) and gold plated test bars were compared with uncoated specimens. Comparison tests were also run in air at 950 K (1250 F), but at ambient pressures. K sub IE and K sub TH were determined on surface flawed specimens in both the air and ammonia in both uncoated and gold plated conditions. Gold plated specimens exhibited better properties than uncoated samples, and aluminized test bars generally had lower properties. The fatigue properties of specimens tested in ammonia were higher than those tested in air, yet the K sub TH values of ammonia tested samples were lower than those tested in air. However, insufficient specimens were tested to develop significant design data.

Curbishley, G.↗
NASA NTRS2021

5 Inch Thick 2219-T87 Plate Low Ductility Investigation - Material Analysis on Broken 2219 Tensile Samples

2219-T87 material used on a LOX tank baffle support on Artemis-I showed unusually low fracture elongation. Recent material data indicated 5” thick 2219-T87 plate stock had very low ductility (<1%) in the short transverse direction at the half-thickness location. Metallurgical analysis performed to understand the root cause that led to the drastic reduction in tensile ductility. The PowerPoint report details the key findings from material analysis by optical and scanning electron microscopy.

aluminum alloy 2219↗
NASA NTRS1978

Morphometric comparison of braided Martian channels and some braided terrestrial features

Large channels on the Martian surface have been variously attributed to erosional, volcanic, and tectonic processes. Morphometric information shows that large braided Martian channels and islands between those channels are similar in their dimensions to channels and islands of large braided fluvial features on earth. The information also suggests that braided fractures in solid materials are fundamentally different in morphometry from braided channels of earth and Mars. Braided tension fractures have characteristically low braiding indices and are much narrower than their irregularly shaped 'midchannel' islands. Terrestrial and Martian channels, in contrast, have high braiding indices, and they are wider than their streamlined midchannel islands. Braided volcanic features are known from the earth and the moon, but the absence of volcanic constructs near the large braided channels on Mars indicates that volcanic origin is unlikely. The morphometric information suggests that braided Martian channels are probably of fluvial origin.

Trevena, A. S.↗
NASA NTRS1990

Tensile and compressive failure modes of laminated composites loaded by fatigue with different mean stress

Laminated composite materials tend to fail differently under tensile or compressive load. Under tension, the material accumulates cracks and fiber fractures, while under compression, the material delaminates and buckles. Tensile-compressive fatigue may cause either of these failure modes depending on the specific damage occurring in the laminate. This damage depends on the stress ratio of the fatigue loading. Analysis of the fatigue behavior of the composite laminate under tension-tension, compression-compression, and tension-compression had led to the development of a fatigue envelope presentation of the failure behavior. This envelope indicates the specific failure mode for any stress ratio and number of loading cycles. The construction of the fatigue envelope is based on the applied stress-cycles to failure (S-N) curves of both tensile-tensile and compressive-compressive fatigue. Test results are presented to verify the theoretical analysis.

Rotem, Assa↗
NASA NTRS2019

Improved Processing Techniques for Inclusion-Free Steel for Bearing and Mechanical Component Applications

High hardness, high carbide powder metallurgy tools steels such as M62 enable the operation of ball bearings at extremely high load and stress levels. Operation under such conditions increases the potential for rolling contact fatigue failure attributed to ceramic particle inclusions. To address this challenge, industry has sought steel made from ever increasing levels of cleanliness but the results have been uneven owing to the random nature of the occurrence of material flaws. One common approach is to rely upon careful ingot inspections prior to bearing manufacture. By selecting the cleanest portion of an ingot, it is expected that bearings relatively free from material flaws will result. This approach is not always successful because detrimental flaws that exist deep within an ingot can pass inspection undetected potentially causing subsequent failure. Recent efforts to commercialize an intermetallic material, 60NiTi, for rolling element bearings demonstrates a pathway to produce bearing steel that is free from unwanted ceramic particle inclusions. In this paper, the process used to make bearing grade ceramic-free NiTi alloys is described and applied to steelmaking. At its core, the NiTi process differs from steel making in one key aspect. NiTi alloys are made from elementally pure starting materials that are melted, blended and processed in equipment absolutely free from exposure to oxygen and ceramics ensuring a ceramic particle-free product. In contrast, the predominant method to make bearing steel is to employ a successive series of purification steps to reduce contamination levels below required thresholds. This paper describes the processes developed and applied to high carbide tool steel, M62. The resulting material and microstructures are evaluated and compared to M62 prepared by conventional powder metallurgy techniques. It is hoped that the application of materials manufacturing techniques used for fracture sensitive ceramics and intermetallic materials like NiTi can provide a pathway to Ultra-Clean, Ceramic-Inclusion free steels for rolling element bearings and other failure critical applications.

Steel↗
NASA NTRS2001

Fracture Test Results for 0.5, 0.7 and 0.9 Inch Thick 2324-T39 Aluminum Alloy Material

A summary of experimental data is presented for a series of fracture tests and tensile tests that were conducted on a 2324-T39 aluminum alloy material. Static tensile tests were performed to determine the stress-strain response for the material. For the fracture tests three thicknesses (B = 0.5, 0.7, and 0.9-inches) and two specimen types (middle crack tension and compact tension specimens) were used. The middle crack tension specimens were 12-inches wide, and the compact tension specimens were 2, 4, and 6-inches wide. The fracture tests were conducted in the L-T orientation (crack perpendicular to the rolling direction and load parallel to the rolling direction).

Johnston, William M.↗
NASA NTRS1976

Ductile fracture theories for pressurised pipes and containers

Two mechanisms of fracture are distinguished. Plane strain fractures occur in materials which do not undergo large-scale plastic deformations prior to and during a possible fracture deformation. Plane stress or high energy fractures are generally accompanied by large inelastic deformations. Theories for analyzing plane stress are based on the concepts of critical crack opening stretch, K(R) characterization, J-integral, and plastic instability. This last is considered in some detail. The ductile fracture process involves fracture initiation followed by a stable crack growth and the onset of unstable fracture propagation. The ductile fracture propagation process may be characterized by either a multiparameter (discrete) model, or some type of a resistance curve which may be considered as a continuous model expressed graphically. These models are studied and an alternative model is also proposed for ductile fractures which cannot be modeled as progressive crack growth phenomena.

Erdogan, F.↗
NASA NTRS1992

Tension fracture of laminates for transport fuselage. Part 1: Material screening

Transport fuselage structures are designed to contain pressure following a large penetrating damage event. Applications of composites to fuselage structures require a database and supporting analysis on tension damage tolerance. Tests with 430 fracture specimens were used to accomplish the following: (1) identify critical material and laminate variables affecting notch sensitivity; (2) evaluate composite failure criteria; and (3) recommend a screening test method. Variables studied included fiber type, matrix toughness, lamination manufacturing process, and intraply hybridization. The laminates found to have the lowest notch sensitivity were manufactured using automated tow placement. This suggests a possible relationship between the stress distribution and repeatable levels of material inhomogeneity that are larger than found in traditional tape laminates. Laminates with the highest notch sensitivity consisted of toughened matrix materials that were resistant to a splitting phenomena that reduces stress concentrations in major load bearing plies. Parameters for conventional fracture criteria were found to increase with crack length for the smallest notch sizes studied. Most material and laminate combinations followed less than a square root singularity for the largest crack sizes studied. Specimen geometry, notch type, and notch size were evaluated in developing a screening test procedure. Traitional methods of correcting for specimen finite width were found to be lacking. Results indicate that a range of notch sizes must be tested to determine notch sensitivity. Data for a single small notch size (0.25 in. diameter) was found to give no indication of the sensitivity of a particular material and laminate layup to larger notch sizes.

Walker, T. H.↗
NASA NTRS1992

Tension fracture of laminates for transport fuselage. Part 1: Material screening

Transport fuselage structures are designed to contain pressure following a large penetrating damage event. Application of composites to fuselage structures requires a data base and supporting analysis on tension damage tolerance. Tests with 430 fracture specimens were used to accomplish the following: (1) identify critical material and laminate variables affecting notch sensitivity, (2) evaluate composite failure criteria, and (3) recommend a screening test method. Variables studied included fiber type, matrix toughness, lamination manufacturing process, and intraply hybridization. The laminates found to have the lowest notch sensitivity were manufactured using automated tow placement. This suggests a possible relationship between the stress distribution and repeatable levels of material inhomogeneity that are larger than found in traditional tape laminates. Laminates with the highest notch sensitivity consisted of toughened matrix materials that were resistant to a splitting phenomena that reduces stress concentrations in load bearing plies. Parameters for conventional fracture criteria were found to increase with the crack length of the smallest notch sizes studied. Most materials and laminate combinations followed less than a square root singularity for the largest crack sizes studied. Specimen geometry, notch type, and notch size were evaluated in developing a screening test procedure. Results indicate that a range of notch sizes must be tested to determine notch sensitivity.

Walker, T. H.↗
NASA NTRS1978

Mechanics of failure of composite materials

Composite materials are both inhomogeneous and anisotropic. Both of these characteristics affect the internal stress distributions since inhomogeneity involves variations in both strength and stiffness. The fracture mechanics of nonuniform materials are considered, taking into account the effect of nonuniformity on stress distributions near the crack tip, predicted yield zones in nonuniform and uniform materials, and the fracture of a center-notched unidirectional specimen. The mechanics of failure of laminated materials is discussed. It is found that the development of damage in a laminate with increasing load and, possibly, increasing numbers of cycles of loading is peculiar to the laminate in question, i.e., the material system, the stacking sequence, and the geometry. Approaches for monitoring damage development are also described.

Reifsnider, K. L.↗
NASA NTRS1993

Time dependent fracture and cohesive zones

This presentation is concerned with the fracture response of materials which develop cohesive or bridging zones at crack tips. Of special interest are concerns regarding crack stability as a function of the law which governs the interrelation between the displacement(s) or strain across these zones and the corresponding holding tractions. It is found that for some materials unstable crack growth can occur, even before the crack tip has experienced a critical COD or strain across the crack, while for others a critical COD will guarantee the onset of fracture. Also shown are results for a rate dependent nonlinear material model for the region inside of a craze for exploring time dependent crack propagation of rate sensitive materials.

Knauss, W. G.↗
NASA NTRS1985

Delamination micromechanics analysis

A three-dimensional finite element analysis was developed which includes elastoplastic, orthotropic material response, and fracture initiation and propagation. Energy absorption due to physical failure processes characteristic of the heterogeneous and anisotropic nature of composite materials is modeled. A local energy release rate in the presence of plasticity was defined and used as a criterion to predict the onset and growth of cracks in both micromechanics and macromechanics analyses. This crack growth simulation technique is based upon a virtual crack extension method. A three-dimensional finite element micromechanics model is used to study the effects of broken fibers, cracked matrix and fiber-matrix debond on the fracture toughness of the unidirectional composite. The energy release rates at the onset of unstable crack growth in the micromechanics analyses are used as critical energy release rates in the macromechanics analysis. This integrated micromechanical and macromechanical fracture criterion is shown to be very effective in predicting the onset and growth of cracks in general multilayered composite laminates by applying the criterion to a single-edge notched graphite/epoxy laminate subjected to implane tension normal to the notch.

Adams, D. F.↗
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 NTRS1969

Abrasion and fracture testing in a high-pressure hydrogen environment

Two devices are necessary for abrasion and fracture testing of materials evaluated for storage of hydrogen at high pressure for long periods. The first device abrades tensile specimens. The second device tests for fracture toughness of metals. Both devices permit testing in both yield and failure modes in high pressure hydrogen.

Sneesby, G. V.↗
NASA NTRS2005

X-Aerogels for Structural Components and High Temperature Applications

Future NASA missions and space explorations rely on the use of materials that are strong ultra lightweight and able to withstand extreme temperatures. Aerogels are low density (0.01-0.5 g/cu cm) high porosity materials that contain a glass like structure formed through standard sol-gel chemistry. As a result of these structural properties, aerogels are excellent thermal insulators and are able to withstand temperatures in excess of l,000 C. The open structure of aerogels, however, renders these materials extremely fragile (fracturing at stress forces less than 0.5 N/sq cm). The goal of NASA Glenn Research Center is to increase the strength of these materials by templating polymers and metals onto the surface of an aerogel network facilitating the use of this material for practical applications such as structural components of space vehicles used in exploration. The work this past year focused on two areas; (1) the research and development of new templated aerogels materials and (2) process development for future manufacturing of structural components. Research and development occurred on the production and characterization of new templating materials onto the standard silica aerogel. Materials examined included polymers such as polyimides, fluorinated isocyanates and epoxies, and, metals such as silver, gold and platinum. The final properties indicated that the density of the material formed using an isocyanate is around 0.50 g/cc with a strength greater than that of steel and has low thermal conductivity. The process used to construct these materials is extremely time consuming and labor intensive. One aspect of the project involved investigating the feasibility of shortening the process time by preparing the aerogels in the templating solvent. Traditionally the polymerization used THF as the solvent and after several washes to remove any residual monomers and water, the solvent around the aerogels was changed to acetonitrile for the templating step. This process took a couple of days. It was experimentally determined that the polymerization reaction could be done in acetonitrile instead of THF with no detrimental effects to the properties of the resulting aerogel. Other changes in the time needed to crosslink the gels in the isocyanate solution as well as changes to the subsequent washing procedure could also shorten the processing time with no effect on the properties. Processing methods were also developed that allowed a variety of shapes as well as sizes of these materials to be formed.

Source record↗
NASA NTRS2010

Resistance Curves in the Tensile and Compressive Longitudinal Failure of Composites

This paper presents a new methodology to measure the crack resistance curves associated with fiber-dominated failure modes in polymer-matrix composites. These crack resistance curves not only characterize the fracture toughness of the material, but are also the basis for the identification of the parameters of the softening laws used in the analytical and numerical simulation of fracture in composite materials. The method proposed is based on the identification of the crack tip location by the use of Digital Image Correlation and the calculation of the J-integral directly from the test data using a simple expression derived for cross-ply composite laminates. It is shown that the results obtained using the proposed methodology yield crack resistance curves similar to those obtained using FEM-based methods in compact tension carbon-epoxy specimens. However, it is also shown that the Digital Image Correlation based technique can be used to extract crack resistance curves in compact compression tests for which FEM-based techniques are inadequate.

Camanho, Pedro P.↗