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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Stress analysis of advanced attack helicopter composite main rotor blade root end lug

Stress analysis of the Advanced Attack Helicopter (AAH) composite main rotor blade root end lug is described. The stress concentration factor determined from a finite element analysis is compared to an empirical value used in the lug design. The analysis and test data indicate that the stress concentration is primarily a function of configuration and independent of the range of material properties typical of Kevlar-49/epoxy and glass epoxy.

Baker, D. J.↗
NASA NTRS2012

Investigation of Effects of Material Architecture on the Elastic Response of a Woven Ceramic Matrix Composite

To develop methods for quantifying the effects of the microstructural variations of woven ceramic matrix composites on the effective properties and response of the material, a research program has been undertaken which is described in this paper. In order to characterize and quantify the variations in the microstructure of a five harness satin weave, CVI SiC/SiC, composite material, specimens were serially sectioned and polished to capture images that detailed the fiber tows, matrix, and porosity. Open source quantitative image analysis tools were then used to isolate the constituents and collect relevant statistics such as within ply tow spacing. This information was then used to build two dimensional finite element models that approximated the observed section geometry. With the aid of geometrical models generated by the microstructural characterization process, finite element models were generated and analyses were performed to quantify the effects of the microstructure and its variation on the effective stiffness and areas of stress concentration of the material. The results indicated that the geometry and distribution of the porosity appear to have significant effects on the through-thickness modulus. Similarly, stress concentrations on the outer surface of the composite appear to correlate to regions where the transverse tows are separated by a critical amount.

Goldberg, Robert K.↗
NASA NTRS2001

Mechanical Behavior of Notched SiC/SiC Composites

Gas turbine components such as combustor liners or turbine vanes are subject to regions of high stress-concentration, e.g., attachment to the frame or at cooling holes. Ceramic matrix composites (CMCs) are potential materials for high temperature applications in gas turbines. They offer some capability to relieve stress at regions of high stress-concentration via matrix damage accumulation. In this study notch sensitivity was examined for woven SiC fiber reinforced, melt-infiltrated SiC matrix composites with a BN interphase, utilizing either Hi-Nicalo(TM) fibers or the stiffer Sylramic fibers. The double-edge notched tensile test approach was used for a wide range of notch sizes and specimen widths. Both composite systems exhibited mild notch sensitivity similar to other CMC systems. Acoustic emission, detected during the tensile tests, indicated that matrix cracking occurred around notches at net-section stresses below the stress where matrix cracking first occurs in unnotched specimens. However, thermoelastic stress analysis did not show any measurable stress relief around notches after the specimens were preloaded.

Morscher, Gregory N.↗
NASA NTRS1990

Heat Shield And Axial Retainer For Turbopump Blade

Modified configuration for axial retention of blades of cryogenic turbopump expected to reduce dynamic and thermal stresses, thereby increasing operating life. Expected to overcome some of structural limitations in which various components of "fir-tree" blade/disk attachments shaped in such way as to concentrate stresses. Advantages include: No overhung loading on attachment faces of disk or blades; No additional stress concentrators added to attachment faces of disk or blades; and No large axial thermal gradients on edges of disk and blades.

Moore, Jerry H.↗
NASA NTRS1959

Fatigue Strengths of Aircraft Materials: Axial-Load Fatigue Tests on Edge-Notched Sheet Specimens of 2024-T3 and 7075-T6 Aluminum Alloys and of SAE 4130 Steel with Notch Radii of 0.004 and 0.070 inch

The present report gives results of axial-load fatigue tests on notched specimens of three sheet materials: 2024-T3 and 7075-T6 aluminum alloys and normalized SAE 4130 steel. Two edge-notched specimens were designed and tested, each having a theoretical stress-concentration factor K(sub t) = 4.0. The radii of the notches were 0.004 and 0.070 inch. Tests of these specimens were run at two levels of nominal mean stress: 0 and 20,000 psi. Results of these studies extended information previously reported on tests of specimens with varying notch severity. They afford data on the variation of fatigue-strength reduction with notch radius and on the potential usefulness of Neuber's technical stress-concentration factor K(sub n).

Grover, H. J.↗
NASA NTRS1993

Probabilistic assessment of composite structures

A methodology and attendant computer code were developed and are used to computationally simulate the uncertain behavior of composite structures. The uncertain behavior includes buckling loads, stress concentration factors, displacements, stress/strain, etc., which are the consequences of the inherent uncertainties (scatter) in the primitive (independent random) variables (constituent, ply, laminate, and structural) that describe the composite structures. The computer code is IPACS (Integrated Probabilistic Assessment of Composite Structures). IPACS can simulate both composite mechanics and composite structural behavior. Application to probabilistic composite mechanics is illustrated by its use to evaluate the uncertainties in the major Poisson's ratio and in laminate stiffness and strength. IPACS' application to probabilistic structural analysis is illustrated by its used to evaluate the uncertainties in the buckling of a composite plate, the stress concentration factor in a composite panel, and the vertical displacement and ply stress in a composite aircraft wing segment. IPACS' application to probabilistic design is illustrated by its use to assess the thin composite shell (pipe).

Chamis, C. C.↗
NASA NTRS1993

IPACS (Integrated Probabilistic Assessment of Composite Structures): Code development and applications

A methodology and attendant computer code have been developed and are described to computationally simulate the uncertain behavior of composite structures. The uncertain behavior includes buckling loads, stress concentration factors, displacements, stress/strain etc., which are the consequences of the inherent uncertainties (scatter) in the primitive (independent random) variables (constituent, ply, laminate and structural) that describe the composite structures. The computer code, IPACS (Integrated Probabilistic Assessment of Composite Structures), can handle both composite mechanics and composite structures. Application to probabilistic composite mechanics is illustrated by its uses to evaluate the uncertainties in the major Poisson's ratio and in laminate stiffness and strength. IPACS application to probabilistic structural analysis is illustrated by its use to evaluate the uncertainties in the buckling of a composite plate, in the stress concentration factor in a composite panel and in the vertical displacement and ply stress in a composite aircraft wing segment.

Chamis, C. C.↗
NASA NTRS2018

A Dual Spring Modeling Approach for Static and Fatigue Failure Assessments of Carbon/Epoxy Composite Sub-Elements

A dual spring model is developed for the static and fatigue damage predictions of laminates interface in composite structures. Stress concentrations can be induced by the defects formed in the fabrication or service process. A conventional S-N based fatigue damage model may not be accurate to predict the fatigue life of a structure with high stress concentration. With the dual spring model, static delamination failure can be simulated using springs of cohesive type material model while fatigue delamination development can be predicted using linear springs, where the crack driving force is computed based on virtual crack closure technique (VCCT). A Paris law type fatigue growth law with its mode mixity is applied for fatigue crack growth prediction. After verified using benchmark examples, including Double Cantilever Beam (DCB), End-Notched Flexure (ENF) and Mix-Mode Bending (MMB), the proposed dual spring model is applied in the static and fatigue damage prediction of NASA/Boeing sub-elements and UTC sub-elements.

Cui, Xiaodong↗
NASA NTRS1978

Finite element microscopic stress analysis of cracked composite systems

This paper considers the stress concentration problems of two types of cracked composite systems: (1) a composite system with a broken fiber (a penny-shaped crack problem), and (2) a composite system with a cracked matrix (an annular crack problem). The cracked composite systems are modeled with triangular and trapezoidal ring finite elements. Using NASTRAN (NASA Structural Analysis) finite element computer program, the stress and deformation fields in the cracked composite systems are calculated. The effect of fiber-matrix material combination on the stress concentrations and on the crack opening displacements is studied.

Ko, W. L.↗
NASA NTRS1986

Fatigue of 1018 steel in hydrogen

Tests are performed at different hydrogen pressures and mechanical load parameter values (applied load, stress concentration factor, applied load frequency) to study enhanced fatigue crack growth in SAE 1018 steel. Oxygen was introduced as a hydrogen-oxygen gas mixture environment in order to compare the role of oxygen in crack inhibition with that previously reported for both static and cyclic loading of higher strength steels. In a number of experiments, a thin film of palladium was applied to the specimen surface to facilitate the study of steel-hydrogen interaction in the absence of any surface impedance to hydrogen entry. The kinetics of the diffusing atoms are found to be influenced by the stress field around the notch tip and the applied frequency. An increment in environmental effect occurred when applied load or stress concentration factor was increased.

Lo, S. H.↗
NASA NTRS2004

Eccentric loading of microtensile specimens

Ceramic materials have a lower density than most metals and are capable of performing at extremely high temperatures. The utility of these materials is obvious; however, the fracture strength of brittle materials is not easily predicted and often varies greatly. Characteristically, brittle materials lack ductility and do not yield as other materials. Ceramics materials are naturally populated with microscopic cracks due to fabrication techniques. Upon application of a load, stress concentration occurs at the root of these cracks and fracture will eventually occur at some not easily predicted strength. In order to use ceramics in any application some design methodology must exist from which a component can be placed into service. This design methodology is CARES/LIFE (Ceramics Analysis and Reliability Evaluation of Structures) which has been developed and refined at NASA over the last several decades. The CARES/LIFE computer program predicts the probability of failure of a ceramic component over its service life. CARES combines finite element results from a commercial FE (finite element) package such as ANSYS and experimental results to compute the abovementioned probability of failure. Over the course of several tests CARES has had great success in predicting the life of various ceramic components and has been used throughout industry. The latest challenge is to verify that CARES is valid for MEMS (Micro-Electro Mechanical Systems). To investigate a series of microtensile specimens were fractured in the laboratory. From this data, material parameters were determined and used to predict a distribution of strength for other specimens that exhibit a known stress concentration. If the prediction matches the experimental results then these parameters can be applied to a desired component outside of the laboratory. During testing nearly half of the tensile Specimens fractured at a location that was not expected and hence not captured in the FE model. It has been my duty to investigate the nature of this phenomenon in hopes of finding a better correlation between theory and empirical results. To investigate I built complete FE models of all of the tensile specimens using ANSYS. It is suspected that some misalignment naturally occurs during testing and thus additional bending stresses are present in the specimens. I modeled this eccentric loading and ran several FE trials using ANSYS/PDS (a probabilistic design system in ANSYS). My objective this summer has been familiarize myself with the CARES/LIFE program in hopes of using it in conjunction with ANSYS to help verify that CARES is applicable to MEMS-scale (greater that 1 micron, less than 1 millimeter) components.

Trapp, Mark A.↗
NASA NTRS2007

Fracture Strength of Single-Crystal Silicon Carbide Microspecimens at Room and Elevated Temperature

Three shapes of tensile specimens were tested--curved with a very low stress concentration factor and straight with either a circular hole or an elliptical hole. The nominal thickness was 125 micron with a net section 100 micron wide; the overall length of these microspecimens was 3.1 mm. They were fabricated by an improved version of deep reactive ion etching, which produced specimens with smooth sidewalls and cross-sections having a slightly trapezoidal shape that was exaggerated inside the holes. The novel test setup used a vertical load train extending into a resistance furnace. The specimens had wedge-shaped ends which fit into ceramic grips. The fixed grip was mounted on a ceramic post, and the movable grip was connected to a load cell and actuator outside the furnace with a ceramic-encased nichrome wire. The same arrangement was used for tests at 24 and at 1000 C. The strengths of the curved specimens for two batches of material (made with slightly different processes) were 0.66+/-0.12 GPa and 0.45+/-0.20 GPa respectively at 24 C with identical values at 1000 C. The fracture strengths of the circular-hole and elliptical-hole specimens (computed from the stress concentration factors and measured loads at failure) were approximately 1.2 GPa with slight decreases at the higher temperature. Fractographic examinations showed failures initiating on the surface--primarily at corners. Weibull predictions of fracture strengths for the hole specimens based on the properties of the curved specimens were reasonably effective for the circular holes, but not for the elliptical holes.

Nemeth, Noel N.↗
NASA NTRS1961

Skin Stresses in an Inflated Sphere During Impact

An analysis is made of the stresses in the skin of an inflated nonstretchable sphere during normal, nonrotating impact with a hard flat surface, assuming infinite modulus of elasticity in the skin and infinite propagation speed of stress waves. The analysis is further applied to the study of the inflated sphere landing vehicle containing a payload suspended at the center. Curves are presented showing the stress distributions during impact for cases corresponding to those calculated in previous reports in which the impact motion and payload landing performance capabilities of the landing vehicle have been studied. It is found, assuming the force from the payload-suspension cords to be distributed continuously on the skin, that is, neglecting stress concentrations,that the skin stresses along a meridian are reduced by the presence of the suspended payload during impact, but that the maximum values of skin stress normal to a meridian are little affected.

Martin, E. Dale↗
NASA NTRS1987

Nodule activity and allocation of photosynthate of soybean during recovery from water stress

Nodulated soybean plants (Glycine max [L.] Merr. cv Ransom) in a growth-chamber study were subjected to a leaf water potential (psi w) of -2.0 megapascal during vegetative growth. Changes in nonstructural carbohydrate contents of leaves, stems, roots, and nodules, allocation of dry matter among plant parts, in situ specific nodule activity, and in situ canopy apparent photosynthetic rate were measured in stressed and nonstressed plants during a 7-day period following rewatering. Leaf and nodule psi w also were determined. At the time of maximum stress, concentration of nonstructural carbohydrates had declined in leaves of stressed, relative to nonstressed, plants, and the concentration of nonstructural carbohydrates had increased in stems, roots, and nodules. Sucrose concentrations in roots and nodules of stressed plants were 1.5 and 3 times greater, respectively, than those of nonstressed plants. Within 12 hours after rewatering, leaf and nodule psi w of stressed plants had returned to values of nonstressed plants. Canopy apparent photosynthesis and specific nodule activity of stressed plants recovered to levels for nonstressed plants within 2 days after rewatering. The elevated sucrose concentrations in roots and nodules of stressed plants also declined rapidly upon rehydration. The increase in sucrose concentration in nodules, as well as the increase of carbohydrates in roots and stems, during water stress and the rapid disappearance upon rewatering indicates that inhibition of carbohydrate utilization within the nodule may be associated with loss of nodule activity. Availability of carbohydrates within the nodules and from photosynthetic activity following rehydration of nodules may mediate the rate of recovery of N2-fixation activity.

Non-NASA Center↗
NASA NTRS2008

Stress Analysis and Fracture in Nanolaminate Composites

A stress analysis is performed on a nanolaminate subjected to bending. A composite mechanics computer code that is based on constituent properties and nanoelement formulation is used to evaluate the nanolaminate stresses. The results indicate that the computer code is sufficient for the analysis. The results also show that when a stress concentration is present, the nanolaminate stresses exceed their corresponding matrix-dominated strengths and the nanofiber fracture strength.

Chamis, Christos C.↗