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NASA NTRS1999

Slow Crack Growth Analysis of Brittle Materials with Finite Thickness Subjected to Constant Stress-Rate Flexural Loading

A two-dimensional, numerical analysis of slow crack growth (SCG) was performed for brittle materials with finite thickness subjected to constant stress-rate ("dynamic fatigue") loading in flexure. The numerical solution showed that the conventional, simple, one-dimensional analytical solution can be used with a maximum error of about 5% in determining the SCG parameters of a brittle material with the conditions of a normalized thickness (a ratio of specimen thickness to initial crack size) T > 3.3 and of a SCG parameter n > 10. The change in crack shape from semicircular to elliptical configurations was significant particularly at both low stress rate and low T, attributed to predominant difference in stress intensity factor along the crack front. The numerical solution of SCG parameters was supported within the experimental range by the data obtained from constant stress-rate flexural testing for soda-lime glass microslides at ambient temperature.

Chio, S. R.↗

NASA NTRS2010

Phenylethynyl Terminated Imide (PETI) Composites Made by High Temperature Vartm

The use of composites as primary structures on aerospace vehicles has increased dramatically over the past decade. As these advanced structures increase in size and complexity, their production costs have grown significantly. A major contributor to these manufacturing costs is the requirement of elevated pressures, during high temperature processing, to create fully consolidated composite parts. Recently, NASA Langley has licensed a series of low viscosity Phenyl Ethynyl Terminated Imide, PETI, oligomers that possess a wide processing window to allow for Resin Transfer Molding, RTM, processing. These resins, PETI-8 and PETI-330, demonstrate void fractions of approx.1% under elevated pressure consolidation. However, when used with a standardized thermal curing cycle in a High Temperature Vacuum Assisted RTM (HT-VARTM) process, they display undesirable void contents in excess of 7%. It was determined previously that under the thermal cycles used for laminate fabrication, the phenylethynyl endcap underwent degradation leading to volatile evolution. Modifications to the processing cycle used in the laminate fabrication have reduced the void content significantly (typically less than 3%) for carbon fiber biaxially woven fabric. For carbon fiber uniaxial fabric, void contents of less than 2% have been obtained using both PETI-8 and PETI-330. The resins were infused into carbon fiber preforms at 260 C and cured between 316 C and 371 C. Photomicrographs of the panels were taken and void contents were determined by acid digestion. Mechanical properties of the panels were determined at both room and elevated temperatures. These include short beam shear and flexure tests. The results of this work are presented herein.

Ghose, Sayata↗

NASA NTRS1981

Testing panels in tension and flexure

Simple jig adapts tensile test machine for simulataneous application of tension and flexure, for evaluating panel composition, processing, and design. Environmental test chamber can be added so that panel properties can be measured at extreme temperatures.

Jing, G. K.↗

NASA NTRS1991

Comparison of Weibull strength parameters from flexure and spin tests of brittle materials

Fracture data from five series of four point bend tests of beam and spin tests of flat annular disks were reanalyzed. Silicon nitride and graphite were the test materials. The experimental fracture strengths of the disks were compared with the predicted strengths based on both volume flaw and surface flaw analyses of four point bend data. Volume flaw analysis resulted in a better correlation between disks and beams in three of the five test series than did surface flaw analysis. The Weibull (moduli) and characteristic gage strengths for the disks and beams were also compared. Differences in the experimental Weibull slopes were not statistically significant. It was shown that results from the beam tests can predict the fracture strength of rotating disks.

Holland, Frederic A., Jr.↗

NASA NTRS1978

Measurement of the time-temperature dependent dynamic mechanical properties of boron/aluminum composites

A flexural vibration test and associated equipment were developed to accurately measure the low strain dynamic modulus and damping of composite materials from -200 C to over 500 C. The basic test method involves the forced vibration of composite bars at their resonant free-free flexural modes in a high vacuum cryostat furnace. The accuracy of these expressions and the flexural test was verified by dynamic moduli and damping capacity measurements on 50 fiber volume percent boron/aluminum (B/Al) composites vibrating near 2000 Hz. The phase results were summarized to permit predictions of the B/Al dynamic behavior as a function of frequency, temperature, and fiber volume fraction.

Dicarlo, J. A.↗

NASA NTRS2017

Flexural Fatigue Behavior of an EBC CMC Composite System In Air and Steam at High Temperature

Both coated and uncoated SiCSiC ceramic matrix composite (CMC) samples were tested in flexure under sustained peak low cycle fatigue (SPLCF) conditions in air or steam at elevated temperatures. The SiCSiC composites were reinforced with 2-D plies of boron nitride coated Hi-Nicalon Type-S SiC fibers which were woven as 5 harness satin (5HS) cloth. The composites were densified by chemical vapor infiltration (CVI) followed by slurry melt infiltration (SMI). A multilayer barium strontium aluminosilicate (BSAS) coating was applied to the samples by a plasma spray method. Fatigue loading limits were determined from monotonic flexure tests at room temperature and 1200oC. Stress levels under the proportional limit of the composite material were selected for the SPLCF tests. After cyclic testing, the composites were evaluated to determine crack propagation and failure modes in the coated and uncoated composites. Microstructural examination was used to identify coating degradation and failure modes of the EBCCMC system.

Steam testing↗

NASA NTRS2004

Foreign Object Damage in Flexure Bars of Two Gas-Turbine Grade Silicon Nitrides

Foreign object damage (FOD) behavior of two commercial gas-turbine grade silicon nitrides, AS800 and SN282, was determined at ambient temperature through strength testing of flexure test specimens impacted by steel ball projectiles with a diameter of 1.59 mm in a velocity range from 220 to 440 m/s. AS800 silicon nitride exhibited a greater FOD resistance than SN282, primarily due to its greater value of fracture toughness (K(sub lc)). The use of an additional equiaxed, fine-grained silicon nitride (NC132) showed that fracture toughness was a key material parameter affecting FOD resistance. The observed damages generated by projectile impact were typically in the forms of well- or ill-developed ring and cone cracks with little presence of radial cracks.

Choi, Sung R.↗

NASA NTRS1986

On the analysis and design of the end notched flexure (ENF) specimen for Mode II testing

The end notched flexure (ENF) specimen is examined as a candidate for measuring interlaminar fracture toughness in skew symmetric loading. A simple design study for sizing the ENF specimen to minimize geometric nonlinear response and to avoid nonlinear material behavior or flexural failure is presented. Results indicate that interlaminar shear effects may be significant for tough resin systems requiring large thickness-to-length geometries. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.

Carlsson, L. A.↗

NASA NTRS2018

A Simple Test Method for Large Deformation Bending of Thin High Strain Composite Flexures

A simple test method for large deformation bending of thin composite laminates is investigated using image processing and full-field strain measurements. The assumptions and kinematic equations that represent the test are used to calculate numerically the laminate bending stiffness and strength as well as the curvature and strains at failure. In order to validate the test methodology, a comparison is performed between analytical model predictions and empirical data in terms of computed surface strains versus digital image correlation data and calculated rotation angles of the fixture arms throughout the test versus measured ones. The new test method is then used to calculate the bending stiffness in the D11 and D22 directions as well as failure strains for various thin-ply laminates of interest. These parameters are ultimately compared with predicted values using micromechanics and classical lamination theory analysis. In general, bending stiffness and strain test results and predictions for 0 degree orientation coupons have a maximum difference of 10% and 35%.

Fernandez, Juan M.↗

NASA NTRS2000

Accelerated Testing of Polymeric Composites Using the Dynamic Mechanical Analyzer

Creep properties of IM7/K3B composite material were obtained using three accelerated test methods at elevated temperatures. Results of flexural creep tests using the dynamic mechanical analyzer (DMA) were compared with results of conventional tensile and compression creep tests. The procedures of the three test methods are described and the results are presented. Despite minor differences in the time shift factor of the creep compliance curves, the DMA results compared favorably with the results from the tensile and compressive creep tests. Some insight is given into establishing correlations between creep compliance in flexure and creep compliance in tension and compression. It is shown that with careful consideration of the limitations of flexure creep, a viable and reliable accelerated test procedure can be developed using the DMA to obtain the viscoelastic properties of composites in extreme environments.

Abdel-Magid, Becky M.↗

NASA NTRS1972

Mechanical component screening for scanner

The critical mechanical components of the scan mirror mechanism are described and their evaluation and screening procedures are discussed. A bumper/damper unit is used in the design to reverse motion of the mirror and effect scan and retrace cycles. A wear evaluation was conducted on the bumper impact surfaces that established nylon 6-10 as an acceptable material. The elastomeric dampers were subjected to thermal vacuum tests for condensables and outgassing as well as parametric life tests. The flexure pivots that support the mirror were tested to establish a curve of stress plotted as a function of cycles to failure for rotational operation. The pivots met the life requirement of 150,000,000 cycles at a + or - 2.9 deg amplitude during fatigue testing. Screening procedures were established for dampers and flexure pivots to obtain flight quality components.

Olson, J. L.↗

NASA NTRS1975

Effects of environment on the fatigue of graphite-epoxy composites

Torsional and flexural fatigue tests were performed on both uniaxial (0 deg) and crossplied (plus or minus 45 deg) graphite-epoxy materials at temperatures of 24 and 74 C in environments of air and water. The results of the torsion testing showed that the number of cycles required to cause an initial decrease in stiffness as well as the rate of stiffness loss was a function of temperature and environment; the most significant losses were noted for tests at the higher temperature in water. The torsional fatigue specimens were subsequently tested in four-point bending to determine the effect of torsional damage on longitudinal properties. This damage caused changes in the flexural stiffness, failure stress, and failure energy, depending on the stress and environmental histories. The flexural fatigue tests also showed a significant effect of water (at 24 C) on the material behavior. These results are compared with the results of previous investigations and are discussed in terms of proposed damage mechanisms.

Sumsion, H. T.↗

NASA NTRS1978

Measurement of the time-temperature dependent dynamic mechanical properties of boron/aluminum composites

A relatively simple flexural vibration test is developed for accurate measurement of the low-strain dynamic modulus and damping capacity of B/Al composite bar specimens from -200 C to over 500 C. The specimens are prepared from 8-ply unidirectional panels containing 50 volume percent fibers composed of 203-micron commercial boron-on-tungsten fibers. The basic test technique consists of the forced flexural vibration of the composite bar specimens at their two lowest free-free symmetrical resonant modes in a high-vacuum cryostat furnace. Specimen damping is determined from oscilloscope photographs of the free decay obtained after simultaneously removing the resonant drive signal and grounding the vibration-drive electrode. The availability of time-temperature dynamic data coupled with the predictive accuracy of composite theory suggests a future potential for using such data in examining environmental effects on composite macrostructure and microstructure.

Dicarlo, J. A.↗

NASA NTRS1994

A high temperature testing system for ceramic composites

Ceramic composites are presently being developed for high temperature use in heat engine and space power system applications. The operating temperature range is expected to be 1090 to 1650 C (2000 F to 3000 F). Very little material data is available at these temperatures and, therefore, it is desirable to thoroughly characterize the basic unidirectional fiber reinforced ceramic composite. This includes testing mainly for mechanical material properties at high temperatures. The proper conduct of such characterization tests requires the development of a tensile testing system includes unique gripping, heating, and strain measuring devices which require special considerations. The system also requires an optimized specimen shape. The purpose of this paper is to review various techniques for measuring displacements or strains, preferably at elevated temperatures. Due to current equipment limitations it is assumed that the specimen is to be tested at a temperature of 1430 C (2600F) in an oxidizing atmosphere. For the most part, previous high temperature material characterization tests, such as flexure and tensile tests, have been performed in inert atmospheres. Due to the harsh environment in which the ceramic specimen is to be tested, many conventional strain measuring techniques can not be applied. Initially a brief description of the more commonly used mechanical strain measuring techniques is given. Major advantages and disadvantages with their application to high temperature tensile testing of ceramic composites are discussed. Next, a general overview is given for various optical techniques. Advantages and disadvantages which are common to these techniques are noted. The optical methods for measuring strain or displacement are categorized into two sections. These include real-time techniques. Finally, an optical technique which offers optimum performance with the high temperature tensile testing of ceramic composites is recommended.

Hemann, John↗

NASA NTRS1993

Machine Tests Optical Fibers In Flexure

Machine repeatedly flexes single optical fiber or cable or bundle of optical fibers at low temperature. Liquid nitrogen surrounds specimen as it is bent back and forth by motion of piston. Machine inexpensive to build and operate. Tests under repeatable conditions so candidate fibers, cables, and bundles evaluated for general robustness before subjected to expensive shock and vibration tests.

Darejeh, Hadi↗

NASA NTRS2000

Slow Crack Growth Behavior and Life/Reliability Analysis of 96 wt % Alumina at Ambient Temperature With Various Specimen/Loading Configurations

Extensive constant stress-rate testing for 96 wt % alumina was conducted in room-temperature distilled water using four different specimen/loading configurations: rectangular beam test specimens under four-point uniaxial flexure, square plate test specimens in ring-on-ring biaxial flexure, square plate test specimens in ball-on-ring biaxial flexure, and dog-boned tensile test specimens in pure tension. The slow crack growth (SCG) parameter n was almost independent of specimen/loading configurations, in either four-point uniaxial flexure, ring-on-ring biaxial flexure, ball-on-ring biaxial flexure, or pure tension, ranging from n = 35 to 47 with an average value of n = 41.1 +/- 4.5. The prediction of fatigue strength/reliability based on the four-point uniaxial flexure data by using the CARES/Life design code as well as a simple PIA model was in good agreement with both the ring-on-ring biaxial and the ball-on-ring biaxial flexure data. A poor prediction using the PIA model was observed for the dog-boned tensile test specimens, presumably due to different flaw population involved in the tensile test specimens.

Choi, Sung R.↗

NASA NTRS2010

Mode II Interlaminar Fracture Toughness and Fatigue Characterization of a Graphite Epoxy Composite Material

Mode II interlaminar fracture toughness and delamination onset and growth characterization data were generated for IM7/8552 graphite epoxy composite materials from two suppliers for use in fracture mechanics analyses. Both the fracture toughness testing and the fatigue testing were conducted using the End-notched Flexure (ENF) test. The ENF test for mode II fracture toughness is currently under review by ASTM as a potential standard test method. This current draft ASTM protocol was used as a guide to conduct the tests on the IM7/8552 material. This report summarizes the test approach, methods, procedures and results of this characterization effort.

O'Brien, T. Kevin↗

NASA NTRS1973

Development of polyphenylquinoxaline graphite composites

The potential of polyphenylquinoxaline (PPQ)/graphite composites to serve as structural material at 316 C (600 F)has been demonstrated using a block copolymer, BlCo(13), PPQ derivative. Initially, thirteen polyphenylquinoxalines were evaluated. From this work, four candidate polymers were selected for preliminary evaluation as matrices for HMS graphite fiber reinforced composites. The preliminary composite evaluation enabled selection of one of the four polymers for advanced composite preparation and testing. Using an experimentally established cure schedule for each of the four polymers, preliminary laminates of 50% resin volume content, prepared without postcure, were tested for flexure strength and modulus, interlaminar shear strength (short beam), and tensile strength and modulus at ambient temperature. A block copolymer (Bl Co 13) derived from one mole p-bis (phenylglyoxalyl) benzene, one fourth mole 3,3'-diaminobenzidine and three-fourths mole 3,3', 4,4'-tetraminobenzophenone was selected for extensive study. Tensile, flexural, and interlaminar shear values were obtained after aging and testing postcured BlCo(13) laminates at 316 C (600 F). The potential of PPQ/graphite laminates to serve as short term structural materials at temperatures up to 371 C (700 F) was demonstrated through weight loss experiments.

Hoggatt, J. T.↗