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NASA NTRS1991

Damage development under compression-compression fatigue loading in a stitched uniwoven graphite/epoxy composite material

Damage initiation and growth under compression-compression fatigue loading were investigated for a stitched uniweave material system with an underlying AS4/3501-6 quasi-isotropic layup. Performance of unnotched specimens having stitch rows at either 0 degree or 90 degrees to the loading direction was compared. Special attention was given to the effects of stitching related manufacturing defects. Damage evaluation techniques included edge replication, stiffness monitoring, x-ray radiography, residual compressive strength, and laminate sectioning. It was found that the manufacturing defect of inclined stitches had the greatest adverse effect on material performance. Zero degree and 90 degree specimen performances were generally the same. While the stitches were the source of damage initiation, they also slowed damage propagation both along the length and across the width and affected through-the-thickness damage growth. A pinched layer zone formed by the stitches particularly affected damage initiation and growth. The compressive failure mode was transverse shear for all specimens, both in static compression and fatigue cycling effects.

Vandermey, Nancy E.↗

NASA NTRS1989

Fatigue loading history reconstruction based on the rain-flow technique

Methods are considered for reducing a non-random fatigue loading history to a concise description and then for reconstructing a time history similar to the original. In particular, three methods of reconstruction based on a rain-flow cycle counting matrix are presented. A rain-flow matrix consists of the numbers of cycles at various peak and valley combinations. Two methods are based on a two dimensional rain-flow matrix, and the third on a three dimensional rain-flow matrix. Histories reconstructed by any of these methods produce a rain-flow matrix identical to that of the original history, and as a result the resulting time history is expected to produce a fatigue life similar to that for the original. The procedures described allow lengthy loading histories to be stored in compact form.

Khosrovaneh, A. K.↗

NASA NTRS1988

Characterization of debond growth mechanism in adhesively bonded composites under mode II static and fatigue loadings

An experimental investigation of adhesively bonded composite joint was conducted to characterize the debond growth mechanism under mode II static and fatigue loadings. For this purpose, end-notched flexure specimens of graphite/epoxy (T300/5208) adherends bonded with EC 3445 adhesive were tested. In all specimen tested, the fatigue failure occurred in the form of cyclic debonding. The present study confirmed the result of previous studies that total strain-energy-release rate is the driving parameter for cyclic debonding. Further, the debond growth resistance under cyclic loading with full shear reversal (i.e., stress ratio, R = -1) is drastically reduced in comparison to the case when subjected to cyclic shear loading with no shear reversal (i.e., R = 0.1).

Mall, S.↗

NASA NTRSDate not supplied

Progressive Damage Mechanisms in Thick 3D Composite Structures under Static and Fatigue Loading

The objective of this work is to demonstrate validation of predictive capabilities capturing both damage initiation and growth under static and fatigue loading in geometrically complex three-dimensional (3D) composite structures. The composite structure of interest in the present study resembles a 3D blade spar used in rotorcraft applications. Under both static and fatigue testing, it was shown that damage initiated at the same ply-drop location(s) and grew in the axial as well as hoop directions. CompDam – a state-of-the-art Progressive Damage Analysis (PDA) tool was used to achieve successful correlations of metrics including load-displacement responses and damage modes.

progressive damage analysis↗

NASA NTRS1986

Mode 1 delamination growth in adhesively bonded joints under static and fatigue loads

The objective of this investigation was to characterize the pure mode 1 delamination growth in metal to metal adhesively bonded joints under static and fatigue loading conditions, using FM 73 adhesive. Double cantilever beam specimens (DCB) were used for pure mode 1 tests. Aluminum 2024-T3 adherends were bonded with FM 73 adhesive. Delamination was introduced during fabrication by inserting a Teflon film between the two adherends. The mode 1 strain energy release rate G sub I sub c was obtained directly from static DCB tests conducted in accordance with the ASTM designation D1876-12. Constant amplitude fatigue tests on DCB specimens were conducted to determine the relationship between delamination growth rate da/dN and strain energy release rate G sub I sub c for a pure mode 1 delamination growth. It is found that the debond propagation rate in adhesive joints using FM 73 is more sensitive to errors in design load than is typical crack growth rate in metallic structures.

Biswas, S. C.↗

NASA NTRS1990

Residual strength of composite laminates subjected to tensile-compressive fatigue loading

Results are presented on the measurements of the residual strengths of T300/934 graphite epoxy laminates, in tension and in compression, after the samples were exposed to tension-compression fatigue loading (R = -1). Four laminate ocnfigurations were tested: unidirectional, cross-ply, angle-ply, and quasi-isotropic. It was found that the fatigue behavior of laminates was dependent on the quasi-static strengths and the specific structure of the laminate. No direct correlation was found between remaining residual strengths and the percentage of average fatigue life. However, a correlation scheme was developed for the individual specimen under test, based on a cumulative damage model and a stiffness change of the material.

Rotem, Assa↗

NASA NTRS1995

Characterization of Two-Dimensionally Braided Composites Subject to Static and Fatigue Loading

This paper presents a characterization of damage processes in four different two-dimensionally braided architectures consisting of graphite fibers in an epoxy matrix, subjected to both static and fatigue loading. Both notched and unnotched specimens were tested from each architecture. Damage mechanisms in static compression testing were found to range from matrix cracking in an architecture controlled by shearing characteristics of the matrix to axial fiber bundle kinking and splitting in architectures controlled by unidirectional fiber bundles. In both static tension and tension-tension fatigue testing, it was found that all architectures sustained the same basic damage types and progression sequences. The first types of damage to occur were splits in the braider bundles and cracks in the resin rich areas. Once a sufficient density of this type of damage had occurred, the axial bundles began to disbond from the surrounding constituents, and continued to do so until ultimate failure. One architecture sustained an additional mode of failure, which involved splitting of the axial bundles along their length. Varied degrees of notch sensitivity were found among the different architectures. In one architecture, this was attributed to differences found in the stacking of the individual plys of the braided architecture. As the density of axial bundles adjacent to the notch increased, the rate of stiffness loss increased and the life decreased.

Burr, Scott T.↗

NASA NTRS1988

The dependence of crack closure on fatigue loading variables

Optical and electron microscopy and stereoimaging techniques are used to determine crack opening loads local to the crack tip in aluminum alloy 7091 and 304 stainless steel. It is found that the crack peels open nonlinearly such that significant load increases are necessary to open the crack the last few microns from the tip. The stress intensity range ratio is shown to depend on fatigue loading variables according to a given relationship.

Hudak, Stephen J., Jr.↗

NASA NTRS1982

The relationship of compliance changes during fatigue loading to the fracture of composite materials

The study outlined here is based on measurements of the change in engineering stiffness values induced by the development of damage in composite laminates during quasi-static or cyclic (fatigue) loading. These changes are found to be related to the individual details of the damage events in the laminates, as well as to the residual strength and life of the laminates. It is believed that the stiffness changes can also be used to relate composite material behavior under cyclic loading to its behavior under quasi-static loading. Results are presented for both notched and unnotched laminates. Compliance changes are found to be caused by damage events that bring about both global and local redistributions of stress. It is also found that the redistributions of stress determine the residual strength of the laminate. The quantitative link between compliance changes and fracture strength is the mechanics of the internal stress redistributions.

Reifsnider, K. L.↗

NASA NTRS2012

Thermography Inspection for Early Detection of Composite Damage in Structures During Fatigue Loading

Advanced composite structures are commonly tested under controlled loading. Understanding the initiation and progression of composite damage under load is critical for validating design concepts and structural analysis tools. Thermal nondestructive evaluation (NDE) is used to detect and characterize damage in composite structures during fatigue loading. A difference image processing algorithm is demonstrated to enhance damage detection and characterization by removing thermal variations not associated with defects. In addition, a one-dimensional multilayered thermal model is used to characterize damage. Lastly, the thermography results are compared to other inspections such as non-immersion ultrasonic inspections and computed tomography X-ray.

Zalameda, Joseph N.↗

NASA NTRS2005

Space Shuttle Partial Stack Rollout Test Analytical Correlation In Support Of Fatigue Load Development

A rollout test with only the Solid Rocket Boosters was conducted in November 2003 to gather structural dynamic response data of the transportation environment from the Vehicle Assembly Building to the Launch Pad. The data was acquired to develop and validate analytical methods used to predict rollout Orbiter fatigue load spectra. Earlier predictions computed by a base drive approach with only 5 input drive degrees-of-freedom raised questions that commissioned the partial stack test. Not only was there a concern because of the input degree-of-freedom omission due to measurement limitations, but there was also a concern with the implementation of the "large mass" itself. Three methods were evaluated with the partial stack test data. The analytical correlations to measured strain derived SRB base loads and acceleration5 showed the earlier 5 degree-of-freedom base drive approach to yield the most conservative results for all quantities monitored except the SRB base moment about the axis in which the input drive was missing. This non-conservative shortcoming led to a recommendation to use either the 6 degree-of-freedom base drive or the 12 degree-of-freedom Craig-Bampton boundary drive methods whose results did not substantially differ.

DelBasso, Steve↗

NASA NTRS2018

A Benchmark Example for Delamination Propagation Predictions Based on the Single Leg Bending Specimen Under Quasi-Static and Fatigue Loading

Benchmark examples based on Single Leg Bending (SLB) specimens with equal and unequal bending arm thicknesses were used to assess the performance of delamination prediction capabilities in finite element codes. First, the development of the quasi-static benchmark cases using the Virtual Crack Closure Technique (VCCT) is discussed in detail. Second, based on the quasi-static benchmark results, additional benchmark cases to assess delamination propagation under fatigue loading are created. Third, the application is demonstrated for the commercial finite element code Abaqus Standard 2018. The benchmark cases are compared to results obtained from VCCT-based, automated quasi-static propagation analysis. A comparison with results from automated fatigue propagation analysis was not performed at this point since the current version of Abaqus does not include this capability under variable mixed-mode conditions. In general, good agreement between the results obtained from the quasi-static propagation analysis and the benchmark results were achieved. Overall, the benchmarking procedure proved valuable for analysis verification.

Krueger, Ronald↗

NASA NTRSDate not supplied

A Benchmark Example for Delamination Propagation Predictions Based on the Single Leg Bending Specimen under Quasi-static and Fatigue Loading

Ronald Krueger↗

NASA NTRS1977

Biaxial fatigue loading of notched composites

Thin-walled, 2.54-cm diameter tubular specimens of graphite/epoxy were fatigue cycled in combinations of axial, torsional, and internal pressure loading. Two different four-ply layup configurations were tested: (0-90)s and (+ or- 45)s; each tube contained a 0.48-cm diameter circular hole penetrating one wall midway along the tube length. S-N curves were developed to characterize fatigue behavior under pure axial, torsional, or internal pressure loading, as well as combined loading fatigue. A theory was developed based on a plane stress model which enabled the S-N curve for combined stress states to be predicted from the S-N data for the uniaxial loading modes. Correlation of the theory with the experimental data proved to be remarkably good.

Francis, P. H.↗

NASA NTRS1954

Axial-Load Fatigue Properties of 24S-T and 75S-T Aluminum Alloy as Determined in Several Laboratories

In the initial phase of a NACA program on fatigue research, axial-load tests on 24S-T3 and 75S-T6 aluminum-alloy sheet have been made at the Battelle Memorial Institute and at the Langley Aeronautical Laboratory of the National Advisory Committee for Aeronautics. The test specimens were polished and unnotched. The manufacturer of the material, the Aluminum Company of America, has made axial-load tests on 24S-T4 and 75S-T6 rod material. The test techniques used at the three laboratories are described in detail; the test results are presented and are compared with each other and with results obtained on unpolished sheet by the National Bureau of Standards. (author)

Grover, H J↗

NASA NTRS1977

Biaxial fatigue loading of notched composites

Thin walled, 2.54-cm (1-in.) diameter tubular specimens of T300/934 graphite/epoxy were fabricated and fatigue cycled in combinations of axial, torsional, and internal pressure loading. Two different four-ply layup configurations were tested: (0/90)S and (+ or - 45)S; all tubes contained a 0.48-cm (3/16-in.) diameter circular hole penetrating one wall midway along the tube length. S-N curves were developed to characterize fatigue behavior under pure axial, torsional, or internal pressure loading, as well as combined loading fatigue. A theory was developed based on the Hill plane stress model which enabled the S-N curve for combined stress states to be predicted from the S-N data for the uniaxial loading modes. Correlation of the theory with the experimental data proved to be remarkably good.

Francis, P. H.↗

NASA NTRS1971

Effects of loading sequence for notched specimens under high-low two-step fatigue loading

The effects of loading sequence on crack-initiation period were investigated for notched aluminum-alloy specimens under high-low two-step loading with special emphasis on local cyclic stresses and strains at the notch root. Local stress and strain were determined by a procedure based on an equation proposed by Neuber which relates elastoplastic stress and strain at a notch. Local stress and strain were also measured experimentally to verify the Neuber equation. The effects of initial high load on the crack-initiation periods were demonstrated with notched specimens and were simulated in unnotched specimens fatigue tested with local stress sequences. An analysis of the results indicated that sequence effects were not caused solely by local residual stresses, as is usually assumed; the existence of a damaging effect, resulting from the high local strain cycles, was demonstrated. The sequence effects observed with notched specimens were interpreted as the combined result of residual stresses and high local strain cycles.

Crews, J. H., Jr.↗

NASA NTRS1989

Characterization of failure processes in tungsten copper composites under fatigue loading conditions

A fractographic and metallographic investigation was performed on specimens of a tungsten fiber reinforced copper matrix composite (9 vol percent), which had experienced fatigue failures at elevated temperatures. Major failure modes and possible failure mechanisms, with an emphasis placed on characterizing fatigue damage accumulation, were determined. Metallography of specimens fatigued under isothermal cyclic loading suggested that fatigue damage initiates in the matrix. Cracks nucleated within the copper matrix at grain boundaries, and they propagated through cavity coalescence. The growing cracks subsequently interacted with the reinforcing tungsten fibers, producing a localized ductile fiber failure. Examinations of interrupted tests before final failure confirmed the suggested fatigue damage processes.

Kim, Yong-Suk↗