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NASA NTRS1990

Probabilistic simulation for flaw acceptance by dye-penetrant inspection

This paper examines the problems encountered in assessing the reliability of dye-penetrant nondestructive inspection (NDI) techniques in preventing failures due to undetected surface flaws, as well as from flaw acceptance (Fitness-For-Purpose). A Monte Carlo simulation procedure which includes the major variables of the problem is presented as a means of quantifying reliability. Some issues associated with distribution selection are examined. A methodology for selecting the penetrant type and flaw acceptance size for the specific components analyzed using the simulation is proposed. Current methodology limitations are discussed along with possible future effort. Penetrant selection and acceptable sizes of detected flaws are based on a probabilistic assessment of the effect of component and dye-penetrant system variables on structural reliability.

Russell, D. A.↗

NASA NTRS1994

Electromagnetic Flaw Detector Is Easier To Use

Electromagnetic flaw detector based on eddy-current principle. It nondestructively detects cracks, voids, and other flaws introducing electrical discontinuities into specimens of electrically conductive materials. Circuitry associated with this flaw detector simpler than older eddy-current flaw detectors. Unlike older eddy-current nondestructive testers, calibration ordinarily not needed, and readings interpreted with relative ease.

Clendenin, C. Gerald↗

NASA NTRS1994

The probability of flaw detection and the probability of false calls in nondestructive evaluation equipment

The space industry has developed many composite materials that have high durability in proportion to their weights. Many of these materials have a likelihood for flaws that is higher than in traditional metals. There are also coverings (such as paint) that develop flaws that may adversely affect the performance of the system in which they are used. Therefore there is a need to monitor the soundness of composite structures. To meet this monitoring need, many nondestructive evaluation (NDE) systems have been developed. An NDE system is designed to detect material flaws and make flaw measurements without destroying the inspected item. Also, the detection operation is expected to be performed in a rapid manner in a field or production environment. Some of the most recent video-based NDE methodologies are shearography, holography, thermography, and video image correlation.

Temple, Enoch C.↗

NASA NTRS2003

Flaw Detection for Composite Materials Improved by Advanced Thermal Image Reconstruction Techniques

The development of advanced composite materials for use in space and propulsion components has seen considerable growth over the past few years. In addition to improvements that have been made in material properties and processing techniques, similar growth must be seen in the development of methods for the detection of flaws, either generated in service or during manufacturing. Thermal imaging techniques have proven to be successful for the nondestructive evaluation (NDE) of composite materials, but their detection capabilities decrease as flaw depth increases. The purpose of this research is to investigate advanced thermal imaging methods and thermal image processing technologies to increase the maximum depth below surface that a flaw can be detected and improve the contrast between flawed regions and sound regions.

Martin, Richard E.↗

NASA NTRS2013

Unit-Sphere Anisotropic Multiaxial Stochastic-Strength Model Probability Density Distribution for the Orientation of Critical Flaws

Models that predict the failure probability of monolithic glass and ceramic components under multiaxial loading have been developed by authors such as Batdorf, Evans, and Matsuo. These "unit-sphere" failure models assume that the strength-controlling flaws are randomly oriented, noninteracting planar microcracks of specified geometry but of variable size. This report develops a formulation to describe the probability density distribution of the orientation of critical strength-controlling flaws that results from an applied load. This distribution is a function of the multiaxial stress state, the shear sensitivity of the flaws, the Weibull modulus, and the strength anisotropy. Examples are provided showing the predicted response on the unit sphere for various stress states for isotropic and transversely isotropic (anisotropic) materials--including the most probable orientation of critical flaws for offset uniaxial loads with strength anisotropy. The author anticipates that this information could be used to determine anisotropic stiffness degradation or anisotropic damage evolution for individual brittle (or quasi-brittle) composite material constituents within finite element or micromechanics-based software

Nemeth, Noel↗

NASA NTRS1969

Flaw Growth of 6Al-4V Titanium in a Freon TF Environment

The plane strain threshold stress intensity and sustained stress flaw growth rates were experimentally determined for 6AI-4V S.T.A. titanium forging and weldments in environments of Freon TF at room temperature. Sustained load tests of surface flawed specimens were conducted with the experimental approach based on linear elastic fracture mechanics. It was concluded that sustained stress flaw growth rates, in conjunction with threshold stress intensities, can be used in assessing the service life of pressure vessels.

Tiffany, C. F.↗

NASA NTRS1972

The elastic analysis of the part-circular surface flaw problem by the alternating method.

This paper summarizes and evaluates the work done on the elastic analysis of the surface flaw problem by the application of the alternating method. An attempt is made to describe the alternating method and to present the history of its application to the surface flaw problem and to related problems in fracture mechanics. Stress intensity factors obtained by this method are summarized and compared. Results are also compared to those obtained by investigators using other methods of analysis. An evaluation of the use of the alternating method is presented with the purpose of pointing out the advantages and disadvantages in the application of this technique to the surface flaw problem.

Smith, F. W.↗

NASA NTRS1973

Fatigue flaw growth behavior in stiffened and unstiffened panels loaded in biaxial tension

The effect was investigated of biaxial loading on the flaw growth rate of 2219-T87 aluminum alloy that would be typical of Space Shuttle cryogenic tankage design. The stress distribution and stress concentration factors for several integrally stiffened panels under various loading conditions were obtained. The flaw growth behavior of both stiffened and unstiffened panels under biaxial loading conditions was determined. The effect of a complex stress state was studied by introducing flaws in fillet areas of biaxially loaded stiffened panels.

Beck, E. J.↗

NASA NTRS1979

Investigation and characterization of constraint effects on flaw growth during fatigue loading of composite materials

An investigative program is presented in an attempt to add to the current understanding of constraint effects on the response of composite materials under cyclic loading. The objectives were: (1) to use existing data and to develop additional data in order to establish an understanding and quantitative description of flaw growth in unidirectional lamina under cyclic loading at different load direction to fiber direction angles; (2) to establish a similar understanding and description of flaw growth in lamina which are embedded in laminates between other unflawed lamina; (3) to determine the nature of the influence of constraint on flaw growth by quantitatively comparing the results of the tests; and (4) to develop a model and philosophy of constraints effects based on our investigative results.

Stinchcomb, W. W.↗

NASA NTRS1979

Characterization of constraint effects on flaw growth

Experimental results have been presented for the case of a flawed unidirectional lamina constrained by off-axis unflawed plies under static and fatigue loading. Flaw growth in the interior of the laminate, around the embedded flaw, was followed by nondestructive testing methods including video-thermographs and ultrasonic C-scan schemes. Two aspects of the results were distinctive. There was an effort of lamination on damage development which was independent of the presence of the notch, and effects which were directly related to the influence of the notch on the local situation. The results are compared with sectioning studies, surface replication, and stiffness change measurements.

Yeung, P. C.↗

NASA NTRS1981

Battery failure model derived from flaw theory

A previously derived failure model for battery lifetime is discussed in terms of growth rate of the flaw, distribution of flaw sizes, and number of flaws. Equations are presented for determining the failure model for a nickel cadmium battery.

Schulman, I.↗

NASA NTRS1981

Eddy current probe response to open and closed surface flaws

A general analysis of eddy current response to certain types of open and closed surface flaws is presented for both standard low-frequency and ferromagnetic-resonance (FMR) probes. It is shown analytically that for two-dimensional and three-dimensional surface flaws interrogated by a uniform probe field, the crack opening sensitivity increases with the operating frequency of the probe, this behavior being due to the Faraday induction effect. Experiments with low-frequency probes operating at or below 1 MHz and with the FMR probe operating at approximately 1000 MHz confirm this increase of the crack mouth opening displacement for practical situations where the probe field is not uniform in the vicinity of the flaw.

Auld, B. A.↗

NASA NTRS1992

Stress analysis and damage evaluation of flawed composite laminates by hybrid-numerical methods

Structural components in flight vehicles is often inherited flaws, such as microcracks, voids, holes, and delamination. These defects will degrade structures the same as that due to damages in service, such as impact, corrosion, and erosion. It is very important to know how a structural component can be useful and survive after these flaws and damages. To understand the behavior and limitation of these structural components researchers usually do experimental tests or theoretical analyses on structures with simulated flaws. However, neither approach has been completely successful. As Durelli states that 'Seldom does one method give a complete solution, with the most efficiency'. Examples of this principle is seen in photomechanics which additional strain-gage testing can only average stresses at locations of high concentration. On the other hand, theoretical analyses including numerical analyses are implemented with simplified assumptions which may not reflect actual boundary conditions. Hybrid-Numerical methods which combine photomechanics and numerical analysis have been used to correct this inefficiency since 1950's. But its application is limited until 1970's when modern computer codes became available. In recent years, researchers have enhanced the data obtained from photoelasticity, laser speckle, holography and moire' interferometry for input of finite element analysis on metals. Nevertheless, there is only few of literature being done on composite laminates. Therefore, this research is dedicated to this highly anisotropic material.

Yang, Yii-Ching↗

NASA NTRS1995

A new merit function for evaluating the flaw tolerance of composite laminates

Advanced composite materials are providing notable savings in numerous aerospace and non-aerospace applications by virtue of their uniquely high strength-to-density ratio and high stiffness-to-density ratio. However, their full potential for high performance has yet to be achieved because of the low tolerance of the 'brittle' material to impact, holes, flaws, and localized discontinuities due to relatively low toughness. In the present paper a new and simple merit function is proposed for maximizing the strength performance of flawed composite laminate for different design requirement. This new merit function takes into account the flaw tolerance of a composite laminated as well as its stiffness. After development of the new merit function, an example is presented of its application to a practical laminate family.

Mikulas, Martin M., Jr.↗

NASA NTRS1997

A New Merit Function for Evaluating the Flaw Tolerance of Composite Laminates: Arbitrary Size Holes and Center Cracks - Part 2

In a previous paper, a new merit function for determining the strength performance of flawed composite laminates was presented. This previous analysis was restricted to circular hole flaws that were large enough that failure could be predicted using the laminate stress concentration factor. In this paper, the merit function is expanded to include the flaw cases of an arbitrary size circular hole or a center crack. Failure prediction for these cases is determined using the point stress criterion. An example application of the merit function is included for a wide range of graphite/epoxy laminates.

Mikulas, Martin M., Jr.↗

NASA NTRS1996

Study Thermographic Flaw Detection

The development of thermographic inspection methods for use on aerospace structures is under investigation. Several different material systems, structural geometries and defect types have been included in this study so as to establish a baseline from which future IRT testing can be made. This study examines various thermal loading techniques in an attempt to enhance the likelihood of capturing and identifying critically sized flaws under 'non-laboratory' actual working conditions. Qualification techniques and calibration standards are also being investigated to standardize the thermographic method. In conjunction with the thermographic inspections, advanced image processing techniques including digital filtering and neural networks have been investigated to increase the ability of 91 detecting and sizing flaws. Here, the digitized thermographic images are mathematically manipulated through various filtering techniques and/or artificial neural mapping schemes to enhance its overall quality, permitting accurate flaw identification even when the signal-to-noise ratio is low.

Walker, James L.↗

NASA NTRS1996

Study Thermographic Flaw Detection

The development of thermographic inspection methods for use on aerospace structures is under investigation. Several different material systems, structural geometries and defect types have been included in this study so as to establish a baseline from which future Infrared Thermography(IRT) testing can be made. This study examines various thermal loading techniques in an attempt to enhance the likelihood of capturing and identifying critically sized flaws under 'non laboratory' actual working conditions. Qualification techniques and calibration standards are also being investigated to standardize the thermographic method. In conjunction with the thermographic inspections, advanced image processing techniques including digital filtering and neural networks have been investigated to increase the ability of detecting and sizing flaws. Here, the digitized thermographic images are mathematically manipulated through various filtering techniques and/or artificial neural mapping schemes to enhance its overall quality, permitting accurate flaw identification even when the signal-to-noise ratio is low.

Walker, James L.↗

NASA NTRS1999

A New Merit Function for Evaluating the Flaw Tolerance of Composite Laminates: Arbitrary Size Holes and Center Cracks - Pt. 2

Mikulas, Martin M., Jr.↗