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  • 1
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    Crack Development in Cross-Ply Laminates Under Uniaxial Tension (1996)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: This study addresses matrix-dominated failures in carbon fiber/polymer matrix composite laminates in a cross-ply lay-up. The events of interest are interlaminar fracture in the form of transverse cracks in the 90' plies and longitudinal splitting in the 0 deg plies and interlaminar fracture in the form of 0/90 delamination. These events were observed using various nondestructive evaluation (NDE) techniques during static tensile tests. Acoustic emission (AE) x radiography, and edge view microscopy were the principal ones utilized in a real-time environment. A comparison of the NDE results with an analytical model based on the classical linear fracture mechanics concept of strain energy release rate as a criterion for crack growth was performed. The virtual crack closure theory was incorporated with a finite element model to generate strain energy release rate curves for the analytical case. Celion carbon fiber/polyimide matrix (G30-500/PMR-15) was the material tested with cross-ply lay-ups of (0(2)/90(6))s and (0(4)/90(4))s. The test specimens contained thermally induced cracks caused by the high-temperature processing. The analytical model was updated to compensate for the initial damage and to study further accumulation by taking into account the crack interactions. By correlating the experimental and analytical data, the critical energy release rates were found for the observable events of interest.
    Keywords: Structural Mechanics
    Type: NASA-CR-195464 , NAS 1.26:195464 , E-9635
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  • 2
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    Damage Assessment of Creep Tested and Thermally Aged Udimet 520 Using Acousto-Ultrasonics (2001)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Due to elevated temperatures and excessive stresses, turbine components may experience creep behavior. As a result, it is desirable to monitor and assess the current condition of such components. This study employed the Acousto-Ultrasonics (AU) method in an effort to monitor the state of the material at various percentages of used up creep life in the nickel base alloy, Udimet 520. A stepped specimen (i.e., varying cross sectional area) was employed which allowed for a postmortem nondestructive evaluation (NDE) analysis of the various levels of used up life. The overall objectives here were two fold: First, a user friendly, graphical interface AU system was developed, and second the new AU system was applied as an NDE tool to assess distributed damage resulting from creep. The experimental results demonstrated that the AU method shows promise as an NDE tool capable of detecting material changes as a function of used up creep life. Furthermore, the changes in the AU parameters were mainly attributed to the case of combined load and elevated temperature (i.e., creep) and not simply because of a timed exposure at elevated temperature (i.e., heat treatment or thermal aging).
    Keywords: Structural Mechanics
    Type: NASA/TM-2001-210988 , E-12842 , NAS 1.15:210988 , Turbo Expo 2001; Jun 04, 2001 - Jun 07, 2001; New Orleans, LA; United States
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  • 3
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    Thermoelastic Stress Analysis: The Mean Stress Effect in Metallic Alloys (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: The primary objective of this study involved the utilization of the thermoelastic stress analysis (TSA) method to demonstrate the mean stress dependence of the thermoelastic constant. Titanium and nickel base alloys, commonly employed in aerospace gas turbines, were the materials of interest. The repeatability of the results was studied through a statistical analysis of the data. Although the mean stress dependence was well established, the ability to confidently quantify it was diminished by the experimental variations. If calibration of the thermoelastic response to mean stress can be successfully implemented, it is feasible to use the relationship to determine a structure's residual stress state.
    Keywords: Structural Mechanics
    Type: NASA/TM-1999-209376 , NAS 1.15:209376 , E-11789
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  • 4
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    Damage Assessment of Aerospace Structural Components by Impedance Based Health Monitoring (2005)
    In:  CASI
    Details
    Publication Date: 2019-07-11
    Description: This paper addresses recent efforts at the NASA Glenn Research Center at Lewis Field relating to the set-up and assessment of electro-mechanical (E/M) impedance based structural health monitoring. The overall aim is the application of the impedance based technique to aeronautic and space based structural components. As initial steps, a laboratory was created, software written, and experiments conducted on aluminum plates in undamaged and damaged states. A simulated crack, in the form of a narrow notch at various locations, was analyzed using piezoelectric-ceramic (PZT: lead, zirconate, titarate) patches as impedance measuring transducers. Descriptions of the impedance quantifying hardware and software are provided as well as experimental results. In summary, an impedance based health monitoring system was assembled and tested. The preliminary data showed that the impedance based technique was successful in recognizing the damage state of notched aluminum plates.
    Keywords: Structural Mechanics
    Type: NASA/TM?2005-213579 , E?15046
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  • 5
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    Quantifying Residual Stresses by Means of Thermoelastic Stress Analysis (2001)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This study focused on the application of the Thermoelastic Stress Analysis (TSA) technique as a tool for assessing the residual stress state of structures. TSA is based on the fact that materials experience small temperature changes when compressed or expanded. When a structure is cyclically loaded, a surface temperature profile results which correlates to the surface stresses. The cyclic surface temperature is measured with an infrared camera. Traditionally, the amplitude of a TSA signal was theoretically defined to be linearly dependent on the cyclic stress amplitude. Recent studies have established that the temperature response is also dependent on the cyclic mean stress (i.e., the static stress state of the structure). In a previous study by the authors, it was shown that mean stresses significantly influenced the TSA results for titanium- and nickel-based alloys. This study continued the effort of accurate direct measurements of the mean stress effect by implementing various experimental modifications. In addition, a more in-depth analysis was conducted which involved analyzing the second harmonic of the temperature response. By obtaining the amplitudes of the first and second harmonics, the stress amplitude and the mean stress at a given point on a structure subjected to a cyclic load can be simultaneously obtained. The experimental results showed good agreement with the theoretical predictions for both the first and second harmonics of the temperature response. As a result, confidence was achieved concerning the ability to simultaneously obtain values for the static stress state as well as the cyclic stress amplitude of structures subjected to cyclic loads using the TSA technique. With continued research, it is now feasible to establish a protocol that would enable the monitoring of residual stresses in structures utilizing TSA.
    Keywords: Structural Mechanics
    Type: NASA/TM-2001-210697 , NAS 1.15:210697 , E-12628 , Nondestructive Evaluation of Aging Materials and Composites IV; Mar 08, 2000 - Mar 09, 2000; Newport Beach, CA; United States
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