Now showing 1 - 10 of 13
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    Observation of two stage dislocation dynamics from nonlinear ultrasonic response during the plastic deformation of AA7175-T7351 aluminum alloy
    (25-06-2009)
    Rao, V. V.S.Jaya
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    Kannan, Elankumaran
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    Systematic nonlinear ultrasonic (NLU) studies using longitudinal and guided surface acoustic waves (SAW) were conducted on AA7175-T735 aluminum alloy specimens that were plastically deformed to varying levels of residual strain. The variation of nonlinear ultrasonic (NLU) parameter (A2 / A12) as a function of residual plastic deformation, suggests an increase in NLU parameter at a slow rate up to a certain level of deformation and thereafter there is a significant increase in NLU parameter with further increase in the strain level. X-ray diffraction methods to characterize line broadening and etch pit dislocation distribution studies suggest that the smaller rate of increase in NLU parameter could be due to the increase in the dislocation density and higher rate of increase could be caused by formation of cellular dislocation pattern. Thus, our study shows that it is possible to study the two stage dislocation dynamics observed during residual plastic deformation of Al alloy 7175-T7351 using nonlinear ultrasonic technique. © 2009 Elsevier B.V. All rights reserved.
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    Thermomechanical studies in glass/epoxy composite specimen during tensile loading
    (01-08-2009)
    Mohamed Muneer, K. M.
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    This paper presents the results of thermo-mechanical characterization of Glass/Epoxy composite specimens using Infrared Thermography technique. The specimens used for the study were fabricated in-house with three different lay-up sequences and tested on a servo hydraulic machine under uni-axial loading. Infrared Camera was used for on-line monitoring surface temperature changes of composite specimens during tensile deformation. Experimental results showed that thermomechanical characteristics of each type of specimens were distinct. Temperature was found to be decreasing linearly with increasing tensile stress in the elastic region due to thermo-elastic effect. Yield point could be observed by monitoring the change in temperature profile during tensile testing and this value could be correlated with the results obtained from stress-strain response. The extent of prior plastic deformation in the post-yield region influenced the slopes of temperature response during tensile loading. Partial unloading and reloading of specimens post-yield results in change in slope in elastic and plastic regions of composite specimens.
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    Fatigue crack growth at stress concentrators under spectrum loading
    (01-02-2005)
    Fatigue cracks initiate at stress raisers such as notches, discontinuities, and surface defects. Many of the field failures that indicate the presence of a fatigue crack at failure can be traced to crack initiation from one or more crack initiation sites and merger of cracks over a period of service. Substantial service life is spent in the growth of small cracks from an initial size of few micrometres before they coalesce and grow to critical dimensions that cause fracture. This paper summarizes research that was carried out in order to understand the kinetics of crack growth of small cracks at notches under simulated FALSTAFF service loading. This paper also presents a method used to understand crack growth kinetics in a pin-loaded lug joint through a crack-front-mapping technique.
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    Investigation of material fatigue behavior through cyclic ball indentation testing
    (01-01-2009) ;
    Bhokardole, Prashant
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    Shin, Chow Shing
    An experimental investigation of cyclic indentation testing to characterize the fatigue response of three different metallic materials: two aluminum alloys (2014-T651 and 7175-T7351) and a duplex stainless steel (2205), was carried out. The force-displacement response during cyclic loading was logged continuously throughout the entire duration of the test and the data were analyzed to identify parameters such as change in total depth of penetration, change in loading and unloading slopes, change in unloading intercept as a function of number of cycles, and change in displacement range as a function of number of cycles. From the results, one could identify the transient response of the material during cyclic loading, and identify some specific points relating to fatigue failure life, such as knee-point response in depth of penetration as a function of number of cycles of loading. It was observed that data on unloading slope plotted as a function of number of cycles also provide an indication of specimen failure in compression-compression fatigue. The responses were found to be similar for all the three materials tested at different maximum compressive force levels. Failure life data in the low cycle fatigue (LCF) region was evaluated for Al-Cu-Mg alloy 7175-T7351 and the data compared with the failure indicators (knee point) during cyclic indentation testing. A reasonable correlation was established between failure life, as indicated by LCF testing and knee point indicated by cyclic indentation. Experiments were also carried out on virgin material of 7175-T7351 alloy and plastically deformed material of the same alloy. Both static and cyclic indentation tests show a difference in material behavior before and after residual plastic deformation. Further work is required to correlate failure life data as obtained from cyclic indentation with specimens having controlled damage levels, before this technique can be used for residual life estimation purposes. Copyright © 2008 by ASTM International.
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    Investigation of material fatigue behavior through cyclic ball indentation testing
    (01-01-2008) ;
    Bhokardole, Prashant
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    Shin, Chow Shing
    An experimental investigation of cyclic indentation testing to characterize the fatigue response of three different metallic materials: two aluminum alloys (2014-T651 and 7175-T7351) and a duplex stainless steel (2205), was carried out. The force-displacement response during cyclic loading was logged continuously throughout the entire duration of the test and the data were analyzed to identify parameters such as change in total depth of penetration, change in loading and unloading slopes, change in unloading intercept as a function of number of cycles, and change in displacement range as a function of number of cycles. From the results, one could identify the transient response of the material during cyclic loading, and identify some specific points relating to fatigue failure life, such as knee-point response in depth of penetration as a function of number of cycles of loading. It was observed that data on unloading slope plotted as a function of number of cycles also provide an indication of specimen failure in compressioncompression fatigue. The responses were found to be similar for all the three materials tested at different maximum compressive force levels. Failure life data in the low cycle fatigue (LCF) region was evaluated for Al-Cu-Mg alloy 7175-T7351 and the data compared with the failure indicators (knee point) during cyclic indentation testing. A reasonable correlation was established between failure life, as indicated by LCF testing and knee point indicated by cyclic indentation. Experiments were also carried out on virgin material of 7175-T7351 alloy and plastically deformed material of the same alloy. Both static and cyclic indentation tests show a difference in material behavior before and after residual plastic deformation. Further work is required to correlate failure life data as obtained from cyclic indentation with specimens having controlled damage levels, before this technique can be used for residual life estimation purposes. Copyright © 2008 by ASTM International.
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    Fatigue crack growth in open and nut-loaded bolts with and without pretension loading
    (01-10-2008) ;
    Bagla, Akash
    Results of an experimental study to understand crack growth in open threaded bolts and nut-loaded bolts with and without pretension are presented in this paper. Experimental crack growth rates are compared with predicted solutions obtained from known stress intensity factor solutions. For the experiment, a specially designed fixture was used for loading the bolt and the level of pretension was varied by controlling the displacement on the arm of the C section of the fixture. Programmed Hi-Lo fatigue load sequence was applied on a bolt to ensure that decodable marker bands are left behind on the fracture surface, to study the crack growth characteristics. Fatigue crack growth rate was estimated using a scanning electron microscope by identifying crack increment during high stress ratio cycles. This has been compared with predicted crack growth rates using available stress intensity factor solutions. Crack growth at the center of the bolt correlates well with Toribio's (Int. J. Fract., Vol. 53, 1992, pp. 367-385) finite element based predictions for open threaded bolts. Crack growth rate is much higher at the surface than at the center. In case of nut-loaded bolts, available K solutions are found to be inadequate to provide an accurate estimate of crack growth rates at the center of the bolt. Further study is required to characterize the effect of friction, pitch angle, and thread geometry to understand crack growth in bolts. Copyright © 2008 by ASTM International.
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    Crack closure stress estimation by decodable marker banding techniques and comparison of results with electron fractographic technique
    (01-04-2007) ;
    Sunder, R.
    A specially programmed marker load sequence was designed that permits unambiguous measurement of crack opening and crack closure stress from marker band spacing using both optical and scanning electron fractography. Crack growth tests using programmed marker loading sequence were carried out on single edge notch tension specimens made out of Al-Cu alloy 2014-T3 (BS: L73 equivalent) from crack initiation to failure and the fracture surface examined under optical and electron microscope. Reproducible marker band patterns on the fatigue fracture surface could be obtained using this method. Measured crack closure/opening stress values are observed to be consistent with known material behaviour. The marker banding technique can be applied over wider crack growth rate ranges compared to striation measurement technique, which is restricted to a small range of crack growth rate and in select materials. Quantitative fractography of the marker bands reveals a degree of load interaction under variable amplitude loading that requires further examination. © 2007, Institution of Mechanical Engineers. All rights reserved.
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    Characterization of fatigue response using infrared thermography on a structural steel
    (01-12-2009) ;
    Patil, P. T.
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    Thiyagarajan, K.
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    Fatigue response was evaluated using an Infrared thermographic technique in a structural steel to study the elastic, elastic-plastic response of the material. Both elastic loading (typical of high cycle fatigue) and elastic-plastic loading (low cycle fatigue) was applied. The temperature profile of the specimen was found to be having three distinct regions - initial steep rise in temperature as a function of cycles (region 1), stable response (region 2), steep increase in temperature at about the failure cycles (region 3). Increased test frequency results in higher temperature for region 2. The slope of the region 1 was found to be similar in case of complete elastic loading; however, in case of elastic-plastic loading, the slope differs as a function of spent life of the specimen. Thus, thermographic technique can be used to characterize the extent of fatigue damage in a material for elastic-plastic loading conditions (typical of low cycle fatigue).
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    Fatigue crack growth behavior in dissimilar metal weldment of stainless steel and carbon steel
    (01-12-2009)
    Krishnaprasad, K.
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    Constant amplitude fatigue crack growth (FCG) tests were performed on dissimilar metal welded plates of Type 316L Stainless Steel (SS) and IS 2062 Grade A Carbon steel (CS). The plates were welded by TIG welding using SS E309 as electrode. FCG tests were carried on the Side Edge Notch Tension (SENT) specimens of 5 mm thickness, with crack initiator (notch) at base metal region (BM), weld metal region (WM) and heat affected zones (HAZ). The tests were performed at a test frequency of 10 Hz and at load ratios (R) of 0.1 & 0.6. FCG rate was found to increase with stress ratio for weld metals and base metals, where as in case of HAZ, FCG rates were almost equal at high ΔK. FCG rate of HAZ of stainless steel was found to be lowest at low and high ΔK. At intermediate ΔK, WM showed the lowest FCG rate. CS showed higher crack growth rate at all ΔK. However, the scatter band of data was found to be narrow. Fracture toughness (Kc) was found to vary in different locations of weldments. Kc was found lowest for the weldment and highest for HAZ of stainless steel. A novel method of characterizing the FCG behavior using an Infrared thermography (IRT) camera was attempted. By monitoring the temperature rise at the fast moving crack tip region, the amount of plastic deformation was estimated.
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    Fatigue damage characterization using surface acoustic wave nonlinearity in aluminum alloy AA7175-T7351
    (01-12-2008)
    Rao, V. V.S.Jaya
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    Kannan, Elankumaran
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    Nonlinear ultrasonic (NLU) harmonic generation system was used to characterize the fatigue damage in a flat hour-glass, high strength Al-Cu-Zn-Mg alloy, AA7175-T7351 specimens. Experiments were carried out to introduce controlled levels of fatigue damage under constant amplitude loading to determine the NLU response using surface acoustic wave (or Rayleigh mode) at regular intervals of fatigue life. The NLU parameter (A2 / 12) plotted as a function of percentage of fatigue life shows two peaks for all the samples tested, independent of the amplitude of fatigue loading. The first peak appeared between 40%-50% of fatigue life and the second peak between 80%-90% of fatigue life. Among the two flat surfaces of the specimen, a higher nonlinearity response was observed on the surface which had the first crack initiation. The appearance of two peaks in the nonlinear response during fatigue damage progression is explained based on the dislocation dynamics and dislocation-crack interaction present in the specimens during the fatigue process. © 2008 American Institute of Physics.