Vikas Kumar

Acoustic Emission During Tensile Deformation of Smooth and Notched Specimens of Near Alpha Titanium Alloy

Acoustic emission (AE) generated during tensile deformation of smooth and notched specimens of near Alpha titanium alloy have been studied. A few tests carried out with both types of specimens were also interrupted in order to correlate with the damage mechanism. The results have shown higher acoustic emission during yielding and reduced AE at higher strain level in the smooth specimen. A peak in the root mean square (RMS) voltage at higher strain was associated with the initiation of microcrack. In the notched specimen, localized deformation at the notch tip gives rise to detectable AE in the region prior to macroscopic yielding and gives intense AE at lower strain compared to net section yielding. This is characterized by peak amplitude distribution of hits in which majority of the hits are shifted to lower amplitudes in notched specimens as compared to the smooth specimen. The AE results have been supported by microstructural investigations.

Key Microstructural Features Responsible for Improved Stress Corrosion Cracking Resistance and Weldability in 7xxx Series Al Alloys Containing Micro / Trace Alloying Additions

The commercial 7xxx series Al alloys are based on medium strength Al-Zn-Mg and high strength Al-Zn-Mg-Cu systems. The medium strength alloys are weldable, whilst the high strength alloys are nonweldable. On the other hand, the Cu-free, weldable alloys suffer from poor SCC resistance. It is the purpose of this article to provide quantitative data and microstructural analysis to demonstrate that small additions of either Ag or Sc to Al-Zn-Mg and Al-Zn-Mg-Cu alloys bring about very significant improvement in SCC resistance and weldability, respectively. The improvement in SCC resistance of the Cu-bearing alloys due to over aging and retrogression and reaging (RRA) is further discussed in light of a similar improvement in the SCC resistance of these alloys, when peak aged, due to Ag and Sc additions.

Evolution of Damage in Near α IMI-834 Titanium Alloy Under Monotonic Loading Condition: A Continuum Damage Mechanics Approach

In the present work, a continuum damage mechanics model, based on Lemaitre’s concept of equivalent stress hypothesis (1986, “Local Approach to Fracture ,” Eng. Fract. Mech., 25, pp. 523–537), has been applied to study the evolution of damage under monotonic loading condition in a near α IMI-834 titanium alloy, used for aeroengine components in compressor module. The damage model parameters have been experimentally identified by in situ measurement of damage during monotonic deformation using alternating current potential drop technique. The damage model has been applied to predict damage evolution in an axisymmetrically notched specimen using finite element analysis. A reasonably good agreement has been observed between numerically simulated and experimentally measured damage behaviors. Damage micromechanisms operative in this alloy have also been identified which show multiple damage events.

Acoustic emission studies for characterization of fatigue crack growth behavior in HSLA steel

Abstract High strength low alloy (HSLA) steels are a group of low carbon steels and used in oil and gas pipelines, automotive components, offshore structures and shipbuilding. Fatigue crack growth (FCG) characteristics of a HSLA steel have been studied at two different stress ratios (R = 0.3 and 0.5). Acoustic emission (AE) signals generated during the FCG tests have been used to understand the FCG processes. The AE signals were captured by mounting two piezoelectric sensors on compact tension specimens in liner location configuration. The AE generated in stage II of the linear Paris region of FCG has been attributed to the presence of two sub-stages with two different slopes. The AE generated at higher values of stress intensity factor is found to be useful to identify the transition from stage II to stage III of the FCG. AE location analysis has provided support for increased damage at the crack tip for higher stress ratio. The peak stress intensity (Kmax) values at the crack tip have shown good correlation with the transitions from stage IIa to stage IIb and stage II to stage III of the FCG for the two stress ratios.

A Modified Theta Projection Model for Creep Behavior of Metals and Alloys

In this work, a modified theta projection model is proposed for the constitutive modeling of creep behavior of metals and alloys. In the conventional theta projection model, strain hardening exponent is a function of time and theta, whereas in the modified theta projection model, the exponent is taken as a function of time, theta, and applied stress. The results obtained by the modified theta projection model for Al 2124 T851 alloy at constant uniaxial tensile stress are compared with the experimental results and with the predictions of the conventional theta projection method. The creep behavior of Al 7075 T651 alloy is also predicted using modified and conventional theta projection model and compared with the available experimental data. It is observed that the modified theta projection model captures the creep behavior more accurately as compared to the conventional theta projection model. The modified theta projection model can be used to predict the creep strain of pure metals and class M alloys (similar creep behavior to pure metals) for intermediate range of stress and temperature.

Programmable controllers for special control applications in rolling mills

Programmable controllers were originally evolved as programmable logic controllers to be used solely for replacement of hardwired logic schemes. In the course of time the PLC hardware has become more powerful and programmable controllers are taking their designated place deservingly in the upper range automation systems and handle some of the special control functions which were hitherto being executed only by process computers or analog controllers. The paper deals with automation of a single stand aluminium cold rolling mill using programmable controllers and PCs. It highlights the various special functions realized using the programmable controllers.<<ETX>>

Microtexture Analysis and Modeling of Ambient Fatigue and Creep-Fatigue Damages in Ti-6Al-4V Alloy

In the present investigation, microtexture analysis using electron back-scattered diffraction technique has been performed to study fatigue- and creep-fatigue damages and associated deformation structures in Ti-6Al-4V alloy. Special emphasis has been given to low-angle grain boundary configuration and its possible application as a damage indicator. Damage is mostly present in the form of voids as investigated through scanning electron microscopy. Stored deformation energies have been evaluated for the strain-controlled fatigue-, the stress-controlled fatigue-, and the creep-fatigue-tested samples. Stored deformation energies have also been analyzed vis-à-vis total damage energies to quantify the contribution of damages to various samples. A relation between the stored deformation energy and the applied strain amplitude has been proposed in this study.

Creep–Fatigue Interactions in Ti-6Al-4V Alloy at Ambient Temperature

Abstract In the present investigation, creep–fatigue interaction was studied in an alpha–beta titanium alloy Ti-6Al-4V at ambient temperature. Stress controlled low cycle fatigue tests were conducted with and without hold times. The hold times were introduced to simulate creep phenomenon. The creep damage was found to be responsible for a large reduction in life of the sample tested with hold time.

Observation of Etch-Pits and LAGB Configurations During Ambient Creep of Ti-6Al-4V Alloy

The present work describes the microstructural features of alloy Ti-6Al-4V during constant stress creep at ambient temperature. Samples tested at 800 and 900 MPa stress levels exhibit the presence of etch-pits and/or voids. The ambient creep strain increases with an increase in applied stress due to higher strain rate sensitivity at higher stresses. A high density of low-angle grain boundaries is noticed in and around etch-pits in the creep-tested specimens due to occurrence of slip. The inverse pole figure obtained by EBSD indicates prismatic texture as the main deformation component in the creep-tested specimens.

Generalization of Semi-Projective Modules

In this paper characterization of pseudo M -p-projective modules and quasi pseudo principally projective modules are given and discussed the various properties of it. It is proved that a pseudo M -pprojective module is Hopfian iff M=N is Hopfian, for each fully invariant small submodule N of M . It is also provided the sufficient condition for pseudo M -p-projective module to be discrete. Finally several equivalent conditions are given for a quasi pseudo principally projective module to have the finite exchange property.