Raghu V. Prakash

Fatigue damage characterization using surface acoustic wave nonlinearity in aluminum alloy AA7175-T7351

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/A12) 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 ...

Influence of Yield Criteria in the Prediction of Strain Distribution and Residual Stress Distribution in Sheet Metal Formability Analysis for a Commercial Steel

In this study, Hill 48 and Barlat 89 yield criteria are evaluated by simulating the Limiting Dome Height test of the Numisheet 96 benchmark problem [1] for a commercial D-grade steel and the results are validated with experimental data. Three strain modes, viz. biaxial, plane strain, and uniaxial strain modes, are used in the simulation studies. The yield criteria are evaluated based on the strain distribution in forming and residual stress distribution after springback. The results show that the Hill 48 and Barlat 89 yield criteria do not show a good match with predicted results of strain distribution in the plane strain mode. The difference between predicted results and experiments can be attributed to the combined influence of friction coefficient and draw bead boundary condition. The prediction of residual stress distribution differed considerably with the yield criteria. The residual stress predicted is validated for selected locations using X-ray diffraction techniques.

Study of Damage and Fracture Toughness Due to Influence of Creep and Fatigue of Commercially Pure Copper by Monotonic and Cyclic Indentation

Fracture toughness is the ability of a component containing a flow to resist fracture. In the current study, the Ball indentation (BI) test technique, which is well acknowledged as an alternative approach to evaluate mechanical properties of materials due to its semi-nondestructive, fast, and high accurate qualities is used to estimate damage and the fracture toughness for copper samples subjected to varying levels of creep and fatigue. The indentation fracture toughness shows the degradation of Cu samples when they are subjected to different creep conditions. Axial fatigue cycling increases the strength at the mid-gauge section compared to other regions of the samples due to initial strain hardening. The advancement of indentation depth with indentation fatigue cycles experiences transient stage, i.e., jump in indentation depth has been observed, which may be an indication of failure and followed by a steady state with almost constant depth propagation with indentation cycles.

Influence of environmental and geometric parameters on the behavior of fastener joints in advanced composites

Single- and double-shear bearing strength of mechanically fastened joints in carbon fiber composite (CFC) systems have been studied and the analysis of results presented. CFC laminates made from unidirectional prepreg tapes as well as bidirectional fabric prepregs were tested in as-received condition at room temperature and under hot/wet environmental conditions after hygrothermal aging. The influence of geometric parameters like the specimen width-to-hole diameter ratio (w/d) and the specimen thickness-to-hole diameter ratio (t/d) on the bearing strength has been investigated. Bearing strength tests were performed with three different fastener bolt materials, and, in all, over 300 specimens were tested. Bearing stresses were evaluated at ultimate failure, at two-percent hole deformation, at onset of nonlinearity, and at first load drop; and two percent offset bearing strength has been selected for the purpose of comparison of data. The influence of lamina configuration, mode of loading, hole tolerance, and fastener bolt material on bearing strength has been investigated. The degradation of bearing strength in hygrothermally aged CFC specimens under hot/wet environmental conditions has been found to be around 25-30 percent, compared to the room temperature values.

Integrating effect of forming in fatigue life prediction: review of present scenario and challenges

Plastic deformation of sheet metals during forming induces complex residual stresses in the component owing to its geometry and material anisotropy. The residual stresses affect the durability of the components. Fatigue life estimation incorporating the forming-induced residual stresses is limited. In the present work, the state-of-the-art in modelling sheet metal forming, residual stress estimation and fatigue life prediction are reviewed. The challenges in integrating the residual stress owing to forming and fatigue life prediction are brought out. The parameters influencing the sheet metal forming, residual stresses and fatigue are discussed to emphasize the complexity of the present problem.

Estimation of Corrosion Fatigue-Crack Growth through Frequency Shedding Method

Corrosion fatigue-crack growth characteristics are important for the design of marine and off-shore structures. Design of critical components requires data on fatigue-crack growth rate at very low frequencies of the order of 10−2 to 10−3 Hz. Experiments at low frequencies pose practical difficulties of enormous test duration. To address this, it is proposed to estimate corrosion crack growth characteristics using a frequency shedding method where the frequency is shed with crack advance using an exponential law. Fatigue-crack growth rate tests have been conducted on Ni–Mn–Cr steel at a constant ΔK range of 18 MPa√m (lower Paris regime) under lab air conditions as well as 3.5 % NaCl solution. Crack growth rate data plotted as a function of test frequency presents a straight-line trend in log–log scale for a frequency range of 1–0.1 Hz; however, there is a change in trend when the frequencies are dropped further, which could be due to domination of corrosion mechanism. To understand the role of crack closure, crack closure estimates were obtained at periodic intervals of crack length and the effective stress intensity graphs suggest acceleration in crack growth rate due to corrosion as the frequency is reduced.

A Study of Naturally Initiating Notch Root Fatigue Cracks Under Spectrum Loading

The kinetics of short cracks at notches were studied in 2014-T6511 Al-Cu alloy extruded bar stock under modified versions of FALSTAFF and TWIST load spectra. Experiments were conducted on test coupons of different thickness and at different stress levels. The effects of these and other parameters on the density and growth rate patterns of multiple cracks were investigated. The quality of fractographic crack growth rate measurements was found to be superior to surface replica technique. It was also observed that artificially initiated fatigue cracks grow slower than naturally initiated ones. Interaction of naturally initiating multiple cracks was quantified.

A Study of Fatigue Crack Growth in Lugs Under Spectrum Loading

Optical fractography and special load sequencing were used to characterize the growth of naturally initiating fatigue cracks under FALSTAFF spectrum loading in lugs with13; bushes fitted to varying degrees of interference. The same technique was used to obtain a stress intensity calibration under constant am plitude.loading. This calibration, corn- bined with crack growth analysis using the hysteretic closure concept and Rainflow cycle count corelated well with experimental observations of growth rate down to crack depth of 100 microns. Available analytical stress intensity solutions do not appear to explain observed high crack growth rates, particularly as crack size diminishes to part through dimensions.

Increasing Hot Rolling Mass of Steel Sheet Products

The article exposes contemporary materials and structures for metallurgy. Feasible increase of dynamic forces on the rolling machinery during rolling of billets with masses up to 20 t (Stand No.1 of CWM 1700 HR, «U.S. Steel Košice», Košice, Slovakia, and PJSC «Illich MMPP», Mariupol, Ukraine) is discussed. It is proved that weight of billet, velocities of metal delivery to working rolls and rolling significantly influence the dynamic loads during metal biting. The technical solutions are suggested which would allow the steady rolling process of billets with masses up to 20 t and prevent accidental failures of frame parts, chocks and main lines.

An Overview of Fretting Aspects Relating to Aero-Engine Dovetail Attachment

An aero-engine fan and compressor rotor design involves dovetail interface between the blade and the disc, which is critical, as early crack initiation occurs at the interface due to fretting. This paper reviews the different methodologies adopted to understand the fretting behaviour at the interface: experimental, analytical and numerical methods. The effects of geometry and material on fretting behaviour are discussed. The results of a 3-Dimensional finite element analysis of a typical dovetail interface with skew, cone and twist angle are presented. Areas that require attention are highlighted at the concluding section of this paper.