Raghu V Prakash

Thin-walled metal tubes are extensively used in aircraft and automobile industries as energy absorbers during collision. When a thin wall tube is subjected to an axial compressive load, lobes are formed sequentially and each lobe undergoes a large plastic deformation without any cracking; this is referred to as progressive buckling. The focus of this paper is to study the displacement and strain fields near the buckled zone of a thin-walled square tube both experimentally and numerically. Quasi-static test was conducted on thin-walled square tubes made of aluminum alloy AA-6063 and the displacement and full-field strain was measured using Digital Image Correlation (DIC) technique. It is noted that while one face of the tube undergoes tensile deformation, the adjacent faces undergo compressive deformation. The strain levels exceed the fracture strain obtained during a tensile test. Strain estimated from DIC was found to be in good agreement with the strain gauge measurements at far field. Further, strain estimations obtained through numerical simulations showed a reasonable agreement with DIC measurements.

Polymer composites have a characteristic, composition specific visco-elastic property which influences the damage progression during fatigue cycling. While some researchers have studied the time dependent constitutive response of polymer composites during the first cycle of fatigue loading, very few have experimentally investigated the dependence of visco-elastic response of built-up polymer composite materials at various stages of fatigue cycling [1]. Our earlier studies on fatigue response of polymer composites focused primarily on the stiffness degradation as a function of applied cycles of loading, which represents the gross response of the material [2]. While doing such an experiment, complimentary experimental techniques to measure the temperature evolution was attempted through the use of infrared thermal imaging technique, which gave some insight into the change in temperature response as a function of fatigue cycling. However, there was no systematic measurement of creep and stress relaxation response of the composite material as a function of induced fatigue damage. The present paper describes the results of creep and stressrelaxation obtained during uni-axial fatigue loading of a hybrid polymer composite material. For this purpose, a woven carbon fiber mat was chosen as the synthetic fiber and Flax fiber in the unidirectional form was chosen as the natural fiber that is laid between the two layers of woven carbon fiber mat. Epoxy LY 556 and hardener Araldite® was used for building up of composite laminate by hand-lay-up technique. Dog-bone shaped tensile specimens with a gage width of 13 mm and gage length of 57 mm were extracted from the 250 x 250 mm sq. plate laminate of 2.1 mm thickness using a numerical controlled milling machine. The specimens were tested at 35% of their median tensile strengths under fatigue at a positive stress ratio (Pmin/Pmax) of 0.1 in tension-tension loading. Prior to start of fatigue loading, the specimens were held in load control and the strain in the gage length was measured for understanding the creep response over 2500 seconds. For stress-relaxation characterization, the specimens were held in extensometer control over a period of 2500 sec. The creep and stress relaxation tests were carried out after periodic intervals of fatigue cycling. It was observed that in the case of un-impacted specimens, the creep rate is consistent with the stiffness variation, which in turn, is dependent on the number of fatigue cycles-till it showed signs of de-lamination. Thereafter it was governed by the woven synthetic fiber response. Similarly, the stress relaxation response was found to decrease with increasing fatigue cycles. In case of impacted specimens, the local deformation had a prominent role in terms of creep and stress relaxation response.

This paper presents the results of corrosion fatigue crack growth behaviour of a Ni-Cr-Mn steel commonly used in marine applications. The effect of mechanical variables such as frequency and load ratio on fatigue crack growth rate at various stages has been studied using compact tension (C(T)) specimens along the rolling direction of steel plate under 3.5% saturated NaCl aqueous environment. The significance of crack closure on corrosion fatigue, and the validity of Elber's empirical linear crack closure model with the ASTM compliance offset method have been examined. Fatigue crack growth rate is higher and threshold stress intensities are lower in aqueous environment compared to the lab air conditions. It is also observed that the crack growth rate increases at lower frequencies. The higher stress ratio promotes the crack growth. The effect of oxidization and corrosion pit formation is very less as the stress ratio is increased. It is observed that as stress ratios are increased, the Elber's crack closure model agrees well with the crack closure estimated by the ASTM compliance offset method for tests conducted at 5Hz frequency compared to tests conducted at 1Hz in corrosive environment.

Extraction of mechanical properties of in-service materials through small specimen test techniques has become attractive in the recent years. Of the available small specimen test techniques, automated ball indentation (ABI) and small punch test (SPT) methods have proved to be more promising. These test methods are basically non-destructive in nature and are proficient enough to extract the flow properties of the materials using small volume specimen. In this work, tensile properties of a pressure vessel steel (P12) have been estimated through ABI and SPT. The objective is to compare the capability of these test methods in determining the tensile properties. The influence of lubrication (between the indenter and the specimen) on the ABI response is also investigated. The ABI response is found to be similar and the effect on the tensile properties was under 2 %. The tensile properties estimated from ABI and SPT are found to be in good agreement with conventional tensile test results. Nevertheless, in case of SPT, the error in the estimation of yield strength and ultimate tensile strength using empirical correlations is significantly high. However, the use of analytical formulations to convert the SPT load–displacement response to stress–strain curve are found to be reliable, since the error in the estimated properties is considerably less. The ABI process is numerically simulated to study the stress–strain field beneath the indenter. The maximum strain occurs at the edge of the contact indicating the material displacement along the radial direction. The plastic zone beneath the indenter resemble hemispherical shape which is in agreement with the expanding cavity model. The nature of stress changes from compressive (right below the indentation axis) to tensile at the edge of contact. This indicates that radial cracks may initiate on the specimen and propagate outwards. The pile-up is significantly higher in the case of frictionless contact between the indenter and the specimen but found to converge for a value of around 0.2.

Fatigue properties of materials is an important input while estimating the residual life of critical components. Fatigue data (stress vs. cycles or strain vs. cycles or fatigue crack growth rate data) are used to predict the residual life. One of the shortcomings of this method is that it relies on data generated from virgin material or surveilance coupons which have been exposed to the harsh environment over a period of time. Often the quantity of material available for fatigue data is small and being probabilistic in nature, fatigue data requires multiple specimens to be tested at any given stress/strain levels. This has prompted us to develop test procedures to determine the fatigue data of materials from small volume of material. In this paper, we present the results of cyclic ball indentation test method as well as cyclic small punch test method that is used to generate the fatigue data at different stress levels. There are several fine details relating to these test technique - viz., establishing a equivalent damage criteria for failure life with standard LCF/HCF test specimens. Apart from this, several variables that influence the testing process needs to be considered. This paper briefly reviews the viability of using miniature specimen test techniques, particularly cyclic ball indentation and cyclic small punch testing for extracting the fatigue data, based on the author's previous work. It is shown that, both the test techniques are capable of detecting and quantifying the prior fatigue damage in the materials.

This paper presents the influence of preloading on a) the contact tractions, b) slip levels and c) stresses at the dovetail blade-disc interface of an aero-engine through a three-dimensional (3D) finite element (FE) modeling and analysis. The preloading is applied by an interference fit at the dovetail interface and the bulk loading is applied through the rotational speed of rotor. Preloading at the dovetail interface reduces the peak contact pressure developed due to bulk loading up to 35%, and reduces the peak contact pressure and stress difference between top and bottom contact edges. Increasing the level of preloading reduces the cyclic stress amplitude at the interface up to certain values of preload and as a consequence, an improvement in fatigue life could be expected. Fretting damage, due to vibration and wind milling effect during engine ground condition, can be minimized by preloading the dovetail interface.

This paper presents the effect of addition of nano-Alumina particles on the fracture properties of glass fiber reinforced plastic (GFRP) composite laminates. Epoxy resin is the most commonly used polymer matrix for advanced composite materials in view of its ability to adhere to a wide variety of fillers; on curing, they provide excellent stiffness and dimensional stability. However the highly cross linked epoxy often behaves undesirably brittle, because, plastic deformation is constrained, leading to poor resistance to crack initiation and propagation. Hence it is necessary to improve the toughness without sacrificing the other important mechanical and thermal properties. In this work, glass-fiber-reinforced composite with nano-Alumina modified epoxy matrix was successfully produced with a hand lay- up process and characterized by EDAX and XRD technique for its composition. The experimental results show that the composites exhibited improvements in inter-laminar toughness values (GIC and GIIC) along with improvements in other mechanical properties, especially in toughness related properties. The Mode-I interlaminar fracture toughness for 2 phr (per hundred gram resin) nano-Alumina was 2.5 times higher than that of unfilled epoxy and the Mode-II inter-laminar fracture toughness improved by 37 %. The significant increase in Mode-I fracture toughness and improvement in Mode II inter-laminar fracture toughness resulting from the nano-particle modification, indicates a pronounced increase in matrix toughness. Impact tests suggest that the energy absorption capability of the GFRP considerably improved with the addition of equi-Axed nano-Alumina particles with epoxy resin. The laminate and fracture surface morphology analysis was done to understand the fracture and toughening mechanisms behind these property changes. The bending characteristic such as ILSS and Flexural properties recorded the maximum improvements of 14 % and 17 % respectively for the laminate with nano-Alumina modified epoxy. A significant improvement in flexural modulus of over 37 % was noticed with respect to unmodified epoxy. The experimental results show that the tensile modulus exhibited 15% improvement compared to laminate without nano-Alumina, while, a modest change was observed in the tensile strength. Copyright © 2012 by ASME.

Occupant safety has become increasingly important in the recent times. At the instance of an accident or collision, structures with high energy absorption can provide better occupant safety. Thin-walled tubes are widely used as energy absorbers in automobiles and other structures. In the present work, crashworthiness characteristics of double wall empty and double wall foam filled tubes are investigated. Thin wall extruded aluminum square tubes are used in this study. Polymer foam of three different densities, viz., 40 kg/m3, 80 kg/m3, 140 kg/m3 was used as filler material between the two tubes to fabricate a double wall foam filled tube. Both parallel and diamond configurations were considered for double walled empty and foam filled configurations.. All the specimens were compressed at a displacement rate of 100 mm/min. Crushing of different configurations was numerically analyzed using nonlinear finite element tool LS-Dyna®. In double wall empty configuration, diamond arrangement absorbed more energy compared to parallel due to the interaction between inner and outer tubes. Results indicate that energy absorption increases with the filling of foam. Compared to double wall tubes, the maximum increase in energy absorption of ~ 50% is observed in foam filled tubes. Using Computed Tomography (CT) scan of specimens, it is observed that foam filling alters the crushing behavior of the inner and outer tubes.

Synthetic fibers are used for critical performance applications of marine rope and cable industries, apart from military applications. The high strength-to-weight ratio and corrosion resistance offered by polymeric synthetic fibers makes them superior alternatives to steel wire ropes particularly in marine applications. These marine ropes and cables are subjected to a complex history of static and cyclic mechanical loading during service, leading to sudden and unexpected failure. The presence of corrosive sea water medium during service adds to the complexity of the problem. Thus, it is essential to study the tensile and fatigue performance of these synthetic fibers in the presence of sea water. In this study, experiments were conducted to examine the effect of marine environment on Vectran (Liquid Crystal Polymer) yarns. The first set of experiments analyzed the tensile strength degradation of Vectran yarns when exposed to simulated sea water for two months. The experiments were performed on Parallel continuous filament yarns and Twisted continuous filament yarns (0.5 twists per centimetre) and the results compared. In the second set of experiments, Vectran yarns were subjected to load controlled fatigue in dry state and continuously wetted state under tension-tension loading at a nominal frequency of 0.1-0.5 Hz. Stress vs. Number of cycles graphs were plotted to compare the fatigue performance in dry and wet conditions. Fatigue experiments were performed on Parallel and Twisted yarns to study the combined effect of twisting and wetting. Scanning Electron Microscopy was used to observe filament surface and failure mechanism. The results indicate that, twisting the continuous filament yarns improves both tensile strength and fatigue performance. Sea water exposure degrades the tensile strength of Parallel yarns. Twisted yarns show no such degradation. The fatigue performance of Parallel yarns appears to be higher in dry state compared to wet state. The fatigue performance of Twisted yarns seems to increase with wetting. SEM images show that failure of filaments is by severe fibrillation.

This paper presents the influence of centrifugal (CF) loading of aero-engine rotor disc on fretting variables at a dovetail interface. A detailed investigation is carried out on the fretting variables such as contact traction, slip, and contact stress at macroscopic level. Three-dimensional (3D) finite element (FE) analysis approach is used for prediction of fretting variables. The study is carried out for frictionless and friction interface (μ=0.7) for the case of Titanium alloy. The slip level increase of about 48 % and 110% is observed for frictional and frictionless condition respectively, due to rotational effect of disc. Different contact traction ratio distribution over the interface is also observed with the CF load of rotating disc. The study suggests the consideration of centrifugal loading effect is important for improved prediction of critical fretting variables, as they would impact the evaluation fretting fatigue and wear characteristics at the interface. Copyright © 2012 by ASME.