Developments in aviation posed requirement of lightweight, high strength and highly damage tolera... more Developments in aviation posed requirement of lightweight, high strength and highly damage tolerant materials. Sandwich structured composites, fulfilling these requirements became the first choice for many applications including structural components for ground transport and marine vessels. Sandwich composites are a special class of composite materials which are widely used because of their high specific strength and bending stiffness. Lower density of these materials makes them well suited for marine and aerospace applications. Syntactic foams are hollow particle filled core materials that have recently emerged as attractive material for use in applications requiring low weight, low moisture absorption and high insulation properties. Quasistatic and dynamic properties of these syntactic foams are very important as these materials are used in various important structural applications in aerospace and marine industry. Even though various destructive techniques such as quasi-static co...
Abstract Laser Shock Peening (LSP) is one of the emerging surface treatment processes being consi... more Abstract Laser Shock Peening (LSP) is one of the emerging surface treatment processes being considered for inducing beneficial compressive surface residual stresses in fatigue critical components. Owing to the ease in handling multiple parameters during optimization of process parameters, simulation based parameterization studies using finite element (FEM) based numerical models are widely gaining importance. Most of the LSP modeling performed so far considered infinite elements in both thickness and lateral directions. However, infinite elements in thickness direction would neglect the deleterious effect of shock wave back reflections for certain sample thicknesses. These back reflections have been reported to result in formation of subsurface cracks in the specimen. Therefore, in this study, using an alternative modeling strategy, effect of thickness on the back reflection of shock waves and its subsequent effect on residual stresses induced are discussed. A 2-D axi-symmetric model with infinite elements in lateral direction and finite elements in thickness direction is developed to simulate a single spot LSP process using ABAQUS/CAE FEM package. It is found that there exists a critical material thickness depending on spot diameter below which the effects of back reflection are predominant.
Single crystal elastic constants (SCEC) of materials occupies an important role in the estimation... more Single crystal elastic constants (SCEC) of materials occupies an important role in the estimation of Young’s modulus (E), bulk modulus (G) and Poisson’s ratio (ν) of materials. SCEC are usually determined destructively by tensile and shear loading a single crystal specimen. However, one of the major limitations in that procedure happens to be the availability of single crystal (in case of destructive testing). Hence, in this study, a nondestructive procedure has been developed to estimate SCEC from polycrystalline specimens. Ultrasonic longitudinal and shear velocities, longitudinal attenuation coefficient and backscattered grain noise (FOM) are measured on pure Copper (forged condition) and pure Aluminium (virgin ingot condition) specimens. In addition to the experimental measurement, ultrasonic parameters are also estimated analytically using the existing relationships involving SCEC and probabilistic grain size. SCEC were further estimated by minimizing the difference between exp...
Probability of detection (POD) , a measure of the reliability of non-destructive testing (NDT) te... more Probability of detection (POD) , a measure of the reliability of non-destructive testing (NDT) techniques, occupies a prominent role in the damage tolerance lifingmethodology for aeroengines. POD is usually dependent on material and geometry of the component, crack characteristics, environment, etc. Ty pi ally, POD curves are plotted for both quantitative as well as qualitative NDT techniques using procedures mentioned in MIL-HDBK1823A (2009). Moreover, POD curves have to be generated for a specific combination, i.e., t ype of crack, location, material and NDT technique and cannot be generalized. This complexity has led to developmen t of model assisted POD (MAPOD). The current study, discusses the MAPOD procedure adopted for d etection of embedded elliptical cracks using ultrasonic testing in a 2-d symmetric plane strain FEM model in freque ncy domain using COMSOL Multiphysics software. As fatigue cracks are expected to follow lognormal distribu tion, model assisted ultrasonic resp...
Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT... more Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT) techniques is essential for implementing damage tolerant (DT) methodology for aero-engines. This POD is usually established with the help of a large number of service expired aero-engine components containing several fatigue cracks. In the absence of such components, artificial defects such as electrical discharge machining (EDM) notches or starter cracks were explored. However, such artificial defects would not meet the key features such as tightness of the fatigue cracks and the possible oxidation in the crack opening and thus, limiting their usage. Therefore, in the current study, an innovative approach of generating fatigue cracks at 650 °C (~ typical aero-engine service temperatures) with key high temperature service degradation aspects of oxidation and fatigue cracking is demonstrated for the first time using Gleeble® test system. Further, POD is estimated by inspecting these lab...
American Society of Mechanical Engineers (Publication) NDE, 2004
ABSTRACT Ultrasonic imaging is a non-destructive evaluation technique, which is used to obtain de... more ABSTRACT Ultrasonic imaging is a non-destructive evaluation technique, which is used to obtain density profile, phase distribution and three-dimensional profiles of cracks and defects in a material. Although this technique is used for a variety of metals and non-metals, it is difficult to use it for testing of porous materials and foams due to high attenuation of ultrasonic waves in these materials. Syntactic foams are hollow particle filled composites that have recently emerged as attractive material for use in applications requiring low weight, low moisture absorption and high insulation properties. The present paper focuses on determining the attenuation coefficient in syntactic foams and its correlation with porosity distribution. Eight types of foam samples are tested in the study. A combination of four types of microballoons and two types of rubber particles is used. Volume fractions of microballoons and rubber particles are maintained at 0.63 and 0.02, respectively, in all samples. Pulse Echo ultrasonic test method is used and results are compared to determine the effect of constituent particles on the ultrasound signal attenuation. Coefficient of attenuation is observed to increase with decrease in density of foam samples and with decrease in size of rubber particles.
Successful implementation of damage tolerance methodology requires estimation of the reliability ... more Successful implementation of damage tolerance methodology requires estimation of the reliability of the currently practiced non-destructive testing ( NDT) techniques in the aero-engine industry. Reliability of the NDT techniques is measured in terms of probability of detection (POD). POD is dependent on m aterial, geometry along with size, shape, location and type of the defect. Experime ntal estimation of POD usually requires large number of service expired aero-engine turbine disks. Alternatively, the POD studies can also be carried out usin g laboratory induced samples (as per MIL-HDBK-1823A) containing various sizes o f defects. In the current study, 55 mm x 10 mm x 10 mm nickel based super alloy s mples were extracted from a mid-life service expired turbine disc. A 2 mm radius circular notch was created at the centre of the specimen so as to represent the true stress concentration factor present near the bolt hole location of a turbine disc. Sa mples were subjected to repeated ...
In order to successfully implement Damage Tolerance (DT) methodology for aero-engines, Non-Destru... more In order to successfully implement Damage Tolerance (DT) methodology for aero-engines, Non-Destructive Testing (NDT) techniques are vital for assessing the remaining life of the component. Probability of Detection (POD), a standard measure of NDT reliability, is usually estimated as per MIL-HDBK-1823A standard. Estimation of POD of any NDT technique can be obtained by both experimental and model assisted methods. POD depends on many factors such as material, geometry, defect characteristics, inspection technique, etc. These requirements put enormous limitations on generating experimental POD curves and hence, Model Assisted Probability of Detection (MAPOD) curves are currently in vogue. In this study, MAPOD approaches were demonstrated by addressing various issues related to selection of crack sizes distribution, challenges involved in censoring and regression, estimation of distribution parameters, etc. Ultrasonic testing on volumetric defects has been identified as a platform to d...
Insight - Non-Destructive Testing and Condition Monitoring
Assessing the reliability of non-destructive testing (NDT) techniques in detecting in-service fat... more Assessing the reliability of non-destructive testing (NDT) techniques in detecting in-service fatigue cracks is vital for ensuring the structural integrity of aero engines. However, the requirement of a large number of in-service failed components with numerous initiating fatigue cracks makes it a cost-intensive methodology. Hence, laboratory samples with electrical discharge machined (EDM) notches representing fatigue cracks have also been used for NDT reliability studies. However, probability of detection (POD), a measure of NDT reliability, is usually a function of all crack dimensions rather than only length. This limits the applicability of EDM notches (minimum width of notch ~0.3 mm) as artificial fatigue cracks for POD studies. The current study demonstrates the methodology of generating cracks under laboratory conditions in nickel-based superalloy samples (representative aero engine material), mimicking in-service conditions such as crack tightness as low as 1 μm, crack tortuosity, transgranular nature, crack branching and multiple initiation sites of cracks. POD curves generated using these samples are demonstrated and the feasibility of this approach is discussed.
Developments in aviation posed requirement of lightweight, high strength and highly damage tolera... more Developments in aviation posed requirement of lightweight, high strength and highly damage tolerant materials. Sandwich structured composites, fulfilling these requirements became the first choice for many applications including structural components for ground transport and marine vessels. Sandwich composites are a special class of composite materials which are widely used because of their high specific strength and bending stiffness. Lower density of these materials makes them well suited for marine and aerospace applications. Syntactic foams are hollow particle filled core materials that have recently emerged as attractive material for use in applications requiring low weight, low moisture absorption and high insulation properties. Quasistatic and dynamic properties of these syntactic foams are very important as these materials are used in various important structural applications in aerospace and marine industry. Even though various destructive techniques such as quasi-static co...
Abstract Laser Shock Peening (LSP) is one of the emerging surface treatment processes being consi... more Abstract Laser Shock Peening (LSP) is one of the emerging surface treatment processes being considered for inducing beneficial compressive surface residual stresses in fatigue critical components. Owing to the ease in handling multiple parameters during optimization of process parameters, simulation based parameterization studies using finite element (FEM) based numerical models are widely gaining importance. Most of the LSP modeling performed so far considered infinite elements in both thickness and lateral directions. However, infinite elements in thickness direction would neglect the deleterious effect of shock wave back reflections for certain sample thicknesses. These back reflections have been reported to result in formation of subsurface cracks in the specimen. Therefore, in this study, using an alternative modeling strategy, effect of thickness on the back reflection of shock waves and its subsequent effect on residual stresses induced are discussed. A 2-D axi-symmetric model with infinite elements in lateral direction and finite elements in thickness direction is developed to simulate a single spot LSP process using ABAQUS/CAE FEM package. It is found that there exists a critical material thickness depending on spot diameter below which the effects of back reflection are predominant.
Single crystal elastic constants (SCEC) of materials occupies an important role in the estimation... more Single crystal elastic constants (SCEC) of materials occupies an important role in the estimation of Young’s modulus (E), bulk modulus (G) and Poisson’s ratio (ν) of materials. SCEC are usually determined destructively by tensile and shear loading a single crystal specimen. However, one of the major limitations in that procedure happens to be the availability of single crystal (in case of destructive testing). Hence, in this study, a nondestructive procedure has been developed to estimate SCEC from polycrystalline specimens. Ultrasonic longitudinal and shear velocities, longitudinal attenuation coefficient and backscattered grain noise (FOM) are measured on pure Copper (forged condition) and pure Aluminium (virgin ingot condition) specimens. In addition to the experimental measurement, ultrasonic parameters are also estimated analytically using the existing relationships involving SCEC and probabilistic grain size. SCEC were further estimated by minimizing the difference between exp...
Probability of detection (POD) , a measure of the reliability of non-destructive testing (NDT) te... more Probability of detection (POD) , a measure of the reliability of non-destructive testing (NDT) techniques, occupies a prominent role in the damage tolerance lifingmethodology for aeroengines. POD is usually dependent on material and geometry of the component, crack characteristics, environment, etc. Ty pi ally, POD curves are plotted for both quantitative as well as qualitative NDT techniques using procedures mentioned in MIL-HDBK1823A (2009). Moreover, POD curves have to be generated for a specific combination, i.e., t ype of crack, location, material and NDT technique and cannot be generalized. This complexity has led to developmen t of model assisted POD (MAPOD). The current study, discusses the MAPOD procedure adopted for d etection of embedded elliptical cracks using ultrasonic testing in a 2-d symmetric plane strain FEM model in freque ncy domain using COMSOL Multiphysics software. As fatigue cracks are expected to follow lognormal distribu tion, model assisted ultrasonic resp...
Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT... more Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT) techniques is essential for implementing damage tolerant (DT) methodology for aero-engines. This POD is usually established with the help of a large number of service expired aero-engine components containing several fatigue cracks. In the absence of such components, artificial defects such as electrical discharge machining (EDM) notches or starter cracks were explored. However, such artificial defects would not meet the key features such as tightness of the fatigue cracks and the possible oxidation in the crack opening and thus, limiting their usage. Therefore, in the current study, an innovative approach of generating fatigue cracks at 650 °C (~ typical aero-engine service temperatures) with key high temperature service degradation aspects of oxidation and fatigue cracking is demonstrated for the first time using Gleeble® test system. Further, POD is estimated by inspecting these lab...
American Society of Mechanical Engineers (Publication) NDE, 2004
ABSTRACT Ultrasonic imaging is a non-destructive evaluation technique, which is used to obtain de... more ABSTRACT Ultrasonic imaging is a non-destructive evaluation technique, which is used to obtain density profile, phase distribution and three-dimensional profiles of cracks and defects in a material. Although this technique is used for a variety of metals and non-metals, it is difficult to use it for testing of porous materials and foams due to high attenuation of ultrasonic waves in these materials. Syntactic foams are hollow particle filled composites that have recently emerged as attractive material for use in applications requiring low weight, low moisture absorption and high insulation properties. The present paper focuses on determining the attenuation coefficient in syntactic foams and its correlation with porosity distribution. Eight types of foam samples are tested in the study. A combination of four types of microballoons and two types of rubber particles is used. Volume fractions of microballoons and rubber particles are maintained at 0.63 and 0.02, respectively, in all samples. Pulse Echo ultrasonic test method is used and results are compared to determine the effect of constituent particles on the ultrasound signal attenuation. Coefficient of attenuation is observed to increase with decrease in density of foam samples and with decrease in size of rubber particles.
Successful implementation of damage tolerance methodology requires estimation of the reliability ... more Successful implementation of damage tolerance methodology requires estimation of the reliability of the currently practiced non-destructive testing ( NDT) techniques in the aero-engine industry. Reliability of the NDT techniques is measured in terms of probability of detection (POD). POD is dependent on m aterial, geometry along with size, shape, location and type of the defect. Experime ntal estimation of POD usually requires large number of service expired aero-engine turbine disks. Alternatively, the POD studies can also be carried out usin g laboratory induced samples (as per MIL-HDBK-1823A) containing various sizes o f defects. In the current study, 55 mm x 10 mm x 10 mm nickel based super alloy s mples were extracted from a mid-life service expired turbine disc. A 2 mm radius circular notch was created at the centre of the specimen so as to represent the true stress concentration factor present near the bolt hole location of a turbine disc. Sa mples were subjected to repeated ...
In order to successfully implement Damage Tolerance (DT) methodology for aero-engines, Non-Destru... more In order to successfully implement Damage Tolerance (DT) methodology for aero-engines, Non-Destructive Testing (NDT) techniques are vital for assessing the remaining life of the component. Probability of Detection (POD), a standard measure of NDT reliability, is usually estimated as per MIL-HDBK-1823A standard. Estimation of POD of any NDT technique can be obtained by both experimental and model assisted methods. POD depends on many factors such as material, geometry, defect characteristics, inspection technique, etc. These requirements put enormous limitations on generating experimental POD curves and hence, Model Assisted Probability of Detection (MAPOD) curves are currently in vogue. In this study, MAPOD approaches were demonstrated by addressing various issues related to selection of crack sizes distribution, challenges involved in censoring and regression, estimation of distribution parameters, etc. Ultrasonic testing on volumetric defects has been identified as a platform to d...
Insight - Non-Destructive Testing and Condition Monitoring
Assessing the reliability of non-destructive testing (NDT) techniques in detecting in-service fat... more Assessing the reliability of non-destructive testing (NDT) techniques in detecting in-service fatigue cracks is vital for ensuring the structural integrity of aero engines. However, the requirement of a large number of in-service failed components with numerous initiating fatigue cracks makes it a cost-intensive methodology. Hence, laboratory samples with electrical discharge machined (EDM) notches representing fatigue cracks have also been used for NDT reliability studies. However, probability of detection (POD), a measure of NDT reliability, is usually a function of all crack dimensions rather than only length. This limits the applicability of EDM notches (minimum width of notch ~0.3 mm) as artificial fatigue cracks for POD studies. The current study demonstrates the methodology of generating cracks under laboratory conditions in nickel-based superalloy samples (representative aero engine material), mimicking in-service conditions such as crack tightness as low as 1 μm, crack tortuosity, transgranular nature, crack branching and multiple initiation sites of cracks. POD curves generated using these samples are demonstrated and the feasibility of this approach is discussed.
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