Gyaneshwar Tandon - Academia.edu (original) (raw)

Papers by Gyaneshwar Tandon

Volume 1: Development and Characterization of Multifunctional Materials; Modeling, Simulation and Control of Adaptive Systems; Structural Health Monitoring, 2012

In the present study, a series of novel linear polyaspartimide-based silane endcapped (cross-link... more In the present study, a series of novel linear polyaspartimide-based silane endcapped (cross-linked) polymers are synthesized using 4-4′ bismaleimidodiphenylmethane, Jeffamine D-400 (BMI-JA-400), and (3-Aminopropyl) trimethoxysilane. To add strength to these systems, the trimethoxysilane moiety is cross-linked with the addition of water to create a thermosetting material with both improved toughness and variable cross-link densities. Thermal analysis is done to evaluate the developed shape-memory polymer (SMP) resin for composite processing feasibility. The solvent content in the resin and thermal stability is monitored using thermogravimetric analysis (TGA) while advanced rheometric expansion system (ARES) with parallel plate geometry is used to measure viscosity variation with temperature. The resin BMI-JA-400-Si-70/30 is chosen for making the composite based on its viscosity, weight change, and kinetic results. Differential scanning calorimetry (DSC) is performed to determine the...

Conference Proceedings of the Society for Experimental Mechanics Series, 2016

In Fused Deposition Modeling (FDM), filaments of heated thermoplastic are extruded from a tip tha... more In Fused Deposition Modeling (FDM), filaments of heated thermoplastic are extruded from a tip that moves in a raster-like motion. Depending on filament material properties and the processing parameters, the extruded beads and the fusion-bond between them could result in anisotropic properties (particularly with regard to strength). In this work, the focus is on characterizing the interlaminar fracture toughness of coupons produced by FDM with varying road-to-road offsets producing positive, zero and negative air gaps between neighboring roads resulting in coupons of varying density (void content). The resulting bond line formation is assessed through measuring and analyzing changes in the mesostructure and the contact attained at the interfaces between the adjoining roads. The bond quality is assessed by evaluating plane-strain fracture toughness using compact tension specimens. Results demonstrate an approximate five-fold increase in fracture toughness with varying air gaps.

Mechanics of Time-Dependent Materials, 2011

Shape memory polymers have attracted great interest in recent years for application in reconfigur... more Shape memory polymers have attracted great interest in recent years for application in reconfigurable structures (for instance morphing aircraft, micro air vehicles, and deployable space structures). However, before such applications can be attempted, the mechanical behavior of the shape memory polymers must be thoroughly understood. The present study represents an assessment of viscoelastic and viscoplastic effects during multiple shape memory cycles of Veriflex-E, an epoxy-based, thermally-triggered shape memory polymer resin. The experimental program is designed to explore the influence of multiple thermomechanical cycles on the shape memory performance of Veriflex-E. The effects of the deformation rate and hold time at elevated temperature on the shape memory behavior are also investigated.

Frontiers in Materials, 2015

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMP... more Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) allow many design possibilities due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be utilized in systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite shape fixity and interfacial stresses. Analytical models show that SMPs can be used to adequately fix the shape of SMA actuators and springs. COMSOL finite element simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP-SMA two-way actuator and the predicted strain is shown to be in good agreement with experimental test results.

Volume 1: Development and Characterization of Multifunctional Materials; Modeling, Simulation and Control of Adaptive Systems; Structural Health Monitoring, 2012

Mechanics of Time-Dependent Materials, 2011

Journal of Intelligent Material Systems and Structures, 2009

The present study is a baseline assessment of the durability of styrene- and epoxy-based shape me... more The present study is a baseline assessment of the durability of styrene- and epoxy-based shape memory polymer resin materials being considered for morphing applications when exposed to service environment. The approach for the experimental evaluation is a measurement of the shape memory properties and elastomeric response before and after separate environmental exposure to (i) water at 49°C for 4 days, (ii) in lube oil at room temperature and at 49°C for 24 h, and (iii) after exposure to xenon arc (63°C, 18 min water and light/102 min light only) and spectral intensity of 0.3—0.4 watts/m2 for 125 cycles (250 h exposure time). Parameters being investigated include modulus in the rubbery and glassy state, stored strain, shape fixity, stress recovery ratio, and linear shape recovery. In addition, we monitor changes in specimen color, weight, and dimensions along with onset of damage due to conditioning and subsequent thermomechanical cycling.

Journal of Intelligent Material Systems and Structures, 2010

This study is a baseline assessment of the environmental durability of current state-of-the-art, ... more This study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape-memory (SM) materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along (0°), perpendicular (90°), and oblique (45°) to the planar orientation of the fabric. The elastomeric response and shape memory properties before and after simulated environmental exposure to moisture, lubrication oil, and UV radiation are measured. Weight loss of the as-received and conditioned specimens is monitored and the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include modulus in the glassy and rubbery state, stored strain, shape fixity, recovery stress, and unconstrained shape recovery. There is a twofold decrease in the composite stiffness as the material is cycled between room and elevated (above the glass transition) temperature. At room temp...

American Society for Composites 2017, 2017

Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrica... more Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrication, an additive manufacturing technique, are typically 20-50% lower than bulk properties. A major contributor to this knockdown is poor raster-toraster and interlayer bond strength driven by incomplete polymer coalescence. The two major steps in the coalescence process responsible for bond strength development are neck growth and interfacial diffusion. The goal of this work is to explore the effects of temperature, time, and particle size on coalescence of ABS polymer in a controlled environment to identify appropriate thermal history to target during the printing process for enhanced raster-to-raster and interlayer bond strength. Particle size was found to have a significant impact on neck growth where doubling the size reduced the normalized neck growth by 58%. Diffusion was qualitatively assessed by tracking absorbance of red pigment from a red colored ABS particle into the neighboring white ABS particle at 550 nm. Smaller particle size held at elevated temperature for longer time had pigment diffusion across longer distances from the neck region. Results suggest that smaller diameter rasters promote faster and more complete coalescence.

Mechanics of Time-Dependent Materials, 2014

This paper presents a three-dimensional (3D) finite deformation thermomechanical model to study t... more This paper presents a three-dimensional (3D) finite deformation thermomechanical model to study the glass transition and shape memory behaviors of an epoxy based shape memory polymer (SMP) (Veriflex E) and a systematic material parameter identification scheme from a set of experiments. The model was described by viscoelastic elements placed in parallel to represent different active relaxation mechanisms around glass transition temperature in the polymer. A set of standard material tests was proposed and conducted to identify the model parameter values, which consequently enable the model to reproduce the experimentally observed shape memory (SM) behaviors. The parameter identification procedure proposed in this paper can be used as an effective tool to assist the construction and application of such 3D multi-branch model for general SMP materials. Keywords Shape memory effect • Shape memory polymer • Constitutive model 1 Introduction Shape memory polymers (SMPs) are polymeric smart materials featured by the ability to recover their permanent shape from one (or sometimes multi-programmed Xie 2010;

American Society for Composites 2020

Design Engineering and Computers and Information in Engineering, Parts A and B, 2006

One of the more formidable problems in composite research is the study of delamination and other ... more One of the more formidable problems in composite research is the study of delamination and other failure modes in the vicinity of a circular hole in a laminate, e.g., a circular cut-out in a structure. In this problem, the singularity varies around the periphery of the hole as well as through the thickness of the laminate. Under tensile loading, the early failure modes in this problem consist of transverse cracks in various layers, so that delamination occurs only after other damage is precipitated, followed by fiber breakage leading to failure. A literature review of past work clearly shows that mechanical testing with simultaneous AE monitoring is a fruitful technique to study damage accumulation in composite systems. The acoustic-ultrasonic (AU) testing combines the high sensitivity of ultrasonics to internal damage and the method of acoustic emission technique to characterize elastic waves. As damage accumulates in the specimen along the wave path, the net internal damping incre...

International Journal for Multiscale Computational Engineering, 2021

Volume 1: Active Materials, Mechanics and Behavior; Modeling, Simulation and Control, 2009

The present study is a baseline assessment of the environmental durability of current state-of-th... more The present study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape memory materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along 0°, perpendicular (90°), and at 45° to the orientation of the fabric. The shape memory properties and elastomeric response before and after relevant environmental exposure to water at 49°C for 4 days, in lube oil at room temperature and at 49°C for 24 hours, and after exposure to Xenon Arc (63°C, 18 minutes water and light/102 minutes light only) and spectral intensity of 0.3 to 0.4 watts/m2 for 125 cycles (250 hours exposure time) are measured. Weight loss of the as-received and conditioned specimens is monitored while the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include stored strain, recovery stress, shape fixity, shape recovery, and modulus in t...

Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials, Volume 3, 2011

Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for reconf... more Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for reconfigurable structures on air vehicles. In this study, epoxy-based SMPs were conditioned separately in simulated service environments relevant to Air Force missions, namely, (1) exposure to UV radiation, (2) immersion in jet-oil, and (3) immersion in water. The mechanical properties and shape recovery abilities of the unconditioned and conditioned SMPs were examined using large-scale compressive tests and small-scale nano-indentation tests. Results show that all the conditioned SMPs exhibit a decrease in Tg as compared to the unconditioned one. Under compressive loading, the SMPs undergo significant plastic deformation prior to fracture, except the UV conditioned sample. Environmentally conditionings generally result in higher moduli and yield strength of the SMPs. Environmental conditionings, in particular the UV exposure and water immersion, also affect the shape recovery abilities of the SMPs if the recovery temperatures are set low.

High Temperature Polymer Matrix Composites (HTPMCs) exposed to oxidizing environments are subject... more High Temperature Polymer Matrix Composites (HTPMCs) exposed to oxidizing environments are subject to mechanical property degradation due to chemical changes in the resin. Long-term degradation rates due to oxidation in a composite structure are controlled by the damage evolution rates in the oxidized zones, which in turn are determined by the composite’s strength and toughness in these regions. The presence of voids, cracks, and holes in the structure alters the local oxidation zone morphologies. In this paper, we describe a methodology for prediction of Open Hole Tensile (OHT) residual strength of isothermally-aged IM7/(bis) maleimide quasi-isotropic composites under static loading. The composite oxidation profiles around holes are explained by monitoring the development and growth of damage through fluorescence imaging in conjunction with optical microscopy and X-ray computed tomography (CT) techniques. Preferred damage initiation directions are investigated for several combinatio...

ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Volume 1, 2011

The aim of this study is to evaluate the influence of microstructure, specifically, variations in... more The aim of this study is to evaluate the influence of microstructure, specifically, variations in foam cell size and density, on the compressive stress-strain behavior of polymeric foam material. Foams with varying densities are examined under incremental loadings to different prescribed strain levels. A comparison is made of the maximum stress level attained during deformation and residual strain on complete unloading and following recovery. Additionally, the durability of the foam material on exposure to service environment, namely, exposure to ultra-violet (UV) light and humidity, is considered. Cylindrical compression samples are exposed to Xenon Arc (63°C, 18 minutes water and light/102 minutes light only) and spectral intensity of 0.3 to 0.4 watts/m2 for 288 cycles. Parameters investigated include changes in modulus, maximum stress, residual strain and linear shape recovery due to conditioning and mechanical cycling.

Volume 1: Development and Characterization of Multifunctional Materials; Modeling, Simulation and Control of Adaptive Systems; Structural Health Monitoring, 2012

In the present study, a series of novel linear polyaspartimide-based silane endcapped (cross-link... more In the present study, a series of novel linear polyaspartimide-based silane endcapped (cross-linked) polymers are synthesized using 4-4′ bismaleimidodiphenylmethane, Jeffamine D-400 (BMI-JA-400), and (3-Aminopropyl) trimethoxysilane. To add strength to these systems, the trimethoxysilane moiety is cross-linked with the addition of water to create a thermosetting material with both improved toughness and variable cross-link densities. Thermal analysis is done to evaluate the developed shape-memory polymer (SMP) resin for composite processing feasibility. The solvent content in the resin and thermal stability is monitored using thermogravimetric analysis (TGA) while advanced rheometric expansion system (ARES) with parallel plate geometry is used to measure viscosity variation with temperature. The resin BMI-JA-400-Si-70/30 is chosen for making the composite based on its viscosity, weight change, and kinetic results. Differential scanning calorimetry (DSC) is performed to determine the...

Conference Proceedings of the Society for Experimental Mechanics Series, 2016

In Fused Deposition Modeling (FDM), filaments of heated thermoplastic are extruded from a tip tha... more In Fused Deposition Modeling (FDM), filaments of heated thermoplastic are extruded from a tip that moves in a raster-like motion. Depending on filament material properties and the processing parameters, the extruded beads and the fusion-bond between them could result in anisotropic properties (particularly with regard to strength). In this work, the focus is on characterizing the interlaminar fracture toughness of coupons produced by FDM with varying road-to-road offsets producing positive, zero and negative air gaps between neighboring roads resulting in coupons of varying density (void content). The resulting bond line formation is assessed through measuring and analyzing changes in the mesostructure and the contact attained at the interfaces between the adjoining roads. The bond quality is assessed by evaluating plane-strain fracture toughness using compact tension specimens. Results demonstrate an approximate five-fold increase in fracture toughness with varying air gaps.

Mechanics of Time-Dependent Materials, 2011

Shape memory polymers have attracted great interest in recent years for application in reconfigur... more Shape memory polymers have attracted great interest in recent years for application in reconfigurable structures (for instance morphing aircraft, micro air vehicles, and deployable space structures). However, before such applications can be attempted, the mechanical behavior of the shape memory polymers must be thoroughly understood. The present study represents an assessment of viscoelastic and viscoplastic effects during multiple shape memory cycles of Veriflex-E, an epoxy-based, thermally-triggered shape memory polymer resin. The experimental program is designed to explore the influence of multiple thermomechanical cycles on the shape memory performance of Veriflex-E. The effects of the deformation rate and hold time at elevated temperature on the shape memory behavior are also investigated.

Frontiers in Materials, 2015

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMP... more Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) allow many design possibilities due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be utilized in systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite shape fixity and interfacial stresses. Analytical models show that SMPs can be used to adequately fix the shape of SMA actuators and springs. COMSOL finite element simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP-SMA two-way actuator and the predicted strain is shown to be in good agreement with experimental test results.

Volume 1: Development and Characterization of Multifunctional Materials; Modeling, Simulation and Control of Adaptive Systems; Structural Health Monitoring, 2012

Mechanics of Time-Dependent Materials, 2011

Journal of Intelligent Material Systems and Structures, 2009

The present study is a baseline assessment of the durability of styrene- and epoxy-based shape me... more The present study is a baseline assessment of the durability of styrene- and epoxy-based shape memory polymer resin materials being considered for morphing applications when exposed to service environment. The approach for the experimental evaluation is a measurement of the shape memory properties and elastomeric response before and after separate environmental exposure to (i) water at 49°C for 4 days, (ii) in lube oil at room temperature and at 49°C for 24 h, and (iii) after exposure to xenon arc (63°C, 18 min water and light/102 min light only) and spectral intensity of 0.3—0.4 watts/m2 for 125 cycles (250 h exposure time). Parameters being investigated include modulus in the rubbery and glassy state, stored strain, shape fixity, stress recovery ratio, and linear shape recovery. In addition, we monitor changes in specimen color, weight, and dimensions along with onset of damage due to conditioning and subsequent thermomechanical cycling.

Journal of Intelligent Material Systems and Structures, 2010

This study is a baseline assessment of the environmental durability of current state-of-the-art, ... more This study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape-memory (SM) materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along (0°), perpendicular (90°), and oblique (45°) to the planar orientation of the fabric. The elastomeric response and shape memory properties before and after simulated environmental exposure to moisture, lubrication oil, and UV radiation are measured. Weight loss of the as-received and conditioned specimens is monitored and the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include modulus in the glassy and rubbery state, stored strain, shape fixity, recovery stress, and unconstrained shape recovery. There is a twofold decrease in the composite stiffness as the material is cycled between room and elevated (above the glass transition) temperature. At room temp...

American Society for Composites 2017, 2017

Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrica... more Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrication, an additive manufacturing technique, are typically 20-50% lower than bulk properties. A major contributor to this knockdown is poor raster-toraster and interlayer bond strength driven by incomplete polymer coalescence. The two major steps in the coalescence process responsible for bond strength development are neck growth and interfacial diffusion. The goal of this work is to explore the effects of temperature, time, and particle size on coalescence of ABS polymer in a controlled environment to identify appropriate thermal history to target during the printing process for enhanced raster-to-raster and interlayer bond strength. Particle size was found to have a significant impact on neck growth where doubling the size reduced the normalized neck growth by 58%. Diffusion was qualitatively assessed by tracking absorbance of red pigment from a red colored ABS particle into the neighboring white ABS particle at 550 nm. Smaller particle size held at elevated temperature for longer time had pigment diffusion across longer distances from the neck region. Results suggest that smaller diameter rasters promote faster and more complete coalescence.

Mechanics of Time-Dependent Materials, 2014

This paper presents a three-dimensional (3D) finite deformation thermomechanical model to study t... more This paper presents a three-dimensional (3D) finite deformation thermomechanical model to study the glass transition and shape memory behaviors of an epoxy based shape memory polymer (SMP) (Veriflex E) and a systematic material parameter identification scheme from a set of experiments. The model was described by viscoelastic elements placed in parallel to represent different active relaxation mechanisms around glass transition temperature in the polymer. A set of standard material tests was proposed and conducted to identify the model parameter values, which consequently enable the model to reproduce the experimentally observed shape memory (SM) behaviors. The parameter identification procedure proposed in this paper can be used as an effective tool to assist the construction and application of such 3D multi-branch model for general SMP materials. Keywords Shape memory effect • Shape memory polymer • Constitutive model 1 Introduction Shape memory polymers (SMPs) are polymeric smart materials featured by the ability to recover their permanent shape from one (or sometimes multi-programmed Xie 2010;

American Society for Composites 2020

Design Engineering and Computers and Information in Engineering, Parts A and B, 2006

One of the more formidable problems in composite research is the study of delamination and other ... more One of the more formidable problems in composite research is the study of delamination and other failure modes in the vicinity of a circular hole in a laminate, e.g., a circular cut-out in a structure. In this problem, the singularity varies around the periphery of the hole as well as through the thickness of the laminate. Under tensile loading, the early failure modes in this problem consist of transverse cracks in various layers, so that delamination occurs only after other damage is precipitated, followed by fiber breakage leading to failure. A literature review of past work clearly shows that mechanical testing with simultaneous AE monitoring is a fruitful technique to study damage accumulation in composite systems. The acoustic-ultrasonic (AU) testing combines the high sensitivity of ultrasonics to internal damage and the method of acoustic emission technique to characterize elastic waves. As damage accumulates in the specimen along the wave path, the net internal damping incre...

International Journal for Multiscale Computational Engineering, 2021

Volume 1: Active Materials, Mechanics and Behavior; Modeling, Simulation and Control, 2009

The present study is a baseline assessment of the environmental durability of current state-of-th... more The present study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape memory materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along 0°, perpendicular (90°), and at 45° to the orientation of the fabric. The shape memory properties and elastomeric response before and after relevant environmental exposure to water at 49°C for 4 days, in lube oil at room temperature and at 49°C for 24 hours, and after exposure to Xenon Arc (63°C, 18 minutes water and light/102 minutes light only) and spectral intensity of 0.3 to 0.4 watts/m2 for 125 cycles (250 hours exposure time) are measured. Weight loss of the as-received and conditioned specimens is monitored while the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include stored strain, recovery stress, shape fixity, shape recovery, and modulus in t...

Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials, Volume 3, 2011

Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for reconf... more Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for reconfigurable structures on air vehicles. In this study, epoxy-based SMPs were conditioned separately in simulated service environments relevant to Air Force missions, namely, (1) exposure to UV radiation, (2) immersion in jet-oil, and (3) immersion in water. The mechanical properties and shape recovery abilities of the unconditioned and conditioned SMPs were examined using large-scale compressive tests and small-scale nano-indentation tests. Results show that all the conditioned SMPs exhibit a decrease in Tg as compared to the unconditioned one. Under compressive loading, the SMPs undergo significant plastic deformation prior to fracture, except the UV conditioned sample. Environmentally conditionings generally result in higher moduli and yield strength of the SMPs. Environmental conditionings, in particular the UV exposure and water immersion, also affect the shape recovery abilities of the SMPs if the recovery temperatures are set low.

High Temperature Polymer Matrix Composites (HTPMCs) exposed to oxidizing environments are subject... more High Temperature Polymer Matrix Composites (HTPMCs) exposed to oxidizing environments are subject to mechanical property degradation due to chemical changes in the resin. Long-term degradation rates due to oxidation in a composite structure are controlled by the damage evolution rates in the oxidized zones, which in turn are determined by the composite’s strength and toughness in these regions. The presence of voids, cracks, and holes in the structure alters the local oxidation zone morphologies. In this paper, we describe a methodology for prediction of Open Hole Tensile (OHT) residual strength of isothermally-aged IM7/(bis) maleimide quasi-isotropic composites under static loading. The composite oxidation profiles around holes are explained by monitoring the development and growth of damage through fluorescence imaging in conjunction with optical microscopy and X-ray computed tomography (CT) techniques. Preferred damage initiation directions are investigated for several combinatio...

ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Volume 1, 2011

The aim of this study is to evaluate the influence of microstructure, specifically, variations in... more The aim of this study is to evaluate the influence of microstructure, specifically, variations in foam cell size and density, on the compressive stress-strain behavior of polymeric foam material. Foams with varying densities are examined under incremental loadings to different prescribed strain levels. A comparison is made of the maximum stress level attained during deformation and residual strain on complete unloading and following recovery. Additionally, the durability of the foam material on exposure to service environment, namely, exposure to ultra-violet (UV) light and humidity, is considered. Cylindrical compression samples are exposed to Xenon Arc (63°C, 18 minutes water and light/102 minutes light only) and spectral intensity of 0.3 to 0.4 watts/m2 for 288 cycles. Parameters investigated include changes in modulus, maximum stress, residual strain and linear shape recovery due to conditioning and mechanical cycling.