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This investigation determined the creep-fatigue cyclic-life (Nf) saturation effect for the DZ445 superalloy with 16–128 min dwell time at 900 °C and total strain range of 1.6%. When the dwell time exceeds 8 min, the mechanical response including maximum stress response, hysteresis loop, plastic strain range, and stress relaxation amount at characteristic cycle are also saturated with dwell time gradually. The Nf saturation effect is also reflected in the creep- and ductile-dominated fracture modes. This critical phenomenon is closely related to the dynamic equilibrium reached between the superdislocations of a/2[101] in γˊ precipitation and the formation of the dislocation networks. While the rupture time is increased continuously with dwell time, which could be better used as an evaluation index when the cyclic life is saturated. The Nf saturation effect can not only reduce the cost of tests with long-dwell time, but also provide the guidance for creep-fatigue safety design.

The present study aims at investigating the effect of organic-modified nickel phyllosilicate (NiPS) particles on the structure–property relationships of epoxy (EP) composites. The organic-modified NiPS particles were synthesized using a facile sol–gel method and subsequently mixed into EP matrix by physical blending to improve its tribological, mechanical and thermal properties. Based on a series of tests and analyses, it was found that organic-modified NiPS particles playing a great role in improving the tribological properties under dry sliding. The steady-state friction coefficient was just decreased moderately, whereas the wear rate was dropped significantly to 0.36 × 10−5 mm3/N·m, showing a reduction up to ca. 95% in comparison with pure EP. According to the analysis of worn surfaces, the main mechanism of wear resistance was discussed. The elastic modulus for the EP composites was increased remarkably while the elongation at break decreased. However, the added organic-modified NiPS particles show a negligible influence on the tensile strength. Moreover, the incorporated NiPS particles could enhance the thermal stability of EP composites, showing the notably suppressed mass loss rate and added residual chars. This study provided a promising way for preparing the high-performance EP composites with organic-modified lamellar particles.

An innovative approach, designated as supported activator (SA), allows preparation of high density polyethylene (HDPE)-based highly performant hybrid materials. This procedure makes use of a nano-sized supported methylaluminoxane (MAO)-activator, based on halloysite natural nanotubes (HNT), combined with an in situ supporting concept. The new protocol when compared with a more conventional approach gives rise to higher polymerization activities as well as ultimate materials with better morphological features, greater crystallinity, thicker crystals, and highly increased stiffness. Moreover, a remarkable synergy between rigidity and toughness is attained. The Young's modulus of a film obtained from the nanocomposite with the highest HNT content increases more than 70 % relatively to a pristine HDPE film, while retaining the limit stretching ability of pristine HDPE (more than 800%). A beneficial impact of using a high aspect ratio support such as HNT in the mechanical properties is also observed, when compared to similar HDPE hybrid materials derived from dendrimer-like silica (DS) nanospheres. Interestingly, polymerization activity, polymer features and derived properties found in the ultimate materials are less impacted by support/filler nature than by preparation method. This fact highlights the crucial role of the synthetic methodology used and corroborates the high potential of the SA route for the preparation of high-performance polyethylene-based nanocomposites with an excellent balance between stiffness and deformability., The authors gratefully acknowledge the funding of this work by Fundação para a Ciência e Tecnologia (Project UIDB/00100/2020, Project UIDP/00100/2020 and Project LA/P/0056/2020), the FCT-CATSUS program and PAUILF (Project TC 04/17). D. M. Cecílio’s PhD scholarship (PD/BD/114580/2016) provided by FCT is gratefully acknowledged. A. Fernandes thanks FCT for law DL/57 contract. This work was also supported by the Agencia Estatal de Investigación (AEI, Spain) (Grant No PID2020-114930 GB-I00).

This paper uses ensemble and automated machine learning algorithms to predict the mechanical properties (tensile and flexural strength) of a three-dimensionally printed (3DP) foamed structure. The closed cell foams were made from the most commonly used thermoplastic, High-Density Polyethylene (HDPE). The hollow glass microspheres are infused in HDPE at varying volume %. The available data on these foams' mechanical properties are used by the chosen machine learning (ML) algorithms to propose the best suited algorithm for such a three-phased microstructure as these closed cell foams exhibit. Finally, the strength predictions from the models were validated using experimental data. The models were trained with nozzle temperature, bed temperature, and force values as input parameters. The output parameters predicted were the tensile and flexural strength. LightGBM outperforms all other models in terms of performance among ensemble-based models, while H2OAutoML outperforms all other models. All the ML algorithms produced models with greater than 95% accuracy. Finally, memory and time consumption for each model are presented. © 2022 The Authors

This paper presents the inclusion of explicit dual poroelastic mechanical behaviour as part of an existing dual porosity numerical model of multiphase, multicomponent chemical-gas transport. The dual poroelastic framework employed considers the pore structure changes occurring as a result of high pressure carbon dioxide injection into coal, particularly the adsorption-induced coal swelling that has been found to limit injectivity in field trials of carbon sequestration in coalbeds around the world. To address this issue, the surface stress of the fluid-solid interface is introduced into the constitutive relation for dual porosity effective stress in order to investigate the coal deformation and porosity changes related to adsorption behaviour. A new porosity model is presented, in which the impacts of gas flow and coal deformation are incorporated, and an interaction coefficient is proposed to explain the effect of fracture-matrix interactions on the porosity evolution. The model is verified and validated in this work against relevant analytical solutions and experimental results, and applied to study the gas flow behaviour and structural changes of coal. The results show that carbon dioxide injection not only causes coal swelling but also has the potential to change the internal pore structure of coal. The variation of fracture porosity is not monotonic as a competing result of effective stress and internal fracture-matrix interactions. However, the matrix porosity is found to increase during carbon dioxide injection, which seems to be a key contributor to the swelling phenomenon.

With the advent of a new arc-based additive manufacturing (AM) process, referred to as Wire-and-Arc Additive Manufacturing (WAAM), the scale of the metal printed parts increased up to several meters, thus becoming suitable for large-scale applications in marine, aerospace and construction sectors. However, specific considerations in terms of geometrical and mechanical properties ought to be made in order to effectively use the printed outcomes for structural engineering purposes. The introduction of the novel printing strategy referred to as “dot-by-dot”, consisting in successive drops of molten metal, enabled the use of WAAM for complex lattice structures, made by continuous grids of WAAM bars. Nevertheless, their proper design requires an accurate evaluation of the influence of the non-negligible inherent geometrical irregularities on the mechanical response of the bars. Hence, extensive experimental work is needed in order to evaluate the mechanical response of WAAM bars with geometrical imperfections. The present study is thus focused on the assessment of the mechanical response in tension of WAAM-produced 304L stainless steel small bars in terms of key effective mechanical parameters. As such, the mechanical characterization through tensile tests is supported by microstructural investigations and detailed studies on the geometrical features. Three batches of bars are studied, each one printed at different build angles representative of limit cases for practical applications. The microstructural analysis confirms the preferential grain orientation typical of WAAM process for all three build angles. The results of the geometrical and mechanical characterization clearly evidence the non-negligible influence of the inherent geometrical imperfections on the mechanical response in tension of the printed bars, with a detrimental effect of the build angle on the main key effective mechanical parameters. Overall, the results highlight the need of specific investigations on both geometrical and mechanical properties of WAAM bars for structural design purposes.

International audience; The shiv derived from the shredded stem of the hemp plant [1], when mixed with a mineral binder and water, produces hemp concrete (or “hempcrete”), which is considered as an eco-material because of its remarkable environmental properties (favourable CO2 balance, easy management and disposal at the end of the product life, etc.) [2]. However, much like any agricultural product, its quality may be affected by the soil where the hemp has been grown, the weather conditions, and a variety of other factors. As these parameters are rather variable, the produced plant matter tends to be somewhat irregular, which can hamper the development of this type of building solution. The study at hand tests the characteristics of the hemp (particle size distribution, water absorption capacity, etc.), to assess the impact of the variability of these factors on the performance of the hempcrete made with the shiv. To begin with, the thirteen types of shiv used in the study were compared by multicriteria analysis, and then classified into three different groups.Next, nine types of shiv were selected, so that each of the three groups was represented. In identical conditions (i.e. the only variable being the type of shiv), specimen of hempcrete were made up in the lab for experimentation. The aim here was to validate the results of the study characterising the different types of shiv. The results show that there are three main types of mechanical response when the hempcrete is subjected to a compression test, where the material behaviour is characterised by small, medium or large strains. The large specific surface area of shiv is known to be responsible for weak binding at the interface between the binder and the plant particles, leading to total failure of the test piece. In addition, its high water absorption capacity, which is the likely cause of the “false set effect” associated with the elongated form of the particles, is identified as being responsible for the high rates of deformability found. It should be noted that in the latter case, the specimens behave like a succession of layers, which accounts for the significant levels of strain.By multicriteria analysis of the results for the hempcrete specimens, we found that they can be also classified into three groups, matching the three detected by the multicriteria analysis of the types of shiv from which they were made. Unlike the previously published literature, this study leads to the conclusion that shiv types with small particle sizes and high water absorption capacities do not produce the best performance.

Based on the experimental method of projectile penetration into the concrete target, different excitation by deceleration of polymer bonded explosives (PBXs) was achieved by controlling the length of steel pillars behind PBXs. A series of impact tests were conducted to study the mechanical response of PBXs with different initial densities under different excitation by deceleration. A physical model was developed to describe the viscoelastic-plastic deformation of PBXs. The plastic part was added on the basis of the viscoelastic statistical crack mechanical model (Visco-SCRAM) model, and the compaction behavior of charge in different environments was characterized based on the Heckel compaction equation. The model was implemented in LS-DYNA, and the simulation results were verified by the experiments. The dynamic mechanical behavior of PBXs under different excitation by deceleration was studied from the macroscopic and mesoscale aspects. The stress curves, strain curves, the density change and the interaction between particles in deceleration environment were predicted. After comparison and analysis, the dynamic response law of the charge in the penetrating process was obtained. Macroscopic responses along with mesoscale modeling results are significantly important to have a better understanding of deformation mechanisms of PBXs.

Biodiesel has emerged as one of the major sources of renewable energy in automotive applications substituting the convention diesel. However, the compatibility of biodiesel and rubber components remains a major concern. Indeed, it is found that biodiesel tends to swell rubber components resulting to their faster degradation. Thus, studies focusing on the compatibility of newly developed biodiesel are essential for rubber durability. Along this line, the main purpose of the present study is to investigate the swelling characteristics of rubber in cottonseed oil biodiesel. Two types of unfilled and filled rubbers are considered: natural rubber (NR) and polychloroprene rubber (CR). For filled rubbers, two carbon black contents are used: 25 and 40 phr. The rubber samples are immersed in conventional diesel fuel, biodiesel and diesel-biodiesel blends for 1000 hours at room temperature (25 °C). The evolutions of rubber mechanical properties such as hardness, tensile modulus and tensile strength are investigated as well as the macroscopic stress-strain characteristics. For both rubbers, it is found that the decrease in the mechanical properties becomes more significant as the biodiesel content increases. Finally, a continuum model predicting equilibrium swelling proposed by Ch’ng [S.Y. Ch’ng. Coupling between diffusion of biodiesel and large deformation in elastomers: from experimental investigation to constitutive modeling. Ph.D. Thesis. University of Malaya, Malaysia. 2014] is adopted to simulate the effect of carbon black on the stress-free equilibrium swelling of rubber.

International audience; Defect-free joints were friction stir welded (FSW) from AZ31 Mg alloy sheets by employing optimum welding parameters for the tool geometry used. Microstructure, texture and mechanical response of the welded joints were studied to identify susceptibility of crack initiation in the different weld zones and to understand the overall fracture behavior of these welds. Uniaxial tensile testing conducted orthogonal to the welding direction at temperatures of 25, 100, 200, and 300 °C revealed that cracks initiated mainly on the tool’s advancing side (AS) of the weld between thermo-mechanically affected zone (TMAZ) and stir zone (SZ). Such cracks were found to propagate preferentially along this interfacial region leading to complete fracture. This finding may be linked to the inhomogeneous plastic deformation in the weld resulting in strong texture and microhardness gradients at the interface region between the TMAZ (AS) and the SZ. On the other hand, in tensile samples machined along the welding direction and containing stir zone microstructure, second phase particles were identified as the preferred sites for cracks initiation.

A simple two-dimensional Unit cell (UC) with three different mesoscopic phases may be geometrically modeled as a combination of two concentric circles or squares embedded in a square. In this paper, a two-dimensional three-phase UC with a similar design is developed and utilized for modeling the full deformation and failure response of cement based quasi-brittle cementitious composites under compression. It was identified from the numerical investigation that the three-phase design greatly improved the prediction capability of the UC in suitably capturing the material nonlinearity observed experimentally. HighlightsSuccessfully developed and implemented the three-phase unit cell approach for the mechanical characterization of cementitious composites.Captured the non-linear deformation behavior in good correlation with experiments.Established the effectiveness of the three-phase unit cell approach in capturing the non-linearity of the materials considered without sacrificing the simplicity of the unit cell model.Different parametric studies were also conducted to identify the influence of certain phase characteristics of the unit cell for the materials considered.

Fibrin is a protein polymer that forms a 3D filamentous network, a major structural component of protective physiological blood clots as well as life threatening pathological thrombi. It plays an important role in wound healing, tissue regeneration and is widely employed in surgery as a sealant and in tissue engineering as a scaffold. The goal of this study was to establish correlations between structural changes and mechanical responses of fibrin networks exposed to compressive loads. Rheological measurements revealed nonlinear changes of fibrin network viscoelastic properties under dynamic compression, resulting in network softening followed by its dramatic hardening. Repeated compression/decompression enhanced fibrin clot stiffening. Combining fibrin network rheology with simultaneous confocal microscopy provided direct evidence of structural modulations underlying nonlinear viscoelasticity of compressed fibrin networks. Fibrin clot softening in response to compression strongly correlated with fiber buckling and bending, while hardening was associated with fibrin network densification. Our results suggest a complex interplay of entropic and enthalpic mechanisms accompanying structural changes and accounting for the nonlinear mechanical response in fibrin networks undergoing compressive deformations. These findings provide new insight into the fibrin clot structural mechanics and can be useful for designing fibrin-based biomaterials with modulated viscoelastic properties.

The translaminar fracture toughness of fibre-reinforced composites is a size-dependent property which governs the damage tolerance and failure of these materials. This paper presents the development, implementation and validation of an original analytical model to predict the tensile translaminar (fibre-dominated) toughness of composite plies and bundles, as well as the associated size effect. The model considers, as energy dissipation mechanisms, debonding and pull-out of bundles from quasi-fractal fracture surfaces; the corresponding lengths are stochastic variables predicted by the model, based on the respective bundle strength distributions and fracture mechanics. Parametric studies show that composites are toughened by stronger fibres with large strength variability, and intermediate values of interfacial toughness and friction. Predictions are validated against four different composite ply systems tested in the literature, proving the model’s ability to capture not only size effects, but also the influence of different fibres and resins.

International audience; The environmental and economic concerns have raised the popularity of biodiesel as a potential replacement for conventional fuel. However, the incompatibility of engineering rubber components with biodiesel affects significantly the performance of the components. Majority of the compatibility studies focus on evaluating the degradation of mechanical properties of the rubbers due to contamination of different types of biodiesel. Nevertheless, the resulting mechanical responses of swollen rubbers, in particularly under cyclic and fatigue loading conditions, are rarely investigated. In engineering applications where elastomeric components are concurrently subjected to fluctuating mechanical loading and contamination of hostile liquids such as biodiesel, it is crucial to investigate the mechanical responses of these components for durability analysis. In this view, the present study aims to investigate the effect of swelling, due to biodiesel diffusion in the elastomers, on the macroscopic mechanical responses under cyclic loading conditions. Simple stress-free immersion tests are conducted on elastomers and the resulting mechanical responses are evaluated. The focus of the present work is on the effect of biodiesel diffusion on the inelastic responses classically observed in elastomers under cyclic loading conditions, i.e. stress-softening, hysteresis and stress relaxation. The results show that the above inelastic responses decrease significantly when the swelling level increases.

cited By 25; International audience; Evolution of atomic mobility in a Ti 40Zr 25Ni 8Cu 9Be 18 bulk metallic glass was studied by dynamic mechanical analysis (DMA) in different states: as-cast, after structural relaxation, after crystallization and after deformation (cold-rolled). Characteristics of Ti 40Zr 25Ni 8Cu 9Be 18 bulk metallic glass are similar to that observed in other based bulk metallic glasses in the amorphous state: at low temperature, the material is mainly elastic and the mechanical response is independent of driving frequency, while on the other hand, the visco-elastic component becomes very large in the glass transition region. Structural relaxation and crystallization induce a decrease in the visco-elastic component, suggesting that atomic mobility is reduced. In contrast, atomic mobility is increased by a plastic deformation (i.e. cold-rolling). The higher is the cold-rolling ratio, the higher is the visco-elastic component. The experimental results were analyzed in the framework of quasi-point defects theory, which is based on the existence of defects in amorphous materials (polymer, bulk metallic glasses and other non-crystalline solids). © 2012 Elsevier Ltd. All rights reserved.

International audience; Motivated by environmental and economic factors, biodiesel has been introduced to partially replace the diminishing stock of petroleum-based fuel. However, the use of biodiesel such as palm biodiesel is placing additional demands on automotive components, e.g. rubber components, due to a compatibility issue in the fuel system. In engineering applications where hostile environments are involved, the long-term mechanical response of rubber components is affected by interactions existed between diffusion of liquids into the material and fluctuating multiaxial loading. Hence, it is crucial to investigate the durability of rubber components in this aggressive environment. A number of studies on static immersion tests to investigate the diffusion of liquids in stress-free rubber can be found in the literature. However, studies focusing on the coupling between diffusion and large deformation in rubber, and the resulting swelling and mechanical response under cyclic loading conditions are less common. In the present work, a compression device for the investigation of interaction between diffusion of palm biodiesel and large compressive strain in rubber is developed. The apparatus is comprised of four rectangular stainless steel plates with spacer bars in between, which are designed such that pre-compression can be introduced on the rubber specimens while they are immersed into biodiesel. This allows diffusion and large strain to take place simultaneously. Different pre-compressive strains and biodiesel blends are considered. At the end of each immersion period, the resulting swelling behavior and mechanical response of rubber specimens under cyclic loading conditions are investigated. The features of this compression device are discussed and perspectives are drawn.

Radiation-induced segregation (RIS) in desensitized type 304 stainless steel (SS) was investigated using a combination of electrochemical potentiokinetic reactivation (EPR) test and atomic force microscopy (AFM). Desensitized type 304 SS was irradiated to 0.43 dpa (displacement per atom) using 4.8 MeV protons at 300 degrees C. The maximum attack in the EPR test for the irradiated desensitized SS was measured at a depth of 70 mu m from the surface. Grain boundaries and twin boundaries got attacked and pit-like features within the grains were observed after the EPR test at the depth of 70 mu m. The depth of attack, as measured by AFM, was higher at grain boundaries and pit-like features as compared to twin boundaries. It has been shown that the chromium depletion due to RIS takes place at the carbide-matrix as well as at the carbide-carbide interfaces at grain boundaries. The width of attack at grain boundaries after the EPR test of the irradiated desensitized specimen appeared larger due to the dislodgement of carbides at grain boundaries. (C) 2011 Elsevier B.V. All rights reserved.

The primary objective of this work is to characterize and compare the dynamic mechanical behavior of asphalt concrete mixes with styrene butadiene styrene (SBS) polymer and crumb rubber modified asphalt binders with the behavior of mixes with unmodified viscosity grade asphalt binders. Asphalt binders are characterized for their physical and rheological properties. Simple performance tests like dynamic modulus, dynamic and static creep tests are carried out at varying temperatures and time. Dynamic modulus master curves constructed using numerical optimization technique is used to explain the time and temperature dependency of modified and unmodified asphalt binder mixes. Creep parameters estimated through regression analysis explained the permanent deformation characteristics of asphalt concrete mixes. From the dynamic mechanical characterization studies, it is found that asphalt concrete mixes with SBS polymer modified asphalt binder showed significantly higher values of dynamic modulus and reduced rate of deformation at higher temperatures when compared to asphalt concrete mixes with crumb rubber and unmodified asphalt binders. From the concept of energy dissipation, it is found that SBS polymer modification substantially reduces the energy loss at higher temperatures. Multi-factorial analysis of variance carried out using generalized liner model showed that temperature, frequency and asphalt binder type significant influences the mechanical response of asphalt concrete mixes. The mechanical response of SBS polymer modified asphalt binders are significantly correlated with the rutting resistance of asphalt concrete mixes. � 2011 Elsevier B.V.

This paper focuses on the development of a plasticity model to describe high rate deformations of metals. Modeling of target mechanical response is performed in frames of continuum mechanics. Plastic flow is described as the result of an over barrier dislocation sliding in specific slip planes. Computations of shock wave propagation in fcc, bcc and hcp metals modeling in comparison with shock wave experiments are performed to verify the model. The model predicts yield strength increase on elastic precursor in aluminum monocrystal and titanium of high purity at high temperatures. The action on a copper target of the electron beams with energy density (the total energy incident on an unit area during an irradiation pulse) 8.6 J cm-2 and varied pulse duration has been investigated. At the considered irradiation regime the target remains in a solid state (maximal temperature is 710 K) and shear stresses can reach values of about 0.72 GPa. Depth distribution of dislocation density after irradiation has a maximum that is localized on a distance of 10 μm from the irradiated surface and the maximum dislocation density is about 6 × 109 cm-2 in the target. The shortening of the exposure time to 1 ns leads to the increase of the dislocation density. Further reduction of exposure time has a weak effect on the dislocation density because the shear stresses reach a limit. © 2011 Elsevier Ltd. All rights reserved.

cited By 8; International audience; Physical ageing of amorphous polymers well below their glass transition temperature leads to changes in almost all physical properties. Of particular interest is the increase in yield stress and post-yield strain softening that accompanies ageing of these materials. Moreover, at larger strain polymers seem to rejuvenate, i.e. aged and non-aged samples have identical stressstrain responses. Also, plastically deforming an aged sample seems to rejuvenate the polymer. In this work we use molecular dynamic simulations with a detailed force field suitable for macromolecular ensembles to simulate and understand the effects of ageing on the mechanical response of these materials. We show that within the timescales of these simulations it is possible to simulate both ageing and rejuvenation. The short range potentials play an important role in ageing and rejuvenation. A typical yield drop exhibited by glassy polymers is a manifestation of a sudden relaxation in the short range structure of an aged polymer. Moreover, the aged polymers are known to be brittle. We show that this is intimately related to its typical stressstrain response which allows it to carry arbitrarily large mean stresses ahead of a notch. © 2010 Elsevier Ltd. All rights reserved.

International audience; Cerium-based bulk metallic glasses are attractive due to their low glass transition temperature, comparable to that of polymers. Their mechanical properties, especially their elastic and viscoelastic ones are reported in a large temperature range. Like in other bulk metallic glasses, the shear modulus, close to 11.5 GPa at room temperature, decreases drastically in the glass temperature range (at about 373 K), while the viscoelastic component becomes very important. Heating to high temperature induces crystallization. As shown by X-ray diffraction experiments performed in situ, the increase of elastic modulus due to the formation of crystalline particles is strongly connected to the volume fraction of these particles. Addition of iron shifts the phenomena towards higher temperatures and affects the crystallization process. (C)

Uniaxial compression stress–strain tests were carried out on three commercial amorphous polymers: polycarbonate (PC), polymethylmethacrylate (PMMA), and polyamideimide (PAI). The experiments were conducted under a wide range of temperatures (−40 °C to 180 °C) and strain rates (0.0001 s −1 up to 5000 s −1 ). A modified split-Hopkinson pressure bar was used for high strain rate tests. Temperature and strain rate greatly influence the mechanical response of the three polymers. In particular, the yield stress is found to increase with decreasing temperature and with increasing strain rate. The experimental data for the compressive yield stress were modeled for a wide range of strain rates and temperatures according to a new formulation of the cooperative model based on a strain rate/temperature superposition principle. The modeling results of the cooperative model provide evidence on the secondary transition by linking the yield behavior to the energy associated to the β mechanical loss peak. The effect of hydrostatic pressure is also addressed from a modeling perspective.

The cochlea of the inner ear transforms the incoming sound pressure into neural excitation. Despite extensive experiments and modeling for the past century, understanding the behavior of the cochlea is far from complete. With an efficient program (Fast4) for shell of revolution structures, all mechanical (elastic) details of the curved cochlear cross-section and the organ of Corti can be computed. Based on the known values for the elastic moduli of the protein fibers and estimates for the geometry, the responses to point and pressure loads have been calculated which are reasonably close to the direct measurements. In the present work, the details of the inner hair cell are included, with a fluid gap between the tip of the cilia and the Hensen stripe of the tectorial membrane. A simple model for the near contact indicates nonlinear response similar to the intracellular recordings. This near contact is included into a more complete elastic model for the organ of Corti that includes three rows of cilia and tip links. The phase of the maximum tension of the tip link, which causes excitation of the cell, is computed for low frequencies for comparison to measurements. For low frequencies the fluid motion in the organ of Corti is approximately two-dimensional. The phase is found to be affected by: (1) geometrical difference between basal and upper turns of the cochlea, (2) initial gap spacing between the tip of the cilium and the Hensen stripe, (3) initial gap spacing between the tip of the cilium and the tectorial membrane, (4) presence of an electrode probe constraint on the motion of the inner hair cell, and (5) the stiffness of the tectorial membrane. The latter is the most significant. For a soft tectorial membrane, the excitation is generally between maximum velocity and displacement of the basilar membrane toward scala vestibuli. However, for a stiff tectorial membrane, the phase changes to an excitation with velocity and displacement toward scala tympani. Thus a possible mechanical reason is offered for the auditory nerve excitation in the base of the chinchilla cochlea for basilar membrane velocity toward scala tympani and in the middle and upper turns of the guinea pig cochlea, excitation for velocity toward scala vestibuli.