Isothermal Loading Research Articles

Mechanistic Approach to Structural Fire Modeling of Composites

In this paper, a framework is presented for the modeling of the response of structural composites subjected to combined mechanical loading and fire. An emphasis is placed on the response of composites at temperatures below the decomposition temperature, where the viscoelastic response of the composite material becomes important. Material property characterization results are presented for an E-glass reinforced vinyl ester composite typical of that used for naval ship applications. Time-temperature equivalence is used in a compression strength model to predict the time to failure of composites subjected to isothermal compression loading (compression creep rupture failure). These predictions are compared with experimentally determined times to failure with good agreement. In particular, shift factors obtained from shear compliance testing are able to collapse the compression creep rupture data at different temperatures, indicating that viscolelasticity is the dominant mechanism driving the failure. This model is combined with a standard diffusion model for heat transfer in the composite to predict the time-dependent failure of composites subjected to simultaneous one-sided heat flux and compression loading. Predicted times to failure are compared with experimental results with good agreement.

Athermal Elastomeric Lens Mount for Space Optics

We investigated the optimum adhesive thickness for athermalizing an elastomeric lens mount in our space optics application. Theoretical results were compared with finite element solutions using two different models; discrete circular pads and discrete circular pads with columns filling the insertion holes reflecting the reality. A noticeable difference between their optimal thicknesses was observed, and physical interpretation revealed the uncertainty of prevailing athermal equations. A pilot sample was made to check our results and thermo-optical stress was assessed using an interferometer after isothermal load. This study presented insight into preliminary design guidance in elastomeric lens mounting.

English

In the present paper, the size and strain rate effects on ultra-thin /{100} Cu nanowires at an initial temperature of 10 K have been discussed. Extensive molecular dynamics (MD) simulations have been performed using Embedded atom method (EAM) to investigate the structural behaviours and properties under high strain rate. Velocity-Verlet algorithm has been used to solve the equation of motions. Two different thermal loading cases have been considered: (i) Isothermal loading, in which Nose-Hoover thermostat is used to maintain the constant system temperature, and (ii) Adiabatic loading, i.e., without any thermostat. Five different wire cross-sections were considered ranging from 0.723 x 0.723 nm(2) to 2.169 x 2.169 nm(2) The strain rates used in the present study were 1 x 10(9) s(-1), 1 x 10(8) s(-1), and 1 x 10(7) s(-1). The effect of strain rate on the mechanical properties of copper nanowires was analysed, which shows that elastic properties are independent of thermal loading for a given strain rate and cross-sectional dimension of nanowire. It showed a decreasing yield stress and yield strain with decreasing strain rate for a given cross- section. Also, a decreasing yield stress and increasing yield strain were observed for a given strain rate with increasing cross-sectional area. Elastic modulus was found to be similar to 100 GPa, which was independent of processing temperature, strain rate, and size for a given initial temperature. Reorientation of /{100} square cross-sectional copper nanowire into a series of stable ultra-thin Pentagon copper nanobridge structures with dia of similar to 1 nm at 10 K was observed under high strain rate tensile loading. The effect of isothermal and adiabatic loading on the formation of such pentagonal nanobridge structure has been discussed.

Influence of heterogeneity on fracture behavior in multi-layered materials subjected to thermo-mechanical loading

Based on the heterogeneous characteristics of materials at mesoscopic level, the stress state and fracture behavior in multi-layered materials are numerically investigated by using RFPA 2D , a numerical code incorporated with thermo-mechanical-damage (TMD) model. We conduct this research through two logical steps. Firstly, we investigate the stress states between two adjacent fractures for a typical three-layer model. Our results show that the critical value of fracture spacing to layer thickness ratio is about 0.9 which is gently lower that of value in latest publications. Secondly, we use the same three-layer model without pre-assigned fractures. An isothermal loading is applied on the model until the state of fracture saturation is achieved. In the stage of initiation, fractures are formed one after another; in the stage of infilling, new fractures are created sequentially between the earlier formed fractures; in the stage of saturation, the formed fractures are spaced so closely that no more new fractures can be filled even with decreasing the temperature. Numerical study show that both the fracture pattern and the critical value of fracture spacing to layer thickness ratio is strongly dependent on the heterogeneous characteristics of the central layer materials. For the cases with a relatively homogeneous central layer, more interface fractures occur and the interface delamination evidently influences the fracture saturation.

Oxidation assisted fatigue crack growth under complex non-isothermal loading conditions in a nickel base superalloy

This paper deals with the prediction of fatigue crack growth at high temperatures in the N18 nickel base superalloy, which is employed by Snecma for turbine disc applications. This material and other nickel base superalloys were widely studied in the past under isothermal conditions and constant amplitude fatigue. Dwell time effects are observed which are attributed, in this material, to grain boundary oxidation. The main objective of this research is to use this knowledge to model the fatigue crack growth rate in the N18 nickel base superalloy when complex “missions” are encountered. This implies variable amplitude and non-isothermal loading conditions (450–650 °C). For this purpose, an incremental fatigue crack growth model which was originally developed for isothermal variable amplitude loading conditions was extended so as to be applicable to non-isothermal loading conditions. In addition, the incremental form of the fatigue crack growth law in this model is very useful to account for the coupling effect between fatigue and time-dependent phenomena such as creep or oxidation. In the present case, the effect of the environment was modelled as a competition between two phenomena: a detrimental effect of grain boundary oxidation ahead of the crack tip and a beneficial effect of the growth of a passivation layer of oxides on the freshly created crack surfaces. The model was used to simulate fatigue crack growth under complex cycles at high temperature and the comparisons with experimental results are satisfactory.

Analysis on Process and Forming Defects of Large-Scale Complex Integral Component Isothermal Local Loading

Isothermal local loading forming is an advanced technology for large-scale complex integral component forming, but the process is with coupling effects of multi-fields and multi-factors due to the local loading mode and high temperature condition, thus makes deformation mechanism extremely complicated and some forming defects may occur. Based on DEFORM3D software, relative FE models for the isothermal local loading forming of large-scale complex integral components of TA15 titanium alloy have been established, and key technologies for modeling have been dealt with properly. Then taking a representative large-scale complex integral component as object the froming process, the typical deformation behavior and metal flow were investigated. And typical forming defects which may occur during the process and causes for them are discussed.

On the nature of anomalously high plasticity of high-strength titanium nickelide alloys with shape-memory effects: II. Mechanisms of plastic deformation upon isothermal loading

Mechanisms of plastic deformation have been studied in detail in the process of isothermal loading at room temperature in a high-purity shape-memory alloy of composition Ti49.4Ni50.6. The alloy was studied in two initial states: usual coarse-grained (with an average grain size of 20–30 μm) and submicrocrystalline (with an average grain size of 0.2–0.3 μm). It has been shown that during tensile tests there occurs a mechanically induced martensitic transformation in the alloy at stresses corresponding to stages I, II, and III in the tensile curve and then elastic and plastic deformation of B19′ martensite is observed at stages IV, V, and VI, respectively. Optical metallography in situ and electron microscopy have been used to study microstructural features and mechanisms of plastic deformation of the alloy up to its failure.

Challenges in the constitutive modeling of the thermo-mechanical deformation and damage behavior of EUROFER 97

EUROFER 97, reduced activation ferritic martensitic (RAFM) steel, is a candidate as structural material for plasma facing components of future fusion reactors where it will be subjected to complex thermo-mechanical loading and high irradiation doses. Correct modeling of its deterioration under these loading conditions is a precondition of a sufficiently reliable lifetime prediction procedure. Therefore a coupled deformation damage model taking into account its complex non-saturating cyclic softening has been developed and successfully applied to describe its creep-fatigue behavior under isothermal cyclic loading. The model has been implemented in the finite element code ABAQUS allowing best estimation of the mechanical behavior of real structures built from EUROFER 97. For verification the implementation has been applied to thermo-mechanical fatigue tests performed on tube specimens. Previous evaluation of these tests showed that thermo-mechanical fatigue loading leads to remarkably reduced lifetime in comparison to isothermal fatigue loading with the same mechanical strain range. In this evaluation changes in the stress range as well as actual test conditions, e.g. temperature gradients, were not considered. However, when applying the coupled deformation damage model changes in stresses between isothermal and thermo-mechanical loadings will be a priori taken into account. Even though the simulations have provided a better description and thus a modified evaluation for the thermo-mechanical tests considered they showed at the same time the lack in describing partial phenomena among others the reduced softening observed in this type of tests. Consequently, further modifications of the deformation constitutive equations are required in order to predict the lifetime correctly. These results underline once more the complexity in modeling the thermo-mechanical material behavior and the need of comprehensive research in this area.

Coupled effect of size, strain rate, and temperature on the shape memory of a pentagonalCu nanowire

A body-centered pentagonal nanobridge structure with lattice constantsc = 2.35 and a = 2.53 Å has been observed under high strain rate tensile loading on an initially constrained Cu nanowire at various temperatures. Extensive molecular dynamics (MD) simulations have beenperformed using the embedded atom method (EAM) for cross-sectional dimensions ranging from0.723 × 0.723 to2.169 × 2.169 nm2, temperature ranging from 10 to 600 K, and strain rates of109–107 s−1. Formations of such pentagonal nanowire are observed for a temperature range 200–600 Kfor particular cross-sectional dimensions and strain rates. A large inelastic deformation of∼50% is obtained under both isothermal loading and adiabatic loading. With very high degree ofrepeatability of such pentagonal nanowire formation, the present findings indicate that theinteresting stability property and high strength of elongated nanowires have variouspotential applications in nanomechanical and nanoelectronic devices. Further,we demonstrate a novel thermomechanical unloading mechanism by which it ispossible to impart recovery from a pentagonal nanowire following a hysteresisloop: .

Large deformation of anisotropic austenitic stainless steel sheets at room temperature: Multi-axial experiments and phenomenological modeling

A newly developed multi-axial testing technique for sheet materials is employed to investigate the inelastic response of a temper-rolled stainless steel 301LN under isothermal quasi-static loading conditions at room temperature. The experimental technique consists of a flat sheet specimen, which is subject to combinations of shear and normal loading using a custom-made dual-actuator system. The large deformation behavior under monotonic loading is determined along more than 20 distinct radial paths in the stress space. The experimental results indicate that Hill's quadratic yield function along with an associated flow rule provides a good approximation of the initial yield behavior of this anisotropic two-phase FCC/BCC sheet material. Based on the experimental data for radial monotonic loading, it is concluded that conventional isotropic-kinematic hardening models cannot successfully describe the strain hardening of this austenitic steel. Instead, a non-associated anisotropic hardening model is proposed that relates the increase in yield strength to an isothermal martensitic transformation kinetics law. The comparison of the model predictions with the experimental results shows very good agreement for all biaxial and uniaxial experiments.

Pseudoelastic Behavior of Cu–Al–Be Shape Memory Wires Subjected to Varied Loading and Unloading Cycles

Cyclic loading and unloading tensile tests were carried out for Cu-11.30wt. %Al-0.32wt. % Be Shape Memory Alloys (SMA) wires to investigate the effect of temperature and cyclic condition on the pseudoelastic behavior. It is shown that σc increases with the isothermal external temperature in single cycle tests and loading stress in constant stress increment tests while tends to a stable level after the 1st cycle in constant maximum strain tests. Both modes of loading and unloading improve the pseudoelasticity obviously.

Comparison of thermo-mechanical behavior of lead-free copper and tin–lead column grid array packages

Thermo-mechanical behavior of the lead (Pb)-free ceramic copper column grid array (CuCGA) package under accelerated thermal cycling is characterized and compared with the conventional tin–lead (Sn–Pb) ceramic column grid array (CCGA). In situ thermal deformations of the highest DNP (distance to neutral point) copper column is measured for an initial isothermal loading of Δ T = −75 °C and subsequent accelerated thermal cycling of −40 °C to +125 °C. The deformed shape of the column and the distribution of inelastic strains are measured from the displacement fields. The dominant deformation mode is bending of the column due to thermal expansion mismatch between the module and the printed circuit board (PCB). The results are compared with those of tin–lead CCGA tested under similar conditions. Unlike tin–lead columns, where the failure occurs at the column near the top of the solder fillet and through the thickness of the column, in the CuCGA, the failure is found to occur first in the solder fillet at the solder/copper column interface and the crack propagates along the periphery of the copper column. The accumulated plastic deformation per cycle is larger in tin–lead columns compared to the copper columns. A deformation mechanism is provided to explain the nature of this failure.

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide under isothermal loading

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide is investigated. Phase transitions are initiated by loading the specimen to 200 MPa under isothermal conditions. It is found out that deformation buildup and acoustic emission in the loading-unloading cycles are observed in the first cycle only, during further cycles the acoustic emission is comparable to the background, while the deformation buildup and recovery are not associated with martensitic transformations. It is shown that recovery of the deformation built up during loading occurs due to heating to 600 °C, with the major part of accumulated deformation undergoing recovery already at 250 °C and recovery of its minor part observed at 400 °C. This behavior of acoustic emission and accumulation and recovery of deformation provide evidence of martensitic phase stabilization during cycling of martensitic transformations under conditions of thermo-mechanical cycling.

Deformation and damage of RAFM steels under thermo-mechanical loading: A challenge for constitutive equations

A coupled deformation damage model recently developed for reduced activation ferritic martensitic (RAFM) steels and implemented in the finite element code ABAQUS was used to simulate thermo-mechanical fatigue tests performed on EUROFER 97 tube specimens. Previous evaluation of these tests showed that thermo-mechanical fatigue loading leads to remarkably reduced lifetime in comparison to isothermal fatigue loading with the same mechanical strain range, but changes in the stress range as well as actual test conditions, e.g. temperature gradients, were not considered. However, by applying the coupled deformation damage model, changes in stresses between isothermal and thermal loadings were taken into account. Results showed a local mechanical strain range that was greater than that obtained experimentally. However, the lifetime due to the calculated fatigue load was still higher than that was found experimentally. Some geometric nonlinear deformation instabilities are suggested, but must be verified.

Progressive Deterioration of Thermal Stability of Nanofilled Polypropylene

This article concentrates on polypropylene stability from the point of structural and thermal characterization of PP/nanocomposites through mechanical testing, X-ray diffraction, transmission electron microscopy (TEM), and thermogravimetric analysis. The composites were prepared from powdered polypropylene (PP), stabilizer, and nano-clays both natural and organophilic using a twin-screw extruder. The results showed that the type of nano-clay affects the morphology and mechanical and thermal properties of the composite, and not only in the expected way. An extreme decrease of PP durability under isothermal load was observed. This article is presented as the first part of a PP/nanocomposites degradation investigation.

Analysis of Multilayered Microelectronic Packaging Under Thermal Gradient Loading

An accurate estimation of interfacial and axial stresses in multilayered structures is important in the design process of microelectronic packaging because these stresses drive the failure modes in the package. During manufacturing, microelectronic packaging devices usually suffer from severe thermal gradients. Design engineers often simplify the thermal gradient case as an isothermal loading case by averaging the temperature of the top and bottom of the microelectronic packaging device. Such simplification usually underestimates the stress level in the devices. With the analytical model presented in this paper, the stresses in multilayered microelectronic packaging devices subjected to thermal gradient loading can easily be predicted. It is shown that ignoring the thermal gradient in the package leads to underestimation of stresses

Three Phase Micromechanical Modeling for a Shape Memory Alloy Reinforced Composite

In this paper, combined the micromechanical and the thermodynamic theory, a three phase model for the SMA composite is developed, in which the composite is considered as the austenitic phase, the product phase (martensite) and the matrix phase. In the present model, the interaction among the three phases is analyzed. From the micromechanical analysis, the macroscopic free energy function is found. Then macroscopic transformation strain, effective elastic compliance, macroscopic constitutive model are derived.Compared with the traditional two-phase method, non-linearity of SMA need not be considered. The method is not only simply but also the interaction among the three phases is considered. As an application of above model, we consider the case of a composite with NiTi/epoxy, illustrate the predicted stress-strain response of it under isothermal loading and unloading conditions and analyses the effects of temperature and fiber volume on macroscopic mechanical property. By comparing with references, it is shown that the results are credible. It is helpful to design the intelligent composite.

Study of Some Parameter Effects on Warpage and Bump-Joint Stresses of COG Packages

The purpose of this paper is to investigate the parameters such as thermal loads, adhesive material properties and fillets, bump materials, and moisture, affecting the warpage and bump-joint stresses of the chip-on-glass (COG) packages with anisotropic conductive film (ACF). Two- and three-dimensional finite-element models (FEMs) are employed for calculating the warpage and bump- joint stresses in terms of such parameters. In addition, the real-time full-field Twyman-Green interferometry is used for measuring the out-of-plane deformations of the specimens and providing validation of the warpage calculated from finite-element models. Also, Suhir's close-form solution of a die-substrate assembly is applied for looking into insight of thermomechanics, especially for adhesive stresses and correlated with finite-element results. The results show that the fillets strongly affect the warpage of the COG specimens by changing the convex shape of warpage for those without fillets to the saddle shape for those with fillets. However, unlike those with fillets, the warpage of the COG is insensitive to those with or without (gold or polymer) bumps. It is also found that the two-dimensional (2-D) FEM in consistence with Suhir's solution can be used for approximately addressing both warpage and bump-joint stress issues. The COG warpage results for thermal loads, from 2-D FEM, indicate that the maximum warpage is very sensitive to the modulus of the ACFs (especially for low modulus) rather than the thermal expansion coefficients of the ACFs, which are consistent with Suhir's solution. The stress results show that the COG with gold bumps provides higher bump-joint stresses (especially for compressive transverse normal stress, which is important for maintaining good electrical contact between bumps and tracks through the conductive particles in the ACF) than that with polymer bumps for the isothermal loaded case. Note that negative isothermal load here applied represents the cooling temperature change from an uniform elevated temperature of the ACF curing in the COG packaging. Furthermore, the transverse normal bump-joint stress increases three fold, when the fillets are present for the COG either with polymer bumps or with gold bumps. The similar mechanism is found in the case of the COGs under thermal-gradient loads. It is also seen that the higher the elastic modulus (E) and thermal expansion coefficient (CTE) of the ACF, the larger the compressive transverse normal bump-joint stress. It is also interestingly found that the volume swelling of the ACF due to moisture absorption provides very relatively large tensile transverse normal bump-joint stresses for the COGs with bumps. These tensile stresses could be one order of magnitude larger than the compressive values resulted from other parameters discussed here. As a result, the approaches to well controlling ACF moisture absorption and selecting low-moisture-expansion ACF materials are very critical to improving the reliability of the COG packages

High temperature deformation and damage behavior of RAFM steels under low cycle fatigue loading: Experiments and modeling

Structural materials of fusion reactors are subjected to complex creep–fatigue loading and high irradiation doses. Correct modeling of their deterioration is a precondition of a sufficiently reliable lifetime prediction procedure. In the continuum mechanics approach selected for lifetime prediction of RAFM steels under creep–fatigue conditions, the inelastic strain rate modified (ISRM) damage model is coupled with a modified viscoplastic deformation model taking into account the complex non-saturating cyclic softening of RAFM steels. The resulting coupled model is a powerful prediction tool, which can be applied to arbitrary creep–fatigue loading provided that the material, temperature and possibly irradiation dose-dependent parameters of the model have been determined. Therefore, a fitting procedure has been developed for the parameters identification on the base of deformation and lifetime data from strain-controlled low cycle fatigue (LCF) tests without and with hold time as well as creep tests. The coupled deformation-damage model has been applied to F82H mod and EUROFER 97 in the reference (unirradiated) state under isothermal cyclic loading at 450, 550 and 650 °C. The comparisons between model and experiment show that the observed lifetimes in the LCF tests could be fairly well calculated even for the tests with hold time, which were not considered for the identification of the damage model parameters.

Isothermal Fatigue Damage Model for Lead-free Solder

This article presents an investigation of lead-free solder material 95.5Sn-3.9Ag-0.6Cu under isothermal fatigue loading. The effects of strain rate on hysteresis loop and fatigue life over the total strain range from 0.4 to 1% at room temperature are investigated. A fatigue damage model is established to characterize the isothermal fatigue behavior of the material with the theory of damage mechanics. A fatigue failure criterion is proposed based on the concept of damage accumulation in solder material. The model takes into account the effect of strain rate in the development of constitutive and damage evolution equations. The life prediction is compared with the testing data and has shown a good agreement.