Measurement of Dynamic Properties of Viscoelastic Materials

An improved method to measure the dynamic viscoelastic properties of elastomers is proposed. The method is based on the analysis of forced oscillation of a cylindrical sample loaded with an inertial mass. No special equipment or instrumentation other than the ordinary vibration measurement apparatus is required. Upper and lower surfaces of the viscoelastic material sample were bonded to a load disc and a rigid base plate, respectively. The rigid base plate was subject to forced oscillations driven by a vibration exciter. Two accelerometers were attached to monitor the displacement of the base plate and the load disc. The recorded magnitude ratio and the phase difference between the load disc and the base plate vibrations represent the axial, dynamic deformation of the sample. The data are sufficient to obtain the dynamic properties of the sample, oscillation properties of vibration exciter, whereas the sensitivity of gauges having no effect on the calculation results. For accurate calculation of the properties, a two-dimensional numerical model of cylindrical sample deformation was used. Therefore, a form factor, which takes into account the sample sizes in one-dimensional models, is not required in this method. Typical measurement of the viscoelastic properties of a silicone rubber Silastic® S2 were measured over the frequency range from 10 Hz to 3 kHz under deformations (ratio of vibration magnitude to sample thickness) from 10^?4% to 5%. It was shown that the modulus of elasticity and the loss tangent fall on a single curve when the ratio of load mass to sample mass changed from 1 to 20. When the sample diameter was varied from 8 to 40 mm, the modulus of elasticity fall on the same curve, but the loss tangent curves showed some degree of scatter. Studied temperature dependence and nonlinear behavior of viscoelastic properties is found not to be associated with this effect.     

On the Failure and Dynamic Performance of Materials

Waves induced by impact initiate deformation mechanisms within a material that precede later flow. An impulse excites a cascade of deformation mechanisms starting with ultrafast and concluding with slower ones. In metals, brittle glasses and polycrystalline ceramics there are a combination of mechanisms with differing relaxation times that condition a loaded target. In the case of ballistic impact, once failure has occurred, long rod penetration can occur and the depth achieved within each target can be scaled with the deformation strengths recorded during the initial high pressure impulse. A review of material shock response and target preconditioning shows a correlation with the ballistic penetration of the target after loading. This indicates that the effect of an initial loading impulse upon material behaviour is a strong feature of the effects observed in many dynamic phenomena.     

Shrinkage Measurement in Concrete Materials using Cure Reference Method

This paper describes the details of using Cure Reference Method (CRM) to determine the shrinkage that develops in concrete materials. The technique involves the replication of diffraction grating on the concrete specimen during curing. After demolded, the specimen is stored in a chamber where a specific drying condition is maintained for 6 days. Every day in this period, the specimen is removed from the chamber and a set of the consecutive moiré fringe patterns are recorded with the help of a specially-designed stage. An automated fringe analysis program is developed to obtain the full-field displacement and strain information. Shrinkage as a function of location, time, and drying conditions is measured. A numerical method is developed in order to obtain material properties from the complex geometry used in the tests. The test in different drying condition and the ring test are performed, and their results are compared with FEA to validate the constructed model.     

Dynamic Measurement of Two Dimensional Stress Components in Birefringent Materials

We adopt classical methods of photoelasticity and Mach–Zehnder interferometry in a combined arrangement in order to determine both principal stresses and their orientations simultaneously. The method is equally applicable to static and dynamic problems. In dynamic problems the measurement may be made with a high-speed photodetector at very high temporal resolution at a single point or a small array of points depending on the detector array and recording device; this eliminates the need for a high-speed photographic system, but more importantly the method described here provides complete, time-resolved evolution of all stress components. Examples of application of the method are demonstrated.     

Dislocation Model of Polysynthetic Shear Bands in Amorphous Materials

A dislocation model for a polysynthetic shear band in an amorphous material is proposed. The stress fields near the polysynthetic shear band are calculated. The distribution of impurities in an amorphous binary Fe–B medium containing a polysynthetic shear band is determined.     

Water Vapor Adsorption in Porous Building Materials: Experimental Measurement and Theoretical Analysis

An experimental and theoretical analysis of the water vapor adsorption in several types of porous building materials is presented. For the measurement of adsorption isotherms, a DVS-Advantage water sorption device is used. The experimental data is analyzed using theoretical formulas based on the BET, BSB, BDDT, and FHH isotherms, assuming a mono as well as multi-layer water vapor adsorption. The BSB equation is found to provide a good approximation for the relative humidities below 0.6–0.7, whereas the FHH equation shows a sufficient accuracy for the relative humidities above 0.4–0.5. Based on a combination of BSB and FHH isotherms, a semi-empirical formula is proposed that allows one to obtain a very accurate approximation of experimental data for all analyzed materials and all values of the relative humidity.     

Measurement of the Gruneisen Coefficient of Some Anisotropic Carbon Materials

Results of measurement of the Gruneisen coefficient of two anisotropic materials (4KMS carbon-carbon composite and UPV-1 pyrolytic carbon) are presented. The values of the Gruneisen coefficient are calculated on the basis of the measured amplitude of mechanical stresses arising in the material with absorption of electron-beam energy. The results for the 4KMS composite include not only a negative value of the Gruneisen coefficient in the range of electron energy density of several tens of joules per square centimeter but also its anomalously low value as compared to other materials. Significant decay of the stress wave propagating over this material is noted. The Gruneisen coefficient of the UPV-1 sample varies depending on the sample orientation. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 6, pp. 171–179, November–December, 2005.     

The Alternative Model of Water Vapour Sorption in Porous Building Materials

Mathematical modelling of water sorption in porous building materials is considered. The explanations of inadequacies of both Brunauer?CDeming?CDeming?CTeller model for the high relative humidities range and Frenkel?CHalsey?CHill model for the low relative humidities range reported by Pavlík et?al. (Transp. Porous Med. 91:939?C954, 2012) are proposed. The generalized D??Arcy and Watt (GDW) model is proposed as a simpler alternative for a procedure of experimental isotherms fitting proposed by Pavlík et?al. The suitability of the GDW equation to describe the water sorption isotherms in the building materials for the whole range of relative humidities is confirmed.     

A Two-Parametric Model for Gas Flow in Low-Permeable Porous Materials

Transport in Porous Media - Capillary flow at low pressure gradients is determined by the pore structure of the matrix and the viscosity of the fluid. Traditionally, the conductivity of porous...     

加筋材料的格形模型和统计数值方法

本文采用格形化方法和统计技术建立加筋复合材料有限元力学模型,使用自动选取载荷步长方法和非平衡迭代技术,对加筋复合材料的宏观等效模量和破坏全过程进行了数值模拟,分析了材料分布的非均匀程度,相对体积比和横截面加筋分布方式对加筋复合材料整体宏观等效模量和承载力的影响。     

Mathematical Model of Bone Regenerate Tissue Obtained as a Result of Bone Growth Under Tension in an External Fixation Apparatus

A mathematical model of the deformation and growth of a bone regenerate is proposed. The model makes it possible to predict the length of the growth zone and to describe the cell composition in the regenerate under different regimes of fixing ring displacement. The elementary dependence of the change in the growth zone length on the problem parameters is studied for the one-dimensional case using a simplified model with the cell activity taken integrally into account. The tissue solidification is related to the intra-tissue calcium deposition. Plots of the length of the growth zone versus time are considered for varying blood calcium content and blood vessel wall permeability.     

A Theoretical Model for the Simulation of Microdamage Accumulation and Repair in Compact Bone*

This paper describes a new theoretical approach to bone microdamage, in which a population of cracks is explicitly modelled. A given sample of bone is assumed to contain a certain number of cracks, whose growth characteristics are described with an equation containing stochastic variables to create statistical differences from one crack to another. This type of model allows us to predict a wide variety of data. The present paper illustrates the different types of prediction which can be made, including: (i) standard damage parameters such as the number and length of cracks and the reduction in stiffness; (ii) fatigue test data such as the number of cycles to failure as a function of stress level, including scatter; (iii) effects due to the living system, including repair, remodelling and adaptation. A useful feature of the model is our ability to examine the statistics of the crack population in detail to find, for example, the number of cracks which are potentially dangerous as opposed to those which are dormant, and to investigate the reasons for increased crack numbers in the bones of older people. The potential also exists to use the model to investigate different theories of bone remodelling and adaptation.     

Towards Criteria Characterizing the Metrological Performance of Full-field Measurement Techniques

Users of full-field measurement methods like Digital Image Correlation (DIC) often aim to perform measurements with the best trade-off between spatial resolution, bias and measurement resolution. Whenever two full-field methods are compared, it is essential that these criteria are taken into consideration. Recently a metrological efficiency indicator for full-field measurements has been proposed and discussed. This indicator combines measurement resolution and spatial resolution. It has been shown to be invariant to the subset size in the case of Local DIC. The goal of this article is to discuss a method, which determines both the spatial and the measurement resolutions for a given bias for two different DIC methods, in order to obtain the metrological efficiency indicator for each of these methods. The benefit of this indicator is that it does not depend on setting parameters such as the subset size, which are chosen by the user. As such, it can be considered as intrinsic to each technique, thus enabling fair comparison. Local DIC and triangular finite element based Global DIC will be the subject of this investigation. With this setting, their respective subset and triangular element sizes will be related to the spatial resolution of both methods for a given acceptable bias. By using the metrological efficiency indicator, the performance of the two methods will be compared and discussed to a new level of detail. Generally speaking, the indicator shows that the metrological performance of both methods is similar, confirming their popularity. However, it will be shown that, depending on the choice of what an acceptable bias is, one of the method may be preferred to another. The results show that for the specific DIC versions used in the study, for cases for which a significant bias is acceptable, Local DIC outperforms Global DIC, while the opposite is true in the case for which the bias requirements are more stringent. Finally, the quadratic versions of both DIC versions are shown to significantly outperform their respective linear versions.

     

Performance Dependent Model for normal and high strength concretes

A new constitutive formulation, the so-called Performance Dependent Model valid for normal and high strength concretes is presented. The distinctive aspect of the proposed model is the consideration of relevant properties of concrete mix components in the evaluation of the involved material performance or quality at the macroscopic stand point. In this way, the composite features of concrete are appropriately taken into account.The model maximum strength surface is defined by means of the Performance Dependent Failure Criterion proposed by the authors in previous works. Concrete behaviors in pre and post peak regimes are modeled with a non uniform hardening law and an isotropic softening rule, respectively. To realistically reproduce the concrete ductility in pre and post peak regimes under different load scenarios, the hardening and softening laws are defined in terms of the acting confining pressure. Concrete dilatancy behavior is approached by means of a volumetric non associative flow rule. The softening law is embedded in fracture energy concepts for mode I and II types of failure. The model considers two main input material parameters: the uniaxial compressive strength and the performance parameter, a quality index defined in the context of the Performance Dependent Failure Criterion.The proposed constitutive model is able to capture the substantial differences in the failure behavior of normal and high strength concretes as well as of concretes with the same compressive strength but different mix components. The predictive capability of the model is demonstrated in the numerical analyses included in this paper where the numerical predictions are compared with experimental results related to concrete specimens of different qualities and subjected to stress histories under both compressive and tensile regimes.     

Methodology for the Stress Measurement of Ferromagnetic Materials by Using Magneto Acoustic Emission

Magneto acoustic emission (MAE) is a magnetic nondestructive testing (NDT) method that has been used in different fields for 30 years. MAE is a promising method for the early characterization of damage and the evaluation of the residual stress of ferromagnetic materials. However, this technique is still in its early stage and requires further development. The mechanism and influence factors of MAE are still under investigation. Quantitative NDT is difficult because of the lack of robust theoretical bases and models. In this study, we investigated the influence factors of MAE signals systematically and established a mathematical model to describe these influences. The special design of the specimen and the precise control of experimental conditions are the key points for obtaining reasonable experimental results and for developing the model. A methodology for stress assessment was developed on the basis of the proposed model and was verified by using the pure bending test. Results show that stresses within a measureable depth of 4 mm can be evaluated and that the maximum testing error is 30 %.     

Thermomechanical Model of Growing Cylindrical Bodies Made of Physically Nonlinear Materials

The finite-element method is used together with the theory of growing bodies to model the residual stress-strain state of cylindrical bodies with built-up layers. The case of two built-up layers is analyzed. Numerical and experimental results are compared __________ Translated from Prikladnaya Mekhanika, Vol. 41, No. 9, pp. 118–126, September 2005.