Dynamic adhesive forces in rough contacting bodies including normal and sliding conditions

Using a high-resolution, high-dynamic bandwidth capacitive force transducer and two piezoelectric actuators, adhesive pull-off forces between nominally flat rough silicon surfaces were measured under various dynamic conditions in normal and tangential directions and environmental humidity levels. The upper specimen approached and retracted with a constant velocity in the vertical (normal) direction, while the lower specimen started moving in the horizontal (tangential) direction during the middle of the contact. The experiments were performed under 35 and 60% relative humidity conditions. It was found that sliding of the contacting surfaces led to a significant reduction in pull-off forces under low-humidity contact conditions, whereas it caused higher pull-off forces under partially wet contact conditions. Comparing the effects of sliding velocity and sliding distance on the measured pull-off force values, it was found that the sliding distance played an important role in the increase in pull-off forces.     

Deformation kinetics of ageing materials

A constitutive equation of time-dependent, chemically stable materials, which stems from the basic ideas of the irreversible thermodynamics of an internal variable and Eyring's absolute reaction rate theory, has been extended to chemically unstable materials. This formulation is quite general and, in principle, can be applied to many types of materials. In this paper, the ageing behaviour of time-dependent network polymers undergoing chain scission is considered. In the network scission process, we postulate that the energy barrier is affected by a changing of the chemical crosslink density. An explicit equation to account for the energy barrier change, which influences the relaxation process, is formulated. For the purpose of illustration, the effect of different chemical crosslink density, ν, on the relaxation rate has been considered, from which the following theoretical expression of relaxation modulus ΔE(t) is obtained:

ΔE = ΔE[t exp (γv/kT)]

It can be seen that a change in v leads to an effective change in the time scale, usually denoted by a_x. Here the analytical expression a_x = exp(γν/kT) correlates quite well with the experimental data.     

Deformation and failure of polycarbonate in an electric field

Electrically induced mechanical stress was produced on a monolithic polycarbonate (PC) film when subjected to an instantaneous direct current voltage using a needle-plane electrode setup. Three different experimental methods were used to investigate the electrically induced mechanical deformation on the glassy PC film, namely, morphological observation, energy loss analysis, and dielectric hysteresis study. It was found that the PC film exhibited elastic behavior at the nominal electric field below 200 MV m⁻¹, showing no indentation on the film surface. When the nominal field was above 200 MV m⁻¹, a spherical indentation was created. The depth and diameter of the deformation increased in response to the applied electric field. Subsequent thermal annealing of the deformed film revealed a recoverable “delayed elastic” and an unrecoverable “plastic” deformation. A three-stage electrically induced mechanical deformation mechanism was proposed based on the experimental results, including a correlation between the energy loss and the deformed volume. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48341.     

Hypervelocity interaction of porous bodies with plates

The interaction of dense and porous cylinders with incident plates is studied experimentally at collision velocities of 3.3–4.6 km/sec. Qualitative differences in the deformation mechanics of compact porous bodies are elucidated. The compression of porous samples is found to take place in two stages. These data can be invoked to explain the structural features of impact craters on planetary surfaces. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 133–135, May–June 1999.     

Deformation kinetics of pH‐sensitive hydrogels

Polymeric gels can undergo large deformation when subjected to external solutions of varying pH. It is imperative to understand the deformation process of pH‐sensitive hydrogels for the effective application of these attractive materials in the biomedical and microfluidic fields. In the modeling of these multi‐phase materials, finite element (FE) modeling is a useful tool for the development of future applications, and it allows developers to test a wide variety of material responses in a cost‐effective and efficient manner, reducing the need to conduct extensive laboratory experiments. Although a FE user‐defined material model is available for the equilibrium state, the transient response of pH‐sensitive gels has not been effectively modeled. Based on our recent work using the heat transfer analogy to tap into the readily available coupled temperature–displacement elements available in the commercial FE software ABAQUS for simulation of the transient swelling process of neutral hydrogels, the transient swelling process of a pH‐sensitive hydrogel is studied and a FE model is further developed to simulate the transient phenomena. Some benchmark examples are investigated to demonstrate the model's capabilities in the simulation of nonlinear deformation kinetics relevant to several applications of pH‐sensitive hydrogels. © 2013 Society of Chemical Industry     

故障模型和影响分析法对偏二甲肼贮罐的安全评价

为确保偏二甲肼(UDMH)贮罐在使用过程中的安全可靠,采用故障模型和影响分析(FMEA)法对其进行了安全分析,找出了贮存过程中可能会出现的故障类型及机理。风险优先数数据显示,装配缺陷、材料缺陷、焊接缺陷以及错误操作等是高风险故障类型。针对风险故障提出改进措施,再一次对改进后的UDMH贮罐进行安全评价,结果表明故障发生的概率及严重度大大降低。     

Rotational components of deformation in metal bodies under dynamic loading

The plane problem of an initially quiescent cylinder subjected to a gradient flow of a highly viscous fluid is solved. It is shown that the cylinder is brought into rotation with angular velocity ω=−εé, where εé in the shear-strain rate of the medium. The solution is used to analyze flows under high-rate loading of metallic bodies that occurs at the level of the microstructure. The appearance of rotation is associated with the presence of fragments in the structure of the substance (grains, fragments, cells, and inclusions) that are unable to change their shape under the given loading conditions and, consequently, begin to rotate in the process of shear strain. Rotations that occur at the microlevel during joint deformation of solid bodies lead to transfer of oxide, hydroxide, and other surface films into the depth of the material, and this contributes to formation of a bond. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 129–133, March–April, 1998.     

Deformation and failure of polymeric matrices under swelling conditions

Polymers in which the diffusion mechanism was characterized by a sharp advancing boundary between the swollen shell and the core showed a highly anisotropic swelling response. The anisotropy of the swelling strains was caused by the mechanical constraints exerted mutually by the two regions of the specimen. The swelling stresses developing during the process eventually led to fracture of the polymer specimen. An analytical model which explained the modes of failure of the polymer under the swelling stresses was developed. The proposed approach was based on the general analogy existing between the studied swollen specimens and composite materials. A model for the prediction of the anisotropic hygroelastic response of the swollen systems was also proposed.     

Deformation kinetics of polypropylene hollow fibers in a continuous drawing process

Effects of isothermal drawing conditions on the deformation kinetics and dimensional change of polypropylene (PP) hollow fibers in a continuous drawing process were investigated. The deformation behavior of solid PP polymers during stretching between two rolls in the isothermal bath was analyzed by a simple model describing the continuous drawing process with a constitutive relation that can express a true (stress–strain–strain rate) surface of solid semicrystalline polymers. Necking profiles during drawing can be calculated from this model without any special assumption for neck criterion, and the calculated results predict that the localization of deformation is promoted with the increase of applied draw ratios. It is also found that at 20°C, the neck is observed apparently both from the calculated and experimental results, and the strain‐rate sensitivity parameter is considered to be a critical factor that determines the intensity of the neck geometry. The calculated drawing forces are shown to increase with increasing the applied draw ratio and decreasing the drawing temperature, and these trends were verified by experimental results. The hollowness, defined as the ratio of inner to total cross‐sectional area, increases as it is drawn at 30°C, but decreases as drawn above this temperature compared with that of the undrawn fiber. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1836–1845, 1999     

Highly efficient phosphate diester hydrolysis and DNA interaction by a new unsymmetrical FeIIINiII model complex

A new heterodinuclear mixed valence complex [Fe^IIINi^II(BPBPMP)(OAc)₂]ClO₄ 1 with the unsymmetrical N₅O₂ donor ligand 2-bis[{(2-pyridylmethyl)-aminomethyl}-6-{(2-hydroxybenzyl)(2-pyridylmethyl)}-aminomethyl]-4-methylphenol (H₂BPBPMP) has been synthesized and characterized. 1 crystallizes in the monoclinic system, space group P2₁/n, a=12.497(2), b=18.194(4), c=16.929(3) Å, β=94.11(3)°, V=3839.3(12) ų and has an Fe^IIINi^II(μ-phenoxo)-bis(μ-carboxylato) core. Solution studies of 1 indicate that a pH-induced change in the bridging acetate occurs and the formation of an active [(OH)Fe^III(μ-OH)Ni^II(OH₂)]⁺ species as the catalyst for phosphate diester hydrolysis and DNA interaction is proposed. In addition, the results presented here suggest that Ni^II would be a good candidate as a substitute of M^II in purple acid phosphatases.     

Spark plasma sintering of B4C and B4C-TiB2 composites: Deformation and failure mechanisms under quasistatic and dynamic loading

Spark plasma sintering (SPS) was employed to consolidate powder specimens consisting of B₄C and various B₄C-TiB₂ compositions. SPS allowed for consolidation of pure B₄C, B₄C-13 vol.%TiB₂, and B₄C-23 vol.%TiB₂ composites achieving ≥99 % theoretical density without sintering additives, residual phases (e.g., graphite), and excessive grain growth due to long sintering times. Electron and x-ray microscopies determined homogeneous microstructures along with excellent distribution of TiB₂ phase in both small and larger-scaled composites. An optimized B₄C-23 vol.%TiB₂ composite with a targeted low density of ～3.0 g/cm³ exhibited 30–35 % increased hardness, fracture toughness, and flexural bend strength compared to several commercial armor-grade ceramics, with the flexural strength being strain rate insensitive under quasistatic and dynamic loading. Mechanistic studies determined that the improvements are a result of a) no residual graphitic carbon in the composites, b) interfacial microcrack toughening due to thermal expansion coefficient differences placing the B₄C matrix in compression and TiB₂ phase in tension, and c) TiB₂ phase aids in crack deflection thereby increasing the amount of intergranular fracture. Collectively, the addition of TiB₂ serves as a toughening and strengthening phase, and scaling of SPS samples show promise for the manufacture of ceramic composites for body armor.     

The sintering kinetics of porcelain bodies made from waste glass and fly ash

Porcelain is a material produced from kaoline, quartz and potassium-feldspar. Recently, research of new materials, for example non-hazardous wastes, that are able to replace traditional fluxing agents without changing the process or quality of the final products has been realized. The aim of this work is to study the possibility of the use of glass powder waste and fly ash together for manufacturing porcelain. Instead of quartz, fly ash was used at the selected porcelain composition. The waste glass was added partially and fully in replacement of potassium-feldspar. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1100, 1150 and 1200 °C for a period of 1, 2, 3 and 5 h. The sintered samples were characterised by XRD (X-ray diffraction) and SEM (scanning electron microscopy). Sintering activation energies were determined based on the bulk density result. At 10, 15, 20 and 25 wt.% glass waste addition, the apparent activation energies were calculated to be 145, 113.5, 70.4 and 53.74 kJ/mol, respectively. It was found that the sintering activation energy decreased with increasing waste glass addition.     

Isothermal and dynamic reaction kinetics of high performance epoxy matrices

The reaction kinetics of the epoxy matrix of a commercial prepreg for high performance composites, based on tetraglycidyl diamino diphenyl methanediamino diphenyl sulfone (TGDDM-DDS) formulation, has been characterized by differential scanning calorimetry (DSC). A phenomenological kinetic model, able to describe the behavior of the system during normal processing operations has been formulated. The diffusion control phenomena, related to the evolution of the glass transition temperature as a function of the degree of polymerization, has been considered in the formulation of the kinetic model. Isothermal and dynamic tests have been used to calculate the model parameters and to verify the proposed model. The model is able to describe incomplete reactions in isothermal tests and heating rate dependence of dynamic test thermograms, and it has been also successfully applied to DSC experiments performed with complex thermal conditions commonly used in the processing of high performance composites.