The brittle compressive failure of orthotropic ice under triaxial loading

Experiments on the brittle compressive failure of S2 fresh-water columnar ice loaded triaxially at −10°C at a strain rate of 6×10⁻³/s have revealed three regimes of Coulombic-like behavior: 1 of lower across-column confinement where the along-column stress has no significant effect on the strength; 2 of higher across-column confinement where the along-column stress increases the strength; and 3 of predominantly along-column loading where the strength increases in proportion to the smaller of the two orthogonal across-column confining stresses. Within each regime the strength of the fresh-water ice is indistinguishable from that of porous salt-water ice (Gratz and Schulson, J. geophys. Res., 1997, 102, 5091) of otherwise similar microstructure, implying similar failure mechanisms. Both kinds of ice fail through macroscopic shear faulting which results from the linking up of cracks formed during the deformation. The principal difference between the two materials is that the deformation damage to the salt-water ice is restricted more to the vicinity of the faults, owing to the crack-stopping influence of pores incorporated during growth. Terminal failure is explained in terms of a new mechanism involving the bending of “splay cracks” under frictional sliding. “Splay cracks” are feather-like deformation features that emanate from one side of parent, sliding cracks. The idea is that slender microcolumns between the “splay cracks” are more likely to bend and break than are microcolumns between adjacent wing cracks because they do not have two fixed ends; instead, the end stemming from the sliding interface is free. A moment is then applied by frictional sliding. A first-order calculation shows that the stress required to break the columns and thus to initiate the fault is of the same order of magnitude as the terminal failure stress.     

Simulating of marble subjected to uni-axial loading using index-parabola damage model

1　ＩＮＴＲＯＤＵＣＴＩＯＮＲｏｃｋｉｓａｈｉｇｈｌｙｎｏｎｌｉｎｅａｒｍｅｄｉｕｍ ,ｗｈｏｓｅｂｅ ｈａｖｉｏｒｃａｎｂｅｃａｔｅｇｏｒｉｚｅｄｉｎｔｏａｎｉｎｉｔｉａｌｌｉｎｅａｒｒｅｓｐｏｎｓｅｆｏｌｌｏｗｅｄｂｙｐｌａｓｔｉｃｏｒｔｈｅｗｅａｋｅｎｅｄｐｈａｓｅａｆｔｅｒａｔ ｔａｉｎｉｎｇｔｈｅｐｅａｋｓｔｒｅｓｓ[1 ] .Ｔｒａｄｉｔｉｏｎａｌｌｙ ,ｔｈｅａｖｅｒａｇｅｍｏｄｕｌｕｓｏｆｒｏｃｋｉｓｔａｋｅｎａｓ 50 %ｏｆｅｌａｓｔｉｃｍｏｄｕｌｕｓｏｆｔｈｅｐｅａｋｓｔｒｅｓｓｉｎｔｒａｄ…     

Closing law and stress intensity factor of elliptical crack under compressive loading

The solution of surface displacement of an elliptical crack under compressive-shear loading was obtained by using the comples function method.The closing mode was established by analyzing the geometrical condition of closing crack,and the corresponding critical stress was solved.The result corrects the traditional viewpoint.in which there exist only open or close states for an elliptical crack,and points out that the local closing is also one of crack states.Based on them,the effect of the closed crack on stress intensity factor was discussed in detail,and its rational formulae are put forward.     

Modeling crack growth from pores under compressive loading with application to metallic glasses

The mechanism of the fatigue-crack growth is essential to understand the fatigue and fracture behavior of bulk metallic glasses (BMGs) and is thus critical to predict the service lifetime of BMGs as potential engineering structural materials. Experiments indicate that fracture under compressive loading exhibits distinct behaviors different from that under tensile loading. A typical compression failure may initiate from micro porosity where cracks propagate in a direction generally parallel to the loading axis. Micromechanical stress analysis shows that pores cause axial tensile microcracks emanating from the pore. A simplified computational model based on the linear elastic fracture mechanics (LEFM) is proposed to investigate crack initiation and subsequent propagation under compressive load, where the effect of crack closure on mode-I fracture is considered. The stable crack length is characterized by a dimensionless fracture-mechanics quantity required to attain the associated crack length. The behavior of crack growth is examined based on the stress-intensity-factor (SIF) calculation, and its dependence on the loading and lateral confinement conditions is discussed.     

The mechanical response of ceramic microballoon reinforced aluminum matrix composites under compressive loading

An investigation is performed on the mechanical response of a family of ceramic microballoon reinforced aluminum matrix composites under both uniaxial compression and constrained die compression loadings. The key material parameters that are varied are the matrix strength and the ratio of wall thickness t to radius R of the microballoons. Uniaxial compressive failure initiates at relatively small strains (≈1–2%) and occurs through a process of crushing and collapse of the material within a localized deformation band. Under constrained die conditions, localization is suppressed and the flow stress increases monotonically with increasing strain. The latter response is well described by Gurson's constitutive law for plastic yielding of porous ductile metals, with an effective strength that depends on the relative wall thickness, t/R. Furthermore, the energy absorption capacity (≈60–70 MJ/m³) is extremely high in comparison with values that are typical of metal foams. The results suggest that the microballoon composites may be attractive for applications requiring a high resistance to penetration by projectiles or other forms of local intrusion, in combination with a high compressive strength.     

Deformation behavior of Zr-based bulk metallic glass in an undercooled liquid state under compressive loading

High temperature deformation behavior of a Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk amorphous alloy has been studied in a temperature range between 355 and 460°C under compressive loading after rapid heating. A transition of flow behavior, viz. from, a Newtonian to a non-Newtonian flow, has been reported by many researchers as the temperature is decreased at a given strain rate. In the present study, two different theoretical relations based on a viscous flow model and a transition state theory have been applied to analyze the transition behavior of deformation in terms of viscosity and flow stress. An experimental deformation map was then constructed to specify the boundaries between Newtonian and non-Newtonian flow, based on the relationship between the flow stress and strain rate in an undercooled liquid state. It has further been confirmed that the stress overshoot phenomena can be observed mostly in a non-Newtonian flow regime appearing in an intermediate temperature and strain rate region in this deformation map.     

Failure of Alzheimer’s Aβ(1–40) amyloid nanofibrils under compressive loading

Amyloids are associated with severe degenerative diseases and show exceptional mechanical properties, in particular great stiffhess. Amyloid fibrils, forming protein nanotube structures, are elongated fibers with a diameter of ≈8 nm with a characteristic dense hydrogen-bond (H-bond)patterning in the form of beta-sheets (β-sheets). Here we report a series of molecular dynamics simulations to study mechanical failure properties of a twofold symmetric Aβ(l–40) amyloid fibril, a pathogen associated with Alzheimer’s disease. We carry out computational experiments to study the response of the amyloid fibril to compressive loading. Our investigations reveal atomistic details of the failure process, and confirm that the breakdown of H-bonds plays a critical role during the failure process of amyloid fibrils. We obtain a Young’s modulus of ≈12.43 GPa, in dose agreement with earlier experimental results. Our simulations show that failure by buck-ling and subsequent shearing in one of the layers initiates at ≈1% compressive strain, suggesting that amyloid fibrils can be rather brittle mechanical elements.     

A study of mechanisms of domain switching in a ferroelectric material via loading rate effect

A method to characterize the strain–electric field butterfly behavior based on the underlying domain switching mechanism is first presented. The effect of loading rate on the different characteristics of the strain–electric field butterfly behavior is then studied. By comparing the changes in these characteristics under different loading rates, it is established that the loading rate dependence of the strain–electric field butterfly behavior is mainly due to two factors: (i) the dependence of the switching of individual domains on the magnitude and duration of the loading time; and (ii) the variation of the transition electric field with the loading rate. Finally, the stability of switched domains is investigated by unloading and reloading the electric field at several predetermined values in the loading cycle. Several interesting attributes of the domain switching behavior that may shed further light on understanding the underlying mechanism of domain switching is illustrated in the present study. The present study also demonstrates that the proposed method of characterizing the strain–electric butterfly behavior based on the underlying domain switching mechanism is very effective for studying ferroelectric behavior under different loading conditions.     

Viscoelastic behavior of HDPE polymer using tensile and compressive loading

Large containers for liquids, exposed to different static loadings, are mainly made of high-density polyethylene (HDPE). The viscoelastic response of HDPE under long-term tensile and compressive creep is investigated. Monotonic experiments under tension are performed over a wide range of strain rates. In these experiments, the transition in the damage mechanisms (development and propagation of contraction in the HDPE specimen) is analyzed. The monotonic tensile behavior of the HDPE is found to be nonlinear and depends on the strain rates. It is observed that both elastic modulus and plastic flow stress present an increase with displacement speed due to the viscoelastic behavior of HDPE. A similar observation can be made for monotonic compressive tests by developing a new experimental device that ensures accurate measurement of the strain. Such a device makes use of an extensometer of compressive displacement of the specimen. In addition, the long-term behavior of HDPE is evidenced through creep and relaxation tests at an imposed range respectively of lower stresses and strains. It is shown that the normalized curves, associated with these tests, can be represented by a single curve characterizing the compressive creep compliance or relaxation stresses versus time. The linearity of the viscoelastic behavior is confirmed within the linear domain of the monotonic compressive and tensile tests.     

Lattice strain evolution during tensile and compressive loading of CP Ti

The micromechanics of textured Grade 1 commercially pure titanium are examined using neutron diffraction, self-consistent modelling and microscopy. It is found that twinning produces greater hardening than slip, that the residual lattice strains produced in the (0 0 0 2) are on the order of 0.001, and that both compression and tension twins are observed for both tensile and compression straining. The critical resolved shear stresses found are consistent with the macroscopic flow curves, lattice strains and textures produced, but are much lower than those found using uncorrected focused ion beam-milled micromechanical testing. The twins observed in axial compression were thicker than those found when compressing in the hoop direction, which is taken to imply that axial compression produced a greater number of twinning dislocations that could result in twin thickening.     

不同加载速率和围压作用下沥青混凝土三轴压缩力学特性数值模拟研究

为研究加载机制下路面沥青混凝土的力学特性,利用颗粒流数值模拟软件PFC3D建立满足实际沥青混凝土粒径级配的数值模型,并对其进行三轴压缩试验,分析不同围压及加载速率下沥青混凝土的力学特性.结果表明:同一围压下,随着加载速率的逐渐增大,试样的峰值应力、峰值应变和弹性模量均呈逐渐递增的趋势,以围压0.5 MPa为例,加载速率...     

Uniaxial step loading test setup for determination of creep curves of oxidation-sensitive high strength materials in vacuum under tensile and compressive load

For long-term applications of components, such as in turbomachinery or automotive engineering, knowledge of creep behavior under increased load and temperature is of interest. Creep tests are commonly used to investigate the creep behavior of materials at a constant test temperature above room temperature under a constant force. The present work describes a so-called uniaxial step loading creep test setup and first results for a WC-Co hard metal under isothermal conditions at 700 °C in vacuum. Heating and temperature control within the tested specimen's gauge length were performed by induced eddy currents and a thermocouple, respectively. In contrast to conventional creep tests, the mechanical load is increased stepwise and the stress at each level is kept constant for 500 s. Displacement of the strain gauge markings was measured contactless with a laser extensometer. First tests were carried out for a WC-Co hard metal under compression and tensile loading. In order to avoid buckling of the high-strength material under compression, a special specimen geometry with non-constant specimen diameter was used. The minimum creep rate was determined for each applied tensile and compressive stress level. Under tensile load, minima of the creep rate were observed above a stress of 500 MPa that are interpreted as the secondary creep rates. Under compressive load, the respective creep rate minima were observed above a stress of −700 MPa.     

Effect of thermal residual stresses on yielding behavior under tensile or compressive loading of short fiber reinforced metal matrix composite

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聚合物膜上的微尺度金属导线在力电耦合作用下的屈曲行为(英文)

研究一种典型的微尺度康铜线/聚合物膜结构在力电耦合作用下的屈曲行为。根据卸载后康铜线从聚合物基底上屈曲的现象,评估康铜线与聚合物基底间的界面韧性。此外,力电耦合作用还会诱发新的失稳模态。在电载荷和拉伸载荷作用下,康铜线易在屈曲时发生断裂。在电载荷和压缩载荷作用下,康铜线易在屈曲区域的顶部发生折叠,偶尔还会向聚合物基底内部方向发生屈曲。分析了这些失稳模态的产生机理。     

Load partitioning during compressive loading of a Mg/MgB2 composite

A composite, consisting of 68 vol.% superconducting continuous MgB₂ fibers aligned within a ductile Mg matrix, was loaded in uniaxial compression and the volume-averaged lattice strains in the matrix and fiber were measured in situ by synchrotron X-ray diffraction as a function of applied stress. In the elastic range of the composite, both phases exhibit the same strain, indicating that the matrix is transferring load to the fibers according to a simple iso-strain model. In the plastic range of the composite, the matrix is carrying proportionally less load. Plastic load transfer from matrix to fibers is complex due to presence in the fibers of a stiff WB₄ core and of cracks produced during the in situ synthesis of the MgB₂ fibers from B fibers. Also, load transfer behavior was observed to be different in bulk and near-surface regions, indicating that surface measurements are prone to error.     

Particle loading effect in cold spray

Cold gas dynamic spray is a line-of-sight, high-rate material deposition process that uses a supersonic flow to accelerate small particles (micron-sized) above a material-dependent critical velocity. When the particles impact the substrate, they plastically deform and bond to form a coating. The objective of this research is to investigate the influence of the particle mass flow rate on the properties of coatings sprayed using the cold spray process. Varying the mass flow rate at which the feedstock particles are fed into the carrier gas stream can change the thickness of the coating. It was shown that poor coating quality (peeling) was not a result of flow saturation but, instead, the result of excessive particle bombardment per unit area on the substrate. By increasing the travel speed of the substrate, this can be overcome and well-bonded dense coatings can be achieved. It has also been shown that by heating the carrier gas flow poor coating quality is avoided. The original version of this paper was published in the CD ROM Thermal Spray Comects: Explore Its Surfacing Potential, Interational Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmBH, Düsseldorf. Germany.     

Superplastic deformation under complex loading conditions

The initial yield surface of a superplastic material was investigated by using a combined loading of axial force and torque. The thin-walled tubular specimen made of Zn-22 wt.%A1 alloy was used in the experimental part of this study. Tests were carried out at room temperature (293 K) and at elevated temperature (523 K). The experimental results show that the material tested did not deform superplastically at 293 K exhibiting a yield surface that can be described by the second invariant of stress deviator (by Mises’ criterion). On the other hand, the material was found to deform superplastically at the higher temperature of 523 K with a yield surface more complex compared to that observed at 293 K. This difference may be attributed to the difference in major deformation mechanism at two applied temperatures; that is, the slip within the grains at 293 K and the grain boundary sliding at 523 K.     

考虑材料拉压异性的三层预应力组合凹模设计

运用统一强度理论,对用于加工废油资源化装备油泵部件等受工作内压作用的三层预应力组合凹模的分层半径、过盈量和弹性极限内压进行详细分析,得到了组合凹模的分层半径、过盈量和弹性极限内压的表达式,并运用所推导公式,在考虑与不考虑拉压异性的两种情况下,计算出组合凹模的分层半径、过盈量和弹性极限内压。结果表明,在考虑拉压异性情况下,组合凹模的分层半径、过盈量有较小差异,而且弹性极限内压有较大提高。