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In this paper, the triaxial compression tests with different confining pressures and strain rates are carried out on polycrystalline ice samples at − 6 °C, and under these conditions all the samples behave strain softening, with volumetric contraction followed by dilatancy at the lower confining pressure, and dilatancy at the higher confining pressure. Based on the framework of breakage mechanics and the above test data, a binary medium constitutive model is established to describe the mechanical properties of polycrystalline ice samples. In this model, polycrystalline ice is composed of two parts: the bonded elements and frictional elements. The bonded elements behave the mechanical properties of elastic–brittle, and the frictional elements have the mechanical properties of elastoplasticity. Under the external loads, the bonded elements gradually break up and transform into frictional elements, and both bear the load together. With the homogenization theory, the definitions of breakage ratio and strain concentration factor are given, which describe the evolution of the polycrystalline ice structure and the uneven distribution of strain on the mesoscopic level. Some parameters of the constitutive model are determined from the triaxial compression test results and the others are determined by trial and error. Meanwhile, the evolution law of the parameters varying with confining pressure is given. Finally, the test curves are compared with the predicted curves to verify the rationality of the model.

When a crack occurs under an installed waterproofing material and moves due to environmental effects (freeze–thaw, settlement, vibration, dead load, etc.), waterproofing materials without adequate elongation or tensile strength properties may break and tear. To enable the selection of materials with proper response against the strain that occur during crack movement, this study proposes and demonstrates a new evaluation method for determining and comparing strain concentration of waterproofing materials under the effect of concrete crack movement. For the proposed testing method and demonstration, three common types of waterproofing material types were selected for testing, poly-urethane coating (PUC), self-adhesive asphalt sheet (SAS) and composite asphalt sheet (CAS). Respective materials are installed with strain gauges and applied onto a specimen with a separated joint that undergoes concrete crack movement simulation. Each specimen types are subject to repeated movement cycles, whereby strain occurring directly above the moving joint is measured and compared with the strain occurring at the localized sections (comparison ratio which is hereafter referred to as strain concentration ratio). Specimens are tested under four separate movement length conditions, 1.5 mm, 3.0 mm, 4.5 mm and 6.0 mm, and the results are compared accordingly. Experimental results show that materials with strain concentration ratio from highest to lowest are as follows: PUC, SAS and CAS.

In this paper, the stress–strain responses of frozen sands and an elastoplastic constitutive model based on the homogenization theory of heterogeneous materials are presented. In the model, frozen soils are conceptualized as binary-medium materials consisting of bonded blocks and weak bands, and their mechanical behavior is described with elastic–brittle and elastoplastic constitutive models, respectively. By introducing two groups of parameters (i.e., the breakage ratio (λv and λs) and strain concentration coefficient (cv and cs) related to the spherical and deviatoric stress components), the proposed model incorporates the breakage process of ice crystals and nonuniform strain distributions between the matrix (bonded elements) and inclusions (frictional elements) of the heterogeneous frozen soil samples. Moreover, an elasticity-based model and a double hardening constitutive model are employed to simulate the mechanical properties of the bonded elements and the characteristics of the frictional elements, respectively. To provide appropriate and quantitative predictions with the binary-medium constitutive model proposed here, triaxial compression tests are performed on the frozen and unfrozen sands to determine the individual parameters at confining pressures of 300–1800 kPa. The model validations demonstrate that the predictions agree well with the available laboratory results.

Alloys used for turbine blades have to safely sustain severe thermomechanical loadings during service such as, for example, centrifugal loadings, creep and high temperature gradients. For these applications, cast Ni-based superalloys characterized by a coarse-grained microstructure are widely adopted. This microstructure dictates a strong anisotropic mechanical behaviour and, concurrently, a large scatter in the fatigue properties is observed. In this work, Crystal Plasticity Finite Element (CPFE) simulations and strain measurements performed by means of Digital Image Correlations (DIC) were adopted to study the variability introduced by the coarse-grained microstructure. In particular, the CPFE simulations were calibrated and used to simulate the effect of the grain cluster orientations in proximity to notches, which reproduce the cooling air ducts of the turbine blades. The numerical simulations were experimentally validated by the DIC measurements. This study aims to predict the statistical variability of the strain concentration factors and support component design.

In this paper, a new technology for on-orbit cold forming of space truss rods is proposed. For the cold roll forming process of asymmetric cross sections of thin strips, the effects of roll gap and roll spacing on the forming of asymmetric cross sections of strips were investigated using ABAQUS simulation + experiments. The study shows the following. When forming a strip with a specific asymmetric cross section, the stresses are mainly concentrated in corners 2/4/6, with the largest strain value in corner 2. With increasing forming passes, when the roll gap is 0.3 mm, the maximum equivalent strain values are 0.09, 0.24, 0.64 sequentially. Roll gaps of 0.4 mm and 0.5 mm equivalent strain change amplitude are relatively similar, and their maximum equivalent strain values are approximately 0.07,0.15, 0.44. From the analysis of the stress–strain history of the characteristic nodes in corners 2/4/6, it can be seen that the stress and strain changes in the deformation process mainly occur at the moment of interaction between the upper and lower rollers, where the stress type of node 55786 shows two tensile types and one compressive type, the stress type of nodes 48594 and 15928 shows two compressive and one tensile type, and the strain of the three nodes is in accordance with the characteristics of plane strain. When the roll gap is about 0.4 mm, the forming of the strip is relatively good. With increased roll spacing, the strip in the longitudinal stress peak through the rollers shows a small incremental trend, but the peak stresses are 380 Mpa or so. When the roll spacing is 120 mm, the longitudinal strain fluctuation of the strip is the most serious, followed by the roll spacing at 100 mm, and the minimum at 140 mm. Combined with the fluctuation in strip edges under different roll spacings, manufacturing cost and volume and other factors, a roll spacing of 100 mm is more reasonable. It is experimentally verified that when the roll gap is 0.4 mm and the roll spacing is 100 mm, the strip is successfully prepared in accordance with the cross-section requirements. When the rolling gap is 0.3 mm, due to stress–strain concentration, the strip is prone to edge waves in forming. The top of corner 2 of the flange triangular region is susceptible to intermittent tear defects, and the crack extension mechanism is mainly based on the cleavage fracture + ductile fracture.

PurposeThe purpose of this paper is to investigate the strain concentration factor in a central countersunk hole riveted in rectangular plates under uniaxial tension using finite element and response surface methods.Design/methodology/approachIn this work, ANSYS software was elected to create the finite element model of the present structure, execute the analysis and generate strain concentration factor (,) data. Response surface method was implemented to formulate a second order equation to precisely compute (,) based on the geometric and material parameters of the present problem.FindingsThe computations of this formula are accurate and in a great agreement with finite element analysis (FEA) data. This equation was further used for obtaining optimum hole and plate designs.Originality/valueAn optimum design of the countersunk hole and the plate that minimizes the (,) value was achieved and hence validated with FEA findings.

This paper investigates the cracking and stress–strain behavior, especially the local strain concentration near the flaw tips, of rock-like material containing two flaws. A series of uniaxial compression tests were carried out on rock-like specimens containing two flaws, with strain gauges mounted near the flaw tips to measure the local strain concentration under the uniaxial compressive loading. Four different types of cracks (wing cracks, anti-wing cracks, coplanar shear cracks and oblique shear cracks) and seven patterns of crack coalescences (T1 and T2; S1 and S2; and TS1, TS2 and TS3) are observed in the experiments. The type of crack coalescence is related to the geometry of the flaws. In general, the crack coalescence varies from the S-mode to the TS-mode and then to the T-mode with the increase of the rock bridge ligament angle. The stress–strain curves of the specimens containing two flaws are closely related to the crack development and coalescence process. The strain measurements indicate that the local tensile strain concentration below or above the pre-existing flaw tip causes wing or anti-wing cracks, while the local compressive strain concentration near the flaw tip is related to the shear crack. The measured local tensile strain shows a jump at the initiation of wing- and anti-wing cracks, reflecting the instant opening of the wing- and anti-wing crack propagating through the strain gauge. During the propagation of wing- and anti-wing cracks, the measured local tensile strain gradually increases with few jumps, implying that the opening deformation of wing- and anti-wing cracks occurs in a stable manner. The shear cracks initiate followed by a large and abrupt compressive strain jump and then quickly propagate in an unstable manner resulting in the failure of specimens.

Electrodermal activity (EDA) is a popular index of mental stress. State-of-the-art EDA sensors suffer from obstructiveness on the palm or low signal fidelity off the palm. Our previous invention of sub-micron-thin imperceptible graphene e-tattoos (GET) is ideal for unobstructive EDA sensing on the palm. However, robust electrical connection between ultrathin devices and rigid circuit boards is a long missing component for ambulatory use. To minimize the well-known strain concentration at their interfaces, we propose heterogeneous serpentine ribbons (HSPR), which refer to a GET serpentine partially overlapping with a gold serpentine without added adhesive. A fifty-fold strain reduction in HSPR vs. heterogeneous straight ribbons (HSTR) has been discovered and understood. The combination of HSPR and a soft interlayer between the GET and an EDA wristband enabled ambulatory EDA monitoring on the palm in free-living conditions. A newly developed EDA event selection policy leveraging unbiased selection of phasic events validated our GET EDA sensor against gold standards. Designing efficient sensing devices for ambulatory use remains a challenge. Here, the authors demonstrate heterogeneous serpentine ribbons enable a stretchable and robust interface between sub-micron thin graphene e-tattoos and thick and rigid printed circuit boards, which allows ambulatory electrodermal activity monitoring on the palm.

To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite.

Stretchable electronic devices have been extensively investigated in healthcare and wearable electronics. The stretchable substrate with strain control is paramount to integrating rigid devices into stretchable electronic systems for high mechanical strain. The common approach utilizes rigid strain control islands integrated on the surface of the soft stretchable matrix, but strain concentration effect and poor adhesion at the interface between the rigid islands and the elastic substrate are challenges. In this work, a stretchable substrate is developed by embedding the bottom strain control islands into the interior of an elastic substrate and the top strain control islands on the substrate surface to minimize strain concentration and improve the adhesion of the interface. The simulation results show that the strain concentration at the interface of the top island is suppressed and the maximum strain on the substrate surface is reduced by more than 30%. The interface between strain control islands of PI film and elastic substrate of Ecoflex can maintain more than 3000 stretch cycles under repeated strain of 100%. A stretchable inverter is fabricated based on this stretchable substrate with stretchable conductive wire. It is shown that the voltage transfer characteristics of the stretchable inverter have no significant change under strain of 100% for more than 3000 stretch cycles, demonstrating the capability of this stretchable substrate to integrate rigid devices for reliable stretchable electronic systems.