Wide Range Of Loading Rates Research Articles

Evolution of the fracture process zone in high-strength concrete under different loading rates

For cementitious materials, the inelastic zone around a crack tip is termed as fracture process zone (FPZ) and dominated by complicated mechanism, such as microcracking, crack deflection, bridging, crack face friction, crack tip blunting by voids, crack branching, and so on. Due to the length of the FPZ is related with the characteristic length of the cementitious materials, the size, extent and location of the FPZ has been the object of countless research efforts for several decades. For instance, Cedolin et al. (1) have used an optical method based on the moire interferometry to determine FPZ in concrete. Castro-Montero et al. (2) have applied the method of holographic interferometry to mortar to study the extension of the FPZ. The advantage of the interferometry method is that the complete FPZ can be directly observed on the surface of the sample. Swartz et al. (3) has adopted the dye penetration technique to illustrate the changing patterns observed as the crack progress from the tensile side to the compression side of the beam. Moreover, acoustic emission (AE) is also an experimental technique well suited for monitoring fracture process. Haidar et al. (4) and Maji et al. (5) have studied the relation between acoustic emission characteristics and the properties of the FPZ. Compared with the extensive research on properties of the FPZ under quasi-static loading conditions, much less information is available on its dynamic characterization, especially for high-strength concrete (HSC). This paper presents the very recent results of an experimental program aimed at disclosing the loading rate effect on the size and velocity of the (FPZ) in HSC. Eighteen three-point bending specimens were conducted under a wide range of loading rates from from 10 -4 mm/s to 10 3 mm/s using either a servo-hydraulic machine or a self-designed drop-weight impact device. The beam dimensions were 100 mm�100 mm in cross section, and 420 mm in length. The initial notch-depth ratio was approximately 0.5, and the span was fixed at 300 mm during the tests. Four strain gauges mounted along the ligament of the specimen were used to measure the FPZ size. Surprisingly, the FPZ size remains almost constant (around 20 mm) when the loading rate varies seven orders of magnitude. This is clearly different from NSC, in which the FPZ size actually decreased with loading rate.

Mechanoenzymatics and Protective Mechanisms of Titins’ Catalytic and IG Domains

The giant titin filament controls many structural and functional properties of the sarcomere. Titin filaments connect M-line and Z-disc of the sarcomere and consist of four regions: the M-line, A-band, I-band, and Z-line. The different domains of titin (mainly immunoglobulin Ig and fibronectin-3 domains, and the catalytic domain titin kinase (TK)) exhibit dramatically different mechanical properties. We used atomistic molecular dynamics simulations to explore the coupling of mechanical stability with the enzymatic activity of titin kinase and the protective properties of Ig-domains. We showed that a unique autoinhibitory mechanism allows TK to act as a molecular force sensor, as relatively low forces already remove the autoinhibitory tail and prime the molecule for ATP binding. At much higher forces, the mechanical stability of Ig27 becomes important: In our studies, extensive dynamic force spectroscopy (DFS), Brownian dynamics, and molecular dynamics simulations worked together to examine mechanical stability of Ig27 under different loading rates. Our results suggest that Ig27 is perfectly suited to act as a molecular force buffer over a wide range of loading rates.

Determination of Tensile Properties of Polymers at High Strain Rates

In the field of high rate testing of polymers the measured properties are highly dependent on the applied methodology. Hence, the test setup as whole but in particular also the geometrical type of specimen plays a decisive role. The widely used standard for the determination of tensile properties of polymers (ISO527-2) was extended by a novel standard (ISO18872:2007), which is targeted on the determination of tensile properties at high strain rates. In this standard also a novel specimen shape is proposed. Hand in hand with the introduction of new specimen geometry the question of comparability arises. To point out the differences in stress-strain response of the ISO18872 specimen and the ISO527-2 multipurpose specimen tensile tests over a wide loading rate range were conducted in this paper. A digital image correlation system in combination with a high speed camera was used to characterize the local material behaviour. Different parameters like nominal stress, true stress, nominal strain, true strain as well as volumetric strain were determined and used to compare the two specimen geometries.

Characterization of the fracture behavior of NBR and FKM grade elastomers for oilfield applications

The monotonic fracture behaviour of two elastomer compounds (HNBR and FKM) was characterized in this study. These elastomeric materials are frequently used in many oilfield applications (e.g., seals and hoses) and are exposed to a complex combination of mechanical, thermal and environmental loads. Furthermore, rapid gas decompression failure of these elastomeric materials was observed under real service conditions. A phenomenon termed rapid gas decompression (RGD) damage occurs when elastomer seals exposed to high gas pressure fail in a brittle manner upon the sudden release of the gas pressure. Hence, to support both material development and design efforts, it is of prime theoretical and practical importance to characterize the fracture behaviour of these materials. Fracture mechanics tests using a faint waist pure shear specimen (FWPS) configuration were performed over a wide loading rate range. First, global force and displacement values were measured and single parameter fracture mechanics values in terms of peak tearing energy, T p values were calculated. Furthermore, the crack growth process and the local crack tip deformation were characterized by non-contact optical devices. The digital image correlation technique used allows for the determination of displacement and full-field strain up to a high strain and crack growth rate. Based on above measurements, crack growth resistance curves in terms of tearing energy ( T − Δ c) and crack tip opening angle ( CTOA − Δ c) functions were derived. While a continuous crack growth was observed for HNBR at all loading rates, a discontinuous crack extension was observed for FKM. Finally a phenomenological model was deduced for describing the rapid monotonic crack extension process for these elastomeric compounds.

Loss of α-Catenin Decreases the Strength of Single E-cadherin Bonds between Human Cancer Cells

The progression of several human cancers correlates with the loss of cytoplasmic protein alpha-catenin from E-cadherin-rich intercellular junctions and loss of adhesion. However, the potential role of alpha-catenin in directly modulating the adhesive function of individual E-cadherin molecules in human cancer is unknown. Here we use single-molecule force spectroscopy to probe the tensile strength, unstressed bond lifetime, and interaction energy between E-cadherins expressed on the surface of live human parental breast cancer cells lacking alpha-catenin and these cells where alpha-catenin is re-expressed. We find that the tensile strength and the lifetime of single E-cadherin/E-cadherin bonds between parental cells are significantly lower over a wide range of loading rates. Statistical analysis of the force displacement spectra reveals that single cadherin bonds between cancer cells feature an exceedingly low energy barrier against tensile forces and low molecular stiffness. Disassembly of filamentous actin using latrunculin B has no significant effect on the strength of single intercellular E-cadherin bonds. The absence of alpha-catenin causes a dominant negative effect on both global cell-cell adhesion and single E-cadherin bond strength. These results suggest that the loss of alpha-catenin alone drastically reduces the adhesive force between individual cadherin pairs on adjoining cells, explain the global loss of cell adhesion in human breast cancer cells, and show that the forced expression of alpha-catenin in cancer cells can restore both higher intercellular avidity and intercellular E-cadherin bond strength.

Failure of Au RF-MEMS switches subjected to dynamic loading

The dynamic failure of Au RF-MEMS was investigated over a wide range of loading rates by three different experimental setups: a drop weight tower, which induced a maximum peak acceleration of 600 g ( g: acceleration of gravity), a Hopkinson pressure bar with a maximum peak acceleration of 300,000 g, and a pulsed laser loading technique with a maximum peak acceleration of 1.8 × 10 8 g. In the drop weight tower the total load pulse duration was in the milliseconds range – much longer than the 28 μs resonant period of the devices – and no failure of any kind occurred in the RF-MEMS devices or their substrate. At 90,000 g (generated in the Hopkinson bar) no damage in either the substrate or the devices was observed. However, at 200,000 g, which corresponds to a loading duration of a few microseconds, i.e., comparable to the device resonant period, 10% of the switches failed although postmortem imaging showed no damage to the substrate. Damage increased after this acceleration and at 300,000 g 20% of the switches failed, but, in addition, significant failure in the quartz substrate was recorded. Lastly, the pulsed laser loading technique, which has a loading pulse duration of a few tens of nanoseconds, was applied to accelerate the Au switches to 1.8 × 10 8 g, and the probability of failure at this loading ranged from 50% to 80%. At even larger accelerations, 10 9 g, the probability of failure was 100%. The results of this study establish the severity of dynamic failure in MEMS, despite their small mass, and its dependence on the level of acceleration which spanned about 7 orders of magnitude.

Fracture behaviour of high-strength concrete at a wide range of loading rates

Three-point bending tests on notched beams of a high-strength concrete have been conducted using both a servo-hydraulic machine and a self-designed drop-weight impact device. The fracture energy and the peak load were measured over a wide range of loading point displacement rates, spanning eight orders of magnitude. Under low displacement rates, from 10 −4 to 10 mm/s, the tests were performed with the servo-hydraulic machine; from 10 2 to 10 3 mm/s we used the drop-weight impact machine. The results show that the fracture energy and the peak load increase as the loading rate increases. Nevertheless, such a trend is relatively slight under low rates and can be attributed to viscous effects mainly originating from the presence of water in the pore structure. Under high rates the increases in the fracture energy and in the peak load are dramatic due to the effect of inertia.

Rate dependent critical strain energy density factor of Huanglong limestone

Critical strain energy density of rock can be defined as a fundamental parameter in rock fracture mechanics, an intrinsic material property related to resistance to crack initiation and propagation. By means of the three-point bending experiments, the critical strain energy density factor of Huanglong limestone was measured over a wide range of loading rates from 8.97 × 10 −4 MPam 1/2 s −1 to 1.545 MPam 1/2 s −1. According to the approximate relationship between static and dynamic critical strain energy density factor of Huanglong limestone, relationship between the growth velocity of crack and magnitude of load is obtained. The main conclusions are summarized as follows: (1) when the loading rate is higher than 0.0279 MPam 1/2 s −1, the critical strain energy density factor of rock increased markedly with increasing loading rate. However, when loading rate is lower than 0.0279 MPam 1/2 s −1, the critical strain energy density factor slightly increased with an increase in loading rate. It is found from experimental results that the critical strain energy density factor is linear proportional to the exponential expression of loading rate, (2) for Huanglong limestone, when the growth velocity of crack is lower than 100 m/s, value of the maximum load was nearly a constant. However, when the growth velocity of crack is higher than 1000 m/s, value of the maximum load dramatically increases with increasing the crack growth velocity, and (3) the critical SED of Huanglong limestone is higher as the loading rate is higher.

Rate-dependent serrated flow and plastic deformation in Ti45Zr16Be20Cu10Ni9 bulk amorphous alloy during nanoindentation

The plastic deformation of Ti45Zr16Be20Cu10Ni9 bulk metallic glass has been investigated by nanoindentation performed with loads ranging from 10 to 200 mN in a wide range of loading rates. The plastic flow in the alloy exhibited conspicuous serrations at low loading rates. The serrations, however, became less prominent as the rate of indentation increased. Atomic force microscopy showed a significant pile-up of materials around the indents, indicating that a highly localized plastic deformation occurred under nanoindentation. The possible mechanism governing the plastic deformation in bulk metallic glass specimens is tentatively discussed in terms of strain-induced free volume.

Bio-inspired design of dental multilayers: Experiments and model

This paper combines experiments, simulations and analytical modeling that are inspired by the stress reductions associated with the functionally graded structures of the dentin–enamel-junctions (DEJs) in natural teeth. Unlike conventional crown structures in which ceramic crowns are bonded to the bottom layer with an adhesive layer, real teeth do not have a distinct “adhesive layer” between the enamel and the dentin layers. Instead, there is a graded transition from enamel to dentin within a ∼10 to 100 μm thick regime that is called the Dentin Enamel Junction (DEJ). In this paper, a micro-scale, bio-inspired functionally graded structure is used to bond the top ceramic layer (zirconia) to a dentin-like ceramic-filled polymer substrate. The bio-inspired functionally graded material (FGM) is shown to exhibit higher critical loads over a wide range of loading rates. The measured critical loads are predicted using a rate dependent slow crack growth (RDEASCG) model. The implications of the results are then discussed for the design of bio-inspired dental multilayers.

Polymer composites for the automotive industry: characterisation of the recycling effect on the strain rate sensitivity

In this paper, the mechanical response of two composites-based polypropylene (PP) is studied. High strain rate tests as well as quasi-static uniaxial tests were performed. Dynamic loadings were carried out using the Split Hopkinson Pressure Bar. Materials used consist of an unfilled high-impact PP (hiPP) and a talc-filled hiPP. For each of these two materials, the recycling effects on the stress–strain response for both quasi-static and dynamic loading were studied. The degradation of the mechanical response due to recycling is then characterised for a wide range of loading rates for both materials. Scanning electron microscopy analyses have been achieved in order to quantify recycling and strain rate effects and also to correlate them with the microstructure evolution for a better understanding of the mechanisms involved in the recycling process and in the dynamic loading. This study has led to a preliminary understanding of the recycling effects, particularly for impact loadings.

Ductile–brittle transitions in the fracture of plastically deforming, adhesively bonded structures. Part II: Numerical studies

To enable the effective and reliable use of structural adhesive bonding in automotive applications, the cohesive properties of a joint need to be determined over a wide range of loading rates. In this paper, a strategy for determining these properties has been described and used to analyze a set of experimental results presented in a companion paper. In the particular system studied, a crack growing in a toughened quasi-static mode could make a catastrophic transition to a brittle mode of fracture. The cohesive parameters for both the toughened and brittle modes of crack growth were determined by comparing numerical predictions from cohesive-zone simulations to the results of experimental tests performed using double-cantilever beam specimens and tensile tests. The cohesive parameters were found to be essentially rate-independent for the toughened mode, but the toughness dropped by a factor of four upon a transition to the brittle mode. The results of wedge tests were used as an independent verification of the cohesive parameters, and to verify that the quasi-static properties remained rate-independent to very high crack velocities corresponding to conditions of low-velocity impact. The effects of friction, and the use of the wedge test to determine cohesive parameters, were also explored.

Structural characterization and mechanical properties of nanocrystal-containing Cu–Ti-based bulk metallic glass-forming alloys

Cylindrical Cu42.5Ti41.5Ni7.5Zr2.5Hf5Si1 bulk metallic glass with a diameter of 2 mm was fabricated by copper-mold casting. X-ray diffraction and differential scanning calorimetry analysis of the material showed that the alloy has a homogenous amorphous structure and high glass-forming ability. However, detailed observation by transmission electron microscopy revealed that a kind of nanocrystal with size of about 20 nm is sparsely distributed in the glass matrix. Nanobeam electron diffraction experiments indicated that the nanocrystal has a face-centered cubic crystalline structure. Room-temperature compression tests revealed that the alloy has a high fracture strength of 2250 MPa and obvious plastic strain of about 5.3%. Nanoindentation tests revealed that the as-cast alloy exhibits obviously serrated flow over a wide range of loading rate from 0.5 to 10 mN/min.

Upgrading secondary biological treatment of wastewater using anoxic/aerobic attached growth systems for effluent quality improvement

A pilot plant was constructed and operated using both Aerobic Submerged Fixed Film (ASFF) and Anoxic/Aerobic Submerged Fixed Film (A/ASFF) processes at different loading rates in the range of 0.03–0.3 g BOD.g-1 BVS.d-1 corresponding to the Hydraulic Retention Times (HRTs) in the range of 0.7 to 8 hr. The results obtained revealed that both the ASFF and the A/ASFF processes are viable options to upgrade the activated sludge process for combined organic and ammonia removal with minimal excess sludge production. High removal efficiencies of up to 98% for BOD, 75% for COD and 97% for ammonia were obtained with both processes over a wide range of loading rates, but the A/ASFF process appears to be more capable of maintaining a stable and efficient treatment at higher loading rates. The treated effluent quality obtained from either upgrading process is considerably better than that achieved by the activated sludge process and satisfies the water quality requirements for reuse in irrigation, thus tertiary filtration may not be needed.

Shear strength of epoxypolysulfone glass-reinforced plastics in a wide range of loading rates

The shear strength of unidirectional epoxypolysulfone glass-reinforced plastics in the range of six decimal orders of loading rates has been measured. ED-22 epoxydiane resin (analog of DGEBA) modified by PSK-1 aromatic polysulfone served as a matrix. It has been shown that the strength values linearly increase with increasing logarithm of the loading rate; the sensitivity of composites to the rate of application of a force increases with increasing quantity of PSK-1 introduced into the matrix, and the fracture mechanism of composites is the thermofluctuation one. The laws revealed do not differ from those established by us earlier for glass-reinforced plastics based on epoxy binders modified by active diluents.

Effect of loading rate on absorbed energy and fracture surface deformation in a 6061-T651 aluminum alloy

The absorbed energy, micro- and macrodeformation on fracture surfaces were evaluated at various loading rates for a 6061-T651 aluminum alloy. The variation of absorbed energy with loading rate was compared with the variation of micro- and macrodeformation features in a wide loading rate range. It was found that there are correlations between loading rate dependences of total absorbed energy, whole fracture surface area, shear lip volume and shear lip shape.

Experimental Characterization of Interlaminar Fracture Behavior in Polymer Matrix Composites under Low-Velocity Impact Loading

Novel experimental methods were proposed to easily and precisely evaluate the mode I, II and I+II fracture toughness of polymer matrix composite laminates under low-velocity impact loading. A ramped incident stress wave was applied to suppress the flexural vibration of the specimen. A strain-based formula was employed to improve the accuracy of evaluation of the dynamic energy release rate. The validity of the proposed method was confirmed by the results of finite element analyses and dynamic experiments. The effects of loading rate and mode mixture on the interlaminar fracture behavior of carbon -fiber/epoxy composite laminates were investigated over a very wide range of loading rate from static to impact. The macroscopic fracture toughness clearly showed loading rate dependence regardless of mode ratio, and consequently the mixed mode fracture criterion varied with loading rate. The microscopic fracture morphology also showed loading rate dependence ; cohesive fracture of matrix resin itself was a dominant fracture mechanism at higher loading rate, whereas debonding of fiber/matrix interface was a dominant fracture mechanism at lower loading rate.

A Piconewton Forcemeter Assembled from Microtubules and Kinesins

A forcemeter capable of detecting piconewton forces was assembled from nanoscale building blocks, using a cantilevered microtubule as a beam of known stiffness, which is loaded by a second microtubule transported by kinesin motor proteins adsorbed to the surface and connected to the cantilevered microtubule by the link whose strength is to be determined. Due to the loading rate of less than 1 pN/s, this forcemeter is ideally suited to study the strength of biological receptor/ligand pairs, such as streptavidin/biotin. Here we introduce a new technique to measure the force necessary to rupture receptor/ligand bonds: a molecular forcemeter self-assembled from microtubules and motor proteins. Measurements of the dynamics of receptor-ligand interactions under an applied external force, termed dynamic force spectroscopy, probe the intricate relationship between mechanical force, bond lifetime, and bond (stereo)chemistry. The distribution of rupture forces provides a fingerprint of the energy barriers traversed in the energy landscape along the unbinding pathway. 1 Typical forces to rupture noncovalent bonds range from 1 pN to 1 nN, depending on the rate at which the force is ramped up (the loading rate). A variety of experimental setups have been developed to measure pN forces over a wide range of loading rates down to 0.1 pN/s, which is typical for many biological systems. 2 Our forcemeter utilizes two microtubules - one microtubule, which is clamped on one side and freely swinging on the other, serves as a molecular cantilever, and a second microtubule moving on a kinesin-coated surface 3 applies the force. The perpendicular movement of the second microtubule bends the freely swinging end of the cantilevered microtubule once a link through the receptor/ligand pair of interest is established. The bending is imaged by fluorescence microscopy, and the applied force is calculated from the known flexural rigidity of the microtubules. 4 In our experiment we utilize streptavidin/biotin as the model receptor/ligand pair, because it has been studied repeatedly by force microscopy. Using established protocols, tubulin was biotinylated and polymerized into microtubules that expose biotin on their surface. Streptavidin-coated beads were used as links between the cantilevered microtubule and

808 炭素繊維強化エポキシ積層板の混合モード破壊じん性に及ぼすひずみ速度の影響

In order to clarify the strain rate effects on mode I, mode II and mixed mode (I+II) fracture behavior of graphite/epoxy laminates, fracture toughness was investigated over a wide range of loading rate from static to impact. MMF (Mixed Mode Flexure) and MMB (Mixed Mode Bending) specimens were used to evaluate the mixed mode fracture toughness. The mixed mode fracture toughness clearly showed strain rate dependence regardless of mode ratio; the mixed mode impact fracture toughness was about 10% lower than the local maximum value under static loading. Local maximum values existed at static loading rate for mixed mode fracture toughness, G_C. The mixed mode fracture toughness conform to the linear criterion for the region of high mode ratio, G_<II>/G_1=13,regardless of loading rates. However, it did not conform to the linear criterion for the region of low mode ratio, G_<II>/G_I&le;3,and high loading rate, G&ge;(10)^2 J/m^2/s.

複合材料 層間高じん化ＣＦＲＰの混合モード（Ｉ＋ＩＩ）破壊じん性およびその負荷速度依存性

Mixed mode (I+II) fracture toughness of the Interlayer-toughened composite laminates, T800H/3900-2 (Toray), which is a newly developed composite material having a tough ‘interlayer’ containing fine polyamide particles, was investigated over a very wide range of loading rate from static to impact. The MMF (Mixed Mode Flexure) specimen and SHPB (Split Hopkinson Pressure Bar) system were employed for measuring the mixed mode fracture toughness under impact loading. The strain rate dependence of fracture behavior was also studied from both macro- and microscopic aspects to clarify the mesoscopic fracture mechanism of the material. The experimental results showed the crack path was inside the toughened interlayer during the initial stage of crack growth, however, it moved from the toughened interlayer to the carbon/epoxy base lamina during the propagation stage of crack growth. The mixed mode fracture toughness was sensitive to loading rate; the impact fracture toughness for the toughened interlayer was about 23% lower than the maximum value under static loading, and the impact fracture toughness for the carbon/epoxy base lamina was about 11% lower than the maximum value under static loading. The effects of mode mixture varied with loading rate for interlayer fracture, though they did not varied for base-lamina fracture. Microscopic observation showed that the fracture morphology was sensitive to loading rate.