Abrupt Strength Research Articles

Dynamics of Strength Gain of an Artificial Stone Based on a Cement Binder

The article discusses the existing theories of physical and chemical processes occurring in artificial stone based on a cement binder, affecting the dynamics of strength gain. A mathematical model of the continuously increasing strength of the cement stone according to the logarithmic law has been investigated. The authors questioned the adequacy of the application of such theories to exclude the factor of hardening time from experiments aimed at identifying the effect of such measures as the use of surfactants, mineral additives, changes in the water-cement ratio, etc., on the strength gain. The authors proposed to study the influence of such factors on the dynamics of changes in the strength of cement stone as a more efficient tool. The main arguments of the theory of abrupt strength gain and destruction of cement systems are presented. The authors planned an experiment to plot the dynamics of the strength gain of an artificial stone based on a cement binder to assess the relevance of the problem. During the experiment, two batches of specimens were tested, six specimens in each at 3-, 7-, 14-, 28-, 56-and 84-day age, respectively. The values obtained during the experiment showed significant differences at an early age of hardening. The theoretical curves of variation of strength in time were obtained based on the experimental data, within the framework of the existing theory of the strength gain of an artificial stone based on a cement binder. The simulation results were compared with the experimental results and with each other. The arguments were put forward for and against existing theories based on the results of the analysis of experimental studies and theoretical calculations.

Broadscale interseismic deformation and fault slip rates in the central Tibetan Plateau observed using InSAR

Knowledge of the number, location, and interseismic slip rate of crustal faults is key to our understanding of the bulk mechanics of continental deformation. However, at present, the sparsity of geodetic velocity observations is a major limiting factor to improving our knowledge of the fault network. In this study, we combine long‐swath interferometric synthetic aperture radar (InSAR) data to form an interseismic displacement rate map of over 1000 km length spanning the central Tibetan Plateau. Our dense geodetic observations indicate that strain varies smoothly in space without major concentration at previously mapped faults. Discrepancy between line‐of‐sight InSAR and GPS observations between 33°N–35°N could indicate ~4 mm/yr of surface uplift in this region. Derived strain rates are less than 5 × 10−8 yr−1 across the plateau. At the E‐W left‐lateral Kunlun fault, postseismic deformation following the 2001 Kokoxili earthquake results in an observed strain rate of 18 ± 4 × 10−8 yr−1 across a 100 km wide zone. Following removal of a viscoelastic model, we estimate an interseismic slip rate of 12 ± 2 mm/yr for the Kunlun fault. In the central Tibetan conjugate fault zone, the NE‐SW striking Amdo‐Sewa and NW‐SE striking Beng Co faults have right‐lateral slip rates of 6 ± 1 mm/yr and 1–4 ± 1 mm/yr, respectively. Meanwhile, the NE‐SW striking Dongqiao fault has a left‐lateral slip rate of 1–2 ± 1 mm/yr. The low slip rates we estimate for these faults indicate that tectonic strain is more broadly distributed across the plateau interior, perhaps due to the lithosphere being weak and without the abrupt strength contrasts that exist at the plateau margins.

Structural Behaviours and Curling Influence of Single Shear Cold-Formed Ferritic Stainless Steel Bolted Connections Fastened with Two Bolts

Experimental studies for single shear bolted connection of cold-formed ferritic stainless steel fastened with two bolts (2×1 bolt array) and 3.0 mm thick plate has been performed by T.S. Kim etc. End distances parallel to the direction of applied force were considered as the main variables of specimens. Curling (out of plane deformation) was observed in bolted connection with 60mm end distance and it caused abrupt strength reduction. The purpose of this paper is to compare the analysis results with test results and investigate the curling influence on ultimate strength through numerical modelling. FE models with restrained curling were also simulated for strength comparison with the curled specimen. Therefore, the validation of finite element(FE) analysis in predicting ultimate strength and curling behaviour was verified and the strength reduction ratio by the curling was estimated.

Analytical Evaluation of Seismic Performance of RC Frames Rehabilitated Using FRP for Increased Ductility of Members

Many reinforced concrete (RC) frame structures designed according to pre1970 strength-based codes are susceptible to abrupt strength deterioration once the shear capacity of the columns is reached. Fiber composites are used to increase the shear strength of existing RC columns and beams by wrapping or partially wrapping the members. Increasing the shear strength can alter the failure mode to be more ductile with higher energy dissipation and interstorey drift ratio capacities. The objective of this study was to analytically evaluate the effect of varying distributions of fiber-reinforced polymer (FRP) rehabilitation on the seismic performance of three existing RC frames with different heights when subjected to three types of scaled ground motion records. The FRP wrapping is designed to increase the displacement ductility of frame members to reach certain values representing moderate ductility and high ductility levels. These values were assumed based on previous experimental work conducted on members wrap...

Caracterização de tubo e adesivo utilizados em tubulações de plástico reforçado com fibras de vidro aplicados em plataformas marítimas

Na indústria de exploração e produção de petróleo, principalmente no setor offshore, os plásticos reforçados por fibras de vidro (PRFV) têm apresentado grande destaque. Isso se deve à intrínseca capacidade destes de resistir à corrosão em ambientes salinos, bem como serem materiais que apresentam elevada resistência mecânica específica. O presente trabalho buscou caracterizar tubo e adesivo utilizados em plataformas marítimas. Para tanto, as técnicas utilizadas foram: espectroscopia na região do infravermelho com transformada de Fourier (FTIR), análise termogravimétrica (TGA), análise termodinâmico-mecânica (DMTA) e temperatura de amolecimento de Vicat. Amostras do tubo de PRFV e dos componentes do adesivo (denominados de A e B) foram estudadas individualmente. Além disso, foram confeccionadas misturas, em massa, nas seguintes proporções: 40%A-60%B, 50%A-50%B e 60%A-40%B. Foi possível determinar a composição e a estrutura química dos materiais, além do comportamento mecânico à degradação térmica. Foi verificado que adesivo e tubo apresentaram desempenho satisfatório à temperatura ambiente quanto à degradação e à rigidez. Por outro lado, quando submetido ao calor, principalmente os adesivos, apresentaram queda brusca de desempenho.

Ridging, strength, and stability in high‐resolution sea ice models

In multicategory sea ice models the compressive strength of the ice pack is often assumed to be a function of the potential energy of pressure ridges. This assumption, combined with other standard features of ridging schemes, allows the ice strength to change dramatically on short timescales. In high‐resolution (∼10 km) sea ice models with a typical time step (∼1 hour), abrupt strength changes can lead to large internal stress gradients that destabilize the flow. The unstable flow is characterized by large oscillations in ice concentration, thickness, strength, velocity, and strain rates. Straightforward, physically motivated changes in the ridging scheme can reduce the likelihood of abrupt strength changes and improve stability. In simple test problems with flow toward and around topography, stability is significantly enhanced by eliminating the threshold fraction G* in the ridging participation function. Use of an exponential participation function increases the maximum stable time step at 10‐km resolution from less than 30 min to about 2 hours. Modifying the redistribution function to build thinner ridges modestly improves stability and also gives better agreement between modeled and observed thickness distributions. Allowing the ice strength to increase linearly with the mean ice thickness improves stability but probably underestimates the maximum stresses.

Cyclic tests on steel and concrete‐filled tube frames with Slit Walls

AbstractCyclic loading tests were performed on three one‐storey steel frames and four three‐storey concrete‐filled tube (CFT) moment frames reinforced with a new type of earthquake‐resisting element consisting of a steel plate shear wall with vertical slits. In this shear wall system, the steel plate segments between the slits behave as a series of flexural links, which provide fairly ductile response without the need for heavy stiffening of the wall. The steel shear walls and the moment frames behaved in a ductile manner up to more than 4% drift without abrupt strength degradation or loss of axial resistance. Results of these tests and complementary analysis provide a basis for an equivalent brace model to be employed in commercially available frame analysis programs. Test and analytical results suggest that the horizontal force is carried by the bolts in the middle portion of the wall–frame connection, while the vertical forces coupled with the moment in the connection are resisted by the bolts in the edge portion of the connection, for which the friction bolts in the connection should be designed. When sufficient transverse stiffening is provided, full plastic strength and non‐degrading hysteretic behaviour can be achieved for this new type of shear wall. Copyright © 2006 John Wiley & Sons, Ltd.

축소모델실험에 의한 철근콘크리트 3층 보통모멘트골조의 구조 성능 평가

초록·키워드 목차 오류제보하기 등록된 정보가 없습니다. ABSTRACT1. 서론2. 실험계획3. 실험결과 및 평가3. 결론참고문헌요약

The effect of grain boundary phase on contact damage resistance of alumina ceramics

The effect of grain boundary phase on contact damage behavior is investigated in alumina ceramics. Four types of aluminas doped with MgO, anorthite (CaO·Al2O3·2SiO2), silica, and with both MgO and anorthite are prepared such that they have similar average grain size by adjusting sintering conditions. MgO-doped alumina composed of equiaxed grains shows brittle fracture behavior, and anorthite-doped alumina composed of elongated grains shows a quasi-plastic response under Hertzian sphere indentation. The co-doped alumina with MgO and anorthite, however, is damage tolerant even with its rounded grains, while silica-doped alumina with similar grain size and shape to anorthite-doped alumina shows abrupt strength degradation with low critical load for cone cracking. The damage behavior is discussed from the viewpoint of residual stress induced by thermal expansion mismatch between the grains and grain boundary phases. The damage tolerant behavior of alumina ceramics is significantly affected by the composition of grain boundary phase.

Evaluation of the seismic performance of a three‐story ordinary moment‐resisting concrete frame

AbstractThis study focuses on the seismic performance of Ordinary Moment‐Resisting Concrete Frames (OMRCF) designed only for gravity loads. For this purpose, a 3‐story OMRCF was designed in compliance with the minimum design requirements in the American Concrete Institute Building Code ACI 318 (1999). This model frame was a regular structure with flexure‐dominated response. A 1/3‐scale 3‐story model was constructed and tested under quasi‐static reversed cyclic lateral loading. The overall behavior of the OMRCF was quite stable without abrupt strength degradation. The measured base shear strength was larger than the design base shear force for seismic zones 1, 2A and 2B calculated using UBC 1997. Moreover, this study used the capacity spectrum method to evaluate the seismic performance of the frame. The capacity curve was obtained from the experimental results for the specimen and the demand curve was established using the earthquake ground motions recorded at various stations with different soil conditions. Evaluation of the test results shows that the 3‐story OMRCF can resist design seismic loads of zones 1, 2A, 2B, 3 and 4 with soil types SA and SB. For soil type SC, the specimen was satisfactory in seismic zones 1, 2A, 2B and 3. For soil type SD, the OMRCF was only satisfactory for seismic zones 1 and 2A. Copyright © 2004 John Wiley & Sons, Ltd.

Model of Strength Degradation from Hertzian Contact Damage in Tough Ceramics

A model of strength degradation for ceramics subject to damage from contact with hard spheres is developed. Primary attention is focused on tough ceramics with heterogeneous microstructures which deform in a quasi‐plastic mode. Brief consideration is also given to ideally brittle ceramics which form classical ring cracks, as a comparative baseline. Strength vs indentation load data from two microstructurally controlled ceramics, silicon nitride and a micaceous glass‐ceramic, illustrate distinctive strength degradation responses: in fine‐grain (F) form, ideally brittle failure from ring cracks, with abrupt strength loss at the critical load for crack initiation followed by a slow falloff at increasing load; in coarse‐grain (C) form, failure from within the quasi‐plastic zone, with continuous strength loss beyond a load well above that for the onset of yield, and with even slower falloff. Failure in the latter materials occurs from contact‐induced microdamage flaws with two essential elements: an inner closed shear crack with frictional sliding faces (“shear fault”), which forms within the confining compression–shear contact field; an outer annular, kinked crack that initiates at the fault edges (“wing crack”), and that extends in tensile local mode. The critical fault–crack is modeled as a virtual crack, with the residual field from the inner fault stabilizing the net driving force on the outer wing crack during ensuing tensile loading. Finite element modeling is used to evaluate the nonlinear elastic–plastic contact fields, and to provide a relationship between residual shear fault stress and contact load. The model accounts for the essential qualitative and quantitative features of the strength–load data, with provision for catastrophic degradation at high fault densities and extreme loads by microcrack coalescence. The model also contains the ingredients for analysis of contact fatigue, via attrition of the frictional tractions on the residual fault.

Slip complexity in earthquake fault models.

We summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes. For models of smooth faults between elastically deformable continua, it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state. That results in a finite nucleation size, or coherent slip patch size, h*. Models of smooth faults, using numerical cell size properly small compared to h*, show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self-similar (power law) Gutenberg-Richter frequency-size statistics. This conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences. In contrast, some models of locally heterogeneous faults with quasi-independent fault segments, represented approximately by simulations with cell size larger than h* so that the model becomes "inherently discrete," do show small event complexity of the Gutenberg-Richter type. Models based on classical friction laws without a weakening length scale or for which the numerical procedure imposes an abrupt strength drop at the onset of slip have h* = 0 and hence always fall into the inherently discrete class. We suggest that the small-event complexity that some such models show will not survive regularization of the constitutive description, by inclusion of an appropriate length scale leading to a finite h*, and a corresponding reduction of numerical grid size.

Crack Growth and Acoustic Emission in Ceramics During Thermal Shock

It is recognized that microcracks may contribute to failure of ceramics undergoing thermal shock, although there is little direct experimental evidence. The combination of acoustic emission (AE) and SEM observations provides such evidence. In this work AE data were analyzed after rapid quenching of samples in silicone oil. A thermal shock resistant material, alumina, and a material resistant to thermal damage, advanced zirconia refractory, have been examined. Cracks were detected by analysis of AE amplitudes and durations and their growth was monitored by systematic SEM observations as thermal shocks of increasing severity were applied. Three‐point bending strengths were determined in air after quenching. For the first time SEM images are presented showing early stages of crack initiation for temperature differences less than ΔTcrit, i.e., where the fracture was not believed to occur. Further development of the cracks leads to abrupt strength reduction in alumina and controlled strength loss in zirconia, although AE data did not indicate any particular pattern of catastrophic crack propagation when substantial loss of strength occurred.

部材の座屈および破断を考慮したトラス構造の崩壊解析

A method and a numerical analysis to the large deformation collapse behavior of truss structures are presented in this paper, with consideration of yielding, buckling and the rupturing of members. Truss members are idealized that the both ends of each member are connected to the related nodes by pin condition and only axial force acts to the each member. Axial force and stiffness of each member are assumed to be expressed as a functions of axial force-deformation hysteresis and current axial deformation. For the purpose of avoiding the difficulties of convergence due to the occurrence of large unbalanced force caused by the rupture of members, a method of repeating load-unload process is used to reduce axial force step-by-step, and to express the rupturing phenomena of members. A shallow truss dome, a plane truss and a double-layer grid space truss are analyzed as examples. In the first example, the result of our analysis is in accord with the other numerical solution. The results of other examples show that gradual strength reduction due to the buckling or yielding of members and abrupt strength dropping due to the rupture of members can be expressed well by using the proposed method.