Compressive Strain Limit Research Articles

Refined deformation limit states for circular reinforced concrete bridge columns

AbstractOne hurdle against the widespread implementation of the performance‐based design (PBD) for bridges is the lack of consensus among practitioners, researchers, and code committees on engineering demand parameter (EDP) limits defining the onset of various types of damage and their variability. This study seeks to bring consistency to the PBD methodology by establishing refined EDP limits at concrete cover spalling and bar buckling for circular reinforced concrete bridge columns. To this end, a database consisting of 118 previously tested flexure‐dominant bridge columns was formulated and analyzed. The EDP limit considered at the member level was drift ratio, whereas, at the sectional level, the EDP limits considered were material strain and curvature ductility. State‐of‐the‐art symbolic regression was adopted to fit the resulting data to mathematical expressions. At the member level, predicted drift ratio limits at the two damage states obtained with the proposed expressions were associated with lower root‐mean‐square error (RMSE) than those obtained from other similar expressions in the literature. At the sectional level, drift ratios at concrete cover spalling were adequately predicted with compressive strain limits in concrete ranging from 0.004 to 0.007 and a curvature ductility limit of 6.3. More accurate predictions of drift ratios at bar buckling were attained with variable sectional EDP limits, particularly for columns subjected to relatively high axial load. Two expressions predicting the tensile strain limit in the rebar and curvature ductility limit at bar buckling were proposed. The ratios of the measured drift ratio at bar buckling to the drift ratio predicted based on the proposed variable tensile strain limit had a mean of 0.99 and a coefficient of variation (COV) of 33%. Corresponding ratios based on the proposed variable curvature ductility limit had a mean of 1.02 and a COV of 31%. Fragility functions relating the likelihood of concrete cover spalling and bar buckling to the considered EDP limits were also developed.

Finite element analysis of an intact lumbar spine model: Effects of loading under different coordinate systems.

Finite element (FE) analysis has been used in studying the biomechanics of the lumbar spine. While some FE studies used a follower load technique that intended to replace the compressing effect of local muscle force, other studies considered satisfying the relationship between the human body's posture and the centre of gravity (CG) for investigating spine biomechanics. However, the above studies did not reveal the importance of a coordinate system that gratifies the posture-CG relation and follower load techniques. The present FE study compares the variation in ranges of motion (ROM) and stress-strain distributions due to the application of loading via different coordinate systems, follower (FCS) and global (GCS). A subject-specific computed tomography scan-based intact spine (L1-L5) FE model was developed and simulated for physiological movements. The FE results indicated a minimum deviation in ROM of 2.7° for the L1-L5 full model at all physiological activities between the defined coordinate systems. The observed variation for the L3-L4 functional spinal unit was between 4.7° and 19°. The von Mises strain in the vertebrae was between 0.0007 and 0.003 for the FCS case. In contrast, the peak von Mises strain for the GCS case was above the compressive yield strain limit of cancellous bone by 38.5%. The GCS model transferred the load unsymmetrically, whereas the distribution was symmetrical for the FCS case, without any potential risk of bone failure. These observations clearly indicate that the selection of the appropriate loading coordinate system is as crucial as the magnitude of loading.

Seismic safety evaluation method based on fiber-element strain for square-section steel piers

Inconsistency and incompatibility exist between the seismic capacity values obtained from shell-element models and the seismic demand results obtained from beam-based models in engineering. In this paper, the improved fiber model which can consider local instability of steel plates is used uniformly to conduct seismic capacity and seismic demand analyses under bidirectional earthquake actions. First, the restoring-force interaction curves of square-section steel piers are obtained under horizontal unidirectional and oblique loading; however, the evaluation method based on the restoring-force interaction curves may produce incorrect evaluation results according to the elastoplastic stability theory. Therefore, a new seismic safety evaluation method, based on the fiber-element strain within the effective damaged length (Led), is further proposed through the quasistatic analysis under the square spiral loading path. Dynamic seismic response analysis is conducted to verify the effectiveness of the proposed strain-based evaluation method. The results show that the ultimate state according to the compressive strain limit obtained via the square spiral loading is consistent with that based on the elastoplastic stability theory under bidirectional earthquake actions. The proposed method can not only coordinate the demand and capacity results, but also avoid misjudgment cases and facilitate engineering application.

Multifaceted Hybrid Carbon Fibers: Applications in Renewables, Sensing and Tissue Engineering

The field of material science is continually evolving with first-class discoveries of new nanomaterials. The element carbon is ubiquitous in nature. Due to its valency, it can exist in various forms, also known as allotropes, like diamond, graphite, one-dimensional (1D) carbon nanotube (CNT), carbon fiber (CF) and two-dimensional (2D) graphene. Carbon nano fiber (CNF) is another such material that falls within the category of CF. With much smaller diameters (around hundreds of nanometers) and lengths in microns, CNFs have higher aspect (length to diameter) ratios than CNTs. Because of their unique properties like high electrical and thermal conductivity, CNFs can be applied to many matrices like elastomers, thermoplastics, ceramics and metals. Owing to their outstanding mechanical properties, they can be used as reinforcements that can enhance the tensile and compressive strain limits of the base material. Thus, in this short review, we take a look into the dexterous characteristics of CF and CNF, where they have been hybridized with different materials, and delve deeply into some of the recent applications and advancements of these hybrid fiber systems in the fields of sensing, tissue engineering and modification of renewable devices since favorable mechanical and electrical properties of the CFs and CNFs like high tensile strength and electrical conductivity lead to enhanced device performance.

Deformation Capacity Limits for Reinforced Concrete Walls

The use of deformation capacity limits is becoming increasingly common in seismic design and assessment of reinforced concrete (RC) walls. Deformation capacity limits for RC walls in existing design and assessment documents are reviewed using a comprehensive database. It is found that the existing models are inconsistent and do not account for variation in deformation capacity with changes in the ratio of neutral axis depth to wall length ( c/ L w) and ratio of transverse reinforcement spacing to longitudinal bar diameter ( s/ d b) at the wall end region. A new mechanics-based model considering strain limits on the concrete and reinforcement is recommended. Concrete compressive strain limits for different levels of wall end region detailing are selected based on curvature ductilities for the walls in the database. Reinforcement tensile strain is limited based on a model for bar buckling. The proposed model, which accounts for c/ L w and s/ d b, is shown to have less dispersion and more accuracy than existing models when compared against experimental data and provides consistency between assessment and design provisions.

Numerical Evaluation of Compressive Strain Capacity for API X100 Line Pipe

In the strain-based design, pipeline steels are required to satisfy not only the stress capacity but also the strain capacity. Thus, considerable efforts have been made to meet the various requirements of the pipe-material properties that might undergo large deformation during the installation stage in various hostile environments. An adequate compressive strain capacity can help effectively avoid local buckling. Hence, highly deformable steel becomes crucial in achieving the required strain capacity. This study proposes a nonlinear finite element procedure based on the commercial software ABAQUS combined with the User-defined Material Module (UMAT), which is created using Fortran language. The Gurson–Tvergaard–Needleman (GTN) model is implemented in UMAT to fully consider the nonlinear behavior of an API X100 pipe subjected to compressive loading. The numerical simulation results of the full-scale bending test were compared with the experimental results to verify the nonlinear finite element procedure equipped with the GTN model. The simulation results are in good agreement with the experimental results. The parametric studies reveal the effects of geometric imperfections and material properties on the compressive strain limit.

Super-elasticity and deformation mechanism of three-dimensional pillared graphene network structures

Covalently bonded graphene/single-walled carbon nanotube (SWCNT) hybrid material can extend the excellent properties of graphene and SWCNT to three dimensions. We perform molecular dynamics simulations to investigate the mechanical properties of pillared vertically aligned carbon nanotube-graphene (VACNT-graphene) structure under uniaxial tensile and compressive loadings. The simulation results demonstrate that VACNT-graphene structures exhibit excellent elasticity with considerable elastic compressive/tensile strain limit. The early small deformation is dominated by the bending of graphene layers. Under larger elastic strain, graphene layers will form a corrugated pattern and shrink laterally in both the compression and tension processes. In the compression process, the failure mode of VACNT-graphene structure with small pillar distance is the bond breaking and reforming on the ridges of corrugated-shaped graphene and junction areas, while the failure mode of the structure with large pillar distance is the structural buckling. The elastic compressive strain limit exhibits a local maxima at the critical point between the two failure modes. In the tensile process, the failure is due to structural damage and elasticity increases with the pillar distance increasing. The essential tendency in our simulated VACNT-graphene structure might be significant to the further design and application of CNT-graphene hybrid materials.

Modified Plastic-Hinge Method for Circular RC Bridge Columns

AbstractThis paper discusses a research program aimed at defining accurate limit-state displacements that relate to specific levels of damage in reinforced concrete bridge columns subjected to seismic hazards. In design, concrete compressive and steel tensile strain limits are related to column deformations through the use of an equivalent curvature distribution. An experimental study was carried out to assess the performance of 30 circular well-confined bridge columns. Material strains, cross-section curvatures, and fixed-end rotations attributed to strain penetration of reinforcement into the adjoining member were quantified by using a three-dimesional (3D) position monitoring system. An equivalent curvature distribution was created that reflects the measured spread of plasticity and components of deformation. When compared with the current approach, the proposed modified plastic-hinge method improved the accuracy of both tensile and compressive strain-displacement predictions, while maintaining similar...

Compressive strain limits of composite repaired pipelines under combined loading states

Recent literature and the industry have widely accepted the benefits of strain-based design in regards to steel pipelines. In accordance with the recent increase in the use of strain-based design for pipelines, in this paper, the authors investigate composite repaired pipelines and its applicability in reference to the strain-based design. In an earlier work, the authors found that under combined loading conditions, a repaired pipe would tend to buckle locally in a location adjacent to the composite repaired wrap. In an attempt to limit the initiation of local buckling response in composite repaired pipelines, the compressive strain limit was investigated. For that, a finite element study was conducted. The results clearly showed that the maximum strains would not occur in the post-repaired defect region. The local buckling would however occur in the unrepaired (undamaged) section of the steel pipelines. An experimental study was also conducted on Fibre Reinforced Polymers (FRP) repaired pipelines to verify the finite element results. A parametric computational investigation was also conducted to understand the limits of composite repair wraps.

Assessment of material strain limits for defining plastic regions in concrete structures

The New Zealand Structural Loadings Standard, until its latest revision, used the structural ductility factor as a measure of the deformation demand of all potential plastic hinges in a structure. In the new version of New Zealand Standard for Earthquake Actions (NZS 1170.5:2004) the detailing of potential plastic regions is determined according to the local deformation demand in these regions. The change has been prompted by evidence that the structural ductility factor gives a poor indication of the demand on individual plastic regions. This new approach has also been adopted by the revised New Zealand Concrete Structures Standard (NZS 3101:2006) which classifies potential plastic regions into three categories (namely ductile, limited ductile and nominally ductile) based upon their inelastic deformation demand specified in terms of material strain limits. The material strain limits currently set in NZS 3101:2006 for the three categories of plastic regions are based on limited experimental evidence and need a closer revision. This paper tries to obtain more justifiable values of material strain limits based on experimental data. In this research, reversed cyclic loading tests of beams are conducted to compensate for a lack of data in the nominally ductile range of detailing. Based on the results of the tests conducted, curvature limits for nominally ductile plastic hinges are derived. Combining the experimental results collected from literature and the tests conducted in this project, updated material strain limits for the three categories of plastic regions are proposed. To unify the design process for all types of plastic regions, curvature limits for nominally ductile plastic hinges are also proposed as the multiple of first yield curvature (similar to the existing approach for the other two categories of plastic regions) rather than the existing approach of specifying allowable compressive (concrete) and tensile (rebar) strain limits for nominally ductile plastic regions. To further simplify the process, the representative value of first yield curvature is approximated as two times the yielding strain to the beam height ratio, thereby relieving the designers from having to conduct section analysis to estimate neutral axis depth.

Metallographic analysis of Cu–Zr–Al bulk amorphous alloys with yttrium addition

Abstract Minor yttrium addition can improve the glass-forming ability of Cu–Zr–Al ternary alloys via suppression of the growth of eutectic clusters. Yttrium addition also makes the room temperature ductility of the alloys decrease, and both the compressive strength and elastic strain limits increase slightly.

Stress Block Parameters for High-Strength Concrete Members

Early spalling of cover concrete has been reported by researchers in high-strength concrete (HSC) column tests. A rational way to incorporate this phenomenon into structural building codes has yet to be introduced. Moreover, use of ACI 318-02 stress block parameters to estimate the strength of HSC columns results in unconservative predictions for moment capacity in some cases. Hence, modification of ACI 318-02 stress block parameters to incorporate early cover spalling in HSC columns is of great significance. With this in mind, experimental results obtained during the course of this research and those available from the literature were used to study the accuracy and conservativeness of the ACI 318-02 stress block parameters. Based on experimental evidence, it was determined to reduce the ultimate compressive strain limit of ACI 318-02 to 0.0025 for HSC members such that the capacity of these members can be evaluated prior to cover spalling. A new set of stress block factors was analytically derived and validated with experimental data.

Proportional Biaxial Cyclic Hardening of Annealed Oxygen-Free High-Conductivity Copper

Abstract Cyclic-dependent hardening of annealed metals is by now a well-known experimental phenomenon. The annealed metal, when cycled between equal tensile and compressive strain limits, develops increasing stresses with each strain reversal. However, most previous experimental work on cyclic hardening has been restricted to axial or torsional loading. In this investigation hollow circular cylinders of annealed oxygen-free high-conductivity copper were subjected to combined axial-torsional cyclic proportional strain histories. The stress-strain histories for different proportional strain paths were correlated on the basis of effective stresses and strains defined previously by Tresca and Mises, and the correlations using Tresca's definitions were shown to be more representative than those of Mises. Power law hardening constants were determined for each stress-strain reversal. The power law hardening constants for axial cycling were predicted with reasonable accuracy from the corresponding torsional constants by employing correlations based on Tresca's definitions of effective stresses and strains.

Influence of Hold-Time and Temperature on the Low-Cycle Fatigue of Incoloy 800

A study has been conducted to determine the low-cycle fatigue behavior of solution-annealed Incoloy 800 bar at temperatures from 800–1400 deg F. The experimental work included evaluation of specimens under both continuous, completely reversed strain cycling and under strain cycling with hold time periods at the strain limits. At 1000, 1200, and 1400 deg F, it was found that 10-min hold-times at the tensile strain limit during every cycle significantly reduced the cyclic fatigue life compared to continuous cycling. However, there was little reduction in cyclic fatigue life when 10-min hold-times were introduced at the compressive strain limits or at both the tensile and compressive limits. The ratio of hold-time cyclic fatigue life to no-hold-time cyclic fatigue life decreased as the length of hold time increased (at constant total strain range) and as the magnitude of strain range decreased (at constant hold-time length).