Reduction In Load Carrying Capacity Research Articles

Effects of stress invariants and reverse loading on ductile fracture initiation

Recent research studies on ductile fracture of metals have shown that the ductile fracture initiation is significantly affected by the stress state. In this study, the effects of the stress invariants as well as the effect of the reverse loading on ductile fracture are considered. To estimate the reduction of load carrying capacity and ductile fracture initiation, a scalar damage expression is proposed. This scalar damage is a function of the accumulated plastic strain, the first stress invariant and the Lode angle. To incorporate the effect of the reverse loading, the accumulated plastic strain is divided into the tension and compression components and each component has a different weight coefficient. For evaluating the plastic deformation until fracture initiation, the proposed damage function is coupled with the cyclic plasticity model which is affected by all of the stress invariants and pervious plastic deformation history.For verification and evaluation of this damage-plasticity constitutive equation a series of experimental tests are conducted on high-strength steel, DIN 1.6959. In addition finite element simulations are carried out including the integration of the constitutive equations using the modified return mapping algorithm. The modeling results show good agreement with experimental results.

Effect of Hydrolytic Ageing on Kevlar/Polyester Using Acoustic Emission Monitoring

When composite materials are exposed to seawater it undergoes various processes of degradation such as matrix cracking, fiber breakage, delamination and interfacial debonding, resulting in the reduction in load carrying capacity. To improve the strength characteristics, Kevlar fibers are used in Marine applications. But Kevlar is prone to lot of moisture absorption and its adhesiveness towards the resin is also very much less. When exposed to seawater, the most noticeable effect is the plasticization of the matrix. This research attempts to investigate the reduction in flexural strength of ASTM D790 Kevlar/polyester specimens by 3 point bending test with AE monitoring using an INSTRON 3367 UTM, which are conditioned in seawater at non-laboratory conditions following ASTM-D5229 standard over different time intervals. The change in magnitude of AE parameters are correlated with the mechanical strength of Kevlar/polyester samples. Since Kevlar offers little resistance to Delamination, the focus of the study is being restricted to the failure modes such as debonding and Delamination.The range of Peak frequencies for various hydrolytically aged states for Kevlar samples are investigated using FFT analysis. Hits chosen randomly from each range of peak frequencies pertaining to each failure mode for various hydrolytically aged specimens are investigated using STFFT analysis.

Hydroelasto-plasticity approach to predicting the post-ultimate strength behavior of a ship’s hull girder in waves

Dynamic collapse behavior of a ship’s hull girder in waves is investigated; post-ultimate strength behavior is the focus. Firstly, a simulation method is proposed. Assuming that a plastic hinge is formed during the collapse of the hull girder, the whole ship is modeled as two rigid bodies connected amidship via a nonlinear rotational spring. The post-ultimate strength behavior, such as the reduction of load carrying capacity due to buckling and yielding, is reflected in the model. Hydrodynamic loads are evaluated by using nonlinear strip theory to account for the effect of large plastic deformations on the loads. A scaled model for validation of the simulation is designed and fabricated. Then a series of tank tests is conducted using the scaled model to validate the simulation results. Post-ultimate strength behavior characteristics in waves are clarified by using the numerical and tank test results. It is shown that the hull girder collapses rapidly after reaching ultimate strength, and then the plastic deformation grows until unloading starts at the collapsed section. Finally, several parametric dependencies of the extent of the collapse behavior are discussed based on a series of the simulations.

Fracture investigations on piping system having large through-wall circumferential crack

In level-3 Leak-Before-Break (LBB) analysis, stability of a postulated through-wall circumferential crack is demonstrated by simplified fracture mechanics calculations. Detailed experimental studies, conducted by the authors, have revealed that the conventional assessment procedure used to demonstrate LBB is too conservative. There is a large factor of safety due to system indeterminacy. It was observed that the critical load of a cracked piping system (with even a large through-wall circumferential crack of about 120°) is of the order of 75–90% of the collapse load of the uncracked piping system. Reduction in load carrying capacity is even less for a piping system having an off-centre crack. This article discusses the above-mentioned aspects in detail. Detailed 3-D elastic-plastic finite element analyses of some of these tests were performed. The suitability of these numerical results to predict crack initiation load in light of the experimental data is discussed.

Behavior of steel-concrete composite cantilever beams with web openings under negative moment

The effect of web openings on the mechanical behavior of composite beams under negative moment was studied through monotonically loading tests. Nonlinear finite element method based analysis was also conducted for cantilever composite beams. The test and the finite element analysis results indicated that the initial cracking loads of composite beams with web opening are lower than that without web opening. The first crack initiated from the concrete slab on the top of the opening. The beams with web opening failed due to shear failure of concrete slab upon the opening. It was also found that the load carrying capacity of beam decreases with the increase of the moment-to-shear ratio at the central line of the opening and the mechanical behavior of beam can be improved significantly by applying stiffening steel plates around the opening. To quantify the reduction of load carrying capacity, a method for calculating the load carrying capacity of beams with web openings under negative moment was derived with consideration of the interaction between moment and shear. Good consistency was obtained between the proposed method, the finite element method and the test results.

Knockdown factors for buckling of cylindrical and spherical shells subject to reduced biaxial membrane stress

Cylindrical shells under uniaxial compression and spherical shells under equi-biaxial compression display the most extreme buckling sensitivity to imperfections. In engineering practice, the reduction of load carrying capacity due to imperfections is usually addressed by use of a knockdown factor to lower the critical buckling stress estimated or computed without accounting for imperfections. For thin elastic cylindrical shells under uniaxial compression and spherical shells under equi-biaxial compression, the knockdown factor is typically as small as 0.2. This paper explores the alleviation of imperfection-sensitivity for loadings with a reduced circumferential (transverse) membrane stress component. The analysis of Koiter (1963) on the effect of an axisymmetric imperfection on the elastic buckling of a cylindrical shell under uniaxial compression is extended to both cylinders and spheres for loadings that produce general combinations of biaxial membrane stresses. Increases in the knockdown factor due to a reduction of the transverse membrane component are remarkably similar for cylindrical and spherical shells.

Ratchet Boundary Determination Using a Noncyclic Method

A simple and systematic procedure is proposed for shakedown analysis using a combination of linear and nonlinear finite element analysis (FEA). The method can identify the boundary between the shakedown and ratcheting domains directly and does not require a cyclic analysis (noncyclic). The proposed method performs an elastic-plastic FEA to determine the reduction in load carrying capacity due to the cyclic secondary loads. An elastic modulus adjustment procedure is then used to generate statically admissible stress distributions and kinematically admissible strain distributions under the applied primary loads. By modifying the local elastic moduli it is possible to obtain inelastic-like stress redistribution. The method is demonstrated with a “two-bar structure” model based on analytical routine. The analysis is then applied to some typical shakedown problems including the “classical Bree problem,” the “bimaterial cylinder,” and the “plate with a hole subjected to radial temperature gradient.”

ASR-Deteriorated Bridges and Rock Type of Reactive Aggregate on Noto Expressway

There exist large numbers of ASR-deteriorated bridges on the Noto expressway through the center of Noto peninsula in Ishikawa Prefecture. In some serious cases, a very severe deterioration which appears to directly be related with the reduction in load carrying capacity of structures has occurred; the significant decrease in compressive strength of concrete, the loss in bonding strength around steel reinforcement, and the fracture of steel reinforcement at the bent and so on. This study aims at investigating both the mix proportions of concrete and the rock type of reactive aggregates used in ASR-deteriorated bridges. Furthermore, the relationships between mechanical properties of cores taken from various portions of bridge pier and ASR degradation ranks classified by the various inspection were discussed for the purpose of the diagnosis and maintenance procedures of these bridges.

Cement-Modified Backfills for Mechanically Stabilized Earth Walls with Built-In Facing

A field study was undertaken to evaluate the performance of model geotextile-reinforced mechanically stabilized earth walls constructed using locally available marginal backfill soils without and with cement modification. Cyclic plate load tests were carried out on the constructed walls both at optimum moisture content and in fully wet conditions in the case of plain backfills and in fully wet condition for cement-modified backfills. The performance and utility of the proposed built-in soil cement facing with folded-back geotextile layers were also examined. The reduction in load-carrying capacity of walls upon wetting could be compensated by cement modification of marginal backfills while maintaining the pullout resistance of reinforcing layers with nominal variation in the flexibility of walls. The performance of model walls constructed using cement-modified marginal backfills is found to be even superior to the wall constructed using free draining sand backfill in terms of higher load-carrying capacity with reduced lateral facing deformations. It was observed that the geotextile-reinforcing layers in marginal backfills also facilitate the internal drainage.

Assessment of the load-carrying capacity of multi-span masonry arch bridges using fibre beam elements

An approach that makes use of non-linear beam elements with fibre cross-section has been used for modelling the ultimate behaviour of multi-span masonry arch bridges. The proposed approach proves able to take into account the interaction among the spans and the non-linear material behaviour with low computational effort. In order to validate the use of the model for the assessment of masonry arch bridges, the load-carrying capacity for typical multi-span railway bridges has been evaluated and the results compared to experimental results and to currently used limit analysis methods. It is shown that, by parity of constitutive assumption, the method provides the same results as limit analysis, both in terms of maximum load prediction and hinges position at collapse; however, taking the effective ductility capacity into account, a strong reduction in load-carrying capacity with respect to classical limit analysis was found, depending on rise-to-span arch ratio, piers slenderness and backfill height. The approach is then applied to a seven span viaduct of the Italian railway network, for which the effective mechanical properties of masonry were identified through an experimental campaign on brickwork samples according to the effective material properties surveyed on site.

Compression and post-buckling damage growth and collapse analysis of flat composite stiffened panels

Experimental and numerical investigations were conducted into the damage growth and collapse behaviour of composite blade-stiffened structures. Four panel types were tested, consisting of two secondary-bonded skin–stiffener designs in both undamaged and pre-damaged configurations. The pre-damaged configurations were manufactured by replacing the skin–stiffener adhesive with a centrally located, full-width Teflon strip. All panels were loaded in compression to collapse, which was characterised by complex post-buckling deformation patterns and ply damage, particularly in the stiffener. For the pre-damaged panels, significant crack growth was seen in the skin–stiffener interface prior to collapse, which caused a reduction in load-carrying capacity. In the numerical analysis of the undamaged panels, collapse was predicted using a ply failure degradation model, and a global–local approach that monitored a strength-based criterion in the skin–stiffener interface. The pre-damaged models were analysed with ply degradation and a method for capturing interlaminar crack growth based on multi-point constraints controlled using the Virtual Crack Closure Technique. The numerical approach gave close correlation with experimental results, and allowed for an in-depth analysis of the damage growth and failure mechanisms contributing to panel collapse. The successful prediction of collapse under the combination of deep post-buckling deformations and several composite damage mechanisms has application for the next generation of composite aircraft designs.

Fire Induced Thermal and Structural Response of the World Trade Center Towers.

Over the past several years, there has been a resurgence of interest in studying the response of building structures to fires. Simulations of the effects of severe fires on the structural integrity of buildings requires a close coupling between the gas phase energy release and transport phenomena and the stress analysis in the load bearing materials. A methodology has been developed for coupling CFD simulations of fire growth with finite element models for thermal analysis and for using the thermal data to compute the demand-to-capacity ratio in a multi-story structure. A simple radiative transport model that assumes the compartment is divided locally into a hot, soot laden upper layer and a cool, relatively clear lower layer is employed to predict radiative fluxes incident on sub-grid scale structural members. Thermal response coupled with realistic fire simulations of various steel structural components on floors of World Trade Center Tower 1 that were subjected to aircraft impact damage and fires are presented. The thermal response was used to compute the reduction in load carrying capacity of the structural components as a function of time, which ultimately results in global collapse of the towers.

Wire fractures in locked coil cables

Wire fractures in the main cables of the Lysefjord Suspension Bridge display an approximate linear relationship with time. Uncertainties related to wire fractures in sub-layers, reduction of load-carrying capacity and lifetime, and future development of wire fractures have prompted extensive investigations of the main cables. Failure analyses of fractured Z-wire samples and fracture mechanics analysis concluded that cracks have initiated and propagated from surface imperfections. The potential for hydrogen atom generation and absorption that causes hydrogen-induced cracking is discussed. Hydrogen atoms result from the hot-dip galvanization process, or due to corrosion. Evaluation of remaining load-carrying capacity and the need for strengthening of the main cables are briefly discussed and methods for surveillance of wire fractures are presented.

Life extension of impact damaged honeycomb sandwich panels

AbstractThe assessment of damage due to impacts in composite sandwich panels and the component life reductions associated with such damage is becoming increasingly important within the aerospace industry. This study analyses minimum gauge, non-metallic honeycomb wing panels subject to impact damage. In all instances the damage is caused by ‘soft body’ impactors travelling at elevated velocities to simulate bird strike and other soft debris. The damage formed during these impacts is shown to be large in plan area but shallow and primarily causes crushing of the core in a thin layer. Loading of the panels after impact has been performed to determine the reduction in load carrying capacity and associated failure mechanisms. Damage due to soft body impact is shown to be very different to the classical rigid body impact upon which current repair schemes are based. As a result in many applications the use of current repair schemes may be inappropriate. The viability of new repair techniques is discussed with particular attention to the prevention of failure mechanisms due to loading after impact.

Piping elbows with cracks Part 1: A parametric study of the influence of crack size on limit loads due to pressure and opening bending

Finite element analyses of cracked short-radius piping elbows are reported. Both axial and circumferential cracks, centred about the bend crown or intrados respectively, are considered. The defects are internal, with depth-thickness ratios a/t in the range 0.25 ≤ a/t ≤ 1.0. Since the aim is to assess limit loads, the analysis assumes elastic-perfectly plastic material behaviour with no geometric non-linearities. The loadings considered are (a) opening bending moment, (b) internal pressure and (c) combined loading of steady pressure, set at the design value of the pipe elbow, with a superimposed monotonically increasing opening bending moment. For each type of loading, the general component behaviour, by consideration of the global deformation with increasing load, is reported. Limit load values for each case are presented and the relative reductions in load-carrying capacity, compared with that predicted for a defect free elbow of the same material and dimensions, are given.

Evaluating Structural Deterioration by Dynamic Response

The dynamic response of two C 3 × 4.1 channel steel beams is evaluated in terms of the reduction in load-carrying capacity of the damaged state versus the undamaged stage. The beam specimens are sized and configured to have similar frequency response observed in actual bridges. Each beam is subjected to different damage scenarios, i.e., various types, locations, and degree of damage. For each scenario, the characteristics of the dynamic response are determined from acceleration measurements made during low-level free-vibration tests. A finite-element model was used to calculate the dynamic response of the simulated damaged structures. The experimental and analytical results are scrutinized to correlate increasing deterioration and changes in the dynamic response. Good correlations are found between the experiments and analysis. For each damage scenario, the yield and ultimate-load capacities of the test beams are defined by a damage index. Both experimental and numerical results indicate that there is less than 10% change in the natural frequencies at critical damage state.

異種強度軸筋の混使用によるRC柱断面曲げ性能の改善

A new method to enhance the moment-curvature relations of reinforced concrete column sections in post-yield range of deformation was developed. The point of the idea is to introduce the sequential yielding of various grade longitudinal bars in a section. The section then does not show any significant reduction in load carrying capacity up to the yielding of highest grade bars. To confirm the advantages of the new method, flexural analyses and loading tests were carried out on reinforced concrete columns with various grade longitudinal bars. The results indicated the advantages of the combined use of various grade longitudinal bars.

Load-carrying capacity of polymer tape rolls under impact loads

The appratus, method, and result of experimental investigation of load-carrying capacity of rolled magnetic tapes subjected to impact and thermal effects are described. Analysis of the basic regularities of loss of load-carrying capacity of the rolls (lamination of turns) has facilitated determination of the safe level of impact accelerations and thermal effects which do not cause damage of rolls under multiple impacts (104...105 cycles). It is established that breakdown in compactness of rolled magnetic tapes under multiple impacts occurs due to reduction in interturn pressures (up to 30 ... 50% of the original level). It is noted that, with increase in environmental temperature, there is sharp reduction in load-carrying capacity of rolls under the action of impact loads.

Combined axial and pressure buckling of end supported shells of revolution

A reduced stiffness theoretical analysis of the imperfection sensitive elastic buckling for end supported shells of revolution is extended to the case of arbitrary combinations of axial and radial pressure loading. Depending upon the shell and loading parameters, the potential reductions in load capacity due to imperfections are shown to involve two distinct forms of post-buckling loss of stiffness. Lower bounds in each of these regimes are provided by appropriate reduced stiffness models, and shown by comparisons with available test data to be reliable even for relatively perfect test models. By attributing reductions in load carrying capacity to weakened end support conditions, it is suggested that past interpretations of these tests may have underestimated the deleterious effects of initial imperfections.

Cyclic creep and endurance of a material with curved loading paths under plane-stress conditions

1. Creep occurs more rapidly in loading along complex paths than in proportional loading, which leads to a reduction in load-carrying capacity. 2. Present design formulas capably describe the cyclic creep and rupture time of isotropic materials only when the loading is close to simple. 3. An expression was obtained for equivalent stress which permits the use of present relations in calculations for cyclic creep conditions with either proportional loading or complex loading along a path of constant curvature.