Assessment of the residual fatigue strength in RC beams

In conventional analysis and design procedures of reinforced concrete structures, the ability of concrete to resist tension is neglected. Under cyclic loading, the tension-softening behavior of concrete influences its residual strength and subsequent crack propagation. The stability and the residual strength of a cracked reinforced concrete member under fatigue loading, depends on a number of factors such as, reinforcement ratio, specimen size, grade of concrete, and the fracture properties, and also on the tension-softening behavior of concrete. In the present work, a method is proposed to assess the residual strength of a reinforced concrete member subjected to cyclic loading. The crack extension resistance based approach is used for determining the condition for unstable crack propagation. Three different idealization of tension softening models are considered to study the effect of post-peak response of concrete. The effect of reinforcement is modeled as a closing force counteracting the effect of crack opening produced by the external moment. The effect of reinforcement percentage and specimen size on the failure of reinforced beams is studied. Finally, the residual strength of the beams are computed by including the softening behavior of concrete.

Experimental investigation on lateral impact response of concrete-filled double-skin tube columns using horizontal-impact-testing system

This paper presents an experimental investigation on the lateral impact performance of axially loaded concrete-filled double-skin tube (CFDST) columns. These columns have desirable structural and constructional properties and have been used as columns in building, legs of off shore platforms and as bridge piers. Since they could be vulnerable to impact from passing vessels or vehicles, it is necessary to understand their behaviour under lateral impact loads. With this in mind, an experimental method employing an innovative instrumented horizontal impact testing system (HITS) was developed to apply lateral impact loads whilst the column maintained a static axial pre-loading to examine the failure mechanism and key response parameters of the column. These included the time histories of impact force, reaction forces, global lateral deflection and permanent local buckling profile. Eight full scale columns were tested for key parameters including the axial load level and impact location. Based on the test data, the failure mode, peak impact force, impact duration, peak reaction forces, reaction force duration, column maximum and residual global deflections and column local buckling length, depth and width under varying conditions are analysed and discussed. It is evident that the innovative HITS can successfully test structural columns under the combination of axial pre-loading and impact loading. The findings on the lateral impact response of the CFDST columns can serve as a benchmark reference for their future analysis and design.

Impact resistance and evaluation of retained strength on geotextiles

Over the last few decades, geotextiles have progressively been incorporated into geotechnical applications, especially in the field of coastal engineering. Geotextile materials often act as separator and a filter layer between rocks laid above and subgrade beneath. This versatile material has gradually substituted traditional granular materials because of its ease of installation, consistent quality and labour costefficiency. However, geotextiles often suffer damage during installation due to high dynamic bulk loading of rock placement. This can degrade geotextiles' mechanical strength. The properties considered in this paper include the impact resistance and retained strength of geotextiles. In general, the greater the impact energy applied to geotextiles, the greater the potential for damage. Results highlight the inadequacy of using index derived values as an indicator to determine geotextile performance on site because test results shows that geotextiles (staple fibre (SF) and continuous filament (CF)) with better mechanical properties did not outperform lower mechanical strength materials. The toughest CF product with a CBR index value of 9696N shows inferior impact resistance compared to SF product with the least CBR strength (2719N) given the same impact energy of 9.02 kJ. Test results also indicated that the reduction of strength for CF materials were much greater (between 20 and 50%) compared to SF materials (between 0 and 5%) when subjected to the same impact energy of 4.52 kJ.

Prediction of fatigue strength in plain and reinforced concrete beams

A fatigue crack propagation model for concrete is proposed based on the concepts of fracture mechanics. This model takes into account the loading history, frequency of applied load, and size, effect parameters. Using this model, a method is described based on linear elastic fracture mechanics to assess the residual strength of cracked plain and reinforced concrete (RC) beams. This could be used to predict the residual strength (load carrying capacity) of cracked or damaged plain and reinforced concrete beams at a given level of damage. It has been seen that the fatigue crack propagation rate increases as. the size of plain concrete, beam increases indicating an increase in brittleness. In reinforced concrete (RC) beams, the fracture process becomes stable only when the beam is sufficiently reinforced.

Single and multi-step phase transformation in CuZr nanowire under compressive/tensile loading

A novel stress induced martenistic phase transformation is reported in an initial B2-CuZr nanowire of cross-sectional dimensions in the range of 19.44 x 19.44-38.88 x 38.88 angstrom(2) and temperature in the range of 10-400 K under both tensile and compressive loading. Extensive Molecular Dynamic simulations are performed using an inter-atomic potential of type Finnis and Sinclair. The nanowire shows a phase transformation from an initial B2 phase to BCT (body-centered-tetragonal) phase with failure strain of similar to 40% in tension, whereas in compression, comparatively a small B2 -> BCT phase transformation is observed with failure strain of similar to 25%. Size and temperature dependent deformation mechanisms which control ultimately the B2 -> BCT phase transformation are found to be completely different for tensile and compressive loadings. Under tensile loading, small cross-sectional nanowire shows a single step phase transformation, i.e. B2 -> BCT via twinning along {100} plane, whereas nanowires with larger cross-sectional area show a two step phase transformation, i.e. B2 -> R phase -> BCT along with intermediate hardening. In the first step, nanowire shows phase transformation from B2 -> R phase via twinning along {100} plane, afterwards the nanowire deforms via twinning along {110} plane which cause further transformation from R phase -> BCT phase. Under compressive loading, the nanowire shows crushing along {100} plane after a single step phase transformation from B2 -> BCT. Proper tailoring of such size and temperature dependent phase transformation can be useful in designing nanowire for high strength applications with corrosion and fatigue resistance. (C) 2009 Elsevier Ltd. All rights reserved.

Stress-strain characteristics of Tress-strain characteristics of reinforcing steel bars under cyclic loading

This paper presents test results for 22 high strength deformed bars and nine mild steel bars subjected to monotonic repeated and reversed axial loading to determine the stress-strain behavior. Equations have been proposed for the stress-strain curves and have been compared with test results. Satisfactory agreement was obtained.

Stress-Strain Characteristics of Reinforcing Steel Bars Under Cyclic Loading

This paper presents test results for 22 high strength deformed bars and nine mild steel bars subjected to monotonic repeated and reversed axial loading to determine the stress-strain behavior. Equations have been proposed for the stress-strain curves and have been compared with test results. Satisfactory agreement was obtained.

Enhancement of fatigue life of Ni-Ti-Fe shape memory alloys by thermal cycling

The effect of thermal cycling on the load-controlled tension-tension fatigue behavior of a Ni-Ti-Fe shape memory alloy (SMA) at room temperature was studied. Considerable strain accumulation was observed to occur in this alloy under both quasi-static and cyclic loading conditions. Though, in all cases, steady-state is reached within the first 50-100 cycles, the accumulated steady-state strain, epsilon(p.ss), is much smaller in thermally cycled alloy. As a result, the fatigue performance of them was found to be significantly enhanced vis-a-vis the as-solutionized alloy. Furthermore, under load-controlled conditions, the fatigue life of Ni-Ti-Fe alloys was found to be exclusively dependent on epsilon(p.ss). Observations made by profilometry and differential scanning calorimetry (DSC) indicate that the 200-500% enhancement in fatigue life of thermally cycled alloy is due to the homogeneous distribution of the accumulated fatigue strain. (C) 2010 Elsevier B.V. All rights reserved.

Fatigue in Ti-6Al-4V-B alloys

The addition of small amounts of B to Ti-6Al-4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress-strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-beta grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti-6Al-4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress-strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening. (c) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Fatigue crack propagation model for plain concrete - An analogy with population growth

In this work, an analytical model is proposed for fatigue crack propagation in plain concrete based on population growth exponential law and in conjunction with principles of dimensional analysis and self-similarity. This model takes into account parameters such as loading history, fracture toughness, crack length, loading ratio and structural size. The predicted results are compared with experimental crack growth data for constant and variable amplitude loading and are found to capture the size effect apart from showing a good agreement. Using this model, a sensitivity analysis is carried out to study the effect of various parameters that influence fatigue failure. (C) 2010 Elsevier Ltd. All rights reserved.

Monitoring Fatigue Crack Propagation in Compact Tension Specimens via Remote Sensing of Back Face Strain

Back face strain (BFS) measurement is now well-established as an indirect technique to monitor crack length in compact tension (CT) fracture specimens [1,2]. Previous work [2] developed empirical relations between fatigue crack propagation (FCP) parameters. BFS, and number of cycles for CT specimens subjected to constant amplitude fatigue loading. These predictions are experimentally validated in terms of the variations of mean values of BFS and load as a function of crack length. Another issue raised by this study concerns the validity of assigning fixed values for the Paris parameters C and n to describe FCP in realistic materials.

Fatigue crack propagation model and size effect in concrete using dimensional analysis

It is well known that fatigue in concrete causes excessive deformations and cracking leading to structural failures. Due to quasi-brittle nature of concrete and formation of a fracture process zone, the rate of fatigue crack growth depends on a number of parameters, such as, the tensile strength, fracture toughness, loading ratio and most importantly the structural size. In this work, an analytical model is proposed for estimating the fatigue crack growth in concrete by using the concepts of dimensional analysis and including the above parameters. Knowing the governed and the governing parameters of the physical problem and by using the concepts of self-similarity, a relationship is obtained between different parameters involved. It is shown that the proposed fatigue law is able to capture the size effect in plain concrete and agrees well with different experimental results. Through a sensitivity analysis, it is shown that the structural size plays a dominant role followed by loading ratio and the initial crack length in fatigue crack propagation. (C) 2010 Elsevier Ltd. All rights reserved.

Elasto-plastic contact stress analysis of joints subjected to cyclic loading

Rigorous elastic-plastic finite element analysis of joints subjected to cyclic loading is carried out. An incremental-iterative algorithm is developed in a modular form combining elasto-plastic material behaviour and contact stress analysis. For the case of the interference fit, the analysis sequentially carries out insertion of the pin and application of the load on the joint, covering possible initiation of separation (and/or yielding) and progressively the receding/advancing contact at the pin-plate interface. Deformations of both the plate and the pin are considered in the analysis. Numerical examples are presented for the case of an interference fit pin in a large plate under remote cyclic tension, and for an interference fit pin lug joint subjected to cyclic loading. A detailed study is carried out for the latter problem considering the effect of change in contact/separation at the pin-plate interface on local stresses, strains and redistribution of these stresses with the spread of a plastic zone. The results of the study are a useful input for the estimation of the fatigue life of joints. Copyright (C) 1996 Elsevier Science Ltd

Study of fracture features in foam bearing glass epoxy composites subjected to repeated impacts

The paper reports the failure features observed in low mass repeatedly (pendulum) impacted glass epoxy composites with and without the mid section having either 2-layers or 3-layers of flexible foam. Features such as through width and inclined cracks as well as adhering of foam observed in the experiments are explained. The significance of the foam material in modifying the impact response of the composite is stressed.

Fatigue crack growth under an eqivalent falstaff spectrum

Optical fractography was used to estimate growth of small cracks at notches under programmed FALSTAFF loading in an Al-Cu alloy. Crack sizes as low as 25 microns and growth rates over two orders of magnitude could be resolved using this technique. Randomized MiniFALSTAFF load sequence was modified into a programmed load equivalent with major loads either preceding or following marker loads. Crack growth rate under programmed FALSTAFF spectrum as estimated by optical fractography conformed to compliance based estimates on a SE(T) specimen. Long crack growth rates under programmed and randomized MiniFALSTAFF spectrum were essentially similar. Spectrum load fatigue crack growth was studied in central hole coupons under notch inelastic conditions. Scatter in growth rates for small notch cracks was found to be of the same magnitude as that of long cracks. Multiple fatigue cracks are observed at the notch root, and they appear to influence each other.