Terms Of Damage Accumulation Research Articles

Coupling effects between wind and train transit induced fatigue damage in suspension bridges

Long-span steel suspension bridges develop significant vibrations under the effect of external time-variable loadings because their slenderness. This causes significant stresses variations that could induce fatigue problems in critical components of the bridge. The research outcome presented in this paper includes a fatigue analysis of a long suspension bridge with 3300 meters central suspended span under wind action and train transit. Special focus is made on the counterintuitive interaction effects between train and wind loads in terms of fatigue damage accumulation in the hanger ropes. In fact the coupling of the two actions is shown to have positive effects for some hangers in terms of damage accumulation. Fatigue damage is evaluated using a linear accumulation model (Palmgren-Miner rule), analyses are carried out in time domain by a three-dimensional non-linear finite element model of the bridge. Rational explanation regarding the above-mentioned counterintuitive behavior is given on the basis of the stress time histories obtained for pertinent hangers under the effects of wind and train as acting separately or simultaneously. The interaction between wind and train traffic loads can be critical for a some hanger ropes therefore interaction phenomena within loads should be considered in the design.

Development of a rocking R/C shear wall system implementing repairable structural fuses

In the last decades, the concept of earthquake resilient structural systems is becoming popular in which the rocking structure is considered as a viable option for buildings in regions of high seismicity. To this end, a novel wall-base connection based on the “repairable structure” approach is proposed and evaluated. The proposed system is made of several steel plates and high strength bolts act as a friction connection. To achieve the desired rocking motion in the proposed system, short-slotted holes are used in vertical directions for connecting the steel plates to the shear wall (SW). The experimental and numerical studies were performed using a series of displacement control quasi-static cyclic tests on a reference model and four different configurations of the proposed connection installed at the wall corners. The seismic response of the proposed system is compared to the conventional SW in terms of energy dissipation and damage accumulation. In terms of energy dissipation, the proposed system depicted better performance with 95% more energy dissipation capability compared to conventional SW. In terms of damage accumulation, the proposed SW system is nearly undamaged compared to the conventional wall system, which was severely damaged at the wall-base region. Overall, the introduced concept presents a feasible solution for R/C structures when a low-damage design is targeted, which can improve the seismic performance of the structural system significantly.

Are microseismic ground displacements a significant geomorphic agent?

This paper considers the role that microseismic ground displacements may play in fracturing rock via cyclic loading and subcritical crack growth. Using a coastal rock cliff as a case study, we firstly undertake a literature review to define the spatial locations that may be prone to microseismic damage. It is suggested that microseismic weakening of rock can only occur in ‘damage accumulation zones’ of limited spatial extent. Stress concentrations resulting from cliff height, slope angle and surface morphology may nucleate and propagate a sufficiently dense population of microcracks that can then be exploited by microseismic cyclic loading. We subsequently examine a 32-day microseismic dataset obtained from a coastal cliff-top location at Staithes, UK. The dataset demonstrates that microseismic ground displacements display low peak amplitudes that are punctuated by periods of greater displacement during storm conditions. Microseismic displacements generally display limited preferential directivity, though we observe rarely occurring sustained ground motions with a cliff-normal component during storm events. High magnitude displacements and infrequently experienced ground motion directions may be more damaging than the more frequently occurring, reduced magnitude displacements characteristic of periods of relative quiescence. As high magnitude, low frequency events exceed and then increase the damage threshold, these extremes may also render intervening, reduced magnitude microseismic displacements ineffective in terms of damage accumulation as a result of crack tip blunting and the generation of residual compressive stresses that close microcracks. We contend that damage resulting from microseismic ground motion may be episodic, rather than being continuous and in (quasi-)proportional and cumulative response to environmental forcing. A conceptual model is proposed that describes when and where microseismic ground motions can operate effectively. We hypothesise that there are significant spatial and temporal limitations on effective microseismic damage accumulation, such that the net efficacy of microseismic ground motions in preparing rock for fracture, and hence in enhancing erosion, may be considerably lower than previously suggested in locations where high magnitude displacements punctuate ‘standard’ displacement conditions. Determining and measuring the exact effects of microseismic ground displacement on damage accumulation and as a trigger to macro-scale fracture in the field is not currently possible, though our model remains consistent with field observations and conceptual models of controls on rockfall activity.

TOF-SIMS with Argon Gas Cluster Ion Beams: A Comparison with C60+

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an established technique for the characterization of solid sample surfaces. The introduction of polyatomic ion beams, such as C(60), has provided the associated ability to perform molecular depth-profiling and 3D molecular imaging. However, not all samples perform equally under C(60) bombardment, and it is probably naïve to think that there will be an ion beam that will be applicable in all situations. It is therefore important to explore the potential of other candidates. A systematic study of the suitability of argon gas cluster ion beams (Ar-GCIBs) of general composition Ar(n)(+), where n = 60-3000, as primary particles in TOF-SIMS analysis has been performed. We have assessed the potential of the Ar-GCIBs for molecular depth-profiling in terms of damage accumulation and sputter rate and also as analysis beams where spectral quality and secondary ion yields are considered. We present results with direct comparison with C(60) ions on the same sample in the same instrument on polymer, polymer additive, and biomolecular samples, including lipids and small peptides. Large argon clusters show reduced damage accumulation compared with C(60) with an approximately constant sputter rate as a function of Ar cluster size. Further, on some samples, large argon clusters produce changes in the mass spectra indicative of a more gentle ejection mechanism. However, there also appears to be a reduction in the ionization of secondary species as the size of the Ar cluster increases.

Understanding the Fatigue Behaviour of NiTiCu Shape Memory Alloy Wire Thermal Actuators

This paper deals with the fatigue behaviour of NiTiCu shape memory alloy (SMA) wire actuators on thermo-mechanical cycling (TMC). Cyclic loading in SMA actuators is invariably associated with both functional and structural fatigue. The characteristic of the actuators such as austenite (hot shape) remnant deformation and recovery strain undergo changes upon TMC. These in turn result in continuous change in strain response (functional fatigue) during application. It has been shown that the functional fatigue can be minimized by adopting TMC at higher stress than that of the working stress prior to the application. On the other hand, failure of the actuators by fracture (structural fatigue) due to cyclic stress/strain is inevitable. Study shows that the fatigue life of the actuators is strongly dependent on the type of loading and the temperature range of operation. This has been explained in terms of damage accumulation, crack initiation and fracture behaviour.

Applications of a non-contacting Kelvin probe during sliding

Sliding contact causes dramatic changes near the interface in metals. These include the development of large plastic strains and associated changes in defect concentration, microstructure, crystallographic texture and phase content. In addition, interactions with the counterface material and the environment and mechanical mixing processes can modify the material on both sliding surfaces. Structural and chemical characterization techniques have provided some insight into friction and wear processes, but many questions remain concerning mechanisms. A non-contacting Kelvin probe which detects changes in electron work function offers a useful tool for in situ monitoring of changes associated with sliding. Use of such a probe allows improved design of friction and wear experiments and provides new information on wear mechanisms. Earlier results reported by Zharin were interpreted in terms of damage accumulation and fatigue-like processes. The present results for the unlubricated sliding of various metal combinations confirm the experimental observations of Zharin but suggest that chemical reactions and changes in composition influence the changes detected by the Kelvin probe. For copper-based alloys, the Kelvin probe signal (KPS) can vary with a well-defined period. Images from a video system and from post-test observations with SEM and EDS indicate that the KPS correlates well with the amount of oxygen incorporated in the surface material. Results from materials which do not exhibit nearly-periodic behavior are also described. Differences in the development of the friction signal and the KPS with time can be correlated with differences in the development, accumulation and removal of material affected by tribochemical processes.