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Fatigue Treatment of Wood by High

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different materials acting on cycle load
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  Fatigue treatment of wood by high-frequency cyclic loading  New scientific tools for investigating the qualitative and quantitative response of wood to mechanical loading under conditions close to those of mechanical pulping are needed to find ways of radically reducing energy consumption. At KCL a modulated loading device was developed to fulfill these requirements. The device allows the static load, the loading frequency and the amplitude to which wood is subjected to be varied over a wide range. Wood samples were cyclically stressed at large strain (1 mm) pulses and at frequencies in the range 100-1000 Hz. The temperature rise in the wood was measured with thermocouples inserted into the wood and on the wood surface using a thermographic camera. It was found that a high static load during cyclic loading increased heat generation in the wood. The cyclic loading frequency had less effect than the level of the static loading. However, low frequencies generated more heat at a specific number of impacts. The responses of heartwood and sapwood were significantly different in the fatigue treatment. The temperature rise was higher in heartwood than in sapwood. The wood underwent irreversible plastic deformations seen as heat generation and increased pore volume in the wood microstructure. Effect of cyclic loading on undrained behavior of compacted sand/clay mixtures Compacted aggregate/clay mixtures are frequently used as the core material of embankment dams all over the word. In the seismic zones, the post earthquake static stability of embankment dams, has great importance for the geotechnical engineers. During a seismic event, the compacted embankment material is expected to experience little, if any, strength and stiffness reduction during and shortly after the design earthquake. A series of undrained post cyclic triaxial compression tests after cyclic loading were performed on a medium plasticity sand/clay mixtures. Testing was performed on isotropically and anisotropically consolidated specimens to investigate the effectiveness of aggregate fraction on the mechanical behavior of the mixtures. In addition, monotonic triaxial compression tests were also performed on the same sand/clay mixtures with the same initial condition. The results point out different peculiarities which can be of interest in assessing the mechanical behavior of the mixtures under post seismic shaking. The results show that effect of cyclic loading on post cyclic pore water  pressure build-up is significant when pore water pressure build-up is considerably lower than the associated value in monotonic loading. The effect of aggregate content on post cyclic pore water  pressure build-up is miner. However, when the aggregate content increases the shear strength increases.  REINFORCED CONCRETE UNDER CYCLIC LOADING In order to contribute to the on going research effort in this field, an experimental working plan with cylinder steel fibre reinforced concrete specimens under compression cyclic loading was carried out. Sets of five specimens were reinforced with conventional transverse reinforcement of 0, 0.57, 1.71 and 4.01 volume percentage of specimen concrete core. To evaluate the fibre reinforcement effect, each of this specimens set was reinforced with 0, 30, 60 and 90 kg/m 3  of hooked-ends steel fibres with an aspect-ratio of 60 and a yield strength of 1250 MPa. A total of eighty tests were carried out. The peak stress and the initial elasticity modulus were not significantly changed by fibre reinforcement. The strain at peak stress and the rigidity of the unloading/reloading branches were marginally increased with the increment of the fibre content. The slope of the softening branch was decreased with the increment of fibre content, revealing a significant increase in the energy absorption capacity. The results have pointed out that conventional transverse reinforcement can  be partially replaced by appropriate fibre content, without loss of ductility and strength. This replacement could be favorable in zones densely reinforced with hoops and stirrups, like beam-column joints of structures submitted to seismic action. CYCLIC LOAD TESTING OF WOOD-FRAMED, PLYWOOD SHEATHED ,SHEAR WALLS USING ASTM E564 AND THREE LOADING SEQUENCES The damage to wood-framed residential buildings in recent earthquakes has raised questions regarding the performance of shear wall assemblies when subjected to cyclic lateral loading. Historically shear wall performance has been evaluated on monotonic testing which has been used to develop model building code provisions for engineered shear walls. Presently no national or international standards for conducting cyclic lateral load testing of wood-framed assemblies are recognized. ASTM E 564, for example, does not specify a cyclic lateral load testing protocol. This study applied ASTM E 564 to four identical sets of plywood shear wall assemblies using three different cyclic lateral load test sequences in order to investigate the effects of loading sequence on test results. The first sequence had a large number of cycles patterned after a sequentially phased displacement test  procedure. The second sequence had only large excursions used to study the effects of large pulses in the near-field of an earthquake. The third sequence had a moderate number of cycles occurring in progressively increasing excursion increments. Twenty-four 2.44 m x 2.44 m (8 foot x 8 foot) shear walls with 9.5 mm (3/8 inch) thick plywood  panel sheathing were tested. Four different nail styles were used for the identically framed and sheathed samples. Two samples of each configuration were tested under each of the three loading sequences to obtain the load displacement and load-capacity characteristics for each assembly. The objective of these shear wall tests was to compare four identical sets of plywood shear wall assemblies, except for nail type, using three different cyclic lateral load test sequences in order to investigate the effects of loading sequence. The first set of tests followed the protocol developed by SEAOSC’s Ad Hoc Committee on Testing Standards (4) , (5) using the Sequentially Phased  Displacement loading sequence (6). The second set of tests followed the same SEAOSC test  protocol, except the first half of the loading sequence was removed to subject the test samples to large excursions as to simulate large pulses in the near-field of an earthquake. The third set of tests also followed SEAOSC’s test protocol, except the final three cycles per displacement increment were removed to minimize nail fracture effects on the sheathing fasteners. Performance of Steel Structures under Fatigue Cyclic Loading  A component or structure, which is designed to carry a single monotonically increasing application of static load, may fracture and fail if the same load or even smaller load is applied cyclically a large number of times. For example a thin rod bent back and forth beyond yielding fails after a few cycles of such repeated bending. The fatigue failure is due to progressive propagation of flaws in steel under cyclic loading. This is partially enhanced by the stress concentration at the tip of such flaw or crack. The presence of a hole in a plate or simply the presence of a notch in the plate has created stress concentrations at the center points. These stress concentrations may occur in the material due to some discontinuities in the material itself. At the time of static failure, the average stress across the entire cross section would be the yield stress. However when the load is repeatedly applied or the load fluctuates between tension and compression, the center points experience a higher range of stress reversal than the applied average stress. These fluctuations involving higher stress ranges, cause minute cracks at these points, which open up progressively and spread with each application of the cyclic load and ultimately lead to rupture. Fatigue failure can be defined as the number of cycles and hence time taken to reach a pre-defined or a threshold failure criterion. Low cycle fatigue could be classified as the failures occurring in few cycles to a few tens of thousands of cycles, normally under high stress/ strain ranges. High cycle fatigue requires about several millions of cycles to initiate a failure. The type of cyclic stresses applied on structural systems and the terminologies used in fatigue resistant design are illustrated in this paper. The common form of presentation of fatigue data is by using the S-N curve, where the total cyclic stress (S) is plotted against the number of cycles to failure (N) in logarithmic scale. The point at which the S- N curve flattens off is called the “endu rance limit”. To carry out fatigue life predictions, a linear fatigue damage model is used in conjunction with the relevant S-N curve. The Effect of the Strain Rate on Soft Soil Behaviour under Cyclic Loading In this paper, cyclic triaxial loading tests were conducted on specimens of soft clay with varying cyclic stresses and frequencies to investigate the performance of soft soil subgrade subjected to cyclic loading. The laboratory results indicate that given the same level of cyclic stress, the stability of clay subgrade is primarily dependent on the loading time with no consistent frequency/train speed effect. There exists a critical level of cyclic stress between 60 and 80% of the monotonic shear strength, above which failure may occur regardless of the loading frequency. The nature that soils behave dependently on cyclic stress level rather than loading frequency was investigated through the strain rate during cyclic loading, which is considered  responsible for the cyclic response of soft clays under various loading conditions. For loading frequencies ranging from 0.1 to 5 Hz, it was found that the strain rate depended on the cyclic stress ratio rather than the loading frequency, which implies that the cyclic stress level plays a more important role in influencing the cyclic performance of soft soil subgrade.

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