Study on metal ejection under laser shock loading
Jianting XinGu Yu-QiuLi PingLuo Xuan蒋柏彬 Jiang BaibinFang TanDan HanYin‐Zhong WuZongqing ZhaoShu Jing-QinBaohan Zhang
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Dynamic response of metal to shock loading plays an improtant role in the fields of civil engineering, aeronautics, etc. The forming process of ejection of metal under shock loading, including micro-jet, fragmentation and micro-spall remain to be studied. The present work is devoted to laser-shock experiments in metal ejection on the SGIII laser facility. And the fragment recovery and post-test evaluation of the fragment-size distribution are achieved.Keywords:
Spall
Fragmentation
This preliminary paper is a progress report on an analytical investigation into the implications of explosive spalling on the fire performance of reinforced concrete structural elements and whole structures. This study does not attempt to predict whether spalling will occur. For accurate prediction of the occurrence of spalling a complete and fully coupled hygro-thermal-mechanical (HTM) analysis is required, as described by a comprehensive review of current research into the parameters and mechanisms that influence spalling, including a review of physical spalling criteria. This paper describes the structural performance of spalled concrete elements, using finite element analysis where spalling is modelled by removing layers of concrete when a set of spalling criteria are met. The method is presented using a case study of a simply supported reinforced concrete beam, where the analytical results indicate that spalling invariably triggers an early failure (well short of the required FRR rating) of a beam exposed to the standard fire. • The moisture content of the member is less than 3% or the member is designed for internal exposure • The tabulated data is used to prescribe generic fire ratings for concrete elements (except for axis distances > 70 mm) However, if the designer expects the moisture content to be greater than 3% for beams, slabs and tensile members the effect of spalling on the load bearing function of the element is checked by assuming local loss of cover to one reinforcing bar or bundle of bars and calculating the reduced load bearing capacity. This check is not deemed necessary where the number of bars is high enough, it is assumed that an acceptable level of redistribution of stress is possible without loss of stability. Examples given of where the number of bars is high enough to allow redistribution of stress are: solid slabs with evenly distributed bars and beams of widths greater than 400 mm and 8 bars in the tensile region. Concrete structures are generally designed for fire using the tabulated data thus it is apparent that there are very few occasions when a designer must consider the possible effects of spalling on structural stability. If checks must be made they are then limited to only considering localised spalling. The tabulated data is compiled from the results of standard fire resistance tests of isolated elements, generally without any spalling having occurred. Therefore continued reliance on this empirical data to account for the possible debilitating effects of spalling masks the mechanisms of how an element or a whole structure truly performs in the event of spalling. Potential alternative load paths or stability mechanisms such as compressive membrane action are ignored. 3 MECHANISMS OF SPALLING Spalling of concrete is generally categorised as pore pressure induced spalling, thermal stress induced spalling or a combination of the two. 3.1 Pore pressure induced spalling As concrete is heated the free water vaporises at 100 o C and expands; thereby resulting in increased pore pressures. Migration of some this vapour to the interior of the concrete member, where it cools and condenses, will result in an increasingly ‘wet’ zone (sometimes referred to as moisture clog). At some distance from the hot surface the vapour front reaches a critical point at which a maximum pore pressure is achieved (further movement will result in a reduction in pressure). The distance of this point from the heated surface will depend on the concrete’s permeability. Pore pressure spalling occurs if the maximum pore pressure is greater than the local tensile strength of the concrete. However, no pore pressures have yet been measured which would exceed the tensile strength of concrete which suggests that pore pressure in isolation does not lead to the occurrence of spalling (Khoury & Anderberg 2000, Jansson & Bostrom 2008). 3.2 Thermal stress induced spalling Strong thermal gradients develop in concrete as it is heated, due to its low thermal conductivity and high specific heat. These thermal gradients induce compressive stresses close to the surface due to restrained thermal expansion and tensile stresses in the cooler interior regions. The surface compression may also be augmented by applied loading or prestress. 3.3 Combined pore pressure and thermal stress induced spalling It is most likely that spalling occurs due to the combination of tensile stresses induced by thermal expansion and increased pore pressure. Much debate still surrounds the identification of the key mechanism (pore pressure or thermal stress) (Khoury & Anderberg 2000). However, it is noted that the key mechanism may change depending upon the section size, material and moisture content (Davie et al. 2008).
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Spalling occurs when high strength concrete(HSC) columns are exposed to fire,which decreases the fire resistance of columns.Studies show that the numerical modeling becomes difficult because of the randomness of spalling.In this paper,the finite element software ABAQUS was used to develop the model for numerical anal ysis of HSC columns related to the spalling and the model was verified by test data.The verified model was then used to analyze the effect of spalling on the temperature field,the effect of the length and depth of spalling on the fire resistance and axial deformation.The result shows that the temperature of cross section rises when the depth of the spalling increases.On condition that keeps the same depth of spalling invariability,with the increase of the spalling length,the fire resistance reduces and the axial deformation increases.On condition that keeps the length of spalling invariability,the fire resistance of HSC columns decreases significantly with the increase of the spalling depth.
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Abstract Different qualities of concrete have been fire tested using different geometries of the specimens as well as different load levels and load configurations. The main objective with the study was to examine a test methodology consisting of a full‐scale test and different small scale‐tests for determining the probability of spalling and the amount of spalling of fire exposed concrete structures. A reference specimen was defined as a one‐sided fire exposed slab with the dimensions 1800 × 1200 mm 2 giving an exposed area of 1500 × 1200 mm 2 . A number of concrete qualities with different probabilities for spalling, were tested using the reference specimen. These tests showed that the reference specimens worked well giving the expected test results. Small specimens were manufactured in different shapes with the same concrete as the one used in the reference tests. These small specimens were tested either at the same time as the reference specimens in the large furnace or afterwards on a small‐scale furnace where the fire exposed surface was 450 × 360 mm 2 . The test results clearly show the increased probability and the increased amount of spalling by using external compressive loading. The results also show that by using pre‐stress through bars or wires the load can be lost due to heating of the bars/wires which results in a decreased amount of spalling. The boundary of the specimen also affects the amount of spalling. The spalling around the edges was in all tests less than the spalling on the central parts of the exposed area. It could also be noted that the spalling did not pass completely through any of the specimens. The reason for this is probably that the water/vapour could migrate out from the unexposed surface of the specimen. Copyright © 2006 John Wiley & Sons, Ltd.
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A state-of-the-art review of experimental and analytical studies on fire-induced spalling in concrete structures is presented. The limitations in current code provisions related to spalling are discussed. A hydrothermal-based spalling model is applied to undertake parametric studies to investigate the effect of critical parameters on fire-induced spalling in reinforced concrete beams. The results of these studies show that fire scenario and concrete permeability largely influence the extent of fire-induced spalling in reinforced concrete beams. Research needed for improving the state-of-the-art on fire-induced spalling in concrete structures is highlighted.
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