舰船玻璃钢夹芯板缺陷和损伤的影响英文文献和中文翻译(3)
2. Determination of a sensitivity factor Sp that ac-counts for the location of the damage in relation to the stress field in the panel for the real loading case. 3. Combination of these factors to give the panel strength reduction factor Rp = Rl Sp. The factor Sp, which is referred to as the local location and load case factor, also provides a means of seeing immediately what are most likely to be critical locations of damage on a panel. For Level 1 damage, it is possible to neglect redistribu-tion of stresses between panels (and other elements) in the structure when estimating the influence of the dam-age on global strength. Level 2 damage: Medium local damage Level 2 damage is confined to one panel but does not meet the requirements for Level 1 damage. However, the damage is not so severe that the stiffness of the panel is significantly influenced by the damage. This means that, as with Level 1 damage, redistribution of stresses in the global structure can be neglected in as-sessing the reduction of global ship strength. Examples of medium local damage include moderately large impact damage, moderately large debonds, and moderate core shear cracking. In such cases the influence of the damage on the panel strength has to be considered for the particular damage size, damage location and load case. Level 3 damage: Large local damage Level 3 damage is confined to one panel but does not meet the requirements for Level 1 or Level 2 damage. The damage is so severe that the stiffness of the panel is significantly influenced by the damage. This means that redistribution of stresses cannot be neglected in assess-ing the reduction of global ship strength. Examples of large local damage include large impact damage, large debonds, and developed core shear crack-ing with debonding. In such cases the influence of the damage on the panel strength has to be considered for the particular damage size, damage location and load case. In extreme cases it may be appropriate to assume that the panel’s contribu-tions to the global strength and stiffness have been com-pletely destroyed. In this case the panel is removed from the global analysis model of the ship. LocalPanelShipLocal LocalPanel PanelShip Ship Level 4 damage: Extensive damage Level 4 damage affects two or more panels and/or sup-porting structure. Each case must be considered indi-vidually. Generally this type of damage leads to redis-tribution of stresses in the remaining structure. It may or may not be relevant to consider the damaged panels as fully removed for the purpose of the analysis. In cases of Level 3 and Level 4 damage it is important to consider carefully whether the damage has led to a change in the external loading on the structure. If the damage is modelled simply by removal of the damaged panel it may be important to retain in some way the external loadings that were applied to the removed panel, and also the masses of the removed elements. Panel Strength Reduction: Local Impact Damage For cases of in-plane compressive loading, local impact damage involving only the face sheet can be analysed using equivalent hole and equivalent crack models as described by Bull and Edgren (2003, 2004) and Edgren et al. (2004). These models are based on the Budi-ansky, Soutis and Fleck model developed by Soutis et al. (1991). For damage involving fibre fracture it may be appropriate to use either the equivalent crack or the equivalent hole model; Bull and Edgren (2003) provide guidance on how to determine the equivalent hole di-ameter or equivalent crack length. For less severe cases where the damage does not in-volve fracture of the fibres, it may still be possible to apply the equivalent hole model, but if there is perma-nent damage to the underlying core it may be necessary to consider an alternative residual dent model, and also to check for delamination buckling. Fig. 2 shows the far-field strain at failure predicted by Bull for panels of differing widths having holes of di-ameter 20, 40 and 60 mm. The laminates have quadr-iaxial, quasi-isotropic, CFRP reinforcement in a vinyles-ter matrix. It is seen that the strain at failure is virtually independent of panel size provided the hole diameter is less than about 25% of the panel width. On this basis it Fig. 2: Far-field strain at failure for panels of differing widths having holes of diameter 2R in the face laminate. appears that impact damage to the face laminate can be taken as Level 1 damage provided the damage diameter is less than this size. Local strength reduction factor, Rl The local strength reduction factor for a Level 1 dam-age, Rl, is defined as follows: damage without failure cause strain to or stress nominaldamage with failure cause strain to or stress nominal= lR For impact damage modelled with the equivalent hole model referred to above, the local strength reduction for quasi-isotropic CFRP laminates is shown in Fig. 3. Note that this applies for all face laminates of this lay-up provided the hole diameter is less than 25% of the panel width. A similar curve can be drawn for the equivalent crack model. Fig. 3: Local strength reduction factor for face lami-nate impact damage, equivalent hole model Estimation of local location factor relative to panel, Sp The local location factor Sp indicates how large a margin exists for accommodating small local damage at a given location on the panel. It is a function of the following: • the position of the damage on the panel • the type and direction of loading on the panel, and the resulting stress distribution • the governing failure mechanisms for the intact and damaged panel, and safety factors associated with these in the design.
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