Critical Damage States Research Articles

Fibre Break Failure Processes in Unidirectional Composites. Part 2: Failure and Critical Damage State Induced by Sustained Tensile Loading

In order to improve the testing and design of filament wound composite structures a model to predict the accumulation of fibre breakages has been developed that takes into account all physical phenomena controlling fibre failure, including the stochastic nature of fibre strength, stress transfer between fibres due to the shear of the matrix, interfacial debonding and viscosity of the matrix. In this, the first of three papers, the damage processes leading up to failure are discussed and quantified in terms of fibre breaks for the case of elastic monotonically tensile loading. It is clearly shown that the failure of a unidirectional composite structure results from the formation of random fibre breaks which at high loads coalesce into clusters of broken fibres. A critical damage state is found.

Fibre Break Failure Processes in Unidirectional Composites. Part 1: Failure and Critical Damage State Induced by Increasing Tensile Loading

Fibre Break Failure Processes in Unidirectional Composites. Part 1: Failure and Critical Damage State Induced by Increasing Tensile Loading

Statistical microdamage mechanics and damage field evolution

Discussed are the underlying background of statistical microdamage mechanics, the fundamental partial differential equation of evolution of microdamage number density, two basic solutions, and the saturation of microdamage number density evolution. Knowledge of microdamage number density evolution is applied to engineering practice by using the field equations of microdamage number density and continuum damage. The addition of continuum equations renders a complete system of field equations of deformation and damage. However, they are open-ended in character at the continuum level although the dynamic damage function is completed from the meso- to the macro-scale level. Once decoupling of the function is made, the system of equations can be connected in an approximate manner. This provides a reasonable approximation to the continuum field of deformation and damage. The open literature prediction based on damage evolution relies on assuming arbitrary critical damage states. In this work, use is made of the criterion for damage localization. Several applications of statistical microdamage mechanics are made. This includes damage evolution in a heterogeneous medium and failure forecast under impact. The results show that statistical microdamage mechanics and the derived closed approximate continuum formulations are physically sound and practically effective.

Evaluating Structural Deterioration by Dynamic Response

The dynamic response of two C 3 × 4.1 channel steel beams is evaluated in terms of the reduction in load-carrying capacity of the damaged state versus the undamaged stage. The beam specimens are sized and configured to have similar frequency response observed in actual bridges. Each beam is subjected to different damage scenarios, i.e., various types, locations, and degree of damage. For each scenario, the characteristics of the dynamic response are determined from acceleration measurements made during low-level free-vibration tests. A finite-element model was used to calculate the dynamic response of the simulated damaged structures. The experimental and analytical results are scrutinized to correlate increasing deterioration and changes in the dynamic response. Good correlations are found between the experiments and analysis. For each damage scenario, the yield and ultimate-load capacities of the test beams are defined by a damage index. Both experimental and numerical results indicate that there is less than 10% change in the natural frequencies at critical damage state.