Predicting excavation damage zone depths in brittle rocks

During the construction of an underground excavation, damage occurs in the surrounding rock mass due in large part to stress changes. While the predicted damage extent impacts profile selection and support design, the depth of damage is a critical aspect for the design of permeability sensitive excavations, such as a deep geological repository(DGR) for nuclear waste. Review of literature regarding the depth of excavation damage zones(EDZs) indicates three zones are common and typically related to stress induced damage. Based on past developments related to brittle damage prediction using continuum modelling, the depth of the EDZs has been examined numerically. One method to capture stress induced damage in conventional engineering software is the damage initiation and spalling limit(DISL) approach. The variability of depths predicted using the DISL approach has been evaluated and guidelines are suggested for determining the depth of the EDZs around circular excavations in brittle rock masses. Of the inputs evaluated, it was found that the tensile strength produces the greatest variation in the depth of the EDZs. The results were evaluated statistically to determine the best fit relation between the model inputs and the depth of the EDZs. The best correlation and least variation were found for the outer EDZ and the highly damaged zone(HDZ) showed the greatest variation. Predictive equations for different EDZs have been suggested and the maximum numerical EDZ depths, represented by the 68% prediction interval, agreed well with the empirical evidence. This suggests that the numerical limits can be used for preliminary depth prediction of the EDZs in brittle rock for circular excavations.

Sensor placement for structural damage detection with modal data

A new method is presented for prioritizing sensor locations for structural health monitoring (SHM). In view of the needs of SHM and damage detection,sensor locations are optimized for the purpose of both sensitivity for local damages and independence of the target mode. However,the two different optimization criterions lead to an inconsistency of the optimal result. Considering the structural response changes that result from damage,the relationship between the structural response and damage is deduced from the structural motion equation by a quasi-analytical mode. Based on the harmony between damage identifiability and mode observability,an object function is set up,including the information of mode independence and damage sensitivity. Utilizing the technique of singular value decomposition,an interior algorithm for the optimum sensor placement is proposed with the multiple objective criterions of minimizing the condition number of coefficient matrix and maximizing the fisher information matrix. A numerical example shows that this approach can effectively avoid the contradiction between the two different optimization criterions. Comparing with the result of single object,the result of damage detection from the optical sensor locations is much more accurate.

Damage sensitivity of a wing-type leading edge structure impacted by a bird

On the windward side of an aircraft,the components with higher probability of impact with birds are the wing-type leading edge structures,such as the wing and tail.A study on the damage sensitivity of a wing-type leading edge structure under bird strikes was presented in this paper.First,a bird strike test was carried out on a wing.The principles of the bird strike test equipment and method were introduced in detail,including the bird strike test system,bird projectile production process and data acquisition system.The dynamic strain measurement results,the high-speed camera videos,and the final deformation and damage morphology observations of the structure were obtained.Based on the coupled Smooth Particle Hydrodynamics(SPH)-Finite Element Method(FEM),the commercial software PAM-CRASH was used to simulate the process of a bird strike with the wing.The good agreement between the finite element simulation results and the experimental results shows that the calculation method and the numerical model presented in this paper were reasonable.On this basis,wing-type leading edge structures can be designed by adding triangular support.The bird strike resistances of an original structure and improved structure were studied by numerical simulation.The calculated results show that the improved wing-type leading edge structure is less damaged than the original structure under bird strike.The improved leading edge structure satisfied the anti-bird strike airworthiness requirements,as the thickness of the triangular support was 1.2 mm,and the weight of the structure was reduced by 0.87 kg compared with the original structure.This indicated that the bird strike resistance of the improved structure is better than that of the original structure,and the improved design of the wing-type leading edge structure presented in this paper is reasonable.

Time Series Analysis for Vibration-Based Structural Health Monitoring:A Review

Structural health monitoring(SHM)is a vast,interdisciplinary research field whose literature spans several decades with focusing on condition assessment of different types of structures including aerospace,mechanical and civil structures.The need for quantitative global damage detection methods that can be applied to complex structures has led to vibration-based inspection.Statistical time series methods for SHM form an important and rapidly evolving category within the broader vibration-based methods.In the literature on the structural damage detection,many time series-based methods have been proposed.When a considered time series model approximates the vibration response of a structure and model coefficients or residual error are obtained,any deviations in these coefficients or residual error can be inferred as an indication of a change or damage in the structure.Depending on the technique employed,various damage sensitive features have been proposed to capture the deviations.This paper reviews the application of time series analysis for SHM.The different types of time series analysis are described,and the basic principles are explained in detail.Then,the literature is reviewed based on how a damage sensitive feature is formed.In addition,some investigations that have attempted to modify and/or combine time series analysis with other approaches for better damage identification are presented.