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In this first part of a two-part paper, the use of two existing algorithms developed for global nondestructive evaluation to locate and evaluate localised damage in timber beams is investigated using a finite element model. These damage localisation algorithms were found, through this investigation, not to be effective in locating multiple damage scenarios and unable to evaluate the severity of damage. Hence, modifications on damage index algorithm as well as a hybrid algorithm are proposed to overcome the problems. In this study, Experimental Modal Analysis (EMA) was used as a tool to extract mode shapes for calculating the damage index in the proposed method which utilises changes in modal strain energy between the undamaged and the damaged timber beam model. The Modified Damage Index method normalises the mode shape curvature and the hybrid algorithm combines the modified index algorithm and changes in flexibility algorithms which reflect the changes of natural frequency and mode shape. Analytical evaluations were performed to compare and verify the ability of existing and modified damage localisation algorithms in locating single and multiple damage in timber beams. The Modified Damage Index algorithm and the hybrid damage algorithm are also used in the companion paper to validate the effectiveness of the methods to

This paper employs modal-based damage detection algorithms to identify location of defects commonly found in timber and to estimate their severities. In this study, the authors propose modifications to an existing damage detection algorithm for locating and evaluating damage by comparing the modal strain energy before and after damage using the first two flexural modes of vibration. Experimental verification was performed on pin-pin supported timber beams by employing the algorithms with extracted modal parameters using Experimental Modal Analysis (EMA). Single and multiple damage simulating pocket(s) of rot with various severities were inflicted by removing section(s) on timber beam specimens. The proposed damage indicator computed from the first two flexural modes was capable of detecting all damage locations. It was also able to estimate, with reasonable accuracy, the severity of damage in term of loss in sectional moment of inertia. The modified damage index method is in general reliable in detecting location as well as estimating the severity of simulated defects in timber beams.

Received as an invited paperThe same fundamental approach that has previously been found reasonably accurate in predicting the tensile behaviour ofbulky singles yams at small extensions of up to 10% is used here to model yarn torsion. The theoretical analysis is based on discrete-fibre-modelling principles, an energy method, and a 'shortest-path' hypothesis. It is applied to the twisting behaviour of bulky singles yarns which may have non-uniform fibre-packing-densify across their cross-sections. The twisting of a yam both under constant yam length and under constant yarn tension is modelled. The contributions to yam torque due to fibre tension, fibre torsion, and fibre-bending can all be calculated. Comparison of the theoretical models with some experimental data will be presented in Part II. INTRODUCTION 1.1 Significance of Yam TorqueTorque has practical significance for a number of the characteristics of yams, fabrics, and carpets. The magnitude of initial torque and the torsional-deformation and torsional-recovery properties of yams influence the twist distribution in singles yams, the tendency of singles yams to snarl, yam-twist instability, the balance of twist in ply yams, skewness of woven fabrics, spirality of knitted fabrics, tuft integrity of cut-pile carpets (this affects carpet appearance-retention) and effects or defects such as pebble or pucker in fabrics and frise tufts and tip-curl tufts in cut-pile carpets.There are three options for the theoretical treatment of freshly twisted yams: snarl; apply constraint; allow to untwist. We know it is impracticable to apply a significant torque to a singles yam in the absence of an axial tension, and a freshly twisted yam will invariably snarl or twist up on itself unless it is constrained from doing so. This phenomenon of yam twisting up on itself has been called 'torsional buckling' [1]. Accordingly, a singles yam with a straight axis can generally only be regarded as subject to a combined-load case of tension and a restraining torque [2].For example, in the cut-pile-carpet trade, the demand for yams of good twist stability is high, as these are essential for piece-dyeable qualities and a high level of tuft integrity in use. For many cut-pile carpets, it is necessary to have a yam which is straight and free from any tendency to untwist, even at the cut ends. Twist stabilisation or setting is an essential process for all yams to be used in cut-pile carpets. It is known that it is impossible to achieve a high degree of set if the fibres in the yam to be set are not significantly strained [3]: this applies to wool and probably to other fibres used in carpet-pile yams. The strains in individual fibres are translated into a residual yarn torque, and the greater the torsional stress in the

SUMMARYThe paper presents a research recently completed by the authors utilising a method of damage evaluation for identifying damage in timber bridges, numerically and experimentally. The method utilises changes in modal strain energy between the undamaged and damaged states of plate-like structures. A finite element model of a laboratory timber bridge was developed to investigate the capabilities and limitations of the method to detect damage. A simple four-girder bridge was fabricated and tested in a laboratory to verify the method. The numerical studies showed that the method can correctly identify single and multiple damage locations within the bridge. The experimental studies also showed promising results for detecting severe damage, but less effective for light and medium damage.

A theoretical analysis has been developed to gain an insight into the torsional behaviour of singles yams, which may be bulky and which may have non-uniform fibre-packlng-deasity distribution. In this paper, a detailed experimental evaluation of the theory for two different cases of yam torsion is reported whereby its predictions of a torque-twist relationship are compared with results obtained for woollen-spun carpet yarns spun at various twist levels. Also presented are typical theoretical results ofthe relative contributions to yarn torque due to fibre tension, fibre-bending, and fibre torsion and of the variations of both fibre-tonsile-strain distribution in the yam and yam axial and lateral dimensions wbich lake place as tbe yam is further twisted.The evaluation shows that, as the yam is twisted at fixed length, it jams progressively from the inside to the outside. Initially the outer fibres remain unstrained. In the theory of Postle et aL, these outer fibres are assumed to carry a real tensile strain. They have a large helix angle and thus a large component of tension-creating torque. This is the main reason why earlier theories of yarn torque have given predictions much higher than the values observed experimentally. In general terms, the new theory gives reasonable predictions of the observed torque behaviour of woollen carpet yarns, given the precision limitations with such a material.Most interestingly, the theory clearly predicts that yams statically twisted at constant load will become hollow in the centre -a feature which has often been reported in yam-crosssectional experiments.

Using vibration methods for the damage detection and structural health monitoring in bridge structures is rapidly developing. However, very little work has so far been reported on timber bridges. This paper intends to address such shortcomings by experimental investigation on a timber beam using a vibration based method to detect damage. A promising damage detection algorithm based on modal strain energy was adopted and modified to locate/evaluate damage. A laboratory investigation was conducted on a timber beam inflicted with various damage scenarios using modal tests. The modal parameters obtained ITom the undamaged and damaged state of the test beam were used in the computation of damage index, were then applied using a damage detection algorithm utilising modal strain energy and a statistical approach to detect location of damage. A mode shape reconstruction technique was used to enhance the capability of the damage detection algorithm with limited number of sensors. The test results and analysis show that location of damage can be accurately identified with limited sensors. The modified method is less dependent on the number of modes selected and can detect damage with a higher degree of confidence.

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