Finite Element Analysis of Adhesively Bonded Lap Joints

Adhesive bonding is a preferred method of joining aerospace structural components, since it provides fewer points of stress concentration compared to fastener joints. Geometrically nonlinear analysis of adhesively bonded lap joints is presented in this paper using both linear and non-linear material properties of the adhesive. The numerical results show beneficial effects of material non-linear behaviour of the adhesive which decrease the stress concentration at the ends of the lap Length. This paper also presents the estimation of strain-energy-release rate components in the presence of debond in the adhesive. The studies have relevance in structural integrity assessment of the joints

Condition Monitoring of Rotating Elements Using Acoustic Emission: Indian Scenario

The first experimental study in the world on the phenomenon of generation of Acoustic Emission dates back to the 1950’s when Josef Kaiser heard these emissions during metal failure. But unfortunately, the work could not get impetus because of the limited means of sensing high frequency waves and the lack of knowledge dissemination. In the 1970’s, the electronic instrumentation improved resulting in the increased activity centers of AE. At that time, Rao initiated work in this area at the Indian Institute of Science, Banglore for the first time in India. With the pioneering efforts of his group at IISc, new vistas were opened in this field. As a consequence of it, the first paper from India on this phenomenon was published in 1977 by Eshwar, et al. [1], The paper dealt with the study of the form of AE observed on the cracking/breaking of plywood with prepared defects

A double exponential model for AE signals

Acoustic emission (AE) signals are conventionally modeled as damped or decaying sinusoidal functions. A major drawback of this model is its negligible or zero rise time. This paper proposes an alternative model which provides for the rising part of the signal without sacrificing the analytical tractability and simplicity of the conventional model. The,signals obtained from the proposed model through computer programs are illustrated for demonstrating their parity with actual AE signals. Also derived are analytic expressions for the time-domain parameters, viz., peak amplitude and rise time used in conventional AE signal processing (4 refs.

Fatigue damage stages in unidirectional glass-fiber-epoxy composites: identification through acoustic emission technique

Fatigue damage in composites occurs in stages. Before attempting to estimate quantitatively the extent of cumulative damage which is necessary to estimate the fatigue life of these materials, it is useful to understand the basic process in relation to the significance of the stages of damage. The acoustic emission (AE) technique, with its on-line monitoring capability, has the potential to be the most suitable candidate as a tool for this purpose. Experimental investigations were carried out on unidirectional glass-fibre-reinforced plastic (GFRP) composite coupons to explore the feasibility of identifying the damage stages using acoustic emission. The coupons were subjected to constant-amplitude load cycles with continuous AE monitoring. The results of the experiments show three distinct stages of damage before the final failure of coupons; these stages could be correlated to different failure mechanisms

A model with nonzero rise time for AE signals

Acoustic emission (AE) signals are conventionally modelled as damped or decaying sinusoidal functions. A major drawback of this model is its negligible or zero rise time. This paper proposes an alternative model,which provides for the rising part of the signal without sacrificing the analytical tractability and simplicity of the conventional model. Signals obtained from the proposed model through computer programs are illustrated for demonstrating their parity with actual AE signals. Analytic expressions for the time-domain parameters, viz., peak amplitude and rise time used in conventional AE signal analysis, are also derived. The model is believed to be also of use in modelling the output signal of any transducer that has finite rise time and fall time

Ultrasonic evaluation of delamination in quasi-isotropic CFRP laminates subjected to low-velocity impact

Ultrasonic C-Scan is used very often to detect flaws and defects in the composite components resulted during fabrication and damages resulting from service conditions. Evaluation and characterization of defects and damages of composites require experience and good understanding of the material as they are distinctly different in composition and behavior as compared to conventional metallic materials. The failure mechanisms in composite materials are quite complex. They involve the interaction of matrix cracking, fiber matrix interface debonding, fiber pullout, fiber fracture and delamination. Generally all of them occur making the stress and failure analysis very complex. Under low-velocity impact loading delamination is observed to be a major failure mode. In composite materials the ultrasonic waves suffer high acoustic attenuation and scattering effect, thus making data interpretation difficult. However these difficulties can be overcome to a greater extent by proper selection of probe, probe parameter settings like pulse width, pulse amplitude, pulse repetition rate, delay, blanking, gain etc., and data processing which includes image processing done on the image obtained by the C-Scan

Characteristics of AE signals obtained during drilling composite laminates

The ultimate goal towards on-line monitoring of composites drilling is in order to produce damage free high quality drilled holes. This however pertains to first understanding the acoustic emission (AE) signal characteristics at different stages as the drilling operation proceeds. To determine the partition function of individual sources contributing to the AE energy release by isolating them from the rest is a formidable task. On the contrary, it would be simpler and practical to monitor if certain identifiable characteristics that distinguishes the different states of drilling mechanisms can be extracted from the time and frequency domain of the AE-composite signal. Experimental observations on drilling specimens of epoxy resin (Araldite) and glass fibre reinforced plastic (GFRP) laminate show AE signal features in time and frequency domain which distinguish different stages as the twist drill tool performs its action from entry to exit

On using peak amplitude and rise time for AE source characterization

Acoustic Emission (AE) signals, which are electrical version of acoustic emissions, are usually analysed using a set of signal parameters. The major objective of signal analysis is to study the characteristics of the sources of emissions. Peak amplitude (P-a) and rise time (R-t) are two such parameters used for source characterization. In this paper, we theoretically investigate the efficiency of P-a and R-t to classify and characterize AE sources by modelling the input stress pulse and transducer. Analytical expressions obtained for P-a and R-t clearly indicate their use and efficiency for source characterization. It is believed that these results may be of use to investigators in areas like control systems and signal processing also

An experimental approach for non-destructive evaluation of fatigue damage in CFRP composites

Study of fatigue phenomenon in composites requires a dynamic tool which can detect and identify different failure mechanisms involved. The tool should also be capable of monitoring the cumulative damage progression on-line. Acoustic Emission Technique has been utilized in the experimental investigations on unidirectional carbon fiber reinforced plastic (CFRP) composite specimens subjected to tension-tension fatigue. Amplitude as well as frequency distribution of Acoustic Emission (AE) signals have been studied to detect and characterize different failure mechanisms. For a quantitative measure of degradation of the material with fatigue load cycles, reduction in stiffness of the specimen has been measured intermittently. Ultrasonic imaging could give the information on the changes in the interior status of the material at different stages of fatigue life