Depth Of Surface-breaking Crack Research Articles

Numerical and Experimental Study of Crack Depth Measurement in Concrete Using Diffuse Ultrasound

This paper presents a combined numerical and experimental study on the diffuse ultrasonic measurement technique for determining the depth of surface breaking cracks in concrete. A finite element (FE) model for the dissipative diffusion in a two-dimensional domain with a surface breaking crack is developed using a commercial FE package; for this purpose, the dissipation term is eliminated by a simple change of variables. Three concrete blocks with a crack depth between 25.4 mm to 101.6 mm are prepared. Diffuse ultrasonic measurements are performed on uncracked and cracked concrete blocks, from which the diffuse energy evolution curves are obtained. The basic material parameters of the hardened concrete, i.e. the diffusivity and dissipation, are retrieved, which are needed for the numerical simulations. The crack depths are then determined by comparing the experimental and numerical arrival times of the average diffuse ultrasonic energy. Various geometrical configurations that arise in real-world concrete structures are simulated including an inclined crack, a partially closed crack, two parallel cracks, and a crack with an underlying reinforcement bar. The objective is to investigate the possible limitations of the diffuse ultrasonic measurement technique when implemented in real concrete structures. Finally, it is shown that the time of flight (TOF) of the average diffuse ultrasonic energy constitutes the theoretical basis of the present diffuse ultrasonic measurement of macroscopic cracks and therefore the present diffuse ultrasonic method forms another kind of TOF technique.

Numerical simulation of evaluation of surface breaking cracks by array-lasers generated narrow-band SAW

Most of the factors limiting the extensive application of laser-based ultrasonic for nondestructive evaluation of surface breaking crack are its poor sensitivity, low efficiency relative to conventional contact ultrasonic methods and limit on the dimension of the cracks. For this reason, a new technique that multiplepulse narrow-band ultrasound generated by laser arrays has been proposed. It is found that crack detection dependent on spectrum of narrow-band ultrasound generated by laser arrays can be operated with low amplitude requirements. In this paper, the narrow-band ultrasound generated by pulse laser arrays interacting with surface breaking cracks has been simulated in detail by the finite element method (FEM) according to the thermoelastic theory. The pulsed array lasers were assumed to be transient heat source, and the surface acoustic wave (SAW) which propagating on the top of the plate was computed based on thermoelastic theory. Then the frequency spectrums of both reflected waves by crack and transmission ones through crack were compared with the direct waves. Results demonstrate that multiple-frequency components of the narrow-band ultrasound were varied with change of the depth of surface breaking cracks significantly, which provides the possibility for precise evaluation of surface breaking cracks.

Depth Measurement of Surface-Breaking Cracks Using Point-Source/Point-Receiver Acoustic Transducer

ABSTRACTA newly developed point-source/point-receiver (PS/PR) acoustic transducer is used for measuring the depth of surface-breaking cracks. The transducer consists of two miniature conical PZT elements which form a transmitter/receiver pair for generating and detecting surface waves. A tone-burst measurement system operates the PS/PR transducer at a fixed frequency. When a surface-breaking crack is present in between the transmitter and receiver, the change in time-of-flight of surface wave propagation caused by the crack is measured and used to estimate the crack depth. To verify the feasibility of this method, machined slots on a carbon steel sample are tested by the PS/PR transducer for crack depth determination. Good experimental results are obtained. Future applications and improvements on the PS/PR transducer system are addressed.

레이저 유도 초음파 및 자기보상 기법을 이용한 재료의 표면균열 깊이 비파괴 평가

It is required to evaluate nondestructively the crack depth of surface-breaking cracks for the assurance of safety of structure. Optical generation of ultrasound produces well defined pulses with a repeatable frequency content, that are free of any mechanical resonances; they are broad band and are ideal for the measurement of attenuation and scattering over a wide frequency range. Self-calibrating surface signal transmission measurement is very sensitive and practical tool for surface-breaking crack depth. In this paper, the self-calibrating technique by laser-based ultrasound is used to evaluate the depth of surface-breaking crack of material. It is suggested that the relationship between the signal transmission and crack depth can be used as a practical model for predicting the surface-breaking crack depths from the signal transmission measured in structure.

The non-destructive detection and mapping of ASR cracking in concrete : Smith, R.L.; Crook, M.J. Transport and Road Research Laboratory, Crowthorne (United Kingdom), TRRL-CR-156, ISSN-0266-7045, 38pp. (1989)

The alkali silica reaction (asr) occurs in concrete when certain siliceous minerals in an aggregate react with the alkaline pore solution in the hardened cement paste. The reaction is expansive and may crack and damage structural components constructed from reinforced and prestressed concrete. This report looks at non-destructive methods for detecting and mapping cracks caused by asr. The aim was to identify a method whch could determine the extent of internal cracking and the orientation and depth of cracks in reinforced concrete. The selected system would be able to scan the surface of the concrete, discriminate between cracks, reinforcement and other embedded materials, and be robust and portable. The investigation took the form of a literature survey and a laboratory demonstration of techniques with potential for development. The report concludes that there are a number of techniques which might be able to provide all or some of the information required. The practical demands for a portable robust instrument rules out x-ray or gamma-ray radiography and tomography which are capable of mapping internal cracks. Ultrasonic time-of-flight methods appear to have the greatest potential for measuring the depth of surface breaking cracks, although radar, acoustic emission, thermography and electrical continuity are thought to be worth exploring in the longer term. (Author/TRRL)

QUANTITATIVE EVALUATION OF DEFECTS BY NEURAL NETWORK

Abstract A neural network is applied to a quantitative evaluation of defects. The training data for learning of neural network are prepared from a numerical analysis based on an elastic wave theory. The question posed here is to. evaluate the depth of surface-breaking cracks in a structural component. The numerical analysis is carried oui for ten cracks ranging from 0.6 mm to 2.4 mm separated by 0.2 mm in depth. Waveforms in the time and frequency domains are calculated by the use of the boundary element method for these ten types of crack depth. The neural network used here is the three layered network of perception type. Each layer is called input, association, and output layer, respectively. In this type of network, we can use a learning algorithm called back error-propagation and the network deduces the waveform characteristics from the training data and induces the evaluation criteria. After the network training by the use of numerical waveforms, the network is used to evaluate the measured data obta...