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Impact-echo measurements of concrete
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Alert
X-ray computed tomography and ceramic/ceramic composites
Impact-echo measure, ments of concrete
A medical computed tomographic imaging system has been evaluated at the Argonne National Laboratory, Illinois, for its ability to detect voids and characterize fibre orientation in ceramic composites consisting of SiC fibres in a matrix of chemical vapour infiltrated SiC.
It has been shown by the US National Bureau of Standards (now National Institute of Standards and Technology) that the impact/echo method could be used for nondestructively monitoring the development of the mechanical properties of concrete from its initial setting to ages of several days.
Specimens in various early stages of processing were examined to see if the presence of a graphite fixture which holds the fibrous preform used in the chemical vapour infiltration process caused image degradation. Specimens examined covered various fibre loadings and different final densities; they were fabricated from random chopped fibres or continuous unidirectional fibres and included layups of various cloth types. The results show that the CT images are able to discriminate
among fabric weaves and show major fabric weave characteristics prior to SiC impregnation, after impregnation to at least 85 % of theoretical density, and in the presence of a graphite holder. It was also shown, however, that small individual voids of the order of 50-250/~m cannot be detected with currently available medical scanners. Special high-resolution scanners may yield information at this level. Partial-volume averaging may cause difficulties, however, as the voids are not necessarily constant in size over the slice thickness.
Further information: a 25 page report, reference 3067-NDT(M), price £10.00 + £1.50 VAT, is available on microfiche from TechAlert. If a paper copy is required the price will be £23.90 + £1.15 postage and packing.
Microstructural characterization with a pulsed laser Work at Idaho National Engineering Laboratory has shown that local heating produced by absorption from a pulsed laser provides an efficient non-contacting source of ultrasonic waves in materials. The work demonstrated the feasibility of using this type of source for the microstructural characterization of metals and ceramics. Higher amplitude longitudinal waves have been recorded than could be produced with conventional electrical excitation of piezoelectric transducers. A report describes the use of a pulsed laser source to record microstructural scattering through attenuation measurements. The detectors used were a capacitive transducer for characterization of the laser pulse and a contact piezoelectric transducer mounted on a delay line for recording multiple echoes through the sample for the attenuation measurements. The laser pulse produces wide
114
bandwidth signals and can be modelled as a point source when small spot sizes are used. As a source, the laser is efficient and easily controlled. Unfortunately, as yet, a laser detector is not as sensitive as the piezoelectric detector.
Further information: a 16 page report, reference 3066-NDT(M), price £10.00 + £1.50 VAT, is available on microfiche from TechAlert. If a paper copy is required the price will be £23.90 + £1.15 postage and packing.
To order reports Unless otherwise indicated, reports are available from TechAlert, Microinfo Ltd, PO Box 3, Newman Lane, Alton, Hants GU34 2PG, UK. Cheques should be in Pounds Sterling and drawn on a UK banlc Non-UK customers should add £2. The report's reference number should always be quoted
In its current state, the method can be used to monitor the setting time of concrete in the laboratory. It can also be used to evaluate admixture performance and could be incorporated into performance acceptance standards. It is hoped that it can be developed into a reliable method for measuring the setting times and estimating the strength of concrete at construction sites. In the impact-echo method a cylindrical specimen of concrete is subjected to a point impact by dropping a small steel sphere onto its surface. The surface displacement adjacent to the impact point is monitored with a receiving transducer. P-wave velocity is determined from the measured displacement waveform and the thickness of the object. Changes in the P-wave velocity with time reveal information on the development of mechanical properties. Setting time can be defined as either the time at the onset of P-wave velocity development or the time when a specified P-wave velocity is reached.
Further information: a 122 page report, reference 2583-NDT(M), price £10.00 + £1.50 VAT, is available on microfiche from TechAlert. If a paper copy is required the price will be £37.90 + £2.00 postage and packing.
Evaluating adhesive bonds The feasibility of using leaky Lamb waves to detect and delineate flaws in the bonded surface of metal/rubber laminates has been
NDT International April 1989
Alert
X-ray computed tomography and ceramic/ceramic composites
Impact-echo measure, ments of concrete
A medical computed tomographic imaging system has been evaluated at the Argonne National Laboratory, Illinois, for its ability to detect voids and characterize fibre orientation in ceramic composites consisting of SiC fibres in a matrix of chemical vapour infiltrated SiC.
It has been shown by the US National Bureau of Standards (now National Institute of Standards and Technology) that the impact/echo method could be used for nondestructively monitoring the development of the mechanical properties of concrete from its initial setting to ages of several days.
Specimens in various early stages of processing were examined to see if the presence of a graphite fixture which holds the fibrous preform used in the chemical vapour infiltration process caused image degradation. Specimens examined covered various fibre loadings and different final densities; they were fabricated from random chopped fibres or continuous unidirectional fibres and included layups of various cloth types. The results show that the CT images are able to discriminate
among fabric weaves and show major fabric weave characteristics prior to SiC impregnation, after impregnation to at least 85 % of theoretical density, and in the presence of a graphite holder. It was also shown, however, that small individual voids of the order of 50-250/~m cannot be detected with currently available medical scanners. Special high-resolution scanners may yield information at this level. Partial-volume averaging may cause difficulties, however, as the voids are not necessarily constant in size over the slice thickness.
Further information: a 25 page report, reference 3067-NDT(M), price £10.00 + £1.50 VAT, is available on microfiche from TechAlert. If a paper copy is required the price will be £23.90 + £1.15 postage and packing.
Microstructural characterization with a pulsed laser Work at Idaho National Engineering Laboratory has shown that local heating produced by absorption from a pulsed laser provides an efficient non-contacting source of ultrasonic waves in materials. The work demonstrated the feasibility of using this type of source for the microstructural characterization of metals and ceramics. Higher amplitude longitudinal waves have been recorded than could be produced with conventional electrical excitation of piezoelectric transducers. A report describes the use of a pulsed laser source to record microstructural scattering through attenuation measurements. The detectors used were a capacitive transducer for characterization of the laser pulse and a contact piezoelectric transducer mounted on a delay line for recording multiple echoes through the sample for the attenuation measurements. The laser pulse produces wide
114
bandwidth signals and can be modelled as a point source when small spot sizes are used. As a source, the laser is efficient and easily controlled. Unfortunately, as yet, a laser detector is not as sensitive as the piezoelectric detector.
Further information: a 16 page report, reference 3066-NDT(M), price £10.00 + £1.50 VAT, is available on microfiche from TechAlert. If a paper copy is required the price will be £23.90 + £1.15 postage and packing.
To order reports Unless otherwise indicated, reports are available from TechAlert, Microinfo Ltd, PO Box 3, Newman Lane, Alton, Hants GU34 2PG, UK. Cheques should be in Pounds Sterling and drawn on a UK banlc Non-UK customers should add £2. The report's reference number should always be quoted
In its current state, the method can be used to monitor the setting time of concrete in the laboratory. It can also be used to evaluate admixture performance and could be incorporated into performance acceptance standards. It is hoped that it can be developed into a reliable method for measuring the setting times and estimating the strength of concrete at construction sites. In the impact-echo method a cylindrical specimen of concrete is subjected to a point impact by dropping a small steel sphere onto its surface. The surface displacement adjacent to the impact point is monitored with a receiving transducer. P-wave velocity is determined from the measured displacement waveform and the thickness of the object. Changes in the P-wave velocity with time reveal information on the development of mechanical properties. Setting time can be defined as either the time at the onset of P-wave velocity development or the time when a specified P-wave velocity is reached.
Further information: a 122 page report, reference 2583-NDT(M), price £10.00 + £1.50 VAT, is available on microfiche from TechAlert. If a paper copy is required the price will be £37.90 + £2.00 postage and packing.
Evaluating adhesive bonds The feasibility of using leaky Lamb waves to detect and delineate flaws in the bonded surface of metal/rubber laminates has been
NDT International April 1989