Studies of Influence of Moisture Content in Cellulose Fiber Cement Boards

on Wave Parameters Using Non-Contact Ultrasound

Radosław DRELICH1, Piotr DOMANOWSKI

2, Tomasz GORZELAŃCZYK

3,

Michał PAKUŁA1, Łukasz RADZIK

3, Krzysztof SCHABOWICZ

3

1 Institute of Mechanics and Applied Computer Science, Kazimierz Wielki University in Bydgoszcz,

85-064 Bydgoszcz, ul. J.K. Chodkiewicza 30, e-mail: michalp@ukw.edu.pl, radeko@ukw.edu.pl 2 Faculty of Mechanical Engineering ,University of Technology and Life Sciences in Bydgoszcz,

85-796 Bydgoszcz Al. Prof. Kaliskiego 7, email:pedom@utp.edu.pl 3 Faculty of Civil Engineering, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27,

50-370 Wrocław, Poland, e-mail: tomasz.gorzelanczyk@pwr.edu.pl, k.schabowicz@pwr.edu.pl

Abstract

The paper presents results of studies focused on evaluation of influence of moisture content of cellulose fiber

cement boards using non-contact (air-coupled) ultrasonic technique. Cellulose fiber cement boards are relatively

new ecological materials commonly used in architectural engineering for many external and internal applications

such as buildings elevation or as walls and roofs covering materials. The theoretical studies of wave propagation

in plates served to identification of the wave mode measured in the boards. Ultrasonic examinations were

performed using Lamb waves. Independently, for the same boards the moisture content was evaluated by

nondestructive testing using dielectric humidity measuring device. The obtained results confirmed applicability

of the non-contact ultrasonic technique using Lamb waves to studies of cellulose fiber cement boards. It was

observed high sensitivity of the method to the moisture content. Based on the obtained results one can conclude

that following appropriate calibration, the method can be potentially applied as a tool for evaluation of moisture

content in cellulose fiber cement boards.

Key words: non-contact ultrasound, nondestructive testing, cellulose fiber cement boards, moisture content

1. Introduction

The building and construction boards are commonly used in many sectors such as the

automotive, electronics, aerospace and mechanical engineering industries. The materials can

be applied both as safety elements of machines and constructions (internal and external e.g.

the aircraft skins, metal tank shells) as well as architectural decoration (e.g. ship building, the

walls, etc.).

The standard inspection methods of the material, specified by the appropriate norms, are

selective (the size of tested specimens is much smaller than the size of the whole element) and

they have destructive character. The abovementioned limitations can be significantly reduced

by application of complementary, non-destructive tests e.g. using ultrasonic methods [1-4],

which are recognized as a powerful tool widely applied in NDT community. One of the

disadvantages of the classical ultrasonic techniques, in context of potential application for

examination of cement based boards, is requirement of direct contact of the transducers with

the material. Considering that most of construction boards are porous, many technical

problems appear including repeatability of the measurements being result of the soaking of

the coupling medium (water, gel or another fluid) into the material, [5]. Thus, application of

such technique may lead to modification of the properties of studied materials and as a

consequence to erroneous interpretation of the obtained results.

The existing nowadays non-contact ultrasonic methods allow overcoming the problems with

penetration of the coupling to the material, but till now they were not commonly applied in

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the NDT community, because of small wave energy transmitted to the material, what

essentially limit their utility in the production conditions, [3].

To answer this call in the present paper the non-contact ultrasonic method using Lamb waves

is developed. The method will be applied to studies of the cellulose fiber cement boards

during their fabrication especially to find delamination into the structure of boards.

Cellulose fibre cement boards are the materials commonly used in architectural engineering

for many external and internal applications such as buildings elevation or as walls and roofs

covering materials, [6-8]. Modern boards are asbestos-free and are classified as eco-friendly

constructional materials [9]. They are characterized by good fire-proof and heat insulation as

well as moisture-proof performance. Moreover, they are easy for construction, lightweight,

stable dimensions durability and long service life, [6, 7].

The main purpose of the paper is to presents results of experimental studies focused on

evaluation of influence of moisture content of cellulose fibre cement boards using non-contact

ultrasound.

2. Experimental Studies

2.1. Ultrasonic studies

The Lamb waves (also called plate waves) can be generated in a plate with free boundaries

with an infinite number of modes for both symmetric and antisymmetric displacements within

the layer, [10-12]. One of the partial aims of the studies was to understand the physics

underlying the interaction and propagation of Lamb wave within the bard. Such knowledge

enables to select diagnostic conditions optimal from the point of view of effective wave’s

introduction to the boards and their reception from the material. The studied parameters were

the centre frequencies of the transducers, their size, angle of incidence, type of generated

signals, etc. The scheme of signal generation, propagation and reception using non-contact

ultrasound was presented in fig. 1.

Fig.1. Scheme of Lamb wave propagation in the cellulose fiber cement boards

Ultrasonic experimental studies of the cellulose fibre cement boards were performed using

non-contact ultrasonic system. Prior to the measurements the preliminary theoretical and

numerical studies were performed aiming at selection of optimal experimental conditions

such as frequency, angle of incidence, shape of generated impulse, distance between

transducers and board. The tests enabled also to evaluate which of considered wave modes

(longitudinal wave, surface wave, Lamb wave) is the most effective from the point of view of

transmitted energy. Moreover, such studies have given possibility to understand if, how and

which of the material properties influence on the measured wave parameters. The non-

contact 1D ultrasonic device is shown in fig. 2. The device allow to studies of materials using

various methods: in transmission mode, using wave reflectometry as well as spectroscopy of

surface waves. Preliminary tests were performed on the device on the specimens having

moisture varying along their length. Following the results of preliminary tests, the Lamb

waves operating at 50 and 100 kHz frequency were chosen as the most effective to study the

material and the simplified version of 1D non-contact ultrasonic device was built.

Fig.2. Photography of non-contact ultrasonic device.

2.2. Moisture Tests

Moisture test of the cellulose fibre cement boards were performed using capacitance dielectric

non-invasive moisture meter Trotec T650 (TROTREC®, Germany). The hand-held device

enables to determine the moisture content distribution in areas up to depth 4 cm from the

surface and is dedicated to measurements of building materials, walls, ceilings, floors etc.

The measurements were performed on the upper part of the whole boards, having length 3050

mm, width 1200mm and thickness varying from 8mm to 14 mm. The distribution of

measuring points during the moisture test and configuration of the corresponding ultrasonic

measurements are presented in fig. 3. The 48 measuring point in the moisture studies were

situated approximately every 250 mm. Moreover, in order to verify the results obtained by

non-invasive dielectric method, destructive moisture content tests of the samples taken

randomly from the boards have been performed. The square pieces of boards of the dimension

about 250 mm (see fig. 3) were cut and the moisture content of the specimens were measured

classically by finding the loss of mass of the sample when dried on a tray over a source of

heat, following the procedure described elsewhere, [13, 14].

Fig.3. Distribution of measuring points for non-invasive moisture measurements.

3. Results and analysis

The fig. 4 presents the results of comparative test performed within the present paper. They

concern the moisture content measurements (fig. 4a) and ultrasonic studies (fig. 4b and

fig. 4c) of in cellulose fibre cement boards. The moisture experiments were performed at 48

measuring points (12 positions along the length and at each of them 4 tests along the width of

the board). The average data obtained is presented (see fig. 4a). It is visible that the values of

moisture content increase from 4.37±0.16% to 7.39±0.18%.

Despite, the Lamb ultrasonic waves were generated in the boards using non-contact technique

for the centre frequencies 50 kHz and 100 kHz, in the fig. 4b and fig. 4c the data only for 100

kHz are presented. The results for the 50 kHz are qualitatively the same, but they exhibit

lower spatial resolution. In the case the ultrasonic studies the speed of the boards movement

enabled to measure wave amplitudes and time of flight of the Lamb wave pulse

approximately every 2.5 mm. The fig. 4b and fig. 4c presents the evolution of Lamb wave

amplitudes and time of flight along the board length.

In the case of wave amplitudes there is observed decrease of the values from 147±15V to

86±6V. It worth noticing increase of wave’s amplitudes, which can be attributed to effects

associated with boundary of the board which can be neglected. The time of flight was

determined based on the time position of the maximum of the amplitude of Lamb wave

transmitted through the board. There is observed increase of the time of flight values from

130±4 µs to 155±4 µs, which is associated to the lower speed of sound of the Lamb wave in

more moisture areas of the boards.

a)

b)

c)

Fig. 4. Values (averaged cross to the board’s thickness) of moisture content along the length of the boards (a)

and the amplitudes (b) and time of flight (c) of Lamb waves propagating in the cellulose fiber cement boards.

4. Conclusions

In the paper experimental comparative ultrasonic and moisture content studies of cellulose

fiber cement boards were performed. Within the ultrasonic test innovative non-contact

airborne technique was applied. The ultrasonic device was built in order to have a possibility

to perform the experiments on the boards in motion i.e. during the boards manufacture

process. The obtained results confirmed applicability of the non-contact ultrasonic method

using Lamb waves to effective studies of cellulose fiber cement boards. The moisture

experiments were performed using capacitance dielectric based non-invasive moisture meter.

The main purpose of the paper was to evaluate the correlation between the ultrasonic and

moisture content in the boards. It was shown significant influence of the moisture content on

the wave parameters (wave amplitude and time of flight). The increase of the moisture content

implies decrease the amplitude and inverse behavior is observed in the case of the time of

flight. It is visible significant sensitivity of the method to the variation of moisture. The

changes of the moisture content of approximately 2.5% gives changes of amplitudes about 60

V, while for the time of flight the changes reach 35 µs.

The obtained results imply high sensitivity of Lamb wave parameters (amplitude and

velocity) to the moisture content in the cellulose fiber boards and usefulness of the non-

contact (air-borne) ultrasonic technique to study of this type of materials.

In the paper only the time domain analysis was performed. Confirmation of the correlation

between the wave parameters and the moisture content is promising, because the small

difference of the moisture content leads to significant changes of wave amplitudes. The

obtained results prove that the non-contact ultrasonic technique is effective tool of evaluation

of the material properties and can be applied to control of the quality of the boards during

their fabrication.

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