Experimental MethodologyThe test setup is illustrated in Figure 3. This setup is often referred to as ‘pitch-catch’ where a piezoelectric transmitter creates a ultrasonic stress wave on one side of the specimen that is measured on the opposite face by a piezoelectric receiver. The fastest stress wave type that exists in a solid is the so-called compression or p-(primary) wave mode. For this study, a wave signal as shown in Figure 4 (black line) with a peak frequency of 150 kHz was used as input signal and repeated several times so that statistical variations of the measurement and material could be established.

Figure 3. Experimental setupResultsTwo parameters were determined from the recorded data using Matlab:• The p-wave amplitudes Ap of the received signals (see

Figure 4.a)• The p-wave velocity cp using the time difference Dt and

thickness d between the transmitted and the received signal (see Figure 4.a)

(a) (b)Figure 4. Example of transmitted (black) and received (red) signal waveforms from (a) an integer area and (b) an area

having a ‘blow-out’.

It was found that the p-wave amplitudes from the OSB (Figure 2.b) had a lower amplitude and a slower velocity than the plywood (Figure 2.a). This can be attributed to a lower density (more voids) of the OSB.

Non-Destructive Testing of Wood-Based Composites Using Ultrasonic Stress Waves

Table 1. Estimated p-wave velocities for all specimens

Figure 5 shows an example of how the data can be represented using a contour plot for the p-wave velocities for specimens 3) and 4):

(a) (b)Figure 5. Contour plots for p-wave velocities in (a) low

density panel and (b) panel with a blow-out board

ConclusionsThe different specimens could be distinguished by the different p-wave velocities. Using ultrasonic NDT, flaws in wood-based composites such as ‘blow-outs’ can be quantified and localized.

Future WorkContinued evaluation of wood-based composites using ultrasonic stress wave propagation and correlation with mechanical properties such as density, glue line content and quality, and tensile and shear strength. AcknowledgmentsThis research work was sponsored by the University of Delaware Research Foundation (UDRF) Research Experience for Undergraduates (REU) Summer Internship Program. Wood composite specimens were provided by Arijit Sinha, Assistant Professor in Wood Science at Oregon State University. The support is greatly appreciated.

Jean Carlo Vilalta, Undergraduate Research Assistant & Thomas Schumacher, Assistant Professor (Adviser)

Civil & Environmental Engineering

Objectives: To detect (using ultrasonic NDT) any flaws or irregularities within the wood specimens otherwise undetectable by visual means.

Introduction and Background• Wood and wood-based composites are more likely than

other materials to decay under certain environmental conditions and exposure.

• Non-destructive testing (NDT) is commonly used to determine the properties of different materials such as metals, polymers, and concrete, without varying their end-use capabilities.

• NDT may be used to analyze physical and mechanical properties of wood-based composites.

• Specifically, ultrasonic NDT may be useful in the evaluation of wood-based composites during the manufacturing process to estimate glue line quality and to detect flaws (see Figure 1) to accomplish quality control.

Figure 1. Example of a ‘blow-out’ (large crack parallel to the surface of the specimen) in a wood-based composite.

Test SpecimensFour engineered-wood-type specimens were tested and are shown in Figure 2:1) 0.75” thick, 16” x 3”, plywood (Figure 2.a)2) 0.75” thick, 16” x 3”, oriented strand board (OSB) (Figure

2.b)3) 0.75” thick, 16” x 16”, low-density particle board with a

‘blow-out’ (Figure 2.c)4) 0.75” thick, 16” x 16”, low-density particle board (Figure

2.d)

(a) (b) (c) (d)Figure 2. Test specimens

Waveform Generator

Data Acquisition

System

Signal Preamplifier

Waveform Amplifier

Computer using wave recording

software Specimen

Transmitter

Receiver

The objective of this study is to evaluate ultrasonic NDT methods for the detection and visualization of flaws in wood-based composites such as ‘blow-outs’ and the distribution and quality of the glue line.

Change in time (s) of wave arrival

Ap

Dt

Dpdct