Frank C. Beall

Overview of the use of ultrasonic technologies in research on wood properties

Abstract Two ultrasonic technologies, acoustic emission and acousto-ultrasonics, are reviewed in terms of characteristics and applications to wood and wood-based materials. The background is given on the two technologies, including the effect of wood on wave propagation and sensitivity to defects and other properties. The factors that affect wave injection and processing are reviewed for each technology. Four applications are given as examples: particleboard springback, creep/creep-rupture, adhesive curing, and deterioration detection. Recent developments in the technologies are discussed.

Use of the cone calorimeter to detect seasonal differences in selected combustion characteristics of ornamental vegetation

The flammability of living vegetation is influenced by a variety of factors, including moisture content, physical structure and chemical composition. The relative flammability of ornamental vegetation is of interest to homeowners seeking to make their homes ‘fire safe’. The relative importance of the factors influencing fire behaviour characteristics, such as flammability, is unknown. In the present study, oxygen consumption calorimetry was used to obtain selected combustion characteristics of ornamental vegetation. Peak heat release rate, mass loss rate, time to ignition and effective heat of combustion of 100 × 100-mm samples of foliage and small branches were measured using a bench-scale cone calorimeter. Green and oven-dry samples of 10 species were collected and tested seasonally for a period of 1 year. Similar measurements were made on whole shrubs in an intermediate-scale calorimeter. The range of cone calorimeter peak heat release rates for green and oven-dry samples was 1–176 and 49–331 kW m−2, respectively. Moisture content significantly reduced heat release rates and increased time to ignition. Peak heat release rates for Olea europea and Adenostoma fasciculatum were consistently highest over the year of testing; Aloe sp. consistently had the lowest heat release rate. The correlation of peak heat release rates measured by the cone calorimeter and an intermediate-scale calorimeter was statistically significant yet low (0.51). The use of the cone calorimeter as a tool to establish the relative flammability rating for landscape vegetation requires additional investigation.

Development of a laboratory protocol for fire performance of landscape plants

Over 2.5 million people and 1 million structures risk destruction from wildland fires in California. One way to mitigate this risk is the manipulation and/or selection of landscape vegetation. In this study, six species were studied for their intrinsic characteristics and tested with a newly-developed laboratory fire protocol at 150 kW to determine heat release rate (HRR). The plants were 2–3 years old and obtained from a local nursery. Whole plants were subjected to desiccation in a dry kiln at 50°C, which was found to be much more effective than simulating fire weather in a greenhouse. This is apparently the first study that has measured plant variables, burned them in a natural vertical position, and related HRR to the plant characteristics. Multiple regression showed the overwhelming importance of foliage and moisture content to peak HRR. Chemical variables were not significant and high moisture contents were found to obscure other plant characteristics.

Classification study for using acoustic-ultrasonics to detect internal decay in glulam beams

Abstract Bayes, k-nearest neighbor (KNN), and neural network classifiers were used to study the decay detection efficiency of acousto-ultrasonics (AU). Brown-rotted Douglas-fir glulam beams removed from service were measured by using through-transmission AU. Single and multiple sets of AU signal features included velocity, attenuation, shape, and frequency content. Although all of the AU signal features were sensitive to decay, they were also affected by natural characteristics of wood. However, it was possible to improve the detection efficiency by using multiple signal feature sets in classification analysis. A 79% efficiency was achieved with the neural network classifier for detecting small levels of decay (10% of the cross section) and a 68% overall correct classification for different degrees of decay when using three or four signal features as inputs. The results of the Bayes and KNN classifiers were quite similar, with 79% KNN and 75% Bayes detection efficiency for small levels of decay, and 67% overall.

INDUSTRIAL APPLICATIONS AND OPPORTUNITIES FOR NONDESTRUCTIVE EVALUATION OF STRUCTURAL WOOD MEMBERS

This paper reviews techniques that can or have been used for determining the key properties of structural wood members, largely softwood lumber and structural panels. For solid wood materials, moisture content, density, and defects are underlying basic properties that must be assessed independently to arrive at structural values. In reconstituted materials, an additional variable, adhesive quality dominates. In addition to reviewing these properties, an assessment is provided for the state of maturity of the relevant technologies.

Subsurface Sensing of Properties and Defects in Wood and Wood Products

The application of subsurface sensing to wood and wood-based materials is covered in four parts: (1) basic properties; (2) mechanical properties; (3) internal defects; and (4) adhesive bonding. The basic properties include moisture content, density, and grain orientation. Mechanical properties depend heavily on detection of basic properties, and emphasize those properties that are needed to predict strength in service. Internal defects relate to both green materials (trees and logs) and materials in service, primarily heavy dimension. Adhesive bonding includes both bond quality and bond formation. This review will address the determination of physical and mechanical properties of these materials; the detection of defects, both growth and processing; and the combined measurements to determine the conditions of such materials in various stages of processing or end-use. Subsurface sensing of wood and wood-based materials can be defined as any technique to acquire properties of these materials in a non-invasive manner and therefore will be considered similar to nondestructive evaluation.

Acoustic monitoring of cold-setting adhesive curing in wood laminates

Among the challenges in manufacturing of wood products such as glue-laminated timbers is the difficulty in controlling process variables and assessing the bond quality. This research project investigated an ultrasonic method as a means of monitoring bonding processes and assessing the quality of the cured bonds in wood laminates. The effects of curing phases and bond types on ultrasonic transmission were studied using paired specimens of clear Douglas fir with a single adhesive bond. Monitoring was performed simultaneously at normal and angular (5° nominal) transmission to the bond plane. Acoustic measurements were supplemented with destructive cure monitoring, gel time measurement and microscopic examination. Angular transmission provided greater sensitivity to bond quality and curing status than did normal transmission. The method was sensitive to curing phases (spread, penetration and hardening) and had a reasonable correlation with bond strength development. Different curing curves were observed for thick (0.5 and 1.0 mm), normal and kissing bonds. Thick bonds caused a greater increase in transmission than did normal bonds, whereas kissing bonds showed the smallest signal increase. In angular transmission, thick-bond curing curves showed a characteristic inflection that may be used to identify thick bonds and measure curing rate.

Monitoring of in-situ curing of various wood-bonding adhesives using acousto-ultrasonic transmission

Abstract Hard maple adherends were bonded with six different types of adhesives while monitoring the curing process by acousto-ultrasonic transmission. Adhesives included hot melt, cellulose nitrate, polyvinylacetate, epoxy, cyanoacrylate, and resorcinol. The adherends were lapped for a 25 × 25 mm bond area and either shimmed or lightly clamped. Standard acoustic emission sensors, 175 kHz transmitter and 75 kHz receiver, were used to generate an RMS voltage output which increased as the adhesive cured. The transmission increase was quantified using a half-time to cure. The diffusion of solvents from cellulose nitrate and cyanoacrylate adhesives appeared to cause complex and variable transmission. Hot melt, polyvinylacetate and resorcinol produced curves similar to those previously reported for epoxies. In all cases, the presence of shims had a dominant effect on the nature of the transmission curves.

Monitoring critical defects of creep rupture in oriented strandboard using acoustic emission: incorporation of EN300 standard

This creep rupture study in commercial oriented strandboard (OSB) used a 4-point flexural test to evaluate the dynamic property changes of a 300×1,000-mm specimen using an acoustic emission (AE) system. Compared to deflection, AE events were more sensitive to damage accumulation than deflection to final failure. Specimens were artificially notched on either the tension- or compression-side and were subjected to 80% stress level at ambient conditions. Defects on the compression side of the bending specimen were found to be more critical than on the tension side in creep-rupture. The in-plane fractures followed patterns of the valleys of low-density spots as defect trenches, demonstrating adverse effects of high variation in horizontal density. An impetus and rationale to incorporate regulatory quality inspection standards and product certification of structural OSB based on the control limits of ±10% panel density as stipulated in EN300 standard is discussed.

Monitoring creep-rupture in oriented strandboard using acoustic emission: Effects of moisture content

Abstract Creep-rupture studies of large specimens of commercial oriented strandboard (OSB) were conducted using a fourpoint flexural test in combination with an acoustic emission (AE) monitoring system. Four equilibrium moisture content (EMC) conditioning and testing regimens were used: ambient room conditions, constantly moderate EMC (12/12), dynamic rising EMC (12/24), and constant high EMC (24/24). Compared with deflection, the cumulative event AE parameter was found to be more sensitive and representative to detect cumulative damage to final failure. Under 80% ultimate stress, the 24/24 specimens had the highest creep rate (1.43 mm h−1) as compared with 0.27 and 0.04 mm h−1 for 12/24 and 12/12 specimens, respectively. However, for the 12/24 specimens, a step-like deflection occurred, accelerating the rupture process and attaining the greatest creep factor of 2.28—a recommended safety factor for prolonged exposure to humid service conditions. The total recovered deflection under ambient conditions was 85% recoverable elastic, 5% recoverable viscoelastic, and 10% permanent irreversible creep. Correspondingly, the total recovered stiffness was 70% recoverable elastic, 10% recoverable viscoelastic, and 20% permanent irreversible creep. The observed creep-rupture limit of the OSB was 24 days to failure.