230 Dahle et al.: Measuring Modulus of Elasticity with Acoustical Stress Waves tive sampling. The DMOE for frozen samples was significantly higher than the warm samples (P < 0.0001, N = 118, Table 1), but this was not the case for the load press where no difference was found between the temperatures (P = 0.0757, N = 118, Table 1). Mean diameter was not found to vary (P = 0.6623, N = 118) between the warm (3.5 ± 0.05 cm) and frozen (3.4 ± 0.04 cm) samples. When comparing E obtained from the UTM, the stress-wave timer system was higher than the load press, whether temperatures were warm (P < 0.0001, N = 58, Table 2), frozen (P < 0.0001, N = 60), or grouped (P < 0.0001, N = 118). These findings are consistent with other research that indicates when wood temperature is below freezing, there is a continual increase of acoustic velocity (Gao et al. 2012). This phe- nomenon can be observed in other naturally occurring materials such as rocks and alumi- num (Timur 1977; Fukuhara and Yamauchi 1993). Moisture content was not found to differ between the warm (55.1% ± 0.9 SE) and cold (53.6% ±1.0 SE) samples (P = 0.2169 N = 118). The MC was lower than reported by the Forest Products Laboratory Wood Handbook at 69% (Glass and Zelinka 2010), yet in the literature there is little difference in material properties when MC is greater than 50% (Lavers 1983; Kretschmann 2010; Spatz and Pfisterer 2013). To measure velocity in the fiber direction, in this study, the pins were placed in the cross section of the samples, rather than radially through the bark. However, the results are expected to be the same when testing radi- ally through the bark, as comparison testing on other tree branch pieces showed little to no difference in stress wave time (less than 1 microsecond) when testing at the ends and radially through the bark with low-angle pin placement. This result is also similar to the manufacturer’s results, as they reported that the angle of the fiber and transducer pins is negligible as long as the pins are not placed at an angle of more than 45 degrees when placing the pins radially through the bark (Fakopp, no date). Hence the reason researchers of the current study believe the test is a suitable approximation of this system. Regression analysis determined that there was a 1:1 relationship between the two mea- surement systems, but that DMOE was higher (ES = 1.01 * DMOE – 272.9, r2 Figure 3). While the r2 = 0.42, N = 118, for this regression is not overly high, the study authors believe this supports further investigation of stress-wave timers as a potential field tool for arborists. Separate regressions were developed between DMOE and ES (not presented here), yet the slopes between the two treatments (warm and frozen) were not significantly different (P = 0.4669). Interestingly, the stress-wave meter had a lower standard error than the destruc- tive testing, which could make this a desir- able method for calculating branch rigidity. No significant differences were found between the diameters (warm = 3.5 ± 0.1 cm, frozen = 3.4 ± 0.1 cm, P = 0.6623, N = 118) or age (warm = 3.2 ± 0.06 years, frozen = 3.4 ± 0.07 years, P = 0.0694). The size and age of the sample sug- gests that the wood is juvenile and thus has a lower modulus of elasticity than mature red oak green wood (E = 9300 MPa) reported by the For- Table 1. Mean flexural modulus of elasticity (E) when tested using a universal testing machine or stress-wave timer. P-values less than 0.05 represent significant differences using a paired t-test. Testing machine UTM Stress wave Ewarm (MPa) ± SE 1120 ± 40.0 1355 ± 20.6 Efrozen (MPa) ± SE 1216 ± 38.9 1495 ± 25.5 N 118 118 P-value 0.0757 <0.0001 ences using a paired t-test. Treatment ES Warm Frozen Grouped Table 2. Mean flexural modulus of elasticity (E) tested at each temperature (warm 21.1°C or frozen -6.7°C) when tested using a stress-wave timer (DMOE) or universal testing machine (ES (MPa) ± SE 1120 ± 36.9 1216 ± 38.8 1169 ± 27.1 DMOE (MPa) ± SE 1355 ± 20.6 1495 ± 25.5 1426 ± 17.6 N 60 58 118 P-value <0.0001 <0.0001 <0.0001 ). P-values less than 0.05 represent significant differ- ©2016 International Society of Arboriculture
July 2016
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