# Parallel-to-Grain Compressive and Tensile Behavior of Paulownia Wood at Elevated Temperatures
## 4 Conclusions  
In this paper, a total of 108 clear Paulownia wood specimens were tested to investigate the parallel-to-grain compressive and tensile properties at elevated temperatures from 20 \({}^{\circ}\)C to 220 \({}^{\circ}\)C. Based on the experimental results, the following conclusions can be drawn:  
(1) The compressive Paulownia wood exhibited ductile failure modes. Kinking, end crushing and splitting failure modes were observed in the compressive specimens, while kinking was the main failure mode. The tensile Paulownia wood mainly exhibited a brittle failure. Transverse cracks were formed in the middle of the specimens, while a few specimens exhibited longitudinal cracks. The failure modes were generally not affected by elevated temperatures.  
(2) For the Paulownia wood in compression, the parallel-to-grain compressive stress increased linearly at the initial stage. Subsequently, the parallel-to-grain compressive stress increased nonlinearly, and then decreased slowly up to the end of the test. For the specimens in tension, the parallel-to-grain tensile stress increased linearly up to the tensile failure. Moreover, the elevated temperatures had limited effect on the nonlinear- (linear-) loading behavior of the compressive (tensile) stress-displacement curve of the Paulownia wood.  
(3) The parallel-to-grain tensile and compressive strengths of the Paulownia wood depended highly on the moisture content and the behavior of polymer components (i.e., cellulose, hemicellulose and lignin) at elevated temperatures from 20 \({}^{\circ}\)C to 220 \({}^{\circ}\)C. Generally, the parallel-to-grain tensile and compressive strengths decreased with the increase in temperature. At 220 \({}^{\circ}\)C, the retention rates of the average parallel-to-grain compressive and tensile strengths were 38% and 42%, respectively. However, due to the moisture evaporation and the hardening of the dry lignin, the average parallel-to-grain strengths significantly increased as the temperature increased from 100 \({}^{\circ}\)C to 140 \({}^{\circ}\)C.  
(4) The design curve suggested by EN 1995-1-2 was very conservative (as much as 76%) at predicting the temperature-dependent compressive strengths of the Paulownia wood. However, the design curve was nonconservative (less than 6%) at estimating the  
Figure 12: Deformation at the peak load: (**a**) compression; (**b**) tension; and (**c**) normalized value.  
tensile strengths of the Paulownia wood when the temperature was below 60 \({}^{\circ}\)C, whereas it was conservative (3-24%) when the temperature was above 100 \({}^{\circ}\)C.  
(5) The maximum deformation (at the peak load) of the Paulownia wood decreased from room temperature to 60 \({}^{\circ}\)C. Subsequently, the maximum deformation increased as the temperature increased from 60 \({}^{\circ}\)C to a temperature of around 120 \({}^{\circ}\)C. The lowest value of the deformation was found within the temperature range of between 180 \({}^{\circ}\)C and 220 \({}^{\circ}\)C. Hence, the deformation capacity of the specimens significantly decreased when the temperature was higher than 180 \({}^{\circ}\)C.  
Conceptualization, L.Z.; methodology, Y.L.; software, B.X.; formal analysis, L.Z., K.C. and B.X.; investigation, L.Z.; data curation, K.C. and B.X.; writing--original draft, L.Z. and K.C.; writing--review and editing, L.Z., Y.L. and K.G.; funding acquisition, L.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 52108253. The data that support the findings of this study are available from the corresponding author upon reasonable request. The authors declare no conflict of interest.