Failure mode
The failure modes of the control and decayed specimens parallel to the grain were all nail withdrawals. An apparent difference in failure mode between the control and decayed specimens was not observed in the main members, as shown in Fig. 7. The deformation of the nails can be classified into two modes, as shown in Fig. 8: the mode that has a plastic hinge in the nail (mode I) and the mode with two plastic hinges (mode II). The all-control specimens parallel to the grain exhibited mode II. Six of the twelve decayed specimens exhibited mode I and six specimens exhibited mode II. Toda et al. [2] reported that the formation of the plastic hinges in the nails was interfered by the decrease of embedding strength caused by wood decay, and a similar tendency was observed in this study.
The failure mode of the nailed joints perpendicular to the grain can be classified into two types: nail withdrawal and main member splitting. No specimen exhibited nail-head pull-through. The failure mode of all the control specimens was nail withdrawal. Five decayed specimens failed by nail withdrawal, and seven exhibited main member splitting, as shown in Fig. 9. It is most likely that the degradation of wood by decay changed the failure mode of nailed joints perpendicular to the grain. Deformation of the nail was by mode II for all the control specimens and 10 decayed specimens. The two decayed specimens deformed by mode I. In the nailed joints perpendicular to the grain, embedding and tensile stress perpendicular to the grain are generated around the nail holes. Degradation of tensile strength perpendicular to the grain would cause main member splitting, and then the formation of plastic hinges in the nails might be interfered by a large decrease in the embedding strength perpendicular to the grain.
Load–slip curve
Figure 10 shows the envelope load–slip curves of the nailed joints parallel and perpendicular to the grain. The load of the control specimens parallel to the grain increased up to 5–8 mm and then decreased owing to nail withdrawal. The decayed specimens parallel to the grain showed a low load during the initial deformation. The load increased from 5 to 18 mm and then gradually decreased. The decayed specimens perpendicular to the grain also exhibited a lower load at the initial deformation than the control specimen. The slip at the maximum load of the control specimens perpendicular to the grain was 5–8 mm and that of the decayed specimens was 5–21 mm. Both nailed joints parallel and perpendicular to the grain with decay showed a low load at the initial deformation and had a large variation in slip at the maximum load.
The nailed joints perpendicular to the grain exhibited two failure modes. In the case of un–decayed nailed joints perpendicular to the grain, nailed joints that failed by the main member splitting exhibited brittle load–slip curves [14]. However, decayed nailed joints perpendicular to the grain that failed by the main member splitting exhibited a somewhat lower load at initial deformation than nailed joints that failed by nail withdrawal, and the shapes of the load–slip curves were slightly different from that of the main member.
Shear properties of the nailed joints
The shear properties of the nailed joints were evaluated based on the initial stiffness (Ks), yield resistance (Py), and maximum resistance (Pmax). A perfect elastoplastic idealization scheme using 10, 40, and 90% of Pmax [11, 20] was applied to the load–slip curves to obtain Ks and Py. Figure 11 shows the values of Ks, Py, and Pmax of the nailed joints parallel and perpendicular to the grain. The values of Ks, Py, and Pmax were all significantly different between the control and decayed specimens in each loading direction at the 5% level. The Ks, Py, and Pmax values of the decayed specimen parallel to the grain were 68.4%, 39.2%, and 31.9% smaller, respectively, than those of the control specimen. For the nailed joints perpendicular to the grain, Ks, Py, and Pmax values of the decayed specimen were 60.2%, 40.3%, and 34.4% smaller, respectively, than those of the control specimen. There was no clear difference in the shear properties degradation of nailed joints depending on the loading directions. The value of Ks exhibited the largest decrease, followed by those of Py and Pmax for both the nailed joints parallel and perpendicular to the grain. On the other hand, some preceding studies on the nailed [2] and screwed [9, 11] joints parallel to the grain have reported that degradation of the initial stiffness was not present regardless of decay. In those studies, decay treatment was conducted on the surface of the specimens similarly as in this study; however, the effective length of the fasteners was 1.3–1.6 times that in this study. Presumably the long effective length of the fasteners maintained the initial stiffness in those studies, and the result in this study implied that degradation of the initial stiffness would be present if shorter fasteners were used. Compared to the failure mode of decayed specimens perpendicular to the grain, the decayed specimens that failed by the main member splitting showed lower Ks values than those that failed by nail withdrawal, and the former had almost the same Py and Pmax as the latter. The failure mode of the nailed joints perpendicular to the grain had little effect on the initial stiffness.
The values of Ks, Py, and Pmax of the decayed specimens were divided by the corresponding mean values of the control specimen, and the ratios of these shear properties were obtained. The relationship between the ratios of Ks and Py and that of the ratios of Pmax and Py are shown in Fig. 12. The ratio of Ks did not exhibit an apparent correlation with the ratio of Py, and the value of the former ranged from close to that of the latter to 73% lower than that of the latter. The ratio of Pmax linearly decreased with decreasing Py, and the value of the former was, on average, 10% higher than that of the latter. These tendencies did not change with the loading direction of the grains. When a wood member subjected to decay was divided into sound and decay regions and the yield mode of nailed joints was assumed according to the regions, the yield resistance of the nailed joints with decay can be estimated using the European yield theory [8]. When calculated from the decayed state, the ratio of yield resistance is obtained from the theory, and the minimum ratio of initial stiffness and ratio of maximum resistance can be estimated from the results shown in Fig. 12.
Pilodyn penetration depth
The relationships between the shear properties of nailed joints parallel and perpendicular to the grain and dp are shown in Fig. 13. The values of Ks, Py, and Pmax of the nailed joints parallel to the grain had a negative correlation with dp, and Py and Pmax had a particularly significant negative correlation at the 5% level. The presence of negative correlation between dp and Py has been reported in preceding studies on nailed [7] and screwed [9, 10] joints parallel to the grain, and some compatibility of Pilodyn for the assessment of residual strength was similarly confirmed.
The dp of the control specimens was ≤ 20 mm. The decayed specimens with dp ≤ 20 mm had smaller Ks, Py, and Pmax values than the control specimens, and the difference was significant at the 5% level. Nishino et al. [7] investigated the shear properties of nailed joints parallel to the grain with decayed main member and sound side member, and reported that the nailed joints with decayed main member exhibited similar Py to those with sound main member at dp below 25 mm. This result is different from this study. Nishino et al. conducted the nailed joint tests using nails that have length of 50 mm and diameter of 2.75 mm, and the nails used in this study have the same length but a larger diameter, which was 2.87 mm. Because the cross-sectional dimensions greatly affect the geometrical moment of inertia and section modulus, those also would affect the change of shear properties of nailed joints by wood decay.
The values of Ks, Py, and Pmax of the nailed joints perpendicular to the grain with decay had no significant correlation with dp at the 5% level, since those shear properties exhibited large variable with dp. The decayed specimens perpendicular to the grain with dp ≤ 20 mm exhibited smaller Ks, Py, and Pmax values than the control specimens, and the difference was significant at the 5% level. The Ks, Py, and Pmax of decayed specimens with dp ≤ 20 mm had large variables, and some of them exhibited significantly small values. For some of the nailed joints perpendicular to the grain failed by the main member splitting, the load–slip curves exhibited large variables as shown in Fig. 10. Even if the dp was within the range of those obtained from sound wood, the shear properties of nailed joints perpendicular to the grain subjected to wood decay might be significantly changed.
Decay depth
The relationships between Ks, Py, and Pmax of nailed joints parallel and perpendicular to the grain and decay depth (dd) are shown in Fig. 14. The values of Ks, Py, and Pmax of the nailed joints parallel to the grain tended to decrease with increasing dd. Those of the nailed joints perpendicular to the grain did not exhibit this tendency. Regardless of the loading direction to the grain, the values of Ks, Py, and Pmax of the decayed nailed joints with a small dd greatly decreased. For example, when dd was less than or equal to 2 mm, which is approximately 5% of the nail penetration length of the main member, the values of Ks, Py, and Pmax of the nailed joints parallel to the grain were 0.28–0.38, 0.63–0.73, and 0.62–0.85 times those of the control specimens, respectively. In the case of the decayed nailed joints perpendicular to the grain, the values were 0.12–0.73, 0.26–0.87, and 0.23–1.04 times the latter, respectively. The shear properties of nailed joints were significantly affected by a small decay depth, and the variable of decrease was larger for the nailed joints perpendicular to the grain than for those parallel to the grain.