In this work, we examined the validity of four methods for measuring the mode I fracture toughness of wood; double cantilever beam (DCB), compact tension (CT), single edge notched tension (SENT), and single edge notched bending (SENB) tests, which have been frequently conducted. In the fracture tests, the crack length was variously changed, and the validity of each method was evaluated by the dependence of fracture toughness on the crack length. From the comparisons of fracture toughnesses obtained by the different four methods, the DCB test was more recommended than any other methods because the fracture toughness could be appropriately obtained in the wide range of crack length.
Softwood and alternately laminated lumber of rubber wood veneer and falcate veneer were compressed transversely. On the assumption that softwood and alternately laminated lumber are recognized as laminated composite structure of soft layer and hard layer, transverse compression strength of them are expected to be larger than that of lumber only of weaker layer (earlywood and falcate veneer). Cross section of transversely compressed softwood was observed using digital image correlation for strain analysis, and large shear strain has progressed on earlywood at edge of the softwood specimen. The cross section of the specimen after compression was examined under an incident-light microscope, so it was found that compression crush lines began on the boundary between earlywood and latewood at the edge of the specimen. It is probably because of shear stress generated at the boundary between soft layer and hard layer, that is, latewood of high-density would prevent earlywood of low-density from deforming under compression stress. However, when alternately laminated lumbers which are a similar structure to softwood were compressed transversely, shear strain was found not at the edge but at the inner part of the specimen. Moreover compression strength of alternately laminated lumber has been no larger than laminate lumber of falcate veneer. The area of large shear strain has formed inclined lines on the laminated lumber under compression stress, and the lines tended to coincident with checks formed by rotary peeling using a lathe. The lathe checks would mitigate the restriction of hard layer (rubber wood veneer), and so the alternately laminated lumber may not behave similar to softwood.
The properties of compression parallel to grain were examined by using the wood compressed perpendicular to the grain with various ratios, and the plastic stress-strain behavior in the compression properties was investigated. The results was summarized that the compression strength parallel to the grain increased with the larger compression ratio in the heat treated condition without dimensional recover. The change of the plastic behavior in the compression stress-strain curves was recognized in the boundary before and after reached to the maximum stress. The plastic behavior after reached to the maximum stress in the stress-strain diagram showed that the effect of the work-softened appeared remarkably with the larger compression ratio. The plastic region before reached to the maximum stress decreased with the larger compression ratio. From this result, it was concluded that distinguish of wooden porous tissue by densification affected to the plastic behavior with yielding in the compression stress-strain diagram.
The commercial grading system for the strength of lumber is divided into visual grading and mechanical grading. Mechanical stress grading can estimate strength more precisely than visual grading, but the mechanical stress method cannot detect local defects such as knots that seriously affect mechanical strength. This study investigated methods of automatically detecting these knots at the time of measuring Young's modulus (E) with a grading machine. The results were as follows. (1) Edgeline of the lumber is extracted from a digital image. Bending deflection curve calculated from the displacement of the edgeline is measured using the digital image correlation technique. (2) Differential profile of the bending deflection curve around knots showed a characteristic peak and the profile pattern depended on the structure of knots, i.e. intergrown knot, encased knot and direction of spike knot. (3) The specimens, with a differential profile that showed a clear peak around knots, also ruptured at the knots. However, other specimens, with a differential profile that did not show a clear peak around knots, did not always rupture at the knot, although many of these specimens ruptured at the area without knots. Therefore, the method using a differential profile of bending deflection curve can distinguish two types of knots that would be evaluated as the same type by visual grading.
Chemical modification of wood with maleic anhydride (MA) was carried out in a vapor phase without catalyst. Weight percent gain (WPG), dimensional stability (ASEm), equilibrium moisture content, and weight loss (WL) in the fungal decay tests according to JIS K1571 were discussed from the aspects of optimization of the reaction conditions, analysis of moisture adsorption characteristics, and practical performance of anti-fungal properties. The results are summarized as follows : 1) water leaching of un-reacted and/or weakly bonded MA was notably marked in the reaction at low temperature and in the early stage, and reaction at elevated temperature was necessary for the stable linkages. 2) WPG increased with increasing ratio of reagent to wood sample (R/S ratio), and the R/S ratio was still insufficient within the condition tested (ca. 30% of theoretically required). 3) Competitive reaction of MA with water molecule and hydroxyl group in wood components was expected because larger WPG was observed at lower initial moisture content (IMC). 4) Treatment for about 10h and 180°C was required for the formation of stable linkage, and satisfactorily high dimensional stability (higher than 50% ASEm) and anti-fungal property (less than 3% WL). Dimensional stabilization efficiency (ASEm per unit WPG) was higher in low R/S ratio, but high R/S ratio was effective for the shortening of the reaction time. On the other hand, the increase of IMC brought the shortened reaction time and high efficiency in dimensional stabilization, although thermal degradation is apprehensive. 5) Formation of cross-linking was suggested from the analysis of moisture adsorption isotherm.
Carbonization of wood loaded with nickel, which aims at co-production of dual functional nano-carbon and fluid fuels, was conducted by two steps process (500°C-carbonization as the first step followed by 900°C-carbonization as the second one) and usual one step carbonization at 900°C to compare them in terms of practicability. The 2nd carbonization of wood char prepared by the 1st carbonization could produce the 2nd char almost equal to char obtained by one step carbonization of wood in both crystallographic structure of carbon and mesoporous structure. Gases produced by the 2nd carbonization contained hydrogen and carbon dioxide in much larger and smaller proportions, respectively, than those obtained by one step process, and the total amount of gases produced by the 1st and the 2nd carbonizations was equal to that by one step process. When considering easier operation of the 2nd carbonization giving no tar, it is concluded that two steps process is superior to one step process in actual operation. In addition, useful findings of macropore formation in wood char were obtained by the 2nd carbonization.
Influence of short-time duplex heat treatment on the fatigue strength of α + β titanium alloy Ti-6Al-4V was investigated. When the alloy was solution-treated for 60s at 1203K and water-quenched, prior β phase was hardened since a part of this phase was transformed to acicular α' martensite phase. With subsequent aging for 40s at 753∼903K, fine α phase was precipitated in retained β phase so that hardness of prior β phase further increased. Such hardening of prior β phase remarkably improved the fatigue strength of the alloy as well as its yield stress and tensile strength. That was because slip in primary α phase, where would be fatigue crack initiation sites, was restricted by hardened prior β phase so that fatigue crack initiation was suppressed to high stress amplitude level. In the examined treatment conditions, the combination of the above short-time solution treatment and aging for 40s at 803K was suitable to improve the strengths. In this case, the tensile strength and fatigue strength were increased by 29% and 22%, respectively. At the same time, ductility was also increased by 10% due to stress-induced martensitic transformation of retained β phase.
Most constitutive laws for transformation plasticity, which are known to contribute a drastic effect on the simulation of thermo-mechanical processes, have been treated to be independent of ordinal thermo-plasticity. Considering that the mechanisms for both strains are essentially with no difference from metallurgical viewpoint, the constitutive equation for transformation plastic strain rate is expected to be described in relation with plasticity theory. This paper motivates to derive a unified constitutive model including transformation-induced and ordinal thermo-mechanical plastic strain rates by introducing the effect of varying phases during phase transformation into yield function. Thus derived model is applied to simulate temperature-elongation diagram by use of a kinetic rate equation of induced phase.
In this study, a computer simulation taking into account the defects size distribution and the micro-crack propagation behavior was conducted by using virtual bending specimens. The defects size distributions in the virtual specimens were estimated by using a Monte-Calro technique based on a large number of defects diameter (approximately 72000) measured on the Si3N4 specimen surface. The effects of two types of scatter−one is defects diameter, the other is micro-crack propagation rate−on fatigue lives were estimated separately. It is proved that the scatter of fatigue life mainly originated from the scatter of defect size. For the cases of the simulation using micro-crack propagation behavior, estimated fatigue lives were always shorter than that of using long-crack propagation behavior. Recognizing simulated fatigue lives Nf as crack propagation lives Np, we can define a crack initiation life Ni. As the applied stress decreases, estimated value of Ni/Nf increases. It is remarkable that this tendency shows a good agreement with the experimantal value of Ni/Nf. It is concluded a crack initiation life and its scatter are very important to discuss the fatigue lives in ceramic materials.
Fatigue tests of carbon fiber made from polyacrylonitrile (PAN) were performed to clear S-N properties and discuss the relationships between the fatigue fracture toughness and the pattern of fatigue fracture surface. The fatigue tests were carried out at a frequency of 10Hz with three stress ratios of 0.1, 0.5 and 0.7. It was found that S-N properties of carbon fibers clearly showed the fatigue behavior, depending on the stress ratio R. The fatigue lives of carbon fiber showed a large scatter. Mirror-mist-hackle patterns were clearly observed on the fracture surface of specimen tested under cyclic loading. As outside length on mirror region decreased, the form of the mirror region in each stress ratio changed from a semicircle to a half-ellipse. Relations between Kmax or ΔK calculated from their area and fatigue life can be approximated in a straight line. Fatigue crack growth behavior is governed by two parameters such Kmax and ΔK, and combined parameter Kmax(1-α)×ΔKα is useful to describe fracture toughness at the different stress ratio.
The single-edge-cracked specimens of MBS resin with two different sizes of rubber particles were fractured under impact tensile loading. The dynamic load and displacement were measured using a piezo sensor and a high-speed extensometer, respectively. The load and displacement diagram, i.e. the external work Uex applied to the specimen was used to determine the elastic energy Ee and the fracture energy Ef for creating a new fracture surface As. Energy release rate was then estimated using Gf = Ef/As. The values of Gf were correlated with the fracture loads and the mean crack velocities determined from the load and time relationships. The effect of the particle size on the impact tensile fracture behavior was then examined. The results showed that the particle size plays an important role in changing the impact fracture behavior of MBS resin.
This paper describes flexural fatigue strength of concrete with high volume fly ash (HVFA concrete). The number of coal-fired power plants has been increasing, and amount of coal ash will increase gradually in Japan. Proper use of coal ash is urgently needed. Long term increase of flexural strength of HVFA concrete is expected. From the life cycle cost performance, HVFA concrete suits for pavement concrete. Since Japanese normal pavement concrete design method requires the flexural fatigue strength, this study aims to investigate the flexural fatigue strength of HVFA pavement concrete. Fatigue test results have large scatter, and this scatter includes a scatter of concrete materials strength mixed in several batches. All test specimens were made from only 2 batches in order to exclude scatter of materials strength. From the flexural fatigue tests, the relation (P-S-N curve) between S-N curve and the life probability are obtained based on the normal and 3 parameter Weibull distributions. Based on the fatigue test results, the strength at 2 million repetitions are determined from these distributions. Resultantly, the S-N curve of HVFA pavement concrete is almost same as the normal pavement concrete. The P-S-N curve of HVFA concrete obtained from the normal distribution does not have a large difference to that obtained from 3 parameter Weibull distribution.