Fatigue strength characteristics were investigated in two different non-combustible Mg-Al-Ca alloys, Mg-9%Al and Mg-4%Al controlled processability and strength by adjusting the amounts of added elements. Fatigue strength of Mg-9%Al with higher Al addition was higher than that of Mg-4%Al with smaller Al addition. According to fracture surface observation, a precipitate was observed at fracture origin in Mg-9%Al. However, precipitate was not observed at fracture origin in Mg-4%Al. Fatigue strength of Mg-9%Al was well evaluated based on fracture mechanics by precipitate size observed at the fracture origin and applied stress amplitude. In case of Mg-4%Al, the minimum size of a precipitate which can be the fatigue fracture origin estimated from the threshold stress intensity factor range obtained with fatigue crack growth test was larger than the maximum size of a precipitate estimated by statistics of extreme value. Therefore, it was able to confirm the validity of the experimental result based on fracture mechanics. Relationship between amount of Al addition and the fatigue strength at 107cycles was estimated by using a crack closure model with assuming fatigue crack initiation from a precipitate. According to the calculation result with the crack closure model, degradation of the fatigue strength ratio (ratio for fatigue strength at 107cycles to the tensile strength) was observed with increase in Al addition. In other words, it is considered that fatigue strength is significantly affected by increasing the precipitate size to play as a fracture origin rather than increasing of the tensile strength due to increase in Al addition. In additionally from the calculation result in the model, it was found that fatigue strength at 107 cycles showed about 40% of the tensile strength when amount of Al addition is larger than 4%.
Recently, a rapid determination method of fatigue limit based on dissipated energy measurement using infrared thermography has been getting an increasing interest in industries. The applicability of this method to the specific materials such as carbon steels and stainless steels has been investigated well by many researchers. On the other hand, the applicability of this method to aluminum alloys is different in case by case. In this study, the relationship between dissipated energy and fatigue strength for aluminum alloy A6061-T6 was investigated. As a result, the relationship between dissipated energy and plastic strain energy was confirmed for A6061-T6. In addition, the dissipated energy values measured during the staircase-like stress amplitude testing were well correlated to the number of cycles to failure obtained from the constant stress amplitude testing. Therefore, the possibility to be able to evaluate the fatigue life for A6061-T6 by measuring the dissipated energy was clarified.
Structural adhesives are used for the purpose of weight saving and rigidity improvement in recent automotive development. In this study, fracture behaviors of a cohesive zone under mixed-mode conditions were focused on, and an evaluation method for the parameters of J-integral using the crack tip opening displacement was proposed. The method is an application of the Dugdale model which is known as an idea to deal with localized plasticity region of a crack tip for adhesive joints. Validity of the method was verified using finite element analysis. Furthermore, the method was applied for adhesive joint on galvannealed steel and its fracture mechanism was investigated.
To clarify the mechanisms of creep crack propagation in metallic thin films (nano-films), in situ FESEM observation/EBSD analysis of creep crack propagation in freestanding gold films with approximately 340 nm in thickness was conducted. We developed an in situ FESEM/EBSD creep testing machine which was able to precisely apply a constant tensile load (stress) to a freestanding film specimen under FESEM observation at tilting angles of up to 70 degrees and EBSD analysis. Creep crack was propagated by repeating the processes of (i) necking-like creep deformation in the thickness direction ahead of the crack tip, (ii) void nucleation at the bottom of the necking-like creep deformation region, and (iii) creep crack propagation through coalescence of voids and/or main crack. Void nucleation and growth occurred along the grain boundaries. Creep deformation and subsequent fracture occurred in a grain between voids or between main crack and a void, resulting in coalescences of voids and/or main crack.
PCD Tools have a high heat resistance and a high abrasion resistance. PCD tools are made by brazing a cutting blank and a cemented carbide base. However, the strength of the brazing is varied. In this paper, joint strength of specimens of cemented carbide brazed with silver filler metal was investigated through tensile tests and Cantilever beam bending tests. The tests revealed that the strength of the joints decreases as the joint gap is narrower. Therefore two methods to control the gap of the brazing joint were developed. One is a chamfering method where the chamfering forms the gap. The other is a powder mixture method in which metal powder is mixed with brazing filler metal as a spacer. The average of joint strength increased approximately 1.5 times when the brazing gap was more than 25 μm to around 40 μm by 75 μm Ni powder spacer. The strength of the Cantilever beam bending was discussed by the finite element analysis.
Fatigue design criterion for members of cast steel is not specified in the design standard of railway bogie frames in Japan. The objectives of the present paper are to clarify the fatigue property of cast steel used in railway bogie frames and to propose a fatigue design criterion for members of cast steel. Fatigue tests are conducted on test specimens with casting or machined surfaces under axial loading or plane bending. It is clarified that the fatigue strength of specimens with casting surfaces is higher than that of specimens with machined surfaces; the fatigue strength under plane bending is extremely higher than that under axial loading. These results are attributed to differences of locations and geometries of cast defects where cracks originate. A fatigue design criterion for members of cast steel is proposed by statistical evaluation of the fatigue data. Moreover the validity of the proposed criterion is demonstrated by the full-scale fatigue test of bogie frames with members of cast steel.
In this study, an embedded fiber is considered in matrix under pull-out force. Then, two intensities of singular stress fields (ISSF) are discussed appearing at the fiber end A and the intersection point E of the fiber and the surface. The analysis method focuses on the FEM stress at points A and E by applying the same FEM mesh pattern to the unknown and reference 2D models. To analyze the ISSF at A, the body force method solution is used as the reference model. To analyze the ISSF at E, the reciprocal work contour integral method (RWCIM) solution is used as the reference model. Then, the two ISSFs are compared by varying the fiber bonded length lin. When lin is shorter, the singular stress at A is larger than the singular stress at E. When lin is longer, the singular stress at E is larger than the singular stress at A.
The hybrid molding method has been developed by combining press molding and injection molding to mold CFRTP with high-strength and high-stiffness for complex shaped products with ribs and bosses. In the hybrid molding, longer fibers such as continuous fibers and cut prepreg are used for the laminate of the press part and short fiber or long fiber pellets are used for the injection materials. It has been reported that the fracture occurs at the interface between the laminate and ribs. In our previous studies, to improve the rib root tensile strength, the supplying method of laminates to the rib structures was proposed. Since the rib root shape affects their flow behavior into the rib structure, it is necessary to clarify the influence of rib root shape on the rib root tensile strength. In this study, to clarify the effects of the molding pressure and rib root shape on the rib root tensile strength of the press and injection hybrid molded products, rib root tensile test of T-shaped specimens were conducted. CF/PA6 random prepreg sheets were used for the materials of the press part, and the molds with two kinds of radius of curvature R0 and R2.5 for rib root shape were used. Their fiber orientation and voids were evaluated by X-ray CT. Since the specimen with R0 has more carbon fibers oriented to the vertical direction to the interface than that with R2.5 at the press pressure of 8 MPa, the specimen with R0 had higher rib root tensile strength than that with R0. In terms of obtaining higher rib root tensile strength, R0 is the optimum rib root shape for the press and injection hybrid molded product.
The tensile strength properties of cross laminated timber (CLT) is one of the indispensable concern for designing the structures using it. For evaluating the strength properties of wood based materials, the strength distribution has to be understood. However, experimental approach to obtain the distribution is limited because of the problem in finance and load capacity of testing machine. This study therefore developed the model for simulating the tensile strength distribution of CLT. First, relationship between tensile strength and maximum strain of virtual laminae was made using Monte-Carlo method. Second, tensile fracture of CLT was modeled and the tensile strength was calculated. In addition, reinforcement effect by the laminating was included to the presented model. The simulated result showed agreement with experimental result. However, the simulated average strength became little lower than the experimental one. This study also discussed the problem to be taken consideration for better simulating accuracy.