Fatigue behavior of a two-pass, double-sided, friction stir welded joint was studied to get a basic understanding on the local fatigue crack propagation (FCP) behaviors of the different zones within the weld. The fatigue crack propagation tests were carried out on center notched specimens with initial notch positions in the different weld regions and longitudinal to the weld direction. The experimental results demonstrated that the microstructure, as dictated by the thermomechanical history, may play an important role in the fatigue crack propagation within the two-pass friction stir welded joint. The FCP rates were found to be higher in the weld affected zones than in the base metal. Within the weld zones, the heat affected zone and the fine-grained double stirred zone (DSZ) had the highest propagation rates. The effect of the second pass was found to be minimal on the fatigue behavior of the areas outside the stirred zones; however, in the DSZ where the two weld passes overlap, the FCP rates were higher than in the single stirred zones. Furthermore, within the heat affected zones the FCP rates were higher on the retreating side than on the advancing sides of both welds indicating sensitive of the FCP to the tool rotation direction.
The many fracture accidents in engineering applications are related to fatigue crack initiation and growth. Therefore, the inspection of fatigue crack initiation and the arrest of crack growth are very important for the safe operation of a machine and the maintenance of a structure. Non destructive inspection has been developed in order to prevent a fracture accident resulting from the cracks or defects. When a crack initiates in machine equipment, continuous observation of the crack is carried out and the crack growth is arrested by some method. To arrest the crack growth, stop-holes are drilled at the crack tips in a case of initiation of a small number of cracks. Drilling the stop-hole is an effective method to prevent crack growth because it reduces stress concentration at the crack tip. Application of residual stress around the stop-holes and making a plastic hardening zone around the crack tips are effective to arrest crack growth. Applying a patch for repairing cracked parts is more effective than drilling stop-holes. In practical cases, patches are applied to repair cracked parts in airplane bodies, bridges, etc. However those methods cannot be used in some cases, because of the conditions of working space, for example. In this paper, an example of the application of a crack arrester at a car facility is introduced. Then, the effectiveness of the crack arrester is discussed. When many cracks are initiated and working space is very small, the crack arrester is a good tool for repairing the cracked parts. It is necessary to develop a method for stopping crack growth in order to replace cracked equipment, because new equipment should be designed carefully for preventing crack initiation again.
This paper presents a finite element method to investigate the hygrothermal effects on the free vibration characteristics of delaminated carbon-epoxy composite pretwisted rotating conical shells. The generalized dynamic equilibrium equation is derived from Lagrange's equation of motion neglecting Coriolis effect for moderate rotational speed. In the present formulation, an eight-noded isoparametric plate bending element is used based on the Mindlin's theory. The multi-point constraint algorithm has been incorporated to ensure the compatibility of deformation and equilibrium of resultant forces and moments at the delamination crack front. Numerical results have been generated to study the combined effects of temperature and moisture concentration on the natural frequencies of delaminated composite conical shells in respect of twist angle, stacking sequence, rotational speed and location of delamination. The mode shapes for a typical laminate configuration in hygrothermal environment is also presented.
Responses of stagnating laminar methane/air premixed flames under fuel concentration oscillation, i.c., equivalence ratio oscillation, were numerically investigated using a one-step overall reaction mechanism and a four-step reaction mechanism that included CO and H2 formation. The flame motion was numerically investigated for three different oscillation cases namely: lean, rich and lean-rich crossover case. Methane/air mixtures with sinusoidal equivalence ratio oscillations were issued from the burner exit with uniform 1.0 m/s velocity profiles. In the steady state condition, the one-step overall reaction mechanism and the four-step reaction mechanism had nearly the same characteristics in the lean region, while in the rich region variations in characteristics such as flame location and flame displacement speed for the four-step model were much more significant than those for one-step model. When the equivalence ratio was oscillated, the flame location oscillated. The amplitude of the flame location oscillation did not change with the equivalence ratio oscillation when the frequency of equivalence ratio oscillation was less than 8Hz, while it decreased monotonically when it exceeded 8 Hz. Here 8 Hz corresponds to a Strouhal number (St) of unity. Thus, this result indicates that the flame was in a quasi-steady state when St<1.0 while it became unstable when St>1.0. The variation in flame location and the flame displacement speed did not follow those for the steady state condition and made a limit cycles. This was due to the back support effect. The cycles were significantly inclined at higher frequencies. In the lean condition, the limit cycle was inclined similarly for both the one-step and four-step reaction mechanisms. In the rich condition, however, the limit cycle for the four-step reaction was more inclined than that for the one-step reaction. These results show that the formation of CO and H2 played an important role in the rich condition.
This paper presents a method to damp sloshing in a liquid container during its transfer in a cart equipped with an active vibration reducer having a six-degree-of-freedom parallel linkage. Sloshing is generated during the liquid container's transfer. To damp this sloshing, the active vibration reducer tilts and horizontally moves the container. The reducer is controlled by a linear quadratic regulator. The weighting matrix of the quadratic performance index is optimized with a genetic algorithm (GA). The amplitude of sloshing is considered for the fitness of the GA. In this study, the cart was driven along a straight path on a horizontal plane. The usefulness of the control system was verified through a simulation and experiment.
Polymeric materials are divided broadly into two categories, amorphous and crystalline polymers. Polypropylene (PP) is widely used from daily necessities to machine parts because of its high formability and light weight. PP that is classified as a crystalline polymer has the following multiscale structures. Lamellae consisting of amorphous and crystalline phases grow radially and spherulites are generated. PP is filled with the spherulites, whose size and crystallinity are determined by molding conditions. Many researchers have studied the deformation of crystalline polymeric materials. However, many points remain unclear, such as the effects of multiscale structures on the material and mechanical properties of the crystalline polymer. A computational model reproducing the properties of PP based on multiscale structures is desired in the materials science and engineering fields. In this study, we perform a polymer plasticity simulation considering a spherulite structure consisting of amorphous and crystalline phases in order to investigate the effects of the amorphous and the crystalline phases of PP on macroscopic stress-strain behavior. PP samples with different crystallinity are prepared by changing the cooling conditions. Information on the crystallinity of specimens based on the experiment results is introduced into a computational model. We thoroughly investigate the effect of spherulite structures on the material properties of PP.
This study proposes a reliability-based multiobjective optimization (RBMO) approach using the satisficing trade-off method (STOM). STOM is a multiobjective optimization method that obtains a highly accurate single Pareto solution, regardless of the shape of the Pareto set. By introducing an aspiration level, STOM transforms the multiobjective optimization problem into the equivalent single objective problem. When the given Pareto solution is not satisfactory, the search process is repeated with a different aspiration level, which is selected using the automatic trade-off method, for example. RBMO considers multiobjective optimization under reliability constraints that consider uncertainties in the design parameters. In this study, the reliability is evaluated by the first-order reliability method. Therefore, the optimization problem is formulated as a conventional double-loop approach. However, the validity of the proposed method can be illustrated without a decoupled reliability-based design approach. Through numerical examples, the proposed method is shown to obtain an accurate Pareto solution for the RBMO problem. In addition, compared to multiobjective particle swarm optimization, parametrically changing the aspiration level produces a more accurate, uniformly distributed, and diverse Pareto set. The tracking ability of Pareto solutions with the same aspiration level is investigated in terms of the target reliability, which clarifies that the shift in the dominant failure mode influences the kink in the tracking trajectory. Finally, an analysis of the automatic trade-off method demonstrates that the desired Pareto solution can be obtained by updating the aspiration level, even when the Pareto surface is nonlinear.
This paper describes a basic study on an emergent design system in which serendipity is exerted from interactions between computer and human. The system is proposed to support idea generation in the early design process. In the early design process, which is composed of conceptual and basic design, new and novel ideas of design should be obtained from global solution search because the design objective and conditions are unclear. Therefore, we propose an emergent design system, which enhances designer's serendipity, intended to support idea generation. Serendipity is a natural ability to unexpectedly make interesting or valuable discoveries. This ability is influenced by designer's experiences and knowledge. The possibility of generating new and novel design ideas will increase if we can utilize serendipity. This emergent design system consists of incidental form generation and representation methods. Based on form organizing phenomenon seen in nature, the system can generate diverse forms. These self-organized forms enable designer's serendipity. Moreover, the designer can add or delete elements of the forms like clay modeling. In this process of modification, the designer will have more chance to get inspiration. We perform elementary experiments of this emergent design system with designers. As a result, we confirm both self-organization and representation methods help designers to generate design ideas with characteristic form by inspiring designer's serendipity. Furthermore, the possibility, that the iterations of these functions propose more opportunities to exert designer's serendipity during the computer-human interactions, is indicated.
Active acoustic shielding (AAS) is a system that can attenuate a sound passing through an open window. An AAS is constructed from a number of AAS cells set in an array having an approximately co-located microphone and speaker system. The concept of AAS was previously demonstrated and some simple simulations and experiments were performed. Moreover, an AAS window with four AAS cells was proved to be effective for not only a single stable noise source but also multiple noise sources and moving noise sources. However, the AAS window could only attenuate noise with frequencies from 500Hz to 2 kHz, and could not attenuate noise below 500 Hz, which is dominant in construction sites and daily life. Therefore, in this paper, a new AAS window consisting of two different AAS units in one window is proposed. One of the units attenuates low-frequency sound and the other attenuates high-frequency sound. These units are controlled independently. The new AAS window was fabricated and experimentally evaluated in an anechoic room. It was found that the new AAS window reduced noise by 5 to 15 dB in the frequency range from 300 Hz to 2 kHz over a wide area in the room.