With the rapid progress of recent industry, conditions for using various machines have become more and more severe than ever before, and requirements for safety and reliability have been increasingly augmented. Under these circumstances, experimental stress analysis has come to play an increasingly important role and more accurate measurements under severe conditions have been required. In the present paper, a selective review of recent development of experimental mechanics in Japan is presented.
It is difficult to solve the problem of stress-strain distribution in plastic regions in comparison with elastic regions. Thus, an attempt to expand and apply stress-strain analysis by photoelasticity, has been promoted so far and many significant achievements have resulted therefrom.
Photoplasticity is a full-field technique that is based on the interpretation of optical patterns in transparent birefringent materials that have been stressed beyond their elastic limit.
Polycarbonate resin, a photoplastic model material, has been subjected to much study. In this paper, we attempted to survey of recent photoplastic experimentation.
There have been numerous research works of reliability analysis whose objectives are concerned with the evaluation of variability of characteristic features of composite materials. Most of these are based on the modelling analysis from the viewpoint of micro yielding behavior or crack propagation phenomena under the idealized condition such as uniaxial tensile load. Though they contribute to clarify the micro behavior of failure phenomena and strength variabilities and to find what is the most sensitive factor among various factors, they can not construct the method of materials design. One of the major objectives of this paper is to discuss some problems for stochastic materials design of composite laminate with material characterization as modulus, lamination angle, various strengths under the generarized load condition.
In design problems of actual engineering systems, many design variables take discrete and not continuous values on account of the inherent characteristics. Hence, the optimization of the design is considered as a combinatorial optimization problem. Branch-and-Bound method has been the traditional technique to solve such problems. Recently, Hopfield, J.J. showed that some combinatorial optimization problems can be solved on an artificial neural network system.
In this paper, an algorithmic procedure with neural networks for solving combinatorial optimization problems is proposed which attains very good (not necessarily best) solutions by systematically changing the Lagrange multipliers vector for the constraint equations in the problem. The proposed method can be applied to both linear and quadratic programming problems with discrete control variables. Numerical examples for optimum design problems of frame structures are provided to illustrate the basic properties and applicability of the proposed method.
The neural network system that can solve combinatorial optimization problems performing parallel computation is extremely attractive as a future CAD system.
This paper is concerned with the properties of sandwiched type laminated steel plate which excels at vibration damping. The laminated steel plate is prepared by sandwiching a viscoelastic resin sheet between two steel plates and leads to the vibration damping. Sufficient weldability and formability are required for the application to structural materials. However, the laminated steel plate is isolated by the resin, and has poor weldability. Furthermore, unfavourable phenomena, such as peeling and wrinkle, occasionally arise in this plate. In spite of the fact that such problems have been not completely solved yet, this plate has been applied as damping materials in automobile, electrical and architectural industries. The market of this plate will certainly expand in future.
By using Reaction Injection Molding (RIM) in which two kinds of chemically active liquids are mixed instantaneously, an integral-skin foam that has a composite foamed structure consisting of an internal foamed layer enclosed by rigid surface layer is obtained by a single injection process.
This paper is concerend with the optimization of mechanical properties of composite foamed structures. Assuming that the composite foamed structure made by rigid polyurethane RIM is a composite beam of sandwich structure, the basic equations for predicting the tensile and flexural properties of RIM moldings have been derived. The validity of these equations has been confirmed by comparing of theoretical results with experimental results obtained by tensile and flexural tests. And the influence of the layer composition on static mechanical properties of such moldings have been studied. Furthermore, the influence of the layer composition on dynamic mechanical properties of such moldings have been investigated by Charpy impact tests and flexural fatigue tests. As results, the method of optimization of the layer composition for the mechanical properties of composite foamed structures could be proposed by using the above-mentioned equations.
A large way of polypropylene resins was found in the field of industrial materials as the substitution of ABS, polystyrene or metal material with their special E/P block copolymer development. By compounding PP with spherical (ex. CaCO3), laminar (ex. talc) or fibril (ex. glass fiber) filler, PP has expanded its market and reached to automobile parts by this time.
In the PP composite with CaCO3 particles are coagulated into one and make a suitable dispersion form for higher impact strength. Talc-filled PP composites shows well-balanced properties between impact strength and stiffness which is competable to ABS resin. This is explained by the structure of talc particles surrounded with ethylene-propylene rubber (EPR) which is contained in E/P block copolymer.
Glass fiber reinforced PP composites attained super high stiffness by improving the adhesion strength between PP and glass fiber. When special type EPR is added to this system, glass fiber reinforced PP composite shows almost equivalent property to Noryl (modified PPO) which is superior to talc-filled PP in the stiffness and impact strength.
An automatic test method to obtain the growth behavior of delamination fatigue cracks near the threshold region was established for unidirectional CFRP laminates by using computer-controlled fatigue testing systems. Loading apparatus for the double cantilever beam specimen and computer software were newly developed in order to make a precise measurement of the crack length by the compliance method. The resolution of the crack length increment reached to 1μm and was high enough to determine the growth threshold. The load shedding was performed in a continuous manner. A normalized gradient of the energy release rate was kept constant between -0.3 and -3mm-1 in CF/Epoxy laminates. The relation between the crack propagation rate and the fracture mechanics parameters was independent of the load-shedding condition from the threshold region to the high rate region. On the other hand, normalized gradient of the energy release rate should be kept grater than -0.9mm-1 in CF/PEEK laminates in order to get identical threshold values. This test method was applied to the CF/Epoxy and CF/PEEK laminates. Test results gave sufficient data near the threshold region for design applications.
The full potential of fiber-reinforced composite laminate can be realized by developing optimum design methodology that is fundamentally different from that with isotropic materials.
In this paper, strength characteristics of multidirectional laminates are studied first. Laminates are assumed to be symmetric about the midplane surface and to consist of plies with 0, 90, 45 and -45 degree fiber orientations. A simple transformation was introduced to normalize the combined loading data and express all of them in the closed stress space. Strength discontinuities of multidirectional laminates are pointed out, which should be taken into consideration during the process of ply thickness optimization of multidirectional laminates. Computer codes were developed to find the optimal combination of a maximum of four ply groups to obtain the maximum strength for a given set of general in-plane stresses.
The numerical results obtained using T300/5208 graphite/epoxy composite material showed the strength of optimized laminate was about six times as high as that of the quasi-isotropic one at the highest point.
The experiments were performed using transverse impact due to an aluminium bar on the center of the simply supported beams and plates which were made of plaster and examed the time dependence of impact load and the position and the time of crack initiation. The fracture stress of impact was estimated by combining the position and the time of crack initiation and the impact load with the theory of flexural motion of an infinite beam and an infinite plate. It was found that the fracture stress was almost equal to the tensile stress obtained from the static tensile experiment.
Fatigue tests of Sheet Molding Compounds (SMC) are carried out for the purpose to investigate the diminution of the stiffness due to cyclic loading, the change of crack situation dependent of the number of cyclic loading and the relation of the current stiffness to the crack length. A linear relationship between the stiffness and the density of surface crack in fatigue process is recognized although the dependence of this relation on the applied stress level clearly exist. From further observation on the crack behavior inside of the specimens, it is found that the applied stress dependence of the relation is caused by the difference of crack behavior due to cyclic loading.