This paper deals with a problem in applying the statistical method to structural design. It is generally recognized that safety of structural system must be statistically evaluated, and reliability or failure probability has been proposed as a reasonable criterion for structural safety. From the point of the above view, the failure probability is used in reliability-based structural design instead of the safety factor. However, the safety factor has been widely used as the traditional structural design code, and has been the most popular design concept. Therefore, some problem still remain unsolved in the probabilistic design of structures. One of the problems for the designer is how to choose an appropriate value of safety factor corresponding to a level of design reliability. This problem stems from the difficulties in relating reliability to safety factor. Several research works concerned with the evaluation method of relating safety factor to structural failure probability are reviewed. Some extended approaches of reliability based design are also introduced. The basic concept and attitudes included the research works presented in this paper will suggest the new contributions on the design philosophy of materials system.

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New design method of composite laminates which have required in-plane elastic properties is proposed in this paper. The materials intended for this paper are multiple balanced angle-plied symmetric laminates with one kind of lamina. A newly constructed diagram for lamination parameter made possible to determine the design specifications based on four in-plane effective engineering constants. The lamination parameter is a function of the angles of fiber orientation. A certain constitution of laminates is indicated as a point in the diagram in which contours of four in-plane effective engineering constants can be drawn. Using these contour lines, the design specification is easily determined as a point on this diagram. Thus, the point can be called as a design point. It became possible to specify the fiber orientation angle of each ply using the lamination parameters indicated as the point in the diagram.

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The influence of matrix resin on the time and temperature dependence of the transverse tensile strength, interlaminar shear strength and longitudinal compressive strength of unidirectional CFRP was investigated. These tests were respectively conducted at various temperatures and strain rates. These strengths of CFRP showed remarkable dependence upon time and temperature in the same manner as the tensile strength of matrix resin. The master curve of these strengths in CFRP could be obtained using their thermorheologically simple properties. The time and temperature shift factors for the fracture behaviors of CFRP were quantitatively in good agreement with that of matrix resin. The relations between the tensile strength of matrix resin and these strengths of CFRP were uniquely determined by the volume fraction of fibers etc. Without the dependence of time and temperatures. The estimation method which was proposed in this paper enables us to evaluated : 1) the transverse tensile strength, 2) the interlaminar shear strength, and 3) the longitudinal compressive strength of unidirectional CFRP at any combination of strain rates and temperatures using : 1) the time - temperature shift factors for matrix resin, and 2) the strength of CFRP on a constant strain rate at several temperatures, unless the matrix resin degradates. It is suggested that the range to apply this method is in the combination of the strain rate from one figure up its maximum to one figure down its minimum of the tensile tests of matrix resin, and the temperature from 10℃ above its maximum to 10℃ below its minimum of these tests.

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The present paper discusses the residual stress and the deformation caused when the thermosetting-resin beam is quenched at various conditions, which are six combinations of two temperature conditions and three constraint conditions. The temperature conditions consist of: 1) both-sides quenching, and 2) single-side quenching, and on the other hand, the constraint conditions are : 1) constraint free, 2) constraint of warp, and 3) constraint of warp and expansion of longitudinal direction, The 4-element viscoelastic model including an element for thermal expansion is proposed in this paper in order to express the viscoelastic behavior of themosetting-resin with the change of temperature. The model which consists of three parallel viscoelastic models are also proposed to express mechanical behavior of quenched beams. The model qualitatively clarifies the influence of the mechanical properties of material and quenching conditions on the residual stress and the deformation of the thermosetting-resin beam caused by quenching. The experimental values of the residual stress and the deformation of thermosetting-resin beams by quenching under the constraint free show a similar tendency along with the results obtained by the proposed model.

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This paper is concerned with the photoviscoelastic analysis in the case where the axes of principal stress, principal strain and polarization of light coincide with each other in spite of the existance a large change of temperature in the body. Concretely, this paper intends to obtain the changes of thermal stress and constraint strain with time in the epoxy resin beam having a rectangular section subjected to the quenching from both upper and lower surfaces, in which the time-temperature equivalent law among stress, strain and birefringence holds. That is, the birefringence and temperature which change with time due to the quenching of the beam were measured, from which the changes of thermal stress and constraint strain were obtained with the hereditary integrations. On the other hand, the changes of thermal stress and constraint strain were also calculated from the linear-viscoelastic theory. As a results, the measured value of thermal stress agrees well with the theoretical value. The measured value of constraint strain agrees well, in its tendency, with the theoretical value. And, this measured value of constraint strain after quenching agrees well with the residual strain obtained by measurement from gauge marks on the specimen in the longitudinal direction at room temperature before and after quenching. Consequently, it was found that the photoviscoelastic technique is effective in solving the problem mentioned above.

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The expedient optical method for the residual stress and strain generated by quenching on both upper and lower surfaces of thermosetting-resin beams has been discussed. The results obtained are summarized as follows: (1) The fundamental expressions, by which the residual stress and strain should be expediently estimated from the residual fringe order observed, were derived by introducing the 4-element photoviscoelastic model. (2)The residual stress and strain obtained by the expedient optical method agree well with the measured values by the layer removal method etc., which are the usual mechanical methods. Therefore, the evidence has been given that the expedient optical method is very successful for estimating the residual stress and strain of quenched thermosetting resin beams.

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