Time dependent damage of structural materials attracts a great deal of attention, because environmental and loading conditions became more severe with the appearance of high efficient and big scale machine and structures. The period of employment is also longer under the present economic situations, which closes up the time dependent fracture of stress corrosion cracking (SCC) and corrosion fatigue (CF). In the present paper, life estimation method of SCC and CF in structural materials will be discussed based upon the experimental results that cracks are mostly initiated from corrosion pits in corrosive environment followed by the growth of cracks to final fracture. The threshold value of crack initiation K_ISCC and ΔK_CF from corrosion pit is almost constant considering the pit being sharp surface crack, and thereafter the growth period can be calculated based upon fracture mechanics approach considering environmental growth characteristics such as short crack growth problems, crack closure due to corrosion products, and so forth. Flow chart to estimate SCC and CF life of structural materials is presented.

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Quasi-static tension tests were conducted on CFRP laminates with an open hole to clarify the effect of lamination configuration on both the behavior of microscopic damages and strain concentration factor at the edge of the open hole. Laminate configurations are angle-ply［±θ]_s(θ＝30,45,60゜) and quasi-isotropic［±30/90]_s and ［±45/90]_s. An open hole was introduced at the center of the specimen. A soft X-ray radiography was used to detect the internal microscopic damage progress. Damage progress was observed in［±30]_s,［±30/90]_s and［±45/90]_s laminate. In the failure process of［±45]_s and［±60]_s laminates, no damage was observed until the specimen final fracture. These laminates failed in a catastrophic manner. Moiré interferometry was applied to measure the microscopic deformation. The observation with a moiré interferometer revealed the whole-field strain distribution on the specimen surface. The effect of laminate configuration on strain concentration factor was clarified experimentally.

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Considering the space environment, it is important to study the effects of thermal cycling on the mechanical properties of composite materials. It is known that composite laminates subjected to thermal cycling suffer from matrix cracking. In the present study, thermal cycling induced matrix cracking in CFRP laminates is investigated experimentally. A carbon/epoxy system, T300/2500, is used. Laminate configurations are (0/90)_s, (90/0)_s and (±45)_s. The specimens are thermally cycled between -196℃ and 100℃ Thermal cycling tests are performed up to 1000 cycles. During the thermal cycling test, the polished edge surfaces of specimens are examined by the replica technique to detect microscopic damages. The matrix crack density is measured as a function of the number of thermal cycles. The number of thermal cycles at initial cracking in (0/90)_s and (90/0)_s are similar and smaller than that in (±45)_s. The matrix crack density increases more rapidly in (0/90)_s than in (90/0)_s. The matrix crack density growth rate in (±45)_s is smaller than those in (0/90)_s and (90/0)_s. Tensile tests are performed on the specimens subjected to the thermal cycling. To discuss the matrix cracking behavior under tension after thermal cycles, the critical energy release rate and the critical stress for matrix cracking are evaluated. The effect of thermal cycles on matrix cracking under tension can be evaluated by the change in the critical energy release rate and the critical stress.

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The present paper reports a simulation of the probabilistic strength of unidirectional composites, where three elements of composites, i.e., fiber, matrix and interface are taken into consideration. The fracture mechanism of fiber reinforced plastics (FRP) includes fiber/matrix interface debonding and matrix cracking in addition to fiber breakage. Fracture toughness values were adopted in deciding the fiber/matrix interface debonding and the matrix cracking. The present simulation can realize a zig-zag mode of the fracture surface in the unidirectional composites. By varying the interfacial adhesive strength (fracture toughness) from very low to very high, the strengths of seven types of unidirectional composites were calculated by a Monte Carlo Simulation. The results showed that the strength of unidirectional composites generally increased with increasing the interfacial strength. This result means that reliable load transmission is essential for FRP, as is widely accepted. However, at the same time, the highest strength of unidirectional composites appeared when the adhesion of the fiber/matrix interface was slightly weak compared with the completely tight adhesion. This result suggests that a material design of the fiber/matrix interface is important to get good-quality composites.

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The fracture mechanisms and residual strength for the non-fatigue and the fatigued specimens of FRPA made with the direct injection molding method are studied by means of Acoustic Emission (AE) technique during the tensile fracture process. It is estimated from the AE frequency analysis that the fiber breaking occurs at the first and the last stages and the fiber debonding occurs at the second stage for the non-fatigue specimen, but that the fiber debonding and the fiber freaking occur at the first stage and the fiber breaking dominates the second and the last stages for the fatigued specimens. The damaged (whitened) zone and the crack propagation at the notch tip are only observed at the last stage of the non-fatigue specimen, but the whitened zone becomes larger and the crack propagates at the all stages of the fatigued specimens. As the correlation coefficient between the normalized loads P_b/P_b(N) and P_max/P_max(N) which the load of fatigued specimen is normalized by the load of non-fatigue specimenm is higher than the correlation coefficient between P_i/P_i(N) and P_max/P_max(N), the load ratio P_b/P_b(N) gives better estimation of the residual strength, where P_i is the load at first AE initiation, P_b is the load at the change in damage mechanisms, P_max is the maximun load and the mark (N) is the value of the non-fatigue specimen.

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The purpose of this study is to investigate the influence of different type of fibers on the rheological properties of hybrid composite materials. Long and short glass fibers (GF), different type of organic fibers, viz・aramid fiber (KF), liquid crystalline polymer (LCP) and vinylon (VF) in hybrid composites, were used to reinforce the high density polyethylene (PE) matrix. Long fiber hybrid composites were prepared by a “fiber separating and flying machine”, while the short fiber hybrid composites were prepared by an “elastic extruder”. The total amount of fibers used in both long and short fiber hybrid composites was filled at 20 vol.%. Viscoelastic properties of these composites in molten state were measured by parallel plate rheometer. The influence of fiber characteristics, length, mixing ratio, and also flow conditions such as angular frequency and strain amplitude on viscoelastic properties was studied. These properties were also compared with the viscoelastic properties of these hybrid composites in the solid states.

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An attempt was made to improve the interfacial affinity between the carbon black (CB) and polypropylene (PP) by introducing the maleic anhydride-grafted-polypropylene (MAH-PP). x weight parts of MAH-PP were added to (100－x) weight parts of PP ; x＝O, 3, 5, 8, 10, 20. Then, 20 weight parts of CB particles were filled to make the composite specimen. Effects of MAH-PP content x on the flexural properties and electrical resistivity of the composite were investigated. Results obtained are as follows: (1) In the range of x≦8, the elastic modulus and strength of the composite have increased and the electrical resistivity has decreased with the increase of x. (2) When x＝20, the elastic modulus and strength have decreased and the resistivity has increased, inversely. (3) The composite with x=8 is annealed at various temperatures for 24 hrs. When the annealing temperatute is around the crystallization temperature (126℃), the annealed composite has exhibited the maximum elastic modulus and strength. Although the electrical resistivity increases, the PTC intensity (resistivity at peak temperature/resistivity at room temperature) has increased 4.5 times larger by the annealing compared to the quenched composite.

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In the history of the study on the fatigue behavior of metallic materials, the fracture mechanics approach on the fatigue crack has given us many important results required to the safety design of mechanical structures. Macroscopic crack propagation behavior has been examined from a view point of Paris' law which gives the relationship between stress intensity factor range ΔK and fatigue crack propagation rate da/dN. But fatigue crack initiation behavior and crack growth behavior at early stage remain unsolved due to the difficulty of the direct observation and influence of the material microstructures. In order to observe the crack initiation behavior, we developed a fatigue-testing machine combined with SEM. By using this machine, fatigue tests were carried out in pulsating tension for 70/30 brass and S 35 C carbon steel. Thus crack initiation behavior was carefully observed together with the microscopic deformation preceding the crack initiation. Based on the SEM observations, the authors have the following findings ; (1) in the case of 70/30 brass, fatigue cracks take place as the slip band crack and the pencil glide crack. (2) in the case of S 35 C carbon steel, fatigue cracks tend to occur along the grain boundaries and then they tend to grow into the crystalline grains. (3) ΔK-da/dN relationship indicates a distinct fluctuation in the small crack region which gives the low stress intensity factor range of ΔK＜1 MPa・m^1/2.

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Combined effects of forming condition on deep-drawability of square cups were studied. In order to investigate the fundamental deep-drawability, drawing of rectangular shell of steel sheet was carried out under varied deep-drawing condition. (1) As for the thickness of square cups, the reduction of blank holding force and the increase of die-radius cause the increase of the thickness. (2) As for the height of square cups, the increase of blank holding force and the reduction of die-radius cause the increase of the height of cup. (3) the shape of square cups is concerned with the improvement of blank shape. For example good deep-drawing of SPCD sheet could be finished, under the condition of blank holding force 5 (kN), die-radius 6.0 (mm), punch speed 0.5 (mm/sec), and optimum blank shape.

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The reliability evaluation of characteristics of the bulk of material and the surface treatment becomes important in order to meet the requirement for diversifying industrial products. As each material has merit and demerit, the material and the surface treatment should be carefully selected. The idea of the right material for the right application is very important, It is under development to compose a consulting system by systematizing know-how on these materials. The system is “the material selection and surface reforming retrieval software” which supports the designer engaged in product development because it is anticipated that the number of skilled designers will decrease in the future.

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