This paper presents the effect of temperature on the nonlinear tensile stress-strain behavior of carbon fiber reinforced epoxy composites (CF/Epoxy). A two-dimensional elastic-plastic constitutive equation is developed using both a fourth-order complimentary energy function and a second-order one-parameter plastic potential, assuming that linear elastic deformation occurs in the fiber direction and anisotropy parameters change with plastic deformation. Monotonic and loading/unloading tensile tests on the on-axis and off-axis specimens at various temperatures are carried out to determine the elastic constants and plastic parameters of the effective stress-effective plastic strain relation. It is proved that nonlinearity in the transverse direction and the transverse-shear coupling is due to plastic deformation, while nonlinearity in the shear direction is ascribed not only to plasticity but also to nonlinear elasticity. The elastic constants in the transverse and shear directions and the plastic parameters are significantly affected by temperature. The off-axis tensile stress-strain curves predicted by using the present model show better agreement with the experimental results than those predicted by using Sun and Chen's model.
A new estimating method for mode I interlaminar fracture toughness of CFRP laminates was proposed with the aim of applying to dynamic fracture test. In this method, a wedge indentor was used to cause mode I interlaminar fracture in a precracked coupon specimen. That is, compressive load, which is convenient to dynamic test, can be used in this method, while tensile load, which is not appropriate to dynamic test, is necessary for the conventional double cantilever beam test. Finite element analyses and quasi-static experiments were carried out to validate an estimating formula for the mode I energy release rate, which is based on the beam theory and the compliance method. As the result, this method is found to be able to estimate the mode I interlaminar fracture toughness of CFRP laminates and to obtain the same stress field as the one formed in the double cantilever beam specimen. But in this method one has to measure accurately the crack length at the fracture initiation, which is difficult under high rate loading. Three types of CFRP laminates were tested in order to investigate the effect of the toughness of matrix resin. They are: T300/2500 (Toray) which has standard epoxy matrix resin, IM600/133 (Q-C133, Toho rayon) which has toughened epoxy matrix resin, and HTA/PEEK (Toho rayon) which has thermoplastic matrix resin of high fracture toughness. The experimental results show that the toughness of the matrix resin scarcely affects the estimation of the mode I interlaminar fracture toughness and that the fracture toughness estimated by the present method is comparable to that estimated by the double cantilever beam test.
The purpose of the present work is to investigate the effects of matrix resin and fiber content on the behavior of fatigue crack propagation in continuous-glass-fiber-mat reinforced CP-resin composites. For this purpose, ductile matrix resin and brittle one are used. These two kinds of resins have the characteristic that the elastic modulus and tensile strength are nearly the same with each other, while the elongation is different. The composite specimens are made of these resins and continuous glass fiber mat of 20wt.% and 60wt.% fiber contents. The fatigue crack propagation test was conducted by using the tapered DCB specimens to control the stress intensity factor range, ΔK, during the test. The results obtained are as follows; (1) The relation between the crack propagation rate, da/dN, and ΔK for all the present materials is shown by a straight line in logarithmic representation. (2) For the composites of 20wt.% fiber content, the da/dN of the ductile matrix composite is lower than that of the brittle matrix one at the same ΔK. (3) For the composites with either one of the matrix resins, the ΔK value of the 20wt.% fiber content composites is about 40 percent less than that of 60wt.% at the same da/dN.
The tensile static and fatigue tests for conical shaped FRP joints using polymeric adhesives (FRP joint) were conducted at wide ranges of loading rate and temperature. The time and temperature dependence on the static and fatigue strengths of the FRP joints were investigated from the viewpoint of viscoelastic behavior of the matrix resin of FRP. The static and fatigue strengths and fracture modes of the FRP joints depended on time to fracture and test temperature. The reciprocation law of time and temperature for the viscoelastic behavior of the matrix resin of FRP holds for the static and fatigue strengths of the FRP joints. Therefore, the master curves of static and fatigue strengths for the FRP joints can be obtained, and this fact enables us to estimate the fatigue strength at an arbitrary temperature and the number of cycles to fracture.
A damage criterion on CFRP under fatigue loading defined in terms viscoelasticity is proposed. In order to elucidate the change in viscoelastic property of CFRP composites under cyclic loading, due to matrix hardening, matrix crack, interface debonding and delamination, a series of creep tests was incorporated during monotonic tensile fatigue test. A generalized Voigt model was applied to describe the creep behavior of the damaged composites. By use of the increment of creep strain induced by internal damage growth, a damage variable was proposed in the framework of Kachanov-Ravotnov's classical damage theory. The evolution of the damage variable in relation to the maximum stress is discussed. Lemaitre-Chaboche's damage evolution equation was employed to estimate the fatigue life of composites successfully.
Generally, to determine the creep property of a composite material, test must be carried out continuously for about 103 hours under a steady condition. However, power failure, earthquake or impact may occur as obstacles. In order to avoid these problems and obtain accurate data, it is necessary to carry the test on each specimen by using a single test device and collect the data in a short term as possible. Therefore, it was tried to evaluate the creep property of a specimen by enhancing the environment temperature in a step wise manner. It was found that the influence of thermal history, such as the environment temperature and hold time of each stepwise temperature change was slight up to the last step where the creep fracture occurred. Consequently, this method is considered valid for understanding the profile of creep property.
Fiber reinforced plastics that have good specific strength and rigidity are used in many fields. Recently, they are applied as components of high speed vehicles such as automobile, aircraft and spacecraft. Such vehicles need a large absorbing capacity of energy to secure passengers from impact caused by collisions with stones, hailstones and birds. In the earlier paper on this work, it was found that, when FW rings having some notches along the internal circumference of the ring were subjected to lateral impact loading, interlarminar fracture appeared but the absorbing energy increased under constant impact load. So such a notched FW ring shows ideal characteristics as the shock absorbing component. However, some problems may occur such as that energy absorption characteristics are influenced by the direction of impact loading. In addition, FRP used as a component of transportation vehicles may be exposed to various environments such as water or thermal environments. So, it is also necessary to investigate the energy absorbing ablity of FRP exposed to such environments. In this paper, FW rings are exposed to water environment and hot wet environment to cause the deterioration of rings, and then the impact loading tests of FW rings are carried out in order to obtain the ideal load-time history and energy absorption as well as the influence of water environment on energy absorption. As a result, the influence of water environment is made clear and the ideal load-time history as shock absorber is obtained in some conditions.
Measurement of interfacial shear stress has been difficult on a composite with short-fiber, since the fiber length is shorter than the critical length. In this study, a mechanical model is proposed for the composites and the interfacial shear stress is evaluated with viscoelastic propeties of the model. Injection molded composite specimens are used in short-term creep tests. The coefficients in a viscoelastic model are evaluated by the optimization analyses of experimental results. The strain shared by fiber for a given macroscopic strain is analysed from the change of the coefficients for elements in the mechanical model. The stress in fiber is obtained by substituting the mean stress into Cox's relation. The interfacial shear stress, from the shear-lag analysis, is compared with the prediction after Cox. As a result, the maximum shear stress was found to be bigger than that of Cox's prediction and also least affected by fiber volume fraction.
In recent years, printed wiring board (PWB) is used in most of electronic products, such as office automation apparatus and home electronics. As these electronic products become smaller in size and higher in function, it has been required for the packaging density of PWB to be improved. So, it is necessary that the hole is made small in drilling and that the pattern is made fine on a circuit. Specially, the small diameter drilling in PWB is one of the important processes in forming circuit, because the drilled hole quality is required for improving the reliability of through-hole plating. Consequently, in order to develop the reliability of smaller diameter drilling in PWB, it was needed to establish a new method to grasp the relation between the drilled hole quality and cutting condition. So, the picture processing method was applied to observe the internal damage by using transmitted light through GFRP. An attention was paid to the relation between the internal damage and surface roughness of the hole, and the relation between the drilled hole quality and internal damage at various drilling conditions was examined. The drilled hole quality has been made clear by this method.
Two types of Al-Zn-Mg-Cu system alloys with different microstructures, i.e., a fully recrystallized fine-grain and an un-recrystallized sub-grain, were prepared to investigate their tensile properties and stress corrosion cracking, SCC, resistance. The former is the superplastic material (SP), and the latter is a typical one of the commercial materials (CM). The effects of grain shapes and orientation on SCC resistance were discussed on the basis of the SCC test and fractography. The main results obtained in the present work are as follows: (1) When the orientations, L and T, of specimens were in agreement to rolling direction, the values of mechanical Young's modulus, tensile strength and 0.2% proof stress of both SP and CM materials were nearly equal. However, the elongation of CM material was higher than SP one. (2) The nucleation time and life of SCC on L-and T-directions of CM material were longer than those of SP one. SCC resistance in the CM material was superior to that in the SP material. (3) Anisotropy of specimen orientation for SCC life was not observed in the SP material. In the CM material, on the other hand, SCC life on L-direction was longer than that of T-direction. The anisotropy on SCC life observed in the CM material seemed to be related to an easy separation of interfaces between the matrix and the inclusions having Fe and Si elements existing continuously along the grain boundaries by hot rolling.
The effects of specimen geometry and loading mode on the resistance to brittle fracture initiation were analyzed by the local approach. The fracture resistance of materials under a large scale yielding condition evaluated by the conventional fracture mechanics parameters, such as crack tip opening displacement (CTOD), depends to a large extent on the specimen geometry and loading mode. This is due to the geometrical constraint effect on the crack tip stress field. The three-dimensional FE analysis figured out that, as plasticity developed, a shallow notch bend specimen and tension specimens (CCP and DECP) showed significant loss of constraint compared to a deep notch bend specimen. This led to a lower stress near the crack tip for the shallow notch bend specimen and tension specimens than the deep notch bend specimen. By contrast, the critical Weibull stress at brittle fracture according to the local approach did not depend on the geometry of specimens used. This paper demonstrates the independence of the critical Weibull stress on the crack depth of 3-point bend specimen. As an advantage, the local approach enabled us to predict the specimen geometry effect on the critical CTOD at fracture. The effects of specimen geometry and loading mode on the critical CTOD at fracture depend on the hardenability of materials. In the case of high hardening materials, the critical CTOD is not very sensitive to the specimen geometry and loading mode excepting the thickness effect on the critical CTOD for materials with a large scatter in toughness. On the other hand, for low hardening materials, the critical CTOD is affected to a large extent by the crack depth and loading mode of specimens.
The cyclic nonproportional plastic-creep constitutive relation was studied by experiments and analysis, in order to estimate the stress response of Type 304 stainless steel which shows a significant additional hardening and fatigue life reduction by nonproportional loading. Mises' equivalent strain controlled push-pull/reversed torsion plastic-creep loading tests were carried out at 923K in air using two proportional and three nonproportional strain paths. Every strain path had the same Mises' equivalent strain wave form but the combination of normal and shear strains was different. A simplified incremental inelastic analysis was made for five different proportional and nonproportional loadings. In the analysis, Mises yield criterion with the Prager kinematic hardening rule was used and the total strain was assumed to be a linear combination of elastic, plastic and creep strains. The simplified analysis predicted well the actual stress response obtained in the experiments.
This paper describes the development of multiaxial creep testing machine using cruciform specimens in a wide range of biaxial stress conditions of 0≤λ≤1 and the multiaxial creep rupture tests at 923K, where λ denotes the ratio of the minimum principal stress to the maximum principal stress. The developed test machine has a loading capacity of 98KN and the maximum temperature is 923K. Three dimensional finite element (FE) creep analyses were made to determine the shape and dimensions of the cruciform specimen having uniform stress distribution along the gage length. The specimen shape determined by the FE analyses has a 32mm×32mm parallel part with 5mm thickness. In the biaxial Mises' stress constant creep tests using type 304 stainless steel cruciform specimens, creep rupture time increased with increasing λ, so that the biaxial stress has an effect on rupture time. The rupture data did not depend on λ when correlated with the equivalent stress based on crack opening displacement, but they depended on λ with Huddleston's stress. Huddleston's stress gave more conservative equi-biaxial tension creep rupture time than the Mises' stress. The Mises' equivalent strain rate decreased with increasing λ in multiaxial creep tests.
By using stress function methods and the conversion theories from isotropy to anisotropy, the torsional rigidity was analyzed, and the theoretical formulas for the anisotropic shear moduli in width and thickness directions were given for 3-ply composite. According to the theoretical analysis, the anisotropic shear moduli in width and thickness directions, and the anisotropic coefficients were obtained from Eq. (36), (37) and (41). The shear modulus in width direction was not related to that in the thickness direction of each ply, and could be calculated by Eq. (42) including secondary moment. There was an effect of the shear moduli in width direction of each ply on the shear modulus in thickness direction for composite, but it was little. In addition, the variations of shear stress distribution factors, which appear as the correction coefficient to shear modulus in the Timoshenko's beam theory including secondary shear deformation effects, were estimated for the composite materials. Using the mixture law of shear modulus obtained herein and the well-known mixture law of Young's modulus in bending, the shear stress distribution factors in thickness direction for 3-ply composite were about 1.0-1.35 except for 90° plywood. There were still the errors between the theoretical shear moduli and the those estimated from the distribution factors (κ44).
Ni-P alloy film was electrodeposited at 5-232A·dm-2 on an alumina substrate by using the electroplating apparatus equipped with a high flow rate circular pump. With increasing current density, the current efficiency for the deposition on Ni-P decreased once. When the current efficiency was minimum, the P content in the film obtained in the electrolytes with various pH was always 7.5wt%. In the X-ray diffraction profile of Ni-P film with 7.5wt% P content, (200) diffraction peak disappeared and only (111) peak became strong. The overpotential of hydrogen evolution on the Ni-P film with 7.5wt% P content became minimum. The current efficiency, P content, crystal structure and overpotential of hydrogen evolution were closely related one another. TCR of Ni-P alloy film was correlate with its P content.
This paper describes the hole quality of small diameter drilling in GFRP using two kinds of laser beam machines in order to invent a no-traditional process of drilling in printed wiring board (PWB) as no contact machining. Drilling is performed by a CO2 laser machine and a YAG laser machines and the drilling conditions (assisting gas, output power, irradiation time, wavelength) are varied in order to assess the effect on the hole quality, especially the damage appearing at the hole wall drilled by laser machines. From these results, it is shown that the application of CO2 laser beam is effective on smaller diameter drilling in GFRP. It is important to decrease the total laser irradiation time in order to improve the hole quality.