The effects of loading type, shape of cross section and finishing conditions of the edge of specimens on fatigue strength were investigated on two types of smooth specimens of low carbon steel, one stress-relieved in vacuum and the other treated with tufftriding. As for the effect of loading type, the difference between the rotating and the plane bending fatigue strengths was studied on the specimens with circular cross section and the relation between the in-plane and the out of plane bending fatigue strengths were examined using the specimens with rectangular cross section. The influences of the shape of cross section and the edge finishing of specimens on the in-plane bending fatigue strength were studied on the specimens having circular, semicircular cross section and three kinds of edge finishing radius, respectively. The results obtained were summarized as follows: (1) On the stress-relieved specimens, a curve of total strain range versus the number of cycles to failure can represent all the results of the in-plane bending test specimens with circular cross section and three kinds of edge finishing radius, of the in-plane and the out of plane bending tests on the specimen with rectangular cross section and of rotating and the plane bending tests on the specimen with circular cross section. (2) As for the material used for the tests, the edge finishing has no effect on the in-plane bending fatigue strength for the stress-relieved and the tufftrided specimens with rectangular cross section.
The bending stress distribution in a sandwich composite beam is different from that derived by the conventional composite beam theory because of the shear effect of core, especially when the ratio of elastic moduli of face to core (k=Ef/Ec) is large. In this study, the stress distributions in house-building sandwich materials under four-point bending are analysed by introducing the multi-layer built-up theory. The bending stiffness of face and core and the slip between both faces are taken into consideration in the present analysis. The photoelastic measurements on model specimens having four kinds of k values are carried out and the applicable ranges of both theories are discussed comparing with the experimental results. The stress distributions in sandwich composite beams having k larger than 120 can be well explained by the multi-layer built-up theory. The ratio of the coupling moment due to axial forces in both faces to the applied total moment in the uniform-bending range, which denotes the sandwich structural efficiency, can be well estimated by the multi-layer built-up theory. This simple one-dimensional theory will be useful in the structural design of sandwich construction.
The tensile properties and fracture behaviors of 32 Cr-8 Ni steel with ferrite matrix and stable austenite phase were studied at room temperature. The specimens were prepared in two different conditions: The one consisting of ductile ferrite and austenite was obtained by quenching and the other having strengthened ferrite was obtained by ageing at 475°C after quenching. The grain size of the specimens varied from 0.8 to 26.9μ. The main results obtained are as follows: (1) For the aged specimens, 0.2 proof stress of the coarse grained specimens is below the value expected from the results of the fine grained specimens by extrapolation using the Petch's relation. This may be caused by the formation of deformation twins in the ferrite of the coarse grained specimens. (2) The fracture surfaces of all the quenched speimens show dimples, while those of the aged ones show both quasi-cleavage and dimples. Quasi-cleavage fracture occurs in the initial stage of fracture irrespective of grain size and is characterized by initiation and propagation of cleavage cracks in ferrite, followed by tear or dimple fracture in austenite. In the specimens with coarse grain, a crack propagates in the same manner as mentioned above until the final fracture. However in the specimens of fine grain the fracture by progress of quasi-cleavage cracks formed in the initial stage changes to dimple fracture in both ferrite and austenite. The size of this dimple region increases with a decrease in grain size and the reduction in area also increases with an increase in dimple region.
The effect of grain size on the relation between non-ductile transition behaviour and bending speed of a low carbon steel has been examined with Charpy type specimens (V-notch). The principal results obtained are summarized as follows: (1) Under a constant bending speed, the relation between non-ductile transition temperature TB and grain size D is represented by the following form 1/TB=A'-B'logD (a) where A' and B' are constants. However, this relation does not hold when the bending speed becomes higher and the grain size is larger. (2) In the regions of higher bending speed and of larger grain size, the non-ductile transition temperature is lower than that estimated from eq. (a) and the difference between them increases with increasing speed and grain size. It may be considered that difference arises because the plastic constraint force is affected by grain size.
It is postulated that yielding of roving glass cloth FRP depends only on the shear stress component τ12 in the principal axis of anisotropy and the yield criterion for the material is expressed as τ12=τy (constant). By using this criterion the elastoplastic behavior under uniaxial tension of an anisotropic plate with a circular hole has been studied by means of the finite-element method. The results obtained are as follows: (1) In the case of 0 degree tension, the angle θ where the shear stress τ12 becomes maximum is about 62∼72 degree. The yeilding zone starts at this point and grows to 0 degree direction. The nominal stress level σ0y when the material begins to yield is about 1.23 τy in the case of an infinite plate. (2) In the case of 45 degree tension, θ is always 90 degree and the yielding zone grows to 45 degree direction. σ0y is 0.74 τy for an infinite plate. (3) The stress concentration factor increases when the yielding zone grows.
The fiber reinforced composites are generally characterized by a considerable spread in mechanical properties, since the variabilities in these materials, which are composed of dispersed phase and matrix phase, are considered to be not only dependent upon the properties of each phase but also upon the interface formed between two phases. Random fiber composites reinforced with chopped strand mat show complex failure behaviors and the prediction of their static strength is considered to be a perplexing problem. The main results obtained in the present study using the strength analysis based on the reliability concept are summarized as follows: (1) The static strength distribution of random fiber composites can be described by Weibull and double exponential distributions under the assumption that laminate is an aggregate of cross sectional layers in series and the weakest link theory can be applied to the specimen. (2) The cross sectional layer is modeled as a quasi-isotropic laminate consisting of unidirectional plies. Three possible failure mechanisms of layer are obtained by the combination of the failures parallel and perpendicular to the fiber direction and the failure in shear, and each failure type proves to be dependent upon the fiber content of laminate. The mean value and the spread of the strength of cross sectional layers can be calculated.
The effect of atmospheric-corrosion on the fatigue crack nucleation and fracture surface morphology in the early stage of fatigue has been investigated by tests in air and vacuum on pure iron. The experiments were conducted under push-pull loading, and both external and fractured surfaces were observed by means of optical and scanning electron microscopy. It was found that the fatigue lives of iron in vacuum were 2.1 times those in air and the fatigue limit was also 1kg/mm2 higher in vacuum than in air. The density of slip lines in the specimens tested in vacuum was much higher than that in air and the cyclic numbers up to fatigue crack nucleation in vacuum were found to be 2.6 times those in air. The morphology of initial transcrystalline fracture surfaces in air was classified in two types and the number of intercrystalline fracture surfaces was decreased as stress amplitude increased. The morphology of initial transcrystalline surfaces in vacuum was similar to that in air. No intercrystalline fracture surfaces were observed in the specimens tested in vacuum. Clear striations were found on the fracture surfaces of the specimens tested in vacuum, though most of the surfaces have a flattened and smeared appearance.
The fatigue crack propagation rate (da/dN) up to 1mm/cycle has been obtained for three-points bent specimens of HT 80 steel subjected to gross cyclic plastic deformation. The effect of incremental plasticity on da/dN was examined in terms of maximum J value (Jm), like in the case of the effect of maximum stress intensity (Kmax) in linear fracture mechanics. ΔJ values were determined from the load versus deflection hysteresis loops and Jm values were estimated based on the three parameters method proposed by Garwood et al using the envelope of load versus deflection hysteresis loops. The fatigue crack propagation rate during incremental plastic deflection cannot be predicted by the ΔJ criterion alone, but it can be done by the combination of ΔJ and Jm criteria, giving an agreement with the extrapolation of linear elastic fatigue crack propagation rate data. These experimental results suggest that Jm is a very important parameter for gross plastic crack problems.
For designing various machinery components or structures, it is important to clarify the condition of crack initiation in notched specimens. Now, the local stress state of any place of the parts, from which a crack may initiate, can be calculated comparatively easily by the elasto-plastic analysis with an electronic computer, so that the fatigue life prediction of the components or structures is achieved more accurately. In this study, fatigue tests have been carried out on both the smooth and notched specimens of annealed S 35 C steel in order to obtain the crack initiation life, and the elasto-plastic calculation has been performed by FEM to find the stress and strain at the notch root. These data were used to discuss the relationship between the stress or strain at the notch root and the crack initiation life. The results show that, except the fatigue limit, the crack initiation life can be predicted from the stress or strain at the notch root, and that an equation Kt-1/Kσ-1-1=2(Kε-1/Kt-1-1) is convenient to estimate the fatigue strength of the notched specimen from both S-N curve and σ-ε curve of the material.
Low-cycle torsional fatigue tests were carried out on annealed carbon steel of S 45 C and the results were examined by taking the mean strain as a parameter. The main conclusions obtained are summarized as follows: (1) The hysteresis energy per cycle is influenced by the mean strain at the beginning of strain cycling, but it saturates approximately to a constant value due to the rapid relaxation of mean stress after progressive strain cycling. (2) On the basis of hysteresis energy, an equation for estimating the effect of mean strain on the crack initiation life is proposed as follows; D=K0/K0=1/K0(We1/2+λ∫N'1/2Wedn) λ=N0'/N' where N' is the number of cycles to crack initiation when mean strain is applied, N0' is that when mean strain is zero, e and K0 are material constant and the average value of the constant K0 obtained from the equation 1/K0∫N0'0Wdn=1 when mean strain is zero, W1/2 and W are the hysteresis energies for the first half cycle and the subsequent cycles, respectively, when mean strain is applied, and dn is the infinitesimal increment of hysteresis energy cycle n. The test results confirms that this equation is well applicable to the estimation of crack initiation life. (3) Crack growth life is not influenced by mean strain, because mean stress is relaxed in a short period after the initial stage of strain cycling.
Push-pull and plane bending fatigue tests were conducted on the sandwich-type composite plates composed of low carbon steel-copper or iron-copper to examine the effects of volume fraction and residual stress on the fatigue strength of composite plates. The composite plate specimens used in this investigation were fabricated by means of a diffusion bonding method. The results of the push-pull fatigue tests indicate that the axial fatigue strength of sandwich plates can be estimated on the basis of the“mixture rule”. On the other hand, the results of the plane bending fatigue tests show that, as a first approximation, the residual compressive stress in the surface iron plate significantly improves the bending fatigue strength of sandwich plates, but the residual tensile stress in the surface copper plate reduces the fatigue strength of sandwich plates in comparison with that of the original copper plate.
The strain distribution and the strain concentration factor of friction welded specimens were examined by applying the copper-electroplating technique. The fatigue cracks, the fatigue limit and the mechanical properties of specimens were also investigated. The results obtained are summarized as follows: (1) The change of the strain distribution at the friction welded specimens subjected to repeated torsion, depends on the condition of welding. The strain becomes maximum at the weld interface. The strain decreases with increasing distance from the interface, and become minimum in the heat affected zone. The strain, however, slightly increases again in the parent material sufficiently distant from the interface. (2) The strain concentration factor decreases in the order of the weld interface, the parent material, and the heat affected zone. (3) The fatigue crack propagates most rapidly in the parent material, and the fracture of the specimens occurs there. (4) The hardness (HV: 200gr) at the surface of the friction welded specimen becomes maximum in the weld interface, and decreases with increasing distance from the interface.
As an abrasive wheel for Taber abraser, the rubber wheel covered with sandpaper manufactured and standardized by Taber Instruments Company is now internationally specified to be used. However, the test results obtained from the sandpaper are noted for poor reproducibility. It is suspected that the glue applied for cementing abrasive grains changes its strength according to relative humidity of testing and storaging conditions. In this paper, the influence of relative humidity (R. H.) on abrasive quality was examined. The results are as follows: (1) The friction between an abrasive wheel and a specimen consists of sliding and rolling. As regards sliding friction which contributes most to the wear of specimen, abrasive grains scratched the specimen very slowly in almost straight line during contact. The velocity was given not by 2πr, but by 2πH mm/sec (Fig. 1). (2) The expressions were derived to relate pf, Wsp, Wgf, Fp and R. H. each other, where pf is the fracture probability of grains, Wsp the weight loss of specimen, Wgf the weight loss of grains by fracture and Fp the peel strength of applied glue. The expressions agreed well with the experiments (Eq.9∼15). (3) k(=dWsp/dWgf) represents a factor of abrasive quality. It is desirable that k takes a large value at the beginning of abrasion and keeps it as long as possible. Under high humidity conditions, the initial k was very small but became larger rapidly. In this case the paper was exhausted easily. (4) It was concluded that 65% R. H. specified in JIS Standard was too high to give reliable data, and 50% R. H. was preferable. When R. H. differs from this, the date must be corrected strictly. From this standpoint Taber Instruments Company also published the diagram for compensation, but some disagreements were recognized between the diagram and the results obtained in this experiment. More experiments are required for more accurate compensation.