Well Over 32 percent of the commercial air carrier fleet, worldwide, are beyond their original 20-year design life goal. In the past, 20-year old aircraft were most often replaced by newer aircraft for airline service. However, this is no longer true, and by the turn of the century, 64 percent of the current fleet will be at least 20 years old. This is because economic and market conditions have resulted in the use of commercial jet airplanes beyond their original economic design life objectives. As aircraft exceed their economic design life objectives, the incidence of fatigue increases and corrosion may become more widespread. The purpose of this paper is to discuss our experience with aging aircraft, the basic considerations of fracture mechanics treatment of cracks initiating at rivet holes (to say Multiple-Site Damage) and some problems due to fatigue cracks in aircraft components from the operators viewpoint.
Ball impact tests were carried out to evaluate the effect of static initial stress field on the foreign object damage of ceramics. Two kinds of balls, steel and Si3N4, the size of which was 3mm in diameter were used as the projectiles, and five kinds of SiC and Si3N4 ceramics were used as the target specimens of 7.5×10×85mm size. After the impact tests, three point bending tests were carried out to evaluate the residual strength of the specimens, and the difference in failure with ball contact was discussed taking the results of numerical analyses into consideration. The results obtained are summarized as follows; (1) From the impact tests, the critical condition to produce the effect of initial stress on foreign object damage became clear. (2) From the numerical impact fracture analyses, it was found that the effect of residual tensile stress was caused by an increase in compressive permanent deformation in the target at impact due to the initially applied static load. (3) The mechanical property of the target material subjected to compressive load is an important parameter to evaluate the foreign object damage of Si3N4 ceramics.
In the development of Fast Breeder Reactors (FBRs), structural integrity must be assured for components subjected to high temperatures up to 550°C, even though possible defects are presumed. Nonlinear fracture mechanics is one of the most effective approaches to evaluate ductile fracture behavior of cracked components. In this study, ductile fracture tests were conducted at room temperature and 550°C for austenitic stainless steel SUS304 and 316FR, which were candidates for FBR structural material. The applicability of fracture parameters was investigated from tests using small CT specimens, small CCT specimens, and wide CCT specimens. Fracture tests under the condition of combined tension and bending loads were also performed to investigate the effect of additional bending stress due to the temperature gradient through thickness. It was ascertained that fracture load could be predicted based on the net section collapse criterion and was not so affected by an additional bending stress.
In our previous study, the mixed mode fracture criterion was established for an interface crack by using aluminum/epoxy circular disk specimens (Brazil-Nut-Sandwich) under compression or tension loading conditions. In the present study, four-point-bending tests are carried out under various loading conditions by using specimens made of the same dissimilar materials with the same adhesive conditions as before to compare with the previous results. The present results well consist with the previous results on the mixed mode fracture criterion for the interface fracture. It is confirmed that this criterion is valid independent of specimen shape, loading type and edge or internal interface crack.
A crack-tip field in particulate-reinforced composite which contains hard particles dispersed homogeneously in a ductile matrix has been analyzed taking account of damage evolution. In such a composite, debonding of particle-matrix interface is a significant damage, because the damaged particles have a weakening effect while the intact particles have a reinforcing effect. A new finite element method is developed based on Tohgo-Chou's constitutive relation which describes the elastic-plastic behavior and the damage behavior of particulate-reinforced composites. In this constitutive relation, it is assumed that the debonding damage is controlled by the stress of the particle and the statistical behavior of particle-matrix interfacial strength, the debonded (damaged) particles are regarded as voids, and a void volume fraction increases with deformation. Analyses of stress/strain field and damage evolution around a crack tip in particulate-reinforced composites are carried out by the FEM. With an increase in stress intensity, the debonding damage spreads out, and the damaged zone is constructed ahead of the crack tip. This debonding damage drastically influences the distributions of macroscopic and microscopic stress/strain fields around the crack tip.
Effects of high humidity and high temperature exposure on polycarbonate (PC) and cellulose-acetate (CA) were investigated. After the specimens were exposed under 85°C 40% or 85°C 85% for 8 days, tension tests and fracture toughness tests were carried out. The fracture behavior of the specimens was studied by fractography using a video microscope. The results were summarized as the tensile strength-failure strain diagram and the fracture toughness-crack extension resistance diagram. The failure strain decreased because a mechanical scratch at the corner of the specimen had grown during the exposure. Specimens which were finshed smoothly after exposure showed an increase in failure strain. The appearance of fracture surface in CA specimens was influenced by the location of initial defects which causes them fracture. The change in tensile strength of PC was small while that of CA decreased with the increase of exposure humidity. For the case of PC, the exposure increased the fracture toughness but diminished the crack extension resistance. For the case of CA, the exposure did not so much influence the fracture toughness while it diminished the crack extension resistance significantly. The difference in fracture behavior between two materials was discussed by use of the crack extended model in an ideally plastic solid.
Propagation Characteristics of rail web-cracking has been discussed in connection with residual stress in rails. To clarify the effect of residual stress on rail web-cracking, several testing methods for rail web-cracking were examined first and a DWWT (Drop Weight Wedge Test) method with high accuracy and good reproducibility was newly proposed. Secondly, by using this test method, the rail web-cracking properties of several kinds of rails were evaluated. DHH rails and NHH rail which were hardened by heat-treatment showed better web-cracking properties than the standard carbon rail and low alloy rail which were straightened by a roller straightenner. This is caused by the differeces in distribution and longitudinal residual stress between the head and the base in rails. Furthermore, the relationship between the propagation length of web-crack and the amount of opening value on cut mouth by saw cutting along the rail web was discussed.
Static and dynamic fracture toughness tests at three strain rates were performed on two ASTM A508Cl. 3 steels by using 1T-CT specimens and fatigue pre-cracked instrumented Charpy specimens. The KJC converted from JC of the small specimens indicated a wide scatter. When the strain rate increased, the fracture toughness transition curves shifted to a higher temperature region. An increase in strain rate reduced the scatter of KJC dramatically, especially in the high temperature region, and decreased the lower bound fracture toughness. Fractographic examination of the fractured specimen surfaces indicated that the relationships between KJC and the stable crack growth, Δa0, distance from stable crack front to trigger point, X, or distance from fatigue crack front to trigger point, Δa0+X, were expressible by a single curve, respectively. The scatter of KJC was caused by the varience of Δa0, X, and Δa0+X. With increasing strain rate, Δa0, X, and Δa0+X decreased significantly, leading to a small scatter of KJC. The KJC value at Δa0=0, X=0, and Δa0+X=0 was proposed as the lower bound fracture toughness of a steel and labeled KJCi. The shape of KJCi versus temperature curve was controlled by the critical stretch zone width, SZWc. The Weibull slope m of KJC became larger with increasing strain rate and decreasing temperature. Higher toughness data with larger stable crack extension than 100μm violated the linearity of Weibull plots. In the statistical approach to determine the lower bound fracture toughness in the transition region, much more analytical development is needed. The KJC value with 3% fracture probability coincided with the KJCi value in the low temperature region even in a high strain rate.
The effect of microstructure on the small fatigue crack growth behavior was studied in Ti-6Al-4V alloy, to which three kinds of heat treatments were conducted in beta field, in comparison with the long crack growth properties. The small crack opening-closing behavior relevant to microstructure was also investigated. The crack closing ratio exhibited a noteworthy behavior near crystallographic orientation boundaries: the ratio gradually increased as the crack tip approached to the grain boundary, and decreased after passing through. This experimental result indicates that the change of the crack closure is one of the important factors which lead to the characteristic behavior of small crack growth near grain boundaries. It was shown from these experimental evidences that the microstructural influence on the small crack growth rate and the difference in growth rate between small and long cracks were intrinsic and not compensated only by the crack closure phenomenon.
The fatigue strength of ductile cast irons is strongly influenced by small defects such as graphite, casting defects, etc. In order to investigate quantitatively the effects of these small defects, rotating bending fatigue tests of specimens containing an artificial small defect (hole or notch) as well as smooth specimens were performed on FCD400 and FCD700. The effect of the interaction between cracks or between a crack and a hole was discussed based on fracture mechanics. The microscopic observation of the surface and interior of specimens revealed that the fatigue limit is the threshold stress for propagation of a small crack emanating from a graphite or an artificial defect. Graphites distributed in the structure of current high-quality ductile cast irons are well separated so that the fatigue limit can be modelled regardless of interaction of graphites. Consequently, the fatigue limit is predicted by the Murakami and Endo's equation, which has been applied to many fatigue problems of metals containing a single defect or crack, in terms of √area for a maximum graphite or defect and the Vickers hardness Hv of matrix.
Reversed plane bending fatigue tests were conducted on extruded Al-15Zn P/M (Powder Metallurgy) aluminum alloy using two types of specimens with different notch dimensions. The initiation and growth of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail. A large difference in crack initiation strength was observed between the extruded (L-C orientation) and transverse directions (C-C orientation). It was found that the strength of shallow notched specimen, when evaluated by true stress at the notch root, was lower than that of deep notched one. If the stress intensity range of inclusion at which the fatigue crack initiated was calculated, the difference in fatigue strength depending on specimen orientation and notch size could be eliminated. And also, a prediction method of fatigue strength in smooth specimen was proposed on the basis of the extreme value statistics. The macroscopic fatigue crack growth rate could be expressed by the Paris equation in terms of the maximum stress intensity factor irrespective of stress level and the notch size, and the growth rate was found faster in C-C orientation specimen than that in L-C orientation specimen.
The effects of loading frequency and test temperature on the delamination crack growth under cyclic loading were investigated with unidirectional CF/epoxy laminates. Tests were conducted in air on double cantilever beam (DCB) specimens. Constant ΔK test were employed for the experiments. The interlaminar crack growth rate was almost constant with crack growth, and the effect of loading frequency was not observed. When the test temperature was changed from lower to higher temperature, the interlaminar crack growth rate was unaltered. On the other hand, the interlaminar crack growth rate drastically decreased after the variation from higher to lower test temperature. The intralaminar crack growth rate decreased with crack growth, and it was higher for higher temperature. In this case also the effect of loading frequency was not observed. The growth rate of an interlaminar fatigue crack was always higher than that of an intralaminar fatigue crack when they were compared at the same value of ΔK.
In order to measure the Mode II fatigue crack growth resistance of SS41 steel and a rail steel, a particular double-cantilever type specimen with side grooves was designed. Mode II fatigue crack growth tests by using the newly designed specimens were conducted by a conventional closed-loop type tension-compression fatigue testing machine under a constant load amplitude which corresponds to ΔKII-decreasing test with increase in crack length. The crack length was measured by the AC potential method. Mode II crack growth rate gradually decreased as the crack grew. The typical fracture surface of Mode II fatigue crack growth which was quite different from Mode I fatigue fracture surface was observed. But the trace of local branching of crack growth mode from Mode II to I was observed on the overall fracture surface of Mode II. The relationship between Mode II crack growth rate (da/dN) and ΔKII was obtained.
An alternating method is developed for the interaction analysis of arbitrary distributed multiple elliptical cracks. The completely analytical solutions (VNA solutions) for a single elliptical crack in an infinite solid, subject to arbitrary crack-face traction, are implemented in the present alternating method, together with the coordinate transformations for stress tensor. Since only the VNA solutions of individual cracks are used as a basic solution of the Schwartz-Neumann alternating technique, the present method leads to a highly accurate and efficient numerical tool for routine evaluation of the stress intensity factors for interacting multiple cracks. Numerical demonstrations are also given for interaction problems of several types.
Three dimensional elastic-plastic analyses are conducted by FEM for several kinds of surface cracked problems. They are: (a) CT specimen with stationary cracks, (b) CT specimen during stable crack growth, (c) surface cracked plates with different aspect ratios subjected to three point bending, (d) a pipe with an inner surface crack subjected to four point bending, and (e) CT- and surface cracked-specimens made of welded plates. The crack tip stress fields are compared with HRR solutions, and the effects of the specimen shape, loading condition and fusion line on the crack tip singular fields are studied. The Q factor, which is defined as the difference of the crack tip field from the HRR solution, is evaluated based on these analyses. It is shown that the crack tip fields deviate from the HRR solution gradually due to the increse of the elastic-plastic deformation. It is also found that the Q factor expressed the stress triaxiality at the crack tip well, and it has a close relation with the dimple fracture of the specimen. In the welded plate, Q factor controls the behavior of stable crack growth locally near the fusion line, though the J integral controls the entire behavior of stable crack growth.
Non-linear fracture mechanics is one of the most effective approaches for the integrity assessment of high temperature components. J-integral is a feasible parameter to represent fracture behavior in the non-linear regime. However, there are many difficulties in calculating an accurate value of J-integral in actual components. Some simplified methods have been proposed to estimate the J-integral, and applicability of those methods has been recognized. Although J-estimation scheme based on the reference stress method has an advantage to be applicable to many problems, the method has not been established yet. This paper presents a detailed procedure and applications for ductile fracture, creep-fatigue inhomogeneous and three-dimensional problems.
This paper discribes a new strain/displacement measuring SSDG system by using laser speckles. An emphasis was put on the development of a real time non-contact system which could be applicable to the measurements of a local strain and a crack opening displacement (COD) around crack tips both at room and elevated temperatures. A conventional double sensor SSDG system, in which two image sensors are required to eliminate the rigid motion of samples, was first discussed. A single sensor SSDG system was newly proposed which was applicable to the COD measurements. A real time processing of speckle movements was realized by an originally developed software. The applicability of the single sensor system was successfully tested in the COD measurements of cracks in ceramics and in the closure measurements of small cracks in steels under cyclic thermal stressing at elevated temperatures. Furthermore an application of this system to a crack tip deformation measurement under mixed loading of Mode I and II was included.
Slip bands formed at the low strain region ranging from about 0.005 to 2% in a polycrystalline Cu-30 mass% Zn alloy were investigated by optical microscopy and crystallographic orientation analysis using an SEM-ECP method. Many grains exhibited slip bands caused by the primary slip system at about 0.005% strain. Some grains in which the secondary slip system had been activated were observed at this stage of plastic deformation. Most of these grains produced slip bands due to the primary slip system with further plastic deformation. A model of Hashimoto and Margolin based on the elastic incompatibility at a grain boundary could explain the secondary slip system becoming active in only about 20% of the grains in which this slip system had been activated initially. In about 60% of such grains, when the secondary slip system produced more similar shape change to that of the surrounding grains than the primary slip system did, the secondary slip system was understood to become preferentially active, taking account of the two-dimensional surface shape change after small plastic deformation.
The effects of strain rate and strain on the structure of bonding interface and the behavior of surface oxide film during a superplastic forming and diffusion bonding processing have been investigated in the process of making laminates of 5083 aluminum alloy foils with conversion coating. It is likely that surface oxide films were broken at the intersections of grain boundary and foil surface, and that grains behind foil surface moved to the bonding interface by grain boundary sliding to contact with grains in the other foil which appeared at the bonding interface by the same way. To get a contact without any flaw between them, a certain surface pressure was required. At a strain rate of 6×10-4s-1, some flaws existed around oxide segments because of low flow stress, whereas at 2×10-2s-1, no flaw was found because of higher flow stress. In the case of large grain size caused by strain-induced grain growth observed at a low strain rate of 6×10-4s-1, the oxide segments were longer and waviness of the bonding interface was increased. It is noted that there were many oxide segments with double layers even after large deformation, which probably have low bonding strength.
In the present study, impact fracture patterns of triangular plates subjected to an impulsive load are investigated by means of experiments and numerical simulations. The specimens of triangular plate are made of plaster and have several height/base ratios (apex angles). The center of the free base edge of a triangular plate with uniform thickness is subjected to an impulsive load in the in-plane direction, while the sloping side edges are free from stress. The impulsive load is given by an input bar colliding with a striker shot out from a compression air gun. The feature of fracture pattens occurred in the triangular plates are discussed in relation to the effect of the apex angle and the duration of an incident stress into the plates. Numerical analyses for a few fracture patterns are carried out based on a dynamic finite element method using the Newmark-β method. Mechanism of the impact fracture initiation and development in triangure plates is made clear by comparing the experimental results with the numerical simulations. It is found that the origin of fracture coincides well with the region of dynamic tensile stress concentration due to stress wave reflection and reinforcement from the base edge and both sloping sides of the plate. The formation of the region of dynamic tensile stress concentration is strongly governed by the dimensions and apex angle of triangular plates and the duration of an incident stress.
Fiber reinforced metal matrix composite materials which consist of the high strength ductile steel wire and aluminium or titanium foil sheets were produced by explosive bonding method. Strength property was evaluated by a tensile test. For the aluminium matrix composite, the tensile strength was 80% of the value predicted by the law of mixture and fracture initiated at the weakly bonded area between the fiber and the matrix. For the titanium matrix composite, the tensile strength was in agreement with the value predicted by the law of mixture and fracture initiated at the originally fused interface between the sheets. A cup and cone type of fracture was observed in the fiber of aluminium matrix composite, and a cup and cone and shear type of fracture were observed in the fiber of titanium matrix composite.
In this paper, the hybrid effect on the flexural behavior of pitch/PAN hybrid unidirectional CFRP was theoretically and experimentally studied according to laminate constitution. First, the stress-strain curves of pitch based unidirectional CFRP (XN40/25CH) and PAN based unidirectional CFRP (T300/25CL) were examined. These stress-strain curves were obtained by tension and 4-point bending tests. Secondly, the hybrid effect of the flexural behavior predicted by the laminate beam theory was confirmed by the experiments. The results are as follows. (1) Both slopes of the stress-strain curves of XN40/25CH and T300/25CL become steeper as the tensile strain increases. They become gradual as the compressive strain increases, and the compressive stresses approach a constant value. So, these CFRP's stress-strain curves indicate non-linear behavior. The constant value of compressive stress for XN40/25CL is nearly half of T300/25CH. (2) The hybrid CFRP of XN40/25CH laminated with T300/25CL on the compressive side shows a little decrease in elastic modulus and large increase in flexural strength compared with the original XN40/25CH. This is a hybrid effect on the flexural behavior.
Small fatigue crack growth tests in 3%NaCl solution were conducted on a solution-treated and aged material and two kinds of annealed materials of Ti-6Al-4V alloy. The crack growth rate in 3%NaCl solution was enhanced above a certain value of δK compared with that in air. The difference in crack growth rate between small and long cracks was not observed for all the microstructures studied, and so no specific chemical behavior appeared in short cracks. The effect of specimen orientation on crack growth was not found in air, while in 3%NaCl solution the crack growth rate in T-orientation for which load was applied normal to the rolling direction was faster than that in L-orientation. From fractographic observations, cleavage fracture was observed remarkably with increasing crack growth rate.
Since low alloy steels SNCM are being used widely for machines and structures operated under extremely severe service conditions, it is important to evaluate their fatigue lives. Although many fatigue data sheets for SNCM have been published, they exhibit large scatter and it seems to be difficult to predict directly the fatigue life from these raw data. In this study, by using four kinds of plain specimens of a low alloy steel (SNCM439), which were subjected to different heat treatments and thus have different strength levels, rotating bending fatigue tests were carried out. The crack initiation and small crack growth behaviors were investigated by the plastic replica technique. By analyzing the fatigue data, a small-crack growth law for SNCM was examined, and then an effective and convenient method based only on tensile strength was presented for estimating the fatigue life of plain members. The validity of this method was checked by applying it to the data sheets published by the National Research Institute for Metals.
The effect of microstructure on the macroscopic low-cycle-fatigue crack growth rate was examined on ductile cast iron with the same chemical compositions but different microstructures, ie., ferritic (FDI), pearlitic (PDI) and austempered (ADI) structures. The crack growth rate was evaluated by measuring crack closure behavior. FDI showed the fastest crack growth rate, irrespective of the assessing fracture mechanics parameter of δK or δKeff. The crack opening ratio was constant in PDI and ADI, irrespective of the crack growth stage, while it suddenly increased in FDI when the value of Kmax exceeded a certain value. Each material microscopically indicated zigzag-like crack growth caused by the effect of spheroidal graphite. In particular, crack branching, which retarded the crack growth, was often observed in ADI. Each material showed no discernible difference in appearance and roughness of fracture surface at the main stage of crack growth.
This paper presents a new method using Monte Carlo simulation to estimate the life distribution of fatigue crack propagation on the basis of crack length vs. striation-spacing data measured by a SEM on a fatigue fracture surface. This method considers the distributions of two parameter estimates of a regression line and the correlation of the two parameter estimates. One cycle of the Monte Carlo scheme generates a set of estimates for the two parameters and they give a life of fatigue crack propagation. For an example, this study takes up the striation-spacing data measured on the fatigue fracture surface of the rod end housing of a hydraulic actuator used for a main landing gear in transport aircraft. The proposed method and the other two methods, which use a deterministic slope and the distribution of intercept estimates in a linear regression model, provide the distribution of fatigue crack propagation. The calculated results are discussed and compared. The proposed method approximately predicts the fatigue life of the rod end housing as the B allowable life when the initial crack length is assumed to be zero mm.
The cryostat for MHD propulsion ship must be equipped with the supports in the vacuum layer for the purpose of transmitting electro-magnetic force from superconducting coils to the outer vessel. These supports also must have the adiabatic properties such as preventing the heat transfer to superconducting coils. Then the adiabatic supports which are made of syntactic foam or FRP cylindrical shells are proposed. This report describes the thermal and mechanical properties of these supports at cryogenic temperatures. The following results are obtained. (1) The syntactic foam has a comparatively large value of overall heat transmission because its cross-sectional area for heat conduction is large. On the other hand, the cylindrical shells show a smaller value of overall heat transmission as the number of layers of the cylindrical shells increases, so that they are useful for the adiabatic supports. (2) The thermal contraction of the cylindrical shells is smaller than that of the syntactic foam. Even though the number of layers of the cylindrical shells increases, the thermal contraction remains about constant.
The high temperature oxidation behavior of pressureless-sintered β-SiC samples containing 0.3wt%B and 1.8wt%C as sintering aids and small amounts of impurities were examined at 1400-1800K for 15h in Ar-O2 atmosphere. The oxidation reaction was followed continuously by measuring the evolved CO2 using a quadrupole mass spectrometer. The weight changes of the samples before and after the oxidation were measured and their oxidized surfaces were analyzed by SEM and ESCA. The oxidation behavior of β-SiC with B and C additives obeyed a parabolic kinetics. The amount of CO2 evolved, and the weight gain of the bodies increased with decreasing temperature. These results indicated that the crystallization of the silica layer formed on the surface of β-SiC occurred at high temperatures.
In works of rock mass excavation such as tunnel constructions etc., the values and directions of initial stresses in the ground and the elastic constants of rock mass are usually decided by back analysis using in-situ measuring data for estimating stability of caverns. The present paper shows a highly accurate analytical method utilizing complex variable method and point matching technique for the problems of two neighboring arbitrary shaped tunnels in an isotropic elastic ground under 2-dimensional in-plane loading conditions, and describes the theoretical process of back analysis using this solution. The complex variable method with conformal mapping transformation is a very useful analytical technique for tunnel excavation problems, because stresses and displacements around a tunnel can easily be obtained as exact closed-form solutions of elasticity. Therefore, it is possible to remove some restrictions on calculating time and computer capacity compared with FEM or BEM. The usefulness of this method is additionally shown through some numerical examples in this paper.