Fatigue Rupture Research Articles

Role of rotational deformations in the formation of spherical particles in a fatigue fracture

Investigations of the fractures of specimens of VT3-1 and VT8 titanium alloys and AVT and D1T aluminum alloys under steadt and nonsteady loading conditions indicate that in the near-threshold area and in the case of partial stoppage of a crack after overloading the processes of transverse (K /SUB III/ ) slip play a significant role in failure. The authors consider a plan of deformation of a material in the elastic area associated with loading of the specimen in compression and tension at a distance from the crack tip and in a crosspice between the microtunnels. It was concluded that the formation of spherical particles must be related to microtunneling of a fatigue crack and rupture of the crosspices between the microtunnels in rotational plastic deformation of the materials, ''nondulizing'', fragmentation of the particles with heating, and subsequent rolling of the particles between the free surfaces of the crack edges.

The effects of sea water and concentrated salt solutions on the fatigue of nylon 6,6 fibres

Cyclic fatigue and creep rupture tests have been run on high-tenacity nylon 6,6 single fibres, yarns and small ropes in air and sea water environments. Fatigue failure in each case is by a creep rupture mechanism; yarns and small ropes show the same fatigue sensitivity as do single fibres. Sea water reduces the strength by approximately 10% under most conditions. Concentrated metallic salt solutions which cause environmental stress cracking in bulk nylon do not degrade the fibres beyond the effect of plain water. Tests on oriented nylon specimens show that environmental stress crack sensitivity is greatly reduced by orientation.

On the Disintegration of Ice Shelves: The Role of Fracture

AbstractCrevasses can be ignored in studying the dynamics of most glaciers because they are only about 20 m deep, a small fraction of ice thickness. In ice shelves, however, surface crevasses 20 m deep often reach sea-level and bottom crevasses can move upward to sea-level (Clough, 1974; Weertman, 1980). The ice shelf is fractured completely through if surface and basal crevasses meet (Barrett, 1975; Hughes, 1979). This is especially likely if surface melt water fills surface crevasses (Weertman, 1973; Pfeffer, 1982; Fastook and Schmidt, 1982). Fracture may therefore play an important role in the disintegration of ice shelves. Two fracture criteria which can be evaluated experimentally and applied to ice shelves, are presented. Fracture is then examined for the general strain field of an ice shelf and for local strain fields caused by shear rupture alongside ice streams entering the ice shelf, fatigue rupture along ice shelf grounding lines, and buckling up-stream from ice rises. The effect of these fracture patterns on the stability of Antarctic ice shelves and the West Antarctic ice sheet is then discussed.

On the Disintegration of Ice Shelves: The Role of Fracture

AbstractCrevasses can be ignored in studying the dynamics of most glaciers because they are only about 20 m deep, a small fraction of ice thickness. In ice shelves, however, surface crevasses 20 m deep often reach sea-level and bottom crevasses can move upward to sea-level (Clough, 1974; Weertman, 1980). The ice shelf is fractured completely through if surface and basal crevasses meet (Barrett, 1975; Hughes, 1979). This is especially likely if surface melt water fills surface crevasses (Weertman, 1973; Pfeffer, 1982; Fastook and Schmidt, 1982). Fracture may therefore play an important role in the disintegration of ice shelves. Two fracture criteria which can be evaluated experimentally and applied to ice shelves, are presented. Fracture is then examined for the general strain field of an ice shelf and for local strain fields caused by shear rupture alongside ice streams entering the ice shelf, fatigue rupture along ice shelf grounding lines, and buckling up-stream from ice rises. The effect of these fracture patterns on the stability of Antarctic ice shelves and the West Antarctic ice sheet is then discussed.

Reliability of Offshore Structures

This paper discusses current practice for protection against potential failure sources. Variability, in loadings and in the strength of structural elements typical for offshore installations is discussed in terms of statistical properties. A reliability analysis based on modern probabilistic methods is conducted. The reliability, of different types probabilistic methods is conducted. The reliability, of different types of structures is compared according to load type, structural elements, materials, and design codes. Introduction The need for risk evaluation and reliability criteria today is gaining recognition in the offshore industry. Ideally, such criteria should be quantitative and should provide for unambiguous interpretation of safety targets and standards.Probabilistic reliability is the only meaningful concept that can be used to obtain a logical and objective distribution of risks and safety requirements. Probabilistic reliability as a tool specifies criteria for how safe offshore engineering equipment should be and for evaluating design and operational requirements from probabilistic criteria for the installation as a whole. The mathematics and concepts used for probabilistic calculations are defined in the Appendix. Failure Sources Failure sources of an offshore structure may be associated with design, choice or use of material, workmanship and maintenance, and operation of the structure. Failures are associated with accidental noncompliance with written or unwritten criteria for safe design, construction, or operation. This noncompliance may be caused by errors that may be classified conveniently as gross errors (blunders), systematic errors, and random errors, as classified by Gauss 150 years ago.Examples of gross errors are decimal points misplaced in the analysis, using low-grade steel where high-grade steel is specified, etc. All gross errors can be eliminated with adequate care. But no design safety factor, however large, will give adequate protection against the effect of gross errors.The only way to eliminate gross errors is with extensive supervision and inspection of all activities involved in the process of design, construction, maintenance, and operation of the structure. A reliable inspection must be based on the contractor's own supervision and control of his activities. In addition, the quality of the control has to be inspected by some independent authority conducting design evaluation and survey of the construction and operation. The expenditure of time and money for these activities contributes more to structural safety than large safety factors.Systematic errors are associated with causes that contribute to deviations on one side. These errors may be associated with empirical relationships between visual wave records compared with significant wave heights, recognized design formulas based on incomplete theory or experiment, strength properties determined by distorting recording devices, etc.Systematic errors associated with ignorance of real physical phenomena may result in fatal structural physical phenomena may result in fatal structural failures. This type of error might be feared especially in connection with rapid technological developments beyond current experience. Examples are the suspension bridge failures caused by ignorance of dynamic effects in the late 1930's and the fatigue ruptures of the Comet airplanes in the 1950's. In this context, ignorance errors are classified as gross errors, and for this reason the inspection authority must have superior scientific competence to discover them. JPT P. 1486

On low- and high-cycle fatigue ruptures at the same load in Ti-6Al-4V

Abstract A series of measurements was made to obtain bending fatigue strength curves of commercial Ti-6Al-4V alloy at different temperatures, using notched and unnotched specimens with random and sinusoidal excitation. The results of sinusoidal fatigue tests show a gap at about 106 cycles between short and long fatigue life, which is explained by brittle nucleation at low cycles originating at preexisting microcavities on the surface caused by removal of crystals; the results also show ductility at high cycles. The nucleation is always brittle with random excitation. In all cases crack propagation is intergranular and takes place to a different extent in both brittle and ductile zones of the specimens.

Time-temperature relationship for long-term and fatigue strength of some partially crystalline polymers

1. The investigation was concerned with the time-temperature relationship for the long-term strength of PEVP. The maximum test length was 1400 h. 2. It was shown that the use of the Manson—Haferd parametric equation and the Kovpak physical similarity method in processing experimental data produced better results than other relations. 3. The observed deviations of experimental data from the parametric relationship observed at medium loads may be attributed to the changes in the mutual relationship of the two steps of the fatigue rupture of PEVP, to the reduction (increase) in the incubation period of crack formation, and to the changing nature of the fracture. 4. In the long-term strength curve characterizing the brittle fracture, two sections were observed which can be attributed to the existence of two mechanisms of rupture of PEVP in this region. 5. For the phenomenological description of two fatigue rupture stages of PEVP two scales of vulnerability were introduced which, in contrast to the theory of cumulative damage, had made possible the consideration of the effect of loading history.

Study on the regularities of the transition from quasistatic to fatigue rupture of light alloys under a few load cycles

Study on the regularities of the transition from quasistatic to fatigue rupture of light alloys under a few load cycles

ELECTRON MICROSCOPIC OBSERVATION OF THE FATIGUE RUPTURE OF NYLON 6•10 FILAMENTS

The fatigue rupture of drawn and undrawn filaments and films of nylon 6•10, especially its relationship with the molecular orientation has been investigated. Flexural and extensional fatigue tests were carried out with the rolled filaments and drawn films in which a cut was introduced with razor perpendicular to the filament axis or draw direction. The fracture surfaces were observed with the optical and electron microscope.In the unoriented specimens crack propagate perpendicular to the direction of stress and striations corresponding to the stress periods are observed on the fractured surfaces. But in the oriented specimens crack propagates to the directions inclined at about 19_??_25 degrees to the fiber axis by the cyclic extension. These cracks propagating obliquely in the drawn specimens may be caused by the shear stress acting parallel to the fiber axis at the crack tip.

Fatigue of aluminum by uniaxially repeated tensile load

Measurements of deformation and internal friction and optical microscopic observation were made on commercial pure aluminum specimens when they were under various uniaxially repeated tensile loads.The uniaxially repeated tensile load was cyclically given by a loading machine with an eccentric cam. It was given by pulling the spring to the stroke of 10mm and various loads were given by the exchange of the spring.The relation between amplitude and repeated number to rupture was nearly identical with the S-N relation in ordinary push-pull fatigue test. Two types of rupture were observed under the uniaxial repeated tensile stress conditions. One was a ductile rupture which gave a large local contraction, but the other would be a fatigue rupture which did not give a so large local contraction. The former ductile rupture occurred under the conditions of higher stress and its total elongation attained to about 5060%. Whereas, the latter fatigue rupture occurred under the conditions of lower stress and its total elongation attained to about 20%. Slip band striations were sharpl yproduced under lower stress conditions and clear caterpillar patterns to be characterized by fatigue rupture also appeared on the fracture. They were much different from those produced in the structure under the conditions of higher stress or tensile test.The change in amplitude-independent internal friction with total plastic strain under various stress conditions was plainly increased with the increase of total plastic strain.

Strength Properties of Glass Fiber Reinforced Polybenzothiazole Composites

A processable, oxidatively stable polybenzothiazole (PBT) resin has been developed for use as a matrix in reinforced plastic struc tural composites subjected to elevated temperature. The complex chemistry of the resin polymerization reaction is presented. Composites using the new resin as matrix and Style 181 E glass cloth as reinforcement have been evaluated in flexure, tensile stress rupture and bending fatigue at temperatures ranging from ambient to 800°F. These composites have exhibited excellent retention of flexural properties after 300 hours heat aging at 600°F and six hours at 800°F in air. Bending fatigue and tensile stress rupture properties at 550°F are superior to those available from other high temperature resin composites, such as polyimides and polybenzimidazoles.

Fatigue in Textile Fibers

Samples of polyester and nylon 66 fibers have been fatigued by cyclic longitudinal tension, each sample at two strokes, at various temperatures from 21° to 90°C. In the polyester, it was found that, in general, at both 4.5 and 5.9% stroke, average logarithmic lifetimes increase with temperature. Straight lines, fitted to the experimental points by the method of least squares and representing logarithmic lifetime as a function of reciprocal temperature, were observed to have negative slopes, the slope at the 5.9% stroke having the larger (absolute) negative value. In the nylon, it was found that, with a 7.2% stroke, logarithmic lifetimes tend to decrease with increasing temperatures, i.e., the slope of the least-squares line for logarithmic lifetime vs 1/ T has a positive value. At a higher stroke (8.1%), for the nylon fibers, the converse is true. Thus —generalizing—in both samples the slope of the curve for logarithmic lifetime vs 1/T decreases with increasing stroke. Correspondingly, energies for the fatigue rupture process, calculated according to the Prevorsek-Lyons theory, were found to be negative for both tests on the polyester and for one on the nylon sample, with the 8.1% stroke. A small positive energy was calculated for the nylon at the 7.2% stroke. These results were, confirmed by the energies calculated from terminal growth rates measured on the polyester at the 5.9% stroke and on the nylon sample at the 7.2% stroke. It is concluded that the energy involved in fatigue rupture is identical with that related to growth during cyclic loading.

Surface Strain Distributions of Rubber Cylinders under Shear or Compressive Shear

Surface strain distributions of rubber cylinders with various shape factors and fillet radii are measured and the relation between these distributions and fatigue rupture is considered. The obtained results are as follows : (1) In the axial strain distribution of the filletted specimen, there exist strain regions of tension and compression and also the strain concentration in both fillets. (2) The maximum surface strain is greater for smaller fillet radius and for larger shape factor but the position of concentration is scarcely influenced by the shape of specimen. (3) In the case of compressive shear, the tensile strain region decreases with the increase of compression and finally wrinkles appear on the compressive side of fillet. (4) The fatigue characteristics of rubber is explained qualitatively by so-called maximum shearing strain theory.

1105 防水下地のキレツ部におけるアスファルト防水層の疲労破断に関する基礎的研究(その1)(材料・施工)

1105 防水下地のキレツ部におけるアスファルト防水層の疲労破断に関する基礎的研究(その1)(材料・施工)

Fatigue Fracture in Fibrous Polymers as a Brittle, Crack-Nucleation Process

An interpretative analysis of experimental results on the failure of synthetic fibers in fatigue under cyclic tension leads to the conclusion that rupture is of a brittle nature. Although molecular rearrangement undoubtedly takes place during longitudinal fatiguing, rupture itself cannot be regarded as a viscous, laminar flow process. Failure is localized; the undamaged sections of the specimens show relatively less important effects of the fatiguing. Lifetime cannot be predicted from measurements of fiber tenacity, modulus, or rate of creep. A theory of fatigue rupture, based on the observation of cracks in specimens strained to breakage, is formulated. The treatment involves the assumption that a crack grows by a nucleation process up to a critical size. At a slightly larger size, the crack becomes unstable and propagates spontaneously, to produce rupture. The theory leads to an expression for lifetime as a function of stress that is in substantial agreement with experimental data on a number of samples.

The Effects of Misalignment on the Forces Acting on the Retainer of Ball Bearings

The forces acting on the retainers of rotating radial ball bearings were measured by wire strain gauge. The bearings were operated under radial and/or thrust load with misalignment, that is, relative inclination between inner ring and outer ring, and also under pure moment load. The mechanism of generating the forces is analyzed theoretically, and the cause of seizure or fatigue rupture of the retainers under such conditions is made clear.

Strength of quenched tool steels under cyclic loading

1. We demonstrated the conditional nature of the socalled fatigue limit for quenched tool steels. The fatigue curves confirm the fact that in the steels there is a typical continuous dependence (500–1000 million cycles) between fatigue strength and time (number of cycles) over which the load is applied. Ten million cycles cannot be taken as a test basis for quenched steel. 2. Tests in which the bend-cyclic effect curve is plotted show that the mechanism of fatigue rupture in quenched steels is different from the failure of viscous materials.

Fatigue in Textile Fibers

The fatigue lifetime of an acrylic sample was found to be expressible by the equation: L = a v ", where v is frequency, a is a function of stroke, and n is 0.72. The parameter a was found to decrease with an increase in stroke. The results are interpretable in terms of an elementary, molecular fatigue process. It is postulated that fatigue rupture occurs when a critical level of extension is reached. A study of the residual mechanical properties of a viscose sample fatigued at various lengths was inconclusive with respect to whether fatigue failure results from the aggravation of weak spots or from over-all deterioration of the fiber substance. Fatiguing the acrylic sample at different frequencies and for various periods of time was found to produce only insignificant differences in residual rupture properties among the fatigued fibers. The properties of these as a group, however, were found to be appreciably different from those of unfatigued fibers. Microscopic examination of viscose fibers broken as a result of fatiguing showed the rupture surfaces to be essentially transverse planes, with no indication of fibrillation. X-ray diffraction data on fatigued nylon fibers revealed an axial lengthening of crystal lites of the material.

Material for making and heat treating crankshafts for high-speed diesel engines

1. The use of high-alloy 18KhNVA steel for manufacturing crankshafts is not advisable: a) the high sensitivity of high-strength steel to stress concentration means frequent fatigue rupture during operation; b) reduction in the strength of the shaft metal through increase in residual austenite cannot be recommended since it involves a reduction in the stability of the shaft (heating during grinding or cooling below 0° may cause the austenite to change into martensite and warp the shaft). 2. The manufacture of engine crankshafts from loweralloyed steels would bring about a considerable saving in alloying elements and improve the machinability of the parts. A sorbite structure is best for parts working under cyclic stress.

A fatigue-free silicon device structure

Germanium devices, when correctly used, are capable of essentially infinite life. It was commonly assumed in early higher temperature semiconductor work that silicon devices and those of materials, such as silicon carbide and III-V compounds would have similar life expectancy. That this is not realized in soft-soldered silicon devices is now well established. Evidence is presented to show that this limitation in life is due to fatigue rupture of the soft-soldered joints when they are subjected to cyclic thermal stress. A new hard-soldered structure is described which has been proved by long experience to eliminate fatigue failure completely.