- Email: [email protected]
Systems reliability of offshore structures including fatigue and extreme wave loading
not give an accurate picture of the performance in situations where cyclic loading is experienced in corrosive environments, that is where corrosion fatigue can occur. Graphs, photomicrographs. 13 refs.
T r a n s i e n t retardations in fatigue crack growth following s single-peek oveHoed. Damri, D. and Knott, J. F. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 709-719 Retardation in the fatigue crack growth rate following the application of a single peak overload in a fatigue loading sequence has been studied for a Iow-cerbon structural steel. Tests have been performed at load ratios of R = 0.2 and R = 0.6 at a baseline stress intensity range, AKb, corresponding to fatigue crack growth rates in the Paris regime. Single-peak overloads were applied st a crack length to specimen width ratio of (~/W= 0.5. At the load ratio of R = 0.6 monotonic or 'static' fracture modes were observed upon application of the overload, and these produced an immediate increase in growth rate. A subsequent retardation is attributed to the presence of a residual compressive stress field ahead of the crack tip. A similar retardation was observed at a load ratio of 0.2. The importance of residual stress was established by performing stress relieving experiments. In addition, removal of the surface deformation after an overload by roaching 'T' sidegrooves resulted in an ex~ended transient, which could not be explained by residual machining stresses. Graphs, photomicrographs. 22 refs.
Environmental fatigue of an AI-Li-Cu alloy. I. Intrinsic crack propagation
kinetics in hydrogenous environments. Piascik, R.S. and Gangloff, R.P. Metall. Trans. A Oct. 1991 22A, (1), 2 4 1 5 - 2 4 2 8 Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak-aged AI-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant-~K and "Kmax loading. Such rates are equally unaffected by a vacuum, by purified helium or by oxygen, but are accelerated in order of decrasing effectiveness by aqueous 1% NaCI with anodic polarization, pure water vapour, moist air, and NaCI with cathodic polarization. While da/dN are proportional to ~/~ for the inert gases, for water vapour and chloride multiple-power laws and a transition growth rate 'plateau' are induced. Environmental effects are strongest at low values of ~K. Crack tip damage is ascribed to hydrogen embrittlement because of accelerated values of da/dN owing to levels of H20 of the order of ppm without condensation, impeded molecular flow model predictions of the measured water vapour pressure dependence of da/dN as affected by mean crack opening, the lack of an effect of film-forming 02, the likelihood for crack tip H production in NaCI, and the environmental and &Kprocess zone volume dependences of the microscopic cracking modes. For NaCt, the growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3 and upon cathodic polarization. These variables increase crack surface film stability to reduce H-entry efficiency. Small-crack effects are not observed for the 2090 alloy; such cracks do not grow at abnormally high rates in single grains or in NaCI, and ere not arrested at grain boundaries. The H environmental FCP resistance of the 2090 alloy is similar to those of other 2000 series alloys and is better than that of the 7075 alloy. Graphs, 86 refs.
Engineering components and structures Damping characteristics o f f l e x u r e l vibration for partially covered b e a m s w i t h constrained viscoelastic layers. Dewa, H., Okada, Y. and Nagai, B. JSME Int. J. Ser. III, J u n e 1991 34, (2), 2 1 0 - 2 1 7 A beam partially covered with a constrained viscoelastic layer shows superior damping capacity to that of a fully covered beam, depending on the beam geometry and the properties of the viscoelastic layer. This damping effect, the so-called partial cover effect, however, had not been examined in detail until now. The strain energy analysis of the three types of partially covered five-layered beam is carried OUt to explain the damping mechanism. The results of analysis for the centre-covered beam and the end-constrained beam indicate that the partial cover effect is caused by the shear strain energy dissipation at the outer edges of the viscoelastic layer, and also, such energy dissipation depends on the tensile stiffness of the constraining layer. The separately covered beam has a superior damping capacity to that of the centre-covered beam with the same cover ratio, owing to the energy dissipation of the inner edges of the viscoelastic layers. These damping characteristics of the three types of beam presented are confirmed by experimental results. Graphs. 14 refs. Rational method for fatigue design end inspection planning of o f f s h o r e structures. Skjong, R. and Torhuag, R. Marine Struct. 1991 4, (4), 3 8 1 - 4 0 6 This paper reviews the current usage of probabilistic fracture mechanics in fatigue assessment of offshore structures and summarizes the practical usage of probabilistic load and response analysis models used in conjunction with fracture mechanics models. These models are then used in conjunction with probabilistic models for inspection methods used in the offshore and marine industries to assess the probability of failure, derive optimum inspection plans, and derive optimum design considering the cost of design, cost of inspection, cost of repair and consequential costs of failure. Comments are made on the status of such reliability-based optimization techniques in design and on the need for further work. Some examples of use are demonstrated. Graphs. 43 refs. S y s t e m s r.eliabi!~f o f o f f s h o r e structures including f a t i g u e end e x t r e m e w a v e ,oeomg. ~aramchandani, A., Dalane, J.I. and Bjerager, P. Marine Struct. 1991 4, (4), 3 5 3 - 3 7 9 Traditionally, the focus of systems reliability analysis has been on estimating the reliability under either fatigue loading or extreme environmental loading. However, for many offshore structures, a combination of these two sources of risk may be more critical, for example, initial failures may occur in fatigue and subsequently collapse under an extreme wave. In this paper a probebilistic formulation for such a sequence of combined failures is presented. A formulation for a sequence of failures under an extreme wave, with realistic failure criteria for offshore structures, is also presented. With these formulations it is possible to account for both fatigue and extreme wave failures. An application of such a unified approach to a steeljacket platform is presented. It is found that, for an individual member, the most probable cause of failure is fatigue, but for overall structural failure, overload and a combination of fatigue and overload are more important. Graphs. 25 refs.
Int J Fatigue May 1992
A n improved fatigue analysis for o f f s h o r e structures. Karadeniz, H. Marine Struct. 1991 4, (4), 3 3 3 - 3 5 2 This paper presents a spectral fatigue damage calculation on offshore structures subjected to non-narrow-banded Gaussian stress processes. The calculation is based on use of the Palmgren-Miner's rule and a multilinear S-N fatigue model. In the calculation, an equivalent stress range, which is defined as dependent on the absolute value of stress maxima and a parameter, is used. This parameter is calculated in terms of the spectral bandwidth measure of the stress process and the slope of the S-N line used. For this purpose, the rainfiow cycle counting algorithm and the Monte-Carlo procedure are used to determine stress ranges and cycles from time simulations of a number of stress spectra of a monopod structure. A relation between the equivalent stress range parameter and the well known damage correction factor is constructed. The cumulative damage arising during a high sea state and over a long period of time is presented. Graphs. 13 refs. Uncertainties in t h e f a t i g u e design of offshore steel structures. Pittaluga,
A., Cazzulo, R. and Romeo, P. Marine Struct. 1991 4, (4), 3 1 7 - 3 3 2 Fatigue is one of the failure mechanisms of offshore steel structures, which must be more carefully examined during the design stage. It demonstrates many aspects of uncertainty, due to the stochastic nature of the wave motion, a considerable dispersion of the physical end mechanical characteristics that govern the phenomenon, and the microscopic way in which it initially develops. The purpose of this study is to discuss to what extent present approaches are actually able to distinguish between the different uncertainty contributions and to calibrate the safety factors to be used in reliability-based codes, from a practical point of view. The conclusion is that considerable advantages, in terms of an increase in the usage factor, can be obtained by improving the quality of the analyses and inspections. Graphs. 13 refs.
General Aluminium alloys, for circumferential face m i l l i n g cutters. Scheffel, R.D. and Huber, H. Werkstattstech. Apr. 1991 81, (4), 2 1 7 - 2 2 0 (in G e r m a n ) Blade holders made of AI alloys (AIMgSil, AIMgSi0.7, AIZnMgCul.s, AICuMgPb, AICuMg2, AICuMgl) are becoming more widely used in the woodworking industry. The most important properties of these alloys as regards safety were investigated, since holder breakage (eg owing to fatigue failure) poses a danger to the operator. The assessment is based on the results of fatigue tests, notched bar impact bending tests, and tension tests. Graphs. 11 refs.
Study rolling Wang, Metall.
on t h e f a t i g u e life o f t h e p i l o t shaft of the charger in roughm i l l II o f t h e A n s h a n Iron and Steel company. Dong, Y., Lu, Y., Y. and Wang, X. Equip. (China) 1991 3 0 - 3 2 (1), 1101-1269 (in Chinese)
It is a key to the extension of the life of a charger that the life of the pilot shaft of the charger is extended. The weak points are found on the basis of testing and computing, thus, the stress spectrum is determined, and the fatigue life is calculated. Both the overhauling period and the fatigue life are extended by improving the design and reducing the peak stress. The cracks which occurred before have not recurred for Wvo years since modification. Graphs. 8 refs. Fatigue prope~t, ies o f l o w - c a r b o n 316 stainless steel under 20 MeV p r o t o n irradiation and t h e r m a l pulses. Mizubayashi, H., Fujita, T., Yen,
S.-C. and Okuda, S. Mater. Trans, JIM J u n e 1991 32, (6), 5 3 9 - 5 4 5
The high-cycle fatigue properties of low-carbon 316 stainless steels, 316F with 0.04 mass% carbon and 316PSI with 0.001 mass% C, were investigated and compared with those of 316(ST-1) with 0.08 roses% C reported previously. Both the fatigue hardening and life decreased with C content, especially in 316PSI, indicating that the fatigue induced precipitates suggested for 316(ST-1) were mainly carbide or some complex of C-solute atoms. The effects of in situ 20 MeV proton irradiation or thermal pulses were further investigated for 316F at 333-573 K. In 316(ST-1), radiation damage, that is the effect of irradiation after the subtraction of the thermal effect, caused an enhancement of the fatigue hardening at both 333 and 573 K. In contrast, in 316F radiation damage caused a suppression of the fatigue hardening at 333 and 453 K, which reversed to the enhancement at 403 and 503 K, and again to the suppression at 573 K. The suppressions can be expected when the effects of dispersion of existing precipitates by irradiation were larger than the other effects of irradiation, and vice versa. Therefore, it is considered that the fatigue-induced precipitates were smaller in their size in 316F than in 316(ST-1), and in 316F the precipitates of various types probably correspond to the suppression at 333, 453, and 573 K. The corresponding results for thermal pulses and the results observed for the effects of in situ irradiation or thermal pulses on the fatigue life were compatible with the view mentioned above. Graphs. 16 refs. The effect of neutron irradiation on t h e f a t i g u e behavlour of A I S 1 3 1 6 L - results o f first in-pile tests. Vandermeulen, W., Hendrix, W., Massaut,
V., de Velde, J. Van J. Nucl. Mater. J u l y 1991 183, ( 1 - 2 ) , 57-61
The effect of neutron irradiation on the fatigue properties of fusion reactor structural materials is mainly studied by means of post-irradiation testing. This means that, for the case of low-temperature irradiations, the fatigue test is started on a material that is already fully hardened by the irradiation damage. The behaviour of a material under simultaneous fatigue and irradiation Ioadings could be completely different. Therefore, a system was developed which makes possible in-pile Ioed-controlled tests. A first irradiation was performed during which the specimens were cycled in the BR2 reactor with a stress range of 580 MPa at 250 °C. Ruptures occurred after 10000 and 12500 cycles, respectively, for doses at ruptures of 0.4 and 0.bdpe. These fatigue lives are comparable with those obtained in out-of-pile tests. To explain this absence of an irradiation effect, it is assumed that either moving dislocations prevent irradiation damage building up or that crack initiation occurs before radiation can cause any hardening and that subsequent crack growth is almost unaffected by irradiation. Graphs. 15 refs.
197
T r a n s i e n t retardations in fatigue crack growth following s single-peek oveHoed. Damri, D. and Knott, J. F. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 709-719 Retardation in the fatigue crack growth rate following the application of a single peak overload in a fatigue loading sequence has been studied for a Iow-cerbon structural steel. Tests have been performed at load ratios of R = 0.2 and R = 0.6 at a baseline stress intensity range, AKb, corresponding to fatigue crack growth rates in the Paris regime. Single-peak overloads were applied st a crack length to specimen width ratio of (~/W= 0.5. At the load ratio of R = 0.6 monotonic or 'static' fracture modes were observed upon application of the overload, and these produced an immediate increase in growth rate. A subsequent retardation is attributed to the presence of a residual compressive stress field ahead of the crack tip. A similar retardation was observed at a load ratio of 0.2. The importance of residual stress was established by performing stress relieving experiments. In addition, removal of the surface deformation after an overload by roaching 'T' sidegrooves resulted in an ex~ended transient, which could not be explained by residual machining stresses. Graphs, photomicrographs. 22 refs.
Environmental fatigue of an AI-Li-Cu alloy. I. Intrinsic crack propagation
kinetics in hydrogenous environments. Piascik, R.S. and Gangloff, R.P. Metall. Trans. A Oct. 1991 22A, (1), 2 4 1 5 - 2 4 2 8 Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak-aged AI-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant-~K and "Kmax loading. Such rates are equally unaffected by a vacuum, by purified helium or by oxygen, but are accelerated in order of decrasing effectiveness by aqueous 1% NaCI with anodic polarization, pure water vapour, moist air, and NaCI with cathodic polarization. While da/dN are proportional to ~/~ for the inert gases, for water vapour and chloride multiple-power laws and a transition growth rate 'plateau' are induced. Environmental effects are strongest at low values of ~K. Crack tip damage is ascribed to hydrogen embrittlement because of accelerated values of da/dN owing to levels of H20 of the order of ppm without condensation, impeded molecular flow model predictions of the measured water vapour pressure dependence of da/dN as affected by mean crack opening, the lack of an effect of film-forming 02, the likelihood for crack tip H production in NaCI, and the environmental and &Kprocess zone volume dependences of the microscopic cracking modes. For NaCt, the growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3 and upon cathodic polarization. These variables increase crack surface film stability to reduce H-entry efficiency. Small-crack effects are not observed for the 2090 alloy; such cracks do not grow at abnormally high rates in single grains or in NaCI, and ere not arrested at grain boundaries. The H environmental FCP resistance of the 2090 alloy is similar to those of other 2000 series alloys and is better than that of the 7075 alloy. Graphs, 86 refs.
Engineering components and structures Damping characteristics o f f l e x u r e l vibration for partially covered b e a m s w i t h constrained viscoelastic layers. Dewa, H., Okada, Y. and Nagai, B. JSME Int. J. Ser. III, J u n e 1991 34, (2), 2 1 0 - 2 1 7 A beam partially covered with a constrained viscoelastic layer shows superior damping capacity to that of a fully covered beam, depending on the beam geometry and the properties of the viscoelastic layer. This damping effect, the so-called partial cover effect, however, had not been examined in detail until now. The strain energy analysis of the three types of partially covered five-layered beam is carried OUt to explain the damping mechanism. The results of analysis for the centre-covered beam and the end-constrained beam indicate that the partial cover effect is caused by the shear strain energy dissipation at the outer edges of the viscoelastic layer, and also, such energy dissipation depends on the tensile stiffness of the constraining layer. The separately covered beam has a superior damping capacity to that of the centre-covered beam with the same cover ratio, owing to the energy dissipation of the inner edges of the viscoelastic layers. These damping characteristics of the three types of beam presented are confirmed by experimental results. Graphs. 14 refs. Rational method for fatigue design end inspection planning of o f f s h o r e structures. Skjong, R. and Torhuag, R. Marine Struct. 1991 4, (4), 3 8 1 - 4 0 6 This paper reviews the current usage of probabilistic fracture mechanics in fatigue assessment of offshore structures and summarizes the practical usage of probabilistic load and response analysis models used in conjunction with fracture mechanics models. These models are then used in conjunction with probabilistic models for inspection methods used in the offshore and marine industries to assess the probability of failure, derive optimum inspection plans, and derive optimum design considering the cost of design, cost of inspection, cost of repair and consequential costs of failure. Comments are made on the status of such reliability-based optimization techniques in design and on the need for further work. Some examples of use are demonstrated. Graphs. 43 refs. S y s t e m s r.eliabi!~f o f o f f s h o r e structures including f a t i g u e end e x t r e m e w a v e ,oeomg. ~aramchandani, A., Dalane, J.I. and Bjerager, P. Marine Struct. 1991 4, (4), 3 5 3 - 3 7 9 Traditionally, the focus of systems reliability analysis has been on estimating the reliability under either fatigue loading or extreme environmental loading. However, for many offshore structures, a combination of these two sources of risk may be more critical, for example, initial failures may occur in fatigue and subsequently collapse under an extreme wave. In this paper a probebilistic formulation for such a sequence of combined failures is presented. A formulation for a sequence of failures under an extreme wave, with realistic failure criteria for offshore structures, is also presented. With these formulations it is possible to account for both fatigue and extreme wave failures. An application of such a unified approach to a steeljacket platform is presented. It is found that, for an individual member, the most probable cause of failure is fatigue, but for overall structural failure, overload and a combination of fatigue and overload are more important. Graphs. 25 refs.
Int J Fatigue May 1992
A n improved fatigue analysis for o f f s h o r e structures. Karadeniz, H. Marine Struct. 1991 4, (4), 3 3 3 - 3 5 2 This paper presents a spectral fatigue damage calculation on offshore structures subjected to non-narrow-banded Gaussian stress processes. The calculation is based on use of the Palmgren-Miner's rule and a multilinear S-N fatigue model. In the calculation, an equivalent stress range, which is defined as dependent on the absolute value of stress maxima and a parameter, is used. This parameter is calculated in terms of the spectral bandwidth measure of the stress process and the slope of the S-N line used. For this purpose, the rainfiow cycle counting algorithm and the Monte-Carlo procedure are used to determine stress ranges and cycles from time simulations of a number of stress spectra of a monopod structure. A relation between the equivalent stress range parameter and the well known damage correction factor is constructed. The cumulative damage arising during a high sea state and over a long period of time is presented. Graphs. 13 refs. Uncertainties in t h e f a t i g u e design of offshore steel structures. Pittaluga,
A., Cazzulo, R. and Romeo, P. Marine Struct. 1991 4, (4), 3 1 7 - 3 3 2 Fatigue is one of the failure mechanisms of offshore steel structures, which must be more carefully examined during the design stage. It demonstrates many aspects of uncertainty, due to the stochastic nature of the wave motion, a considerable dispersion of the physical end mechanical characteristics that govern the phenomenon, and the microscopic way in which it initially develops. The purpose of this study is to discuss to what extent present approaches are actually able to distinguish between the different uncertainty contributions and to calibrate the safety factors to be used in reliability-based codes, from a practical point of view. The conclusion is that considerable advantages, in terms of an increase in the usage factor, can be obtained by improving the quality of the analyses and inspections. Graphs. 13 refs.
General Aluminium alloys, for circumferential face m i l l i n g cutters. Scheffel, R.D. and Huber, H. Werkstattstech. Apr. 1991 81, (4), 2 1 7 - 2 2 0 (in G e r m a n ) Blade holders made of AI alloys (AIMgSil, AIMgSi0.7, AIZnMgCul.s, AICuMgPb, AICuMg2, AICuMgl) are becoming more widely used in the woodworking industry. The most important properties of these alloys as regards safety were investigated, since holder breakage (eg owing to fatigue failure) poses a danger to the operator. The assessment is based on the results of fatigue tests, notched bar impact bending tests, and tension tests. Graphs. 11 refs.
Study rolling Wang, Metall.
on t h e f a t i g u e life o f t h e p i l o t shaft of the charger in roughm i l l II o f t h e A n s h a n Iron and Steel company. Dong, Y., Lu, Y., Y. and Wang, X. Equip. (China) 1991 3 0 - 3 2 (1), 1101-1269 (in Chinese)
It is a key to the extension of the life of a charger that the life of the pilot shaft of the charger is extended. The weak points are found on the basis of testing and computing, thus, the stress spectrum is determined, and the fatigue life is calculated. Both the overhauling period and the fatigue life are extended by improving the design and reducing the peak stress. The cracks which occurred before have not recurred for Wvo years since modification. Graphs. 8 refs. Fatigue prope~t, ies o f l o w - c a r b o n 316 stainless steel under 20 MeV p r o t o n irradiation and t h e r m a l pulses. Mizubayashi, H., Fujita, T., Yen,
S.-C. and Okuda, S. Mater. Trans, JIM J u n e 1991 32, (6), 5 3 9 - 5 4 5
The high-cycle fatigue properties of low-carbon 316 stainless steels, 316F with 0.04 mass% carbon and 316PSI with 0.001 mass% C, were investigated and compared with those of 316(ST-1) with 0.08 roses% C reported previously. Both the fatigue hardening and life decreased with C content, especially in 316PSI, indicating that the fatigue induced precipitates suggested for 316(ST-1) were mainly carbide or some complex of C-solute atoms. The effects of in situ 20 MeV proton irradiation or thermal pulses were further investigated for 316F at 333-573 K. In 316(ST-1), radiation damage, that is the effect of irradiation after the subtraction of the thermal effect, caused an enhancement of the fatigue hardening at both 333 and 573 K. In contrast, in 316F radiation damage caused a suppression of the fatigue hardening at 333 and 453 K, which reversed to the enhancement at 403 and 503 K, and again to the suppression at 573 K. The suppressions can be expected when the effects of dispersion of existing precipitates by irradiation were larger than the other effects of irradiation, and vice versa. Therefore, it is considered that the fatigue-induced precipitates were smaller in their size in 316F than in 316(ST-1), and in 316F the precipitates of various types probably correspond to the suppression at 333, 453, and 573 K. The corresponding results for thermal pulses and the results observed for the effects of in situ irradiation or thermal pulses on the fatigue life were compatible with the view mentioned above. Graphs. 16 refs. The effect of neutron irradiation on t h e f a t i g u e behavlour of A I S 1 3 1 6 L - results o f first in-pile tests. Vandermeulen, W., Hendrix, W., Massaut,
V., de Velde, J. Van J. Nucl. Mater. J u l y 1991 183, ( 1 - 2 ) , 57-61
The effect of neutron irradiation on the fatigue properties of fusion reactor structural materials is mainly studied by means of post-irradiation testing. This means that, for the case of low-temperature irradiations, the fatigue test is started on a material that is already fully hardened by the irradiation damage. The behaviour of a material under simultaneous fatigue and irradiation Ioadings could be completely different. Therefore, a system was developed which makes possible in-pile Ioed-controlled tests. A first irradiation was performed during which the specimens were cycled in the BR2 reactor with a stress range of 580 MPa at 250 °C. Ruptures occurred after 10000 and 12500 cycles, respectively, for doses at ruptures of 0.4 and 0.bdpe. These fatigue lives are comparable with those obtained in out-of-pile tests. To explain this absence of an irradiation effect, it is assumed that either moving dislocations prevent irradiation damage building up or that crack initiation occurs before radiation can cause any hardening and that subsequent crack growth is almost unaffected by irradiation. Graphs. 15 refs.
197