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Fatigue life prediction of heat-treated carbon steels and low alloy steels based on a small crack growth law
378
Abstracts
Fatigue crack growth in ductile materials under cyclic compressive loading. Hermann. R. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 (1), 93-103
Influence of mixed-mode loading on fatigue-crack propagation. Guo. Y.H. and Srivatsan, T.S. Eng. Fract. Mech. (Apr, 1994) 47 (6), 843-866
Mode I fatigue crack growth has been studied in notched specimens of 7017T651 A1 alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload, which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field, and on subsequent compressive cyclic loading in laboratory air a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented. Graphs, photomicrographs, 20 ref.
The subject of fatigue, which comprises crack nucleation, crack propagation and final failure or fracture, has over the years been the subject of numerous theoretical and experimental studies. These studies have highlighted the extrinsic influence of mixed-mode loading in governing the fatigue behaviour of a wide spectrum of engineering materials and structures. The basis criteria and models that have been proposed and used to predict crack behaviour and response for structures containing large cracks and subject to mixedmode loading are reviewed. Since the aspect of crack growth is the focus of this review, the effects or contributions from intrinsic microstructural effects are largely excluded. Specific criteria discussed are the maximum tangential stress, minimum strain-energy density and the maximum energy-release rate criteria. The use of the criteria to predict the bchaviour of structures is examined based on results published in the open literature for e.g. aluminium alloy 2124. The characteristics and implications of each criterion are examined and discussed with particular relevance to threshold conditions on crack growth, direction of crack growth and crack-growth rate. The limitations of each criterion are highlighted. Graphs, photomicrographs, 52 rcf.
Fracture and life prediction under thermal-mechanical strain cycling. Ellison, E.G. and Al-Zamily, A. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 ( l ) , 53-67 Thermal-mechanical cyclic strain tests were carried out under in-phase and out-of-phase conditions on a ferritic 1CrMoV steel in the temperature range 315-565°C and two different batches of a 316 stainless steel in the temperature range 400-625°C. The results were compared with isothermal data. In general, lives for in-phase thermal-mechanical tests were lower than for equivalent isothermal tests. This was substantiated by metallographic examination, which indicated greater integranular damage for the in-phase specimens. The addition of a tensile dwell to the in-phase condition caused even greater differences. Three life-prediction methods were used: the ductility exhaustion and a modified strain range partitioning approach gave reasonable estimates of life, while the linear life fraction method produced poor prediction. Graphs, photomicrographs, 21 ref.
Stress response under thermal-mechanical strain cycling for a ICrMoV ferritic steel and two 316 stainless steels. Ellison, E.G, and AI-Zamily, A. Fatigue Fract. Eng, Mater. Struct. (Jan. 1994) 17 (1), 39-51 Isothermal and thermal-mechanical strain fatigue tests were conducted in air on representative service alloys: a 1CrMoV steel, and two batches of 316 stainless steel. Data were obtained for thermal-mechanical in-phase and outof-phase cycles, and also for isothermal tests at the maximum, minimum, and mid-temperature of the thermal-mechanical cycle. Dwell periods were also incorporated in the cycle to assess their effects. A comparative evaluation has been made on the basis of the materials' cyclic stress response. In general, the results have shown that the thermal-mechanical strain cycling tests cause a large increase in stress over those tested under isothermal conditions at maximum temperature. In addition, mean stress and strain offsets were developed in continuous cycle thermal-mechanical tests, whereas negligible offsets occurred in isothermal tests. It appears that the response of the materials could not always be explained simply by reference to the temperature change itself. Graphs. 16 ref.
Two-parameter model of fretting fatigue crack growth. Troshchenko. V. T,, Tsybanev. G. V. and Khotsyanovsky, A.O. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 (1), 15-23 A fretting fatigue crack growth model is proposed accounting for the effects of tribological parameters of the contacting materials and the biaxiality of contact together with bulk stresses, upon fretting fatigue crack growth rate and direction of growth in the fretting-zone subsurface layers. Based on this model a new technique is developed to determine fatigue life and predict the fatigue limit in an AI alloy, AMg6N, and a Ti alloy, VT9, under fretting conditions. For the above cases, fretting fatigue crack growth behaviour predicted by the proposed model is in good agreement with the experimental results. Graphs, 8 ref.
Short fatigue crack path determinants in polycrystalline nickel-base superalloys. Boyd-Lee, A. and King, J.E. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 (1), 1-14 Fifty-seven short fatigue cracks in the Ni-base superalloy API have been examined, to ascertain how the paths taken by growing fatigue cracks are determined. The observations were made on the surface of a smooth specimen. and on the exposed fracture surfaces. Three-dimensional reconstructions of the vulnerable microstructures in the vicinity of the cracks were produced. Initiation occurred in mode If, with the lines of intersection of the initiation sites with specimen top surface oriented at approx 45 ° to the tensile axis. These initiation sites developed in slip bands, which cross a large grain and at least one other grain via a grain boundary with a low angle of misorientation. "River markings' on one of the initiation facets indicated that the crack first opened from the top centre of the initiation grain. Subsequent to initiation, the growth paths of these cracks are related to the misorientations of the grains and the progress of the crack front. Photomicrographs, graphs, 18 ref.
A new mechanistic approach to analysing LEFM fatigue crack growth behaviour of metals and alloys. Lal, D.N. Eng. Fract. Mech. (Feb. 1994) 47 (3), 379-401 Basic features of a new mechanistic approach developed for allowing an improved physical understanding and prediction of the variable-influenced cyclic growth hehaviour of LEFM (long) cracks in metals and alloys (A533B, 7075, 2024, T i - 6 A I - 4 V ) are presented, The approach is essentially an extension of the one adopted recently for an accurate description of the intrinsic LEFM fatigue-threshold condition. The analysis conditions assumed here are low-strength (or coarse grain ) material, room temperature, laboratory air environment, and stress ratio R = 0, so that the fracture mechanics parameter AK o r Kma x is the only dominant variable. Three distinct micromechanical modes of fatigue fracture,namely a Kma:Controlled 'submicroscopic cleavage" a AK-controlled 'reversed shear', and a Km~x-controlled 'microscopic cleavage' or 'static' mode operating in a critically stressed zone VC ahead of the crack tip, are postulated to predominate respectively in Stages 1, II and I1 of the log da/dN vs. log AK fatigue crack growth curve. The two observed transition points between stages are postulated to correspond to a transition in growth mechanisms. The mechanistic differences necessitate description of each stage separately. Assuming Paris-type power-law relations, growth equations for Stages I and II are deduced wherein constants A and m can be estimated theoretically. Model-predicted crack growth curves showed good agreement with experimental data for several steels and aluminium alloys. The approach may be found useful in design situations where a quick, fairly accurate and conservative estimate of fatigue crack growth life is desired. Graphs, 88 ref,
Crack growth and closure behavior of short fatigue cracks. Pang. C.-M. and Song. J.-H. Eng. Fract, Mech. (Feb. 1994) 47 (3), 327 343 Crack growth and closure behaviour of short cracks are investigated for various stress ratios, using in-plane bending specimens of 2f124-T351 aluminium alloy. Artificially prepared two-dimensional, short through-thickness cracks are used. Crack length and closure of short cracks are measured continuously during the test by employing an unloading elastic compliance technique and a personal computer system. For most of the stress ratios tested, short cracks grow faster than long cracks in the low stress intensity factor range region, and the growth rates of short cracks merge with the long crack growth curve with increases in the value of stress intensity fractor range. The variation in growth rates between short and long cracks is reduced as the value of stress ratio is increased, and finally disappears at a stress ratio of R - 0.5. The growth rates of short cracks are well described by the effective stress intensity factor range . The growth rates of short cracks in terms of the effective stress intensity factor range coincide well with the long crack data. However, the closure behaviour of short cracks is significantly different from that of long cracks. The crack opening ratio of a short crack decreases from an initial high value corresponding to a fully open crack linearly with the value of effective stress intensity factor range, and merges with the long crack results. Based on the test results obtained, a relatively simple procedure is proposed to predict closure and growth behaviour of short cracks. The validity of the proposed procedure is examined using the test data of other workers. In addition, some methods proposed by other workers are also discussed. Graphs, 42 ref.
Fatigue life prediction of heat-treated carbon steels and low alloy steels based on a small crack growth law. Goto. M. and Nisitani, H. Fatigue Fract. Eng. Mater. Struct. (Feb. 1994) 17 (2), 171-185 Since heat-treated high-strength steels are often used as materials for machines and structures that operate under severe service conditions, it is important to evaluate their fatigue life. Hence the growth law of a small fatigue crack must be known in order to estimate the fatigue life of machines and structures, since the life of such memembers is controlled mainly by the behaviour of a small crack. The growth rate of a small crack cannot usually be predicted by linear elastic fracture mechanics, but can he determined uniquely by the term sigmao'~ ~/, where sigmao', is stress amplitude, / is crack length and n is a
Abstracts material constant. In this paper, the small-crack growth law of heat-treated carbon steels and low alloy steels was studied. An effective and convenient method based on a small-crack growth law, dl/dN = C3 sigma~r,/ sigmatru~ / is proposed, where ~ru is the ultimate tensile strength, for predicting the small crack propagation life of heat-treated steels with different tensile strength levels, together with a method for determining the fatigue life of plain members. Graphs, photomicrographs, 27 ref.
High strain biaxial fatigue of a structural steel. Shatil, G.. Smith. D.J. and Ellison. E. G. Fatigue Fract. Eng. Mater. Struct. (Feb, 1994) 17 (2), 159-170 t h e biaxial fatigue behaviour of two batches of a structural steel (EN15R), termed 'isotropic" and 'anisotropic' batches, is examined. Tests were carried out using thin-walled hollow specimens subjected to axial load and internal and external pressure. The fatigue life of the isotropic batch is found to be about two to three times greater than the fatigue life of the anisotropic material. The ratio, however, is found to be dependent on the degree of biaxiality. Three equivalent strain parameters were used to correlate the fatigue lives.
379
Fatigue crack growth of neturon irradiated V-15Cr-5Ti and V - 3 T i - I S i . van Witzenburg, W. and de Bruyne, H.J. Proc. Conf. Effects of Radiation on Materials: 16th International Symposium, Aurora, CO, USA, 23-25 June 1992, pp. 1201-12[4 Compact-tension (CT) specimens of the alloys V - 1 5 C r - 1 5 T i and V - 3 T i - l S i were irradiated to 3.8 x 1025 n.m -2 (E > 0.1 MeV), corresponding to 4.0 dpa, at the temperatures 500, 600 and 700°C. Some specimens of the alloy V - 3 T i - I S i weer doped with 100 appm 10B, which during the irradiation was almost entirely transmuted to helium. Following irradiation, constant-loadamplitude crack growth was measured at 200°C in air. All tests were performed at one R-ratio of 0.1 and a frequency of 2 Hz. Although both alloys in unirradiated condition showed similar crack growth rates as a function of the stress intensity factor range, the irradiated alloys behaved differently. The fatigue crack growth rate of V - 3 T i - I S i irradiated at 500° was slightly higher, with a lower dependence on the stress intensity factor range, compared with the unirradiated material, while after irradiation at 600 700°C the fatigue crack growth rate approached thai of the unirradiated material. On the other hand, the alloy V - 1 5 C r - 5 T i irradiated at 500 and 600°C showed irregular crack growth and a higher growth rate, indicative of local unstable crack growth. After irradiation at 700~C the fatigue crack growth rate of this alloy also approached that of the unirradiatcd reference material. Limited data indicate that helium, generated by (n, c0-reaction with 10B, had no effect on the crack growth ratc.
Abstracts
Fatigue crack growth in ductile materials under cyclic compressive loading. Hermann. R. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 (1), 93-103
Influence of mixed-mode loading on fatigue-crack propagation. Guo. Y.H. and Srivatsan, T.S. Eng. Fract. Mech. (Apr, 1994) 47 (6), 843-866
Mode I fatigue crack growth has been studied in notched specimens of 7017T651 A1 alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload, which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field, and on subsequent compressive cyclic loading in laboratory air a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented. Graphs, photomicrographs, 20 ref.
The subject of fatigue, which comprises crack nucleation, crack propagation and final failure or fracture, has over the years been the subject of numerous theoretical and experimental studies. These studies have highlighted the extrinsic influence of mixed-mode loading in governing the fatigue behaviour of a wide spectrum of engineering materials and structures. The basis criteria and models that have been proposed and used to predict crack behaviour and response for structures containing large cracks and subject to mixedmode loading are reviewed. Since the aspect of crack growth is the focus of this review, the effects or contributions from intrinsic microstructural effects are largely excluded. Specific criteria discussed are the maximum tangential stress, minimum strain-energy density and the maximum energy-release rate criteria. The use of the criteria to predict the bchaviour of structures is examined based on results published in the open literature for e.g. aluminium alloy 2124. The characteristics and implications of each criterion are examined and discussed with particular relevance to threshold conditions on crack growth, direction of crack growth and crack-growth rate. The limitations of each criterion are highlighted. Graphs, photomicrographs, 52 rcf.
Fracture and life prediction under thermal-mechanical strain cycling. Ellison, E.G. and Al-Zamily, A. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 ( l ) , 53-67 Thermal-mechanical cyclic strain tests were carried out under in-phase and out-of-phase conditions on a ferritic 1CrMoV steel in the temperature range 315-565°C and two different batches of a 316 stainless steel in the temperature range 400-625°C. The results were compared with isothermal data. In general, lives for in-phase thermal-mechanical tests were lower than for equivalent isothermal tests. This was substantiated by metallographic examination, which indicated greater integranular damage for the in-phase specimens. The addition of a tensile dwell to the in-phase condition caused even greater differences. Three life-prediction methods were used: the ductility exhaustion and a modified strain range partitioning approach gave reasonable estimates of life, while the linear life fraction method produced poor prediction. Graphs, photomicrographs, 21 ref.
Stress response under thermal-mechanical strain cycling for a ICrMoV ferritic steel and two 316 stainless steels. Ellison, E.G, and AI-Zamily, A. Fatigue Fract. Eng, Mater. Struct. (Jan. 1994) 17 (1), 39-51 Isothermal and thermal-mechanical strain fatigue tests were conducted in air on representative service alloys: a 1CrMoV steel, and two batches of 316 stainless steel. Data were obtained for thermal-mechanical in-phase and outof-phase cycles, and also for isothermal tests at the maximum, minimum, and mid-temperature of the thermal-mechanical cycle. Dwell periods were also incorporated in the cycle to assess their effects. A comparative evaluation has been made on the basis of the materials' cyclic stress response. In general, the results have shown that the thermal-mechanical strain cycling tests cause a large increase in stress over those tested under isothermal conditions at maximum temperature. In addition, mean stress and strain offsets were developed in continuous cycle thermal-mechanical tests, whereas negligible offsets occurred in isothermal tests. It appears that the response of the materials could not always be explained simply by reference to the temperature change itself. Graphs. 16 ref.
Two-parameter model of fretting fatigue crack growth. Troshchenko. V. T,, Tsybanev. G. V. and Khotsyanovsky, A.O. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 (1), 15-23 A fretting fatigue crack growth model is proposed accounting for the effects of tribological parameters of the contacting materials and the biaxiality of contact together with bulk stresses, upon fretting fatigue crack growth rate and direction of growth in the fretting-zone subsurface layers. Based on this model a new technique is developed to determine fatigue life and predict the fatigue limit in an AI alloy, AMg6N, and a Ti alloy, VT9, under fretting conditions. For the above cases, fretting fatigue crack growth behaviour predicted by the proposed model is in good agreement with the experimental results. Graphs, 8 ref.
Short fatigue crack path determinants in polycrystalline nickel-base superalloys. Boyd-Lee, A. and King, J.E. Fatigue Fract. Eng. Mater. Struct. (Jan. 1994) 17 (1), 1-14 Fifty-seven short fatigue cracks in the Ni-base superalloy API have been examined, to ascertain how the paths taken by growing fatigue cracks are determined. The observations were made on the surface of a smooth specimen. and on the exposed fracture surfaces. Three-dimensional reconstructions of the vulnerable microstructures in the vicinity of the cracks were produced. Initiation occurred in mode If, with the lines of intersection of the initiation sites with specimen top surface oriented at approx 45 ° to the tensile axis. These initiation sites developed in slip bands, which cross a large grain and at least one other grain via a grain boundary with a low angle of misorientation. "River markings' on one of the initiation facets indicated that the crack first opened from the top centre of the initiation grain. Subsequent to initiation, the growth paths of these cracks are related to the misorientations of the grains and the progress of the crack front. Photomicrographs, graphs, 18 ref.
A new mechanistic approach to analysing LEFM fatigue crack growth behaviour of metals and alloys. Lal, D.N. Eng. Fract. Mech. (Feb. 1994) 47 (3), 379-401 Basic features of a new mechanistic approach developed for allowing an improved physical understanding and prediction of the variable-influenced cyclic growth hehaviour of LEFM (long) cracks in metals and alloys (A533B, 7075, 2024, T i - 6 A I - 4 V ) are presented, The approach is essentially an extension of the one adopted recently for an accurate description of the intrinsic LEFM fatigue-threshold condition. The analysis conditions assumed here are low-strength (or coarse grain ) material, room temperature, laboratory air environment, and stress ratio R = 0, so that the fracture mechanics parameter AK o r Kma x is the only dominant variable. Three distinct micromechanical modes of fatigue fracture,namely a Kma:Controlled 'submicroscopic cleavage" a AK-controlled 'reversed shear', and a Km~x-controlled 'microscopic cleavage' or 'static' mode operating in a critically stressed zone VC ahead of the crack tip, are postulated to predominate respectively in Stages 1, II and I1 of the log da/dN vs. log AK fatigue crack growth curve. The two observed transition points between stages are postulated to correspond to a transition in growth mechanisms. The mechanistic differences necessitate description of each stage separately. Assuming Paris-type power-law relations, growth equations for Stages I and II are deduced wherein constants A and m can be estimated theoretically. Model-predicted crack growth curves showed good agreement with experimental data for several steels and aluminium alloys. The approach may be found useful in design situations where a quick, fairly accurate and conservative estimate of fatigue crack growth life is desired. Graphs, 88 ref,
Crack growth and closure behavior of short fatigue cracks. Pang. C.-M. and Song. J.-H. Eng. Fract, Mech. (Feb. 1994) 47 (3), 327 343 Crack growth and closure behaviour of short cracks are investigated for various stress ratios, using in-plane bending specimens of 2f124-T351 aluminium alloy. Artificially prepared two-dimensional, short through-thickness cracks are used. Crack length and closure of short cracks are measured continuously during the test by employing an unloading elastic compliance technique and a personal computer system. For most of the stress ratios tested, short cracks grow faster than long cracks in the low stress intensity factor range region, and the growth rates of short cracks merge with the long crack growth curve with increases in the value of stress intensity fractor range. The variation in growth rates between short and long cracks is reduced as the value of stress ratio is increased, and finally disappears at a stress ratio of R - 0.5. The growth rates of short cracks are well described by the effective stress intensity factor range . The growth rates of short cracks in terms of the effective stress intensity factor range coincide well with the long crack data. However, the closure behaviour of short cracks is significantly different from that of long cracks. The crack opening ratio of a short crack decreases from an initial high value corresponding to a fully open crack linearly with the value of effective stress intensity factor range, and merges with the long crack results. Based on the test results obtained, a relatively simple procedure is proposed to predict closure and growth behaviour of short cracks. The validity of the proposed procedure is examined using the test data of other workers. In addition, some methods proposed by other workers are also discussed. Graphs, 42 ref.
Fatigue life prediction of heat-treated carbon steels and low alloy steels based on a small crack growth law. Goto. M. and Nisitani, H. Fatigue Fract. Eng. Mater. Struct. (Feb. 1994) 17 (2), 171-185 Since heat-treated high-strength steels are often used as materials for machines and structures that operate under severe service conditions, it is important to evaluate their fatigue life. Hence the growth law of a small fatigue crack must be known in order to estimate the fatigue life of machines and structures, since the life of such memembers is controlled mainly by the behaviour of a small crack. The growth rate of a small crack cannot usually be predicted by linear elastic fracture mechanics, but can he determined uniquely by the term sigmao'~ ~/, where sigmao', is stress amplitude, / is crack length and n is a
Abstracts material constant. In this paper, the small-crack growth law of heat-treated carbon steels and low alloy steels was studied. An effective and convenient method based on a small-crack growth law, dl/dN = C3 sigma~r,/ sigmatru~ / is proposed, where ~ru is the ultimate tensile strength, for predicting the small crack propagation life of heat-treated steels with different tensile strength levels, together with a method for determining the fatigue life of plain members. Graphs, photomicrographs, 27 ref.
High strain biaxial fatigue of a structural steel. Shatil, G.. Smith. D.J. and Ellison. E. G. Fatigue Fract. Eng. Mater. Struct. (Feb, 1994) 17 (2), 159-170 t h e biaxial fatigue behaviour of two batches of a structural steel (EN15R), termed 'isotropic" and 'anisotropic' batches, is examined. Tests were carried out using thin-walled hollow specimens subjected to axial load and internal and external pressure. The fatigue life of the isotropic batch is found to be about two to three times greater than the fatigue life of the anisotropic material. The ratio, however, is found to be dependent on the degree of biaxiality. Three equivalent strain parameters were used to correlate the fatigue lives.
379
Fatigue crack growth of neturon irradiated V-15Cr-5Ti and V - 3 T i - I S i . van Witzenburg, W. and de Bruyne, H.J. Proc. Conf. Effects of Radiation on Materials: 16th International Symposium, Aurora, CO, USA, 23-25 June 1992, pp. 1201-12[4 Compact-tension (CT) specimens of the alloys V - 1 5 C r - 1 5 T i and V - 3 T i - l S i were irradiated to 3.8 x 1025 n.m -2 (E > 0.1 MeV), corresponding to 4.0 dpa, at the temperatures 500, 600 and 700°C. Some specimens of the alloy V - 3 T i - I S i weer doped with 100 appm 10B, which during the irradiation was almost entirely transmuted to helium. Following irradiation, constant-loadamplitude crack growth was measured at 200°C in air. All tests were performed at one R-ratio of 0.1 and a frequency of 2 Hz. Although both alloys in unirradiated condition showed similar crack growth rates as a function of the stress intensity factor range, the irradiated alloys behaved differently. The fatigue crack growth rate of V - 3 T i - I S i irradiated at 500° was slightly higher, with a lower dependence on the stress intensity factor range, compared with the unirradiated material, while after irradiation at 600 700°C the fatigue crack growth rate approached thai of the unirradiated material. On the other hand, the alloy V - 1 5 C r - 5 T i irradiated at 500 and 600°C showed irregular crack growth and a higher growth rate, indicative of local unstable crack growth. After irradiation at 700~C the fatigue crack growth rate of this alloy also approached that of the unirradiatcd reference material. Limited data indicate that helium, generated by (n, c0-reaction with 10B, had no effect on the crack growth ratc.