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The effect of grain size on the fatigue of commercially pure aluminium
184
Fatigue Abstracts
Spot-welded double-hat specimen under torsional loading [Punktgeschweisste Doppelhutprofilprobe unter Torsionsbelastung]. Radaj. D. and Zhang, S. Moterialprufung (1995) 37, (11 12), 458~162
Behaviour of initiation and growth of short fatigue cracks. Wu, Z aml Xu, H. Acta Metall. Sini('a (English Lett) (1996) 9 (1), 65 70
The local parameters (structural stress, stress intensity factors and notch stresses) at the nugget edge of the spot-welded double-hat specimen under torsional loading are numerically determined based on engineering formulae and finite element analysis dependent on the geometrical design parameters and their production-related deviations. The paper presents an assessment of the t~ltigue-relevant sustainable local parameters. The diagrams and formulae allow one to express the test results from specimens with special dimensions in terms of a uniform reference specimen. Graphs, 9 ref.
Rotary bending fatigue tests were carried out using smooth specimens ol ~l medium carbon steel, Fe~).43C-0.66Mn-0.23Si-/).I4Cu4).07Ni~0.05Cr, with two different grain sizes. The process of early crack development was observed by the replica method, and the effects of grain size and microstructure on short crack development were studied, It was shown that the initiation process. of fatigue cracks is that the damaged region is intensified gradually, and the growth of short cracks relates to the coalescence of cracks. The grain size and microstructure have a great influence upon the initiation and growth of shola cracks. A reasonable definition was given in relation to shorl fatigue crack initiation and propagation. Graphs, Photomicrographs, 5 ref.
Low activation braze joint of dispersion-strengthened copper. Chen. S. and Chin. B.A. .L Nucl. Mater. (15 Aug. 1995) 225 (I 3), 132-136 This paper presents the results of an investigation of a commercial braze alloy (BCuP-3), which was chosen because of its low activation characteristics. The braze could be used to join alumina dispersion-strengthened copper alloys in fusion reactor applications. The brazes were made using a Gleeble 1500 system with which time, temperature and stress could be precisely controlled. Excellent braze strengths were obtained by controlling the heating rate, braze hold time and cooling process. However, the brazed joints exhibited low ductility in both tensile and fatigue tests. The investigation has shown that a redistribution of the strengthening alumina particles occurs in the region near the brazed joint. This redistribution results in a higher density of alumina particles building up in a layer near the braze joint interface. Fractography showed an intergranular fracture pattern across this transition zone indicating that it is this interface that is responsible for the observed low ductility of the braze joint.
Study of fatigue and fracture of poly-, bi- and single-crystals of C u 18.2Zn-7.0AI shape memory alloys [Estudio de fatiga y fractura de monocristales, bicristales y policristales de la aleacion Cu-18.2Zn-7.0AI con memoria de forma]. Gil, F.X. and Guilemany. J.M. Revista Metal. (1995) 31, 222-227 Mechanical properties--tensile, compression and fatigue stresses <)f poly , biand single-crystals of Cu-18.2Z-7.0AI have been studied. The relationship between mechanical behaviour and microstmcture has been determined. Photo micrographs, 15 ref.
Fatigue design d a t a and concepts of aluminium structures. V. Takeuchi, K. Kei Kinzoku Yosetsu (J. Light Metal Welding Construction) (May 1995) 33 (5), 37-43 The fatigue strength of aluminium alloys under different roughness and surface damage is reviewed. Various A1 alloys (2000, 5000, 6000, and 7000 series) are used as examples and their S-N curves are given. The influence of cycling rate on fatigue strength is also discussed. Graphs, 11 ref.
Effects of roughness-induced crack closure on fatigue crack growth in AC4CH cast aluminium alloys. Kumai, S., Sekikawa, A., Hu, J.Q., Higo, Y. and Nunomura, S. J. Japan Inst. Light Metals (1995) 45 (4), 204-208 The role of extrinsic mechanism which improves fatigue crack resistance of the cast alloy has been examined. A constant load amplitude fatigue crack growth test and a crack closure measurement have been performed using AC4CH cast aluminium alloys. The da/dN- A K relation and the crack closure levels were compared between the cast alloys and squeeze cast or powdermetallurgy processed 6061 Al alloys. Improved fatigue crack growth resistance at the low A K range was shown for the cast alloy in comparison with the powder-metallurgy alloy with finer microstructure, when evaluated by the da/dN- A K relation. While, the difference was diminished by evaluation based on the da/dN- A K~n relation. This indicates that the roughness-induced crack closure effect is large in the cast alloys to improve the crack growth resistance in the low A K region. This extrinsic mechanism is considered to be originated from the deflected fatigue crack growth path and the resultant rough fatigue fracture surface, which are attributed to the dentritic solidification structure and the eutectic Si particle distribution in the cast alloy. Graphs, Photomicrographs, 11 ref.
Effects of solidification structure on fatigue crack growth in AC4CH cast aluminium alloys. Kumai, S., Hu, J.Q., Higo, Y. and Nunomura, S. J. Jap. Inst. Light Metals (1995) 45 (4), 198-203 The fatigue crack growth behaviour of a AI-Si-Mg base cast aluminium alloy (AC4CH) has been investigated with special reference to the effects of solidification stmcture. Fatigue crack growth tests under the constant load amplitude condition have been performed using CT type specimens of systematically controlled solidification structures with dendrite a n n spacing, size and morphology of eutectic Si particles and matrix aging conditions. The results were discussed from viewpoints of microstructures and fracture mechanics. The da/dN- A K relationship reveals that refining and spheroidizing of eutectic Si particles improve fatigue crack growth resistance in the high A K range, while the crack growth rate at the lower limit range is not influenced by morphology of the particles. Matrix aging condition influences little the nearthreshold and the Paris regime fatigue crack growth. The crack growth resistance in the high A K range is improved by increase in ductility of matrix. It is, however, deteriorated by the coarser dendritic structure which yields the higher crack growth resistance near the threshold level. Graphs, Photomicrograbs, 12 ref.
A micro-meso-process theory of fatigue crack initiation and theory of internal fatigue limit. Wang, R.Z.. Ra, J.L.. Li, X.B., Yao, M.. Li, .I.K.. Wang. S.P. and Shao, P.G. Trans. Metal Heat Treatment (China)(1995) 16 (4), 26-34 Analyzing for various models of fatigue crack initiation and observing t\~r the phenomena of fatigue crack initiation of surface strengthened metals, it was shown that existing models of crack initiation are not very perfect. Therefore a new "micro-meso-process theory of fatigue crack initiation" and 'theory of internal fatigue limit' (a new characteristic parameter in the classical theory of fatigue limit) have been proposed. According to the theories mentioned above a lot of existing phenomena and problems difficult to understand on the fatigue could be explained satisfactorily. 20Cr, 40Cr, 30CrMo, GC4, LC9 arc discussed. Photomicrographs. 8 ref.
Fatigue lifetime prediction methods: an analysis of the different approximations involved in local approaches. Chaudonneret, M. and Robert, M. Int. J. Pressure Vessels Piping (1996) 66 (1-3), 113 123, Conferem'e: 3rd International Colloqium." Ageing of Materials and Methods ./~r Assessing th~ Lifetime Extension ~f Engineering Plants March 1995 This paper deals with different aspects included m fatigue life prediction algorithms of metallic components under complex uniaxial Ioadings: plastic. damage and loading analysis. It attempts to identify and analyse the various models generally used at each of these steps. Some comparisons arc made with experimental results. Graphs, 15 ref.
Microstructural effects on small fatigue crack initiation and growth in Ti6AII4V alloys. Demulsant, X. and Mendez, J. Fatigue Fracture Engng Mater. Structures (1995) 18, 1483 1497 The nucleation and growth of small naturally initiated cracks have been investigated on Ti 56 AI 4V alloys, with bimodal or globular microstructures. Tests have been performed in air at a stress amplitude near 0.75 of the yield stress. The influence of microstructure on fatigue damage is described and the differences in fatigue life of some titanium alloys are explained. The effect ol microstmcture on fatigue resistance is mainly related to the early stages of damage including initiation and small crack growth. Coalescence processes favoured by a high density of initiated surface microcracks in fine oL grain material, or rapid initiation of large cracks in coarse o~ colonies, explains low fatigue resistance or lifetime scattering in globular alloys. Small crack growth retardation due to od/3 barriers associated with a low surface crack density, limiting coalescence processes, explains the highest resistance of bimodal structures. Photomicrographs, Graphs, 32 ref.
Predicting fatigue life in commercially pure aluminium using a short crack growth model. Turnbull, A. and de los Rios, E.R. Fatigue Fracture Engng Mater. Struct. (1995) 18, 1469-1481 Advances in manufacturing techniques and improvements in non-destmctive inspection procedures have reduced the size of initial defects in fatigue-critical components to the scale of the microstmcture. Unfortunately, the growth of these so-called short fatigue cracks is not properly characterized by conventional linear elastic or elastic-plastic fracture mechanics. However, the Navarro-de los Rios crack growth model does account for many aspects of their growth and has proved successful in predicting fatigue life of medium carbon and C Mn steels. In consequence, the growth of short fatigue cracks in commercially pure Al, under fully reversed uniaxial loading, has been studied, using a plastic replication technique and modelled, using the Navarro~le los Rios crack growth equation. The predicted lifetimes are in good agreement with the actual experimental lifetimes, and compare favorably with predictions based on two previously reported crack growth models. Graphs, Photomicrographs, 35 ref.
The effect of grain size on the fatigue of commercially pure aluminium. Turnbull, A., de los Rios, E.R. Fatigue Fracture Engng Mater. Struct. (1995) 18, 1455-1467 Fully reversed uniaxial fatigue tests were performed on polished hour-glass specimen of commercially pure aluminium with three different grain sizes, in order to examine the effect of grain sizes on fatigue. The growth of surface cracks was monitored by a plastic replication method. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. Multiple crack initiation and growth is a feature of the fatigue of AI, while the grain size influences the specific detail of crack coalescence. Crack path deviation
Fatigue Abstracts is greatest in the coarse grained microstmcture and crack surface roughness is more pronounced. SEM fractography reveals that crack initiation and early crack growth takes place along crystallographic slip planes, and that fatigue striations, characteristic of stage II cracking, extend to the very edge of the specimen section, suggesting extensive crack tip blunting. Photomicrographs. Graphs, 32 ref
Fatigue crack growth under Mode II loading. Wang, M.O., Hu, R.H.. Qmn, C.F. and Li. .I.C.M Fatigue Fracture t'.'ngng Mater. Struct. 11995) 18, 1443-1454 The behaviour of iatigue crack growth for low and medium carbon steels, an austenitic stainless steel and an aluminium alloy under pure Mode II loading was investigated experimentally, using cruciform specimens. The results show that under pure Mode II loading, fatigue crack propagation has three possi bilities, namely, bifurcation into two branches, propagation along the original Mode I1 direction, and the mixture of these two situations, depending on the material. The growth rate daMN vs A K H relation for Mode 11 propagation i~ similar to a Paris type law for Mode 1 growth. Fractographic observations by optical microscopy and SEM were made also on all specimens tested. When a crack branched, striations parallel to the crack front which were often associated with Mode 1 fatigue crack growth were observed and long marks parallel to the crack propagation direction were also found for slanted fracture surfaces. When a crack propagated along the original Mode II direction, many frictional marks parallel to the crack propagation direction were observed Photomicrographs, 29 ref.
Effect of internal creep mierocracks on fatigue macrocrack propagation at high temperatures. Ohtani, R., Kitamuru, T. and Zhou, W. Mater. Sci. Res. Int. (1995) 1, 238 246 The cycle-dependent pure fatigue crack propagation and the time-dependent creep-fatigue one, in general, were scarcely accompanied with microcracks and voids or cavities in the vicinity of the tip of propagating cracks inside the material when subjected to relatively high strain rate cycles at intermediatc temperatures. In very low strain rate and/or high temperature conditions, however, diffusion-controlled creep cavities and microcracks tend to be gener ated along grain boundaries at the mid-thickness. These cavities and microcracks affect the macrocrack propagation in high temperature fatigne m two ways: the decrease in the crack propagation resistance and the decrease in the crack driving force, i.e. the J-integral. Consequently, the creep-fatigue crack propagation is accelerated by several times and the succeeding fatigue crack propagation in the pre-creep damaged material by ten times at most for a given J-integral range, A J. Photomicrographs, 17 ref.
Ductile crack initiation in steels, b3,rahimi, F. and Seo. H.K. Acta Mater. il996) ,14, 831 843 The process of ductile crack initiation has been investigated for a ferritic. pearlitic and a bainitic structural steel. A three-dimensional analysis has revealed that the ductile crack initiation occurs by formation of disconnected microcracks along the crack front. While the distribution of inclusions and highly strained regions governs the microcracking in the ferritic-pearlitic steel, the geometrical inhomogeneities produced by fatigue precracking act as the sites for tbrmation of microcracks in the bainitic steel. In the latter steel, the size of the carbides is two orders of magnitude smaller than the crack tip opening produced by fatigue precracking, and hence, ductile crack initiation can not be modelled as the coalescence of the first microvoid with the crack tip. Photomicrographs, Graphs, 8 ref.
Contribution to the comparative analysis of some periodical multiaxial criteria [Contribution a I'analyse comparative de criteres de fatigue multiaxiaux periodiques. Fogue, M., Robert, J.-L. and Bahuaud, J. Mecanique lndust Materiau_r (1995) 48, 136,139 Periodical multiaxial latigue criteria for long and limited life, are generally based on one of these two models: the global concept characterized by a damage function issued from an energizing (quadratic) formulation as Von Mises; or the local concept, characterized by the search of the facet where the damage function is the highest one. After presentation of the two concepts, a method of comparison is developed; it is based on simple experimental observations of material fatigue behaviour. Thereafter, the comparison of a set of eight fatigne criteria is realized by the use of experimental test results obtained by man 3 laboratories. 16 ref.
Response spectrum methods in tank-vehicle design. Ol@son, U., Sven~so,t, T. and Torstensson, H. E.W. Mech. (1995) 35. 345 351 Existing design rules for aluminium tank vehicles have proved insufficient, because vibrations often cause fatigue cracks. Measurements have been performed on the tank to provide a picture of the influence of different road types and filling ratios. Shock response spectrum analysis (SRS) was used to obtain a measure of single dynamic events. To get a basis for dimensioning against fatigue a calculation of fatigue~lamage response spectra (FDRS) has been performed. This relates the vibrations in the tank to the risk of fatigue damage In lieu of cycle counting by rain-flow count techniques, which has certain disadvantages, a new model, the so-called HdM model, for fatigue-fife assessment based on level crossing has been used. Further development of the results of the analysis can he used to improve design criteria in transport tank regulations, and Io determine uptimum inspection intervals for, in particular, tanks for hazardous materials. Spectra, 17 ref.
185
Failure analysis of aluminium wire bonds in high power IGBT modules. Dittmer, K.J., Poech, M.H., WMFF, F.W.. and Krumm, M. Materials Research SocieO'. Conference: Electronic Packaging Materials Science VIII, San Francisco, Call fornia, USA (17-20 Apr. 1995), pp. 251 256 The reliability of high power insulated gate bipolar transisto~ (IGBT) modules is mainly limited by the aluminium wire bonds. Bond failure occurs due to fatigue fracture close to the welded interface inside the large bonding wire caused by thermal cycling at extreme operating conditions. This paper presents the results of the metallographic and electron microscopic examinations of the welded interface and fracture morphology investigations of failed parts after power cycling. Infrared thermal imaging and investigations of the stress/strain relationship analyze the differential expansion caused by temperature distri bution and thermal expansion mismatch between large bonding wire and semiconductor. Photomicrographs, Graphs, 6 ref
Fatigue crack growth behaviour of small Sn-Bi-Ag solder ,joints. Liu, 1', Korhonen, M.A., Ting, S., Kim. D. and Li, C. Y. Materials Research Society Confi'rence. Electronic Packaging Materials Sciem e VIII, San Francisco, Cali fornia, USA (17-20 Apr. 1995), pp. 177 182 Reliability of solder interconnects is normally characterized by extrapolation of bulk solder reliability data, such as crack growth measurements of solder materials in the bulk form. In the present study, experimental evidence is presented to show that crack growth, which is the most commonly observed failure mode in small solder joints strongly depends on the size and shape of the joint cross-section. Furthermore, the crack growth rate can be appropriately correlated to the size of the intense plastic yielding zone at the crack tip using a plastic decohesion argument. The solder tested in this study was a lead-free 57Bi/41SIU2Ag alloy used to join pairs of Cr/Cu/Au metallized contact pads patterned onto silicon. Graphs, Photomicrographs. 5 t e l
Mechanical behaviour of eutectic Sn-Ag a n d S n - Z n solders. Mavoori, tt., Vaynman, S.. Chin. J., Moran, B., Keer, L.M. and Fine, M.E. Materials Research Society Conj~rence: Electronic Packaging Materials Science VIIL San Francisco, California, USA il7 20 Apr. 1995), pp. 161 175 Eutectic Sn Ag and Sn Zn solders are currently being investigated as the basis for replacement of Sn-Pb solders. Some mechanical properties ot these solder systems: fatigue, tensile, stress relaxation and creep are presented here along with some damage feature observations [i~r eutectic Sn-Ag solder. Senti empirical modeling, constitutive modeling and numerical simulation based on damage models have been carried out for lifetime prediction. Pivliminary agreement of this numerical simulation with experimental results appears encouraging, but the models need fnrlher relinement Graphs, Photomicrographs, 21 ref.
A novel method for determining fatigue behaviour of notched concrete beams [Neues Prufverfahren zur Ermittluug des Ermudungsverhaltens yon gekerbten Betonbalken]. Krupke, H.-J. Materialprufi4n~ (1995) 37, 464-467 Within the framework of a researcll program sponsored b~ Ihe I)eutsche Forschungsgemeinschafl, an experimental method Iot characterizing fatigue behaviour of concrete beams was to be experimentally accomplished to given criteria. Considering features of fracture mechanics by means of this procedure. it shall be attempted to describe hehaviour of notched specimens under dynamic bending stress 8 ref.
Testing fatigue of thin-sheet specimens, structural stress as a basis for improvement of the comparability of fatigue test results [Schwingfestigkeit gefugter Dunnblechproben prnfen. Die Strnkturspannung als Grundlage fur die Verbesserung der Vergleichbarkeit von Wohlerlinien]. Weihel. K. P.. and Wecker. A. Materialpr@tng (1995~ 37. 452M-56 Today powerful computers and software are available Ibr designer and CAEengineers. But the operating strength data are frequently not available. So lar test results exist, an unique presentation is missed. This paper deals with the structural stress as ordinate of the Wohler-('urve. It ma) contribute to an unique presentation of test results and impro,.ed use ot them h} the designer Graphs, 8 ref.
Spring elements in car production, fatigue under the aspect of total quality management [Federnde Elemente im Farhzeugbau. Sehwingfestigkeit nnter dem Aspekt yon 'Total Quality M a n a g e m e n t ' ] Bep:~mann..I.W. and ftuek, M MaterialprtCung (19q5) 37, 448M-51 Total quality management as an overall instrument of quafit~ as~.urancc requires a close cooperation of material, spring and car producer. Th~s paper deals with tile assurance of the strength of springs during development and pmducfion. The rotating bending fatigue test is snitable for ewduafion ol the original material. A single spring coil is used effectively in order to tesf the total spring. Particular attention should be directed Io protection againsl stone impact and corrosion. Photomicrographs, Graphs, 8 rel
Fatigue performance of old bridge steel and the procedures for life prediction with given survivability. Zhem,,. X l i, Z ~,'tli. ) . ?'ant:, Y. and Shi, Z. Engng FrucL Mech. tJall. 1996) 53. 251 262 In the present study, the fatigue life o| the steel of an old bridge on the lille from Beijmg to Shenyang, which was constructed hi the 102(is and replaced
Fatigue Abstracts
Spot-welded double-hat specimen under torsional loading [Punktgeschweisste Doppelhutprofilprobe unter Torsionsbelastung]. Radaj. D. and Zhang, S. Moterialprufung (1995) 37, (11 12), 458~162
Behaviour of initiation and growth of short fatigue cracks. Wu, Z aml Xu, H. Acta Metall. Sini('a (English Lett) (1996) 9 (1), 65 70
The local parameters (structural stress, stress intensity factors and notch stresses) at the nugget edge of the spot-welded double-hat specimen under torsional loading are numerically determined based on engineering formulae and finite element analysis dependent on the geometrical design parameters and their production-related deviations. The paper presents an assessment of the t~ltigue-relevant sustainable local parameters. The diagrams and formulae allow one to express the test results from specimens with special dimensions in terms of a uniform reference specimen. Graphs, 9 ref.
Rotary bending fatigue tests were carried out using smooth specimens ol ~l medium carbon steel, Fe~).43C-0.66Mn-0.23Si-/).I4Cu4).07Ni~0.05Cr, with two different grain sizes. The process of early crack development was observed by the replica method, and the effects of grain size and microstructure on short crack development were studied, It was shown that the initiation process. of fatigue cracks is that the damaged region is intensified gradually, and the growth of short cracks relates to the coalescence of cracks. The grain size and microstructure have a great influence upon the initiation and growth of shola cracks. A reasonable definition was given in relation to shorl fatigue crack initiation and propagation. Graphs, Photomicrographs, 5 ref.
Low activation braze joint of dispersion-strengthened copper. Chen. S. and Chin. B.A. .L Nucl. Mater. (15 Aug. 1995) 225 (I 3), 132-136 This paper presents the results of an investigation of a commercial braze alloy (BCuP-3), which was chosen because of its low activation characteristics. The braze could be used to join alumina dispersion-strengthened copper alloys in fusion reactor applications. The brazes were made using a Gleeble 1500 system with which time, temperature and stress could be precisely controlled. Excellent braze strengths were obtained by controlling the heating rate, braze hold time and cooling process. However, the brazed joints exhibited low ductility in both tensile and fatigue tests. The investigation has shown that a redistribution of the strengthening alumina particles occurs in the region near the brazed joint. This redistribution results in a higher density of alumina particles building up in a layer near the braze joint interface. Fractography showed an intergranular fracture pattern across this transition zone indicating that it is this interface that is responsible for the observed low ductility of the braze joint.
Study of fatigue and fracture of poly-, bi- and single-crystals of C u 18.2Zn-7.0AI shape memory alloys [Estudio de fatiga y fractura de monocristales, bicristales y policristales de la aleacion Cu-18.2Zn-7.0AI con memoria de forma]. Gil, F.X. and Guilemany. J.M. Revista Metal. (1995) 31, 222-227 Mechanical properties--tensile, compression and fatigue stresses <)f poly , biand single-crystals of Cu-18.2Z-7.0AI have been studied. The relationship between mechanical behaviour and microstmcture has been determined. Photo micrographs, 15 ref.
Fatigue design d a t a and concepts of aluminium structures. V. Takeuchi, K. Kei Kinzoku Yosetsu (J. Light Metal Welding Construction) (May 1995) 33 (5), 37-43 The fatigue strength of aluminium alloys under different roughness and surface damage is reviewed. Various A1 alloys (2000, 5000, 6000, and 7000 series) are used as examples and their S-N curves are given. The influence of cycling rate on fatigue strength is also discussed. Graphs, 11 ref.
Effects of roughness-induced crack closure on fatigue crack growth in AC4CH cast aluminium alloys. Kumai, S., Sekikawa, A., Hu, J.Q., Higo, Y. and Nunomura, S. J. Japan Inst. Light Metals (1995) 45 (4), 204-208 The role of extrinsic mechanism which improves fatigue crack resistance of the cast alloy has been examined. A constant load amplitude fatigue crack growth test and a crack closure measurement have been performed using AC4CH cast aluminium alloys. The da/dN- A K relation and the crack closure levels were compared between the cast alloys and squeeze cast or powdermetallurgy processed 6061 Al alloys. Improved fatigue crack growth resistance at the low A K range was shown for the cast alloy in comparison with the powder-metallurgy alloy with finer microstructure, when evaluated by the da/dN- A K relation. While, the difference was diminished by evaluation based on the da/dN- A K~n relation. This indicates that the roughness-induced crack closure effect is large in the cast alloys to improve the crack growth resistance in the low A K region. This extrinsic mechanism is considered to be originated from the deflected fatigue crack growth path and the resultant rough fatigue fracture surface, which are attributed to the dentritic solidification structure and the eutectic Si particle distribution in the cast alloy. Graphs, Photomicrographs, 11 ref.
Effects of solidification structure on fatigue crack growth in AC4CH cast aluminium alloys. Kumai, S., Hu, J.Q., Higo, Y. and Nunomura, S. J. Jap. Inst. Light Metals (1995) 45 (4), 198-203 The fatigue crack growth behaviour of a AI-Si-Mg base cast aluminium alloy (AC4CH) has been investigated with special reference to the effects of solidification stmcture. Fatigue crack growth tests under the constant load amplitude condition have been performed using CT type specimens of systematically controlled solidification structures with dendrite a n n spacing, size and morphology of eutectic Si particles and matrix aging conditions. The results were discussed from viewpoints of microstructures and fracture mechanics. The da/dN- A K relationship reveals that refining and spheroidizing of eutectic Si particles improve fatigue crack growth resistance in the high A K range, while the crack growth rate at the lower limit range is not influenced by morphology of the particles. Matrix aging condition influences little the nearthreshold and the Paris regime fatigue crack growth. The crack growth resistance in the high A K range is improved by increase in ductility of matrix. It is, however, deteriorated by the coarser dendritic structure which yields the higher crack growth resistance near the threshold level. Graphs, Photomicrograbs, 12 ref.
A micro-meso-process theory of fatigue crack initiation and theory of internal fatigue limit. Wang, R.Z.. Ra, J.L.. Li, X.B., Yao, M.. Li, .I.K.. Wang. S.P. and Shao, P.G. Trans. Metal Heat Treatment (China)(1995) 16 (4), 26-34 Analyzing for various models of fatigue crack initiation and observing t\~r the phenomena of fatigue crack initiation of surface strengthened metals, it was shown that existing models of crack initiation are not very perfect. Therefore a new "micro-meso-process theory of fatigue crack initiation" and 'theory of internal fatigue limit' (a new characteristic parameter in the classical theory of fatigue limit) have been proposed. According to the theories mentioned above a lot of existing phenomena and problems difficult to understand on the fatigue could be explained satisfactorily. 20Cr, 40Cr, 30CrMo, GC4, LC9 arc discussed. Photomicrographs. 8 ref.
Fatigue lifetime prediction methods: an analysis of the different approximations involved in local approaches. Chaudonneret, M. and Robert, M. Int. J. Pressure Vessels Piping (1996) 66 (1-3), 113 123, Conferem'e: 3rd International Colloqium." Ageing of Materials and Methods ./~r Assessing th~ Lifetime Extension ~f Engineering Plants March 1995 This paper deals with different aspects included m fatigue life prediction algorithms of metallic components under complex uniaxial Ioadings: plastic. damage and loading analysis. It attempts to identify and analyse the various models generally used at each of these steps. Some comparisons arc made with experimental results. Graphs, 15 ref.
Microstructural effects on small fatigue crack initiation and growth in Ti6AII4V alloys. Demulsant, X. and Mendez, J. Fatigue Fracture Engng Mater. Structures (1995) 18, 1483 1497 The nucleation and growth of small naturally initiated cracks have been investigated on Ti 56 AI 4V alloys, with bimodal or globular microstructures. Tests have been performed in air at a stress amplitude near 0.75 of the yield stress. The influence of microstructure on fatigue damage is described and the differences in fatigue life of some titanium alloys are explained. The effect ol microstmcture on fatigue resistance is mainly related to the early stages of damage including initiation and small crack growth. Coalescence processes favoured by a high density of initiated surface microcracks in fine oL grain material, or rapid initiation of large cracks in coarse o~ colonies, explains low fatigue resistance or lifetime scattering in globular alloys. Small crack growth retardation due to od/3 barriers associated with a low surface crack density, limiting coalescence processes, explains the highest resistance of bimodal structures. Photomicrographs, Graphs, 32 ref.
Predicting fatigue life in commercially pure aluminium using a short crack growth model. Turnbull, A. and de los Rios, E.R. Fatigue Fracture Engng Mater. Struct. (1995) 18, 1469-1481 Advances in manufacturing techniques and improvements in non-destmctive inspection procedures have reduced the size of initial defects in fatigue-critical components to the scale of the microstmcture. Unfortunately, the growth of these so-called short fatigue cracks is not properly characterized by conventional linear elastic or elastic-plastic fracture mechanics. However, the Navarro-de los Rios crack growth model does account for many aspects of their growth and has proved successful in predicting fatigue life of medium carbon and C Mn steels. In consequence, the growth of short fatigue cracks in commercially pure Al, under fully reversed uniaxial loading, has been studied, using a plastic replication technique and modelled, using the Navarro~le los Rios crack growth equation. The predicted lifetimes are in good agreement with the actual experimental lifetimes, and compare favorably with predictions based on two previously reported crack growth models. Graphs, Photomicrographs, 35 ref.
The effect of grain size on the fatigue of commercially pure aluminium. Turnbull, A., de los Rios, E.R. Fatigue Fracture Engng Mater. Struct. (1995) 18, 1455-1467 Fully reversed uniaxial fatigue tests were performed on polished hour-glass specimen of commercially pure aluminium with three different grain sizes, in order to examine the effect of grain sizes on fatigue. The growth of surface cracks was monitored by a plastic replication method. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. Multiple crack initiation and growth is a feature of the fatigue of AI, while the grain size influences the specific detail of crack coalescence. Crack path deviation
Fatigue Abstracts is greatest in the coarse grained microstmcture and crack surface roughness is more pronounced. SEM fractography reveals that crack initiation and early crack growth takes place along crystallographic slip planes, and that fatigue striations, characteristic of stage II cracking, extend to the very edge of the specimen section, suggesting extensive crack tip blunting. Photomicrographs. Graphs, 32 ref
Fatigue crack growth under Mode II loading. Wang, M.O., Hu, R.H.. Qmn, C.F. and Li. .I.C.M Fatigue Fracture t'.'ngng Mater. Struct. 11995) 18, 1443-1454 The behaviour of iatigue crack growth for low and medium carbon steels, an austenitic stainless steel and an aluminium alloy under pure Mode II loading was investigated experimentally, using cruciform specimens. The results show that under pure Mode II loading, fatigue crack propagation has three possi bilities, namely, bifurcation into two branches, propagation along the original Mode I1 direction, and the mixture of these two situations, depending on the material. The growth rate daMN vs A K H relation for Mode 11 propagation i~ similar to a Paris type law for Mode 1 growth. Fractographic observations by optical microscopy and SEM were made also on all specimens tested. When a crack branched, striations parallel to the crack front which were often associated with Mode 1 fatigue crack growth were observed and long marks parallel to the crack propagation direction were also found for slanted fracture surfaces. When a crack propagated along the original Mode II direction, many frictional marks parallel to the crack propagation direction were observed Photomicrographs, 29 ref.
Effect of internal creep mierocracks on fatigue macrocrack propagation at high temperatures. Ohtani, R., Kitamuru, T. and Zhou, W. Mater. Sci. Res. Int. (1995) 1, 238 246 The cycle-dependent pure fatigue crack propagation and the time-dependent creep-fatigue one, in general, were scarcely accompanied with microcracks and voids or cavities in the vicinity of the tip of propagating cracks inside the material when subjected to relatively high strain rate cycles at intermediatc temperatures. In very low strain rate and/or high temperature conditions, however, diffusion-controlled creep cavities and microcracks tend to be gener ated along grain boundaries at the mid-thickness. These cavities and microcracks affect the macrocrack propagation in high temperature fatigne m two ways: the decrease in the crack propagation resistance and the decrease in the crack driving force, i.e. the J-integral. Consequently, the creep-fatigue crack propagation is accelerated by several times and the succeeding fatigue crack propagation in the pre-creep damaged material by ten times at most for a given J-integral range, A J. Photomicrographs, 17 ref.
Ductile crack initiation in steels, b3,rahimi, F. and Seo. H.K. Acta Mater. il996) ,14, 831 843 The process of ductile crack initiation has been investigated for a ferritic. pearlitic and a bainitic structural steel. A three-dimensional analysis has revealed that the ductile crack initiation occurs by formation of disconnected microcracks along the crack front. While the distribution of inclusions and highly strained regions governs the microcracking in the ferritic-pearlitic steel, the geometrical inhomogeneities produced by fatigue precracking act as the sites for tbrmation of microcracks in the bainitic steel. In the latter steel, the size of the carbides is two orders of magnitude smaller than the crack tip opening produced by fatigue precracking, and hence, ductile crack initiation can not be modelled as the coalescence of the first microvoid with the crack tip. Photomicrographs, Graphs, 8 ref.
Contribution to the comparative analysis of some periodical multiaxial criteria [Contribution a I'analyse comparative de criteres de fatigue multiaxiaux periodiques. Fogue, M., Robert, J.-L. and Bahuaud, J. Mecanique lndust Materiau_r (1995) 48, 136,139 Periodical multiaxial latigue criteria for long and limited life, are generally based on one of these two models: the global concept characterized by a damage function issued from an energizing (quadratic) formulation as Von Mises; or the local concept, characterized by the search of the facet where the damage function is the highest one. After presentation of the two concepts, a method of comparison is developed; it is based on simple experimental observations of material fatigue behaviour. Thereafter, the comparison of a set of eight fatigne criteria is realized by the use of experimental test results obtained by man 3 laboratories. 16 ref.
Response spectrum methods in tank-vehicle design. Ol@son, U., Sven~so,t, T. and Torstensson, H. E.W. Mech. (1995) 35. 345 351 Existing design rules for aluminium tank vehicles have proved insufficient, because vibrations often cause fatigue cracks. Measurements have been performed on the tank to provide a picture of the influence of different road types and filling ratios. Shock response spectrum analysis (SRS) was used to obtain a measure of single dynamic events. To get a basis for dimensioning against fatigue a calculation of fatigue~lamage response spectra (FDRS) has been performed. This relates the vibrations in the tank to the risk of fatigue damage In lieu of cycle counting by rain-flow count techniques, which has certain disadvantages, a new model, the so-called HdM model, for fatigue-fife assessment based on level crossing has been used. Further development of the results of the analysis can he used to improve design criteria in transport tank regulations, and Io determine uptimum inspection intervals for, in particular, tanks for hazardous materials. Spectra, 17 ref.
185
Failure analysis of aluminium wire bonds in high power IGBT modules. Dittmer, K.J., Poech, M.H., WMFF, F.W.. and Krumm, M. Materials Research SocieO'. Conference: Electronic Packaging Materials Science VIII, San Francisco, Call fornia, USA (17-20 Apr. 1995), pp. 251 256 The reliability of high power insulated gate bipolar transisto~ (IGBT) modules is mainly limited by the aluminium wire bonds. Bond failure occurs due to fatigue fracture close to the welded interface inside the large bonding wire caused by thermal cycling at extreme operating conditions. This paper presents the results of the metallographic and electron microscopic examinations of the welded interface and fracture morphology investigations of failed parts after power cycling. Infrared thermal imaging and investigations of the stress/strain relationship analyze the differential expansion caused by temperature distri bution and thermal expansion mismatch between large bonding wire and semiconductor. Photomicrographs, Graphs, 6 ref
Fatigue crack growth behaviour of small Sn-Bi-Ag solder ,joints. Liu, 1', Korhonen, M.A., Ting, S., Kim. D. and Li, C. Y. Materials Research Society Confi'rence. Electronic Packaging Materials Sciem e VIII, San Francisco, Cali fornia, USA (17-20 Apr. 1995), pp. 177 182 Reliability of solder interconnects is normally characterized by extrapolation of bulk solder reliability data, such as crack growth measurements of solder materials in the bulk form. In the present study, experimental evidence is presented to show that crack growth, which is the most commonly observed failure mode in small solder joints strongly depends on the size and shape of the joint cross-section. Furthermore, the crack growth rate can be appropriately correlated to the size of the intense plastic yielding zone at the crack tip using a plastic decohesion argument. The solder tested in this study was a lead-free 57Bi/41SIU2Ag alloy used to join pairs of Cr/Cu/Au metallized contact pads patterned onto silicon. Graphs, Photomicrographs. 5 t e l
Mechanical behaviour of eutectic Sn-Ag a n d S n - Z n solders. Mavoori, tt., Vaynman, S.. Chin. J., Moran, B., Keer, L.M. and Fine, M.E. Materials Research Society Conj~rence: Electronic Packaging Materials Science VIIL San Francisco, California, USA il7 20 Apr. 1995), pp. 161 175 Eutectic Sn Ag and Sn Zn solders are currently being investigated as the basis for replacement of Sn-Pb solders. Some mechanical properties ot these solder systems: fatigue, tensile, stress relaxation and creep are presented here along with some damage feature observations [i~r eutectic Sn-Ag solder. Senti empirical modeling, constitutive modeling and numerical simulation based on damage models have been carried out for lifetime prediction. Pivliminary agreement of this numerical simulation with experimental results appears encouraging, but the models need fnrlher relinement Graphs, Photomicrographs, 21 ref.
A novel method for determining fatigue behaviour of notched concrete beams [Neues Prufverfahren zur Ermittluug des Ermudungsverhaltens yon gekerbten Betonbalken]. Krupke, H.-J. Materialprufi4n~ (1995) 37, 464-467 Within the framework of a researcll program sponsored b~ Ihe I)eutsche Forschungsgemeinschafl, an experimental method Iot characterizing fatigue behaviour of concrete beams was to be experimentally accomplished to given criteria. Considering features of fracture mechanics by means of this procedure. it shall be attempted to describe hehaviour of notched specimens under dynamic bending stress 8 ref.
Testing fatigue of thin-sheet specimens, structural stress as a basis for improvement of the comparability of fatigue test results [Schwingfestigkeit gefugter Dunnblechproben prnfen. Die Strnkturspannung als Grundlage fur die Verbesserung der Vergleichbarkeit von Wohlerlinien]. Weihel. K. P.. and Wecker. A. Materialpr@tng (1995~ 37. 452M-56 Today powerful computers and software are available Ibr designer and CAEengineers. But the operating strength data are frequently not available. So lar test results exist, an unique presentation is missed. This paper deals with the structural stress as ordinate of the Wohler-('urve. It ma) contribute to an unique presentation of test results and impro,.ed use ot them h} the designer Graphs, 8 ref.
Spring elements in car production, fatigue under the aspect of total quality management [Federnde Elemente im Farhzeugbau. Sehwingfestigkeit nnter dem Aspekt yon 'Total Quality M a n a g e m e n t ' ] Bep:~mann..I.W. and ftuek, M MaterialprtCung (19q5) 37, 448M-51 Total quality management as an overall instrument of quafit~ as~.urancc requires a close cooperation of material, spring and car producer. Th~s paper deals with tile assurance of the strength of springs during development and pmducfion. The rotating bending fatigue test is snitable for ewduafion ol the original material. A single spring coil is used effectively in order to tesf the total spring. Particular attention should be directed Io protection againsl stone impact and corrosion. Photomicrographs, Graphs, 8 rel
Fatigue performance of old bridge steel and the procedures for life prediction with given survivability. Zhem,,. X l i, Z ~,'tli. ) . ?'ant:, Y. and Shi, Z. Engng FrucL Mech. tJall. 1996) 53. 251 262 In the present study, the fatigue life o| the steel of an old bridge on the lille from Beijmg to Shenyang, which was constructed hi the 102(is and replaced