Cyclic Loading and Fracture Toughness of Steels

Fatigue crack growth in the intermediate range can be described by the Paris law of linear elastic fracture mechanics. However, if gross plastic deformation is involved, the crack growth rate is often underestimated by LEFM. In the present study the increased crack growth rate due to plastic deformation is determined by using the J-R curve. This model was found to give very good results over a wide range of high crack growth rates for three different geometries of two different steels. A way to predict instability under controlled cyclic load is discussed and is proved to work well in practice, Instability under displacement control is also discussed.

“…The influence of the cyclic loading on the fracture mechanics properties themselves [9], is also of interest when coming close to instability.…”

Section: Discussionmentioning

confidence: 99%

On the Relation Between Stable Crack Growth and Fatigue

Kaiser

1983

“…The paper presents a brief account of the results of studying this problem. First experimental results were published in Refs [1,2], a generalization of the results of investigations of many years was performed in Refs [3,4] and the results of studying this problem by other authors are presented in Refs [5–7] and in other works.…”

Section: Introductionmentioning

confidence: 99%

Fatigue fracture toughness of metals

Troshchenko

2009

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A B S T R A C T The paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.Keywords crack jumps; fatigue; fracture toughness. I N T R O D U C T I O NThe analysis of fractures of machine parts and structures has revealed that in most cases, their fracture is due to material fatigue, which is known to be responsible for the initiation and propagation of fatigue cracks in the course of cyclic loading. Such cracks ultimately lead to complete failure of a component. In the literature, there are many reported investigations related to the fatigue crack development. The peculiarities of the fatigue crack growth at the stage preceding final fracture are the least studied. Knowledge of this behaviour is of particular importance when final fracture of a component is by a brittle mode and with a fatigue crack of small size. In this case, an important characteristic is the fatigue fracture toughness, which is determined directly under cyclic loading. The paper presents a brief account of the results of studying this problem. First experimental results were published in Refs [1,2], a generalization of the results of investigations of many years was performed in Refs [3,4] and the results of studying this problem by other authors are presented in Refs [5][6][7] and in other works. E X P E R I M E N T A L P R O C E D U R E A N D M A T E R I A L SInvestigations involved testing of compact tension specimens, 7.5-150 mm in thickness, on electrohydraulic machines of various capacities. The machines were equipped with systems for specimen cooling and heating that provided variation of the test temperature from 77 to 623 K. The specimen preparation, crack growing procedures and Correspondence: V.T. Troshchenko. E-mail: vtt@ipp.kiev.ua processing of results were in conformity with the accepted standards. Cyclic loading of specimens was performed at a constant load, with the loading frequency varying in the range from 10 to 15 Hz. The stress intensity factors were calculated with the use of the formulaewhere P is the load, a is the crack size, t is the specimen thickness and w is the specimen width. The plane strain conditions and the size of the plastic zone at the crack-tip were determined by the following formulae:2r y = 1 3πwhere σ 0.2 is the material offset yield strength. The materials under study were different heat-resistant steels used for manufacturing high-pressure vessels, including those for nuclear reactors, high-strength chromemolybdenum steels, ductile austenitic steel and titanium alloys. The mechanical properties and fracture toughness characteristics of some of the materials studied are shown in Table 1. Heat-resistant steel II was subjected to a special heat treatment that simulated radiation embritt...

“…In the literature there is conflicting evidence on the proportion of static and fatigue fracture toughnesses of materials [1][2][3][4][5][6][7][8][9][10][11][12]. Cyclic fracture toughness is frequently shown to be higher than the static value [ 1-31.…”

Section: Introductionmentioning

confidence: 99%

Unstable Fatigue Crack Propagation and Fatigue Fracture Toughness of Steels

Troshchenko

Pokrovsky

Ясній

1994

Self Cite

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions.It has been shown that steels in an embrittled state, caused primarily by thermal treatment and lowtemperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature.An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour. NOMENCLATURE a,, of= initial, and final, crack length B = specimen thickness d = crack jump (cleavage zone) length da/dN = fatigue crack growth rate ep = preliminary plastic strain F = specimen cross-sectional area after plastic prestraining F, = specimen original cross-section K,,, K,,, = minimum, and maximum, stress intensity factor (SIF) KO =initial maximum SIF KO, = SIF corresponding to crack opening Kth = A&/( 1 -R) = threshold SIF K , = dynamic fracture toughness K,,, K , = static plane-strain, and plane-stress, fracture toughness Kit, KE = fatigue fracture toughness corresponding to the first, and final, crack jump Pmi,, P, , , = minimum, and maximum, load PM = maximum load under static loading R = stress ratio (R = Kmin/K,.,) T = temperature W = specimen width Aa = fatigue crack extension between two sequential cleavage zones AK = K,, -Kmin = SIF range AK,,, = range of the threshold SIF = 0.2% offset yield stress AK,,,,, = K,,, -KO, = effective range of the threshold SIF o, = ultimate strength (I/ = reduction of area 6 = relative elongation SIF = stress intensity factor