Yozo Sawaki

Delayed failure in silica glass

Stable crack-propagation behaviour in silica glass as a raw material for optical fibres is studied under static tensile stress in various environments such as distilled water, NaCl aqueous solution, air and dry nitrogen gas, and the influence of these environments is discussed. The crack-growth rate in distilled water is obtained qualitatively as a function of the stress intensity factor and temperature, and the activation energy of the cracking process is determined as 97.6 kcal mol−1. The growth rate seems to be unaffected by Na+ and Cl− ions in an NaCl acqueous solution, but is influenced significantly by the humidity in the atmosphere. In a dry atmosphere, the growth rate in Region II cannot be expressed as a single function of the stress intensity factor. A plot of the log of time to failure against the initial stress intensity factor reveals a linear relationship in the environments tested. The critical fracture stress of an optical fibre is evaluated taking account of the crack size on the basis of fracture mechanics concept.

Fracture toughness and fatigue crack propagation in high strength steel from room temperature to −180°c

Abstract Fracture toughness under tensile test and fatigue test on high strength steel at temperature ranging from room temperature to −180°C were experimentally studied. The value of fracture toughness under fatigue test is considerably tower than that obtained under tensile test. Within the range from room temperature to −100°C the following results were obtained: the power coefficient δ of the fatigue crack propagation rate [(dc)/(dN)] = AΔK 5 is related with [(1)/( T )] as: δ = b 1 + [(a 1 )/(kT)] . [(dc)/(dN)] shows Arrhenius type, and, however, different equation from usual stress dependent rate process equation. The trend is in good agreement with the dislocation dynamics theory of fatigue crack propagation.

Criterion for brittle fracture of notched or cracked specimens based on combined micro- and macro crack mechanics—II

Abstract Brittle fracture criterion based on combined micro- and macro crack mechanics was proposed in the previous paper. In the present paper more improved criterion for the brittle fracture based on this approach has been obtained as a function of ferrite grain diameter, over much wider range of crack tip radius including the case of small crack tip radius. The criterion is in good agreement with the experimental results on brittle fracture of notched or cracked specimens of low carbon steel.

Determination of fatigue fracture toughness by conventional rotary bending specimen

Determination of fatigue fracture toughness,Kfc, is made by rotary bending specimen considering partial contact of fatigue cracked surfaces in the compression side of the beam specimen. It is shown thatKfc is a material constant independent of the nominal stress at the notch section, the specimen geometry, and the shape of the final fracture area.RésuméOn détermine la ténacité à la rupture par fatigueKfc à laide déprouvetes soumises à flexion rotative, en considérant le contact partiel entre les surfaces fissurées par fatigue, au cours de la phase de compression. On montre queKfc est une constante du matériau, indépendante de la tension nominale au droit de la section entaillée, de la géométrie de léprouvette, et de la forme de la surface finale de rupture.

Fatigue fracture toughness and crack propagation rate

Fatigue crack growth rate, da/dN, of two high strength steels were examined in a laboratory air at different stress ratios, covering almost the entire range of stress intensity, ΔK, from nearly threshold value, ΔKth, to final fracture. The fatigue fracture toughness, ΔKfc, corresponding to the final fracture in fatigue, was also determined. The lower the ΔKfc, the higher da/dN and reduced ΔKth are revealed.This correlation was analyzed quantitatively based on the four parameter Weibull function. And the stress ratio dependency of the fatigue crack propagation curve can be cleared in a successful manner.The fatigue characteristic stress intensities, Ke and Kv, are proposed to define the transition behaviour in fatigue crack growth curve, from so called region 1 to 2, and from region 2 to 3, respectively. Especially the Kv valua can be specified to be the 0.63Kfc.RésuméOn a étudié la vitesse de propagation de fissure en fatigue da/dN de deux aciers à haute résistance dans un atmosphère de laboratoire sous des sollicitations couvrant toute la gamme des intensités de contraintes variables ΔK, depuis une valeur voisine de la valeur du seuil ΔKth jusquà celle correspondant à rupture finale.La ténacité à la rupture par fatigue ΔKfe correspondant à la rupture finale par fatigue a été également déterminée. II savère que plus ΔKfe est faible, plus élevée est da/dN et plus ΔKth est réduite. Cette correlation est analysée quantitativement en se basant sur la fonction de Weibull à quatre paramètres. On peut ainsi clarifier la manière dont le rapport de contraintes influe les courbes de propagation des fissures de fatigue.On propose de définir pas les facteurs caractéristiques dintensité de contrainte Ke et Kv les comportements de transition de la courbe de vitesse de propagation de la fissure entre respectivement les régions dénommées 1 et 2, et 2 et 3.En particulier, on peut spécifier que la valeur Kv vaut 0,63 Kfe.

Environmental fatigue fracture toughness and crack propagation behaviour.

The effect of stress ratio, frequency and temperature on the corrosion fatigue crack growth rate of high strength steel has been examined in a 3.5% sodium chloride solution. Corrosion fatigue crack growth data (da/dN - ΔK) observed in the middle range of stress intensity are well expressed by the parallel curve of the data obtained under the conditions of the laboratory air. It is found that the parallel data obtained between these two environments are the results of the reduction of fatigue fracture toughness in corrosive environment. The decreased fracture toughness value is named as an environmental fatigue fracture toughness, Kfe. The hydrogen permeating into the plastic zone at the crack tip is thought to be the main reason for this toughness reduction. It is shown that the Kfe successfully explains the acceleration of fatigue crack growth rate in aqueous environment.