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A shot‐peened specimen with a precrack exhibits the following peculiar fatigue fracture behaviour. (1) Precracks can be rendered harmless by peening, even though the precrack reduced the fatigue limit by approximately 20%–60% of the nonpeened specimen. (2) When shot‐peened specimens were fractured at a higher stress than the fatigue limit, most specimens were fractured from outside the precracked part. (3) In most run‐out specimens, stage II (tensile type) nonpropagating cracks were observed. Why were nonpropagating cracks initiated and arrested? To explain the above fatigue behaviours, a new threshold stress intensity factor range ( ) equation is necessary. In this paper, a equation as a function of crack size was proposed using a nonlinear zone size criterion. By using the equation, the above peculiar fatigue fracture behaviours (1) and (2) and the stage II nonpropagating crack arrest condition could be evaluated quantitatively. Stage II nonpropagating crack initiation could be explained by considering the microdistribution of residual stress due to peening.

Fatigue life and penetration behaviour of cracks in HT80 steel plates were examined experimentally at room temperature for both large and small specimens containing a surface crack. Application of stress intensity factor solutions from small specimens, as proposed by the present authors, to large specimens was investigated. Crack growth behaviour, crack shape and crack opening displacement after penetration can be satisfactorily determined using the K solutions proposed by the authors. NOMENCLATUREa, = one half of the initial surface crack length ab = one half of the back surface crack length a, = one half of the thickness centre crack length aL = one half of the front surface crack length at penetration a, = one half of the front surface crack length b =crack length in the depth direction da/dN = crack growth rate da,/dN = crack growth rate on the front surface dab/dN = crack growth rate on the back surface AK = range of stress intensity factor AKA = range of stress intensity factor on the front surface before penetration AKL, = range of stress intensity factor on the front surface after penetration AKb =range of stress intensity factor on the back surface after penetration N = number of cycles Nab = number of cycles at the first stage after penetration NL = number of cycles at penetration 2 W = plate width 6(a) = crack opening displacement Aaba = crack length at the first stage after penetration Aabb =crack length at the second stage after penetration Y = measure of crack opening displacement

The present study is to investigate the fatigue crack propagation behaviour of surface cracks subjected to combined tension and bending stress. An estimation of fatigue crack growth behaviour employed the Newman-Raju formula before penetration, and a K solution, proposed by the authors, after penetration. Crack length aL at penetration increases with an increase in bending stress. The calculated fatigue crack shape was in good agreement with the experimental shape. It was also found that the crack growth behaviour and the crack shape after penetration can be satisfactorily evaluated using the K solution proposed by the authors. NOMENCLATURE a, = one half of the initial surface crack length aba = crack length at the first stage after penetration a, = one half of thickness centre crack length aL = one half of front surface crack length at penetration a,, ab = one half of front, and back, surface crack lengths, respectively b = crack depth of surface crack e = eccentricity da,/dN, dab/dN = crack growth rate on front, and back, surfaces, respectively F(q) = compensation factor for the tensile stress AKB = stress intensity factor range in the thickness direction AKA, AKD = stress intensity factor range on the front, and back, surface, respectively under combined AKE, AKS = stress intensity factor range on the front, and back, surface, respectively after penetration AK;, AKb, = stress intensity factor range on the front, and back surface, respectively after penetration A,,,, Ab = displacement between central points of centre cracked plates distance Y away from tensile and bending stress under tensile stress under bending stress the crack under tensile and bending stress, respectively Nab = number of cycles at the back surface after penetration 2W= plate width N, NL = number of cycles, and number of cycles at penetration respectively c( = compensation factor 6 =crack opening displacement Auk, Auk = measured tensile and bending stress, respectively Au,,,, Aub = nominal tensile, and bending stress, respectively A u~, dugs = calculated elastic stress on the front, and back surface, respectively @ ( I ) = influence function due to plate thickness Abbreviations AP, BP = after and before penetration, respectively

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Recently, surface crack nondamaging technology has been developed to render harmless surface cracks that can reduce the fatigue limit of metals by 20-60%. This is due to the effects of compressive residual stress induced by shot peening.To improve the reliability of high-strength steel equipment designed against fatigue limit using surface crack nondamaging technology, the following characteristics were determined: (1) the effects of surface crack aspect ratio on maximum harmless crack depth (a hlm ) at high-strength steel for three residual stress distributions; (2) the relationship between the maximum allowable crack depth (a crN ) and a hlm assuming the value of safety factor; (3) the relationship between minimum detectable crack depth via nondestructive inspection (a NDI ) and a hlm , assuming that nondestructive inspection could detect the crack with sufficient probability; and (4) the relationships between a hlm , a NDI , and a crN .

Precipitation behavior of metastable phases in an Al 1.94atCu alloy during isothermal aging at 373 K was investigated by means of Vickers microhardness tests, DSC measurements and TEM observations. The size distribution of the precipitates was quantitatively investigated based on the TEM, HRTEM and HAADF STEM images, and statistical parameters that fit the precipitate size distribution were determined under a log normal distribution approximation. We have successfully estimated the volume fraction of copper in precipitates, and found that the G.P.(II) formation results in increases of volume fraction of metastable particles, mean size and hardness.