Susumu Machida

On the effect of cyclic stress ratio on the fatigue crack propagation

Abstract A new model of fatigue crack propagation is proposed which takes the effect of cyclic stress ratio into account. In the model it is assumed that the fatigue crack propagation rate is proportional to the absorbed hysterisis energy per stress cycle at the tip of a crack. The energy is calculated from stress field resulting from the Dugdak-Barenblatt Model and strain field from an experimental result. The model was applied to analyse the experiments on several materials.

Study on fast fracture and crack arrest: Experimental results of brittle-crack propagation and arrest on structural steel are analyzed using the finite-difference method

In this paper, the experimental results of brittle-crack propagation and arrest on structural steels are analyzed using the finite-difference method. The test specimens used are double-tension and double-cantilever type (DCB).It is found that dynamic consideration is indispensable for general interpretation of fast fracture and crack arrest and material toughness can be defined as a function of temperature and crack velocity. Using the defined fracture toughness, crack-propagation-arrest behavior can be predicted by crack-propagation simulation if the condition of boundary loading of a specimen is known. This implies a possibility to develop simple methods for crack-arrester design.

Study on fast fracture and crack arrest

In this paper, the experimental results of brittle-crack propagation and arrest on structural steels are analyzed using the finite-difference method. The test specimens used are double-tension and double-cantilever type (DCB).

Preliminary Approaches to Experimental and Numerical Study on Fast Crack Propagation and Crack Arrest

Within the limit of relatively short arrested crack, static approximation by applying linear fracture mechanics has yielded useful results for theoretical interpretation and practical application of currently used brittlefracture propagation arrest test (the concept of arrest toughness). But later experimental investigations using very wide specimens (1300 to 2500-mm-wide plates) have revealed that this simple interpretation fails to have a consistency with the case of a long arrested crack. For the inadequacy of static approximation based on arrest toughness concept, the concept of an effective stress intensity factor has been introduced from a practical point of view without a clear quantitative explanation for it. In search of a more relevant theory for fast crack propagation and to see how the neglect of dynamic aspects affects the interpretation of unstable crack propagation arrest test and philosophy of crack arrest design, a dynamic fracture mechanics analysis was made with the use of the finite-difference method to solve the equation of motion for the two-dimensional elastic problem. Results of a numerical experiment are presented, and experimental results for polymethylmethacrylate specimen and structural steels are discussed in terms of dynamic fracture mechanics analysis with a focus on energetic aspect of the crack propagation processes.

Fatigue crack propagation under various types of loading

ABSTRACT A model of fatigue crack propagation was developed with consideration of the behaviour of crack opening and closing during the part of cycles and the limiting stress at which no cumulative cyclic plastic energy has occured, namely “consuming stress”. The effects of stress ratio and over- and/or under-spike load on fatigue crack propagation were analyzed by using the model. An example of δKth value under zero-to-tension stress condition was also calculated by using the model. The results show the good agreement with expriments.

Recent Studies on Brittle Crack Propagation and Arrest in Japan

This report reviews studies made in Japan recently on predicting the brittle crack propagation and arrest behaviors using the dynamic or simplified dynamic technique. Also reviewed in the last chapter of this report is an analysis, though statical in technique employed, made of an upper deck plating brittle failure experienced by a 10379 G.T. bulk carrier.

Fatigue crack propagation in welded joints under variable-amplitude loading in synthetic sea water

Abstract Fatigue crack propagation rates and the fatigue threshold in transverse butt-welded joints of centre crack type were measured in synthetic sea water under variable-amplitude loading. The fatigue threshold was about a half of that in air, and the propagation rates near the fatigue threshold were high compared with those in air. The propagation rate for each cycle under variable-amplitude loading could be estimated from the constant-amplitude data, which were obtained in the crack-closure-free condition.

Very low cycle fatigue properties of butt welded joints containing weld defects. Study of acceptable size of defects in girth welds of gas pipelines

Summary This paper describes an investigation of the very low cycle fatigue strength of HT570 steel welded joints containing weld defects to determine the acceptable size of weld defects in girth welds of underground gas pipelines subjected to cyclic ground displacements due to earthquakes. Butt welded joints containing incomplete penetration (IP), blowholes (BH), lack of fusion in theintermedi‐ate pass (LF), and cracks in the penetration bead (CR) were prepared and tested under strain‐controlled conditions. All specimens tested in the present research study satisfy the fatigue design curve for girth welds of underground gas pipelines. The fatigue strength of specimens containing weld defects generally decreases with an increasing equivalent defect size. The shape of a crack initiated from a defect is affected by the reinforcement, with the surface crack propagating rapidly along the weld toe of the penetration weld over the width of the specimen. The relationship between the J‐integral range and the crack propagation rate under a very low cycle fatigue load is virtually the same as the extension of the relationship in the low J‐integral range. A crack propagation analysis based on the defect being regarded as a crack is performed to determine the relationship between the defect size and the number of cycles over the plate thickness. The critical size of defects in welds of gas pipelines under cyclic ground displacements is proposed by the analysis. The critical crack size of surface defects is applicable to buried defects, because surface defects give a more conservative evaluation than buried defects. The analytical results provide more conservative estimations than the experimental ones.

Fatigue crack propagation in welded joints under random loading in an alternating dry/wet environment

Summary A 62 level block fatigue loading pattern approximating the peak Rayleigh distribution was applied at the centre of cracked transverse butt‐welded specimens in which the tensile residual stress was always induced at the crack tip. The test environment alternated between synthetic sea water and ambient air. The fatigue crack propagation rate da/dn obtained in this environment was made lower than that in synthetic sea water by increasing the time in ambient air. The decrease in da/dn was caused by crack closure due to oxide debris despite the high stress ratio conditions around the crack tip.

Fatigue Crack Propagation in Welded Joints in Environment Periodically Changed between Synthetic Sea Water and Ambient Air.

62 level block fatigue loadings approximating to the Rayleigh distribution of peak were applied on center cracked transverse butt welded joint specimens in which tensile residual stress was always induced at the crack tips. The test environment was periodically changed between synthetic sea water and ambient air. The fatigue crack propagation rate, da/dn, obtained in this environment became low compared with da/dn in synthetic sea water by increasing the period in the ambient air. The decrease of da/dn occurred from the crack closure due to oxide debris in spite of the high stress ratio condition around the crack tips.