Hitoshi Yoshinari

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.

Computer Simulation of Fast Crack Propagation and Arrest in Steel Plate with Temperature Gradient Based on Local Fracture Stress Criterion

A fracture mechanics model for fast crack propagation and arrest is proposed based on the local fracture stress criterion. Dynamic fracture toughness (K{sub D}) for a propagating crack is calculated as a function of crack velocity and temperature. The model is extended to incorporate the effect of unbroken ligament (UL) formed near the plate surfaces and crack-front-tunneling. The model simulates acceleration, deceleration and arrest of a crack in a ESSO or a double-tension test plate with temperature-gradient. Calculated arrested crack lengths compare well with experimental results. It is shown that the conventional crack arrest toughness calculated from applied stress and arrested crack length depends on temperature-gradient and the toughness is not a unique material property.

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.

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.

Prediction of the fatigue crack propagation behavior under various random loading using crack closure concept.

Structures are often subjected to various types of random loading in service. Hence fatigue crack propagation under random loading is one of practically interesting topics. This study aims at finding empirically a practical relation for predicting the fatigue crack propagation rate under different types of random loading from constant amplitude loading tests.The specimen used was CCT (center crack tension) one with 3.2mm thick, made of JIS SPCC mild steel. Several patterns of power spectrum with different band widths in frequency were used, and the mean stress level was changed from compression to tension to identify the crack closure effect. Cycle counting was made using “hysterisis loop method”. The constant amplitude loading test was carried out to compare with the random loading test. The stress range ΔSr of random loading was converted into the fixed value ΔSeq (equivalent stress range). By using this ΔSeq and considering the crack closure effect, a possibility has been shown that the fatigue behavior under random loading may be predicted from the constant amplitude loading test.

Monte Carlo Simulation of the Structural Reliability of Offshore Structure with Preexisting Defects

It is indispensable for the material selection for offshore structures, particularly used in low temperature environments, to take good care of preventing brittle fracture initiation. Fatigue crack growth from the initial defects such as weld defects in highly strain concentrated region may cause an extensive brittle fracture. Thus, the toughness of the material must be high enough to be safe from brittle crack initiation from the most critical crack which is anticipated to exist in the structure. The information about defect size in structures is very important for rational procedures to make a relevant material selection.

Crack arrest behavior of fast fracture in steel.

The concept of crack arrester, such as the insertion of a high toughness plate, is an effective means to avoid the catastrophic brittle fracture in welded steel structure. Many researches have been made to clarify the arrest behavior of brittle fracture and to establish the arrester design concept, but an inadequacy still exists in the arrest theory.In this study, three types of crack arrest tests, i.e. temperature gradient type double-tension test, stress gradient type double-tension test and surface notched double-tension test were carried out by using mild steel. The data obtained were analyzed from the standpoint of the energy balance concept on the basis of dynamic fracture mechanics.Shear lips which are related to plastic deformation around a propagating crack tip were found to contribute to crack arrestability of steel because they can absorb rather a large amount of energy. And the dynamic fracture toughness KDb corresponding to a brittle fracture surface, which can be evaluated by subtracting the contribution of shear lips from the total absorbed energy, seemed to be defined as a function of crack speed and temperature. Thus, KDb is a material quantity and the arrest behavior of brittle crack may be interpreted generally by this value.Further work to find out the conditions controlling shear lip formation is necessary for the prediction of crack run/arrest phenomenon.