Akio Otsuka

The condition of fatigue crack growth in mixed mode condition

The condition of the initiation of fatigue crack growth in mixed mode conditions has been investigated by using precracked low carbon steel specimens. It is pointed out that, firstly, the critical condition of crack growth should be defined with regard to the modes of fatigue crack growth, i.e. shear mode and tensile mode. Secondly, it is proposed that the critical condition of fatigue crack growth is given by the local tensile stress and shearing stress at the notch tip determined by stress intensity factors KI and KII, and that this criterion is generally applicable to in-plane-loading conditions, i.e. Mode I, Mode II and Mixed Mode conditions.

Relationship between ductile crack initiation and void volume fraction

Abstract An investigation on ductile crack initiation in structural steel has been made, based on the concept of Gursons yield function for porous material. First, the condition of ductile crack initiation in the uniform stress field has been investigated. The condition of ductile crack initiation under various stress triaxiality obtained from the tests on axisymmetric notched tensile specimens is well expressed by the condition of constant void volume fraction analytically obtained from Gursons model. This result means that the condition of constant void volume fraction may be used as the criterion of ductile crack initiation. Secondly, the behavior of void growth and ductile crack initiation in the area near the notch tip under mode I and mode II loading has been investigated. Under mode I loading, the increase in void volume fraction around the notch with an increase in applied load agrees well with the behavior of porous material predicted by the finite element analysis based on Gursons yield function, and the ductile crack initiation can be predicted by the concept of critical void volume fraction as in the case of uniform stress-strain field given above. The same criterion is not applicable to the crack initiation under mode II loading and further study is needed.

Fatigue Crack Initiation and Growth under Mixed Mode Loading in Aluminum Alloys 2017-T3 and 7075-T6

Abstract Fatigue crack initiation and growth characteristics under mixed mode loading have been investigated on aluminum alloys 2017-T3 and 7075-T6, using a newly developed apparatus for mixed mode loading tests. In 2017-T3, the fatigue crack initiation and growth characteristics from a precrack under mixed mode loading are divided into three regions—shear mode growth, tensile mode growth and no growth—on the ΔK I - ΔK II plane. The shear mode growth is observed in the region expressed approximately by ΔK II > 3MPa√m and ΔK II / ΔK I > 1.6. In 7075-T6, the condition of shear mode crack initiation is expressed by ΔK II > 8 MPa√m and ΔK II / ΔK I > 1.6, and continuous crack growth in shear mode is observed only in the case of ΔK I / ΔK II , ≅ 0. The threshold condition of fatigue crack growth in tensile mode is described by the maximum tensile stress criterion, which is given by Δσ θmax √2π r ≅ 1.6MPa√m, in both aluminum alloys. The direction of shear mode crack growth approaches the plane in which K I decreases and K II increases towards the maximum with crack growth. da / dN - ΔK II relations of the curved cracks growing in shear mode under mixed mode loading agree well with the da / dN - ΔK II relation of a straight crack under pure mode II loading.

Crack initiation from a sharp notch and stretched zone

Abstract The “crack tip COD” at the fracture initiation mainly consists of the stretched zone, the new surface which appeared due to the slip deformation at the notch tip. Therefore, the COD or the stretched zone width at the fracture initiation is a very important parameter which reflects the notch tip behavior until fracture initiation. In the case of fibrous crack initiation, the stretched zone width ( S i ) and COD ( δ i ), where δ i ≈ √2 S i , take almost constant values regardless of temperature, specimen geometry, preloading (if the total δ i is taken), slit angle (in the case of mixed mode condition) and so on, while, in the case of cleavage fracture initiation, COD and the stretched zone width take various values between the value at the fibrous crack initiation ( δ i or S i ) and almost zero, depending on temperature and plastic constraint.

Mode II fatigue crack growth characteristics and mechanism in aluminum alloy 7N01-T4 weldments under mode II loading

ABSTRACT Mode II fatigue crack growth behavior has been investigated on 7N01-T4 aluminum alloy weldments by four-point shear loading. da/dN-δKII relations and δKII-threshold for mode II fatigue crack growth have been obtained for base metal-, HAZ-, and weld metal-specimens. HAZ-specimens showed very low δKIIth for mode II growth , compared with those of base metal- and weld metal-specimens . Based on the observations of the deformation near the crack tip at the successive stages of a loading cycle (namely at Kmin, Kmax and Kmin), a mechanism of mode II fatigue crack growth has been proposed.

Static and cyclic fatigue of glass and silicon nitride under tensile and tension-compression fatigue tests

Abstract Static and cyclic fatigue tests were made under static tensile loading and tension-compression cyclic loading at the stress ratios of R = 0 and − 1 on two kinds of silicon nitride (silicon nitride A and B) and borosilicate glass. The tests were made with a specially designed apparatus which was equipped with a device to minimize the load eccentricity. It worked satisfactorily in performing cyclic tension-compression tests at arbitrary stress ratios as well as static tensile tests. The effects of cyclic stress on fatigue strength were investigated. Scarcely any effects of cyclic loading were observed on the strength of silicon nitride A and borosilicate glass, while appreciable degradation in strength due to cyclic loading was observed on silicon nitride B. From the fractographic observation and EDX analysis of the fracture initiation areas, it was shown that the fracture in silicon nitride B occurred by crack growth from defects formed during sintering, while in silicon nitride A, fracture initiation occurred by the cracking formed along the grain boundaries by stress corrosion cracking in the surface layer, and then these cracks linked up to form small cracks and grew by slow crack growth to final failure. The difference between the effect of stress cycling on the strength of silicon nitride A and on that of silicon nitride B, mentioned above, seemed to result from this difference in the fracture initiation process in these materials.

Prediction of fracture toughness by local fracture criterion

Incorporation of micromechanistic criteria of failure to analytical or numerical crack tip stress and strain solutions, known as the RKR (Ritchie, Knott, and Rice) model, derives a certain relation between the fracture toughness and flow/fracture properties of materials. Based on the above analytical prediction, correlations between the fracture toughness and yield stress, cleavage fracture stress, and critical plastic strain of the materials have been investigated for cleavage and ductile fracture. Nine types of low carbon structural steel were tested that had various microstructures with yield strengths of 280 to 1110 MPa. Good correlations according to analytical predictions have been obtained for both cleavage and ductile fracture, and the possibility of quantitative prediction of the fracture toughness from a conventional round bar tensile test can be shown. Effects of metallurgical and mechanical factors on the fracture toughness, and temperature dependence of the fracture toughness, can be explained from their effects on the flow and fracture properties.

Study on Fracture Toughness of Steels by Local Stress Criterion

ABSTRACT Based on the deterministic and statistical local criterion approach, correlations between the cleavage fracture toughness and the yield stress, cleavage fracture stress of the materials have been investigated. Over twenty types of low carbon structural steels in all were tested that had various microstructures, grain size and carbide size with yield strength of 250 to l,110MPa. General formulation to correlate the cleavage fracture toughness and the tensile properties of the materials has been successfully derived. Specimen size effect on the cleavage fracture toughness was also investigated by the statistical weakest link analogy and the possibility of prediction of the KIC from round bar specimens can be shown.

STATIC AND CYCLIC FATIGUE OF SMOOTH TENSILE SPECIMENS AND CRACK GROWTH IN GLASS AND SILICON NITRIDE

ABSTRACT Static and cyclic fatigue tests were made on glass and silicon nitride under static tensile loading and tension-compression cyclic loading at the stress ratios between R = -1 and 1, by using an apparatus specially designed with a device to minimize the load eccentricity. Crack growth tests were also made under static loading and cyclic loading in order to obtain the crack growth rate da/dt, as a function of stress intensity factor K. Test results on smooth round bar specimens show that almost no effects of stress cycling were seen on silicon nitride A and borosilicate glass, while appreciable degradation in strength due to cyclic loading was observed on silicon nitride B. The relation, da/dt versus K, of small cracks obtained from the tensile test on smooth round bar specimens showed much higher values than those of large cracks obtained from the crack growth tests. The increase in crack growth rate due to cyclic stress was observed in both silicon nitride B and A, in the latter of which no effects of stress cycling were observed in the tests on smooth round bar specimens.

Cycle and Crack Length Dependence of Crack Growth Characteristics in Silicon Nitride

Crack growth tests on silicon nitride have been made to clarify the crack growth characteristics under static and cyclic loading. The test results have shown that, under static loading, the crack growth rate decreases with an increase of crack extension until the crack is arrested, while under cyclic loading, the crack grows with almost constant velocity without being arrested, even in the range of K values where no growth is observed under static loading. Crack length dependence of “K-V relation” (K: stress intensity factor, V: crack velocity) and crack velocity dependence of “K-Δa relation” (Δa: crack extension) have been obtained from the crack growth data concerning the relations between crack velocity and crack extension under static loading and cyclic loading. Crack velocity is higher for smaller cracks than larger cracks at the same K values and the effect of stress cycling in accelerating crack growth is much larger at smaller K values. The effect of stress cycling is obvious in crack growth tests even in the material in which no effect of stress cycling is observed on cyclic fatigue tests on smooth specimens.