Notch Root Stress Research Articles

Specimen and automated test method development for investigation of fatigue crack initiation

An economical and effective test specimen has been developed for use in fatigue crack initiation studies. A dynamic materials-testing system was similarly developed for this purpose, consisting of a servohydraulic dynamic testing unit, high-resolution photomacrographic equipment, and computer control systems. The specimen and system permit fully-automated fatigue crack initiation testing, and crack lengths of approximately 0.001 inches have been detected with the peak load amplitude applied to the specimen. This paper presents the development of the specimen, the design constraints imposed by the system and the required test results, and the prototype test program and subsequent specimen modifications. Final specimen dimensions, the correlation between notch root stress and applied load, and recommendations for specimen use are also provided.

Notch root strain and stress distributions in power hardening materials

Based on the results of an elastoplastic analysis for a pure bending V-notched bar, computations of the effective plastic strains at 11 hardening exponents are carried out. It is found that a further modified form of the slip-line solution can satisfactorily describe in the plastic zone the variation of the maximum principal stress below the notch root in power hardening materials.

Effect of notch root radius on fatigue crack initiation

The initiation of a fatigue crack terminates the useful life of small components. The initiation process is intimately related to cyclic plastic deformation and thus occurs mostly at notches where stresses are elevated appreciably. The root radius of such notches dictates the severity of stress and strain concentration. Using notched specimens of varying root radii the initiation lives were determined experimentally. Both the fictitious elastic notch root stress and the local strain at the notch root have been used to correlate with the initiation life. The local strain approach, together with Neuber's rule, is observed to give less scatter. The design-oriented nominal mean stress method of Juvinall yields results comparable with the local strain approach.

Monte Carlo simulation of weldment fatigue strength

Abstract The fatigue strength of constructional steel weldments is predicted using a Monte Carlo simulation which estimates the mean fatigue strength and its expected uncertainty at long lives. The underlying analytical model is at present solely based on fatigue crack initiation and early crack growth and is restricted to long-life applications. Several different kinds of fatigue variables are considered: applied or induced bending stresses, notch severity, notch-root residual stresses, and notch-root mechanical properties. Each of the fatigue variables is treated as a random variable, and its uncertainty is modeled by an appropriate probability distribution function based on laboratory observations. Simulations of constructional steel weldment fatigue strength are compared with laboratory test results on cruciform weldments and similar recent fatigue data reported in the literature. The Monte Carlo simulation also permits one to estimate the contribution of each fatigue variable to the overall uncertainty in fatigue strength and thus determine the economic merits of reducing the uncertainty in any one of the fatigue variables to improve the design fatigue strength.

Simple Methods of Estimating the Notch Root Stress and Strain under Biaxial Load Conditions

切欠き底の応力およびひずみにおよぼす荷重の二軸性の影響を検討し,互いに直交する二方向に引張力あるいは圧縮力の二軸負荷をうける中央切欠き平板の,弾塑性域における切欠き底の応力・ひずみの簡便推定法を提案した.本法によれば,材料の応力-ひずみ関係式,切欠きの形状係数,縦方向と横方向の荷重比などが任意に与えられても,任意の負荷のもとで切欠き底に生ずる応力およびひずみの値を,実用上十分な精度で簡便に予測できた.

Effect of hydrogen on fracture of U-notched bend specimens of quenched and tempered AISI 4340 steel

U-notched bend specimens of quenched and tempered AISI 4340 steel were deformed in the uncharged condition and after either precharging or dynamically charging with hydrogen. In the uncharged condition fracture initiated at the notch surface and progressed in mode II along a characteristic slip trace. For precharged specimens, strain to fracture was markedly reduced, cracks nucleated internally in a mode I manner and the crack progressed to the surface in mode II. Dynamic charging reduced plastic strain to essentially zero at crack nucleation, which occurred when the notch root stress reached the yield stress, and the crack grew by mode I. The results are compared to those on lower strength steels and are discussed in view of theories for hydrogen degradation of mechanical properties.