R. Stickler

Contribution of the cyclic loading portion below the opening load to fatigue crack growth

A simple test procedure involving stress ratio changes at the fatigue threshold is proposed to reveal the role of the lower portion of the loading cycle below Kop in the fatigue crack growth behaviour. It is observed for both Al 2024-T3 and Al 7475-T761 alloys that the initially non-propagating fatigue crack at the fatigue threshold resumes growth upon diminishing Kmin while keeping Kth,max constant. These experimental findings can be interpreted by means of a modified crack closure concept in which the contribution of the lower portion of the loading cycle below Kop (including also a part of compressive loading if R < 0) to the variation in the stress state experienced by the fatigue crack tip is taken into account.

A model for crack closure

The current status of the knowledge about the fatigue crack closure effect is briefly reviewed. A nodel for fatigue crack closure process is proposed, which involves an elastic wedge placed into an elastic crack to simulate the obstruction of crack closure due to asperities or oxide debris. Linear elastic fracture mechanics is applied to calculate the relations between the externally applied stress and the crack opening displacement (COD) for both the closure-free and the closure-affected cases. The slope of the stress-COD response for the closure-affected case is indicated to be larger than that for the closure-free case, depending on the fracture surface topography. Based on the difference in the stress-COD responses, an alternative definition of the fatigue crack closure effect is proposed. The difference in the externally applied stress intensities for the closure-free and closure-affected cases at the minimum applied stress is defined as shielding stress intensity range †Ksh. The actual stress intensity transmitted to the crack tip at the minimum applied stress is used to replace the conventional Kop value. The theoretical calculation indicates that the conventionally defined Kop value merely expresses an extreme case of Kmin,act for an infinitely hard or infinitely wide wedge, which is practically non-existent in engineering materials. Thus, the application of the Kop value underestimates the effective stress intensity range experienced by the fatigue crack tip. The results obtained from the current considerations are discussed and compared with those described in the literature.

An alternative presentation of the effects of the stress-ratio on the fatigue threshold

Abstract The effect of the stress ratio on the threshold value for fatigue crack growth was investigated over a wide range of mean stresses ( − 10 ) for pure Cu, the age-hardened alloy Al-2024 and the mechanically alloyed pm Al-alloy IN-905XL. In particular the influence of the compressive portion of the loading cycle on the closure behavior was studied. The experimental results indicate that depending on the material differing compressive stresses have to be applied to achieve a complete closure of the cracks. These compressive stresses must not be neglected when threshold values are computed. On the other hand, the results support the assumption that the effective threshold value is not affected by mean stress and is characteristic for the materials tested. To reveal more clearly the relation between mean stresses and the threshold values we propose to present the data ( K max ( K min ( K open and ( K closed ) in a modified Smith diagram, i.e. a plot of the K- values vs ( K mean

A NEW EVALUATION PROCEDURE FOR CRACK CLOSURE

Abstract Bending fatigue tests can be used as a simple method for the determination of load-displacement curves near threshold for specimens containgng surface cracks. The test method involves a strain gauge technique that yields highly reproducible results for cyclic loading from positive to negative stress ratios. Applying a new evaluation procedure one obtains ΔKth,eff values that are considered to be more realistic compared with those determined by conventional methods.

A new approach for the determination of stress intensity factors for finite width plate

Abstract A new approach—force balance method—is developed for the calculation of precise geometric correction functions regarding stress intensity factors. The proposed method is based on the consideration that the externally applied loading is equilibrated by the internal stress existing in the elastic material ahead of the crack tip in the crack-line direction. The obtained geometric correction factors in various cases are found to be in excellent agreement with the results reported in the literature. In particular, the application of the force balance method gives rise to a fairly simple geometric correction factor formula for the commonly-used center cracked tensile (CCT) specimen (i.e. Y = 1√(1 − ( 2a W ) 2 )).

Effect of stress ratio and loading condition on the fatigue threshold

Abstract The fatigue threshold behaviour of cold-worked copper tested under different fatigue loading modes (axial, bending) and conditions (various test frequencies and stress ratios) has been investigated. Emphasis was placed on the determination and evaluation of crack-closure effects as well as the computation of the effective fatigue threshold stress intensity. A special strain gauge technique was used to determine the load displacement curves. A newly proposed closure evaluation method was introduced which permits the explanation of the dependence of the fatigue threshold on stress ratio and loading conditions. The effective fatigue threshold of about 3 MPa√ m was determined for cold-worked copper. Various evaluation procedures are discussed.

The effective fatigue threshold: significance of the loading cycle below the crack opening load

Abstract Experiments were carried out to demonstrate the validity of a new concept for the evaluation of the effective fatigue threshold values for loading conditions with K min below the crack opening value. For this, load-CTOD curves were determined for different R -ratios keeping K th, max constant and reducing K min below K th, min , which resulted in a resumption of the fatigue crack growth. This observation is explained on the basis of a new closure evaluation, which takes into account the total range CTOD to affect the driving force for fatigue crack growth. Three methods to measure the effective threshold values are proposed and critically compared. Experimental data are given for Cu, high-strength Al alloys and a dual-phase steel.

Effect of mean stress on the low-amplitude cyclic stress-strain curve of polycrystalline copper

Abstract Cyclic stress-strain curves (CSSCs) were determined for polycrystalline copper specimens for various mean stresses (both tensile and compressive). The effect of the mean stress on the CSSC was found to be very strong over the whole range of amplitudes and mean stresses used. An explanation of this is to be found in the specific features of the dislocation structures produced by cyclic creep and by cyclic plastic deformation.

Correlation between precipitation reactions and bulk density changes in type 18-12 austenitic stainless steels

Dimensional changes in austenitic stainless steels exposed to fast neutron fluxes have been attributed mainly to void formation and thermal and irradiation induced creep. In this in-vestigation, immersion bulk density measurements were performed on variously preheated AISI Types 304, 316L and 316 stainless steels. The results show that the density changes accompanying sequential precipitation of the various carbide (M23C8, M6C) and intermetallic phases (sigma, chi, eta) during thermal aging can be comparable to those attributed to void formation at low fast neutron fluences. Vacuum melted Type 316 stainless steel always den-sified with aging at 725°C or 810°C and exhibited higher rates of precipitation and recovery of cold work during aging than the equivalent air melted steel. The direction of the density change in the air melted material was dependent upon the prior thermomechanical treat-ments. This behavior can be rationalized on the basis of a higher impurity content in the air melted material. A mathematical model was constructed which successfully explains the variation of bulk density due to sequential precipitation in Type 18-12 austenitic stain-less steels.

High-cycle fatigue Behaviour of iron-based PM-materials

The effect of pores and inclusions on the high-cycle fatigue and threshold behaviour of PM iron and PM steel specimens containing porosities of up to 22% has been investigated. HIPed specimens have been prepared to obtain base-line data from practically pore-free materials. The distribution, size and morphology of the inhomogeneities was characterised as function of porosity by quantitative metallography and fractography, in addition to measurements of the Youngs modulus and the electrical resistivity. The results show that the properties for low-porosity material can be interpreted on the basis of the appropriate Kitagawa-diagram, taking into account the effective threshold stress intensity and the magnitude and extent of the stress concentration at the perimeter of the pores or inclusions. For high-porosity material the properties appear to be related to the actual load-bearing area, ie the sum of the cross-sections of the fractured sintering necks as revealed by fractography.