A.J. McEvily

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth.

An analysis of the growth of short fatigue cracks

Abstract The behavior of short fatigue cracks cannot be analysed by linear elastic methods because of large scale plasticity effects as well as a breakdown in the stress intensity factor correlation of fatigue crack growth rates for crack sizes less than a millimeter in length. In the present paper a number of modifications are introduced to the linear elastic approach to establish a new parameter which is capable of correlating both long crack and short crack fatigue crack growth data. These modifications include the use of a material constant, r e ; an allowance for large scale plasticity effects; allowance for the development of crack closure; and the incorporation of the fatigue crack growth threshold. The new parameter is used with reasonable success in calculating fatigue crack growth behavior of short cracks in notched and unnotched specimens, and comparing with experimental data. In addition the analysis leads to a rationale for the dependence of the stress amplitude at the endurance level on the mean stress in a manner consistent with Gerbers parabolic relationship.

The influence of a duplex microstructure in steels on fatigue crack growth in the near-threshold region

The influence of a duplex microstructure on fatigue crack growth in the near threshold region has been studied for AISI types 1018, 1045, and 10B35, as well as 2.25 Cr-1 Mo steels. For a duplex microstructure which consists of a continuous martensitic phase encapsulating ferrite, an increase in threshold level and yield strength in both the AISI 1018 and 2.25 Cr-1 Mo steels was observed. However, with increase in carbon content and consequent decrease in the volume of ferrite, the threshold levels were not as significantly affected, although the yield strengths were higher.

Low cycle fatigue test for solders using non-contact digital image measurement system

In a strain controlled low cycle fatigue test it is customary to use a contacting extensometer. However the extensometer can cause premature fracture at the contact points by inducing localized plastic deformation, particularly in a soft metal such as a lead– tin solder. In order to avoid this problem, a non-contacting, digital-image measurement system has been developed to measure the displacement of the specimen gage length during a cyclic loading. The capability of this system for strain-controlled fatigue testing was studied in preliminary experiments, and the effect of variables in the digital image measurement system on the accuracy and the reliability were established. The results confirmed that this system was capable of measuring the displacements in straincontrolled fatigue tests. Low cycle fatigue tests on three solder materials at 20°C were then carried out using this system. The solders were: Ag–Sn eutectic solder (3.5Ag/96.5Sn), Sn–Pb eutectic solder (63Sn/37Pb) and Sn–Pb solid solution solder (5Sn/956Pb). It was found that at a given strain range the Sn–Pb eutectic solder (63Sn/37Pb) had the lowest low cycle fatigue resistance. The lead free eutectic alloy (3.5Ag/96.5Sn) has the highest low cycle fatigue resistance in low strain range regime (p 1%). At higher strain range regime (p 1%), the 5Sn/95Pb alloy has better low cycle fatigue resistance. For all three types of solder, the initial failure mode at the surface of specimens was intergranular. For the 63Sn/37Pb alloy the subsequent fracture path was also intergranular, but for the 3.5Ag/96.5Sn and the 5Sn/95Pb alloys, the subsequent crack paths were generally transgranular.  2001 Elsevier Science Ltd. All rights reserved.

The high-cycle fatigue behavior of Ti-6Al-4V alloy

Abstract The cyclic stress-strain behavior and the S/N behavior of a “pseudo-elastic” alloy, Ti-6Al-4V of coarsened microstructure and of a “plastic” material, copper, are compared. In accord with views recently put forth by Freudenthal, important differences are noted between these two types of materials. These differences include the mode of crack initiation, the intensity of the Bauschinger effect, and the level below macroscopic yield at which long-life fatigue data lie.

On crack closure and the notch size effect in fatigue

Abstract A method for determining the fatigue notch-size-effect is presented based upon the development of closure in the wake of a newly formed crack growing from a notch. A comparison with experimental results is in accord with predictions. The effect of crack closure on the formation of non-propagating cracks at notches and on notch sensitivity is also discussed.

The influence of load ratio on fatigue crack growth in 7090-t6 and in9021-t4 p/m aluminum alloys

The rate of fatigue crack growth has been studied over a wide range of growth rates in two high strength P/M aluminum alloys as a function ofR, with particular attention given to crack closure and near-threshold behavior. A principal difference between the two alloys was grain size, being less than oneµm for the alloy IN9021* and greater than five µm for the alloy 7090. No closure was found in the fine-grained alloy nor was there any dependency of the threshold level onR. In contrast, closure was observed in the coarser grained alloy and there was also a dependency of the threshold level onR. The observations are discussed in terms of current models for threshold behavior. Consideration is also given to the overall fatigue crack growth behavior of these P/M in comparison to that of a conventional high strength I/M aluminum alloy.

Analysis of short fatigue crack growth in cast aluminum alloys

Abstract An analysis of the rate growth of short, surface fatigue cracks in both a coarse-grained and a fine-grained cast 319 aluminum alloy and squeeze-cast aluminum alloy has been carried out using a modified linear elastic approach. The modified LEFM approach includes consideration of elastic–plastic behavior, the development of crack closure, and the relation between the threshold for fatigue crack growth and the endurance limit. A reasonable degree of agreement has been achieved between the predicted behavior and experimental result.

Fatigue crack tip deformation

The shape of a fatigue crack tip as influenced by an air or a vacuum environment has been investigated in two stainless steels and an aluminum alloy. Under plane strain conditions and at crack growth rates in the Paris region, the crack tip opening displacement (CTOD) is much larger in vacuum than in air, a circumstance attributed to strain localization in air due to the presence of moisture and the absence of strain localization in vacuum. In type 304 stainless steel, a strain-induced transformation from austenite to martensite occurs at the crack tip, and the extent of this strain-induced transformation in type 304 stainless steel is consistent with the degree of blunting taking place at the crack tip as influenced by the environment. In air, the extent of transformation is a function of the ΔK level, and as a result, the crack opening level is found to differ in a ΔK decreasing test as compared to aAK increasing test. Fatigue striations are observed in air but are absent in vacuum. It is proposed that the greater extent of blunting in vacuum is responsible for the absence of striations in vacuum.

A coaxing effect in the small fatigue crack growth regime

In fatigue parlance, coaxing refers to an increase in the fatigue strength of a susceptible metal brought about by prior cycling at a lower stress level. The purpose of the present paper is to describe a coaxing effect observed during two-step loading of a 0.46 w/o carbon steel tested at stress levels above the endurance limit. An explanation of this coaxing effect based upon an analysis of the growth of small cracks will also be given.