Showing papers on "Stress concentration published in 1975"

Uniaxial Failure of Composite Laminates Containing Stress Concentrations

Abstract: Two previously developed failure criteria for predicting the uniaxial tensile strength of a laminated composite containing through-the-thickness material discontinuities (notches) are subjected to further experimental scrutinization. In particular, the two-parameter (unnotched tensile strength of the laminate and a characteristic length) models, which are capable of predicting observed discontinuity size effects without resorting to the concepts of linear elastic fracture mechanics, are based on limited experimental verification. In the present paper, and experimental program is presented which examines the effect of changes in the material system, the laminate fiber orientations, and the notch shape and size (stress gradient), on the model predictions. This is accomplished by obtaining experimental data on two material systems, glass/epoxy and graphite/ epoxy, in conjunction with two orientations of fiber-dominated laminates containing through-the-thickness circular holes and sharp tipped cracks of several sizes. In addition to the test results, two observations based on the models are presented. First, the statistical failure distribution for a composite containing a circular hole is predicted using the models and shown to agree well with experimental observations. Second, an Irwin type correction factor applied to the stress intensity factor is shown to result in nearly constant values of the critical stress intensity factor for all values of crack length. The correction factor is shown to be related to the characteristic length of the present models.

Hydrogen-controlled cracking—An approach to threshold stress intensity

Abstract: Threshold stress intensities, KTH, below which no cracking will occur under stress-corrosion cracking or hydrogen-assisted cracking conditions, are discussed. A quantitative correlation, which suggests why KTH may exist, is evolved. The controlling mechanism is proposed to be a critical concentration of hydrogen, its propensity to form being governed by yield strength, initial concentration, state of stress and temperature variables. The general trends of all variables are reviewed for high strength AISI 4340, maraging and 9 Ni4 Co steels. Specifically it is proposed that Kapplied=KTHCeq=Ccr which means that the applied stress intensity equals the threshold when the equilibrium concentration of hydrogen at the crack tip just achieves the critical concentration. Based upon this concept, correlations are derived for elastic, plane-strain plastic and plane-stress plastic conditions.

An analysis of fatigue cracks in fillet welded joints

Abstract: In most of the lower fatigue strength welded joints failure occurs by the propagation of a semi-elliptical surface crack which initiates at the weld toe. In order to analyse the progress of these cracks using fracture mechanics techniques, the solution for the stress intensity factor, K, is required. Fatigue cracks in most welded joints adopt shapes which give low a/2c values (up to approximately 0.3) while solutions in the literature are more applicable to a/2c values close to 0.5. Therefore, results in the literature were used to estimate the stress intensity factor for cracks with low a/2c values. Furthermore, the effect of the weld stress concentration factor was incorporated in the solution. The accuracy of the resulting solution was confirmed by using it to determine ΔK values of weld toe cracks for which crack propagation data were available. The results agreed with the expected da/dN vs. ΔK scatterband obtained from centre-notched specimens.

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Abstract: Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.

Optimization of shape to minimize stress concentration

Abstract: The problem of minimizing stress concentrations in machinery components is formulated as one of unconstrained minimization by incorporating all ‘side’ constraints on design variables by use of penalty functions. Design parameters describing the transition are determined for an optimal fillet in a tension bar, as well as for a piston-rod ‘eye’. The procedure is generally applicable.

Elastic-Plastic Analysis of a Propagating Crack under Cyclic Loading

Abstract: Fatigue cracks have been experimentally shown to close at positive stresses during constant-amplitude load cycling. The crack-closure phenomenon is caused by residual plastic deformations remaining in the wake of an advancing crack tip. This paper is concerned with the development and application of a two-dimensional finiteelement analysis to predict crack-closure and crack-opening stresses during specified histories of cyclic loading. An existing finite-element computer program which accounts for elastic-plastic material behavior under cyclic loading was modified to account for changing boundary conditions—crack growth and intermittent contact of crack surfaces. This program was subsequently used to study the crack-closure behavior under constantamplitude and simple block-program loading. a [B]

Slow crack growth in ceramic materials at elevated temperatures

Abstract: Techniques for studying slow crack growth at high temperatures are described. The techniques are used to obtain crack growth data for a range of silicon nitrides between 1100 and 1400°C. For these materials the data suggest that the slow crack growth may be effectively characterized by the relation between crack velocity and stress intensity factor. Data obtained for mechanical and thermal fatigue modes indicate that these behaviors can be predicted with moderate accuracy from the isothermal, static stress parameters (for the range of conditions investigated). Finally, the application of slow crack growth data to failure prediction is described and illustrated using data for one of the materials tested.

Metallurgical factors affecting high strength aluminum alloy production

Abstract: With the advent of linear elastic fracture mechanics, the detailed effects of processing and microstructure on toughness can be evaluated. The effect of microstructure on plane stress and plane strain fracture toughness is considered in detail together with strength, fatigue behavior and corrosion resistance. It is concluded that second phase particles in all size ranges can influence toughness. Increasing the size and amount of particles or decreasing precipitate coherency all lead to decreases in toughness. Grain structure is also shown to play a prominent role in determining plane stress fracture toughness ; at a given strength level, a fibrous grain structure and the prevention of recrystallization are desirable. The ability to influence fatigue crack propagation by control of processing is more remote though relatively little systematic work has been carried out in this field. Thermomechanical processing is considered to offer another possible route to achieving a desirable balance of toughness, strength and corrosion resistance.

On the crack propagation with variable velocity

Abstract: The plane problem of propagation of a straight crack in an elastic medium under arbitrary variable loading is considered. The locations of the edges of the crack are specified as arbitrary smooth functions of time under the only restriction that crack speed at any instant of time is less than the velocity of Rayleigh wave. Solution for the distribution of plane stress components near the crack tip is obtained. In particular, expressions for stress intensity factors at the crack are given, which thus makes it possible to deduce the crack motion under given loading conditions.

The effect of mean stress on fatigue crack propagation a literature review

Abstract: Most investigations of fatigue crack propagation have been carried out using pulsating tension loading and rate of crack propagation (da/dN) expressed in terms of range of stress intensity factor (ΔK). However, mean stress can have a significant effect on da/dN, so that generally it should be expressed in terms of mean and range of stress intensity factor. Literature relating to the effect of mean stress on da/dN, mainly in steels and aluminium alloys, is reviewed in the present report. Empitical relationships which allow the correlation of crack propagation data obtained at different mean stresses, crack propagation theories which predict the effect of mean stress, and fundamental explanations of the effects of mean stress are discussed. Suggestions for future research are made.

Some Considerations of the Influence of Sub-Critical Cleavage Growth during Fatigue-Crack Propagation in Steels

Abstract: The results are interpreted in terms of a 3-stage relationship between log growth-rate and log δK, and show that, for intermediate and high growth-rates, effects of mean stress are caused directly by the occurrence of ‘static’ modes. The final acceleration in rate near instability is considered to be associated with the critical linkage of such modes. During this stage it is inappropriate to relate the crack growth-rate to the alternating stress intensity, δK, because the maximum value of stress intensity, Kmax, is critical in determining how much of the total crack front is composed of transgranular cleavage facets and hence how fast the crack is growing. Discussion is also included on the difference between the terminal stress intensity during fatigue and the K Ic value (fracture toughness) under monotonic loading. A possible explanation for such differences is proposed in terms of plasticity-induced heating at the crack tip.

Nonuniformly moving shear crack model of a shallow focus earthquake mechanism

Abstract: A shallow earthquake focus mechanism is modeled by a plane strain shear crack extending at a nonuniform rate under the action of general loading. An equation of motion of the crack tip is determined in terms of a specific fracture energy, i.e., a surface energy, by applying an overall energy balance fracture criterion. If the specific fracture energy is specified, the motion can be determined. On the other hand, if any particular motion of the fault boundary is specified, the requisite specific fracture energy can be calculated from the equation of motion. A process of destruction of unbroken material is included in which the relative sliding of the faces of the fracture surface leads to complete removal of stress in the fractured area. The strength of the dynamic stress singularity at the crack tip is written as a product of an equivalent static stress singularity multiplied by a universal function of crack velocity, the universal function decreasing with increasing crack tip velocity.

Crack growth as an interpretation of static fatigue

Abstract: Static fatigue is due to water in the environment which accounts for substantial reductions in the strength of glass. Extensive experimental investigations have shown that static fatigue results from surface cracks that grow when glass is subjected to tensile loads. In this paper evidence for this conclusion is reviewed. First, the earlier studies of strength of glass are summarized. Then, supporting evidence for crack growth obtained from the newer discipline of fracture mechanics is given. Finally, the influence of ion exchange and chemical reactions at crack tips is discussed with regard to crack growth. The chemical composition of the crack-tip solution is shown to play an important role in controlling crack growth.

The effect of pre-strain on fatigue crack growth and crack closure

Abstract: A single pre-strain ( ϵ = 0.03 ) applied to 2024-T3 material raised the static yield strength from 428 to 480 MN/m 2 . The growth rate of a fatigue crack in the pre-strained material was about twice as large as in the material that was not pre-strained. A pre-strain followed by 1000 high, but still elastic pre-stress cycles did not further increase the growth rate. It was shown by COD measurements that crack closure occurred to a lesser extent in the pre-strained material as compared to the original material. The significance of the increased yield strength for fatigue crack growth is briefly discussed.

Using the scanning infrared camera in experimental fatigue studies

Abstract: Materials under cyclic loading dissipate energy in the form of heat due to hysteresis effects in the material. At locations of high stress levels, more heat is released than elsewhere, resulting in a local temperature rise in those areas. The scanning infrared camera has been used in this study to visualize the surface-temperature field on steel and fiberglass-epoxy composite samples during fatigue tests. The information achieved in this manner allows one to predict the probable location of the greatest fatigue damage well before such damage becomes visible in the form of a crack. The use of the scanning infrared camera for monitoring traveling cracks and mapping the temperature fields resulting from stress concentrations in cyclically loaded materials is also demonstrated. The results indicate that this instrument is of value in both nondestructive testing and crack-propagation studies.

Nucleation of submicroscopic cracks in stressed solids

Abstract: The study of very fine cracks formed in stressed solids was carried out using different types of solids, such as polymers, ionic crystals, and metals. Such methods as small-angle X-ray scattering, light scattering, and electron microscopy were used. Size, form, orientation and concentration of submicrocracks have been estimated. A study of the connection between submicrocracks and the structure of solids and of the kinetics of crack accumulation has been started. The development of the fracture process in solids on the basis of nucleation and successive coalescence of submicrocracks is discussed.