Showing papers on "Stress concentration published in 1981"

Oxide-Induced Crack Closure: An Explanation for Near-Threshold Corrosion Fatigue Crack Growth Behavior

Abstract: The concept of oxide-induced crack closure is utilized to explain the role of gaseous and aqueous environments on corrosion fatigue crack propagationat ultralow, near-threshold growth rates in bainitic and martensitic 2 1/4 Cr-1 Mo pressure vessel steels. It is shown that at low load ratios, near-threshold growth rates are significantly reduced in moist environments (such as air or water), compared to dry environments (such as hydrogen or helium gas), due to the formation of excess corrosion deposits on crack faces which enhances crack closure. Using Auger spectroscopy, it is found that at the threshold stress intensity, ΔKo, below which cracks appear dormant, the maximum thickness of excess oxide debris within the crack is comparable with the pulsating crack tip opening displacement. The implications of this model to near-threshold fatigue crack growth behavior, in terms of the role of load ratio, environment, and microstructure are discussed.

Fatigue growth threshold of small cracks

Abstract: The effects of grain size and crack length on the threshold condition of fatigue growth of small cracks are analysed both theoretically and experimentally. The theoretical model is based on the assumption that the threshold condition is determined by whether the crack-tip slip band blocked by the grain boundary propagates into an adjacent grain or not. By comparing the model analysis with the experiments of mild steel, it was found that the threshold stress estimated from the fatigue limit of the smooth specimen gave the effective components of the applied stress. When the threshold stress normalized by the fatigue limit of the smooth specimen was correlated to the crack length normalized by a certain crack length, the theoretical relation was found to agree well with the experimental data. From a further analysis of the published data of various materials, the theoretical normalized relation was found to give a lower bound of the threshold values of the stress and the stress intensity factor. A deviation from the theoretical relation seen in the cases of hard metals was explained through an extension of the model by regarding that hard metals contained initial, inherent flaws even in the smooth specimen.

Failure of materials in mechanical design : analysis, prediction, prevention

Abstract: The Role of Failure Prevention Analysis in Mechanical Design. Modes of Mechanical Failure. Strength and Deformation of Engineering Metals. State of Stress. Relationships Between Stress and Strain. Combined Stress Theories of Failure and Their Use in Design. High-Cycle Fatigue. Concepts of Cumulative Damage, Life Prediction, and Fracture Control. Use of Statistics in Fatigue Analysis. Fatigue Testing Procedures and Statistical Interpretations of Data. Low-Cycle Fatigue. Stress Concentration. Creep, Stress Rupture, and Fatigue. Fretting, Fretting Fatigue, and Fretting Wear. Shock and Impact. Buckling and Instability. Wear, Corrosion, and Other Important Failure Modes. Index.

Stress-intensity factor equations for cracks in three-dimensional finite bodies

Abstract: Empirical stress intensity factor equations are presented for embedded elliptical cracks, semi-elliptical surface cracks, quarter-elliptical corner cracks, semi-elliptical surface cracks at a hole, and quarter-elliptical corner cracks at a hole in finite plates. The plates were subjected to remote tensile loading. Equations give stress intensity factors as a function of parametric angle, crack depth, crack length, plate thickness, and where applicable, hole radius. The stress intensity factors used to develop the equations were obtained from three dimensional finite element analyses of these crack configurations.

Anticrack model for pressure solution surfaces

Abstract: We propose that discrete solution surfaces originate at stress concentrations and propagate through rock as anticracks . As material is dissolved and removed, the anticrack walls move toward each other; stress and displacement fields are identical to those for the conventional opening crack, but with a change of sign. Observations of entire traces of solution surfaces are consistent with the anticrack concept: (1) the surfaces are bounded in extent; (2) the dissolved thickness varies from a maximum near the center to zero at the tips; and (3) the maximum dissolved thickness is proportional to the length of the surface. Local dissolution and in-plane propagation are suggested by the large isotropic compressive stress at the anticrack tip. Propagating solution surfaces will interact to form a regular array corresponding to some bulk strain rate. Anticracks may also interact with opening and shear cracks; observations of interacting solution surfaces, veins, and faults illustrate these configurations. Intersecting arrays of cracks, anticracks, and shear cracks operate to yield a mode of bulk deformation similar to diffusion-accommodated grain-boundary sliding in polycrystalline solids.

Effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel

Abstract: A study has been made on the effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel. The coarse dual phase structure obtained by continuously annealing in the two phase region directly from the austenite region results in poor elongation ductility with relatively high strength. However, upon obtaining a fine fibrous or fine globular dual phase structure by following different transformation paths, significant improvements occur in elongation ductility without much sacrifice in strength. The poor elongation ductility of the coarse dual phase structure is due to the initiation of cleavage cracks in the ferrite region where maximum localized stress concentration took place. But, in steels with both fine fibrous or globular morphologies, fracture occurred by void nucleation and coalescence after large amounts of plastic deformation.

Fatigue crack propagation behaviour of short cracks; the effect of microstructure

Abstract: — –Fatigue cracks shorter than some critical length tend to propagate anomalously quickly. This paper examines the concept of a ‘critical length’, identifying three regimes of behaviour for different crack lengths. Some published work is examined, covering a wide range of different materials. It is concluded that there is an approximate correlation between the critical length for short crack behaviour and the scale of the microstructure. LEFM is difficult, if not impossible, to apply to cracks shorter than this critical length because the material surrounding a crack cannot be assumed to approximate to a homogeneous continuum. Suggestions are made for a fatigue design philosophy which incorporates short crack behaviour.

Toughening of Glasses by Metallic Particles

Abstract: The role of elastic, thermoelastic, and interfacial properties in the toughening of a brittle matrix by metallic second-phase particles was studied. Two composites were studied: glass+partly oxidized Ni particles (thermal expansion coefficient of the glasses lower than, equal to, and higher than that of Ni) and glass+partly oxidized Al particles (thermal expansion and elastic moduli equal). Weak interfacial bonding between the nickel and its oxide and developed stress concentrations are the major toughness limitations found in the glass/Ni composites. When the thermal expansion coefficient and elastic modulus of the second phase are sufficiently greater than that of the glass matrix, a propagating crack will bypass the particles. When the thermal and elastic stresses are minimized and satisfactory bonding is achieved (glass/Al composites), a 60x toughness increase was realized.

Methods and models for predicting fatigue crack growth under random loading

Abstract: Papers are presented in the volume summarizing the baseline data, methodology, procedures, and results of a round-robin analysis which was conducted to predict the fatigue crack growth in 2219-T851 aluminum center-cracked specimens subjected to flight loading in random cycle-by-cycle format. The objective of the analysis was to assess whether data from constant-amplitude fatigue crack growth tests on center-cracked specimens can be used to predict fatigue crack growth lives under random loading. The following approaches are discussed in detail: a root-mean-square approach, a crack-closure model, a multi-parameter yield zone model, and a load-interaction model.

The Penny-Shaped Crack and the Plane Strain Crack in an Infinite Body of Power-Law Material

Abstract: A study is carried out of the problem of a penny-shaped crack in an infinite body of power-law material subject to general remote axisymmetric stressing conditions. The plane strain version of the problem is also examined. The material is incompressible and is characterized by small strain deformation theory with a pure power relation between stress and strain. The solutions presented also apply to power-law creeping materials and to a class of strain-rate sensitive hardening materials. Both numerical and analytical procedures are employed to obtain the main results. A perturbation solution obtained by expanding about the trivial state in which the stress is everywhere parallel to the crack leads to simple formulas which are highly accurate even when the remote stress is perpendicular to the crack.

A Simple Crack Closure Model for Prediction of Fatigue Crack Growth Rates Under Variable-Amplitude Loading

Abstract: A model for the prediction of growth rates of fatigue cracks in aluminium alloys is presented. The model is based on an approximate description of the crack closure behavior and can be used to predict effects of crack growth acceleration and retardation observed experimentally under variable-amplitude loading. A computer program was developed for analysis of fatigue crack growth. It was used to analyze the effect of certain parameter variations in a flight simulation load spectrum on the crack growth rate. For 7075-T6 thin sheet material the results are compared with experimental data.

A critical review of crack tip stress intensity factors for semi-elliptic cracks*

Abstract: Several crack tip stress intensity factor solutions have been published for semi-elliptic, surface breaking cracks in plates subjected to tension or bending forces. These solutions do not agree with each other particularly well and the basis for choosing which one is the best has not been established. In this paper, the development of fatigue crack shape is used as a diagnostic tool to test the accuracy of these theoretical stress intensity solutions in predictive fatigue crack growth calculations. Those solutions giving the best engineering estimate of crack tip stress intensity factors are identified. Single equations are also given for each loading case at the deepest point or surface intersection point of semi-elliptic cracks in order to facilitate calculations on programmable calculators. A rational basis for calculating the progress of a crack which snaps through the thickness and continues to propagate in a stable way by fatigue is suggested.

Effect of Density on Tensile Strength, Fracture Toughness, and Fatigue Crack Propagation Behaviour of Sintered Steel

Abstract: The tensile strength, fatigue crack propagation behaviour, and fracture toughness of a low-alloy sin tered steel were determined for the porosity range 11–17%. Static and cyclic strength were found to increase with density in a non-linear fashion. The pores both exerted a stress-concentrating influence and reduced the load-bearing section. The micromechanism of failure was always ductile fracture in the necks between sintered steel particles. It was concluded that the stress state at the tips of cracks subjected to static or cyclic loading was closer to plane stress than to plane strain. Retardation of fatigue crack propagation appeared to occur due to the blunting action of the pores. The presence of a wear mechanism had little influence upon fatigue crack growth rates. A companion paper (following) attempts to model the static and cyclic behaviour of the steel, based on the known micromechanisms of failure. PM/0172

Viscoelastic Analysis of Adhesively Bonded Joints

Abstract: In this paper an adhesively bonded lap joint is analyzed by assuming that the adherends are elastic and the adhesive is linearly viscoelastic. After formulating the general problem a specific example for two identical adherends bonded through a three parameter viscoelastic solid adhesive is considered. The standard Laplace transform technique is used to solve the problem. The stress distribution in the adhesive layer is calculated for three different external loads namely, membrane loading, bending, and transverse shear loading. The results indicate that the peak value of the normal stress in the adhesive is not only consistently higher than the corresponding shear stress but also decays slower.

Off-Axis Fatigue of Graphite/Epoxy Composite

Abstract: Off-axis static and fatigue behavior of AS/3501-5A graphite/epoxy was studied in an effort to characterize the matrix/interface-controlled failure. Seven different off-axis angles were tested: 0, 10, 20, 30, 45, 60, and 90 deg. Initial (static) and post-fatigue residual strength were obtained together with S-N relationships. Fracture surfaces were examined through photomicrographs and stereo (three-dimensional) scanning electron microscope (SEM) photographs, in order to delineate failure modes, and the results of these inspections are discussed. The off-axis static strength, including scatter, was fully characterized by a polynomial and a nondimensional strength parameter. Essentially, no strength or modulus degradation was observed in the specimens surviving fatigue loading of 10 6 cycles regardless of the off-axis angle or fatigue stress level. When fatigue stress level is normalized with respect to static strength, all data seem to fall on the same S-N curve. Fatigue failure occurred without any warning or visible damage. Matrix failure characteristics vary with off-axis angle and appear in the form of serrations and axial and transverse cracks. Large scatter in life was observed at all off-axis angles; however, since the number of specimens employed in the present study is not sufficient to provide meaningful statistical S-N data, a more detailed investigation of the off-axis (and angle ply) behavior of graphite/epoxy composites is warranted.

The Threshold Stress Intensity for Hydrogen- Induced Crack Growth

Abstract: The crack growth rates and threshold stress intensities,K TH, for a 3 1/2 NiCrMoV steel (0.2 pct proof stress 1200 MPa) have been measured in a hydrogen environment at various temperatures and hydrogen pressures. Fractographic evidence and the observation of alternating fast and slow crack growth nearK TH suggests that the crack advances by the repeated nucleation of microcracks at microstructural features ahead of the main crack. Transient crack growth is observed following load increases just belowK TH. Using the idea, from unstable cleavage fracture theory, that for fracture a critical stress must be exceeded over a critical distance ahead of the crack, and assuming that this critical stress is reduced in proportion to the local hydrogen concentration (in equilibrium with the external hydrogen atK TH), a theoretical dependence ofK TH on hydrogen pressure is derived which compares well with the experimental evidence.

Stress-Concentration Factors for Finite Orthotropic Laminates with a Pin-Loaded Hole.

Abstract: Stresses were calculated for finite size orthotropic laminates loaded by a frictionless steel pin in a circular hole of the same diameter. The calculations were based on finite element analyses for six laminates. Stress concentration factors, based on nominal bearing stress, were determined for wide ranges of the ratios of width to diameter, w/d and edge distance to diameter, e/d. An infinite laminate case was analyzed for each laminate. Orthotropy had a significant influence on the tensile stress concentration at the hole. For example, the stress concentration factors for the infinite laminate cases ranged from 0.82 to 2.16, compared with 0.87 for the quasi-isotropic laminate. The finite widths and edge distances strongly influenced the tensile stress concentration. For the practical range w/d or = 3, the peak tensile stresses were as much as 80% larger than the infinite laminate reference value. For e/d or = 3, these stresses were amplified by as much as 50%. In contrast, the finite width and edge distance had little effect on shear-out and bearing stress concentrations.

Linear elastic fracture mechanics computations of cracked cylindrical tensioned bodies

Abstract: The variation of the stress intensity factor along the crack front and the strain energy release rate were computed, by the boundary integral equation method, for semi elliptical cracks on the surface of cylindrical bodies strained in tension or in bending. The results were checked by the compliance method. The fracture toughness of high strength steel wires was measured on notched cylindrical specimens. The value which was found to be 80 MPa √m, was used to predict the fracture loads of cracked wires with various crack configurations, and they compared well with the experimental results. The evolution of the cracks during fatigue was followed as a function of the number of cycles and the fatigue life could be predicted making use of Paris law.

A probabilistic theory for the strength of short fibre composites

Abstract: This paper presents a probabilistic theory for predicting the strength of unidirectional short fibre composites. It is assumed that the failure of the composite occurs due to the inability of the short fibres bridging a critical zone to carry the load. The stress concentrations on the fibres bridging a fibre end gap are evaluated as a function of the number of fibre ends forming the gap. The sizes of the gaps are predicted from a probabilistic approach. The short fibre composite strength is then estimated from the gap size and the corresponding stress concentration factor. Comparisons of the present work with existing theories and experiments have been made.

Compression Fatigue Behavior of Graphite/Epoxy in the Presence of Stress Raisers

Abstract: Graphite/epoxy composites are being applied to aircraft structures because of their demonstrated capability to reduce weight and increase life. Although composites have better fatigue properties than metals, their behavior differs significantly. Unlike metals, composites exhibit excellent tensile fatigue behavior-the constant amplitude fatigue strength at 10 7 cycles being close to static ultimate. Compression and reversed loading fatigue behavior in the presence of stress raisers, however, has not been well characterized. This program was therefore undertaken to determine the characteristics of composites under these conditions. Two stress raisers were considered; the first was an open hole and the second was damage resulting from low velocity, hard object impact. For the open hole specimen, constant and variable amplitude tests were performed to determine the significance of compression fatigue and to investigate the failure mechanism. These tests indicated that the fatigue life under compression and reversed loading is less than for tension-tension loading and will be an important design consideration in future composite applications. The failure mechanism appeared to be local progressive failure of the matrix near the stress raiser resulting in delamination, and final failure by fiber buckling. The variable-amplitude loading results also demonstrated the unconservativeness of Miner's rule for making analytical predictions. In order to determine the nature and significance of impact damage to generic composite structural elements, low velocity impact and residual properties tests were conducted on solid laminate specimens of 42 and 48 plies thick and on honeycomb sandwich specimens with 12-ply composite face sheets. Results of post impact properties tests indicate subvisual damage can degrade compression static and fatigue strength, although subvisual damage will not propagate under moderate (0.003) cyclic strain.