Regions Of High Stress Concentration Research Articles

Analysis and design of adhesively bonded modified double containment corner joints -II

This study comprises the stress and stiffness analyses of a second type of modified double containment corner joint which is presented as an alternative to two previous designs in order to reduce the effect of bending moment on the adhesive stresses. Plates are bonded at a right angle into slots of a corner support and the vertical slot depth is kept as large as possible in order to produce a joint which is stiffer and sustainable to high loads, provided that high stress concentration regions are under compression, and to obtain savings of the corner joint volume. The analyses were carried out using the finite element method and assuming that the adhesive, plates, and corner support had linear-elastic properties. Since the geometry of the adhesive free end has an important effect on the high adhesive stresses, the adhesive spew fillet arising from the applied pressure to provide the physical contact between the adhesive and plates was taken into account. In order to show the effect of boundary and loading...

Development of a finite element based strain accumulation model for the prediction of fatigue lives in highly stressed Ti components

Abstract Low-cycle fatigue studies have been conducted on the near-alpha IMI 829 Ti alloy under constant-amplitude and variable-amplitude load- and strain-controlled loading conditions, and fundamental data have been generated relating to the effects of cyclic loading on elastic modulus, yield stress and plastic strain development. The results of these studies have been used to develop a finite element based damage accumulation model, which is capable of predicting both the stress and strain distributions in cyclically loaded components, and the time to failure (time to first crack) in critical regions of high stress and/or strain concentration. The model has been successfully validated using LCF tests conducted on notched specimens (Kt = 2.8 and 3.2) at ambient (20 °C) temperature.

Fatigue failure of SUS 304 caused by vibratory cavitation erosion

Abstract The mechanism of cavitation erosion is investigated in detail for SUS 304 stainless steel, a typical erosion-resistant material. Systematic observations of eroded surfaces and dislodged particles show that the predominant failure mode in cavitation erosion is fatigue. Characteristic features of such failures include (1) packets of slip bands, (2) cracks that nucleate at regions of high stress (strain) concentration, (3) cracks that propagate first at an inclination to the surface and then parallel to the surface, and (4) fatigue striations and ‘tire tracks’ on the rupture surfaces. These last two features are the most indicative of a fatigue process. It is also deduced that massive voids accompanying pressure shock-rings are the most erosive kind of cavitation.

Effect of partial thickness actuation on stress concentration reduction near a hole

Recently, there has been much interest in adaptive structures that can respond to a varying environment by changing their properties. Piezoelectric materials and shape memory alloys (SMA) are often used as partial thickness actuators to create such adaptivity by applied energy, usually electric curent. These actuators can be used to inducce strains in a structure and reduce stresses in regions of high stress concentration. Two of the present authors show that axisymmetric actuation strains applied troughout the thickness of a plate with a hole can reduce the stress concentration factor (SCF) in an isotropic plate from 3 to 2. However, in most cases actuators are expected to be bonded to or embedded in the plate, so that the actuation strains are applied in the actuators and not directly in the plate. The objective of this note is to show that such partial-thickness actuation cannot be used to reduce the stress concentration factor with axisymmetric actuations strain distribution.

The assessment of flaws in a high stress concentration region

The assessment of flaws in a stress concentration region (SCR) has been a difficult problem; there are a few references which deal with the problem in detail. CEGB/H/R6-Rev. 3 generalized the Ainsworth's J- integral method of reference stress to assess flaws in SCR. The present paper presents in detail FEM results on the double-edge cracks along a central hole. Based on the above results, we argue that R6-Rev. 3 option 1 and 2 curves are dangerous when they are used in the analysis of cracks in SCRs. The assessment of such structures should be carried out making use of option 3. Options 2 and 1 should be modified according to a proposal made in this paper (factor modified method) to assess any cracks in the SCR.

Investigation of the production of vacancies in fatigue and of their possible role in fatigue fracture using a unidirectionally solidified composite

Abstract Owing to the presence of the fibres which provide a homogeneous reinforcement, it has been possible to combine, in the whole nickel-base matrix of a unidirectionally solidified composite, fatigue life and fatigue strength conditions which cannot be attained in an ordinary material except in high-stress concentration regions where the observation is usually impossible. Taking advantage of this ideal material, two complementary observation techniques, the high-voltage electron microscope (1 MV) and the field-emission gun scanning electron microscope, have not only revealed deformation microstructures such as dislocation multipoles which may be responsible for vacancy production, but also some unusual aspects of the fatigue fracture surfaces which may be most probably attributed to the presence of a supersaturation of vacancies and to their further condensation. These last results, which were obtained at room temperature, may be correlated with the observations performed on ordinary materials but at...

On the use of betti's reciprocal theorem for computing the coefficients of stress singularities in anisotropic media

A method based on Betti's reciprocal theorem is used in deriving the singular stress field in the neighbourhood of notches and corners in anisotropic media. The method is to be used, in conjunction with numerical methods or other approximate methods, to carry out accurate stress analysis of regions of high stress concentration near notches or corners in finite bodies. A numerical application of this method is made by studying an orthotropic material welded to a rigid body and the stress concentration factor thus obtained.

An investigation into the fatigue crack growth behaviour of an IMI 829 titanium rotating disc material. (Dissertation): Hull, D.A. Diss. Abstr. Int. Mar. 1990 50, (9)

Low-cycle fatigue studies have been conducted on the near-alpha IMI 829 Ti alloy under constant-amplitude and variable-amplitude load- and strain-controlled loading conditions, and fundamental data have been generated relating to the effects of cyclic loading on elastic modulus, yield stress and plastic strain development. The results of these studies have been used to develop a finite element based damage accumulation model, which is capable of predicting both the stress and strain distributions in cyclically loaded components, and the time to failure (time to first crack) in critical regions of high stress and/or strain concentration. The model has been successfully validated using LCF tests conducted on notched specimens (Kt = 2.8 and 3.2) at ambient (20 °C) temperature.

Fracture failure prediction using a moving finite element mesh refinement scheme

A two-dimensional fracture mechanics finite element computer program using an automated mesh refinement scheme in regions of high stress concentration is presented. Constant stress-strain triangular elements are used to discretize the structural system so as to predict the initiation and propagation of fracture, yield patterns, load-displacement history and failure load under monotonically increasing loads. The material stress-strain curve is idealized by selecting yield points on the nonlinear portion of the curve and joining them by linear segments. For each yield point taken, a minimum element area is specified which is gradually decreased as one moves from a lower yield point stress (or strain) to a higher yield point stress (or strain). As loads are incremented, regions of high stress/strain concentration in an input coarse finite element mesh are detected by using either the von Mises or the St. Venant yield criterion. Then the elements in these regions are repeatedly refined until their areas reach the prescribed minimum area requirements. A five-noded triangular element is used to confine the mesh refinement locally. To redistribute the energy of the fractured element into the unfractured media the zero modulus unload-reload method is used. The method is demonstrated for a center-cracked rectangular panel under tensile load.

Slow crack propagation in a heavily‐filled particle reinforced epoxide resin

AbstractThe fracture properties of two proprietary composite dental restorative materials and a model composite system were studied to determine the effects of filler concentration, exposure to water, and particle/polymer adhesion on subcritical crack propagation. Particle content ranged from 36 to 60 volume percent. The double torsion (DT) test was used to measure relationships between the stress intensity factor (K1) and the speed of decelerating cracks or the rate of loading in dry and wet materials in air at laboratory conditions. Materials with weak particle/polymer interfaces fractured by continuous crack growth in both dry and wet conditions. In dry and wet materials with strong interfaces, continuous cracking also occurred at the low end of the range of speeds observed (10−7 to 10−3 m/s), but under test conditions of high crack speeds unstable (stick‐slip) crack propagation was found in dry specimens and in wet model composites with 41 percent vol, filler. Water had a corrosive effect lowering K1c for continuous crack propagation. The exponential dependence of K1c on crack velocity, representing the viscoelastic response of the materials, was positively correlated to the filler concentration and the plasticizing effect of water. Observations on fracture surfaces indicate that low velocity cracks (<10−5 m/s) propagate through regions of high stress concentrations (interfaces, corners, pores) while at higher crack velocities failure occurs by a combination of interparticle and transparticle fracture.

Electron‐beam‐induced fracture of polymers

AbstractWhen a notched polymeric material is stressed, the notch opens into a wide crack tip, exposing a region of high stress concentration. The consequences of electron bombardment of the tip of such a stressed material under vacuum are explored here for the first time. Evidence is presented for electron‐induced crack growth at stress far below that needed for crack growth due to stress alone. The electron current densities used in these experiments are sufficiently small that thermal heating of the zone near the crack tip is minimal. To provide information on the phenomena involved, we present simultaneous measurements of electron current, gas pressure, and sample load in response to periodic bombardment of the sample. Experiments involving the bombardment of un‐notched polymers under stress are also described. Fractography of the unique structures obtained by fracture due to the combination of electron bombardment and stress are presented and interpreted in terms of a crosslinking mechanism.

Slow Crack Growth in Dental Composites

ABSTRACTThe fracture properties of selected commercial composite dental restorative materials and a model composite system were studied to determine the influences of the reinforcing phase, exposure to water, and particle/polymer adhesion on crack propagation. The content of inorganic fillers ranged from 36 to 62 volume percent. In the model system the polymer phase approximated that of the commercial products, a constant size distribution of quartz fillers was used, and polymer/particle adhesion was varied. The double torsion test method was employed to measure relationships between applied stress intensity factor and velocity of crack propagation during stable crack growth. In all systems, cracks propagated through regions of high stress concentration at the low end of the velocity range studied (10−7 m/sec to 10−3 m/sec). Wet materials fractured at lower stress intensities than dry materials at all velocities. At high velocities unstable (stick-slip) growth occurred in dry materials with strong filler/matrix interfaces and in wet specimens with initially strong interfaces and less than 41 volume percent filler. In wet conditions, materials with poorly bonded fillers fractured by slow crack growth at stress intensities 10% to 30% below the levels of composites with strong interfaces.

Acoustic excitation of a panel having a central opening under biaxial tension

The behaviour of a thin tensioned sheet with an opening has been the subject of a number of recent investigations. In this type of problem, as in an aircraft structure, a skin carries all or some of the in-plane load as well as a lateral pressure load. Failure often results from the development of fatigue cracks which propagate from a stress concentration at the cut-out being subjected to structural vibration excited by acoustic pressure fluctuations. The Comet disaster in 1954 was due to the structural failure of the pressure cabin, brought about by the development of fatigue cracks from high stress concentration regions around the corners of the windows. The cases of statically tensioned, uniaxially and biaxi-ally loaded plates having a central crack subjected to acoustic excitation have been studied. These investigations were concerned with the basic features of plate vibration; i.e., mode shapes at a resonance frequency, the resonance frequency versus load behaviour and the crack propagation rates.

Acoustic emission generated during the tensile testing of aluminium alloys

The acoustic emission of AlCuMg and AlMg alloys during plastic deformation was studied. The emission characteristics of each alloy throughout the deformation range were significantly different. The Kaiser effect was invariably observed with an obvious correlation with previous stress. The 5086 alloy exhibited a “delayed Kaiser effect” due to strain-enhanced grain boundary segregation of excess magnesium. During plastic deformation of the 2024 alloy and the prestressed 5086 alloy, jerky discontinuous yielding was observed, with several plateaux of jerks appearing in the stress-strain curve. High acoustic bursts were observed at the onset of a new plateau of jerks owing to catastrophic breakaway of dislocations at regions of high stress concentration.

Efficiency of double-lapped composite joints in bending

An analytical and experimental study was conducted to examine the mechanical behaviour of double-lapped adhesive-bonded joints subjected to bending loads. Adherends consisted of a U-shaped high-strength steel hub and a unidirectional composite of either B/AI or B/epoxy. Adhesives with a range of moduli and peel strengths were evaluated with and without bolts in regions of high stress concentration. Finite element analyses indicated high shear and normal stress concentrations at the end of the steel hub legs. Experimental testing of sample joints revealed that failure modes are controlled by the composite adherend material. Joints incorporating a B/AI adherend failed by adhesive peel in the region of high stress concentration. Delamination of the B/epoxy composite was the predominant failure mode of joints utilizing a B/epoxy adherend. Failure by delamination was suppressed by the insertion of high-strength bolts into the stress concentration region.

Investigation into the Consequences of the Failure of a Turbine-Generator at Hinkley Point ‘A’ Power Station

The further investigation was extensive and is continuing but the present account is limited to what throws light on the Hinkley Point ‘A’ incident and its immediate consequences. There were three main areas of investigation: (1) laboratory and on-site exploration of conditions for stress corrosion cracking of relevant steels; (2) a survey of all relevant rotors to explore the effects of applying a purely fracture mechanics approach in ignorance of the mechanism causing the initial crack or defect; (3) an examination of rotors withdrawn from machines of similar mechanical design and of properties deemed to be in the highest risk category. Interim conclusions have been drawn. They suggest that in certain non-reheat turbines it is possible to generate high chemical concentrations in regions of high stress and thus to promote stress corrosion cracking. The rate of propagation of such cracks is sufficient to produce critical stress intensities in discs of low fracture toughness in a region of high stress concentration.

Wave velocities and hysteresis in Solenhofen limestone for pressures up to 12 kilobars

Precision shear (S) and longitudinal (P) ultrasonic velocity measurements were made on jacketed and unjacketed Solenhofen limestone as a function of hydrostatic pressure to 12 kb. Measurements of P velocities were also made in the C direction of calcite to 12 kb and in fused quartz to 14 kb. For constant pressure below approximately 5 kb the P and S velocities in jacketed Solenhofen limestone are found to be directly proportional to initial specimen density. After subjecting specimens to pressures greater than 5 kb, they exhibit features attributed to the polycrystalline nature and porosity of the limestone: (1) a permanent decrease in the zero-pressure velocity is observed which is explained by microfractures at regions of high stress concentration, (2) a ‘negative velocity hysteresis’ is observed; i.e., velocities measured with increasing pressure are less than the velocities measured with decreasing pressure; this feature can be explained on the basis of either phase transitions or porosity changes accompanying permanent deformations. Unjacketed Solenhofen limestone exhibited a ‘positive velocity hysteresis’ that can be explained on the basis of trapped fluid in the pores. The P and S velocities for jacketed Solenhofen limestone falls sharply at pressures exceeding about 8 kb. Comparison with data of unjacketed specimens provides experimental evidence that the often observed sharp decrease in the velocity occurring below 15 kb is a result of local stress concentrations in the specimen.

Effect of stress on the diffusion of electrochemically produced hydrogen in steel

1. Elastic strains activate hydrogen diffusion through a steel membrane. 2. Intense hydrogen charging leads to an increase in hydrogen collector size and to local plastic deformation of the metal in the vicinity of the collectors. The reduction in hydrogen permeability of steel under these conditions is attributable to a reduction in the permeable cross-section of the metal and to an increase in hydrogen concentration in regions of high stress concentration around the collectors. 3. General plastic deformation of steel causes a sharp reduction in its hydrogen permeability owing to the formation of a large number of structural defects which act as stress raisers and internal hydrogen collectors.

Fracture in reactor graphite

A microstructural examination of reactor graphite is described. Thin plates of graphite bonded to a thick beam have been subjected to a uniform strain field by bending the beam and simultaneously examined by a light microscope. From such an examination it has been established that the characteristic striations in grist (calcined coke) particles are main lines of weakness while binder bridges are regions of high stress concentration. Such localized cracking appeared at stresses lower than those required for complete fracture. Further evidence of the presence of localized cracking was provided by the rapid decrease in the apparent modulus of the material in bulk (determined by a resonance frequency method) if specimens were prestressed to values approaching the fracture stress.