Effect Of Stress Redistribution Research Articles

Numerical Simulation of Rock Mass Damage Evolution During Deep-Buried Tunnel Excavation by Drill and Blast

Presence of an excavation damage zone (EDZ) around a tunnel perimeter is of significant concern with regard to safety, stability, costs and overall performance of the tunnel. For deep-buried tunnel excavation by drill and blast, it is generally accepted that a combination of effects of stress redistribution and blasting is mainly responsible for development of the EDZ. However, few open literatures can be found to use numerical methods to investigate the behavior of rock damage induced by the combined effects, and it is still far from full understanding how, when and to what degree the blasting affects the behavior of the EDZ during excavation. By implementing a statistical damage evolution law based on stress criterion into the commercial software LS-DYNA through its user-subroutines, this paper presents a 3D numerical simulation of the rock damage evolution of a deep-buried tunnel excavation, with a special emphasis on the combined effects of the stress redistribution of surrounding rock masses and the blasting-induced damage. Influence of repeated blast loadings on the damage extension for practical millisecond delay blasting is investigated in the present analysis. Accompanying explosive detonation and secession of rock fragments from their initial locations, in situ stress in the immediate vicinity of the excavation face is suddenly released. The transient characteristics of the in situ stress release and induced dynamic responses in the surrounding rock masses are also highlighted. From the simulation results, some instructive conclusions are drawn with respect to the rock damage mechanism and evolution during deep-buried tunnel excavation by drill and blast.

Effect of dissolution/precipitation on the residual stress redistribution of plasma-sprayed hydroxyapatite coating on titanium substrate in simulated body fluid (SBF)

The residual stress distributions in hydroxyapatite (HAp) coating with and without mixed hydroxyapatite/titanium (HAp/Ti) bond coating on commercially pure Titanium substrate (cp-Ti) were evaluated by Raman piezo-spectroscopy analysis. The Raman shifted position 962cm−1, which is the symmetrical stretching of surrounded oxygen atoms with phosphorous atom (PO43−), was referred to analyses of stress dependency. The piezo-spectroscopic coefficient, which is a Raman shift value per stress (cm−1/GPa), was fitted from the result of four-points bending test of rectangular HAp bar and as-sprayed HAp on Zn plate. The calculated values were 3.89cm−1/GPa for the former and 7.11cm−1/GPa for the latter. By using these calibrations, the compressive residual stress in HAp coating with HAp/Ti bond coating (HA-B) has been found to be distributed in the range of −137MPa to −75MPa. For the heat-treated HAp coating (HA-B-HT) specimen, the compressive residual stresses placed in the range of −40–−22MPa. The changes in the values of residual stress of the HAp coating after immersion in SBF were also evaluated. The residual stress in HA-WB specimens tend to change from compressive to tensile after 30 days immersion. The HA-B-HT specimens exhibited similar behavior and reached to zero stress after the immersion. The mechanism of the changes in residual stress would be the effect of stress redistribution around melted calcium phosphate particles to remained HAp splats.

Prestress Loss of a New Vertical Prestressing Anchorage System on Concrete Box-Girder Webs

A new, double-tensioned prestressing steel strands anchorage system (DTPSSAS), characterized by low prestress loss caused by prestressing tendon retraction, is described. When it is used for short prestressing tendons in concrete box-girder webs, this anchorage system significantly reduces prestress loss. To investigate its efficiency in reducing prestress loss, a rectangular thin plate using the DTPSSAS was devised and tested; the results demonstrated that the average rate of instantaneous prestress loss decreased from 32.35% after the first tension to 2.68% after the second tension. Further, long-term observations of prestress loss with the DTPSSAS were conducted in the laboratory and in a field test. Based on the field test of a continuous concrete box-girder bridge, the average loss rate of instantaneous prestress loss to the control tensile stress of two strands in the web is 7.61%, and the average rate of prestress loss at 150 days is 10.53%. Concurrently, the time-dependent prestress loss was calculated with Models GL2000 and CEB-FIP90 for different concrete shrinkage and creep prediction by the step-by-step calculation method. In comparison with the laboratory test results, the calculated results with the two models were greater than the test values. In addition, the effect of stress redistribution caused by the constraint of reinforcing bars to concrete shrinkage and creep was evaluated. The results revealed that the stress redistribution effect increased the vertical compressive prestress loss of concrete, indicating that the time-dependent compressive prestress loss rate of concrete was greater than the tensile prestress loss rate of prestressed tendons. In this regard, bridge engineers may need to pay attention to the stress redistribution phenomenon in vertical prestressing design.

Elastic-Plastic Analytical Solution of Deep-Buried Circle Pervious Tunnel Based on Unified Strength Theory

Mohr-Coulomb strength theory has been used to analyses the elastic-plastic analytical solution of pressure tunnel in the field of rock engineering for many years. Because it could only solve the problems with one kind of material, the application of the theory was greatly restricted. In order to reflect the effects of stress redistribution, the stress adjustment coefficient is introduced in this paper to develop the elastic-plastic analytical solution of deep buried circle tunnel based on the unified strength theory. The applicability formulas is gained based on the critical pressures, which can be obtained in other situations or strength thoeries simplified by assuming the parameter value in the formula. An example is analyzed, and the yield range versus the inner pressure and stress distribution are presented. The results show that the parameter in unified strength theory has significant effects on the radius of plastic zone and the magnitude of stresses of the wall rock of tunnel, comparing with theory of Mohr-Coulomb.

A fully coupled solid and fluid model for simulating coal and gas outburst with DEM and LBM

A fundamental challenge to effectively predict and control the outburst is to understand its physical mechanism and factors contributing to its initiation and development. Field observations and laboratory studies reveal that the occurrence of the outburst is the result of combined effects of stress redistribution, gas desorption, coal property and time effects [1–4]. The use of numerical simulations can provide useful insights on outbursts. Along this line, there are already some attempts [5–11]. However these models do not model solid fracture and fragmentation explicitly. Free flow of fluid and two-way interactions between the solid and fluid are also missing. In this study, a new outburst model which includes the most recognized effecting factors of outbursts is presented. The model couples two well developed numerical approaches: the Discrete Element Method (DEM) and the Lattice Boltzmann Method (LBM).

Computational and Experimental Studies of High Temperature Crack Initiation in the Presence of Residual Stress

Residual stresses have been introduced into a notched compact tension specimen of a 347 weld material by mechanical compression. The required level of compressive load has previously been determined from finite-element studies. The residual stress in the vicinity of the notch root has been measured using neutron diffraction and the results compared with those obtained from finite-element analysis. The effect of stress redistribution due to creep has been examined and it is found that a significant reduction in stress is measured after 1000h at 650°C. The implications of these results with regard to the development of damage in the specimen due to creep relaxation are examined.

Effects of the Load History on the Residual Stress Distribution in Welded Components

The lifetime of a welded component is finally influenced by the various service conditions. The stress redistribution effect of subsequent load histories is fundamental to the load-bearing capacity of a structure later in service. Since every welded structure additionally experiences external shrinkage restraint, the resulting structural loads are very important for mechanical assessment. Forward-looking development of materials furthermore requires particular consideration of in-service load histories and of their effects on the global structural loads of a construction under shrinkage restraint. Using a large-scale test facility, stress and residual stress investigations have been performed relating to the subject area of residual stress redistribution resulting from a simulated load history. Based on the presented results, which were all carried out with matching filler materials, further research work concerning the influence of the variation of the mismatch factor on the residual stress distribution has to be done.

Three-dimensional stress analysis and Weibull statistics based strength prediction in open hole composites

Abstract The critical failure volume (CFV) method is proposed. CFV is defined as a finite subvolume in a material with general nonuniform stress distribution, which has the highest probability of failure, i.e. loss of load carrying capacity. The evaluation of the probability of failure of the subvolumes is performed based on the lowest stress and thus provides an estimate of the lower bound of the probability of local failure. An algorithm for identifying this region, based on isostress surface parameterization is proposed. It is shown that in the case of material with strength following Weibull weak link statistics such a volume exists and its location and size are defined both by the stress distribution and the scatter of strength. Moreover the probability of failure predicted by using the CFV method was found to be close to that predicted by using traditional Weibull integral method and coincide with it in the case of uniform stress fields and in the limit of zero scatter of strength. Experiments performed on homogeneous epoxy resin plaques with and without holes showed that the predictions bound the experimentally measured open hole strength. The Weibull parameters used for prediction were obtained from testing only unnotched specimens of different dimensions. The effect of the hole size on tensile strength of heterogeneous materials such as quasi-isotropic carbon–epoxy composite laminates was considered next. Fiber failure was the only failure mechanism taken into account and a strain-based failure criterion was used in the form of a two parameter Weibull distribution. The stacking sequence was selected to minimize the effect of stress redistribution due to subcritical damage. Not unexpectedly an up to 30% underprediction of the strength of the laminates with small (2.54 mm diameter) holes was observed by using classical Weibull integral method as well as Weibull based CFV method. It was explained by examining the size of the CFV, which appeared to be below Rosen’s ineffective length estimate. The CFV method was modified to account for the presence of a limit scaling size of six ineffective lengths, consistent with recent Monte-Carlo simulations by Landis et al. [Landis CM, Beyerlin IJ, McMeeking RM. Micromechanical simulation of the failure of fiber reinforced composites. Mech Phys Solids 2000;48:621–48] and was able to describe the experimentally observed magnitude of the hole size effect on composite tensile strength in the examined range of 2.54–15.24 mm hole diameters.

Stress Concentrations and Notch Sensitivity in Woven Ceramic Matrix Composites Containing a Circular Hole—An Experimental, Analytical, and Finite Element Study

In this paper, the stress concentrations associated with a circular hole and subjected to uniform tensile loading in woven ceramic matrix composites (CMCs) have been determined experimentally through strain gauge measurements. The stress concentrations were calculated from the measured strain distributions surrounding the holes during the initial loading period. Then, the load‐carrying capacity of notched CMCs with various hole radius to width ratios (a/w) has been determined. The objective is to study the stress concentrations due to circular notch, and then evaluate the notch sensitivity in woven CMCs. Both quasi‐isotropic [0/90/+45/−45]s and cross‐ply [0/90/0/90]s laminates of woven SiC/SiNC composites were considered. The experimental data of stress concentrations were compared with the results obtained from a complex variable method and finite element analysis. The notched strength data were also analyzed on the basis of some existing theories. The results show significant stress concentrations in the range of 3.75–4.75 near the notch edges during the early stage of loading. But in most cases, the notched strength data show notch insensitivity in CMCs, particularly with small hole diameters. This occurs mostly due to stress redistribution effects during inelastic deformation in ceramic matrix composites. The degree of notch insensitivity in CMCs is observed to decrease with comparatively larger diameter holes. For notch insensitivity, the limiting value of a dimensionless parameter n, comprising of fracture toughness, un‐notched strength, and width of the specimen, has been determined on the basis of existing notch sensitivity theories. The stress concentration factors are seen not to be influenced by the lay‐up sequence of the laminate. Reasonable agreement was observed between the theory and experimentally determined stress concentration factors, particularly for small hole diameters.

Excavation-induced damage studies at the Underground Research Laboratory

The Underground Research Laboratory (URL) is situated in the Lac du Bonnet granite batholith, in southeastern Manitoba, Canada. The URL was developed to study issues related to the deep geologic disposal of used fuel from nuclear reactors and has developed into an International Atomic Energy Agency recognized geotechnical center of excellence. Extensive rock mechanics research has been conducted, including work to understand the character and extent of excavation damage. Experience at the URL has shown that damage exists around underground openings and that the damage develops from the energy imparted to the rock by the excavation method and by redistribution of the in situ stress field around the excavated openings. Subsequent near-by excavations, removal of loose material from the existing tunnel, thermal loads and pore pressure changes will all influence the development and extent of rock damage, as does the rock type and its fabric. Studies at the URL have shown that in highly stressed rock, damage will develop around underground openings even when a low-energy excavation method is used and that properly designing the excavation geometry and orientation plays a major role in constructing stable openings. Two zones of excavation-affected rock have been shown to exist from studies at the URL; (1) a zone of irreversibly damaged rock surrounding the excavation, which may include failed zones, inner and outer damage zones and (2) a zone of excavation disturbance where the in situ stresses are influence by the excavation no damage is measurable. Measurements of the properties of the damaged rock (ultrasonic velocity, transmissivity) using a variety of instruments have shown that a less intensively damaged outer zone surrounds a more highly damaged inner zone. The inner damage zone may have zones of failed rock depending on stress concentrations relative to rock strength and damage imparted by excavation method. The inner damage zone is attributed to the effects of the excavation method and stress redistribution and the outer damage zone is attributed to the effects of stress redistribution alone. In lower stressed rock the extent of the outer zone is much less than for an opening in highly stressed rock with the same excavation shape and orientation relative to in situ stresses. The excavation disturbed zone describes the volume of rock surrounding the damage zone where the stress redistribution is too small to permanently change rock properties (unless some other stress increase, such as thermal loading, is added), and it is also characterized non-permanent but potentially long-term changes in hydraulic head.

Seismicity and mean magnitude variations correlated to the strongest earthquakes of the 1997 Umbria‐Marche sequence (central Italy)

Nearby faults can interact, affecting the timing of a future earthquake. A large earthquake can alter the static stress surrounding faults, possibly activating an aftershock sequence. Here we test the hypothesis of earthquake triggering by examining the earthquakes that affected the Umbria‐Marche region (central Italy) during September 1997 to April 1998. The analysis was performed combining information contained in parameters a and b of the Gutenberg‐Richter relationship, before and after the strongest main shocks, using a catalog of 6211 earthquakes (M ≥ 1.5). By analyzing the changes in space of the a parameter, we found that Colfiorito main shocks (Mw 5.7 and 6.0) on 26 September 1997 increased the seismicity rate within 40–50 km of the epicenter. We interpret this behavior as the effect of stress redistribution. After analyzing the b value changes in space and time we found the strongest increase in mean magnitude in the Sellano area, between the Colfiorito shocks and the two strong events (Mw 5.2 and 5.6, respectively) which hit this region on 12 and 14 October 1997. This area was also the location of the strongest increase in change of the local occurrence rate density. On the basis of these observations, we postulate that temporal variations in both a and b values indicate a probable causal connection between the largest earthquakes in the sequence.

The effect of residual thermal stresses on the fatigue crack growth of laser-surface-annealed AISI 304 stainless steel: Part I: computer simulation

The effect of residual thermal stresses on the fatigue crack growth of the laser-surface-annealed AISI 304 stainless steel, especially the effect of stress redistribution ahead of the crack tip was extensively evaluated in the study. Based on the finite element simulation, the longitudinal residual tensile stress field has a width of roughly 20 mm on the laser-irradiated surface and was symmetric with respect to the centerline of the laser-annealed zone (LAZ). Meanwhile, residual compressive stresses distributed over a wide region away from the LAZ. After introducing a notch perpendicular to the LAZ, the distribution of longitudinal residual stresses became unsymmetrical about the centerline of LAZ. High residual compressive stresses exist within a narrow range ahead of notch tip. The improved crack growth resistance of the laser-annealed specimen might be attributed to those induced compressive stresses. As the notch tip passed through the centerline of the LAZ, the residual stress ahead of the notch tip was completely reverted into residual tensile stresses. The existence of unanimous residual tensile stresses ahead of the notch tip was maintained, even if the notch tip extended deeply into the LAZ. Additionally, the presence of the residual tensile stress ahead of the notch tip did not accelerate the fatigue crack growth rate in the compact tension specimen.

A Method for Determining the Elastic-Plastic Response Ahead of a Notch Tip

The stress distribution ahead of a notch tip is the prerequisite to calculating the driving force for cracks emanating from notches. This article first examines whether two commonly used engineering methods, which are often employed to determine the response at a notch tip, can be applied to evaluate the elastic-plastic response ahead of a notch tip. It is found that both methods would significantly underestimate the stress-strain distribution ahead of a notch tip. Based deformation theories of plasticity, and analytical method is then developed, taking into account of the effects of stress redistribution induced by notch plasticity and the in-plane and through-thickness constraints near the notch root. Predictions are shown to be in close correlation with finite element results.

Damage Development in Woven Fabric Composites during Tension-Tension Fatigue

Impacted woven fabric composites were tested in tension-tension fatigue. In contrast to results from static testing, the effects of low energy impact damage in a fatigue environment were found to be the critical element leading to failure of the specimen. This difference emphasizes the need to identify and understand the fatigue damage mechanism. A relatively new non-destructive inspection technique using infrared thermography was found to be a very useful tool in detecting damage initiation and growth. Furthermore, this technique supplies valuable information to the characterization of the operating fatigue damage mechanism(s). Fatigue leads to a degradation of material properties. Consequently, in connection with impact induced local stress raisers, fatigue produces continuously changing non-uniform stress fields because of stress redistribution effects. Other models addressing evolution of fatigue damage in composite materials have not been able to simulate evolving non-uniform stress fields. Therefore. in the second part of this paper. an analytical/numerical approach capable of addressing these issues is also proposed.

Approximate method for the analysis of components undergoing ratchetting and failure

An approximate method has been presented for the design analysis of engineering components subjected to combined cyclic thermal and mechanical loading. The method is based on the discretization of components using multibar modelling which enables the effects of stress redistribution to be included as creep and cyclic plasticity damage evolves. Cycle jumping methods have also been presented which extend previous methods to handle problems in which incremental plastic straining (ratchetting) occurs. Cycle jumping leads to considerable reductions in computer CPU (central processing unit) resources, and this has been shown for a range of loading conditions. The cycle jumping technique has been utilized to analyse the ratchetting behaviour of a multibar structure selected to model geometrical and thermomechanical effects typically encountered in practical design situations. The method has been used to predict the behaviour of a component when subjected to cyclic thermal loading, and the results compared with those obtained from detailed finite element analysis. The method is also used to analyse the same component when subjected to constant mechanical loading, in addition to cyclic thermal loading leading to ratchetting. The important features of the two analyses are then compared. In this way, the multibar modelling is shown to enable the computationally efficient analysis of engineering components.

A new analytical model of inelastic local flange buckling

This paper deals with analytical modelling of inelastic local flange buckling of compressed I-beam flanges. A short discussion of the relevance of different constitutive models for the inelastic material behaviour is carried out. It is claimed that what is known as the ‘plastic buckling paradox’ is not at all a paradox but a result of improper use of plasticity theory. An analytical model for the inelastic local buckling of an I-beam flange is proposed. The model considers the buckling process as being composed of two parts. The first is associated with inelastic torsional buckling of a compressed flange and the second part corresponds to a yield line plate buckling configuration which includes the effect of stress redistribution due to large deformations. The transition between these phases is left out in the model. The model is capable of predicting approximately the force-deformation relation of a locally buckling stocky flange for different stress-strain relations. The model is evaluated against experiments and the agreement is found quite reasonable.

Long-term strength of ceramics under interrupted bending loading conditions

Mathematical modeling is used to study the effect of stress redistribution within the cross section of a bent rod and the relaxation of residual stresses on the nature of the long-term-strength curves for structural ceramics when the load operates with interruptions.

The Use of Thermoelasticity to Evaluate Stress Redistribution and Notch Sensitivity in Ceramic Matrix Composites

ABSTRACTA new generation of relatively ‘ductile’ CMCs is being developed that expands the general utility of structural ceramic composites. These new materials rely on inelastic mechanisms such as interface failure, matrix cracking, fiber failure and fiber pullout to redistribute stress away from locations of stress concentration.1 The combined effect of these mechanisms can be summarized in three macroscopic damage classifications: Class I damage is the development of a single matrix crack bridged by fibers; Class II damage involves the development of multiple cracks in the matrix; and Class III damage involves the development of a shear damage zone. The operative damage mechanism depends upon the composite constituent properties, while the extent of stress redistribution depends upon the damage mechanism. Recent experiments have identified these damage mechanisms and have also quantified the effect of stress redistribution.2,2,4

The thermomechanical integrity of thin films and multilayers

Thin films and multilayers comprised of different classes of material are often used for various functional requirements. As these become relatively large in section and geometrically more complex, thermomechanical integrity is a major concern. It influences performance, yield and reliability. A methodology for thermomechanical design is needed that complements procedures used for circuit design. This article elaborates the principles of fail-safe thermomechanical design, based on the damage mechanisms known to occur in these systems. Among the important mechanisms are delamination and crazing of brittle layers. thermomechanical fatigue of metallic constituents and interface decohesion. The damage mechanisms are generally activated by residual stress, both thermal and ‘intrinsic’. The origins of these stresses are discussed, as well as stress redistribution effects that arise because of bending, discontinuities, etc. Emphasis is given to measurement methods which provide those data needed for implementation of the fail-safe design methodology.

Stress redistribution in ceramic/metal multilayers containing cracks

Multilayer systems comprising either Al or Cu bonded to sapphire have been used to evaluate stress redistribution effects around cracks, caused by slip in the metal. Single precracks have been introduced into the outermost sapphire layer and strain distributions measured around those cracks upon loading. Both moiré interferometry and (Cr 3+) fluorescence piezospectroscopy have been used for that purpose. Finite element calculations of the plastic zone size, L s, the crack opening displacement, δ, and the strains have been made, with the yield strengths of the metals, σ o, regarded as unknowns. Comparison of L s or δ with the measurements establishes σ o. Once the relevant yield strengths have been determined, the calculated stresses and strains (as redistributed by slip in the metal layer) are found to be in reasonable correspondence with the values measured by either moiré interferometry or piezospectroscopy. Some small discrepancies remain, which have yet to be explained.