Peak Normal Stresses Research Articles

Stress Distributions in Brazed Single-Lap Joints Under Tensile Loading

The most common method for characterizing the strength of brazed joints is uniaxial tension testing of single lap joints (SLJs). Standard interpretations depend on the assumption that the average shear stress at failure is the key metric in determining joint strength. However, it is evident from the geometry that the stress distributions must be inhomogeneous with shear lag type stress concentrations at the ends of the overlap regions. Eccentric loading causes overlap rotation and bending stresses that amplify the stress concentrations and result in geometric nonlinearity. Unfortunately, details of the distributions of normal and shear stresses on the braze needed to understand failure have not been presented. Thus, finite element analysis was used to quantify these stress distributions using 2D elastic and elasto-plastic models of monolithic stainless steel SLJs. Bending stresses and normal and shear stresses acting on the braze were determined over a wide range of overlap ratios and applied stresses. For all conditions, stresses are highly concentrated in a narrow region at overlap ends with peak normal stresses exceeding peak shear stresses. Variations in peak stresses with applied stress and overlap ratio were found to fully explain experimental joint strength data. Common interpretations based on the average shear stress at failure are found to be incorrect. Implications for testing, interpretation, and joint design are discussed.

Laboratory study of the combined wave and surge overtopping-induced normal stress on dike

Normal stress on dikes is one of the most critical parameters for a sound dike design. With more rapidly rising sea levels due to global warming, dikes are seriously threatened by overtopping induced by the combination of wave and storm surge. Compared with wave overtopping on positive freeboard, the curling breaking wave on dikes induced by the combined wave and surge overtopping may destroy the weakly protected dike crest and landward slope. Thus, in order to prevent severe damage to dikes, it is necessary to fully understand the normal stress induced by the combined wave and surge overtopping. In this paper, physical model tests were carried out to study the normal stress on dike induced by the combined wave and surge overtopping. Two characteristics of normal stress on dike were observed. The spatial distribution of normal stress on dike was also analyzed. It was found that the Weibull distribution can be used to effectively describe the statistical distribution of peak normal stresses. Furthermore, by curve fitting of the laboratory measured data, the Weibull factors on the part of the crest and the upper part of the landward slope were obtained.

Shaking table test and numerical analysis of nuclear piping under low- and high-frequency earthquake motions

A nuclear power plant (NPP) piping is designed against low-frequency earthquakes. However, earthquakes that can occur at NPP sites in the eastern part of the United States, northern Europe, and Korea are high-frequency earthquakes. Therefore, this study conducts bi-directional shaking table tests on actual-scale NPP piping and studies the response characteristics of low- and high-frequency earthquake motions. Such response characteristics are analyzed by comparing several responses that occur in the piping. Also, based on the test results, a piping numerical analysis model is developed and validated. The piping seismic performance under high-frequency earthquakes is derived. Consequently, the high-frequency excitation caused a large amplification in the measured peak acceleration responses compared to the low-frequency excitation. Conversely, concerning relative displacements, strains, and normal stresses, low-frequency excitation responses were larger than high-frequency excitation responses. Main peak relative displacements and peak normal stresses were 60%–69% and 24%–49% smaller in the high-frequency earthquake response than the low-frequency earthquake response. This phenomenon was noticeable when the earthquake motion intensity was large. The piping numerical model simulated the main natural frequencies and relative displacement responses well. Finally, for the stress limit state, the seismic performance for high-frequency earthquakes was about 2.7 times greater than for low-frequency earthquakes.

Experimental validation for the quasi-shear wave behavior of LiF single crystal along a low-symmetry orientation under uniaxial shock loading

To investigate the quasi-shear wave behavior and the underlying microscopic mechanism of an anisotropic solid under dynamic deformation beyond its Hugoniot elastic limit, LiF single crystals are shock-compressed along the [310] low-symmetry crystallographic orientation via normal plate-impact method. Interfacial velocity profiles are measured with a Doppler pin system. Peak normal stresses in samples vary between 1.91 GPa and 3.23 GPa. Under the lowest stress in this study, the resultant wave profile shows typical elastoplastic two-wave structures. In the second lowest stress experiment, an irregularity of the plastic wave or the inelastic deformation wave appears in the wave profile. At two higher stresses, a third wave is found following the elastoplastic two waves propagating along the normal direction. Our observations of three-wave structures in the [310] LiF are in excellent agreement with the simulation result of literature. This fact confirms that the immobilization of dislocations and rotation of slip planes are responsible for the microscopic mechanism of the three-wave propagations in the [310] LiF under uniaxial shock loading. The mechanism of the elastoplastic two-wave to anomalous three-wave structures evolution of material under different peak normal stresses will also be discussed.

An accurate solution for stress fields strongly varied across the adhesive thickness in soffit plate-strengthened beams

A closed form solution and a finite difference formulation are developed for an accurate prediction of adhesive shear and normal stresses in soffit plate flexural-strengthened beams. First, three statically admissible stress fields (i.e., longitudinal normal, transverse shear and normal stresses) are derived and expressed in terms of four unknown interfacial shear and normal stresses. Then, a modified complementary strain energy is written in which the energy contributions of the transverse normal stress fields in the beam and in the plate, and the transverse shear stresses in the beam are omitted. Based on an energy variational principle, compatibility equations and corresponding boundary conditions are obtained. A closed form solution and a finite difference formulation are finally developed. Through validations, the adhesive shear and normal stresses near the plate ends predicted by the present study are in excellently agreements with those provided in several experimental and numerical studies. Also, the present study excellently captures stress fields strongly varied across the adhesive thickness, especially the transverse normal stresses. Based on the present theory, a parametric study is conducted to quantify the effects of the plate longitudinal modulus and the thickness on the interfacial shear and normal stresses. The parametric study indicates that the peak normal stresses are significantly higher than the peak shear stresses in the adhesive layer. The present study is applicable to predict the adhesive shear and normal stresses highly concentrated near the plate ends in plate-strengthened beams with general applied loads and force boundary conditions.

An experimental study on lap joining of multiple sheets of aluminium alloy (AA 5754) using friction stir spot welding

Friction stir spot welding (FSSW) process is widely used in the automotive industry for a range of applications such as battery components, standard wire connectors and terminals. This manuscript addresses two grand challenges in the arena of FSSW, hitherto, unaddressed in the extant literature: (i) lap joining of thin sheets (0.3 mm thickness) of AA 5754 alloy and (ii) lap joining of more than two sheets using FSSW. To accomplish this task, a novel pinless convex-shaped tool was designed to alter the stress state while gradually advancing the tool which led to achieving stress state necessary for obtaining defect-free lap joints. The weld joints were inspected by optical microscopy, SEM imaging and analysed by nanoindentation tests and Vickers microindentation tests for assessment of the quality of the weld interface (WI). Process parameters of FSSW such as torque on the tool and axially applied load were used to analytically obtain the average local measure of peak normal and axial stresses as well as the coefficient of friction in the contact zone. In samples welded at low rotational speeds, the strain-hardening mechanism was seen dominating in contrast to samples welded at higher rotational speeds, which showed thermal softening. As a direct consequence of this, the samples welded at low rotational speeds showed much higher hardness at the weld surface than the samples welded at higher speeds. A strong transition of strain hardening to thermal softening was noticeable beyond an applied strain rate of 400 s−1.

Relating grass failure on the landside slope to wave overtopping induced excess normal stresses

A high quality safety assessment of levee systems requires a good prediction of when the grass cover of levees fail. Current methods relate the onset of failure to the peak in momentum or energy of the flow, instead of the peak in momentum transfer or energy transfer to the grass cover. The critical velocity necessary in the current methods is thereby difficult to quantify. In line with determining the peak in momentum transfer to the grass, here is shown that the onset of damage of the grass cover can be related to the peak normal stresses acting on the grass cover during wave overtopping. The peak in momentum transfer is thereby assumed to be located at the point of reattachment of the flow with the landside slope. The method is validated against the results of two wave overtopping experiments and benchmarked against the cumulative overload method. An advantage of this method is thereby that both the time and location of the onset of damage can be predicted.

Melt Welding and Its Role in Fault Reactivation and Localization of Fracture Damage in Seismically Active Faults

AbstractLow displacement fracture damage plays an important role in influencing the behavior and mechanical evolution of faults. Fracture damage zones influence slip behavior through changing near‐field stress orientations, altering fluid pathways and modifying fault structure. Here we use small displacement triaxial experiments to explore the development of fault zone damage, frictional lock‐up, and the generation of new faults using samples with preground faults, oriented in 5° increments between 25° and 65° relative to the shortening direction. With increasing reactivation angle, faults support higher peak normal stresses (104–845 MPa) and behavior transitions from stable sliding to stick slip. Frictional melting occurs on surfaces where stick slip is initiated, forming micron‐thick layers that locally weld asperity contacts. The extent of melt welding is correlated with normal stress and melt‐welded zones increase fault cohesion. Distribution of fracture damage adjacent to the fault is spatially correlated with melt‐welded zones and the corresponding concentrations of stress and elastic strain. In a process referred to as “adhesive wear,” fractures bypassing welded zones transfer melt‐adhered material from one side of the fault to the other, forming new geometric asperities. On faults with high reactivation angles (55°–60°) the increase in cohesive strength resulting from melt‐welded contacts drives fault lock‐up after an initial slip event; subsequent slip localizes on new, favorably oriented faults. Given their size, melt‐welded zones are likely to be short‐lived in nature but may play a significant and previously unrecognized role in the development of fault‐related damage.

Flexural behavior of metallic fiber-reinforced adhesively bonded single lap joints

ABSTRACTIncorporation of additives into the adhesive layer in adhesively bonded joints can improve the stress distriution in the adhesive layer and increase adhesive toughness. In this paper, the geometric and material parameters of metal fibers utilized for strengthening adhesively bonded single lap joints under flexural loading were investigated by using experimental investigations and finite element modeling. According to the experimental results, incorporating metal fibers in the adhesive layer of a bonded joint can have a significant impact on the flexural load bearing of the joint. This was in relationship with the numerical results foreseeing enhanced stress distributions of the adhesive layer, when the metal fibers were added to the adhesive layer. Some important parameters in the design of metal fiber-reinforced adhesive joints include the volume fraction (the distance between the fibers and the fiber diameter), orientation, and mechanical properties of the fibers. It was concluded that the peak normal stresses in the adhesive layer can be reduced, and consequently the load bearing of the joint can be improved by reducing the distance between the fibers, increasing the fiber diameter and choosing a stiffer material for the fibers in the longitudinal direction.

Viscoelastic Properties of Dental Pulp Tissue and Ramifications on Biomaterial Development for Pulp Regeneration

IntroductionA critical step in biomaterial selection effort is the determination of material as well as the biological properties of the target tissue. Previously, the selection of biomaterials and carriers for dental pulp regeneration has been solely based on empirical experience. MethodsIn this study, first, the linear viscoelastic material functions and compressive properties of miniature pig dental pulp were characterized using small-amplitude oscillatory shear and uniaxial compression at a constant rate. They were then compared with the properties of hydrogels (ie, agarose, alginate, and collagen) that are widely used in tissue regeneration. ResultsThe comparisons of the linear viscoelastic material functions of the native pulp tissue with those of the 3 hydrogels revealed the gel-like behavior of the pulp tissue over a relatively large range of time scales (ie, over the frequency range of 0.1–100 rps). At the constant gelation agent concentration of 2%, the dynamic properties (ie, storage and loss moduli and the tanδ) of the collagen-based gel approached those of the native tissue. Under uniaxial compression, the peak normal stresses and compressive moduli of the agarose gel were similar to those of the native tissue, whereas alginate and collagen exhibited significantly lower compressive properties. ConclusionsThe linear viscoelastic and uniaxial compressive properties of the dental pulp tissue reported here should enable the more appropriate selection of biogels for dental pulp regeneration via the better tailoring of gelation agents and their concentrations to better mimic the dynamic and compressive properties of native pulp tissue.

Wind Load Combinations of Atypical Supertall Buildings

AbstractWind loads are generally divided into along-wind, across-wind, and torsional loads, but in reality they occur simultaneously in time and space. Furthermore, because they do not show their maximums at the same instant, they need to be combined appropriately. In this study, pressure measurements were conducted on atypical supertall buildings to investigate wind load combinations. Using a simplified reference model with four columns, analyses in the time domain were conducted, and this paper proposes combination rules based on the concept of combination factor that can be applied to atypical supertall buildings. The combination factor for a square model is approximately 0.5, and those for atypical models differ depending on building shape. Two methods are discussed in the determination of combination factors, and comparisons between peak normal stresses by analysis and combination factor show relatively good agreement, ranging mostly within ±5%.

Wind-induced responses of super-tall buildings with various atypical building shapes

A wind tunnel tests were conducted on 13 super-tall building models with atypical building shapes under an urban area flow. The primary purpose of the present study was to directly compare the wind load effects on atypical super-tall buildings. Time history analyses were conducted using a frame model by inputting local wind forces at the center of each floor. The results show that the peak normal stresses on a square model are the largest among all the models tested, the setback model shows the smallest peak normal stresses of the single modification models tested, and CC+TP+360Hel shows the smallest peak normal stresses of the multiple modification models tested. The contributions of bending moments are about 20% of the total, and most of the peak normal stresses resulted from axial force. The increase in bending moment in the across-wind direction becomes significant as the damping ratio decreases, and the sensitivity of the peak normal stresses for the helical and multiple modification models to damping ratio is smaller than those of the other models. From the analyses for the various loading conditions, it was found that the contribution of bending moment in the along-wind direction is the largest and that of torsional moment is almost negligible.

Time History Analysis of Steel Tall Buildings

The results of wind tunnel experiments were used to conduct time history analyses of three conventional square cross-section tall buildings with different structural systems. The primary purpose of the study was the direct comparison of the effects of the wind loads on the steel tall buildings. Time history analyses were conducted by applying local wind forces to the center of each floor. The results showed that, although the bending moments in the ground-level column on the two principal axes were different, the peak normal stresses were almost the same regardless of the structural systems. Similar observations were made regarding the tip displacements. Furthermore, analyses for the various loading conditions revealed that the contribution of the bending moment in the across-wind direction was the largest, followed by that in the along-wind direction. The ratio of the peak normal stresses for different loading conditions were observed to be almost the same regardless of the structural systems.

Application of the Point Stress Criterion to Assess the Bond Strength of a Single-Lap Joint

Finite element analysis has been carried out to obtain the interfacial stresses in a single lap joint using a special 6-node isoparametric element for adhesive layer. The analysis results are found to be in good agreement with the closed-form solution of Goland and Reissner. The peak normal and shear stresses found in the adhesive layer at the edges of the joint are due to stress singularity. The bond strength of the single-lap joint is estimated considering one of the stress fracture criteria known as the point stress criterion. Bond strength estimates are found to be reasonably in good agreement with existing test results.

비정형 초고층 건물의 합리적인 풍하중 산정

In international codes and standards, the wind forces are classified into the two translational forces and torsional moment, but in reality these forces act simultaneously in time and space. And as the wind forces, hence wind loads, do not show their largest values at the same time instant, the wind loads should be combined adequately. In this study, pressure measurements were conducted on the 6 atypical super-tall buildings to investigate the wind load combinations. Time history analysis was conducted using wind pressures, and using the peak normal stresses in columns, new combination rule based on the combination factor was proposed to be applicable to atypical super-tall buildings. Combination factor of square model is about 0.5, and those of atypical models show different values depending on building shapes. The comparisons between peak normal stresses by analysis and combination factor show a good agreement, ranging peak normal stresses within <TEX>${\pm}5%$</TEX>.

Correlation and combination of wind force components and responses

This paper summarizes the findings from extensive wind tunnel tests carried out by the authors' group for the evaluation of wind load combination effects for various types of building models. Characteristics of correlations of wind force components are examined using the absolute ratio of wind forces, phase–plane trajectories and (absolute) cross-correlation, and then wind load combinations are examined. The necessity to consider wind load combinations is inferred from the instantaneous pressure distributions, and the cross-correlation coefficients of the absolute values of wind force components are found to be more important when examining wind force combinations. Wind load combination effects are directly examined using frame models. Based on the peak normal stresses in columns under various loading conditions, combination factors for low-, middle- and high-rise buildings are proposed. Lastly, effects of wind direction on wind load combinations are discussed.

Analysis of deformation twinning in tantalum single crystals under shock loading conditions

The competition between dislocation slip and twinning in tantalum single crystals has been investigated utilizing a crystal level twinning model and the results from gas gun recovery experiments conducted at peak normal stresses of 25 and 55 GPa. The recovered samples were characterized using electron back scattered diffraction, and the observed twinning fractions were compared with the model. The experimental results show very low twin fractions in all orientations at 25 GPa, and that among (100), (110), (111), and (123) crystals, the (110) crystals had the largest amount of twinning at 55 GPa. The analysis shows that the general trends observed in the experimental data can be reproduced by the model when an orientation dependent dislocation evolution is used. This analysis gives insight into the possible influence of the dislocation density and its evolution on the observed twinning behavior.

Shielding effects on wind force correlations and quasi-static wind load combinations for low-rise building in large group

Systematic wind pressure measurements were conducted in order to investigate the effect of a large group of surrounding buildings on wind pressures on a typical low-rise building. The primary purpose of the present work was to understand the characteristics of wind force correlations and quasi-static wind load combinations of a target model in a large group. Fluctuating pressures were integrated over the surfaces and results were obtained of along-wind force, across-wind force, uplift force, along-wind overturning moment, across-wind overturning moment and torsional moment. First, the characteristics of wind force correlations including the force coefficients, phase-plane expression and cross-correlation coefficients were investigated for an isolated model and for a target model in a large group. Then, in order to directly check the wind load combination effects, peak normal stresses in columns of a simple frame model were examined. Next, a shielding factor of a combination factor was proposed in the form of an exponential function.

Effect of the interpass temperature on the predicted residual stress distributions in a multipass welded piping branch junction

A sequentially coupled three‐dimensional thermomechanical finite element model has been developed to predict residual stress distributions in a multipass welded piping branch junction. The residual stresses at the branch and run pipe cross‐sections, as well as along the circumferential weldlines on the outer surfaces of both the run and the branch pipes and on the inner surface of the branch pipe, are predicted. Three levels of interpass temperature have been selected to investigate their effect on the peak residual stresses. It is revealed that the interpass temperature has a significant effect on the residual stresses. As the interpass temperature is increased, both the peak hoop and the axial residual stresses at the run and branch cross‐sections decrease. The peak normal stresses along the circumferential weldline on the outer surface of the run pipes are also reduced. However, increasing the interpass temperature had a negligible effect on the peak tangential residual stresses along the circumferential weld line on the inner surface of the branch pipe. The results presented and the modelling technique described in the current study can be used towards formulating a recommendation to optimize residual stress profiles in multipass welded complex geometries through better interpass temperature control.

An Experimental Investigation into the Influence of Fillers on the Development of Normal Stresses and Stress Relaxation in Asphalt Mixtures Due to Torsion

This study documents the measurement of normal stresses and stress relaxation in sand-asphalt mixtures fabricated with different fillers and asphalts during torsion. Hydrated lime and limestone fillers and asphalts graded as PG64-22 and AC-30 (from Sinclair (Wyoming), Crown (Nevada), and Crown (Canada)) are used in the fabrication of the sand-asphalt mixtures. The specimens are tested in a torsional rheometer. The experimental results clearly show that the normal stresses that are developed are quite significant even for specimens tested at very low rotational rates. Also, asphalts from different sources show differences in peak normal stresses and in their relaxation pattern. The measurement of significant normal stresses is a reflection of the nonlinear character of the material and warrants the development of nonlinear constitutive models for describing their behavior.