Peak Normal Stresses Research Articles

Peak normal stresses and effects of wind direction on wind load combinations for medium-rise buildings

In order to examine wind load combinations, fluctuating surface pressure measurements are made for flat roof building models with square or rectangular plans. The fluctuating pressures are integrated over all their surfaces and results are obtained of along-wind force, across-wind force, vertical force, along-wind overturning moment, across-wind over-turning moment and torsional moment on their frames. The maximum wind force component and other simultaneously observed wind force components are examined, and some interesting facts regarding the effects of wind direction are discussed. In order to directly check the wind load combination effects, the maximum normal stresses in column members of simple frame models are examined, and a wind load combination factor is proposed.

Stress analysis of bonded lap joints using fracture mechanics and energy balance

This paper uses fracture mechanics and energy balance to develop simple closed-form equations for the peak adhesive shear and normal stresses at the end of the bondline for flat and curved lap joints with adhesives with linear and non-linear material behaviour and variable adherend thickness. By using energy balance, explicit relationships between the strain energy release rate, mode ratio, and adhesive shear and normal stress are developed. The approach is shown to give identical results to the traditional beam-on-an-elastic-foundation approach for adhesive stresses in the special cases where closed-form solutions exist in the literature. The simplicity and ability to obtain closed-form expressions for joints with non-linear adhesives and variable adherend thickness makes the approach suitable for engineering analysis and preliminary design.

Machined surface properties determined by nanoindentation: Experimental and FEA studies on the effects of surface integrity and tip geometry

An accurate characterization of subsurface properties is essential to understand machining induced surface integrity. However, little research has been conducted to investigate significant influences of surface integrity (hardness and residual stress) and tip geometry on the indentation process and the resulting load–depth curves. Nanoindentation was used to measure hardness and modulus in the subsurface of machined components by turning, grinding, and honing. The machining induced residual stresses can be estimated by matching the FEA simulated load–depth curves with the experimental ones. The simulation sensitivity analysis shows that the load–depth curves are significantly affected by residual stress and modulus, while friction, tip rounding, and yield strength have much smaller effects. The peak normal stresses occur on the top surface, while the peak shear stress is slightly under the surface. All peak strains are in the subsurface. The onset of material yielding during loading may be determined by tracing the evolution of loading/unloading loops at different indentation depths. A 3D simulation of nanoindentation with a spherical tip has shown that the indentation force induced by the spherical tip is less than 2% of the total force provided that the indentation depth is sufficiently large.

Turbulent Characteristics of a Transverse Supersonic Jetin a Subsonic Compressible Crossflow

Fluctuating velocity fields have been determined from particle image velocimetry data acquired in the farfield of the interaction generated by an overexpanded axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible crossflow. Peak magnitudes of the turbulent stresses were found to be larger and located further from the wall for greater values of the jet-to-freestream dynamic pressure ratio J while the crossflow Mach number M∞ remained constant. These stress magnitudes diminish with downstream distance as their peak location moves further from the wall. The vertical positions of the peak normal stresses and shear stress inflection point coincide with the maximum mean streamwise velocity deficit induced by the jet. Instantaneous realizations of the velocity fluctuation fields reveal large-scale structures whose mean diameter is greater for larger J and decreases with downstream distance. The integral length scale calculated from profiles of the correlation coefficient instead shows an increase downstream; the discrepancy between the two length scales results from a low-pass filter effect of the correlation coefficient. Similarity was demonstrated for constant J at two transonic M∞, but not at two lower M∞ where the flowfield does not distinguish between the jet and its wake.

Finite element modeling of the contact interface between trans-tibial residual limb and prosthetic socket

Finite element method has been identified as a useful tool to understand the load transfer mechanics between a residual limb and its prosthetic socket. This paper proposed a new practical approach in modeling the contact interface with consideration of the friction/slip conditions and pre-stresses applied on the limb within a rectified socket. The residual limb and socket were modeled as two separate structures and their interactions were simulated using automated contact methods. Some regions of the limb penetrated into the socket because of socket modification. In the first step of the simulation, the penetrated limb surface was moved onto the inner surface of the socket and the pre-stresses were predicted. In the subsequent loading step, pre-stresses were kept and loadings were applied at the knee joint to simulate the loading during the stance phase of gait. Comparisons were made between the model using the proposed approach and the model having an assumption that the shape of the limb and the socket were the same which ignored pre-stress. It was found that peak normal and shear stresses over the regions where socket undercuts were made reduced and the stress values over other regions raised in the model having the simplifying assumption.

Integration of CT, CAD system and finite element method to investigate interfacial stresses of resin-bonded prosthesis

The question of whether prosthesis/abutment tooth interface debonding is associated with inappropriate occlusal force is investigated in this study. A new modeling approach was employed to perform the interfacial stress analyses. Solid models of resin-bonded (RB) prosthesis and abutment teeth were constructed by stacking serious section contours that were obtained from CT images. A 3-D finite element (FE) model of RB prosthesis/abutment teeth was generated in a CAD system after assembling and meshing procedures. An in-house program was developed to combine the FE package (ANSYS) to calculate the interfacial (normal and shear) stresses at the prosthesis/molar interface with the bonding and debonding conditions. After 10 different occlusal force(s) evaluations, three initial opening gaps at the distal margin of the retainer were assumed to examine the possible interface debonding mechanism under the worst loading case. The results indicated that a more accurate FE model of the posterior RB prosthesis/abutments could be generated through combining several computer-aided techniques. The maximum interfacial stresses were obtained when the occlusal force was applied on the buccal slope of the distolingual cusp of the molar. For interface debonding simulations, peak normal and shear stresses were found near the debonding areas and stress values increased gradually with small to large initial opening gaps. From these results, prosthesis/abutment tooth interfacial fatigue damage might arise or accelerate the interface to debond under adequate bonding or initial gap opening conditions after long-term repeated inappropriate occlusal force actions.

Finite element estimates of interface stress in the trans-tibial prosthesis using gap elements are different from those using automated contact

When compared with automated contact methods of finite element (FE) analyses, gap elements have certain inherent disadvantages in simulating large slip of compliant materials on stiff surfaces. However, automated contact has found limited use in the biomechanical literature. A non-linear, three-dimensional, geometrically accurate, FE analysis of the trans-tibial limb–socket prosthetic system was used to compare an automated contact interface model with a gap element model, and to evaluate the sensitivity of automated contact to interfacial coefficient of friction (COF). Peak normal stresses and resultant shear stresses were higher in the gap element model than in the automated contact model, while the maximum axial slip was less. Under proximally directed load, compared with automated contact, gap elements predicted larger areas of stress concentration that were located more distally. Gap elements did not predict any relative slip at the distal end, and also transmitted a larger proportion of axial load as shear stress. Both models demonstrated non -linear sensitivity to COF, with larger variation at lower magnitudes of COF. By imposing physical connections between interface surfaces, gap elements distort the interface stress distributions under large slip. Automated contact methods offer an attractive alternative in applications such as prosthetic FE modeling, where the initial position of the limb in the socket is not known, where local geometric features have high design significance, and where large slip occurs under load.

The effect of insoles in therapeutic footwear—A finite element approach

Current practice in the prevention of recurrence of neuropathic foot ulcers is to prescribe accommodative in-shoe orthoses or insoles which reduce plantar pressure levels at locations of bony prominences, particularly under the metatarsal heads. To date, design of these orthoses has largely been a trial and error process. There is little quantitative information available regarding the effects of thickness and the influence of soft tissue characteristics on the cushioning effect of such interventions. The current paper investigated alterations in pressure under the second metatarsal head as a function of insole thickness and tissue thickness. Both experimental and quasi-static plane strain finite element approaches were employed. The orthoses chosen reduced plantar pressure by a maximum of approximately 30% and were more effective (on a percentage basis) in the setting of reduced sub-metatarsal tissue thickness. Peak normal stresses predicted by the FE models were, on average, within 5.9% of experimentally measured values for the normal tissue case and 8.1% for the reduced tissue case. The techniques presented represent a promising approach to understanding plantar cushioning and the principles involved in the design of therapeutic footwear for insensate feet.

Compaction of an earthy sand by rubber tracked and tired vehicles

Normal stresses exerted by two agricultural vehicles (a rubber tracked Caterpillar Challenger 65 and a Steiger Tiger 550 4WD tractor fitted with low pressure dual wheels) were measured with electronic earth pressure cells at four depths (150, 300, 400 and 500 mm) in a sandy soil located in the West Australian wheatbelt. Cone penetration resistance and dry bulk density were also measured at these depths before and after traffic treatments. Uniaxial soil compression testing was done on soil cores sampled at the 300 mm depth where the peak normal stresses were measured during vehicle passage. The Cat 65 exerted less normal stress on the soil than the Steiger 550 at 400 and 500 mm depth. However, the soil had a higher penetration resistance after the passage of the Cat 65 relative to the Steiger 550. Profiles of cone penetration resistance were more discriminating in distinguishing soil physical changes among the trafficked and untrafficked treatments than dry bulk density and most of the parameters derived from compression tests.

Tractive performance of a tread design for improved flexibility

The tractive performance of an 18.4R38 radial-ply tractor tire with increased flexibility in the tread area was compared to that of a standard tread design. Normal soil-tire interface stresses were measured at four locations on the lug surfaces of both tires operating on Decatur clay loam and Norfolk sandy loam soils. There was a tendency for the increased flexibility in the tread area to provide a higher net traction ratio at the same tractive efficiency as the standard tread design, especially on Decatur clay loam soil. The more flexible tread design reduced the magnitude of peak normal contact stresses across the tire width, which may have implications for reducing soil compaction without compromising tractive performance. The more flexible tire reduced the average normal contact stress by approximately 15% in the sandy loam soil and 23% in the clay loam soil for the range of operating conditions investigated.

Two-dimensional creep analysis of structural adhesive joints

A two-dimensional finite element program was developed to study the creep behaviour of bonded joints for two structural adhesives. The program used multiaxial viscoplasticity theory, and modelled creep by reducing the yield stress to zero. The necessary parameters for the finite element model were obtained from creep tests on beam specimens of the bulk adhesives. The results showed that creep led to a more even distribution of the stresses with the peak normal and shear stresses being reduced. However, the shear strain showed a large increase. The former effect could be beneficial in terms of joint strength, but the latter effect would be detrimental.

Soil-Tire Interface Stress Measurements

ABSTRACT NORMAL and tangential stress vectors were measured at the soil-tire interface of a pneumatic tractor tire on firm and soft soil. Data are presented from tests conducted at zero input torque, zero net traction, and 0.15 net traction ratio. Peak normal stresses occurred before peak tangential stresses as the tire rotated through the contact area at 0.15 net traction ratio. A force balance was performed on the tire at each level of tractive performance.