Induced Stress Concentrations Research Articles

Biomechanical analysis of proximal tibia bone grafting and the effect of the size of osteotomy using a validated finite element model.

Harvesting bone graft from the proximal tibia is gaining popularity, with lower complication rates and adequate quantity of cancellous bone. The amount of harvested bone is dependent on the size of the cortical window introduced via osteotomy onto the proximal tibia, and its mechanical strength after surgery could be compromised. The aim of the study was to investigate the proximal tibia's mechanical stability after bone harvesting and the effect of varying window sizes using a validated finite element model. Two cadaveric tibiae were tested with bone strains measured for different circular cortical window diameters (10-25mm). Sixteen finite element models of the intact and harvested tibia were simulated and validated with experimental data. The experimental and predicted max/min principal bone strains were fitted into regression models and showed good correlations. It was predicted the maximum principal bone stresses were greatest and concentrated at postero-inferior and antero-superior regions of the cortical window. A stress line progressed from the edge of the window to the posterior side of the tibia, which became more prominent with the increase of size of the cortical window. It was found that large circular osteotomies for bone harvesting at the proximal tibia induced stress concentrations and stress lines which could lead to eventual failure. Graphical abstract The formation of a stress line in the harvested tibia initiated from the postero-inferior edge of the window and progressed to the medial side. The stress line became more prominent with the increase of the size of the cortical window from 10 to 25mm in diameter.

Effect of notches on the deformation behavior and damage evolution of additively manufactured 316L specimens under uniaxial quasi-static and cyclic loading

Powder bed based additive manufacturing techniques enable for fabrication of filigree and geometrically complex structures. These structures typically are characterized by many cross section transitions, leading to notch induced stress concentration. Furthermore, the process induced surface roughness cannot be always mechanically removed as the surfaces are not accessible using conventional tools. Although these factors are critical for many different applications and materials, previous works mainly focused on notch effects in brittle materials, such as the common Ti-6Al-4V alloy. Systematic examination of notch induced stresses in ductile materials, especially under the constraints of process induced surface roughness and small part dimensions, was not conducted. Thus, the present study addresses these issues by evaluation of the mechanical performance of notched samples made of 316L stainless steel under quasi-static and cyclic loading. By means of digital image correlation insights into the local deformation mechanisms and the damage evolution, respectively, were obtained. Microstructural analysis by electron backscatter diffraction provided profound explanation for the phenomena observed.

Computational analysis of thermally induced stress concentration in structures with geometric constraints

Fracture and fatigue life of an engineering component is highly sensitive to stress concentrations that can arise at sharp corners. Stress concentration can be caused by impact, pressure, and high temperatures, among other factors. High operating temperatures are common in internal combustion engines, batteries, computers, and additive manufacturing. In this work, we aim to provide systematic notch design guidelines for thin metallic plates subjected to strict geometrical boundary conditions under high temperatures. Computational analysis is used to investigate how geometrical configurations, near critical areas, affect the magnitude of thermally-induced stress and strain under the influence of uniform heating conditions. Results show that tuning the curvature of sharp corners can allow for the mitigation of potentially destructive thermally-induced stresses. Additionally, this study shows that the use of functionally graded materials (with varying values of coefficient of thermal expansion and modulus) can redistribute thermally-induced stresses, improving the structural performance of metallic materials in the presence of high temperature environments.

Fatigue reliability analysis of butt welded joints with misalignments based on hotspot stress approach

Fatigue reliability of butt welded joints with misalignments is assessed, based on the structural hotspot stress approach. The unsymmetrical weld profile induced stress concentration is evaluated using the finite element and Monte Carlo methods based on a series of simulated weld profiles from published statistical data. The Monte Carlo method is also used to assess the uncertainty on the critical stress magnification factor proposed to represent the margin left for the misalignment effect to trigger fatigue failure. A limit state function is formulated based on the critical and actual stress magnification factors. The first order reliability method is applied to evaluate the reliability levels of different welding processes, reliability levels of standards and the lateral distortion limits for a target reliability index. The results show the advantage of the laser welding over the conventional arc welding regarding fatigue reliability. The tolerance limits of misalignments specified by standards lead to different reliability levels for different welded joint plate thicknesses. The thin-plate joints show a low reliability level and the reliability index increases with the plate thickness.

Lithiation Induced Stress Concentration for 3D Metal Scaffold Structured Silicon Anodes

Silicon is a promising material for next generation lithium ion battery anodes, because of its high theoretical specific capacity and relatively low operating voltage. However, due to the significant volume change during its lithiation/de-lithiation, the cycle life of Si-based anodes is restricted. A novel Si-coated inverse opal-structured anode, designed to mitigate the negative impacts of cycling-induced volume changes, is investigated in this study. The lithiation-induced stresses in the anode are predicted via coupled electrochemical- and mechanics-based finite element (FE) models. In this study, the effects of various cycling conditions and anode design parameters on the lithiation-induced stresses are explored, including the charging C-rate, thickness of the Si coating layer, and structural and mechanical properties of the supporting scaffold. It is found that the inverse opal anode structure could improve the uniform distribution of lithium in Si host material. It is also found that the anode structural design parameters have large influences on the stress concentrations in the Si coating layer, as well as in the supporting scaffold of the anode.

A Numerical And Statistical Approach For Optimization Of Tab Design For Non-Crimp Fabric Composites

In standard practice, testing of composites in tension requires the use of stress-inducing serrated grips. The low transverse compressive strength of unidirectional non-crimp fabric composites limits the application of high clamping forces. Tabs are therefore essential as they ensure a reduction in grip pressure transmission, surface damage and induced stress damage. Tabs, however, tend to introduce induced stress concentrations at the tab termination region. The objective of this study was to minimise stress concentration by varying tab design configurations to determine the optimal design most suitable for tensile testing of non-crimp fabric composites using finite element and statistical tools. Finite element models generated from experimental data were used for accessing the stress concentrations. A two (2)-level full factorial design was adopted and utilised for statistical analysis. Results revealed that tab stiffness, tab taper angle, adhesive thickness and manufacturing process (bonded or molded) were statistically significant for minimising stress concentration. Molded tabs were found to be acceptable if the stiffness of tab was significantly lower than test specimen.

Microstructures and stress rupture properties of pulse laser repaired Inconel 718 superalloy after different heat treatments

Long-striped Laves phases are considered to be crack sources in laser repaired Inconel 718 during stress rupture test. In order to obtain fine Laves phases, Nd:YAG pulse laser is used to fabricate Inconel 718 superalloy on a wrought Inconel 718 substrate in this study. The microstructures and stress rupture properties of pulse laser repaired Inconel 718 superalloy after DA treatment (720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) and 900STA treatment (900 °C × 20 min/air cooling+720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) are investigated by using SEM and TEM. The results shows that granular Laves phases can be obtained when pulse laser is used. The formation of granular Laves phase is related to high cooling rate (G·R) and low ratio of temperature gradient to growth rate (G/R). The critical values of G·R and G/R are 104 and 107, respectively. After 900STA treatment, δ phase precipitates around the granular Laves phases. But there is no δ phases existing in the DA samples. The surrounding δ phases can induce stress concentration, resulting in that the elongation of the 900STA samples is worse than that of the DA samples. However, the regions denuded in Nb element around the δ phases can delay the formation of the vacancies, which leads to that the stress rupture life of the 900STA samples is better than that of the DA samples.

Damage in Creep Aging Process of an Al-Zn-Mg-Cu Alloy: Experiments and Modeling

In creep age forming (CAF), large integral panel components of high-strength aluminum alloy can be shaped and strengthened under external elastic loading at an elevated temperature through creep deformation and age hardening, simultaneously. However, the high ribbed structure on panel may induce stress concentration, inhomogeneous plastic deformation and even damage evolution on the bending rib, leading to the difficulty in controlling forming precision and material properties. Therefore, the generation and evolution of damage are necessary to be considered in the design of CAF. Taking 7050 aluminum alloy as the case material, the continuous and interrupted creep aging tests at 165 °C and three stress levels (300, 325, and 350 MPa) were conducted, and the corresponding material properties, precipitate, and damage microstructures were studied by mechanical properties tests, transmission electron microscope (TEM) and scanning electron microscope (SEM) characterizations. With the increase of stress level, the creep deformation occurs easier, the precipitates grow up faster, the creep damage occurs earlier, the growth rate and the size of microvoids increase, the mechanical properties decrease more rapidly, and the dominant mechanism of creep fracture changes from shear to microvoid coalescence. To simulate creep aging behavior with damage, a continuum damage mechanics (CDM) based model is calibrated and numerically implemented into ABAQUS solver via CREEP subroutine. The CAF of 7050 aluminum alloy panels with different height ribs were conducted by experiment and FE simulation. The forming process presents a typical stress relaxation phenomenon. The creep damage mainly occurs on the bending rib due to the severe stress concentration. With the increase of rib height, the creep strain and damage degree increase, but the springback decreases.

Theory and Experimental Verification on Cymbal-shaped Slotted Valve Piezoelectric Pump

Valve piezoelectric pumps usually have larger flow rate than that of valveless ones. However, the traditional cantilever valve easily induces stress concentration which impacts the reliability of pumps. Therefore, a cymbal-shaped slotted check valve is proposed to be applied in a piezoelectric pump in order to reduce the stress concentration of the valve and thus improve the reliability of the piezoelectric pump. The structure and working principle of the piezoelectric pump are analyzed; the stress analysis of the cymbal-shaped slotted valve diaphragm is conducted. In addition, finite element software is employed to analyze the difference of the Von-Mises stress between the cymbal-shaped slotted diaphragm and the slotted flat diaphragm. The simulation results show that, the Von-Mises stress of cymbal-shaped slotted diaphragm is smaller than that of the slotted flat one. Furthermore, the cymbal-shaped slotted valve piezoelectric pump is also fabricated, and flow rate experiment is performed. The experimental results indicate that the flow rate of piezoelectric pump working in low frequencies (0 Hz < f < 50 Hz) is larger than that working in high frequencies (200 Hz < f < 2000 Hz). When driven at voltage of 160 V and frequency of 5 Hz, the pump reaches its maximum flow rate of 6.6 g/min. The experimental results validate the feasibility of the cymbal-shaped slotted check valve. This research can effectively solve the problem of stress concentration of valve piezoelectric pumps and is helpful for improving the reliability of them.

How Compression Inside a Delivery System can Degrade the Cover of Aortic Endografts

The goal of the present study was to identify the potential degradations undergone by textile endoprostheses (EPs) over the crimping process related to the catheter insertion purpose. In particular, we studied how the device design parameters can influence the wrinkling of the textile material, assuming that wrinkling induces stress concentration and may jeopardize the lifetime of the device. Custom-designed EPs were obtained from various stent designs and textile constructions. Monofilament and multifilament materials were considered for the cover. Stent segment size, distance, and wire diameter were considered as variable for the stent. The EPs of 26-mm diameter were then crimped in a mock transparent 6-mm diameter catheter sheath for 8 and 30days duration. After releasing the EPs from the sheath, the textile cover was characterized for roughness properties to identify the crease level induced on the surface by crimping. Results brought out that the monofilament material was characterized by a larger number of deeper creases in the zones where the stent was in contact with the cover. Conversely, the multifilament was more folded in the zones between stent segments. Moreover, it appeared that the stent design influenced the creases' topography. The textile seemed to be less prone to heavy wrinkling with stent segments made from larger wire diameter and larger segment size. Regarding the crimping duration, it came out that a longer stay in the sheath tends to promote more significant wrinkling. In this work, it was shown that wrinkling of the textile cover occurs in the EPs already at crimping level. However, an appropriate design of the EPs should limit the phenomenon and improve the performances of the EPs.

Predicting mechanical properties of polyvinylidene fluoride/carbon nanotube composites by molecular simulation

Pristine polyvinylidene fluoride (PVDF) and its CNT composites (PVDF/CNT) at three CNT weight fractions, 8 wt%, 16 wt% and 24 wt%, were used to investigate the influence of CNT fraction on the mechanical properties of PVDF. Molecular dynamics (MD) simulation was utilized to predict the mechanical properties of PVDF/CNT composites. The tensile stress–strain profiles indicate that the Young’s modulus and tensile strength of pristine PVDF can be significantly improved by the embedded CNTs. However, these CNTs induce stress concentration within the composites, resulting in increased brittleness and a fracture at lower strains. By taking advantage of this molecular simulation procedure, fast comparable predictive properties of PVDF/CNT composites could be performed, with the simulation results providing atomistic-level insights into this new pathway to reduce the cost and research time in related experiments.

Relation between crack initiation-damage stress thresholds and failure strength of intact rock

The analysis of a wide literature dataset of mechanical parameters related to intact rocks from more than 480 unconfined compression tests, coupled with new laboratory tests on 132 specimens, is proposed herein with the aim of analyzing the mechanical behavior of a great variety of rock types, mainly focusing on their crack initiation (σci) and crack damage (σcd) stress levels. These thresholds can be employed as warning indicators for rock mass damage and breakouts and represent important input parameters for numerical models. International literature lacks in a detailed analysis on the mutual dependence existing between the main mechanical properties of intact rocks and their crack stress thresholds. In this paper, the study of the correlation between crack initiation-crack damage stress levels and the failure strength of sedimentary, metamorphic and igneous rocks is carried out through single and multiple regression approaches aimed at finding reliable prediction models, which can be useful when time-consuming laboratory experimental procedures need to be avoided. The correlation between predicted and measured values demonstrates that defined models represent a good tool for the empirical estimation of σci and σcd, and can be useful for preliminary engineering design dealing with stress-induced brittle fracturing, especially when the definition of warning indicators for rock mass damage and breakouts is needed. In fact, it is known that underground instability mainly depends on the redistribution of stresses around the excavation, which can produce induced stress concentrations, resulting in sudden release of stored energy and causing stress-induced brittle failure phenomena.

Influence of Chemical Treatment on the Flexural Properties of Sisal Fiber Reinforced Resin Composites

Environmental awareness and an increasing concern with sustainable development have stimulated many industries including ground engineering to replace the conventional synthetic fibers. In this work, flexural strength and flexural modulus of chemically treated random short sisal fiber reinforced polypropylene and unsaturated polyester composites were investigated over a range of fiber content. Flexural strength of the composites was found to decrease with increased fiber content; however, flexural modulus increased with increased fiber content. The reason for this decrease in flexural strength was found to be due to fiber defects which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fiber treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fiber/matrix adhesion.

Adhesion characteristics of fiber-exposed glass composites

Joining of composite materials has been an issue ever since composites were introduced. Among various joining methods, adhesive joining has been widely adopted because the adhesive joining uniformly distributes stresses over larger area, while the mechanical joining of composites induces stress concentration and fiber breakout. However, adhesive joining requires the surface treatment of composite adherends to increase the bonding strength; this treatment is not only costly but also generates much environmental pollution.In this study, a composite adherend that does not require any surface treatment to constitute an adhesive joint is developed. The composite adherends are manufactured by a “soft layer method” to expose bare fibers on the surface during composite fabrication. The bonding strength was significantly improved due to the exposed bare fibers, which was comparable to that of the adhesive joint treated by the conventional peel ply method without generating waste materials.

Validation of Numerical Modeling for the Prediction of Elastic and Failure Behavior of Diamond Powder Filled Woven Composites

A numerical investigation was carried out to examine the role of micro-sized diamond powder filler on the on-axis tensile stiffness properties of the standard modulus T300 and the high modulus YS90A woven fabric composite plates by progressive damage modeling. Finite element modeling (FEM) results for the T300 composite with and without diamond powder predicted a specific case of fiber failure in all the plies showing the characteristics of brittle failure. Static tensile tests were carried out on the YS90A composite coupons containing no diamond powder (DP) and filled with 6% and 12% volume fractions of DP. A higher content of diamond powder in the coupons led to agglomeration. This induced stress concentrations and subsequently reduced the mechanical properties. FEM was carried out considering specimens with and without an induced stress concentration geometry in the YS90A coupons filled with DP. The results of the on-axis tensile tests indicated a delamination type of failure in both cases with additional fiber fracture in the Open Hole Tensile (OHT) coupons.

Stress concentration of transition groove induced by a press-fitted part in railway axles

The design guides of railway axles prescribe these axles to have an infinite life based on fatigue limit and to estimate the local stresses in transition fillets or grooves by using the nominal stresses derived by beam calculations. Our previous report described that the press-fitted wheel induced stress concentration on the adjacent transition. In this study, the effect of interference fitting on stress concentration of the transition was experimentally and analytically evaluated using scaled specimens. Finite element analyses of realistic wheelsets based on Japanese and European axle transition geometries were conducted to assess the interference fitting effect on full-scale axles.

Causes of the abnormal seepage field in a dam with asphaltic concrete core

Asphaltic concrete core (ACC) dams are widely built in China. Many ACC dams perform well, but others have experienced significant leakage including the case in western China studied herein. A numerical model of saturated-unsaturated water flow was adapted to simulate the seepage through the dam. By comparing the normal and abnormal seepage fields under different conditions, the main causes for the actual abnormal seepage field were identified and attributed to a defect in the ACC and an unintended, low permeability layer (LPL) in the transition zone (TZ) and the downstream dam shell. These conclusions are consistent with the situation and performance of the dam. Inadequate ACC construction processes might have caused defects in the ACC. An abrupt change of the ACC thickness probably induced stress concentrations and caused the ACC to fail. Material sources for the TZ and dam shell were complex and varied from specifications, and soil gradation for the TZ was inadequately controlled. In particular, tests show that the permeability varies over large ranges in these two parts of the dam. An unexpected LPL probably exists in both areas, and extends continuously.

Distortion induced fatigue of deck plate at rib intersection with diaphragm in orthotropic steel deck

Toe-deck (TD) fatigue due to wheel induced distortion of orthotropic steel decks (OSDs) was reported by laboratory tests. The existing OSDs could probably suffer the TD fatigue and is evaluated using the hot spot stress (HSS) method. The wheel induced stresses of the trough-deck welded joint are analyzed using the shell model. The influences of HSS with mesh size, stress extrapolation, element shape function and the hot spot location variation are checked. TD fatigue durability using the monitored traffic data and the design specifications are discussed. Results demonstrate that the weld toe of the joint suffers wheel induced stress concentration and frequent stress cycles. The refined shell model using the 4-node-shell elements results in nearly constant HSS. TD fatigue durability of the 12mm deck with 50mm asphalt pavement falls below the designed service life that might explain the observed fatigue cracks in the vicinity of the deck toe by laboratory tests and the in-site observation. The TD fatigue evaluation using the HSS method may be utilized to get a conservative OSD design.

Initiation of a secondary crack across a frictional interface

The propagating crack interacts with an inclined frictional interface. The experiments focus on how a crack reaching the interface would further form a secondary crack on the opposite side after traveling down some distance along the interface. This distance increases with the inclination angle of the interface. This effect is related to an influence of shear along the contact plane caused by the initial crack approaching the contact plane. A simplified model explains the secondary crack development as a result of slip zone propagation along the contact plane, which induces stress concentration at the end of the slip zone.

層間高靭性型CFRP積層板の力学的特性に及ぼす繊維不連続部の影響

In fabricating complicated laminated composite structures by using prepregs, various kinds of fiber discontinuities such as ply drop-off and seams can be formed. Special care must be taken for the fiber discontinuity since they can induce stress concentration, and damage onset and growth originated from them may lead to catastrophic failure of the structure. The present study evaluates mechanical behaviour in unidirectional CFRP laminates that contain centered or dispersed fiber discontinuities fabricated by using interlaminar-toughened CFRP prepregs. Tensile testing has shown that higher onset stress of the interlaminar delamination between continuous and discontinuous plies can be achieved by interlaminar-toughened CFRP with centered fiber discontinuity compared to that for conventional CFRP prepregs. As for the specimen with dispersed fiber discontinuities, fracture stress keeps almost constant with more than 20 mm of interval of the dispersed fiber discontinuities. By applying the onset stress of the delamination that is experimentally obtained using centered discontinuity to an analytical model, critical energy release rate by the delamination onset is calculated. Then it is indicated that one can predict the fracture stress of the specimen with the dispersed fiber discontinuities with more than 20 mm interval of the discontinuities by using the critical energy release rate. This indicates that fracture of the laminate is dominated by the interlaminar delamination between discontinuous plies with more than 20 mm of interval.