High Interface Stresses Research Articles

Enhancement of high-temperature oxidation resistance of CoCrNiAlY–yttria-stabilized zirconia–LaMgAl11O19 dual-ceramic coating by arc Al plating

A novel Al plating was developed to enhance the oxidation resistance of CoCrNiAlY–yttria-stabilized zirconia (YSZ)–LaMgAl11O19 dual-ceramic coating. The results indicated that the lanthanum magnesium hexaaluminate (LMA) layer without the Al plating sustained severe volume shrinkage, consequently initiating the formation and propagation of longitudinal micro-cracks. These micro-cracks behaved as channels that accelerated the inner diffusion of oxygen. The rapid growth of the thermally grown oxide (TGO) layer induced high interface stress, resulting in the severe fracturing of the interface between the CoCrNiAlY and YSZ layers. However, the Al plating not only effectively suppressed the volume shrinkage of the LMA layer and the initiation of large-size longitudinal micro-cracks but also induced the formation of an in-suit Al2O3 barrier layer. The strong synergistic effect of the dense ceramic layer and Al2O3 layer effectively inhibited the inner diffusion of oxygen, consequently reducing the interface stress and rapid TGO layer growth. Accordingly, a considerably effective anti-oxidation performance was achieved by the dual-ceramic coating with the Al plating.

Constrained Liners Implanted Simultaneously at the Time of Acetabular Shell Revision with a Highly Porous Implant: Surprisingly Good Fixation at 10 Years.

Many surgeons are reluctant to use a constrained liner at the time of acetabular component revision because of concerns that doing so might result in early acetabular component loosening related to high bone-implant interface stresses transmitted from the constrained liner. We hypothesized that, with appropriate initial implant stabilization using highly porous acetabular components with robust supplemental screw fixation, constrained liners could be safely used at the time of acetabular revision. We retrospectively identified 148 revision total hip arthroplasties (THAs) in which a constrained liner of 1 design was cemented into a newly placed highly porous acetabular component fixed with supplemental screws (mean, 5 screws). The mean age at the time of revision THA was 69 years, and 68% of the patients were female. The most common indications for revision were 2-stage reimplantation (33%), recurrent dislocation (30%), and aseptic loosening of the acetabular component (22%) or both the acetabular and femoral components (9%). The mean duration of follow-up was 7 years. There were no failures at the bone-implant interface. No cases of acetabular metal shell loosening were identified on radiographic analysis. The 10-year survival rates with acetabular revision and any reoperation as the end points were 75% and 67%, respectively. Overall, 33 hips (22%) required revision or reoperation because of infection or wound complications (n = 12), dislocation or mechanical failure of the constrained liner (n = 11), periprosthetic femoral fracture (n = 4), femoral loosening (n = 3), or other reasons (n = 3). The 10-year survival rate with re-revision for instability as the end point was 88% overall, which was similar to the 85% 10-year survival rate with re-revision for instability as the end point among hips that were specifically revised because of instability during index revision THA (p = 0.9). Implanting a constrained liner at the time of acetabular revision in high-risk patients resulted in no cases of aseptic metal acetabular component loosening in this large series. This finding is likely related to the fact that a highly porous acetabular component was utilized with robust supplemental screw fixation in each case. Such information is valuable as these data favor a paradigm shift when compared with some traditionally held tenets. Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Flexible Interface Design for Stress Regulation of a Silicon Anode toward Highly Stable Dual-Ion Batteries.

Dual-ion batteries (DIBs) have attracted increasing attention due to their high working voltage, low cost, and environmental friendliness, yet their development is hindered by their limited energy density. Pairing silicon-a most promising anode due to its highest theoretical capacity (4200 mAh g-1 )-with a graphite cathode is a feasible strategy to address the challenge. Nevertheless, the cycling stability of silicon is unsatisfactory due to the loss of electrical contact resulting from the high interface stress when using rigid current collectors. In this work, a flexible interface design to regulate the stress distribution is proposed, via the construction of a silicon anode on a soft nylon fabric modified with a conductive Cu-Ni transition layer, which endows the silicon electrode with remarkable flexibility and stability over 50 000 bends. Assembly of the flexible silicon anode with an expanded graphite cathode yields a silicon-graphite DIB (SGDIB), which possesses record-breaking rate performance (up to 150 C) and cycling stability over 2000 cycles at 10 C with a capacity retention of 97%. Moreover, the SGDIB shows a high capacity retention of ≈84% after 1500 bends and a low self-discharging voltage loss of 0.0015% per bend after 10 000 bends, suggesting high potential for high-performance flexible energy-storage applications.

Application of fast multipole boundary element method for two-dimensional nonlinear interface debonding of particulate composites

A fast multipole boundary element method (BEM) is used herein to simulate the two-dimensional interfacial debonding of particulate composites. The behavior of the interface between the inclusion and the matrix is modeled using a nonlinear constitutive relationship. Interface debonding is implemented by decoupling the node pair. In the proposed method, the degree of freedom (DOF) of the interfacial traction changes to that of displacement of the free surface; thus, the number of DOFs of each node pair prior to and after decoupling is the same, which facilitates the simulation better than the finite element method. In simulating the softening stage of the nonlinear interfacial deformation, the stress in the node pair is assumed to be unloaded to zero and then reloaded to a new equilibrium state. A fast multipole expansion technique is applied within the developed BEM to solve the large-scale problem of debonding particulate composites whereby millions of DOFs can be calculated via a step-wise calculation method that provides high precision interface stress results. A comparison of these results with the analytical solution for a single inclusion case demonstrates the high stress accuracy and effectiveness of the developed fast multipole BEM to solve large-scale nonlinear interface problems.

Morphometric and Mechanical Analyses of Calcifications and Fibrous Plaque Tissue in Carotid Arteries for Plaque Rupture Risk Assessment.

Atherosclerotic plaque rupture in carotid arteries is a major source of cerebrovascular events. Calcifications are highly prevalent in carotid plaques, but their role in plaque rupture remains poorly understood. This work studied the morphometric features of calcifications in carotid plaques and their effect on the stress distribution in the fibrous plaque tissue at the calcification interface, as a potential source of plaque rupture and clinical events. A comprehensive morphometric analysis of 65 histology cross-sections from 16 carotid plaques was performed to identify the morphology (size and shape) and location of plaque calcifications, and the fibrous tissue fiber organization around them. Calcification-specific finite element models were constructed to examine the fibrous plaque tissue stresses at the calcification interface. Statistical correlation analysis was performed to elucidate the impact of calcification morphology and fibrous tissue organization on interface stresses. Hundred-seventy-one calcifications were identified on the histology cross-sections, which showed great variation in morphology. Four distinct patterns of fiber organization in the plaque tissue were observed around the calcification. They were termed as attached, pushed-aside, encircling and random patterns. The stress analyses showed that calcifications are correlated with high interface stresses, which might be comparable to or even above the plaque strength. The stress levels depended on the calcification morphology and fiber organization. Thicker calcification with a circumferential slender shape, located close to the lumen were correlated most prominently to high interface stresses. Depending on its morphology and the fiber organization around it, a calcification in an atherosclerotic plaque can act as a stress riser and cause high interface stresses. This study demonstrated the potential of calcifications in atherosclerotic plaques to cause elevated stresses in plaque tissue and provided a biomechanical explanation for the histopathological findings of calcification-associated plaque rupture.

Relationship between electrical properties and interface structures of SiO2/4H-SiC prepared by dry and wet oxidation

We have investigated the relationship between the electrical properties and interfacial atomic structure of SiO2/4H-SiC interfaces, prepared by dry and wet thermal oxidation procedures with 4H-SiC (0001) and 4H-SiC (000-1) substrates, using extended x-ray absorption fine structure (EXAFS) spectroscopy and electrical methods. From the current–voltage (I–V) and capacitance–voltage (C–V) measurements, the gate leakage current onset and density of interface states were shown to depend on the thermal oxidation procedure used for creating SiO2/4H-SiC (000-1) interfaces. This dependence was not observed for SiO2/4H-SiC (0001) interfaces. From EXAFS oscillations, we found that C and Si vacancies formed at the SiC side of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces, respectively. Compressive stress at the SiC sides of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces prepared with the dry thermal oxidation procedure caused decreases in bond lengths. Stress at the interface was smaller in the case of wet oxidation. Wet oxidation released the interface stress more effectively for 4H-SiC (000-1) substrates than for 4H-SiC (0001) substrates. A high gate leakage current onset related to a high interface charge, while a high density of interface states related to a high interface stress and a high interface roughness.

FE modelling stress response for evaluation and design of PCC overlay on bridges

Portland cement concrete overlay on concrete bridges is a special pavement structure subjected to distresses of cracking and interface de-bonding. Most research has focused on materials to improve overlay performance, while there are no systematic mechanistic analysis and structural design methods available. This paper studies the mechanical behaviours and structural designs of four typical bridge structures under traffic loading using finite-element modelling. Results indicate that stress response is highly dependent on bridge structure. The maximum tensile stress occurs in the top overlay at the positions of bridge rib, joint and bearing. Generally, increasing overlay thickness reduces critical stresses, but a 4–5-cm overlay thickness induces the maximum interface shear stress. It was found that a rubber interlay design could effectively relieve flexural stress at the negative moment regions. Cracked overlay poses much higher interface stresses than continuous overlay, but steel reinforcements can effectively reduce critical stresses.

Interface Failure Mechanics of Elastically (Advanced Composite) Reinforced Steel Members

Tests on steel beams with adhesively bonded elastic reinforcing strips are used to deduce the mechanics of stress build up in the steel-to-strip interface up to failure. The solely elastic nature of the strips is a key concept in this study, as this elasticity leads to high interface stresses due either to nonlinear stiffness disparities between the steel and the reinforcing material when the steel yields, or to nonlinear geometric effects which are parasitic on this elasticity, including buckling of the strips and sharp section changes along the reinforced beam. Four features have been used in the tests to study these nonlinear effects, namely, multi-layer multi-length strips, imperfections in the adhesive, strips in both compression and tension, and tapering of the strip. The test results suggest that interface stresses may be sensitive to through-thickness variations of axial strain in the strips and to local variations of interface quality. Data from a nonlinear finite-element analysis, based on a novel two-layer interface element, are given for further insight into interface stress profiles. Conclusions are drawn on interface performance in the tests. Improved measurement strategies are suggested for shedding further light on the stress-transfer mechanics of this interface. The term connection is used synonymously with interface in this paper.

Histological and biomechanical analysis of bone and interface reactions around hydroxyapatite-coated intramedullary implants of different stiffness: a pilot study on the goat

We hypothesized that reduced stem stiffness of orthopaedic implants contributes to a high risk of loosening, since interface stresses and relative motions may exceed a tolerable range. To study this hypothesis, three types of load-bearing implant with different stiffnesses were inserted into the tibia of the goat. Histological analysis was performed of bone repair after insertion of the implant, bone ingrowth, interface disruption and loosening. A finite element model of the configuration provided the quantitative range of interface stresses and relative motions for the present experiment. The implants were made out of stainless steel, hollow titanium and a thin titanium core covered with a polyacetal coating. The stiffness ratios of these implants were approximately 10:4:1, respectively. All implants were coated with a layer of hydroxyapatite (HA) in order to minimize the possible biological effects of the different implant materials. Irrespective of the type of implant, there was a repair phase that lasted 6–12 weeks. The stiff implants functioned well. Large areas of bone bonding to the HA layer were found after the repair phase at 12 weeks postoperatively. After 24 weeks, some signs of loosening were observed. More loosening occurred with the hollow titanium and polyacetal implants, mainly during the repair phase. Three hollow titanium and three polyacetal coated implants survived this period, and were killed after 24 weeks. The integrity of the HA layer at the bone-implant interface of the titanium implants was good. In the polyacetal implants, the repair reaction of the cortical bone was incomplete. Bone ingrowth into HA was largely lacking. In conclusion, we found significant differences in the repair and interface reactions around implants of different stiffness. Stiff implants showed favourable initial interface conditions for bone ingrowth. Intermediate and flexible implants provoked unfavourable interface conditions for initial bone ingrowth. The finite element study showed that the flexible stems produce larger micromotions and higher interface stresses at the bone-prosthesis interface than the stiff stems, indicating an explanation for the histological findings.

Characteristics of polycrystalline silicon thin-film transistors with thin oxide/nitride gate structures

Polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with thin oxide/nitride (O/N) structures as gate dielectrics are fabricated. Various gate dielectrics, i.e., high-temperature thermal oxides with different thicknesses, low-pressu re chemical-vapor-deposited silicon nitrides, and different combinations of O/N structures with various thicknesses, are performed to study their effects on poly-Si TFTs. The effective carrier mobility of devices with thin gate oxides is several times larger than of those with thick gate oxides. However, the breakdown voltages of thin gate oxides are too low to satisfy the requirements of TFT applications. Silicon nitrides can be substituted because of the high breakdown voltage and the smooth dielectric/poly-Si interfaces. A problem in adopting silicon nitride is the large interface stress between the silicon nitride and the poly-Si. A thin thermal pad oxide beneath the silicon nitride is therefore grown to reduce the high interface stress. Finally, the equivalent oxide thickness effect of the O/N gate structures on the electrical characteristics of TFTs is systematically investigated.

Three-dimensional effects in thin film fracture mechanics

This paper presents the results of a three-dimensional finite element analysis of the mechanics of crack growth and decohesion in a highly compliant thin film bonded to a rigid substrate. Essential features of the model are a surface-breaking initial crack which has penetrated to the film-substrate interface, and a remote biaxial residual stress field in the film. Computational analyses show that, in the absence of decohesion, the stress intensity factor along the leading edge of the crack reaches a steady state value when the crack is about twice the film thickness. This steady state condition gives rise to a nearly parabolic crack front along the leading edge. The computed results also confirm the existence of high interface stresses that are the driving force for film decohesion. When decohesion is permitted, the stress intensity factor is observed to increase with debond opening angle. The equilibrium state between crack propagation and decohesion may be determined from the respective stress intensity factors and fracture toughness of the film and the interface.

Prediction of Muscle and Joint Loads After Segmental Femur Replacement for Osteosarcoma

The authors designed a mathematical model that simulated 28 proximal and distal femoral resections and reconstructions for tumor and then predicted muscle and joint forces during gait and stair climbing. Such models will become increasingly useful as improvements are available, as tumor patient longevity increases, and as the design and manufacture of custom devices become more common. Inertial property changes with surgery were minor and can be ignored for most purposes. For most simulations, physiologically reasonable muscle forces were predicted, suggesting that patients would be able to walk in a normal or near-normal fashion. Large joint and muscle loads were predicted after extensive muscle resections, particularly during stair climbing, suggesting that patients would limp. The quadriceps muscle group was sufficiently strong to allow normal gait with partial excisions. With complete quadriceps excision and transfers, however, high joint loads were predicted. Furthermore, if an endoprosthesis had inherent varus-valgus stability in such cases, the moments to be satisfied by the prosthesis would predispose to high interface stresses and loosening. If, conversely, the endoprosthesis did not have inherent stability, muscle forces in the remaining muscles would be unrealistically high, suggesting that such a patient would limp or require ambulation assists.

A new approach to the fixation of a metacarpophalangeal joint prosthesis.

A significant problem with rigid prosthetic stems applied in the finger bones, as well as in other bones of the upper and lower extremity, is resorption of bone at the interface. An investigation was carried out using a plastic plug which would more evenly distribute the stresses to the bone, with fine ridges to produce enhanced fixation by bony ingrowth. A total knee prosthesis in the cat was used as the model, radiographic and histological studies being made at up to one year. A finite element analysis identified areas of high interface and material stresses. With a finely grooved plug, bone ingrowth occurred in all regions except for the region near the bone entry, where the finite element analysis showed the highest interface shear stresses and bone material stresses to occur.