Fracture Of Struts Research Articles

Influence of pore structures on deformation behavior and mechanical properties of porous tantalum scaffolds fabricated by electron beam powder bed fusion

Porous tantalum scaffolds with the dodecahedron, G7, and cubic structures were fabricated by electron beam powder bed fusion (EB-PBF). The influence of pore structures on the deformation behavior and mechanical properties of porous tantalum scaffolds was investigated by compressive testing, tensile testing, three-point bending testing, and finite element analysis (FEA). In the compressive test, the deformation of G7 and cubic scaffolds was dominated by buckling. The dodecahedron scaffold was deformed mainly by bending, leading to its lower compressive strength. On the other hand, the dodecahedron scaffold had superior tensile and bending properties. The uniform maximum stress distributions of the dodecahedron scaffold in the tensile and bending tests contributed to the homogeneous structural deformation, which delayed the fracture of struts; whereas, the maximum stresses of G7 and cubic scaffolds were concentrated in the transverse struts, resulting in the fracture of transverse struts in the early deformation stage. Therefore, pore structures should be properly selected for EB-PBF manufactured porous tantalum scaffolds based on the requirements for mechanical properties.

Compression and energy absorption characteristics of short fiber‐reinforced 2D composite lattices made by material extrusion

AbstractWith the advent of additive manufacturing, lattice structures have been of increasing interest for engineering applications involving light‐weighting and energy absorption. Several studies have investigated mechanical properties of various lattices made up of mostly unreinforced polymers and lack numerical analysis for reinforced lattice structures. In this paper, mechanical response of short fiber reinforced lattice structures under compression is investigated through experiments and numerical simulations. Three different 2D lattices namely, square grid, honeycomb, and isogrid along with their rotated counterparts were fabricated using 15% wt carbon fiber‐acrylonitrile butadiene styrene and experimentally evaluated through uniaxial compression testing up to 30% strain. As simulations on fiber‐reinforced lattices under large compressive strains are rarely performed and published, finite element models accounting for fiber orientation induced anisotropic mechanical properties and geometrical imperfections were developed to predict the stress–strain characteristics up to 30% compressive strains. The stress–strain curves predicted from the numerical simulations matches well with the experimental responses for various lattice geometries. Various failure mechanisms such as thin strut buckling, contact within the lattice, and fracture of struts under large deformation were investigated. Analyzing energy absorption characteristics of these lattices revealed that the honeycomb structures in horizontal configuration exhibits superior energy absorption capability.

Experimental Validation and Evaluation of the Bending Properties of Additively Manufactured Metallic Cellular Scaffold Structures for Bone Tissue Engineering.

The availability of additive manufacturing enables the fabrication of cellular bone tissue engineering scaffolds with a wide range of structural and architectural possibilities. The purpose of bone tissue engineering scaffolds is to repair critical size bone defects due to extreme traumas, tumors, or infections. This research study presented the experimental validation and evaluation of the bending properties of optimized bone scaffolds with an elastic modulus that is equivalent to the young’s modulus of the cortical bone. The specimens were manufactured using laser powder bed fusion technology. The morphological properties of the manufactured specimens were evaluated using both dry weighing and Archimedes techniques, and minor variations in the relative densities were observed in comparison with the computer-aided design files. The bending modulus of the cubic and diagonal scaffolds were experimentally investigated using a three-point bending test, and the results were found to agree with the numerical findings. A higher bending modulus was observed in the diagonal scaffold design. The diagonal scaffold was substantially tougher, with considerably higher energy absorption before fracture. The shear modulus of the diagonal scaffold was observed to be significantly higher than the cubic scaffold. Due to bending, the pores at the top side of the diagonal scaffold were heavily compressed compared to the cubic scaffold due to the extensive plastic deformation occurring in diagonal scaffolds and the rapid fracture of struts in the tension side of the cubic scaffold. The failure in struts in tension showed signs of ductility as necking was observed in fractured struts. Moreover, the fractured surface was observed to be rough and dull as opposed to being smooth and bright like in brittle fractures. Dimple fracture was observed using scanning electron microscopy as a result of microvoids emerging in places of high localized plastic deformation. Finally, a comparison of the mechanical properties of the studied BTE scaffolds with the cortical bone properties under longitudinal and transverse loading was investigated. In conclusion, we showed the capabilities of finite element analysis and additive manufacturing in designing and manufacturing promising scaffold designs that can replace bone segments in the human body.

Compression Performance and Deformation Behavior of 3D-Printed PLA-Based Lattice Structures.

The aim of this study is to fabricate biodegradable PLA-based composite filaments for 3D printing to manufacture bear-loading lattice structures. First, CaCO3 and TCP as inorganic fillers were incorporated into a PLA matrix to fabricate a series of composite filaments. The material compositions, mechanical properties, and rheology behavior of the PLA/CaCO3 and PLA/TCP filaments were evaluated. Then, two lattice structures, cubic and Triply Periodic Minimal Surfaces-Diamond (TPMS-D), were geometrically designed and 3D-printed into fine samples. The axial compression results indicated that the addition of CaCO3 and TCP effectively enhances the compressive modulus and strength of lattice structures. In particular, the TPMS-D structure showed superior load-carrying capacity and specific energy absorption compared to those of its cubic counterparts. Furthermore, the deformation behavior of these two lattice structures was examined by image recording during compression and computed tomography (CT) scanning of samples after compression. It was observed that pore structure could be well held in TPMS-D, while that in cubic structure was destroyed due to the fracture of vertical struts. Therefore, this paper highlights promising 3D-printed biodegradable lattice structures with excellent energy-absorption capacity and high structural stability.

The incidence and risk factors of stent fracture in patients treated for proximal common carotid artery stenosis

ObjectiveOur aim was to identify the incidence of and predictors for common carotid artery (CCA) stent fractures (SFs) and to examine the effect of SFs on the development of in-stent restenosis (ISR). MethodsSeventy patients (37 women; median age, 60.9 years) who were stented for significant (≥60%) proximal CCA stenosis from 2006 to 2016 and revisited us to determine SF using fluoroscopy in 2018 were evaluated. Seventy stents were deployed; among them 87.1% were balloon-expandable and 12.9% were self-expandable. SFs were classified as type I (fracture of one strut), type II (fracture of multiple struts without stent deformity), type III (fracture of multiple struts with stent deformity), type IV (complete fracture of the stent without a gap), and type V (complete fracture of the stent with a gap). Duplex ultrasound examination was used for monitoring stent patency. Mann-Whitney U and Fisher's exact tests, Kaplan-Meier and logistic regression analyses, and a log-rank test and a gamma correlation analysis were applied as statistical methods. ResultsThe patients were followed for 75.5 months (range, 47-109 months). Significant (≥70%) ISR was observed in eight patients (11.4%). Reintervention was performed in four cases (5.7%). Twenty-seven SFs (38.6%; type I, 8; type II, 10; type III, 4; type IV, 2; and type V, 3) were found. Calcification was shown to be a significant predictor for SF (odds ratio, 13.2; 95% confidence interval, 3.9-45.1; P < .001). There was no significant difference between the fractured and the nonfractured group regarding the number of patients with ISR and reintervention (P = .701 and P = .636, respectively). Neither did the primary patency rates differ significantly (P = .372) in patients with and without SF. ConclusionsFractures frequently occur in a wide variety of stent devices deployed in the proximal CCA, but SFs seem to have no effect on ISR and reintervention.

Mechanical Behavior of Natural Fiber-Based Bi-Directional Corrugated Lattice Sandwich Structure.

In this study, 11 kinds of composite material were prepared, and the compression behavior of a bi-directional corrugated lattice sandwich structure prepared using jute fiber and epoxy resin was explored. The factors affecting the mechanical behavior of single and double-layer structures were studied separately. The results shows that the fiber angle, length-to-diameter ratio of the struts, and the type of fiber cloth have the most significant influence on the mechanical behavior of the single-layer lattice structure when preparing the core layer. When the fiber angle of the core layer jute/epoxy prepreg is (90/90) the compressive strength and Young’s modulus are 83.3% and 60.0% higher than the fiber angle of (45/45). The configuration of the core and the presence of the intermediate support plate of the double-layer structure have a large influence on the compression performance of the two-layer structure. After the configuration was optimized, the compressive strength and Young’s modulus were increased by 40.0% and 28.9%, respectively. The presence of the intermediate support plate increases the compressive strength, and Young’s modulus of the double-layer structure by 75.0% and 26.6%, respectively. The experimental failure is dominated by the buckling, fracture, and delamination of the core struts.

Structural Loading of Cellular Metals: Damage Mechanisms and Standardization Concepts

Structural loading of cellular metals is strongly affected by brittle fracture of cell struts and walls that exhibit tensile loads, e.g., during fatigue loading. The present paper summarizes results of compression, tension and cyclic loading experiments on various closed-cell metal foams and metal foam sandwiches (Alulight, Alporas, Foamtech, AFS) using various mechanical testing systems. The results were correlated with a thorough analysis of the cellular mesostructure and the cell strut/wall microstructure by means of scanning electron microscopy revealing defects, such as casting porosity and large Si precipitates in the Al-Si eutectic of aluminum cast alloy. The results of the work served for the definition of testing standards for compression testing (ISO 13314) and tensile testing (DIN 50099), which are outlined in the paper. Such standards and design guidelines are crucial for a successful implementation of cellular metals in innovative products in mechanical, automotive and energy engineering as well as in bioengineering.

The safety and effectiveness of celect inferior vena cava filter evaluated by CT

Objective To investigate the safety and effectiveness of the Celect inferior vena cava filter (IVCF) in the treatment of deep venous thrombosis (DVT). Methods CT follow-up examinations were performed in 72 patients who underwent Celect inferior vena cava filter placement between August 2015 and September 2017. Forty-five patients (62.5%) presented with DVT alone, 27(37.5%) with DVT and pulmonary embolism (PE). With filters in place, each patient was followed up with CT at least once, to visualize new pulmonary embolism, inferior vena cava occlusion, filter tilting, fracture of struts, migration and penetration of struts through the inferior vena cava. Caval penetration was graded on a five-point scale. Results All Celect filters were successfully deployed. The interval between the filter placement and the CT was 5.0 to 695.0 days, mean (42.4±84.6)days. No new pulmonary embolism, cava occlusion, filter fracture and migration were found. Forty-two (58.3%) filters tilted, including tilt>15 degrees in 8 (11.1%) and tilt ≤15 degrees in 34 (47.2%). Caval penetration with 0 grade was observed in 47 (65.3%) and beyond grade 1 was observed in 25 (34.7%), including grade Ⅰ in 12 (16.7%), grade Ⅱ in eight (11.1%), grade Ⅲ in four (5.6%) and grade Ⅳ in one (1.4%). No patient presented with symptoms associated with caval penetration. Filter retrieval was attempted in 54(75.0%) patients,the indwell time was 5.0 to 111.0 days, mean(32.0±23.9) days. There were 52 (96.3%) filters successfully removed, including four filters tilted lead retrieval hook embedded the caval wall, which used the adjunctive retrieval techniques. Filter retrieval failure in two (3.7%) cases, including one case retrieval hook embedded the caval wall and the other with retrieval hook penetration. Conclusions Celect inferior vena cava filter can effectively prevent PE, but had a tendency to tilt and to penetrate caval wall, which affected the filter retrieval, and had the potential risk of serious complications such as abdominal aortic pseudoaneurysm. The filter should be removed as soon as the risk of PE is manageable. We advocate CT follow-up for patients with long-term inferior vena cava filter placement. Key words: Vena cava filters; Venous thrombosis; Lower extremity; Tomography, X-ray computed

Deformation behavior and energy absorption capability of polymer and ceramic-polymer composite microlattices under cyclic loading

Abstract

Long-fiber reinforced thermoplastic composite lattice structures: Fabrication and compressive properties

Recyclable lightweight materials with advanced processing techniques are essential for the sustainable development of future transportation. Thermoplastic composites lattice structures were developed to meet this demand. An additive manufacturing method is presented here to fabricate such lattice structures by reversible assembly of several long-fiber reinforced thermoplastic composite parts (LFTCPs) which were economically processed by injection molding. The resulting thermoplastic lattice structures (density of 30kg·m−3) assembled with different sequences and connections are structurally evaluated and compared. Out-of-plane compression tests revealed that their mechanical properties were more sensitive to the presence of the connections rather than their assembly sequence, although their structural failure mode was always brake of inclined struts followed by fracture of the horizontal struts. Potential solutions to the problem of internal stresses, induced during assembly, are also explored by designing novel LFTCPs. The novel fabrication route for thermoplastic lattice structures will improve the prospects for their industrial application.

Modeling the mode I fracture toughness of anisotropic low-density rigid PUR and PIR foams

Low-density foams have to possess a sufficient resistance to cracking in order to ensure the mechanical integrity of foam materials in service, even when not intended for load-bearing applications. In this study, mode I fracture toughness in the foam rise direction has been experimentally characterized for anisotropic rigid commercial polyurethane foams as well as for polyisocyanurate foams produced using polyols derived from rapeseed oil and filled with a montmorillonite nanoclay. Rectangular parallelepiped unit-cell based scaling relations expressing foam toughness via its relative density, cell dimensions, geometrical anisotropy, and the ultimate tensile stress of the base polymer have been employed for prediction of foam toughness. Assuming a brittle fracture of foam struts, a conservative estimate of toughness is obtained. It is demonstrated that considering the yielding of foam struts at the crack front as the criterion of crack extension provides a closer estimate of foam toughness.

Two-scale study of the fracture of an aluminum foam by X-ray tomography and finite element modeling

An aluminum foam can be characterized by its architecture and by the solid phase’ microstructure. Our aim is to link the foam's morphological and microstructural features with its mechanical properties thanks to X-ray tomography and finite element (FE). An approach combining X-ray tomography at different resolutions, image processing, and FE modeling was developed to take into account the influence of the intermetallics on the foam's fracture. First, the samples were scanned with “local” tomography, where the specimen is placed close to the X-ray source. These images allowed for observing intermetallics. Then an in situ tensile test was performed in the tomograph to follow the sample's deformation at low resolution. The images obtained from local tomography were processed to create one low-resolution image of the initial sample including details from high resolution. This was done by a series of thresholding and scaling of the high-resolution images. This image was used to generate a FE mesh. A FE input file was obtained thanks to Java programs associating the elements to the phases. At the local scale, the calculated stress distribution and the images of the struts were analysed. Our work confirms that the presence of inclusions can explain the fracture of struts.

Compressive deformation behaviour of coral Porites Cylindrica

Porites Cylindrica was used to understand their compressive deformation behaviour along the growth direction. Powder X-Ray diffraction (XRD), scanning electron microscope (SEM) and uniaxial compression testing were employed to study microstructure, phase analysis and mechanical properties, respectively. An average strength of 8.85±1MPa and modulus of 12.22±2.4GPa were measured. Failure analysis showed the influence of nanoscale microstructural features on fracture surface with steps and striations on flat fracture of struts.

Deformation and failure behaviour of open cell Al foams under quasistatic and impact loading

Metal foams show efficient and almost isotropic energy absorption during compression making them interesting materials for the design and development of impact resistant components. The (mechanical) properties of metal foams are determined (i) by the properties of the alloy they are made of and (ii) by their complex meso- and microstructures. While the dynamic mechanical properties of Al foams have been investigated by several groups, the deformation mechanisms at higher strain rates has been considered to a much lesser extent.To gain a deeper understanding of the relationship between the specific properties of the hierarchical structural elements (e.g. cell, strut, microstructure of the struts) and the macroscopic properties we combine mechanical testing with different imaging methods. Compression tests on open cell Al (A356) foams, which have been produced by investment casting and which have very low relative densities (1.7–2.2%), were performed under quasistatic (dε/dt=5×10−4s−1) and dynamic (10s−1) loading conditions. Video recordings allow us to evaluate the failure mechanisms in the foams on a macroscopic scale, while in-situ testing in the µCT or SEM provides important information on the deformation behaviour as well as the crack formation and development on the microscale. Metallographic sections of specimens deformed to defined strain values are furthermore investigated to gain insight into the role of microstructural features with higher resolution.The deformation behaviour and the mechanical properties of the quasistatically and the dynamically tested specimens are very similar. Higher strain rates result in a more homogeneously distributed deformation, however with a negligible influence on the plateau stress, densification strain and energy absorption characteristics. The highly efficient energy absorption during the plateau phase is due to microstructural mechanisms such as plastic bending of struts (mostly C- and S-curved) and different types of crack formation and growth in the struts. Local differences of the strut microstructure cause locally different mechanical behaviour of struts: while high plastic deformation leads to a more homogeneous macroscopic deformation, we assume brittle fracture of single struts promotes the formation of deformation bands.

Fracture and microstructure of open cell aluminum foam

SEM and EDS measurements were used to scrutinize the microstructure of Duocel open cell 6101 aluminum foam in relation to its fracture properties. In-situ SEM tensile tests on the open cell aluminum foam were performed to investigate the different fracture modes of struts and Aramis/Digital Image Correlation software was used to map the strain in individual struts. Observations during tensile tests showed that the microstructure of the struts has a great influence on the fracture behaviour of the foam. In particular AlFeSi-precipitates, which are due to the casting of the 6101 aluminum alloy, and the morphology of the foam alters the fracture mode of the struts in the foam from transgranular to intergranular. Less energy is needed for intergranular fracture of struts and the strain to failure of the foam is decreased due to weak individual struts.

Endoscopic Removal of Metallic Airway Stents

Complications of metallic airway stents include granulation tissue formation, fracture of struts, migration, and mucous plugging. When these complications result in airway injury or obstruction, it may become necessary to remove the stent. There have been few reports detailing techniques and complications associated with endoscopic removal of metallic airway stents. We report our experience with endoscopic removal of 30 such stents over a 3-year period. We conducted a retrospective review of 25 patients who underwent endoscopic stent removal from March 2001 to April 2004. The patients ranged in age from 17 to 80 years (mean, 56.3 years). There were 10 male and 15 female patients. The stents had been placed for nonmalignant disease in 20 patients (80%) and malignant disease in 5 patients (20%). All procedures were done under general anesthesia with a rigid bronchoscope. Special attention was focused on the technique of stent removal and postoperative complications. Thirty metallic airway stents were successfully removed from 25 consecutive patients over a 3-year period. The basic method of removal involved the steady application of traction to the stent with alligator forceps. In all cases, an instrument such as the barrel of the rigid bronchoscope or a Jackson dilator was employed to help separate the stent from the airway wall before removal was attempted. In some instances, the airway wall was pretreated with thermal energy prior to stent removal. Complications were as follows: retained stent pieces (n = 7), mucosal tear with bleeding (n = 4), re-obstruction requiring temporary silicone stent placement (n = 14), need for postoperative mechanical ventilation (n = 6), and tension pneumothorax (n = 1). Although metallic stents may be safely removed endoscopically, complications are common and must be anticipated. Other investigators have described airway obstruction and death as a result of attempted stent removal. Placement and removal of metallic airway stents should only be performed at centers that are prepared to deal with the potentially life-threatening complications.

STRUT FRACTURE IN BJORK-SHILEY PROSTHETIC CARDIAC VALVES.

A rare but well-recognized problem with the Bjork- Shiley tilting disc valve is the fracture of the struts supporting the disc. This retrospective