Compression Collapse Research Articles

Compressive deformation of Rohacell foams: Effects of strain rate and temperature

Uniaxial compression experiments have been performed on four different densities of Rohacell foam. The experiments explored the sensitivity of the response to the imposed strain rate (in the range 10−3 to 5 × 103 s−1) and temperature (203–473 K). The compressive collapse stress is generally found to increase with increasing strain rate and decreasing temperature; however this tendency is inverted at very low temperatures or very high strain rates. This behaviour is mainly due to embrittlement of the parent polymer but is also related to the details of the foams' microstructures. Time–temperature superposition is employed to map the temperature sensitivity of the foams to their strain rate dependence. A simple design formula is provided to predict the foam stiffness as a function of temperature and relative density.

Topology optimization of hyperelastic bodies including non-zero prescribed displacements

Stiffness topology optimization is usually based on a state problem of linear elasticity, and there seems to be little discussion on what is the limit for such a small rotation-displacement assumption. We show that even for gross rotations that are in all practical aspects small (<3 deg), topology optimization based on a large deformation theory might generate different design concepts compared to what is obtained when small displacement linear elasticity is used. Furthermore, in large rotations, the choice of stiffness objective (potential energy or compliance), can be crucial for the optimal design concept. The paper considers topology optimization of hyperelastic bodies subjected simultaneously to external forces and prescribed non-zero displacements. In that respect it generalizes a recent contribution of ours to large deformations, but we note that the objectives of potential energy and compliance are no longer equivalent in the non-linear case. We use seven different hyperelastic strain energy functions and find that the numerical performance of the Kirchhoff---St.Venant model is in general significantly worse than the performance of the other six models, which are all modifications of this classical law that are equivalent in the limit of infinitesimal strains, but do not contain the well-known collapse in compression. Numerical results are presented for two different problem settings.

Thermal, mechanical, and moisture absorption properties of wood-TiO2 composites prepared by a sol-gel process

Wood-TiO2 (titania) composites were prepared by a sol-gel process, in which wood was impregnated with the precursor solutions prepared from tetrabutyl titanate (TBT), followed by a curing step. The surface morphology and moisture absorption behavior of the wood composites, as well as their thermal and mechanical performances, were examined. Environmental scanning electron microscopy (ESEM) analysis revealed that TiO2 gels were deposited principally in the cell lumens and partly in the cell walls, as confirmed by the energy dispersive X-ray (EDX) analysis. By this inorganic modification, the hygroscopicity of wood was significantly reduced and its dimensional stability was improved consequently. Greater amounts of TiO2 gel deposited in the cell lumens were not helpful in enhancing the hygroscopicity of wood. Thermal analysis (TG-DTA) showed that the incorporation of TiO2 gel retarded the thermal decomposition of wood matrix and improved the thermal stability of wood. The incorporated inorganic gel seemed to stiffen the wood cell walls, as indicated by the increased resistance of the wood composites to deformation and collapse in compression.

THERMAL, MECHANICAL, AND MOISTURE ABSORPTION PROPERTIES OF WOOD-TiO2 COMPOSITES PREPARED BY A SOL-GEL PROCESS

Wood-TiO2 (titania) composites were prepared by a sol-gel process, in which wood was impregnated with the precursor solutions prepared from tetrabutyl titanate (TBT), followed by a curing step. The surface morphology and moisture absorption behavior of the wood composites, as well as their thermal and mechanical performances, were examined. Environmental scanning electron microscopy (ESEM) analysis revealed that TiO2 gels were deposited principally in the cell lumens and partly in the cell walls, as confirmed by the energy dispersive X-ray (EDX) analysis. By this inorganic modification, the hygroscopicity of wood was significantly reduced and its dimensional stability was improved consequently. Greater amounts of TiO2 gel deposited in the cell lumens were not helpful in enhancing the hygroscopicity of wood. Thermal analysis (TG-DTA) showed that the incorporation of TiO2 gel retarded the thermal decomposition of wood matrix and improved the thermal stability of wood. The incorporated inorganic gel seemed to stiffen the wood cell walls, as indicated by the increased resistance of the wood composites to deformation and collapse in compression.

The use of Diagnostic Imaging for Identifying Abnormal Gas Accumulations in Cetaceans and Pinnipeds

Recent dogma suggested that marine mammals are not at risk of decompression sickness due to a number of evolutionary adaptations. Several proposed adaptations exist. Lung compression and alveolar collapse that terminate gas-exchange before a depth is reached where supersaturation is significant and bradycardia with peripheral vasoconstriction affecting the distribution, and dynamics of blood and tissue nitrogen levels. Published accounts of gas and fat emboli and dysbaric osteonecrosis in marine mammals and theoretical modeling have challenged this view-point, suggesting that decompression-like symptoms may occur under certain circumstances, contrary to common belief. Diagnostic imaging modalities are invaluable tools for the non-invasive examination of animals for evidence of gas and have been used to demonstrate the presence of incidental decompression-related renal gas accumulations in some stranded cetaceans. Diagnostic imaging has also contributed to the recognition of clinically significant gas accumulations in live and dead cetaceans and pinnipeds. Understanding the appropriate application and limitations of the available imaging modalities is important for accurate interpretation of results. The presence of gas may be asymptomatic and must be interpreted cautiously alongside all other available data including clinical examination, clinical laboratory testing, gas analysis, necropsy examination, and histology results.

Compressive strength assessment of a moderately corroded box girder

This paper presents the results of the experimental test of a multispan corroded stiffened box girder subjected to moderate corrosion deterioration. The experiment is carried out to study the compressive collapse strength of a box girder subjected to four-point loading, which results in a constant vertical bending moment. The box girder reproduces the behaviour of a midship hull structure. The collapse modes are discussed, showing the effect of corrosion on the final collapse mode and the release of the residual stresses. The load-displacement, moment-curvature as well as the strain readings are presented and analysed. The ultimate bending moment results have been compared with some existing formulas, showing the lack of the prediction of the behaviour of the corroded structures.

Structure and mechanical properties of gelatin/sepiolite nanocomposite foams

Biocompatible gelatin foams were found to exhibit significantly improved mechanical properties by the additions of sepiolite nanofibres. Gelatin nanocomposite foams containing various contents of sepiolite were prepared by lyophilization. With the presence of 9.1 wt% sepiolite, the Young's modulus and compressive collapse strength of the gelatin foam with a cell size of 159 μm and a porosity of 98% were increased by 288% and 308%, respectively. Such enhancements were attributable to the strong interactions between hydrophilic gelatin and sepiolite as suggested by the results from Fourier transform infrared spectroscopy and differential scanning calorimetry which show substantial shifts in the characteristic absorption peaks and the glass transition temperature, to the reduced cell sizes characterised by scanning electron microscopy, and to the reinforcement effects of sepiolite on the gelatin solid material. The mechanical properties of the low-density closed-cell gelatin/sepiolite nanocomposite foams were well predicted using the Gibson–Ashby model for open cells after normalisation of the Young's modulus of the neat polymer foam.

Collapse mechanism maps for a hollow pyramidal lattice

Cellular materials with hollow lattice truss topologies exhibit higher compressive strengths than equivalent structures with solid trusses owing to their greater resistance to plastic buckling. Consequently, hollow trusses have attracted interest as the cores for sandwich panels. Finite-element calculations are used to investigate the elastic–plastic compressive collapse of a metallic sandwich core made from vertical or inclined circular tubes, made from annealed AISI 304 stainless steel. First, the dependence of the axial compressive collapse mode upon tube geometry is determined for vertical tubes with built-in ends and is displayed in the form of a collapse mechanism map. Second, the approach is extended to inclined circular hollow tubes arranged as a pyramidal lattice core; the collapse modes are identified and the peak compressive strength is determined as a function of geometry. For a given relative density of hollow pyramidal core, the inclined tube geometry that maximizes peak strength is identified. The predicted collapse modes and loads for the pyramidal core are in excellent agreement with measurements for the limited set of experimentally investigated geometries.

Finite element modelling of compressive mechanical behaviour of high and low density polymeric foams

AbstractIn this work, different methodologies were studied by means of finite element modelling (FEM) with the aim of predicting the mechanical behaviour of high and low relative density (ρr) polymeric foams. Virtual structures which resemble the real ones were created by various computer‐based tools. These tools were employed to make up both “unit cells” and random structural units so‐called “Representative Volume Elements” (RVE). Low ρr foams are usually modelled as regular (tetrakaidecahedron) and irregular (Voronoi tessellations) 3D structures made of structural elements (beams and shells). These types of finite elements are only applicable if the ratio between longitudinal and axial dimensions exceeds a certain value and therefore, there is a practical relative density limit above which these elements are not suitable. Alternatively, virtual low ρr foams were created by means of cellular automata which allow a close control of bubble growth and final cell character and do not show the previous limitation. Additionally, virtual high ρr structures (ρr &gt; 0.5) consisting of isolated bubbles or cells were created by random incorporation of cell sets whose size distributions adjust to experimentally measured ones. A random sequential adsorption algorithm (RSA) which accurately controls final thickness of ligaments between cells was programmed for this purpose. FEM results of this kind of virtual foams are compared with experimentally tested mechanical properties. Moreover, impact of structural parameters (mean cell size) on elastic modulus and compressive collapse stress is critically assessed.

On how degeneration influences load-bearing in the cartilage–bone system: a microstructural and micromechanical study

This study investigated the microanatomical response to compression of intact and degenerate cartilage-on-bone samples with the aim of elucidating the functional consequences of articular surface disruption and related matrix changes. Two groups of mature bovine patellae were identified at the time of harvest; those with intact cartilage and those with cartilage exhibiting mild to severe degeneration. Cartilage-on-bone samples were statically compressed (7 MPa) to near-equilibrium using an 8-mm diameter cylindrical indenter, and then formalin-fixed in this deformed state. Following mild decalcification full-depth cartilage-bone sections, incorporating the indentation profile and beyond, were studied in their fully hydrated state using differential interference contrast optical microscopy (DIC). Differences in matrix texture, degree of disruption of the articular surface layer (or its complete absence), number of tidemarks and absence or presence of vascularization of the calcified cartilage zone were all observable features that provided clear differentiation between the normal and degenerate tissues. Under load a chevron-type shear discontinuity characterized those samples in which the strain-limiting surface layer was still largely intact. The extent to which this shear discontinuity advanced into the adjacent non-directly loaded cartilage continuum was influenced by the integrity of the cartilage general matrix. For those tissues deficient in a strain-limiting articular surface there was no shear discontinuity, the cartilage deformation field was instead shaped primarily by its osteochondral attachment and a laterally-directed compressive collapse of a much weakened matrix. In the degenerate samples the altered matrix textures associated with different regions of the deformation field are interpreted in terms of an intrinsic fibrillar architecture that is weakened by two fundamental processes: (1) a de-structuring resulting from a reduction in connectivity between fibrils and (2) subsequent aggregation of these now disconnected fibrils. DIC microscopy provides a high-resolution description of the integrated osteochondral tissue system across the full continuum of matrices, from normal to severely degenerate. Our study demonstrates the important functional role played by the strain-limiting articular surface, the consequences associated with its disruption, as well as the loss of effective stress transmission associated with a 'de-structured' general matrix. The study also provides new insights into the integration of cartilage with both its subchondral substrate and the wider continuum of non-directly loaded cartilage.

Compression testing of periodic cellular sandwich cores

Periodic cellular metal (PCM) hybrid sandwich cores with 95 pet open porosity have been constructed from perforated 3003 aluminum alloy (AA3003) sheets using perforation-stretching methods. Two compressive collapse mechanisms (i.e., plastic hinging and plastic buckling) were studied using two limiting test conditions: first, where the PCM nodes were restricted only by interfacial friction (i.e., free compression) and compressive forces were resisted primarily through strut bending and plastic hinging mechanisms; and, second, where the PCM nodes were laterally confined (i.e., confined compression) and compressive forces were resisted primarily through strutbuckling mechanisms. The contribution of each collapse mechanism to the overall truss core performance was studied. The strut bending during free compression was tracked by a nodal displacement mapping (NDM) technique, while the progression of confined compression strut buckling was correlated to the truss core stress-strain profile. The present data can be used to illustrate the different strengths between strut bending (free-PCM) and strut buckling (confined-PCM) collapse mechanisms.

The effect of strut geometry on the yielding behaviour of open-cell foams

A micromechanical analysis was carried out to investigate the effect of strut geometry on the yielding behaviour of open-cell foams. Different strut cross sections, in rectangular, circular and equilateral triangular shapes, were investigated. It was found that the strut geometry significantly affects the plastic-yielding behaviour of open-cell foams. The shape of the plastic-yield surface was found to depend not only on relative density but also on the cross-sectional shape of the struts. Numerical results show that even though the material of the struts is perfectly plastic, open-cell foams with asymmetrical sectional struts will exhibit different tensile and compressive collapse strength.

The toucan beak: Structure and mechanical response

The structure and mechanical response of a Toco toucan ( Ramphastos toco) beak were established. The beak was found to be a sandwich composite with an exterior of keratin scales (50 μm diameter and 1 μm thickness) and a core composed of fibrous network of closed-cells made of collagen. The tensile strength of the external shell is about 50 MPa. Micro- and nanoindentation hardness measurements corroborate these values. The keratin shell exhibits a strain-rate sensitive response with a transition from slippage of the scales due to release of the organic glue, at a low strain rate (5 × 10 − 5 s − 1 ) to fracture of the scales at a higher strain rate (1.5 × 10 − 3 s − 1 ). The closed-cell foam consists of fibers having a Young's modulus (measured by nanoindentation) of 12.7 GPa. This is twice as high as the keratin shells, which have E = 6.7 GPa. This is attributed to their higher calcium content. The compressive collapse of the foam was modeled by the Gibson–Ashby constitutive equations. There is a synergistic effect between foam and shell evidenced by a finite-element analysis. The foam stabilizes the deformation of the keratin shell by providing an internal support which increases its buckling load under compressive loading.

Percutaneous vertebroplasty in the management of vertebral lesions

Debilitating backache due to different types of vertebral lesions is a common cause of morbidity in all age groups. Percutaneous vertebroplasty (PV) gives substantial pain relief and stabilizes the weak vertebrae. Most of the information regarding PV comes from the Western literature. The effect of PV in our population should be studied. The primary objective is to assess the therapeutic benefit of PV in alleviating back pain and improving the functional status in patients with painful pathologic vertebrae. The secondary objectives are to study the technical aspects of the procedure and their relation to outcome and complications. This is a retrospective hospital-based (tertiary teaching hospital) study. From January 2001 to December 2004, 46 patients underwent PV procedures. Sixty-five vertebroplasties were done in 13 males and 33 female patients. Twenty-four (36.92%) procedures were done for osteoporotic compression collapse, 26 (40.0%) for hemangioma, and 15 (23.07%) for different vertebral body tumors and metastasis. The Wilcoxon signed rank test was used to evaluate the statistical significance of differences between the preoperative and postoperative levels of pain, mobility and analgesic usage. Most of the patients had pain relief within 48 h. Only minor side effects were encountered. No patient had any deficit related to the procedure. On follow up of 3-48 months, all patients had statistically significant improvement in clinical condition (P < 0.001). Percutaneous vertebroplasty is a safe and effective procedure in relieving debilitating backache and can be used to treat vertebral lesions in selected cases.

The Study of Buffer Structure of On-Board Test's Circuit Module in High Shock

In order to measure the acceleration of a projectile penetrating hard aims, we adopt on-board test technology. When the projectile penetrates hard targets, such as concrete, rock, and metal targets, the on-board test's circuit module suffers very high shock. After having analyzed related technology references, and compared different kinds of buffer materials and structures, we performed some experiments, then drew the conclusion that it is a valid method to use nonlinear buffer structures, such as plastic springs, to absorb the energy of the on-board test circuit module in the process of the buffer structure's compressive collapse.

Improved prediction of simultaneous local and overall buckling of stiffened panels

In this paper, improved expressions for elastic local plate buckling and overall panel buckling of uniaxially compressed T-stiffened panels are developed and validated with 55 ABAQUS eigenvalue buckling analyses of a wide range of typical panel geometries. These two expressions are equated to derive a new expression for the rigidity ratio ( EI x / Db) CO that uniquely identifies “crossover” panels—those for which local and overall buckling stresses are the same. The new expression for ( EI x / Db) CO is also validated using the 55 FE models. Earlier work by Chen (Ultimate strength analysis of stiffened panels using a beam-column method. PhD Dissertation, Department of Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, 2003) had produced a new step-by-step beam-column method for predicting stiffener-induced compressive collapse of stiffened panels. An alternative approach is to use orthotropic plate theory. As part of the validation of the new beam-column method, ABAQUS elasto-plastic Riks ultimate strength analyses were made for 107 stiffened panels—the 55 crossover panels and 52 others. The beam-column and orthotropic approaches were also used. A surprising result was that the orthotropic approach has a large error for crossover panels whereas the beam-column method does not. Some possible reasons for this are suggested.

Energy absorption of an egg-box material

Conical frustra made from leaded gun-metal have been compressed axially. Collapse is either by a travelling plastic hinge or by tearing. An analytical model is developed for the travelling plastic hinge in a rigid, ideally plastic solid; its predictions are compared with the observed response, and with those of an axisymmetric finite element analysis. The travelling hinge mechanism is also observed in the compressive collapse of an egg-box material comprising a square array of conical frustra. Collapse mechanism maps are constructed for the egg-box material, and they show the regimes of dominance of elastic buckling, material tearing and the travelling plastic hinge. The maps are useful for selecting egg-box geometries that maximise the energy absorption per unit mass at any prescribed value of collapse stress. The optimisation indicates that the egg-box material has a similar energy absorption capacity to that of hexagonal honeycombs and is superior to that of metal foams.

617 角部に開口部を有する薄肉角筒の軸圧壊挙動とエネルギー吸収特性

617 角部に開口部を有する薄肉角筒の軸圧壊挙動とエネルギー吸収特性

A Case of Primary Pericardial Hemagiopericytoma

Intracardiac hemangiopericytomas are rare tumors which originates from the pericyte in the external wall of capillaries. 1 The tumors are known to usually develop in the lower extremities, pelvic cavity and retroperito- neum, 2 but are very rare in the heart. 3 The symptoms and signs of a hemagiopericytoma depend on the size and location of the tumor. 2 A hemagiopericytoma has a high potential for local recurrence and metastasis, so regular follow-up is needed following surgical excision. 2 A 36-year-old man presented with shortness of breath and chest discomfort. Before operating, a chest CT scan showed that a compressive collapse of the left lung had developed next to a large mediastinal tumor. Because of impending respiratory failure due to collapse of the left lung, an operation was performed. The operation showed that the mediastinal tumor was a large loculated hemopericardium accompanied by pericardial bleeding. A hematoma evacuation with a pericardiectomy was performed, and the pathology of the thickened pericardial wall revealed a malignant hemangiopericytoma. The patient has followed up for 6 months without symptoms or sign of tumor recurrence following the radiation therapy. (Korean Circulation J 2002;32(10 :922-926

Compressibility and phase transition of δ-UO 3 under high pressure

A ‘compressibility collapse’ transition of δ-UO 3 (ReO 3 structure type) was detected at 0.3 GPa and room temperature using X-ray diffraction in a diamond anvil cell. The compressibility of UO 3 increased from 6.3×10 −3 to 9.4×10 −2 GPa −1 at the transition point. The p– V relationship up to 2.2 GPa is presented. An amorphisation was observed at 2.2 GPa. The investigation of bulk amorphous samples obtained under high pressure at room temperature showed that the amorphisation is connected with the kinetics of the α-to-β UO 3 phase transformation.