Semi-confined Compression Research Articles

On the contribution of solid and fluid behavior to the modeling of the time-dependent mechanics of tendons under semi-confined compression

The present work aims to investigate whether it is possible to identify and quantify the contributions of the interstitial fluid and the solid skeleton to the overall time-dependent behavior of tendons based on a single mechanical test. For this purpose, the capabilities of three different time-dependent models (a viscoelastic, a poroelastic and a poroviscoelastic) were investigated in the modeling of the experimental behavior obtained from semi-confined compression with stress relaxation tests transverse to collagen fibers. The main achieved result points out that the poroviscoelastic model was the only one capable to characterize both the experimental responses of the force and volume changes of the tissue samples. Moreover, further analysis of this model shows that while the kinematics of the sample are mainly governed by the fluid flow (pore pressure contribution of the model), the behavior intrinsically associated with the viscoelastic solid skeleton makes a significant contribution to the experimental force response. This study reinforces the importance of taking both the experimental kinematics and kinetics of tendon tissues into account during the constitutive characterization procedure.

Correlating the microstructural architecture and macrostructural behaviour of the brain

The computational simulation of pathological conditions and surgical procedures, for example the removal of cancerous tissue, can contribute crucially to the future of medicine. Especially for brain surgery, these methods can be important, as the ultra-soft tissue controls vital functions of the body. However, the microstructural interactions and their effects on macroscopic material properties remain incompletely understood. Therefore, we investigated the mechanical behaviour of brain tissue under three different deformation modes, axial tension, compression, and semi-confined compression, in different anatomical regions, and for varying axon orientation. In addition, we characterised the underlying microstructure in terms of myelin, cells, glial cells and neuron area fraction, and density. The correlation of these quantities with the material parameters of the anisotropic Ogden model reveals a decrease in shear modulus with increasing myelin area fraction. Strikingly, the tensile shear modulus correlates positively with cell and neuronal area fraction (Spearman’s correlation coefficient of rs=0.40 and rs=0.33), whereas the compressive shear modulus decreases with increasing glial cell area (rs=−0.33). Our study finds that tissue non-linearity significantly depends on the myelin area fraction (rs=0.47), cell density (rs=0.41) and glial cell area (rs=0.49). Our results provide an important step towards understanding the micromechanical load transfer that leads to the non-linear macromechanical behaviour of the brain. Statement of significanceWithin this article, we investigate the mechanical behaviour of brain tissue under three different deformation modes, in different anatomical regions, and for varying axon orientation. Further, we characterise the underlying microstructure in terms of various constituents. The correlation of these quantities with the material parameters of the anisotropic Ogden model reveals a decrease in shear modulus with increasing myelin area fraction. Strikingly, the tensile shear modulus correlates positively with cell and neuronal area fraction, whereas the compressive shear modulus decreases with increasing glial cell area. Our study finds that tissue non-linearity significantly depends on the myelin area fraction, cell density, and glial cell area. Our results provide an important step towards understanding the micromechanical load transfer that leads to the non-linear macromechanical behaviour of the brain.

Age-dependent mechanical and microstructural properties of the rabbit soleus muscle

During growth there are serious changes in the skeletal muscles to compensate for the changed requirements in terms of body weight and size. In this study, the age-dependent (between 21 and 100 days) mechanical and microstructural properties of rabbit soleus muscle tissue were investigated. For this purpose, morphological properties (animal mass, soleus muscle mass, tibial length) were measured at 5 different times during aging. On the other hand, fibre orientation-dependent axial and semi-confined compression experiments were realised. In addition, the essential components (muscle fibres, extracellular matrix, remaining components), dominating the microstructure of muscle tissue, were analysed. While the mechanical results show hardly any age-dependent differences, the morphological and microstructural results show clear age-dependent differences. All morphological parameters increase significantly (animal mass by 839.2%, muscle mass 1050.6%, tibial length 233.6%). In contrast, microstructural parameters change differently. The percentage of fibres (divided into slow-twitch (ST) and fast-twitch (FT) fibres) increases significantly (137.6%), while the proportions of the extracellular matrix and the remaining components (48.2% and 46.1%) decrease. At the same time, the cross-sectional area of the fibres increases significantly (697.9%). The totality of this age-dependent information provides a deeper understanding of age-related changes in muscle structure and function and may contribute to successful development and validation of growth models in the future. Statement of significanceThis article reports the first comprehensive data set on age-dependent morphological (animal mass, soleus muscle mass, tibial length), mechanical (axial and semi-confined compression), and microstructural (muscle fibres, extracellular matrix, remaining components) properties of the rabbit soleus muscle. On the one hand, the results of this study contribute to the understanding of muscle mechanics and thus to understanding of load transfer mechanisms inside the muscle tissue during growth. On the other hand, these results are relevant to the fields of constitutive formulation of age-dependent muscle tissue.

Experimental evaluation of the conjugate anvil hammer mill – Comparison of semi-confined to confined particle breakage

The Conjugate Anvil Hammer Mill (CAHM) was identified as a novel energy efficient technology by the Canadian Mining Innovation Council that has the potential to replace conventional crushers and Semi-Autogenous Grinding (SAG) mills. Discrete Element Method (DEM) modelling showed that the CAHM requires significantly less energy in comparison to existing comminution technologies including High Pressure Grinding Rolls (HPGR). However, experimental evidence was required to validate this potential and to address operational questions.In order to evaluate the CAHM, piston-die testing was conducted to represent the comminution conditions in the CAHM for comparison to the HPGR. Three piston-die conditions were tested including low pressure semi-confined compression as generated in the CAHM, low pressure confined compression, and high pressure confined compressions as generated in the HPGR. The particle breakage was investigated and compared in terms of applied force, energy consumption, product particle size reduction and particles’ surface area generation. Results showed that the CAHM test apparatus required a much lower force and produced a finer product particle size distribution and higher surface area when compared to low pressure confined compression. When compared to high pressure confined compression (HPGR), up to 37% and 22% energy savings were achieved by the CAHM for a soft gold ore and a harder copper ore, respectively. The results demonstrated that the CAHM technology has potential to improve energy efficiency in the comminution process.

Finite element simulation of plate sinkage, confined and semi-confined compression tests: A comparison of the response to yield stress

Pre-compression stress (σpc) has been widely used as a criterion for assessing the susceptibility of soil to compaction. Confined compression (CCT), semi-confined compression (SCCT) and plate sinkage (PST) tests are the most practical procedures to estimate σpc. Discrepant estimates of σpc may be obtained from the above tests mainly due to differences in boundary conditions. The aim of this study was to simulate these compaction tests using the finite element method (FEM) to compare σpc estimated from stress-strain curves with the simulated yield stress. It was hypothesized that for a given simulated yield stress, σpc estimated either at the point of maximum curvature or the point of Casagrande's method is not the same as simulated yield stress for different tests. FEM models were developed in ABAQUS with elastic-perfectly plastic law and Drucker-Prager yield criterion for the soil material. Yield stresses of 20, 50, 75, 100, 125, 150, 200 and 300 kPa were examined. The results showed that the stress at the point of maximum curvature was close to the simulated yield stress in CCT but smaller than that in SCCT and PST. Casagrande's method overestimated σpc (averagely by 40%) for all the tests. A more precise estimate of σpc was obtained at the point of maximum curvature than the Casagrande’s method in experimental CCT and PST on remolded soils. Cyclic loading-unloading CCT and PST to different stress levels showed that a safe threshold for preventing a severe plastic strain is significantly smaller (with a factor of 0.5 and 0.8 for CCT and PST, respectively) than σpc in either of the tests.

English

Soil compaction due to traffic is the process whereby soil bulk density is increased. The compaction level depends on the weight of the vehicle, and the soil susceptibility to compaction, which is characterized by its mechanical properties (that is, the stress-strain relationships). The plate penetrometer can be used to quantify the levels of soil compaction in-situ and supply a larger number of information on the control of this phenomenon, in real time. This paper evaluates the use of this tool with data filtering system assistance on semi confined compression assay, for predicting soil compaction in-situ, through parameters pre-compaction stress (spc) and soil compression Index (Cc). The tests were carried out on Experimental Agricultural Engineering Nucleus (NEEA in Portuguese) at West Parana State University - UNIOESTE - Brazil. The sample collection was carried at points along the tire track, 24 h after soil compaction induced by the rear tractor wheel with one, five and ten passages, on the same track. The information system is able to read all data derived from the compression test and carry out a filtering on the data by means of pre-selected parameters, plotting the compression curves and regression equations. The analysis, carried out “in situ”, avoids laboratorial procedures and gives the farmer more agility in the decision process on the management of the agricultural machinery system. Key words: Semi-confined compression test, information system, soil compaction in-situ.

Long-term mechanical behaviour of skeletal muscle tissue in semi-confined compression experiments

In this study, porcine skeletal muscle tissue was tested until 112 hours post mortem using a semi-confined compression device that induces fascicles to enter one of the states of compression (mode I), tension (mode II), or constant length (mode III). Based on the authors׳ previous studies (Böl et al., 2014, 2015a), the anisotropic mechanical behaviour of the tissue was analysed, with a special focus on the testing time post mortem. The results suggest that the tissue exhibits significant anisotropic behaviour during the first hours of post mortem but that this anisotropy becomes insignificant at later time points. Interestingly, the compressibility of the tissue is more or less unaffected by the testing time. These results are discussed especially with respect to tissue microstructure.

Finite element models of the human tongue: a mixed-element mesh approach

One of the key factors for obtaining accurate and reliable results using the finite element method is the discretisation of the domain. Traditionally, two main types of elements are used for three-dimensional mesh generation: tetrahedral and hexahedral elements. Tetrahedral meshes are automatically generated but standard displacement-based tetrahedral elements generally suffer from performance issues in terms of convergence rate and accuracy of the solution associated with volumetric and shear locking. Because of these distinct disadvantages, hexahedral meshes have been used up until now for the design of biomechanical models of the orofacial system in particular for medical applications. However, hexahedral meshing is very costly and labour-intensive when the mesh must be hand-made. The aim of the present contribution is to evaluate the performance of mixed element meshes as an alternative to all-tetrahedral or all-hexahedral meshing for the analysis of problems involving nearly incompressible materials at large strains. The case study of a semi-confined compression experiment of an elastic cylindrical specimen was analysed. The theoretical expression of deformation was derived from the literature. We observed that linear mixed element meshes allowed results very close to those obtained using hexahedral ones. As a second experiment, we generated a mixed element mesh of the tongue and analyse its simulated response to activation of the posterior Genioglossus muscle. Overall, our results show that mixed element meshes can be used as computationally less demanding alternative to all-hexahedral meshes for the analysis of problems involving nearly-incompressible materials at large strains.

Tissue-scale anisotropy and compressibility of tendon in semi-confined compression tests

In this study, porcine tendon tissue was tested with a dedicated semi-confined compression set-up that enables us to induce states of either fibrils in compression (mode I), tension (mode II) or at constant length (mode III), respectively. The results suggest that tendon tissue is compressible and demonstrates a significantly stiffer response in mode I than in mode III. This implies that the fibril direction remains the axis of transverse isotropy in compression and that it provides an anisotropic contribution to the tissue stress. These results, which are important for the development of constitutive models for tendon tissue, are discussed with respect to the hierarchical structure of the extracellular matrix.

Assessing confined and semi-confined compression curves of highly calcareous remolded soil amended with farmyard manure

Confined and semi-confined compression curves of a highly calcareous remolded soil amended with farmyard manure (FYM) from dairy cattle were investigated. The samples were collected from the topsoil of the experimental plots where FYM had been added to the soil at four application rates (0, 25, 50 and 100Mgha−1) for 7years. Large remolded specimens were prepared by applying 100kPa load at 17.1% and 20.9% dry basis moisture contents. The center section of the preloaded soil specimen was firstly submitted to a 50mm diameter plate for plate sinkage test (PST). One cylindrical sample was also immediately cored for confined compression test (CCT). The S-shaped curves were observed with the CCT, whereas the bi-linear form was obtained with the PST. In the CCT, the S-shaped form was more obvious at the higher moisture content. The results revealed that the moisture content and the application rate of FYM influenced the start and end points and precompaction stress of compression curve for PST. Moreover, the moisture content and the application rate of FYM affected the end point and the compression index (Cc) in the case of CCT. However, it was concluded that the overall trend in compression curve only depends on the type of test conducted.

Stress relaxation of swine growth plate in semi-confined compression: depth dependent tissue deformational behavior versus extracellular matrix composition and collagen fiber organization

Mechanical environment is one of the regulating factors involved in the process of longitudinal bone growth. Non-physiological compressive loading can lead to infantile and juvenile musculoskeletal deformities particularly during growth spurt. We hypothesized that tissue mechanical behavior in sub-regions (reserve, proliferative and hypertrophic zones) of the growth plate is related to its collagen and proteoglycan content as well as its collagen fiber orientation. To characterize the strain distribution through growth plate thickness and to evaluate biochemical content and collagen fiber organization of the three histological zones of growth plate tissue. Distal ulnar growth plate samples (N = 29) from 4-week old pigs were analyzed histologically for collagen fiber organization (N = 7) or average zonal thickness (N = 8), or trimmed into the three average zones, based on the estimated thickness of each histological zone, for biochemical analysis of water, collagen and glycosaminoglycan content (N = 7). Other samples (N = 7) were tested in semi-confined compression under 10% compressive strain. Digital images of the fluorescently labeled nuclei were concomitantly acquired by confocal microscopy before loading and after tissue relaxation. Strain fields were subsequently calculated using a custom-designed 2D digital image correlation algorithm. Depth-dependent compressive strain patterns and collagen content were observed. The proliferative and hypertrophic zone developed the highest axial and transverse strains, respectively, under compression compared to the reserve zone, in which the lowest axial and transverse strains arose. The collagen content per wet mass was significantly lower in the proliferative and hypertrophic zones compared to the reserve zone, and all three zones had similar glycosaminoglycan and water content.Polarized light microscopy showed that collagen fibers were mainly organized horizontally in the reserve zone and vertically aligned with the growth direction in the proliferative and hypertrophic zones. Higher strains were developed in growth plate areas (proliferative and hypertrophic) composed of lower collagen content and of vertical collagen fiber organization. The stiffer reserve zone, with its higher collagen content and collagen fibers oriented to restrain lateral expansion under compression, could play a greater role of mechanical support compared to the proliferative and hypertrophic zones, which could be more susceptible to be involved in an abnormal growth process.

Semi-confined compression of microfabricated polymerized biomaterial constructs

Mechanical forces are critical parameters in engineering functional tissue because of their established influence on cellular behaviour. However, identifying ideal combinations of mechanical, biomaterial and chemical stimuli to obtain a desired cellular response requires high-throughput screening technologies, which may be realized through microfabricated systems. This paper reports on the development and characterization of a MEMS device for semi-confined biomaterial compression. An array of these devices would enable studies involving mechanical deformation of three-dimensional biomaterials, an important parameter in creating physiologically relevant microenvironments in vitro. The described device has the ability to simultaneously apply a range of compressive mechanical stimuli to multiple polymerized hydrogel microconstructs. Local micromechanical strains generated within the semi-confined hydrogel cylinders are characterized and compared with those produced in current micro- and macroscale technologies. In contrast to previous work generating unconfined compression in microfabricated devices, the semi-confined compression model used in this work generates uniform regions of strain within the central portion of each hydrogel, demonstrated here to range from 20% to 45% across the array. The uniform strains achieved simplify experimental analysis and improve the utility of the compression platform. Furthermore, the system is compatible with a wide variety of polymerizable biomaterials, enhancing device versatility and usability in tissue engineering and fundamental cell biology studies.

Compression testing of very soft biological tissues using semi-confined configuration—A word of caution

We analyse semi-confined (i.e. using no-slip boundary conditions) compression experiment of very soft tissue sample using finite element method. We show that the assumption that the planes perpendicular to the direction of the applied force remain plane during the experiments is not satisfied for compression levels lower than previously stated in Miller [2005. Method for testing very soft biological tissues in compression. Journal of Biomechanics 38, 153–158]. Therefore, we recommend that the parameters for constitutive models of very soft tissues be determined by fitting a solution of the finite element models of the experimental set-up to the measurements obtained using semi-confined compression experiments.

Suitability of pre-compression stress as the real critical stress of unsaturated agricultural soils

This study was conducted to examine if the pre-compression stress σpc is the major critical stress influencing soil physical quality indices, i.e. air permeability Kg and air-filled porosity (AFP). Topoils of five soil series were collected from the Isfahan Province in central Iran. Treatments consisted of: five soil types (sandy loam to clay), four matric suctions (10, 20, 50 and 80 kPa), three values for the maximum axial stress σpc (200, 400 and 600 kPa), and three loading types with 10 loading cycles. The loading types were a confined compression test (CCT), a semi-confined compression test (SCCT) and a kneading compression test (KCT). Soil type and matric suction, loading type, maximum applied stress and number of loading cycles significantly affected log [Kg], void ratio e, and AFP. The Significance of the loading cycle's number implies that though the σpc of the soil was not exceeded, the soil physical properties were considerably changed. A gradual transition from elastic-to-plastic deformation was observed on the stress–strain curves obtained from CCT. The stress−strain curves of the SCCT were sharp at σpc due to the semi-confined condition, which allows lateral deformation during loading. Cyclic loading was not always accompanied by significant irreversible strain but this could result in up to 10 times the decrease in Kg. For the fine-textured soils, CCT resulted in a significantly greater reduction in Kg, e and AFP when compared with KCT. The opposite trend was observed for the coarse-textured soils. The KCT homogenised the pore system and resulted in more decrease of Kg when compared with CCT at matric suctions of 10 and 20 kPa. However, at the matric suctions of 50 and 80 kPa, KCT created an open microstructure, which led to relatively higher values of Kg. The results show that the σpc might not be a real critical stress from a view of soil physical quality indices (i.e. Kg and AFP), especially at low matric suctions. Thus, the characterisation of soil compaction may not be completely accounted by a bulk property such as void ratio, but additional information about pore characteristics are needed to describe the effect of compaction on soil physical quality.