Effect of preload and shim types on the mechanical properties of composite-aluminium bolted joints

The influence of both preload and the presence of shim types on the mechanical properties of composite-aluminium single-bolt, single-lap joints were studied in this paper. The load-displacement curve and surface strain field of joints in different shim types and preloads were obtained through tensile experiments. A progressive damage model was established using the UMAT subroutine in ABAQUS. A hybrid failure criterion and a linear continuous degradation model were used to describe the progressive damage of composite laminates. The results show that for joints with no shim and for those with various types of shims, the tensile stiffness, peak load and initial damage load could be reduced when the preload is insufficient or too large. Compared with joints with no shims or with peelable fibreglass shims, joints with liquid shims required a larger preload to achieve the best mechanical properties. As the proportion of peelable fibreglass shim increased, the tensile stiffness and peak load continued to increase in joints with a mixed shim of liquid and peelable fiberglass shim. Shims can serve as tension bearings, but have little effect on the initiation and development of bearing failure.

Regulating the Deformation Properties of Paper by Varying the Degree of Its Anisotropy

Mechanical properties are crucial in assessing the paper quality. Deformation and strength properties of paper are determined by the strength and stiffness of the interfiber and intermolecular hydrogen bonds. The contribution ratio of interfiber and intermolecular hydrogen bonds to the strength of paper can be changed by adjusting the degree of its anisotropy. The article presents the results on a study of the deformation properties of laboratory anisotropic paper samples from kraft bleached softwood pulp with a beating degree of 30 °SR. The samples had basic weight of 90 g/m2 and the degree of stiffness anisotropy TSIMD/CD of 1.75–4.08. They were made by using Techpap SAS automatic dynamic handsheet former (Grenoble, France), with varying forming parameters – diameter of the nozzle, motion speed of the forming wire, and injecting speed of pulp. Deformation properties were determined by tensile test and processing of the stress-strain dependence (σ−ε). The outcomes have shown that, an increase of the fiber orientation degree in paper structure by changing the forming parameters caused a change in the nature of the paper deformation under tension. Increasing the fiber orientation degree in the structure of paper made it possible to increase the strength by 55 %, tensile stiffness by 63 % in the machine direction, while reducing the extensibility by 10 %. In the cross direction, it was possible to decrease tensile stiffness by 33 %, strength by 55 %, and increase the extensibility by 5 %. Anisotropy of tensile strength was 1.73–6.00. The greatest effect was obtained for the elasticity modulus in the pre-failure zone E2 (2.8–38.6). It means that, fiber orientation had a key importance when large deformations in the samples took place. The established quantitative regularities allowed optimizing the values of the deformation and strength properties of paper, and their ratio in the machine direction and cross direction due to the variation of the forming parameters. For citation: Rech D., Potasheva A.N., Kazakov Ya.V. Regulating the Deformation Properties of Paper by Varying the Degree of Its Anisotropy. Lesnoy Zhurnal [Russian Forestry Journal], 2021, no. 5, pp. 174–184. DOI: 10.37482/0536-1036-2021-5-174-184

A Time-Zero Biomechanical Study of the Effective Length of the Graft in Posterior Cruciate Ligament Reconstruction

Background The effect of effective length on the biomechanical properties of the graft is regarded as an essential variable influencing the posterior cruciate ligament reconstruction. However, the effect has not been fully studied. The purpose was to compare the effects of different effective graft lengths (35 mm, 55 mm, 65 mm) on the time-zero biomechanical properties of the graft in posterior cruciate ligament (PCL) reconstruction.Methods Bovine digital flexor tendons and porcine tibias were used to establish in-vitro PCL reconstruction models. Tensile strength testing was performed at 3 different effective lengths of the graft: short (35 mm, n = 10, group 1), medium (55 mm, n = 10, group 2), and long (65 mm, n = 10, group 3). A computer software (Trapezium X; Shimadzu) was used to record the load-elongation curve, ultimate load (N), the elongation of the graft during the test (mm), tensile stiffness (N/mm), and energy absorption (J). The failure pattern was evaluated by visual observation.Results All the samples failed because the grafts slipped out from the bones, except two grafts ruptured in group 1. The tensile stiffness and ultimate load in group 1 were significantly higher than those in group 2 and group 3 (tensile stiffness, 50.49 ± 11.43 N/mm in group 1 vs 31.20 ± 10.44 N/mm in group 2[P < 0.001] and 19.18 ± 6.18 N/mm in group 3 [P < 0.001]; ultimate load, 452.40 ± 54.52 N in group 1 vs 338.50 ± 26.79 N in group 2 [P < 0.001]and 268.70 ± 28.30 N in group 3 [P < 0.001]). There were significant differences between group 1 and group 3 in energy absorption(9.61 ± 3.25 J vs 5.22 ± 2.43 J, P = 0.002). At 50 N and 100 N of applied load, no statistically significant differences were detected on the elongation of the grafts (P > 0.05). The elongation of the short grafts under 150 N and 200 N of applied load was significantly less than that of the long grafts (150 N, 1.77 ± 0.83 mm in group 1 vs 4.14 ± 1.75 mm in group 3, P = 0.047; 200 N, 2.62 ± 1.10 mm in group 1 vs 7.06 ± 3.20 mm in group 3, P = 0.006).Conclusions This study demonstrated the time-zero biomechanical properties of the graft with short effective length were superior to those of the graft with medium and long effective lengths in PCL reconstruction.

A biphasic approach for characterizing tensile, compressive and hydraulic properties of the sclera

Measuring the biomechanical properties of the mouse sclera is of great interest: altered scleral properties are features of many common ocular pathologies, and the mouse is a powerful tool for studying genetic factors in disease, yet the small size of the mouse eye and its thin sclera make experimental measurements in the mouse difficult. Here, a poroelastic material model is used to analyse data from unconfined compression testing of both pig and mouse sclera, and the tensile modulus, compressive modulus and permeability of the sclera are obtained at three levels of compressive strain. Values for all three properties were comparable to previously reported values measured by tests specific for each property. The repeatability of the approach was evaluated using a test–retest experimental paradigm on pig sclera, and tensile stiffness and permeability measurements were found to be reasonably repeatable. The intrinsic material properties of the mouse sclera were measured for the first time. Tensile stiffness and permeability of the sclera in both species were seen to be dependent on the state of compressive strain. We conclude that unconfined compression testing of sclera, when analysed with poroelastic theory, is a powerful tool to phenotype mouse scleral changes in future genotype–phenotype association studies.

Thickness-dependent stiffness of wood: potential mechanisms and implications

When wood is split or cut along the grain, a reduction in tensile stiffness has been observed. The averaged mechanical properties of wood samples, veneers or splinters therefore change when their thickness is less than about 1 mm. The loss of stiffness increases as the thickness approaches that of a single cell. The mechanism of the effect depends on whether the longitudinal fission plane is between or through the cells. Isolated single cells are a model for fission between cells. Each cell within bulk wood is prevented from twisting by attachment to its neighbours. Separation of adjacent cells lifts this restriction on twisting and facilitates elongation as the cellulose microfibrils reorientate towards the stretching direction. In contrast when the wood is cut or split along the centre of the cells, it appears that co-operative action by the S1, S2 and S3 cell-wall layers in resisting tensile stress may be disrupted. Since much of what is known about the nanoscale mechanism of wood deformation comes from experiments on thin samples, caution is needed in applying this knowledge to structural-sized timber. The loss of stiffness at longitudinal fracture faces may augment the remarkable capacity of wood to resist fracture by deflecting cracks into the axial plane. These observations also point to mechanisms for enhancing toughness that are unique to wood and have biomimetic potential for the design of composite materials.