Load-elongation Curve Research Articles

Biomechanical Comparison of Anatomic Versus Lower of Anteromedial and Anterolateral Tibial Tunnels in Posterior Cruciate Ligament Reconstruction.

In order to reduce the "killer turn" effect, various tibial tunnels have been developed. However, few studies investigated the biomechanical effects of different tibial tunnels during PCL reconstruction. This study aims to compare the time-zero biomechanical properties of anteromedial, anterolateral, lower anteromedial, and lower anterolateral tibial tunnels in transtibial posterior cruciate ligament (PCL) reconstruction under load-to-failure loading. Porcine tibias and bovine extensor tendons were used to simulate in vitro transtibial PCL reconstruction. Forty bovine extensor tendons and 40 porcine tibias were randomly divided into four experimental groups: anteromedial tunnel group (AM group, n=10), anterolateral tunnel group (AL group, n=10), lower anteromedial tunnel group (L-AM group, n=10), and lower anterolateral tunnel group (L-AL group, n=10). The biomechanical test was then carried out in each group using the load-to-failure test. The ultimate load (in newtons), yield load (in newtons), tensile stiffness (in newtons per millimeter), load-elongation curve, failure mode, and tibial tunnel length (in millimeter) were recorded for each specimen. One-way analysis of variance (ANOVA) was used to compare the mean differences among the four groups. The biomechanical outcomes showed that there were no differences in the mean tensile stiffness and failure mode among four groups. The ultimate load and yield load of the L-AM group were significantly higher than those of other three groups (P < 0.05). For the AM group, its ultimate load is significantly higher than that of the L-AL group (P < 0.05), and its yield load is higher than that of the AL group and L-AL group (P < 0.05). However, we found no significant differences in either ultimate load or yield load between AL group and L-AL group (P > 0.05). There was significant statistical difference in the length of tibial tunnel between anatomic groups (AM and AL) and lower groups (L-AM and L-AL) (P < 0.05). Compared with the anteromedial, anterolateral, and lower anterolateral tibial tunnel, the lower anteromedial tibial tunnel showed better time-zero biomechanical properties including ultimate load and yield load in transtibial PCL reconstruction.

Anterior to Posterior Bone Plug Suture Tunnels Provide Optimal Biomechanics for Bone−Patellar Tendon−Bone Anterior Cruciate Ligament Graft

PurposeTo evaluate different bone−patellar tendon−bone (BPTB) plug suture configurations for pull through strength, stiffness, and elongation at failure in a biomechanical model of suspensory fixation.MethodsForty nonpaired, fresh-frozen human cadaveric BPTB allografts with an average age of 65.6 years were tested. Tensile testing was performed with the use of a custom-designed fixture mounted in a dynamic tensile testing machine. A preload of 90 N was applied to the graft and held for 5 minutes. Following this, a tensile load-to-failure test was performed. The ultimate failure load, elongation at failure, and mode of failure were recorded, and the resulting load–elongation curve was documented.ResultsThe drill tunnel through the cortical surface (anterior to posterior) was found to be significantly stronger than the drill tunnel through the cancellous surface (medial to lateral). There were no significant differences found when comparing the strength of the suture augmentation through the tendon and the drill tunnel alone (P = .13 among cancellous groups, P = .09 among cortical groups). The cortical drill tunnel with suture augmentation through the tendon showed significantly greater elongation values (13.7 ± 3.2) at failure when compared with either the cancellous or cortical drill tunnel only test groups (P = .0003 compared with cancellous alone, P = .009 when compared with cortical alone). Conclusions: The BPTB suture configuration with an anterior to posterior–directed suture tunnel without a suture through tendon augmentation provides the optimal strength and stiffness while minimizing graft elongation after fixation in a biomechanical model. This configuration is best for preventing suture pull through and failure when passing sutures through the BPTB plug.Clinical RelevanceThis study biomechanically evaluates the optimal suture configuration in the proximal bone plug for suspensory fixation in the setting of BPTB grafts.

Testing, numerical modelling and design of S690 high strength steel channel-to-plate connections

This paper describes experimental and numerical studies of S690 high strength steel channel-to-plate connections failing by net section fracture. Experiments were conducted on 27 S690 high strength steel channel-to-plate connection specimens, with each consisting of a channel section member bolted to two gusset plates by web or flange. The test ultimate loads, load–elongation curves, failure modes and strain distribution at critical cross-sections were fully reported and analysed. The influences of the number of bolt holes, longitudinal pitch, connection length and out-of-plane eccentricity on net section efficiency were discussed. The experimental programme was followed by a numerical modelling programme, where finite element models were firstly developed and validated against the test results and then used to conduct parametric studies to generate further numerical data. Based on the test and numerical data, the accuracy of the relevant design rules, as set out in the European code, American specification and Australian standard, was assessed. The assessment results revealed that the European code leads to overly conservative and scattered ultimate load predictions, while the American specification and Australian standard result in unsafe and scattered ultimate load predictions. Finally, a new design approach was proposed and shown to provide substantially improved ultimate load predictions for S690 high strength steel channel-to-plate connections over the design codes. The reliability of the new design approach was also demonstrated by means of statistical analyses.

Experimental and numerical study of stainless steel channel-to-gusset plate connections

With the advantages offered by stainless steel in terms of sustainability, mechanical performance and superiorserviceability, structural application of stainless steel is prevalent. This paper presents experimental and numerical studies of stainless steel channel-to-gusset plate connections failing by net section fracture. The experimental programme was carried out on 16 stainless steel channel-to-gusset plate connections, with each consisting of a channel section member bolted to two gusset plates by web or flange. The test setup and procedures as well as the key test results, including the failure loads, load–elongation curves, failure modes and strain distribution at critical cross-sections, were reported in detail. The effects of connection length, out-of-plane eccentricity and in-plane eccentricity on net section efficiency were discussed. Following the experimental programme, a numerical modelling programme was performed, where finite element models were firstly developed to replicate the tests and then employed to conduct parametric studies to generate further numerical data over a wide range of cross-section dimensions and connection lengths. Based on the test and numerical data, the existing design rules for stainless steel channel-to-gusset plate connections failing by net section fracture, as given in the European code and American specification, were evaluated. Generally, the European code yields excessively conservative and scattered failure load predictions, while the American specification leads to unsafe and scattered failure load predictions. An improved design approach was proposed and shown to provide substantially improved failure load predictions over the design codes.

Investigation on tool positioning in friction stir lap welding of AA6061 to polycarbonate sheets using force-torque signals

The lightweight high-performance thermoplastics are often replaced the metallic parts in aircraft and automobile industries due to high corrosion resistance along with upgraded strength to weight ratio. However, the joining of dissimilar non-ferrous aluminium to amorphous polymers is highly difficult caused by a drastic change in thermal properties which indicates the necessity of solid state welding processes. Therefore, the current work reports the practicality of friction stirred welding unlike AA6061 aluminium alloy and polycarbonate sheets for overlapped conditions by a cylindrical tool having a taper pin of H13 steel. The prime objective was to analyse the influence of tool tilt angle at different plunging depths and tool rotational speeds on the dissimilar weldability using a load-elongation curve. The aluminium to polycarbonate weld interface has also been studied to correlate with mechanical behaviour due to tensile load. The axial thrust force with tool stirring torque signals has been acquired for these parametric conditions for further process monitoring by segregating consecutive stages in the welding cycle. The weld bead profile along welding direction has been monitored using time domain thrust-force signals in the welding phase whose extracted significant features were compared. Finally, a sensitivity analysis was also processed to investigate the degree of parametric impact on thrust-torque as well as weld quality features. The highest joint strength efficiency was found to be 39.4% at higher tool rotational speed (1400 rpm) with reduced plunge depth (0.1 mm) without any tool tilting whereas weld ductility was significantly improved at reversed parametric settings with tool inverted position. The tool plunging force is a better indicator whereas both plunge depth as well as tilt angle were almost equally important for the prediction of lap weld quality.

An Efficient Needleless Grasping Suture Technique for Graft Preparation in Anterior Cruciate Ligament Reconstruction.

ObjectiveSeveral needleless techniques have been developed to outcome the inherent disadvantages of the traditional needle stitching technique for graft preparation, such as tendon damage through the needle, time consumption, and the potential risk of needlestick injury. The purpose of the present study is to compare the graft preparation time and the biomechanical performance between an efficient needleless technique and the traditional needle stitching technique for graft preparation in anterior cruciate ligament reconstruction (ACLR).MethodsThe time required to perform a complete suture on 20 hamstring tendons during ACLRs was measured. The grafts from one side were prepared using the needle stitching technique. The grafts from the other side used the needleless grasping suture technique. For the second part of the study, 12 fresh-frozen porcine flexor tendons were divided into two groups using two techniques and were mounted in an electric tensile test system. Each group was pretensioned to 100 N to simulate the maximum initial graft tension. The suturing state of sutures and graft (intact and damaged) and the load-elongation curve were recorded for each group. A Student’s t-test was used to compare the means of the two groups.ResultsIn operation, the needleless grasping suture technique group (19.8 ± 4.8, range: 13.5–32.9 s) was significantly faster (p < 0.05) than the needle stitching technique group (52.7 ± 12.7, range: 36.0–87.5 s). The state of sutures in each group was intact. The mean elongation was 11.75 ± 1.38 (range: 9.47–12.99) mm and 10.59 ± 1.02 (range: 9.12–11.76) mm in the needleless stitching technique group and the needle grasping suture technique group, respectively. There was no statistically significant difference in the elongation between the two groups (p > 0.05).ConclusionThe needleless grasping suture technique was a convenient and efficient method for graft preparation in ACLR.

Experimental and numerical studies of stainless steel angle-to-plate connections

This paper presents experimental and numerical studies of stainless steel angle-to-plate connections failing by net section fracture. Tension tests were conducted on 16 austenitic stainless steel angle-to-plate connections, with each consisting of an equal-leg or unequal-leg angle section member bolted to gusset plates by one leg. The specimens were designed with different angle section sizes, number of bolt holes and connection lengths. The key test observed results, including the load–elongation curves as well as the failure loads and modes, were fully reported. The strain distribution at critical cross-sections and the effects of connection length, out-of-plane eccentricity and in-plane eccentricity on net section efficiency were discussed. The experimental programme was supplemented by a numerical modelling programme, where finite element models were firstly developed and validated against the test results and afterwards adopted to perform parametric studies to generate further numerical data. On the basis of the test and numerical data, the accuracy of the design rules for stainless steel angle-to-plate connections failing by net section fracture, as set out in the European code and American specification, was assessed. The assessment results revealed that the European code leads to excessively conservative and scattered failure load predictions, while the American specification yields unsafe failure load predictions. Therefore, an improved design approach was proposed and shown to provide substantially improved predictions of failure load over the design codes; the reliability of the proposed design approach was also demonstrated by statistical analyses. Besides, the existing codified and proposed design methods for carbon steel angle-to-plate connections were also assessed for their applicability to stainless steel angle-to-plate connections. It was found that the method specified in the American specification for carbon steel angle-to-plate​ connections can be safely extended to cover the design of their stainless steel counterparts.

Impact of Electrospun Piezoelectric Core-Shell PVDFhfp/PDMS Mesh on Tenogenic and Inflammatory Gene Expression in Human Adipose-Derived Stem Cells: Comparison of Static Cultivation with Uniaxial Cyclic Tensile Stretching.

Specific microenvironments can trigger stem cell tenogenic differentiation, such as specific substrates or dynamic cell cultivation. Electrospun meshes composed by core–shell fibers (random or aligned; PDMS core; piezoelectric PVDFhfp shell) were fabricated by coaxial electrospinning. Elastic modulus and residual strain were assessed. Human ASCs were seeded on such scaffolds either under static conditions for 1 week or with subsequent 10% dynamic stretching for 10,800 cycles (1 Hz, 3 h), assessing load elongation curves in a Bose® bioreactor system. Gene expression for tenogenic expression, extracellular matrix, remodeling, pro-fibrotic and inflammatory marker genes were assessed (PCR). For cell-seeded meshes, the E modulus increased from 14 ± 3.8 MPa to 31 ± 17 MPa within 3 h, which was not observed for cell-free meshes. Random fibers resulted in higher tenogenic commitment than aligned fibers. Dynamic cultivation significantly enhanced pro-inflammatory markers. Compared to ASCs in culture flasks, ASCs on random meshes under static cultivation showed a significant upregulation of Mohawk, Tenascin-C and Tenomodulin. The tenogenic commitment expressed by human ASCs in contact with random PVDFhfp/PDMS paves the way for using this novel highly elastic material as an implant to be wrapped around a lacerated tendon, envisioned as a functional anti-adhesion membrane.

Block tearing of S700 high strength steel bolted connections: Testing, numerical modelling and design

The block tearing behaviour and resistance of S700 high strength steel (HSS) bolted connections in tension have been investigated through tests and numerical simulations and reported in this paper. The experiments were performed on sixteen S700 high strength steel bolted connections, including seven 2-bolt connections (with each comprising two bolts arranged perpendicular to the loading direction) and nine 4-bolt connections (with each comprising four bolts arranged in two rows). All specimens were carefully designed, with a series of geometric parameters, including the end and edge distances as well as the longitudinal and transverse pitches, varied and studied. The experimental setup and procedures, together with the key observed results, including the failure loads, the load–elongation curves and the block tearing failure modes, are fully reported and discussed. The experiments were supplemented by numerical simulations; finite element models were firstly developed to replicate the experimental behaviour and then employed to perform parametric studies to generate further numerical data on S700 high strength steel bolted connections susceptible to block tearing over a wide range of geometric dimensions. On the basis of the test and numerical data, the existing design methods for high strength steel bolted connections susceptible to block tearing, as given in the European code, American Specification and Australian Standard, were evaluated. The evaluation results revealed that all three considered design codes lead to consistent and safe-sided but conservative predictions of the failure loads, due principally to the lack of appropriate consideration of the location and area of the shear failure plane. The proposals of Teh and Uz [1], developed for normal strength steel bolted connections, were then investigated and shown to result in substantial improvements of block tearing resistance predictions, although some of the resistances are unsafe. Finally, the reliability of the proposal of Teh and Uz [1] when used for S700 high strength steel bolted connections was demonstrated by means of statistical analyses.

Modeling tensile behavior of 3D orthogonal woven green composites considering variability of natural fibers

Over the past few decades, ecological concerns resulted in an interest in using natural fibers such as flax and hemp as a reinforcement in composite materials. However, the inherent variability of their mechanical and physical properties complicates predicting the mechanical performance of their preforms and hence composites. This research introduces a novel approach in modeling natural fiber composites. To the best of the author's knowledge, this is the first model that considers the variability of natural fibers' properties used in 3D orthogonal woven composites. Measured properties of fibers, yarns and resin in addition to geometrical parameters were used as inputs while the entire Load-Elongation curve of the composite was the output of the model. This model is generalized for any weave design and can be applied for hybrid structures. The model was verified experimentally using preforms from flax and hemp fibers. The model verification showed that there is a better agreement between the predicted and experimental results when using the properties of yarns as input rather than fibers.

A novel and efficient surgical knotting technique for high-tension closures.

BackgroundThe closure of high-tension incisions without any assistance can be difficult and challenging for surgeons. A common practice is to fix the first knot with a clamp and then tie a reverse locking knot; however, this practice has certain disadvantages. The aim of this study was to introduce a novel and efficient surgical knotting technique with various advantages.MethodsThe two knotting methods used in this study were the absorbable braided suture where the first suture was fixed with a clamp (with assistance) and the SH-9Hospital knotting technique (without assistance) applied on the smooth surface of a cylinder. Mechanical testing was performed using a universal material testing machine. The load-elongation curve and ultimate tensile load (UTL) were recorded.ResultsThe mean knotting time was 36.40±1.50 s (range, 32–41 s) and 24.80±1.16 s (range, 21–28 s) in the clamp and SH-9Hosptial groups, respectively. The mean UTL was 120.8±10.14 N (range, 81.11–136.55 N)and 126.5±6.29 N (range, 104.88–139.56 N) in the clamp and SH-9Hospital groups, respectively. The knot strength of the SH-9Hospital technique was not inferior to traditional clinical practice.ConclusionsThe SH-9Hospital knotting technique was a secure, convenient, and efficient method for high-tension closure.

Hardening model of anisotropic sheet metal during the diffuse instability necking stage of uniaxial tension

In order to determine the stress–strain response in the diffuse necking for low carbon steels, the evolution of the diffuse necking region and the distributions of stress and strain in the narrowest cross-section are analyzed comprehensively based on 3D-DIC and FE simulation. Three parabolic functions are adopted to simplify the diffuse necking deformation: the shape of the necking region is parabolic, the distributions of stress and strain in the narrowest cross-section are also parabolic. A hardening model for solving the uniaxial tensile true stress and strain in the diffuse necking is established. Uniaxial tensile tests of four kinds of low carbon steel sheets and three typical anisotropic directions have been carried out to verify the reliability of the developed model. The numerical simulation results show that the uniaxial tensile load–elongation curves based on the hardening model are in good agreement with the experimental results, with an error of less than 1.3%. The true stress and strain in the diffuse necking can be determined easily by interpreting the tensile curve and measuring the shape of necking region. It avoids the difficulty and time-consuming of the inverse methods. This study can provide a valuable reference for establishing theoretical models to acquire true stress–strain curves over large range of strains for anisotropic sheet metals.

Comparative Study of Long- and Short-Stretch Woven Compression Bandages

Abstract Compression therapy using bandages or socks is the most common treatment for venous leg ulcers and edema. This article aims to compare the compression between long- and short-stretch bandages (LSB and SSB, respectively). Load-elongation curves, cyclic loading, and elastic recovery are investigated for both Cotton/Polyamide/Polyurethane and 100% bleached cotton bandages as LSB and SSB, respectively. Static (resting) and dynamic (working) pressures are measured on seven male legs, 31 ± 3.6 years old, using both two and three layers bandaging. Picopress pressure tests are performed on the ankle and mid-calf positions at gradual decreasing compression from the ankle to the knee. The deviation percentage between the experimental results by Picopress and theoretical calculations using Laplace's law and Al-Khaburi equations is compared. LSB recovered approximately 99% of its original length after stress-relaxation whereas SSB recovered only 93% of its original length after 5 days of cyclic load-relaxation. Moreover, SSB lost approximately 28.6% of its activity after wearing on the human leg for 5 days.

Experimental and numerical study of S700 high strength steel double shear bolted connections in tension

This paper presents a thorough experimental and numerical study of the structural behaviour and resistance of S700 high strength steel double shear bolted connections subjected to tension. A total of 38 S700 high strength steel double shear bolted connections were adopted in the tests, and included seven 1-bolt connections (with each containing one bolt), ten 2-pe-bolt connections (with each containing two bolts arranged perpendicular to the load transfer direction), eight 2-pa-bolt connections (with each containing two bolts arranged parallel to the load transfer direction) and thirteen 4-bolt connections (with each containing four bolts arranged in two rows). The specimens were carefully designed with a series of geometric parameters, including the end distance, the edge distance, the longitudinal spacing and the transverse spacing, varied and examined. The test setup, procedures and key results, including the failure loads and modes as well as the load–elongation curves, were fully reported. Two types of failure modes, namely net section fracture and bearing failure, were observed from the tests, with their underlying mechanisms discussed. The effects of various geometric parameters on the failure loads and modes of the S700 high strength steel double shear bolted connection specimens were studied. It was generally found that the net section fracture behaviour and resistance are sensitive to the edge distance and transverse spacing, while the bearing behaviour and resistance are more dependent on the end distance and longitudinal spacing. The experimental programme was supplemented by a numerical modelling programme, where finite element models were initially developed and validated against the test results, and then used for conducting parametric studies to generate further numerical data over a wide range of geometric dimensions. The experimentally and numerically obtained failure loads and modes were then adopted to assess the accuracy of the relevant design rules for high strength steel double shear bolted connections in tension, as given in the European code, American specification and Australian standard. The results of the assessment revealed that (i) the American specification yields the most accurate failure load predictions on average, but with many over-predicted failure loads and inaccurate failure mode predictions, (ii) the Australian standard leads to less accurate but more consistent and safe predictions of failure load than the American specification, and (iii) the European code results in the most conservative and scattered failure load predictions among the three considered design codes, but with the best failure mode predictions. The development of improved design approaches for S700 high strength steel double shear bolted connections is underway.

A Study on Mechanical Properties of Dissimilar Steels Welded with Electric Arc Welding

Different materials may be used together in order to obtain materials having desired properties. The main objective of this study is to investigate weldability and the mechanical properties of welded AISI 316 and AISI 4140 obtained by the electric arc technique. Mechanical properties were determined by tensile and impact tests. The critical points and Charpy energy value have been determined by means of load-elongation curves and impact test, respectively. The information about mechanical properties has been obtained by determining the hardness values at welding points and points which are closest to them. Microstructure investigations have been carried out using optical microscopy. The obtained results indicate that AISI 316 and AISI 4140 steels will be successfully combined with electric arc welding and used safely in engineering processes.

An SEM compatible plasma cell for in situ studies of hydrogen-material interaction.

An in situ hydrogen (H) plasma charging and in situ observation method was developed to continuously charge materials, while tensile testing them inside a scanning electron microscope (SEM). The present work will introduce and validate the setup and showcase an application allowing high-resolution observation of H-material interactions in a Ni-based alloy, Alloy 718. The effect of charging time and pre-straining was investigated. Fracture surface observation showed the expected ductile microvoid coalescence behavior in the uncharged samples, while the charged ones displayed brittle intergranular and quasi-cleavage failure. With the in situ images, it was possible to monitor the sample deformation and correlate the different crack propagation rates with the load-elongation curves. H-charging reduced the material ductility, while increasing pre-strain decreased hydrogen embrittlement susceptibility due to the possible suppression of mechanical twinning during the tensile test and, therefore, a reduction in H concentration at grain and twin boundaries. All the presented results demonstrated the validity of the method and the possibility of in situ continuously charging of materials with H without presenting any technical risk for the SEM.

Does Manual Drilling Improve the Healing of Bone-Hamstring Tendon Grafts in Anterior Cruciate Ligament Reconstruction? A Histological and Biomechanical Study in a Rabbit Model.

Background:Heat necrosis due to motorized drilling during anterior cruciate ligament (ACL) reconstruction could be a factor in delayed healing at the bone–tendon graft interface.Hypothesis:The process of osteointegration could be enhanced using manual drilling. It reduces the invasiveness of mechanical-thermal stress normally caused by the traditional motorized drill bit.Study Design:Controlled laboratory study.Methods:ACL reconstruction using semitendinosus tendon autografts was performed in 28 skeletally mature female New Zealand white rabbits, which were randomly divided into 3 groups. In group A (n = 12), the tunnels were drilled using a motorized device; in group B (n = 12), the tunnels were drilled using a manual drill bit; and group C (n = 4) served as a control with sham surgical procedures. The healing process in the tunnels was assessed histologically at 2, 4, 8, and 12 weeks and graded according to the Tendon–Bone Tunnel Healing (TBTH) scoring system. In addition, another 25 rabbits were used for biomechanical testing. The structural properties of the femur–ACL graft–tibia complex, from animals sacrificed at 8 weeks postoperatively, were determined using uniaxial tests. Stiffness (N/mm) and ultimate load to failure (N) were determined from the resulting load-elongation curves.Results:The time course investigation showed that manual drilling (group B) had a higher TBTH score and improved mechanical behavior, reflecting better organized collagen fiber continuity at the bone–fibrous tissue interface, better integration between the graft and bone, and early mineralized chondrocyte-like tissue formation at all the time points analyzed with a maximum difference at 4 weeks (TBTH score: 5.4 [group A] vs 12.3 [group B]; P < .001). Stiffness (23.1 ± 8.2 vs 17.8 ± 6.3 N/mm, respectively) and ultimate load to failure (91.8 ± 60.4 vs 55.0 ± 18.0 N, respectively) were significantly enhanced in the specimens treated with manual drilling compared with motorized drilling (P < .05 for both).Conclusion:The use of manual drilling during ACL reconstruction resulted in better tendon-to-bone healing during the crucial early weeks. Manual drilling was able to improve the biological and mechanical properties of bone–hamstring tendon graft healing and was able to restore postoperative graft function more quickly. Tunnel drilling results in bone loss and deficient tendon-bone healing, and heat necrosis after tunnel enlargement may cause mechanical stress, contributing to a delay in healing. Manual drilling preserved the bone stock inside the tunnel, reduced heat necrosis, and offered a better microenvironment for faster healing at the interface.Clinical Relevance:Based on study results, manual drilling could be used successfully in human ACL reconstruction, but further clinical studies are needed. A clinical alternative, called the original “all-inside” technique, has been developed for ACL reconstruction. In this technique, the femoral and tibial tunnels are manually drilled only halfway through the bone for graft fixation, reducing bone loss. Data from this study suggest that hamstring tendon–to–bone healing can be improved using a manual drilling technique to form femoral and tibial tunnels.

Количественная характеристика полей локальных деформаций в образце картона топлайнер при одноосном растяжении

The paper presents the method for quantifying the distribution of local tensile (longitudinal), compressive (transverse) and shear strains in a sample of pulp and paper material. The results of its use for the sample of top-liner board when uniaxial tensile testing are provided. Applying this method, we have determined the values, the degree of heterogeneity and change patterns of local strains in inhomogeneous and anisotropic structure of the sample, depending on the total strain and the direction of fiber orientation. The method is based on constant-speed tensile testing of samples with a pre-applied dot matrix to their surface. The test is accompanied with photo fixing and recording a load-elongation curve. The coordinates and displacements of dots are determined by means of the specially designed software. The analysis of data and calculation of local deformations are performed by finite elements method (FEM) algorithms. The outcomes have shown that the local longitudinal deformations increased, transverse – decreased, and shear – had their maximum at 45° to machine direction (MD) when increasing the angle of preferential fiber orientation to MD in the board samples. The rise of the average absolute values of all components of local strains when increasing total deformation in the sample was confirmed. As it has been found, the heterogeneity of local strains, estimated through the standard deviation, rose when increasing their absolute value; the trends of changing depend on the fiber orientation direction in the sample structure. For citation: Romanova A.N., Kazakov Ya.V., Malkov A.V. Quantitative Characteristics of Local Strain Fields in a Top-Liner Board Sample under Uniaxial Tension. Lesnoy Zhurnal[Russian Forestry Journal], 2020, no. 1, pp. 180–189. DOI: 10.37482/0536-1036-2020-1-180-189

Mechanical properties of low-density paper

Abstract For the mechanical properties of paper, tensile testing has been widely used. Among the tensile properties, the tensile stiffness has been used to determine the softness of low-density paper. The lower tensile stiffness, the greater softness of paper. Because the elastic region may not be clearly defined in a load-elongation curve, it is suggested to use the tensile modulus which is defined as the slope between the two points in the curve. The two points which provide the best correlation with subjective softness evaluation should be selected. Low-density paper has a much lower tensile strength, but much larger elongation at the break. It undergoes a continuous structural change during mechanical testing. The degree of the structural change should depend on tensile conditions such as the sample size, the gauge length, and the rate of elongation. For low-density paper, the tensile modulus and the tensile strength should be independent of each other. The structure efficiency factor (SEF) is defined as a ratio of the tensile strength to the tensile modulus and it may be used a guideline in developing superior low-density paper products.

Experimental and numerical studies on ductile-fracture-controlled ultimate resistance of bars in aluminum alloy gusset joints under monotonic tensile loading

Aluminum alloy gusset (AAG) joints have been applied extensively in single-layer latticed shells. This paper investigates the resistance of the aluminum alloy bar (AL bar) in AAG joints under monotonic tensile loading. The main failure mode is the fracture of the I-shaped AL bar, accompanied by local buckling of the web. The fracture modes of the flanges differ: block tensile–shear fracture in the small plate (SP) group, whereas net-section fracture in the big plate (BP) group. Compared with traditional simulation, the simulation which considers the fracture can yield a load-elongation curve with a declining stage so that the ultimate resistance can be obtained straightforward. This simulation is based on the modified Mohr–Coulomb (MMC) fracture criterion with damage-induced softening, which is achieved by VUHARD and VUSDFLD subroutines implemented into ABAQUS/Explicit. The parameters in this criterion are calibrated on smooth plate specimen tests. The results from the finite element models are in excellent agreement with the experimental results in terms of the web buckling, fracture path, and ultimate resistance governed by the crack initiation and succeeding propagation. This simulation, particularly the fracture model and its detailed calibration procedures, have a broad prospect of applications for evaluating the resistance of aluminum components that are governed by various ductile fracture modes under monotonic loadings, such as the rupture of bolt connections and block tearing of AAG joint plates.