Biaxial Testing Machine Research Articles

Experimental study of the shear failure of granite based on full-field-strain monitoring using digital image correlation

Shear failure is a common failure form of engineering rock mass. As a typical engineering rock mass, granite is widely present in reservoir dams, mine slopes, mine roadways, and other projects in China. Therefore, in this study, the direct shear testing of granite was conducted by taking granite as the research object and using an RLW-3000 shear creep biaxial testing machine. The complete process of shear failure was monitored by using digital image correlation non-contact full-field strain measurement technology, and the spatial evolution of the shear-failure process of granite was assessed by analyzing the main strain nephogram in combination with the main strain–time variation curves. The spatiotemporal evolution of strain during the shear-failure process of granite was studied to reveal the shear-failure mechanism of the material. Results show that the strain nephogram of granite during shear failure could be divided into four stages: uniform strain distribution, strain-localization band generation, strain band expansion, and strain-band expansion acceleration. Strain localization is an important feature of the shear failure of a rock mass; it is characterized by the uneven distribution of strain on the surface of the specimen and the appearance of strain-concentration bands, which indicate the initiation and propagation of macroscopic shear cracks. According to the principal strain–time curve obtained, the principal strain curve exhibits a sharp upward trend when strain localization occurs, and the stress is approximately 76%–79% of the peak stress. The space–time evolution characteristics of the above strain reflect the spatiotemporal variations in the stress field on the surface of the specimen, and the spatial extension of the banded strain-concentration area corresponds to the formation and development of shear failure in the rock. Therefore, the direction of the spatial development of shear microfractures can be predicted by capturing the spatial development of strain-localization bands. The results of this work provide an important theoretical basis and guidance for reservoir dam seepage prevention, highway and mine slope support, and goaf face instability early-warning systems.

Comparison of two techniques (in vivo and ex-vivo) for evaluating the elastic properties of the ascending aorta: Prospective cohort study

IntroductionAneurysms of the ascending aorta (AA) correspond to a dilatation of the ascending aorta that progressively evolves over several years. The main complication of aneurysms of the ascending aorta is type A aortic dissection, which is associated with very high rates of morbidity and mortality. Prophylactic ascending aorta replacement guidelines are currently based on maximal AA diameter. However, this criterion is imperfect. Stretching tests on the aorta carried out ex-vivo make it possible to determine the elastic properties of healthy and aneurysmal aortic fragments (tension test, resistance before rupture). For several years now, cardiac magnetic resonance imaging (MRI) has provided another means of evaluating the elastic properties of the aorta. This imaging technique has the advantage of being non-invasive and of establishing aortic compliance (local measurement of stiffness) without using contrast material by measuring the variation of the aortic surface area during the cardiac cycle, and pulse wave velocity (overall stiffness of the aorta).Materials and methodsProspective single-center study including 100 patients with ascending aortic aneurysm requiring surgery. We will perform preoperative cine-MRI and biomechanical laboratory stretching tests on aortic samples collected during the cardiac procedure. Images will be acquired with a 3T MRI with only four other acquisitions in addition to the conventional protocol. These additional sequences are a Fast Low Angle Shot (FLASH)-type sequence performed during a short breath-hold in the transverse plane at the level of the bifurcation of the pulmonary artery, and phase-contrast sequences that encodes velocity at the same localization, and also in planes perpendicular to the aorta at the levels of the sino-tubular junction and the diaphragm for the descending aorta. For ex-vivo tests, the experiments will be carried out by a biaxial tensile test machine (ElectroForce®). Each specimen will be stretched with 10 times of 10% preconditioning and at a rate of 10 mm/min until rupture. During the experiment, the tissue is treated under a 37°C saline bath. The maximum elastic modulus from each sample will be calculated.ResultsThe aim of this study is to obtain local patient-specific elastic modulus distribution of the ascending aorta from biaxial tensile tests and to assess elastic properties of the aorta using MRI, then to evaluate the correlation between biaxial tests and MRI measurements.DiscussionOur research hypothesis is that there is a correlation between the evaluation of the elastic properties of the aorta from cardiac MRI and from stretching tests performed ex-vivo on aorta samples collected during ascending aorta replacement.

P9. A distraction-based bench top protocol for the evaluation of growing rod concepatients

P9. A distraction-based bench top protocol for the evaluation of growing rod concepatients

Fracture resistance parameters for the compressor disk imitation model

This paper presents a calculation and experimental technique for determining stress intensity factors in an imitation model of a titanium alloy disk. We studied a low-pressure compressor disk of a gas turbine engine (GTE) D-36. During operations, there occur fatigue cracks initiated and developed in the slot fillet under the blade at the place of transition of the bottom to the lateral surface of the inter-groove projection, which lead to a separation of the disk’s part within its rim. The mixed-mode crack growth occured in the compressor disks. Based on the principles of imitation modeling, the geometry and loading condition of the imitation model of the compressor disk was developed. The fatigue test of the imitation model was carried out with a frequency of 5 Hz, at room temperature and with stress ratio Rc = 0.1, by means of a biaxial testing machine. The crack growth was monitored using an optical microscope. The criterion for failure was the condition for reaching a growing crack of the compensation hole. During the test, the positions and sizes of the crack fronts were fixed, which are the basis for the numerical calculation of the fracture resistance parameters. In the order of the numerical studies, six three-dimensional finite element models with different positions and sizes of the crack fronts are considered. The results of the numerical calculations based on the finite element method were used to determine the distributions of elastic and plastic stress intensity factors along each crack front. We demonstrated the advantages of the calculation and experimental methods for solving the problems of interpretation and prediction of the crack growth in the rotating disks of turbomachines using the methods of fracture mechanics.

Biomechanical comparison of ex vivo lumbar vertebral fracture luxations stabilized with tension band or polymethylmethacrylate in cats.

To evaluate spinal stabilization with tension band stabilization (TS) in cats compared to screw and polymethylmethacrylate fixation (SP). Ex vivo study. Sixteen feline thoracolumbar spinal specimens. The intact specimens were mounted in a six-degree-of-freedom biaxial testing machine for nondestructive testing to obtain the neutral zones (NZ) and range of motion (ROM) in flexion and extension. Thereafter, nondestructive testing was consecutively performed after destabilization by disc fenestration and partial L1 corpectomy and after treatment with either TS or SP. Load to failure was compared after surgical treatment in flexion. Significance was assessed by Student's t test or Wilcoxon signed-rank test. Range of motion was 26.4° ± 2.2° in TS constructs and 13.4° ± 2.1° in SP constructs (P = .0005). When flexion and extension were analyzed separately, no difference was found for ROM in flexion (SP, 7.0° ± 3.7°; TS, 8.3° ± 2.1°; P = .38). In extension, the mean displacement was 6.4° ± 2.7° and 18.1° ± 5.1° in SP and TS constructs, respectively (P = .0001). Neutral zone was 2.9° ± 0.6° and 7.5° ± 0.8° for the SP and TS groups, respectively (P = .0003). Screw and polymethylmethacrylate fixation constructs were two times stiffer (P = .045). Tension band stabilization provided stability comparable to SP in flexion. In extension, ROM of SP constructs was half that of TS constructs. The mode of failure of TS was related to the limited dorsal bone stock of feline lumbar vertebrae. Surgeons should be aware of the limited stability in extension provided by TS when it is used to stabilize thoracolumbar spinal injuries. Our results provide evidence to justify additional studies to clarify the type of fractures amenable to TS.

Biomechanical study on ascending aortic aneurysms associated with quadricuspid aortic valve

The quadricuspid aortic valve (QAV) is a rare anatomical situation and the biomechanical properties are not well known when it is associated with ascending aortic aneurysms (AsAA). The objective is to find out what is the biomechanical properties in such situation and to compare it with the existing data. In a sixty-three-years-old female (BMI 26,4) with hypertension disease, QAV, AsAA of 52 mm, an aortic valve and ascending aorta replacement were performed. The aortic wall sample was collected within 30 mins after replacement, partitioned related to medial, posterior, lateral, and anterior quadrants. The sample was cut in square size (15 mm × 15 mm, n = 13) with marking the circumferential and longitudinal directions. An average thickness was measured, using an electronic micrometer (Litematic VL-50, Mitutoyo®). The experiments were carried out by a biaxial tensile test machine (ElectroForce®). Each specimen was stretched at a rate of 10 mm/min until rupture. Maximum Elastic Modulus was calculated for each specimen. The CFD simulation was carried out by ANSYS® R1 2019, with the input of inlet measured in MRI sequence. The longitudinal direction of the aorta is stiffer than the circumferential direction except in the medial quadrant in both tissue types. The lateral quadrant of the aorta shows a higher stiffness (0.672 MPa), the double value of the medial quadrant (0.308 MPa). This result matches our previous study of AsAA on the bicuspid aortic valve. In the CFD simulation, it shows a higher velocity in the laterial quadrant of the acsending aorta near the aortic arch ( Fig. 1 ). In this study, the aortic biomechanical properties are similar regardless of quadricuspid or bicuspid aortic valve.

Optimization Design of a Small-Sized Cruciform Specimen for Biaxial Fatigue Testing

The in-plane biaxial specimen can well reflect the complex stress state of sheet metal. However, there is no standard for small-sized specimens used in the low-power biaxial fatigue testing machine. The main goal of this paper is to apply the finite element method and orthogonal experiment method to design the cruciform specimen, considering the influence of three main parameters including the diameter of the central semispherical thinning area, the minimum thickness of the center and the arm thickness of the specimen. According to the central strain dispersion coefficient and the strain concentration coefficient proposed in this paper, we ensured that the distribution of strain in the gauge area is uniform and the strain it at its maximum value at the same time. The optimized specimen is verified by a biaxial fatigue test with the digital image correlation (DIC) technique. It is found that the fatigue crack appears in the center region, which proves that the optimized specimen can be effectively used for biaxial fatigue test.

Biaxial Testing Machine: Development and Evaluation

Biaxial mechanical testing gained increased importance for characterization of materials that present anisotropic behavior and/or different responses when subjected to tensile and compression loadings. In this work, a new biaxial testing machine was developed. The various systems and components were designed, manufactured, assembled, and assessed. Uniaxial tensile tests were performed to validate the device, showing results consistent with those obtained on a universal testing machine. Finally, biaxial tensile tests were also performed on polypropylene cruciform specimens. The results revealed high precision levels, thus showing the potential of this new machine.

Osteosynthesis of Phalangeal Fractures: Biomechanical Comparison of Kirschner Wires, Plates, and Compression Screws

The aim of this study was to compare several osteosynthesis techniques (intramedullary headless compression screws, T-plates, and Kirschner wires) for distal epiphyseal fractures of proximal phalanges in a human cadaveric model. A total of 90 proximal phalanges from 30 specimens (index, ring, and middle fingers) were used for this study. After stripping off all soft tissue, a transverse distal epiphyseal fracture was simulated at the proximal phalanx. The 30 specimens were randomly assigned to 1 fixation technique (30 per technique), either a 3.0-mm intramedullary headless compression screw, locking plate fixation with a 2.0-mm T-plate, or 2 oblique 1.0-mm Kirschner wires. Displacement analysis (bending, distraction, and torsion) was performed using optical tracking of an applied random speckle pattern after osteosynthesis. Biomechanical testing was performed with increasing cyclic loading and with cyclic load to failure using a biaxial torsion-tension testing machine. Cannulated intramedullary compression screws showed significantly less displacement at the fracture site in torsional testing. Furthermore, screws were significantly more stable in bending testing. Kirschner wires were significantly less stable than plating or screw fixation in any cyclic load to failure test setup. Intramedullary compression screws are a highly stable alternative in the treatment of transverse distal epiphyseal phalangeal fractures. Kirschner wires seem to be inferior regarding displacement properties and primary stability. Fracture fixation of phalangeal fractures using plate osteosynthesis may have the advantage of a very rigid reduction, but disadvantages such as stiffness owing to the more invasive surgical approach and soft tissue irritation should be taken into account. Headless compression screws represent a minimally invasive choice for fixation with good biomechanical properties.

Stress intensity factors for mixed-mode crack growth in imitation models under biaxial loading

In this study a procedure to stress intensity factors calculation for imitation models made of titanium alloy is described. In service fatigue cracks are detected in a disk and blade “dovetail type” attachment. On the base of this attachment dimensions and with taking into account the biaxial loading conditions of rotating compressor disk the two geometries of imitation model of gas turbine engine compressor disk are developed. To accurate verification of biaxial loading conditions, the first imitation model of constant thickness is used. In order to fully reproduce the geometry of the compressor disk and conditions of mixed mode crack growth, the second imitation model with reduced cross section is proposed. The fatigue crack growth experiments of imitation models were carried out at room temperature on a biaxial testing machine. Two different stress ratio values are applied several times to each imitation model in order to fix the experimental crack front positions. The elastic and plastic stress intensity factors used for the representation of the experimental results are computed by using full-size 3D Finite Element analysis of the imitation models with surface quarter elliptical and through-thickness cracks.

Development of miniature cruciform specimen and testing machine for multiaxial creep investigation

High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to extreme temperature conditions and their complex shapes. A multiaxial creep investigation is required for the safety of high temperature components in the design phase of the manufacture of such a component. However, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. A miniature cruciform specimen having a plane stress condition gauge section that is 5 mm square was designed using finite element (FE) analysis. Further, a biaxial testing machine capable of tensile loading was designed to conduct multiaxial creep tests. The testing machine had a 2 kN loading capacity and a 1 kW furnace. We also developed a non-contact displacement measuring method for the miniature specimen, which had an excessively small gauge section to permit the attachment of a mechanical extensometer. The proposed method uses a conventional optical camera to capture some images of the surface. We obtained the displacement value of the specimen, by tracking the trace of the target marks painted on the surface of the specimen. The measured strain value that was obtained from the non-contact displacement measuring method corresponds to the strain obtained by the mechanical method at room and high temperatures. By using the developed multiaxial creep testing machine and the non-contact observation system, we can investigate not only the deformation of the testing specimen, but also the surface conditions of materials during the creep test.

Application and practical validation of topology optimization technology for the frame of biaxial tensile testing machine

Biaxial tensile testing machines are used to improve accuracy of yield loci and constitutive model for sheet materials. The frame of these machines is the main load-bearing structure significantly affecting the machine accuracy. However, the available loading frames are too heavy to be practical for popularization and application. To reduce both the total weight of the frame and install consumption, this study develops a design method of stepwise multi-stage topology optimization based on the solid isotropic microstructures with penalization (SIMP) method. First, the validity of the finite element model during the optimization is verified by experiments. Next, the design method of stepwise multi-stage topology optimization is proposed based on the results of conventional topology optimization and the influence law of key parameters. This method is then applied to the topological optimization design of frames for a biaxial tensile testing machine. Finally, the optimized frame is manufactured, and biaxial tensile tests are conducted on the designed machine. The measurement of frame deformations shows that the frame weight is decreased by 19.5% compared with that of the initial design and that the frame stiffness meets design requirements. These results prove the effectiveness of the proposed method.

Experimental and computational study on mechanical behaviour of carpentry corner log joints

This work concerns experimental and numerical research on carpentry joints used in historic wooden buildings in southeastern Poland and western Ukraine. These structures are mainly sacred buildings, and the types of corner log joints characteristic of this region are primarily saddle-notch and dovetail joints; thus, these two types of joints were analysed in this study.The modelling of historic timber structures is a complex issue, so the following steps are necessary to obtain accurate solutions: verification, validation and uncertainty quantification. The first and third steps were performed in a previous study, so the current research aimed to validate the numerical models and perform simulations of carpentry joints. Herein, the authors created finite element models of two types of joints and subsequently analysed the mechanical behaviour of these joints.Due to issues concerning model validation, the authors designed a testing stand for corner joints, which formed a part of a biaxial testing machine. The joints were subjected to horizontal loads (deformations), which may cause damage to the connection. Thus, special parts were designed for the stand, which made it possible to fix the joint and prevented eccentric forces that could possibly damage the machine during testing.The authors presented the differences and similarities in the behaviour of both types of joints, emphasizing the corresponding advantages and disadvantages. In addition, the authors determined which type of joint was the most susceptible to damage and what elements failed first.This paper also showed the complexity of modelling timber structures and the accuracy of the proposed numerical models for both types of joints through comparisons of the numerical and experimental results. This work primary addressed the problems in accurately reflecting material, load and boundary conditions in numerical modelling of tested carpentry corner log joints.

Local ex-vivo evaluation of the biomechanical properties of the ascending aortic aneurysms

Currently, surgical recommendations for aneurysm of the ascending aorta (AsAA) are based on the maximum diameter of the ascending aorta, but this factor is not reliable. Understanding the biomechanical properties of the aorta could lead to improve the prediction of the development of an AsAA. The aim of this study is to obtain the local patient specific elastic modulus distribution of the AsAA from a biaxial tensile test. Pathologic ascending aortic tissue samples ( n = 10) were obtained from patients undergoing elective surgical repair of AsAA ( Table 1 ). All the aortic wall samples were partitioned related to medial, posterior, lateral, and anterior quadrants. Each AsAA sample was cut in square size (15 mm × 15 mm, n = 89) with marking the circumferential and longitudinal directions. An average thickness was measured, using an electronic micrometer (Litematic VL-50, Mitutoyo®). The experiments were carried out by a biaxial tensile test machine (ElectroForce®). Each specimen was stretched at a rate of 10 mm/min until rupture. Maximum Elastic Modulus was calculated for each specimen. The longitudinal direction of the aorta is stiffer than the circumferential direction except in the medial quadrant in both tissue types. The lateral quadrant (outer curvature) of the aorta shows a higher stiffness, roughly the double value of the medial quadrant (inner curvature) ( Fig. 1 ). The thickness of TAV and BAV tissues did not show a significant difference in the four regions of the AsAA wall. Among the ten AsAA tissues, there are variations in the tissue stiffness according to their quadrants and the direction. It is expected to observe the regional variability in the thickness of the dilated samples corresponding the change of mechanical properties caused by valve disease. However, there was no significant correlation between the regional thickness and mechanical properties, neither TAV ( r =−0.3584) nor BAV ( r =−0.3107).

Experimental and numerical FEM-based determinations of forming limit diagrams of St14 mild steel based on Marciniak-Kuczynski model

Forming Limit Diagrams (FLD) are used to design metal sheet processes and to choose appropriate material according to their formability. Development of simple and robust methods to determine the formability domain is then a major industrial issue. Marciniak-Kuczynski (M-K) plastic instability criterion is a classical and common approach used to predict numerical FLD. A convergence of the experimental conditions and theoretical M-K hypotheses is investigated. This analysis leads in the proposition of a simple experimental procedure based on classical instrumented biaxial testing machines to reproduce M-K in-plane loading conditions. Experimental results are compared with numerical simulations based on a finite element method implantation of M-K model. This versatile implementation allows the coupling of the plastic instability criterion in ABAQUS with a large range of material behaviors. Application of these experimental and numerical procedures on commercial St14 mild steel leads to the relative differences between lower experimental points and numerical forming limits predictions that do not exceed 4% and 10% in the case of uniaxial and equibiaxial tension for initial prescribed defect ratio of 0.99 and 0.95, and 22% in the case of plane tension. These procedures constitute ways easily implementable in the industry to obtain FLD at reduced costs that show good correlations between experimental results and numerical M-K predictions.

A new type of flat-bar specimen for non-proportional mixed-mode crack tip loading under uniaxial tension

Non-proportional cyclic loading is a challenging issue for numerical crack path predictions. Principal stress axes and the vector of the J-integral rotate during loading and the mixed-mode ratio of stress intensity factors changes within a load period. Reliable criteria for crack deflection, accounting for arbitrary loading and boundary conditions under non-proportional loading, have not yet been found. Experimental research in this field nowadays requires expensive set-ups and multiaxial testing equipment such as hydraulic biaxial or tension-torsion testing machines. In this paper, a specimen is presented, suitable for uniaxial testing, however providing conditions of non-proportional loading. Finite element simulations show that the variance of mixed-mode parameters is sufficiently large to cover the range of KII/KI-ratios required for investigating crack deflection criteria.

Characterization and modelling the mechanical behaviour of poly(l-lactic acid) for the manufacture of bioresorbable vascular scaffolds by stretch blow moulding

Bioresorbable Vascular Scaffolds (BVS) manufactured from poly (l-lactic acid) (PLLA) offer an alternative to metal scaffolds for the treatment of coronary heart disease. One of the key steps in the manufacture of these scaffolds is the stretch blow moulding process where the PLLA is biaxially stretched above glass transition temperature (Tg), inducing biaxial orientation and thus increasing ductility, strength and stiffness. To optimise the manufacture and performance of these scaffolds it is important to understand the influence of temperature and strain rate on the constitutive behaviour of PLLA in the blow moulding process. Experiments have been performed on samples of PLLA on a custom built biaxial stretch testing machine to replicate conditions typically experienced during blow moulding i.e. in a temperature range from 70 °C to 100 °C and at strain rates of 1 s−1, 4 s−1 and 16 s−1 respectively. The data is subsequently used to calibrate a nonlinear viscoelastic material model to represent the deformation behaviour of PLLA in the blow moulding process. The results highlight the significance of temperature and strain rate on the yielding and strain hardening behaviour of PLLA and the ability of the selected model to capture it.

A Review of Cruciform Biaxial Tensile Testing of Sheet Metals

Biaxial testing technology enables the comprehensive study of sheet-forming performance and has great potential developmental and commercial value. This work is a review of cruciform biaxial tensile testing technology, as used in actual production, and provides evidence for sheet metal forming processes to describe problems that remain to be solved and to suggest new ideas for future study. The testing apparatus including cruciform biaxial tensile testing machines, heating devices, and optical strain measurement systems are discussed. Some typical descriptions of material tests to achieve stress and strain curves, the yield loci, and limit strains relating to both ambient and high temperatures are also presented. Moreover, finite element modeling as applied to specimen design, material deformation prediction, and material model determination is investigated. Furthermore, the sample design methods and a few examples are summarized in spite of the lack of geometry standard. Future prospects are mainly focused on the establishment of specimen geometry standard, reverse modeling technique for high temperature tests, and the actual applications of biaxial tensile tests.

Development of a biaxial tensile testing machine for pulsed neutron experiments

A biaxial tensile testing method has been used to get macroscopic information on elastoplastic deformation of a thin steel specimen and improve the accuracy of plastic processing of steel materials. We newly developed a biaxial tensile testing machine for pulsed neutron experiments (BTM-NEU) to provide the microscopic crystallographic information of steel materials under biaxial load and correlate it with the macroscopic mechanical properties of the materials. The performance of the BTM-NEU was experimentally evaluated with cold-rolled mild steel and hot-rolled high-tensile-strength steel materials and compared with that of a standard biaxial tensile testing machine (BTM-std) as follows.•The BTM-NEU can test an ISO-standardized cruciform specimen as the BTM-std and its performance is equivalent to that of the BTM-std.•The BTM-NEU has excellent long-time reliability and stability necessary for pulsed neutron experiments, especially Bragg-edge neutron imaging experiments.•The BTM-NEU can be applied to pulsed neutron experiments using a Bragg-edge imaging method.

Development of Multiaxial Creep Testing Machine for Miniature Specimen

High temperature components such as boiler tube and jet engine turbine blade undergo multiaxial creep damage. Although multiaxial creep testing investigation is required for safety high temperature components designing, there are few commercial testing machines which can conduct multiaxial loading at high temperature. A new miniature cruciform specimen, which has a 5 mm square size plane stress gauge part, was designed by using FE analysis. A biaxial tensile loading creep testing machine was also designed to conduct multiaxial creep testing. The testing machine has 2 kN loading capacity and 1 kW furnace. We had also developed a non-contact displacement measuring method for the miniature specimen. This method uses conventional optical camera to get the surface observation photograph. Chasing the trace of the target mark painted at surface of the specimen, we can get the displacement of the specimen. The calculated strain value obtained from the non-contact displacement measuring method corresponds with the strain gauge value at room temperature. By using the multiaxial creep testing machine and the non-contact observation system, we can investigate not only deformation of the testing specimen but also surface conditions of materials during the creep testing.