Biaxial Servohydraulic Testing Machine Research Articles

Development of a novel testing device for fabric reinforced carbon fibre composites under cyclic biaxial load applications

In this paper, a novel testing device has been developed and adopted to a planar- biaxial servo hydraulic testing machine to examine the multiaxial fatigue behaviour of woven CFRP under superimposed interlaminar cyclic shear and static or cyclic through-thickness compression stresses.

Pullout failure strength of the posterior horn of the medial meniscus with root ligament tear

To evaluate the reparability of the posterior horn of the medial meniscus with root ligament tear by measuring the actual pullout failure strength of a simple vertical suture of an arthroscopic subtotal meniscectomized posterior horn of the medial meniscus. From November 2009 to May 2010, nine posterior horns of the medial meniscus specimens were collected from arthroscopic subtotal meniscectomy performed as a treatment for root ligament rupture of the posterior horn of the medial meniscus. Simple vertical sutures were performed on the specimens, and pullout failure load was tested with a biaxial servohydraulic testing machine (Model 8874; Instron Corp., Norwood, MA, USA). The degree of degeneration, extrusion, and medial displacement of the medial meniscus were evaluated with magnetic resonance imaging (MRI). The Kellgren-Lawrence classification was used in standing plain radiography, and mechanical alignment was measured using orthoroentgenography. Tear morphology was classified into ligament proper type or meniscoligamentous junctional type according to the site of the torn root ligament of the posterior horn of the medial meniscus during arthroscopy. The mean pullout failure strength of the posterior horn of the medial meniscus was 71.6 ± 23.2 N (range, 41.4-107.7 N). The degree of degeneration of the posterior horn of the medial meniscus on MRI showed statistically significant correlation with pullout failure strength and Kellgren-Lawrence classification. Pullout failure strength showed correlation with mechanical alignment and Kellgren-Lawrence classification (P < 0.05). The measurement of pullout failure strength of the posterior horn of the medial meniscus with root ligament tear showed a degree of repairability. The degree of degeneration of the posterior horn of the medial meniscus on MRI showed a significant correlation with the pullout failure strength. The pullout failure strength was also not only correlated with the degree of degeneration of the posterior horn of the medial meniscus, but also with mechanical alignment and Kellgren-Lawrence classification, which represent bony degenerative change.

A device for performing whole bone torsional testing in a single-axis linear motion testing machine

To design a device for the conversion of linear to rotational motion and to use it in order to determine torsional properties of canine tibiae in a universal tension/compression testing machine. A fixture incorporating a cable and pulley at one end and a guide block and rail assembly at the other was used to test 10 whole canine tibiae to failure in torsion at 1 degrees/s. The device produced spiral fractures in all tibiae. Torsional strength and stiffness were 13.9 +/- 1.4 N m and 0.53 +/- 0.07 N m/degree, respectively. This inexpensive device can be used to evaluate the mechanical properties of long bones after various interventions to improve fracture healing. The device can be adapted for use with any single-axis linear motion testing machine. Assessment of fracture healing often includes loading to failure in torsion (1-5). Torsional testing is typically performed using a biaxial servohydraulic testing machine or similar sophisticated electromechanical equipment. Due to its high cost, many researchers do not have such equipment at their disposal. However, uniaxial tension-compression testing machines are widely available. Therefore, a device that can be used to perform torsional testing in a tension-compression machine would be beneficial. In this study we present a custom fixture for converting the vertical linear motion of a universal testing machine to rotational motion and validate its performance in torsional testing of canine tibiae.

Numerical and Experimental Investigations of Curved Fatigue Crack Growth under Biaxial Proportional Cyclic Loading

The paper presents numerical and experimental results of continued studies of curved fatigue crack growth in arbitrarily pre-cracked isotropic sheets under biaxial proportional plane loading. The predictor-corrector method (PCM) was extended in order to analyse the growth of multiple crack systems. As a result, the program PCCS-2D was written to run within ANSYS without any user interaction. In order to check the accuracy and efficiency of the method biaxial crack growth simulations were carried out for a fracture mechanics cruciform specimen. The results are compared with experimental findings obtained by specimens made of 6061 aluminium alloy in T651 condition using a 250kN biaxial servohydraulic testing machine. From the numerical and experimental results, we conclude, that the proposed predictor-corrector method can be used in curved crack growth simulation also under biaxial proportional loading conditions.

Torsional stability of intramedullary compression nails: Tibial osteotomy model

Studies comparing intramedullary compression nailing to conventional dynamized intramedullary nailing contend that better clinical outcomes of intramedullary compression techniques result from greater rotational stability of fracture or osteotomy sites. However, there appears to be no experimental evidence that rotational stability is improved with intramedullary nail compression. This study evaluated the effect of intramedullary nail compression on cadaveric and composite tibial fragment rotation above and below a mid-tibial transverse osteotomy. Twelve composite and four matched pairs of human cadaveric tibiae were randomized into either a compression group or a non-compressed dynamized group. A bi-axial servo-hydraulic testing machine applied 5Nm of internal and external torque (2Nm/s) under constant axial loads of 375N and 750N. Rotation along each tibia and at the osteotomy site was recorded from the trajectories of infrared emitting diodes rigidly attached along the tibial shaft. In comparison to dynamized nails, intramedullary nail compression significantly reduced the rotation at the osteotomy site in both human (8.9 degrees reduction; P=0.007) and composite tibiae (2.5 degrees reduction; P=0.039) under 750N of axial load. A similar result was noted for simulated half-body weight loads (375N) for both composite (3.0 degrees reduction; P=0.009) and human tibiae (10.5 degrees reduction; P=0.003). Intramedullary nail compression more effectively reduced tibial fragment rotation about a mid-tibial osteotomy than conventional dynamized intramedullary nails because intramedullary nails created more osteotomy site compression than the application of body weight to tibiae instrumented with dynamized intramedullary nails.

Comparative Study of the Additional Hardening Effects of Three Structural Steels

For a safe and reliable design of components, it is needed to study the effects of multiaxial loading and particularly the non-proportional loadings on the fatigue damage. The objective of this paper is to evaluate and compare the additional hardening effects of proportional and non-proportional loading paths. Low-cycle fatigue behaviour of three structural steels: CK45 (ferritic-perlitic microstructure) normalized steel, 42CrMo4 (bainitic microstructure) quenched and tempered steel and stainless steel (austenitic microstructure) X10CrNiS 18 9 are studied under different proportional and non-proportional loading paths and different levels. A series of tests of biaxial low-cycle fatigue composed of tension/compression with static or cyclic torsion were carried out on a biaxial servo-hydraulic testing machine Instron 8088. The experiments showed that the three materials studied have very different additional hardening behaviour, under multiaxial cyclic loading paths. The local cyclic stress/strain states are influenced by the multiaxial loading paths due to interactions between the normal stress and shear stress during cyclic plastic deformation. The microstructure is an important key and has a great influence on the additional hardening. The additional hardening effect is dependent of the loading path and also the intensity of the loading.

Simulation of cyclic stress/strain evolutions for multiaxial fatigue life prediction

This study deals with simulation for cyclic stress/strain evolutions and redistributions, and evaluation of fatigue parameters suitable for estimating fatigue lives under multiaxial loadings. The local cyclic elastic–plastic stress–strain responses were analyzed using the incremental plasticity procedures of ABAQUS finite element code for both smooth and notched specimens made of three materials: a medium carbon steel in the normalized condition, an alloy steel quenched and tempered and a stainless steel, respectively. Emphasis is on the studying of ‘intelligent’ material behaviors to resist fracture, such as stress redistribution and relaxation through plastic deformations, etc. For experimental verifications, a series of tests of biaxial low cycle fatigue composed of tension/compression with static and cyclic torsion were carried out on a biaxial servo-hydraulic testing machine (Instron 8800). Different multiaxial loading paths were used to verify their effects on the additional cyclic hardening. The comparisons between numerical simulations and experimental observations show that the FEM simulations allow better understanding on the evolutions of the local cyclic stress–strain and it is shown that strong interactions exist between the most stressed material element and its neighboring material elements in the plastic deformations and stress redistributions. Based on the local cyclic elastic–plastic stress–strain responses, the energy-based multiaxial fatigue damage parameters are applied to correlating the experimentally obtained lives. Improved correlations between the predicted and the experimental results are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elasto-plastic stress/strain responses.

Biaxial fatigue for proportional and non‐proportional loading paths

ABSTRACTIn order to study the use of a local approach to predict crack‐initiation life on notches in mechanical components under multiaxial fatigue conditions, the study of the local cyclic elasto‐plastic behaviour and the selection of an appropriate multiaxial fatigue model are essential steps in fatigue‐life prediction. The evolution of stress–strain fields from the initial state to the stabilized state depends on the material type, loading amplitude and loading paths. A series of biaxial tension–compression tests with static or cyclic torsion were carried out on a biaxial servo‐hydraulic testing machine. Specimens were made of an alloy steel 42CrMo4 quenched and tempered. The shear stress relaxations of the cyclic tension–compression with a steady torsion angle were observed for various loading levels. Finite element analyses were used to simulate the cyclic behaviour and good agreement was found. Based on the local stabilized cyclic elastic–plastic stress–strain responses, the strain‐based multiaxial fatigue damage parameters were applied and correlated with the experimentally obtained lives. As a comparison, a stress‐invariant‐based approach with the minimum circumscribed ellipse (MCE) approach for evaluating the effective shear stress amplitude was also applied for fatigue life prediction. The comparison showed that both the equivalent strain range and the stress‐invariant parameter with non‐proportional factors correlated well with the experimental results obtained in this study.

A biomechanical evaluation of cortical onlay allograft struts in the treatment of periprosthetic femoral fractures

Periprosthetic femoral fractures are increasingly addressed through the use of cortical onlay allografting. This study was designed to determine the effect of allograft cortical strut length, configuration, cable number, cable tension and the use of wire or cable on the fixation of periprosthetic femoral fractures. Ten cadaveric femora-strut constructs were tested using anteroposterior and axial loads to simulate the forces at the hip during gait. A transverse fracture at the level of the tip of the femoral stem was simulated. A biaxial servohydraulic testing machine was used to apply one hundred cycles of craniocau-dal load of 1.53 × bodyweight at a frequency of one Hz, along with an anteroposterior load of 0.15 × bodyweight at one half Hz. Variables for different constructs included the strut length (twelve cm, sixteen cm, or twenty cm), the number of cables (two, three, or four above and below the fracture site), cable tension, strut configurations and orientation (single strut or two struts, adjacent or opposite), and the use of wires instead of cables. Cable tension was measured using a calibrated tensioner. Movement at the fracture site was measured using a precision optoelectronic camera system. There was significantly less motion when cables were used rather than wires (p&lt;0.05). Increasing the number of cables decreased fracture motion in some directions (p&lt;0.05) and increasing cable tension showed a trend towards decreased fracture motion. We observed strut fractures in four cases when a single strut alone was used to stabilise the fracture. There was a significant decrease in fracture motion if two struts were used rather than one (p&lt;0.01), but there was no significant difference between the anterior and lateral, and the medial and lateral strut configurations. Decreasing the strut length from twenty cm to twelve cm led to a significant decrease in axial rotation (p&lt;0.05). Our data strongly favour the use of two struts, rather than a single strut alone. Cables enhance fracture stability compared to wires, presumably due to increased tension and to different surface characteristics. Increasing the cable tension gave greater stability although this may not fully translate to the clinical situation because the cable may garrotte or fracture the strut. Increased cable number and decreased strut length also enhanced fracture stability. The cortical struts essentially represent biological bone plates. If appropriately selected and prepared they can be customised to fit any femur. Our improved understanding of this technique should contribute to high rates of fracture union with an increase in bone stock and overall bone strength.