Servo-hydraulic Testing Machine Research Articles

Dynamic intralaminar fracture toughness characterisation of unidirectional carbon fibre-reinforced polymer composites using a high-speed servo-hydraulic test set-up

This paper presents an experimental investigation on the intralaminar fibre-dominated fracture of unidirectional (UD) IM7/8552 carbon-epoxy composites, loaded under quasi-static (QS) and intermediate strain rates (IR). Dynamic tensile testing is carried out using a previously proposed test methodology involving a high-speed servo-hydraulic test machine, equipped with a slack adapter, and Digital Image Correlation (DIC), with the aim of providing an alternative means to Split-Hopkinson pressure bar testing for the characterisation of similar composite materials at intermediate-to-high strain rates. Three sizes of Double-Edge Notched Tension (DENT) specimens are tested at QS and IR to obtain the mode I intralaminar fracture toughness. The results of the fracture toughness tests compare well with literature data, supporting the proposed methodology. This study is a first step towards standardising a more easily accessible test method for characterising the mechanical properties of fibre-reinforced polymer (FRP) composites over a broad range of dynamic strain rates.

Experimental Analysis of Matrix Cracking in Glass Fiber Reinforced Composite Off-Axis Plies under Static and Fatigue Loading.

The inter-fiber failure of glass fiber-reinforced epoxy specimens with four different fiber angles was analyzed. Flat specimens were subjected to static and fatigue loading considering different load levels and load ratios. Damage investigation in terms of crack density measurement was performed by transmitted white light imaging using a digital camera and LED illumination from the back of the specimen on a servo-hydraulic testing machine. Static and fatigue results were examined with respect to crack initiation and crack growth, considering the special case of bonding yarns parallel to the fiber directions. The bonding yarns act as stress concentrations, influencing the early cracking behavior, and complicate the detectability of cracks growing underneath or next to the bonding yarns. In cyclic loading, the influence of load level, load ratio, mean stress, fiber orientation, and ply thickness was the focus of the experimental campaign. Cyclic cracking behavior in terms of initiation and growth was analyzed based on the applied loading conditions and laminate configurations. It was found that halving the ply thickness nearly doubled the amount of microcracks in case of high loads. For low loads, no such effect was observed up to loading cycles. Experimental findings on individual crack growth confirmed that crack interaction started for crack spacings less than four times the ply thickness and that subsequent crack growth shifted into regions of larger local crack spacing.

A study on the fatigue crack growth behaviour of GTM718 nickel based super alloy under cold-TURBISTAN spectrum loads

In this study, the fatigue crack growth (FCG) behavior of GTM718 nickel base super alloy subjected to cold-TURBISTAN spectrum load sequence was determined by three different methods i.e., (i) Experimental, (ii) Analytical prediction using cycle-by-cycle method and, (iii) Prediction by finite element analysis (FEA). In the experimental method, a standard compact tension (CT) specimen was prepared and pre-cracked to produce a sharp fatigue crack at the notch root. Then, the specimen was subjected to spectrum load blocks repeatedly and crack growth was measured as a function of applied number of flights. Two such tests were carried out in a servo-hydraulic universal test machine with triangular waveform and at an average frequency of 2 Hz. In the second analytical method, the individual load cycles in cold-TURBISTAN spectrum block were separated through rain flow cycle counting method. For each of this counted cycle, the crack extension was determined using crack growth law which was based on two-parameter crack driving force, ΔK*. An in-house MATLAB code was written and used to determine the FCG behavior by cycle-by-cycle method. In the third FEA method, a global model of CT specimen of GTM718 was created in HYPERMESH and a local model with a pre-crack was created in FRANC3D. The FCG behavior under spectrum load was determined using FRANC3D in conjunction with MSC NASTRAN. Two different types of ‘FCG law - stress ratio effect’ combinations were employed i e., Paris’-Walker (P-W) and Bi-linear-Walker (B-W). The required constants for these laws were derived from the available experimental data of the material. It was observed that the predicted FCG behavior by analytical and FEA methods, although conservative, were fairly good when compared to experimental results.

Development of a Novel Passive-Dynamic Custom AFO for Drop-Foot Patients: Design Principles, Manufacturing Technique, Mechanical Properties Characterization and Functional Evaluation

Ankle foot orthoses (AFOs) are medical devices prescribed to support the foot and ankle of drop-foot patients. Passive-dynamic AFOs (PD-AFOs) are an effective solution for less severe cases. While off-the-shelf PD-AFOs are rather inexpensive, they provide poor anatomical fit and do not account for the required patient-specific biomechanical support. Three-dimensional (3D) scanning and manufacturing technologies allow manufacturing PD-AFOs customized for the patient’s anatomy and functional needs. This paper aimed to report the overall procedure for designing and manufacturing a novel, fiberglass-reinforced polyamide, custom PD-AFO. The feasibility of the proposed procedure was tested in a case study. The methodology can be divided into the following steps: (i) foot and leg scanning, (ii) 3D design, and (iii) additive manufacturing via selective laser sintering. A custom PD-AFO was designed and manufactured for a 67-year-old male drop-foot patient following paraparesis in severe discarthrosis after spine stabilization surgery. AFO mechanical properties were measured via an ad hoc setup based on a servohydraulic testing machine. The functional outcome was assessed via gait analysis in three conditions: shod (no AFO), wearing an off-the-shelf PD-AFO, and wearing the patient-specific PD-AFO. As expected, wearing the PD-AFO resulted in increased ankle dorsiflexion in the swing phase with respect to the shod condition. Sagittal rotations of the hip, knee, and ankle joints were similar across PD-AFO conditions, but the custom PD-AFO resulted in faster walking speed with respect to the off-the-shelf (walking speed: 0.91 m/s versus 0.85 m/s). Additionally, the patient scored the custom PD-AFO as more comfortable (VAS score: 9.7 vs. 7.3). While the present analysis should be extended to a larger cohort of drop-foot patients, the novel PD-AFO seems to offer a valid, custom solution for drop-foot patients not satisfied with standard orthotics.

Unified validation of a refined second-generation HR-pQCT based homogenized finite element method to predict strength of the distal segments in radius and tibia

IntroductionHR-pQCT based micro finite element (μFE) analyses are considered as “gold standard” for virtual biomechanical analyses of peripheral bone sites such as the distal segment of radius and tibia. An attractive alternative for clinical use is a homogenized finite element method (hFE) based on constitutive models, because of its much shorter evaluation times and modest computational resource requirements. Such hFE models have been experimentally validated for the distal segment of the radius, but neither for the distal segments of the tibia nor for both measurement sites together. Accordingly, the aim of the present study was to refine and experimentally validate an hFE processing pipeline for in vivo prediction of bone strength and stiffness at the distal segments of the radius and the tibia, using only one unified set of material properties. Material and methodsAn existing hFE analysis procedure was refined in several aspects: 1) to include a faster evaluation of material orientation based on the mean surface length (MSL) method, 2) to distinguish cortical and trabecular bone compartments with distinct material properties and 3) to directly superimpose material properties in mixed phase elements instead of densities. Based on an existing dataset of the distal segment of fresh-frozen radii (double sections 20.4 mm, n = 21) and a newly established dataset of the distal segment of fresh-frozen tibiae (triple sections, 30.6 mm, n = 25), a single set of material properties was calibrated on the radius dataset and validated on the tibia dataset by comparing hFE stiffness and ultimate load with respective experimental results, obtained by compressing the samples on a servo-hydraulic testing machine at a monotonic and quasi-static displacement rate up to failure. ResultsUsing the identified set of material properties, the hFE-predicted stiffness and failure load were in excellent agreement with respective experimental results at both measurement sites (radius stiffness R2 = 0.93, slope = 1.00, intercept = 479 N/mm2/radius ultimate load: R2 = 0.97, slope = 1.00, intercept = 679 N; tibia stiffness R2 = 0.96, slope = 1.01, intercept = −1027 N/mm2/tibia ultimate load: R2 = 0.97, slope = 1.04, intercept = 394 N; combined dataset stiffness R2 = 0.95, slope = 1.01, intercept = −230 N/mm2/combined dataset ultimate load: R2 = 0.97, slope = 1.03, intercept = 495 N). Discussion and conclusionIn conjunction with unified BV/TV calibration, the established hFE pipeline accurately predicts experimental stiffness and ultimate load of distal multi-sections at the radius and tibia. Processing time for non-linear analysis was substantially reduced compared to previous μFE and hFE methods but could be further minimized by estimating bone strength based on a fast and linear analysis like as is currently done with μ FE.

Cyclically Loaded Copper Slag Admixed Reinforced Concrete Beams with Cement Partially Replaced with Fly Ash.

Generally, the concrete with higher strength appears to produce brittle failure more easily. However, the use of mineral admixture can help in enhancing the ductility, energy dissipation, and seismic energy in the designed concrete. This paper presents energy absorption capacity, stiffness degradation, and ductility of the copper slag (CS) admixed reinforced concrete with fly ash (FA) beams subjected to forward cyclic load. The forward cyclic load was applied with the help of servo-hydraulic universal testing machines with 250 kN capacity. Twenty-four beams with a size of 100 mm × 150 mm × 1700 mm made with CS replaced for natural sand from 0% to 100% at an increment of 20%, and FA was replaced for cement from 0% to 30% with an increment of 10% were cast. Beams are designed for the grade of M30 concrete. Based on the preliminary investigation results, compressive strength of the concrete greatly increased when adding these two materials in the concrete. Normally, Grade of concrete can change the behaviour of the beam from a brittle manner to be more ductile manner. So, in this work, flexural behaviour of RC beams are studied with varying compressive strength of concrete. Experimental results showed that the RC beam made with 20% FA and 80% CS (FA20CS80) possesses higher ultimate load-carrying capacity than the control concrete beam. It withstands up to 18 cycles of loading with an ultimate deflection of 60 mm. The CS and FA admixed reinforced concrete composite beams have excellent ultimate load carrying capacity, stiffness, energy absorption capacity, and ductility indices compared to the control concrete beam.

Strain rate effect on tensile strength of glass fiber-reinforced polymers

This study presents the tensile properties of glass fiber-reinforced polymer (GFRP) composites under dynamic loadings. The dynamic tensile tests were performed via a high-speed servo-hydraulic testing machine with a velocity ranging from 1 m/s to 15 m/s, corresponding to a strain rate range between 4.9 s−1 and 93.7 s−1. The effects of the strain rate on the tensile strength and dynamic increase factor (DIF) were considered. The results showed that the tensile strength and DIF of GFRP increased with the strain rate. The failure pattern of GFRP was discussed via a group of high-speed camera images. The results of this research can provide useful information for future construction, e.g., protective structures under extreme impacts.

Derivation and Validation of Linear Elastic Orthotropic Material Properties for Short Fibre Reinforced FLM Parts

Additively manufactured parts play an increasingly important role in structural applications. Fused Layer Modeling (FLM) has gained popularity due to its cost-efficiency and broad choice of materials, among them, short fibre reinforced filaments with high specific stiffness and strength. To design functional FLM parts, adequate material models for simulations are crucial, as these allow for reliable simulation within virtual product development. In this contribution, a new approach to derive FLM material models for short fibre reinforced parts is presented; it is based on simultaneous fitting of the nine orthotropic constants of a linear elastic material model using six specifically conceived tensile specimen geometries with varying build direction and different extrusion path patterns. The approach is applied to a 15 wt.% short carbon-fibre reinforced PETG filament with own experiments, conducted on a Zwick HTM 5020 servo-hydraulic high-speed testing machine. For validation, the displacement behavior of a geometrically more intricate demonstrator part, printed upright, under bending is predicted using simulation and compared to experimental data. The workflow proves stable and functional in calibration and validation. Open research questions are outlined.

Mode III testing of structural adhesive joints at elevated loading rates

The Out-of-plane loaded Double Cantilever Beam (ODCB) test has proven to be a well-suited test method for determining both energy release rate and cohesive law of adhesive layers in mode III under quasi-static conditions. As the rate-dependency of the fracture behaviour of adhesive layers in shear is still a topic of ongoing research, we aim to investigate, whether the ODCB test can be modified to be applicable at elevated loading rates. For this purpose, the experimental setup of the ODCB test was modified to allow testing at elevated loading rates. Tests were performed on an epoxy adhesive at outer loading rates over several orders of magnitude, ranging from quasi-static conditions up to the maximum possible test speed of the used servo-hydraulic test machine. The results of these experiments are discussed thoroughly to gain insight into the applicability of the ODCB test at moderate loading rates. Furthermore, the experimental challenges associated with an increase in loading rate are highlighted to identify limitations of the experimental setup that need to be addressed when transitioning to impact testing.

A lateral fracture step-off of 2mm increases intra-articular pressure following tibial plateau fracture

IntroductionThe aim of this study was to evaluate the effects of increasing posttraumatic step-offs after lateral tibial plateau fracture reduction on the intra-articular pressure. Materials and MethodsIn eight fresh-frozen human cadaveric knees with intact menisci, a standardized sagittal osteotomy of the lateral tibial condyle was performed as an OTA/AO type 41-B1 fracture-model. The fragment was fixed by a customized sled including an angular stable tibia plate to evaluate step-offs from 0 mm to 8 mm in 1mm increments. In a servo-hydraulic testing machine, an axial force was applied to the tibial plateau in 0° (700N), 15° (700N), 30° (700N), 60° (350N), and 90 ° (350N) of flexion while the joint pressure was recorded by two pressure sensors. ResultsA 1mm step-off did not result in an increased joint pressure. At 60° of flexion a 2mm step-off increased the lateral joint pressure by 61.84kPa (P = 0.0027). In 30° of flexion, a 3mm step raised the lateral joint pressure by 66.80kPa (p = 0.0017), whereas in 0°, 15° and 90° of flexion, a 4mm step increased the pressure by >50kPa (P < 0.05). Concomitant medial joint pressure increments were lower than those in the lateral plateau. A significant increase of 19-24kPa in the medial joint pressure was detected in 90° of flexion with a 1mm lateral step (P = 0.0075), in 15° and 60° of flexion with a 2mm step (P < 0.05), in 0° of flexion with a 4mm step (P = 0.0215) and in 30° of flexion with a 7mm step (P = 0.0487). ConclusionLateral fracture step-offs of 2mm or larger should be reduced intraoperatively to avoid large increases in lateral joint pressure.

An Industrial Approach to High Strain Rate Testing

Some industrial applications require properties of materials to be determined to evaluate components’ safety in the event of loading and impact. Understanding the behaviour of materials subjected to extreme dynamic loading will aid in enhancing their design. This work is based on developing methods to appraise high loading rate measurements. Different approaches to quantify material properties such as finite element method (FEM), instrumented Charpy testing, and impact testing using servo-hydraulic testing machines are included. Testing is performed at various loading rates, extending existing quasi-static fracture toughness determination to higher loading rates, and accounting for strain-rate dependent properties. The high loading rate servo-hydraulic test machine located at TWI, Cambridge has the capacity to test up to a displacement rate of 20 m/s. The force, displacement, and time parameters are captured by Digital Image Correlation (DIC), which improves the accuracy of the results obtained from experiments. Moreover, the underlying plasticity theory to capture the influence of the strain rate is presented, along with damage constants for FEM calculations adopting the Johnson-Cook model. In addition to the Johnson-Cook approach, analytical solutions using dislocation evolution theory were applied which features the effects of phonon drag and dynamic recovery coefficient in body-centered cubic materials of which X65 grade steel was applied. Also, a deep learning framework was built to predict the tensile curves when given specific test conditions and sample specifications. It was found that high strain rate tests lead to local change at the crack tip which increases plasticity and reduces fracture toughness with single-edged notched three-point bend specimen. The yield strength of the material increased with loading rates during tensile testing leading to a ductile to brittle transition of metals. These strategies were used to establish a revised approach for high strain rate testing and predicting stress-strain curves with a machine learning algorithm.

Compressive behavior of a natural core

Composite sandwich structures with core cellular materials are widely used in diverse industries such as the aerospace, marine, car, and wind industries due to their good mechanical properties on bending. The growing environmental awareness has increased the interest in the use of materials from biodegradable and renewable sources. During recent years, there has been an increasingly growing interest in the use of agglomerated cork as a core material in sandwich structures, replacing synthetic polymeric foams in specific applications. Among its remarkable combination of properties, we can find a hyperelastic behaviour, low thermal conductivity, low permeability, and good energy absorption and vibration damping properties. Additionally agglomerated cork is produced from the waste of the cork industry, being a renewable and biodegradable material that is environmentally sustainable. The aim of this work is to address the quasi-static and dynamic compression testing of two types of agglomerated cork of different densities, using a servo-hydraulic testing machine and drop weight tower. The mechanical behaviour and energy absorption properties at different strain rates are evaluated from the contact force measurements and estimations of impact kinetic energy. Additionally, Digital Image Correlation (DIC) is used to estimate the variability in the local strain through the test specimen.

Propagation of corrosion induced fatigue crack in aluminum alloy

&lt;abstract&gt; &lt;p&gt;Aluminium is considered a green metal due to its environmental responsive characteristics. The 7475-T7351 aluminum alloy is extensively used in automotive and aerospace applications due to its light weight and high strength. In the present work, the effects of the corrosive environment on the high cycle fatigue (HCF) behaviors of the 7475-T7351 aluminum alloy was investigated. The aqueous solution of sodium chloride was used for solution treatment. The HCF test was performed on pre-cracked specimens using a servo-hydraulic universal testing machine, Instron 8800. The fractured specimens were characterized using a scanning electron microscope. It was observed that the crack propagation occurred through anodic dissolution at high stress and a significant crack tip blunting and crack extension occurred. However, no appreciable change in crack growth was noticed over the lower frequency range of 0.1 to 0.9 Hz. The slower growth rate envisages oxide debris formation between the cracked faces. When the alloy was treated under corrosive environments, the HCF tests depicted that the fatigue life reduces up to two orders of magnitude. The corrosion pits induced the crack initiation in stage-I at lower alternating stress; however, the fatigue crack growth rate (FCGR) was increased in the corrosive environment. The transition from stage-I to stage-II occurred at a lower stress intensity range (∆&lt;italic&gt;K&lt;/italic&gt;) level; it was due to the combined effects of corrosion, hydrogen embrittlement, active path dissolution, and stress concentration. The corrosion fatigue test at low frequency also depicted a slower FCGR as compared to its moderate frequency counterpart and showed an irregular crack growth behavior.&lt;/p&gt; &lt;/abstract&gt;

Numerical and Experimental Investigation on Small Scale Magnetorheological Damper

This paper presents the design of an Magnetorheological (MR) damper that includes an arrangement of a piston and cylinder. This study developed a 3-D model based on the finite element method (FEM) concept on the COMSOL Multiphysics to analyze and investigate the MR damper characteristics. A prototype of the MR damper is being fabricated based on the FEM model and is put through a series of experiments using the Servo-Hydraulic material testing machine (MTS). Maximum and minimum forces, 171.5235N and 249.2749N, were measured at 0.1Hz and 1Hz, respectively, for the FEM model. The fabricated model obtained similar results at 0.1Hz and 1Hz, with maximum and minimum forces of 175.9103N and 252.7765N, respectively. Comparing these two model analyses reveals that the FEM-based model accurately depicts the experimental behaviour of the MR damper in terms of its damping force, although there is minor variation. The findings of this paper will be helpful for designers in creating MR dampers that are more efficient and reliable, as well as in predicting the characteristics of their damping force.

A systematic experimental study on the impact of multiaxiality on fatigue life of cast steels at high temperature

Rising demands on material performance at high temperature in components under complex loading such as gas turbine housings demand an increase in versatility and precision of component life modelling approaches. However, the database to train those models is commonly derived from uniaxial testing. The impact of multiaxial loading, both proportional and non-proportional, is usually addressed theoretically by the use of equivalent stress/strain formulations or reduction ratios derived from selective validation testing.In this study, the fatigue life of a cast steel is investigated systematically using a servo-hydraulic bi-axial test machine with induction heating. Each experiment is accompanied with finite element simulations before and after the test to parametrize the loading condition and derive hot-spot equivalent loading parameters. The impact of different load axis ratios is investigated systematically under displacement control as well as the effect of hold times.The results show that the modelled strain hot spots are coinciding with the observed crack initiation. A reoccurring sequence can be found in the impact of strain axis ratios on specimen life. Also, curve shift factors to compare the different combinations of axis ratio, temperature and hold time debit with uniaxial fatigue life data, have been identified.

Hybridization effect on water absorption and flexural properties of E-glass/banana fibre/epoxy composites

Hybrid composites occupying the area of standard materials by satisfying the necessities of various sectors like part industries, automobile industries, ship building and numerous bio-medical sectors. the most reason to substitute standard materials is that the hybrid composites giving same or additional needed properties with less weight and value than standard materials and most output in minimal consumption with higher lifespan to seek out the economical suggests that of utilizing the technology for various applications. Mats were fancied and stratified up with rosin matrix. The laminate is factory-made exploitation hand lay-up technique followed by compression moulding. Critical properties of the fancied material like flexure, enduringness, square measure through an experiment all over and results square measure recorded. The aim of the current analysis work was experimental investigation to judge numerous mechanical properties of hybrid fiber compound composite (E-glass fiber and epoxy, Banana fibres) at totally different weight percentages with epoxy. The properties of E-glass fiber and epoxy, Banana, fibres were found to be sensible large to be used as reinforcement in composite materials. The results of the experiments were per shaped on one hundred kN servo hydraulic universal testing machine (UTM) (50% E-glass, 100% of banana fiber and therefore the four-hundredth of epoxy resin) will study associate optimum results of the composite. In water absorption take a look at C-1(20% E-glass, four-hundredth of banana fiber and therefore the four-hundredth of epoxy resin) will observe the absorption of water. and at last experimental study the water absorption depends on amount of banana fiber.

Localization of Plastic Deformation in Ti-6Al-4V Alloy

This article investigated the mechanical behavior of Ti-6Al-4V alloy (VT6, an analog to Ti Grade 5) in the range of strain rates from 0.1 to 103 s−1. Tensile tests with various notch geometries were performed using the Instron VHS 40/50-20 servo hydraulic testing machine. The Digital Image Correlation (DIC) analysis was employed to investigate the local strain fields in the gauge section of the specimen. The Keyence VHX-600D digital microscope was used to characterize full-scale fracture surfaces in terms of fractal dimension. At high strain rates, the analysis of the local strain fields revealed the presence of stationary localized shear bands at the initial stages of strain hardening. The magnitude of plastic strain within the localization bands was significantly higher than those averaged over the gauge section. It was found that the ultimate strain to fracture in the zone of strain localization tended to increase with the strain rate. At the same time, the Ti-6Al-4V alloy demonstrated a tendency to embrittlement at high stress triaxialities.

MODELING AND CALCULATION OF FATIGUE CRACK GROWTH LIFE IN STEELS

In this work, we studied the kinetics of fatigue crack growth on compact steel tensile specimens (C (T)-type), in the middle section of crack growth diagram under regular and irregular loading with different asymmetries and maximum load. The crack growth kinetics was obtained by the authors experimentally on modern servo-hydraulic testing machine. Irregular loading was carried out using samples of standard loading spectra characteristic of various technical objects experiencing variable loading during operation. The values of the crack growth rate were obtained. Parameters that evaluate the character of irregular loading and crack closure, namely, irregularity factor and crack closure ratio were suggested. When calculating the effective value of the magnitude of the stress intensity factor (SIF) at the crack mouth, it is proposed to consider in addition to the closure coefficient and cracks also measure irregular loading. The fatigue crack growth life was predicted taking into account its “closure” and the nature of loading according to the approach proposed by the authors and the cyclic calculation method (cycle-by-cycle), all the data obtained are tabulated and distributed according to the type of loading. The results obtained showed good convergence of the calculated and experimental data, which confirms the high values of the correlation.

Assessing the magnetic flux leakage contraction parameters for the fatigue life prediction of SAE1045 steel specimens

The aim of this paper is to assess the stress concentration zone (SCZ) of a circular smooth specimen at an early stage. Following the detection of the stress concentration zone, the specimen residual life was calculated and compared to the experimental result. Uniaxial loading on the servo-hydraulic testing machine was used for the laboratory testing. The stress concentration tester was used to detect the SCZ on the specimen, based on spontaneous surface micro magnetic signals on the ferromagnetic surface. In addition, cyclic testing for fatigue characteristics was conducted at 70% and 75% of ultimate tensile stress (UTS). The results indicated that the estimated residual life for the 70% UTS was nearly identical to the experimental value, based on two magnetic parameters, i.e, Kmax and the contraction value. Meanwhile, the estimated residual life for the 75% UTS was equivalent to the experimental findings. The minimum and maximum error values for life estimation using both parameters were 0.47% and 24.35%, respectively, for the contraction value. The correlation value, R2, between the estimated life using MMM and experimental life was 0.95, showing good estimation. All the estimated life data using this method was distributed within the range of a 95% to 90% confidence interval. Hence, the proposed method can be used to predict the fatigue life of ferromagnetic components through magnetic flux leakage signals, especially in the stress concentration region.

Effect of Bidirectional Insertion of External Skeletal Fixation Pins on Axial Pullout Strength in Canine Cadaveric Bone.

The aim of this study was to evaluate the effect of bidirectional insertion on axial pullout strength of tapered run out (TRO), traditional negative profile (TNP) and positive profile (PP) pins. Cadaveric adult canine tibiae were harvested. Tapered run out pins (Group 1) were inserted unidirectionally to the desired position; bidirectionally past the desired position, then withdrawn to the desired position (Group 2); and bidirectionally as described for Group 2, repeated twice (Group 3). Traditional negative profile pins (Group 4-6) and PP pins (Group 9-11) were placed in the same manner. Tapered run out (Group 7), TNP (Group 8) and PP pins (Group 12) were driven unidirectionally such that the shaft of the pin violated the cis-cortex. A servohydraulic testing machine extracted the pins and measured axial peak pullout strength. Positive profile pins had significantly greater pullout strength than TRO and TNP pins placed unidirectionally to the desired position. Method of insertion had no effect on peak pullout strength of TNP pins. TRO and PP pins inserted unidirectionally to the desired position had significantly greater peak pullout strengths than insertion bidirectionally or if the shaft of the pin violated the cis-cortex. The authors recommend that pins used for external skeletal fixation should be placed unidirectionally to the desired position with fluoroscopic guidance, intra-operative depth gauge measurements or measurements from preoperative radiographs. Repositioning pins results in loss of peak pullout strength with TRO and PP pins.