Servo-hydraulic Testing Machine Research Articles

Microstructure, mechanical properties and fatigue behaviour of a new high-strength aluminium alloy AA 6086

This study presents the comprehensive experimental investigation of the microstructure, mechanical and fatigue properties of a new high-strength aluminium alloy AA 6086, which was developed from a commercial aluminium alloy AA 6082. The new alloy possesses a higher content of Si, and, it also contains Cu and Zr. The alloy was characterised in the as-cast condition after homogenisation, extrusion, and T6 heat treatment. Light microscopy, scanning and transmission electron microscopy with energy dispersive spectrometry were used to analyse the microstructure and the fractography of broken specimens. The quasi-static and fatigue tests were performed on the MTS Landmark 100 kN servo-hydraulic test machine, controlled with a mechanical extensometer with a 25 mm gauge length. The quasi-static strength of the analysed aluminium alloy AA 6086 was found to be significantly higher if compared to some other AA 6xxx alloys, while the ductility was kept almost the same. The experimental results of the comprehensive fatigue tests in a Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) regime showed a good fatigue resistance, and represent a good basis for engineering design applications of the newly developed aluminium alloy AA 6086.

Guided Double Cantilever Beam test method for intermediate and high loading rates in composites

Although the rate-dependent behaviour of the interlaminar fracture toughness of fibre reinforced composites has been a matter of ongoing research in recent decades, further research is required into characterising these properties and, consequently, developing more reliable constitutive models to simulate dynamic events. Furthermore, despite the various studies that have been carried out, no consensus on which test method to use to characterise these properties has yet been reached. This paper presents a new test method for measuring the dynamic mode I interlaminar fracture toughness in composite laminates by using a novel device based on a modified Double Cantilever Beam test method with a guided tensile configuration. In contrast to other methods reported in the literature, the proposed device guarantees a symmetric crack opening and, thus, pure mode I propagation during high loading rate testing. When used in a dynamic servo-hydraulic testing machine with controllable displacement rate, a constant opening velocity can be achieved. The Guided Double Cantilever Beam testing method is validated against a quasi-static Double Cantilever Beam test, showing good agreement between the results. The testing method has been satisfactorily tested under intermediate/high loading rates, from 0.1 to 10 m/s.

Plasticity, ductile fracture and ballistic impact behavior of Ti-6Al-4V Alloy

The strength and fracture behavior of a metal are crucial to its response in impact events. In the present paper, a hybrid experimental-numerical study was conducted to characterize the plasticity and ductile fracture behavior of Ti-6Al-4V alloy. To this end, ten types of specimens were utilized to cover a range of stress states from uniaxial tension to high stress triaxiality, shear, plane strain as well as elevated temperatures and strain rates. Especially, a slightly modified in-plane shear specimen suggested by Roth and Mohr was proposed and adopted to prevent premature fracture initiation from the free boundaries at a tension stress state. Aside from a Split Hopkinson Pressure Bar (SHPB) system, a high-speed servo-hydraulic testing machine was applied to conduct high-rate tests. A Modified Johnson-Cook (MJC) plasticity model and an extended Xue–Wierzbicki (XW) fracture criterion with strain rate and temperature effects were used and calibrated using a combined experimental-numerical approach. It was found that the fracture strain of the metal is sensitive to both the stress triaxiality and the deviatoric state parameter, and reduced ductility is reached at increasing stress triaxialities and a plane strain as well as shear stress states. The yield strength increases with increasing strain rate and decreasing temperature. An enhanced ductility is achieved at elevated temperatures and lower strain rates, except for at the highest strain rate of 750 /s, at which a higher ductility is observed compared with that at slightly lower strain rates. Ballistic impact tests on 4 mm thick Ti-6Al-4V alloy targets were conducted using 5.95 mm diameter blunt rigid projectiles in an impact velocity range of 167.9 – 321.6 m/s. Parallel finite element simulations were performed for the ballistic tests and the validity of the calibrated material models was confirmed by comparing the predicted ballistic resistance and failure pattern of the target with the experimental ones. In addition, it was found that a finite element prediction on the ballistic resistance of the Ti-6Al-4V alloy targets struck by blunt projectiles is highly mesh size sensitive and a converged solution was obtained by using a mesh size of 20 μm.

Fatigue analysis of electro discharge machined Nitinol 60

Nitinol 60 (NiTi60) is a shape memory alloy (SMA) where the atomic percentage of nickel is slightly higher than titanium. It advantages the material to have higher hardness and better sensitivity to phase transformation. One of the machining techniques that is preferred to cut hard material is electrical discharge machining (EDM). This study aims to examine the fatigue characteristics of a NiTi60 alloy that has undergone electro-discharge machining and examine the impact of machining parameters on microstructural changes. In this investigation first, the influence of EDM process parameters such as pulse current, voltage, pulse on time, and pulse off time on surface integrity aspects was investigated. Second, the impact of these process variables on fatigue strength was examined. The surface integrity parameters of EDM-machined specimens, such as microcracks, surface crack density (SCD), white layer thickness (WLT), and residual stress formation have been examined by various characterization techniques. The obtained results show that pulse current and voltage are dominating factors affecting SCD. The thickness of the white layer seems to be increased with the rise in the pulse current and pulse on time, and tensile kinds of residual stresses are present in the WLT region, whose magnitude is dependent on process parameters. The fatigue tests were performed using a servo hydraulic testing machine at a frequency of 20[Formula: see text]Hz for 106 number of cycles. The fatigue crack initiation, propagation, and effects of process parameters have been examined. It has been found that an increase in pulse current and voltage leads to the generation of microvoids in the WLT region and thereby causes fatigue cracks to take birth. Later on, a correlation between WLT and SCD was observed by implementing an artificial neural network (ANN) model. The accuracy of ANN model prediction is reported to be high, where WLT and SCD have a 0.98 observed correlation coefficient.

Compressive Behavior of Interlocking Plastic Blocks Structural Elements Having Slenderness

Earthquakes are among of the most harmful and potentially fatal natural disasters. Masonry structures in seismic zones of urban and rural areas around the world pose a threat to human life. Housing that is both affordable and earthquake-resistant in earthquake-prone areas is currently in demand in developing countries. For affordable earthquake-resistant structures in earthquake-prone areas, numerous researchers have studied mortar-free interlocking structures. Plastic blocks are used in order to reduce the mass of the overall structure. To start with, structures under gravity are explored first because more than 95% of its design life, any structure has to withstand gravity. Prototypes of interlocking plastic-block columns, solid walls, and walls with an opening are considered for making the mortar-free structures. In this study, the effect of slenderness on the behavior of interlocking-plastic-block structural elements is investigated under compressive loading by a servo-hydraulic testing machine in the laboratory. The effect of slenderness on the behavior of one and two-block-wide structural elements was investigated in terms of the stress–strain curve, energy absorption, and toughness index under compressive loadings. Correlations between the compressive strength of interlocking-plastic-block structural elements with varying thicknesses were found. Scaled-down prototypes of interlocking-plastic-block structural elements having two-block wide depicted more resistance to compressive loads than one block wide structural elements. The correlations among the one and two block wide interlocking-plastic-block columns, single and double-block-wide solid walls, and single and double-block-width walls with an opening found in this analysis were Pdc = 2.2 Psc, Pdsw = 2.9 Pssw, and Pdwo = 3.5 Pswo. This study can be applied in the future to better understand the detailed behavior of interlocking plastic blocks.

Fatigue crack growth kinetics in titanium alloys at different interaction of cyclic loading amplitudes

Fatigue crack growth kinetics of various titanium alloys BT6 (α+β structure) and 3M (α-structure) under constant amplitude loading with different types of cyclic disturbances in the form of over- and underloading at a loading frequency of 25 Hz was investigated.The tests were conducted on a servohydraulic testing machine under controlled load with simultaneous automatic recording of loading parameters and readings from a crack opening gauge. Tests were carried out on compact specimens with an edge crack which were subjected to off-centre loading. The crack opening data was converted into crack length using a yield strength measurement method. Experimental results, namely fatigue crack growth curves and fatigue fracture kinetic diagrams, were analyzed. It is shown that crack growth kinetics depends mainly on the type of alternating loading amplitude sequences. Reliability estimation of crack growth on the basis of crack opening gauge and study of fractographic features of materials fracture has been carried out.

Experimental determination of cyclically stabilized material properties of the Mo-based alloy MHC in stress-relieved condition from room temperature to 1400 °C

The aim of the current study was the determination of the time-dependent visco-plastic material behavior of the molybdenum alloy MHC (Molybdenum-Hafnium-Carbon) in stress-relieved condition with cyclic strain-controlled low cycle fatigue experiments at room temperature, 800 °C and 1400 °C. To ensure high data quality, a servohydraulic testing machine modified with a vacuum chamber was utilized to avoid material oxidation. The long-time strain-controlled experiments were conducted with a laser extensometer with high accuracy up to the highest applied test temperature of 1400 °C. The determined data were appropriate to generate cyclic stress-strain curves, strain-Wöhler curves, and their descriptive parameters. Furthermore, the strain rate dependency of the cyclic stress response and the stress relaxation behavior of the cyclically stabilized material were also investigated at the mentioned temperatures. The current study indicates that MHC in stress-relieved condition has a softening ability during cyclic loading, especially at 800 °C and 1400 °C. The strain rate sensitivity of the stress amplitude for the case of cyclic stabilization shows similar behavior as described in the literature for the case of monotonously increasing loading conditions, with a minimum at 800 °C. An interesting effect was identified in relaxation tests with loading at different strain rates. A kind of rebound effect was observed at the highest utilized strain rate of 10−2 s−1, which disappears with decreasing strain rate. Elastic strain proportions can be converted into plastic strain proportions during the application of the slow strain rate.

Effect of Fluoride Content of Mouthwashes on Superelastic Properties of NiTi Orthodontic Archwires.

The influence of sodium fluoride (NaF) concentration in mouthwashes on the properties of superelastic NiTi orthodontic wires has been studied. In this work, 55.8%Ni and 44.2%Ti (in weight) wires were introduced in commercial mouthwashes with different NaF contents (0, 130, 200 and 380 ppm). The release of Ni2+ and Ti4+ ions was by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) at 1, 4, 7 and 14 days. Superelastic orthodontic wires present at oral temperature the austenitic phase which is transformed into a plastic phase (martensite) by cooling. The temperatures at which this occurs are influenced by the chemical composition. The release of ions from the wire will produce variations in the temperatures and stresses of the stress-induced martensitic transformation. Ms, Mf, As, Af were determined by Differential Scanning Calorimeter (DSC). The transformation stresses (austenite to stress induce martensite) were determined with a servo-hydraulic testing machine at 37 °C. The surfaces for the different times and mouthwash were observed by Scanning Electron Microscope (SEM). The release of Ni2+ in mouthwashes with 380 ppm NaF concentrations reaches 230,000 ppb in 14 days and for Ti4+ 175,000 ppb. When NaF concentrations are lower than 200 ppm the release of Ni and Ti ions is around 1500 ppb after 14 days. This variation in compositions leads to variations in Ms from 27 °C to 43.5 °C in the case of higher NaF concentration. The increasing immersion time and NaF concentrations produce a decrease of Ni in the wires, increasing Ms which exceed 37 °C with the loss of superelasticity. In the same way, the stresses (tooth position corrective) decrease from 270 MPa to 0 MPa due to the martensitic phase. The degradation can produce the growth of precipitates rich in Ti (Ti2Ni). These results are of great interest in the orthodontic clinic in order to avoid the loss of the therapeutic properties of superelastic NiTi due to long immersion in fluoride mouthwashes.

Mechanical behavior of CP-Ti at high strain rates and under stress triaxiality

In this study, we investigated the influence of stress triaxiality on the mechanical behavior of commercially pure (CP-Ti) VT1-0 titanium (Grade 2) in the range of strain rates from 0.1 to 103 s−1. Tensile tests were carried out on flat notched and smooth specimens using an Instron VHS 40 / 50–20 servo-hydraulic testing machine. Phantom V711 was used to capture the deformation process of the specimens. The digital image correlation (DIC) method was employed to investigate the evolution of local fields in the gauge section of the specimen. Fracture surface topology analysis was performed using the Keyence VHX‐600D digital microscope. The results revealed that an increase in the strain rate from 102 to 103 s−1 leads to a significant discrepancy between the relative residual elongation δ and the strains that occur in the localization bands prior to crack initiation. CP-Ti undergoes fracture due to nucleation, growth, and coalescence of damage in bands of localized plastic deformation oriented along the maximum shear stresses. Fracture surface analysis indicated an increase in the surface roughness parameter with increasing stress triaxiality. The results confirm that the fracture of commercially pure titanium exhibits ductile behavior at strain rates from 0.1 to 103 s−1, for triaxiality stress parameter 0.333 ≤ η < 0.5, and at a temperature close to 295 K. The least influence of stress triaxiality on the strain-to-fracture appeared at a strain rate of 103 s−1.

An Automated Optical Strain Measurement System for Estimating Polymer Degradation under Fatigue Testing.

(1) Background: this study deals with design of an automated laboratory facility based on a servo-hydraulic testing machine for estimating parameters of mechanical hysteresis loops by means of the digital image correlation (DIC) method. (2) Methods: the paper presents a description of the testing facility, describes the grounds for calculating the elastic modulus, the offset yield strength (OYS) and the parameters of the mechanical hysteresis loops by the DIC method. (3) Results: the developed hardware-software facility was tested by studying the fatigue process in neat polyimide (PI) under various amplitude tension-tension loadings. It was found that the damage accumulation was accompanied by the decrease in the loop areas, while failure occurred when it reduced by at least ~5 kJ/m3. (4) Conclusions: it was shown that lowering the loop area along with changing the secant modulus value makes it possible to estimate the level of the scattered damage accumulation (mainly at the stresses above the OYS level). It was revealed that fractography data, namely the pattern and sizes of the fatigue crack initiation and propagation zones, did not correlate well with the dependences of the parameters of the hysteresis loops.

A novel apparatus to assess the mechanical properties of Ankle-Foot Orthoses: Stiffness analysis of the Codivilla spring

Ankle-Foot Orthoses (AFOs) are the most common devices prescribed to support the ankle and restore a quasi-normal gait pattern in drop-foot patients. AFO stiffness is possibly the main mechanical property affecting foot and ankle biomechanics. A variety of methods to evaluate this property have been reported, however no standard procedure has been validated and widely used. This study is reporting the repeatability of a novel apparatus to measure AFO stiffness in ideal frictionless conditions.The apparatus is based on a servo-hydraulic testing machine and allows to apply a displacement-controlled rotation of the AFO shell, simulating the physiological ankle dorsi/plantarflexion movement. The repeatability of the apparatus in measuring AFO stiffness in dorsiflexion and plantarflexion was assessed intra- and inter-session in a sample of standard polypropylene AFOs of different sizes (Codivilla spring).The repeatability of the apparatus in measuring the AFO stiffness was high. The Intra- and Inter-session Coefficient of Variation ranged between 0.02 ÷ 1.3 % and 1.3 ÷ 5 %, respectively. The Intra Class Correlation Coefficient ranged between 0.999 ÷ 1 intra- and 0.993 ÷ 0.997 inter-session. AFOs stiffness was observed to increase with the AFO size.The setup is easy to replicate and can be implemented with any torsion-controlled servo-hydraulic testing machine and has resulted simple to use and flexible enough to adapt to AFOs with different sizes. The frictionless contacts characterizing the apparatus make it possible to measure the ideal AFO stiffness by excluding the effect of the fixation methods to the leg and help to improve the repeatability of measurements.

Mechanical Behavior of Alpha Titanium Alloys at High Strain Rates, Elevated Temperature, and under Stress Triaxiality

The paper presents the experimental results of the mechanical behavior of Ti-5Al-2.5Sn alloy at high strain rates and elevated temperature. Flat smooth and notched specimens with notch radii of 10 mm, 5 mm, and 2.5 mm were used. The experimental studies were carried out using the high-velocity servo hydraulic test machine Instron VHS 40/50-20. The samples were heated with flat ceramic infrared emitters on average between 60 s and 160 s. The temperature control in the working part of specimens was carried out in real time using a chromel-alumel thermocouple. The digital image correlation (DIC) method was employed to investigate the evolution of local fields in the gauge section of the specimen. Data on the influence of stress triaxiality on the ductility of Ti-5Al-2.5Sn alloy were obtained under tension with strain rates ranging from 0.1 to 103 s−1 at a temperature of 673 K. It was found that, at 673 K, the ductility of Ti-5Al-2.5Sn alloy increases with the increasing strain rate for both smooth and notched specimens.

A Comparison Study of Fatigue Behavior of S355J2+N, S690QL and X37CrMoV5-1 Steel

Steel of the mild-strength S355J2+N steel grade is the most often used steel for manufacturing carrying sections of constructions exposed to fatigue loads. The use of high-strength steels, such as S690QL, allows for the creation of structures that are light and simple to construct. However, increasing the yield strength of high-strength steels does not result in a corresponding increase in fatigue resistance. As a result, using high-strength steels for constructions subjected to fatigue loading can be a major design concern, raising the question of whether high-strength steels should be used at all. Most of the experimental investigations regarding the hot work tool steel X37CrMoV5-1 found in the literature are focused on its machining and wear resistance, with insufficient attention paid to the cyclic loads. This article evaluates the fatigue properties of mild-strength S355J2+N, high-strength S690QL, and X37CrMoV5-1 steel grades. A SHIMADZU servo-hydraulic testing machine is used to perform uniaxial tensile tests under uniaxial fatigue stress-controlled, fully reversed conditions (tensile–compression testing with R = −1 stress ratio) in accordance with EN ISO and ASTM standards. The aim of this paper is to highlight the fatigue characteristics of these three steels that are among the most used in their respective groups. Steel S355J2+N belongs to the group of hot-rolled normalized steels, S690QL belongs to the group of improved (quenched + tempered) steels with increased strength, and X37CrMoV5-1 belongs to the group of high-alloyed tool steels for hot work. This choice was made as the tested steels can be considered typical representatives of their groups. Based on the test results of these three steels, which are organized in S–N curves, the fatigue behavior of the entire mentioned groups of steels can be foreseen.

Designing of double-threaded intramedullary pin and evaluation of ex vivo biomechanical resistance to axial compression load on the canine long bone fracture gap model

Intramedullary (IM) pinning is a biological method of fracture fixation; but is associated with complications ofpin migration, proximal fragment collapse and rotational instability. The study aimed to design double threaded(DT) IM pin and evaluated its biomechanical resistance to axial compression load in comparison to end-threaded(ET) and simple Steinman (SS) IM pin on the canine long bone fracture gap model. The DT IM pin was designedconsidering various morphometric measurements (17 femur and 8 tibia bones) on lateral radiographs of the routinely done healthy dogs. For ex vivo biomechanical study, a distal third fracture was created in the 17 (8 femoral, 9 tibial) canine cadaveric bones. A normograde IM pinning using SS, ET and DT was done keeping atleast 10 mm gap at the fracture site (fracture gap-model). A pin occupying 60-70% of the narrowest medullary canal was used for ETand DT, and &gt;80% for SS models. The bone-implant constructs were subjected to axial compression load (N) till5 mm displacement of the proximal bone fragment or till dislodgement of the implant using 0.5 mm/min velocity ona servo-hydraulic testing machine. The magnitude of the axial compression load/mm displacement was influencedby the bone (femur vs tibia) and implant types. In femur, the DT pins sustained higher compression loads followedby SS and ET. However, for tibia, the load required was highest with DT followed by ET and SS pin. The studyreports the first of its kind indigenously designed biomechanically superior double threaded pin for the canine longbone fracture fixation

On the transition of failure control from material-intrinsic defects to defects forming during monotonically increasing and cyclic mechanical loading in WC-Co hard metal at elevated temperature

WC-Co hard metals are composite materials with extraordinary mechanical properties especially at elevated temperatures which make them common materials for metalworking tools. The current work investigates damage and fracture behavior of hourglass-shaped specimens made of a WC-Co hard metal with submicron-sized WC grains and 12 wt.% Co binder. All investigated specimens were isothermally heated inductively to 700°C in a servo-hydraulic testing machine equipped with a vacuum chamber to avoid surface oxidation. The nature of origins of fracture and the evolution of bulk material damage features was studied via scanning electron microscopy for experiments under monotonically increasing load as well as cyclic loading under a stress ratio R = σmin / σmax = -1. A formation of small cavities that formed in the material bulk during loading was observed for all investigated loading conditions. For monotonically increasing load, the coalescence of these cavities was identified as the dominant damage mechanism. For cyclic loading, the number and size of the mentioned cavities did rise with the applied number of load cycles and rising stress amplitude. The coalescence of the cavities was found to be a main mechanism controlling the fatigue crack propagation process at elevated temperature. The found results imply that a certain combination of stress amplitude and temperature exists, at which a transition of failure control occurs from: (i) the size of material-inherent defects present prior to loading, to (ii) the kinetics of the coalescence of cavities that form due to plastic deformation and creep in the Co matrix during loading.

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.