Compressive Preloads Research Articles

Nonlinear vibrations and imperfection sensitivity of a cylindrical shell containing axial fluid flow

The high imperfection sensitivity of cylindrical shells under static compressive axial loads is a well-known phenomenon in structural stability. On the other hand, less is known of the influence of imperfections on the nonlinear vibrations of these shells under harmonic axial loads. The aim of this work is to study the simultaneous influence of geometric imperfections and an axial fluid flow on the nonlinear vibrations and instabilities of simply supported circular cylindrical shells under axial load. The fluid is assumed to be non-viscous and incompressible and the flow to be isentropic and irrotational. The behavior of the thin-walled shell is modeled by Donnell's nonlinear shallow-shell equations. It is subjected to a static uniform compressive axial pre-load plus a harmonic axial load. A low-dimensional modal expansion, which satisfies the relevant boundary and continuity conditions, and takes into account all relevant nonlinear modal interactions observed in the past in the nonlinear vibrations of cylindrical shells with and without flow is used together with the Galerkin method to derive a set of eight coupled nonlinear ordinary differential equations of motion which are, in turn, solved by the Runge–Kutta method. The shell is considered to be initially at rest, in a position corresponding to a pre-buckling configuration. Then, a harmonic excitation is applied and conditions for parametric instability and dynamic snap-through are sought. The results clarify the marked influence of geometric imperfections and fluid flow on the dynamic stability boundaries, bifurcations and basins of attraction.

Osteochondral microdamage from valgus bending of the human knee

Background Valgus bending of the knee is promoted as an anterior cruciate ligament injury mechanism and is associated with a characteristic “footprint” of bone bruising. The hypothesis of this study was that during ligamentous failure caused by valgus bending of the knee, high tibiofemoral contact pressures induce acute osteochondral microdamage. Methods Four knee pairs were loaded in valgus bending until gross injury with or without a tibiofemoral compression pre-load. The peak valgus moment and resultant motions of the knee joint were recorded. Pressure sensitive film documented the magnitude and location of tibiofemoral contact. Cartilage fissures were documented on the tibial plateau, and microcracks in subchondral bone were documented from micro-computed tomography scans. Findings Injuries were to the anterior cruciate ligament in three knees and the medial collateral ligament in seven knees. The mean (standard deviation) peak bending moment at failure was 107 (64) N m. Valgus bending produced regions of contact on the lateral tibial plateau with average maximum pressures of approximately 30 (8) MPa. Cartilage fissures and subchondral bone microcracks were observed in these regions of high contact pressure. Interpretation Combined valgus bending and tibiofemoral compression produce slightly higher contact pressures, but do not alter the gross injury pattern from isolated valgus bending experiments. Athletes who sustain a severe valgus knee bending moment, may be at risk of acute osteochondral damage especially if the loading mechanism occurs with a significant tibiofemoral compression component.

Modeling of Magnetomechanical Actuators in Laminated Structures

A magnetomechanical plate model (MMPM) has been developed to predict the elastic and magnetostrictive strains and mechanical stress in laminated structures with magnetostrictive and non-magnetic layers under the simultaneous effect of quasi-static mechanical stress and magnetic field. This model was obtained by combining an energy-based statistical magnetomechanical model with the classical laminated plate theory. The magnetomechanical plate model was used to study a unimorph structure having a magnetostrictive iron—gallium (Galfenol) patch attached to different non-magnetic substrates. The actuation response from the patch was obtained for in-plane axial magnetic field acting on the unimorph. The MMPM was used to predict the normalized tip displacement due to induced-strain actuation in a unimorph cantilever beam and the results were compared with existing modeling techniques. The model was used to study the effect of tensile and compressive axial pre-loads on the actuation response of the structure. A study was also performed to understand the effect of total thickness of the structure, the ratio of the active/ passive layer thickness and the effect of the mechanical properties of different substrate materials on the actuator performance. A non-dimensional parameter ST i (percentage strain transfer) was introduced to explain the behavior of the actuator in extension and bending dominated regimes and a critical thickness ratio (trc) was defined to demarcate these two regimes. The results demonstrate that the model captures the non-linearity in the magnetomechanical process and the different structural couplings.

105. Effect of Compressive Preload Magnitude on the Quality and Quantity of Motion of Lumbar Spine after Total Disc Arthroplasty Using a Compressible Disc Prosthesis

BACKGROUND CONTEXT: The lumbar spine experiences compressive preloads up to 800N during activities of daily living (ADL). However, kinematics of lumbar disc prostheses under large preloads have not been reported. In our experience, disc prostheses with articulating bearings tend to “bind” under large preloads, resulting in degradation of motion quantity and quality.

Head-Turned Postures Increase the Risk of Cervical Facet Capsule Injury During Whiplash

In vitro experiments using cadaveric cervical spine motion segments to quantify facet capsular ligament strain during whiplash-like loading. To quantify facet capsule strains during whiplash-like loading with an axial intervertebral prerotation simulating an initial head-turned posture and to then compare these strains to previously-published strains for partial failure and gross failure of the facet capsule for these specimens. Clinical data have shown that a head-turned posture at impact increases the severity and duration of whiplash-related symptoms. Thirteen motion segments were used from 7 women donors (50 +/- 10 years). Axial pretorques (+/-1.5 Nm), axial compressive preloads (45, 197, and 325 N), and quasi-static shear loads (posteriorly-directed horizontal forces from 0 to 135 N) were applied to the superior vertebral body to simulate whiplash kinematics with the head turned. Three-dimensional displacements of markers placed on the right facet capsular ligament were used to estimate the strain field in the ligament during loading. The effects of pretorque direction, compression, and posterior shear on motion segment motion and maximum principal strain in the capsule were examined using repeated-measures analyses of variance. Axial pretorque affected peak capsule strains more than axial compression or posterior shear. Peak strains reached 34% +/- 18% and were higher for pretorques toward rather than away from the facet capsule (i.e.-, head rotation to the right caused higher strain in the right facet capsule). Compared to previously-reported data for these specimens, peak capsule strains with a pretorque were double those without a pretorque (17% +/- 6%) and not significantly different from those at partial failure of the ligament (35% +/- 21%). Thus a head-turned posture increases facet capsular ligament strain compared to a neutral head posture-a finding consistent with the greater symptom severity and duration observed in whiplash patients who have their head turned at impact.

Evaluation of Residual Stress Effects on Brittle Fracture Strength Based on Weibull Stress Criterion

Abstract This paper studies the method for estimating the residual stress effects on brittle fracture of structural steel based on the Weibull stress criterion. First, brittle fracture tests were conducted for four-point bend notched specimens with and without compressive preloading. It is shown that the compressive preloading apparently decreases the critical crack tip opening displacement (CTOD). The critical CTOD of the preloaded specimen can be predicted from monotonically loaded test results based on the Weibull stress criterion. It has been found that the Weibull stress is an effective fracture parameter for brittle fracture initiation under tensile residual stress. Second, using wide plate specimens with and without welding residual stress subjected to uniform tension and three-point bend fracture toughness specimens, welding residual stresses effect on the brittle fracture strength is investigated. Experiments show that the critical CTOD of wide plate with welding residual stress is significantly smaller than that without residual stress. Constraint loss effect on CTOD of wide plate can be assessed by the equivalent CTOD ratio β. β is defined as β=δ3PB/δWP, where δ3PB and δWP are CTODs of the standard fracture toughness specimen and wide plate, respectively, at the same level of the Weibull stress. Fracture assessment result using β is shown within the context of failure assessment diagram. An excessive conservatism observed in the conventional procedure (β=1) is reasonably reduced by applying the equivalent CTOD ratio β.

Thermodynamic Modeling of Hysteresis Effects in Piezoceramics for Application to Smart Structures

W ITH the development of piezoceramic sensors and actuators of varying shapes and sizes for use in structural applications, the field of smart structures has emerged as an area of research of great importance [1]. The mechanical, thermal, and electrical behavior of piezoceramics has been studied extensively by physicists and material scientists [2–9]. The introduction of these materials in structural applications has created a necessity to review the traditional structural modeling and analysis. Under cyclic variation of the applied electric field, piezomaterials exhibit polarization-electric (P-E) field hysteretic losses, as shown in Fig. 1. The points indicated by the symbols Ps, Pr, and EC represent saturation polarization, remnant polarization, and the coercive electric field, respectively. The saturation polarization Ps corresponds to the value of maximum polarization, which shows negligible change with further increase in electric field. Remnant polarization Pr is the value of polarization when the electric field becomes zero. Coercive electric fieldEC corresponds to the points of zero polarization. The observed phenomenon is due to the delay in polarization switching with variation in electric field. P-E hysteresis effect leads to an interesting variation of strainwith respect to electric field ( E), and it is denoted as butterfly loop. The hysteresiswill be affected by various parameters such as temperature, amplitude of oscillating electric field, frequency of oscillation, and external mechanical preloading. The effect of the amplitude of the electric field on the hysteresis loop has been studied experimentally by Nalwa [2]. It is observed that the remnant polarization and coercive electric field are functions of amplitude of the electric field. With a decrease in amplitude of the electric field, there is a reduction in the values of maximum polarization, remnant polarization, and coercive electric field. Viehland and Chen [3] experimentally studied the effects of frequency of oscillation of the electric field on the hysteresis loop. From the experiments, it was observed that if the amplitude of the electric field is above the coercive electric field ECmax corresponding to the case with saturation polarization, then the dissipation energy increases with an increase in frequency, whereas if the amplitude of the electric field is belowECmax, then the dissipation energy increases with a decrease in frequency. The effect of mechanical preloading on hysteresis has been studied experimentally by Arndt et al. [4]. It is observed that when a compressive mechanical preloading is applied parallel to the electric field, the reduction in polarization is found to be higher than that corresponding to the case with mechanical loading applied perpendicular to the electrical field. With the application of compressive mechanical preloading, the remnant polarization and coercive field show a reduction in magnitude. Mathematical modeling of hysteresis was approached at two different levels: 1) at the microscopic level and 2) at the macroscopic level of the piezomaterial. The microscopic models of hysteresis can be categorized as 1) polarization switching based on the Eshelby inclusion model [10], 2) crystal-plasticity-based nonlinear switching models [11], 3) free-energy-based domain-switching model [12], and 4) dipole–dipole interaction models with threshold-switching energy given by the time-dependent Ginzburg–Landau model [13,14] or the Landau–Devonshire model [15,16]. Macroscopic models can be categorized as either empirical models or thermodynamically consistent models. Empirical models are based on either introducing an additional variable in the constitutive relations [17], by representing the P-E curve by a tanhyperbolic function [18], or by using the Presaich model [19,20]. Bassiouny et al. [21–24] developed a thermodynamic phenomenological model for capturing the electromechanical hysteresis effects based on the work-hardening plasticity model. A similar approach was followed by McMeeking and Landis [25] in modeling domain switching in ferroelectric materials. A model based on extended irreversible thermodynamics was proposed by Lu and Hanagud [26]. Presented as Paper 1743 at the 15-th AIAA/ASME/AHS Adaptive Structures Conference, Honolulu, HI, 23–26 April 2007; received 1 May 2007; accepted for publication 25 August 2007. Copyright © 2007 by V. L. Sateesh, C. S. Upadhyay, and C. Venkatesan. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0001-1452/08 $10.00 in correspondence with the CCC. ∗Graduate Student, Department of Aerospace Engineering. Student Member AIAA. Associate Professor, Department of Aerospace Engineering. Pandit Ramachandra Dwivedi Chair Professor, Department of Aerospace Engineering. Senior Member AIAA. AIAA JOURNAL Vol. 46, No. 1, January 2008

Enhancing the Stability of Anterior Lumbar Interbody Fusion

Biomechanical study using human cadaver spines. To assess the stabilizing effect of a supplemental anterior tension band (ATB, Synthes) plate on L5-S1 anterior lumbar interbody fusion (ALIF) using a femoral ring allograft (FRA) under physiologic compressive preloads, and to compare the results with the stability achieved using FRA with supplemental transpedicular instrumentation. Posterior instrumentation can improve the stability of ALIF cages. Anterior plates have been proposed as an alternative to avoid the additional posterior approach. Eight human specimens (L3 to sacrum) were tested in the following sequence: (i) intact, (ii) after anterior insertion of an FRA at L5-S1, (iii) after instrumentation with the ATB plate, and (iv) after removal of the plate and adding transpedicular instrumentation at the same level. Specimens were tested in flexion-extension, lateral bending, and axial rotation. Flexion-extension was tested under 0 N, 400 N, and 800 N compressive follower preload to simulate physiologic compressive preloads on the lumbar spine. Stand-alone FRAs significantly decreased the range of motion (ROM) in all tested directions (P < 0.05); however, the resultant ROM was large in flexion-extension ranging between 6.1 +/- 3.1 degrees and 5.1 +/- 2.2 degrees under 0 N to 800 N preloads. The ATB plate resulted in a significant additional decrease in flexion-extension ROM under 400 N and 800 N preloads (P < 0.05). The flexion-extension ROM with the ATB plate was 4.1 +/- 2.3 under 0 N preload and ranged from 3.1 +/- 1.8 to 2.4 +/- 1.3 under 400 N to 800 N preloads. The plate did not significantly decrease lateral bending or axial rotation ROM compared with stand-alone FRA (P > 0.05), but the resultant ROM was 2.7 +/-1.9 degrees and 0.9 +/- 0.6 degrees , respectively. Compared with the ATB plate, the transpedicular instrumentation resulted in significantly less ROM in flexion-extension and lateral bending (P < 0.05), but not in axial rotation (P > 0.05). The ATB plate can significantly increase the stability of the anterior FRA at L5-S1 level. Although supplemental transpedicular instrumentation results in a more stable biomechanical environment, the resultant ROM with the addition of a plate is small, especially under physiologic preload, suggesting that the plate can sufficiently resist motion. Therefore, clinical assessment of the ATB plate as an alternative to transpedicular instrumentation to enhance ALIF cage stability is considered reasonable.

Failure Analysis of Bolted Aluminum Sandwich Composite Plates under Compressive Preload

The aim of this study is to examine the failure load and failure mode in an aluminum— glass-epoxy sandwich composite plate fastened mechanically by a bolt and washers on a circular hole under a preload exerted by a moment. Parametric studies were performed by experiments to find out the effects of joint geometry and preload moment on the failure strength and failure mode. The preload moments chosen (M) were 0, 3 and 6 Nm while the end distance to hole diameter (E/D) and width to diameter (W/D) ratios were chosen as 1—5 and 2—5, respectively. It is concluded from these experiments that the failure load increases with the increase of M, E/D and W/D. When M, E/D and W/D ratios reach 3—6, 2—5 and 5 Nm, full bearing strengths occur, respectively. When E/D = 1 for all the widths and preload moments, the shear-out failure mode is observed. The mixed modes, especially bearing and net-tension, are observed for E/D and W/D equal to 2—5 and 3—4, respectively. The preload moments increase the failure strengths of the composite laminate.

Novel model to analyze the effect of a large compressive follower pre-load on range of motions in a lumbar spine

A 3-D finite element model (FEM) of the lumbar spine (L1–S1) was used to determine the effect of a large compressive follower pre-load on range of motions (ROM) in all three planes. The follower load modeled in the FEM produced minimal vertebral rotations in all the three planes. The model was validated by comparing the disc compression at all levels in the lumbar spine with the corresponding results obtained by compressing 10 cadevaric lumbar spines (L1–S1) using the follower load technique described by Patwardhan et al. [1999. A follower load increases the load-carrying capacity of the lumbar spine in compression. Spine 24(10), 1003–1009]. Further validation of the model was performed by comparing the lateral bending and torsion response without pre-load and the flexion–extension response without pre-load and with an 800 N follower pre-load with those obtained using cadaver lumbar spines. Following validation, the FEM was subjected to bending moments in all three planes with and without compressive follower pre-loads of up to 1200 N. Disc compression values and the flexion–extension range of motion under 800 N follower pre-load predicted by the FEM compared well with in vitro results. The current model showed that compressive follower pre-load decreased total as well as segmental ROM in flexion–extension by up to 18%, lateral bending by up to 42%, and torsion by up to 26%.

Ballistic Penetration of Compressively Loaded Composite Plates

The effects of ballistic penetration on woven E-glass fiber-reinforced epoxy laminates subjected simultaneously to in-plane compressive loading were investigated. A portable hydraulic press and fixture was designed to apply compressive preloads during ballistic penetration. A civilian version of the US Army ’s M4 carbine was used for the ballistic tests where penetration velocities were well above the ballistic limit for 5.56 mm projectiles. Compression-only data were initially obtained to characterize the various modes of damage. Samples without simultaneous compressive preload were subjected to ballistic penetration alone and then compressively loaded to failure. Finally, samples that had previously undergone ballistic testing at varying levels of compressive preloads were loaded in compression to determine the residual compressive failure strength. For simultaneous loading, samples were subjected to a specified compressive preload while a projectile was fired through the center of the sample. This study finds that the effect of simultaneous preload and ballistic penetration results in a greater extent of damage propagation than for ballistic penetration alone, increasing the possibility of initiating buckling failure of the loaded plate. The equivalent ballistic limit velocity for a given laminate and thickness is also likely to be reduced with increasing preload in composite laminates.

Cyclic actuation of Ni–Mn–Ga composites

Planar (2-2) and fibular (1-3) composites were fabricated by embedding single crystal Ni49.9Mn29.1Ga21.0 in a polyurethane resin. Magnetic and mechanical testing was performed on all samples. Mechanical testing showed the twin boundary yield stress to be 1.0 and 0.3MPa for the monolithic and composites, respectively. Magnetic testing was performed to determine the magnetic field induced strain as a function of increasing compressive preloads (from 0to2.0MPa). Actuation strains without an external preload are 4.9% for the monolithic and 1.0% for the composite samples. The composite samples show considerable cyclic strain without the application of a compressive loading, unlike the monolithic material. A rule of mixture model is presented to predict the strain output of the composite samples under a magnetic field and explain the reduced twin boundary yield stress.

Response of Charité total disc replacement under physiologic loads: prosthesis component motion patterns

Background context Total disc replacement (TDR) has been recommended to reduce pain of presumed discogenic origin while preserving spinal motion. The floating core of Charité TDR is professed to allow the replication of the kinematics of a healthy disc under physiologic loads. While segmental motion after Charité TDR has been measured, little is known about the effects of a physiologic compressive preload on vertebral motion and the motion of prosthesis components after TDR. Purpose (1) Does Charité TDR allow restoration of normal load-displacement behavior of a lumbar motion segment under physiologic loads? (2) How do the prosthesis components move relative to each other under physiologic loads when implanted in a lumbar motion segment? Study design A biomechanical study using human lumbar spines (L1-sacrum). Methods Five lumbar spines (age: 52±9.3) were used. Specimens were tested under flexion (8 Nm) and extension (6 Nm) moments with compressive follower preloads of 0 N and 400 N in the following sequence: (i) intact, (ii) Charité TDR at L5-S1, (iii) simulated healed fusion at L5-S1 with Charité TDR at L4-L5. Segmental motion was measured optoelectronically. Motions between prosthesis end plates and core were visually assessed using sequential digital video-fluoroscopy over the full range of motion. Here we report on kinematics of 10 Charité TDRs: 5 at L5-S1 and 5 at L4-L5. Results Charité TDR increased the flexion-extension range of motion of lumbar segments (p<.05). At 400 N preload, the range of motion increased from intact values of 6.8±4.4 to 10.0±2.4 degrees at L5-S1 and from 7.0±2.6 to 10.8±2.9 degrees at L4-L5. Charité TDR increased segmental lordosis by 8.1±6.9 degrees at L5-S1 (p<.05) and 5.4±3.5 degrees at L4-L5 (p<.05). Four patterns of prosthesis component motion were noted: (1) angular motion only between the upper end plate and core, with little or no visual evidence of core translation (9 of 10 TDRs at 0 N preload and 5 of 10 TDRs at 400 N preload); (2) lift-off of upper prosthesis end plate from core or of core from lower end plate (observed in extension in 9 of 10 TDRs under 0 N preload only); (3) core entrapment, resulting in a locked core over a portion of the range of motion (observed in extension in 8 of 10 TDRs under 400 N preload); (4) angular motion between both the upper and lower end plates and core, with visual evidence of core translation (1 of 10 TDRs at 0 N preload, 5 of 10 TDRs at 400 N preload). The pattern of load-displacement curves was substantially changed under a physiologic preload in 8 of 10 TDRs; instead of a relatively gradual change in angle with changing moment application as seen for an intact segment, the TDR displayed regions of both relatively small and relatively large angular changes with gradual moment application. Conclusions Charité TDR restored near normal quantity of flexion-extension range of motion under a constant physiologic preload; however, the quality of segmental motion differed from the intact case over the flexion-extension range. Whereas some TDRs showed visual evidence of core translation, the predominant angular motion within the prosthesis occurred between the upper end plate and the polyethylene core. Likely factors affecting the function of the Charité TDR include implant placement and orientation, intraoperative change in lordosis, and magnitude of physiologic compressive preload. Further work is needed to assess the effects of the prosthesis motion patterns identified in the study on the load sharing at the implanted level and polyethylene core wear.

Analysis of large compression loads on lumbar spine in flexion and in torsion using a novel wrapping element

Axial compression on the spine could reach large values especially in lifting tasks which also involve large rotations. Experimental and numerical investigations on the spinal multi motion segments in presence of physiological compression loads cannot adequately be carried out due to the structural instability and artefact loads. To circumvent these problems, a novel wrapping cable element is used in a nonlinear finite element model of the lumbosacral spine (L1–S1) to investigate the role of moderate to large compression loads on the lumbar stiffness in flexion and axial moments/rotations. The compression loads up to 2700 N was applied with no instability or artefact loads. The lumbar stiffness substantially increased under compression force, flexion moment, and axial torque when applied alone. The presence of compression preloads significantly stiffened the load-displacement response under flexion and axial moments/rotations. This stiffening effect was much more pronounced under larger preloads and smaller moments/rotations. Compression preloads also increased intradiscal pressure, facet contact forces, and maximum disc fibre strain at different levels. Forces in posterior ligaments were, however, diminished with compression preload. The significant increase in spinal stiffness, hence, should be considered in biomechanical studies for accurate investigation of the load partitioning, system stability, and fixation systems/disc prostheses.

Flexion–Extension Response of the Thoracolumbar Spine Under Compressive Follower Preload

The authors conducted an in vitro biomechanical flexibility study of T2-S1 specimens in flexion-extension under compressive follower preloads of physiological magnitudes. The objectives of this study were to test the hypotheses that 1) the thoracolumbar spine will support compressive preloads of in vivo magnitudes and 2) allow physiological mobility under flexion-extension moments if the preload is applied along an optimized follower load path that approximates the kypholordotic curve of the thoracolumbar spine. In the absence of muscle forces, the ligamentous thoracolumbar spine specimens cannot support the compressive loads expected in vivo. As a result, the flexibility of the thoracolumbar spine in flexion-extension has not been studied in vitro under physiological compressive preloads. Seven human thoracolumbar spines (T2-sacrum) were subjected to flexion and extension moments (up to 8 and 6 Nm, respectively) under compressive preloads from 0 to 800 N applied along an optimized follower preload path. The experimental technique applied the compressive preload such that: 1) it minimized the internal shear forces and bending moments resulting from the preload application, 2) made the internal force resultant compressive, and 3) caused the preload path to approximate the tangent to the curve of the thoracolumbar spine. The range of motion was measured in the T2-sacrum, T2-T11, T11-L1, and L1-sacrum regions. All thoracolumbar specimens supported the compressive follower preload up to 800 N without damage or instability. At 800 N preload, the total flexion-extension range of motion of the T2-sacrum region decreased by 22%, from a mean of 73 degrees to 57 degrees (P < 0.05). The range of motion of the T2-T11 and L1-sacrum regions decreased from the baseline value by 23% and 30%, respectively, at a preload of 800 N. The sagittal mobility of the thoracolumbar junction (T11-L1) was not affected by the preload. The follower preload did not significantly affect the proportion of the total T2-sacrum flexion-extension range of motion contributed by the T2-T11 and L1-sacrum regions of the thoracolumbar spine. The optimized follower preload vector minimizes the effects of artifact moment and shear force on the range of motion of the thoracolumbar spine in flexion-extension. This model allows the entire thoracolumbar spine to be investigated under physiological loading for different clinical applications.

Effect of supplemental translaminar facet screw fixation on the stability of stand-alone anterior lumbar interbody fusion cages under physiologic compressive preloads.

A biomechanical study of lumbar threaded interbody cage construct under varying compressive preloads of similar magnitudes to those experienced in vivo during daily activities. To test the hypothesis that supplemental translaminar facet screws would enhance the stability (ability to reduce segmental angular motion) of threaded interbody cages in flexion-extension during activities in which the spine is subjected to low compressive preloads, and therefore the stand-alone interbody cage construct is least stable. Controversy exists over whether threaded anteriorly placed interbody cages can be routinely used as "stand-alone" devices or whether they require supplemental posterior stabilization to achieve successful fusion. Biomechanical studies suggest that under conditions of low preloads, the motion segment treated with stand-alone cages might be less stable, particularly in extension. METHODS.: Eight human lumbar spine specimens (from L1 to sacrum) were tested intact, after insertion of 2 threaded cylindrical cages (BAK) at L5-S1 and after supplemental translaminar facet screw fixation. They were subjected to flexion and extension moments under progressively increasing magnitude of externally applied compressive follower preload from 0 to 1200 N. The range of angular motion in flexion-extension at L5-S1 was analyzed to assess the effect of translaminar facet screws on the stability of the cage construct for different compressive preloads. In flexion, over 0 to 400 N preload, the supplemental translaminar facet screw fixation reduced the L5-S1 angular motion relative to intact by 71% to 74% as compared to 40% to 44% for the cages alone. This difference was statistically significant (P < 0.05). In extension at 0 N preload, the cages allowed more angular motion than the intact segment, whereas with translaminar facet screw fixation, the motion was reduced to the level of the intact segment. At 400 N preload, supplemental TLFS fixation significantly increased the stability of the cages, reducing the extension angular motion by 60% of intact (P = 0.04). Supplemental translaminar facet screw fixation did not significantly increase the stability provided by the cages in flexion or extension at the 1200 N preload magnitude. In vivo during activities of daily living, interbody cage constructs are subject to varying compressive preloads due to external loads generated by paraspinal musculature, and our results suggest that the stability created by the cage (reduction in segmental angular motion) is not constant. The cage construct is likely to be least stable in extension during activities that impart low compressive preloads to the lumbar spine. Supplemental translaminar facet screw fixation will enhance stability of the motion segment treated with threaded cages, particularly during conditions of low compressive preloads, the very condition in which the cage alone is least effective in providing stability.

Effects of uniaxial prestress on the ferroelectric hysteretic response of soft PZT

This paper deals with the influence of preload stress on the ferroelectric hysteretic behavior of piezoelectric ceramics. The polarization and strain versus electric field hysteresis loops were measured for soft lead zirconate titanate (PZT) piezoceramic material under various uniaxial compressive stress preloads of up to −400 MPa. The investigation revealed that the superimposed compression load reduced the remnant polarization, decreased the coercive field, and also had a significant impact on the dielectric and piezoelectric properties. With increasing mechanical load, dielectric hysteresis and butterfly hysteresis became less and less pronounced, as the compressive stress prevented full alignment of the domains and induced mechanical depolarization. The slopes of the polarization and strain curves at zero electric field were measured to evaluate the dependence of permittivity and piezoelectric coefficients on the prestress. The experimental results were interpreted in terms of the non-180° domain switching process under combined electromechanical loading.

The effect of compressive axial preload on the flexibility of the thoracolumbar spine.

An in vitro flexibility study of the human thoracolumbar spine under compressive preload. To attain kinematics descriptive of the thoracolumbar spine in vitro by applying a pure bending moment under a range of physiologic compressive preloads. Many studies on the mechanical behavior of the spine under pure moment have been conducted; however, little is known regarding variations in the range of motion of the thoracolumbar spine attributable to simulated body weight and other physiologic load conditions. Five fresh human cadaveric thoracolumbar spine specimens (T9-L3) were used. Five compressive axial preloads ranging from 75 to 975 N were applied to each specimen along the spinal curvature through four adjustable brackets attached to each vertebral body. Flexibility measurements were taken by applying a maximum of 5 Nm pure bending moment to the specimen in flexion and extension. The flexibilities in flexion and extension for each loading case were compared. The thoracolumbar spine supported compressive preloads as much as 975 N without damage or instability in the sagittal plane when the preload was applied along the natural curvature of the spine through estimated centers of rotation. The flexibility in bending (flexion/extension) of the ligamentous thoracolumbar spine decreased with increasing compressive preload. A higher bending stiffness was reached after the compressive load exceeded 500 N. Such knowledge could be used to establish better testing guidelines for implant evaluation and more realistic loading conditions.

Torsional Fatigue Characteristics of Aluminum–Composite Co-Cured Shafts with Axial Compressive Preload

Power transmission shafts such as driving shafts or automotive propeller shafts should transmit static and dynamic torques with vibrational stability. Hybrid shafts made of unidirectional fiber-reinforced composite and metal have high fundamental bending natural frequency as well as high torque transmission capability: composite increases the fundamental bending natural frequency due to its high specific stiffness and metal such as aluminum or steel transmits the required torque. However, fabrication-induced thermal residual stresses due to the coefficient difference of thermal expansion of the composite and the metal are developed during manufacturing hybrid shafts so that the high residual stresses decrease fatigue resistance of the hybrid shafts, especially at low operating temperatures. In this paper, the torsional fatigue characteristics of aluminum–composite co-cure joined shafts with axial compressive preload were investigated. To change the thermal residual stresses, an axial compressive preload was given to the aluminum tube by a compressive jig during the co-cure bonding operation. In order to determine the thermal residual stresses with respect to the preload and temperature difference, stress analyses were performed by simple equations from mechanics of materials and finite element method. Then the static torque capacities and fatigue strengths of the hybrid shafts from static and fatigue torsional tests were correlated with the calculated thermal residual stresses. The fatigue strength of the hybrid shaft was much improved by the axial compressive preload, exceeding that of a pure aluminum shaft. Also, the degradation of the fatigue resistance of the hybrid shaft at subzero operating temperature was overcome by the axial compressive preload.

A mission synthesis algorithm For fatigue damage analysis

This paper presents a signal processing based algorithm, the Mildly Nonstationary Mission Synthesis (MNMS), which produces a short mission signal from long records of experimental data. The algorithm uses the discrete Fourier transform, orthogonal wavelet transform and bump reinsertion procedures. In order to observe the algorithm effectiveness a fatigue damage case study was performed for a vehicle lower suspension arm using signals containing tensile and compressive preloading. The mission synthesis results were compared to the original road data in terms of both the global signal statistics and the fatigue damage variation as a function of compression ratio. Three bump reinsertion methods were used and evaluated. The methods differed in the manner in which bumps (shock events) from different wavelet groups (frequency bands) were synchronized during the reinsertion process. One method, based on time-synchronized section reinsertion, produced the best results in terms of mission signal kurtosis, crest factor, root-mean-square level and power spectral density. For improved algorithm performance, bump selection was identified as the main control parameter requiring optimization.