Ben M. Hillberry

Measurement of the total motion between two body segments—I. Analytical development

Abstract All joints of human and animal bodies permit six degrees of freedom to varying extents. Thus to accurately measure the motion permitted by any anatomical joint, all six degrees of freedom must be taken into account. The objective of Part I of this paper is to present the analytical basis for a measurement system which incorporates an instrumented spatial linkage capable of doing this. The system permits both the study of the overall relative motion between two adjacent body segments (e.g. bones) as well as the detailed study of the relative motion between the two complementary articular surfaces of the intervening joint. The mathematical development is carried out in matrix algebra to facilitate data reduction using the digital computer.

Finger joint force minimization in pianists using optimization techniques

A numerical optimization procedure was used to determine finger positions that minimize and maximize finger tendon and joint force objective functions during piano play. A biomechanical finger model for sagittal plane motion, based on finger anatomy, was used to investigate finger tendon tensions and joint reaction forces for finger positions used in playing the piano. For commonly used piano key strike positions, flexor and intrinsic muscle tendon tensions ranged from 0.7 to 3.2 times the fingertip key strike force, while resultant inter-joint compressive forces ranged from 2 to 7 times the magnitude of the fingertip force. In general, use of a curved finger position, with a large metacarpophalangeal joint flexion angle and a small proximal interphalangeal joint flexion angle, reduces flexor tendon tension and resultant finger joint force.

Impact of exercise on bone health and contraindication of oral contraceptive use in young women.

PURPOSE The effect of quantified resistance and high impact exercise training on bone mass as modified by age and oral contraceptive (OCont) use in young women was studied. METHODS Women were categorized by age (18-23 vs 24-31 yr) and OCont use, and were then randomized into either three sessions of resistance exercise plus 60 min.wk-1 of jumping rope or a control group for 24 months. Total body, spine, femoral neck, greater trochanter, Wards area, and radial bone mineral density (BMD) and/or content (BMC), biochemical markers of bone turnover, dietary intake of calcium, lean body mass, maximal oxygen uptake, and strength were determined at baseline and every 6 months. RESULTS Total body (TB) BMC percent change from baseline was higher in exercisers compared with nonexercisers at 6 and 24 months. OCont users had lower bone turnover at baseline and a decrease in TBBMC from baseline compared with non-OCont users at 24 months. Spine BMC and BMD decreased in the exercise and OCont group at 6 months and remained significantly below nonexercisers who used oral contraceptives at 2 yr. Femoral neck BMD also decreased in the exercise and oral contraceptive group at 6 months. CONCLUSIONS Exercise prevented a decline in TBBMC seen in the nonexercisers. On the other hand, exercise in oral contraceptive users prevented the increase observed in the spine of the nonexercise plus OCont group.

Measurement of the total motion between two body segments — II Description of application☆

The total motion between two body segments can be measured accurately if all six possible degrees of freedom between the two body segments are considered. Part I of this paper (Kinzel et al. 1972) described the analytical basis for a measurement scheme capable of considering all six degrees of freedom. The objective of part II is to describe an application of the measurement system to a study of the motion in the shoulder joint of a dog. The design of the instrumented spatial linkage used to make the measurements and the use of the linkage data to determine both the gross relative motion of the scapula with respect to the humerus and the detailed relative motion between the two articulating surfaces of the shoulder joint are described.

Load history effects on fatigue crack growth threshold for Ti–6Al–4V and Ti-17 titanium alloys

Abstract Load history effects on room temperature fatigue-crack-growth threshold measurement are evaluated for titanium alloys Ti–6Al–4V and Ti-17. Baseline thresholds are determined by the use of a conventional load-shedding technique. Load history is synthesized by precracking at Δ K levels with K max greater than the subsequently measured threshold. Threshold for the latter is defined as the first notice of crack propagation under increasing Δ K , constant R , step loading. An empirical overload model is developed to account for precracking history on the threshold. Stress relief annealing after precracking and prior to threshold measurement is demonstrated to eliminate load history effects on rising Δ K , constant R , step loading threshold measurement, and to be comparable to that made using load shedding, but with considerable time saving.

Effect of stress ratio and overload ratio on fatigue crack delay and arrest behavior due to single peak overloads

In this investigation stress interaction effects on fatigue crack propagation following single peak overloads in 2024-T3 aluminum alloy were studied. The two parameters investigated were the overload stress ratio, R O L =K m i n /K O L , and the overload ratio, K O L /K m a x . Tests were run with quasi-constant stress intensity fatigue loading following the overload. The effect of overload on the crack growth rate through the overload plastic zone was observed. Additional tests were run to determine only whether or not crack arrest would occur. The delay effect due to single peak overloads was found to increase with overload stress ratio, R O L , and overload ratio, K O L /K m a x . The higher values of both R O L and K O L /K m a x produced nonpropagating fatigue cracks, and the delay/arrest boundary was defined.

The effect of low cycle fatigue cracks and loading history on high cycle fatigue threshold

The high cycle fatigue (HCF) resistance of Ti-6Al-4V for gas turbine engine applications is studied when the material is first subjected to low cycle fatigue (LCF). The high cycle fatigue (HCF) threshold is determined after small LCF surface cracks are formed in notch tension specimens. LCF loading at two stress ratios, R = 0.1 and R -1.0, is used to initiate the LCF cracks, which are detected using direct current potential difference (DCPD). The surface crack sizes are measured under load using a static loading fixture and a scanning electron microscope (SEM). In addition to the SEM surface measurements, heat tinting is used to mark the crack profiles before HCF testing so that fractography can be used after failure to measure the 2D crack geometry. The LCF surface-cracked specimens are tested at room temperature in lab air at 600 Hz using a step-loading procedure at two stress ratios, R = 0.1 and R = 0.5. The LCF loading history is found to affect the HCF threshold compared to what is predicted from long crack threshold values obtained from other crack geometries. Variations in HCF crack growth thresholds obtained on specimens with LCF crack sizes from 25 to 175 μm are attributed to overload and underload effects from the LCF precracking.

Force platform for rats measures fore and hind forces concurrently

Animal models are commonly used to test the efficacy of impact loading regimens on bone strength. We designed an inexpensive force platform to concurrently measure the separate peak vertical impact forces produced by the fore and hindfeet of immature F-344 rats when dropped onto the platform. The force platform consisted of three load cells placed in a triangular pattern under a flat plate. Rats were dropped from heights of 30, 45 and 60 cm onto the platform so that they landed on all four feet concurrently. The peak vertical impact forces produced by the feet of the rats were measured using a sampling frequency of 100 kHz. The location of each foot at landing relative to the load cells, and the force received by each load cell were combined in a series of static equations to solve for the vertical impact forces produced by the fore and hindfeet. The forces produced by feet when rats stood on the single platform were similarly determined. The forces exerted separately by the fore and hindfeet of young rats when landing on the plate as a ratio to standing forces were then calculated. Rats when standing bore more weight on their hindfeet but landed with more weight on their forefeet, which provides rationale for the greater response to landing forces of bones in the forelimbs than those in the hindlimbs. This system provided a useful method to simultaneously measure peak vertical impact forces in fore and hindfeet in rats.

Determining the effect of microstructure and heat treatment on the mechanical strengthening behavior of an aluminum alloy containing lithium precipitation hardened with the δ′ Al3Li intermetallic phase

The effect of the thermal treatment and composition on microstructure and subsequent mechanical behavior of an Al-2.6 wt.% Li-0.09 wt.% Zr alloy that was solution heat treated (SHT) and artificially aged for a series of aging times and temperatures was studied. The underaged, peakaged, and overaged thermal heat treatments were studied to determine the effect of the microstructure and processing on the mechanical properties. The precipitates in the microstructure, which impede dislocation motion and control the precipitation strengthening response as a function of aging practice, were analyzed as the basis for controlling the strengthening depending on their size distribution, average size, and interparticle spacing. The average particle size, spacing, and size distribution were determined from the microstructure as a function of the thermal processing and composition. For the demonstration alloy, the primary strengthening was a direct consequence of ordered coherent Al3Li (δ′) intermetallic precipitates, which are uniformly distributed throughout the microstructure and restrict the glide motion of dislocations during plastic deformation. The Al3Li average particle size, distribution, spacing, and volume fraction are closely related to the overall mechanical behavior and are a result of the heat treating practice and composition. Consequently, a micromechanical model was developed for predicting the precipitation hardening response in terms of the variation in polycrystalline strength with aging time, aging temperature, and composition. The overall micromechanical model, which was determined from the particle coarsening kinetics, dislocation mechanics, thermodynamics, resolved shear stress, as well as the dislocation particle shearing and bypassing mechanisms, accurately predicted the mechanical strength in the underaged, peak-aged, and overaged tempers of the demonstration alloy.

Prediction of Matrix Fatigue Crack Initiation from Notches in Titanium Matrix Composites

Fatigue crack initiation from notches in titanium matrix composites (TMCs) is of interest in the design of TMC structures. A model is presented for predicting the fatigue crack initiation life of longitudinal, transverse,and symmetric cross-ply laminates using the strain-life behavior of the matrix material. The effective strain parameter, Δ∈ e f f , proposed by Hillberry and Johnson is used to account for the residual stress state imposed during composite processing. Since the effective strain parameter requires knowledge of the local residual and applied strain in the matrix, methods for calculating these strains are developed. Model predictions were verified with crack initiation tests of [0] 4 , [90] 4 , and [0/90] s SCS-6/ TIMETAL 21S center-hole specimens at room temperature and [0 4 /90] s SCS-6/Ti-15-3 center-hole specimens at 300 and 550°C.