Ralph I. Stephens

The influence of wrist position on the force produced by the finger flexors

Abstract This paper describes the design, instrumentation, procedures and results of a study of the influence of wrist position on the forces produced by the finger flexors at the middle and distal phalanges. Measurement at each phalangeal level was made on all four fingers simultaneously in each of five different wrist positions. Results of the study suggest that the percentage distribution of the total force produced by the finger flexors to each individual finger bear a constant relationship regardless of wrist position. The magnitude of the total force produced does vary with wrist position.

Fatigue crack growth with negative stress ratio following single overloads in 2024-T3 and 7075-T6 aluminum alloys

Modified pre-cracked compact specimens of 2024-T3 and 7075-T6 aluminum alloys were subjected to four different overload patterns followed by subsequent constant-amplitude steady-state loading with R =P l i n /Pl m a x equal to 0, -½, -1, and -2. The overload patterns were tension, compression-tension, tension-compression, and compression. Cyclic loading with negative stress ratio, R, drastically reduced crack-growth retardation. The higher the negative R ratio the greater the reduction in retardation. Overload ratios, OLR =P h m a x /Pl m a x , ranging from 2.0 to 3.0 were used. For compression overloads, the OLR ranged from -2.0 to -4.0. High compression overloading was detrimental and dependent upon subsequent R ratio loading. Substantial fracture surface abrasion near the mid-thickness occurred for higher negative R ratios. Striations were not readily found in this region, however, they were quite evident near the edges, which indicated crack closure was greater near the mid-thickness. The 2024-T3 gave better crack growth life than 7075-T6 in some loading conditions, while the opposite was true for other loadings. The results indicate negative R ratio must be considered in retardation models and that retardation life cannot be modeled based solely on overload plastic zone sizes.

Overload Effects on Subcritical Crack Growth in Austenitic Manganese Steel

Single and intermittently repeated tensile overloads were applied to precracked compact type specimens using austenitic manganese steel which strain hardens under cyclic loading. Delayed crack retardation following overloading was observed macroscopically and verified microscopically with a scanning electron microscope. Crack growth during delayed retardation varied up to 0.10 in. or up to about 30 percent of the overload reversed plane stress plastic zone size and involved up to 6 x 10 3 cycles. Total crack retardation life varied from 50-6000 percent. Single tensile overloads, in general, tended to produce the greatest crack growth retardation compared with intermittently repeated tensile overloads. This behavior is best explained by comparing ΔN with ΔN a g . The greater the single overload, the larger the crack growth retardation. Likewise, for a given number of cycles between overloads, the greater the overload force, the larger the crack growth retardation. Use of the crack growth equation da/dN = A(DK) n for single and periodic intermittent overloads is conjectural since n takes on negative, zero, and positive values, and a substantial percentage of the total life involves negative and zero values of n. Single compressive overloads decreased crack growth life up to 23 percent.

Experimental strain analysis of infant, adolescent and adult miniature swine skulls subjected to simulated mastication forces

Abstract An experimental strain analysis was performed on three miniature swine skulls to obtain a better insight into the mechanism of bone remodelling. An infant, adolescent and adult swine skull were instrumented with electrical resistance foil strain gage rosettes. The skulls were loaded through the muscles of mastication with applied forces proportional to the muscle-masses. Tooth force reactions were obtained at the molar and incisors. Measured strains were converted to principal strains using Mohrs strain circle. The principal strain variation and magnitude was greatest in the infant. less in the adolescent and least in the adult. The strain variation results can be attributed to a negative feedback system modifying a genetically derived bone structure and form. For given masticatory muscle forces, the molar forces were 50–60% greater than incisor forces. The largest strains due to mastication occurred on the side of the skull where chewing takes place.

A Finite Element Model for Predicting Time-Dependent Deformations and Damage Accumulation in Laminated Composite Bolted Joints

To assess and predict the long-term durability of advanced composite joints, a general, nonlinear finite element program was developed and implemented to model the localized time-dependent deformations and damage accumulation in the vicinity of a fastener hole in a polymer matrix composite (PMC) laminate. This code incorporates an elastic-viscoplastic constitutive model for unidirectional, orthotropic laminae, coupled with classical lamination theory, to determine the time-dependent stresses and deformations in the laminate. The accumulation of damage in the composite, and the subsequent deterioration of mechanical properties, is predicted by a set of ply failure criteria and an associated property degradation model. The results from this analysis were compared to experimental data and were found to provide reasonable correlation with time-dependent bolt-hole elongation measurements obtained from a graphite fiber reinforced PMC.

Constant and variable amplitude fatigue behavior and modeling of an SRIM polymer matrix composite

Strain-controlled fatigue behavior of smooth specimens of an SRIM polymer matrix composite under constant and variable amplitude loading was investigated, including the effects of mean stresses/strains. Significant degradation of the macroscopic stiffness was observed during cyclic loading, and SEM examination of the failed specimens revealed the degradation was due to a variety of damage mechanisms, including matrix cracking, fiber/matrix debonding, fiber fracture, and fiber buckling. Fatigue life predictions made using common strain-based models overestimated the experimental results for both constant amplitude and variable amplitude loading. An improved strain-based model for making life predictions using an effective strain amplitude was proposed which was in much closer agreement with experimental results.

Influence of Cold Rolling Threads before or after Heat Treatment on the Fatigue Resistance of High Strength Fine Thread Bolts for Multiple Preload Conditions

SI class 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment at five different preload values. Bolts used were 3/8 UNRF-24 (fine) and preloads were taken as 1, 50, 75, 90, and 100 % of roll before heat treatment proof stress. Since proof stress was lowered 10 % for roll after heat treatment, these preloads for roll after heat treatment bolts were then actually 1.1, 55, 83, 100, and 110 % of proof stress (to keep load the same). The tests produced a range of R ratios (R=Smin/Smax) between 0.03 and 0.92. Maximum near surface residual compressive stresses, obtained via x-ray diffraction, ranged from −500 to −1000 MPa. Axial loads were applied through the nut and all fatigue failures occurred at the first thread of the nut/bolt interface with crescent shaped cracks dominating in most tests. Multiple ratchet marks (separate crack nucleation sites) occurred for roll before heat treatment bolts, while fewer or no ratchet marks, were evident for the roll after heat treatment. Scanning electron microscopy evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions contained ductile dimpling. Cyclic creep/ratcheting was monitored and little, or none, was observed for preload tests equal to or less than 75 %. Replication data indicated a log-normal distribution on life was very reasonable. The fatigue resistance for fine threads rolled after heat treatment with preload stresses of 1 % (R ratio less than 0.05) caused very large increases (158 %) in 107 cycles fatigue strength compared to roll before heat treatment. This is in agreement with other roll before/after low R-ratio results. The roll after heat treatment bolts when tested at the higher proof loads had 107 fatigue strengths of 69 to 30 % increase. These increases are much less than the 158 % at 1 % preload, but still significant. Constant life Haigh diagrams at 105 and 107 cycles were in qualitative agreement with VDI 2230 bolt preload guidelines.

Fatigue Crack Growth and Life Predictions in Man-Ten Steel Subjected to Single and Intermittent Tensile Overloads

Fatigue crack growth behavior was investigated in Man-Ten steel under conditions of constant load amplitude, single tensile overloads followed by constant load amplitude and intermittent single tensile overloads. Constant-amplitude ratios, R l = P l m i n /P l m a x , were + ½, 0, -½, -1 and -2. The single overload test program excluded R l = +½, and the intermittent overload program was for R l = 0 only. A new simplified mathematical model was introduced which assumes that fatigue crack growth retardation is primarily a function of the overload ratio and the load ratio. The model predicted intermittent tensile overload life within 57 to 112 percent of the experimental life. The average prediction was 77 percent of experimental life. For Man-Ten steel, constant-amplitude crack growth rate versus positive ΔK fell within a reasonable scatter band for R l = ½ to - 1. However, substantial decrease in life occurred with R 1 = -2. Negative values of R 1 following a single tensile overload substantially reduced retardation life. Thus compressive stress, either large or small, cannot be neglected in spectrum loading fatigue crack growth life predictions.

Subcritical crack growth under single and multiple periodic overloads in cold-rolled steel

Fatigue crack growth retardation, delayed retardation, initial acceleration, and optimum fatigue crack growth life under single, single periodic, and multiple periodic tensile overloads were investigated in cyclic strain softening AISI 1020 cold-rolled steel using compact type specimens. Macroscopic delayed crack growth retardation was observed following both single and multiple periodic overloads and was best observed at high overload ratios and larger crack lengths. Initial crack growth acceleration at the beginning of an overload interval was not quantitatively evident, however initial acceleration could have occurred before the crack extended to the least measurable scale division. Under constant load range testing, multiple periodic overloads of about 10 to 30 gave optimum or maximum fatigue life in most cases, however values of 1 gave maximum life in some cases. The largest increase in fatigue life was 453 percent. Maximum or optimum fatigue life obtained from multiple periodic overloads was always greater than that obtained from a single overload. In general, the results suggested maximum fatigue life can be obtained by applying a few multiple tensile overloads at high overload ratios only when accelerated crack growth begins following low load crack retardation.

Subsurface Fatigue Crack Initiation and Propagation Behavior of Induction-Hardened Shafts under the Effect of Residual and Applied Bending Stresses

The Society of Automotive Engineers Fatigue Design and Evaluation (SAEFDE) Committee has been conducting a long-term program aimed at the development of a predictive capability for fatigue life of SAE 1045 induction-hardened shafts. As a part of a larger-scale investigation provided by the SAEFDE committee, this research provided an analytical model capable of predicting the total fatigue life, both crack initiation and crack propagation, of an induction-hardened shaft under applied bending stress. The analysis procedure incorporated the effects of residual stresses. Total stress intensity factors were calculated and superimposed using applied bending stress intensity factors and residual stress intensity factors along the subsurface elliptical crack front. Fatigue tests were conducted using SAE 1045 induction-hardened shafts to verify the analytical models of subsurface fatigue crack growth. The total fatigue life calculations of subsurface failure showed a factor from 0.6 to 0.8 compared with the experimental results. The analytical model and experimental data confirmed that the majority of the total fatigue life is spent in the crack propagation phase.