Stripping torques in human bone can be reliably predicted prior to screw insertion with optimum tightness being found between 70% and 80% of the maximum

Aims To devise a method to quantify and optimize tightness when inserting cortical screws, based on bone characterization and screw geometry. Methods Cortical human cadaveric diaphyseal tibiae screw holes (n = 20) underwent destructive testing to firstly establish the relationship between cortical thickness and experimental stripping torque (Tstr), and secondly to calibrate an equation to predict Tstr. Using the equation’s predictions, 3.5 mm screws were inserted (n = 66) to targeted torques representing 40% to 100% of Tstr, with recording of compression generated during tightening. Once the target torque had been achieved, immediate pullout testing was performed. Results Cortical thickness predicted Tstr (R2 = 0.862; p < 0.001) as did an equation based on tensile yield stress, bone-screw friction coefficient, and screw geometries (R2 = 0.894; p < 0.001). Compression increased with screw tightness up to 80% of the maximum (R2 = 0.495; p < 0.001). Beyond 80%, further tightening generated no increase in compression. Pullout force did not change with variations in submaximal tightness beyond 40% of Tstr (R2 = 0.014; p = 0.175). Conclusion Screw tightening between 70% and 80% of the predicted maximum generated optimum compression and pullout forces. Further tightening did not considerably increase compression, made no difference to pullout, and increased the risk of the screw holes being stripped. While further work is needed for development of intraoperative methods for accurate and reliable prediction of the maximum tightness for a screw, this work justifies insertion torque being considerably below the maximum. Cite this article: Bone Joint Res 2020;9(8):493–500.

Description of Residual Stress and Strain Fields in FGM Hollow Disc Subject to External Pressure

Elastic/plastic stress and strain fields are obtained in a functionally graded annular disc of constant thickness subject to external pressure, followed by unloading. The elastic modulus and tensile yield stress of the disc are assumed to vary along the radius whereas the Poisson’s ratio is kept constant. The flow theory of plasticity is employed. However, it is shown that the equations of the associated flow rule, which are originally written in terms of plastic strain rate, can be integrated with respect to the time giving the corresponding equations in terms of plastic strain. This feature of the solution significantly facilitates the solution. The general solution is given for arbitrary variations of the elastic modulus and tensile yield stress along the radial coordinate. However, it is assumed that plastic yielding is initiated at the inner radius of the disc and that no other plastic region appears in the course of deformation. The solution in the plastic region at loading reduces to two ordinary differential equations. These equations are solved one by one. Unloading is assumed to be purely elastic. This assumption should be verified a posteriori. An illustrative example demonstrates the effect of the variation of the elastic modulus and tensile yield stress along the radius on the distribution of stresses and strains at the end of loading and after unloading. In this case, it is assumed that the material properties vary according to power-law functions.

Algebraic Convexity Conditions for Gotoh's Nonquadratic Yield Function

A necessary and sufficient condition in terms of explicit algebraic inequalities on its five on-axis material constants and a similarly formulated sufficient condition on its entire set of nine material constants are given for the first time to guarantee a calibrated Gotoh's fourth-order yield function to be convex. When considering the Gotoh's yield function to model a sheet metal with planar isotropy, a single algebraic inequality has also been obtained on the admissible upper and lower bound values of the ratio of uniaxial tensile yield stress over equal-biaxial tensile yield stress at a given plastic thinning ratio. The convexity domain of yield stress ratio and plastic thinning ratio defined by these two bounds may be used to quickly assess the applicability of Gotoh's yield function for a particular sheet metal. The algebraic convexity conditions presented in this study for Gotoh's nonquadratic yield function complement the convexity certification based on a fully numerical minimization algorithm and should facilitate its wider acceptance in modeling sheet metal anisotropic plasticity.

On the nature of steel fatigue fracture

Experimental data confirmed that if steel cyclic stress reduces to less than tensile yield stress values, i.e., in case of high-cycle fatigue, the mechanism of fracture changes from dislocation to vacancy one. The authors based their findings on the fact that steel density determined by the method of liquid displacement is less than that of steel in both initial conditions and after fracture under the cyclic loads exceeding tensile yield stress values. In the latter case steel hardness increases, whereas the specimens fractured under the cyclic stresses less than their tensile yield stress values show no change in hardness. It means that in such a case metal fractures without strain hardening, i.e., undergoes brittle fracturing developing by vacancy mechanism rather than by dislocation one. As a result, such steel obtains the structure and properties similar to those appearing after its exposure to radiation, i.e., friability and brittleness.

Temperature, Strain Rate and Pressure Sensitivity of the Polyetheretherketone’s Yield Stress

The polymer yield behavior is affected by temperature, strain rate and pressure. In this work, tensile yield stress of polyetheretherketone (PEEK) is characterized for temperature ranging between [Formula: see text] and [Formula: see text] ([Formula: see text]C and [Formula: see text]C). The tensile yield stress is decreasing in terms of temperature. Two temperature transitions are observed: [Formula: see text] ([Formula: see text]C) and the glass transition temperature. The temperature sensitivity is well captured by the modified-Eyring equation proposed by the authors. This paper completes three previous works where the PEEK’s yield behavior was described under compression on wide ranges of strain rate and temperature and under tension on a wide range of strain rates. Thus, the pressure effect is analyzed in terms of temperature and strain rate. Using either the experimental data or the modified-Eyring equation, the effect of the hydrostatic pressure is increasing with temperature and decreasing with strain rate.

Tension and Shearing Characteristics of Magneto Rheological Fluid under Magnetic Fields

The mechanical properties of a magneto rheological (MR) fluid in tension and shearing have been studied in the magnetic field which is generated by a coil carrying different magnitudes of DC electrical current. An experiment setup was designed and fabricated to perform this operation. The magnetic behavior of the equipment was analyzed using software of ANSYS/Multiphysics. The shear yield stress represents the effect of the interaction force along the shear direction (perpendicular to the direction of the magnetic field). The tensile yield stress of MR fluids represents the effect of the interaction force between the polarized particles along the direction of the applied magnetic field. The shearing tests showed that the shear yield stress is proportional to the external magnetic flux density with an exponent of 1.19. The tensile yield stress is about four times of shear yield stress.

A Study of Polyvinyl Chloride Modified by Barium Sulfate

PVC/BaSO4 composites were prepared by melt blending method. The mechanical properties, microstructure and thermal stability of the composites were investigated. The results indicated that BaSO4 decreased the tensile yield stress and improved the elongation at break of PVC composites. Ductile fracture characteristics such were observed in the tensile fracture surface of PVC/BaSO4 composites. The toughening mechanism was cavitations toughening mechanism and shear zone toughening mechanism. The reaction of dehydrochlorination was limited by the addition of BaSO4.