Force-elongation Curve Research Articles

A nanoscopic view of structure and deformation of hard elastic polypropylene with scanning force microscopy

Scanning force microscopy (SFM) was used to visualize the surface of hard elastic polypropylene (HEPP) film. The surface morphology of unstrained HEPP shows crystalline and noncrystalline rows oriented parallel to the extrusion direction. The crystalline rows are composed of lamellar blocks. The dimensions of crystalline and noncrystalline regions are determined. The structural surface changes induced by stepwise elongation of the film with a home-built stretching device are documented by SFM. Stretching of HEPP perpendicular to the extrusion direction causes cracks advancing through several crystalline rows. During elongation parallel to the extrusion direction the separation of adjacent lamellae by their translatory displacement occurs. Deformation-induced structural changes of HEPP on the nanometer scale are compared with proposed deformation models. Nanostructural changes are correlated with characteristic variations in the force-elongation curve. © 1996 John Wiley & Sons, Inc.

A nanoscopic view of structure and deformation of hard elastic polypropylene with scanning force microscopy

Scanning force microscopy (SFM) was used to visualize the surface of hard elastic polypropylene (HEPP) film. The surface morphology of unstrained HEPP shows crystalline and noncrystalline rows oriented parallel to the extrusion direction. The crystalline rows are composed of lamellar blocks. The dimensions of crystalline and noncrystalline regions are determined. The structural surface changes induced by stepwise elongation of the film with a home-built stretching device are documented by SFM. Stretching of HEPP perpendicular to the extrusion direction causes cracks advancing through several crystalline rows. During elongation parallel to the extrusion direction the separation of adjacent lamellae by their translatory displacement occurs. Deformation-induced structural changes of HEPP on the nanometer scale are compared with proposed deformation models. Nanostructural changes are correlated with characteristic variations in the force-elongation curve. © 1996 John Wiley & Sons, Inc.

Engineering and experimental analyses of the tensile loads applied during strength testing of direct bonded orthodontic brackets

The stress levels within the cement layer (hence, the apparent strength) of a direct bonded orthodontic bracket depends, to a large extent, on the alignment of the tensile loads that are applied to the specimen. The purpose of this analysis was to determine how the construction of a ligature wire harness affects the alignment of the applied loads. Tensile tests conducted on a modified bracket/cement system showed large variations in the force-elongation curve profiles. An engineering model was developed to explain these deviations. The results indicate that it is virtually impossible to evenly apply tensile loads to the bracket. It was also proposed that long harnesses constructed with thin ligature wire, prestressing the harness, and lubrication may reduce some of the effects of unavoidable load-bracket misalignment. (A M J O RTHOD D ENTOFAC O RTHOP 1994;106:167-74)

The effects of 4 mrad of γ irradiation on the initial mechanical properties of bone-patellar tendon-bone grafts

Pairs of frozen human patellar tendon-bone (PTB) ligament allografts were exposed to either 0 or 4 Mrad of gamma irradiation sterilization, the latter value based on recent reports suggesting higher dosage levels for adequate sterilization against the human immunodeficiency virus. All specimens were subjected to three levels of loading: lower functional loads, higher functional loads, and failure. Lower functional loads were simulated by performing in vitro static and cyclic creep tests, similar to loads that the surgeon and patient would apply before and after implantation, respectively. Higher functional loads, simulating moderate activities of daily living, were represented by the slope of the linear portion of the force-elongation curve or linear stiffness. Failure or trauma was then simulated by failing the grafts in tension at a high strain rate. We found that the irradiation treatment shortened the tendon by only 0.6 mm, which was nevertheless statistically significant (p < 0.01). By contrast, 4 Mrad did not significantly alter either static or cyclic creep (p > 0.05) at lower functional loads. Instead, irradiation produced the greatest changes during failure testing, reducing both the graft's linear stiffness by 12% (p < 0.025) and maximum force by 26% (p < 0.001). Although our data do not describe how an allograft might perform during the early healing and later collagen-remodelling phases, such in vitro studies remain important if we are to optimize allograft properties before arthroscopic anterior and posterior cruciate ligament reconstruction.

Stiffness of Canine Stifle Joint Ligaments at Relatively High Rates of Elongation

A falling-weight impact tester was developed specifically to evaluate the tensile characteristics of animal joints at high elongation rates. The tensile force and elongation in the ligaments of the canine stifle joint were measured using a force transducer, a linear variable displacement transducer, and a digital storage oscilloscope. Stiffness and elongation rates were determined at 1 mm and at 2 mm elongation. The stiffness was calculated as the slope of the force-elongation curve, and the elongation rate was calculated as the slope of the elongation-time curve. Results demonstrated that stiffness in the canine stifle joints was not affected by the rate of elongation in the 0.1 to 1.0 m/s range.

In situ mechanical behavior of posterior spinal ligaments in the lumbar region. An in vitro study

The posterior ligaments: ligamentum flavum, articular, interspinous and supraspinous ligaments of twenty five fresh cadaveric intervertebral segments, from T11–T12 to L4–L5, extracted from fourteen spines were tested in tension. A progressive dissection method was used, that is, each segment was tested after first resecting the disk with the ligaments intact and a force-elongation curve obtained. Then one ligament was cut and the test repeated, and so on. The most restrictive ligament was found to be the ligamentum flavum followed by the articular, interspinous, and supraspinous ligaments.

The Force-Elongation Curve of a Thin Fibrous Network.

Specimens from low-density weblike handsheets were tested in a tensile tester. In a test the direction of extension was frequently reversed and the specimen reextended to obtain a series of force-elongation curves. For Kraft woodpulp specimens the force-elongation behavior was well represented by an exponential equation involving three parameters: a modulus of elasticity C 2, a length parameter x c related to average segment length between network bonds, and an elongation value x 0 at which the curve starts. The unstrained length of a specimen l increases, and the parameters x c and C 2 tend to decrease with each successive reextension curve. For a series of specimens of increasing area density representative values of x c /l tend to decrease and C 2 to increase. For a series of specimens made from pulps beaten increasing amounts representative values of x c /l tend to decrease and C 2 to increase. Some features of the tensile behavior can be modeled by a system of parallel Filaments of equal length to which longer parallel filaments with an exponential length distribution have been added. Upon extension the filaments assume load successively, thus simulating the force-elongation behavior of a paper network. By thinking in terms of this model it is possible to anticipate intuitively much of the behavior of a paper network.

An Analysis of Tearing Failure

The tearing failure of materials depends very strongly on the conditions of use or testing which produce the non-uniform tensile stress which causes such failure. In the trapezoid tear test. the stress pattern is determined largely by variables at the disposition of the operator. The effect of these variables on the tearing strength and the relationship of the latter to the shape of the force-elongation curve have been determined. The tear load is given by an equation of the form where g is the modulus. h is the thickness. L o is the gauge length, α is the trapezoid angle. and E is the elongation of the material at break. The form of the term f( E) is deter mined by the shape of the force-elongation curve and has been calculated for the linear case and for cases yielding convex and concave curves. The validity of this equation has been evaluated with experiments on cotton and rayon fabries in which gauge length, trapezoid angle, thickness and elongation were varied The data are reasonably well tit by the equation using the form of f( E) calculated for a linear force-elongation curve.