Peter Popper

The Theoretical Behavior of a Knitted Fabric Subjected to Biaxial Stresses

Abstract : The theoretical mechanical behavior of a plain knitted fabric subjected to biaxial stresses is derived. An approximate mathematical model of the fabric structure was established from which the stress vs fabric-geometry and stress vs strain relationships were determined. The work was done by considering only the properties of the fabric structure, completely independent of the fiber properties. The results may be used for such applications as predicting the performance of a plain knitted fabric in situations where it will be stressed biaxially. (Author)

Mechanics of a Hybrid Circular Braid with an Elastic Core

A mathematical model based on four modes of operation is developed to predict the tensile response of a hybrid circular braid encompassing an elastic core. Model inputs include parameters characterizing the initial geometric configuration and ma terial properties of the hybrid. Output parameters are forces, strains, modes of operation, and hybrid geometry. The results show that a hybrid braid can display a unique syn ergism when the braid is in contact with the core but not yet locked, since its tensile response exceeds the sum of its components acting alone. While this analysis idealizes braid geometry and excludes explicit treatment of yam crimp, it incorporates primary influences including nonlinear yam tensile response, large deformations, yam failure, and varying hybrid deformation modes.

Simulating the Mechanics of Human Falls

Several estimates indicate that over 10,000 people 65 and older die annually from fallrelated injuries, making falls the leading cause of injury-related death for this age group. At present, no standard test or devices exists that can be used to simulate human falls, which makes research to undertand the mechanics of falls very difficult. To address the need for a test method and protocol for studying falls, a project was initiated to develop a device that simulates human falls and the resulting hip-floor impact. The device is designed to approximate the kinematics and kinetics of a fall event, and is configured to accept full-scale physical models of the hip, femur, fat and tissue. Key outputs of the test device include the force vs. time profile exerted on the ball of the hip during impact with the floor, the force exerted on the end (knee joint) of the femur, deformation of the hip model vs. time, and the velocity of the hip at impact. Simulations and preliminary tests with the prototype configured for an average-sized person indicate the device is capable of accurately simulating a fall and the resulting hip-floor impact. Peak force exerted on the ball of the hip during impact is similar to values from simulations and to those reported from previous studies. Preliminary tests on protective garments does reveal a significant reduction in the force exerted on the hip during fall.

Vibration Transmission in the Hand and Arm from Composite Driveline Components for Impact Tools

A multi-year study was initiated to evaluate the performance of conventional impact tools and develop improved designs that reduce vibration and sound emission. The initial focus was to improve hand struck and power tool designs using reinforced engineering polymers between metal impacting components. Non-linear models describing the equations of motion and resulting output forces were developed. In addition, several experiments with a high frequency Instron test machine and prototype tools were performed to validate the model and compare performance of conventional power tools to the new polymer based designs. The results show that although adding a polymer does significantly reduce vibration, the change in cutting (output) force is relatively small and statistically insignificant. Thus far, polymers with mineral reinforcements demonstrate higher effective stiffness and durability compared to fiber reinforced composites.

A Reinforced Polymer Hammer Cap for Eliminating Metal-to-Metal Contact and Reducing Hand-Transmitted Vibration

A hammer fitted with a polymer cap on the impact face has been developed and successfully used to drive nails. As configured, the hammer-cap assembly eliminates direct metalto- metal contact during nail driving. Unlike non-marring and other protective hammers, the design utilizes a high performance reinforced polymer which does not reduce hammer performance. Experiments to measure the nail driving characteristics of conventional hammers and hammers fitted with a polymer cap revealed no significant difference in driving capability. Vibration measurements on the handle of several hammers fitted with caps revealed that vibrations at the handle-hand interface were significantly reduced compared to conventional hammer designs. Sound pressure levels were also significantly reduced. Unlike previous work with hand-struck chisels fitted with a cap, the polymer modulus had much less effect on performance.