No abstract

This study considers laminate warpage (or spring-in) associated with the curing of reinforced thermoset composites. This processing induced spring-in shows up in flat parts as well as those with curved geometry, and is most prevalent with parts that do not have a closed cross section. Three contributions to spring-in were considered, namely: thickness cure shrinkage, mold expansion, and fiber volume fraction gradients. These effects were combined into a predictive finite element model (FEM). Mold stretching and thickness shrinkage were described through a modified in-and out-of-plane material thermal expansion, respectively, while fiber volume fraction gradients were accounted for by scaling the thermal and elastic properties of the composite through the thickness with the fiber volume fraction. For thin parts (< 2 mm) spring-in was dominated by fiber volume fraction gradient and mold stretching effects. For thick parts (> 2 mm) spring-in was dominated by thickness cure shrinkage. The FEM was able to account for 80% of the observed spring-in for parts ranging between 1 and 5 mm thick and having a 3 to 13 mm bend radius. This study shows that a relatively simple, linear-elastic analysis, can accurately describe competing contributions to spring-in over a range of material types and thicknesses.

Numerical simulation of sport ball impacts is challenging due to the varied contact conditions involved and the difficulty in characterizing nonlinear materials at high strain rates. The following considers rigid polyurethane foam used in softballs. Past works have shown that load displacement curves from viscoelastic material models do not completely agree with experiment, suggesting incorrect mechanisms of compressive deformation. Additionally, dynamic testing using a pressure bar apparatus was unable to achieve strain rates low enough, and dynamic mechanical analysis was unable to achieve strain magnitudes large enough to represent play conditions. A method was developed to impact polyurethane foam samples over a range of displacement rates and magnitudes representative of play conditions. A characteristic stress–strain loading curve of polyurethane foam was produced and incorporated into a finite element model. Comparison of instrumented and numerical impact properties produced agreeable results. The foam material model was then applied to a simulated softball which impacted surfaces of varying geometry. Results showed that the foam material model better predicted softball deformation mechanisms than previous linear–viscoelastic models both as a function of speed and surface geometry. Persistent discrepancies in rate dependence indicate a lack of complete characterization, however.

Head impact sensors are increasingly used to quantify the frequency and magnitude of head impacts in sports. A dearth of information exists regarding head impact in un-helmeted sport, despite the substantial number of concussions experienced in these sports. This study evaluated the performance of one small form factor head impact sensor in both laboratory and field environments. In laboratory tests, sensor performance was assessed using a Hybrid III headform and neck. The headform assembly was mounted on a low-friction sled and impacted with three sports balls over a range of velocities (10-31 m/s) at two locations and from three directions. Measures of linear and angular acceleration obtained from the small form factor wireless sensor were compared to measures of linear and angular acceleration obtained by wired sensors mounted at the headform center of mass. Accuracy of the sensor varied inversely with impact magnitude, with relative differences across test conditions ranging from 0.1% to 266.0% for peak linear acceleration and 4.7% to 94.6% for peak angular acceleration when compared to a wired reference system. In the field evaluation, eight male high school soccer players were instrumented with the head impact sensor in seven games. Video of the games was synchronized with sensor data and reviewed to determine the number of false positive and false negative head acceleration event classifications. Of the 98 events classified as valid by the sensor, 20.5% (20 impacts) did not result from contact with the ball, another player, the ground or player motion and were therefore considered false positives. Video review of events classified as invalid or spurious by the sensor found 77.8% (14 of 18 impacts) to be due to contact with the ball, another player or player motion and were considered false negatives.

Current timber waterfront structures require the use of treated lumber to reduce environmental damage. Wood–plastic composites (WPCs) have been proposed as an alternative to treated timber due to their potential for reduced water absorption and degradation without chemical treatment. Even though WPCs are primarily made of wood (58% wood flour in the current case) their mechanical response to applied loads is quite different. The current study attempts to describe the time and temperature dependence of a commercial WPC formulation. Characterization of the formulation was undertaken by means of a series of creep and recovery tests. A Prony Series was used to describe the material’s time dependent compliance, where time was shifted with stress and temperature to describe the observed nonlinear response and temperature dependence. Damage effects were successfully correlated isothermally using an effective stress. The model was evaluated by comparing the behavior predicted by the model with experimental results in 3-point bending and fatigue.