Prediction of Delamination Onset and Critical Force in Carbon/Epoxy Panels Impacted by Ice Spheres

Polymer matrix composite structures are exposed to a variety of impact threats including hail ice. Internal delamination damage created by these impacts can exist in a form that is visually undetectable. This paper establishes an analysis methodology for predicting the onset of delamination damage in toughened carbon/epoxy composite laminates when impacted by high velocity ice spheres (hailstones). Experiments and analytical work focused on ice sphere impact onto composite panels have determined the failure threshold energy as a function of varying ice diameter and panel thickness, and have established the ability to predict the onset of delamination using cohesive elements in explicit dynamic finite element analysis. A critical force associated with damage onset was found to be independent of the ice diameter and thus can be expressed as a function of basic panel-describing parameters, namely bending rigidity and interlaminar fracture energy. Critical force can be used as a failure criterion in simpler models (e.g., shell elements) when predicting the onset of delamination by high speed spherical ice impact.     

The effects of explicit timing on mathematics performance in second-grade Caucasian and African American students.

Investigated and compared the effects of explicit timing on mathematics problem completion rates in African American and Caucasian 2nd-grade students. During explicit timing, students were told they were being timed and instructed to circle the last problem completed at each 1-min interval. A repeated measures ANOVA showed that both African American and Caucasian 2nd-grade students increased their problem completion rates when explicit timing procedures were implemented, but no significant cross-group differences were found in the increase in problem completion rates or problem accuracy. Although some have suggested that African American students have different learning styles, which are related to their conceptualization of time, this study showed that an explicit timing procedure was equally effective for increasing problem completion rates among African American and Caucasian students. Discussion focuses on applied and theoretical implications of culture-treatment interactions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Student performance on and preferences for mathematics word problems: An investigation of the effects of explicit timing and interspersing procedures.

A within-subjects design was used to compare two instructional methods--explicit timing and interspersing--with 187 second-grade students. Students completed two mathematics word problem assignments under differing conditions. In the explicit timing condition, students were told they would be timed while working long mathematics word problems. In the interspersing condition, students were presented with brief mathematics problems between longer target mathematics word problems, but were not told they would be timed. Students then completed a preference survey. Results indicated that both assignments produced similar problem completion rates and the number of target problems and accuracy remained constant. Students reported that the explicit timing assignment required the most time and effort to complete and was the most difficult; however, there was no preference for homework choice. The significance of the current findings in relation to the explicit timing and interspersing literature, the continued need to compare two or more interventions directly, and the significance of student preference data are discussed. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

The damage resistance of quasi-isotropic carbon/epoxy composite tape laminates impacted by high velocity ice

Transverse impact from hail ice can create internal damage to composite structures that is not visually detectable and is therefore a damage tolerance concern. This paper focuses on the experimental characterization of the damage resistance of laminates made from T800/3900-2 carbon/epoxy tape material to impact by high velocity ice spheres, i.e., simulated hail ice (SHI). The failure threshold energy (FTE) defining the onset of damage was found for three panel thicknesses (1.59, 3.11, and 4.66 mm), each impacted by three ice diameters (38.1, 50.8, and 61.0 mm). Non-destructive investigation techniques were used to detect, map, and characterize the delaminated area. A regression analysis was used to quantitatively determine the FTE of the tape laminates, which was found to closely match previous woven carbon/epoxy FTE data. Both data sets were found to exhibit a linear and common relationship to the ratio of panel thickness to ice diameter (H/D). The resulting delamination patterns of the current and previous panels were found to be similar at damage initiation, but to differ for higher damage states.     

Stress intensity solutions of thermal fatigue induced cracks in a thin plate hot spot using LEFM and finite element analysis

The chief aim of this research is to marry elegant LEFM methods, published material data, and a commercially available FEA code together to represent complex crack growth and arrest phenomena in a thin plate with residual viscoplastic stresses. Additionally, representing the variation in these stress intensities across a thermal gradient to include ultimate fatigue crack arrest is of specific interest. To the authors’ knowledge, modeling fracture mechanics due to these hot spots with viscoplastic residual stresses using LEFM methods on this type structure has not been achieved. With these stress intensities, predictions are to be made regarding not only the FCG rate but the probable radial arrest distance of a nominal crack from the hot spot center.     

A comparison of two instrumental techniques used to discriminate the cooking quality of spaghetti

Pasta is a popular food whose quality can be measured by appearance, flavour and texture. Several instruments have been devised to measure texture but there is little comparative information. This study compared the TA.XT2i texture analyser with the viscoelastograph of thirty spaghetti samples. There was a high correlation between these instruments and good agreement in ranks. While both instruments provide comparable data it is not the same. Two laboratories used the texture analyser to measure cooked spaghetti firmness using their own procedures. There was good agreement in firmness, however; there were differences in the ranks for samples that fell between the extremes in firmness. We attributed these differences to variations in the instrument settings, cooking method and sample presentation used by the laboratories indicating the need to standardise the method. Using a standard method greatly improved the correspondence between the laboratories improving the r ² to 0.99 with excellent agreement in the ranking of ten samples.     

Effectiveness of a multi-component treatment for improving mathematics fluency.

An alternating treatments design was used to evaluate the effectiveness of an educational program that combined timings (via chess clicks), peer tutoring (i.e., peer-delivered immediate feedback), positive-practice overcorrection, and performance feedback on mathematics fluency (i.e., speed of accurate responding) in 4 elementary students with mathematics skills deficits. Results showed that both serving as a tutee (i.e., overt computation responding) and as a tutor (i.e., delivering feedback) resulted in increased rates of accurate responding for 3 of the students. Furthermore, all 4 students showed additional increases in fluency when performance feedback was added. Results are discussed in terms of combining intervention components designed to increase accuracy and fluency. (PsycINFO Database Record (c) 2010 APA, all rights reserved)