Philosophiae Doctor - PhD / Mental skills are critical to the development of a positive mind-set in the athlete in order to develop the ability to concentrate more, focus attention, manage arousal, develop self-esteem and thereby enhance self-confidence in competitive sport. This study was an exploratory study to investigate how well Zimbabwean inter-collegiate athletes were prepared mentally and how this was handled during their participation in inter-collegiate games. The study focused specifically on just one group of athletes, netballers.

The phase separation process of a rubber modified epoxy system during cure was analyzed by a model developed on the basis of a thermodynamic description of binary mixture and constitutive equations for nucleation and growth rates. As epoxy resins are cured, rubber molecules are precipitated from the epoxy matrix to a non-equilibrium composition due to the decrease in the configurational entropy and the increase in the viscosity with conversion. If phase separation takes place in a metastable region, this model can monitor the changes of rubber compositions in both phases as well as the changes in the number and size of rubber particles upon conversion of polymerization. The particle size distribution at the completion of phase separation was also calculated. The effect of cure temperature on the final morphologies of a rubber modified epoxy system was discussed. The computed particle size distributions for piperidine and diaminodiphenyl sulfone cured systems showed good agreements with experimentally measured values. Depending on the activation energy for viscous flow of the epoxy matrix relative to that for the polymerization, the particle size distribution may show bimodal or unimodal distribution. The size of rubber rich phase increases to a maximum and then decreases with an increase in cure temperature. However, due to limitations of temperature range to probe in an actual experiment, one may observe only either decreasing or increasing particle size as cure temperature increases. The number of rubber particles per unit volume increases for the DGEBA/DDS/ETBN system as cure temperature increases in the temperature range of 30 °C to 220 °C. Fracture toughness of cured DGEBA/DDS/ETBN system was analyzed in terms of morphologies generated by the temperature variation. Since the volume fraction of rubber particles did not change with cure temperature, the critical stress intensity factor did not vary significantly with cure temperature as expected. However, increases in cure temperature produced smaller but more numerous particles. The critical stress intensity factor normalized by the number density of particles exhibited dependence on the radius of particles to the third power. On the other hand, the critical stress intensity factor normalized by the radius of particles showed a linear dependence with respect to the number density of particles. / Ph. D.

Indiana University-Purdue University Indianapolis (IUPUI) / Since the conception of three dimensional (3D) printing circa 40 years ago, there has been the proliferation of several additive manufacturing (AM) technologies that enable its use in everyday applications such as aerospace, medicine, military, oil and gas and infrastructure. In order to improve its applicability and growth, 3D printed materials are subjected to the same or even higher levels of scrutiny for its mechanical behavior as its conventionally manufactured counterpart. One of the most important mechanical properties is toughness or the ability of a material to undergo large strain prior to fracture when loaded. The toughness of a material can be correlated to its impact energy or the increase in internal energy due to impact. In this study, the impact properties, including the toughness of 3D printed 15-5 stainless steel were investigated at low temperature (77 K), room temperature (298 K) and high temperature (723 K) using experimental and numerical modeling of the Charpy impact test. In addition, ballistic impact simulations were performed to determine the applicability of 3D printed 15-5 stainless steel in the defense industry. The 15-5 stainless steel specimens were printed (horizontal-build) using the direct metal laser sintering (DMLS) technique, cooled or heated to the specified temperature, then tested in accordance with the ASTM E23-2016b [1] standard. The Johnson-Cook (J-C) phenomenological material model and fracture parameters were used in the numerical modeling. The cross-sectional microstructures of surfaces and impact energies of the Charpy impact test were examined. For the ballistic impact simulations, a 3D printed 15-5 stainless steel typical plate was investigated at the same temperatures as the Charpy impact test. A typical missile using the J-C properties at room temperature (298 K) was assigned an initial velocity of 300 ms-1 for each plate temperature. The fracture surface investigation (microsurface analysis as well as visual inspection) and impact energy values of the Charpy impact test show that the 3D printed 15-5 stainless steel exhibited brittle behavior at low and room temperatures, but transitioned into a more ductile behavior at high temperature. At 77 K, 298 K and 723 K, the experimental Charpy impact test results were 0.00 J/cm2, 6.78±4.07 J/cm2 and 50.84±3.39 J/cm2 respectively; whereas the simulated impact energy were 1.05 J/cm2, 10.46 J/cm2 and 47.07 J/cm2 respectively. Hence, the impact energy for the experimental and numerical simulations were in good agreement; especially at higher temperatures. Consistent with the results from the Charpy impact test, the ballistic impact simulations show an increase in the impact energy, elastic plastic strain and deflection of the plate with an increase in temperature indicating brittle-to-ductile behavior. The high exit velocity at low and room temperature may not make the plate attractive in defense in its current configuration; however, at the high temperature, the exit velocity reduction was significant.

Composite materials with laminated structure provide advantages which are utilised during component design. Low density, temperature and chemical stability are the profitable properties predetermining application of ceramic laminates. The main obstacle for wide spread of ceramic materials is their inherent brittleness. Therefore, in this thesis, the crack propagation in ceramics laminates has been extensively studied. Laminated structures with various volume fractions of components (alumina and zirconia) were prepared by electrophoretic deposition. Evaluation of crack propagation through the interface and determination of basic mechanical properties was conducted on the basis of extensive literature search. Crack deflection originated in both presence of internal stresses and differences in elastic modulus during the crack interface passing was monitored. A special type of specimen geometry was employed with the aim to set arbitrary angle between crack and interface. It was experimentally found that the degree of crack deflection is dependent on entering angle and volume fraction of components. Higher crack deflection was already found in the bulk of the test piece comparing to the test piece surface. The 3D fracture surface reconstruction generated using laser confocal microscopy was used in this detailed crack propagation study. Further basic elastic and strength characteristics of laminates were determined and compared to those obtained from monolithic materials. The validity of the mix rule for elastic characteristics was confirmed by comparing of elastics modulus. The most reliable method for elastic modulus determination was marked the dynamic resonance method due to low scatter and consistency in measurement. The flexural strength of all laminates tends to be close to the flexural strength of the weakest component. Therefore the mix rule is not applicable for flexural strength estimation on the contrary of elastic characteristics. The change of component volume fraction leads only to change of flexural strength scatter. Thanks to gained knowledge about crack propagation and basic characteristic determination will be possible to design ceramic laminates more efficiently for given needs of application.

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The obesity epidemic in youth is increasing, due to the decreasing levels of physical activity within the youth population. One variable that has never been examined as a correlate of physical activity is mental toughness. Mental toughness is a psychological trait characterized by determination, resiliency, and perform optimally regardless of the circumstances. In order to evaluate this trait the MeBActive-Youth was developed. The purpose of this study was to first, evaluate the psychometric properties of the newly developed measure and secondly, to assess the relationship of mental toughness, social support and self-efficacy to physical activity. Participants (N = 106) completed a demographic survey, the MeBActive-Youth, Social Support and Exercise Survey (SSES), Physical Activity Self-Efficacy Scale (PASES), and Physical Activity Questionnaire for Adolescents (PAQ-A). The Rasch Rating Scale Model showed that the MeBActive-Youth had appropriate items (range of 0.53 – -0.64 logits) for the sample and measured mental toughness appropriately. All but four of the items had a fit statistic within the acceptable range, but only one item had a much higher statistic (infit = 1.68 logits). Although the four response options were all utilized appropriately, it may be beneficial to reduce them to three. There was a significant positive correlation between MeBActive-Youth and physical activity (ρ = .52, p ≤ .01) and PASES (ρ = .30, p ≤ .01). The correlation between social support from friends and family and MeBActive-Youth was not significant (ρ = .12, p ≥ .05; ρ = .17, p ≥ .05). There was a positive significant correlation between, physical activity and familial and friend social support (ρ = .47, p ≤ .01; ρ = .27 p ≤ .05), PASES (ρ = .34, p ≤ .01) and mental toughness. The MeBActive-Youth is a valid and reliable instrument yet can be improved with slight changes. This study showed that there is a strong positive correlation between mental toughness, self-efficacy and physical activity. Positive correlations were also found between physical activity and all the measured variables.

The aim of the present study was to review the elite Crossfit athletes’ opinions of the importance of mental toughness to achieve optimal performance. A number of other research studies looking at psychological factors such as grit and mental toughness supports the idea that elite athletes should develop their mental capacity to improve their individual performance.  The method and research design consists of a qualitative interview study where four elite crossfit athletes have been interviewed. The athletes participated in a semi-structured interview about mental toughness, the attributes of what it means to be a mentally tough athlete and if it is important to be mentally tough to become successful in the sport of Crossfit. A semi-structured interview is where a list of questions are prepared at the same time as giving the interviewee room to elaborate and raise issues that the researcher may have missed (Clarke & Braun, 2013). The interviews where then analysed through a thematic analysis. The participants that were included in this study consistedof Swedish elite Crossfit athletes.  The results were divided into four themes which were: control commitment, challenge and confidence. These themes are from the 4 C model which is the definition of mental toughness by Clough et al. (2002). Challenge seems to be something that motivates the athletes to reach higher and something they also seek out. All the athletes agreed that commitment and confidence was crucial for having success in the sport and they also mentioned that mental toughness was important to be able to perform at your absolute best. The feeling of being in control was also mentioned as something that they strived for in the competition environment.  The interview results together with the presented existing research indicates that according to elite Crossfit athletes, mental toughness is very valuable for a professional Crossfit athlete to both develop and maintain to become and stay successful in the sport. This could be valuable information for both coaches and athletes in the sport. / Det övergripande syftet med denna studie var att granska Crossfit-elitidrottares åsikter om vikten av mental toughness för att uppnå optimal prestation. Ett antal andra olika forskningsstudier som tittar på psykologiska faktorer som grit och mental toughness stödjer tanken att elitidrottare bör utveckla sin mentala förmåga för att förbättra sin individuella prestation.   Metoden och forskningsdesignen består av en kvalitativ intervjustudie där fyra elit Crossfit atleter har intervjuats. Atleterna deltog i en semistrukturerad intervju om mental tuffhet, egenskaperna för vad det innebär att vara en mentalt tuff idrottare och om det är viktigt att vara mentalt tuff för att bli framgångsrik i sporten Crossfit. En semistrukturerad intervju är där en intervjuguide förbereds samtidigt som intervjupersonen får utrymme att utveckla sina svar och lyfta frågor som forskaren kan ha missat (Clarke & Braun, 2013). Intervjuerna analyserades sedan genom en tematisk analys. Deltagarna som ingick i denna studie bestod av fyra svenska elit Crossfit atleter.  Resultaten delades sedan in i fyra teman: kontroll, engagemang, utmaning och självförtroende. Dessa teman är enligt 4 C-modellen vilket är Clough et al., (2002) definition av mental toughness. Utmaningar verkar vara något som motiverar idrottarna att sträva högre och något de också söker sig till. Alla idrottare var överens om att engagemang och självförtroende var avgörande för att nå framgång i sporten och de nämnde också att mental toughness var viktigt för att kunna prestera på topp som atlet. Känslan av att vara i kontroll nämndes också som något man strävade efter i tävlingssammanhang.  Intervjuresultaten tillsammans med den befintliga forskningen indikerar att mental toughness enligt elit Crossfit atleter, är mycket värdefullt för en elit Crossfit-atlet att både utveckla och bibehålla för att både bli och förbli framgångsrik inom sporten. Detta kan vara värdefull information för både coacher och atleter i sporten.

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HSLA-65 steel (6.4 mm thick) was friction stir welded at various welding speeds and spindle speeds. Varying weld parameters provided a range of heat inputs. Impact toughness was evaluated as a function of the different weld parameters and corresponding weld heat inputs. Charpy V-Notch (CVN) tests were conducted in parent material and at both the weld nugget centerline and heat-affected zone (HAZ) locations. The upper shelf CVN impact energy of the weld nugget was above that of the base metal for all weld parameters. The upper shelf impact toughness in the HAZ was largely unaffected by changing weld parameters. The nil-ductility transition (NDT) temperature in the weld nugget increased with increasing heat input. The toughness, with respect to the ductile-to-brittle transition, was negatively affected by the increase in heat input. The NDT temperature in the HAZ did not correlate with heat input. The microstructures and microhardness data were examined. Aspects of variation in the impact energy results were identified as the inhomogeneity of the weld microstructure and the placement of the V-notch. Weld nugget microstructures were more inhomogeneous than base metal. Hardness results showed varying values of hardness from the weld crown to the root, transversely across the weld, and longitudinally along the length. Variation due primarily to the inhomogeneity of the weld microstructure is compounded by the location of the V-notch.