EXPERIMENTS AND MODELING OF PLASTIC ANISOTROPY AND DUCTILE FRACTURE UNDER MULTIAXIAL LOADING

The implementation of new materials for light-weighting purposes in the automotive industry has often been hindered due to the low ductility of these materials, as well as inadequate empirical knowledge about their fracture behavior and inadequate material modeling techniques. This thesis addresses these issues through extensive experimental and numerical study of plastic anisotropy and ductile fracture of several aluminum alloys and a stainless-steel. The test materials used for this study include AA365 die-casting, AA6013 and AA6111 aluminum sheets, AA6260 aluminum tube and SS304L stainless-steel microtube. The plastic anisotropy is assessed using uniaxial tension, plane-strain tension and disc compression experiments for the die-casting and the sheets; and using biaxial experiments for the tubes. These experiments are then used to model the anisotropic plastic behavior of the test materials using advanced non-quadratic anisotropic yield criteria including Yld2000-2D and Yld2004-3D. The fracture behavior of the casting and sheets is investigated using conventional notched tension and central hole specimens, as well as novel specimen designs for shear and biaxial stress states. These improved specimen designs exhibit stress states that develop at the neighborhood of the fracture initiation point to remain proportional throughout the loading history. Likewise, the fracture behavior of the tubes is assessed by loading them under axial force and internal pressure along different stress paths. The ability to control the force/pressure ratio enables probing the fracture behavior under proportional and non-proportional loading paths. Fracture oftentimes initiate at the interior (for example through-thickness mid-plane) of the specimens and thus direct measurement of fracture parameters i.e., stress triaxiality, Lode angle parameter and equivalent plastic strain is not possible from experiments alone. Instead, these parameters at the onset of fracture are obtained in this work using finite element modeling with the material modeling framework using anisotropic yield criteria described above. The loading path and the resulting fracture locus are found to be sensitive to the constitutive model employed, which underscores the importance of an appropriate modeling of plastic anisotropy in ductile fracture studies. Based on the finite element results, the fracture locus is represented by numerous fracture initiation criteria common in literature (e.g., Oyane, Johnson-Cook, Hosford-Coulomb and DF2015), as well as a newly proposed one, created during the course of this research, that is shown to offer better agreement with the experiments, without additional calibration or implementation cost

Experimental Investigation of Plastic Anisotropy of Commercially-Pure Titanium

The plastic anisotropy of Commercially-Pure Titanium (CP-Ti, or Grade 2 Ti) is probed with an extensive set of experiments, performed on a hot-rolled CP-Ti plate of 12.7 mm thickness. The experiments reported here are: 1) uniaxial tension at 15o angles to the rolling direction (RD) of the plate, 2) uniaxial tension in the thickness, or normal direction (ND) of the plate, 3) uniaxial compression at 15o angles to the RD, as well as in ND, 4) plane-strain tension at 15o angles to the RD, 5) plane-strain compression at 15o angles to the RD. A total of 30 experiments (and two repetitions for each) were performed for this research. The uniaxial tension and compression experiments involve standard specimen geometries, except for the uniaxial tension in the ND, which required the creation of a custom, miniature tensile specimen. A novel method for determining the stresses in a plane-strain tension experiment was devised using finite element analysis. The experiments reveal the plastic anisotropy and tension-compression asymmetry of CP-Ti. Furthermore, they are used to calibrate 3 commonly used constitutive models (yield functions). While not pursued here, it is expected that this improved understanding and representation of the behavior of CP-Ti can lead to improved numerical simulations of material response during manufacturing or service

Accuracy of Glucose Meter Among Adults in a Semi-urban Area in Kathmandu, Nepal

Introduction: Glucose meters are gaining popularity in monitoring of blood glucose at household levels and in health care set-ups due to their portability, affordability and convenience of use over the laboratory based reference methods. Still they are not free of limitations. Operator’s technique, extreme temperatures, humidity, patients’ medication, hematocrit values can affect the reliability of glucose meter results. Hence, the accuracy of glucose meter has been the topic of concern since years. Therefore, present study aims to evaluate the analytical and clinical accuracy of glucose meter using International Organization for Standardization 15197 guideline. Methods: A community based descriptive cross-sectional study was conducted in Kapan, Kathmandu, Nepal in April 2018. Glucose levels were measured using glucose meter and reference laboratory method simultaneously among 203 adults ≥20 years, after an overnight fasting and two hours of ingestion of 75 grams glucose. Modified Bland-Altman plots were created by incorporating ISO 15197 guidelines to check the analytical accuracy and Park error grid was used to evaluate the clinical accuracy of the device. Results: Modified Bland-Altman plots showed>95% of the test results were beyond the acceptable analytical criteria of ISO 15197:2003 and 2013. Park Error Grid-Analysis showed 99% of the data within zones A and B of the consensus error grid. Conclusions: Glucose meter readings were within clinically acceptable parameters despite discrepancies on analytical merit. Possible sources of interferences must be avoided during the measurement to minimize the disparities and the values should be interpreted with caution.

Acoustic Emission Sensors to Monitor Early Onset of Necking During Uniaxial Tension

Elastic waves are generated and propagate when a material undergoes plastic deformation and can be detected by acoustic emission (AE). In this work, AE measurements are obtained during a uniaxial tension (UT) test using a custom-made sensor employing piezoelectric crystals. The UT tests are performed on an MTS machine with two AE sensors clamped on each end of the specimen gage section. A low pass Butterworth filter is designed to attenuate the high frequency noise from the AE signals. Also, full-field strain measurements on the specimen surface are acquired using the 2-D digital image correlation (DIC) method. A typical result from a UT test reveals, as the plastic deformation increases, the AE signals from each sensor increase until they reach a maximum value followed by a drop of signal until the specimen fractures. It is found through interrogation of the DIC images that the maximum amplitude from the AE signals corresponds to the early onset of localized necking. The goal of this work is to implement the UT findings in an actual forming process (e.g., cup drawing) and monitor the event in real time using closed loop control to achieve improved formability

Assessment of High-Risk Human Papillomavirus Infections Using Clinician- and Self-Collected Cervical Sampling Methods in Rural Women from Far Western Nepal

IntroductionNepal has one of the highest cervical cancer rates in South Asia. Only a few studies in populations from urban areas have investigated type specific distribution of human papillomavirus (HPV) in Nepali women. Data on high-risk HPV (HR-HPV) types are not currently available for rural populations in Nepal. We aimed to assess the distribution of HR- HPV among rural Nepali women while assessing self-collected and clinician-collected cervico-vaginal specimens as sample collection methods for HPV screening.MethodsStudy participants were recruited during a health camp conducted by Nepal Fertility Care Center in Achham District of rural far western Nepal. Women of reproductive age completed a socio-demographic and clinical questionnaire, and provided two specimens; one cervical-vaginal specimen using a self-collection method and another cervical specimen collected by health camp auxiliary nurse midwives during a pelvic examination. All samples were tested for 14 different HR-HPV mRNA and also specific for HPV16/18/45 mRNA.ResultsOf 261 women with both clinician- and self-collected cervical samples, 25 tested positive for HR-HPV, resulting in an overall HR-HPV prevalence of 9.6% (95% confidence Interval [CI]: 6.3-13.8). The overall Kappa value assessing agreement between clinician- and self-collected tests was 0.62 (95% CI: 0.43-0.81), indicating a "good" level of agreement. Abnormal cytology was reported for 8 women. One woman identified with squamous cell carcinoma (SCC), and 7 women with high grade squamous intraepithelial lesions (HSIL). Seven of the 8 women tested positive for HR-HPV (87.5%) in clinician-collected samples and 6 in self-collected samples (75.0%).ConclusionThis is the first study to assess HR-HPV among rural Nepali women. Self-collected sampling methods should be the subject of additional research in Nepal for screening HR-HPV, associated with pre-cancer lesions and cancer, in women in rural areas with limited access to health services