Characterising resistance to fatigue crack growth in adhesive bonds by measuring release of strain energy

Measurement of the energy dissipation during fatigue crack growth is used as a technique to gain more insight into the physics of the crack growth process. It is shown that the amount of energy dissipation required per unit of crack growth is determined by Gmax, whereas the total amount of energy available for crack growth in a single cycle is determined by (Δ √g)2

Prognostic value of early, conventional proton magnetic resonance spectroscopy in cooled asphyxiated infants

BACKGROUND: Neonatal hypoxic-ischemic encephalopathy (HIE) commonly leads to neurodevelopmental impairment, raising the need for prognostic tools which may guide future therapies in time. Prognostic value of proton MR spectroscopy (H-MRS) between 1 and 46 days of age has been extensively studied; however, the reproducibility and generalizability of these methods are controversial in a general clinical setting. Therefore, we investigated the prognostic performance of conventional H-MRS during first 96 postnatal hours in hypothermia-treated asphyxiated neonates. METHODS: Fifty-one consecutive hypothermia-treated HIE neonates were examined by H-MRS at three echo-times (TE = 35, 144, 288 ms) between 6 and 96 h of age, depending on clinical stability. Patients were divided into favorable (n = 35) and unfavorable (n = 16) outcome groups based on psychomotor and mental developmental index (PDI and MDI, Bayley Scales of Infant Development II) scores (>/= 70 versus < 70 or death, respectively), assessed at 18-26 months of age. Associations between 36 routinely measured metabolite ratios and outcome were studied. Age-dependency of metabolite ratios in whole patient population was assessed. Prognostic performance of metabolite ratios was evaluated by Receiver Operating Characteristics (ROC) analysis. RESULTS: Three metabolite ratios showed significant difference between outcome groups after correction for multiple testing (p < 0.0014): myo-inositol (mIns)/N-acetyl-aspartate (NAA) height, mIns/creatine (Cr) height, both at TE = 35 ms, and NAA/Cr height at TE = 144 ms. Assessment of age-dependency showed that all 3 metabolite ratios (mIns/NAA, NAA/Cr and mIns/Cr) stayed constant during first 96 postnatal hours, rendering them optimal for prediction. ROC analysis revealed that mIns/NAA gives better prediction for outcome than NAA/Cr and mIns/Cr with cut-off values 0.6798 0.6274 and 0.7798, respectively, (AUC 0.9084, 0.8396 and 0.8462, respectively, p < 0.00001); mIns/NAA had the highest specificity (95.24%) and sensitivity (84.62%) for predicting outcome of neonates with HIE any time during the first 96 postnatal hours. CONCLUSIONS: Our findings suggest that during first 96 h of age even conventional H-MRS could be a useful prognostic tool in predicting the outcome of asphyxiated neonates; mIns/NAA was found to be the best and age-independent predictor

Cyclic fatigue fracture of composites: What has testing revealed about the physics of the processes so far?

Where for metallic materials fatigue fracture testing has contributed significantly to understanding macroscopic and microscopic fatigue failure, the understanding of fatigue fracture in fibre reinforced polymers (FRP) still seems limited. It appears that the research on fatigue in FRPs raises more questions, rather than providing a framework for understanding the underlying mechanisms. Open questions include for example the correct formulation of the driving force and fatigue damage resistance, how to account for mixed mode loading and fibre bridging? But also the question to what extent averaging and homogenising over time- and length scales hinders the understanding in relation to the microscopic mechanisms? To contribute to the development of understanding, this paper discusses several of these open questions. The selected questions were discussed at a recent workshop on ‘Physics of fatigue damage growth’ at TU Delft with participants from several laboratories covering expertise on fatigue fracture testing of both metals and polymer composites. The discussion focuses on potential experimental and simulation approaches that may lead to a better understanding of the physics of the fatigue fracture process

The effect of temperature on fatigue crack growth in FM94 epoxy adhesive bonds investigated by means of energy dissipation

The effect of temperature on fatigue crack growth in epoxy adhesive bonds was investigated for a range of temperatures from −55 to 80 °C. The fatigue crack growth behaviour was characterised using both strain energy release rate (SERR) and by measurements of energy dissipation. It was found that for a given maximum SERR, or a given energy dissipation per cycle, crack growth rate was higher at higher temperatures. The resistance to crack growth (in terms of energy dissipation per unit crack growth) was linearly related to the maximum SERR, and this relationship was not affected by temperature. A number of tests did show anomalous behaviour, which could be linked to differences on the fracture surfaces. Previous work had found a power-law relationship between the amount of available energy and the applied cyclic work. This relationship was found to be insensitive to temperature changes in the range of 0 °C to 40 °C, but at −55 °C and −20 °C, as well as at 60 °C and 80 °C, the behaviour was affected by temperature. This could again be linked to differences on the fracture surfaces. It was concluded that temperature does not affect crack growth by directly affecting the failure mechanisms themselves, but rather by affecting which mechanisms are active