The role of the skin and core regions in controlling the effects of V-notches, on the fracture behavior of PET injection-moldings, was correlated with their tensile and impact properties. Investigations revealed that there were three distinct fracture behaviors: ductile, semiductile, and brittle fracture transitions. The notch sensitivity factor for strength (K S ) in the ductile and semiductile transitions indicates that the fracture strength was not sensitive to 1.5 mm deep notches, which is considered the skin region. The introduction of core-deep notches (>1.5 mm) resulted in a rapid increase in K S . On the other hand, the notch sensitivity factor for energy (K T ) shows that the fracture energy was not sensitive at 0.5 mm deep notches. However, K T increased drastically when notches >0.5 mm deep were introduced. The development of an anisotropic skin-core structure in injection moldings is well acknowledged. This is revealed in a constant fracture behavior between 0.6 and 1.0 mm deep notches. Notably, there was a drastic change in the fracture pattern from ductile to semiductile at a critical 0.6 mm deep notch. The specimens experienced a mixed fracture behavior at 1.5 mm deep notch, which marks a transitional fracture pattern at the interface between the skin and core regions. Lastly, a constant fracture behavior was observed at notch depths !1.5 mm. Results show that crack opening, in the samples that had semiductile fracture, was a postnecking phenomenon. Before shear yielding, two shear lines that intersected at 45 were seen to originate from the crack root when a 1.2 kN load was applied. Conversely, crack opening and failure occurred simultaneously in brittle fractures. It is obvious that V-notches provided a gradual transition in fracture behavior from the skin to the core regions, which confirms that the fracture behavior of PET injection moldings can be dependent on the skin and core structure.
Hairline cracks are microscopic defects found in most injection-molded components. The effects of such hairline cracks on the fracture modes of injection moldings, especially polyethylene terephthalate (PET), are still not well understood. The slow crystallizing nature of PET results in a distinct skin-core structure when it is injection molded. By controlling the hairline crack depth, the crack tip can be situated within the skin region of the moldings. The fracture behavior of hairline-cracked PET specimens during tensile loading was found to be dependent not only on the crack depth but also on the region in which the crack tips were situated. In contrast to our previous studies on a standard v-notch, very shallow hairline cracks resulted in catastrophic fracture of PET components. Crystallinity distribution along the skin core was found to affect the notch sensitivity of the materials. Annealing the specimens in hot oil was expected to create a crystallized outermost skin (referred to as the pseudo-skin) in the samples and remove internal stresses. This was intended to alter the skin-core ratio to evaluate its effect on the toughness of hairline-cracked specimens. The results revealed that annealing created a pseudo-skin layer and improved the toughness of the materials. Keywords: annealing; hairline crack; stress concentrators; notch sensitivity factor for energy; notch sensitivity factor for strength; toughness INTRODUCTION Very sharp microscopic defects are known to affect the fracture modes of engineering components. Unlike v-notches, 1 crack initiation and propagation in sharp microscopic cracks (also known as hairline cracks) is very rapid and catastrophic. It is believed that a shallow hairline crack can cause a drastic change in the fracture behavior of polyethylene terephthalate (PET) injection moldings. The skin-core structure of injection moldings is believed to affect their fracture modes. Another element of interest in the fracture modes of hairlinecracked injection-molded PET components is the possible effect of annealing. It is believed that annealing the components in hot oil creates a crystallized outermost skin (referred to as the pseudo-skin). Annealing is performed uniformly to remove internal stresses, thus improving the fracture toughness of the materials. A study revealed that the differences in the percent crystallinity and residual stress levels between the skin and core layers decreased considerably after annealing. 2 However, in another study, injection-molded PET was annealed at 120 1C for 4 h to increase its crystallinity. 3 The effect of annealing on the skin-core morphology and fracture behavior of hairline-cracked samples was correlated with the toughness and notch sensitivity of virgin polyethylene terephthalate (VPET) and recycled polyethylene terephthalate (RPET) products. The effects of hairline cracks on the notch sensitivity factors for strength (k S ) and energy (k T ) at the successive crack depths before and after annealing the samples were
Abstract-Recovering deleted or hidden data is among most important duties of forensics investigators. Extensive utilisation of smartphones as subject, objects or tools of crime made them an important part of residual forensics. This chapter investigates the effectiveness of mobile forensic data recovery tools in recovering evidences from a Samsung Galaxy S2 i9100 Android phone. We seek to determine the amount of data that could be recovered using Phone image carver, Access data FTK, Foremost, Diskdigger, and Recover My File forensic tools. The findings reflected the difference between recovery capacities of studied tools showing their suitability in their specialised contexts only.
Polyethylene terephthalate (PET) is often thought to be extremely notch sensitive, as other amorphous or slow-crystallizing polymers. However, our previous studies demonstrated three distinct fracture modes during tensile loading of PET when notch depth was gradually increased, namely ductile, semiductile and brittle. Therefore, the notch sensitivity of PET is thought to depend on its morphology and notch depth, whereas linear elastic fracture mechanics would be inadequate to characterize the fracture toughness of PET, especially when the material is injection molded and possesses the characteristic skin-core structure. This study revealed that the temperature at the crack tip significantly increased because of local plastic deformation when the crack tip was situated within the skin region. As such, full ligament yielding was obtained, which allowed characterization of the fracture toughness of PET using the principles of essential work of fracture (EWF). However, no significant temperature increase occurred when specimens fractured in a brittle manner as the notch penetrated through the skin and into the core. At this point, the material is said to be notch sensitive and is no longer able to undergo any plastic deformation before failure.
The accurate prediction of scientific and statistical parameters depends, largely, on the functional correlation between the variables involved, as well as their definite and predictable behaviour around the line of best fit (the regression line). Hence, an accurate and reliable prediction of the fracture toughness and mechanical properties of glass fibre reinforced polypropylene (GFPP) composites is very important in order to prevent sudden failure of components fabricated from them. It also gives the opportunity to save cost on experimentation. Three grades of polypropylene (PP) of different molecular weights (MW) had glass fibre reinforcements each. The incorporation of maleic anhydride grafted PP (MA-g-PP) into the composites was for compatibilisation between the matrix and reinforcements. There was tensile testing on both the un-notched and notched dumbbell samples. The use of J-Integral analysis was to investigate the fracture toughness of the composites. There was an evaluation of the effect of different MW of PP on the fracture toughness and impact properties of the composites. The examination of the morphology of the materials was by (SEM), while charge coupled device (CCD) camera monitored crack propagation behaviour under tensile loading. There was further morphology study to analyse the fibre length distribution and interfacial shear strength of the composites. Investigations revealed that the composite with the lowest MW PP consistently showed better mechanical, impact and fracture toughness properties than those with higher MW PP. It was also observed that incorporating MA-g-PP as compatibilisers had greater interfacial bonding effects, resulting in higher interfacial shear strength in GFPP of lower MW and high melt flow rates than those of higher MW and low melt flow rate. The investigation further revealed that as the MW of PP decreased, the fracture toughness and other mechanical properties increased in a linear correlation. Crack propagation trailed the interface between PP matrix and GF because of weak bonding in the composite with higher MW PP. This study revealed that incorporating MA-g-PP into GFPP composites achieved better properties with PP of lower MW and higher melt flow rate. Hence the mechanical and fracture properties of GFPP depend largely on the MW of the polymer PP matrix. There was a strong linear correlation observed between the MW of PP, fracture toughness and other impact properties thereby making it possible to make a reliable prediction of any property of the composites from another.
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