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2004
iii In memory of Grandma (" Grannie ") and Buck Dedicated to Jordan iv ACKNOWLEDGEMENTS First of all, I want to thank the Lord Almighty for giving me the ability to complete such an educational task and for giving me the strength and courage to go on day by day. This experience has shown me that through God, all things are possible. Next, I would like to thank the National Textile Center for providing the funds to do this research. I would also like to thank my advisor, Dr. Mary Lynn Realff for giving guidance and providing me with the opportunity to work on this project. I also appreciate the long and arduous work of Dr. Stephen Michielsen, who believed that this research could be accomplished, despite what was unsuccessful in the past. In addition, I would like to thank my co-chair, Dr. Richard Neu for helping me with technical related questions, even when he was busy, and for convincing me that this fracture mechanics research would work. I would also like to thank Dr. ...
Journal of Applied Polymer Science, 1998
The effect of relative humidity on the fracture energy release rate, G Ic , for single nylon 6,6 fibers has been determined previously. In this article, it is shown that G Ic is independent of relative humidity for moisture contents of ú 2.3% once the plastic zone correction is made. G Ic is compared with various proposed mechanisms to account for fracture energy. It is shown that the energy required to disrupt or ''melt'' the crystals in the plastic zone accounts for the majority of the energy required to break the specimen, and should be considered explicitly in future analyses of fracture in semicrystalline polymers.
Polymer Engineering and Science, 1978
Nylon 6 fibers which had been relaxed to different extents by annealing were examined at fixed strains by small angle and wide angle X-ray techniques. It was found that the strain of the long period of the semicrystalline microfibrils is identical to the macroscopic fiber strain. Approximately % of the tensile deformation results from molecular shear of imperfectly oriented crystalline chains. Virtually no evidence for intercrystalline slip is found; the orientation of the intercrystalline amorphous regions results in a low compliance for the shear of crystals past one another. The majority of the microfibril deformation occurs by stretching these intercrystalline amorphous regions, accompanied by the flow of extrafibrillar amorphous material to maintain constant volume. In highly annealed fibers this "filling" mechanism is less efficient, as the amount of extrafibrillar material has been reduced during shrinkage. This effect leads to a decrease in Poisson's ratio after increasingly severe annealing. A related result of annealing is the dehomogenization of the microstructure, leading to the presence of more stress-induced "microcracks" during the stretching of annealed fibers.
Polymer, 2001
The impact fracture parameters of blends of nylon 6 and maleated ethylene±propylene rubber (EPR-g-MA) reinforced with glass ®bers as a function of glass ®ber and EPR-g-MA content were examined. Both the linear elastic fracture mechanics (LEFM) model and a modi®ed essential work of fracture (EWF) model were used to analyze the data. It was found that the addition of EPR-g-MA to unreinforced nylon 6 increased the EWF parameters u o and u d de®ned by U=A u o 1 u d`; where U/A is the total fracture energy per unit area and l is the ligament length. Beyond a critical rubber content, which coincided with the ductile-to-brittle transition, there was a large increase in u d. When glass ®ber reinforcement was used without rubber toughening, the EWF model was unable to model the observed fracture response. On the other hand, the LEFM model adequately described the fracture behavior, and it was found that the critical strain energy release rate, G IC , increased with increasing glass ®ber content. When both glass ®ber reinforcement and rubber toughening were used, the u o increased with increasing EPR-g-MA or glass ®ber content; whereas, u d increased with increasing ERR-g-MA content or decreasing glass ®ber content.
Polymer Engineering and Science, 1990
We have extended the essential work of fracture technique to allow for the determination of the plane-strain essential work of fracture. The new technique is to measure the specific work of fracture as a function of ligament length in deeply double edge notched samples. This type of data is then experimentally corrected to remove the plastic work of fracture and leave only the essential work of fracture as a function of ligament length. By extrapolating the essential work of fracture to zero-ligament length, we claim to be measuring the plane-strain essential work of fracture. This new technique was applied to two rubber toughened nylons and to a series of polyethylenes. The plane-strain essential work of fracture was found to be independent of thickness. Where comparison can be made to J-integral testing, the plane strain essential work of fracture was similar to the critical J-integral, J Ic. * Analysis done using full set of experimental data spanning both the mixed-mode and the plane-stress regions ** Analysis done by the double extrapolation procedure
Plane-stress fracture toughness of amorphous copolyester (COP) sheets of different composition and molecular mass characteristics was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. It was determined that these COPs meet the basic requirement of the EWF concept since their yielding along the full ligament preceded crack growth. A drop in load in the corresponding load-displacement (F − x) curves indicated yielding and allowed us to split both the specific essential and non-essential work of fracture (w e and w p , respectively) into their contributing terms based on yielding (w y ) and necking including fracture (w n ). Development and size of the plastic zone were studied by light microscopy (LM) and infrared thermography (IT).
International Journal of Solids and …, 2010
The micromechanisms of deformation and fracture in tension were analyzed in a commercial polypropylene nonwoven geotextile material in a wide range of strain rates. Two different loading scenarios (smooth and notched specimens) were considered to study how these mechanisms are modified in presence of a stress concentration. The nonwoven fabric presented significant deformability and energy-absorption capability, which decreased with the strain rate, together with a high level of strength, which increased with strain rate. In addition, the material was notch-insensitive as the stress concentration around the crack tip was relieved by marked nonlinear behavior, which induced crack blunting. Different experimental techniques (standard mechanical tests, in situ testing within the scanning electron microscope, digital image correlation, etc.) were used to establish the sequence of deformation and failure processes and to link these micromechanisms with the macroscopic behavior.
2016
Results from an experimental program to investigate the propagation of damage in 2D triaxially braided carbon fiber textile composites (2DTBC) under static conditions are reported. A methodology is presented in which classical concepts from fracture mechanics are generalized to address damage growth in an orthotropic and heterogeneous structural material. Along with results from the experimental program, a novel numerical technique that employs ideas from cohesive zone modeling and implemented through the use of finite element analysis is also presented. The inputs that are required to implement such a discrete cohesive zone model (DCZM) are identified. Compact tension specimen (CTS) fracture tests were carried out by loading 2DTBC coupons cyclically and monotonically. Load and load point displacement were measured. The crack initiation, propagation and crack path history was recorded using high resolution digital photography. The measurements were used to extract the fracture energ...
Polymer Engineering & Science, 2000
The plane strain fracture toughness of two ductile polymers, ultra high molecular weight polyethylene (UHMWPE) and acrylonitrile-butadiene-styrene (ABS), was measured by using the essential work of fracture approach. Truly plane strain fracture toughness (Wre) was measured for ABS at quasi-static and impact rates of loading. For UHMWPE, the measured values were only "near" plane strain values (Wre*). It was confirmed both w,~* and wIe were independent of specimen type but dependent on strain rate. For UHMWPE, there was a negative strain rate effect, i.e., wIe* decreased with increasing loading rate. At low quasi-static loading rate (v 5 10 mm/min), wr,* was constant at 55 kJ/m2. It then decreased to 15 W/m2 when the loading rate was increased to 100 mm/min, and remained at that value even up to impact rate of loading (u = 3.7 m/s). For ABS, a mild positive strain rate effect was observed. w,, increased from 13 kJ/m2 at u = 10 mm/min to 17 kJ/m2 at u = 3.7 m/s.
1999
In this study, the microstructure of injection-molded polypropylene reinforced with cellulose fiber was investigated. Scanning electron micros-The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service. copy of the fracture surfaces and X-ray diffraction were used to investigate fiber orientation. The polypropylene matrix was removed by solvent extraction, and the lengths of the residual fibers were optically determined. Fiber lengths were reduced by one-half when compounded in a high-intensity thermokinetic mixer and then injection molded. At low fiber contents, there is little fiber orientation; at high fiber contents, a layered structure arises. To better understand mechanisms of fracture under impact loading, dynamic fracture analysis was performed based on linear elastic fracture mechanics. Dynamic critical energy release rates and dynamic critical stress intensity factors were deduced from instrumented Charpy impact test measurements. Dynamic fracture toughness increased with cellulose content and with orientation of fibers perpendicular to the crack direction. A preliminary evaluation of a simple model relating the microstructure to the dynamic fracture toughness shows promise, but further work is needed to assess its validity.
Polymer Bulletin, 1999
The essential work of fracture (EWF) fails for the toughness determination of polymers showing a decrease of the specific work of fracture, as a function of the specimen ligament. This type of behaviour was observed for poly(butylene terephthalate) (PBT) and its core/shell rubber modified blend (PBT/CS). It was found that this peculiar behaviour is due to a ductile-to-brittle transition (DBT) in the crack propagation phase. Experimental data were corrected by considering only the ductile-fractured specimen area. When a non linear function of the type y=a+bx -1 was applied for the corrected specific work of fracture and ligament data, the specific essential work of fracture (w e ) could be deduced. The latter being an inherent material toughness parameter was compared with the critical J-integral (J c ) values and a good correlation was found between them.
2009
The research purposes to implement fracture based approach in failure analysis of nylon 600 fiber and also partly embedded nylon 600 fiber in cementitious matrix. The methods are experiment and analytical by modeling. The experiment activities consist of tension test of nylon 600 fiber and pullout test of partly embedded nylon 600 fiber in cementitious matrix with length 150 mm and 180 mm. The research meets conclusions: (1) Whenever fracture takes place, it is always an unstable crack; (2) Stable cracks are established by the presence of crack arrester; (3) After the establishment of stable cracks, increasing strain beyond strain 1 will not increase stress 1 , hence do not induce additional fracture; (4) Increasing of strain after the establishment of stable cracks in point g will increase stress, the second slip will not take place; (5) Broken nylon fibers have a longer embedded length because of the possibility of crack arrester presence is bigger than the shorter ones; and (6) Since the middle right side of matrix is at the intersection point with fiber acts as crack arrester in the beginning of pull-out process, then the load-displacement (P- and stress-strain () curves of pull-out test will be the same as the load-displacement (P-and stress-strain () curves of fiber tension test.
Composites Part A: Applied Science and Manufacturing, 1999
The Mode I interlaminar fracture toughness properties of vinyl ester-based composites reinforced with fibreglass manufactured by the advanced textile technologies of braiding, knitting, stitching and through-the-thickness weaving are assessed in comparison to a variety of traditional composites made from fibreglass such as unidirectional or woven rovings. The interlaminar fracture toughness (G Ic) of braided and knitted composites are higher than traditional composites by factors of more than two and four, respectively. Toughening in these textile composites was caused by extensive crack branching as the interlaminar crack was forced to follow a tortuous path through the complex fibre architectures. The G Ic values of the composites reinforced in the through-thickness direction by weaving or stitching were higher than traditional composites by factors of nearly two and three, respectively, with the main toughening mechanism being crack bridging by the through-thickness binder yarns/stitches. A review of Mode I interlaminar fracture data collected from papers shows that advanced textile techniques are capable of manufacturing composites with substantially improved delamination resistance.
Macromolecular Materials and Engineering, 2008
Journal of Materials Science, 1984
Nylon 6 fibres were irradiated with up to 60 Mrad of cobalt-60 gamma radiation in air or up to 76 #C of 4.5 MeV cyclotron-accelerated protons in vacuum. The products of the irradiations were then characterized by the resulting changes in ultimate mechanical properties and in fracture morphology when tested at 21~ under selected conditions of fibre moisture content. In general, the results show a progressive deterioration of tensile properties, an increase in torsional brittleness, and a decreasing flex life, with increasing dose, and display some dependence onfibre moisture content. The effects are most pronounced in the first interval of dose; i.e. 10 Mrads gamma irradiation and 12/~C of high energy protons. The rate of deterioration of these mechanical properties generally slows with subsequent dose increases. An annulus/core fracture morphology was found in the gamma-irradiated material which was not in evidence for material irradiated with protons in vacuum. The morphology of the proton-irradiated tensile breaks are characteristic of a brittle material; flex fatigue fracture surfaces were often characterized by step-like formations reminiscent of kink bands. Discussion of the data in terms of radiochemical processes, structure-property relations, and implications for accelerated ageing programs are included.
Polymer, 2005
This paper presents a new test method that measures fracture toughness of polymeric materials when subjected to in-plane shear loading (mode II), and compares the toughness with that in tension mode (mode I). The new test method uses an Iosipescu device to apply the shear load, and determines the toughness based on the concept of essential work of fracture (EWF). Three physical-based criteria were used to verify the occurrence of mode II fracture. The new test method was then used to evaluate toughness of poly(acrylonitrile-butadiene-styrene) (ABS). The results suggest that for the ABS, the ratio of toughness in mode II to mode I is about 2.5 which leads to the dominance of mode I fracture in most loading conditions. The results also showed that for ABS in mode I fracture, the specific work of fracture (defined as the absorbed energy for fracture divided by the cross sectional area of the ligament between the notch tips) depends on ligament length; while in mode II fracture, it depends on ligament thickness. The study concludes that the new test method has a good potential for evaluation of mode II fracture toughness of polymers, though further study using polymers of different characteristics will be needed to confirm universality of the test method in the measurement of mode II fracture toughness.
Journal of Materials Science, 2004
Hybrid composite systems consisting of liquid crystalline polymer (LCP), short glass fibers and toughened nylon in varied ratios were studied. Dynamic mechanical results indicated that, elastomeric phase in toughened nylon 6,6 promoted a better compatibilization between nylon 6,6 and LCP in a hybrid system containing short glass fibers in comparison with one without glass fibers. Improved compatibility facilitated fibrillation of LCP phase in the skin region of the hybrid composite, thereby providing superior tensile strength. Without the presence of LCP, glass fiber reinforced toughened nylon 6,6 exhibited the least tensile strength. J-integral analysis and essential work of fracture (EWF) method were used to compare the fracture behavior of composites. Results showed that specific essential work of fracture were consistent with the critical J-integral. Matrices reinforced by LCP alone showed (Shing-Chung Wong) 2 best crack initiation and propagation toughnesses, followed by glass fiber reinforced and hybrid composites. The better compatibility between nylon 6,6 and LCP appeared to inhibit the interfacial debonding process, resulting in brittle fracture.
Engineering Fracture Mechanics, 2002
Composites Part B: Engineering, 2002
The mode I inter-laminar fracture toughness of advanced knitted textile composites was investigated. Two complex weft-knitted glass fabrics were selected for the study: a triple rib knit and a Milano knit were impregnated with a tough epoxy resin and tested using a double cantilever beam geometry. For both knitted composites, the influence of the growth direction was studied by investigating crack propagation in both the wale and course directions. The fracture toughness was quantified by determining the critical strain energy release rate ðG IC Þ using the modified beam theory. The specimens had to be stiffened with layers of glass woven composites added on top and bottom of the beams. This was necessary in order to avoid plastic deformation of the beams and crack deviation out of the inter-laminar plane. The results clearly showed that knitted fabric composites have exceptional inter-laminar fracture toughness properties, namely, more than 7000 J/m 2. The origin of the high G IC values, which are superior to woven or UD laminates, lies in the very complex fabric architecture. The threedimensional loop structure induces various energy consuming mechanisms, which do not occur in other composites. Toughening mechanisms such as crack branching, friction, yarn bridging and breakage were identified using scanning electron microscopy.
Journal of Materials Science, 1975
Axial compression fracture of carbon fibres was studied by embedding single fibres in epoxy resin and compressing the specimens parallel to the fibre axis. By careful optical monitoring of the fibre surface the earliest stages of fracture were identified leading to estimates of the fibre axial compression failure strengths. Compression strength decreases markedly from about 2.2 GN m-~ for moderately oriented fibres to < 1 GN m-= for highest modulus filaments. The trend towards decreasing compression strength with increasing anisotropy is explained on the basis of an increasing fibre microfibrillar nature. However fracture morphology studies show that the unduly rapid strength decrease results from an increasing degree of fibre outer layer ordering which accompanies increasing axial anisotropy in carbon fibres since cracking occurs first on the more highly aligned filament surfaces. It is suggested that fibre compression fracture changes from a shear to a microbuckling or kinking mode with increasing fibre anisotropy, where the latter initiates in individual, well-aligned but uncoupled microfibrils. The similarity of fine axial compression fractures in oriented carbon fibres to those found in elastica loop experiments is noted as are the possible implications which the low strain-to-failure in compression of very high modulus fibres might have for practical composites.
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