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Fig. 6. Load versus displacement curves for NT fiber composites tested at 0.5 and 1.0 mm/min.

Figure 6 Load versus displacement curves for NT fiber composites tested at 0.5 and 1.0 mm/min.

Related Figures (9)

4. Results and discussion Fig. 1. (a) Compact tension (CT) specimen dimensions. (b) Detail of the pre-cracking in a neat polyurethane specimen.
4. Results and discussion Fig. 1. (a) Compact tension (CT) specimen dimensions. (b) Detail of the pre-cracking in a neat polyurethane specimen.
Fig. 2. Specimen orientations related to the molded plate for both short fiber and fabric composites with translaminar crack (L—length longitu- dinal, 7—width, S—thickness).
Fig. 2. Specimen orientations related to the molded plate for both short fiber and fabric composites with translaminar crack (L—length longitu- dinal, 7—width, S—thickness).
Fig. 3. Load versus displacement curves for the polyurethane matrix at two distinet loading rates and also for short fiber composites, as example of non-linear behavior.
Fig. 3. Load versus displacement curves for the polyurethane matrix at two distinet loading rates and also for short fiber composites, as example of non-linear behavior.
Fig. 5. Load versus displacement curves for NT and T fibers composites tested at 0.5 mm/min. Load versus displacement curves of the composites are shown in Figs. 5 and 6. All the composites, regardless the fiber treatment and applied strain-rate, showed extensive stable crack growth. In general, the curves rise in a non-lin- ear way until their maximum value. In the case of sisal fab- ric, the first load drop-off is followed by a sequence of ups and downs on load, sometimes with the curves displaying constant load shelves. It can be argued that sudden load drop-offs result from successive fiber breaks inside the yarns, whereas partial untwisting of the yarns, before frac- ture, delays the crack growth as the load is sustained. This behavior disagrees from the commonly observed in ther- mosetting resin composites (e.g. polyester and epoxy) and
Fig. 5. Load versus displacement curves for NT and T fibers composites tested at 0.5 mm/min. Load versus displacement curves of the composites are shown in Figs. 5 and 6. All the composites, regardless the fiber treatment and applied strain-rate, showed extensive stable crack growth. In general, the curves rise in a non-lin- ear way until their maximum value. In the case of sisal fab- ric, the first load drop-off is followed by a sequence of ups and downs on load, sometimes with the curves displaying constant load shelves. It can be argued that sudden load drop-offs result from successive fiber breaks inside the yarns, whereas partial untwisting of the yarns, before frac- ture, delays the crack growth as the load is sustained. This behavior disagrees from the commonly observed in ther- mosetting resin composites (e.g. polyester and epoxy) and
Fig. 4. CT specimens after test. (a) Sisal short fiber composite. (b) Sisal fabric composite.
Fig. 4. CT specimens after test. (a) Sisal short fiber composite. (b) Sisal fabric composite.
Fig. 7. Fracture toughness of NT fiber composites tested at 0.5 and 1.0 mm/min. The values in brackets in the x-axis correspond to the number of fabric layers. The fracture toughness results of the not-treated fibers composites tested at 0.5 and 1.0 mm/min are shown in Fig. 7. It is not possible to recognize any defined pattern related to s was observed are noted particularly for some fabric composites, for treated fibers composites. High d rain-rate effect on Gpmax values. The same eviations probably as a consequence of the complex translaminar crack growth mec observed tha is locally acc hanism acting in the material. It has been t inside resin rich layers the crack growth rate elerated as a result of the relatively low tenac- ity of the po ymeric phase; on the other hand, in he fibers rich layers the crack is deflected and pinned by the reinforc- synthetic fibers. In these, a linear increase of load with increasing applied displacement until the maximum load attainment is followed by a single and abrupt load drop- off, which typically describes the fracture behaviors [4,7]. In the load—displacement curves showed in Figs. 5 and 6, a slight deviation from linearity occurs as a result of matrix ductility and/or as an indicative of matrix cracking initia- tion. Regarding short fiber composites, load drops gently and continuously as a result of fiber failure and matrix cracking. Although an increase on the maximum load with increasing strain-rate is observed in Fig. 6, which has already been observed in Fig. 3 regarding neat polyure- thane, this was not a general trend for the fracture tough- ness tests conducted in this study. Figs. 5 and 6 also summarize the effects of the alkaline treatment and
Fig. 7. Fracture toughness of NT fiber composites tested at 0.5 and 1.0 mm/min. The values in brackets in the x-axis correspond to the number of fabric layers. The fracture toughness results of the not-treated fibers composites tested at 0.5 and 1.0 mm/min are shown in Fig. 7. It is not possible to recognize any defined pattern related to s was observed are noted particularly for some fabric composites, for treated fibers composites. High d rain-rate effect on Gpmax values. The same eviations probably as a consequence of the complex translaminar crack growth mec observed tha is locally acc hanism acting in the material. It has been t inside resin rich layers the crack growth rate elerated as a result of the relatively low tenac- ity of the po ymeric phase; on the other hand, in he fibers rich layers the crack is deflected and pinned by the reinforc- synthetic fibers. In these, a linear increase of load with increasing applied displacement until the maximum load attainment is followed by a single and abrupt load drop- off, which typically describes the fracture behaviors [4,7]. In the load—displacement curves showed in Figs. 5 and 6, a slight deviation from linearity occurs as a result of matrix ductility and/or as an indicative of matrix cracking initia- tion. Regarding short fiber composites, load drops gently and continuously as a result of fiber failure and matrix cracking. Although an increase on the maximum load with increasing strain-rate is observed in Fig. 6, which has already been observed in Fig. 3 regarding neat polyure- thane, this was not a general trend for the fracture tough- ness tests conducted in this study. Figs. 5 and 6 also summarize the effects of the alkaline treatment and
Fig. 8. Fracture toughness for all short fiber and fabric composites tested at 0.5 mm/min. The percentile values in the columns correspond to the fabric volume fraction. (Note that the volume fraction of the treated fabric composites is higher than the corresponding not-treated ones. This occurs due to the fabric shrinkage during the drying stage of the alkaline treatment that promotes a more closed woven and consequently larger volume.)
Fig. 8. Fracture toughness for all short fiber and fabric composites tested at 0.5 mm/min. The percentile values in the columns correspond to the fabric volume fraction. (Note that the volume fraction of the treated fabric composites is higher than the corresponding not-treated ones. This occurs due to the fabric shrinkage during the drying stage of the alkaline treatment that promotes a more closed woven and consequently larger volume.)
Fig. 9. Fracture surface morphology of the sisal fabric composites in the stable crack propagation region. (a) NT fibers. (b) T fibers. The bubbles are formed mainly due to hydrophilic charac- teristic of the natural fibers and the high reactivity of the polyurethane with water or moisture. The presence of
Fig. 9. Fracture surface morphology of the sisal fabric composites in the stable crack propagation region. (a) NT fibers. (b) T fibers. The bubbles are formed mainly due to hydrophilic charac- teristic of the natural fibers and the high reactivity of the polyurethane with water or moisture. The presence of
Fig. 10. (a) Fracture surface morphology of treated coconut fiber composites in the stable crack propagation region. (b) Detail of the fibrillation process.
Fig. 10. (a) Fracture surface morphology of treated coconut fiber composites in the stable crack propagation region. (b) Detail of the fibrillation process.
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EngineeringEnergy absorptionFracture MechanicFracture ToughnessCastor Oil
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