Composite Structures 56 (2002) 157–164 www.elsevier.com/locate/compstruct Fiber micro-buckling of continuous glass-ﬁber reinforced hollow-cored recycled plastic extrusions under long-term ﬂexural loads Zhiyin Zheng *, John J. Engblom Department of Mechanical and Aerospace Engineering, Florida Institute of Technology, Melbourne, FL 32901, USA Abstract Experimental results on ﬁber micro-buckling of continuous glass-ﬁber reinforced hollow-cored recycled plastic extrusions under creep tests are introduced in the paper. The full size specimens with dimensions in 2:5 Â 3:5 Â 42 in:3 were submerged in warm water at a temperature of 125 °F when they were under a four-point bending creep test. The results show that the micro-buckling of the embedded glass-ﬁber roving occurs along 90% the length of the specimen on the upper inner surface (compressive side) and mainly during the time between 5 and 100 h from the initial loading moment. The micro-buckling causes the steady-state apparent ﬂexural modulus of the composite drop faster, and it also causes the plastic matrix local crackling which subsequently leads to the structural failure of the composite. The stress level has little eﬀect on the steady-state creep rate. The results also show some evidence that the plastic matrix becomes more brittle when it is submerged in warm water for certain long time. From the results, it is indicated that the pattern or distribution of the micro-buckling is signiﬁcantly diﬀerent from that of short-term four-point bending test for the same composite materials, for which the ﬁber micro-buckling occurs locally only on the middle section of the specimen. Ó 2002 Published by Elsevier Science Ltd. Keywords: Micro-buckling; Glass-ﬁber roving; Plastic composite extrusion; Creep 1. Introduction characterization of their behavior not only in short-term but also in long-term ﬂexural/compressive properties, The commingled recycled plastic lumbers (CRPLs) especially under warm and wet environments. are being more and more used in civil engineering ap- Results and analysis  from the short-term four- plications, e.g., marine docking/piling, home/outdoor point bending tests for the same batch of composite decking, walking overpasses/army bridges [2,3], electric extrusions indicated that occurring of ﬁber micro- transmission tower , etc., for their light weight, long buckling can signiﬁcant reduce the load bearing capa- life-span, excellent resistant to chemical hazards as well bility of the composite beam, and progressively may as environment beneﬁts from recycling the plastic waste cause catastrophic structural failure. and reducing the usage of chemical-treated pinewood. Theoretical studies of short-term compressive These applications are to design the composite beams to strength of unidirectional ﬁbrous composites by Rosen resist compressive and/or ﬂexural bending loads. But the  and Sadowsky et al.  have indicated a sinusoidal design loads are very limited since CRPL has inherited micro-stability failure mode and a strong dependence of low strength and high creep rate compared with steel, composite strength on the stiﬀness of the supporting concrete and wood. Glass-ﬁber reinforced version of the matrix. Few analytical or experimental research reports CRPL with hollow-cored cross-section was introduced about the long-term characteristics of unidirectional ﬁ- by Chasewood Industries, TX for the purpose of com- brous composites have been found in the literature re- peting with the pressure-treated wood lumber. A struc- garding the ﬁber micro-stability failure mode. tural design using these new materials requires the The work in this paper was initiated from and is a part of a cooperative project with Chasewood Indus- * Corresponding author. Tel.: +61-408-258-8280; fax: +61-321-674- tries, for characterizing one of its newly introduced 8813. products, i.e., the continuous glass-ﬁber (roving) rein- E-mail address: zheng@ﬁt.edu (Z. Zheng). forced, double hollow-cored, commingled recycled 0263-8223/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 2 6 3 - 8 2 2 3 ( 0 1 ) 0 0 1 8 6 - 6 158 Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164 plastic extrusions, for prospective applications in con- ment through an oﬀset extrusion process. Fig. 1 shows structions. The full size product is a 2:5 in: Â 3:5 in: Â the product’s cross-section conﬁguration, which is 42 in: extruded beam, double hollow-cored, with 39 double hollow-cored with 39 glass-ﬁber roving embed- continuous glass-ﬁber roving(s) embedded into the top ded in top and lower sections as layers. The length of the and bottom layers of the matrix proﬁle. More details product used in the tests is about 42 in., and the loading about the speciﬁcations and conﬁgurations of this support span L is 39 in. in Fig. 2. product are described in Section 2. The actual specimens, as manufactured, are shown in Fig. 3. The photo of these specimens was taken after the test, so they are in residue-deformed shapes. Table 1 2. Specimen conﬁgurations and test set-up gives the material contents of the product. Fig. 4 shows the schematic diagram of the complete The product is manufactured using blends of poly- set-up for the submerged, temperature controlled creep ethylene with continuous glass-ﬁber roving reinforce- test. This system is designed for automatic data collec- tion and can handle 16 full-sized lumber specimens at once time. The specimens are submerged in hot water which temperature is controlled by circulating the water through a water heater. A speciﬁc designed mechanism connected to an extensometer is attached to each spec- imen for measuring the mid-span displacement, and then the data are wired to a transition box and a lap-top computer which utilizes the Lab-View software for data logging. Fig. 5 presents the photos of the test set-up. On the left picture, the computer, data transition box, wiring and specimens submerged in the water are visible. The picture on the right-hand side in Fig. 5 shows a creep test in progress with dead loads applied. The specimens Fig. 1. Cross-section of double hollow-cored extruded plastic proﬁle are placed under the water, and the surface is covered by with continuous ﬁber reinforced. plastic sheets, which keep the water warm and assure an uniform temperature distribution within the water trough. Fig. 2. Supports and load on the specimen of the test. Fig. 4. Schematic of the test set-up for submerged and temperature Fig. 3. A set of four after-test specimens marked as 242-i. controlled creep study. Table 1 Constitution of the specimen designated as 242 Batch HDPE (%) Glass ﬁber (%) Additives Coupling agent (%) Epolene G-3003 242 92 5 UV, AO, pigment (yellow color) 3 Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164 159 Fig. 5. Test set-up (left) and a creep test in progress (right). 3. Results and discussions micro-buckling of the embedded ﬁber roving which is supposed to act as a reinforcement of the composite. The tests were conducted under the following condi- The size of an average matrix ﬂake is about a quarter tions: (1) the dead load applied on each set of specimen inch. Since the length of the specimen is 42 in. and the is 200 and 140 lb, respectively; (2) the specimens were matrix cracking ﬂakes are compared too small, it was totally submerged in the water; (3) the water tempera- not able to take photo evidence of the buckling distri- ture is 125 Æ 3 °F; and (4) when started, each load is bution of the entire specimen. A schematic show of the totally released in 5 s. distribution of the ﬁber micro-buckling along the entire Fig. 6 shows the photographic evidence of the surface specimen is presented in Fig. 7. Unlike the results from bubbling and ﬂaking of the plastic matrix caused by short-term testing  where the ﬁber micro-buckling Fig. 6. Fiber micro-buckling and resulted matrix crack and local failure. 160 Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164 Fig. 7. A schematic of typical distribution of ﬁber micro-buckling across the inner surface of the upper (compressive) half of the double hollow-cored specimen under four-point bending. Fig. 8. Time–displacement plot of the composite 242 under the load of 140 lb (specimens named CHANNEL_6, CHANNEL_8, and CHAN- NEL_10). Fig. 9. Time-dependent ﬂexural modulus of the hollow-cored specimen, corresponding to the displacement results in Fig. 8. Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164 161 only occurs on the middle section of the specimen, under CHANNEL_8 and CHANNEL_10, which consist of the long-term and lower load conditions the ﬁber micro- the ﬁrst set of the specimen for the composite 242. The buckling and matrix ﬂaking are resulted and distributed load applied on this set of specimens is 140 lb. The re- all-over the compressive side of the specimen. Basically sults indicate that the steady-state creep rate of the there exists a diﬀerence of deformed shape of specimens composite is about 8.05e ) 4 in./h, which is about 30% between creep test represented by lower and constant higher than that of the matrix itself and much higher loading and short-term test represented by higher and than that of a kind of chemical-treated pinewood. The faster loading. It was observed that the deformed shape ﬁber micro-buckling is probably a major factor aﬀecting of the composite lumber under a lower load, long-term the creep rate of the reinforced commingled recycled four-point bending is arc-like, while that under a higher plastic (CRP) since that not only causes ﬁber compres- load and short-term four-point bending is more like sive failure but also makes the matrix cracking. Besides, parabolic shaped. From appearance of the matrix diﬀusion and sliding on the ﬁber/matrix interfaces may cracking, it seems that after long time submerged in also have eﬀect on the creep rate since the incremental warm water the additive added HDPE is more brittle plastic strain by plastic matrix introduces Somigliana’s than it is in normal environment. dislocation on the matrix/ﬁber interfaces. The corre- Fig. 8 shows the mid-span deﬂection–time relation- sponding ﬂexural modulus of the 242 specimens is as ship of three specimens, named as CHANNEL_6, shown in Fig. 9. The reinforcement embedded in the Fig. 10. A logarithm plot of the ﬂexural modulus of the reinforced specimens (CHANNEL_6, CHANNEL_8, CHANNEL_10). Fig. 11. Time–displacement plot of the composite 242 under the load of 200 lb (specimens named CHAN_7, CHAN_9, and CHAN_11). 162 Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164 matrix can raise the overall ﬂexural modulus, but it also creep law. These unusual drops of ﬂexural modulus are can damage the integrity of the micro-structure in some most probably caused by the ﬁber micro-buckling and areas inside the matrix. As a result, according to the test induced matrix cracking, which are shown in Fig. 6. data obtained for both the composite and its matrix, the Fig. 11 shows the displacement history of specimens reinforcement material in a composite may cause the in a set consisting of CHAN_7, CHAN_9 and ﬂexural modulus of the composite drop faster than CHAN_11 under a higher load of 200 lb but still under the sole matrix. the same water temperature as for specimen set #1. The A logarithm plot of the time-dependent ﬂexural results indicate that the averaged steady-state creep rate modulus in Fig. 10 gives a better look of the modulus for this set of specimens is about 8.85e ) 4 in./h, which is change when the time is before 100 h. The ﬁgure shows slightly higher than that of CHANNEL_6, CHAN- something unusual happened during the time between 5 NEL_8, and CHANNEL_10. It is diﬃcult to state from and 100 h because the plotted curves should be ap- here that the stress level has eﬀect on the steady-state proximately straight lines based on the exponential creep rate for the studied composite 242. Compared Fig. 12. Time-dependent ﬂexural modulus of the hollow-cored specimen, corresponding to the displacement results in Fig. 11. Fig. 13. A logarithm plot of the ﬂexural modulus vs. time of CHAN_7, CHAN_9, CHAN_11. Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164 163 those two sets of specimens, CHANNEL_6, CHAN- NEL_8, CHANNEL_10 and CHAN_7, CHAN_9, CHAN_11, are presented in Fig. 15. The ﬁtted linear lines show that the load or stress level has little eﬀect on the steady-state creep rate for the studied composite. 4. Conclusions Submerged in 125 °F water and under certain constant load for certain time, the studied composite material, i.e., continuous ﬁber reinforced double hol- low-cored recycled plastic extrusions, demonstrates a Fig. 14. Comparison of the averaged creep responses between the kind of local material failure mode featured by ﬁber specimen set of CHANNEL_6, CHANNEL_8, CHANNEL_10 and micro-buckling and its resulted matrix cracking. The the set of CHAN_7, CHAN_9, CHAN_11. ﬁber micro-buckling, assuming together with the ﬁber/ matrix interfacial diﬀusion and sliding, adversely af- fects the steady-state creep rate of the studied com- posite. The ﬁber micro-buckling occurs during the process of creeping, but have apparently impact on the ﬂexural modulus during the time between 5 and 100 h after the initial loading. Fiber micro-buckling appears on the entire upper inner surface of the hol- low-cored double-layer-ﬁber reinforced extruded spec- imens. Fiber micro-buckling induces the plastic matrix to have local cracking and ﬂake-like peering, which could lead to structural failure of the composite. Stress level has little eﬀect on the steady-state creep rate. The experimental results also show that the ad- ditives added plastic matrix CRP has exhibited more brittle characteristics when it is submerged in warm water for a certain period of time than in the normal Fig. 15. Comparison of the averaged steady-state creep rates between atmosphere. the specimen set of CHAN_7, CHAN_9, CHAN_11 and the set of CHANNEL_6, CHANNEL_8, CHANNEL_10. Acknowledgements with results in Fig. 8, this set of specimens demonstrates certain instability in the creep process due to the higher This research has been funded through an NSF/Lu- load, especially the CHAN_7. It was in abnormal and cent Technologies Industrial Ecology Research Fellow- ruptured at 500 h. The ﬂexural modulus of CHAN_7, ship under the Bioengineering and Environmental CHAN_9 and CHAN_11 are illustrated in Fig. 12. Fig. Systems Division (Contract No. BES-9727144). Dr. 13 is the logarithm plot, which shows again an abrupt Charles R. Lockert, President of Chasewood Industries, curve bend between 5 and 100 h that is probably resulted is also acknowledged for his contribution to this re- from a massive ﬁber micro-buckling and matrix crack- search. He developed the extrusion process and super- ing during that period of time, as the same phenomena vised the processing of the hollow-cored specimens used illustrated in Fig. 10. in this work. Fig. 14 presents the averaged creep results for the two sets of specimens diﬀerentiated by load levels. The re- sults show that the two sets of specimens have very References similar creep characteristics and about the same steady- state creep rate. The only diﬀerence demonstrated is the  Rosen BW. Mechanics of composite strengthening. 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