Fiber micro-buckling

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					                                              Composite Structures 56 (2002) 157–164

           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

    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 [6] 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 [5], 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                  [1] and Sadowsky et al. [4] 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: (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
                                                                             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

        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 [6] 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-

           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

                                                                        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

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                [1] Rosen BW. Mechanics of composite strengthening. In: Fiber
displacement level, which is caused by the different load                    composite materials. Cleveland, OH: American Society of Metals;
                                                                            1964. p. 27–56.
levels.                                                                 [2] Riggle D. New look in recycled plastic. Bio-cycle 1994:39–42.
    The sections of steady state or secondly creep, which               [3] Lampo R. Standards boost an industry. ASTM Stand News
approximately runs from 200 to 1000 h in the tests, for                     1999;(7):22–6.
164                                Z. Zheng, J.J. Engblom / Composite Structures 56 (2002) 157–164

[4] Sadowsky MA, Pu SL, Hussain MA. Buckling of microfibers. J        [6] Zheng Z, Engblom J. Fiber micro buckling of glass-fiber reinforced
    Appl Mech 1967;(12):1011–6.                                          hollow-cored recycled plastic extrusions under ¼ 20 short-term
[5] Goldsworthy WB, Hiel C. Composite structures. SAMPE J                bending. J Reinforced Plastics Compos 2001, in press.