Modeling the hygrothermal creep behavior of wood plastic composite (WPC) lumber made from thermally modified wood

Abstract

The viscoelastic behavior of an extruded wood plastic composite (WPC) made from thermally modified wood under hygrothermal treatment was studied and modeled. Multiple three-point bending creep/recovery tests were carried out using a dynamic mechanical thermal analyzer (DMTA) equipped with a submersible clamp. WPC specimens with a 15-mm span were subjected to two initial applied stresses; 9% and 14% of the flexural strength in 30 min of creep and 30 min of creep recovery under the combined effects of temperature (25°C, 35°C, and 45°C) and water immersion (saltwater (SW) and distilled water). A dry condition WPC control was used to compare the hygrothermal effects with respect to the control conditions. The WPC material in this article exhibited a linear viscoelastic behavior under the effect of temperature, whereas a nonlinear viscoelastic behavior was observed under immersion conditions. A power law model is considered a useful model to describe the creep behavior of WPC specimens with a 15-mm span in the control and the SW conditions and at 45°C. A power law model was used to describe 180-day creep deflection of WPC lumber beams with an 853-mm span subjected to 12 MPa of the flexural strength in four-point bending at 50% relative humidity and at 21°C. The power law model predicts that the WPC lumber will reach a flexural strain in outer fiber of 1% in approximately 150 years.

Keywords

WPC hygrothermal creep viscoelasticity saltwater DMTA water immersion power law modeling

Introduction

Wood plastic composites (WPCs), unlike elastic materials, exhibit viscoelastic behavior. Gibson¹ reported that thermoplastic composites, in general, exhibit a linear viscoelastic behavior and that can be represented as a physical model; for instance, spring-dashpot in a series arrangement. The spring represents the elastic behavior (i.e. following Hooke’s law) of thermoplastic composites, whereas the dashpot represents the “Newtonian fluid” viscosity. However, other researchers² reported that WPC materials exhibit nonlinear viscoelastic behavior that can be related to the effect of the filler (i.e. the wood flour). Barbero³ distinguished between the linear and the nonlinear viscoelasticity behavior of viscoelastic materials via the stress dependency of the models’ parameters.

To better understand the viscoelastic behavior of WPCs, different models have been proposed in the literature; Maxwell, power law, generalized Burger’s, standard linear solid, standard nonlinear solid, Maxwell–Kelvin, linear solid Zener, and improved Zener (a.k.a. Prony Series) models. The time-dependent viscoelastic behavior of the material based on these models is governed either by an empirical (mathematical) equation similar to the power law model or by the number of the spring and dashpot elements in the model (physical model), and on the arrangement of these elements (parallel or in series).^1,3
–5 Certain models can describe the short-term time-dependent behavior of the viscoelastic material, whereas other models can describe the long-term time-dependent behavior of viscoelastic materials.

Dynamic mechanical thermal analysis (DMTA) instruments and techniques have helped researchers to conduct a variety of short-term experiments, to predict or evaluate the time-dependent behavior of WPC specimens in a short period of time, and hence, to provide a better understanding of the time-dependent behavior of the material over a longer period of time. Tamrakar et al.⁶ applied a generalized Burger’s model to describe the strain of WPC and polyvinyl chloride specimens under a short-term (100 min) tensile creep test. Pooler⁷ used a power law and Prony Series models to describe the 10- and 600-min creep behavior of WPCs. Xu et al.⁸ implemented a three-parameter power law model to investigate the creep behavior of wood filled polystyrene/high-density polyethylene. Slaughter used a three parameter power law model to describe the creep behavior of wood polypropylene plastic composites.⁹ Hamel¹⁰ implemented a two-parameter power law model on WPC for seven different formulations in axial creep experiments.

WPC flexural creep experiments have not been implemented solely for short-term (below 24 h) behavior using small specimens (DMTA specimens that have the span ≤50 mm), but also long-term (>24 h) creep implemented on large-scale specimens (WPC specimens have spans > 100 times the size of DMTA specimens).¹¹ For instance, Brandt and Fridley¹² conducted 90-day creep experiments on specimens with a span length of 1830 mm in flexure. Likewise, Hamel¹⁰ conducted creep experiments for 3 years in compression, and tension on specimens with gage lengths of 12.7 and 57.1 mm, respectively. Hamel¹⁰ further predicted the creep behavior of WPC in flexure for 90 days for WPC specimens with a span of 2130 mm.

The objective of the research presented here was to characterize the hygrothermal creep response of a WPC material evaluated under water immersion and compare it with the creep responses (displacements) published in the literature for other exposure conditions.

In this study, a DMTA instrument with a three-point bending submersible clamp was used to conduct short-term creep and creep recovery experiments under the combined effects of water immersion and temperature on preconditioned WPC specimens with a 15-mm span and to compare that with the control (reference) dry state of the specimens. The WPC materials exhibit different time-dependent behavior, attributable to their different formulations (i.e. different type of plastics and different types and quantity of wood flour). A further description on the creep behavior of the WPCs was conducted using 250-min and 180-day creep experiments using power law models. The WPC material used in this study has potential for application in submerged marine structures,¹³ and hence, an understanding and investigation of the time-dependent behavior of this material under the effect of water immersion and temperature is necessary.

Experiment

Materials and equipment

30- and 250-min creep experiments

WPC specimens with dimensions (L, w, h), 15.00 mm, 7.24 ± 0.18 mm, and 2.69 ± 0.24 mm were cut and machined from extruded WPC lumber with width and thickness equal to 139 and 34 mm, respectively. A dynamic mechanical thermal analyzer (DMTA; TA Q800, New Castle, DE, USA) instrument was used to conduct the creep and creep recovery experiments. A three-point bending submersible clamp was used in these experiments as shown in Figure 1(a). This clamp has the ability to conduct a three-point bending test on a specimen submerged in a fluid environment over a range of temperatures from room temperature (ca. 20°C) to 80°C. To ensure the temperature measurement in the liquid environment is the specimen’s temperature, an extended thermocouple in the liquid environment of the clamp was located at 1 mm distance from the tested specimen. The WPC lumber was produced using a counter rotating twin screw Davis-Standard Woodtruder^TM in the Advanced Structures and Composites Center at the University of Maine, Orono, Maine, USA.¹⁴

Figure 1.

(a) Three-point bending DMTA submersible clamp and (b) a schematic of the 180-day creep experiment in four-point bending.

180-day creep experiments

Five WPC lumber specimens with a span length of 853 mm were loaded in a 180-day creep experiment in four-point bending, as shown in Figure 1(b). The 180-day experiment was conducted in the creep room at the Advanced Structures and Composites Center at the University of Maine. The relative humidity (RH) and temperature were controlled during the 180 days to be 50 ± 5%, and 21 ± 2°C. The WPC examined here is based on a patent-pending formulation that combines thermally modified wood flour that was produced at Uimaharju Sawmill in Finland with a high-strength styrenic copolymer system in an equivalent weight ratio to each of the two constituents. WPC specimens were preconditioned for 1 month in both saltwater (SW) and distilled water (DW).

Mechanical testing

DMTA creep experiments

To obtain the flexural strength WPC used in the DMTA creep experiments (the cut and machined specimens), three-point bending tests were performed according to ASTM D790 using an Instron dual column tabletop electromechanical testing machine, Norwood, MA, USA. A 10-kN load cell was used in displacement control method of testing with an average crosshead speed motion of 1.59 ± 0.17 mm/min¹⁵ was applied during the flexural tests of the five WPC specimens.¹⁶ Prior to the testing, the five WPC specimens with dimensions (L, w, h) (59.68 ± 0.39 mm, 7.25 ± 0.25 mm, 3.73 ± 0.39 mm) were oven-dried at 50 ± 3°C for 24 h.¹⁷

The applied initial flexural stress levels of the WPC specimens in the DMTA experiments were selected to be 9.20% and 13.8% of the ultimate flexural strength. This selection of the stress levels was made based on the recommendation from previous studies; for instance, Hamel¹⁰ recommended studying stress levels that are below 20% of the ultimate stress. Thus, the stress levels in this study were selected to be below 15%. The average ultimate flexural strength was 27.22 ± 3.88 MPa with a coefficient of variation of 13%, and hence, the applied initial flexural stresses in the DMTA experiments were 2.5 and 3.75 MPa, respectively. Three different temperatures (25, 35, and 45°C) were evaluated and five replicates to each stress level were studied on the reference (dry) condition and the preconditioned WPC specimens (1-month immersion) in both DW and SW immersions, respectively. Ten minutes of soaking time was applied prior to the DMTA flexural creep and creep recovery experiments.

180-day creep experiments

The 180-day creep experiments of the WPC lumber (with 853-mm length) was conducted in accordance with ASTM D6112.¹⁸ The applied initial flexural load, (P) as shown in Figure 1(b), in the 180-day creep experiment was 2243.2 ± 15.2 N (12-MPa flexural stress). In accordance with the loading rate mentioned in ASTM D6109, the maximum applied flexural load was applied to the WPC specimen during the instantaneous loading phase (4 min) with a crosshead rate motion of 39.4 mm/min. This crosshead rate motion was computed using the equation stated in the ASTM D6109 by considering a strain of 1% at the outer fiber. This loading rate was similar to the quasi-static rate so that the instantaneous duration is maintained within the short time stated in the ASTM D6109 (not less than 10 s and not more than 10 min).

Results and discussion

Power law model

The relationship between the creep compliance of a material and the stress is shown in equation (1). Furthermore, in accordance with the ASTM D6109, the creep displacement of the WPC lumber in four-point bending test was computed using equation (2)

S (t) = \frac{∊ (t)}{σ_{0}}

d (t) = \frac{0.21 ε (t) l^{2}}{D}

where S(t) is the creep compliance, ∊(t) is the creep strain, σ ₀ is the applied stress, d(t) is the midspan creep deflection (vertical displacement), ε(t) is the creep strain (mm/min), l is the span length, and D is the depth (thickness) of the WPC lumber.

An empirical power law model was used to describe the creep compliance of the WPC in dry, DW, and SW conditions. Prior to and during the creep test, both the DW and SW immersion WPC specimens were subjected to flexural stresses and range of temperatures in the immersed environment. The power law, as shown in equation (3),³ is suggested for these experiments based on the assumption of the linear viscoelastic behavior of the WPC specimens in this study.¹ The initial value of the compliance represents the reciprocal of the modulus of elasticity (E). The power law model was implemented on the creep compliance vectors obtained from the 30-min and 250-min creep experiments on WPC specimens tested in the DMTA submersible clamp. Parameters of the model and the initial experimental compliance values are reported in Tables 1 to 3

S (t) = S_{0} + S_{1} t^{n}

where S(t) is the total creep compliance of the model, S ₀, S ₁, and n represent the model’s parameters that can be found from the experimental data fitting using a written code in MATLAB,¹⁹ and t is the time of the creep experiment. Parameters of the model for each condition (stress, temperature, and environmental condition) are reported in Tables 1 to 3. In addition to reporting the coefficient of determination (R²) that shows the degree of agreement between the model and the experimental data, and to quantify the degree of the agreement between the model and the experimental data to each condition, the summation of the square error (SSE) vector was also reported in Tables 1 to 3. Figure 2 illustrates the viscoelastic linearity and nonlinearity of the WPC specimens used in this study, whereas Figures 3 and 4 illustrate the combined effect of the high value of temperature in this study (45°C) and the water immersion (SW and DW) on the creep compliance values of the 30-min of WPC specimens under the 2.5 and 3.75 MPa of the applied initial flexural stresses in logarithmic scale. Tables 1 to 3 show that the WPC specimens for the both selected initial flexural stress levels exhibit a reduction in creep compliance at a temperature of 35°C, compared with creep compliance of the WPC specimens at 25 and 45°C. This reduction in the creep compliance at temperatures below the glass transition temperature (T _g) can be related to the developed interfacial bonding between the amorphous polymer and the thermally modified wood particles of the WPC of this study. A similar trend was noticed on the neat and sisal fibers reinforced polystyrene composites that was studied by Nair et al.,²⁰ when they investigated the variation in the storage modulus with respect to temperature to polystyrene reinforced with different percentages of sisal fibers. According to a study reported elsewhere²¹ and according to the constituents of the WPCs used in this study, this reduction in the creep compliance at 35°C is related to the properties of the amorphous polymer of the WPC and its ability to maintain modulus of elasticity (E) at elevated temperatures below the T _g and that was observed in storage modulus versus temperature relationship.

Table 1.

Power law model parameters of the creep compliance curves of WPC specimens tested in the D condition.

Condition (flexural stress (MPa)—temperature (°C))	Initial compliance (GPa⁻¹)	Power law model parameters (S ₀, S ₁, n)	SSE (%)	R2
2.5—25	0.65	0.3485, 0.2813, 0.0649	0.03	0.99
2.5—35	0.90	0.1252, 0.7294, 0.0527	0.08	0.99
2.5—45	0.68	0.0950, 0.5232, 0.0638	0.20	0.99
3.75—25	0.66	2.72e−04, 1.0798, 0.0360	0. 13	0.98
3.75—35	0.85	5.082e−04, 0.8014, 0.0409	0.10	0.99
3.75—45	0.73	0.1512, 0.5250, 0.0666	0.11	0.99

WPC: wood plastic composite; D: dry; SSE: summation of the square error.

Table 2.

Power law model parameters of the creep compliance curves of WPC specimens conditioned and tested in DW condition.

Condition (flexural stress (MPa)—temperature (°C))	Initial compliance (GPa⁻¹)	Power law model parameters (S ₀, S ₁, n)	SSE (%)	R2
2.5—25	1.42	0.1308, 1.2577, 0.0297	0.14	0.98
2.5—35	1.58	0.1178, 1.3859, 0.0481	0.24	0.99
2.5—45	1.90	0.0193, 1.7394, 0.0605	0.82	0.99
3.75—25	0.97	9.192e−04, 0.9389, 0.0446	0.17	0.98
3.75—35	1.52	1.102e−04, 1.4215, 0.0597	0.39	0.99
3.75—45	1.42	0.6509, 0.69, 0.1027	0.42	0.99

WPC: wood plastic composite; DW: distilled water; SSE: summation of the square error.

Table 3.

Power law model parameters of the creep compliance curves of WPC specimens conditioned and tested in SW condition.

Condition (flexural stress (MPa)—temperature (°C))	Initial compliance (GPa⁻¹)	Power law model parameters (S ₀, S ₁, n)	SSE (%)	R2
2.5—25	1.58	0.1111, 1.4370, 0.0316	0.29	0.98
2.5—35	1.89	0.0947, 1.6918, 0.0529	0.43	0.99
2.5—45	2.10	1.3114, 0.6977, 0.1162	0.63	0.99
3.75—25	1.11	0.0280, 0.5871, 0.0432	0.16	0.98
3.75—35	1.33	7.239e−04, 1.2307, 0.0550	0.17	0.99
3.75—45	1.24	0.8332, 0.3509, 0.1398	0.28	0.99

WPC: wood plastic composite; SW: saltwater; SSE: summation of the square error.

Figure 2.

Isochronous curves of the WPC specimens at the creep time; 5, 15, and 30 min at the three different conditions; D, SW, and DW at 25°C.

Figure 3.

30-min creep compliance of WPC specimens (with 15-mm length) subjected to a maximum flexural stress of 2.5 MPa at 45°C in three different testing conditions; D, SW, and DW conditions.

Figure 4.

30-min creep compliance of WPC specimens (with 15-mm length) subjected to a maximum flexural stress of 3.75 MPa at 45°C at three different testing conditions; D, DW, and SW conditions.

The 30-min creep of WPC

To investigate the viscoelastic behavior of the WPC, isochronous curves were constructed at 5, 15, and 30 min to the two applied creep flexural stresses for the purpose of evaluating whether the WPC exhibits a linear or nonlinear viscoelastic behavior and these constructed isochronous curves were reported elsewhere²¹ and as shown in Figure 2. Three conditions were selected for the purpose of illustration and comparison, the dry (control) and the SW and the DW immersion conditions at 25°C. The WPC specimens show a linear viscoelastic behavior under the dry condition and nonlinear viscoelastic at the other immersion conditions. Isochronous curves at different immersion conditions and at different values of temperature were reported elsewhere.²¹

A preliminary study was conducted to apply different models to describe the behavior of WPC, and the power law model exhibited the lowest SSE among the other models examined. For all the cases of the applied flexural stress and hygrothermal conditioning, two distinctive regions in the creep compliance curve can be observed; a high creep compliance rate region at the time below 5 min, and a steady-state rate response for a time greater than 5 min. These two distinctive regions are attributable to the behavior of the WPC material by maintaining its creep compliance and, hence, decreasing the deformation under the sustained applied flexural stress.²¹ This can be observed in dry conditions where the moisture degradation is not considered. Once the hygrothermal effect is considered, the creep compliance rate in the steady-state region started to increase and it was higher in SW, compared with the DW, because it was found that the degradation of the SW is higher than the degradation of the DW.

Application of the power law model: 250-min creep of WPC specimen

To verify the power law model used in this study to describe the 30-min creep compliance of WPC specimens, a 250-min creep experiment (stress = 2.5 MPa and temperature = 45°C) was conducted in dry and immersion environments (DW and SW) using the DMTA submersible clamp. The power law model shows good agreement with the experimental data at all three testing conditions; dry, DW, and SW, as shown in Figure 5.

Figure 5.

250-min creep compliance values and models’ data fitting of WPC specimens under σ = 2.5 MPa tested at 45°C in D, DW, and SW conditions.

Application of the power law model: 180-day creep of WPC lumber

For the purpose of evaluating the extension of the application of the power law model from the short term to include the long-term creep experiments, the power law model was used to describe the 180-day creep behavior of WPC lumber with a span length 853 mm, tested in creep under a flexural load. The power law model showed good agreement with experimental creep displacement data, as shown in Figure 6. According to ASTM D6109²² and to equation (2), the flexural yield strength is computed for the stress corresponding to 1% of the flexural strain. Thus, in this study, a prediction of the creep displacement of the WPC lumber is presented using the power law model. Based on the assumption that the WPC should fail at a flexural strain in outer fiber of 1% (similar to the failure strain value mentioned in ASTM D6109), the computed midspan creep deflection will be 44.9 mm. The WPC lumber in this study will reach this creep midspan deflection under a sustained flexural stress (11.8 ± 0.08 MPa) after approximately 150 years, as shown in Figure 7.

Figure 6.

180-day creep displacement of WPC specimens under 2243 ± 15.20 N of applied flexural load in a four-point bending test configuration.

Figure 7.

Implementation of the power law model that was used in the description of creep behavior of 30-min creep, 250 min, and 180-day creep to predict the time-dependent displacement of WPC lumber in flexure.

Comparison of WPC creep response and the predicted creep lifetime of previous studies

To evaluate the time-dependent behavior of the WPC used in this study, a comparison to the predicted creep displacement of the WPC studied in this article and the WPCs studied by other researchers is reported in Table 4. The short-term viscoelastic extrapolated creep techniques, for instance, the time–temperature or time–temperature–stress superposition^6,23

–26 are used to have an understanding of the long-term behavior of the viscoelastic material (WPC) by applying the principle of superposition and superimpose the short-term relationships to construct the master curve that describes the viscoelastic behavior of the WPC at longer duration of testing. However, these extrapolated experiments encompassed exposing the WPC specimens to high temperature (above the glassy region of the material), whereas the material exposure temperature in service could be lower than the T _g. Thus, the prediction of the long-term viscoelastic behavior of these extrapolated experiments is not representative of the viscoelastic behavior of the full-scale WPC members in service.^{2,5,6,23,24,26

–30} Furthermore, these techniques (extrapolated experiments) are implemented on the assumption of the linear viscoelasticity and hence the principle of superposition on the short creep curves can be applied, whereas the WPCs in this study showed a nonlinear viscoelastic behavior under the combined effect of temperature and water immersion. Thus, the extrapolated techniques cannot be used to predict the hygrothermal behavior of the WPC specimens in this study.

Table 4.

Comparison of the experimental and predicted creep lifetime of WPCs.

Source	This article	Alvarez-Valencia et al.¹¹	Tamrakar et al.⁶
Loading configuration	Four-point bending	Four-point bending	Tensile
Value of the applied stress (MPa) or load (N)	12 MPa, 2243 N	11,700 N	5.45 MPa
Length of the WPC specimen (mm)	853	4700	246
Condition of testing and/or the specimen conditioning	21°C at 50% relative humidity	∼20°C at ∼3–50%	Dry at 21°C
Predicted failure occurrence (years)	154	—	∼1.5
Maximum experimental and predicted creep displacement value (mm), respectively	14.60, 44.9	ca.86, n.a.	4.77, n.a.

WPC: wood plastic composite.

Viscoelastic models are another tool (approach) in addition to the extrapolated techniques that are utilized to describe the viscoelastic behavior of the WPC for longer durations than the experiments’ durations. However, both the extrapolated techniques and the viscoelastic models do not describe the viscoelastic behavior in the tertiary region. Likewise, the model used in this study did not consider the prediction of the creep behavior in the tertiary region, even though it predicted the creep behavior for 150 years. The creep lifetime prediction can be used to evaluate the WPC in the primary and the secondary regions. Barbero³ emphasizes the importance of studying the time dependence of the viscoelastic behavior of the material in structural applications in the primary and the secondary regions. The power law model, in this study and unlike the previous studies, was applied for different; time durations, specimen length, and testing conditions (temperature only or hygrothermal). The model showed good agreement with the different hygrothermal conditions in addition to the different test durations and for different specimen lengths, compared with previous studies, as shown in Table 4. In accordance with ASTM D6815,³¹ the WPC in this study should attain acceptable fractional creep limit in approximately 10 years, whereas the deflection limit L/20^9,32 of the WPC of this study should be reached in approximately more than 150 years. Furthermore, and according to Shao,³² when a 70% increase in the initial creep deflection is expected to occur during the service life of the pultruded FRP sheet pile, the expected 70% increase to the initial deflection of the WPC in this study is predicted to occur in 4 years.

For most of the experimental conditions, the parameter (n) in the power law model of this study was dependent on hygrothermal effects (i.e. the value of n increases with the increase of temperature). However, power law models used in previous studies had an independent parameter (n), as was found in the models used by Hamel.^10,33 However, a similar temperature dependency to the power law model parameter (n) was observed in Pooler’s study.⁷

The WPC material in this study has the potential to be used in dry applications where the values of the temperature are below the maximum temperature used in this experiment (45°C) and subjected to low levels of applied stresses. Whereas the time-dependent performance of the WPC materials used in the studies reported by Tamrakar et al.⁶ and Alvarez-Valencia et al.¹¹ has shown a potential to be used in structural applications, but with the belief of having a shorter lifetime as shown in Table 4, attributable to the predicted creep tensile displacement and the experimental creep midspan deflection, respectively.

In accordance with ASTM D790, ASTM D2915,³⁴ and ASTM D6109,²² the shear deformation can be ignored in the computation of the total midspan deflection when the span to depth ratio (L/h) is greater than or equal to 16. According to DMTA instrument model Q800, it is suggested that the shear deformation can be ignored for L/h greater than or equal to 10.³⁵ However, to illustrate the difference between accounting for or ignoring the shear deformation in the DMTA experiments for the L/h = 6 ratio, an investigation was conducted to quantify the difference between including or ignoring the shear deformation using Timoshenko’s theory of beam deflection.³⁶ This investigation required adopting a value of Poisson’s ratio of 0.33 from previous studies.^37,38 These computations were conducted with the assumption that the WPC behaved as an isotropic linearly elastic material. The computed values of the elastic moduli for the WPC specimens with L/h = 16 and L/h = 6 with the inclusion of the shear deformation were 3% and 12.5% higher than the computed moduli when the shear deformation was ignored.

Conclusions

The hygrothermal creep behavior of WPC has been characterized as a function of temperature and water immersion (distilled and SW) using the DMTA three-point submersible clamp. The power law model is considered a useful tool to describe the hygrothermal creep behavior of the WPC in 30- and 250-min DMTA creep experiments. Furthermore, the power law model was implemented to describe the 180-day creep behavior of WPC specimens with a longer span (853 mm) in four-point bending. The WPC in this study showed a linear viscoelastic behavior under the effect of temperature, whereas a nonlinear viscoelastic behavior under the combined effects of temperature and water immersion.

According to the predicted flexural strain limit (1%) at the outer fiber, the WPC in this study has the potential to be used in long-term applications where low sustained flexural stresses (below 15% of the flexural strength) are applied.

Limiting the increase in creep compliance in the range of temperatures between 25°C and 45°C (at 35°C) in the dry condition to be lower than 39% at both the stress levels 9% and 14% of the flexural strength (related to the formulation of the WPC in this study) is useful for considering this material in environments over a range of temperatures below 45°C.

Footnotes

Authors’ note

The work described in this document was conducted at the Advanced Structures and Composites Center at the University of Maine, Orono, Maine. The thermally modified wood fiber used in this research was supplied by Stora Enso (Finland).

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The first author was supported through a scholarship provided by the Higher Committee for Education Development (HCED) in Iraq. The University of Maine research reinvestment funds (RRF) Seed Grant entitled (Development of structural wood plastic composite (WPC) timber for innovative marine application) and the United Stated Department of Agriculture (USDA)-the agricultural research service (ARS) Funding Grant Number (58-0204-6-003) have provided the financial support for this project. The WPCs is based on a patent-pending formulation that has the publication number (WO2018/142314).

ORCID iD

Murtada Abass A Alrubaie

References

Gibson

. Principles of composite material mechanics. Boca Raton: CRC Press, 2016.

Pooler

Smith

. Nonlinear viscoelastic response of a wood–plastic composite including temperature effects. J Thermoplast Compos 2016; 17(5): 427–445.

Barbero

. Finite element analysis of composite materials using AbaqusTM. Boca Raton: CRC Press, 2013.

Tajvidi

Simon

. High-temperature creep behavior of wheat straw isotactic/impact-modified polypropylene composites. J Thermoplast Compos 2015; 28(10): 1406–1422.

Nuñez

Marcovich

Aranguren

. Analysis of the creep behavior of polypropylene-wood flour composites. Polym Eng Sci 2004; 44(8): 1594–1603.

Tamrakar

Lopez-Anido

Kiziltas

, et al. Time and temperature dependent response of a wood–polypropylene composite. Compos Part A Appl S 2011; 42(7): 834–842.

Pooler

. The temperature dependent non-linear response of a wood plastic composite. Pullman: Washington State University, 2001.

Simonsen

Rochefort

. Creep resistance of wood-filled polystyrene/high-density polyethylene blends. J Appl Polym Sci 2001; 79(3): 418–425.

Slaughter

. Design and fatigue of a structural wood-plastic composite. Master Thesis, Washington State Uuniversity, USA, 2006.

10.

Hamel

. Modeling the time-dependent flexural response of wood-plastic composite materials. PHD Thesis, The University of Wisconsin, Madison, 2011.

11.

Alvarez-Valencia

Dagher

Davids

, et al. Structural performance of wood plastic composite sheet piling. J Mater Civil Eng 2010; 22(12): 1235–1243.

12.

Brandt

Fridley

. Load-duration behavior of wood-plastic composites. J Mater Civil Eng 2003; 15(6): 524–536.

13.

Gardner

Han

(eds) Towards structural wood-plastic composites: technical innovations. In: Proceedings of the 6th meeting of the Nordic-Baltic network in wood material science and engineering (WSE), Tallinn, Estonia, 21–22 October, 2010.

14.

Davis-Standard Woodtruder Advanced Structures and Composites Center. Orono: University of Maine, 2018, https://composites.umaine.edu/equipment-and-facilities/thermoplastic-composite-extrusion-lab/ (accessed August 2018).

15.

ASTM International. Standard guide for evaluating mechanical and physical properties of wood-plastic composite products. D7031-11. West Conshohocken: ASTM International, 2011.

16.

ASTM International. Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials. D790-17. West Conshohocken: ASTM International, 2017.

17.

ASTM International. Standard test method for water absorption of plastics. D570-98 (reapproved 2010). West Conshohocken: ASTM International, 2010.

18.

ASTM International. Standard test methods for compressive and flexural creep and creep-rupture of plastic lumber and shapes. D6112-13. West Conshohocken: ASTM International, 2013.

19.

MATLAB. Computer software. Natick: The MathWorks, Inc: MathWorks®, 1994–2018.

20.

Nair

KCM

Thomas

Groeninckx

. Thermal and dynamic mechanical analysis of polystyrene composites reinforced with short sisal fibres. Compos Sci Technol 2001; 61(16): 2519–2529.

21.

Alrubaie

MAA

Lopez-Anido

Gardner

, et al. Experimental investigation of the hygrothermal creep strain of wood plastic composite (WPC) lumber made from thermally modified wood. J Thermoplastic Compos Mater. 2018.

22.

ASTM International. Standard test methods for flexural properties of unreinforced and reinforced plastic lumber and related products. D6109-13. West Conshohocken: ASTM International, 2013.

23.

Tajvidi

Falk

Hermanson

. Time–temperature superposition principle applied to a kenaf-fiber/high-density polyethylene composite. J Appl Polym Sci 2005; 97(5): 1995–2004.

24.

Chang

Lam

Kadla

. Application of time–temperature–stress superposition on creep of wood–plastic composites. Mech Time-Depend Mat 2013; 17(3): 427–437.

25.

Hadid

Rechak

Tati

. Long-term bending creep behavior prediction of injection molded composite using stress–time correspondence principle. Mater Sci Eng: A 2004; 385(1): 54–58.

26.

Pedrazzoli

Pegoretti

. Long-term creep behavior of polypropylene/fumed silica nanocomposites estimated by time–temperature and time–strain superposition approaches. Polym Bull 2014; 71(9): 2247–2268.

27.

Stark

Gardner

. Outdoor durability of wood-polymer composites. Philadelphia: Woodhead Publishing, 2008.

28.

Struik

. Mechanical behaviour and physical ageing of semi-crystalline polymers: 3. Prediction of long term creep from short time tests. Polymer 1989; 30(5): 799–814.

29.

Mehndiratta

Bandyopadhyaya

Kumar

, et al. Experimental investigation of span length for flexural test of fiber reinforced polymer composite laminates. J Mater Res Technol 2018; 7(1): 89–95.

30.

Garoushi

Lassila

Vallittu

. The effect of span length of flexural testing on properties of short fiber reinforced composite. J Mater Sci: Mater Med 2012; 23(2): 325–328.

31.

ASTM International. Standard specification for evaluation of duration of load and creep effects of wood and wood-based products. D6815-09 (reapproved 2015). West Conshohocken: ASTM International, 2015.

32.

Shao

. Characterization of a pultruded FRP sheet pile for waterfront retaining structures. J Mater Civil Eng 2006; 18(5): 626–633.

33.

King

Hamel

The tensile creep response of a wood-plastic composite in cold regions. In: ISCORD 2013: Planning for Sustainable Cold Regions 2013, pp. 771–778. https://ascelibrary.org/doi/pdf/10.1061/9780784412978.073.

34.

ASTM International. Standard practice for sampling and data-analysis for structural wood and wood-based products. D2915-10. West Conshohocken: ASTM International, 2010.

35.

TA Instruments. Dynamic mechanical analysis: basic theory and applications training, http://people.clarkson.edu/∼skrishna/DMA_Basic_Theory_Applications.pdf (accessed August 2018).

36.

Timoshenko

Goodier

. Theory of elasticity, vol. 412. New York: McGraw-Hill Book Company, 1951, p. 108.

37.

Dura

. Behavior of hybrid wood plastic composite-fiber reinforced polymer structural members for use in sustained loading applications. Orono: University of Maine, 2005.

38.

Hamel

Hermanson

Cramer

. Predicting the flexure response of wood-plastic composites from uni-axial and shear data using a finite-element model. J Mater Civil Eng 2014; 26(12):04014098.