Fiber-reinforced-polymer composites (FRPs) possess superior mechanical
properties and formability, making them a desirable material for construction
of large optimized mechanical structures, such as aircraft, wind turbines,
and marine hydrokinetic (MHK) devices. However, exposure to harsh marine
environments can result in moisture absorption into the microstructure of the
FRPs comprising these structures and often degrading mechanical properties.
Specifically, laminate static and fatigue strengths are often significantly
reduced, which must be considered in design of FRP structures in marine
environments. A study of fiberglass epoxy unidirectional and cross-ply
laminates was conducted to investigate hygrothermal effects on the mechanical
behavior of a common material system used in wind applications. Several
laminates were aged in 50
Fiber-reinforced polymers (FRPs) offer desirable properties for development of large mechanical structures, such as wind turbines and more recently marine hydrokinetic (MHK) devices. High specific strength and stiffness, the ability to tailor the anisotropic properties, and low costs make FRPs the primary choice for optimizing the design of energy-harvesting devices (Samborsky et al., 2012). Additionally, the formability of FRPs enables the manufacture of complex geometry and structures necessary in the wind and MHK industry. As wind energy becomes a more dominant energy source, and as MHK technology progresses, it is paramount to understand how FRPs perform throughout a device's designed lifetime. Inherently, this means characterizing material degradation from environmental exposure. This research explores how exposure to a wet environment affects the mechanical response of fiberglass epoxy laminates. Effort is made to characterize the effect of moisture on composites on a coupon level, which can provide insight to subsequent component and structure design.
FRPs typically exhibit Fickian diffusion, primarily through the matrix material as fiberglass typically absorbs a negligible amount of moisture. Other modes are possible, but defect driven; poorly bonded fiber–matrix interfaces and porosity provide these secondary diffusion pathways (Sun et al., 2011; Tsenoglou et al., 2006). At ambient design temperatures moisture diffusion occurs over time periods impractical for study in the laboratory setting. The rate–temperature relationship of the Arrhenius law applies for moisture diffusion in FRPs (Tsai et al., 2009), and allows test specimens to be aged at elevated temperatures to increase diffusion rates. Elevated aging temperatures are chosen to increase diffusion rates without degrading the temperature-sensitive matrix material. Exceedingly high temperatures can alter the material physically or chemically as evidenced by non-Fickian diffusion and/or changes in glass transition temperature (Tsai et al., 2009; Grammatikos et al., 2016).
Past research has shown significant reductions in both static and fatigue performance due to moisture absorption (Mourad et al., 2010; Miller et al., 2012; Liao et al., 1999; Komai et al., 1991; Siriruk and Penumadu, 2014; Nunemaker, 2017, 2016). MHK devices and wind turbines fall into the fatigue loading regime, but material characterization through static tests is essential to understanding the changes in performance anticipated under cyclic loading and the mechanism responsible. Changes in material performance due to conditioning are a consequence of changes in the mechanical behavior of the matrix and quality of the fiber–matrix interface. Water is a known plasticizer in polymers; water molecules disrupt interactions between polymer chains, resulting in increased chain mobility and ultimately leading to degraded mechanical properties. Moisture-induced plasticization has been observed experimentally through changes in viscoelastic properties and glass-transition temperature of neat resin (Zhou and Lucas, 1999a, b; Nogueira et al., 2001). Interface integrity is more challenging to determine experimentally, but scanning electron microscope (SEM) images of fracture surfaces of aged composites have provided evidence of a weakened interface bond (Mourad et al., 2010; Liao et al., 1999).
Understanding matrix and interface response to hygrothermal aging is essential to better understanding of how moisture absorption affects FRPs at a micromechanical level. However, the changes in matrix and the interface properties can be difficult to translate to the mechanical response of the laminate and do not fully describe the macroscopic damage progression of the material. Thus, laminate-scale testing is still necessary to characterize mechanical behavior of these materials in hygrothermal environments. Unidirectional tension and transverse tension are commonly used to characterize the change in mechanical response due to hygrothermal aging (Miller et al., 2012). These tests can be applied to predict behavior of more complex laminates; however, this approach does not incorporate hygrothermal effects on ply interactions. This research expands this idea to experimentally examine cross-ply laminates and unidirectional laminas in both unconditioned and fully conditioned states and to investigate the effects of hygrothermal aging on the mechanical response and damage progression of multi-angle laminates.
Damage progression at the micromechanical level is difficult to experimentally observe. In situ acoustic emission (AE) monitoring is a technique that enables real-time observation of damage events using piezoelectric sensors mounted on the surface of the test specimen to record damage-initiated elastic waves propagating through the material. These waveforms can provide useful information for both structural health monitoring and material characterization purposes. For this research, material characterization is the focus, with the intent of using acoustic events to help characterize changes in damage progression due to environmental aging.
AE is an indirect method of measurement and therefore requires thorough analysis to correlate AE activity to micromechanical damage within the material. Many analysis techniques exist, including single-parameter analysis (Bourchak et al., 2007; Suzuki et al., 1987; deGroot et al., 1995; Ramirez-Jimenez et al., 2004), clustering and artificial neural networks (ANNs) (Gutkin et al., 2011; Suresh Kumar et al., 2017; Pashmforoush et al., 2012), and detailed waveform analysis (Ni and Iwamoto, 2002; Surgeon and Wevers, 1999; Voth, 2017). In this study, single-parameter methods of fast-Fourier-transform (FFT) peak frequency and event energy are used to characterize the damage progression of the material. Past research has attempted to correlate FFT peak frequencies in AE waveforms to specific damage mechanisms, while energy has been shown to correlate with damage accumulation in the material (Bourchak et al., 2007; Kumar et al., 2017). When used as parameters for ANN, energy, and other parameters indicative of damage event magnitude, such as counts and number of hits, have successfully been used to predict residual strength (Suresh Kumar et al., 2017). Although multivariable techniques permit relationships to be made among many AE parameters in the analysis, differentiating among damage mechanisms remains a challenge. Clustering studies have shown that frequency content of AE waveforms persists as a dominant parameter for differentiating types of damage events (Gutkin et al., 2011; Pashmforoush et al., 2012).
Previous research incorporating AE monitoring in the evaluation of hygrothermally aged composites has revealed that moisture uptake frequently causes a decrease in AE response in terms of energy, amplitude, number of events, and counts (Assarar et al., 2011; Czigány et al., 1995; Garg and Ishai, 1985a, b; Hamstad, 1983; Komai et al., 1991). However, this may not always be the case: a recent delamination study showed an increase in magnitude of AE response in terms of energy and amplitude for conditioned delamination samples (Liu et al., 2017). Changes in the magnitude of the AE response due to moisture have not been thoroughly explored or understood, but have been theorized to be an effect of matrix plasticization and its effect on signal attenuation and damage source behavior. Consideration of this change in AE response is important when analyzing AE results, and can help differentiate changes in AE response associated with damage behavior from effects of moisture on the AE propagation behavior.
Vectorply E-LT 3800, a stitched E-glass fabric consisting of
1138 g m
Samples were hygrothermally aged by immersion in distilled water maintained
at 50
Moisture uptake curves.
Five tensile tests were conducted for each laminate in both the dry and
saturated conditions, in accordance with ASTM D3039 (2014). Testing took
place using an Instron 8562 100 kN servomechanical load frame with a
crosshead speed of 1.5 mm min
All tests were conducted with in situ AE monitoring, using a Mistras PCI-8 Micro-II SAMOS system and two Physical Acoustics WD sensors. The sensors were connected in line with 40 dB external preamplifiers and featured a 50–1000 kHz operating range. A linear array was used to record acoustic activity throughout the tensile tests. The sensors were spaced 128 mm apart, each 64 mm from the longitudinal center of the coupon as seen in Fig. 2. A thin layer of vacuum grease was applied to ensure proper acoustic contact between the sensor and the coupon.
The MISTRAS system operated at a sampling rate of 3 MHz. A band-pass
filter of 50 to 400 kHz to eliminate noise of undesired frequencies and
ensure the collected data fell in the operating range of the sensors was
used (2011). The timing parameters peak definition time, hit definition time,
hit lockout time, and max duration were set to 50, 100, 300, and
99
During post processing, acoustic activity was truncated beyond the maximum load. This was for two reasons: damage after maximum load is not pertinent to damage progression in the material and extensive damage present in the sample significantly affects event propagation and attenuation. The latter may also affect AE events immediately preceding final failure, which is an important consideration when evaluating final damage progression.
Coupon layout geometry and sensor layout
Average ultimate strengths of the cross-ply laminates and unidirectional
laminates in each environmental condition are displayed in Fig. 3. Standard
deviation error bars are added to each test group. All laminates experienced
significant moisture-induced strength reductions. Cross-ply laminate
strengths were reduced by 54 %, while the [0]
Average ultimate stresses of dry and saturated samples.
However, it is important to note that comparing changes in strengths of
unidirectional laminates to cross-ply laminates does not fully describe the
lamina-level changes. Thus, ultimate unit loads were also used to compare
laminates since the 90
Ultimate loads per unit width of dry and saturated samples.
Stress–strain response of dry and saturated laminates.
The typical stress–strain response for dry and saturated laminates are
displayed in Fig. 5. The [90]
The stress–strain behavior of the [0]
Transverse cracking in failed [90]
Failed coupons were inspected to provide insight into changes of damage progression with moisture absorption. Images were captured with a high-resolution flat-bed scanner or digital single-lens reflex camera; Table 1 summarizes the results from the failure inspection of each laminate.
Failure inspection results summary.
Inspection of failed [90]
Failed [0]
Failed [0]
Failed [0/90]
Failed [90/0]
Summary of changes in AE response from dry conditions to saturated conditions.
In the saturated cross-ply coupons, damage was much more localized, producing
a neat transverse failure. This is shown in both the [0/90]
Both frequency and energy were evaluated for all the laminates tested and are discussed in the subsequent section. Table 2 summarizes the changes in AE frequency and energy results due to hygrothermal conditions for all laminates.
Previous work has shown that these FFT peak frequencies of AE waveforms correlate to the damage mechanisms (matrix, fiber, interphase, etc.) that caused the acoustic event (Suzuki et al., 1987; Ni et al., 2001; Bohse, 2000; deGroot et al., 1995; Ramirez-Jimenez et al., 2004). The results of these works identified discrete frequency ranges, or bins, which correlate to specific damage modes shown below in Fig. 10. Based off these works, changes in AE frequency content with conditioning can provide insight into whether the damage-present mechanisms change due to hygrothermal degradation.
Frequency-damage mode correlations from various works.
The FFT peak frequency of each damage event was plotted against strain to illustrate the damage progression throughout loading. FFT peak frequency results for unidirectional and cross-ply laminates are shown in Figs. 10 and 11. Saturated samples accumulate significantly fewer events than the respective dry samples. Despite fewer events, dry and saturated samples of the same layups exhibit similar prevalent frequencies throughout the progression of damage. Distinct frequency bands are present during the loading process as can be seen in Figs. 11 and 12. In Fig. 11, the main frequency bands are located at 100, 230, and 290 kHz for the dry laminates, which do not change with conditioning. Similarly, frequency spectra of cross-ply laminates were unaffected by conditioning as shown in Fig. 12. These results would suggest that the damage mechanisms present do not change with conditioning; the micromechanical damage progression was not affected by conditioning. However, a notable difference in damage onset was evident in the AE response. The acoustic events begin to occur at lower strain levels in the saturated laminates compared to their respective dry laminates.
Typical event peak frequency versus strain for two-ply laminates.
Typical event peak frequency versus strain for four-ply laminates.
The event energies are plotted against strain in Figs. 13, 14 for the
unidirectional and cross-ply laminates, respectively. There is a substantial
change in event energies between dry and saturated samples. Acoustic events
recorded from dry samples consistently reach 10
The event energy AE results of the cross-ply laminates are shown in Fig. 14.
Both laminates show an increase in number of events near failure for both
conditions. Interestingly, the dry laminates feature many high-energy events
with less than 1 % strain that are not present in the saturated samples. The
[0/90]
Typical event energy versus strain for two-ply laminates.
Typical event energy versus strain for four-ply laminates.
The substantial reduction in laminate strength (Fig. 3) shows moisture uptake
to be particularly detrimental for this material system. Unidirectional
laminate strengths were reduced by 40 %, cross-ply laminate strengths by
54 %. The greater strength reduction present in [0/90]
The variation in damage tolerance can partially be explained by examination
of damage initiation. Changes in damage initiation were most evident in the
[90]
Despite a moisture-induced change in damage initiation of the transverse
plies, the subsequent damage progressions of dry and saturated laminates were
remarkably similar. Stress–strain and AE response are the tools used to
compare damage progression. Stress–strain responses of dry and saturated
[0]
As mentioned previously, a decrease in AE events and energy has been associated with moisture absorption. This effect was present in this study as well; however, general trends in AE response can still be used for comparison. Dry and saturated samples of both unidirectional and cross-ply laminates experienced the same frequency spectra (Figs. 10 and 12) throughout the loading cycle, suggesting that the individual damage mechanisms present were consistent regardless of condition. Event energies (Figs. 11 and 13), despite being lower in saturated samples, followed similar patterns, pointing to a consistent damage progression in both dry and saturated samples.
The similar damage progression between dry and saturated samples suggests that
the reduced ultimate strength was in fact due to reduced damage tolerance,
with moisture uptake increasing sensitivity to damage, leading to premature
growth. The damage growth attributed to ultimate failure can be observed by
inspection of failed coupons. The transverse cracking in [90]
Swelling due to hygrothermal conditioning may also contribute to the changes in damage tolerance observed in this work. Specifically, moisture-induced swelling may alter the internal stress state of the material without any mechanical loads. The effects of this internal stress state may be twofold: first, the internal stress reduces the mechanical loading required to cause failure; second, the increased internal stress state may cause damage or stress concentration, which would initiate and propagate at lower strain energy levels.
This work focused on characterization of hygrothermal effects on composites subjected to a static loading; further testing would be necessary to characterize fatigue performance, which would provide useful data for MHK design. However, from the results collected in this work, it is anticipated that fatigue performance would also be significantly reduced due to hygrothermal conditioning. The premature damage initiation and reduced damage tolerance after conditioning are both attributes which could be particularly detrimental to the material lifespan in a cyclic loading regime.
The fiberglass–epoxy system tested in this research experienced significant strength reductions after hygrothermal aging. Unidirectional laminates experienced static strength reductions of 40 % while cross-ply laminates experienced a more substantial reduction of 54 %. Larger strength reduction in cross-ply laminates compared to unidirectional laminates suggests the reduction in damage tolerance in a multi-angle composite is not reflected by the lamina behavior, with interacting ply behavior increasing the severity of hygrothermal effects.
Mechanical response of unidirectional and cross-ply laminates, supplemented by acoustic emission data, shows changes in the damage initiation in aged samples, as well as damage growth at final failure. When saturated, damage in transverse plies initiates at lower strains; however both the stress–strain response and AE response show consistency of subsequent damage progression compared to the unconditioned specimens. Although these data suggest that the damage progression is largely unaffected by moisture ingress into the composite, they reaffirm that moisture in fact reduces the damage tolerance. Examination of failed test specimens supports a change in damage tolerance, with saturated specimens experiencing more localized damage growth than unaged specimens. The more notable reduction in the strength of cross-ply laminates can be explained by an increased sensitivity to damage growth from transverse ply failures. This change in behavior cannot adequately be captured by lamina-based tests, emphasizing the importance of laminate characterization and qualification for wind energy and MHK application.
Data are available from the Montana State University Composite Material Technologies Research Group upon request (David A. Miller, davidmiller@montana.edu).
The authors declare that they have no conflict of interest.
This research was funded by the Water Power Technologies group at Sandia National Laboratories. Edited by: Lars Pilgaard Mikkelsen Reviewed by: two anonymous referees