Hygrothermal Damage Research Articles

Water Intrusion: An Analysis of Water Sources, Categories, and the Degradation Science of Building Materials

Water intrusion into a building envelope describes the unwanted movement of water or vapor into a building, where it causes damage. Various factors dictate water intrusion category determination and classification. These factors include, but are not limited to, the type and degree of water intrusion, the source and route of the contamination, and exposure time, as well as geographical environmental conditions. This comprehensive research paper looked at the literature and the science to explore the bases for indoor environmental professional (IEP) classification and category determination, but also the science behind the effects of water intrusion on building materials (BM). The efficacy of building materials once degradation has occurred and any accelerating effects impacting the efficiency of building materials and their loss of integrity were closely examined in terms of material microstructural and compositional changes. The damaging effects of moisture and heat on building materials are called hygrothermal damage, which compromises the properties and use of materials. Both aspects of building integrity, i.e., water intrusion and structural deterioration, should be of concern when mitigating and remediating the intrusion of moisture. Previous research on the clarification of water categories for water intrusions is lacking. Past approaches to water classification have lacked universal scientific clarity and understanding. In addition to a need to understand the effects that water category might have on building materials and their corresponding degree of degradation, more science and reviews are needed. The need for proper class and category determination for the remediation of water intrusion within buildings is the first step toward achieving correct mitigation to ensure human health and safety. The possible adverse health effects of water intrusion need focus and cohesion for the determination of categories. We know that the final determination of water categories differs according to the degree of contamination over time and the degree of a given class of water intrusion; however, what role do the route and initial water contamination play in the determination of the category? The following paper aims to provide not only a review of the science but also an elaboration of the category determination process and the degradation effects on building materials which should be considered, as well as possible avenues of scientific research.

Interlaminar Shear Strength Change and Storage Life Prediction of Carbon Fiber/Epoxy Composites with Hygrothermal Accelerated Aging.

In order to investigate the durability of fiber-reinforced polymer composites in hygrothermal environments, hygrothermal accelerate aging tests, for 360 days at 70 °C, RH70%; 70 °C, RH85%; 85 °C, RH70%; and 85 °C, RH85% and natural storage for 2 years in Guangzhou, China, were carried out for composite laminates. Then, the moisture absorption and interlaminar shear strength were measured. The hygrothermal damage mechanism of the composite was studied by Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and field emission scanning electron microscopy (FSEM). A dual stress storage life prediction model and the equivalent relationship between natural storage and hygrothermal acceleration were established. The results show that the order of moisture absorption rates, moisture absorption contents, and the severity effect order on the interlaminar shear strength is RH85%; 85 °C > 70 °C; RH85% > 85 °C; RH70% > 70 °C; and RH70%. The time to achieve an effective moisture absorption balance is opposite to this. The moisture absorption rate meets Fick's law before the effective moisture absorption balance, and then shows a linear trend. The interlayer shear strength still decreases exponentially with aging, which is mainly caused by the resin plasticization and interface weakening. Hygrothermal accelerated aging for 13.4831 days at 85 °C; RH85% is equivalent to that for one-year actual storage in Guangzhou. According to the failure criterion of shear strength decreasing to 77%, the storage life of T700/epoxy in Guangzhou is 14.4661 years.

Assessment of damage in composites using static component generation of ultrasonic guided waves

Static component (SC) generation of guided waves (GWs), which combines the high sensitivity of acoustic nonlinearity to micro-damage and low attenuative effect, has great potential for damage assessment in large composite structures. The present work explores the use of SC generation of GWs for assessing damages in carbon fiber reinforced polymer (CFRP) composite laminates. The features including mode, waveform, and cumulative effect of the generated SC in composites are numerically investigated by three-dimensional finite element modeling and simulation. A dynamic displacement measurement method based on piezoelectric transducers is accordingly proposed and experimentally verified. The cumulative SC pulse generated from primary GW tone-burst with a finite duration, is observed and verified numerically and experimentally. It is found that the magnitude of the generated SC pulse is linearly proportional to the quadratic material nonlinearity. Experimental results demonstrate that the generated SC pulse of GW under group velocity matching condition, is an effective means to assess the hygrothermal damage and low-velocity impact damage in CFRP composite plates. The performed experimental examination validates the feasibility of the proposed approach for damage assessment in CFRP composites.

Hygrothermal Damage Monitoring of Composite Adhesive Joint Using the Full Spectral Response of Fiber Bragg Grating Sensors.

Adhesive joints in composite structures are subject to degradation by elevated temperature and moisture. Moisture absorption leads to swelling, plasticization, weakening of the interface, interfacial defects/cracking and reduction in strength. Moisture and material degradation before the formation of defects are not readily revealed by conventional non-destructive examination techniques. Embedded fiber Bragg grating (FBG) sensors can reflect the swelling strain in adhesive joints and offer an economical alternative for on-line monitoring of moisture absorption under hygrothermal aging. Most of the available works relied on the peak shifting phenomenon for sensing. Degradation of adhesive and interfacial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. It is reasoned that the full spectral responses may be more revealing regarding the joint’s integrity. Studies on this aspect are still lacking. In this work, single-lap joint composite specimens with embedded FBGs are soaked in 60 °C water for 30 days. Spectrum evolution during this period and subsequent tensile and fatigue failure has been studied to shed some light on the possible use of the full spectral response to monitor the development of hygrothermal degradation.

Hygrothermal aging mechanism of carbon fiber/epoxy resin composites based on quantitative characterization of interface structure

The in-situ quantitative characterization of the interface structure and properties of carbon fiber/epoxy resin composites was carried out by using peak force quantitative nanomechanical mapping (PF-QNM) technology in this study. The mechanism of hygrothermal damage was developed. It is found that the hygrothermal aging can be divided into three stages. In the first stage, the resin matrix and the interface absorb simultaneously moisture and the interface size increases linearly from 45.2 ± 4.1 nm to 76.2 ± 10.3 nm. The increase of the “soft interface” area makes the bending strength and interlaminar shear strength decrease by 17.3% and 15.8%, respectively. In the second stage, resin matrix undergoes secondary curing and the interface size continues to increase up to 110.8 ± 13.8 nm, resulting in the increase of the bending strength, interlaminar shear strength and impact resistance. The impact peak load and strain energy increase by 56.1% and 1286.3%, respectively. In the third stage, interface area is saturated with moisture and the size changes little and the resin continues to absorb moisture, resulting in the swells and plasticization under high temperature and high humidity. The bending strength and interlaminar shear strength of the material decrease respectively to 1185.9 ± 13.1 MPa and 53.2 ± 3.2 MPa, with a reduction of 27.6% and 28.0%.

Environmental performance assessment using monitoring and DVS testing

It is now widely accepted that the adverse effects of climate change have become more visible over recent last decades, and current climatic trends suggest that more severe conditions are to follow. In relation to extreme weather events due to climate change, various material degradation phenomena and loss of structural integrity, due mostly to moisture-related deteriorating agents, have been observed on the existing building stock, especially historic buildings. The identification and quantification of such adverse effects is therefore of utmost importance, and environmental monitoring is one of the strongest tools to achieve this aim. This paper provides detailed information regarding on-site environmental monitoring work carried out on a sixteenth century timber frame cottage in Tewkesbury, Gloucestershire, England, along with hygroscopic characterisation using the dynamic vapour sorption (DVS) technique. The results obtained from monitoring and DVS testing are discussed to give detailed insight into hygrothermal damage mechanisms and the resulting loss of strength/physical integrity due to adverse environmental impacts on historic building envelopes.

Anisotropy of hygrothermal damage in fiber/polymer composites: Effective elasticity measures and estimates

Measured elasticity moduli of a highly (68%) glass-fiber reinforced epoxy matrix for different amounts of fiber/matrix interface weakening and debonding, due to different hygrothermal ageing stages, are compared to estimated ones. Ultrasonic measurements provide seven of the nine elasticity moduli of the orthotropic material samples, including all the moduli significantly affected by damage. Theoretical estimates combine homogenization modeling techniques and Finite Element (FE) calculations, the latter when the effect of observed partial debonding on effective moduli is to be specified. These estimates are performed under different assumptions for the composite structure, with special attention to the existence of a fiber–matrix interphase. Analytical comparisons for the undamaged composite establish that matching US measurements with estimates cannot be obtained, regardless of the chosen model, without the assumption of an interphase layer of modified resin coating the fibers. This coating resin, when in relevant concentration with regard to literature data about fiber coating thickness, typically conserves the epoxy moduli transversally to the fiber orientation, while, in the fiber direction its moduli approach those of the fibers. The comparison of the US measurements on damaged samples to FE calculations assuming progressive one-directional debonding shows that most of the composite stiffness loss can preferentially be due to an initial interphase weakening, while the fiber/matrix debonding seems more likely delayed to long H-ageing times. This is consistent with physical interpretation of damage by water pooling through silane bridges bonding epoxy to glass. The calculations also provide the effective stiffness, at different damage stages, of the “Undamaged Equivalent Inhomogeneity” for this damaged inclusion type.

Effects of Moisture on the Durability of a Wood/Thermoplastic Composite

The effects of simulated marine environments on the performance of wood/thermoplastic composites under quasi-static and fatigue loading is investigated. The material under consideration consists of wood flour embedded in a thermoplastic matrix. The effects of additives and processing methods were included in the durability study. Moisture sorption studies are presented for these formulations and are found to compare favorably with a linear Fickian diffusion model. Qualitative and quantitative evidence of hygrothermal damage was nevertheless consistently observed. The combination of Fickian diffusion and damage is explained from moisture-induced swelling considerations of the constituents. The influence of marine environments on fatigue response was examined by immersing the coupons in seawater during the fatigue test. The synergistic effects of moisture and fatigue are discussed.

Hygrothermal Damage in Graphite/Epoxy Laminates

The use of micromechanics equations for moisture diffusivity shows that the in situ diffusivity is slightly lower than the bulk diffusivity for matrix resins, thereby indicating absence of any matrix damage in virgin composites. When exposed to hygrothermal environments, however, composites undergo degradation which manifests itself in anomalous moisture diffusion behavior and reduced structural performance. The hygrothermal degradation is the result of matrix plasticization, microvoid formation, and microcracking. The time dependence of plasticization as well as the tensile stress resulting from steep moisture gradient is responsible for the damage induced by thermal spiking of wet composites. Swelling of neat resins is frequently less than predicted by the volume additivity. A simple micromechanics analysis provides a good estimate of composite swelling strain from resin properties. The bilinearity and the hysteresis observed in relations between swelling and moisture content are attributed to the existence of a threshold moisture concentration below which swelling is negligible. Relaxation of residual stresses is a long-term process under nonhostile environments. However, it is accelerated considerably around and above the glass transition temperature. The fast stress relaxation can change the transverse ply stress from compressive to tensile after thermal spiking, thereby inducing ply cracking and accelerating the subsequent moisture absorption. How residual stresses can affect ply cracking and delamination is shown through a fracture mechanics analysis.