Moisture-induced Stresses Research Articles

Analysis of moisture-induced stresses in an FRP composites reinforced masonry structure

It has been shown in many studies that fiber reinforced polymer (FRP) composite laminates can be used to effectively strengthen structures constructed by unreinforced masonry units. For practical application of this reinforcement method, the long-term interface bonding degradation due to moisture and temperature environmental effects needs to be addressed further. In this study, a finite-element modeling procedure for analyzing moisture-induced stresses in a multi-layered structure constructed with distinct permeable materials was developed. The modeling procedure was used to analyze moisture-induced stresses in a concrete block reinforced with a unidirectional glass–epoxy FRP composite laminate partially covering one lateral surface. The nonlinear humidity transport properties of both the concrete and FRP materials were taken into account in the analysis, and the convergence issue of the interfacial shear stress components associated with the free edge effect was addressed by the use of the submodeling technique. It was demonstrated that the moisture-induced stresses at the FRP–concrete interface critical to structural integrity could be determined for any time instant. The results showed that the interfacial stresses increased with the increase of the humidity diffusion time and monotonically approached the stress level at the steady-state condition of the humidity diffusion. It was also shown that the analysis by assuming constant humidity transport properties resulted in a significant underestimation on the maximum interfacial stresses. The current finite-element analysis procedure provides a general method for determining moisture-induced stresses in a multi-layered permeable structure. It can be used to aid with the design of FRP–masonry structures or other similar structures for minimizing interfacial stresses induced due to the mismatch of moisture swelling properties of the constituent materials.

Transient hygrothermal stresses in fiber reinforced composites: a heterogeneous characterization approach

Transient hygrothermal stresses induced in fiber-reinforced composites are studied in detail by adopting a novel heterogeneous characterization approach. This approach incorporates two distinct features: transient moisture absorption analysis of actual composite materials exposed to a humid environment, and highly detailed computational analyses that capture the actual heterogeneous microstructure of the composite. The latter feature is carried out by modeling a uniaxial laminate having more than one thousand individual carbon fibers that are randomly distributed within an epoxy matrix. Results indicate that these computational models are essential in capturing the accurate moisture absorption process of the actual specimen. In the analysis, the evolutions of thermal residual stresses and moisture-induced stresses within the humidity and thermal exposed composites have been analyzed. It was observed that high stress concentration develops in the epoxy phase where high fiber density or fiber clustering exists and its magnitude increases as the moisture content saturates. Large stresses can potentially initiate epoxy damage or delamination of epoxy and fibers. Furthermore, due to opposing effects of thermal and moisture exposure, lower stresses are found in the laminate when both are considered simultaneously.

Stress and moisture-sorption in ozone-annealed films of zirconium oxide obtained from sol-gel

The stress development in zirconium oxide films prepared by sol-gel and annealed under ozone-enriched oxygen has been investigated. The organic compounds are mostly oxidized at 200°C with ozone and the stress rises up to higher values than the stress in films annealed with pure oxygen between 200 to 400°C. During exposure to humid air of films annealed at 150°C under pure or ozone-enriched oxygen chemical transformations of organic compounds have been observed. The moisture-induced stress change during film exposure to the ambient air also has been investigated. The competition between hydration of ZrO2, H-bound formation, surface ionization and capillary condensation mechanisms is discussed.

A Growing-Season Hydroclimatology, Focusing on Soil Moisture Deficits, for the Ohio Valley Region

A hydroclimatology, or description of long-term means and interannual variation, that focuses on soil moisture deficits was constructed for the period of 1895‐1998 for a six-state region composing the Ohio Valley. The term ‘‘deficit’’ is considered from an agricultural point of view whereby moisture-induced crop stress is a combination of insufficient precipitation and soil moisture. Of particular concern are deficits that occur during the growing season (May‐September) when vegetation is most susceptible to moisture-induced stress. Evidence suggests that there is considerable temporal variability but no long-term trend toward either wetter or drier conditions in the Ohio Valley. The pattern of growing-season deficit is characterized by multiyear and multidecadal cycles of wet and dry periods. Decreases in precipitation during years with anomalously large growing-season deficits, however, are associated more with the reduced frequency of precipitation events than with any changes in intensity. These variations in precipitation frequency and the conditions conducive to droughts are intimately linked with largescale atmospheric conditions, including the low-level and upper-level flow patterns.

Densification and aging of ZrO 2 films prepared by sol–gel

The densification of ZrO 2 films prepared by sol–gel synthesis has been investigated by stress measurements, infrared spectroscopy, and spectroscopic ellipsometry after annealing at 150–400°C. It is shown that the tensile stress is limited by viscous flow accompanying the densification, and that the main organic compounds reducing the rate of condensation reactions are acetate groups. The oxidation of these groups at 350°C induces a jump in both the stress and the refractive indices of the films. The effect of film aging in air on the stress has also been studied. The moisture-induced stress change follows a logarithmic time dependence, most likely due to hydration of ZrO 2. The sign of stress change is determined by the amount of organic compounds in the gel.

Influence of moisture-induced stress on in situ fiber strength degradation of unidirectional polymer composite

Abstract A three-phase concentric cylinder model is used to estimate moisture-induced internal stress in unidirectional polymer composite. It is found that moisture-induced tensile stress on the fiber is substantial when absorbed moisture level in the polymer matrix is high. It is also shown that the stress-state of the fiber is not significantly changed by inclusion of an interphase layer with properties similar to the matrix. The consequence of moisture-induced internal stress is that glass fibers are subjected to stress corrosion, even under no externally applied stress. It is found, by measuring the size of the fracture mirrors of failed glass fibers in model composites, that the in situ tensile strength of E-glass fibers in an epoxy matrix composite was degraded at a faster rate compared to the case of stress-free aging of glass fibers. Thus it is concluded that moisture-induced tensile stress in glass fibers plays an important role in strength degradation of unidirectional composite under environmental aging.

Analysis of dimensionally stable copolyimide with a low-level residual stress

Copolyimide thin film, which has low-level stress and stress relaxation induced by water sorption, was characterized for potential applications as an encapsulant, a stress-relief buffer, and in interlayer dielectrics. The polyimides examined were poly(p-phenylene pyromellitimide) (PMDA-PDA) and poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) as well as their random copolyimides with various compositions. These copolyimide films exhibited good combinations of physical and mechanical properties with low thermal expansion coefficients, residual stress, and moisture-induced stress–relaxation behavior by appropriately selecting the ratios of the dianhydride component. For these polyimides, the residual stress increased in the range of −8.1–7.5 MPa, whereas stress relaxation induced by water uptake decreased in the range of 10.3–4.7 MPa with increasing BPDA contents, respectively. The major factor in determining the magnitude of the stress behavior induced by both the thermal mismatch and water uptake in films should be the morphological factors such as chain rigidity, chain orientation, crystallinity, and microvoids. Their morphological structures were examined by wide angle X-ray diffraction and a prism coupler, and the thermal properties were measured using a dynamic mechanical thermal analyzer as well as thermomechanical analysis. Overall, the candidate for the low level stress buffer application from the PMDA/BPDA-PDA copolyimide was the 30/70 (= PMDA/BPDA in molar ratio) copolyimide. This copolyimide showed no residual stress after curing at 400 °C and relatively insensitive stress relaxation to ambient humidity. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 796–810, 2001

Moisture-induced stress relaxation of polyimide thin films

Four different polyimides: rodlike poly(p-phenylene pyromellitimide) (PMDA-PDA), pseudo-rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semiflexible poly(4,4-oxydiphenylene pyromellitimide) (PMDA-ODA) and poly(4,4-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA) were prepared from two different precursors, poly(amic diethyl ester) (PAE) and poly(amic acid) (PAA). For polyimides prepared from the PAE precursors, stress relaxation coefficient at 25°C, 100% RH was 14.76×10−10cm2/s for PMDA-PDA, 3.29×10−10cm2/s for BPDA-PDA, 24.91×10−10cm2/s for PMDA-ODA and 9.34×10−10cm2/s for BPDA-ODA. For polyimides prepared from the PAA precursors, stress relaxation coefficient at 25°C, 100% RH was 10.50×10−10cm2/s for PMDA-PDA, 1.91×10−10cm2/s for BPDA-PDA, 28.84×10−10cm2/s for PMDA-ODA and 9.10×10−10cm2/s for BPDA-ODA. The effect of bulky ethyl ester group on the stress relaxation behavior is significantly high in rodlike PMDA-PDA or pseudo-rodlike BPDA-PDA having high chain rigidity and intermolecular packing order. The decreased birefringence, crystallinity and the looser packing in amorphous region of rigid polyimide derived from PAE may induce fast stress relaxation compared to flexible polyimide. Specially, the different chain mobility of polyimide also induced significant effect on the stress relaxation.

The effects of flux materials on the moisture sensitivity and reliability of flip-chip-on-board assemblies

This paper discusses the nonproprietary aspects of the work-in-progress regarding developments in flux materials to improve the moisture-induced stress sensitivity of flip chip plastic ball grid array devices. Studies addressing the moisture characterization and preconditioning, using JEDEC Level 3, of assemblies built with four proprietary fluxes and two alternative underfill materials will be presented. This report includes the evaluation of the weight loss during a simulated die attach reflow profile for each flux type using thermogravimetric analysis. Also, the underfill adhesion strength to the die surface after assembly, after moisture preconditioning and after 48 h of autoclave stressing will be discussed. The integrity of the solder joint corresponding to each flux type and measured using die pull techniques will also be presented. The effects of the experimental variables on underfill to die/substrate adhesion will also be reviewed using C-SAM imaging methods. Proprietary aspects of Motorola's FC-PBGA materials and assembly technology will not be disclosed.

Models of Moisture Transport and Moisture-Induced Stresses in Epoxy Composites

Mathematical models are presented to describe moisture transport in epoxy composites using concentration and stress dependent diffusion coefficients. These models take into consideration the stresses induced due to moisture and their influence on the transport mechanism. All governing differential equations are solved with appropriate boundary conditions to obtain concentration and stress profiles. It is concluded that non- Fickian and anomalous transport of moisture in epoxy composites can occur under certain conditions, leading to stress accumulation close to the fibers of the composite.

Effect of precursor history on residual stress and relaxation behaviour of high temperature polyimides

Using a wafer bending technique, residual stress and its relaxation were investigated for four different kinds of high temperature polyimides: rodlike PMDA-PDA, semi-rigid BPDA-PDA, semi-flexible PMDA-ODA, and flexible BTDA-ODA. Residual stress was measured in situ on silicon wafers during thermal imidization of the polyimide precursors and subsequent cooling of the resulting polyimides as a function of temperature over the range of 25–400°C. The stress of the cured films at room temperature was significantly relaxed by the moisture uptake of the films rather than their creep behaviour. Both residual stress and moisture-induced stress relaxation in the cured polyimide films were strongly dependent upon the molecular nature (that is, molecular chain rigidity, degree of molecular orientation and packing), as well as the precursor origin. In addition, the diffusion coefficients of water in the cured polyimide films were estimated by best-fitting stress relaxation against time profiles.

Wellbore stress distribution produced by moisture adsorption : Yew, C H; Chenevert, M E; Wang, C L; Osisanya, S O SPE Drilling EngngV5, N4, Dec 1990, P311–316

Evaluation of the stress distribution produced by moisture adsorption around a wellbore by applying the mechanics of deformable solids. The process of moisture adsorption is governed by a diffusion equation, and the equations governing moisture-induced stresses around the hole, are similar to those used in thermoelasticity theory. A computational method is developed for calculating the stress distribution around the hole, and an experimental procedure for obtaining the material constants needed in the computation (adsorption characteristics, moisture-induced modulus change of the material) is presented.

Wellbore Stress Distribution Produced by Moisture Adsorption

Summary For this study, we evaluated the stress distribution produced by moisture adsorption around a wellbore by applying the mechanics of deformable solids. This paper shows that the moisture-adsorption process is governed by a diffusion equation and that the equations governing the moisture-induced stresses around the hole are similar to those equations used in thermoelasticity. A computational method is developed for calculating the stress distribution around the borehole, and the experimental procedure for obtaining the material constants needed in the computation is presented. This paper also studies the effect of borehole pressure on the stress distribution around the hole. Results show that the magnitude and distribution of stresses around the borehole are very sensitive to the adsorption characteristics and the moisture-induced modulus change of the material.

The Effect of Moisture on the Structural Stability of a Coal Cavity

The drying of coal and the associated thermal and moisture-induced stresses are examined with a numerical model to estimate the rate of surface regression in underground coal gasification. The model includes internal convection, diffusion, conduction and flow of liquid, vapor and gas. The structural failure is modeled by three different mechanisms based upon a strength reduction due to heating, drying, or crack formation. Using properties and boundary conditions appropriate to Western United States coal, the model predicts a regression rate which is in qualitative agreement with measured results. Using the model, it is possible to examine different thermal and failure modes and to gain an understanding of some of the mechanisms which may control the surface regression of coal during gasification. These results may be applicable to the formation of rubble during gasification and to internal cavity collapse.

Consideration of moisture effects on the upstream face of concrete dams

1. Investigations of the saturation of concrete of the Sayano-Shushenskoe dam showed that during five years of operation of the structure moisture penetrated into the concrete to a depth of 4–6 m, and the maximum change in the moisture content of the concrete of the upstream face was 1.5%. The moisture-induced strains at a depth of 0.5 m from the upstream face reached a value of 4.5–5.5×10−5 and at a distance of 4.5–5 m a value of 0.6–1.0× 10−5. The change both in the depth of penetration of moisture into the structure and in the degree of saturation occurred most intensely during the first 1–1.5 years after a rise of the head. 2. Calculation of moisture-induced stresses on the basis of the data of on-site investigations of the moisture content and moisture-induced strains with the use of the physical and mechanical characteristics of concrete determined under laboratory and on-site conditions showed that compressive stresses, which reached 0.95–1.52 MPa, occurred on the upstream face of the dam. The width of the zone of compression is 3–4 m for concrete located below the seasonal drawdown of the reservoir and up to 2 m in the zone of the variable water level. 3. Consideration of the additional compressive stresses on the upstream face of the Sayano-Shushenskoe dam during the 1982 flood made it possible to raise the elevation of filling the reservoir and to increase the production of electric energy by 127 million kWh. 4. A generalization of the results of investigating saturation of concrete in the zone of the upstream face of the Sayano-Shushenskoe dam and also of the additional data obtained at the Krasnoyarsk and Zeya dams made it possible to establish a general regularity for determining stresses occurring on the upstream face of dams of various types due to swelling of concrete, which can be recommended for inclusion in SNiP 2.06.06-85 “Concrete and Reinforced-Concrete Dams.”

Analytical/Experimental Correlations of Stiffness Properties of Unidirectional Composites

A finite element micromechanics analysis is used to predict the longitudinal and transverse moduli of both graphite/epoxy and glass/epoxy unidirectional composites, as a function of temperature, for various moisture preconditionings. These predictions are then correlated with corresponding experimental data. The micromechanics analysis includes temperature- and moisture-dependent matrix material properties, inelastic matrix stress-strain response, and anisotropic fibers. Thermal residual stresses caused by cooldown from the cure temperature and moisture-induced swelling stresses are included in the analysis. Good correlation is obtained between theory and experiment.