Hygrothermal stress

Based on the authors experience, membrane fracture frequently occurs during events when membrane is dried out the in-plane hygrothermal stress is a critical driving force in membrane mechanical failure. The in-plane hygrothermal stress can be lowered by reducing the swelling/shrinkage ratio of the membrane, i.e., the dimensional instability of the membrane. 

The absorption of moisture in underfill adhesives induces a tensile hygrothermal stress on the solder-ball connections causing electrical opens in the connections and cracking in the adhesive. These tensile stresses offset the compressive stresses that underfill adhesives provide in improving the reliability of flip-chip and ball-grid-array devices. 

D.Y. Perera, M. Oosterbroek, Hygrothermal stress evolution during weathering in organic coatings, J. Coating Technol. 66 (1994) 83-88. 

As a result of in the changes in temperature and moisture, the membrane, GDL and bipolar plates will all experience expansion and contraction. Because of the different thermal expansion and swelling coefficients between these materials, hygrothermal stresses are expected to be introduced into the unit cell during operation. In addition, the non-uniform current and reactant flow distributions in the cell result in non-uniform temperature and moisture content of the cell which could in turn, potentially causing localized increases in the stress magnitudes. 

Vaddadi P, Nakamura T, Singh RP. Transient hygrothermal stresses in fiber reinforced composites a heterogeneous characterization approach. Compos. A Appl Sci Manufac 2003 34 719. 

The effect of nmiuniform fiber distribution was shown (Tsotsis and Weitsman 1990) to introduce a stress concentration factor of 3, indicating that residual hygrothermal stresses are likely to exceed the strength of polymers at certain locatimis within the composite. 

Hygrothermal stresses, induced in the laminate when it absorbs moisture, should also be considered. These are generally factored in by most computer analysis programs. 

Perera DY, Eynde DV (1987) Moisture and temperature induced stresses (hygrothermal stresses) in organic coatings. J Coat Technol 59(748) 55-63... 

At the microscopic scale, mechanical stress can also develop due to the property-mismatch of the electrode and the membrane. The porous electrode is typically much weaker than the membrane and has lower hygrothermal expansion coefficient than the membrane hence it can develop mud-cracks and delaminates from the membrane after RH cycling. This will perturb the local electrochemical processes and results in non-uniform decay of membranes. 

K A Kasturiarachchi Hygrothermal Degradation of Fibre Reinforced Epoxide Resins under Stress, PhD Thesis, Kingston Polytechnic, UK, 1983. 

K E Stansfield, The Effects of Stress and Thermal Spiking on the Hygrothermal Response of Carbon Fibre Reinforced Plastics, PhD Thesis, Kingston Polytechnic, 1989. 

A fiber reinforced composite material with a polymer matrix will typically absorb moisture in a humid environment and at elevated temperatures. Combined exposure to heat and moisture affects reinforced plastics in a variety of ways. First, the hygrothermal swelling causes a change in the residual stresses within the composite that could lead to microcrack forma-... 

Various types of coupled non-linear Fickian diffusion processes were numerically simulated using the free-volume approach given by equation [12.8], as well as non-Fickian transport. The non-Fickian transport was modeled as a stress-induced mass flux that typically occurs in the presence of non-uniform stress fields normally present in complex structures. The coupled diffusion and viscoelasticity boundary value problems were solved numerically using the finite element code NOVA-3D. Details of the non-hnear and non-Fickian diffusion model have been described elsewhere [14]. A benchmark verification of the linear Fickian diffusion model defined by equations [12.3]-[12.5] under a complex hygrothermal loading is presented in Section 12.6. 

Figure 12.4 depicts the comparison of the measured moisture uptake history for the laminate with NOVA-3D predictions. In general, the model prediction agrees reasonably well with test data over the 112 day period used for this comparison, especially during the absorption cycles. Figure 12.5 shows the predicted evolution of the in-plane stress at the exposed laminate surface with hygrothermal cycling. The increase in in-plane tensile... 

A comprehensive analytical model for predicting long term durability of resins and of fibre reinforced plastics (FRP) taking into account viscoelastic/viscoplastic creep, hygrothermal effects and the effects of physical and chemical aging on polymer response has been presented. An analytical tool consisting of a specialized test-bed finite element code, NOVA-3D, was used for the solution of complex stress analysis problems, including interactions between non-linear material constitutive behavior and environmental effects. 

Typically, the adhesive and/or the matrix in FRP retrofitting applications transfers three different stress categories. These are shear, peel and thermal residual stresses. The latter occur in FRP composite joints either upon fabrication due to mismatch in the hygrothermal and elastic properties of the fibres, matrices/adhesives and adherends or due to the difference between curing and operating temperatures of the FRP material. These three stress categories can be referred to as the good, the bad and the unavoidable, respectively. 

Compatibility of repair materials and existing substrate is another important condition for a successful repair. Compatibility should be investigated analytically and then experimentally verified both in the laboratory and in situ. The interface between new and old material should ensure that all stresses that unavoidably appear due to mechanical and hygrothermal actions are safely transferred. 

G. Yaniv and O. Ishai, Hygrothermal effects on stresses and deformations in a bonded fiber-reinforced plastic/aluminum system. Compos. Technol. Rev. 6, 63-73 (1984). 

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