Moisture induced change of the viscoelastic material properties of adhesives for SHM sensor applications

Abstract

This paper addresses the influence of the viscoelastic material properties of adhesives on the functionality of SHM (Structural Health Monitoring) sensor applications. Epoxy adhesives behave viscoelastically and show a strong temperature and time dependency of their mechanical properties. Creep processes increase the deformation under mechanical load and relaxation can decrease the stress in the adhesives (polymer) with time. These processes are most effective in the temperature range of glass transition (Tg), at which the material behavior switches between the glassy and rubbery state and all material parameters change drastically. Additional the viscoelastic material properties are influenced by environmental loading like moisture as it behaves like a plasticizer in the epoxy matrix. In this study the moisture influence on the viscoelastic behavior of structural adhesives was investigated by DMA (Dynamic Mechanical Analysis) methodology for harsh environmental loading conditions like DI water immersion and relative humidity (r.h.) environment. For testing under RH (Relative Humidity) conditions, a novel DMA-RH equipment was applied, which allows online testing at different temperatures and RH levels between 0%r.h. and 90%.r.h. Commercially available two component epoxy adhesives (unfilled, high filled) with high potential of acceptance in avionic applications were studied. For the unfilled adhesive a significant material property change due to moisture absorption occured. The modulus decreased down to about 25 percent of the dry state modulus E'. As expected the Tg was reduced for "wet" samples. Additional the change in the time dependence was seen. The high filled adhesive showed much lower diffusion constants and the modulus change was not that remarkable

Long-term stability of guided wave structural health monitoring using distributed adhesively bonded piezoelectric transducers

“…The observed losses of performance that were related to adhesive property changes are expected to be greater due to effects of harsher operational exposures (e.g. humidity adsorption and sensor fatigue 35,37,40,45 ). Thus, it is recommended that research be used to investigate the potential of devising bonding strategies that better exploit and control adhesivedependent properties such as those being developed by some researchers.…”

“…Thus, it is recommended that research be used to investigate the potential of devising bonding strategies that better exploit and control adhesivedependent properties such as those being developed by some researchers. 45 Additionally, it may be worthwhile updating the signal processing methods so that they can cope with expected variations in the properties of the adhesives used to bond transducers to specimens. Unless the issues with drifts in baseline subtraction results can be resolved, long-term monitoring of such structures to detect the growth of small defects is not feasible.…”

“…44,[46][47][48][49][50][51][52] Measurements of dynamic viscoelastic parameters suggest that the dynamic moduli of epoxies are particularly sensitive. 37,42,44,45,48,52 The density of these materials may also change. [47][48][49][50] The investigation of the complex mechanisms underlying these effects lies outside the scope of this work.…”

Lead Zirconate Titanate Transducers Embedded in Composite Laminates: The Influence of the Integration Method on Ultrasound Transduction

“…Indeed, the thickness and stiffness of the bonding layer affect the coupling properties, and consequently the transduction efficiency [12,13]. Moreover, the bonding layer is very sensitive to environmental conditions (moisture [14] and temperature [7,15]) and deteriorates with aging [16,17]. To overcome such bonding layer issues under environmental conditions and also provide protection from impacts, one solution is the integration of the SHM system directly into the material.…”

Reliability and functionality investigation of CFRP embedded ultrasonic transducers supported by FEM and EFIT simulations

“…The adhesive layer is one potential weak point in the design [1,2], mainly due to material degradation of the adhesive layer [3,4]. Improved integration concepts are required.…”