Modelling Moisture Effects
Finite Element Approaches for Modelling Interfaces
Strength of Materials Approach
Environmental considerations are a key factor in the design of bonded structures for outdoor applications, particularly for load-bearing structures. Adhesives used in aircraft structures may be exposed to extreme temperatures, ranging from –50°C at high altitudes to 50°C (or more) sitting on the runway. In tropical environments, such as SE Asia, high temperatures are accompanied by high humidity (almost 100% relative humidity under certain circumstances). At high altitudes the low temperatures accompanied by very low pressures can cause rapid moisture loss resulting in possible blistering and void formation in the adhesive. Adhesives used in airframes may also be exposed to jet fuel, chemical solvents (e.g. paint stripper), de-icing fluids, and salt spray in the case of sea rescue helicopters. Harsh environments are not limited just to aerospace applications. Automotive components will often be exposed to harsh environmental conditions (high temperatures and humidity) and chemical agents, including salts and other chemicals for melting ice, air pollutants (ozone, sulphur dioxide and other smog constituents).
Although the list of environmental factors is too numerous to elaborate, the common denominator with all these factors is the possible detrimental effect on the adhesive properties (particularly creep). It is therefore of paramount importance, to select an adhesive appropriate to the service application (see Adhesive Selector ). This applies equally to the selection of adherends. Many structural adhesive systems are far more chemical resistant to strong acids, salt solutions and oxidative agents than stainless steel or aluminium alloys and it is not unusual in laboratory tests for the adhesive to outlast the adherend. Mild steel exposed to a hot/humid environment will undergo thinning due to corrosion whilst the adhesive if sufficiently resistant remains unaffected. Certain combinations of adhesive and adherends may pose chemical incompatibility problems.
The effect of simultaneous exposure to both mechanical stress and a chemical environment is often more severe than the sum of each factor taken separately. The combination of mechanical stress, elevated temperature and high relative humidity can prove catastrophic. The environmental consequences can be so severe that it is advisable to trial bonded components under actual service conditions (i.e. field trials). Factors that will most likely affect the durability of the joint are listed below .
· Maximum stress and average constant stress levels
· Environmental conditions (e.g. temperature and humidity)
· Cyclic effects of stress and environment (including rate and period)
· Exposure time
Another factor for consideration is the surface treatment. Good design can be easily undermined by poor surface preparation. Most failures occur as result of interfacial degradation. Moisture and other chemicals can diffuse rapidly along the adhesive-adherend interface through capillary action and trigger chemical reactions with the adherend surface resulting in debonding.
In the case of poorly prepared metal surfaces, hydrated oxides will form on the surface displacing the chemical bonds between the adhesive and adherend, and as a consequence the adhesive bond is compromised (failing prematurely). Chemicals, such as moisture, may also permeate through the adherend (e.g. fibre-reinforced polymeric materials) entering the interfacial zone from beneath leading to adverse effects on interfacial bond strength. For an adhesive bond to be durable, the surface should be free of contaminants, sufficiently chemically active to enable the formation of chemical bonding between adhesive and adherend and resistant to environmental attack.
Note: It is recommended to seek the advice of the adhesive manufacturer as to adherend-adhesive-compatibility. This also applies to surface preparation of the adherends.
The following notable generalisations on designing a joint for outdoor applications are :
· The most severe locations are those with warm temperatures and high humidity (e.g. tropical climates);
· Stressed joints deteriorate more rapidly than unstressed joints (unstressed bonds are relatively resistant to severe outdoor weathering, although all joints will eventually exhibit some strength loss);
· Heat cured adhesive systems are generally more environmental resistant than room-temperature cured systems; and
· Stainless steel joints are more resistant to corrosion than aluminium joints;
Bond strength is affected by internal stresses arising from the combined effect of shrinkage during cure, differences in the coefficient of thermal expansion (CTE) and thermal conductivity between the adhesive and the adjoining adherends (see Table 1). This effect is exacerbated at low temperatures (i.e. sub-zero).
Table 1:Coefficients of Thermal Expansion
CTE (10-6/ ° C)
At cryogenic temperatures, the elastic modulus of the adhesive may increase to the point where the adhesive can no longer effectively relieve concentrated stresses. It is important that the adhesive retains a degree of resiliency where the CTE of the adherends and adhesive cannot be matched. Joints with thinner bond-lines and adhesives with a higher level of thermal conductivity have better cryogenic properties than joints with thicker bond-lines.
Most conventional low modulus adhesives, such as flexible epoxies and toughened acrylics, are sufficiently flexible to use at low temperatures (-40°C (-40°F) to -73°C (-100°F)). Structural adhesives can operate as low as –240°C (-400°F). Epoxy based systems are brittle at low temperatures (i.e. low peel and impact strength).
Prolonged, or even short term, exposure to elevated temperatures will often produce irreversible chemical and physical changes within adhesives. Increasing the temperature will make the adhesive more ductile and as a result the bond strength decreases. Above the glass-transition temperature Tg the behaviour will change dramatically. All adhesives degrade at elevated temperatures. Thermal ageing can compromise the ability of an adhesive to absorb stresses due to thermal expansion or impact. Oxidation is the primary cause of adhesive degradation at elevated temperatures with the rate of degradation increasing with the amount of oxygen present. Oxidation can result in a loss of strength and discolouration. Metallic substrates are impermeable to oxygen, thereby providing a barrier to the gas. In contrast, fibre-reinforced polymer composites are permeable to atmospheric gases, and hence the rate of degradation can be expected to be higher. Pyrolysis (thermal destruction through scission (breaking) of the molecular chains) can also occur at elevated temperatures resulting in a reduction in cohesive strength and brittleness of the adhesive.
It is important to identify the maximum temperature that the bonded structure will experience during its service life. For an adhesive to withstand exposure to elevated temperatures it must possess the following:
· A high melting or softening point
· Resistance to oxidation and pyrolysis
· Creep resistance (decreases at elevated temperatures and high loads)
Moisture Effects« BACK TO DESIGN, PREPARATION AND TESTING