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Tapered-strap joint
Residual Shear Strength
The extent of creep damage and its importance is dependent primarily on the stress level at which irreversible damage occurs relative to the stress for complete failure (i.e. ultimate static strength). The degradation process is exacerbated under hot/wet conditions with the rate of degradation increasing with increasing temperature, humidity and mechanical stress. The dynamic rupture strength of a bonded joint, particularly under ambient conditions gives no indication of bond durability to sustained loading. It is therefore necessary to subject bonded joints to creep resistance tests in order to assure long-term durability. The test conditions should be controlled and relevant to the in-service conditions to be experienced (e.g. hot, high humidity for South East Asia and sub-zero for Northern Europe or North America).
Often tests will be conducted under severe conditions (i.e. high temperature, humidity and stress) in order to accelerate ageing of the material. Mechanical acceleration methods tend to use stress levels that are significantly higher than stress level limits used in design, thus the limiting design strains are reached in shorter times than in actual service.
Two approaches have been adopted for assessing the degree of degradation under combined static load and environment:
Rate of strength loss with time (i.e. residual strength): This approach determines the time taken for the strength of the joint to decline to a design stress limit, below which the joint is no longer considered safe. Specimens are removed at regular intervals to assess strength reduction.
Time-to-failure: This approach attempts to determine the probable average life expectancy of a bonded joint at a prescribed stress level or to determine the percentage of failures that can be expected to occur within a given exposure period.
The residual shear strength data (i.e. time-to-failure) for aluminium alloy tapered strap joint specimen bonded with an epoxy film adhesive loaded in tension is shown in Figure 4.
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Tapered-strap joint |
Residual Shear Strength |
Figure 4: Residual shear strength data for a tapered-strap joint
At elevated temperatures, the time-to-failure at any given stress level will tend to decrease. It is important when conducting test with self-stress mechanisms to constantly check applied loads. Stress relaxation is a common occurrence, which is more pronounced at elevated stress levels and temperature.
A number of key observations, listed below, can be made in regard to both the test results and test procedure for creep rupture testing using self-stressing fixtures.
An alternative approach to using the self-stressing fixtures employed in this programme would be load specimens individually in smaller tubes, which will add to the overall costs of testing, but would ensure a more reliable method of determining average life expectancy. Ideally, the loading chain should be instrumented to ensure an accurate indication of time to failure. The usual approach is to carry out a routine visual inspection for failed specimens.
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Miniature T-Peel Joint |
Residual Peel Strength |
Figure 5: Residual peel strength vs exposure time for a miniature T-peel joint
Caution is needed in interpreting creep rupture data as shown by the residual strength data presented in Figure 5 for a miniaturised mild steel T-peel joint bonded with an epoxy adhesive and exposed to a hot/humid environment. The curve (peak load) indicates two competing mechanisms: (i) interfacial degradation; and (ii) plasticisation (softening) of the epoxy adhesive. The increase in “apparent” strength (i.e. hump in the downward sloping curve) indicates that stress relaxation is occurring with increasing exposure time as the adhesive absorbs moisture. The stress concentrations in the adhesive fillet are reduced, and thus the load to initiate failure increases. This is counteracted by degradation at the interface, which increases with exposure time. The miniaturised T-peel specimen has been designed to specifically to fit the self-stressing tube shown in Figure 2. These results emphasise the need to carry out environmental tests on bonded structures, and not rely solely on design data generated on bulk adhesive samples.
