Failure of Rubber Components under Fatigue
Abstract
Rubber components under cyclic loading conditions often are considered to
have failed as a result of the stiffness changing to an amount that makes the part no
longer useful. This thesis considers three distinctive but related aspect of the fatigue
failure exhibited by rubber components. The first considers the reduction in stiffness
that can result from a phenomenon known as cyclic stress relaxation. The second
considers fatigue crack growth encountered resulting in potentially catastrophic failure.
The final issue relates to the complex topography of the resulting fatigue fracture
surfaces.
Previous work has shown that the amount of relaxation observed from cycle to
cycle is significantly greater than that expected from static relaxation tests alone. In this
thesis the reduction in the stress attained on the second and successive loading cycles as
compared with the stress attained on the first cycle in a stress strain cyclic test of fixed
strain amplitude has been measured for elastomer test pieces and engineering
components. Adopting the approach of Davies et al. (1996) the peak force, under cyclic
testing to a specific maximum displacement, plotted against the number of cycles on
logarithmic scales produces a straight line graph, whose slope correlates to the rate of
cyclic stress relaxation per decade. Plotting the rate of stress relaxation per decade
against the maximum average strain energy density attained in the cycle reduces the
data measured in different deformation modes for both simple test pieces and
components to a single curve. This approach allows the cyclic stress relaxation in a real
component under any deformation to be predicted from simple laboratory tests (Asare
et al., 2009).
Earlier work (Busfield et al., 2005) has shown that a fracture mechanics
approach can predict fatigue failure in rubber or elastomer components using a finite
element analysis technique that calculates the strain energy release rate for cracks
introduced into bonded rubber components. This thesis extends this previous work to
examine real fatigue measurements made at both room temperature and 70±1ºC in both
tension and shear using cylindrical rubber to metal bonded components. Dynamic
testing of these components generated fatigue failures not only in the bulk of the
component but also at the rubber to metal bond interface. The fatigue crack growth
characteristics were measured independently using a pure shear test piece. Using this
independent crack growth data and an accurate estimate for the initial flaw size allowed
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the fatigue life to be calculated. The fracture mechanics approach predicted the crack
growth rates accurately at both room temperature and 70±1ºC (Asare et al., 2011).
Fatigue crack growth often results in rough fatigue crack surfaces. The rough
fatigue crack surface is, in part, thought to result from anisotropy being developed at
the front of a crack tip. This anisotropy in strength whereby the material is less strong
in the direction that the material is stretched might allow the fatigue crack to grow in an
unanticipated direction. It might also allow the crack front to split. Therefore the final
part of this thesis examines how, once split, the strain energy release rate associated
with growth of each split fatigue crack develops as the cracks extend in a pure shear
crack growth test specimen. The aim being to understand how the extent of out of plane
crack growth that results might allow a better understanding of the generation of
particular crack tip roughness profiles. Using a method of extending one split crack at a
time, whilst keeping a second split crack at a constant length, it has been possible to
evaluate the initial strain energy release rates of split cracks of different configurations
in a pure shear specimen. It was observed that, for a split crack in a pure shear
specimen, the initial strain energy release rate available for crack growth depends on
the precise location of the split crack. It is also clear that the tearing energy is shared
evenly when the crack tip is split into two paths of equal length, but as one crack
accelerates ahead it quickly increases in tearing energy and leaves the slower crack
behind. It is thought that this phenomenon is responsible for a lot of the roughness
observed on the resulting fracture surfaces.
Authors
Asare, SamuelCollections
- Theses [3702]