Polymers ductile failure of brittle

Ductile Failure of Brittle Polymers under Compressive Shear Stresses... 

In recent experiments, the application of stress orthogonally to a shearing surface caused a ductile failure of brittle polymer (5, 6). In the first series (5), a variety of plastomers and elastomers were made to slide one on the surface of another, at a constant velocity of 215 cm/sec, under increasing normal loads. The wear characteristics of polymers, including several brittle ones such as PMMA and PS, depend on the applied normal stress. At relatively low pressure, almost no wear was observable, even under magnification the little observed was apparently brittle, ill-defined, microparticulate debris. At intermediate normal loads, 3 to 20 kg/cm2, roll formation was the dominant mode of wear. Such rolls appear on the surfaces of all uncrosslinked polymers whose Tg is below the test temperature and on amorphous and semicrystalline polymers whose Tg is above... 

Craze Initiation. Although the effect of multiaxial states of stress on the brittle and ductile failure of isotropic polymers is sufficiently well represented by the above classical failure criteria, this is not the case for crazing or the failure of anisotropic polymers, ie, oriented sheets, fibers, single crystals, etc. For craze initiation we will cite the stress-bias criterion as proposed by Stemstein (54) ... 

Failure Morphologies. Ductile failure of notched polycarbonate specimens has long been recognized to occur with shear yielding from the notch tip (6). This occurs for the block polymers for all rates of test. Hull and Owen (5) recently reported from micrographic studies of impact fracture surfaces that the brittle failure of polycarbonate involves the formation and breakdown of a craze at the notch tip. The ductile-... 

Figure 7, Temperature dependence of failure stresses in Instron three-point bend tests on Vs inch notched Izod bars cut from (a) extruded polycarbonate sheet and (b) compression molded block polymer B. Crosshead rate = 0,02 inch /min. Span = 2 inches, o = net section stress = force/net cross-section at notch root, O, Craze initiation , ductile failure X, brittle failure ...

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Unlike ductile metals, composite laminates containing fiber-reinforced thermosetting polymers do not exhibit gross ductile yielding. However, they do not behave as classic brittle materials, either. Under a static tensile load, many of these laminates show nonlinear characteristics attributed to sequential ply failures. One of the difficulties, then, in designing with laminar composites is to determine whether the failure of the first ply constitutes material failure, termed first-ply failure (FPF), or if ultimate failure of the composite constitutes failure. In many laminar composites, ultimate failure occurs soon after first ply failure, so that an FPF design approach is justified, as illustrated for two common laminar composites in Table 8.9 (see Section 5.4.3 for information on the notations used for laminar composites). In fact, the FPF approach is used for many aerospace and aircraft applications. 

For a given rate of chain scission, the failure properties of an initially brittle polymer decrease at a considerably slower rate than for an initially ductile polymer. 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

Figures 13.16 and 13.17 are plots of the compressive stress-strain data for two amorphous and two crystalline polymers, respectively, while Figure 13.18 shows tensile and compressive stress-strain behavior of a normally brittle polymer (polystyrene). The stress-strain curves for the amorphous polymers are characteristic of the yield behavior of polymers. On the other hand, there are no clearly defined yield points for the crystalline polymers. In tension, polystyrene exhibited brittle failure, whereas in compression it behaved as a ductile polymer. The behavior of polystyrene typifies the general behavior of polymers. Tensile and compressive tests do not, as would normally be expected, give the same results. Strength and yield stress are generally higher in compression than in tension.

Since the carbon-fluorine bond is very stable and since the only other bond present in PTFE is the stable C-C bond, the polymer has a high stability, even when heated above its melting point. Its upper-use temperature is given as 260°C. It is reported to give ductile rather than brittle failures at temperatures just above absolute zero, signifying a useful temperature range of more than 500°C. In many instances PTFE has been used satisfactorily as a totally enclosed gasket for considerable periods of time at temperatures well above the recommended upper-use temperature. 

There is, however, no generally accepted theory for predicting the brittle-ductile transition or relating it to other properties of the polymer, although for some polymers it is closely related to the glass transition. The type of failure is also affected by geometrical factors and the precise nature of the stresses applied. Plane-strain conditions, under which one of the principal strains is zero, which are often found with thick samples, favour brittle fracture. Plane-stress conditions, xmder which one of the principal stresses is zero, which are often found with thin samples, favour ductile fracture. The type of starting crack or notch often deliberately introduced when fracture behaviour is examined can also have an important effect ... 

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