Brittle: behaviour

The stress-strain curve in Fig. 7.24a shows a linear region corresponding to enthalpic elasticity, followed by a slightly curved region in which local deformation processes (referred to as preplastic) lead to a reduction in modulus. Brittle fracture then occurs, perpendicular to the load direction. 

The fragile character of rigid amorphous polymers is an important limitation in many potential applications. The problem is circumvented by blending with an elastomer phase. This can produce materials capable of resisting impact. Hence, car rear lights are made of poly(methyl methacrylate) strengthened by addition of a polyacrylate with Tg around —40°C. 

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It should, however, be realised that this lower value (2.5 ftlbf per in notch) is still high compared with many other plastics. Such values should not be considered as consistent with brittle behaviour. Comparatively brittle mouldings can, however, be obtained if specimens are badly moulded. 

Such degradation of the surface causes little effect on either flexural strength or flexural modulus of elasticity but the influence on the impact properties is more profound. In such instances the minute cracks form centres for crack initiation and samples struck on the face of samples opposite to the exposed surface show brittle behaviour. For example, a moulded disc which will withstand an impact of 12 ftlbf without fracture before weathering will still withstand this impact if struck on the exposed side but may resist impacts of only 0.75 ftlbf when struck on the unexposed face. 

It may be seen from Fig. 2.80 that some plastics experience the change from ductile to brittle behaviour over a relatively narrow temperature range. This permits a tough/brittle transition temperature to be quoted. In other plastics this transition is much more gradual so that it is not possible to attribute it to a single value of temperature. In these circumstances it is common to quote a Brittleness Temperature, rg(l/4). This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4 mm tip radius) equals 10 kJ/m. This temperature, when quoted, gives an indication of the temperature above which there should be no problems with impact failures. It does not mean that the material should never be used below Tb(1/4) because by definition it refers only to the sharp notch case. When the material has a blunt notch or is un-notched its behaviour may still be satisfactory well below Tb(1/4). 

Steels are normally ductile at ambient temperatures, although they are often close to brittle behaviour, as is indicated by the ductile-brittle transition temperature. If the conditions at the tip of a sharp crack are considered, it can be seen that brittle fracture will occur if it is easier to break the atomic bond at the tip of the crack than it is to emit a dislocation to blunt the crack (see Thompson and Lin ). As dislocation emission is more temperature sensitive than the bond strength it becomes more difficult at low temperatures and brittle fracture occurs. The very severe effects of hydrogen on the performance of steels can be attributed to its role in allowing brittle fracture... 

Again because of the crosslinks, such brittle behaviour occurs whatever the temperature unlike brittle materials based on linear polymers, there is no temperature at which molecular motion is suddenly freed. In other words, the Tg, if there is one, does not produce dramahc changes in mechanical properties so that the material is changed from one that undergoes brittle behaviour to one that exhibits so-called tough behaviour. 

The brittle behaviour of the silica aerogel remains upon the introduction of TMS-modihed PNPs. However, even a low polymer content promotes a clear improvement of the mechanical properties of the material Young s modulus increases and the maximum compression strength and the corresponding strain become three to five times higher. Furthermore, the improvement in mechanical behaviour is noticeable... 

The mechanical properties are generally fair with low elongations at break and a brittle behaviour at room temperature. Moduli and hardnesses are higher than those of polyolefins. 

The properties of a material must dictate the applications in which it will best perform its intended use. All materials made to date with polymerized sulphur show time-dependent stress-strain behaviour. The reversion to the brittle behaviour of orthorhombic sulphur is inevitable as the sulphur transforms from the metastable polymeric forms to the thermodynamically stable crystalline structure. The time-span involved of at most 15 months (to date) would indicate that no such materials should be used in applications dependent on the strain softening behaviour. Design should not be based on the stress-strain relationships observed at an age of a few days. Since the strength of these materials is maintained, however, uses based on strength as the only mechanical criterion would be reasonable. 

Sulphur concretes have undergone substantial development in the last six years. Recent effort has been directed towards improved durability and less brittle stress-strain behaviour has been achieved. A technology has been developed to produce a material (Sudicrete) with the same stress strain behaviour after three years as that observed soon after casting. Other materials show a consistent reversion to brittle behaviour with time. Nevertheless, there is considerable room for improvement in mix design. 

Depending on the chemical structure of the polymer and on the experimental conditions (T and e), polymer solids can present a brittle behaviour, a ductile behaviour, or an intermediate fracture behaviour. 

In most thermoplastics, transitions from ductile to brittle behaviour may be induced by increasing the test speed. For the reasons already invoked in the introduction, this is of particular concern in iPP, whose impact proper-... 

Swain, M.V. (1991), Quasi-brittle behaviour of ceramics and its relevance for thermal-shock , Eng. Fract. Mech., 40, 871-877. 

Although fracture toughness can be increased, particle- or whisker-reinforced sialon composites generally show brittle behaviour and low damage tolerance this is in contrast to fibre-reinforced sialons which exhibit non-catastrophic failure. 

Polypropylene homopolymer (PP) is a widely used thermoplastic material, despite its brittle behaviour at either low temperature or high loading rates. Improvement in the fi acture toughness of PP can be achieved by either modifying the crystalline structure, or addition of a second phase material [16], The toughening effect and mechanisms of different second phase materials such as stiff fibres, soft rubbery inclusions (EPR, EPDM), and some mineral fillers have been analysed. Recent developments concern the effect of hybrid system consisting of rigid and rubbery inclusions. 

Under test conditions (23°C, quasi-static), polypropylene has a semi-brittle behaviour, to a crack extent of I mm, and is brittle at larger crack growth. 

In case of elastic linear brittle behaviour the straight line described by the experimental points runs through the origin (see Fig. 4(i)) and its slope gives Kimax (= Kic) [8-10]. 

Tests were made with a polyurethane resin used in the wood industry (Young s Modulus K O MPa) displaying a quasi-brittle behaviour, and substrates made from Norway Spmce (14 ... 

When thermoplastics are reinforced with short fibres, both stiffness and strength may be increased, but these improvements are accompanied by a reduction of the ultimate strain for high concentrations of fibres [1-5]. This brittle behaviour is incompatible with growing engineering applications in which tlie parts are subjected to cyclic loadings or impacts, as in automotive under-the-hood applications. Thus, very tough nylon thermoplastics with enhanced creep and impact resistances have been produced by the introduction of a rubbery phase to the... 

Linear Elastic Fracture Mechanics (LEFM) describes the brittle behaviour of a material in term of the critical value of the stress intensity factor at the crack tip, Kq, at the onset of propagation at a critical load value Pc ... 

Ti5Si3 fibers was estimated to 2530 MPa. This value is higher than the fiber strength reported in the paper of Crossman et al. [24], The estimated stress is of the order of the strength of titanium silicide whiskers prepared by chemical vapor deposition. However, it should be noted that it is very difficult to determine the exact flow stress and tensile strength at room and somewhat higher temperatures because of the brittle behaviour of the Ti5Si3 fibers. 

The volumetric behaviour is given by equation 19. But in order to reproduce brittle behaviour at low confinement and ductility at high confinement, the dilatancy angle depends on the stress state according to equation 20. 

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