Stress-cracking

The cause of stress cracking is not yet fully understood. It is established that there is still amorphous material in the cracks. This material can be deformed by cold flow. Of course, the extent of the cold flow is also determined by the diffusion and degree of swelling of the surrounding medium into the material. Wetting substances can build up a swelling pressure at the weak points. 

The stress here can be either static or oscillating. In static tests (creep strength tests) the sample is, for example, subjected to a specific force, and 

Tobolsky, Properties and Structures oj Polymers, Wiley, New York, 1960. 

Oberst, Elastische und viskose Eigenschaften von Werkstoffen, Beuth-Vertrieb, Berlin, 1963. L. E. Nielsen, Crosslinking—effect on physical properties of polymers. Rev. MacromoL 

Mechanical Properties oj Solid Polymers, Wiley-Interscience, London, 1971. A. Peterlin, Mechanical properties of polymeric solids, Ann. Rev. Mater. Sci. 2, 349 (1972). 

The extent of the stress corrosion cracking varies according to the 

Figue 11-23. Tensile stress at break (tensile strength) ob of at-poly(styrenes) of narrow molar mass distribution and various number-average molar masses M . Measurements were made at 23°C and 50% relative humidity. Processing was by compression molding ( ) or injection molding (O ) Injection-molded samples are oriented. (After H. W. McCormick, F. M. Brower, and L. Kin.) 

Stresses induced by the threads in addition to stress from uncured adhesive outside the joint produce very high stress levels in the part. Surface preparation methods to alleviate stress cracking include abrading the surface with sandpaper, cleaning with isopropyl alcohol, and assembhng the parts immediately after application of alcohol.The use of an incompatible adhesive for the substrates can produce stress cracks. The part surface softens and weakens, creating a crack. Joining materials with different 

A good source for solubility parameters of a wide range of materials has been published by CRC Press. 

---

In the sheeting market, the low density polyethylenes are less important than the high density resins. The high density resins have excellent chemical resistance, stress-crack resistance, durabiUty, and low temperature properties which make them ideal for pond liners, waste treatment faciUties, and landfills. In thicker section, HMW-HDPE sheet makes good containers, trays, tmck-bed liners, disposable items, and concrete molds. The good durabiUty, abrasion resistance, and light weight are critical elements for its selection. 

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

The high MV resia is used as liners for process equipmeat. Its melt viscosity is significantly higher than that of other resias and therefore it is unsuitable for conventional iajection molding. Stress-crack resistance and mechanical properties are superior to those of the other three products (52)... 

A combination of excellent chemical and mechanical properties at elevated temperatures results in rehable, high performance service to the chemical processing and related industries. Chemical inertness, heat resistance, toughness and flexibiUty, stress-crack resistance, excellent flex life, antistick characteristics, Htfle moisture absorption, nonflammability, and exceptional dielectric properties are among the characteristics of these resins. 

Modifiers. Latices are added to bitumens, mortars, and concrete to improve impact resistance and reduce stress cracking. Key to the use of latices in these technologies is compatibiHty between the latex and the constmction materials. 

Fast reactor fuel assembhes are shrouded with a relatively heavy metal envelope. This envelope is removed before shearing by either laser cutting (14) or stress cracking (15). 

Chemical Properties. LLDPE is chemically stable. Very few analyses and tests related to its chemical properties are carried out routinely. Resistance to thermal stress-cracking is determined by exposing film wrapped on a metal mandrel to hot (100°C) air for 48, 96, and 168 hours (ASTM D2951-71). 

The principal advantage of plastic dmms and liners is their resistance to corrosion. This aspect of their performance requires the lading to be investigated in terms of capacity for chemical attack on the dmm. Stress-cracking tests should be performed in all instances where the compatibiUty of jading and dmm material has not been estabUshed (6). 

MSTM D1693, ed. Test Methodfor Environmental Stress-Cracking of Ethylene Plastics, Vol. 8.01, ASTM, Philadelphia, Pa., 1988. 

ISO 4599, Plastics Determination of Resistance to Environmental Stress-Cracking, Pent Strip Method, ISO, Geneva, Swit2edand, 1986. 

In addition to the semicrystalline nylons, which comprise the vast majority of commercial resins, nylon is also available in an amorphous form that gives rise to transparency and improved toughness at the expense of high temperature properties and chemical stress crack resistance. Table 2 shows the properties of some different polyamide types. 

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

Stress cracking resistance is substantially enhanced in the presence of reinforcement such as glass fibers. 

For reasons that are not fiiUy understood, PPSF exhibits generally improved compatibiUty characteristics over either PSF or PES in a number of systems. An example of this is blends of PPSF with polyaryletherketones (39,40). These blends form extremely finely dispersed systems with synergistic strength, impact, and environmental stress cracking resistance properties. Blends of PPSF with either PSF or PES are synergistic in the sense that they exhibit the super-toughness characteristic of PPSF at PSF or PES contents of up to 35 wt % (33,34). The miscibility of PPSF with a special class of polyimides has been discovered and documented (41). The miscibility profile of PPSF with high temperature (T > 230° C) polysulfones has been reported (42). 

Pacemaker Interfaces and Leads. Problems of existing pacemaker interfaces and pacemaker lead materials made from siUcones and standard polyurethanes are environmental stress cracking, rigidity, insulation properties, and size. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

---