Reduction of residual stress in montmorillonite/epoxy compounds

An epoxy resin was cured while in intimate contact with small amounts of epoxyphilic montmorillonites. It was determined that cured epoxy exists within the montmorillonite interlayer by the observation of very high interlayer spacings, even greater than 8 nm, Generally, epoxy compounds containing montmorillonites that had been swollen in the curing agent prior to curing exhibited larger interlayer spacings, especially among the non-dispersed montmorillonite layers. The maximum observed residual stress was reduced by greater than 50% in the epoxyphilic montmorillonite/epoxy compounds over that of the pure epoxy. The epoxyphilic montmorillonite/epoxy compounds generally exhibited higher values of glass transition temperature, flexural modulus, and ultimate flexural strength than the pure epoxy. The tyramine-montmorillonite compounds typically had the highest values overall.     

Mechanistic analysis of fatigue crack initiation in medium-density polyethylene

Kinetics parameters of craze evolution preceding fatigue crack initiation (FCI) in mediumdensity polyethylene (MDPE) pipe materials were determined and analysed within fracture mechanics theory. A single craze initially preceded the notch tip, a root craze, which subsequently became accompanied by a few side crazes. Crack initiation transpired after the craze-zone growth had reached its fully developed configuration. The length of the root craze of the fully developed zone was found to be equal to the length of the first discontinuous crack band on the fracture surface. The growth of the root-craze length and the crack-tip opening displacement (CTOD) followed a power law over the major portion (94%) of the FCI time. Measurable rupture of the craze material was only noted within the final portion of the FCI time and was associated with exponential increase of the CTOD. The Dugdale/Barenblatt model overestimated the craze length by 30% and underestimated the CTOD by 50% which was hypothesized to be due to multiple crazing at the notch tip.     

The specific enthalpy of fracture for beta Ti-15-3 alloy

Fatigue crack propagation (FCP) experiments were conducted on beta Ti-15-3 alloy under various loading conditions to examine the constancy of the specific enthalpy for fracture, advanced by the Crack Layer (CL) theory as a material parameter characteristic of its intrinsic toughness. The energy release rate and the irreversible work were determined from load-displacement curves during crack propagation. Microscopic and diffraction analyses were conducted to identify the damage mechanisms ahead of the crack tip. A damage zone whose geometry exhibited plane strain character at the initial stage of crack propagation was observed optically. The damage zone transformed into plane stress configuration when the crack reached half its critical length. Damage mechanisms involved slip lines and microcracking which is believed to ensue from intense accumulation of slip processes. The magnitude of microcracking became more weighty as the crack moved deeper into plane stress dominance. The damage preceding crack advance was quantitatively assessed as the crack resistance moment which is the volume of transformed material per unit crack extension. Application of the CL theory to the data generated under a wide range of applied stress levels gave rise to a constant value of the specific enthalpy of fracture, 20 MJ/m³. This value is in close agreement with the specific energy of slip lines computed from microstructural considerations.     

Effect of residual stress on crack propagation in MDPE pipes

Crack propagation behaviour in single edge notched specimens prepared from medium-density polyethylene (MDPE) pipe is examined under creep condition. The crack grown from an exterior notch (inbound) initiated faster than that grown from an interior notch (outbound). Subsequently, the outbound crack propagated monotonically to ultimate failure. The inbound crack showed anomalous behaviour involving two arrest stages prior to ultimate failure. The pipe is found to possess substantial residual stresses. The energy release rate for each case was calculated taking into account the respective residual stream distribution. The fact that the rates of crack propagation are not a unique function of the energy release rate indicates that the fracture is also influenced by morphological gradients imposed by processing conditions.     

Crack tip damage analysis in fatigue fracture of epoxy resin

The application of the crack layer theory to fatigue crack propagation (FCP) in epoxy is discussed. A crack tip damage evolution coefficient μ is introduced to assess the extent of damage as a fraction of the damage associated with critical crack propagation. The results can be expressed in the form where dl/dN is the rate of FCP, G₁ is the energy release rate whose critical value is G_1c, and β is a phenomenological constant. Although no damage was detected from microscopic analyses, μ increases fivefold during stable crack propagation. Fractal analysis of fracture surface profiles provides a quantitative measure of the roughness associated with crack advance. The fractural measure d is found to evolve in a similar fashion as μ, suggesting the applicability of d to quantify crack tip damage evolution.     

Dissipative Processes in Interfacial Failure

An investigation of the failure behavior of pressure sensitive adhesive tape was performed utilizing the constrained blister test. The constrained blister test was designed to measure the energy of interfacial adhesion of thin polymeric coatings. A constant energy of interfacial adhesion of 1.8J/m² was determined for a rubber based pressure sensitive adhesive on a copper substrate. An active zone was visualized through the transparent backing. The deformation within the active zone was found to consist of cavitation and deformation of ligaments. Fracture of the ligaments causes the detachment front to advance. It was proposed that the rate of energy dissipation, D, reflects the resistance of the bond to time dependent deformation, and therefore dictates the lifetime for this particular specimen geometry. A direct relationship was established between lifetime and the inverse of the rate of energy dissipation in the active zone, D.     

An evaluation of the use of infrared heating for contouring 30% short carbon-fibre-reinforced PEEK

The ability to contour reinforced composites at specific points would allow these low modulus, high strength materials to be utilized in a wide variety of new applications. Contourable materials of this type would be especially suitable for orthopaedic applications such as internal fixation of fractures where plate removal due to stress shielding and bone resorption are major concerns. To this end, the high-temperature contourability of 30% short carbon-fibre-reinforced PEEK was investigated. The use of infrared radiation to heat this material to temperatures suitable for contouring was developed and refined into a protocol for heating and bending 12.4×6.4 mm injection-moulded flexural testing bars. To evaluate the heating, contouring and cooling effects of this processing on the material's mechanical and physical properties, five specimen groups were tested: control, extended heat only, cyclical heat only, repeated contouring, and repeated contouring-quenched. Each group's flexural properties, fracture toughness, post-contouring matrix crystallinity and extent of fibre-matrix wetout were monitored.The two heat only groups showed no significant changes in any properties compared to control. For the contoured groups, the quenched subgroup showed a minimal 0–9% decrease in all categories. The non-quenched group showed only a 0–6% change. The resilience of short carbon-fibre-reinforced, PEEK after severe thermo-mechanical cycling is convincing evidence of this material's suitability for structural applications, especially in which contourability and fast processing are required. This study demonstrates the thermoductility of CFR PEEK.