J.T. Evans

Laminate Theory Analysis of Composites under Load in Fire

Laminate analysis is used to model a loaded composite plate under one-sided heat flux. The input to the laminate analysis comes from a thermal/ablative model, which predicts the temperature evolution through the thickness. It also gives the profile of residual resin content, which reflects the extent of thermal damage. Relationships are proposed to enable the computation of the elastic constants and other mechanical properties as functions of temperature and resin content. The model was applied to a 12 mm thick woven glass/polyester laminate exposed to a heat flux of 75 kW m 2. The laminate A, B, and D matrices were modeled, along with the variation of failure loads in compression and tension. The predictions agreed well with experimental values for compression of a constrained plate. Both the local buckling load, which is proportional to √D1 D2, and the compressive failure load fall rapidly on exposure to heat flux. The bending/tensile coupling matrix, B, which is zero initially, becomes finite due to the asymmetric thermal profile, then declines as the thermal front progresses. For tensile loading, the residual properties after fire were accurately modeled, but the fall in tensile failure load was somewhat over-predicted.

The effect of surface roughness, oxide film thickness and interfacial sliding on the electrical contact resistance of aluminium.

Contact resistance measurements were made on aluminium alloy sheet with a configuration relevant to electrical resistance spot welding. Previous work has shown that a small amount of sliding is required at the interface to break down contact resistance when the material has a thin, insulating coating. Sliding on a macroscopic scale occurs at the electrode-sheet interface but not at the faying surface. For this reason, the contact resistance at the faying surface is high and its behaviour with applied force is observed to be anomalous. However, local sliding on a microscopic scale can occur at the faying surface, depending on the surface roughness. The effect of surface roughness and oxide film thickness was investigated in the present work. In addition, the degree of sliding required to break down contact resistance was quantified in experiments in which relative rotation was induced at the faying surface. It is estimated that a sliding displacement of only about 10 μm is required to produce a dramatic reduction in contact resistance. The results are interpreted in terms of Holms constriction resistance theory of microscopic spots of metal-to-metal contact within a mechanical contact area largely insulated by the presence of the surface oxide films.

Environmentally enhanced fatigue damage in glass fibre reinforced composites characterised by acoustic emission

A series of tests has been conducted to investigate the effect of sea water absorption on fatigue damage accumulation in a glass fibre reinforced polyester laminate of the type widely used in the marine and offshore industries, using four-point bend flexural loading to ensure peak strain in the outer layers of the material most subject to seawater absorption. Pre-exposure was found to reduce the flexural strength and enhance damage accumulation in fatigue by stimulating matrix cracking, fibre debonding and delamination. Acoustic emission (AE) was used to characterise damage accumulation. These results were found to correlate well with independent measurements of changing bending stiffness and with microstructural observations of the damaged sections.

The effect of mechanical loading on the contact resistance of coated aluminium

Abstract Measurements of contact resistance, related to resistance spot welding, were made using pre-treated (coated) and abraded aluminium alloy strip. With conventional domed electrodes, the contact resistance was much larger at the sheet-to-sheet (faying) contact than at the electrode-sheet contact. The effect is believed to be associated with the role of sliding in breaking down contact resistance of sheet with an insulating surface film. When the coating was removed the difference between faying surface and electrode-sheet contact was much smaller. Macroscopic shear stresses are developed by electrode-sheet contact whereas no shear stresses are present at the faying surface. The hypothesis is supported by experiments made with asymmetrical electrode pairs which give rise to varying shear stresses in faying surface contact. Some implications for the control of spot welding of different aluminium surfaces are discussed.

The Effect of Matrix Microcracks on the Stress-Strain Relationship in Fiber Composite Tubes

A model for the stress-strain behavior of fiber composites with matrix cracks is presented and applied to the deformation of fiber reinforced composite pipes under different combinations of hoop and axial stress. The model provides a good description of the nonlinear stress-strain relationship that develops in composites when the matrix is damaged by the progressive nucleation of microcracks during loading. Ply properties are expressed as a function of crack density, calculated as a function of increasing stress using the shear-lag approximation. The predictions are in very good agreement with data from multiaxial tests on ±55° filament wound glass-reinforced epoxy pipes.

The effect of the white layer on micro-pitting and surface contact fatigue failure of nitrided gears:

Abstract The effect of the ‘white layer’ in nitrided steel on micro-pitting of gears has been investigated by comparing the performance of case carburized and nitrided gears fabricated from modern, low-oxygen steels. It is shown that the white layer, produced during nitriding, imparts a high resistance to micro-pitting. Because surface contact fatigue is nucleated at micro-pits in low-oxygen steels, the resistance to micro-pitting also manifests itself in a higher fatigue resistance for the nitrided material.

Microcrack nucleation and sustained-loading crack growth in AlLi alloys

Abstract Sustained-loading crack growth has been observed in two experimental AlLi alloys at 20 and 80 °C in circumstances where there is no strong connection between fracture and conventional stress corrosion cracking or hydrogen embrittlement. The simple ternary alloy examined (AlLiZr) was found to be much more susceptible to slow crack growth than the more complex alloy (AlLiCuMgZr) which contains additional hardening precipitates. This difference is associated with the different creep response of the alloys. Water vapour in the atmosphere was found to assist crack growth in the simple ternary alloys, but a consideration of equilibrium conditions at the crack tip shows that the crack tips are “dry” in all atmospheres apart from water-saturated air. Assistance of slow crack growth by the adsorption of water molecules is considered in terms of the formation of a Thomson-type molecular wedge.

Strain concentration at grain boundaries in AlMgSi alloys

Abstract Grain boundary sliding has been observed in an AlMgSi alloy deformed at room temperature. The extent of sliding was primarily a function of the macroscopic strain whether this was produced by creep or in slow-strain-rate tensile tests but was more intense at low strain rates that at high strain rates. Grain boundary microcracks were observed to form after a critical amount of sliding at low strain rates. At high strain rates, microvoids nucleated at larger second-phase particles rather than in the grain boundaries. A consideration of the mechanisms which could operate to produce localized deformation indicates that the phenomenon is due initially to slip on the grain boundary plane itself but is quickly intensified by plastic flow within the precipitate-free zone after microvoids have nucleated. Plastic flow without the prior nucleation of holes cannot be sufficiently concentrated to account for the observed strain localization.

Modal approach to assurance of structural integrity

A smart system for monitoring the structural integrity of pressure vessels throughout service life might consist of a limited number of piezoelectric transducers permanently bonded to the vessel, together with a system for intelligent real-time data processing. In the present work it is shown that the growth of subcritical cracks can be detected using the modal properties of a vibrating cylindrical tube. Finite element analysis and experimental measurements are in agreement in showing that the growth of cracks in a variety of locations can be detected using various strategies based on modal analysis. One strategy corresponds closely with a whole- field method of detection in that measurements made at a single point are capable of revealing crack growth in a number of different locations in the cylinder.

Pre-exposure embrittlement of an Al-Cu-Mg alloy, AA2024-T351

Abstract When Al-Cu-Mg (AA2024-T351) alloy plate is immersed in 0.53 m NaCl solution, intergranular corrosion occurs, which initially takes the form of narrow fissures at the surface. Although growth of these fissures is initially rapid, it soon slows and the penetration depth is limited. However, these intergranular fissures can give rise to intergranular subcritical crack growth and eventual catastrophic fracture if specimens are strained slowly in laboratory air after pre-exposure to the NaCl solution. In our experiments, tensile specimens of the alloy were strained at strain rates in the range 10-7–10-4/s, and the cracking propensity was found to increase with pre-exposure times up to around 300 h. However, cracking was much reduced with longer pre-exposure times and became negligible after 500 h. This rather unexpected result can be explained by the observation that the intergranular corrosion fissure becomes increasingly broad and un-crack-like with extended pre-exposure times, failing to meet the prerequisite initiation conditions required for intergranular stress corrosion cracking. A further unexpected result was that straining specimens in the same 0.53 m NaCl solution after pre-exposure produced no intergranular crack growth. To investigate this further, tensile tests were conducted under electrochemical control, and this showed that the free corrosion potential in the test solution was too low in 0.53 m NaCl to allow cracking to proceed.