Discrete element modelling of unidirectional fibre-reinforced polymers under transverse tension

The mechanical behaviour of unidirectional fibre-reinforced polymer composites subjected to transverse tension was studied using a two dimensional discrete element method. The Representative Volume Element (RVE) of the composite was idealised as a polymer matrix reinforced with randomly distributed parallel fibres. The matrix and fibres were constructed using disc particles bonded together using parallel bonds, while the fibre/matrix interfaces were represented by a displacement-softening model. The prevailing damage mechanisms observed from the model were interfacial debonding and matrix plastic deformation. Numerical simulations have shown that the magnitude of stress is significantly higher at the interfaces, especially in the areas with high fibre densities. Interface fracture energy, stiffness and strength all played important roles in the overall mechanical performance of the composite. It was also observed that tension cracks normally began with interfacial debonding. The merge of the interfacial and matrix micro-cracks resulted in the final catastrophic fracture.     

Failure of spheroplastics under axial tension and hydrostatic compression

The problem of determination of the external pressure and axial tensile force, which fail spheroplastics — a structurally inhomogeneous material consisting of hollow glass microspheres of different diameters, the latter being distributed in a random manner in a continuous epoxy matrix — is examined.Translated from Problemy Prochnosti, No. 9, pp. 59–61, September, 1990.     

Failure behavior of quasi-isotropic carbon fiber-reinforced polyamide composites under tension

In this paper, the microscopic failure behavior of quasi-isotropic carbon fiber-reinforced polyamide-6 (CF/PA6) laminates under tension was investigated experimentally. Laminates of two layups, namely [45°/0°/?45°/ 90°]_s and [45°/0°/?45°/ 90°]_2s, were made from CF/PA6 tapes of two different manufacturers and then subjected to tensile testing. Crack initiation and progression on the polished free edge of specimens were examined using optical microscopy, under several load levels. Crack growth behavior through the specimen width was also traced by observing the crack configurations in different sections in the specimen width direction. The effects of the spatial distribution of fiber on the microscopic damage events were elucidated. The difference in failure behavior between the present CF/PA6 laminates and conventional thermosetting CF/Epoxy laminate is discussed.     

Failure behaviour of particulate-reinforced aluminium alloy composites under uniaxial tension

Tensile tests were carried out at room temperature on 6061-aluminium alloy reinforced with SiC and Al₂O₃ particulates. Although a significant increase in strength could be achieved by introducing ceramic reinforcements into the aluminium alloy matrix, it is associated with a substantial decrease in fracture strain. In order to understand the reason for the inferior ductility of such composites, analytical solutions were obtained using a simple composite model. SEM studies were carried out on the side surfaces of the fractured specimens to verify the proposed failure behaviour. Failure modes observed to operate in such composites under uniaxial tension are described.     

Stability of void growth in an incompressible solid under uniaxial tension and shear

Stability of a growing single prolate spheroidal cavity in linear elastic incompressible solid is considered. It is shown from the classical theory of elasticity that a single void becomes unstable in pure shear loading. Considering finite incremental deformation of the matrix in the presence of a void, it is proved that a prolate spheroidal cavity can become unstable under the application of an uniaxial initial tensile stress. The applied tensile stress produces only a pure torque on the void for such unstable (internal shear buckling) mode of deformation. For an incompressible solid, the condition of instability in shear loading depends entirely on the shape of the spheroid (i.e. the ratio of major axis to minor axis). On the contrary, in uniaxial tension, there exists a single shear property of the material which decides the instability for a given void size. Plane strain longitudinal (and radial growths of the void are also studied considering finite simple shear and finite pure shear deformations. These theoretical analyses are in excellent agreement with the experimental observations of other people.     

Failure of pultruded GRP single-bolt tension joints under hot–wet conditions

An experimental study of the effects of hot–wet conditions on the load carrying capacity of pultruded GRP (glass reinforced plastic) single-bolt tension joints has been carried out. The bolted joints were failed in tension after being immersed in water at three temperatures for two periods of time. Two joint geometries, defined in terms of end distance and width to bolt diameter, were tested with the pultrusion and tension axes coincident. The reductions in the load capacities of the joints due to the hot–wet conditions were quantified and shown to be very large. For example, it was found that more than 60% of the load carrying capacity of a single-bolt tension joint was lost after being immersed in water for 6.5 weeks at 60 °C. This temperature is lower than the manufacturer’s recommended maximum service temperature for this type of pultruded GRP material.     

Hydrostatic extrusion of solid polymers

The hydrostatic extrusion of high density polyethylene rod was studied over a wide range of temperatures. The extrudates had smooth surfaces. Those with high extrusion ratios possessed unusual properties being highly transparent and fibrous. The molecular orientation and structural changes due to the hydrostatic extrusion were investigated using wide- and small-angle X-ray diffraction techniques and birefringence. The partial transformation from the orthorhombic to the monoclinic structure occurred in the extrudate obtained at low temperatures (especially at room temperatures). The small-angle X-ray scattering pattern of extrudate for low extrusion ratio showed a diagram with six intensity maxima. On the other hand, for the high extrusion ratio, a diagram with two meridional intensity maxima elongated perpendicular to meridian was observed. At the extrusion ratio R _E?6, the chain axes of polyethylene became aligned to the extrusion direction. Small-angle X-ray scattering patterns showed that the new fibre structure began to appear at an extrusion ratio as low as R′_E=3.0. The structure of the extrudate at a high extrusion ratio was accounted for by the highly-oriented fibre structure consisting of stacks of folded chain crystallites and the interfibrillar amorphous chain bundles.     

Hydrostatic extrusion of solid polymers

Hydrostatic extrusion of a high density polyethylene rod was carried out. Both the process of plastic deformation during hydrostatic extrusion and the changes of properties of the polyethylene with extrusion were studied. Particular attention was paid to the process of molecular orientation and the destruction of the original lamellae. To obtain a better understanding of the deformation process, the lateral dimension of crystallite and the long period were obtained from the X-ray diffraction data. Furthermore, the influence of extrusion conditions on the structure and properties of the extrudate were investigated. The Vickers hardness number and thermal shrinkage of the extrudates were affected considerably by the extrusion conditions, i.e. extrusion temperature and extrusion ratio. In order to determine the relationship between the properties and extrusion conditions, it is necessary to consider not only the molecular orientation but also the crystallinity, fibrosity and the fine-structure of the extrudates.     

Failure behavior of E-glass fiber- and fabric-reinforced IPC composites under tension and compression loading

This paper analyzes the failure behavior of E-glass reinforced inorganic phosphate cement composites in tension and compression. The first part of the study experimentally evaluates the behavior of different composite systems resulting from three different manufacturing processes (hand lay-up, pultrusion and pultrusion/compression). Tensile and compression tests are performed on fiber orientations ranging from 0° to 90°, and the mechanical tests are analyzed in terms of failure strength and failure modes. The prediction of failure strength is approached using mechanical models found in the literature. The comparison of experimental and modeling results is unsatisfactory, confirming the necessity of establishing adapted fracture criteria. The second part of this paper concerns the establishment of a failure envelope based on the Tsai–Wu and Mohr–Coulomb criteria. The anisotropic nature of the composite is accounted for by the Tsai–Wu criterion, while the Mohr–Coulomb criterion allows for the modeling of the mineral matrix’s brittle behavior. The maximum stress criterion is also compared to the obtained envelope curves.     

Failure in laminates in tension under increasing stress,constant stress,and cyclic stress

Both monotonic and cyclic tension experiments have been carried out to fracture on transparent laminates made from flat ribbon glass and polyethylene sheets by heat bonding in a vacuum. The distribution of the measured tensile fracture stresses in monotonic loading correlates very well with the distribution of fracture stresses calculated from the measured distribution of element fracture stresses and the yield strength in shear of the polyethylene, according to a detailed statistical theory worked out earlier by Scop and Argon. Although the expected mode of fatigue damage by propagation of delamination cracks emanating from isolated stable fractures in reinforcing elements was observed, actual fatigue failure was a result of a more rapid mechanism of continued fracturing in reinforcing elements by a humidity-induced time-integrated static fatigue process. While laminates subjected to static stresses of the same magnitude as those in the dynamic experiments failed by the development of an identical form of damage during the same length of time under stress in laboratory air, other laminates tested in the same manner in dry air had 5 to 10 fold increased lives. In addition re-testing of individual elements of delaminated composites showed that elements can often be damaged during lamination, which must be taken into account in any quantitative study.     

Failure of cadmium plated maraging steel tension bolt

Cadmium electroplated (EP) high strength steel fasteners are widely used for assembling threaded joints as cadmium gives lubrication and provides excellent corrosion resistance. However, in-house-failure case histories revealed that in some cases delayed failure of cadmium EP high strength steel fasteners occurred. Recently, one such premature failure of a cadmium EP 18 Ni 1800 MPa maraging steel tension bolt occurred at a sustained stress of about 40% of the ultimate tensile stress (UTS), after 11,500 h. For their successful functional application, cadmium EP fasteners require proper and adequate baking treatment after electroplating. This paper highlights details of analysis carried out on the failed bolt to find out the reason for the failure, using standard metallographic and NDT techniques. Remedial measures to minimise, if not totally prevent, such failure are also discussed.     

The stress intensity factor for a flat elliptical crack in an elastic solid under uniform tension

Making use of simple results concerning 2-dimensional Fourier transforms an expression is derived for the stress-intensity factor at the rim of a flat elliptical crack in an infinite elastic solid under uniform tension.     

Failure of brittle polymers by slow crack growth

The variation of crack velocity (V) with stress intensity factor (K _I) at the tip of a crack has been measured for an epoxy resin containing 42% by volume of irregularly-shaped silica particles. It has been found that at crack velocities above 10^–5 m sec^–1 the crack propagates primarily through the silica particles, whereas at velocities below this value, failure occurs primarily by particle pull-out. This variation in fracture mode is accompanied by a corresponding change in slope of the V(K) curve. Using data obtained from creep rupture experiments and the derived V(K) relationship, it has been possible to estimate the size of the inherent flaw in the composite. This was found to be approximately twice the average particle diameter which is also equal to the size of the largest particles (140 m). Fracture of the unfilled epoxy resin and the effect of environment upon slow crack growth in the composite have also been investigated.     

Failure of brittle polymers by slow crack growth

The mechanical properties of a silica particle-filled epoxy resin composite system have been investigated in air as a function of volume fraction of particles for volume fractions ranging from 0 to 0.52. The Young's modulus and the compressive yield stress both increase as the volume fraction of silica particles is increased and various models of particle strengthening have been used to explain this behaviour. Slow crack growth in the various particulate composites has been studied using a fracture mechanics approach. The variation of crack velocity (V) with stress intensity factor (K _I) has been measured for each of the compositions investigated. In each case, a unique relationship between V and K _I has been found with K _I increasing with volume fraction of particles at a given value of V. The failure mechanisms and the variation of other fracture mechanics parameters, for example, crack opening displacement and plastic zone size with increasing particle volume fraction have been discussed.     

Failure analysis of naval vessel's mooring system and suggestion of reducing mooring line tension under ocean wave excitation

A moored ship can experience problems when subjected to heavy environmental loads. In most cases, wind and current loads are the only design considerations for the mooring situation in the harbor, and wave loads are not considered due to the low wave amplitudes in harbors. However, many new harbors are constructed near the open sea, and entrance channels are being built deeper and wider than the classical harbors. So, many cases of mooring failure due to heavy wave loads are recently being reported in harbors. In this paper, failure analysis has been performed on a ship in harbor with mooring failure. The failure analysis results demonstrated that the wave drift forces can cause failure of the mooring system. Therefore, consideration for new design of mooring system, which considers the wave loads, is suggested.     

Singular stresses in an infinite solid with a flat annular crack around a spherical cavity under tension

The problem of singular stresses in an infinite elastic solid containing a spherical cavity and a flat annular crack subjected to axial tension is considered. By application of an integral transform method and the theory of triple integral equations the problem is reduced to that of solving a singular integral equation of the first kind. The singular integral equation is solved numerically, and the influence of the spherical cavity upon the stress intensity factor and the influence of the annular crack upon the maximum stress at the surface of the spherical cavity are shown graphically in detail.