Change In Failure Mechanism Research Articles

Stacking sequence effect of aramid–UHMPE hybrid composites by flexural test method: MATERIAL PROPERTIES

Aramid fibre–ultra-high modulus polyethylene (UHMPE) fibre interply hybrid composites were fabricated with changes in the stacking sequence. The flexural strength and modulus of hybrid composites were measured in order to investigate the effect of stacking sequence. Scanning electron microscopy (SEM) was used to examine the fracture surface of interply hybrid composites. When aramid fibre was at the compressive side and dispersion extent of fibers was small, the higher flexural strength and positive hybrid effect were observed. In addition, the different stacking sequence resulted in a change in flexural failure mechanism which had an effect on the flexural strength. As the dispersion extent of fibers decreased, the introduction of cohesive failure in aramid–aramid interface and PE–PE interface improved the flexural strength of hybrid composites.

Maximum depth of seismicity and thermal parameter of the subducting slab: general empirical relation and its application

The maximum depth of seismicity ( D m) in subduction zones is limited by some physical mechanisms related to the rheological strength of slab material, metastability conditions, failure strength etc. The main factors controlling these properties are temperature and pressure within the lithosphere sinking into the mantle. These P-T conditions may be coarsely assessed from thermal models of slab subduction. The observed relationship between D m, the age of the subducted lithosphere ( A) and the convergence rate ( V) can be used to test the theoretical models and reveal inconsistencies which may indicate changes in failure mechanisms with depth. We analyze the relation between D m and the thermal parameter of the descending slab (ϕ), which is a product of the age of the subducted lithosphere and the vertical component of convergence rate ( V ⊥). Seismicity profiles across the subduction zones of Mexico, Chile, Kamchatka, Kuriles, Japan, Sumatra, New Hebrides, Aleutians, Tonga and Marianas are used to estimate D m. The quasi-linear part of the dependence ( D m ≤ 240 km, and ϕ ≤ 2000 km) corresponds to relatively young and slowly descending slab and is in general agreement with the ‘critical temperature’ models of deep earthquakes. For ϕ > 2000 km, which corresponds to the relatively older lithosphere subducting at a higher rate, the D m = f( ϕ) dependence is nonlinear. The flattening of the empirical D m = f( ϕ) curve in the range of 2000 km < ϕ < 3500 km is found to be a direct indication of the influence of the equilibrium Ol-Sp phase transition on D m. Models invoking the metastable Ol-Sp phase transition as a mechanism which controls the deepest seismicity cannot be definitely constrained by the results of this study. The general dependence D m = f( ϕ) may be applied as a standard relation between D m, A, and V for the analysis of deep earthquakes and seismotectonic studies of the subduction zones.

Probability of corrosion induced cracking in reinforced concrete

A probabilistic approach to corrosion induced cracking in reinforced concrete was investigated. Hydraulic pressurization at the reinforcement/concrete interface was used to simulate corrosion of reinforcement and cause fracture. The failure pressure data was analysed statistically and found to have right hand skew. Correlation between failure pressure and visual observations of fracture surfaces suggested a possible change in failure mechanism depending on the presence of voids near the reinforcement. The failure pressure distributions were analysed to determine whether Weibull statistics could provide an adequate description. The effects of variables such as toughness, flaw size, corrosion products and corrosion rate on the shape of failure probability-time relationships were predicted.

Dynamic behaviour of concretes: from the material to the structure

The dynamic behaviour of concrete involves both material and structural aspects. Concerning the material level, the Laboratoire Central des Ponts et Chaussees has cooperated with Delft University of Technology within the framework of a European project. Two problems were studied: the influence of free water in the concrete pores on the rate effects (dynamic strength and static strength) observed in the mechanical behaviour of the material, and the influence of the water/cement ratio on these rate effects. It was found that the rate effects are due to the influence of free water in the hydrate pores. Also, the rate effects depend on the water/cement ratio. They decrease with increasing ratio, but are independent of the absolute value (dynamic strength—static strength). Concerning the structural level, the LCPC has cooperated with LEA (Shelter Testing Laboratory) to carry out a study on the shock behaviour of concrete slabs using a shock tube. At a structural level, the rate effects on the mechanical behaviour of the slabs seem to be due to the presence of water in the material. It was further found that there is a change of failure mechanism in the slabs, i.e., a transition from a bending failure mechanism to a shear failure mechanism, with increasing loading rate and with decreasing water/cement ratio.

Detailed Study of Electromigration Induced Damage in Al and AlCuSi Interconnects

ABSTRACTBecause of the continuing miniaturization, electromigration (EM) phenomena are still a key issue in reliability of VLSI metallizations. The present study of EM induced voiding and hillocking was performed on unpassivated conductor lines with various widths and current densities. Stressed and unstressed interconnects were carefully examined with SEM and TEM techniques, especially with regard to void densities, void sizes and characteristic lengths between void and hillock. The fatal void shape was related to current density and line width indicating that the failure mechanism changes with decreasing line width and decreasing current density.

Degradation of TAB outer lead contacts due to the Au-concentration in eutectic tin/lead solder

The influence of the Au-concentration in OLB solder fillets on the contact reliability is shown. Quantitative analysis of the Au content shows a good accordance with the estimated concentrations using the geometrical data. The Au-concentration is the major influence factor on the thermal aging behavior of OLB contacts. A change in the failure mechanism due to Kirkendall porosity is observed if a Au-concentration of 9 wt% in the solder fillet is reached. The pores formed during diffusion at the interface between copper and the ternary intermetallic compound Cu/sub 3/Au/sub 3/Sn/sub 5/ cause a strong degradation of pull forces. For low Au-concentrations, the degradation of pull-forces and the growth of the ternary compounds CuAu/sub 5/Sn/sub 5/ and CuAu/sub 4/Sn/sub 5/ have comparable activation energies in the range of 0.3 eV. For higher Au-concentrations the two effects show different values. The activation energy of pull-force degradation increases significantly up to 0.67 eV, whereas the growth constant of the ternary compound shows a small increase to 0.4 eV. The investigations show evidence of a critical Au-concentration in OLB contacts. Kirkendall pore formation causes higher activation energies for the pull-test degradation compared to the activation energy for the growth rate of the ternary compound. >

Mechanical Properties of Continuous‐Fiber‐Reinforced Carbon Matrix Composites and Relationships to Constituent Properties

The tensile properties of three carbon matrix composites reinforced with SiC (Nicalon) fibers (materials A, B, C) have been measured with and without notches. One of the three materials (material B) had a relatively low strength and exhibited notch brittleness. This material had both a high interface sliding stress and a low fiber bundle strength, caused by particulates in the matrix. These characteristics have been shown to result in a change in failure mechanism that leads to the inferior properties exhibited by material B. The notch properties of the higher‐toughness materials were shown to involve splitting, which alleviates the notch stress concentration and diminishes the notch sensitivity.

An extended mathematical model for failure kinetics

In practice we often observe some kind of ‘self-acceleration’ when degradation causes the local temperature to increase, and in turn the temperature accelerates degradation. In this paper an extended model for failure kinetics is proposed to describe the phenomenon mentioned above. It is a common belief that a non-linear relation between logarithmic lifetime and inverse temperature reveals a change of failure mechanisms. The results of the paper tell us that non-linearity may be produced by a single failure process with more complex failure kinetics than the simple reaction of the first order.

BEHAVIOUR AND PUNCHING STRENGTH OF RC SLABS DAMAGED BY CORROSION OF REINFORCEMENT

The failure mechanism and the punching strength of RC slabs damaged by corrosion of reinforcement are not only experimentally but also theoretically studied in order to obtain useful information for repair or maintenance. In the experiments, loading tests are conducted for RC slabs damaged with an accelerated galvanostatic corrosion method. It is found that the reduction in punching strength and the change of failure mechanism occurred in the damaged slabs. The modeling of failure mechanism of the damaged slabs is investigated by experimental and theoretical consideration using elastoplastic finite element analysis. From these consideration, it could be indicated that the punching strength of the damaged slabs is composed of the ultimate shear strength in compression zone under the loaded area and the bending strength of the main reinforcement with the bottom covers.

Novel polyfunctional silanes for improved hydrolytic stability at the polymer–silica interface

Polyfunctional silanes were designed to promote the formation of a stable multisilane layer on the silica surface. The hydrolytic stability of polyfunctional silanes at the interface between silica and polymeric resin was investigated using conventional tensile testing. The specimens were immersed in boiling water and a decrease was observed in the tensile bonding strength of polymeric resin against the silane-treated silica surface as the immersion time increased. The failure mechanism changes from cohesive failure of the matrix resin to failure of the interface. The time necessary to start the interfacial failure is dependent on the silane concentration in the treating solution and the number of silicone functional groups within the silane molecules. The silane with two silicone functional groups is more stable at the interface between silica and polymeric resin than those which have mono- or trisilicone functional groups.

Minimum Reinforcement in High‐Strength Concrete

A dimensional analysis criterion is proposed to compute the minimum amount of reinforcement for high‐strength concrete members in flexure. Such an amount is assumed to be provided by the condition of simultaneous first cracking aand steel yielding. The fracture mechanics model utilized defines a brittleness number NP, which is revealed to be a measure of the experimental brittleness or ductility of the test. NP is a function of steel‐yield strength fy, concrete fracture toughness KIC, steel percentage AS/A, and beam depth. The brittleness of the structural member increases by increasing the size and/or decreasing the steel content. On the other hand, a physically similar behavior is revealed in the cases where the brittleness number NP is the same. The experimental investigation carried out by the writers shows that the failure mechanism changes completely when the beam depth is varied, the steel percentage being the same. Only when the steel percentage is inversely proportional to the square root of the ...

Features of failure for ferritic cast iron with spheroidal graphite

1. The effect of spheroidal graphite inclusions on the failure of ferritic cast iron depends on the condition of the matrix: in a ductile matrix spheroidal graphite particles are centers of microcavity generation and they lead as a result of this to a reduction in the level of cast iron properties; in a brittle matrix graphite particles have a favorable tendency of retarding brittle crack development and under certain conditions they lead to a change in failure mechanism. 2. The effect of crack retardation at spheroidal graphite particles develops particularly clearly at low loading rates thus providing properties in cast iron comparable with steel.

Fatigue crack propagation in glass-fibre and glass-sphere filled PBT composites

Fatigue crack propagation has been studied in injection-moulded plates of unfilled PBT and composites filled with short glass fibres of varying length and in different volume fractions. Materials filled with glass spheres and short glass fibres have also been investigated. The influence of crack propagation direction and of the microstructural composition on crack speed is discussed. It is found that a model which accounts for the fibre orientation distribution, the skin — core morphology and the change in failure mechanisms due to variations in the fibre length and aspect ratio can explain the differences observed in crack propagation behaviour.

A proposed model for accelerated‐stress life‐test data exhibiting two failure modes

AbstractAccelerated‐stress life‐testing involves testing of a material or system at a series of stresses higher than that which is normally seen in service. An estimate of the service life is then obtained by extrapolating the failure time vs stress relationship to use stress. A common problem with this methodology is that a change in failure mechanism may occur if stresses become too high. When this occurs, the data above and below the failure mode transition point are generally treated separately. This paper discusses a model that allows for a change in failure mechanism and characterizes the transition by a probability density function over stress. Advantages of the model are: it allows representation of all test data by a single equation; it allows probability statements to be made about the probability of failure of either failure mode; it allows optimal experiments to be designed; and it allows data in the area of the failure mode transition to contribute to the precision of the estimated service life.

A 3-D fracture mechanics approach to the strength of composite materials

The first ply failure in composite laminates are studied by 3-D fracture mechanic approach. The fracture model is based on a crack embedded in one of the inner layers. Numerical results for the case of “angle ply” laminates are based on a general 3-D finite difference program. Stress intensity factors and fracture energy are evaluated by the J- integral 2-D and 3-D methods as well as by the stresses and energy balance. It was found that the J(2- D) method can be applied to 3-D problems while the J(3- D) method is not suitable for the numerical computation. The influence of geometrical properties of the crack and the layers on the various stress intensity factors is presented and discussed. Change in failure mechanism of the weakest ply was proved by experiments to fit with numerical results. It was found that the weakest layer is strengthened by the others, a fact which has been verified experimentally and hasn't been explained by other approaches.

Soil failure with narrow tines

A force prediction model has been developed for tines of a wide range of working depth/width ratios. The soil worked by the tines is assumed to obey the Mohr-Coulomb failure criterion. The predictions show useful agreement with experimental data for the horizontal force components in two soil conditions with high angles of shearing resistance. A method is proposed for estimating the critical depth of a tine, the depth below which the soil failure mechanism changes.

Effect of morphology on biaxial stress rupture of polyethylene

AbstractThe relationship between biaxial stress‐rupture behavior and polymer morphology has been investigated for a series of compression‐molded disks of linear polyethylene. Rupture data were obtained over a range of temperatures on polymers of several melt indices that had been solidified at two rates. Two failure mechanisms were observed: one at high stress levels, which was ductile; the other at low stress levels, which was brittle. The stress level at which the mechanism changed from ductile to brittle decreased as the measurement temperature increased, as the melt index of the polymer increased, and as the rate or solidification decreased. It was shown that initial pressurization of the disks causes the formation of microscopic surface fissures along spherulite boundaries as well as within individual spherulites. The extensive growth of these fissures at high stress levels sufficiently raises the stress on the sound portions of the polymer to cause large‐scale macroscopic drawing of the spherulites. At low stress levels the initially formed fissures grow by a localized drawing process at their ends. A macroscopic crack forms by a chance coalescence of a number of individually growing fissures. This process is more rapid in disks of polymer that form large spherulites when cooled slowly from the melt. The spherulite size in disks of polymer of low melt index is much smaller under the same conditions of cooling and is less sensitive to the cooling rate than in disks of polymer of high melt index. Annealing of rapidly cooled disks whose microstructure contains relatively small spherulites reduces the stress at which the failure mechanism changes without appreciably altering the spherulite size. Oxidation of the polymer also reduces the transition stress.