S. Mall

An evaluation of parameters for predicting fretting fatigue crack initiation

Abstract There are numerous fatigue parameters that can be used to determine the onset of crack initiation in a component subjected to constant amplitude plain fatigue. This study evaluated how well some of these parameters predict fretting fatigue crack initiation in titanium alloy, Ti–6Al–4V. The following crack initiation parameters were evaluated; the strain-life parameter, the maximum strain corrected for strain ratio effects, the maximum principal strain corrected for principal strain ratio effects, the Smith–Watson–Topper (SWT) parameter, the critical plane SWT parameter and the Fatemi and Socie (F–S) parameter. The Ruiz parameters, which are specific to fretting fatigue condition, were also evaluated. The evaluation was based on the parameters ability to predict the number of cycles to initiation and location for crack initiation. The results indicated that the maximum strain amplitude at the contact interface was an important parameter for fretting fatigue crack initiation. Furthermore, the results indicated that when the applied loading was corrected for the effects of contact and mean stress or strain ratio, titanium alloy, Ti–6Al–4V exposed to the fretting fatigue condition behaved in a manner similar to the plain fatigue condition.

A cohesive zone model for fatigue crack growth in quasibrittle materials

Abstract A cohesive zone model for fatigue crack initiation and growth in quasibrittle materials is proposed in the present paper. While bulk material is modeled to be linearly elastic, the softening material in the cohesive zone and cracks are modeled to be internal singular surfaces in the elastic body. The interactions of the singular surfaces are described in a cohesive force law and a Coulomb-type friction law if in contact. The cohesive zone material is modeled to accumulate damage not only along the damage locus but also along an unloading path underneath it, enabling a simulation of fatigue damage and crack growth without the ad hoc imposition of a law of growth rate within the cohesive zone model. The maximum principal stress criterion is used to advance a tip of the cohesive zone in the direction of the maximum principal stress when it reaches the critical value of material strength. The physical crack tip is grown as a natural process of debonding of the cohesive zone under cyclic loading, which, in contrast, may be subcritical with energy dissipation less than the material toughness under static loading. The boundary value problem formulated for fatigue crack growth incorporating the cohesive zone model is nonlinear due to the history dependence of the cohesive zone, and is solved efficiently using the iterative single-domain dual-boundary-element method of successive over-relaxation. It is demonstrated through examples that the present model is capable of predicting fatigue crack initiation as well as growth in a unified way. It is also shown that the cohesive zone model is more advantageous and flexible in handling fatigue cracks under arbitrary loading than the classical singularity-based fracture mechanics approach.

Modeling of a cracked metallic structure with bonded composite patch using the three layer technique

Abstract Due to the high computational cost of three-dimensional finite element analysis, the two-dimensional finite element analysis involving the three layer technique is introduced to investigate the repair of cracked metallic structures using an adhesively bonded composite patch. In the three layer technique, two-dimensional Mindlin plate elements with transverse shear deformation capability are used for all three layers; cracked plate, adhesive and composite patch. The accuracy of the three layer technique to compute the stress intensity factor for the metallic crack is demonstrated by a comparison with available two- and three-dimensional models. The strain energy release rates of the debond at the adhesive interfaces are also examined and compared with the previous studies. The three layer technique provides an efficient and accurate alternative model which is capable of investigating in depth the adhesive effects on the bonded patch repair of cracked metallic structures.

Combined experimental–numerical investigation of fretting fatigue crack initiation

Abstract This study investigated the fretting fatigue crack initiation behavior of titanium alloy, Ti–6Al–4V. Tests were conducted to generate fretting fatigue failures from 2×104 to 5×107 cycles at 200 Hz. Fractography was employed to determine number of cycles to crack initiation, crack location and angle of crack orientation. Finite element analysis was conducted based on the experimental information in order to assess the ability of two critical plane approaches to predict fretting fatigue crack initiation behavior; the Smith–Watson–Topper critical plane parameter and the maximum shear stress range critical plane parameter. When properly formulated, these parameters predicted number of cycles to crack initiation and location of crack initiation which were in agreement with the experimental counterparts. However, these two parameters predicted different orientation angles of crack initiation at the contact surface. Based on the observations of orientation angles, the combined experimental–numerical approach showed that the mechanism for fretting fatigue crack initiation was governed by the maximum shear stress range on the critical plane.

Contact resistance study of noble metals and alloy films using a scanning probe microscope test station

The proper selection of electrical contact materials is one of the critical steps in designing a metal contact microelectromechanical system (MEMS) switch. Ideally, the contact should have both very low contact resistance and high wear resistance. Unfortunately this combination cannot be easily achieved with the contact materials currently used in macroswitches because the available contact force in microswitches is generally insufficient (less than 1mN) to break through nonconductive surface layers. As a step in the materials selection process, three noble metals, platinum (Pt), rhodium (Rh), ruthenium (Ru), and their alloys with gold (Au) were deposited as thin films on silicon (Si) substrates. The contact resistances of these materials and their evolution with cycling were measured using a specially developed scanning probe microscope test station. These results were then compared to measurements of material hardness and resistivity. The initial contact resistances of the noble metals alloyed with Au a...

Effects of slip on fretting behavior: experiments and analyses

The role of relative slip on fretting behavior was investigated by conducting experiments and analyses. The first series of tests was conducted at different substrate applied stress amplitudes, and the second series at different applied fretting pad displacements with a constant substrate stress amplitude. A fretting map was developed using fretting fatigue life, relative slip range and normal force. This map provides useful information about the inter-relationships between these parameters and contact conditions (partial and gross slip) as well as about damages induced by these conditions. Partial and gross slip conditions were modeled using finite element analysis (FEA). FEA showed that gross slip simply shifted the stress distribution and the location of the trailing edge without any appreciable change in magnitude. Modified shear stress range (MSSR) parameter, a critical plane-based multi-axial fatigue model, was used to characterize fretting fatigue crack initiation behavior. With increasing applied relative slip, the MSSR parameter initially increased and the fatigue life decreased; thereafter the MSSR remained constant but the fatigue life increased. This increase in the fatigue life appears to be due to an increased wear rate. There was thus a competition between crack initiation and material removal during fretting as the contact condition changed from partial slip to gross slip.

Modeling damage in unidirectional ceramic-matrix composites

Abstract This paper extends the modified shear-lag model developed previously to analyze the damage progression within a unidirectional fiber-reinforced ceramic-matrix composite subject to quasi-static loading. The damage mechanisms considered in this paper are matrix cracking, fiber/matrix interfacial debonding, interfacial slip and fiber failure. Crack density is determined analytically through the introduction of a ‘critical matrix strain energy’. A priori knowledge of the composites ‘proportional limit’ yields a complete closed-form stress/strain solution. The influence of the interfacial shear stress, the interfacial bond strength and the composite proportional limit on the progression of matrix cracking and interfacial debonding are reported. The unloading behavior, including stress/strain hysteresis, is also modeled in terms of interfacial frictional slip within debonded regions.

Characterization of metal and metal alloy films as contact materials in MEMS switches

This study presents a basic step toward the selection methodology of electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This involves the interrelationship between two important parameters, resistivity and hardness, since they provide the guidelines and assessment of contact resistance, wear, deformation and adhesion characteristics of MEMS switches. For this purpose, thin film alloys of three noble metals, platinum (Pt), rhodium (Rh) and ruthenium (Ru) with gold (Au), were investigated. The interrelationship between resistivity and hardness was established for three levels of alloying of these metals with gold. Thin films of gold (Au), platinum (Pt), ruthenium (Rh) and rhodium (Ru) were also characterized to obtain their baseline data for comparison. All films were deposited on silicon substrates. When Ru, Rh and Pt are alloyed with Au, their hardness generally decreases but resistivity increases. This decrease or increase was, in general, dependent upon the amount of alloying.

Dynamic fracture toughness of Homalite-100

Dynamic fracture toughness of Homalite-100 determined by T. Kobayashi and Dally are compared with those previously obtained by the authors where similarities in the two results for single-edged-notch specimens of various configurations are noted. Dynamic fracture toughness of Araldite B obtained by Kalthoff, Beinert and Winkler and those of Homalite-100 obtained by the authors are then compared and, again, similarities in the two results and, in particular, the scatters in experimental data for wedge-loaded DCB specimens of different sizes are found. All three teams of investigators used static near-field solution to compute the dynamic stress-intensity factors from recored dynamic isochromatics or dynamic caustics. Errors generated through this use of static near-field solutions, as well as through the use of larger isochromatic lobes, are thus discussed.

Modeling of cracked thick metallic structure with bonded composite patch repair using three-layer technique

A finite element analysis involving three-layers of two-dimensional Mindlin plate elements, to model cracked plate, adhesive, and composite patch, was developed to characterize fatigue crack growth behavior of a thick metallic panel repaired with an adhesively bonded composite patch. Also, fatigue experiments were conducted with 6.35 mm thick specimens with a pre-crack repaired asymmetrically with adhesively bonded unidirectional boron/epoxy patch. Fatigue crack growth rates on the unpatched and patched faces (PF) were measured along with debond of the composite patch. Stress intensity factors obtained from the analysis were combined with the fatigue crack growth relationship for the unrepaired cracked material to obtain the analytical fatigue crack growth rates. The experimental and analytical fatigue crack growth rates on the unpatched face (UPF) were in a good agreement with each other when the proper consideration of the effective crack length and debond were incorporated in the analysis. Thus, the three-layer technique was found to be capable of characterizing the fatigue crack growth behavior of the repaired thick panels as in the case of repaired thin panels shown in the previous studies.