Effect of interfacial debonding and sliding on matrix crack initiation during isothermal fatigue of SCS-6/Ti-15-3 composites

Direct observation of initial damage-evolution processes occurring during cyclic testing of an unnotched SCS-6 fiber-reinforced Ti-15-3 composite has been carried out. The aligned fibers break at an early stage, followed by debonding and subsequent sliding along the interface between the reaction layer (RL) and Ti-15-3 alloy matrix. Matrix cracking initiation from the initial broken fiber and RL was avoided. This fracture behavior during cyclic loading is modeled and analyzed by the finite-element method, with plastic deformation of the matrix being considered. The plastic strain in the matrix at the initial crack and at the deflected crack tips, when the interface crack is deflected into the RL after extensive interface debonding propagation, is characterized. The effects of interfacial debond lengths and test temperatures on the matrix cracking mechanism are discussed, based on a fatigue-damage summation rule under low-cycle fatigue conditions. The numerical results provide a rationale for experimental observations regarding the avoidance and occurrence of the matrix cracking found in fiber-reinforced titanium composites.     

Thermal fatigue of Ti-24Ai-11NB/SCS-6

The effect of thermally cycling in air with no applied load was studied using the Ti-24A1-11Nb (atomic percent)/SCS-6 composite system. Mechanisms of damage determination were observable cracking and residual tensile properties. Either the number of cycles or the temperature range was varied from specimen to specimen. Effects of number of cycles were investigated using a temperature range of 150 °C to 815 °C. Comparisons of temperature range effects were made at a constant cycle count of 500. Matrix cracking was observed at a †T of 450 °C and greater when oxidation was significant. Transverse cracking was not observed in specimens that were cycled to maximum temperatures where oxidation was insignificant, even for †T's as large as 500 °C. A decrease in tensile properties coincided with the observed transverse matrix cracking. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Interface-controlled fatigue cracking of SCS-6/Ti-22Al-23Nb “orthorhombic” titanium aluminide composite

The effect of aging at elevated temperature on interfacial stability and fatigue behavior of a SCS-6/Ti-22Al-23Nb “orthorhombic” (O) titanium aluminide composite is investigated. The composite was heat treated in vacuum at 900 °C for up to 250 hours to change the microstructural characteristics. The stability of the matrix alloy and interfacial reaction zone after extended thermal exposure was analyzed. The effect of interface on fatigue behavior, including stiffness degradation, evolution of fatigue damage, and crack growth rates, was characterized. Finally, a modified shear-lag model was used to predict the saturated matrix crack spacing in the composite under fatigue loading. The results demonstrate that aging at elevated temperature affects the stability of the interfacial reaction zone, which, in turn, degrades the fatigue properties of the composite. However, fatigue crack will not develop from the ruptured interfacial reaction layer until the thickness of the reaction zone or the maximum applied stress exceeds a critical value.     

Effect of environment on the thermal fatigue response of an SCS-6/Ti-24Al-11Nb composite

A study has been conducted examining the thermal fatigue characteristics of an α₂/SiC composite; in particular, SCS-6 reinforced Ti-24Al-11Nb (at. pct). The effort included the investigation of the effect of the environment by cycling coated and uncoated specimens in air and in an inert environment. Damage assessment was determined by postcycling room-temperature tension testing as well as by microstructural examination, including both optical microscopy and scanning electron microscopy (SEM). Significant reductions in postcycling tensile strength were observed for coated and uncoated specimens thermally cycled in air from 150 °C to 815 °C for 500 cycles, while no measurable loss of strength was found for specimens cycled in a low-pressure inert environment under otherwise identical conditions. The synergistic effect of residual stresses due to a coefficient of thermal expansion (CTE) mismatch and environment on the degradation of tensile properties of the thermally cycled composite is found to be the critical damage evolution mechanism for both coated and uncoated composites cycled in air. Residual stresses alone were found not to be critical in creating damage that could be tracked by a loss in residual strength.     

Strength of SCS-6 fibers in a Tiβ21s/SCS-6 composite before and after fatigue

The strength distributions of SCS-6 fibers extracted from both as-received (AR) and fatigued Tiβ21s/SCS-6 composite specimens have been characterized by single-fiber-tension tests and have then been analyzed statistically. With the assistance of fractography, the whole population of SCS-6 fibers in the composite can be cataloged into three subpopulations with fracture strengths, σ _F , of <2000, 2000 ≤ σ _F <3700, and ≥ 3700 MPa, respectively. Each subpopulation has distinctive statistical parameters, which do not appear to change markedly after fatigue. However, cyclic loading can reduce the average strength of the whole population by decreasing the percentage of high-strength fibers present. In addition, cyclic loading can also break some of the low-strength fibers. By using a trimodal Weibull function, the degradation of the strength of the fibers after fatigue, as well as the influence of such degradation of fiber strength on the fiber-bundle strength and then on the predicted fatigue crack growth resistance of the composite, have been analyzed.     

Observation of short fatigue crack-growth process in SiC-fiber-reinforced Ti-15-3 alloy composite

The microscopic fatigue damage characteristics and short fatigue crack growth of an unnotched SiC(SCS-6) fiber-reinforced Ti-15-3 alloy composite were investigated in tension-tension fatigue tests (R = 0.1) carried out at room temperature for applied maximum stress of 450, 670, and 880 MPa.In situ observation of the damage-evolution process was done using optical and scanning laser microscopies, which were attached in the fatigue machine. The first damage for the composite started from a cracking of the reaction layer followed by fiber fracture. The matrix cracking initiated near the broken fiber when the microhardness of the matrix just to the side of the fracture fiber reached ≈6 GPa, and the number of cycles for the initiation of this cracking decreased with the increase of applied stress. The slope of the relation of surface crack growth lengthvs number of cycles fell into two characteristic stages; in the first stage, the rate was lower than the second stage and accelerated. The surface crack growth rate,d(2c)/dN,vs surface crack length relation also fell into two stages (stages I and II). With the increase in surface crack length, the crack-growth rate,d(2c)/dN, decreased in stage I and increased in stage II. The transition from stage I to stage II occurred due to the fracture of fibers located around the first fractured fiber. It was concluded that the fatigue crack growth resistance of the composite in the short-crack region was controlled by the fiber fracture and matrix work hardening near the fractured fiber. When the fiber fracture occurred, the surface crack growth rate was accelerated and became faster than that of the monolithic matrix.     

The micromechanics of fatigue crack growth at 25 °C in Ti-6Al-4V reinforced with SCS-6 fibers

Micromechanics parameters for fatigue cracks growing perpendicular to fibers were measured for the center-notched specimen geometry. Fiber displacements, measured through small port holes in the matrix made by electropolishing, were used to determine fiber stresses, which ranged from 1.1 to 4 GPa. Crack opening displacements at maximum load and residual crack opening displacements at minimum load were measured. Matrix was removed along the crack flanks after completion of the tests to reveal the extent and nature of the fiber damage. Analyses were made of these parameters, and it was found possible to link the extent of fiber debonding to residual COD and the shear stress for fiber sliding to COD. Measured experimental parameters were used to compute crack growth rates using a well-known fracture mechanics model for fiber bridging tailored to these experiments.     

Observation of fatigue damage process in SiC fiber-reinforced Ti-15-3 composite at high temperature

Unnotched SiC (SCS-6) fiber-reinforced Ti-15-3 alloy composite is subjected to a tension-tension fatigue test in a vacuum of 2×10⁻³ Pa at 293 and 823 K with a frequency of 2 Hz and R=0.1. Direct observation of the damage evolution process during the test is carried out by scanning electron microscopy (SEM). Test temperature dependent and independent fatigue damage behaviors are observed. The early stage fiber fractures observed at the polished surface are not influenced by the test temperature; however, matrix crack initiation and propagation behaviors differ greatly with temperature. The evolution of interface wear damage also differs with temperature, becoming more severe at 823 K, and the interface wear damage zone increases with the increase of the number of fatigue cycles. The macroscopic fatigue damage appears as a modulus reduction associated with interface sliding, matrix crack propagation, and plastic deformation of the matrix. The deformation zone of the composite tested at 823 K spreads more than that at 293 K. The fatigue life of the composite tested at 823 K is longer than that at 293 K. This behavior is related to the difference in spread of the damage zone in the matrix.     

Texture and residual strain in two SiC/Ti-6-2-4-2 titanium composites

Residual strain and texture variations were measured in two titanium matrix composites reinforced with silicon carbide fibers (Ti/SiC) of similar composition but fabricated by different processing routes. Each composite comprised a Ti-6242 α/β matrix alloy containing vol 35 pct continuous SiC fibers. In one, the matrix was produced by a plasma sprayed (PS) route, and in the other by a wiredrawn (WD) process. The PS and WD composites were reinforced with SCS-6 (SiC) and Trimarc (SiC) fibers, respectively. The texture in the titanium matrices differed significantly. The titanium matrix for the PS material exhibited random texture pre and post fabrication of the composite. For the WD material, the starting texture of the monolithic titanium matrix was ≈17 times random, but after consolidation into composite form, it was ≈6 times random. No significant differences were noted in the fiber-induced matrix residual strains between the composites prepared by the two procedures. However, the Trimarc (WD) fibers recorded higher (≈1.3 times) compressive strains than the SCS-6 (PS) fibers. Stresses and stress balance results are reported. Plane-specific elastic moduli, measured in load tests on the unreinforced matrices, showed little difference. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Texture and residual strain in two SiC/Ti-6-2-4-2 titanium composites

Residual strain and texture variations were measured in two titanium matrix composites reinforced with silicon carbide fibers (Ti/SiC) of similar composition but fabricated by different processing routes. Each composite comprised a Ti-6242 α/β matrix alloy containing vol 35 pct continuous SiC fibers. In one, the matrix was produced by a plasma sprayed (PS) route, and in the other by a wiredrawn (WD) process. The PS and WD composites were reinforced with SCS-6 (SiC) and Trimarc (SiC) fibers, respectively. The texture in the titanium matrices differed significantly. The titanium matrix for the PS material exhibited random texture pre and post fabrication of the composite. For the WD material, the starting texture of the monolithic titanium matrix was ≈17 times random, but after consolidation into composite form, it was ≈6 times random. No significant differences were noted in the fiber-induced matrix residual strains between the composites prepared by the two procedures. However, the Trimarc (WD) fibers recorded higher (≈1.3 times) compressive strains than the SCS-6 (PS) fibers. Stresses and stress balance results are reported. Plane-specific elastic moduli, measured in load tests on the unreinforced matrices, showed little difference. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Transverse and cyclic thermal loading of the fiber reinforced metal-matrix composite SCS6/Ti-15-3

The transverse properties of a SiC fiber reinforced Ti alloy matrix composite subjected to transverse mechanical and cyclic thermal loading have been investigated. Fibers and matric have a mismatch in the coefficients of thermal expansion that induces thermal stresses in addition to those caused by mechanical loading. When fluctuations occur in the operating temperature the thermal stresses change and this could cause an incremental accumulation of plastic strain or increase in creep rate. The composite under consideration has a modest mismatch and it was found that the strain accumulation is caused by creep deformation in the matrix at the high temperature portion of the thermal cycles. In the early stages of the deformation for low transverse loading the interface is in compressive contact and the creep rate is accelerated by the cyclic thermal stresses. After debonding has occurred the cyclic thermal stress component is diminished and the creep rate is given by a matrix with holes.     

Thermal fatigue of a SiC/Ti-15Mo-2.7Nb-3Al-0.2Si composite

The influence of thermal cycling and isothermal exposures in air on the residual ambient temperature strength of SCS-6/Ti-15Mo-2.7Nb-3Al-0.2Si (weight percent) metal-matrix composites comprised of [0]₄ and [0/90]_s laminates has been determined. A maximum temperature of 815 °C was used in thermal cycling and isothermal exposure. Temperature range, cycle count, maximum/minimum temperature, environment, and hold time at temperature were systematically varied. Postexposure ambient-temperature tension testing, scanning electron and optical microscopy, and fractography were performed on selected specimens to determine the degree of damage. A reduced residual strength was noted in thermal fatigue with increasing cycle count, maximum temperature, and hold time for all specimens tested in air. Isothermal exposures at 815 °C also substantially reduced residual ambient-temperature strength. Considerably less reduction in strength occurred in inert environment than in air. Damage processes included matrix cracking, fiber/matrix interface damage, matrix embrittlement by interstitials, and oxide scale formation at specimen surfaces and, in some cases, at matrix/fiber interfaces. Fiber orientations which allowed rapid ingress of oxygen lead to greater matrix embrittlement and resulted in more pronounced reductions in strength. Formerly with the Materials Directorate, Wright Laboratory, Wright Patterson AFB, Dayton, OH 45433     

Evolution of fretting fatigue damage and relaxation of residual stress in shot-peened Ti-6Al-4V

The evolution of fretting fatigue damage was investigated in shot-peened Ti-6Al-4V samples, by measuring the changes in the surface residual stress, using the X-ray diffraction technique. The surface residual stress was found to relax as the number of fretting fatigue cycles increased. The relaxation behavior of the residual stress with the increasing number of fretting fatigue cycles was observed to occur in three stages. In the first 20 pct of the fretting fatigue life, a drastic relaxation was observed. In the second part (between 20 and 70 pct), a gradually increasing behavior was observed. During the last 20 to 30 pct of the fretting fatigue life, a dramatic relaxation of the residual stress was found to occur. A complete relaxation of the residual stress occurred in the fracture region. A scanning electron microscope observation of the microstructure of the damaged region was used to examine the mechanisms leading to the relaxation of the residual stress. The development of delaminations at the early stages of the accumulation of the fretting fatigue damage was observed to be the main cause of the initial relaxation. The generation of microcracks from the voids left behind by the delaminations is responsible for the additional relaxation of the residual stress. The coalescence of the microcracks generated from different delaminated regions produced yet more relaxation of residual stress and, ultimately, the final fracture of the specimen.     

Surface residual stresses,surface topography and the fatigue behavior of Ti-6AI-4V

Fatigue tests have been conducted on Ti-6Al-4V from 293 to 589 K to determine the influence of surface residual stresses and surface topography on low and high cycle fatigue properties. Four types of machined surfaces as well as shot peened surfaces were included in the investigation. It was found that surface residual stresses play a key role in controlling the development of microcracks and, therefore, overall fatigue lives at both room and elevated temperature. X-ray measurement of the stability of surface residual stresses under thermal activation and/or cyclic loading demonstrated that, for the conditions studied, cyclic loading was primarily responsible for residual stress decay. In addition, the magnitude of the decay was dependent on the relationship between the sign of the residual stress and the sign of the imposed mean strain. Finally, it was demonstrated that the sharpness of machining grooves is more important than their depth in controlling fatigue resistance. The work was performed when all authors were affiliated with Pratt and Whitney Aircraft, Middletown, Connecticut.     

Interface effects on the micromechanical response of a transversely loaded single fiber SCS-6/Ti-6Al-4V composite

The ability of a fiber-matrix interface to support a transverse load is typically evaluated in straight-sided composite specimens where a stress singularity exists at the free surface of the interface. This stress singularity is often the cause of crack initiation and debonding during transverse loading. In order to develop a fundamental understanding of the transverse behavior of the fiber-matrix interface, it is necessary to alter the crack initiation site from the free surface to an internal location. To achieve this objective, a cross-shaped specimen has been recently developed. In this study, based on the experimentally observed onset of nonlinearity in the stress-strain curve of these specimens and finite element analysis, the bond strength of the SCS-6/Ti-6Al-4V interface was determined to be 115 MPa. The micromechanical behavior of these specimens under transverse loading was examined by finite element analysis using this interface bond strength value and compared with experimental observations. Results demonstrate that the proposed geometry was successful in suppressing de-bonding at the surface and altering it to an internal event. The results from numerical analysis correlated well with the experimental stress-strain curve and several simple analytical models. In an attempt to identify the true bond strength and the interface failure criterion, the present study suggests that if failure initiates under tensile radial stresses, then the normal bond strength of the SCS-6/Ti-6A1-4V composites is about 115 MPa; under shear failure, the tangential shear strength of the in-terface is about 180 MPa.     

The effect of heat treatment on the fatigue strength of microknurled Ti-6Al-4V

Microknurling, a high pressure surface indentation technique, was devised as an alternative to traditional heat-bonded porous coatings found on many orthopedic implants designed for fixation by tissue ingrowth. Heat-bonded porous coating can cause at the surface of an implant stress concentrations that reduce fatigue strength. However, microknurling may reduce stress intensification without eliminating it. Thus the purpose of this work was to explore surface thermal/mechanical processing of Ti-6Al-4V to improve the fatigue strength of microknurled specimens via the production of a Ti-6Al-4V dual microstructure. The latter consists of a surface layer of equiaxed grains known to be effective against crack initiation and a bulk microstructure of lamellar grains that possesses optimum fatigue crack propagation resistance. Rotating-bending fatigue tests showed that such a microstructure had some benefits, but this was offset by the reduction in compressive strains imparted to the surface by the heat treatments needed to obtain this microstructure.