Linear Elastic Fracture Mechanics Techniques Research Articles

Corrosion modified fatigue analysis for next-generation damage-tolerant management

Abstract Although a great deal of success in structural integrity management has been achieved, it may be realized by excessively conservative assumptions, high inspection burdens, and aggressive maintenance and repair strategies. Corrosion has been found to reduce fatigue life, but methods to account for it in fatigue modeling are still limited. There is a recognized need to improve structural integrity calculations through the inclusion of the effect of existing corrosion damage on fatigue. The feasibility of a corrosion modified fatigue analysis process that uses finite element analysis and linear elastic fracture mechanics techniques to predict fatigue life of a corroded aircraft component based on the corrosion damage location is demonstrated in this work. A corrosion modified equivalent flaw size was successfully used to predict fatigue crack growth from AA7075-T651 specimens with two different notch geometries and corrosion damage at the notch center.

Crane girder fatigue life determination using SN and LEFM methods

Abstract This paper examines the fatigue life of a Melt Shop crane girder with over 30 years of operation. Research indicates that the most frequent failures in these structures is related to fatigue in welded joints, thus an initial approach was performed according to AASHTO standard for calculating the remaining fatigue life of the structure. The alternating stress at the most severe loading conditions was obtained by numerical simulation in ANSYS software. Based on history and production forecasts, each critical welded joint fatigue life was determined. To calibrate the model, a field test was performed with strain gages to determine the real stress variation during regular plant operation. At this stage, crane load was parameterized in the computational model and varied to simulate the real behavior of the structure under the measured stresses. The analysis pointed out the most critical points which guided a visual inspection that identified cracks that could lead to a structural collapse. Identified crack growth was simulated through FRANC2D software that applies the LEFM method to determine the stress intensity factor. The calculated values were used to reach fatigue life for crack growth from an initial to a critical value which is expected to lead to a brittle fracture. It was concluded that the S-N method associated with finite element analysis has proven to be an efficient technique for determining the most critical structure joints damage reducing inspection work significantly. The application of linear elastic fracture mechanics techniques reached more conservative results. This technique can assist the maintenance crew in determining inspection intervals and the best moment for intervention to repair existent cracks based on the calculated remaining life.

Comparison of fracture behavior of nylon 6 versus an amorphous polyamide toughened with maleated poly(ethylene-1-octene) elastomers

The fracture behavior of an amorphous polyamide (Zytel 330 from DuPont), a-PA, and nylon 6 toughened by maleated poly(ethylene-1-octene) elastomers are reported. The deformation mechanisms during fracture were verified by examining an arrested crack tip and the surrounding regions using transmission electron microscopy analysis. a-PA blends show higher levels of impact strength and lower ductile–brittle transition temperatures than nylon 6 blends. Fracture toughness, characterized by both linear elastic fracture mechanics techniques in terms of the critical strain energy release rate, GIC, and the essential work of fracture methodology, i.e. the limiting specific fracture energy, uo, and the dissipative energy density, ud, using thick (6.35mm) samples with sharp notches, depends on ligament length, rubber content, rubber particle size and test temperature. In general, a-PA blends show larger values of ud than do nylon 6 blends while the opposite is seen for uo. The amorphous polyamide shows a similar critical upper limit on rubber particle size, or interparticle distance, for toughening as the semi-crystalline nylon 6; thus, it is clear that the crystal morphology around the rubber particles must not be the dominant cause of this critical size scale. The deformation mechanisms involved include cavitation of rubber particles followed by some crazing and then massive shear yielding of the matrix.

Fracture loads for ceramic samples with rounded notches

Fracture loads of ceramic components with rounded notches cannot be computed by linear elastic fracture mechanics techniques because no stress singularity exists. We propose a procedure to estimate such fracture loads, which is based on the cohesive zone model and supported by experimental evidence with alumina, zirconia and silicon ceramics. Data from 18 ceramic materials and different notched geometries were used. The only material parameters needed were the tensile strength and the fracture toughness.

Fracture toughness of nylon‐6 blends with maleated rubbers

AbstractThe fracture toughness of blends of nylon‐6 with maleated ethylene–propylene rubber and maleated styrene/hydrogenated butadiene/styrene triblock copolymer was investigated with a single‐edge‐notched three‐point‐bending instrumented Dynatup test. The blends for which the rubber particle size was less than 0.7 μm fractured in a ductile manner over the whole range of ligament lengths, whereas the blends with particles larger than 0.7 μm showed a ductile‐to‐brittle transition with the ligament length. In this regime, ductile fracture was observed for specimens with short ligaments, whereas brittle fracture was seen for those with long ligaments. The ductile fracture behavior was analyzed with the essential‐work‐of‐fracture model, whereas linear elastic fracture mechanics techniques were used to analyze the brittle fracture behavior. The fact that the ductile fracture energy was larger for the blends with the styrene/hydrogenated butadiene/styrene triblock copolymer than for those with ethylene–propylene rubber was due to the larger dissipative energy density of the blends based on the styrene/hydrogenated butadiene/styrene triblock copolymer. Both the critical strain energy release rate (GIC) and the plane‐strain critical stress intensity factor (KIC) increased as the rubber particle size decreased for both blend systems. The GIC and KIC parameters had similar values, regardless of the rubber type, when the rubber particle size was fixed. The transition ligament length was near the size criterion for plane‐strain conditions for both blend systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1739–1758, 2004

Effect of porosity and environment on the mechanical behavior of acrylic bone cement modified with acrylonitrile-butadiene-styrene particles: I. Fracture toughness.

The elastomeric copolymer acrylonitrile-butadiene-styrene (ABS) was added to a conventional acrylic bone cement matrix. The results obtained show that although strength and stiffness decreased with an increasing second phase volume fraction, ductility and toughness both increased. The crack propagation became stable for specimens containing over a 5% volume fraction of the second phase. The fracture toughness increased up to 60% when the amount of ABS reached 20% (v/v). For larger amounts linear elastic fracture mechanics techniques could not be used properly. The effects of porosity and environmental conditions on the mechanical behavior were also studied. The mechanisms that control the fracture process were investigated by means of scanning electron microscopy.

Reliability of Electroless Processed Thin Layered Solder Joints

AbstractThere has recently been an increasing interest in using electroless processes to produce the metal thin film stacks that make up the bond pads in solder ball grid array (BGA) packages. Electroless processes produce a more uniform and better controlled film thickness, but the resulting metal stacks have exhibited less mechanical reliability than that of traditional electrolytic stacks. This paper addresses a layered system consisting of a eutectic Sn-Pb solder sandwiched between stacks of Cu, Ni, and Au thin films. The samples were tested to determine cyclic fatigue behavior and fracture toughness values using traditional linear elastic fracture mechanics techniques. Surprisingly, fatigue crack propagation occurred in the middle of the solder layer even though the electroless interface was expected to be very weak. In contrast, fracture tests did appear to cause failure near an interface, but not necessarily at the electroless interface. Fracture toughness values measured for these samples were much lower than those reported for bulk metals. These results are discussed in terms of the microstructures present in order to determine possible relationships between the microstructure of the solder joint and its fracture behavior.

The fracture of poly(hydroxybutyrate)Part II Fracture mechanics study after annealing

The fracture behaviour of poly(hydroxybutyrate) (PHB) after high-temperature annealing has been studied using linear elastic fracture mechanics techniques. The effect of the annealing temperature on the polymers' fracture toughness both initially and after re-ageing is examined. Annealing at temperatures of 120°C or above is found to result in an improvement in both the critical stress intensity factor and the strain energy release rate measurements of fracture toughness which is largely maintained on re-ageing. A more detailed study of the re-ageing behaviour after annealing at 130°C finds Gc to remain approximately constant but Kc to increase slightly with age. © 1998 Chapman & Hall

The fracture of poly(hydroxybutyrate)Part I Fracture mechanics study during ageing

Linear elastic fracture mechanics techniques have been used to study the fracture behaviour of the biodegradable thermoplastic poly(hydroxybutyrate). Both the compact tension and double cantilever beam test geometries have been used and give toughness values in good agreement. The variation in fracture toughness with age after moulding has been monitored. It has been found that the strain energy release rate is reduced during ageing, but the stress intensity factor is increased. The primary change during ageing is found to be in the polymers’ stiffness rather than its resistance to fracture. © 1998 Chapman & Hall

Fracture toughness of Kevlar 29/poly(methyl methacrylate) composite materials for surgical implantations.

A study of the fracture behaviour of Kevlar 29 reinforced dental cement is undertaken using both linear elastic and nonlinear elastic fracture mechanics techniques. Results from both approaches--of which the nonlinear elastic is believed to be more appropriate--indicate that a reinforcing effect is obtained for the fracture toughness even at very low fibre content. The flexural strength and modulus are apparently not improved, however, by the incorporation of Kevlar 29 fibres in the PMMA cement, probably because of the presence of voids, the poor fibre/matrix interfacial bonding and unsatisfying cement mixing practice. When compared to other PMMA composite cements, the present system appears to be probably more effective than carbon/PMMA, for example, in terms of fracture toughness. More experimental and analytical work is needed so as to optimize the mechanical properties with respect to structural parameters and cement preparation technique.

Concrete Fracture in CLWL Specimen

Results are reported of a series of tests on concrete crack‐line wedge loaded double cantilever beams. Those tests provide a data base for fracture parameters characterizing mode I cracking in concrete. The development is reported of a replica technique that provides an easy and accurate method for determining the total extent of cracking in concrete specimens, and it is shown that the fracture process zone, which can be determined with that technique, continues to increase in length with increasing load. From analysis of the results, it is concluded that linear elastic fracture mechanics techniques are not suited to predicting unstable cracking in concrete members of the proportions normally used in buildings.

Experimental analysis of crack propagation in granite

An experimental approach is used to demonstrate the concept of an effective crack length with double-edge-notched specimens of Charcoal and Rockville granite. Crack propagation in rock is characterized by microcracking around the crack tip and interlocking along a portion of the crack; this region is called the fracture process zone, and together with the fraction free length defines the effective crack length.Twenty closed-loop, strain-controlled fracture tests were conducted on Charcoal and Rockville granite. Crack growthwas monitored with a travelling behaviour of both granites, it is suggested that the process zone is larger for the larger grain-sized rock (Rockville). Conventional linear elastic fracture mechanics techniques are used to calculate the apparent fracture toughness of Charcoal granite at various crack lengths. In addition, a J-integral expression is derived for the double-edge-notched geometry in terms of the area under the load-displacement record. The shortcomings of both analyses are discussed. An attempt is made to explain the inadequacies by including the process zone in the calculations of the fracture toughness.

EFFECT OF TEMPERATURE ON THE FATIGUE-CRACK PROPAGATION BEHAVIOUR OF INCONEL X-750

Abstract— Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.

Techniques Developed for Elevated Temperature Fracture Toughness Testing of Irradiated Materials in Thin Sections

Large changes in strength and ductility of metals after irradiation are known to occur. The fracture toughness of irradiated metals, which is related to the combined strength and ductility of a material, may be significantly reduced and the potential for unstable crack extension increased. For relatively thin materials which retain substantial ductility, even after irradiation, the fracture behavior cannot be analyzed by linear elastic fracture mechanics techniques. Instead, the multispecimen R-curve method and J-integral analysis were used to develop an experimental approach to evaluate the fracture toughness of thin breeder reactor materials irradiated at elevated temperatures. Alloy A-286 was chosen for this work because the alloy exhibits elastic/plastic behavior and the fracture toughness data of thicker (12 mm) specimens were available for comparison. Technical problems associated with specimen buckling and remote handling were treated in this work. The results are discussed in terms of thickness criterion for plane strain.

Fatigue-crack propagation behaviour of Inconel 600

The techniques of linear-elastic fracture mechanics were employed to characterise the effects of several parameters upon the fatigue-crack propagation behaviour of Inconel 600. The parameters studied included temperature, cyclic frequency, stress ratio, thermal ageing and a limited amount of testing in a liquid sodium environment.

The effect of temperature upon fatigue-crack growth behavior in nimonic PE16

The techniques of linear-elastic fracture mechanics were employed to characterize the effect of temperature upon the fatigue-crack propagation behavior of Nimonic PE16 in an air environment over the range 24–649°C. In general, crack growth rates increased with increasing test temperature. Curves of log(da/dN) versus log(ΔK) were generally sigmoidal in shape and in general did not obey a simple linear power law.

Stress Corrosion Crack Growth in Austenitic Stainless Steel

Abstract Stress corrosion crack velocity and stress corrosion threshold stress intensity have been measured for Type 304L austenitic stainless steel in water with 42% MgCl2 at 130 C. The stress corrosion crack velocity vs stress intensity curve has a shape similar to the one for other austenitic steels and many other metallic and nonmetallic materials. The threshold stress intensity (KISCC is 8 ± 1 MN·m−3/2, and the plateau crack velocity is 3 to 6 × 10−8 m/s. Linear elastic fracture mechanics techniques can be applied to SCC testing of soft and tough materials, provided the KISCC is low enough, and crack branching can be avoided. Criteria for the formation of single, branched, and circular stress corrosion cracks are indicated.

The effect of fast neutron irradiation upon the fatigue-crack propagation behavior of two austenitic stainless steels

The effect of fast neutron irradiation ( 454° < T irr < 477° C ) to a fluence of 9 × 10 21 n/cm 2 ( E > 0.1 MeV) on the fatigue-crack growth behavior was investigated for annealed Type 304 and 20% coldworked Type 316 stainless steels using linear-elastic fracture mechanics techniques. Irradiation to this fluence had little or no effect upon the crack growth behavior of annealed Type 304 at a test temperature of 427° C, nor upon the behavior of 20% cold-worked Type 316 at test temperatures of 427° C and 538° C. Irradiation to this fluence did tend to decrease crack growth rates slightly, relative to unirradiated material, in annealed Type 304 at a test temperature of 538° C.

Estimation of Crack Extension in a Piping Elbow Using Fracture Mechanics Techniques

The extension of a hypothetical crack in a piping elbow under fatigue loadings at elevated temperature is estimated using the analysis techniques of linear-elastic fracture mechanics. The assumptions and approximations concerning the stress and loading conditions, expected in-service operational environment, and material properly behavior that must be made in order to complete the analysis are discussed in detail.

Fatigue-Crack Propagation Behavior of Incoloy 800 at Elevated Temperatures

Linear-elastic fracture mechanics techniques were used to characterize the fatigue-crack propagation behavior of Incoloy 800 in an air environment over the temperature range 75 to 1200 deg F (24 to 649 deg F). Crack growth rates were measured over the range 5×10−7 to 5×10−5 in./cycle. Material Grades 1 and 2 were found to exhibit essentially the same behavior over this range. In general, crack growth rates increased with increasing test temperature, although the increases were less then previously noted for austenitic stainless steels. This difference is probably related to the superior oxidation resistance of Incoloy 800.