Showing papers on "Paris' law published in 1971"

Extensive Study of Low Fatigue Crack Growth Rates in A533 and A508 Steels

Abstract: For two heats of ASTM A533 Grade B Class 1 pressure vessel steel and an ASTM A508 Class 2 forged steel, an extensive study of fatigue crack growth rate characteristics was conducted within the framework of linear elastic fracture mechanics. Experimental data obtained covered a wide variation in cyclic growth rates ranging from those associated with fatigue threshold (order of 0.000000001 in./cycle) up to rapid fracture. The bulk of experimental information obtained was at very low cracking rates, usually 0.000001 in./cycle and below. Considered in the investigation were: (1) an evaluation of the effect of stress ratio on very slow rates of fatigue crack extension; (2) an evaluation of temperature (75 to 650 deg F) on very slow rates of fatigue cracking; (3) an examination of the combined effects of temperature and stress ratio and their influence on fatigue crack extension; (4) an evaluation of the effect of a distilled water environment on fatigue crack propagation rates; (5) an examination of the effects of specimen thickness; and (6) a comparison of results for two separate heats of the same alloy. /Author/

New method for the statistical evaluation of constant stress amplitude fatigue-test results

Abstract: REGRESSION MODELS, BASED ON LINEAR ELASTIC FRACTURE MECHANICS, ARE USED TO INTERPRET THE EFFECT OF STRESS RATIO ON FATIGUE CRACK GROWTH RATE IN 7075-T6 ALUMINUM ALLOY. IT IS SHOWN THAT FATIGUE CRACK GROWTH RATE CAN BE RELATED TO RANGE AND MAXIMUM VALUES OF STRESS-INTENSITY FACTOR. STATISTICAL TECHNIQUES ARE USED TO EXAMINE THE ERROR WHICH MAY ACCUMULATE WHEN REGRESSION EQUATIONS ARE USED TO PREDICT CRACK GROWTH VERSUS CYCLES CURVES. /ASTM/

Fatigue-crack growth in polymethylmethacrylate

Abstract: Linear-elastic fracture mechanics is used to interpret observations of through-thickness fatigue-crack growth in sheet specimens of polymethylmethacrylate. The paper shows how statistical techniques have been used to design and interpret experiments to determine the interactions and main effects of frequency, mean and range of stress-intensity factor on fatigue-crack-growth rate.

Mode II Fatigue Crack Propagation

Abstract: Fatigue crack propagation rates were obtained for 2024-T3 bare aluminum plates subjected to in-plane, mode I, extensional loads and transverse, mode II, bending loads. These results were compared to the results of Iida and Kobayashi for in-plane mode I-mode II extensional loads. The engineering significance of mode I-mode II fatigue crack growth is considered in view of the present results. A fatigue crack growth equation for handling mode I-mode II fatigue crack growth rates from existing mode I data is also discussed.

An exploratory study of delay in fatigue-crack growth

Abstract: Compression, creep, stress relaxation and overloading effects on delay in fatigue crack growth and structural life predictions

Fatigue Fracture in Polyethylene

Abstract: The application of non-classical fracture mechanics to the growth of dynamic fatigue cracks in a visco-elastic solid is discussed. The ideas developed are used to characterize the results of fatigue crack growth measurements on a range of low-density polyethylenes. The results can be expressed in the form [Note: See the image of page 57 for this formatted text]d$c$/d$N$ = B $\scr{J}^{n}$, where d$c$/d$N$ is the growth of the crack each cycle,[Note: See the image of page 57 for this formatted text]$\scr{J}$ is a fracture mechanics parameter and B, n are constants. Most of the materials studied reveal two distinct regions (with differing values of the constants) linked by a transition zone, and corresponding to brittle and ductile crack propagation respectively. The fatigue life of virgin specimens can be predicted from the crack growth characteristics assuming the existence of intrinsic flaws which (when the predictions are matched to actual fatigue data) are found to correspond in size to the polyethylene spherulites.

Fatigue crack growth in poly (vinyl chloride)

Abstract: Our measurements on fatigue cracks grown in PVC under plane-strain conditions shows that the width of the observed arrest lines is in agreement with the theoretical results obtained from the Dugdale-Muskhelishvili model. Further, we have shown that the fatigue of one arrest line before rupture requires a large number of loading cycles this number of cycles is constant for every arrest line of a given specimen. Brittle fracture of the specimen occurs when the “stress-intensity factor” K first introduced by Irwin reaches a given value.

The effect of frequency upon the fatigue-crack growth of type 304 stainless steel at 1000f

Abstract: The fatigue-crack growth rate of Type 304 Stainless Steel was studied at 1,000 deg F with cyclic frequencies ranging over approximately 4-1/2 orders of magnitude It was found that as frequency decreased, the fatigue-crack growth rate showed a significant increase An equation was given which characterized this behavior /Author/

Stress intensity factors for surface cracks in bending

Abstract: Fracture mechanics solutions applicable to surface cracked plates in bending are studied. The flawed plate is modeled as a two-dimensional beam containing a spring to find the stress intensity factor at the point of intersection of a semielliptical surface crack with the free boundary. Results are given for cracks of various shapes and sizes. These findings, along with previous analytical solutions obtained from the literature, are then compared with experimental data. Since it is difficult to measure surface cracks in metals, polymethylemethacrylate, a transparent polymer, was selected for the test material. First, the relation between fatigue crack growth rate and range in stress intensity is calibrated for polymethylmethacrylate. Measured fatigue crack growth rates for surface flaws in bending are then used to calculate experimental values for the stress intensity factor which are compared with theory. /Author/

Fatigue Threshold Crack Propagation in Air and Dry Argon for a Ti-6Al-4V Alloy

Abstract: Fatigue crack propagation experiments on a Ti-6A1-4V alloy were carried out within the framework of linear elastic fracture mechanics. Crack propagation results encompassing a wide range of fatigue crack growth rates and stress intensity range were plotted in the conventional manner for studies conducted in room temperature air and dry argon environments. Corresponding fatigue striations formed in both environments were measured and compared with macroscopic crack extension rates. Examination of very low fatigue crack propagation rates at low stress intensity range levels disclosed a stress intensity threshold level below which Mode I fatigue crack propagation became insignificant. In a limited investigation at very low fatigue crack growth rates, the fatigue threshold levels were observed to decrease with increasing stress ratio. The influence of stress ratio also was observed at stress intensity range levels above the threshold; cyclic crack growth rates increased for the increased values of stress ratio. However, for the ranges of stress intensity, stress ratio, and growth rate considered in this work, the stress ratio influence appeared most pronounced at the lowest growth rates and lessened with increasing stress intensity range. Comparable fatigue crack growth tests (stress ratio equals 0.33) conducted on the Ti-6A1-4V alloy in air and inert dry argon at room temperature yielded almost identical results for cylic crack growth rates encompassing those associated with the fatigue threshold upward past 0.00001 in./cycle. Within this range, the fracture surface morphology changed from a cleavage-like appearance at threshold levels to a typical straited and dimpled appearance at higher growth rates. No significant difference in fatigue crack growth rate behavior was reported for Ti-6A1-4V specimens tested in either the STA (solution treated and aged) or STA plus annealed heat treated condition. /Author/

Fatigue crack growth data for various materials deduced from the fatigue lives of precracked plates

Abstract: Data previously obtained for a wide range of metallic materials, using thin plates containing edge cracks between 0.3 and 7.5 mm (0.01 and 0.3 in.) long, have shown for both zero mean load and the general tensile stress cycle of mean stress plus or minus alternating stress where the mean stress is greater than the alternating stress that whether a crack grows or remains dormant can be predicted from the value of the range of stress intensity factor during the fatigue cycle. A critical value of the range of stress intensity factor during the fatigue cycle is necessary for crack growth; its value in general depends on the ratio mean stress/alternating stress. An equation is given which conveniently represents the fatigue crack growth data for many materials. If this equation is integrated the initial rate of crack growth can be calculated from the total fatigue life of a cracked specimen. Data for low crack growth rates, obtained in this way from the broken plate specimens, are in good agreement with data obtained from conventional crack growth tests. The method has the advantage that no crack growth monitoring instrumentation is necessary, so that data on crack growth in difficult environments can readily be obtained. /Author/

Fatigue crack propagation growth rates under a wide variation of delta k for an astm a517 grade f(t-1) steel

Abstract: Fatigue crack propagation experiments on an ASTM A517 F(T-1) steel were carried out within the framework of linear-elastic fracture mechanics. At low stress intensity levels an examination of very slow fatigue crack propagation rates of the order of 0.000000001 in./cycle revealed the existence of a stress intensity threshold level below which fatigue crack propagation rates become diminishingly small (order of 0.000000001 in./cycle). Stress ratio was found to alter the threshold stress intensity with higher stress ratios resulting in somewhat depressed threshold values. The influence of stress ratio was also felt at stress intensity levels above the threshold with increased growth rate being associated with increased values of stress ratio. Low stress intensity level fatigue crack growth rate tests were conducted at room temperature in air, distilled water, and dry hydrogen gas. These environments did not affect the fatigue threshold. Identical values of the threshold stress intensity were recorded in each environment studied. Furthermore, for the range of slow growth rates, order of 0.000000001 to 0.0000001 in./cycle, and frequencies, 120 to 180 Hz, considered, these environments seemed to produce little effect on fatigue crack growth rate results. For higher cyclic growth rates (greater than 0.0000001 in./cycle) and stress intensity levels, the effect of material condition and heat treatment was evaluated. Plate material rated as ultrasonically good and poor was evaluated in two heat treated conditions. Examination of fatigue crack growth rates indicated that ultrasonic ratings do not necessarily give good indications of crack growth resistance. /Author/

Experimental observation of a stress intensity history effect upon fatigue crack growth rate

Abstract: : Experimental results illustrate a previously unreported stress intensity history effect following a step increase in stress intensity. Data for 12 of 24 cases of step increases in stress intensity are presented which show an initially enhanced crack growth rate of two to four times the steady state growth rate. The effect diminishes until the normal rate is reached after 0.025 to 0.150 inch of crack extension and 200 to 600 cycles. An associated study of the causes of this behavior is being undertaken. (Author)

Crack initiation in PVC for subsequent linear elastic fracture mechanics analysis

Abstract: Reproducible starter-cracks for subsequent linear elastic fracture mechanics analysis have been grown in PVC by fatigue cycling at 80 Hz. The crack growth rate has been related to the fracture surface markings and to the opening mode stress intensity factor (KI) of the fatigue cycle. Termination of the fatigue crack growth when crack growth rate is constant ensures a smooth mirror fracture surface and a sharp crack tip.

Crack propagation in aluminum alloys under high-amplitude cyclic load

Abstract: : Crack propagation studies were conducted on a variety of structural aluminum alloys under high stress-intensity amplitude cyclic load (delta K > 10 ksi square root of in.). The alloys under investigation included 2219-T87, 5456-H321, 6061-T651, 7005-T63, 7039-T6X31, and 7106-T63. The yield strengths of these alloys ranged from 34 to 55 ksi. Tests were conducted in ambient room air and in 3.5-percent NaCl saltwater environments. Data are presented on log-log coordinates in terms of fatigue crack growth rate (da/dN) as a function of the stress-intensity factor range (delta K).

The influence of temperature on fatigue-crack growth in a mill-annealed Ti-6Al-4V alloy

Abstract: To understand the influence of temperature on the rate of fatigue crack growth in high strength metal alloys, constant load amplitude fatigue crack growth experiments were carried out using a 1/4 inch thick (6.35 mm) mill-annealed Ti-6Al-4V alloy plate as a model material. The rates of fatigue crack growth were determined as a function of temperature, ranging from room temperature to about 290 C and as a function of the crack tip, stress intensity factor K, in dehumidified high purity argon environment. The dependence of the rate of fatigue crack growth on K appears to be separable into two regions. The transition correlates with changes in both the microscopic and macroscopic appearances of the fracture surfaces, and suggests a change in the mechanism and the influence of microstructure on fatigue crack growth.

Environmental cracking in high-strength materials

Abstract: The major evidence bearing on environment and crack growth in high-strength structural materials is reviewed and discussed. The effect of environment on subcritical crack growth and fracture is considered for stresses, both cyclic and static, less than the net section yield strength. The applicability of fracture mechanics concepts to environmental cracking is considered, and it is shown that the stress-intensity factor may be viewed as the driving force for environmental flaw growth under both cyclic and static load. In terms of threshold values for crack growth, this allows a correlation of test results from different specimen geometries and also a correlation between test results and service failures. The significance of time-to-failure tests is considered briefly. Environmental cracking under static load is considered. Steel and titanium alloys in various environments of water, water vapor, hydrogen, and oxygen are discussed. For steels, crack growth in water and saturated water vapor is a thermally activated process with an activation energy that agrees well with the measured value for diffusion of hydrogen in a high-strength steel. The role of oxygen in inhibiting environmental cracking in vapor environments is discussed. The combined effects of cyclic stress and aggressive environment are considered. Many of the behavior patterns in static loading are observed to carry over to cyclic loading. One exception is that the protective role of oxygen in vapor environments is no longer observed. A lack of data for stress-intensity ranges less than the threshold is noted. Engineering applications are emphasized. Relationships among flaw size, threshold stress intensities in different environments, and proof and operating stresses are presented graphically and their interpretation is discussed. For 4340 steel, the role of yield strength level is summarized in a diagram that indicates that environmental cracking is the major problem at yield strengths in excess of 180 ksi, whereas fatigue crack growth is the major problem at lower strength levels.

The Effect of Microstructure on Fatigue Crack Propagation in Ti-6Al-6V-2Sn Alloy.

Abstract: : The fatigue crack growth behavior of Ti-6Al-6V-2Sn was investigated for five different microstructures, using contoured double-cantilever beam specimens The fatigue crack growth behavior was compared with the mechanical strength, toughness, and ductility properties for each microstructure The fatigue fracture suffaces were examined with a scanning electron microscope Annealed structures consisting of equiaxed primary alpha have the lowest crack growth resistance, while the acicular alpha microstructure resulting from beta annealing has a growth resistance four times greater in the intermediate growth regime No simple model of crack growth rate using gross mechanical properties describes the relative magnitude nor the ralative order of crack resistance among the different microstructures (Author)

Regression Models for the Effect of Stress Ratio on Fatigue Crack Growth Rate

Abstract: USING THE EXPERIMENTAL AND THEORETICAL RESULTS OF PREVIOUS RESEARCH WORK, THE AUTHOR PRESENTS A PROBABILISTIC DESCRIPTION OF CONSTANT STRESS AMPLITUDE FATIGUE-TEST RESULTS. THIS DESCRIPTION INCLUDES THE S-N CURVES (OR EQUIPROBABILITY OF FRACTURE CURVES), P-S CURVES (OR STRESS-RESPONSE CURVES), P-N CURVES (CUMULATIVE DISTRIBUTION FUNCTIONS OF FATIGUE ENDURANCES), AND ACCOUNTS FOR THE OCCURRENCE OF RUNOUTS. A METHOD OF ESTIMATION OF THE MODEL COEFFICIENTS WHICH USES THE INFORMATION PROVIDED BY ALL THE SPECIMENS TESTED (BROKEN OR UNBROKEN) IS ALSO PRESENTED. THE APPLICATION OF THIS METHOD IS DEMONSTRATED THROUGH FIVE EXAMPLES FOR EACH OF WHICH SEVERAL HUNDRED TEST RESULTS ARE AVAILABLE. /ASTM/

Ultrasonic detection of high-temperature fatigue-crack growth

Abstract: An ultrasonic technique is described for continuously monitoring fatigue-crack growth at temperatures up to 300°C in wedge-opening-load type fracture-toughness specimens The sound-wave energies reflected from the leading edge of the fatigue crack and from the bottom surface of the specimen are electronically compared and used to control the crack-measurement system

The Influence of Curvature on Stress Intensity at the Tip of a Circumferential Crack in a Cylindrical Shell

Abstract: Curvature effect on cylindrical shell circumferential crack tip stress intensity, using fatigue crack growth tests

Estimation of the Fatigue Life of a Thick-Walled Cylinder Subjected to Repeated Internal Pressure

Abstract: Experimental data available on fatigue crack growth may be described by an equation of the form da/dN = B(ΔK)m, where da/dN is the rate of crack growth per stress cycle, ΔK is the range of stress intensity factor, and B and m are material properties.This relationship has been used to estimate the fatigue crack propagation lives of thick-walled cylinders subjected to repeated internal pressure. By using an approximate value for the stress intensity factor, and considering the high local stresses at the crack due to pressurized oil, close correlation is obtained between the estimated and experimental fatigue lives.Design curves are obtained, by which the fatigue life of such a cylinder may be estimated from a knowledge of the transverse uniaxial fatigue limit and fracture toughness properties of the cylinder material, as well as the diameter ratio of the cylinder and the repeated pressure.