Richard Roberts

The Effect of Biaxial Stresses on Fatigue and Fracture

The results are presented of an analytical and experimental program which was conducted to determine if a biaxial stress field produces a significant effect on the fatigue and fracture behavior of thin plates. The materials tested were 6061-T4 and 6061-T6 aluminum sheets and plexiglas sheets. The experimental program included fracture tests with various magnitudes of biaxial load at fracture and fatigue tests with sinusoidal loading normal to the crack and either constant or sinusoidal stresses applied parallel to the crack. The effect of nonsingular stresses on the behavior of a crack is examined from both a linear elastic and an elastic-plastic viewpoint. The experimental study indicates that a biaxial stress field does affect the behavior of a crack in a thin sheet. An increase in the apparent fracture toughness with increasing biaxial load was observed experimentally but as yet cannot be adequately explained using linear fracture mechanics theory. Biaxial stresses were found to produce a shift in the fatigue crack growth rate data, and it is shown that this shift can be predicted using several empirical fatigue crack propagation models.

Effect of Multiple Overloads on Fatigue Crack Propagation in 2024-T3 Aluminum Alloy

The effect of multiple overloads on subsequent fatigue crack propagation in 2024-T3 aluminum alloy specimens was studied, and 1, 2, 10, 100, 1000, and 5000 overload cycle tests were run. Typical tests consisted of runningconstant AK tests where AK = 15 ksi √in. (16.5 MN/m 3 / 2 ) and λ = 1.05 (λ = K m a x /ΔK). The maximum overload level was 50 percent ΔK or K m a x = 22.5 ksi √in. (24.7 MN/m 3 / 2 ). The usefulness of closure concepts in aiding the understanding of fatigue crack propagation due to overloads is considered. Evidence is given to demonstrate the general applicability of closure concepts for analysis of fatigue crack propagation results.

Plastic enclave sizes for internal cracks emanating from circular cavities within elastic plates

Abstract The size of the plastic enclave at the tip of a crack within an externally loaded elastic medium is important for the complete understanding of the stress distribution in the cracked medium. One of the first useful approximations of zone size was given by Irwin[1]. Later, Dugdale[2] significantly aided in the estimation of plastic enclave lengths by introducing his elastic-plastic strip model of the enclave region. Experimental results from tests made by him and others [3] with mild steel substantiate the models use to reduce the analytical complexity of the non-linear plasticity effect in the enclave problem for materials with similar mechanical properties.

Some aspects of fatigue crack propagation

The models of fatigue crack propagation proposed by Forman et al. and Roberts and Erdogan were studied in this paper. By applying these models to existing data in the literature for thin 2024-T3 and 7075-T6 aluminum plates subjected to fluctuating tensile loads, it was found that both models gave comparable results when one considered just a gross correlation of the experimental data. By modifying Formans model to incorporate the ideas of Roberts and Erdogan, a model was produced which appeared to be a more rational basis for studying the problem of fatigue crack propagation in thin plates and shells subjected to tensile loads, bending loads, or a combination of both. This fact was demonstrated for the case of thin plates subjected to fluctuating bending loads and for the case of thin cylindrical shells subjected to fluctuating internal pressure. This paper also presents the large quantity of data relating the rate of fatigue crack propagation in thin plates subjected to fluctuating bending loads collected at Lehigh University.

An alternate measure of fracture toughness

Linear elastic fracture mechanics has been primarily used for very high strength materials. For such applications it is adequate to measure KIc, the plane-strain fracture toughness. It is the objective of this paper to discuss a method for making dynamic fracture toughness measurements, KId, which are appropriate for structural grade steels. A simplified scheme for measuring KId is also proposed.

Fatigue crack propagation for defects near a free surface

Abstract The fatigue crack growth associated with an internal flaw as it approaches a free surface was studied. Previous researchers recommended when the plastic zone at the crack tip nearest the free surface reaches one-tenth of the ligament size which exists between the free surface and the crack tip that the flaw be treated as an edge crack. The results of this study show that the assumption of an edge crack can prove to be exceptionally conservative when employed for design purposes. Underestimates of fatigue life of as much as 1000% were observed. Improved methods for predicting the fatigue growth of near surface defects are offered in the paper.

Fracture Behavior of Bridge Steels

The results of a recent study of the dynamic and static fracture toughness behavior, over a wide-temperature range, of eight different bridge steels are presented. These results are discussed in connection with current toughness requirements for bridge steels. Strain rate and temperature effects on fracture toughness are examined.

The 2003 Benjamin Franklin Medal in civil engineering presented to Charles H. Thornton

The world is a more interesting and safer place to live because of the efforts of Dr. Charles H. Thornton. Throughout his career Dr. Thornton has been able to translate simple structural elements into unique solutions for the construction of long-span structures and high-rise buildings. His leadership role in the analysis of structural failures and his translation of knowledge obtained from the analysis of these events has clearly provided the public with safer structures. Dr. Thornton has also been a leader in the motivation of high school students to pursue careers in engineering, architecture and construction.

Analysis of stress intensity factors for an edge cracked T- section in tension

Using a finite element model to generate compliance values, stress intensity solutions were developed for an edge cracked T-section in tension. The finite element model was developed specifically for T-section geometries with thin web sections. Several T-section geometries were analyzed. A limited set of experiments were performed to validate some of the finite element modeling using compliance specimens. The experimental results were in good agreement with the predictions made using the finite element model. Dimensional similarity was investigated and a set of dimensionless modeling parameters were identified.