R. W. Hertzberg

DELAY EFFECTS IN FATIGUE CRACK PROPAGATION

The fatigue crack propagation behavior resulting from variations in load is examined for 2024-T3 aluminum alloy, from both a macroscopic and a fractographic point of view. A peak load is found to cause retardation of the crack growth rate, which becomes more pronounced as the percentage overload or baseline stress intensity level or both is increased. The delaying effect of the overload is observed to exist for a calculated crack length increment equivalent to the plastic zone size formed during the peak load. Multiple overloads and high-low block loading sequences are found to result in additional retardation. It is observed that the macroscopic fracture surface appearance (that is, transition to plane stress) is a function of the crack growth rate. From fractographic examination it is found that the initiation of microvoid coalescence during fatigue occurs when plane stress conditions are achieved; this limits the extent of the stretch zone associated with an overload cycle. As a result, the stretch zone is found to be followed by striations in plane strain and by dimples under plane stress conditions. The size of the stretch band is observed to depend on the stress intensity level during the overload cycle. The usefulness of closure concepts in aiding the understanding of fatigue crack propagation under uniform and nonuniform loading conditions is considered. Evidence is given to demonstrate the general applicability of closure concepts for analysis of macroscopic and fractographic fatigue crack propagation results. /Author/

On the generality of discontinuous fatigue crack growth in glassy polymers

Fatigue fracture surface characteristics of five commercially available amorphous polymers [poly(methylmethacrylate) (PMMA), polycarbonate (PC), poly(vinyl chloride) (PVC), polystyrene (PS), and polysulphone (PSF)] as well as bulk-polymerized PMMA prepared over a wide range of molecular weights were studied to determine if common mechanisms of fatigue crack propagation prevail among these glassy polymers. In those polymers with viscosity-average molecular weight ¯Mv≲2×105, the macroscopic appearance of the fracture surface showed the presence of a highly reflective mirror-like region which formed at low values of stress intensity and high cyclic test frequencies (∼100 Hz). The microscopic appearance of this region revealed that many parallel bands exist oriented perpendicular to the direction of crack growth and that the bands increase in size with ΔK. In all instances, the crack front advanced discontinuously in increments equal to the band width after remaining stationary for hundreds of fatigue cycles. Electron fractographic studies verified the discontinuous nature of crack extension through a craze which developed continuously with the load fluctuations. By equating the band size to the Dugdale plastic zone dimension ahead of the crack, a relatively constant yield strength was inferred which agreed well with reported craze stress values for each material. At higher stress intensity levels in all polymers and all values of ¯Mv, another series of parallel bands were observed. These were also oriented perpendicular to the direction of crack growth and likewise increased in size with the range in stress intensity factor, ΔK. Each band corresponded to the incremental advance of the crack during one load cycle, indicating these markings to be classical fatigue striations.

Fatigue of rubber-modified epoxies: effect of particle size and volume fraction

A change in crack-tip plastic zone/rubber particle interactions induces a transition in the fatigue crack propagation (FCP) behaviour of rubber-modified epoxy polymers. The transition occurs at a specific K level, KT, which corresponds to the condition where the size of the plastic zone is of the order of the size of the rubber particles. At ΔK>ΔKT, rubber-modified epoxies exhibit improved FCP resistance compared to the unmodified epoxy. This is because the size of the plastic zone becomes large compared to the size of the rubber particles and, consequently, rubber cavitation/shear banding and plastic void growth mechanisms become active. At ΔK>ΔKT, both neat and rubber-modified epoxies exhibit similar FCP resistance because the plastic zone size is smaller than the size of the rubber particles and hence, the rubber cavitation/shear banding and plastic void growth mechanisms are not operating. As a result of these interactions, the use of smaller 0.2 μm rubber particles in place of 1.5 μm rubber particles results in about one order of magnitude improvement in FCP resistance of the rubber-modified system, particularly near the threshold regime. Such mechanistic understanding of FCP behaviour was employed to model the FCP behaviour of rubber-modified epoxies. It is shown that the near threshold FCP behaviour is affected by the rubber particle size and blend morphology but not by the volume fraction of the modifiers. On the other hand, the slope of the Paris-Erdogan power law depends on the volume fraction of the modifiers and not on the particle size or blend morphology.

The effect of sheet thickness on fatigue crack retardation in 2024-T3 aluminum alloy

Abstract Variable-amplitude fatigue studies of 2024-T3 aluminum alloy were performed to examine the effect of sheet thickness on fatigue crack growth rate retardation. Results indicated that the amount of retardation increased with decreasing specimen thickness. This phenomenon was attributed to enhanced plastic strains under plane stress conditions (i.e. in a thin sheet) which formed ahead of the advancing crack tip as a result of a high load excursion. These strains are believed to produce both crack closure and a favorable compressive residual stress field around the crack tip. Evidence of increased crack surface interference under plane stress situations was verified with electron fractographic observations.

Mechanisms of fatigue fracture in short glass fibre-reinforced polymers

Fatigue-crack profiles and fracture surfaces of several short glass fibre-reinforced polymers were examined to gain insight into the mechanisms of cyclic damage and fatigue-crack propagation in these materials. Several distinctly different features were noted between fracture surfaces generated by stable fatigue crack growth and those produced by monotonic or unstable fracture. Among the most significant differences were the higher degree of single and multiple fibre fracture generally observed on stable fatigue-crack growth fracture surfaces, and the variations in the interfacial failure site in well-bonded systems. While the former effect is attributed to the occurrence of crack closure and the build-up of compressive stresses in the crack-tip damage zone during unloading, the differences in the interfacial failure mode are related to the adverse effect of fatigue loading on the interfacial bond strength. No features could be identified that would allow a quantitative correlation between the applied stress intensity factor level or the crack growth rates and characteristic fracture surface details.

Fatigue fracture processes in polystyrene

Fatigue crack growth characteristics in polystyrene were studied as a function of stress intensity factor range and cyclic frequency. Precracked single edge notched and compact-tension type specimens made from commercially available polystyrene sheet (mol.wt. =2.7×105) were cycled under constant load at frequencies of 0.1, 1, 10 and 100 Hz, producing growth rates ranging from 4×10−7 to 4×10−3 cm/cycle. For a given stress intensity level, fatigue crack growth rates were found to decrease with increasing frequency, the effect being strongest at high stress intensity values. The variable frequency sensitivity of this polymer over the test range studied was explained in terms of a variable creep component. The macroscopic appearance of the fracture surface showed two distinct regions. At low stress intensity values, a highly reflective, mirror-like surface was observed which transformed to a rougher, cloudy surface structure with increasing stress intensity level. Raising the test frequency shifted the transition between these areas to higher values of stress intensity. The microscopic appearance of the mirror region revealed evidence of crack propagation through a single craze while the appearance of the rough region indicated crack growth through many crazes, all nominally normal to the applied stress axis. Electron fractographic examination of the mirror region revealed many parallel bands perpendicular to the direction of crack growth, each formed by a discontinuous crack growth process as a result of many fatigue cycles. The size of these bands was found to be consistent with the dimension of the crack tip plastic zone as computed by the Dugdale model. At high stress intensity levels a new set of parallel markings was found in the cloudy region which corresponded to the incremental crack extension for an individual loading cycle.

The role of mechanical properties in low-stress fatigue crack propagation

An experimental investigation was undertaken to study the relationship between mechanical properties and low stress fatigue crack propagation. Attention was focused on the “fatigue” or “reversed plastic zone” at the crack tip, since it was felt that material properties in this region were of prime importance in the crack propagation process. An effort was made to simulate this region through fully reversed strain-cycling tests on tensile specimens. Mechanical properties obtained from a number of materials before and after strain cycling were correlated with crack propagation data from the same materials. Evidence indicated that while monotonic tensile properties are inadequate for correlation purposes, the cyclic strain-hardening coefficient, the cyclic yield strength, and the elastic modulus appear to be important parameters. This was felt to be an indication of the importance of strain cycling in the reversed plastic zone in influencing the rate-governing mechanisms in fatigue crack growth.

Micromechanisms of fatigue-crack advance in PVC

The fatigue-crack propagation characteristics in poly(vinyl chloride) (PVC) are examined in terms of fracture mechanics concepts where the crack growth rate is related to the applied stress intensity factor range. The microscopic details of fatigue crack extension are examined with the aid of light optical, scanning and transmission electron microscopes. The mechanism of crack advance is found to be that of void coalescence through craze material generated in advance of the crack tip. While the craze is shown to grow continuously with cyclic loading, the crack is found to grow discontinuously in several hundred cycle increments.

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.

Spontaneous intravenous catheter fracture and embolization from an implanted venous access port and analysis by scanning electron microscopy

A spontaneous fracture occurred in the midportion of a catheter that was indwelling in the left subcla‐vian vein and embolized to the pulmonary artery. Scanning electron microscopy studies were performed upon the fractured catheter. Possible reasons for the fracture are discussed. Sudden chest pain in a patient with an implanted catheter may be the first clinical manifestation of an embolized catheter.