Fatigue crack retardation and closure in polymethylmethacrylate

The presented results are from an investigation of crack surface profiles and the influence of intermittent overloads on fatigue crack growth in polymethylmethacrylate, a transparent polymer. Fatigue cracks were grown in compact tension specimens under conditions of constant range in stress intensity factor. Tensile overloads were found to perturb subsequent crack growth in polymethylmethacrylate under certain conditions.

Examination of monochromatic light interference fringe patterns emanating from the fatigue cracks indicates that the crack perimeter was closed at zero load. The crack surfaces were displaced, however, in the interior of the specimen at zero load. Measurements of the crack opening displacements during loading revealed that a significant tensile load was required to displace the portions of the crack surfaces which Were initially closed. These observations are discussed in light of the Elber concept of crack closure.     

Fatigue crack propagation in steels

From previous investigations of the mechanisms of both fracture and fatigue crack propagation, the static fracture model proposed by Lal and Weiss may be thought as reasonable for describing fatigue crack propagation in metals at both low and intermediate stress intensity factor ranges $\text{ΔK}$. Recent progress in fatigue crack propagation indicates that it is not only possible, but also necessary, to modify this static fracture model. Based on the modified static fracture model, the effective stress intensity factor range $\text{ΔK}{}_{\mathrm{eff}}$, which is defined as the difference between $\text{ΔK}$ and the fatigue crack propagation threshold value $\text{Δ}{}_{\mathrm{th}}$, is taken as the governing parameter for fatigue crack propagation. Utilising the estimates of the theoretical strengths of metals employed in industry, a new expression for fatigue crack propagation, which may be predicted from the tensile properties of the metals, has been derived. The correlation between the fatigue crack propagation rate and the tensile properties is thus revealed. The new expression fits the test results of fatigue crack propagation of steels below 10^?3 mm/cycle and indicates well the effect of stress ratio on the fatigue crack propagation rate.     

Fatigue crack propagation in polyacetal

The fatigue crack propagation characteristics of a typical commercial homopolymer and copolymer polyacetal were determined. These materials were found to be the most fatigue resistant plastics examined to date, thus confirming the generally high fatigue resistance of all crystalline polymers. A discontinuous fatigue cracking process was identified at all test frequencies in the acetal copolymer and at high frequencies in the homopolymer, while continuous crack propagation occurred at low test frequencies in the homopolymer. The discrete advance increments of the crack in the discontinuous mode were equal to the dimension of the prevailing crack-tip plastic zone. On a more local scale, the crack path is seen to be mainly trans-spherulitic in nature.     

Fatigue crack propagation in cellular metals

Two types of cellular metals were investigated: a closed-cell aluminium foam with a cell size of about 3.5 mm and densities ranging from 0.25 to 0.40 g/cm³ and hollow sphere structures made of a stainless steel (316L) with sphere sizes of 2 and 4 mm and a density of about 0.3 g/cm³. Fatigue and fatigue crack propagation tests were performed on these materials using an electro-dynamic resonance fatigue testing machine. The crack extension was monitored by a potential drop technique. Additionally, investigations were carried out inside the scanning electron microscope (SEM) using an in situ loading device. All tests were accompanied by local deformation measurements and fracture surface analyses. From the fatigue crack propagation tests it is evident that these materials show a relatively high Paris-Exponent m in the range of 6 to 25 compared to common ductile solid metals. Additional tests were performed to estimate the influence of crack closure, crack bridging and micro cracking on the da/dN versus ΔK curve for these materials. The in situ fatigue tests and the fracture surface analyses revealed a difference in the fatigue crack propagation mechanisms between the closed-cell foam and the hollow sphere structure: in the closed-cell foam a contiguous fatigue crack can be found, where in the case of the hollow sphere structure the fatigue crack propagation is concentrated in the vicinities of the sintering necks.     

Fatigue crack propagation in microcapsule-toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔK _T, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.     

Fatigue crack propagation in polymeric materials

In order to gain a better understanding of matrix-controlled fatigue failure processes in non-metallic materials a series of fatigue tests were performed on several different polymer materials representing different classes of mechanical response. Fatigue crack propagation rates between 5×10^–6 in. cycle^–1 (127 nm cycle^–1) and 4×10^–4 in. cycle^–1 (10 300 nm cycle^–1) were measured in nylon, polycarbonate, ABS resin, low-density polyethylene and polymethyl methacrylate. A strong correlation was found between the fatigue crack propagation rate and the stress intensity factor range prevailing at the advancing crack tip. Whereas metals exhibit comparable fatigue growth rates for a given stress intensity range when normalised with respect to their static elastic modulus, the polymer materials exhibited a 1300-fold difference in crack growth rate for a given normalised stress intensity range. This observation dramatically illustrates the importance of understanding molecular motion and energy dissipation processes in polymer materials as related to their chemistry and architecture. The relative behaviour of the different polymer materials could be generally correlated with their reported damping characteristics.     

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.     

Fatigue crack propagation in austenitic stainless steels

Fatigue crack growth rate was found to be independent of grain size in 309S stainless steel, for grain sizes of 45 and 480 μm. The data were compared to literature results for a variety of stable and unstable austenitic alloys, and were shown to agree well.     

Fatigue crack propagation in an epoxy polymer

This experimental study investigates the fatigue crack growth rate behavior in an epoxy resin polymer using the tapered-double-cantilever-beam (TDCB) specimen in ambient and elevated temperature environments. An empirical crack growth rate model based on the change in crack extension force, Δ?, is proposed. Its relation to current $\text{ΔK}$ models is discussed and observations on the fatigue surface morphology presented.     

Fatigue crack propagation under variable-amplitude loading

A finite-element analysis has been performed to investigate quantitatively the level at which fatigue crack surfaces come into contact during fatigue cycles. The results agreed well with experimental observations and suggest that quantitative design rules may be developed, based upon the crack-closure hypothesis.A summary of remarks made during a seminar held at the Institute for Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev, October 21, 1976.NASA-Langley Research Center, Hampton, Virginia 23665. Published in Problemy Prochnosti, No. 10, pp. 26–29, October, 1977.     

The character of crack propagation during fatigue failure of polymethylmethacrylate

Photographs of the surface of specimens fatigued to fracture in bending at various stress amplitudes were taken. A similarity in the character of the formation and propagation of cracks in material deformed in bending and in tension was observed. It was shown that the crack size increases with rising temperature and with increasing loading time.