Fatigue crack propagation in poly(methyl methacrylate): Effect of molecular weight and internal plasticization

Abstract

In spite of the importance of fatigue behavior in engineering plastics, relatively few fundamental studies have been made of the effects of polymer structure, molecular weight, composition, and morphology on fatigue crack propagation (FCP). As, part of a broad program for the study of such effects, the role of molecular weight and internal plasticization has been studied in poly(methyl methacrylate) (PMMA) which had been specially prepared and characterized with respect to molecular weight, dynamic mechanical behavior, and, in some cases, stress‐strain response. As expected, values of fracture toughness, Kc, varied considerably as the molecular weight was rai ed, from 0.7 MPa, √m at Mv = 1.0 × 105 to 1.1 at Mv, = 4.8 × 106. However, a specific effect of fatigue was noted: over the same range of Kc, values of FCP rate decreased by two orders of magnitude as molecular weight was; increased. It is proposed that this high sensitivity is due to differences in the degree of chain disentanglement effected by the cyclic loading, with consequent differences in the strength of the craze preceding the crack. With PMMA plasticized internally with a low level (10 percent) of n‐butyl acrylate (nBA), the FCP rate and Kc, were similar to those of controls, with very high rates shown. At higher nBA levels (up to 30 percent), the sensitivity of FCP rate to stress intensity factor range decreased considerably, Kc, increased by 30 percent and the pre‐exponential constant in the growth rate law increased. Plasticization weakens the polymer but at high degrees leads to enough hysteretic heating to induce local creep and crack blunting.

Influence of molecular weight on the dynamic mechanical properties of poly(methyl methacrylate)

“…44, and 66 -t-5 ~ for f= 1, 10, and 100 Hz, respectively. These and other similar results [9,[11][12][13]15] indicate that the current data and those of Manson et al [14] were consistent within the context of the frequency shift factor.…”

“…Although numerous values have been documented for T~ ranging from 78-125 ~ (e.g., refs. 9, 13, 15, 16 and 32), the lowest detailed T~ determinations were observed both for the present DMA (e, heating rate = 10 ~ f~ 3 Hz, and the syr~diotactic fraction = 0.80), and for the Rheovibron measurements (L:l, f = I10 Hz) of Manson et al [14]. While these latter investigators stated that materials with an M, < 3.5 x 104 were too brittle to evaluate, nevertheless, they were able to obtain valuable data in the high molecular weight region.…”

“…Accordingly, the calculated modulus was not truly the pure Comparison of T, and Tp (cf. Table 2) with several earlier investigations are shown in Figure 4 [4,14,21,32]. In ~ereral the data separated into three broad bands that were discriminated primarily by the measurement technique, and, to a lesser extent, by the degree of stereoregularity.…”

Fatigue of polymers

“…reaches a value somewhere between 100,000 and 200,000. While this may be true for some properties, such as Tg or possibly modulus, it does not hold for ultimate properties such as fatigue endurance [14,15,[74][75][76]. Also, various literature references indicate that increasing molecular weight also increases craze strength, creep resistance, and endurance under long-time steady loading [3,77].…”

Fatigue crack propagation in amorphous poly(ethylene terephthalate)

“…Fatigue tests were performed in air on notched compact-tension specimens (3.2 mm x 7.5 cm x 7.5 cm) at a sinusoidal frequency of 10 Hz, and a ratio of minimum to maximum load of 0.1, using an electrohydraulic closedloop test machine and standard procedures (4). Values of the stress intensity factor range, AK, were calculated fromthe equation AK=YAa, where Y is a geometrical factor, Aa the stress load and a the crack length (15).…”