Cracks In Polymethyl Methacrylate Research Articles

Re-directing slow cracks in PMMA

Crack re-direction is a little explored area in the management of structural integrity. We have examined this subject by performing experiments with PMMA (polymethylmethacrylate), working mainly with quasi-static (slow) cracks, whose mean crack speed varies between 0.1 and 0.4 mm s−1. We observed that in this regime, secondary thermal and acoustic sources of relatively low power (of the order of 2 and 10 W, respectively) could be used to achieve significant crack re-direction. We argue that this effect is due to an interplay between macro- and micro-scale phenomena. We show that micro-photographs of quasi-static cracks present a fishbone structure and that the side lobes may be micro-cracks inside the shear bands. The respective fracture surfaces appear rippled (hackled). We also show that, in our experiments, the angle of re-direction decreases with the mean crack speed, and above a mean crack speed of 0.4 mm s−1—when the fracture surface becomes mirror smooth—no crack re-direction is observed. It is conceivable that the micro-cracks act as notches and alleviate crack re-direction. Therefore, one of our conclusions is that it might be possible to reproduce the effect in an entirely different speed regime, that is use relatively weak secondary sources to re-direct fast cracks that propagate at super-critical speed. This hypothesis is advanced because fast cracks are known to possess a fishbone structure and a hackled fracture surface.

An experimental study of the interaction of internal cracks in PMMA

Internal cracks in polymethyl methacrylate (PMMA) specimens were generated by pulsed laser light. The interactions of coplanar cracks, parallel cracks, and cracks in T and H configurations were investigated. The tensile strength of specimens with a single internal crack decreased with increasing crack size, and the strength correlated well with the initial crack and unstable crack size in the Griffith relationship. For specimens with coplanar and parallel cracks, the strength increased and decreased with the crack distance, respectively. For the T and H crack configurations, the presence of delamination cracks decreased the strength, and the reduction in strength became more significant when the crack distance was small. All fracture surfaces showed similar fracture morphology in the sequence of laser-generated crack, smooth fracture mirror, mist with hyperbolic markings, and rough hackle region with rib markings. Examination of the fracture surfaces revealed crack arrest by the delamination cracks in both T and H configurations, and crack bowing between delamination cracks in the H configuration. The propagating crack was eventually able to circumvent the delamination cracks. The experimental results are compared with the available theoretical analyses, and the relevance of the present study to the toughening of brittle materials is discussed.

Influence du frottement sur le branchement de fissures à partir de défauts obliques soumis à une compression uniaxiale

In order to understand the influence of friction on crack extension in brittle materials, we studied a set of wing cracks in PMMA (polymethylmethacrylate) plates, formed under constant uniaxial loading at the tip of preexisting oblique closed slots of various angles. It was found that: 1.- as for an open slot (no friction), the wing cracks were initiated at a certain distance from the initial crack tip; 2.- for a given friction coefficient, this distance and the branching crack angle (which is less than 90°) increased with the slot-loading axis angle.

The number of radial cracks in polymethylmethacrylate in explosion of a borehole charge

The data available on the number of main radial cracks formed in a solid coarsely fracturing medium(PMMA) upon weak and strong explosion of borehole charges are generalized. A dependence of the number of radial cracks on the loading rate of the borehole walls is obtained.

On the interaction of internal cracks in polymethyl methacrylate

On the interaction of internal cracks in polymethyl methacrylate

Controlled generation of internal cracks in polymethyl methacrylate

A simple technique is presented for generating internal cracks in polymethyl methacrylate (PMMA) using laser irradiation. With precise control of the laser beam, a well-defined single crack can be produced in the matrix. The shape of the crack is flat and regular, close to an ellipse with nearly equal major and minor axes. The size and orientation of the crack can be controlled through the laser energy and the adjustment of the optical system. The laser-induced crack in PMMA consists of three regions: the central damage region caused by direct energy absorption, the intermediate cracking region, and the outer circumferential region associated with crazing. These controlled internal cracks can be used to study crack propagation, interaction, and related problems.

Investigation of the kinetics of cracks in polymethyl methacrylate by dynamic photoelasticity

Methods have been developed for experimental investigation of the kinetics of fast cracks in plates of optically active model material making it possible to load by pulsed pressure varying the amplitude and length of the pulse and also to use the photoelastic method for recording dynamic processes with use of a superfast motion picture camera. The relationship of the dynamic stress intensity factor to crack growth rate and plate thickness was obtained taking into consideration the influence of longitudinal and transverse elastic waves. A clear relationship between stress intensity factor and vcr was established. It was shown that with a fast crack growth rate (vcr < 270 m/sec) the uniqueness between stress intensity factor and vcr is disturbed by the action of reflected waves. The character of action on the stress intensity factor and crack growth rate of longitudinal and transverse waves reflected from the boundaries of the plate and also the influence of inertial effects caused by stress waves, high crack growth rates, and a change in the stressed and strained state with an increase in plate thickness were determined. It was determined that in polymethyl methacrylate in incidence on a crack at an angle of ∼90° a longitudinal wave increases the stress intensity factor and vcr (with vcr<625 m/sec) while a transverse wave decreases these characteristics.

Holographic recording of caustic light beams of fast propagating cracks.

A pulsed holographic technique is described which records and reconstructs caustic light beams of fast propagating cracks in polymethylmethacrylate (PMMA). A caustic light beam of a fast propagating crack is recorded as a hologram at an instant during its propagation with a pulsed holographic recording system. Illuminated with reconstruction light, the hologram reconstructs the caustic light beam, and diameters of caustics at various distances from the specimen are measured from the reconstructed caustic light beam. Values of dynamic stress intensity factor K1 and initial curve radii are calculated from the caustic diameters. The initial curve radius at which calculated K1 values deviate from the real K1 value gives the scale of the three-dimensional stress field which appears around the fast propagating crack tip. In the case of cracks propagating through specimens 3mm in thickness, the 3-D stress field spreads as far as the distance of half of the specimen's thickness from the tip.

Measurement of crack tip displacement field using laser speckle photography

It is shown how double exposure laser speckle photography, when used with an automatic image processing system, can provide detailed information on both in-plane displacement components around the tip of a crack. This paper describes the results of the analysis of six speckle photographs of sharp cracks in polymethylmethacrylate (PMMA), each on a mesh of 16 × 16 datapoints. Two numerical methods for calculation of the mode I stress intensity factor ( K I ) from the measured displacement field have been investigated. Least-squares fitting of the theoretical displacement field gave good estimates of K I , with low systematic and random errors (~1%). Evaluation of the J-integral around square paths enclosing the crack tip was found to result in larger errors (up to 5%). It is thought that laser speckle photography together with J-integral calculations may find applications in nondestructive testing, as a method of automatically detecting the presence of a crack.

Dependence of crack acceleration on the dynamic stress-intensity factor in polymers

The caustics method in combination with high-speed photography was employed to study velocity effect on the dynamic-stress-intensity factor of fast cracks in polymethyl methacrylate and in Araldite D. The specimen geometry was so determined that both the accelerating and decelerating crack propagation occurred noticeably in one fracture event. Instantaneous crack velocity as well as its acceleration were expressed as a function of the crack length by using polynomials of the ninth order which were given on the basis of the least-square method. The results show that the dynamic-stress-intensity factor depends not only on the crack velocity but also on crack acceleration, and that the accelerating crack has a smaller value stress-intensity factor than the decelerating crack at the same velocity.

Microscopic observation of the tips of fast running cracks in PMMA

Although various theoretical models exist, little experimental data is available on the material behaviour in the ultimate vicinity of the tip of fast running cracks. Using a microscope coupled image converter camera, the tips of cracks running in PMMA (polymethylmethacrylate) at speeds between 250 m s −1 and 680 m s −1 were photographed. Due to the high aperture of the optical set-up, shadow optical effects could be greatly reduced. Thus it was possible to observe the contour of the crack flanks up to the crack tip, revealing the existence of fibrils in between the flanks. Seemingly the appearance of these fibrils is connected with the onset of crack branching. Having the crack pass a microlattice, which had been vapor deposited onto the specimen surface, the displacements around the crack tip could be determined. The recorded plastic zone is of triangular shape. Experimental results are compared with theoretical predictions.

Development of fatigue cracks in polymethyl methacrylate

Development of fatigue cracks in polymethyl methacrylate

Growth criteria for solvent crazes

Single methanol crazes are grown from sharp cracks in polymethylmethacrylate. Double exposure holographic interferometry is used to determine the sequential strain energy release rates G and opening displacement profile of the craze from the initiation of growth to its cessation. The craze stress profile is determined at various points in its growth from the opening displacement profiles using a Fourier transform method. The rapid increase in G observed just before the craze ceases to grow demonstrates that craze growth criteria based on the concept of a constant critical total strain energy release rate cannot be correct. Similarly, the large stress concentration which develops just behind the craze tip at growth cessation is incompatible with the assumption that the craze grows to a length that just eliminates a stress singularity at its tip (Dugdale model). This feature, however, would be expected if sufficient methanol cannot reach the fibrils just behind the tip of the craze to plasticize them fully.

Centers of generation of destructive mechanical cracks in polymethyl methacrylate

Centers of generation of destructive mechanical cracks in polymethyl methacrylate

A molecular weight dependent fracture transition in polymethylmethacrylate

A novel method which uses an electro-optical arrangement to measure crack velocities in transparent materials is described. It is applied to fast cracks in polymethylmethacrylate of different molecular weights; in polymethylmethacrylate of weight average molecular weightMw=163 000 there is a transition in the appearance of the fracture surface, which changes from smooth to coarse-structured rib-like roughness, associated with a transition in the measured crack velocity. This effect and possible causes are discussed.

Autovibrational development of laser cracks

The authors experimentally investigate the discontinuous ("jumping") character of the development of laser cracks in polymethyl methacrylate. The discontinuities arise in such an order that the crack remains rounded, even if the laser beam is rectangular in cross section. The authors estimate the pressure drop in the crack during a "jump."

Crack propagation in polymeric composites

The velocities of rapidly moving cracks in polymethylmethacrylate, an epoxy resin, a rubber modified epoxy resin, coupled and decoupled glass bead filled epoxies and randomly oriented glass fiber reinforced epoxies were measured with a crack propagation gage that was electrolissecally plated on the surfaces of the materials. When fracture was initiated from a natural crack it was found that the velocity conformed to Mott's equation a ̇ 2 = a ̇ T 2[1− (a 0/a)] , while fracture initiated from a blunted notch resulted in a velocity that conformed to Dulany and Brace's equation a ̇ 2 = a ̇ T 2[1− (a 0/a) 2] . A general energy balance was used to show how one could develop these two equations as bounds to the velocity of catastrophic crack propagation. The terminal crack velocity in the unfilled materials and the glass bead filled materials was a ̇ T = 0.28√E/ρ , where E/ ρ was the modulus to density ratio of the matrix phase at the macroscopic strain rate of the fracture test. The proportionality constant of 0.28 was independent of matrix type, temperature and degree of adhesion. Cracks in the rubber reinforced epoxies always tended to become blunt, resulting in breaking loads that were higher than that expected for materials possessing a natural crack. In addition, the average terminal velocity was less than 0.28√ E/ ρ, indicating the retardation effects of the rubber particles. These facts were used to explain the higher fracture toughness of these composites. Fracture surface roughness was primarily a function of crack extension and breaking stress and was less sensitive to crack velocity. An empirical modification of the Mott energy balance was used to qualitatively explain this behavior.

Mechanism of polymethylmethacrylate cracking in various working media

The authors studied the growth of rupture cracks in polymethylmethacrylate (PMM) specimens, the nucleation and growth of a pencil of cracks at the tip of a rupture crack under the influence of various working media, the kinetics of stress relaxation in bent specimens and the dependence of the critical breaking stress of PMM specimens with stress raisers on the time during which this polymer is held under stress in various working media. The results obtained were interpreted on the basis of an assumption that the adsorption-effect is mainly manifested in the initial stages of the action of a working medium; the variation in the above-listed properties due to protracted action of the working media is produced by penetration of these media into the supermolecular polymer structures. Dipole moment, polarizability, and the degree of association of medium molecules were chosen as the criteria of the influence of working media on PMM.

Fracture-mechanics analysis of fatigue-crack propagation in polymethylmethacrylate

Repeated tension tests on centre-notched sheet specimens show that the rate of growth of fatigue cracks in polymethylmethacrylate is constant if the range of crack-tip stress-intensity factor is constant. Within the limits considered fatigue-crack growth rate was independent of specimen thickness and loading history.