Craze Region Research Articles

Thermal de-cohesion model for time of craze failure in cohesive zone

The cohesive parameter corresponding to craze failure time is predicted for thermoplastics material. A craze failure separation criterion is proposed for a cohesive zone subjected to a melt layer formed and thickened by adiabatic deformation heat from a craze drawing. The numerical simulation of cohesive zone separation is based on non-linear thermal conduction and convection in the craze region and bulk region around the active layer, associated with a mechanical craze fibrils drawing in an uniaxial direction. The craze failure time is predicted with the assumption of the constant craze thickening rate and cohesive stress for a pipe-grade polyethylene. The numerically computed model reveals the inverse power law decay of the craze failure time, t f, with increasing in craze thickening rate, v c, (almost, t f∝ V c −1) for the thermoplastics. The full notch impact test experimental results are consistent with the analysis prediction. It is concluded that the craze failure time can be theoretically predicted using the numerical modeling.

Crazing Micromechanism in Glassy Atactic Polystyrene and Its Blends with Poly(2,6-dimethyl-1,4-diphenyl oxides) by AFM

Crazing in glassy polymer thin films was found to follow a micronecking process similar to that of local shear deformation in ductile polymers. The void fraction in the fully necked craze region was determined, and a close-packed fibril structure was concluded. The local stress and strain within the craze were obtained from AFM topographic data by the Bridgman's plasticity analysis. The stress/strain curve of craze fibrillation was subsequently determined where an apparent strain softening was found in the initiation of fibrillation, which was then followed by strain hardening as fibrils were drawn into the neck region. Strain rate was found to peak at the craze boundaries, consistent with the surface drawing mechanism from TEM results. During craze fibrillation, the local strain rate of the drawn polymer increased with drawing strain until a critical strain εh was reached; beyond that, the strain rate decreased with the strain. The critical strain εh, identified as the onset of strain hardening, was found to decrease with entanglement density νe in the low-νe, craze-forming regime but become a constant in the high-νe regime where crazing was replaced by shear yielding. The transition between crazing and shearing is controlled by the tendency of strain localization in the entanglement network.

Effect of Molecular Weight and Orientation on Fatigue Crack Propagation of Polyimide Resin.

The effects of molecular weight and orientation on crack propagation property of polyimide (PI) are investigated. The specimens which have four kinds of molecular weight (Mw) from 2.5×104 g/mol to 3.8×104 g/mol are prepared. The crack propagation resistance increased with increasing Mw. The discontinuous crack growth (DCG) bands were observed on the fracture surfaces except for that of the specimen with the highest Mw. The reason that the DCG disappears for high Mw is attributable to the decrease of ductility. The width of DCG bands is almost constant until transition of DCG to continuous crack growth. The critical craze stress for cyclic loading given by Dugdale model decreases with the increase of Mw since the damage of the craze region is accumulated until breaking down. From the results of miniature CT specimens, the crack propagation resistance increased when the direction of crack propagating was perpendicular to the orientation of PI molecular chains.

Crack propagation in the power-law nonlinear viscoelastic material

An analysis on crack creep propagation problem of power-law nonlinear viscoelastic materials is presented. The creep incompressilility assumption is used. To simulate fracture behavior of craze region, it is assumed that in the fracture process zone near the crack tip, the cohesive stress sigma(f) acts upon the crack surfaces and resists crack opening. Through a perturbation method, i. e., by superposing the Mode-I applied force onto a referential uniform stress state, which has a trivial solution and gives no effect on the solution of the original problem, the nonlinear viscoelastic problem is reduced to linear problem. For weak nonlinear materials, for which the power-law index n similar or equal to 1, the expressions of stress and crack surface displacement are derived. Then, the fracture process zone local energy criterion is proposed and based on which the formulas of cracking incubation time t

微少量のガラス短繊維を含んだＰＭＭＡ材のき裂成長 光干渉縞を利用したき裂開口の観察

In order to investigate the effect of short glass fiber on crack growth, DCB samples were machined from injection-molded PMMA plates with a very little content of short glass fibers and the crack tip in the sample was observed by means of optical Interferometry. It was shown that (1) the interference fringes were distorted around the fiber crossing the crack planes over the region 2-3 times larger than the fiber diameter and (2) the fiber depressed the crack opening due to bridging between the crack planes, lessened the craze region in front of the crack and reduced the crack growth rate leaving the arrest line on the fracture surface.

Stress analysis around a three‐dimensional craze in glassy polymers

AbstractStress analysis around a three‐dimensional craze and a three‐dimensional craze containing a penny‐shaped crack has been made. The craze is treated as a transversely isotropic, oblate spheroid embedded in an isotropic glassy polymer. The craze is assumed to consist of primary fibrils and cross‐tie fibrils, such that a penny‐shaped crack may form at the central regime of the craze. The craze surface stress, the stress field near the craze tip outside the craze region, and the stress intensity factor in the crack tip are determined by using Eshelby's equivalent inclusion method. Numerical values of the fracture toughness and the stress needed to sever a craze fibril at the crack tip are calculated and the results appear to be in good agreement with the data given in the literature.

The J integral fracture toughness and damage zone morphology in polyethylenes

The J integral analysis of compact tension samples has been used to evaluate plane strain fracture toughness of various polyethylenes at room temperature. Crack propagation commences from a razor notch in high-density polyethylene at J Ic = 1.7 kJ m −2. Plastic deformation is confined to a small craze region (about 300 μm long, 20 μm high) through which the crack subsequently propagates. A tough copolymer of PE3408 material resists crack advance until J Ic = 8.2 kJ m −2. Here again crack propagation is through a craze, though craze length exceeds 1 mm. Toughness is also imparted by the formation of shear bands near the notch tip. Low-density polyethylene does not really fracture under the present test conditions; this material responds by general yielding and blunting of the notch tip.

Fracture toughness and crack growth mechanism for multiphase polymers

AbstractThe fracture behavior of acrylonitrile‐butadiene‐styrene (ABS) was investigated using the J‐integral method under monotonic loading. Two ways were used to monitor the onset of crack growth: the dyeing method and the length of craze region ahead of initial notch. The blunting at the crack tip and crack growth mechanism for ABS Was observed using a scanning electron microscope. Before the onset of crack growth, the energy put into material was dissipated to create crazes ahead of the initial notch and to deform the material at the crack tip. A part of the energy was released to create a new crack surface after the onset, of crack growth.

Energy absorption in polymer crazing

AbstractThe total energy absorbed by a craze during its development in creep is analyzed and calculated on the basis of a time‐dependent theory of crazing. Experimental measurements of the craze length have been utilized in the energy calculations. For polystyrene the initial energy absorption in the craze region is found to be several hundred times that in the uncrazed medium. This ratio decreases sharply in a short period of time to about 50 to 1 and less and remains low afterward. For polycarbonate, somewhat similar behavior has been found. The initial strain energy absorption by crazing is about 200 times that in the uncrazed region. The energy ratio reduces rapidly to about 55 to 1 and tends to level off thereafter. However, in general, the amount of strain energy absorbed does increase as a function of time, as it should.

Stress and displacement fields at the tip of a craze containing a crack

AbstractPlane elasticity theory is utilized to obtain expressions for the stress and displacement fields at the tip of a craze containing a crack. The craze is modeled as a very thin elliptical inclusion with different elastic properties from hat of the surrounding bulk polymer. Problem is solved by superimposing the solution of a crack problem onto the solution for a uniformly loaded homogeneous craze. Invoking stress free boundary conditions on the crack surface provides a singular integral equation of Hilbert type with a unique solution. Contour lines of constant hydrostatic stress and constant maximum shear stress around the craze tip are shown graphically. These two stress combinations have played prominent roles in a number of proposed craze growth criteria. Results show that even for relatively long cracks within the craze, very little stress enhancement at the craze tip occurs. Only as the crack tip approaches the craze tip does the enhancement become significant, tending to drive the craze region ahead of the crack.

The stress and displacement fields at the tip of crazes in glassy polymers

This analysis models a craze region in glassy polymers as an elastic transversely isotropic homogeneous inclusion of thin elliptical shape with different elastic properties from the bulk polymer. The plane elasticity problem for an applied uniform stress field is solved and the results dimensionalized with respect to the craze tip radius. Stress and strain enhancements of several times far field values are found to occur at the craze tip and are independent of craze tip radius. These results are consistent with experimentally observed characteristics of craze growth and should be important in assessing the relative merits of different criteria that have been proposed for craze growth in glassy polymers.

A time dependent theory of crazing behavior in polymers

The development of crazing is not only a function of stress, but also a function of time. Under a simple state of tension, a craze opening displacement is closely associated with the viscoelastic behavior of the original bulk polymer medium in which individual crazes initiate and develop. Within each craze region, molecular orientation takes place when conditions permit, and a new phase of rearranged molecules governs its local behavior. Based upon a time-dependent viscoelastic two-dimensional model, using a computer program the craze opening displacement field has been calculated, time-dependent craze length was also computed by taking into consideration the molecular orientation mechanism and large deformations in the craze region. Examples are given for simple viscoelastic media with simplified stress distributions. It is interesting to find out that the occurrence of crazing may be interpreted in terms of the stability or instability of the constitutive behavior of the bulk polymer.

The effect of rate on the impact fracture toughness of polymers

The determination ofG cfor a polyethylene and PTFE from a conventional energy measuring impact test is described, together with the use of plastic zone and slow crack growth corrections. The rate of the test is changed by two decades by varying the specimen dimensions andG cas a function of loading time is determined. A remarkably strong time dependence (t −0.42) is observed and this is discussed in terms of loss processes in the craze region at the crack tip.