C.R. Joe’s research while affiliated with University of Massachusetts Amherst and other places

It is shown that the tear resistance in a pure-shear tear specimen can be determined in terms of G̃ as a function of crack growth using a single specimen. The load-displacement curves of pure-shear tear specimens are analyzed based on the generalized locus method which determines G̃ using a locus line of characteristic points on the load-displacement records of specimens which differ only in initial crack size. Based on this analysis, it is shown that the constant G̃ locus lines on the load-displacement records of pure-shear tear specimens are vertical lines. The tear resistance in a pure-shear tear specimens are vertical determined from a single specimen utilizing these predictable locus lines. The effectiveness of this method is illustrated by determining an R-curve of a pure-shear tear specimen made of a thermoplastic rubber. The R-curve determined in this study for the pure-shear specimen is valid for the given loading configuration.

For fracture toughness measurements, accurate identification of crack initiation is of critical importance. Maximum load point criteria can work well for brittle materials but may be inappropriate for other materials. The 5% compliance intercept criterion stipulated in the ASTM E399 standard for metals gives reproducible data but is not necessarily related to the actual fracture process in polymers. Visual observation techniques are inherently related to the fracture process but may be somewhat subjective. Crack resistance values based on the above criteria are measured for polystyrene, polypropylene, rubber modified polymethylmethacrylate, and a nylon 6/ABS blend. It is found that the method of determining characteristic points can affect the generated fracture characterizing parameters.

It is shown that the R-curve of flexible materials can be determined using a monotonic loading of a single trouser tear specimen. The load-displacement records of trouser tear specimens are analyzed based on the locus method which determines the critical J-integral value (J c)) using the locus line of crack initiation points on load-displacement curves of specimens which differ only in initial crack length. Based on this analysis, it is shown that the crack resistance (R) during crack growth including crack initiation and steady state crack propagation can be expressed in terms of quantities that are directly related to the load-displacement plane, and that R can be simplified to a linear function of load. It is also shown that the crack growth (C) can be expressed as a linear function of load-point displacement. The load-displacement record of trouser tearing is then converted to an R-curve simply by changing the scale on each axis. The effectiveness of this method is illustrated experimentally by constructing the R-curve directly from the load-displacement record of a trouser tear specimen made from a thermoplastic rubber sheet.

The resistance to crack initiation and quasi-static crack propagation is investigated for a polybutylene terephthalate (PBT) using annealed and unannealed three-point bend specimens. The resistance to crack initiation (Ri) is determined based on the generalized locus method which determines the resistance to crack growth including crack initiation utilizing the locus of characteristic points on the load against load-point displacement curves of specimens which differ only in initial crack length. This generalized locus method also enables us to investigate the invariance of the crack resistance value along the locus line. The steady state crack resistance (Rp) during quasi-static crack propagation is determined utilizing the functional relation between total essential energy (Uf) for complete fracture and the initial ligament length. The total essential energy is the sum of the blunting energy and the integration of the resistance to crack propagation with respect to the cracking area. The invariance of the crack initiation resistance Ri, and the steady state resistance Rp is discussed based on the experimental results of the annealed and unannealed specimens which show different sizes of crack tip plastic deformation.

Etude du travail specifique de la rupture dans les materiaux ductiles en utilisant des specimens, dans lesquels la taille de la zone plastique externe est proportionnelle au carre de la longueur du ligament initial

The effect of remote energy absorption in determining the critical value () which characterizes the crack initiation is investigated for several methods. It is shown that the remote energy absorption can affect the accuracy of the experimentally determined values and that eliminating the remote energy term in the early stage of the procedure can help to yield consistent values. Transformation of the loading lines due to the remote energy absorption shows that the enclosed area between the loading curve and the crack initiation locus line, which is the essential energy for crack propagation, is independent of remote energy absorption. Consistent values can be determined utilizing the enclosed area between the loading curve and the locus line regardless of the amount of remote energy absorption. It is verified experimentally by determining values for two different loading configurations which have different amounts of remote energy absorption.

The ratio (Φ) of the J-integral to the total work done per unit uncracked area is shown to be dependent upon Poisson's ratio (v) in both plane stress and plain strain conditions. An expression for Φ in three-point-bending of linear elastic materials is derived analytically as a function of a/W (initial crack size/width), S/W (span/width) and v. The variation of Φ on the Poisson's ratio and a/W is illustrated in both graphical and tabulated fonns for S/W = 4. Error margins are discussed when Φ is taken as 2.00 universally for all materials exhibiting linear elastic behavior when a/W exceeds 0.5. The Φ value is experimentally detennined for several a/W ratios using a polymeric material. This experimental result is compared with an analytic prediction shown in the present study. The prediction is in good agreement with the experimental result.

The resistance to crack propagation starting from crack initiation to complete fracture is discussed. A method which determines crack growth resistance utilizing the locus of any set of characteristic points on the load vs load-point displacement curves of specimens which differ only in initial crack length is presented. This generalized locus method can also be used to investigate the invariance of the crack resistance along any set of characteristic points. Experimental evaluation of the crack resistance from crack initiation to steady state propagation is performed using a polymeric material. The resistance to crack initiation is determined in terms of the critical value (). The resistance to crack propagation at maximum load point is also investigated utilizing the locus of the maximum load points on the load-displacement curves. The plateau resistance () during extensive crack propagation is determined utilizing the total essential energy for a complete fracture which is the sum of the blunting energy and the integration of the resistance to crack propagation with respect to the cracking area. The experimental result supports the assumption that the crack initiation resistance () and the plateau resistance () values are invariant regardless of the initial crack length provided the irreversible deformation zone size around the crack tip is not confined by specimen geometry.

A single specimen fracture-test method for determining the fracture energy of highly deformable materials is presented. This test method is based on the locus method, which determines the critical value () by partitioning the essential energy for crack propagation from the total absorbed energy. The locus method uses the locus of crack initiation points on the load-displacement record of specimens which differ only in initial crack size. With a proper specimen shape and loading condition, the locus line is predictable. When the locus line is predictable can be evaluated from a single specimen. Experimental results show that the fracture energy determined by the single specimen method agrees well with those by the tearing energy approach and multiple specimen locus method.

Two methods that determine the critical () for crack initiation in the presence of remote energy dissipation are compared. The locus method of partitioning the energy for crack propagation from the total deformation energy is based on energy release rate interpretation of . This method is a reliable means to measure value in the presence of remote energy dissipation. On the other hand, the extrapolation method is based on an assumption that the total energy absorbed can be thought of as the sum of the energies absorbed in the crack tip region and the region away from it, and thus the effect of remote energy dissipation on the critical value can be eliminated by evaluating the value using the crack tip energy only. However, there exists a theoretical problem in the extrapolation method, and the problem is discussed. A comparison of these two methods is attempted based on the same load-displacement records of a highly deformable thermoplastic rubber, and advantages of the locus method are presented.