Viscoelastic Fracture Mechanics Research Articles

A viscoelastic fracture mechanics assessment of slow crack growth in polyethylene gas distribution pipe materials

A substantial amount of the plastic gas distribution pipe currently in service, as well as that anticipated for future use, is polyethylene (PE). While this material has an excellent record, due to a variety of abnormal loadings that can arise in long time service, some slow crack growth (SCG) related field failures have occurred. Accelerated test procedures and analysis methodologies to predict accurately the long term performance of PE gas pipes are therefore required. One such procedure is the three-point bend test conducted on a pre-cracked pipe segment that is known as the SCG test. The research reported here examines the validity of this test through the application of advanced fracture mechanics principles. Specifically, considering the viscoelastic behavior of PE materials and using the region of dominance concept, the delimitation of a linear elastic fracture mechanics (LEFM) interpretation of the SCG test was developed. Excluding the time spent in crack initiation, the long-term performance of the category of materials for which the assumptions of LEFM are valid can therefore be effectively predicted from SCG test results. Through illustrative examples, this paper demonstrates the veracity of the approach by comparing time to failure predictions for abnormal service loadings with actual gas distribution service experiences.

Duration of load effects in lumber. Part II: Experimental data

The viscoelastic fracture mechanics model developed in Part I is calibrated using a 70-day duration of load test on commerical lumber. The model is then verified using another similar test on a different population of boards, and on a unique 6-year test just terminated. All three test programs are described. The model is used to tentatively project results beyond the period of testing. The strength predictions of the model compare well with those obtained experimentally during the tests, the coefficient of correlation being 0.95. Implications for design practice are outlined in Part III of this paper.

Duration of load effects in lumber. Part III: Code considerations

The implications for practical structural design with wood of a visco-elastic fracture mechanics model, recently developed and verified experimentally by the authors, are presented. These are compared with those of the presently used Madison concept, and with those of the cumulative damage models. Following discussion of present practice and the consequences of the fracture mechanics model, specific design recommendations are made as to how this more complicated model might be incorporated into design codes.

The Engineering of Polymers for Mechanical Behavior

Abstract The broad subject matter in this paper is treated in only cursory fashion. Our intent was to present an approach to the systematic analysis of the relationships between engineering properties and molecular variables for viscoelastic materials. Literature omissions will undoubtedly be numerous, but the basic concept of the Interaction Matrix allows for continual upgrading and refinement. The examples chosen were based on the stress relaxation modulus which provided an ideal starting point due to the wide use of this property in viscoelastic stress analysis and fracture mechanics. Other important engineering parameters which might be selected for similar examination include bulk compressibility and dilation for example; and, of course, the rupture properties under various imposed stress fields.