The comeback of the instrumented charpy impact test and its relationship with the split hopkinson pressure bars to assess for rapid crack initiation of pe resins

摘要

Nowadays resistance to rapid crack propagationrn(RCP) of PE pipes is of primary importance on therninternational stage as proved by the tremendousrnliterature on this subject [1][2].rnAttempts for correlation between Full Scale andrnSmall Scale tests are under the sunlights since arnlong period now and gives very interesting - butrntime consuming - debates.rnThe real problem- one more time - is thernunavailability of a correct rheological behavior ofrnthe material and the related modellization underrnsuch constraints which would allow one torntranspose the Full Scale conditions to the SmallrnScale ones.rnApart from these neverending story regarding arncorrelation between both tests there exists a crucialrnproblem totally hidden which is that of therninitiation.rnInitiation is far more important since it representsrnthe "trigger" which will make possible the RCP orrnnot [3-5].rnTherefore it is not only convenient but alsornnecessary to evaluate not solely the immediaterneffect but also the delayed one of an external shockrn- applied during e.g. handling of a pipe - in a widernrange of loading speeds typically 2-to-20m/s.rnThen calculation of a critical threshold forrninitiation taking account of both temperature effect,rnloading speed and material characteristics becomesrnof primary importance to evaluate the residualrnlifetime of a PE specimen in terms of both RCPrnand Slow Crack Growth (SCG).rnSuch an approach allows one to evaluate thernconditions for which a PE specimen which wouldrnhave been declared safe according to a pure propagation test (e.g. S4) presents some risks of further delayed propagation either rapid or - more tricky - slow.rnThe instrumented Charpy test has proved to be a natural and a very attractive candidate to assess for such phenomena although presenting some limitations with regards to both the range of accessible loading speeds - typically limited to 3m/s - and the lack of reliable theoretical treatment of the results based on a consistent behaviorial law. To go over these limitations the Charpy method has been associated with the well-known Split Hopkinson Pressure Bars (SHPB) test which has permitted to impact PE specimens in a range of loading speeds complementary to that of the Charpy test, meaning up to 15-20m/s, after some modifications of the classical system to adapt the impedance of the bars to that of the PE material. Thus a rheological model could be derived from the Hopkinson data because of the available theoretical description of the test for a series of materials including polymers. Tn a second step, notched specimens were broken under both Charpy conditions and Hopkinson modified ones.rnThe data obtained after a specific treatment of the loading history on the Charpy specimens on the one hand, and after a modification of the Hopkinson device on the other hand, would allow one to evaluate more accurately the real energy for rupture - and the related criteria - in a range of loading speeds and temperatures - typically 1-to-20m/s and -20℃-to-30℃ - representative of the field conditions.