Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 284 DuPont [44] further provide the following advice regarding the design of springs: “ In the design of springs in Delrin® acetal resin, certain fundamental aspects of spring properties of Delrin® acetal resin should be recognized.  The effect of temperature and the chemical nature of the environment on mechanical properties.  Design stresses for repeatedly operated springs must not exceed the fatigue resistance of Delrin® acetal resin under the operating conditions.  Sharp corners should be avoided by provision of generous fillets. ” This advice indirectly points out the shortcomings of the above analysis:  The morphology can in no way be taken into account o The self-heating is not considered  Local stress raisers are not taken into account  SN data is provided for one temperature only and needs to be shifted for other relevant temperatures using an appropriate technique, unless more data can be measured.  The approach becomes increasingly harder to justify with increasing geometrical complexity SN analysis can in our opinion be considered a valid tool for comparing the infinite life margin of initial design variations, after which dedicated component analysis needs to follow once a design decision has been reached. That being said, without a proper temperature estimation technique, the concept will bear limited merit only. Exemplary eN analysis: Rather than trying to replicate the SN study, two strong limitations need to be pointed out. The first concerns the matter of cyclically stable stress-strain data. Through personal experience, the authors have been able to conclude that in spite of trying to resort to controlling measures during testing, the hysteresis loops in cyclic testing of plastics 13 do not stabilize, even after intensely long periods of time. Kitagawa et. Al [46] report similar observations for PE, PP and POM. The second limitation pertains to the process of deducing a Coffin-Manson type strain-life curve. To produce this diagram, uniaxial strain-controlled tests are necessary, where the strains are recorded for the most part using tactile devices. Uniaxial testing for anything but load-controlled pulsating tensile fatigue testing requires purpose-made specimens, which greatly increases cost and that are not available as off-the-rack supply from plastics manufacturers. In addition to the former, trying to measure strain using glued on gauges, results in the test recording a compound of the plastic and the adhesive (typically epoxy resin) [47]. Non-tactile measurements devices such as laser extensometers, or techniques such as speckled patterns and digital image correlation need to be applied to correctly capture the strains on the surface of the specimen. Herein lies the nature of the second impediment. The temperature on the surface will be drastically different from that of the bulk and will not be detected. We have so far concluded that cracks are more likely to form inside the bulk than initiate on the surface, and we therefore need to know the temperature at the core in order to feed any simulation with accurate material data. Due to the temperature gradient being very steep, the monotonic and cyclically stable (if indeed possible to obtain) stress-strain response would need to be captured at a multitude of temperatures and a sensible interpolation technique utilised that is capable of coping with the glass transition phenomenon in plastics. Adding to the above limitations is the influence of self-heating and how this affects the test results, a fact that would also need to be taken into account. It is therefore our opinion that eN analysis is the most difficult concept to adapt or apply to non-reinforced plastics. Exemplary da/dN analysis: A plastic component made of POM is subjected to a cyclic pulsating load ranging between 1.0 and 0.1. The load applied causes stresses in a critical design element of maximum 50 MPa in magnitude. The stress cycle would therefore range from 0.1  50 to 1.0  50 MPa (  =  max –  min = 50 – 5 = 45 MPa). The requirement is that the component must display a minimum endurance of 5  10 5 cycles. We want to estimate the maximum acceptable internal flaw size resulting from the manufacturing process for the component to fulfil this requirement. For the following experiment, we will use Irwins theory, which states that there is only a small annulus around the crack which is plastic. This allows us to use the idea of the stress intensity factor, although in simplified form, meaning that for a physically true description of the problem, we will still need to look at actual energy release rates. The first step is to calculate the critical flaw size (which would cause fracture to occur in a single cycle). This may be obtained from the following equation: 13 PA46, PA66 and POM