in Vol. 20 - January Issue - Year 2019
Experimental Investigations on The Hole-Drilling Test Method for Determining Residual Stresses in Polymeric Materials
Figure 1: Schematic of the automatic hole-drilling measurement setup
Figure 2: Bending test bench to simulate a known reference residual stress
Over the last fifty years, the plastics industry has developed greatly, outstripping the steel industry in technical applications as well. Increasingly accurate and in-depth mechanical characterization is therefore necessary, and it is in this context, that the need arises to know and study the value of residual stresses induced by machining processes in these materials.
The hole-drilling strain gauge method allows residual stress to be measured in a wide range of materials. It has the advantage that the measurements can be made over a small area. A special strain gage rosette is bonded to the surface of the specimen and a hole is drilled precisely through the center of the rosette. The strains measured at the surface correspond to the stresses relaxed during the drilling process. Using the measured strains and appropriate models (e.g. ASTM E837-13), it is possible to calculate the stresses that exist in the material.
This technique can be successfully applied for the measurement of residual stresses in polymeric materials. The application of the hole-drilling method to polymers is certainly more complex than the same application to metals due to the higher coefficients of expansion, and to the viscoelastic behavior of polymeric materials.
When applying the hole-drilling technique to polymers, it is essential to minimize the thermal and mechanical effects due to both temperature variations and hole-drilling procedures; also avoiding the rise in temperature near the strain gauges because of the electrical resistance heating.
Polymers have linear thermal expansion coefficients that vary significantly, but strain gauges for residual stress analysis available on the market are self-compensated for some metallic materials only (carbon steel, stainless steel and aluminum alloys). When temperature variations occur during the test, it is advisable to perform the thermal compensation using the dummy gauge technique in order to minimize significant errors due to apparent deformation (temperature-induced apparent strain).
The low conductivity of the polymeric material can result in a substantial temperature rise in the vicinity of the gauge because of the electrical resistance heating of the gauge. Since the creep and stress relaxation properties of such materials are very sensitive to temperature, such interferences by the measuring strain gauges in the properties to be measured can introduce serious errors. It is, therefore, essential to make sure that the bridge excitation level is not excessive to avoid the consequent rise in temperature near the strain gauges.
The tests should be carried out in a temperature-controlled test environment and drilling is performed using the automatic drilling technique with remote control rather than the manual drilling technique: using manual drilling, the operator's body heat affects the local strain gauge temperature, causing additional apparent thermal strains. It is, therefore, necessary to wait after each drilling step, before taking the measurements used for the residual stress calculation, for the length of time necessary to obtain a regular trend of the measured strain signal.
Using the SINT Technology EVAL7 software, the strain measurements can be interpolated and the strain measurements are processed in conformity with ASTM E837-13 or, alternatively, using different calculation algorithms, such as the Influence Functions method proposed by the University of Pisa. The latter method allows more accurate calculations using a database of Influence Functions, which takes into account the variation of the Poisson’s ratio in the range from 0.20 to 0.40 (suitable for polymers) and the measured eccentricity.
There are also additional important factors that should be taken into account when making this type of test. In particular:
Influence of the rotational speed
Influence of the feed rate
Effect of the type of end mill (number of flutes)
Effects introduced by the drilling operations
For verifying the best measuring parameters, the results have been checked and validated by using a special apparatus developed by SINT Technology, which applies a known bending stress on a flat uniform-resistance cantilever beam, fixed at one end, and loaded at the other end by a pneumatic actuator. This procedure makes it possible to separate the bending stress from the residual stress present in the material, and to compare them with the expected ones (obtained from the beam theory). The results are found to be in good accordance, with a maximum difference within 10-15%.