New Testing Method to determine the Yield Stress of Polymeric Materials (19-Dec-2009)

New Test Method to determine the Yield Stress of Polymeric Materials

By Howard Hong, Lee Kim Kheng

Motorola, Inc.

Mobile Device Business (iDEN^TM)

ABSTRACT

This paper describes a new test method to determine the yield stress of polymeric materials. The determination of the correct yield stress for a particular material is crucial to defining the range of nonlinear elastic curve defined by the hyperelastic material model and the initiation of post-yield data defined by plastic material deck in finite element software (Abaqus™).   The benefits entailed from this work would be a better prediction of the material under deformations in simulations.

PROBLEM

Presently, there is no appropriate method or standard to find the yield point of polymeric (polycarbonate) materials as they behave non-linearly when they are tensile-loaded in a universal testing machine (UTM).  Unlike the metallic materials which have an obvious yield point, the non-linear response of polymeric material does not have a distinct point demarcated from the line of proportionality as shown in Fig. 1, thereby not enabling the yield stress location to be determined on the stress vs. strain curve; in this case Lexan^TM EXL1414. In most cases, the lower yield stress is used in numerical simulations, which results in a large plastic strain predicted in simulations when the polymeric materials are deformed much during highly nonlinear drop. Unnecessary follow-up work could entail to tackle the over-conservative predictions.

Fig. 1: Stress vs Strain for Lexan^TM EXL 1414 Polycarbonate

Hence, a new test method is proposed to determine the correct yield stress of polymeric materials.

SOLUTION

The dogbone sample would need to be experimentally pulled in the UTS at a quasi-static speed, and determine the amount of residual strain (or plastic strain in a more familiar term) at a lower load.  It is then relaxed for a suitable period before pulling it at next higher load and determining the strain again.  This process is continuing for a few cycles till the residual strain is explicitly identified from the experimental curve seen in the UTS.  The determination of the yield point is then obtained from the residual strain vs engineering stress curve.

OPERATION

The method involves the use of a test profiler feature available in in-house UTM (INSTRON^TM model 5544).  This feature is basically controlling how the sample is to be tested with force or displacement control at a specified load for a particular duration.  The sample, whi...