Computer-based solutions offer enhanced efficacy and ease of automation in every walk of life. Computer-based systems for real time monitoring and control of corrosion testing equipment provide significantly improved measurement precision as well as a basis for improved accuracy in data acquisition for complex tests. This paper discusses one such system called a Constant Extension Rate Tester (CERT) used to automate slow strain rate (SSR) testing. SSR testing is an effective means to assess material susceptibility to environmentally assisted cracking (EAC) in corrosive systems.
INTRODUCTION AND OVERVIEW
Evaluation and testing of materials in corrosive laboratory environments is an essential aspect of designing materials for safe utilization. Slow Strain Rate (SSR) testing has proven to be a widely used, convenient methodology to assess material susceptibility to environmentally assisted cracking (EAC). Uniform, constant displacement (strain) is imposed dynamically by external means on the gage section of a uniaxial tensile specimen and the stress/strain data is monitored.
The imposed strain should be slow enough for certain corrosive reactions to occur and fast enough to produce failure in a reasonable period of time 1. Typical evaluation times for SSR testing range from a day to a week depending upon the conditions of the test.
This paper describes a system designed to automate SSR testing. The system is called a Constant Extension Rate Test (CERT) system. This system represents the next generation in slow strain rate test machines. This computer controlled machine facilitates stress/strain data generation over a wide range of testing speeds (strain rates) without the need for cumbersome gear changes that are required for conventional SSR testing machines. The benefit of a computer controlled system lies in its ability to offer very fine strain/displacement resolutions in an automated mode.
Typical set up for a CERT system consists of a test specimen placed in a controlled environment (eg. in a glass vessel), fitted to a pull rod by means of clevice pins. The pull rod is connected to a stepper motor that provides the drive control for imparting different strain rates. The system provides an easy set up to that may be used to subject materials to rapid deformation as a function of time. The methodology also reduces the incubation time to onset of cracking in susceptible materials through application of dynamic plastic strains. The plastic strain causes an accelerated disruption of surface films thereby overcoming the initial period of incubation that can result in unacceptably long test duration for corrosion tests. A schematic of an SSR set up is shown in Figure 1.
SSR testing yields several measures of susceptibility such as time to failure, maximum stress or load achieved during the test as well as ductility parameters such as 6 reduction in area or elongation 2.
A schematic of the computerized CERT system is shown in Figure 2. The stepper motor and gear reducer attached to the top of the external load frame provide the required precision motion (See Figure 1). The drive assembly provides a controlled shaft rotation rate which is used to drive the precision ball screw assembly. This, in turn, provides the axial displacements used to load the test specimen. The stepper motor utilizes signals from the drive controller housed in the control instrumentation box to deliver precise up or down motion sequences.
The instrumentation panel is controlled through the computer. Various system parameters, such as displacement, load, temperature and pressure are monitored and controlled through analog voltage signals generate