Canadian Plastics

Setting a new standard

"Instrumented impact testing -- that really is the future," says Gerard Nelson, sales manager, CEAST USA. This type of impact testing incorporates a strain gauge or piezoelectric cell at the point of ...

July 1, 2000   By Cindy Macdonald, associate editor



“Instrumented impact testing — that really is the future,” says Gerard Nelson, sales manager, CEAST USA. This type of impact testing incorporates a strain gauge or piezoelectric cell at the point of impact to produce a force/time curve. Typical impact testing does not incorporate a time factor, it simply produces a number that reflects a material’s ability to withstand shock.

“Instrumented testing allows you see fracture initiation and fracture propagation. It really gives a clear picture of how that material responds under those conditions,” he explains.

But, it will not likely become common for quality control testing at the processor level, says Allen Zielnik, general manager of the polymer evaluation products division of Atlas Electric Devices Co.

“Instrumented impact testing will probably always remain in the research and development realm. It does not give a nice clean, simple answer; it gives a curve of varying quality (depending on your instrument) that requires some level of mathematical interpretation and some subjective interpretation. Any time you have data that does not provide a black or white, pass or fail answer it rarely finds its way into test methods for quality control.”

Tinius Olsen has released an instrumented impact testing system for use with its Model 92T plastics impact tester. Olsen’s design has high-speed system response of at least 100kHz. A high-speed system response is necessary to accurately record rapid load change events, such as the accelerations of the specimen up to the speed of the striker (inertial rise) and brittle crack propagation.

The additional data collected by instrumented testing can be used in fracture mechanics models to determine static and dynamic fracture toughness correlation. According to Olsen, such data were not commonly measured in the past because earlier instrumented striker systems required a correction factor, and the use of a correction factor distorted the characteristic load data, making it unacceptable for use in engineering correlation.

Olsen’s instrumented system consists of an instrumented striker, a power supply/signal amplifier, an oscilloscope card, data acquisition and analysis software, and a computer.

The new Advanced Pendulum Impact Tester (API) from Atlas Electric is a classic design with features optimized to match current global standards for impact testing. An instrumented version is available.

The API offers a wide spectrum of energy ranges with minimal hardware changes. Interchangeable sample supports, mountings, pendulums and weights allow for Izod and Charpy versions.

The API is also capable of determining the resistance of notched plastics and composites to breakage by flexural shock.

The API uses an integrated microprocessor system and menu-driven test prompts to guide the user through the test. An advanced electronic braking system provides variable pendulum starting angle, and catches the arm to prevent double strikes.

Will it withstand wear?

Sample size and shape are no longer a problem in scratch and abrasion testing. Taber Industries has introduced a Linear Abraser that can test curved, rounded, large or small surfaces.

The Linear Abraser can test wear resistance in a linear motion, and with a quick head change can also perform scratch tests. The unit is suitable for testing molded plastic parts, membrane switches, paints and coatings, and optical products.

Pinpoint the problem with micro thermal analysis

Micro thermal analysis is a new development in the field of materials characterization that gives polymer scientists an additional research and problem solving tool. It combines the visualization capability of an atomic force microscope (AFM) with the material characterization ability of a thermal analyzer. In TA Instruments’ -TA 2990, the AFM measurement head is fitted with an ultra-miniature probe that functions as a programmable heat source and temperature sensor.

“Micro thermal analysis is particularly interesting for multi-layer films and polymer blends,” explains George Dallas, of TA Instruments. “You can scan a few hundred micrometers at a time, and then place the probe and take thermal measurements on an area as small as 10 micrometers at a time. In a seven or eight layer film you can pinpoint and characterize just one layer.”


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