A tensile strength is a common materials test. Typically, a sample is subjected to controlled tension until it fails, providing valuable data for fundamental materials development or quality control.

The key data acquired include maximum elongation, reduction in cross-section and ultimate tensile strength. Derived from these are a host of properties: Young’s modulus, yield strength, Poisson’s ratio and strain-hardening characteristics.

Nanovea, Irvine, Calif., has developed an alternative to traditional tensile testing methodology. The breakthrough reliably acquires both yield strength (YS) and ultimate tensile strength (UTS) through the indentation technique and has significant advantages when compared to traditional tensile testing.

The YS is the stress at which a material begins to deform plastically and won’t return to its original shape when removed. The UTS is the maximum stress that a material can withstand while being pulled before breaking.

These two properties indicate the upper limit to the load that can be applied to mechanical parts and structure, and play important roles in materials production including annealing, forging, rolling and pressing.

Traditionally, YS and UTS have been tested using a tensile testing machine, a large instrument requiring enormous strength to pull apart the test specimen. It’s both costly and time consuming to properly machine many test coupons for a material—each sample can only be tested once.

Small defects in the sample could possibly create noticeable variance in the test result. The different configuration and alignment of the tensile testers in the market often result in substantial variations in the mechanics of testing and their outcomes.

In contrast, Nanovea provides a novel solution of measuring both YS and UTS values by an indentation technique using the Nanovea Mechanical Tester. As a cylindrical 200-µm flat diamond tip is driven into the flat test specimen at a controlled loading rate, the in situ force and displacement of the indenter are recorded.

The position of the indenter relative to the sample surface is precisely monitored with a patented optical non-contact depth sensor, while the load is applied and measured using precision load cell in closed-loop control with the Z motor.

Figure 1 shows the load-displacement curve of stainless steel SS304 and Al6061 alloy samples measured during indentation. Through analysis of the “S” shape of the loading curve using a set of special algorithm developed by Nanovea, the YS and UTS are automatically calculated by the software as summarized in Table 1. The YS and UTS values obtained by conventional tensile tests are listed for comparison.

The indentation test is traditionally used for hardness and elastic modulus measurements. However, a reliable indentation technique that allows direct correlation of macro tensile properties to indentation results has not been available.

Nanovea’s new indentation method directly provides YS and UTS values comparable to that measured by conventional tensile tests. Such a measurement opens a new realm of applications for industries including construction, automotive, biomaterials, and nanotechnology/microtechnology.

The simple experimental setup significantly cuts the time and cost for sample preparation, compared to the typical tensile test coupons which feature a complex shape. The small indentation size makes it possible to perform multiple measurements on one single sample, and can prevent the influence of defects in the tensile test coupons created during sample machining.

Moreover, it allows YS/UTS measurements on small samples and localized areas, which is vital for YS/UTS mapping and local defect detection of pipelines or automotive structures.

The new method allows reliable YS and UTS results on an endless list of materials.

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Filed Under: Materials • advanced

 

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