Plastics News

Issue 3

Are You Experiencing Variation or Unexpected Calculation Results?

Experiencing variation or unexpected results in calculations can cause significant concern and problems for a QC lab. Before troubleshooting results in Bluehill® or any testing software, we recommend to first indicate the calculation on the graph. This will serve as a good visual representation and first pass to ensure that the algorithm is picking the point or region you are expecting.

Troubleshooting Calculations

In the case of a calculation, like modulus, simply making the calculation visible can highlight the problem. In a case like the graph depicted below, the reported values from the red curve could be lower than the expected value (blue) by a factor of two!

Troubleshooting Calculations in a QC Lab

If the algorithm for modulus is picking the area of the curve you are expecting and you’re still getting unexpected results, please use the following chart as a guide to troubleshooting:

Modulus Causes Why Solutions
Values Too High Wrong micrometer anvil Spherical anvils = lower cross-sectional area when sink exists ASTM D638 - Use flat anvil
ISO 527 - Ensure both parties use the same anvil
Values Too Low Too much compliance or initial comp load Unbending as opposed to tensile Use a preload or slack correction and specimen protect
  Knife-edge bite Penetrating skin may weaken the material More care when attaching or changing to an automatic extensometer
Too Much Variation Inconsistent gripping force Wedge grips: operator Use side-acting grips or preload+auto balance strain
  Insufficient data rate Increased chance of incorrect slope Increase data rate or modify calculations (regions/overlap)
  Inconsistent specimen alignment More common with older grips Upgrade grips or incorporate homemade alignment aid
  Inconsistent extensometer alignment Not accurately measuring the axial strain More care when attaching the clip-on or upgrade to an automatic extensometer

Diagnose an Issue

Determining the Correct Prestress (ISO-527) or Toe Compensation (ASTM D638)

The toe region is the first region of the curve with a low slope before the initial linear portion of the curve. If the strain in this region is not handled properly, calculations—such as modulus, strain at yield, and strain at break—will be incorrect.

Prestress (ISO-527) or Toe Compensation (ASTM D638)

ISO and ASTM, while similar, are not technically equivalent. The determination of the prestress value as described in ISO-527 and the toe compensation method as described in ASTM D638 is another key difference between the two methods.

The strain or extension seen in this region must not be considered when analyzing the curve. Failure to correct this artifact will result in incorrect values of parameters such as modulus and strain at any point. It is important to note that this compensation is only applied to the strain or extension axis. The stress or load seen during this region is not discarded.  It is important to remember when comparing results, the load or stress threshold used to compensate for the toe region should be the same for a given material.

Prestress with ISO 527-2: 2012

When testing to ISO 527-2, it is important to ensure that the initial stress (σ0) falls within the ranges stated in the equations below. The initial prestress value is material dependent; a prestress value that is appropriate for one material is not necessarily appropriate for another.

Prestress: ISO 527-2 2012

Take an example where the max stress (σ*) is 65MPa and the modululs (Et) is 2950MPa. The initial prestress value (σ0) must be greater than zero but less than 1.5MPa, per equation 1. Applying equation 2 initial prestress value (σ0) must be greater than zero but less than 0.65MPa. The information indicates that the initial prestress value must be less than 0.65MPa.

Toe Compensation with ASTM D638-14

If testing to D638-14, the following procedure must be used to determine the appropriate toe compensation.

First, determine whether the material being tested has a linear region, similar to the graph shown on the right or if no linear region exists, similar to the graph shown on the left.

Toe Compensation: ASTM D638

In the case of a material with a linear region, the slope line of this region is extended until it crosses the strain access, point B. The graph is then shifted such that the stress value associated with point B now corresponds with the corrected zero-strain point.

In the case of a material that does not exhibit a linear region, the same kind of toe correction can be made by constructing a tangent to the maximum slope at the inflection point (H'). This is extended to intersect the strain axis at point B', the corrected zero-strain point.

Plastic adhesives for automotive electronics

Plastics in electronic components are heavily used to optimize performance and design of final products. In the automotive industry, the ever-decreasing space makes it difficult to accommodate as many electronic components as needed. For this reason, a variety of structural adhesives, for metal and low-surface substrates, are used to put electronic components in place thanks to their capabilities of reducing weight while increasing the long-term component performance.

Typically, adhesive’s applications are for airbags, power locks, roof panels, display units, speakers and wire harnesses, as well as for lighting systems.

Measuring the strength of adhesives used for electronic components is essential to properly select the adhesive that can withstand heat generation and mechanical stresses and has good chemical stability.

An impact wedge peel test is used to measure the cleavage resistance of high-strength adhesives. According to ISO 11343, the cleavage corresponds to the separation of the adherends by a wedge, moving at high speed, whose displacement is initiated by an impact. The specimen is shaped like a tuning fork, and a wedge is drawn through the bonded portion of the specimen. The substrates should be bonded over a length of 30mm and the unbonded arms being formed to give the 'tuning fork' profile. The free arms of the specimen are clamped and the wedge is drawn through the bonded portion.

Impact Wedge Peel

A drop tower allows an operator to measure the cleavage resistance under impact loading of high-strength adhesive bonds between two metallic adherends, at different speeds and temperatures. The wedge velocities recommended by the International Standard are 2 m/s and 3 m/s. The clamping for adhesives bonds is directly connected to a 15 kN strain gauge sensor in order to acquire the force as a function of time and displacement. Furthermore, the Instron Data Acquisition System allows easy and efficient acquisition of force and thanks to VisualIMPACT Software it is possible to manage instruments and measurement acquisition simultaneously, to analyze final results and automatically report or export data.

AUTOMATION FOR OPTIMIZATION

The requirements for testing plastic materials involve a combination of:

  • Accuracy: Composite materials are stiff, often hard, and fail at comparatively low strains
  • Robustness: Many rigid plastics exhibit high energy breaks, which is very hard on the equipment
  • Repeatability: Given the tightness of controls, any source of external error must be minimized
  • Safety: It is common for reinforced plastics to shatter and launch projectiles when they fail, requiring significant shielding and protection for the operator

Based on these requirements, it is not uncommon for labs to pursue ways that best remove the operator from the test cycle without compromising accuracy and repeatability, while producing the kinds of results necessary to support production or R&D.

When exploring the various levels of automation in your lab, the key areas to focus on are a combination of process and product:

  • Specimen identification and data entry (bar-coding)
  • Specimen measurement (digital measurement devices with direct input to the computer)
  • Gripping (automatic pneumatic and hydraulic powered grips)
  • Extensometry (automatic hands-free strain measurement devices, contacting, and non-contacting)
  • Specimen Handling Systems (Cartesian, robotic)
AT3 Specimen Pick-Up

Combining process and product into an automated solution contributes toward minimizing variability of results, making better use of skilled labor in the lab, increasing safety, and increasing laboratory throughput.

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