A pendulum impact tester looks simple: lift a hammer, release it, and read how much energy the specimen absorbed. In practice, many small factors affect the reading, from friction in the bearings to the shape of the striker. That is why standards require the machine to be verified, both by measuring its parts and by testing reference specimens. For Charpy pendulum machines, the verification method is set out in ISO 148-2, with corresponding Indian standards in the IS 1757 series.
Two kinds of verification
Direct verification checks the machine's construction and measuring system against the dimensional and mechanical requirements of the standard. Indirect verification tests certified reference specimens and compares the measured energy with their certified values. Both are needed: direct verification shows the machine is built and set up correctly; indirect verification shows that, as a whole, it gives correct results.
What direct verification covers
- Foundation and installation: the machine must be rigidly mounted and level, because frame movement absorbs energy.
- Pendulum: potential energy, effective length, centre of percussion and the position of the striker.
- Striker: geometry and radius of the striking edge, which differ between strikers defined in ISO 148 and ASTM E23.
- Anvils and supports: span, radii, angle and alignment relative to the striker.
- Impact velocity: derived from the drop height.
- Friction and windage losses: measured by free swings of the pendulum and used to correct readings.
- Indicating device: accuracy of the dial or encoder reading of absorbed energy.
What indirect verification covers
Certified reference specimens, typically at low, medium and high energy levels within the machine's range, are tested under the conditions specified on their certificate. The mean result and spread at each level are compared with the certified value and the tolerances in the standard. If the machine fails, the cause is found through direct checks and corrected before the machine is used for reportable tests.
Why striker geometry matters
ISO 148 and ASTM E23 define strikers with different edge radii (commonly referred to as 2 mm and 8 mm strikers). Results with different strikers are not interchangeable, especially at higher energies. Make sure your machine has the striker your customer's standard requires, and that reports state which one was used.
Day-to-day checks
| Check | When |
|---|---|
| Free swing (zero) reading | Daily or before each test series |
| Visual check of striker and anvils for damage or debris | Daily |
| Fixings and foundation bolts | Periodically |
| Guarding and safety interlocks | Daily |
| In-house control specimens | At a frequency set by your quality system |
Factors that cause drift
- Wear or chipping of the striker edge or anvil radii
- Loose foundation bolts or a cracked base
- Increased bearing friction
- Damage to the indicator or encoder
- Specimens not centred on the anvils, which is a procedural rather than machine fault
Low-temperature testing
Many specifications ask for impact tests at sub-zero temperatures. The time between removing the specimen from the cooling bath and impact is limited by the standard, so the layout of the bath, tongs and centring device matters. A centring tool helps place the notch correctly and quickly.
Equipment
Akuracy offers a pendulum impact testing machine, a Charpy impact testing machine and the digital impact testing machine RI-300 E. For machines that need checking or calibration, see calibration of material testing machines.
Preparing for verification
Before the verification visit, clean the machine and anvils, check the foundation bolts, confirm the free-swing reading is stable, and make sure the specimen centring device and cooling equipment are available. Have the previous verification report at hand so the results can be compared. If you test to both ISO and ASTM methods, tell the verifier which strikers and energy ranges are in use so that each is covered.
Interpreting the report
The verification report should identify the machine, the standard and edition, the direct verification results for each checked item, the reference specimen lots and certified values used, the measured results and whether the machine met the tolerances. Note any items marked as adjusted or repaired. If the indirect verification bias is close to its limit, plan a closer look at the striker, anvils and bearings before the next interval.
Record keeping between verifications
Daily free-swing readings and periodic results on in-house control specimens, plotted over time, give an early warning of changes in the machine. This is simple to set up and helps demonstrate control to auditors.
Key takeaways
- ISO 148-2 requires both direct and indirect verification.
- Striker geometry must match the standard you test to.
- Daily free-swing checks and control specimens catch drift early.
- Re-verify after relocation, repair or striker change.
Frequently asked questions
Is indirect verification enough on its own?
No. ISO 148-2 requires direct verification of the machine as well. Indirect verification confirms the overall result, but direct checks show the machine meets its construction requirements.
How often should an impact tester be verified?
Follow the interval in the standard and your accreditation requirements; many labs verify annually, and after relocation or repair.
Can I use ASTM and ISO strikers interchangeably?
No. They have different edge radii and can give different results. Use the striker required by the test standard.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.