Ultimate Guide to Measuring Hardness: Best Practices for Reliable Almen-Strip Testing

Author:  Kurumi Narasaki
Source:  The Shot Peener magazine, Vol 40, Issue 2, Spring 2026
Doc ID:  2026014
Year of Publication:  2026
Abstract:  
INTRODUCTION Accurate hardness testing is critical for quality control in shot peening and related processes. SAE-J422 dictates using Rockwell to measure hardness (HRc for A strips and HRa for N strips) but provides no detail on how to measure hardness. Recent studies and production data reinforce three core principles for obtaining consistent and specification-compliant results on Almen strips: 1. Effect of measurement surface orientation 2. Influence of anvil size on hardness readings 3. Validity of hardness conversion methods SURFACE ORIENTATION: CONCAVE OR CONVEX UP? Dr. David Kirk (2024) evaluated the pre-bow of Almen strips to understand how strip orientation affects hardness readings. His study examined multiple production lots, testing 10 strips per lot with three hardness measurements taken on each strip. Measurements were compared between the convex and concave sides of the strips. Based on this work, Kirk concluded that Almen strips should be placed curve-up, with the indenter contacting the concave side. This orientation reduces elastic flattening during the test and leads to more accurate and repeatable hardness readings. To further confirm these findings, additional testing was conducted on over 100 samples. For each strip, 3 hardness measurements were taken under four conditions: