CASE STUDY

From Fragments to Answers:

An Insightful Alternative to Hardness
Numbers for Failure Analysis
In collaboration with
Based on the published paper “Use of hardness, PIP and tensile testing to obtain stress-strain relationshipsfor metals”,
by T.J.F. Southern, J.E. Campbell, C. Fang, A. Nemcova, A. Bannister, & T.W. ClyneAs hardness numbers cannot give full stress-strain curves, finite element modellingcannot always be conducted accurately without further information.

Challenge
Where machining tensile specimens for testing
is not an option, hardness is often considered

as a solution. This poses a significant limitation;
while hardness testing offers advantages
such as affordability, speed, ease of use,and suitability for testing small specimens, ahardness number is not a fundamental material
property. Furthermore, practitioners are limited

These standards contain warnings for the user,

property values, as hardness numbers cannot‘Conversion from one hardness value to a tensile

to conversions into a limited set of material
give full stress-strain curves, which meansthat without further information finite element
modelling cannot always be conductedaccurately.

Hardness numbers are produced by measuring
the lateral extent or depth of an indent

such as in BS EN ISO 18265:2013 where it states,strength value involves uncertainties whichmust be taken into account.’ These uncertainties
include the relationship between hardness andUTS, which is only available for certain specificcategories of alloy, which limits the use of theseprocedures to these metals.

performed at a known load and applying a

Despite this, hardness data is often used

These results are then frequently converted into

as a material quality release test. This reliancestandard equation for the specific indenter used.
UTS (ultimate tensile strength) and sometimes,albeit less frequently, yield stress values.

Conversions can only be performed using

empirical relationships, some of which can befound in standards published by ASTM and ISO.

extensively to inform design/safety decisions andon hardness numbers alone poses a problem

for failure analysis and end users alike as it canlead to the need for approximations as well as aloss of potential significant information about thematerials’ behaviour.

2 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

Objectives

The aim of this case study, conducted inconjunction with HSE Science Division, was todetermine whether PIP testing could provide

the insights required for better understandingof the reasons for failure of components, aswell as overcoming the challenge of testing offragments from the failed component due tolimited availability of the material.

In this study, we compare hardness, PIP, andtensile testing to obtain mechanical properties

and stress-strain relationships for a wide rangeof metals. Tensile testing is often considered thebenchmark for determining these properties,

however it does come with some drawbacks;these include the size of sample required, theuniaxial nature of the results and the need tomachine specific geometries for testing. In

comparison to this, hardness testing is much

more convenient and can be performed quickly
on a wide array of samples. These advantagesof hardness testing are shared by PIP. The

difference between these techniques is in theresults produced; with PIP producing a full stressstrain curve, similar to tensile testing, while

hardness testing generates a single number forthe material, from which other parameters canbe determined based on empirical relationships.
The ability for mechanical characteristics tobe extracted from this hardness number has

been an area of substantial research with many
empirical relationships found and implemented
in different situations.

3 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

Materials

12 different materials were used in this work:
A. Waspaloy
B.

Aluminium 7075

C. Martensitic Steel
D. Carbon SteelE.

316L Stainless SteelF.

S355 Low Alloy steel

G. 4340 Low Alloy SteelH. 2D Steel
I.

Rail SteelJ.

Plain Carbon SteelK.

As-received Copper

L.

Annealed Copper

Tensile specimens had 5 mm diameter and weretested with an extensometer gauge length of

25 mm. Indentation samples were in the form ofsmall samples, with a thickness of at least about
5 mm and lateral dimensions of the order of 1020 mm. Surfaces for indentation were polished
to at least a 6 µm finish.

4 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

Measurements

The mechanical properties were measured using
an Instron 3369 loading frame for uniaxial testing
specimens and a PLX-Benchtop, a compact

indentation-based device for PIP testing. PIP uses
an accelerated inverse finite element method
to infer accurate stress-strain curves fromindentation test data.

The PLX-Benchtop comes with both 2 mm and
1 mm diameter indenter tips, allowing stress-strain measurements to be taken as close as

5 mm and 2.5 mm apart, respectively. The test
itself is fully automated and takes less than 5

minutes without the need for separate samples
or extensive sample preparation. The larger
diameter indenter was used in this work fortesting all samples.

Hardness testing was carried out using aBuehler Wilson VH3300 automatic Vickers

hardness tester, with a load of 10 kg. The indent

diagonal was taken to be the average of the twomeasurements (made via the built-in optical
microscopy system). In general, these two

diagonals were very close, reflecting the isotropyof these materials, and also their relatively

fine grain structures – even with the hardermaterials, the indents still tended to straddle

at least several grains. These measurements

were repeated in 5 different locations for eachmaterial.

Plastometrex PLX-Benchtop

5 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

Results

In this case study, results from uniaxial tensile

testing were compared with those obtained fromPIP tests and with hardness tests converted toUTS values. PIP testing provides the full stressstrain curve of each material directly as partof its data output, however hardness numberscannot directly be compared to full tensile tests

method can only be used on specific subsets ofhardness values to UTS values.

whereas PIP can be applied to any metallic

as empirical relationships are used to convert

One example of these empirical relationships isfound in BS EN ISO 18265:2013 for unalloyed andlow alloy steels and cast steels. It is important toalloys, such as ‘unalloyed and low alloy steels’material and can account for the effects of

microstructural changes which result in different
mechanical responses of the material.

note the text of this standard which describes

This standard method was applied to the subset

this relationship: “While hardness-to-hardness

is permitted. PIP tests were carried out on all

the caution that should be used when applying
conversion involves considerable scatter andsystematic errors, conversion of hardness totensile strength values produces even greater
scatter. One reason for this is that a great

uncertainty can be affected by microstructuralchanges (e.g. resulting from heat treatment
or cold working) within even the same typeof steel.” This is described in the standard as

scatter bands of ± 25 Hv10 for the hardness and± 85 MPa for the UTS. In addition, this standardof materials in this study for which the conversion
the samples, and the UTS values obtained fromthese two methods are compared on a unity

plot with the tensile measurements in Figure 1.

Perfect agreement with tensile testing is indicatedby the solid unity line with deviations of ±10%shown by dashed lines. PIP values of UTS align2.96 percentage points more closely with tensile

measured values than hardness converted values,
having a mean absolute percentage error of 3.8%
compared to 6.8% for hardness testing.

Figure 1: Plot of inferred UTS from PIP andhardness testing using BS ISO-18265:2013
against corresponding tensile tests.

6 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

Results

While hardness conversions can be used onlyto estimate UTS values, PIP can also provide thefull stress-strain curve including yield stress andhardening behaviour. Figure 2 demonstrates theagreement between the full stress-strain curves
from tensile testing and PIP for a subset of thematerials in this work covering a wide varietyof properties. In general, the level of agreement
between PIP-derived and directly measured

tensile curves is very good. This full stress-strain

curve cannot be obtained from a hardness test,

which can result in blind spots when trying to use
hardness values to understand the mechanicalproperties of metallic components.

Figure 2: Nominal stress – nominal strain curves for selectedmaterials, from tensile testing and from PIP. The latter are
shown only up to the onset of necking (peak in the plot).

7 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

Outcomes

As demonstrated, hardness testing

Find out more about PIP Testing

within failure analysis, may be used as

a complementary technique to tensile

testing. When use of tensile testing is notpossible, hardness numbers can provide

useful indicators of the resistance to plastic
deformation. Empirical correlations fromstandards can be used to convert thesehardness numbers to UTS values, for a limited
subset of alloys. These standards cannot beapplied universally and detail that conversions
should ‘be used with caution’.

In contrast to hardness testing, PIP testing

provides the full stress-strain curves of themetal being tested, including the yield stress.

This results in detailed quantitative informationand higher levels of confidence in the yieldstress, ultimate tensile strength and complete

stress-strain curve of metal parts. Furthermore,when it comes to applying the learnings fromfailure analysis work, the PIP data can also beused as a reliable and important input for finite
element modelling to understand if high stressareas are at risk of failure.

8 ] From Fragments to Answers: An Insightful Alternative to Hardness Numbers for Failure Analysis

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