Failure analysis
The extensive skills and equipment of Unilab Laboratori Industriali s.r.l. are at your service for the performance of component failure analysis.
Failure analysis: what’s the purpose of failure analysis?
Failure Analysis is an analysis process that aims to determine the causes of malfunction or failure of a product or component. The aim of this process is to provide information to the customer to prevent a failure from reoccurring.
Failure analysis: who requests this service?
Fenomeno corrosivo su struttura di carpenteria verniciata
Following the failure of a mechanical component, the user incurs damage. This damage then needs to be quantified and the cause of it needs to be determined.
A failure analysis may therefore be requested:
- by the designer seeking to establish the design fault that has caused the failure;
- by the component manufacturer seeking to establish whether their manufacturing methods may have introduced critical factors capable of causing the failure;
- by the supplier of the raw material wishing to evaluate the quality of the material used in the production of the failed component;
Failure analysis: what does failure analysis consists of?
These types of investigations are typically multidisciplinary and can involve not only an examination of production processes but also design and installation, requiring analysis ranging from the study of the process to the characterisation of the materials, up to and including analysis of the operating conditions.
We begin our investigation by collecting information about operating conditions and information relating to the failure of the component at the moment it failed. Knowing the load and work history of the failed component is essential to help guide our engineer’s investigation and understanding of the nature of the breakage. On-site photos, analysis of designs, study of manufacturing processes, collection of operating data.
The next step is to carry out non-destructive analyses which allow us to obtain important information without in any way altering the state of the faulty parts (broken surfaces, corroded, degraded or worn parts). PND and tomography techniques are the main methods we use in this phase.
We then move on to destructive analyses, which enables us to gain a deeper understanding, but which require adequate preparation of test samples; cross-sectional metallographic examinations, mechanical tests, hardness tests, surface or full-depth chemical analyses, etc.
Most of the time, the sum of all these analyses provides enough information to reveal the reason for the failure.
Failure analysis: laboratory tests and the tools used to analyse failures
The primary tools for conducting failure analysis are:
- Tomography, radioscopic inspections, inspections using penetrating liquids, visual examinations, if necessary with macrographs for non-destructive investigations
- Electron microscopy (SEM/EDX) for the morphological analysis of fracture surfaces and precise chemical analysis in areas of interest (for example when corrosion is a factor)
- Optical microscopy, for the performance of fractographic analysis and the microstructural characterisation of the material to detect any anomalies or defects that may be present
The result of these activities is presented as a technical report that describes the reason for the damage and, where possible, presents potential resolutions/improvements or simply usage precautions to ensure the phenomenon doesn’t happen again.
CASE STUDIES: ALCUNI ESEMPI DI FAILURE ANALYSIS
Shell and tube heat exchanger
Cosa è successo:
Cedimento di scambiatore di calore a fascio tubiero
Analysis performed/conclusions drawn:
Chemical analysis of the material: In both cases the material is identified as AISI 347 (UNS S34700) according to ASTM A240
SEM-EDS microanalysis: SEM-EDS analysis conducted on the products present on the surface of the retaining plate indicated an operating environment rich in chlorides.
Metallographic analysis: base material – anomalous plastic deformation of the retaining plate
Metallographic inspection: analysis of the damage – Presence of ramified hairline cracks, typical of stress corrosion cracking (SCC)
Report on the cause of failure:
Failure caused by stress corrosion resulting from the simultaneous presence of anomalous internal stresses (high plastic deformation found in the plate) and a harsh working environment (products rich in chlorides).
Possible solutions
– Adequate storage
– Improved working environment
– Elimination of internal stresses (stress relieving or solubilisation)
What information do we have:
● Material: UNS S34700 (AISI 347)
● Type of damage: breakage of a retaining plate (6 mm thick)
● Additional information:
– 3 months in operation
– no heat exchanger leaks,
no external stresses
Worm screw failure
Cosa è successo:
Failure analysis di vite senza fine
What information do we have:
● Material: 44SMn28 steel
● Type of damage: brittle breakage of a screw thread
Analysis performed/conclusions drawn:
Chemical analysis of the material: Material steel identified as 44SMn28 according to EN 10087
Fractographic examination: Strongly oriented ductile fracture
Tensile test: Material complies with the customer’s technical specifications (44SMn28 cold drawn, untempered steel)
Metallographic tests:
● inclusional state – Presence of non-metallic MnS inclusions of considerable size oriented in the direction of mechanical processing
● microstructure of the material – Microstructure consisting of ferrite and perlite arranged in bands. Absence of a neutral hardening heat treatment
● Fracture profile without metallographic etching – «Linear» fracture in the same direction as the non-metallic inclusions
● Fracture profile with metallographic etching – Plastic deformation in the fracture zone (mechanical failure due to overload). The fracture appears to cross the ferritic/pearlitic bands
Report on the cause of failure:
Metallurgical factor: Ferritic/pearlitic band structure (absence of a neutral hardening treatment) and high level of inclusions (large and oriented inclusions).
– Excellent longitudinal resistance of the material but poor transversal resistance
Geometric factor: particular shape of the thread (high thread height, large pitch) with consequent orthogonal stresses to the mechanical machining direction of the component
– high flex stresses on the thread
Possible solutions
– Evaluate whether the geometry of the component (with consequent flex stresses on the thread) allows free-machining steels to be used.
– Carry out in any case a neutral hardening heat treatment (to provide a more homogeneous structure and enhanced material toughness).
Fractured luminaire
Cosa è successo:
Corpo illuminante fratturato
What information do we have:
● Material:
Die-cast aluminium alloy
● Type of damage:
failure in the vicinity of the screw fixing holes of the luminaire support
Analysis performed/conclusions drawn:
Chemical analysis of the material: Material identified as aluminium alloy EN AC-46500 AlSi9Cu3 (Fe) (Zn).
Fractographic examination: Brittle fracture with the presence of oriented facets
Metallographic tests:
● base material – Microstructure consisting of alpha phase surrounded by a eutectic and the presence of large needle-shaped precipitates
● fracture profile – Morphology of the failure profile typical of a fragile fracture, with no plastic deformation and evidence of the presence of needle-shaped platelets
Report on the cause of failure:
Excessive intrinsic fragility of the material the support is made of due to its metallurgical state (presence of coarse, needle-shaped crystals).
– the presence of these very fragile crystals constitutes a dangerous discontinuity in the metallic matrix. During production, this alloy requires a “modification” treatment designed to reduce the size and to round off the silicon crystals in the matrix; in the case in question, this treatment doesn’t seem to have been completely successful.
Broken spring
Cosa è successo:
Molla fratturata
Analisi eseguite / cosa abbiamo capito:
Esami macrografici:
● Presenza di abrasioni in corrispondenza dell’intradosso della curvatura originate nel processo
di produzione
● Aspetto macrografico della frattura tipico di una rottura per fatica
● Linee di spiaggia chiaramente visibili.
● Presenza di una discontinuità centrale (ratchet mark).
Esame frattografico al SEM:
● Analisi della superficie di frattura a elevati ingrandimenti.
● Individuazione di due punti di innesco principali in corrispondenza dell’intradosso e di due
inneschi secondari all’estradosso.
Esame micrografico:
● Microstruttura fortemente incrudita e orientata nel senso di trafilatura, priva di anomalie
metallurgiche.
Relazione sulla causa del cedimento:
– Rottura innescata all’intradosso a causa della presenza di irregolarità superficiali dovute al
processo di fabbricazione.
– Sviluppo di due fronti di cricca che hanno generato la discontinuità centrale (ratchet mark)
con propagazione a fatica della rottura.
– La diminuzione della sezione resistente ha causato un innalzamento dello stato di tensione
con conseguente innesco di cricche anche all’estradosso.
Analysis performed/conclusions drawn:
Macrographic examinations:
● Presence of abrasions in correspondence of the intrados curve caused during the production process
● Macrographic appearance of the fracture typical of a failure caused by fatigue
● Clearly visible beach marks.
● Presence of a central discontinuity (ratchet mark).
SEM fractographic examination:
● Analysis of the fracture surface at high magnification.
● Identification of two main trigger points at the intrados and two secondary triggers on the extrados.
Micrographic examination:
● Strongly work hardened microstructure oriented in the direction of wire drawing, no metallurgical anomalies.
Report on the cause of failure:
– Breakage triggered in the intrados due to the presence of surface irregularities caused during the manufacturing process.
– Development of two crack fronts that generated the central discontinuity (ratchet mark) with fatigue propagation of the break.
– The decrease in the size of the resistant section has caused an increase in the state of tension with consequent triggering of cracks along the extrados as well.
What information do we have:
● Material: AISI 302 stainless steel
● Type of damage: failure at one of the curvatures
● Additional information: failure after 3700 km; component obtained by drawing and bending; presence of components with similar breaks
Corroded tank
Cosa è successo:
Serbatoio corroso
What information do we have:
● Material:
5xxx series wrought aluminium alloy
● Type of damage: Extensive corrosion of the metal underneath the installation sheet
● Additional information: Tank installed on a wheeled vehicle for industrial transport by means of an EPDM sheet
Analysis performed/conclusions drawn:
Visual and macrographic tests:
● Presence of violent corrosion in the form of pitting of variable shapes and extensions
● Presence of evident dark-coloured deposits caused by corrosion (in relief) and opaque white colouring (inside the pitting)
SEM observations and EDS microanalysis:
● Following chemical analysis, the material was confirmed to be EN AW-5754 aluminium alloy, as stated.
● The deposits inside the pits were found to be linked to aluminium sulphate Al 2 (SO 4) 3.
● At some sites within the pits, it could be seen that corrosion was not purely selective in nature.
Micrographic analysis:
● The corrosion presented as subsurface and undercutting pitting with development in all directions and with a depth that in some cases was found to be almost half the thickness of the wall.
● The microstructure of the base material was found to be a typical aluminium alloy of the 5000 series free from metallurgical anomalies.
● The observations confirmed that the corrosive attack was not selective in nature.
Report on the cause of failure:
– In polluted environments typically found of many industrial areas, the sulphur dioxide (SO2) present in the environment plays an important role in corrosion processes and gives rise to the formation of aluminium sulphate (a salt which is highly damaging for aluminium and which tends to increase the rate of corrosion). It’s also possible that the sheet (sulphur vulcanised EPDM) has released substances and compounds that have intensified the observed phenomenon.
– In addition, the presence of contact areas between the sheet and tank have favoured the corrosion mechanism known as Crevice Corrosion (corrosion in confined spaces), which has led to a severe and rapid degradation of the material, especially in curved areas (greater state of deformation).
