There are various reasons for fasteners failures during application. Design defects, unqualified materials, bad manufacturing or assembling, wrongly use&maintenance, environment influence etc are the most common reasons for fasteners failures.How to find the specific reason for parts failure? It is necessary to know the right analysis method for them.

Many reason may caused the failure of fasteners, some are primary reasons and some are secondary reasons.It is necessary to find all the reasons lead to the failure by proper analysis. The main focus is to analysis the application force to the broken points and find typical characters and related factors, then validate whether they are the main reasons for fasteners failure, and eventually confirm the reasons for the failure.
The main analysis methods for fasteners failure can be concluded into 5 aspects below
1. Fracture points points analysis
Through visual inspection to the failure parts fracture points, we can find the typical failure characters and have initial judge on the main factors lead to the failure. The failure type can be confirmed though broken points analysis in Labs.
2. Fasteners failures reasons analysis method
It is not adequate to find failure reasons from material, working process, using etc. Other factors like design,manufacturing, application force etc should be also included.
3. Application force analysis
The first thing to consider for fasteners failure is the application force of the parts. The main force include design stress and practical assembly force, the tension force during application, alternating stress etc, the design force&actual force compliance, whether assembly preload is proper or beyond range, whether proper spanners had been applied during assembly should be paid high attention to. Before assembling by spanner, we can simulate assembly environment and estimate the difference between assembly force and design force.
The parts are influenced by tension force, torque force during assembly and shear force, vibration force, alternating force during application. The failure analysis shall be made to tension force, torque force during assembly and shear force, vibration force, alternating force during application one by one including force types, amplitude value and comprehensive force influence etc to make sure a full analysis is proceeded. So that we can find the main force caused to the fasteners failure.
4. Fracture microscopical characters analysis
The main fracture for fasteners include plastic fracture, brittle fracture, fatigue fracture, embrittlement fracture etc.
A. Plastic fracture
The main consideration for plastic fracture should be whether the design strength is too low to fulfil the strength force required by assembly and application;whether material strength grade and comprehensive mechanical properties are qualified; Whether the temperature during quench&temper is too low or too high,; whether the heat treatment period is too long. They are the most common issues lead to decrease of parts performance, thus causing fasteners failures like not proper assembly force, plastic overload fracture or plastic torque fracture etc.
B. Brittle fracture analysis
  The main reasons for brittle fracture are material coarse by too high quench temperature, insufficient temper by too low tempering temperature, temper brittleness by not proper operation, wrongly materials adopted etc. We can find the reasons for brittle fracture from above reasons. 
C. Fatigue fracture analysis
Fatigue fracture is kind of dangerous fastener failure. It happens in seconds and may cause big loss to people and equipment. The development of fatigue fracture are cracks formation, cracks enlargement and sudden breaking. For failure parts with fatigue fracture, careful analysis shall be lead to the fracture points and fracture characters and find the right reason.
  The possible reasons for bolts head fracture include lower strength than required; bad head streamlines caused lower head strength; bolts head round corner not being rolled; 
  The possible reasons for bolts shank body fatigue fracture include lower strength material be selected, dimensions beyond tolerance, folded threads etc.
D. Embrittlement fracture analysis
Embrittlement fracture belongs to brittle fracture. Some alloy steel made fasteners has hydrogen embrittlement sensitivity which has risk of embrittlement fracture. The main reasons for embrittlement fracture are not dehydrogenation or not completely dehydrogenation after zinc plating and other surface finishes. Through microscopical analysis of the fracture, we can find the difference between embrittlement fracture and normal brittle fractures. And we can further judge whether it’s Hydrogen induced cracking or Hydrogen induced delayed fracture by analysis of the Hydrogen content.
5. Comprehensive analysis method
A. Fracture source analysis: The fracture might be caused by material defects like deoxidization of surface, folding, impurity etc or processing defects like cutting trace, cold forming defects, broken, cracks etc.
B. Analysis manufacturing process like designing, material selection, working process etc and see which process caused the fasteners failure. Judging from the fracture characters, the main reasons for parts failure can be found among designing, material, heat/cool working processes, assembly, application force etc.
C. Lead comparison check on performance, organization, composite etc to the parts made from the same batch of the failure fasteners. In this way, we can confirm it’s individual case or batch cases, thus help further analysis of the fasteners failure reasons.
D. Fully check whether the temperature, period, environment etc during heat treatment meet the requirements;whether protection gas has be adopted for heat treatment for alloy steel and whether the gas flow rate meet the requirements;whether there is coarse organization; whether transformation is completed etc
The heat treatment process is critical to the fasteners quality.Failure analysis shall be carefully lead on heat treatment processes including temperature, period, protection gases, gas flow&purity etc. Besides, it is necessary to have metallographic microstructure analysis on the parts to make sure grain size and metallographic microstructure are qualified after heat treatment.

Peak Fasten is leading strictly standard during material selection, manufacturing and surface finish etc to avoid fasteners failure to the most.We are serving over 50 professional fasteners dealers for various industries around the world. For more information about their technique points and solutions, please feel free to contact us via sales@peakfasten.com. All your request shall be responded properly within 12 hours.