Fastener Failure Cause Analysis Method

There are many reasons for the failure of fasteners. Due to improper design, poor materials, poor manufacturing and assembly, improper use and maintenance, and the influence of environmental factors, parts will fail. How to find the cause of failure, it is necessary to master the failure analysis method of fasteners. Hex head self drilling screw, Hex flange head self drilling , Hex washer head self drilling screw, Truss head self drilling screw, Walfer head self drilling screw, Pan head self drilling screw, Countersunk self drilling screw .

There are many reasons for the failure of fasteners. Some failure reasons play a major role in a certain process, and some play a secondary role. The main reason must be found in the analysis. The focus is on the fracture of the failed part and the use Analyze the stress state, find out the typical characteristics and related factors, and conduct verification to prove the main factors that play a role in the failure process, and finally determine the cause of the failure.

The failure analysis method of fasteners can be summed up in the following aspects:

(1) Appearance and Fracture Analysis

Through the visual inspection of the failed parts, find out the typical failure characteristics, and preliminarily judge a series of factors and main factors that cause the failure. Through macroscopic and microscopic fracture inspection and analysis, it is preliminarily judged which type of failure it belongs to.

(2) Failure cause analysis

In the failure analysis, it is not enough to simply find the cause from one aspect of material, processing, and use. It should be multi-faceted, and the design, material, manufacturing, use, and assembly force of the failed part should be investigated in detail as much as possible.

(3) Force cause analysis

The failure of fasteners first considers the stress situation, and analyzes from the design stress and actual assembly stress of fasteners, the tensile stress received in service, and the tension-tension alternating stress. Whether the actual service stress is consistent, whether the assembly pre-tightening force is appropriate and too large, whether the on-site assembly uses a force-limited wrench and whether the force-limited wrench exceeds the standard, and when the force-limited wrench is not used for assembly, the assembly situation can be simulated to estimate the assembly stress and find out The difference between assembly stress and design stress.

The bolts are subjected to tensile stress and torsional stress after assembly, and may also be subjected to shear stress, vibration, alternating stress, etc. during use. Failure analysis requires a comprehensive analysis of the installation force, torsional stress, tensile stress, and alternating stress during the assembly process and use process, including stress type, amplitude, and various stress composite effects, to find out the causes of tightness. Dominant stress mechanism for firmware failure.

(4) Fracture microscopic feature analysis

The fractures of fasteners mainly include plastic fractures (including torsion fractures), brittle fractures, fatigue fractures, and hydrogen embrittlement fractures, which are analyzed according to the fracture characteristics.

A. Plastic Torsional Fracture Analysis

Consider whether the design strength is too low to meet the requirements of assembly and service stress; whether the material strength level is suitable and whether the performance indicators of raw materials are qualified, which will reduce the performance indicators of fasteners. Whether the heat treatment quenching temperature is too low, the time is too short, and whether the tempering temperature is too high, resulting in a decrease in the performance of the fastener. Comprehensive analysis of the main causes of ductile fracture and torsional fracture in assembly and use. Low design strength, low strength grade of material selection, excessive metallurgical defect grade of raw materials and unqualified surface quality, low heat treatment quenching temperature for short time and/or low part strength due to high tempering temperature, etc., are the most common The low strength cannot meet the requirements of assembly and service stress, resulting in several failure situations of plastic overload fracture or plastic torsional fracture.

B. Brittle Fracture Analysis

Consider whether the quenching temperature is too high to cause coarse structure, whether the tempering temperature is too low to cause insufficient tempering, whether the tempering operation is improper to cause tempering brittleness, whether the material is used wrongly to cause improper heat treatment process, and find out the brittle fracture from these reasons the main reason. After excluding the design and material selection errors, the quenching temperature is too high, the holding time is too long and/or the tempering operation is improper (low temperature, insufficient time, resulting in incomplete structural transformation, incomplete stress release, that is, insufficient tempering, tempering Temper brittleness due to improper cooling), among others, is the most common cause of brittle fracture.

C. Fatigue Fracture Analysis

Fatigue fracture is a dangerous failure mode, and the final fracture occurs instantaneously, which may cause equipment and personnel losses for important components. Fatigue fracture is composed of three processes: crack nucleation initiation, propagation and final instantaneous fracture. Therefore, for fatigue fracture bolts, it is necessary to carefully analyze the fracture site and fracture characteristics to find out the cause of fatigue fracture.

The reasons for the fatigue fracture at the bolt head stem may be: the reasons listed in A cause the strength of the fastener to be low; The surface strength is low. The reasons for the fatigue fracture of the bolt shank or thread may be: low strength grade of material selection, size out of tolerance, thread folding, thread on the stage, etc.

D. Hydrogen embrittlement fracture analysis

Hydrogen embrittlement fracture is a kind of brittle fracture, especially fasteners made of some high alloy materials are sensitive to hydrogen embrittlement. The reason for hydrogen embrittlement fracture may be that after surface treatment such as electroplating, galvanizing and \/surface treatment, the hydrogen is not removed or the hydrogen is not removed completely. Through the analysis of microscopic fracture characteristics, find out the difference between hydrogen embrittlement fracture and general brittle fracture, and carry out hydrogen content detection to further judge hydrogen embrittlement cracking or "hydrogen-induced hysteresis fracture".

(5) Comprehensive cause analysis method

A. Fracture source analysis: The source of crack nucleation may be material defects such as surface decarburization and oxidation, inclusions, and folding; it may also be processing defects such as tool marks, upset forging defects, damage, cracks, and folding.

B. According to the fastener production process, analyze the failure-causing links in each process such as design, material selection, and processing technology.

According to the analysis of fracture characteristics, the main cause of failure is found out in factors such as design, material, cold and hot processing technology, assembly and service stress.

C. Conduct a comparative inspection of the performance, organization, and composition of the faulty part and the same batch of products, determine whether it is a product case or a batch of product problems, and further analyze the leading cause of failure.

D. Whether the heat treatment process of fasteners, including temperature, time and atmosphere, meets the process requirements, such as whether protective gas is used in the solution treatment or quenching heating process of stainless steel superalloy fasteners, whether the purity flow rate of the protective gas meets the process requirements, and the structure Check and analyze whether the quenched and tempered structure of steel fasteners is tempered sorbite, whether the microstructure is coarse, whether the structure transformation is sufficient, and so on.

The heat treatment of fasteners is of special significance to the product quality level. Therefore, the failure analysis should be in-depth and detailed for the inspection of the heat treatment process, including: temperature, time and other process parameters; stainless steel, high-temperature alloy fasteners solution treatment or quenching heating process Process parameters such as purity and flow rate of the protective atmosphere; process parameters such as tempering temperature and time of structural steel fasteners. To investigate the heat treatment process, in addition to checking the equipment and process, it is also necessary to analyze the metallographic microstructure of the product to determine that the heat treatment process has reached a qualified grain size and metallographic structure.