A frequently encountered type of mechanical failure is fracture – the breaking of one unit into two or more pieces.  When fractures happen they are often surprising, seeming to occur without warning.  Indeed they may be the result of sudden catastrophic events, but they can also be the result of extended periods of progressive and often hidden cracking.  Many categories of materials (e.g. metals, polymers, ceramics, glasses) can suffer cracking and fracture. Through our fracture analysis, we can help identify the root causes of your issue.

Crack initiation may be mechanical or chemical in nature, or a combination of these.  Once initiated, the modes and mechanisms that drive cracks to grow and break an item into pieces leave characteristic features on the fracture surfaces.  The nature of the material itself (soft and ductile versus hard and brittle, for example) plays a role in forming fracture features as well.  Studying these features on the macroscopic (visual observation) and microscopic (optical microscopy, scanning electron microscopy (SEM)) levels provides valuable information needed to trace the mechanisms, modes, origins, and crack propagation directions after a specimen has failed.  Understanding how to read a fracture helps to determine the cause of a mechanical failure and how to avoid it in the future.  Some examples of our fracture analysis are shown here. 

Two fractured threaded metal fasteners are shown.  Both materials displayed ductile properties but very different crack propagation mechanisms.  Left: This fastener suffered fatigue cracking over a period of time.  Multiple small cracks formed first with origins at a thread root, shown here around the bottom.  These cracks joined to form a single crack plane that progressed by cyclic fatigue through most of the fastener body as evidenced by the curved bands.  The arrow indicates the crack propagation direction.  Right: This threaded fastener suffered fracture relatively quickly due to mechanical overload.  The fastener was aggressively overtightened creating torsional stress that was relieved by fracture at a thread root.  The arrow shows the rotation direction at the time of failure. 

On rare occasions tempered plate glass may shatter due to changes in the volume of internal nickel sulfide inclusions.  In this example of fracture analysis, a macroscopic “butterfly” crack pattern (upper image) shows the location of the failure origin.  Microscopic brittle chipping of the glass surrounding a nearly spherical nickel sulfide inclusion is shown in these SEM images of mating butterfly fracture faces (inclusion pocket shown at the left, actual inclusion shown at the right).  Longer cracks emanated essentially instantaneously from the microscopic chip cracks resulting in the sudden catastrophic failure of this glass windowpane.

This image shows the fracture face of a “clipped” natural ruby.  Rough rubies are clipped to remove unwanted features such as visually unappealing inclusions.  The clippers were applied to the right side as oriented here.  Rubies are hard and brittle; they fracture rapidly under sufficient stress and may cleave into two or more relatively large pieces or shatter.  In this case the larger piece is shown, the remainder shattered into numerous tiny pieces.  The white arrow points to a small chip (“clamshell” shape), the base of which appears to be the origin for most of this fracture face.  Crack propagation directions (black arrows) emanating from that origin are identified by “twist hackle” features. There are two more chips to the right of the one indicated by the white arrow. These appear to be origins for smaller areas of the fracture at the right side of the clip face. There also an origin on the bottom edge; some hackle features are emanating from an iron oxide inclusion (red arrow).

SEM images of two positions along the edge of a fractured stainless steel tool are shown, illustrating locally ductile and brittle fracture zones.  Ductile fracture is characterized by deformation of grains (e.g. lengthening and thinning) and microvoid or dimple formation before separation; example ductile zones are indicated here by the yellow arrows.  Brittle fracture can be transgranular (through a grain) or an intergranular (between grains) event.  Both types of brittle fracture are visible here.  Example grains exhibiting transgranular fracture are indicated by red arrows.  These grains sit in transition areas between ductile zones and brittle intergranular cracking zones.  Intergranular cracking is defined by clear angular cracking along grain boundaries, and undistorted grain faces.  Highly localized and irregularly distributed ductile/brittle zones can occur in materials subjected to improper heat treatments or embrittling environments, for example.  

Polycarbonate is frequently used in laminated glazing for anti-blast and anti-ballistic applications, zoo environment enclosures, etc.  While its transparency and mechanical properties make it an excellent choice for these uses, it can be degraded by exposure to a number of chemicals.  A result may be local swelling or microcracking (crazing) of the material, for example.  Swelling and crazing can create localized stress or can unite with existing applied or internal stress to initiate cracking further into the polycarbonate body.  This is commonly referred to as environmental stress cracking.  The fracture face shown here began as a series of cracks along the bottom edge as well as a “craze” zone along the face (left side as shown here) near the bottom, caused by a damaging chemical exposure.  The enlarged image shows a transition region far from the origins, where the relationship between applied or internal stresses and the aggressive chemical agent changed, resulting in new fracture features but continuing progression of the crack.

Wire ropes, ribbons, and braids are often used for high strength reinforcement applications.  These stainless steel wires were removed from a braided tube reinforcement in a flexible pipe connector.  The reinforcement failed by fracture of numerous individual wires.  These wires should be ductile, and the “necking down” elongation demonstrated by most of the fractured ends shows that most were.  Examples are circled in the upper left image.  However, in the location where the failure initiated the wires had become brittle.  Brittle failure is characterized by cracking rather than flexing when the wire is stressed, as shown in the upper right image.  Fractured brittle wires have sharp flat ends with little or no elongation, as shown in the bottom images.  In this case a sufficient number of wires eventually fractured due to embrittlement, compromising the strength of the overall reinforcement and leading to catastrophic failure. 

Please contact us about your fracture analysis concerns.